Magnetic and structural properties of L10 Mn-Ga epitaxially grown islands

Abstract Magnetic and structural properties of L 1 0 Mn 53 Ga 47 islands epitaxially grown onto MgO(1 0 0) substrates are discussed. The samples were fabricated by sputter-deposition at a substrate temperature T s during deposition. The samples deposited at T s above 500 °C were found to form the island structure of Volmer-Weber type. The average size of the islands fabricated at T s  = 600 °C is around 200 nm in width and 60 nm in height. The XRD and TEM analyses indicate that those islands are of the L 1 0 phase with the 〈0 0 1〉 axis perpendicular to the substrate surface. The lattice constants a and c are 3.91 ± 0.01 A and 3.65 ± 0.02 A, respectively, which are larger by 0.8%, and smaller by 1.5% respectively, as compared to the bulk values of the same composition. The order parameter of these islands is found to increase from 0.7 to 0.8 for T s  = 450–500 °C, and then remains nearly constant (≈0.8) for higher T s . The M-H hysteresis loops exhibit a high squareness for all the samples for T s  = 450–600 °C. The saturation magnetization M s is about 220 and 350 emu/cm 3 for T s  = 450 and 600 °C, respectively. The out-of-plane coercivity H c is 5 and 13 kOe for T s  = 450 and 600 °C, respectively. The temperature dependence of M s for the sample made at T s  = 600 °C was measured and fitted to an empirical relation in order to estimate T c . The estimated T c around 600 K is in reasonable agreement with the reported value of bulk L 1 0 Mn 53 Ga 47 . The uniaxial perpendicular magnetic anisotropy constant K u estimated by the out-of-plane torque curves is around 1.1 × 10 7 and 8.3 × 10 6  erg/cm 3 at 5 and 300 K, respectively. The initial magnetization curves of these islands exhibit a typical pinning mode for the coercivity mechanism. A phenomenological discussion of the coercivity is presented on the basis of Kronmuller’s formula. It is found that the coefficient α and effective demagnetizing factor N eff in the formula by Kronmuller are approximately 0.38 and 12. For the magnetic anisotropy mechanism, the power law relation between K u (T) and M s (T) is examined. It is found that the exponent n for K u (T)∝M s (T) n varies from 2.3 to 3.5 for T from 350 to 5 K. However, the n in a whole temperature range from 5 to 350 K is 2.6. The n = 2.6 suggests a deviation from the single-ion mechanism, and is consistent with the theoretical predictions of Ga contribution.