Beller Lectureship: Ultrahigh anisotropy Heusler alloys for THz spin-torque oscillators

We investigate the potential of the recently discovered class of almost compensated ferrimagnetic Mn-Ga pseudo-Heusler alloys¹ for high frequency applications. To estimate the frequency ranges where such devices could operate, we first conducted high-field magnetotransport measurements² on selected films with different composition and, therefore, different compensation temperatures (T_c) and anisotropies. In manganese ruthenium gallium (MRG), for instance, both the transverse Hall resistivity and longitudinal resistivity were recorded in magnetic fields up to 58T, at variable temperature. MRG exhibits a large spontaneous Hall angle of ~2%, coercivity exceeding 1T at room temperature (RT) and very low net magnetisation of 25kA/m. Despite having no net moment at T_c, the magnitude of the Hall signal does not become zero, indicating both a half-metallic nature of the material and that the magnetotransport is dominated by one sublattice only. By comparison to analytical and mean-field calculations of the sublattice magnetisation directions, we can estimate both the sublattice anisotropy and interlayer exchange coupling. From these values, the out-of-phase and in-phase resonance modes are expected to lie around 0.3THz and 2THz, respectively. The out-of-phase resonance mode was directly measured for ferrimagnetic Mn_3-xGa thin films as function of anisotropy and applied magnetic fields up to 10T. At low applied fields, we find that the resonance frequency ranges between 200 and 350GHz for films with different compositions/anisotropy³. Furthermore, MRG-based magnetic tunnel junctions demonstrate magnetoresistance ratios as high as 40% at 4.2K and 12% at RT⁴, providing proof of concept for efficient on-chip emitters of coherent, narrow-band light pulses in the THz gap.

1. Kurt H. et al., PRL 112, 027201 (2014)
2. Fowley C. et al., J. Phys. D 48, 164006 (2015)
3. Awari N. et al., APL 109, 032403 (2016)
4. Borisov K. et al., APL 108, 192407 (2016)