The Magnetic, Magneto-transport and Magneto-optical Properties of Mn4-xGaxN Thin Films

Abstract

Thin film Mn4N has recently gained a lot of attention for possible use in spintronic applications, owing to its perpendicular magnetic anisotropy (PMA), low saturation magnetisation and large anomalous Hall effect (AHE). It also has a large predicted ferrimagnetic Neel temperature of 750 K [1] and does not contain expensive and scarce rare-earth elements. It is a tetragonal, non-compensating Q-type ferrimagnet with two Mn sublattices and a N atom at the body center position. The bulk form is cubic but the thin films have a slight tetragonal distortion with c/a = 0.99. This is enough to alter the easy direction of the system which is along [111] in the bulk but [001] in (001)-oriented films. In addition, the bulk form has a triangular ferrimagnetic spin structure that was experimentally measured by neutron diffraction [2] and supported by calculations [3]. Mn4N thin films are predicted to retain this noncollinearity [1].

In its pure form Mn4N does not exhibit a compensation temperature. However, induced compensation in thin films has been shown in the past by doping with various elements including Ni, Co and Sn [4, 5, 6]. This thesis work investigates the use of Ga as a potential dopant for inducing compensation close to room temperature in sputter-deposited Mn4N thin films. The basic growth conditions of Mn4-xGaxN thin films are established, with x ranging from 0 to 0.27. The addition of Ga does not affect the PMA of the system but decreases the net moment in a linear manner by 3.1 muB/Ga atom. This confirms that Ga replaces atoms from only one of the Mn sublattices, a desirable trait of a dopant for inducing controlled compensation. Compensation close to 235 K for Mn3.82Ga0.18N is confirmed by magnetometry, AHE and spectral magneto-optical Kerr effect measurements, while compensation close to room temperature is found for x = 0.17. The magnetic, magneto-transport and magneto-optical properties of Mn4N and the solid solution of Mn4-xGaxN on MgO are discussed and compared to the bulk material.

References [1] He, Y. et al. Noncollinear ferrimagnetism and anomalous Hall effects in Mn4N thin films. Phys. Rev. B 106, L060409 (2022). [2] Fruchart, D., Givord, D., Convert, P., l'Heritier, P. & Senateur, J. P. The non-collinear component in the magnetic structure of Mn4N. J. Phys. F: Met. Phys. 9, 2431-2437 (1979). [3] Uhl, M., Matar, S. F. & Mohn, P. Ab initio analysis of magnetic properties in non-collinearly ordered Mn4N. Phys. Rev. B 55, 2995-3002 (1997). [4] Ghosh, S. et al. Current-driven domain wall dynamics in ferrimagnetic nickel-doped Mn4N films: very large domain wall velocities and reversal of motion direction across the magnetic compensation point. Nano Lett. 21, 2580-2587 (2021). [5] Mitarai, H. et al. Magnetic compensation at two different composition ratios in rare earth-free Mn4-xCoxN ferrimagnetic films. Phys. Rev. Mater. 4, 094401 (2020). [6] Yasuda, T. et al. Sign reversal in anomalous Hall effect at two Sn compositions in Mn4-xSnxN films on MgO(001) substrates. AIP Adv. 13, 015119 (2023).