Patent Description:
The field of spin transport electronics (also known as spin electronics or spintronics) has a number of practical applications, non-limiting examples of which include magnetoresistance devices for reading magnetically-encoded data storage media; and magnetoresistive random-access memory for computers.

The Spin Hall Effect (SHE) has been exploited for electrically-manipulating electron spin in a variety of spintronics applications. Spin current involves an alignment of intrinsic electron spin, and has a specified orientation. Notable materials which exhibit strong SHE behavior are the heavy metals.

The publication entitled "<NPL>, discloses a Pt/Cu junction with a strong spin-orbit coupling.

Patent documents <CIT> and <CIT> reveal devices or circuits based on spin torque transfer (STT) and Spin Hall effect by using a spin Hall effect (SHE) metal layer coupled to a ferromagnetic layer.

Currently, Spin Hall Effect devices rely on heavy metals to provide spin current. In some cases, however, it is desirable to reduce the dependence on heavy metals by utilizing ordinary metals to provide spin current in devices, while reducing the extent of heavy metal required. This goal is met by embodiments of the present invention.

Embodiments of the present invention provide devices for sensing and manipulating magnetic fields based on spin current interactions independent of the Spin Hall Effect (SHE) in heavy metal. According to various embodiments of the invention spin current is generated by conversion of out-of-plane orbital current arising from the Orbital Hall Effect (OHE) in ordinary metals. The OHE does not rely on spin-orbital coupling and orbital currents are produced in ordinary metals.

The term "heavy metal" herein denotes metallic elements having 5d electron shells, particularly including, but not limited to: Platinum (Pt); Tungsten (W); and Tantalum (Ta). In contrast, the term "ordinary metal" herein denotes metallic elements lacking 5d electron shells and having at most 3d or 3d/4d electron shells, or lacking d subshells altogether. Ordinary metals particularly include, but are not limited to: Copper (Cu); and Aluminum (Al). Heavy metals exhibit strong spin-orbital coupling (SOC), whereas ordinary metals exhibit weak spin-orbital coupling.

Orbital current involves an alignment of the orbital motion of atomic electrons and is distinct from the intrinsic spin of the electron. Orbital current has a specified orientation. Embodiments of the invention provide conversion from orbital current to spin current via thin layers of heavy metal, typically only several atomic layers thick (in the nanometer range) capping ordinary metal planar components, thereby substantially reducing heavy metal requirements by replacing most of the heavy metal with ordinary metal. The term "capping" herein denotes that the thin layers adjoin the surfaces of the metal planar components and are in electrical contact therewith. As disclosed herein, non-limiting applications for devices according to the present invention include magnetoresistive sensors for detecting and measuring magnetic fields, and magnetic tunnel junction data storage.

"Charge current" herein refers to the flow of electric charges in a conductor, as encountered in ordinary electrical circuits.

Thus, embodiments of the present invention substantially reduce the requirements for heavy metals by utilizing ordinary metals to provide orbital current, and then converting the orbital current to spin current using a minimal extent of heavy metal.

Therefore, in the present invention according to claim <NUM> there is provided a device for generating a spin current.

The subject matter disclosed may best be understood by reference to the following detailed description when read with the accompanying drawings in which:.

For simplicity and clarity of illustration, items shown in the figures are not necessarily drawn to scale, and the dimensions of some items may be exaggerated relative to other items. In addition, reference numerals may be repeated among the figures to indicate corresponding or analogous items.

<FIG> conceptually illustrates a prior-art device configuration <NUM> utilizing a heavy metal planar component <NUM> for generating an out-of-plane spin current <NUM> from an in-plane charge current <NUM> flowing through heavy metal planar component <NUM>. As noted previously, Platinum, tungsten and tantalum are often used as the heavy metal for planar component <NUM>.

The term "planar component" herein denotes a three-dimensional component with at least one face which is substantially in the form of a plane and which has a surface area sufficient for adjoining to a corresponding face of another component. A planar component has an "in-plane" axis which lies inside the component and is oriented parallel to the surface of the face, and an "out-of-plane" axis which is at least partly outside the component and is oriented perpendicular to the surface of the face. Charge current flow direction within a planar component is herein referenced to the in-plane axis of the planar component; spin current orientation and orbital current orientation are herein referenced to the out-of-plane axis of the planar component.

<FIG> conceptually illustrates a device component configuration <NUM> in which the extent of heavy metal is substantially reduced to a thin layer <NUM>. A metal planar component <NUM> is made of ordinary metal and constitutes most of the bulk of device <NUM>, such that layer <NUM> is in electrical contact with the top face of planar component <NUM>. In-plane charge current <NUM> flowing through ordinary metal <NUM> generates an out-of-plane orbital current <NUM> via the Orbital Hall Effect (OHE), and thin layer <NUM> converts out-of-plane orbital current <NUM> into out-of-plane spin current <NUM> via spin-orbital coupling. In a related example, heavy metal layer <NUM> is only a few atoms thick - a total thickness of <NUM> nanometer is typically sufficient. (As noted previously, the drawings are not intended to be at scale, and thin layer <NUM> is exaggerated in size in the drawings for clarity. ) Thus, by utilizing ordinary metal for conducting charge current <NUM>, examples of the present invention provide a substantial reduction in the amount of heavy metal required to generate out-of-plane spin current. In other examples, copper and aluminum are used for ordinary metal <NUM>.

<FIG> conceptually illustrates a prior-art device configuration <NUM> utilizing heavy metal planar component <NUM> for providing out-of-plane spin current <NUM> from in-plane charge current <NUM>, as illustrated in <FIG>. In this example, out-of-plane spin current <NUM> controls magnetic field orientation <NUM> of a permanent magnet <NUM> situated above heavy-metal planar component <NUM>.

<FIG> conceptually illustrates a prior-art three-terminal magnetic tunneling junction device <NUM>. A magnetic tunneling junction <NUM> (MTJ) incorporates a ferromagnetic reference layer <NUM> above a thin insulating layer <NUM>, below which is a ferromagnetic free layer <NUM>. Insulating layer <NUM> is typically only a few nanometers thick, so that electrons can tunnel through insulating layer <NUM> and pass between reference layer <NUM> and free layer <NUM>. Reference layer <NUM> remains magnetized in a direction <NUM>, but the magnetization of free layer <NUM> can be switched between a direction <NUM> (which is parallel to direction <NUM>) and a direction <NUM> (which is anti-parallel to direction <NUM>). When free layer <NUM> is magnetized in direction <NUM> parallel to direction <NUM>, electrons tunnel with high statistical probability across insulating layer <NUM>, thereby establishing a low resistance electrically-conductive path between reference layer <NUM> and free layer <NUM>. However, when free layer <NUM> is magnetized in direction <NUM> anti-parallel to direction <NUM>, electrons tunnel with low statistical probability across insulating layer <NUM>, thereby establishing a high electrical resistance between reference layer <NUM> and free layer <NUM>. Thus, switching the magnetization direction of free layer <NUM> effectively switches MTJ <NUM> between an electrically-conducting state and a substantially non-conducting state. In device <NUM>, switching the magnetization direction of free layer <NUM> is accomplished by switching an out-of-plane spin current <NUM> provided by a heavy metal planar component <NUM> as a result of an in-plane charge current <NUM> injected through heavy metal planar component <NUM> in response to the Spin Hall Effect. The direction of charge current flow <NUM> governs the direction of out-of-plane spin current <NUM>, and hence controls the conductivity state of MTJ <NUM>. A first electrical terminal T<NUM> <NUM> connects to reference layer <NUM>; a second electrical terminal T<NUM> <NUM> connects to one side of heavy metal planar component <NUM>; and a third electrical terminal T<NUM> <NUM> connects to the other side of heavy metal planar component <NUM>. Thus, the magnitude and direction of in-plane charge current <NUM> are given by the electrical current flowing between terminal T<NUM> <NUM> and terminal T<NUM> <NUM>. The conductivity state of MTJ <NUM> can be detected externally by sensing the electrical resistance between terminal T<NUM> <NUM> and either of terminal T<NUM> <NUM> or terminal T<NUM> <NUM>. A non-limiting application of device <NUM> is for data storage, where a single bit of data can be represented by the conductivity state of MTJ <NUM>. The bit is written by passing a pulse of current between terminal T<NUM> <NUM> and terminal T<NUM> <NUM> in a left-or-right direction according to the desired bit value to be written; and the bit is read by sensing the electrical resistance between terminal T<NUM> <NUM> and either of terminal T<NUM> <NUM> or terminal T<NUM> <NUM>.

<FIG> conceptually illustrates a two-terminal device <NUM> according to an embodiment of the present invention for detecting and measuring the rotation of a permanent magnetic field <NUM> in a ferromagnetic material <NUM> by an external magnetic field <NUM>. In a non-limiting example, external magnetic field <NUM> corresponds to a state of a magnetic data storage medium, and the device illustrated in <FIG> and described herein reads data from the magnetic data storage medium.

This embodiment provides a magnetic field sensor, described as follows: As illustrated in <FIG> and disclosed herein above, a planar component <NUM> of ordinary metal is capped with a thin layer <NUM> of heavy metal (only a few atomic layers thick). In <FIG> are shown a first electrical terminal T<NUM> <NUM> connected to one side of planar component <NUM> and a second electrical terminal T<NUM> <NUM> connected to the other side of planar component <NUM> which allow sensing the electrical resistance of planar component <NUM> when an in-plane charge current <NUM> passes through the ordinary metal of planar component <NUM> and establishes an out-of-plane orbital current <NUM>. Because of the high spin-orbital coupling of layer <NUM>, orbital current <NUM> is coupled to an out-of-plane spin current <NUM>. Spin current <NUM> is affected by rotated magnetic field <NUM>, which in turn affects coupled orbital current <NUM>, and this in turn affects in-plane charge current <NUM>. The affect on in-plane charge current <NUM> is detected and measured by measuring the electrical resistance in ohms (Ω) across terminals T<NUM> <NUM> and T<NUM> <NUM>, such as with electrical resistance-measuring apparatus <NUM>. Since rotation of magnetic field <NUM> is a function of the strength and orientation of external magnetic field <NUM>, the strength and orientation of external magnetic field <NUM> may also be measured in terms of the electrical resistance across terminals T<NUM> <NUM> and T<NUM> <NUM> (such as with apparatus <NUM>), and hence the device of <FIG> provides a magnetic field sensor.

<FIG> conceptually illustrates establishing a magnetic field <NUM> having an upward orientation in a permanent magnet <NUM> by a device <NUM> When an in-plane charge current <NUM> flows through ordinary metal planar component <NUM> in a left-to-right direction, an out-of-plane orbital current <NUM> is established, oriented in an upward direction through heavy metal layer <NUM>. Heavy metal layer <NUM> converts orbital current <NUM> into an out-of-plane spin current <NUM>, also oriented in an upward direction. Out-of-plane spin current <NUM> in turn creates magnetic field <NUM> likewise oriented in an upward direction.

<FIG> conceptually illustrates establishing a magnetic field <NUM> having a downward orientation in permanent magnet <NUM> by device <NUM>. When an in-plane charge current <NUM> flows through ordinary metal planar component <NUM> in a right-to-left direction, an out-of-plane orbital current <NUM> is established, oriented in downward direction through heavy metal layer <NUM>. Heavy metal layer <NUM> converts orbital current <NUM> into an out-of-plane spin current <NUM>, also oriented in a downward direction. Out-of-plane spin current <NUM> in turn creates magnetic field <NUM> likewise oriented in a downward direction.

<FIG> conceptually illustrates a magnetic tunneling junction device <NUM> switched by an out-of-plane orbital-to-spin current conversion according to an embodiment of the present invention. MJT <NUM> incorporates a ferromagnetic reference layer <NUM> above a thin insulating layer <NUM>, below which is a ferromagnetic free layer <NUM>. Insulating layer <NUM> is typically only a few nanometers thick, so that electrons can tunnel across it between reference layer <NUM> and free layer <NUM>. Reference layer <NUM> remains magnetized in a direction <NUM>, but the magnetization of free layer <NUM> can be switched between a direction <NUM> (which is parallel to direction <NUM>) and a direction <NUM> (which is anti-parallel to direction <NUM>). When free layer <NUM> is magnetized in direction <NUM> parallel to direction <NUM>, electrons tunnel with high statistical probability across insulating layer <NUM>, thereby establishing a low resistance electrically-conductive path between reference layer <NUM> and free layer <NUM>. However, when free layer <NUM> is magnetized in direction <NUM> anti-parallel to direction <NUM>, electrons tunnel with low statistical probability across insulating layer <NUM>, thereby establishing a high electrical resistance between reference layer <NUM> and free layer <NUM>. Thus, switching the magnetization direction of free layer <NUM> effectively switches MTJ <NUM> between an electrically-conducting state and a substantially non-conducting state.

Device <NUM> as illustrated in <FIG> and described herein according to this embodiment of the present invention is notably distinct over similar device <NUM> of <FIG>, in that the switching mechanism of <FIG> includes an ordinary metal planar component <NUM> capped by a thin heavy metal layer <NUM>, and in that an injected charge current <NUM> flows substantially through ordinary metal rather than heavy metal, thereby substantially replacing heavy metal with ordinary metal. Whereas device <NUM> relies on the Spin Hall Effect in heavy metal planar component <NUM> for generating spin current <NUM>, device <NUM> does not rely on the Spin Hall Effect, but instead utilizes the Orbital Hall Effect in ordinary metal planar component <NUM>, and then converts a resulting out-of-plane orbital current <NUM> into an out-of-plane spin current <NUM> via a thin heavy metal layer <NUM>.

A non-limiting application of device <NUM> is for data storage, where a single bit of data can be represented by the electrical conductivity state of MTJ <NUM>.

A first electrical terminal T<NUM> <NUM> is connected to reference layer <NUM>; a second electrical terminal T<NUM> <NUM> is connected to one side of ordinary metal planar component <NUM>; and a third electrical terminal T<NUM> <NUM> is connected to the other side of ordinary metal planar component <NUM>. Thus, the magnitude and direction of in-plane charge flow <NUM> is controlled by the electrical current flowing between terminal T<NUM> <NUM> and terminal T<NUM> <NUM>, such as from a bi-directional electrical source <NUM>, which changes direction of the current to selectably write the data bit value of <NUM> or <NUM>. The conductivity state of MTJ <NUM> can be detected externally by sensing the electrical conductivity between terminal T<NUM> <NUM> and either of terminal T<NUM> <NUM> or terminal T<NUM> <NUM>, such as by an electrical conductivity detector <NUM>, which detects the data bit value. The single bit is written by passing a pulse of current between terminal T<NUM> <NUM> and terminal T<NUM> <NUM> in a left-or-right direction according to the desired bit value to be written; and the single bit is read by determining the electrical conductivity state of MTJ <NUM>, such as by sensing the electrical resistance between terminal T<NUM> <NUM> and either of terminal T<NUM> <NUM> or terminal T<NUM> <NUM>.

Claim 1:
A device (<NUM>, <NUM>) for generating a spin current, the device comprising:
a planar component (<NUM>, <NUM>) for generating an out-of-plane orbital current via Orbital Hall Effect (OHE) of an in-plane charge current flow within the planar component, wherein the planar component (<NUM>, <NUM>) is substantially constituted of an ordinary metal lacking a 5d electron shell;
a heavy metal layer (<NUM>, <NUM>) in electrical contact with a face of the planar component, for converting the out-of-plane orbital current to an out-of-plane spin-current, wherein the heavy metal has a 5d electron shell;
characterized by a ferromagnetic component (<NUM>, <NUM>) above the heavy metal layer (<NUM>, <NUM>); by two electrical terminals (<NUM>, <NUM>; <NUM>, <NUM>) connected to different sides of the planar component (<NUM>, <NUM>); and by an electrical resistance measuring apparatus (<NUM>, <NUM>) connected between the two electrical terminals.