Perpendicular magnetic properties of ultrathin L10-Mn1.67Ga films grown by molecular-beam epitaxy

Abstract

Materials with large perpendicular magnetic anisotropies (PMAs) have drawn great attention because of their potential applications in advanced spintronic devices such as spin-transfer-torque magnetic random access memory (STT-MRAM) and ultrahigh-density perpendicular magnetic recording. To date, a large variety of PMA materials have been investigated, such as L10-ordered FePt, CoPt granular films, Co/(Pt,Pd,Ni) multilayers, ultra-thin CoFeB alloys and perpendicularly magnetized Co2FeAl films. Among the various kinds of materials with PMA, MnGa film with L10-structure has received the most attention because it has large PMA (Ku～107 erg/cm3), ultralow Gilbert damping constant (0.008) and theoretically predicted high spin polarization (more than 70%). All these properties make L10-ordered MnGa a good candidate for spintronic devices such as STT-MRAM and spin-torque oscillators. Meanwhile, from the viewpoint of STT related spintronic device, it is necessary to fabricate ultrathin perpendicularly magnetized L10-MnxGa films to lower the critical current for magnetization reversal. However, up to now, in the main researches the ultrathin L10-MnxGa films have been grown on MgO substrates, which makes it difficult to integrate the MnGa-based magnetic tunnel junctions into the semiconductor manufacturing process.In this work, ultrathin L10-Mn1.67Ga films with different thickness values (1-5 nm) are grown on traditional GaAa (001) substrates by a molecule-beam epitaxy system. During the deposition, in situ streaky surface reconstruction patterns are observed from reflection high-energy electron diffraction, which implies high crystalline quality of the L10-Mn1.67Ga film. Only MnGa superlattice (001) and MnGa fundamental (002) peaks can be observed in the X-ray diffraction patterns in a range from 20 to 70, which means that the L10-Mn1.67Ga film is a good single-crystalline with c-axis along the normal direction. The magnetic properties of these films are measured by superconductor quantum interference device magnetometer in a field range of 5 T. The perpendicular M-H curves are almost square, while the in-plane curves are nearly hysteresis-free, each with a remnant magnetization (Mr) of around zero, which clearly evidences the PMA of the ultrathin L10-Mn1.67Ga film. Moreover, as the thickness of L10-Mn1.67Ga film decreases from 5 nm to 1 nm, the ratio of Mr/Ms also decreases from 1 to 0.72, which indicates that the PMA loses as thickness decreases. We also estimate the perpendicular anisotropy constant (Ku) from the relation Ku=Keff+2 Ms2, and the maximum Ku of 14.7 Merg/cm3 is obtained for the 5 nm MnGa film. Although the Ku decreases with thickness decreasing, a Ku value of 8.58 Merg/cm3 is observed in a 2 nm thick film. The obtained results are important for developing the L10-MnGa-based spin-transfer torque Gbit class magnetic random access memory.