Spin Hall magnetoresistance

Last updated February 23, 2019

Spin Hall Magnetoresistance (SMR) is a transport phenomenon that is found in some electrical conductors that have at least one surface in direct contact with another magnetic material due to changes in the spin current that are present in metals and semiconductors with a large spin Hall angle.^[1] It is most easily detected when the magnetic material is an insulator which eliminates other magnetically sensitive transport effects arising from conduction in the magnetic material.

Origins

Spin Hall magnetoresistance is one of many ways in which the electrical resistance of a material is influenced by the spin Hall effect. An electron moving through a conductor is scattered by the spin Hall effect in a direction determined by its spin orientation which induces a net accumulation of spin at the conductors edge.^[2] The spin polarized electrons at the conductors surface are able to interact with the magnetization of a magnetic material in close proximity through a spin-transfer torque. When the conduction electrons spin is aligned parallel to the magnetization direction the electron reflects from the conductor surface with no change in its spin, however, when there is a component of the magnetization that is normal to the spin orientation, the spin can be flipped to its opposite state transferring angular momentum into the magnetic material. This results in a spin current that travels at a normal to the direction of the charge current that can be altered by changing the direction of magnetization.^[3] This spin current is deflected through the inverse spin Hall effect which adds or subtracts from the electrons momentum in the direction of the charge current depending on the size and sign of the conductors spin Hall angle. This deflection provides an addition to the conductors resistivity allowing the spin current to be estimated by the change in the electrical resistivity.^[4]

Spin-transfer torque is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.

Description

To construct a device that exhibits the spin Hall magnetoresistance, a multilayer of conductor and magnetic material is needed. Platinum is commonly used as a conductor due to its large spin Hall angle and YIG is used as a magnetic material with the conductor being deposited on top with a clean interface. The magnetization of the YIG can be rotated by an applied magnetic field strong enough to saturate it which results in a change in the conductors resistivity. The scale of the resistance change observed depends on the conductors spin Hall angle and the ratio of the spin diffusion length and the conducting materials thickness. As most spin diffusion lengths are short, the effect is only significant in materials that are only several nanometers thick.

Yttrium iron garnet (YIG) is a kind of synthetic garnet, with chemical composition Y₃Fe₂(FeO₄)₃, or Y₃Fe₅O₁₂. It is a ferrimagnetic material with a Curie temperature of 560 K. YIG may also be known as yttrium ferrite garnet, or as iron yttrium oxide or yttrium iron oxide, the latter two names usually associated with powdered forms.

Angular dependency

One of the signatures of the spin Hall magnetoresistance is that the change in resistance is observed when the magnetization of the insulator is rotated with respect to the spin axis and not to the direction of the charge current as is seen in anisotropic magnetoresistance.^[5] The change in resistivity follows a squared sine wave pattern when the magnetization vector is rotated about an axis that has a component normal to the spin axis. Platinum has been observed to have maximum resistivity changes of up to 0.12%.^[1]

Temperature dependence

In platinum the maximum resistance change is found to reach a maximum at approximately 120K for all thicknesses^[6]

Applications

Due to the spin-transfer torque at the interface of the conductor and magnet, a spin current can be injected from the metal into the insulator. This allows for new spintronics experiments to investigate the possibility of transmitting spin information through an insulator which would have the advantage of no power loss due to Joule heating.^[3]

Spintronics, also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. The field of spintronics concerns spin-charge coupling in metallic systems; the analogous effects in insulators fall into the field of multiferroics.

Joule heating, also known as Ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor produces heat.

Related Research Articles

Anisotropy, is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties

Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished. Ferromagnetism is the strongest type and is responsible for the common phenomena of magnetism in magnets encountered in everyday life. Substances respond weakly to magnetic fields with three other types of magnetism, paramagnetism, diamagnetism, and antiferromagnetism, but the forces are usually so weak that they can only be detected by sensitive instruments in a laboratory. An everyday example of ferromagnetism is a refrigerator magnet used to hold notes on a refrigerator door. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today".

The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was discovered by Edwin Hall in 1879. For clarity, the original effect is sometimes called the ordinary Hall effect to distinguish it from other "Hall effects" which have different physical mechanisms.

Magnetoresistance is the tendency of a material to change the value of its [electrical resistance] in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance: some occur in bulk non-magnetic metals and semiconductors, such as geometrical magnetoresistance, Shubnikov de Haas oscillations, or the common positive magnetoresistance in metals. Other effects occur in magnetic metals, such as negative magnetoresistance in ferromagnets or anisotropic magnetoresistance (AMR). Finally, in multicomponent or multilayer systems, giant magnetoresistance (GMR), tunnel magnetoresistance (TMR), colossal magnetoresistance (CMR), and extraordinary magnetoresistance (EMR) can be observed.

Electrical resistivity is a fundamental property of a material that quantifies how strongly that material opposes the flow of electric current. A low resistivity indicates a material that readily allows the flow of electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-metre (Ω⋅m). As an example, if a 1 m × 1 m × 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.

In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism.

In physics, a ferrimagnetic material is one that has populations of atoms with opposing magnetic moments, as in antiferromagnetism; however, in ferrimagnetic materials, the opposing moments are unequal and a spontaneous magnetization remains. This happens when the populations consist of different materials or ions (such as Fe²⁺ and Fe³⁺).

A Hall effect sensor is a device that is used to measure the magnitude of a magnetic field. Its output voltage is directly proportional to the magnetic field strength through it.

Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.

Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough, electrons can tunnel from one ferromagnet into the other. Since this process is forbidden in classical physics, the tunnel magnetoresistance is a strictly quantum mechanical phenomenon.

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Exchange bias or exchange anisotropy occurs in bilayers of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film. The exchange bias phenomenon is of tremendous utility in magnetic recording, where it is used to pin the state of the readback heads of hard disk drives at exactly their point of maximum sensitivity; hence the term "bias."

Spin pumping is a method of generating a spin current, the spintronic analog of a battery in conventional electronics.

The spin Hall effect (SHE) is a transport phenomenon predicted by Russian physicists Mikhail I. Dyakonov and Vladimir I. Perel in 1971. It consists of the appearance of spin accumulation on the lateral surfaces of an electric current-carrying sample, the signs of the spin directions being opposite on the opposing boundaries. In a cylindrical wire, the current-induced surface spins will wind around the wire. When the current direction is reversed, the directions of spin orientation is also reversed.

Spin-polarized scanning tunneling microscopy (SP-STM) is a specialized application of scanning tunneling microscopy (STM) that can provide detailed information of magnetic phenomena on the single-atom scale additional to the atomic topography gained with STM. SP-STM opened a novel approach to static and dynamic magnetic processes as precise investigations of domain walls in ferromagnetic and antiferromagnetic systems, as well as thermal and current-induced switching of nanomagnetic particles.

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Pulsed electron paramagnetic resonance (EPR) is an electron paramagnetic resonance technique that involves the alignment of the net magnetization vector of the electron spins in a constant magnetic field. This alignment is perturbed by applying a short oscillating field, usually a microwave pulse. One can then measure the emitted microwave signal which is created by the sample magnetization. Fourier transformation of the microwave signal yields an EPR spectrum in the frequency domain. With a vast variety of pulse sequences it is possible to gain extensive knowledge on structural and dynamical properties of paramagnetic compounds. Pulsed EPR techniques such as electron spin echo envelope modulation (ESEEM) or pulsed electron nuclear double resonance (ENDOR) can reveal the interactions of the electron spin with its surrounding nuclear spins.

References

1 2 Nakayama, H (2013-05-17). "Spin Hall Magnetoresistance Induced by a Nonequilibrium Proximity Effect". Phys. Rev. Lett. 110. Retrieved 19 September 2017.
↑ M. I. Dyakonov and V. I. Perel,; Perel' (1971). "Possibility of orientating electron spins with current". Sov. Phys. JETP Lett. 13: 467. Bibcode:1971JETPL..13..467D.
1 2 Chen, Y (2016-02-16). "Theory of spin Hall magnetoresistance (SMR) and related phenomena". J. Phys. Condens. Matter. 28. Retrieved 19 September 2017.
↑ Marmion, S (2014-06-13). "Temperature dependence of spin Hall magnetoresistance in thin YIG/Pt films". Phys. Rev. B. 89. Retrieved 19 September 2017.
↑ McGuire, T.; Potter, R. (1975). "Anisotropic magnetoresistance in ferromagnetic 3d alloys" (PDF). IEEE Transactions on Magnetics. 11 (4): 1018–1038. Bibcode:1975ITM....11.1018M. doi:10.1109/TMAG.1975.1058782.
↑ Marmion, S. "Comparison of spin Hall magnetoresistance temperature dependence in YIG/metal systems". White Rose eTheses. Retrieved 19 September 2017.

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[ref1-1] 1 2 Nakayama, H (2013-05-17). "Spin Hall Magnetoresistance Induced by a Nonequilibrium Proximity Effect". Phys. Rev. Lett. 110. Retrieved 19 September 2017.

[2] M. I. Dyakonov and V. I. Perel,; Perel' (1971). "Possibility of orientating electron spins with current". Sov. Phys. JETP Lett. 13: 467. Bibcode:1971JETPL..13..467D.

[ref3-3] 1 2 Chen, Y (2016-02-16). "Theory of spin Hall magnetoresistance (SMR) and related phenomena". J. Phys. Condens. Matter. 28. Retrieved 19 September 2017.

[4] Marmion, S (2014-06-13). "Temperature dependence of spin Hall magnetoresistance in thin YIG/Pt films". Phys. Rev. B. 89. Retrieved 19 September 2017.

[5] McGuire, T.; Potter, R. (1975). "Anisotropic magnetoresistance in ferromagnetic 3d alloys" (PDF). IEEE Transactions on Magnetics. 11 (4): 1018–1038. Bibcode:1975ITM....11.1018M. doi:10.1109/TMAG.1975.1058782.

[6] Marmion, S. "Comparison of spin Hall magnetoresistance temperature dependence in YIG/metal systems". White Rose eTheses. Retrieved 19 September 2017.