Magnetism of metastable γ-Fe85Pd15 nanowire arrays across an unusually broad temperature range (5 K to 800 K).

Abstract

Arrays of 50 nm diameter Fe85Pd15 cylindrical nanowires were electrochemically grown, crystallizing in a metastable γ-Fe(Pd) fcc A1 disordered solid solution. After performing a heating-cooling thermal cycle between 300 K and 1000 K, the γ-Fe(Pd) fcc metastable phase still predominates (97%), coexisting with a not-fully-identified minority phase. The thermal cycling induces a moderate increase in the crystallite size and a reduction of the lattice parameter although leading to a significant heating-cooling magnetic hysteresis. No further changes in temperature-dependent magnetization, M(T), are observed during subsequent cycling. The full-range (5 K to 800 K) saturation magnetization Ms(T) curve is quite accurately described by a phenomenological expression, which provides a Bloch-type contribution as T → 0 and undergoes the critical behavior near the Curie temperature TC. An upturn in Ms(T) is observed below 100 K which is described by a spin-glass-like second contribution, with freezing temperature Tf = (80 ± 2) K, and kBTf comparable to the exchange interactions in Fe-Pd systems. A Curie temperature of TC = 830 K, and a critical exponent value β = 0.42 ± 0.05 are estimated. These regimes (below and above 100 K) are also observed in the magnetization process. The temperature dependence of coercivity between 100 K and 800 K is consistent with a nucleation/propagation remagnetization mechanism, with activation energy of (320 ± 20) kJ mol-1 and critical field for magnetization reversal of (65 ± 1) mT, at 0 K. The analysis of the effective magnetic anisotropy as a function of temperature allows us to conclude that it essentially arises from the balance between different magnetostatic contributions.