Patent Application
20060202292
The present invention relates to a magnetoresistive medium comprising a crystalline substrate and a thin film thereon.
Magnetoresistive media are media whose resistance to an electric current is sensitive to an external magnetic field. Such media are widely used in information and communication technologies e.g. in disk drive read heads, magnetic tape read heads, random access memory devices and in numerous other applications. Magnetoresistive media are also commonly used as sensors for a magnetic field in applications that are not directly related to the domain of information and communication technologies, e.g. in the automotive and aviation industries, security devices, goods labelling, position encoders, medical devices and numerous other applications. The best known magnetoresistive material is permalloy.
U.S. Pat. No. 4,949,039 (Gruenberg) describes a magnetic field sensor. The sensor comprises of a stack of ferromagnetic layers separated in such a way that the magnetisation of the layers can be rotated with respect to each other. Typically magnetisation of the layers is switched from mutually parallel to mutually anti-parallel. As the direction of the layers' magnetisation is reversed, the resistance of the stack changes. At present this sensor known as the spin valve, is commonly used in read heads of computer disk drives. U.S. Pat. No. 5,159,513 (Dieny et al) and U.S. Pat. No. 5,206,590 (Dieny et al) describe improvements to the magnetoresistive sensor based on the spin valve effect utilising two ferromagnetic layers. U.S. Pat. No. 5,422,571 (Gurney et al) also describes an improvement of the spin valve sensor. U.S. Pat. No. 5,578,385 (Saito et al) describes yet another improvement to the spin-valve sensor based on a stack of magnetic and nonmagnetic layers.
Virtually any spin valve sensor utilises coupling of one of the ferromagnetic layers to an antiferromagnetic layer. Such coupling, known as exchange pinning or exchange bias, has been well known for decades. It has been utilised in magnetoresistive sensors preceding the spin valve. For example, U.S. Pat. No. 4,755,897 (Howard) describes such a magnetoresistive sensor. This patent teaches the issues related to biasing a ferromagnetic layer by means of exchange interaction coupling with an antiferromagnetic layer.
There are inventions focusing on the utilisation of exchange bias in spin valve sensors. For example, U.S. Pat. No. 5,958,611 (Ohta et al) describes the magnetoresistance effect element based on a multilayered film. In this invention one of the ferromagnetic layers is pinned by the exchange interaction to a layer of antiferromagnetic oxide. It is interesting to note that according to the inventors, the roughness of the antiferromagnetic layer must be small in order to achieve good magnetoresistance sensitivity.
U.S. Pat. No. 5,923,504 (Araki et al.) is another invention describing exchange pinning in a spin-valve-like magnetoresistance device. In this invention one of two ferromagnetic layers is pinned by exchange to an antiferromagnetic layer of FeOx. In some embodiments of this invention there is an oxygen-blocking layer between the pinning layer and the pinned ferromagnetic layer.
There are also inventions in which magnetisation in one of the two layers is fixed without the use of the antiferromagnetic layer. For example, U.S. Pat. No. 5,867,025 (Allenspach et. al.) describes a magnetic spin valve utilising a terraced substrate. The approach in this U.S. Pat. No. 5,867,025 capitalises on the finding by the inventors that when a Co film is deposited on a Cu terraced substrate there is a critical film thickness above which the direction of magnetisation of the film is pinned. This is known as step-induced anisotropy. Therefore, the magnetic spin valve proposed in this U.S. Pat. No. 5,867,025 comprises of two ferromagnetic layers separated by a nonmagnetic metal film like in a conventional spin valve with the difference that one of the two ferromagnetic layers is thinner than the critical thickness and the other one is thicker than it. In this arrangement, magnetisation in the first of the two layers can freely rotate and in the second layer it is pinned.
It is commonly held that surface roughness in a spin valve magnetoresistive medium should be small (Conference of the Materials Research Society (Dec. 1-5, 2003, Boston, Mass., USA, talk FF 5.3 by D. Zhou et. al).
Another recent class of devices sensitive to the external magnetic field are called magnetic tunnel junctions. U.S. Pat. No. 5,629,922 (Moodera et al) and subsequently U.S. Pat. No. 5,835,314 (Moodera et al) describe such electron tunnel junction devices. The devices include two ferromagnetic electrodes separated by a dielectric layer to form a tri-layer tunnel junction. Magnetisations of one of the ferromagnetic electrodes can be reversed with respect to the other. As the electric current passes between the two magnetic electrodes, the current value is sensitive to the relative orientation of the magnetisation directions in them. Therefore, the direction of magnetisation of one of the layers with respect to the other one can be identified. U.S. Pat. No. 5,835,314 further suggests that the greatest magnetoresistance effect is obtained when the tunnelling resistance is comparable to the electrode resistance. U.S. Pat. Nos. 5,734,605 and 5,978,257 (Zhu et al.) describe a tunnel junction element similar to the one described in U.S. Pat. No. 5,629,922 and further teaches how it could be utilised in a memory cell. U.S. Pat. No. 6,335,081 (Araki et al) describes an improved tunnel magnetoresistance effect element based on a multilayered film with a tunnel barrier having reduced the roughness of the layers. In most magnetic tunnel junction devices magnetisation of one of the two ferromagnetic layers is pinned by exchange to an antiferromagnetic layer. There are inventions that deal with improvements of the pinning characteristics. For example, U.S. Pat. No. 5,764,567 (Parkin) describes a magnetic tunnel junction device consisting of two ferromagnetic layers separated by a dielectric barrier layer. Magnetisation in one of the ferromagnetic layers is pinned to the antiferromagnetic layer. This invention teaches that an extra non-ferromagnetic layer should be added between the dielectric barrier layer and the second ferromagnetic layer in order to reduce the coupling between the fixed and free ferromagnetic layers.
U.S. Pat. No. 6,365,286 (Inomata et al) describes a magnetic element and magnetic memory device utilising spin dependent tunnelling between a ferromagnetic metal and a ferromagnetic-dielectric mixed layer. The tunnelling occurs through a dielectric layer of Al2O3. The tunnel current depends on the orientation of magnetisations in the two layers: the ferromagnetic metal layer and the ferromagnetic-dielectric mixed layer. This patent also describes structures comprising of three ferromagnetic layers: ferromagnetic metal layer, ferromagnetic-dielectric layer and again ferromagnetic metal layer all separated from each other by dielectric layers.
Yet another group of magnetoresistive media includes media in which the magnetoresistive effect is enhanced by utilising the array of particles or grains of different materials. The inventions that fall within this group are usually concerned with ways of forming such particulate media. For example, U.S. Pat. No. 6,015,632 (Chambliss et al) describes a self-assembled lateral multilayer for a magnetoresistive sensor. This invention utilises the finding that in the case of growth on Mo(110) substrate by simultaneous co-deposition of a ferromagnetic metal such as Co or Fe and nonferromagnetic metal (Ag) stripes of different materials are formed by self assembly. Although the patent contains no magnetic measurements, the inventors expect that neighbouring stripes of ferromagnetic metal should be aligned antiferromagnetically in the absence of the external magnetic field. The patent disclosure is limited to analysis of STM topography data. It remains to be seen what magnetoresistance effect if any could be achieved in such a lateral multilayer. The invention is further limited to co-deposition of immiscible materials. U.S. Pat. No. 5,858,455 describes the method of forming of such a multilayer by simultaneous co-deposition.
Another representative invention related to the same group of magnetoresistive devices and materials is described in U.S. Pat. No. 5,818,323 (Maeda et al.). According to the invention, the device is comprised of Ag/Co composite. The composite is a nonmagnetic conducting matrix of Ag that includes ferromagnetic anisotropic grains. The grains and the matrix are made of immiscible metals. The magnetoresistance material is produced by simultaneous co-deposition of the two metals. Further stripes are formed in the composite film by means of photolithography. The purpose of forming the stripes is to reduce the coercivity field and enhance magnetoresistance of the film. The invention further describes a multilayer of Cu and another Co/Cu composite. In the Co/Cu composite the Co grains are embedded into the Cu matrix and again the grains are anisotropic. Another U.S. Pat. No. 5,738,929 (Maeda et al.) describes a magnetoresistance effect element. The element comprises of ferromagnetic areas formed by photolithography covered by a non-magnetic metal overlayer. Again the magnetic areas and the nonmagnetic overlayer are made of non-soluble materials, e.g. Co and Cu. In a further U.S. Pat. No. 5,656,381 (Maeda et al.), the inventors utilise a similar composite film of immiscible materials. The film forms the magnetoresistive element. To reduce the operating magnetic field of the element, there are at least three areas of soft magnetic films magnetically coupled to the composite film. The areas are typically formed by means of photolithography. In U.S. Pat. No. 5,736,921 (Maeda et al), the magnetoresistive element is based on a similar composite material of immiscible nonmagnetic and magnetic materials. The inventors further suggest that forming a gradient of concentration of magnetic particles in the nonmagnetic matrix creates a positive effect on the magnetoresistance properties of the element. U.S. Pat. No. 5,585,196 (Koichiro et al.) is another example of a magnetoresistance effect element that utilises the particles of magnetic metals (Fe, Co or Ni) dispersed in a matrix of noble metal. The magnetic and nonmagnetic metals are again immiscible and again the composite is produced by co-deposition of the two metals.
U.S. Pat. No. 6,168,845 (Fontana at al) describes another class of patterned magnetic media and method of making the same. The media is comprised of magnetic and nonmagnetic zones. These are obtained by selective oxidation process. Selective oxidation is achieved by subjecting a magnetic layer to oxygen plasma through voids in a patterned mask made by embossing and the reactive ion etching process. The media are not used as a sensor of magnetic field but rather as an information carrier for storage disks of high density. One of the advantages is that the media has minimal topography.
U.S. Pat. No. 6,387,530 (Chunling Liu et al) also describes a patterned magnetic media for high-density information storage. The media is formed by thermally induced phase transition in an initially amorphous layer of e.g. Ni—P. By heating the layer using a heat/light source the particles of at least a partially crystalline ferromagnetic material are formed in it. It is proposed that the multiplicity of light sources can be used to create an array of particles simultaneously.
Usually substrates for most magnetoresistive devices preferably need to be flat. However, there are inventions in which the magnetoresistive media are grown on non-flat and vicinal substrates. For example, U.S. Pat. No. 6,330,135 (Manako et al.) describes a magnetoresistance effect element based on a ferromagnetic oxide thin film grown on a stepped layer oxide. This invention utilises the fact that the crystal structure of some magnetic oxides (e.g. SrFeO3) is such that antiphase boundaries are formed at the step edges of the substrate. The antiphase boundary is a crystal defect associated with the break in the translation symmetry of the material. The invention suggests that the antiphase boundaries result in additional magnetoresistance of the film.
U.S. Pat. No. 5,589,278 (Kamijo) describes magnetoresistive thin film and device. The patent teaches how to grow ferromagnetic film so that it forms pillars or staircase facets. It is suggested that such film has greater magnetoresistance. The patent further teaches how to control the width of the staircase facets and make them more homogeneous.
It should be appreciated that the key motivation for the development of materials and elements with improved magnetoresistance response is to further utilise them for numerous applications, e.g. in magnetic read heads, non-volatile memory elements, random access memory elements, encoders, security devices, etc. Some of these applications are described in the patents listed above. There are also numerous patents focusing on the development of specific devices utilising magnetoresistive materials. For example, a memory cell utilising a magnetoresistance element is described in U.S. Pat. No. 6,480,411 (Koganei).
In principle, the structures of magnetic layers described above are not limited to utilisation as passive magnetoresistive elements sensing the value of the external magnetic field. Some of these can also be used in various switching devices controlled by means of the current driven in the device or current injected in the structure. For example, U.S. Pat. No. 4,823,177 (Prinz et al.) describes a method and device for magnetising thin films by injecting spin polarised current. According to the invention, the device has two electrodes deposited at two different locations on a material. One of the electrodes is ferromagnetic. When a spin polarised current is passed in between the electrodes, the magnetisation of the material changes.
An object of the present invention is to develop a magnetoresistive medium for applications in information and communications technologies and also for a range of sensor applications.
Another object of the present invention is to provide materials other than those sensitive to magnetic field in general or magnetoresistance in particular. More specifically these materials could be called directional nanowires. These can be thought of as an array of sub-micron size rods with strongly anisotropic shapes: they are elongated in one direction but in two other orthogonal directions the dimensions of the material are limited to typically between 0.2 nm and 50 nm only. Thus, the term “nanowires” is used for rods with small dimensions along two essentially orthogonal directions in the nanometer range combined with elongation along the third essentially orthogonal direction. Unlike conventional nanowires, these nanowires do not have to entirely consist of metal atoms. The nanowire could consist of atoms of any sort, e.g. atoms of semiconductor elements and oxygen atoms forming oxide, or metal atoms and atoms of the sulphur element group or indeed any combination of atoms. The term nanowire, therefore, refers to the shape rather the electric conductance. However, it should be appreciated that if nanowires consist of e.g. metal atoms then their assembly will have strongly anisotropic electric conductance provided most of the nanowires are aligned along the same direction. Nanowires may have strongly anisotropic optical characteristics: e.g. for the polarisation of electric field along—and perpendicular to the direction of the elongation of the nanowires, optical and optoelectronic characteristics may be different. Magnetic susceptibility along—and perpendicular to the elongated direction may also differ. These materials are very new and their range of applications is yet to be fully explored.
The present invention is further directed to a method of fabrication of arrays of nanowires.
An objective of the invention is to develop a magnetoresistive medium, i.e. the material the resistance of which changes in response to changes in the value of an external magnetic field.
Yet another objective is to provide a method of fabrication of magnetoresistive media according to the invention.
Another objective is to develop a method for forming arrays of nanowires for a range of different applications including magnetic, electronic, optical and opto-electronic.
The invention provides a magnetoresistive medium having a crystalline substrate and an upper film on the substrate. In general, the substrate is a single crystal, while the film can be a single or polycrystal. There are stepped terraces of atomic and nanometer scale formed on the substrate by vicinally treating the substrate prior to application of the thin upper film. Many ways of treatment can be used to obtain the necessary stepped terraces. At least some of the crystalline surface terminations formed on the terraces are non-equivalent to the crystalline surface terminations of other terraces such that at least two separate sets of upper nanowires, having a different response to an external magnetic field, are formed in the upper film. Terraces may be formed of a different width so as to cause one or more of lattice mismatch, step induced anisotropy, antiphase boundaries and misfit dislocations between the substrate and the upper thin film. The same effect may be achieved by having a crystal substrate which has a crystalline lattice which comprises a stack of at least two non-equivalent alternating crystallographic layers which will therefore interact differently with the upper film on the substrate and thus provide the separate sets of nanowires. One way of ensuring that there are different surface terminations is to provide terraces of different widths. By having different widths, it is possible to cause antiphase boundaries between the film and the substrate in one set of terraces, and to substantially suppress it in another set. Similarly, this could be arranged to cause misfit dislocations or to cause step induced anistrophy.
The invention also provides a substrate which is a composite substrate comprising one of a vicinally treated antiferromagnetic and a vicinally treated ferromagnetic upper substrate and a vicinally treated non-magnetic lower substrate. Needless to say, these can be reversed so that you have a non-magnetic upper substrate and then either a vicinally treated antiferromagnetic or ferromagnetic film on the lower substrate. There is also provided a substrate which is one of an antiferromagnetic material, ferromagnetic material and a semiconductor material, each material with a relatively high resistance to electric current, whereby the resistance of the substrate is sufficiently greater than that of the film to prevent, in use, the substrate shunting the current in the film.
The substrate can be one of:
The substrate may be pre-annealed in an externally applied magnetic field or may indeed be a composite substrate comprising a base of non-magnetic material carrying a layer of one of antiferromagnetic and ferromagnetic material deposited thereon. This latter material will then have the vicinal surface formed on it. A buffer layer may be interspersed between them.
It will be appreciated that a final stabilising and protective layer may also be used which can be one of:
Similarly, the substrate can form a surface with bunched atomic terraces. There may also be provided additional magnetic biasing layers in the medium in order to achieve smoother response of the resistance change on the value of the external magnetic field changing.
The magnetoresistive medium may be used to form a composite medium, namely, a stack of these media, one on top of the other. They can be oriented in various ways, the miscut angles of the vicinal substrates different, with the miscut angles offset with respect to each other, and so on.
The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which:
a,b,c,d illustrate magnetoresistance of Fe3O4 film grown on MgO(100) measured for current along the miscut direction for sample temperatures of 293, 130, 101.5 and 100 K respectively. The magnetic field is parallel to the current direction. The miscut angle is 1 degree and the miscut direction is along <110>;
a,b,c,d show the magnetoresistance measured for current directed perpendicular to the miscut direction (i.e. parallel to the terrace steps) for the same film of
a and b illustrate the magnetoresistance of Fe3O4 film grown on MgO(100) measured for current along the miscut direction. The miscut angle is 0.4618 degrees and the miscut direction is along <110>. The magnetic field is parallel to the current direction.
a and b show magnetoresistance measured for current directed perpendicular to the miscut direction for the same film as of
a and b illustrate magnetoresistance of Fe3O4 film grown on MgO(100) (sample Mg14M2) measured for current along the miscut direction for sample temperatures of 109 and 105 K respectively. The magnetic field is parallel to the current direction;
a and b illustrate magnetoresistance measured for current directed perpendicular to the miscut direction for the same film as in
a and b illustrate magnetoresistance of Fe3O4 film grown on MgO(100) (sample Mg14M3) measured for current along the miscut direction for sample temperatures of 109 and 105 K respectively. The magnetic field is parallel to the current direction;
a and b illustrate magnetoresistance measured for current directed perpendicular to the miscut direction for the same film as of
a and b illustrate magnetoresistance of Fe3O4 film grown on MgO(100) (sample Mg14M3) measured for current along the miscut direction for sample temperatures of 109 and 105 K respectively. The current polarity is reversed with respect to the one in
a and b illustrate magnetoresistance measured for current directed perpendicular to the miscut direction for the same film as of
In this specification, the term “vicinal” is used not simply in its common meaning of “neighbouring” or “adjacent” but also as a reference to the characteristics of the terraces formed by subsequent treatment of a miscut substrate. Thus, the phrase “the extent to which the substrate is vicinal” implies, as will be appreciated by those skilled in the art, how formed; what size; how mutually arranged; and so on. There is no one term which can describe how the vicinal surface is treated to achieve the desired terraces, so much depends on the substrate material. This is explained in detail in the specification. Again, as explained throughout the specification, the treatment is not uniform and indeed it is not always a treatment as such, but a selection process.
It appears that much attention has been paid to electron transport in films grown on vicinal substrates. There are several studies that address the issue of electron transport and even magneto-transport in low-dimensional vicinal systems. Usually they deal with the fundamental questions of physics and quantum mechanics as opposed to tackling the practical issues of increasing magnetoresistance in films. These studies often relate to semiconductor systems performed under the presence of very high magnetic fields of up to 30 Tesla and very low temperature. A study by RTF van Schaijk et al. published in Physics B 256-258 (1998) 243-247 is a good example of such an investigation. It deals with Shubnikov-de Haas oscillations. However, their use for the purpose of the present invention is not described in the literature.
Accordingly, “the extent to which the surface is vicinal” means the materials are chosen, various miscut angles are used, various treatments of the cut surface are performed and an optimum cut angle and treatment is determined to provide the necessary interaction between the film and the substrate to achieve the objects of the invention. Because the materials will change and the treatments will vary, all one can state is that the optimum cut angle and treatment is used to provide this vicinal surface, as again described in the specification. A convenient term for this could be “vicinal treatment” or “vicinally treated” to cover choosing for the combination of film and substrate, the correct miscut angle and miscut direction and the subsequent treatment of the substrate to provide the necessary upper nanowires in accordance with the invention.
In this specification, the term “film” and “layer” are used interchangeably. There is a difficulty in nomenclature when one refers to “vicinal surfaces”, “atomic terraces” and “terrace steps”. “The vicinal surface consists of “atomic terraces. Therefore, each atomic terrace is a relatively flat area of the vicinal surface. As it will be explained below, in practice atomic terraces are not perfectly flat and contain atomic corrugations, defects, adsorbates and atomic-scale surface reconstructions, however, at this point this is not essential. The separation between the neighbouring atomic terraces in the vertical direction, i.e. in the direction perpendicular to the atomic terraces is called terrace step. The dimension of the terrace step is typically comparable to the separation between the layers of atoms forming crystal lattice (typically 2 Å the same as 0.2 nm the same as 2×10−10 m), i.e. it is comparable to the interatomic distance although it can also be a small integer multiple of this in the case of bunched steps or multiple steps. For example, it could be double or triple or quadruple of the separation between the layers of atoms in the crystal structure. On the other hand, the width of the atomic terraces is typically considerably greater than the interatomic distance, e.g. it would be at least 1 nm or more typically 10 to 50 nm or even greater than 100 nm. This is shown schematically in
It should be noted that, strictly speaking, while in the present specification the reference is to a magnetoresistive medium, all that is illustrated is portion of the magnetoresistive medium.
In this specification, the term “ferromagnetic”, as is often the case is used, to encompass both ferromagnetic and ferrimagnetic materials. As those skilled in the art will know, the difference in the materials is that the ferrimagnetic material has more than one magnetic sub-lattice. Both materials have net spontaneous magnetic moments below their Curie temperatures. Sometimes the Curie temperature of a ferrimagnetic material is called Neel temperature and the ferrimagnetic materials are sometimes called ferrites. A well-known example of a ferrimagnetic material is Fe3O4.
The term “crystalline” is used somewhat loosely in relation to the magnetoresistive medium according to the invention. In general, in relation to the substrate, the word “crystalline” means a single crystal, that is to say, a crystal with axes that have the same direction at different parts of the crystal. In relation to some of the films used, the word “crystalline”, as well as a single crystalline, i.e. an epitaxial film, can also mean “polycrystal”, that is to say, crystal with axes which may change direction at different parts of the film.
Methods of forming vicinal surfaces have been extensively described in the literature. Generally, the methods are based on cutting the surface at a desired angle with respect to the low index direction by diamond saw, spark erosion or another suitable technique and polishing the surface, e.g. by using diamond paste, or by means of electrochemical polishing. Then the surface is characterized by means of a High Resolution X-Ray Diffractometer (HRXRD). For the present invention, a HRXRD instrument from Bede Scientific Instruments Ltd (UK) was used. The miscut angle and the direction of the miscut, i.e. the direction on the surface perpendicular to the average terrace step is identified using the HRXRD.
To establish terraces on a miscut substrate, treatment leading to the atomic scale rearrangement is often required. There are numerous approaches resulting in such a rearrangement. According to one method, the surface may be annealed in vacuum or in ultra high vacuum. In between the annealing sessions it can be characterized by using in-situ scanning tunnelling microscopy, STM, i.e. the STM located inside the vacuum system. Another method includes ion etching of the surface kept at an elevated temperature by means of e.g. Ar ions in vacuum [J. Naumann, J. Osing, A. Quinn, I. V. Shvets, Morphology of sputtering damage on Cu(111) studied by scanning tunnelling microscopy, Surface Science 388 (1997) 212-219] which is included in this specification by way of reference. Alternatively, a chemical reaction can be set up on the surface such that the reaction speed is dependent on the Miller indexes of the atomic terraces. As a result, well-defined terraces can be formed. Other possible methods also include subjecting the surface to chemical or electrochemical reaction. There is no general hard rule of finding the conditions for the preparation of a vicinal surface with well-defined terraces. The conditions are generally optimised for any given material and desired Miller indexes of the atomic terraces.
Miscut and polished SrTiO3(100) substrate is first washed in distilled water in another method. Then it is subjected for about 30 seconds exposure in a buffered hydro-fluoric acid at room temperature. The substrate is then rinsed in distilled water. This procedure results in well-defined atomic terraces. To make the edges of the terraces straight, the substrate may be further subjected to between 1 and 4 hours of anneal in oxygen atmosphere at 1 Bar pressure at a temperature of 10000° C.
A vicinal substrate of Si (111) can also be formed by means of anneal. Typical preparation includes annealing of a polished miscut substrate in an ultrahigh vacuum chamber at a temperature of 1050° C. for some 1 hour. Then the substrate is quickly annealed (flashed) at 1250° C. for 20 seconds also in an ultra high vacuum.
Vicinal substrates of another commonly used material: α-Al2O3 with a (0001) orientation can also be prepared by means of anneal. For this material the anneal needs to be carried out in air and the anneal temperature is 10000° C.
It is known that hysteresis loops of a miscut surface depend on the direction of the magnetic field with respect to the miscut direction. Measurements were performed by means of longitudinal magneto-optical Kerr effect. The sample was epitaxial film of Fe grown on the single crystal Mo(110) vicinal substrate. The miscut direction is <1-1-1> and the miscut angle is 4.6 degrees. We established that atomic terrace steps induce noticeable contribution to the magnetic anisotropy of Fe film for the film thickness of up to 10-12 Å.
In one representative experiment described below, the magnetoresistance of thin films of Fe3O4 (magnetite) deposited on MgO (100) vicinal substrates was measured. In this case the vicinal MgO (100) substrates had a miscut direction along <110> and varying miscut angles. The accuracy of the surface orientation was within ±0.5 degrees. The film thickness was 70 nm.
Prior to insertion into the MBE chamber, substrates were chemically cleaned and mounted on a molybdenum sample holder. The film was deposited using Oxygen-Plasma-Assisted Molecular Beam Epitaxy (MBE) using the MBE system manufactured by DCA (Finland). The MBE system was equipped with facilities for sample heating, Reflection High Energy Electron Diffraction (RHEED), residual gas analyser (RGA), molecular beam sources for deposition of materials, deposition rate monitors as well as an Electron Cyclotron Resonance (ECR) oxygen plasma source. The base pressure in the system was lower than 5×10−10 Torr. The oxygen source used for the growth was the Radio Frequency (RF) OS Pray Plasma source from Oxford Scientific (UK). The RF power supplied to the source was 80 W during the deposition. The MgO substrate was annealed at 600° C. for up to 4 hours in a plasma oxygen environment at the pressure of 1*10−5 Torr prior to the deposition. In most cases the same level of RF power was applied to the plasma source during the substrate annealing. For some samples no RF power was supplied to the plasma source during the substrate annealing. Out of the samples referred to in this specification, sample Mg14M2 and Mg14M3 were annealed without any RF power and all the other samples were annealed at 80 W of RF power.
The structural characterization was done using the high-resolution X-ray diffraction (HRXRD) measurements. With the HRXRD, when operated in a triple axis configuration one can detect lattice constant variations as low as 2×10−5. This enables one to determine the status of the strain relaxation very precisely. The crystalline alignment of the film with respect to the substrate can also be established. The in-plane (a∥) and out-of-plane (a⊥) lattice parameters of the Fe3O4 thin films grown epitaxially on (100) MgO substrates were measured by performing symmetric (400), (800) and asymmetric (622) and (420) Bragg reflections.
We have established that for Fe3O4 films grown on MgO (100) substrates under the growth conditions described are fully strained to achieve one-to-one registry with the substrate for film thickness up to 100 nm and even greater. The representative results of the HRXRD measurements of a 70 nm thick film at room temperature are shown in
The results of the HRXRD characterization can be summarized as follows: the film is single crystalline with the tetragonally distorted unit cell. The strain is tensile in the film plane and the film is fully strained. The volume of unit cell is a good indication of the film stoichiometry and is consistent with the bulk magnetite suggesting that the film is stoichiometric.
The Raman optical spectrum for the film is shown in
This invention is not limited to magnetic oxides or to stoichiometric magnetite in particular. This specification describes a general phenomenon and how it can be exploited to form a magnetoresistive medium. Magnetite is just one example of magnetic material that utilizes the phenomenon.
The present invention can be summarised as follows. The magnetoresistance of a film can be controlled and enhanced by the miscut of the substrate on which the film is grown. Generally, other operations are also required and are encompassed within the term “vicinal treatment”. Results of one representative experiment are shown in
Different values of the magnetoresistance can be found along different crystallographic directions in an epitaxial film. For example, on the (110) or (111) surface one may expect to find some difference between the magnetoresistance values measured along two orthogonal directions as they are crystallographically not equivalent. However, the results presented in
It can be further demonstrated that the miscut angle is an important factor in defining the value of the magnetoresistance.
The specification emphasizes the importance of the difference between the magnetoresistance of films grown on vicinal and non-vicinal substrates. The difference is highly beneficial as magnetoresistance has increased considerably. It appears that the magnetoresistance values shown in
a, b show the results of magnetoresistance measurements for the sample Mg14M3 for the current along the miscut direction at the temperatures of 109 and 105 K respectively.
In Table 5 below, the difference in MR on the Mg14M2 and Mg14M3 are summarized which shows the significant effect of surface preparation conditions on magnetoresistive properties.
The results suggest that the dependency of the magnetoresistance of a film on the miscut of the substrate on which the film is grown is the general property that is not limited to just one substrate-film combination. We have confirmed that similar results are observed for other materials, e.g. thin epitaxial Fe films grown on MgO(100). The effect observed on this material was smaller than in Fe3O4/MgO(100) films but the conclusion is the same: the magnetoresistance of the film can be enhanced by growing it on a miscut substrate forming atomic terraces. The effect observed depends on the angle and direction of the miscut and also on the film thickness.
In order to explain why miscut substrates affects the magnetoresistance of the film grown on it and how the magnetoresistance can be maximized through the choice of the vicinal substrate, it is important to consider under what conditions one may achieve substantial magnetoresistance. Only magnetoresistance caused by a moderate magnetic field is considered in this document. For example, some materials may have a massive magnetoresistance in a large field of say 20 Tesla. In some materials such as MnSe, this magnetoresistance could be so large, that effectively the material undergoes through the metal-insulator transition caused by the magnetic field. This magnetoresistance was termed as colossal magnetoresistance. The present invention relates to magnetoresistance that can be caused by a relatively small field of say a few 10 s of Oe and up to some 10 kOe (10 kOe=1 Tesla). These values are given here as a rough indication. When spin polarized electrons traverse between areas with nonparallel direction of spins, the spin scattering is different from the situation when electrons traverse between the areas with the parallel direction of spins. Therefore, if the external magnetic field allows switching between the two situations, substantial values of magnetoresistance will result. In terms of a thin film medium this teaching can be interpreted as follows. If one is capable of creating the situation in which the medium comprises numerous areas of substantially unparallel direction of magnetization and then with the imposition of the magnetic field all the areas become substantially magnetized along the same direction, this results in resistance change, i.e. magnetoresistance, provided the current flows in the medium between these areas or alternatively electrons come in contact with boundaries between different areas and suffer additional spin scattering.
The above explains, in a relatively theoretical way, how the vicinal treatment can be carried out to prepare a substrate to allow the correct miscut angle and subsequently to provide the necessary change in magnetoresistance.
Referring to
Before describing the operation of the magnetoresistive medium 1, it should be noted that the substrate 2 can be manufactured of an antiferromagnetic material and preferably one with a large resistance to electric current e.g. NiO, FeO, CoO etc. The upper nanowires 10(a) and 10(b) are magnetic. The magnetic nanowires can be either ferromagnetic or ferrimagnetic. They could consist, for example, of Fe, Ni, Co or permalloy. They could also be formed by a layer of magnetic oxide, e.g. Fe3O4 or a layer of another magnetic material such as Heusler alloy, Fe3Si, rare earth doped manganites or many other materials.
The terrace steps 5 may be just one atomic layer high, but could also be more than one atomic layer high if the substrate has so-called bunched terrace steps. Bunched steps(or step bunching) are known to exist on some substrates when height separation between the terraces 4(a) and 4(b) is greater than one atomic layer. Bunched steps are formed when the energy associated with the terrace steps is too high and it is energetically favorable for the surface to reduce the number of the terraces while maintaining the overall miscut angle. Step pairing is a common type of step bunching forming terrace steps with an effective height equal to two regular separations between atomic planes. Mechanisms of step bunching are well understood. Reference is made to the state-of-the-art description of the topic published e.g. in K. Yagi, H. Minoda, M. Degawa, Step bunching, step wandering and faceting: self organization at Si surfaces, Surface Science Reports, 43 (2001) 45-126; or O. Pierre-Louis, Step bunching with general kinetics: stability analysis and macroscopic models, Surfaces Science 529 (2003) 114-134; B. S. Swartzentruber, Y-W. Mo, R. Kariotis, M. G. Lagally, M. B. Webb, Direct determination of Step and kink energies on vicinal Si(001) surface, Physical Review Letters, 65 (1990) 1913-1916. All are included herein by way of reference.
The protective layer 15 could be e.g. a layer of Al2O3, MgO, SrTiO3 or a thin gold or platinum layer. It could also be a layer of a magnetic oxide with large resistance, e.g. NiO. The protective layer, as well as protecting the film 11 forming the upper nanowires 10(a) and 10(b) from the ambient conditions, may also act as a stabiliser, i.e. effectively dampening the surface diffusion so that the pattern of the areas forming the upper nanowires 10(a) and 10(b) of ferromagnetic material does not change over time.
To summarise, in accordance with the invention, the vicinal surface, after it has been vicinally treated, ensures that the upper nanowires form two subsets of upper nanowires which are positioned substantially differently with respect to the atomic steps of the substrate.
It should again be noted that the dimensions in
It should also be kept in mind that the nanowires 10(a) and 10(b) do not have to be positioned in strict alteration, i.e. each nanowires 10(a) does not have to be flanked by two nanowires 10(b) and vice versa. However, preferably a significant extent of intermixing between the two groups of nanowires is desirable so as to provide the situation when a sizeable fraction of the nanowires 10(a) has nanowires 10(b) as their neighbours and vice versa.
The operation of the magnetoresistive medium 1 can be understood from
Therefore, as an electric current passes through the magnetoresistive medium in the direction along the substrate miscut, in a zero external magnetic field(
If an electric current passes perpendicular to the miscut direction, additional spin scattering may still be created. When current passes perpendicular to the miscut direction, i.e. along the upper nanowires 10(a) and 10(b) there is scattering of the charge carriers along the boundaries between each two neighbouring nanowires. The scattering depends on the relative orientation of magnetisations in the neighbouring upper nanowires. With the application of a magnetic field the relative directions of magnetisation in the upper nanowires change with respect to each other thus changing the scattering of electrons and leading to magnetoresistance.
As stated,
It should also be noted that the directions of magnetization in the areas forming the upper nanowires 10(a) and 10(b) do not have to rotate by the full 180 degrees once the external magnetic field is applied. They could rotate by a smaller angle still resulting in change of spin scattering and therefore in resistance change. Moreover, for certain applications it is desirable to have a gradual rotation of magnetization in response to change in external magnetic field as opposed to a sudden flip over of the magnetization. This is particularly important in sensors of magnetic field. To achieve more gradual deviation of magnetization in response to the magnetic field, an additional ferromagnetic biasing layer could be deposited e.g. on top of the upper film 11. This layer could be composed of a hard ferromagnetic material and could function by creating a stray field sensed by upper nanowires 10(a) and 10(b) and acting as an offset field.
Alternatively, the difference in the magnetic response of the upper nanowires 10(a) and 10(b) could be based on the fact that the upper nanowires 10(a) and 10(b) contain different densities of structural defects. In this embodiment the materials forming the upper nanowires 10(a) and 10(b) and of the substrate are selected in such a way that there is lattice mismatch between them. The lattice mismatch results in the formation of misfit dislocations that release strain energy in the film. There is the critical width of the terrace, below which the formation of misfit dislocations is considerably suppressed. This issue has been studied for the Fe/Mo(110) system where the lattice mismatch between the film and the substrate is some 9% [S. Murphy, D. M. McMathuna, G. Mariotto, I. V. Shvets, Physical Review B, 66(19) 195417 (2002), Morphology and strain induced defect structure of ultrathin epitaxial Fe films on Mo(110)]. The study demonstrated that for narrow terraces of Fe/Mo(110), the formation of misfit dislocations is suppressed. Alternatively, a medium utilizing another type of film growth defect called, antiphase boundaries, could be employed. The density of antiphase boundaries at more narrow terraces should be lower as this should be proportional to the density of the film nucleation centers during the film growth, which should be smaller for the more narrow terraces.
As stated, it should be noted that the wide and narrow atomic terraces 4(a) and 4(b) do not have to be in strict alternation with each other, i.e. each wide terrace does not have to neighbour two narrow terraces and vice versa. Indeed, it is sufficient to get a mix of wide and narrow atomic terraces on the substrate 2. For example the surface where two wide terraces are followed by one or two narrow ones is a good example of such a mix.
The same comment applies as in relation to
Referring again to
If there are more than two types of atomic terminations (say, 2(a), 2(b) and 2(c)) then we identify more than two types of nanowires(in this case 10(a), 10(b) and 10(c)). We do not show this on the drawings as it can be readily understood.
To make the difference between the terraces more profound and amplify the difference between magnetic responses of the nanowires 10(a) and 10(b) the vicinal substrate could be subjected to a plasma or reactive gas or another treatment prior to the deposition of the material for the upper nanowires 10(a) and 10(b), optimised in such a way that the reaction on atomic terraces of different kinds takes place differently, e.g. reaction takes places on the terraces 4(a) but not on the 4(b) terraces. This reaction could lead to a selective deposition of an adsorbate material or inducement of atomic disorder or inducement of surface reconstructions on either of the terraces 4(a) and 4(b). In this situation, the nanowires 10(a) and 10(b) are grown on significantly different templates and the conditions can be selected in such a way that the magnetic response of the nanowires 10(a) and 10(b) is different. The difference in magnetic response could be based on any of the factors discussed in detail below. For example, the nanowires 10(a) could be epitaxial and the nanowires 10(b) not. Alternatively, both nanowires 10(a) and 10(b) could be epitaxial but could be of different crystallographic orientations. This is schematically shown in
Another mechanism that may result in a different response to the magnetic field between the areas forming the upper nanowires 10(a) and 10(b) is step-induced anisotropy. Step-induced anisotropy results from breaking the rotational symmetry at the terraces 4. Step-induced anisotropy is usually uniaxial for a surface with well-aligned steps 5. The anisotropy energy depends on the electronic structure of the film that in turn depends on the type of substrate, strain, crystallographic orientation of the film surface and crystallographic orientation of the step edges. Therefore, the step induced-anisotropy will differ for the areas forming the upper nanowires 10(a) and 10(b). The influence of step-induced anisotropy will diminish as the film thickness increases. Therefore, depending on the type of materials for the substrate 2, and upper nanowires 10(a) and 10(b) and depending primarily on the thickness of the upper nanowires 10(a) and 10(b), the step-induced anisotropy may or may not play a significant role in determining the difference between the magnetic responses of the nanowires 10(a) and 10(b). However, this does not fundamentally change the substance of the operation of the embodiment in
It should be pointed that in some cases further layers may need to be added in the magnetoresistive medium 1. For example a buffer layer may need to be added between the substrate 2(a) and substrate 2(b). The purpose of buffer layer is to prevent diffusion of atoms of material of substrate 2(a) into that of the substrate 2(b) and vice versa. This will be readily appreciated by those skilled in the art of heteroepitaxy. For example, very thin buffer layers of S, Ge, Cr, Ag, Au are often used with the substrates of GaAs and Ge. Buffer layers of Cu and Cr are commonly used with Si substrates. Similarly a thin seed layer may need to be added e.g. to improve the growth or adhesion of the key functional layers. These additional layers are not included in the drawings as they are secondary and will complicate understanding the invention.
The material forming the layer 2(a) could also be a ferromagnetic material preferably having high coercivity. The principle of operation of such a magnetoresistive medium is the same as the one of the medium shown in
Referring again to
Again referring to
One can also construct a magnetoresistive medium where due to the lattice mismatch between the materials forming the substrate 2 and the upper nanowires 10(a) and 10(b), the upper nanowires 10(a) and 10(b) are both epitaxial but are characterized by different amounts of strain as they grow on, what are in effect, different templates. This will lead to the same result: their response to the external magnetic field will be different. Consequently, the relative orientations of magnetisations in the upper nanowires 10(a) and 10(b) with respect to each other can be altered by the external magnetic field thus leading to magnetoresistance when current passes through the film of the nanowires 10(a) and 10(b) as described above.
Alternatively, one can also provide a magnetoresistive medium in which the upper nanowires 10(a) are epitaxial and the upper nanowires 10(b) are not.
Alternatively there can be provided a magnetoresistive medium where the upper nanowires 10(a) and 10(b) are both epitaxial but have different crystallographic orientations: e.g. the surface of upper nanowires 10(b) has (100) orientation and one of the upper nanowires 10(a) has (110) orientation. In this case the upper nanowires 10(a) and 10(b) will also respond differently to external magnetic field as they have different surface anisotropy due to different surface terminations as well as different effective fields of the magnetocrystalline anisotropy.
The magnetoresistive medium can be readily easily provided and it's construction will be appreciated by those skilled in the art. The reader is presumed to know, in general terms, how full and fractional layers can be formed and be familiar with the terms “flux” and the use of evaporators. Once the magnetoresistive medium has been formed, then electrodes are deposited on top to apply the current/voltage to the medium and to measure resistivity of the magnetoresistive medium or voltage across it. The application of electrodes is not described in detail in this document, as this aspect of magnetoresistive medium is standard and well described in literature, neither are the electrodes or the rest of the magnetoresistive medium illustrated. The optional protective layer 15 can also be deposited and this, also, is not described in detail in the specification, as such layers are common for thin film devices used in many applications.
Referring again to
In yet anther embodiment related to
Referring again to
The magnetic response of the areas(nanowires) 10(a) and 10(b) is different. This difference can be achieved in a number of ways. The first one utilizes step-induced magnetic anisotropy. The nature of this anisotropy is that each terraces step makes additional contribution to the magnetic anisotropy energy The contribution of the terrace step-induced anisotropy energy is the same for the wide and narrow terraces. However, as the effective anisotropy field is inversely proportional to the magnetic moment and the moment is in turn proportional to the width of the terrace, magnetizations in the nanowires 10(a) and 10(b) of the upper film 11 will respond differently to the magnetic field. Therefore, once again one forms the situation whereby the relative directions of magnetisation in the upper nanowires 10(a) and 10(b) can change with the change in the value of the external magnetic field. This results in the resistance change and therefore, in magnetoresistance.
Referring to
The substrate 2(a) can be formed as previously described or may be formed by growing the wedge 202(a) on the flat essentially rectangular portion 201(a) by carrying out growth of a film with a shadow mask in a normal incident condition and withdrawing the mask gradually over the substrate. Alternatively, this may be provided by moving the substrate away from the mask to create the wedge shaped film 202(a) forming part of the substrate 2(a). Then the remainder, for example, with the deposition of the film 11(a), can be carried out, as previously described. Then the first part 201(b) of the substrate 2(b) of the next magnetoresistive medium 1(b) may be formed in the same way and then subsequently the remainder of the substrate 2(b), namely the portion 202(b) may again be formed and finally the next film 11(b) may be deposited. Obviously, the two vicinal surfaces so formed, that is to say, on the substrate 2(a) and the substrate 2(b), may have different miscut angles α1, and α2, as shown.
The speed of mask withdrawal and growth rate provides an efficient method of controlling the variation in thickness of the film or the vicinal angle. The method is standard and well known in the art [V. I. Nikitenko, V. S. Gornakov, A. J. Shapiro, R. D. Shull, K. Liu, S. M. Zhou and C. L. Chien, Asymmetry in Elementry Events of Magnetisation Reversal in Ferromagnetic/Antiferromagnetic Bilayer, Phys. Rev. Lett. 84(4) 765(2000)].
It will be appreciated that that portion 201(b) of the second substrate 2(b) provides a protection for the first magnetoresistive medium 1(a). As illustrated in
Referring to
It will be appreciated that various forms of composite magnetoresistive medium may be used. A large number of magnetoresistive media according to the present invention may be stacked one on top of the other. It will also be appreciated that the other combination of miscut angle ratios and miscut directions may also be possible.
Such composite magnetoresistive media, formed by the stacking of magnetoresistive media, one on top of each other, may be advantageous for using the magnetoresistive media in memory devices where increasingly the density of the memory cells is a prime requirement.
It will be appreciated that for certain applications, it may be advantageous to have the two miscut directions of substrates, which are adjacent to each other, offset through 90°. As the directions of the upper nanowires in the magnetoresistive media 1(a) and 1(b) are not collinear, the cross-talk between them(influence of the sensing current in one media on the other one) is reduced. Also, any desired pattern of sensitivity to the magnetic field can be developed as each of the media 1(a) and 1(b) is anisotropically sensitive to the magnetic field.
In accordance with the present invention, while it has been described that the substrate can be cut along the required miscut direction, one could equally well deposit a wedge-shaped film, as described with reference to
It is envisaged, for example, that when a substrate is manufactured in accordance with
Clearly, stacks of more than two magnetoresistive media can be formed in the same way. Another situation which can be considered here is the formation of a multilayered superlattice structure based on magnetoresistive medium. This can be realized by producing in sequence a number of e.g. 10-100 repeat units of the magnetoresistive medium 1, as shown in
It will be appreciated that the magnetoresistive medium according to the present invention can be used for many devices which require the use of such magnetoresistive material. The invention is not limited to the use of this material in any particular device.
In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms “include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.
The invention is not limited to the embodiment hereinbefore described, but may be varied in both construction and detail within the scope of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/IE04/00034 | Mar 2005 | US |
| Child | 11078416 | Mar 2005 | US |
