Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single-crystal films (invited)

Abstract

Single‐crystal Fe₁₆N₂ films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density B_s of Fe₁₆N₂ films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of B_s has been measured. B_s changed with temperature reversibly up to 400 °C, while beyond 400 °C, B_s decreased irreversibly. X‐ray diffraction showed that Fe₁₆N₂ crystal is stable up to 400 °C, while beyond 400 °C, Fe₁₆N₂ dissolves into Fe and Fe₄N, and also some chemical reactions between Fe₁₆N₂ and the substrate occurs. This caused the temperature dependence of B_s mentioned above. From the temperature dependence of B_s up to 400 °C, the Curie temperature of Fe₁₆N₂ is estimated to be around 540 °C by using the Langevin function. The above mentioned B_s of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic moment of 3.2μ_B per Fe atom and 3.5μ_B, respectively. These values of the magnetic moment of Fe atoms are literally giant, far beyond the Slater–Pauling curves. The origin of the giant magnetic moment has been discussed based on the calculation carried out by Sakuma. However, there was a significant disagreement between experimental values and calculated ones, so the origin remained to be clarified. Also, magneto‐crystalline anisotropy of Fe₁₆N₂ films has been investigated.