Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator

Abstract

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 ¼ 3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results. DOI: 10.1103/PhysRevLett.116.186601 Pure spin current phenomena and devices are new advents in spin electronics [1,2]. A pure spin current has the unique attribute of delivering spin angular momentum without a net charge current thus with higher energy efficiency. A pure spin current can be generated by several mechanisms, including the spin Hall effect [1][2][3], lateral spin valves [4,5], spin pumping [6,7], and longitudinal spin Seebeck effect (LSSE) [8,9]. The inverse spin Hall effect (ISHE) in a metal can detect a pure spin current by converting it into a charge current with a resultant charge accumulation [3,10]. Inevitably, a spin current decays as it traverses through a material on the scale of the spin diffusion length λ SF , which depends on the strength of the intrinsic spin orbit interaction and the quality of the material [5]. The transmission of a spin current across an interface between two materials, such as a ferromagnet and a nonmagnetic material, is further limited by the spin-mixing conductance at the interface [7]. The rapidly diminishing spin current has severely hampered its exploitation. It is highly desirable to explore ways to enhance pure spin current.Pure spin current phenomena and devices have employed ferromagnetic (F) metals [3][4][5]10], F insulators [8,9], and normal metals (NMs) [3,[8][9][10], where the F magnetization sets the spin index of the spin current injected from the F material, light NM (e.g., Cu) and heavy NM (e.g., Pt), respectively, transmits and detects the spin current. Very recently, spin current exploration involves antiferromagnetic (AF) materials [11][12][13][14][15][16][17][18]. The employment of antiferromagnets in spintronic devices is particularly attractive for terahertz (THz) devices [19]. Recently, spin pumping experiment in Pt=YIG (where YIG ¼ Y 3 Fe 5 O 12 ) shows enhanced spin transport through an intervening AF NiO layer between YIG and Pt at room temperature [13,14]. It was suggested that the spin transport through the AF insulators is related to AF magnons and spin fluctuations [13,14], where the AF spins, strongly coupled to the precessing YIG magnetization, transport the spin current [13,14]. However, thus far, spin transport through AF insulators has only employed ferromagnetic resonance measurements (FMR) at the GHz frequency range [11,[13][14][15]18], which is far less than the characteristic frequencies (up to 1 THz) of the AF NiO...

Effect of NiO inserted layer on spin-Hall magnetoresistance in Pt/NiO/YIG heterostructures

“…Pt. 18,19,22 For Pt/ YIG, as shown in Fig. 2(b), a conventional AMR (CAMR) induced by the magnetic proximity effect (MPE) always coexists with the SMR and its maximum amplitude of 2.2 Â 10 À4 is comparable to the SMR.…”

“…[12][13][14][15][16][17][18][19][20][21][22] Especially, the thermally injected or dynamically pumped spin current from ferromagnetic (FM) Y 3 Fe 5 O 12 (YIG) layer can flow into the NiO or CoO antiferromagnetic insulator (AFMI) layer and reach the Pt or Ta nonmagnetic metal (NM) layer where it can be converted into charge current by means of the inverse spin-Hall effect (ISHE). [18][19][20][21][22] By introducing a thin AFMI layer, the ISHE voltage is largely enhanced and exhibits a nonmonotonic temperature or an AFMI thickness dependence, with a maximum value appearing near the antiferromagnetic (AFM) ordering temperature of AFMI or at the AFMI thickness of $1-2 nm, respectively. [18][19][20][21][22] Several theoretical models have been proposed for the propagation of injected spin current through AFMs in NM/ AFM/FM heterostructures.…”

“…[18][19][20][21][22] By introducing a thin AFMI layer, the ISHE voltage is largely enhanced and exhibits a nonmonotonic temperature or an AFMI thickness dependence, with a maximum value appearing near the antiferromagnetic (AFM) ordering temperature of AFMI or at the AFMI thickness of $1-2 nm, respectively. [18][19][20][21][22] Several theoretical models have been proposed for the propagation of injected spin current through AFMs in NM/ AFM/FM heterostructures. [23][24][25][26] These models describe the spin-current transport and its enhancement by assuming that the AFMs are ordered at room temperatures, while the ordering temperatures of thin AFM films are well below the room temperature.…”

“…27 In most of these experimental or theoretical investigations, the spin current carried by spin waves is produced in YIG layer and flows into the AFMI layer. [18][19][20][21][22][23][24][25][26] In this paper, we provide an alternative method of spin current source, namely, the spin current is generated in the Pt layer via SHE [9][10][11] and the YIG layer serves merely as a spin current absorber. Specifically, we study the magnetoresistance in Pt/NiO/YIG heterostructures in which the spin-Hall magnetoresistance (SMR) has been correlated with the spin current absorbed by the YIG layer.…”

Enhanced Spin Conductance of a Thin-Film Insulating Antiferromagnet

“…Thus, to realize spin transport by thermally generated spin waves, the symmetry between the antiferromagnetic sublattices must be lifted. One means of achieving this is to employ a ferromagnetic layer, controlled by a magnetic field [6]. Alternatively, the magnetic field itself suffices to break the sublattice symmetry, eliminating the need for a ferromagnetic component.…”