Mn4N thin films for spintronics applications based on current-induced domain wall motion

Abstract : A new spintronic material Mn4N has been investigated. Ferrimagnetic Mn4N thin films possess relatively small spontaneous magnetization Ms and strong perpendicular magnetic anisotropy (PMA) and thus are suitable for spin-torque based memory devises. In addition, Mn4N is composed of only cheap, light and abundant elements without any rare-earth nor noble metals., thus free from material criticality. In this work, magnetic and magneto-transport properties of Mn4N grown on SrTiO3 (STO) substrate have been evaluated.First, the magnetic and magneto-transport properties of Mn4N thin films are evaluated, resulting in finding out dramatically improvement of them by replacing conventional MgO substrates by STO substrates. This Mn4N/STO system exhibits astonishing properties: a millimeter-sized domain structure, fully remnant magnetization at zero field and a sharp magnetization switching caused by scarce nucleation of reversed domain and smooth DW propagation. These properties, associated to a very small Ms and a large PMA, underline its potential for spintronic applications.Second, the generation efficiency of spin-transfer torque (STT) in Mn4N thin film has been measured by measuring the speed of domain wall (DW) driven by current pulses. The DW velocity reaches record values of 900 m/s for a current density of 1.3×10^12 A/m2. This value is the highest in all STT-driven systems and is comparable to the highest speed obtained using SOTs. The DW mobility η is also very large, the highest in all STT-based systems too. Fit of our data using a 1D analytical model allows extracting a spin polarization of the conduction electrons of 0.81, suggesting that Mn4N could be suitable to obtain large magnetoresistances. In addition, these amazing properties have been achieved without any rare earth elements, stack structures, nor external assistance such as magnetic/electric field or mechanical stress.At last, the magnetic properties have been tuned by a small amount of Ni-introduction to Mn4N. The spontaneous magnetization of Mn4N on STO has been reduced by Ni-introduction with keeping strong PMA and full remanence. This result indicates the ferrimagnetic Mn4N system might be compensated by substituting Ni atoms. Recently compensation of ferrimagnet has been actively studied because the compensated ferrimagnet provides infinite spin-torque efficiency. The three evidences of the compensation have also been demonstrated, the reversal of anomalous Hall angle, Kerr rotation chirality, and the temperature dependence of magnetization. The compensation point of composition has been estimated around Mn3.82Ni0.18N. We suggested the compensation model of Mn4N by Ni introduction which is consistent with the MS reduction, the reversal of AHE, Kerr and M-T curves.In summary, a potential of Mn4N and Mn4-xNixN films has been demonstrated as a promising candidate for spintronic applications such as current induced DW motion devices with great properties: scares domain nucleation and smooth DW propagation, ultrahigh STT generation efficiency, and tunability of magnetization by Ni-introduction. These amazing properties have been achieved without rare-earth nor noble metal, which can be a milestone for replacement of rare-earth-based materials by abundant elements.
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Toshiki Gushi. Mn4N thin films for spintronics applications based on current-induced domain wall motion. Materials Science [cond-mat.mtrl-sci]. Université Grenoble Alpes; Université de Tsukuba, 2019. English. ⟨NNT : 2019GREAY007⟩. ⟨tel-02280481⟩

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