Patent Application
20170279040
This application claims the benefit of priority of Singapore Patent Application No. 10201406062S, filed 25 Sep. 2014, the content of which being hereby incorporated by reference in its entirety for all purposes.
The present invention generally relates to a magnetic element and a method of fabrication thereof, and more particularly, to a spin current driven magnetic element with perpendicular magnetic anisotropy (PMA) for a magnetic memory device, such as a Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) device.
Magnetic Random Access Memory (MRAM) is a strong competitor of existing memory technologies such as Flash, DRAM and SRAM because of their potential in offering reduction of power consumption through non-volatility, high number of read/write cycles and high speed performance.
A basic MRAM cell (magnetic element) includes a magnetic tunnel junction (MTJ) whereby two ferromagnetic layers, namely a free layer (FL) and a reference layer (RL), are separated by a thin non-magnetic insulating layer (spacer layer). The RL has a fixed magnetization due to a high magnetic anisotropy or a strong exchange coupling to another layer. The FL has a magnetization that is free to rotate in space. The resistance of the cell depends on the relative orientation of the magnetization of the FL and the magnetization of the RL due to the Tunneling Magnetoresistance (TMR) effect. In recent years, it has been found that magnetic momentum can be transferred from the RL to the FL and vice versa, depending on the polarity of the electrical current, across a thin oxide layer (the thin non-magnetic insulating layer). This effect is known as the Spin Transfer Torque (STT). An MTJ therefore forms the basic structure of a MRAM cell for a STT-MRAM, in which the data bit is written by STT and read by TMR.
STT-MRAM devices with perpendicular magnetic anisotropy (PMA) are more advantageous than those based on in-plane magnetic anisotropy in that they provide more efficient current switching, higher thermal stability factor and basically no limitation on the cell aspect ratio. Therefore, improving the STT switching in perpendicularly magnetized MTJ magnetic elements is extremely important for high performance MRAM applications.
An advanced STT-MRAM magnetic element may include additional layers such as a compensating layer and/or a coupling layer, with the former used to cancel the stray field emanating from the bottom pinning layer and the later used to increase the magnetic anisotropy of the FL/RL in order to improve thermal stability. In STT-MRAM applications, PMA materials such as Co/Pt and Co/Pd with strong effective magnetic anisotropy energy density (Keff) have been used conventionally in the MTJ. However, they may not be the best candidates because they face issues of poor growth quality and high damping constant, which can significantly reduce the TMR and increase the critical switching current density of the MTJ, respectively.
A need therefore exists to provide a spin current driven magnetic element with PMA that seeks to overcome, or at least ameliorate, one or more of the deficiencies of conventional magnetic elements, such as improved thermal stability, high TMR ratio and/or low critical switching density. It is against this background that the present invention has been developed.
According to a first aspect of the present invention, there is provided a magnetic element comprising:
In various embodiments, the at least one multilayer is disposed on the magnetic tunnel junction such that the at least one multilayer constitutes a compensating layer configured to cancel an external magnetic field towards the ferromagnetic free layer emanating from outside the magnetic tunnel junction.
In various embodiments, the magnetic element further comprises a pinning layer ferromagnetically coupled to the ferromagnetic reference layer for pinning the magnetization direction of the ferromagnetic reference layer to the pinning layer, wherein the pinning layer comprises one or more multilayers, each multilayer includes Co/Ni/Pt which exhibits perpendicular magnetic anisotropy.
In various embodiments, at least one of the compensating layer and the pinning layer is configured such that the switching field of the compensating layer is less than the switching field of the pinning layer.
In various embodiments, the ferromagnetic reference layer comprises a first ferromagnetic material layer and the at least one multilayer.
In various embodiments, the ferromagnetic reference layer further comprises:
In various embodiments, the at least one multilayer is disposed on the first ferromagnetic material layer such that the at least one multilayer constitutes a diffusion barrier for preventing diffusion of one or more elements of the first ferromagnetic layer to the insulating spacer layer.
In various embodiments, the first ferromagnetic material layer comprises at least one of (Co/Pd)n, (CoFe/Pd)n, (Co/Pt)n, and (CoFe/Pt)n, where n is a stacking number and is at least 1.
In various embodiments, the ferromagnetic free layer comprises a first ferromagnetic material layer and the at least one multilayer.
In various embodiments, the ferromagnetic free layer further comprises an interlayer disposed between the first ferromagnetic material layer and the at least one multilayer for exchange coupling between the first ferromagnetic material layer and the at least one multilayer.
In various embodiments, the first ferromagnetic material layer comprises at least one of CoFeB.
In various embodiments, the at least one multilayer is a multilayer stack including (Co/Ni/Pt)n, where n is a stacking number and is at least 1.
In various embodiments, n is a number from 2 to 30.
In various embodiments, the thickness of Pt in the at least one multilayer is greater than 0 Angstrom to about 10 Angstrom.
According to a second aspect of the present invention, there is provided a method of fabricating a magnetic element, the method comprising:
In various embodiments, forming the at least one multilayer comprises forming the at least one multilayer on the magnetic tunnel junction such that the at least one multilayer constitutes a compensating layer configured to cancel an external magnetic field towards the ferromagnetic free layer emanating from outside the magnetic tunnel junction.
In various embodiments, forming the ferromagnetic reference layer comprises forming a first ferromagnetic material layer, and forming the at least one multilayer comprises forming the at least one multilayer as part of the ferromagnetic reference layer.
In various embodiments, forming the ferromagnetic reference layer further comprises:
In various embodiments, forming the ferromagnetic free layer comprises forming a first ferromagnetic material layer and forming the at least one multilayer comprises forming the at least one multilayer as part of the ferromagnetic free layer.
According to a third aspect of the present invention, there is provided a magnetic memory device comprising an array of magnetic elements according to the above-mentioned first aspect of the present invention.
Embodiments of the present invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:
Embodiments of the present invention provide a magnetic element for a magnetic memory device (such as a STT-MRAM device) that seeks to overcome, or at least ameliorate, one or more of the deficiencies of conventional magnetic elements.
As described in the background, perpendicular magnetic anisotropy (PMA) material layer(s) has been conventionally incorporated in magnetic elements in an attempt to improve the performance of certain aspect(s) of the magnetic element. For example, PMA materials such as Co/Pt and Co/Pd have typically been used conventionally in the magnetic tunnel junction (MTJ) for their strong effective magnetic anisotropy energy density (Keff). However, simply incorporating such a PMA material with strong Keff to improve the PMA of the MTJ may in turn cause other issues, such as poor growth quality and high damping constant (e.g., if used in contact with the free layer), which can significantly reduce the Tunneling Magnetoresistance (TMR) and increase the critical switching current density of the MTJ, respectively.
Embodiments of the present invention provide one or more multilayers (or multilayer stack) with PMA that can be incorporated into a spin current driven magnetic element to seek to address one or more of the above-mentioned deficiencies of conventional magnetic elements. In particular, the multilayer includes Cobalt/Nickel/Platinum (Co/Ni/Pt) exhibiting PMA. The multilayer stack can be defined as (Co/Ni/Pt)n, where n is the stacking number and is at least 1. As will be described later according to various embodiments of the present invention, the one or more multilayers are incorporated in the magnetic element for improving the performance(s) of the magnetic element while minimizing or preventing the introduction of undesirable side effects such as a reduction in TMR or an increase in the critical switching current density of the MTJ.
The multilayer(s) 130 has a net effective PMA energy and was surprisingly found by the present inventors to possess various advantageous magnetic properties over conventional PMA material layers. For example, the multilayer(s) 130 has a lower damping factor compared to conventional PMA materials such as Co/Pt multilayer or Co/Pd multilayer. Furthermore, it was surprisingly found by the present inventors that the effective magnetic anisotropy energy density (Keff) of the multilayer(s) 130 can advantageously be tuned by varying the thickness of Pt, for example and without limitation, from greater than 0 to about 10 Angstrom (Å) according to various embodiments of the present invention, while remaining relatively stable against annealing up to 300° C. As an exemplary illustration, Table 1 below summarizes the magnetic properties of various conventional PMA materials used in typical MTJs and the magnetic property of the present multilayer(s) 130 according to an example embodiment of the present invention, as measured by Alternating Gradient Magnetometry.
As can be seen from Table 1, the present multilayer(s) 130 advantageously enables a tunable value of Keff at a constant saturation magnetization Ms. It can also be observed that the Keff of the present multilayer(s) 130 is in the intermediate range between that of conventional multilayer Co/Ni and conventional Co/Pt-based multilayers and superlattices. This shows that the multilayer(s) 130 is able to have a value of Keff between that of Co/Ni and that of Co/Pt.
For illustration purposes only,
In the magnetic element 100, the magnetization of the free layer 104 can be switched/reversed using spin transfer torque so that the magnetization of the reference layer 102 and the free layer 104 can be substantially aligned in either a parallel or an antiparallel manner. The resistance of the magnetic element 100 will be low when the orientation of their magnetization is aligned parallel and will be high when the orientation of their magnetization is aligned antiparallel. This variation in the resistance of the magnetic element 100 can thus be used to indicate the state of the magnetic element 100 and can therefore store data. For example, data “0” may correspond to a low resistance state while data “1” may correspond to a high resistance state. When a write current passes through the magnetic element 100, the magnetization of the free layer 104 can be switched or maintained, depending on the direction of the spin angular momentum of the electrons incident on the free layer 104.
Throughout the present specification, it can be understood that when a layer or element is referred to as being “on” another layer or element, the layer or element can be directly on another layer or element (i.e., without any intermediate/intervening layers or elements therebetween) or indirectly on another layer or element (i.e., with one or more intermediate layers or elements therebetween). Therefore, unless stated otherwise, such an expression should be interpreted to cover both cases.
According to various embodiments, the multilayer(s) 130 can be incorporated in the magnetic element 100 in various ways to improve the performance(s) of the magnetic element 100. Hereinafter, various exemplary ways of incorporating the multilayer(s) 130 in the magnetic element 100 and the associated improve in performance(s) will be described according to example embodiments of the present invention. However, it will be understood that the multilayer(s) 130 may be incorporated in the magnetic element 100 in various other ways as suitable/appropriate and the present invention is not limited to the example embodiments described hereinafter. Furthermore, the present invention may be embodied in many different forms and should not be construed as limited to the example embodiments described hereinafter. Rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.
The first example embodiment illustrates an example application of providing the at least one multilayer 130 with a configurable/tunable range of perpendicular magnetic anisotropy as described hereinbefore at relatively constant saturation magnetization, and relatively lower damping constant for an advanced MRAM structure. In particular, in the first example embodiment, the at least one multilayer 130 is disposed on the MTJ such that the at least one multilayer 130 constitutes a compensating layer (the top compensating layer or field cancellation layer) 122 configured to cancel an external magnetic field towards the ferromagnetic free layer 104 emanating from outside the MTJ. In order for the top compensating layer 122 to be able to cancel the external magnetic field, the magnetization of the top compensating layer 122 and/or the magnetization of the pinning layer have to be configured to be in opposite but with equal magnetic moment. In this regard, this is possible if the switching field of the top compensating layer 122 is smaller than the pinning layer 116 such that an applied magnetic field may be applied to only switch the magnetization of the top compensating layer 122 (i.e., without also causing the magnetization of the pinning layer 116 to switch) as shown in
In order to only switch the magnetization of the top compensating layer 122, it is thus necessary to ensure that the switching field of the top compensating layer 122 is smaller than the pinning layer 116. In this regard, as described hereinbefore, the effective magnetic anisotropy energy density (Keff) of the multilayer(s) 130 forming the top compensating layer 122 can advantageously be tuned by varying the thickness of Pt. Therefore, the multilayer(s) 130 makes it possible to obtain a top compensating layer 122 to achieve the above-mentioned technical effect of cancelling an external magnetic field towards the ferromagnetic free layer 104 emanating from outside the MTJ.
In various embodiments, the top compensating layer 122 comprises (Co(t1)/Ni(t2)/Pt(t3))n, where n is the stacking number ranging from 2 to 30. In an embodiment, it has been found by the present inventors that the larger the value of n, the larger the overall stray field, therefore n can be used/selected to tune the strength of the stray field for compensating the above-mentioned external magnetic field. The thickness (t2) of Ni is preferably greater than the thickness (t1) of Co, and the thickness (t3) of Pt is preferably greater than 0 to about 10 Å. In an embodiment, the pinning layer 116 may also comprise one or more multilayers including Co/Ni/Pt, such as (Co(t1)/Ni(t2)/Pt(t3))n, where n is the stacking number ranging from 2 to 30. The number n of stacks/repeats in the top compensating layer 122 need not be the same as that of the pinning layer 116 and the thicknesses t1, t2 and t3 can be configured/varied to fine-tuning the switching field of each layer. For example, as shown in
As an example, the reference layer 402 may be provided in the form of a multi-layered structure whereby a multilayer(s) 130 of (Co/Ni/Pt)n, is formed on top of a strongly perpendicularly magnetized multilayer (first ferromagnetic material layer) 403 including at least one of (Co/Pt)n, (CoFe/Pd)n, (Co/Pt)n, (CoFe/Pt)n, where the subscript n denotes the stacking number and is at least 1. The interlayer 406 may for example be made of Ta to provide exchange coupling between the multilayer(s) of (Co/Ni/Pt)n 130 and CoFeB (second ferromagnetic material layer) 404 lying adjacent to the MgO tunnel barrier 106.
For illustration and comparison purposes only in the second example embodiment,
In the magnetic element 500 according to the third example embodiment, for example, the multilayer(s) layer 130 may act as a seed layer for the first ferromagnetic material layer 403 (e.g., (Co/Pt)n)) to mitigate the problem of difficulty in growing the first ferromagnetic material layer 403 by sputtering in forming the reference layer 402 in a top pinned MTJ structure 500.
In the fourth example embodiment, the free layer 604 may be provided in the form of a multi-layered structure 130 with a multilayer(s) of (Co/Ni/Pt)n, and CoFeB (first ferromagnetic material layer) 606 sandwiching a Ta interlayer 608 therebetween. Incorporating the multilayer(s) 130 of (Co/Ni/Pt)n, in the manner as described in the fourth example embodiment advantageously improve the thermal stability of the free layer 604 of the magnetic element 600 without undesirably affecting the critical switching current density of the magnetic element 600 due to its smaller damping constant.
In this regard, one of the present challenges in MRAM technology is to be able to maintain a high thermal stability factor (Δ) of the free layer at reduced dimension. By defining Δ=KeffV/KbT, where Keff denotes the effective magnetic anisotropy energy density, V denotes the volume of the free layer, Kb denotes the Boltzmann constant, and T denotes the absolute temperature. Conventionally, the free layer may comprise of a single CoFeB layer. However, material characteristic and thickness limitation to attain PMA restricts the value of Δ for CoFeB. For example, in the magnetic element disclosed in Lam et al. (“MgO overlayer thickness dependence of perpendicular magnetic anisotropy in CoFeB thin films”, Journal of the Korean Physical Society, Vol. 62, No. 10, pp. 1461-1464 (2013)), a value of 1.74 erg/cm2 was reported for the magnetic anisotropy of the CoFeB free layer. This value is lower than that achieved using the multilayer(s) 130 of (Co/Ni/Pt)n, as for example illustrated in
As mentioned hereinbefore, it will be understood by a person skilled in the art that the multilayer 130 can be incorporated in the magnetic element 100 in various ways to improve the performance(s) of the magnetic element 100, and the present invention is not limited to incorporating the multilayer 130 in the magnetic element 100 in the manner as described hereinbefore in the exemplary embodiments. It will also be understood that the magnetic elements as described hereinbefore with reference to
It can be understood that a person skilled in the art would be able to apply appropriate/suitable deposition techniques and conditions known in the art to form the magnetic elements described hereinbefore according to various embodiments of the present invention. Therefore, it is not necessary to describe the specific deposition techniques and conditions herein. For example and without limitation, a direct current magnetron sputtering method may be used for depositing the multilayer according to various embodiments of the present invention, where the base pressure is less than 5×10−9 Torr. The thickness of each layer can be controlled based on the deposition time. The Co, Ni and Pt layers may be repetitively deposited to form (Co/Ni/Pt)n, where n denotes the stacking number.
While embodiments of the invention have been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10201406062S | Sep 2014 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2015/050345 | 9/25/2015 | WO | 00 |
