IN THE ABSENCE OF EXTERNAL MAGNETIC FIELDS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of co-pending U.S. provisional patent application Serial No. 62/158,805, SWITCHING OF

PERPENDICULARLY MAGNETIZED NANOMAGNETS WITH SPIN-ORBIT TORQUES IN THE ABSENCE OF EXTERNAL MAGNETIC FIELDS, filed May 8, 2015, which application is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government Support under Contract

GR526115 and GR526116 awarded by the Intelligence Advanced Research Projects Activity (IARPA). The Government has certain rights in the invention.

FIELD OF THE APPLICATION

[0003] The application relates to switching the magnetization of nanomagnets and particularly to a base element structure and method for switching the magnetization of nanomagnets.

BACKGROUND

[0004] Complementary metal-oxide-semiconductor (CMOS) technologies are prevalent today in memory and logic systems. However, CMOS technologies no longer provide a desired balance of fast operation, high density integration, and energy efficiency.

SUMMARY

[0005] According to one aspect, a base element for switching a

magnetization state of a nanomagnet includes a heavy-metal strip having a surface. A ferromagnetic nanomagnet is disposed adjacent to the surface. The ferromagnetic nanomagnet has a first magnetization equilibrium state and a second magnetization equilibrium state. The first magnetization equilibrium state or the second magnetization equilibrium state is settable in an absence of an external magnetic field by a flow of electrical charge through the heavy-metal strip.

[0006] In another embodiment, the flow of electrical charge in a first direction through the heavy-metal strip causes the first magnetization equilibrium state and the flow of electrical charge in a second direction through the heavy-metal strip causes the second magnetization equilibrium state.

[0007] In yet another embodiment, the nanomagnet includes an elliptical shape having a long axis and a short axis.

[0008] In yet another embodiment, the long axis is about parallel to the surface of the heavy-metal strip.

[0009] In yet another embodiment, a direction of flow of the electrical charge through the heavy-metal strip includes an angle ξ with respect to the short axis of the nanomagnet.

[0010] In yet another embodiment, the angle ξ determines an energy of switching.

[0011] In yet another embodiment, the angle ξ determines a speed of switching.

[0012] In yet another embodiment, the base element provides a bit of an integrated memory device.

[0013] In yet another embodiment, the base element provides a bit of an integrated logic device.

[0014] In yet another embodiment, the base element provides a bit of an integrated pipelined microprocessor device.

[0015] In yet another embodiment, the heavy-metal strip includes tungsten or tantalum.

[0016] In yet another embodiment, the heavy-metal strip includes Aluminum

(Al) or Gold (Au).

[0017] In yet another embodiment, the heavy-metal strip includes Bismuth

(Bi) or Molybdenum (Mo). [0018] In yet another embodiment, the heavy-metal strip includes Niobium

(Nb) or Palladium (Pd).

[0019] In yet another embodiment, the heavy-metal strip includes Platinum

(Pt).

[0020] In yet another embodiment, the heavy-metal strip includes an alloy of copper (Cu) and Bi, or an alloy of Cu and iridium (Ir).

[0021] According to another aspect, a method for switching a magnetization state of a nanomagnet includes the steps of: providing a heavy-metal strip having a surface and a ferromagnetic nanomagnet disposed adj acent to the surface, the ferromagnetic nanomagnet having a first magnetization equilibrium state and a second magnetization equilibrium state; flowing an electrical charge through the heavy-metal strip in an electrical charge direction to set the magnetization state of the nanomagnet in an absence of an external magnetic field to the first magnetization equilibrium state, or to set the magnetization state to the second magnetization equilibrium state.

[0022] In another embodiment, the step of flowing an electrical charge includes flowing an electrical charge through the heavy-metal strip in an electrical charge direction to set the magnetization state within a time period of less than about 50 picoseconds.

[0023] In yet another embodiment, the step of flowing an electrical charge includes flowing an electrical charge through the heavy-metal strip in an electrical charge direction to set the magnetization state where the magnetization state corresponds to setting a bit of a memory device.

[0024] In yet another embodiment, the step of flowing an electrical charge includes flowing an electrical charge through the heavy-metal strip in an electrical charge direction to set the magnetization state where the magnetization state corresponds to setting a bit of a logic device.

[0025] The foregoing and other aspects, features, and advantages of the application will become more apparent from the following description and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The features of the application can be better understood with reference to the drawings described below, and the claims. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles described herein. In the drawings, like numerals are used to indicate like parts throughout the various views.

[0027] FIG. 1 is a drawing showing a ferromagnetic layer adjacent to a heavy-metal nonmagnetic nanostrip;

[0028] FIG. 2 is a drawing showing how the dynamics of the magnetization motion can be captured by ϋ and φ ;

[0029] FIG. 3 A is a drawing showing that the charge current ( J _e ) injected through the nonmagnetic heavy-metal induces spin current ( J _s );

[0030] FIG. 3B is a drawing showing an exemplary elliptical ferromagnet having an in-plane anisotropy H _fa ;

[0031] FIG. 3C is a drawing that shows the charge current direction and orientation of the spin polarization σ with respect to H _t ;

[0032] FIG. 4 is a drawing showing the motion of magnetization under the influence of spin-torques and anisotropies; and

[0033] FIG. 5 is a drawing showing the trajectory of the magnetization switching of a ferromagnetic layer using spin-orbit torques in the absence of an external magnetic field.

DETAILED DESCRIPTION

[0034] Magnetization switching of ferromagnets using spin-orbit torques provides opportunities to introduce nanomagnets into high performance logic and memory applications requiring low power consumption. Nanomagnets with perpendicular-to-the-plane anisotropy have recently attracted a considerable attention due to their high thermal stability. High stability against thermal fluctuations allows nanomagnets to be deeply scaled down, resulting in dense logic and memory systems with ultra-low power consumption. However, due to the symmetric energy landscape experienced by the magnetization of a nanomagnet with perpendicular-to-the-plane anisotropy, spin-orbit torques induced by an in- plane current pulse cannot switch the magnetization. An external magnetic field is, therefore, required to assist spin-orbit torques by breaking the symmetry. Although the energy dissipated by switching a nanomagnet could be small, the energy necessary to generate the required magnetic field makes the overall memory or logic scheme uncompetitive as compared to complementary metal-oxide-semiconductor (CMOS) counterparts. Additional metals are also necessary to produce the required magnetic field, significantly decrease the number of devices which can be integrated over a given area. Therefore, the need for an external magnetic field is an obstacle for developing dense low power memory and logic systems. Furthermore, fast switching requires higher energy to be injected through the ferromagnet and/or metals producing magnetic field. Since the required energy grows significantly as the desired switching speed increases, fast operation compromises the energy efficiency.

[0035] A solution to the problems described hereinabove switches the magnetization of a nanomagnet with perpendicular-to-the-plane anisotropy using spin-orbit torques induced by an in-plane current pulse without the presence of an external magnetic field.

[0036] The solution includes a scheme to switch the magnetization of a nanomagnet with perpendicular-to-the-plane anisotropy using spin-orbit torques induced by an in-plane current pulse without the presence of an external magnetic field. It was realized that magnetization switching can be achieved by breaking the symmetry by introducing an in-plane anisotropy into the nanomagnet. We describe how spin orbit torques induced by an in-plane current pulse of appropriate amplitude and duration are sufficient to switch the magnetization of the nanomagnet in absence of an external magnetic field. For a given ratio between the in-plane and perpendicular-to-the-plane anisotropies, high switching probability (deterministic switching) is achievable for current pulses of significantly short duration by balancing the spin-orbit and damping torques, resulting in ultra-fast switching. Furthermore, since external magnetic field is not required for magnetization switching within the described scheme, energy efficiency and integration density is significantly improved, resulting in ultra-fast dense memory and low power consumption logic systems. [0037] FIG. 1 shows an exemplary ferromagnetic layer including a perpendicular- to-the-plane anisotropy ( H _fc ) and an in-plane anisotropy ( H _fa ) is situated on a heavy-metal nanostrip. In one exemplary embodiment, the proposed structure of base element 100, FIG. 1 shows a ferromagnetic layer represented by a Stoner-Wohlfarth monodomain magnetic body 101 with magnetization M , situated at a heavy-metal nonmagnetic nanostrip 102 with strong spin-orbit coupling. The ferromagnetic layer, as shown in FIG. 1, includes a perpendicular-to-the-plane uniaxial anisotropy H _fc along the e _z axis and an in-plane uniaxial anisotropy H _fa along the e axis.

[0038] FIG. 2 shows how the dynamics of the magnetization motion can be captured by and φ . As shown in FIG. 2, the motion of M is represented by a unit vector n _m , which makes an angle with e _z axis, while the plane of M and e _z makes an angle φ with e .

[0039] FIG. 3 A shows that the charge current ( J _e ) injected through the nonmagnetic heavy-metal induces spin current ( J _s ). As shown in FIG. 3 A, a charge current J _e , injected through the heavy-metal nanostrip, produces a traverse spin current ^~ @m( ^{c x} '$ _e ) d _ue to the spin-orbit interaction, where J _e is the charge current density, σ is the spin polarization unit vector, and Θ _8Η is the material dependent spin Hall angle.

[0040] FIG. 3B shows an illustration of an exemplary elliptical ferromagnet having an in-plane anisotropy H _fa .

[0041] FIG. 3C shows the charge current direction 301 and orientation of the spin polarization σ with respect to the H _fa . As shown in FIG. 3C, the direction of the charge current J _e makes an angle of ξ with e axis. Spin polarized current transports spin angular momentum into the nanomagnet, exerting a torque on the magnetization. The dynamics of M under the influence of torques and anisotropy fields is described usin the Landau-Lifshitz-Gilbert (LLG) equation as where γ is the gyromagnetic ratio, a is the damping factor, T _ST is the spin torque, and H _e is the effective field experienced by the magnetization of the

ferromagnetic layer. H _e /f is ^a function of H _kx and H _fcz . The spin torque has two components, referred to as the in-plane and out-of-plane torques: T _ST = T _jp + T _00P .

[0042] FIG. 4 shows the motion of magnetization under the influence of spin-torques and anisotropies. As demonstrated in Fig. 4, the in-plane torque T^, lies in the plane defined by M and H _eff , and the out-of-plane torque T _00p points out of the plane defined by M and H _eff .

[0043] FIG. 5 shows the trajectory of the magnetization switching of a ferromagnetic layer using spin-orbit torques in the absence of any external magnetic field. As shown in FIG. 5, by injecting charge current J _e through the heavy-metal nonmagnetic nanostrip, produced spin torques derive M out of the equilibrium position (also called an equilibrium state) toward the in-plane of the nanomagnet. By turning the charge current J _e off after t _e seconds, spin torque reduces to zero and M is close to the x-y plane and away from the e _z axis by an angle of ϋ . At this zone, referred here to as the critical zone, H _e// is significantly dominated by H _fa .

Therefore, M passes the hard axis by precessing around the H _e// . By passing the hard axis, H _e// is dominated by H _fe . Hence, M is pulled towards the new equilibrium state by precessing and damping around H _e// , completing the magnetization switching.

[0044] The duration t _e of the applied current pulse is as short as the time which causes the magnetization M to move from the equilibrium state to the critical zone. The magnetization switching can be performed using current pulses of a duration of sub-50 ps. Therefore, the proposed scheme significantly improves the switching speed and/or reduces the energy consumption, resulting in ultra-highspeed spin-torque memory and logic systems which have significantly low energy consumption. Furthermore, as no extra metal is required for producing an external magnetic field, integration density is considerably enhanced. [0045] It is contemplated that both switching energy and switching speed can be determined by the angle ξ. It is also contemplated that there is a tradeoff between switching energy and switching speed as can be set by the angle ξ.

[0046] Heavy-metals as used hereinabove include any suitable transition metals having a large atomic number, such as, for example, tungsten (W), tantalum (Ta), Aluminum (Al), Gold (Au), Bismuth (Bi), Molybdenum (Mo), Niobium (Nb), Palladium (Pd), or Platinum (Pt). Also included are any suitable metal alloys, such as, for example, an alloy of copper (Cu) and Bi, or an alloy of Cu and iridium (Ir). By injecting a charge current through a heavy-metal thin film of any suitable metal or metal alloy as listed hereinabove, a traverse spin current is produced due to strong spin-orbit coupling. As described hereinabove, the produced spin current may be used to switch the direction of the magnetization of a nanomagnet. By injecting a charge current through a heavy-metal thin film, a traverse spin current is produced due to strong spin-orbit coupling. The produced spin current may be used to switch the direction of the magnetization of a nanomagnet. The magnitude of the produced spin current is directly proportional to the spin Hall angle of a thin film heavy-metal. Large spin Hall angles have been observed in some high resistivity thin films of heavy-metals. It has been shown both experimentally and theoretically that the magnitude of the spin Hall angle in some thin film heavy-metals such as, for example, thin films of W is directly proportional to the resistivity (thickness) of the thin film. For example, it has been observed that by increasing the thickness of a thin film of tungsten from 5.2 nm to 15 nm, the spin Hall angle drops from 0.33 to less than 0.07.

[0047] The magnitude of the produced spin current is directly proportional to the spin Hall angle of a thin film heavy-metal. Large spin Hall angles have been observed in some high resistivity thin films of heavy-metals. It has been shown both experimentally and theoretically that the magnitude of the measured (calculated) spin Hall angle in thin film heavy-metals is directly proportional to the resistivity (thickness) of the thin film. For example, it has been observed that by increasing the thickness of a thin film of tungsten from 5.2 nm to 15 nm, the spin Hall angle drops from 0.33 to less than 0.07. [0048] In summary with reference to the exemplary embodiment of FIG. 1 , a base element 100 for switching a magnetization state of a nanomagnet 101 includes a heavy-metal strip 102 having a surface 103. The ferromagnetic nanomagnet 101 is disposed adjacent to the surface 103 of the heavy-metal strip 102. The

ferromagnetic nanomagnet 101 as a first magnetization equilibrium state 501 and a second magnetization equilibrium state 502 (also referred to in some embodiments as an upward equilibrium state and a downward equilibrium state). The first magnetization equilibrium state 501 or the second magnetization equilibrium state 502 is settable by a flow of electrical charge having an electrical charge current direction 301 through the heavy-metal strip. The ferromagnetic nanomagnet can also be a feature of a magnetic layer in an integrated device incorporating the base element.

[0049] In some embodiments, by causing a flow of charge (current) in the heavy-metal strip as described hereinabove the magnetization of the nanomagnet can be switched between a first equilibrium state and a second equilibrium state, such as by reversing the direction of the flow of charge. In some contemplated applications, such as, for example where the structure is a base element of a memory or a logic system, the first equilibrium state can be assigned to either a Boolean "0" or a "1" and the second equilibrium state can be assigned to the other Boolean number different from the first equilibrium state. In such contemplated applications, the method to change the magnetization as described hereinabove is analogous to a "write" operation.

[0050] Also, in such contemplated applications, methods for reading the magnetization state of a base element are known, such as, for example, by adding an insulating layer over the nanomagnet and another magnetic layer having a fixed magnetization over the insulating layer. When the nanomagnet is switched to a magnetization equilibrium state about parallel to the magnetization of the fixed magnetization magnetic layer, there will be a low electrical resistance between the magnetic layer having a fixed magnetization and the magnetic layer having a switchable magnetization. Conversely, when the nanomagnet is switched to a magnetization equilibrium state about anti-parallel to the magnetization of the fixed magnetization magnetic layer, there will be a high electrical resistance between the magnetic layer having a fixed magnetization and the magnetic layer having a switchable magnetization. Thus, in some embodiments, a "read" operation to determine the magnetization state of the base element (e.g. a single "bit") can be performed by sensing a low resistance or a high resistance.

[0051] It is contemplated that the base element described hereinabove can be used as a bit of an integrated device, such as, for example, a memory device or a logic device. In such applications, techniques of integration known in the art can be used to form and interconnect a plurality of such base elements. It is contemplated that billions of such base elements with nanomagnets of an integrated magnetic layer can be integrated into a single integrated device. Internal integrated electrical connections between base elements can be made using integrated circuit interconnection techniques known in the art.

[0052] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.