Magnetic properties and structure of Mn-x alloy thin films
Among the rare earth free/less permanent magnet candidates, the L10 MnAl, L10 MnGa and D022 Mn3-δGa (δ=0~1) exhibit high magnetic anisotropy energy, which is of the order of 10^7 erg/cm3, despite the spin orbit coupling (SOC) of Mn atom is small. The present project aims to understand the magnetic anisotropy Ku and coercivity Hc mechanism of Mn-X alloy thin films in conjunction with structure. The polycrystalline samples of L10 Mn52Al48, L10 Mn52Ga48 and D022 Mn71Ga29 were sputter-deposited onto silica glass substrates, and the epitaxial L10 Mn53Ga47 and L10 Mn53Ga47Alx (x=0~6) of island structure were deposited onto MgO (100) and SrTiO3 (100). The initial curves and “apparent Hc” were measured, and the relation between Hc(T)/Ms(T) and 2Ku(T)/Ms^2(T) were fitted linearly to Kronmuller’s empirical equation to estimate the pinning size r0 and effective demagnetizing factor Neff, in order to understand the Hc mechanism. The Hc in out-of-plane directions for polycrystalline L10 Mn52Al48, L10 Mn52Ga48, and both in-plane and out-of-plane directions for polycrystalline D022 Mn71Ga29 and epitaxial L10 Mn53Ga47 are governed by domain wall pinning mechanism, while the in-plane directions for polycrystalline L10 Mn52Al48 and L10 Mn52Ga48 by a mixture of domain wall pinning and nucleation modes. The Neff for polycrystalline L10 Mn52Al48, polycrystalline L10 Mn52Ga48, and epitaxial L10 Mn53Ga47 are 0.1, -2, and 12, respectively, while the r0 are 1.1~1.9 (non-magnetic), 2.2~3.3 (non-magnetic), and 2.0~2.8 nm (magnetic), respectively. The normalized Ku(T) as a function of Ms(T) was fitted to Ku(T) ≈ Ms(T)^n. The power law exponent n for the epitaxial L10 Mn53Ga47 and L10 Mn53Ga47Alx is found to be n ≈ 1.6~3.9, which is temperature dependent and larger at lower temperatures. The results show discrepancy with the single-ion (n=3) or two-ion mechanism (n=2) predictions for Ku1 based on a localized model. At the current stage, the theory for the temperature dependence of SOC and Ku is not well established, and the Ku for transition metal systems is based on itinerant models. Therefore, it is not understood which model (either single- or two-ion mechanism) is responsible for the magnetic anisotropy. A more detailed theory of Ku(T) for transition metal systems is needed.