Patent Application
20100188773
Attempts to increase the capacity of magnetic data storage devices must balance writability, grain size and magnetic anisotropy in the magnetic data storage media. Write heads can only generate a limited magnetic field, and this limit is set by the maximum volume magnetization that can be achieved in a material, the maximum current density that can be put through a conductor, and the head-to-media separation. If the anisotropy in the media is lowered to the point where it can be written by the write head and the grains are made small enough to maintain an acceptable signal-to-noise ratio, the media may not be thermally stable for large areal densities. This is referred to as the superparamagnetic limit.
Ferroelectric (FE) data storage media has the advantage that it is written using an electric field, and very large electric field values can be generated with a thin-film device. Thus, FE media with a very large anisotropy can be written by a thin-film device, and a thermally stable FE media with very small domains (and narrow domain walls) can be written.
Multiferroics (materials with multiple order parameters like spontaneous FE distortion and magnetic ordering) are attractive materials because several functionalities can be integrated in the same device. The multiferroic materials which simultaneously show FE and magnetic ordering are also called ferroelectromagnets. Single phase multiferroics usually exhibit transition temperatures well below room temperature, and weak remanent FE and magnetic polarization which make them impractical. One exception is BiFeO3 which possesses transition temperatures well above room temperature, a high switchable ferroelectric polarization but a vanishing remanent magnetization due to the antiferromagnetic ordering. A reasonably large remanent magnetization is required either for magnetoresistive readback from a media disc, or to polarize the current carrying electrons for magnetization orientation detection in a solid state memory device. Composite multiferroics such as the vertical, self-assembled, epitaxial three dimensional heterostructures show strong ferroic properties, but are difficult to fabricate and lack long range order as needed, for example, for data storage media. Further, the typical domain size in composite multiferroics is about 100 nm, while a bit size of less than 10 nm is required for high density data storage. Domain refers to a single ferroic inclusion (either FE or magnetic) in a single ferroic matrix (either FE or magnetic). Thus, it is highly desirable to create single phase multiferroic materials with robust FE and magnetic ordering at room temperature.
An aspect of the present invention is to provide a data storage medium that includes a multiferroic thin film and ferromagnetic storage domains formed in the multiferroic thin film. The multiferroic thin film may be formed of at least one of BiFeO3, or any other ferroelectric and antiferromagnetic material. The ferromagnetic storage domains may be formed in the multiferroic thin film by an ion implantation process.
Another aspect of the present invention is to provide a data storage system that includes a recording head and a data storage medium adjacent to the recording head. The data storage medium includes a multiferroic thin film and ferromagnetic storage domains formed in the multiferroic thin film. The multiferroic thin film may be formed of at least one of BiFeO3, or any other ferroelectric and antiferromagnetic material. The ferromagnetic storage domains may be formed in the multiferroic thin film by an ion implantation process.
A further aspect of the present invention is to provide a bit patterned multiferroic storage medium that includes a layer of multiferroic material and ferromagnetic storage domains formed in the layer of multiferroic material.
These and various other features and advantages will be apparent from a reading of the following detailed description.
In one aspect, the invention relates to switchable ferroelectric and spontaneous magnetization from single-phase multiferroic materials such as, for example, BiFeO3 by local ion-implantation. Ion implantation of a thin multiferroic film through a lithographic mask will create a pattern of high remanent magnetization pillars or domains that may be used for data storage. The impinging ions would disrupt the spiral antiferromagnetic order (G-type) and would give rise to patches of non-zero net magnetization in the form of embedded pillars. The strong coupling between ferroelectricity and antiferromagnetism in the parent multiferroic material will lead to a strong coupling between ferroelectricity in the unimplanted matrix and magnetic order in the implanted regions. This coupling effect is mediated by the strong exchange interaction between the localized spins in the unimplanted and implanted areas.
In one aspect, the invention provides for the fabrication of multiferroic materials with high ferroelectric and magnetic polarizations from single-phase antiferromagnetic and ferroelectric films by an ion implantation process. Such engineered materials can be employed, for example, as storage media, where each of the implanted areas will store information in the form of e.g. up or down magnetization as in bit-patterned media. The patches are lithographically defined and therefore long-range order can be readily achieved, in contrast to the self-assembled ferroelectric-ferrimagnetic bit-patterned media. The long-range order in bit-patterned media is desirable for any storage scheme involving a spinning disk (e.g. hard disk drive) or fixed medium (e.g. solid state disk). The invention also provides for eliminating the need for highly magnetostrictive materials for an EAMR (electrically-assisted magnetic recording) scheme, since the magnetoelectric coupling is in this case mediated by the exchange interaction and not by stress. Further, given the possibility of tuning the implantation conditions, one may also create embedded magnetic spheres in the ferroelectric-antiferromagnetic matrix. In addition, since the pillars magnetization is pinned by the matrix antiferromagnetic order, there is no requirement for the minimum pillar size and Ku to stabilize the ferromagnetic order.
In accordance with an aspect of the invention, a single-phase material with room temperature ferroelectric and antiferromagnetic order could be engineered to exhibit enhanced spontaneous magnetization through an appropriate ion implantation process. First, ion implantation may break the antiferromagnetic order by breaking the transition metal-oxygen-transition metal bonds which are responsible for the onset of the super-exchange interaction. Second, new structural/chemical phases with large room temperature net magnetization can form by a post implantation process such as thermal annealing, or during the implantation process. If this is performed with e.g. Fe, Ni, Co, or Mn ions, or any other suitable ions, the desired magnetic phase may form without relying on a post-implantation process. Furthermore, ion implantation with additional non-magnetic species such as Pt, or Cr may be beneficial for the stabilization of the desired ferromagnetic phases. A uniform concentration profile of the implanted species along the implantation direction is needed, which can be achieved by varying the ion implant parameters.
Since the high net magnetization phase (“pillars”) is fabricated from, and adjacent to the “bulk” antiferromagnetic material, they may strongly interact with each other through magnetic exchange. This will result in a pinning of the pillars' magnetization, whose switching can be assisted by locally altering the surrounding antiferromagnetic configuration through magnetoelectric coupling to the ferroelectric order. The application of an electric field changes the direction of ferroelectric polarization, which alters the antiferromagnetic configuration. A weak magnetic field can be superimposed on the electric field to align the pillars magnetization along the desired direction.
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As stated, the ferromagnetic storage domains 14 are implanted in the layer 12 of multiferroic material by an ion implantation process. Ion implantation is a non-equilibrium technique in which atoms are introduced into the surface region of a target (substrate) material through irradiation with charged particles accelerated to hyperthermal energies. The process is unlimited by thermodynamic considerations allowing the introduction of dopants (or defect/damage centers) at concentrations and distributions that would otherwise be unattainable, offering potentially unique materials engineering capabilities. Upon implantation, ions are brought to rest by losing their translational energy through a series of independent binary interactions with substrate atoms. Energy is essentially lost elastically through collisions between atomic nuclei and inelastically to their electron clouds. The distribution of implanted or displaced substrate atoms produced by ion implantation is described statistically by the projected range and straggle defined by the peak and width of a Gaussian distribution respectively.
Accordingly, it will be appreciated that in accordance with an aspect of the invention, the recording head 20 illustrated in
The implementation described above and other implementations are within the scope of the following claims.
