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

Abstract

Ferrimagnetic Mn 2 Fe x Ga (0.26 x 1.12) thin films have been characterized by x-ray diffraction, magnetometry, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and Mössbauer spectroscopy with the aim of determining the structure and site-specific magnetism of this tetragonal, D0 22 -structure Heusler compound. High-quality epitaxial films with low root-mean-square surface roughness (∼0.6 nm) are grown by magnetron cosputtering. The tetragonal distortion induces strong perpendicular magnetic anisotropy along the c axis with a typical coercive field μ 0 H ∼ 0.8 T and an anisotropy field ranging from 6 to 8 T. On increasing the Fe content x, substantial uniaxial anisotropy, K u 1.0 MJ m −3 , can be maintained over the full x range, while the magnetization of the compound is reduced from 400 to 280 kA m −1 . The total magnetization is almost entirelygiven by the sum of the spin moments originating from the ferrimagnetic Mn and Fe sublattices, with the latter being coupled ferromagnetically to one of the former. The orbital magnetic moments are practically quenched and have negligible contributions to the magnetization. The films with x = 0.73 exhibit an anomalous Hall angle of 2.5% and a Fermi-level spin polarization above 51%, as measured by point contact Andreev reflection. The Fe-substituted Mn 2 Ga films are tunable with a unique combination of high anisotropy, low magnetization, appreciable spin polarization, and low surface roughness, making them strong candidates for thermally stable spin-transfer-torque switching nanomagnets with lateral dimensions down to 10 nm.

Tunable magnetic properties in tetragonal Mn-Fe-Ga Heusler films with perpendicular anisotropy for spintronics applications

“…5(a) antiferromagnetic interactions, whereas the Fe substitution leads to increased ferromagnetic coupling in the tetragonal Heusler cell, as suggested by ab initio calculations [46]. We emphasize that, for direct comparison, the Ga content has to be kept constant, as any extra Mn on the 2b sites would reduce the total magnetization due to its antiferromagnetic coupling to the Mn/Fe on 4d site, as observed in Mn 2 Fe x Ga by Betto et al [40]. The Ga excess seems to allow for a larger substitution range of Fe in the Mn 2.6−x Fe x Ga 1.4 formula, which extends the tunability of the magnetic properties.…”

“…ρ r xy / ρ sat xy is found to be larger than the M r /M s ratio obtained in the M (H ) loops. A possible explanation given by Betto et al [40] is, that the Mn on the 2b position with a lower anisotropy does not significantly contribute to the Fermi level density of states. The AHE loops are therefore substantially influenced by the Mn/Fe species on the 4d site, which is recognized as the site contributing the most to the magnetocrystalline anisotropy in D0 22 Mn 3 Ga [42,50,51].…”

“…Interestingly, this is not the case for the Mn 2.6 Ga film, where the tilt mosaicity appears to be scarcely prevailing. The matching of the in-plane lattice parameters of film and substrate leads to improved epitaxial growth and thus lower mosaicity as on MgO [40,41]. For example, the film with composition Mn 2 Fe 0.6 Ga 1.4 presents the lowest FWHM values of 0.40 • and 0.32 • for the symmetric (004) and asymmetric (112) reflections, respectively, as shown in Table I.…”

“…The presence of only (002) and (004) reflections demonstrates the (001) orientation of the Ga would result in Ga on the 2a (0, 0, 0) site, Mn1 on 2b (0, 0, 1 2 ), and Mn2 on 4d (0, 1 2 , 1 4 ). From Mößbauer measurements conducted on similar compositions, it was concluded that Fe would preferentially occupy the 4d position, with approximately 20% antisite disorder on 2b [40,43]. Further characterization of the ordering of the films is challenging because of the similar scattering factor of Mn and Fe in standard XRD, therefore limiting any precise determination of site occupancy between those two elements.…”

“…Thin films of Mn-Fe-Ga were recently reported to show high magnetocrystalline anisotropy; however, these films display high roughness [39]. Thin films of Mn 2 Fe x Ga grown on MgO substrates display a slight decrease of saturation magnetization M s with Fe concentration x, however, with nonfixed Ga concentration [40]. Such films are proposed as strong candidates for STT-MRAM applications.…”