Spin Hall magnetoresistance in a low-dimensional Heisenberg ferromagnet

Abstract

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...

Differences in the magnon diffusion length for electrically and thermally driven magnon currents in Y3Fe5O12

“…Therefore, SMR only depends on the Pt parameters and the spin transmission efficiency of the interface. In the particular case of YIG, 𝐺 𝑟 ≫ 𝐺 𝑖 [38,43], and thus 𝐺 ↑↓ is governed by 𝐺 𝑟 , which is temperature independent [13,44] for the range explored in this experiment [42,45]. This means that the temperature dependence of the SMR is related to the spin Hall efficiency of the excitation and detection at the Pt/YIG interface [4], which would explain the behavior of the non-local signal observed in Figs.…”

Focused ion beam modification of non-local magnon-based transport in yttrium iron garnet/platinum heterostructures

“…27,28 From the theory side, microscopic models to describe the temperature dependence of the SMR have been put forward and good agreement with experiments was found for different material systems. [29][30][31] In particular, the SMR is expected to scale (in leading order) with the square of the saturation magnetization as found experimentally. 18,19,29,32 Theoretical studies of the MMR showed that it should depend on temperature with a power law T α , where the magnon generation gives rise to a leading expontent of α em = 3/2 and the magnon detection to an additional exponent of α me = 1.…”

“…Consequently, the drop and disappearance of the SMR well below T C might be related to a "dirty" (intermixed) interfacial layer with a lowered Curie temperature and/or modified spin transparency, since the magnetic properties of the interface critically affect the SMR. 29,31 Such an interface layer with modified magnetic properties could also be at the origin of the scaling with temperature discussed above. Note, that the sample remained at high temperatures for extended periods of time, since the stabilization of the temperature and the angle resolved magnetoresistance measurement require roughly 1.5 h for each temperature.…”

Nonlinear $σ$-model for disordered systems with intrinsic spin-orbit coupling

“…The intrinsic spin-Hall effect is a magnetoelectric coupling between spin and charge degrees of freedom, which arises from a geometric property of the electron bands caused by the spin-orbit coupling (SOC). [1][2][3] The spincharge interconversion is the basis of the spin-Hall magnetoresistance [4], the Edelstein [5,6] and spin-galvanic effects [7], observed in a wide variety of systems [8][9][10][11].…”