Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature that increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to the Pt/YIG bilayer, which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of the NiO layer. We explain the negative SMR by the NiO "spin flop" coupled with YIG, which can be overridden at higher temperature by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.

The longitudinal spin Seebeck effect refers to the generation of a spin current when heat flows across a normal metal/magnetic insulator interface. Until recently, most explanations of the spin Seebeck effect use the interfacial temperature difference as the conversion mechanism between heat and spin fluxes. However, recent theoretical and experimental works claim that a magnon spin current is generated in the bulk of a magnetic insulator even in the absence of an interface. This is the so-called intrinsic spin Seebeck effect. Here, by utilizing a non-local spin Seebeck geometry, we provide additional evidence that the total magnon spin current in the ferrimagnetic insulator yttrium iron garnet (YIG) actually contains two distinct terms: one proportional to the gradient in the magnon chemical potential (pure magnon spin diffusion), and a second proportional to the gradient in magnon

The use of magnetic insulators is attracting a lot of interest due to a rich variety of spindependent phenomena with potential applications to spintronic devices. Here we report ultrathin yttrium iron garnet (YIG) / gadolinium iron garnet (GdIG) insulating bilayers on gadolinium iron garnet (GGG). From spin Hall magnetoresistance (SMR) and X-ray magnetic circular dichroism measurements, we show that the YIG and GdIG magnetically couple antiparallel even in moderate in-plane magnetic fields. The results demonstrate an allinsulating equivalent of a synthetic antiferromagnet in a garnet-based thin film heterostructure and could open new venues for insulators in magnetic devices. As an example, we demonstrate a memory element with orthogonal magnetization switching that can be read by SMR.

We explore the possibility to detect spin accumulation due to the spin Hall effect (SHE) by means of the magneto-optical Kerr effect (MOKE). For this purpose, we utilize the generalized magneto-optical ellipsometry (GME), which enables the disentanglement of different magnetization components contributing to the resulting MOKE signal, and perform measurements for three different materials: Pt, W, and Ta. Although we observe a current-induced effect in the light intensity in our polarization sensitive GME setup, it does not arise from a SHE-induced light polarization signal in any of the materials, but from a change in reflectivity due to heating effects. Based on the sensitivity achieved in our experiments, we conclude that state-of-the-art magneto-optical methods utilizing linear optics are not sufficiently sensitive to detect SHE-induced spin accumulation in these metals.

We observe an unusual behavior of the spin Hall magnetoresistance (SMR) in Pt deposited on a tensile-strained LaCoO3 (LCO) thin film, which is a ferromagnetic insulator with the Curie temperature Tc=85K. The SMR displays a strong magnetic-field dependence below Tc, with the SMR amplitude continuing to increase (linearly) with increasing the field far beyond the saturation value of the ferromagnet. The SMR amplitude decreases gradually with raising the temperature across Tc and remains measurable even above Tc. Moreover, no hysteresis is observed in the field dependence of the SMR. These results indicate that a novel lowdimensional magnetic system forms on the surface of LCO and that the LCO/Pt interface decouples magnetically from the rest of the LCO thin film. To explain the experiment, we revisit the derivation of the SMR corrections and relate the spin-mixing conductances to the microscopic quantities describing the magnetism at the interface. Our results can be used as a technique to probe quantum magnetism on the surface of a magnetic insulator.Introduction.-Magnetoresistance has been key for understanding spin-dependent transport in solids [1]. In the last years, new magnetoresistance phenomena were discovered in thin ferromagnetic/normal metal(FM/NM)-based heterostructures [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], which originate from the interplay of the spin currents generated in the heterostructure (via the spin Hall effect [19][20][21][22] or the Rashba-Edelstein effect [23,24]) with the magnetic moments of the FM layer. Among 2 many applications, these magnetoresistance effects have been used for quantifying spin transport properties such as the spin diffusion length  and the spin Hall angle SH of different NM layers, or the spin-mixing conductance ↑↓ of FM/NM interfaces. More interestingly, unlike other surface-sensitive techniques that suffer from a bulk contribution due to a finite penetration depth, the spin Hall magnetoresistance (SMR) [4][5][6][7][8][9][10][11] uses the spin accumulation at interfaces for sensing the magnetic properties of the very first atomic layer of magnetic insulators (MIs) [25,26]. For instance, SMR has been employed for probing the surface of complex magnetic systems such as ferrimagnetic spinel oxides [11,27], spin-spiral multiferroics [28,29], canted ferrimagnets [30], Y3Fe5O12/antiferromagnetic (YIG/AFM) bilayers [31,32], and synthetic AFMs [33].LaCoO3 (LCO) presents an intriguing magnetic behavior, which has been studied for decades and is still under debate [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. Bulk LCO is a diamagnetic insulator at low temperature, owing to the low-spin (LS) configuration of Co 3+ . The relatively small crystal-field splitting of the Co 3+ 3d-shell results in an increasing population of high-spin (HS) Co 3+ with temperature, reaching 1:1 (LS:HS) above ~150K. The close proximity between crystal-field splitting and exchange energy makes the magnetic properties of LCO particularly susceptible to small c...

Recent demonstration of efficient transport and manipulation of spin information by magnon currents have opened exciting prospects for processing information in devices. Magnon currents can be excited both electrically and thermally, even in magnetic insulators, by applying charge currents in an adjacent metal layer. Earlier reports in thin yttrium iron garnet (YIG) films suggested that the diffusion length of magnons is independent on the excitation method, but different values were obtained in thicker films.Here, we study the magnon diffusion length for electrically and thermally excited magnons in a 2-m-thick YIG film as a function of temperature and magnetic field. Our results evidence that the diffusion length depends on the generation mechanism, suggesting that magnons of different energies are excited -sub-thermal and thermal magnons for electrically and thermally driven magnon currents, respectively-and, consequently, indicating that the magnon diffusivity is frequency dependent. Moreover, we show that the damping of the thermally driven magnons with magnetic field is weaker than for those excited electrically. Finally, we demonstrate that the magnon diffusion length for thermally excited magnons is independent of the YIG thickness and material growth conditions, confirming that this quantity is an intrinsic parameter of YIG.

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 interfacial spin 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 (G s , G r , and G i ), disentangling the contribution of field-like torque (G i ), damping-like torque (G r ), and spin-flip scattering (G s ). An interfacial exchange field of the order of 1 meV acting upon the conduction electrons of Pt can be estimated from G i , which is at least three times larger than G r below the Curie temperature. Our work provides an easy method to quantify this interfacial spin-splitting field, which plays a key role in emerging fields such as superconducting spintronics and caloritronics as well as topological quantum computation.

Anomalous Hall-like signals in platinum in contact with magnetic insulators are common observations that could be explained by either proximity magnetization or spin Hall magnetoresistance. In this work, longitudinal and transverse magnetoresistances are measured in a pure gold thin film on the ferrimagnetic insulator Y3Fe5O12 (Yttrium Iron Garnet, YIG). We show that both the longitudinal and transverse magnetoresistances have quantitatively consistent scaling in YIG/Au and in a YIG/Pt reference system when applying the Spin Hall magnetoresistance framework. No contribution of an anomalous Hall effect due to the magnetic proximity effect is evident.