Dynamic, magnetic and electronic properties of the C14 Laves phase Nb0.975Fe2.025 compound: Mössbauer-effect study

Abstract: C14 Laves phase Nb0.975Fe2.025 compound was investigated by means of the M\"ossbauer spectroscopy. Spectra were recorded in the temperature range of 5-300K. Their analysis in terms of three sub spectra yielded information on magnetic and lattice dynamical properties of Fe atoms regularly occupying 2a and 6h lattice sites and, excessively, 4f sites. No indication of magnetism was observed down to the temperature of T=ca.50K, and spectral parameters viz. center shift, CS, and the main component of the electric field gradient, Vzz, behave regularly. In particular, analysis of CS(T) in terms of the Debye model yielded the following values of the Debye temperature, T_D: 453(5) K for the site 6h, 544(10) K for the site 2a, 479(4) K for the weighted average over 6h and 2a sites, and 363(35) K for the site 4f. Below ca.50K anomalous behavior was observed: a broadening of the spectrum appeared indicating thereby a transition into a magnetic phase. Analysis of a temperature dependence of the hyperfine field, B, associated with the 6h and 2a sub spectra yielded the magnetic ordering temperature T_C1=ca.50K for the former and T_C2=ca.31K for the latter. The maximum values of the hyperfine field at 5K, Bo, were 8.2 kGs and 3.3 kGs, respectively. The Bo-values were used to estimate values the underlying magnetic moments, m_Fe(6h)=0.055-0.065 Bohr magneton and m_Fe(2a)=0.02-0.025 Bohr magneton. Noteworthy, they are over one order of magnitude lower than those theoretically calculated. The CS and Vzz parameters showed anomalies in the magnetic phase, in particular the former exhibited a strong departure from the Debye model prediction testifying to a significant effect of magnetism on the lattice vibrations.