High temperature magnetic and structural transformations in Fe-Pd nanowires

Abstract

The microstructure and the hysteresis properties of Fe-Pd nanowire arrays were studied in the as-electrodeposited condition at 300 K, and as functions of temperature after a thermal cycle between 300 K and 950 K. Fe36Pd64 and Fe65Pd35 nanowires, 200 nm in diameter and 7-10 m long, were electrodeposited into commercial 26 % porosity alumina templates. Initially, the main magnetic phase in both arrays is the metastable γ-Pd(Fe) disordered phase. After a thermal cycle up to 950 K the microstructures transform into a majority ferromagnetic ordered phase (FePd3 in Fe36Pd64 and Fe3Pd in Fe65Pd35) embedding γ-Pd(Fe) remaining grains. Curie temperatures result 500 K, 550 K and 900 K for Fe3Pd, FePd3 and γ-Pd(Fe), respectively. The temperature dependence of the magnetic properties exhibits two stages: one below 500-550 K, when the two phases are ferromagnetic and another one above this temperature when only the minority γ-Pd(Fe) is ferromagnetic. Coercivity and remanence exhibit a maximum near the Curie temperature of the corresponding ordered phase. The effective magnetic anisotropy Keff remains high up to the magnetic transition (ferro to paramagnetic) of the ordered phase, due to the contribution of the magnetocrystalline anisotropy of these relatively hard phases. At higher temperature, the effective anisotropy mainly arises from the magnetostatic (shape and inter-wire dipolar interactions) contribution of the minority phase. The polarization reversal mechanism in the γ-Pd(Fe) phase is found to be the nucleation of inverse magnetic domains and the further expansion of the domain walls into the ferromagnetic grains, surrounded by the paramagnetic ordered phase.