Termination layer compensated tunnelling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy

Abstract

Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn–Ge termination layers that are oppositely magnetized to the higher moment Mn–Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.

Structural and Magnetic Properties of Mn3Ge Grown on a Thin Polycrystalline MgO Seed Layer

“…Ono et al succeeded in growing MnGa with PMA on a 10 nm polycrystalline MgO seed layer with Cr(40 nm)/CoGa(30 nm) buffer layer on top . Jeong et al deposited MTJ stacks with D 0 22 ‐Mn 3 Ge as a part of the RL by physical vapor deposition (PVD) . Although with low tunnel magnetoresistance (TMR), they realized the growth of D 0 22 structure in Mn 3 Ge on Si wafers with a 20 nm IrMn 3 seed layer.…”

“…The saturation magnetization ( M s ) is around 175 kA m −1 , which is close to the theoretical value (183 kA m −1 ) . Both M s and are in agreement with literature values . Its cannot to be estimated by measuring the in‐plane saturation field, because its effective anisotropy field is beyond the available magnetic field.…”

Structure, site-specific magnetism, and magnetotransport properties of epitaxial D022 -structure Mn2FexGa thin

“…If ultrathin films can be grown on amorphous SiO 2 or Si [50,51], while achieving high TMR ratios [31] and efficient spin-transfer-or spin-orbit torque-induced switching [52,53], this novel material/heterostructure with perpendicular anisotropy will be an exceptional candidate for spin-torque devices such as memories and oscillators. They will exhibit high thermal stability down to lateral sizes ∼10 nm and a ferromagnetic resonance frequency of about 150 GHz, thus allowing spin electronics to step into the terahertz gap.…”

“…The extracted RMS roughness of about 0.6 nm is in good agreement with the XRR fits. The surface roughness of the films might be further optimized by using lattice-matched substrate (e.g., SrTiO 3 ) or appropriate seed layers [e.g., Pt(001) and IrMn(001)] [10,31]. We note that the low roughness of MFG is in contrast with D0 22 -Mn 3 Ga films grown directly on MgO under similar conditions, which exhibit discontinuous islandlike morphology [10].…”

Isotropic, high coercive field in melt-spun tetragonal Heusler Mn3Ge

“…High coercivity in epitaxial thin films is usually attributed to the preferential tetragonal c-axis orientation along the film normal, 11 leading to perpendicular magnetic anisotropy. Magnetocrystalline anisotropy of 1.1 MJm −312,13 and coercivity up to µ 0 H C = 3.5 T, 14 and more a Electronic mail: kalache@cpfs.mpg.de b Electronic mail: felser@cpfs.mpg.de recently over 6 T on appropriate buffer layers, 15 have been reported. Here we focus on structural, microstructural, and magnetic properties of bulk melt-spun D0 22 Mn 3 Ge ribbons.…”