We report anisotropic magnetoresistance in $\mathrm{Pt}|{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ bilayers. In spite of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ being a very good electrical insulator, the resistance of the Pt layer reflects its magnetization direction. The effect persists even when a Cu layer is inserted between Pt and ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, excluding the contribution of induced equilibrium magnetization at the interface. Instead, we show that the effect originates from concerted actions of the direct and inverse spin Hall effects and therefore call it “spin Hall magnetoresistance.”

We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and ferromagnetic insulator/nonferromagnetic metal/platinum hybrid structures. For the ferromagnetic insulator, we use either yttrium iron garnet, nickel ferrite, or magnetite and for the nonferromagnet, copper or gold. The spin Hall magnetoresistance effect is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the platinum metal top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet and prevail even if an additional, nonferromagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the spin Hall magnetoresistance theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in platinum. For a spin-mixing conductance of $4\times {10}^{14}{\Omega }^{-1}{\mathrm{m}}^{-2}$, we obtain a spin Hall angle of $0.11\pm 0.08$ and a spin diffusion length of $(1.5\pm 0.5)\mathrm{nm}$ for Pt in our thin-film samples.

A scannable laser beam is used to generate local thermal gradients in metallic (${\mathrm{Co}}_2\mathrm{FeAl}$) or insulating (${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) ferromagnetic thin films. We study the resulting local charge and spin currents that arise due to the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE), respectively. In the local ANE experiments, we detect the voltage in the ${\mathrm{Co}}_2\mathrm{FeAl}$ thin film plane as a function of the laser-spot position and external magnetic field magnitude and orientation. The local SSE effect is detected in a similar fashion by exploiting the inverse spin Hall effect in a Pt layer deposited on top of the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Our findings establish local thermal spin and charge current generation as well as spin caloritronic domain imaging.

We investigate the spin Hall magnetoresistance in thin-film bilayer heterostructures of the heavy metal Pt and the antiferromagnetic insulator NiO. While rotating an external magnetic field in the easy plane of NiO, we record the longitudinal and the transverse resistivity of the Pt layer and observe an amplitude modulation consistent with the spin Hall magnetoresistance. In comparison to Pt on collinear ferrimagnets, the modulation is phase shifted by ${90}^{\circ }$ and its amplitude strongly increases with the magnitude of the magnetic field. We explain the observed magnetic field dependence of the spin Hall magnetoresistance in a comprehensive model taking into account magnetic-field-induced modifications of the domain structure in antiferromagnets. With this generic model, we are further able to estimate the strength of the magnetoelastic coupling in antiferromagnets. Our detailed study shows that the spin Hall magnetoresistance is a versatile tool to investigate the magnetic spin structure as well as magnetoelastic effects, even in antiferromagnetic multidomain materials.

The manipulation of the ferromagnetic magnetization via electric fields is investigated in $\text{Ni}/{\text{BaTiO}}_3$ hybrid structures. The application of an electric field to the ferroelectric ${\text{BaTiO}}_3$ induces elastic strain. In the $\text{Ni}/{\text{BaTiO}}_3$ hybrids, this strain is transferred into the ferromagnetic Ni layer, affecting its magnetization due to inverse magnetostriction. Two approaches to electrically alter the Ni magnetization are investigated. One approach exploits the strain-induced change of the nickel magnetic coercivity, and allows for both reversible and irreversible magnetization control. The other is based on irreversible ferroelectric domain effects in ${\text{BaTiO}}_3$ , and yields two different electro-remanent magnetization states.

The control of magnetic properties by means of an electric field is an important aspect in magnetism and magnetoelectronics. We here utilize magnetoelastic coupling in ferromagnetic/piezoelectric hybrids to realize a voltage control of magnetization orientation at room temperature. The samples consist of polycrystalline nickel thin films evaporated onto piezoelectric actuators. The magnetic properties of these multifunctional hybrids are investigated at room temperature as a function of the voltage controlled stress exerted by the actuator on the Ni film. Ferromagnetic resonance spectroscopy shows that the magnetic easy axis in the Ni film plane is rotated by 90 degree upon changing the polarity of the voltage Vp applied to the actuator. In other words, the in-plane uniaxial magnetic anisotropy of the Ni film can be inverted via the application of an appropriate voltage Vp. Using SQUID magnetometry, the evolution of the magnetization vector is recorded as a function of Vp and of the external magnetic field. Changing Vp allows to reversibly adjust the magnetization orientation in the Ni film plane within a range of approximately 70 degree. All magnetometry data can be quantitatively understood in terms of the magnetic free energy determined from the ferromagnetic resonance experiments. These results demonstrate that magnetoelastic coupling in hybrid structures indeed is a viable option to control magnetization orientation in technologically relevant ferromagnetic thin films at room temperature.