Correlation between the spin Hall angle and the structural phases of early 5d transition metals

Abstract

We have studied the relationship between the structure and the spin Hall angle of the early 5d transition metals in X/CoFeB/MgO (X=Hf, Ta, W, Re) heterostructures. Spin Hall magnetoresistance (SMR) is used to characterize the spin Hall angle of the heavy metals.Transmission electron microscopy images show that all underlayers are amorphous-like when their thicknesses are small, however, crystalline phases emerge as the thickness is increased for certain elements. We find that the heavy metal layer thickness dependence of the SMR reflects these changes in structure. The spin Hall angle largest | SH | of Hf, Ta, W and Re (~0.11, 0.10, 0.23 and 0.07, respectively) is found when the dominant phase is amorphous-like. We find that the amorphous-like phase not only possesses large resistivity but also exhibits sizeable spin Hall conductivity, which both contribute to the emergence of the large spin Hall angle.*Email: hayashi.masamitsu@nims.go.jp 2 Heavy metals with strong spin orbit coupling are attracting great interest recently 1-3 as such elements can generate significant amount of spin current via the spin Hall effect 4 . In heterostructures that contain a heavy metal layer and a magnetic layer, the spin current generated within the heavy metal layer can diffuse into the magnetic layer and trigger current induced magnetization switching 2, 5 and domain wall motion [6][7][8][9] . Key to the development of technologies that make use of such current induced effects is the large spin Hall angle of the heavy metal layer 10, 11 .The spin Hall angle, which defines the amount of spin current generated within the layer, is known to be element specific. First principle calculations 12 show that the spin Hall angle of the transition metals depends on the electron filling of the d-orbitals if the mechanism behind it is due to an intrinsic origin (i.e., band structure related) 13 . In addition, the spin Hall effect can originate from spin dependent scattering of electrons, commonly referred to as an extrinsic effect 14,15 . Recently large spin Hall angle has been reported for the heavy transition metals 2,[16][17][18][19] and/or its alloy 20 . The large spin Hall angle for the early 5d transition metals (e.g. Ta and W) has been associated 2,16,21 with the appearance of the distorted tetragonal phase, often known as the A-15 -phase [22][23][24][25] . In order to develop viable devices utilizing the spin Hall effect, it is thus essential to understand how the structure of the heavy metal influences, if any, the size of the spin Hall angle. Identifying the relationship between the structural phase of the heavy metal layer and its spin Hall angle, however, requires thorough and systematic studies.Here we show studies on the spin Hall angle of the early 5d transition metals (Hf, Ta, W, Re) in magnetic heterostructures. Spin Hall magnetoresistance (SMR) 18,[26][27][28] and transmission electron microscopy (TEM) are used to evaluate, respectively, the spin transport properties and the structural phase of the films. We find that t...

Spin–Orbit Effects in CoFeB/MgO Heterostructures with Heavy Metal Underlayers

“…The highly textured hcp phase appears beyond d N $6 nm for¯lms with and without the Ta seed layer. From transmission electron microscopy studies, 62 we alsō nd that the surface roughness of the Re underlayer is much larger than the other elements: this is likely to do with the poor wetting of Re on SiO 2 . It turns out that inserting the Ta seed layer can signi¯cantly improve the surface roughness, 62 thus improving values of M/V and K EFF at small d N .…”

“…From transmission electron microscopy studies, 62 we alsō nd that the surface roughness of the Re underlayer is much larger than the other elements: this is likely to do with the poor wetting of Re on SiO 2 . It turns out that inserting the Ta seed layer can signi¯cantly improve the surface roughness, 62 thus improving values of M/V and K EFF at small d N . For the thick Re underlayer¯lms, the di®erence in M/V and K EFF between the¯lms with and without the Ta seed layer becomes negligible.…”

“…These changes in the resistivity re°ect the structural phase transition that takes places in the HM layer. [59][60][61][62] The corresponding structure of the HM layer, determined using crosssectional transmission electron microscopy, 62 is listed in each panel of Figs. 3(e)-3(h). For Hf, the growth mode changes at d N $2 nm: the thin region remains amorphous-like whereas the thicker region grows as hcp.…”

“…For the W underlayer¯lms, K EFF abruptly decreases at d N $5 nm for¯lms without the Ta seed layer and at a much smaller thickness (d N <4 nm) when the Ta seed layer is introduced. Such drop in K EFF is associated with a structural phase transition of W [59][60][61][62] : W forms an amorphous phase when its thickness is small but transforms to a highly textured body centered cubic (bcc) phase when d N exceeds a certain thickness.…”

Fieldlike spin-orbit torque in ultrathin polycrystalline FeMn films

“…To have a quantitative correlation of this thickness dependence to that of H DL , one needs to look into their expressions, respectively. Firstly, the SMR ratio can be expressed as 34 …”