Surface-assisted spin Hall effect in Au films with Pt impurities

Anatomy of nanomagnetic switching at a 3D topological insulator PN junction

A P-N junction engineered within a Dirac cone system acts as a gate tunable angular filter based on Klein tunneling. For a 3D topological insulator with a substantial bandgap, such a filter can produce a charge-to-spin conversion due to the dual effects of spin-momentum locking and momentum filtering. We analyze how spins filtered at an in-plane topological insulator PN junction (TIPNJ) interact with a nanomagnet, and argue that the intrinsic charge-to-spin conversion does not translate to an external gain if the nanomagnet also acts as the source contact. Regardless of the nanomagnet's position, the spin torque generated on the TIPNJ is limited by its surface current density, which in turn is limited by the bulk bandgap. Using quantum kinetic models, we calculated the spatially varying spin potential and quantified the localization of the current versus the applied bias. Additionally, with the magnetodynamic simulation of a soft magnet, we show that the PN junction can offer a critical gate tunability in the switching probability of the nanomagnet, with potential applications in probabilistic neuromorphic computing.

Regulating Interfacial Spin Hall Conductivity with Ferroelectricity

We report a new physical phenomenon of active and enhanced control of the spin Hall conductivity (SHC) in a family of metal-ferroelectric multilayers. Rather than the direction of the built-in electric field, such ferroelectric regulation of SHC originates from the drastic change of the interfacial electronic state along with its intrinsic Berry phase near the Fermi level due to the distinct hybridization between the metal film and substrate when the ferroelectric polarization reverses. Using Pt/PbZrTiO₃ multilayers as a representative model, we demonstrate the controllability of a large magnitude and even the sign of SHC in the Pt film at the two interfaces with an antitype conducting carrier in a ferroelectric substrate. The interfacial Rashba effect plays a role in contributing to the change of SHC through spin-projected band analysis. The present work makes a fundamental theoretical discovery and opens up a new direction to manipulate spin-charge conversion of thin-film layered structures by ferroelectricity, which is crucial for designing future electric and spintronic devices.

Tuning the interfacial spin-orbit coupling with ferroelectricity

Detection and manipulation of spin current lie in the core of spintronics. Here we report an active control of a net spin Hall angle, θ_SHE(net), in Pt at an interface with a ferroelectric material PZT (PbZr_0.2Ti_0.8O₃), using its ferroelectric polarization. The spin Hall angle in the ultra-thin Pt layer is measured using the inverse spin Hall effect with a pulsed tunneling current from a ferromagnetic La_0.67Sr_0.33MnO₃ electrode. The effect of the ferroelectric polarization on θ_SHE(net) is enhanced when the thickness of the Pt layer is reduced. When the Pt layer is thinner than 6 nm, switching the ferroelectric polarization even changes the sign of θ_SHE(net). This is attributed to the reversed polarity of the spin Hall angle in the 1^st-layer Pt at the PZT/Pt interface when the ferroelectric polarization is inverted, as supported by the first-principles calculations. These findings suggest a route for designing future energy efficient spin-orbitronic devices using ferroelectric control.

The spin Hall angle (SHA) is a measure of the efficiency for converting a charge to a spin current is still challenging to tune in situ. Here, the authors demonstrate by introducing a ferroelectric (FE) material in a ferromagnetic/heavy metal stack the SHA can be voltage controled via the polarization of the FE layer.

Critical Spin Fluctuation Mechanism for the Spin Hall Effect

We propose mechanisms for the spin Hall effect in metallic systems arising from the coupling between conduction electrons and local magnetic moments that are dynamically fluctuating. Both a side-jump-type mechanism and a skew-scattering-type mechanism are considered. In either case, dynamical spin fluctuation gives rise to a nontrivial temperature dependence in the spin Hall conductivity. This leads to the enhancement in the spin Hall conductivity at nonzero temperatures near the ferromagnetic instability. The proposed mechanisms could be observed in 4d or 5d metallic compounds.

Interface induced enhancement of inverse spin Hall voltage in NiFe/Pt bilayers capped by MgO layer

In ferromagnet/heavy metal bilayers, a spin current can be generated under the ferromagnetic resonance (FMR) condition, and then converted into a charge current in adjacent nonmagnetic metals through inverse spin Hall effect (ISHE). Here, we report an experimental observation of interface induced ISHE enhancement in NiFe/Pt bilayers covered by MgO layer. Compared to bare NiFe/Pt bilayers, Pt/MgO interface induces an enhancement of the spin-charge conversion in the NiFe/Pt/MgO trilayers with very thin Pt layers, in agreement with the corresponding trend of Gilbert damping enhancement. When the thickness of Pt is below 1.6 nm, the ISHE induced charge current has about 70% enhancement. These results open a new pathway to improve the spin-charge conversion efficiency by interface engineering.

CuPt Alloy Thin Films for Application in Spin Thermoelectrics

Spin thermoelectrics represents a new paradigm of thermoelectricity that has a potential to overcome the fundamental limitation posed by the Wiedmann-Franz law on the efficiency of conventional thermoelectric devices. A typical spin thermoelectric device consists of a bilayer of a magnetic insulator and a high spin-orbit coupling (SOC) metal coated over a non-magnetic substrate. Pt is the most commonly used metal in spin thermoelectric devices due to its strong SOC. In this paper, we found that an alloy of Cu and Pt can perform much better than Pt in spin thermoelectric devices. A series of CuPt alloy films with different Pt concentrations were deposited on yttrium iron garnet (YIG) films coated gadolinium gallium garnet (GGG) substrate. Through spin Seebeck measurements, it was found that the Cu0.4Pt0.6/YIG/GGG device shows almost 3 times higher spin Seebeck voltage compared to Pt/YIG/GGG under identical conditions. The improved performance was attributed to the higher resistivity as well as enhanced spin hall angle of the CuPt layer.

Tuning Spin Hall Angles by Alloying

Within a combined experimental and theoretical study it is shown that the spin Hall angle of a substitutional alloy system can be continuously varied via its composition. For the alloy system Au_{x}Pt_{1-x} a substantial increase of the maximum spin Hall angle compared to the pure alloy partners could be achieved this way. The experimental findings for the longitudinal charge conductivity σ, the transverse spin Hall conductivity σ_{SH}, and the spin Hall angle α_{SH} could be confirmed by calculations based on Kubo's linear response formalism. Calculations of these response quantities for different temperatures show that the divergent behavior of σ and σ_{SH} is rapidly suppressed with increasing temperature. As a consequence, σ_{SH} is dominated at higher temperatures by its intrinsic contribution that has only a rather weak temperature dependence.

Spin-torque generator engineered by natural oxidation of Cu

The spin Hall effect is a spin–orbit coupling phenomenon, which enables electric generation and detection of spin currents. This relativistic effect provides a way for realizing efficient spintronic devices based on electric manipulation of magnetization through spin torque. However, it has been believed that heavy metals are indispensable for the spin–torque generation. Here we show that the spin Hall effect in Cu, a light metal with weak spin–orbit coupling, is significantly enhanced through natural oxidation. We demonstrate that the spin–torque generation efficiency of a Cu/Ni₈₁Fe₁₉ bilayer is enhanced by over two orders of magnitude by tuning the surface oxidation, reaching the efficiency of Pt/ferromagnetic metal bilayers. This finding illustrates a crucial role of oxidation in the spin Hall effect, opening a route for engineering the spin–torque generator by oxygen control and manipulating magnetization without using heavy metals.

In thin film spintronic devices, heavy metals with strong spin-orbit coupling are required to achieve a sizeable spin Hall effect. Here, the authors demonstrate an enhancement of the spin Hall effect in Cu, a material with weak spin-orbit coupling, via natural oxidation.

Observation of temperature-gradient-induced magnetization

Applying magnetic fields has been the method of choice to magnetize non-magnetic materials, but they are difficult to focus. The magneto-electric effect and voltage-induced magnetization generate magnetization by applied electric fields, but only in special compounds or heterostructures. Here we demonstrate that a simple metal such as gold can be magnetized by a temperature gradient or magnetic resonance when in contact with a magnetic insulator by observing an anomalous Hall-like effect, which directly proves the breakdown of time-reversal symmetry. Such Hall measurements give experimental access to the spectral spin Hall conductance of the host metal, which is closely related to other spin caloritronics phenomena such as the spin Nernst effect and serves as a reference for theoretical calculation.

Rashba spin–orbit coupling enhanced anomalous Hall effect in MnxSi1−x/SiO2/Si p–i–n junctions

The Mn_0.48Si_0.52/SiO₂/Si p–i–n junction shows greatly enhanced negative anomalous Hall effect in the high temperature range due to the interfacial Rashba spin–orbit coupling.

Sign change of the spin Hall effect due to electron correlation in nonmagnetic CuIr alloys

Recently, a positive spin Hall angle (SHA) of 0.021 was observed experimentally in nonmagnetic CuIr alloys [Niimi et al, Phys. Rev. Lett. 106, 126601 (2011)] and attributed predominantly to an extrinsic skew scattering mechanism, while a negative SHA was obtained from ab initio calculations [Fedorov et al, Phys. Rev. B 88, 085116 (2013)], using consistent definitions of the SHA. We reconsider the SHA in CuIr alloys, with the effects of the local electron correlation U in 5d orbitals of Ir impurities, included by the quantum Monte Carlo method. We found that the SHA is negative if we ignore such local electron correlation, but becomes positive once U approaches a realistic value. This may open up a way to control the sign of the SHA by manipulating the occupation number of impurities.

Giant spin Hall effect induced by skew scattering from bismuth impurities inside thin film CuBi alloys

We demonstrate that a giant spin Hall effect (SHE) can be induced by introducing a small amount of Bi impurities in Cu. Our analysis, based on a new three-dimensional finite element treatment of spin transport, shows that the sign of the SHE induced by the Bi impurities is negative and its spin Hall (SH) angle amounts to -0.24. Such a negative large SH angle in CuBi alloys can be explained by applying the resonant scattering model proposed by Fert and Levy [Phys. Rev. Lett. 106, 157208 (2011)] to 6p impurities.

Extrinsic spin Hall effect induced by iridium impurities in copper

We study the extrinsic spin Hall effect induced by Ir impurities in Cu by injecting a pure spin current into a CuIr wire from a lateral spin valve structure. While no spin Hall effect is observed without Ir impurity, the spin Hall resistivity of CuIr increases linearly with the impurity concentration. The spin Hall angle of CuIr, (2.1±0.6)% throughout the concentration range between 1% and 12%, is practically independent of temperature. These results represent a clear example of predominant skew scattering extrinsic contribution to the spin Hall effect in a nonmagnetic alloy.

Spin-torque ferromagnetic resonance induced by the spin Hall effect

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance dynamics. The Oersted field from the current also generates a ferromagnetic resonance signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle.