Spin colossal magnetoresistance in an antiferromagnetic insulator

Journal of Applied Physics, 2020

Antiferromagnetic materials promise improved performance for spintronic applications, as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. Here, we show that the spin Hall magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments by investigating the easy-plane antiferromagnetic insulators α-Fe2O3 (hematite) and NiO in bilayer heterostructures with a Pt heavy metal top electrode. While rotating an external magnetic field in three orthogonal planes, we record the longitudinal and the transverse resistivities of Pt and observe characteristic resistivity modulations consistent with the SMR effect. We analyze both their amplitude and phase and compare the data to the results from a prototypical collinear ferrimagnetic Y3Fe5O12/Pt bilayer. The observed magnetic field dependence is explained in a comprehensive model, based on two magnetic sublattices and taking into account magnetic field-induced modifications of the domain structure. Our results show that the SMR allows us to understand the spin configuration and to investigate magnetoelastic effects in antiferromagnetic multi-domain materials. Furthermore, in α-Fe2O3/Pt bilayers, we find an unexpectedly large SMR amplitude of 2.5 × 10 -3 , twice as high as for prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for room-temperature antiferromagnetic spintronic applications.

Physical Review Letters, 2016

We report a large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet (AF)/yttrium iron garnet (YIG), where a thin AF insulating layer of NiO or CoO can enhance the spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the Néel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM/YIG interface for NM=3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results.

Physical Review B, 2018

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 • 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 multi-domain materials.

Physical Review B, 2013

We report on a comparative study of spin Hall related effects and magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements allow to estimate the characteristic transport parameters of both Pt and Ta layers juxtaposed to YIG: the spin mixing conductance G ↑↓ at the YIG|normal metal interface, the spin Hall angle ΘSH, and the spin diffusion length λ sd in the normal metal. The inverse spin Hall voltages generated in Pt and Ta by the pure spin current pumped from YIG excited at resonance confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we extract the spin diffusion length in Ta, found to be λ Ta sd = 1.8 ± 0.7 nm. Both the YIG|Pt and YIG|Ta systems display a similar variation of resistance upon magnetic field orientation, which can be explained in the recently developed framework of spin Hall magnetoresistance.

Physical Review Letters, 2013

The inverse spin Hall effect (ISHE) has been observed only in nonmagnetic metals, such as Pt and Au, with a strong spin-orbit coupling. We report the observation of ISHE in a ferromagnetic permalloy (Py) on ferromagnetic insulator yttrium iron garnet (YIG). Through controlling the spin current injection by altering the Py-YIG interface, we have isolated the spin current contribution and demonstrated the ISHE in a ferromagnetic metal, the reciprocal phenomenon of the anomalous Hall effect. A large spin Hall angle in Py, determined from Py thin films of different thicknesses, indicates many other ferromagnetic metals may be exploited as superior pure spin current detectors and for applications in spin current.

Applied Physics Letters, 2013

The effective field torque of an yttrium-iron-garnet (YIG) film on the spin accumulation in an attached platinum (Pt) film is measured by the spin-Hall magnetoresistance (SMR). As a result, the magnetization direction of a ferromagnetic insulating layer can be measured electrically. Experimental transverse and longitudinal resistances are well described by the theoretical model of SMR in terms of the direct and inverse spin-Hall effect, for different Pt thicknesses [3, 4, 8, and 35 nm]. Adopting a spin-Hall angle of Pt h SH ¼ 0:08, we obtain the spin diffusion length of Pt (k ¼ 1:1 6 0:3 nm) as well as the real (G r ¼ ð7 6 3Þ Â 10 14 X À1 m À2 ) and imaginary part (G i ¼ ð5 6 3Þ Â 10 13 X À1 m À2 ) of the spin-mixing conductance and their ratio (G r =G i ¼ 16 6 4).

Spin-dependent transport at heavy metal/magnetic insulator interfaces is at the origin of many phenomena at the forefront of spintronics research. A proper quantification of the different spin-dependent interface conductances is crucial for many applications. Here, we report the first measurement of the spin Hall magnetoresistance (SMR) of Pt on a purely ferromagnetic insulator (EuS). We perform SMR measurements in a wide range of temperatures and fit the results by using a microscopic model. From this fitting procedure we obtain the temperature dependence of the spin conductances (Gs, Gr and Gi), disentangling the contribution of field-like torque (Gi), antidamping-like torque (Gr), and spin-flip scattering (Gs). An interfacial exchange field of the order of 1 meV acting upon the conduction electrons of Pt can be estimated from Gi, which is at least three times larger than Gr below the Curie temperature. Our work provides an easy method to quantify this interfacial spin-splitting field, which play a key role in emerging fields such as superconducting spintronics and caloritronics, and topological quantum computation.

AIP Advances, 2017

Applied Physics Letters, 2021

We report that Joule heating can be used to enhance the interfacial spin conductivity between a metal and an oxide. We observe that local annealing of the interface at about 550 K, when injecting large current densities (>1012A/m2) into a pristine 7 nm thick Pt nanostrip evaporated on top of yttrium iron garnet (YIG), can improve the effective spin transmission up to a factor of 3. This result is of particular interest when interfacing ultrathin garnet films to avoid strong chemical etching of the surface. The effect is confirmed by the following methods: spin Hall magnetoresistance, spin pumping, and non-local spin transport. We use it to study the influence of the YIG|Pt coupling on the non-linear spin transport properties. We find that the crossover current from a linear to a non-linear spin transport regime is independent of this coupling.

It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current through the excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice, and, depending on the excitation conditions, can be either attenuated or enhanced. When the phase difference between the excited evanescent modes is close to π/2, there is an optimum AFM thickness for which the output spin current reaches a maximum, that can significantly exceed the magnitude of the input spin current. The spin current transfer through the AFM depends on the ambient temperature and increases substantially when temperature approaches the Neel temperature of the AFM layer. Progress in modern spintronics critically depends on finding novel media that can serve as effective conduits of spin angular momentum over large distances with minimum losses [1–3]. The mechanism of spin transfer is reasonably well-understood in ferromagnetic (FM) metals [4, 5] and insulators [3, 4, 6–9], but there are only very few theoretical papers describing spin current in an-tiferromagnets (AFM) (see, e.g., [10]). The recent experiments [11–13] have demonstrated that a thin layer of a dielectric AFM (NiO, CoO) could effectively conduct spin current. The transfer of spin current was studied in the FM/AFM/Pt trilayer structure (see Fig. 1). The FM layer driven in ferromagnetic resonance (FMR) excited spin current in a thin layer of AFM, which was detected in the adjacent Pt film using the inverse spin Hall effect (ISHE). It was also found in [13] that the spin current through the AFM depends on the ambient temperature and goes through a maximum near the Neel temperature T N. The most intriguing feature of the experiments was the fact that for a certain optimum thickness of the AFM layer (∼ 5 nm) the detected spin current had a maximum [11, 12], which could be even higher than in the absence of the AFM spacer [12]. The spin current transfer in the reversed geometry, when the spin current flows from the Pt layer driven by DC current through the AFM spacer into a microwave-driven FM material has been reported recently in [14]. The experiments [11–14] posed a fundamental question of the mechanism of the apparently rather effective spin current transfer through an AFM dielectric. A possible mechanism of the spin transfer through an easy-axis AFM has been recently proposed in [10]. However, this uniaxial model can not explain the non-monotonous dependence of the transmitted spin current on the AFM layer thickness and the apparent " amplification " of the spin current seen in the experiments [11, 12] performed with the bi-axial NiO AFM layer [15]. In this Letter, we propose a possible mechanism of spin current transfer through anisotropic AFM dielectrics, which may explain all the peculiarities of the experiments FIG. 1. Sketch of the model of spin current transfer through an AFM insulator based on the experiment [11]. The FM layer excites spin wave excitations in the AFM layer. The output spin current (at the AFM/Pt interface) is detected by the Pt layer through the inverse spin Hall effect (ISHE). [11, 12, 14]. Namely, we show that the spin current can be effectively carried by the driven evanescent spin wave excitations, having frequencies that are much lower than the frequency of the AFM resonance. We demonstrate that the angular momentum exchange between the spin subsystem and the AFM lattice plays a crucial role in this process, and may lead to the enhancement of the spin current inside the AFM layer. We consider a model of a simple AFM having two magnetic sublattices with the partial saturation magnetiza-tion M s. The distribution of the magnetizations of each sublattice can be described by the vectors M 1 and M 2 , |M 1 | = |M 1 | = M s. We use a conventional approach for describing the AFM dynamics by introducing the vectors of antiferromagnetism (l) and magnetism (m) [16–19]: l = (M 1 − M 2)/(2M s), m = (M 1 + M 2)/(2M s). (1) Assuming that all the magnetic fields are smaller then the exchange field H ex and neglecting the bias magnetic field, that is used to saturate the FM layer, the effective AFM Lagrangian can be written as [16, 18, 19]: