Magnetic hardening and spin-glass phenomena in nanocrystalline FeNbB at low temperatures

Abstract

The soft nanocrystalline alloy ${\mathrm{Fe}}_{80.5}{\mathrm{Nb}}_7{\mathrm{B}}_{12.5},$ containing 25 vol % of bcc Fe, displays a rapid magnetic hardening below 20 K. This interesting process is accompanied by a strong irreversibility between field- and zero-field-cooled magnetizations and by a maximum of the magnetic viscosity near 8 K, both determined in a field of 1 Oe. We investigated the magnetization dynamics in more detail by measuring the linear ac susceptibility ${\chi }^{\prime }+i{\chi }^{″}$ between 30 mHz and 100 kHz. The shifts of the ${\chi }^{″}$ maxima towards low temperatures with decreasing frequency reveal a critical slowing down of the characteristic time, characterized by a critical exponent $z\nu =6.9\mathrm{}\left(2\right),$ and a collective freezing temperature of $T_g=7.5\mathrm{}\mathrm{K}.$ The collective nature of the freezing is supported by a dynamical scaling analysis of ${\chi }^{″}(T,\omega ),$ yielding for the order parameter exponent $\beta =0.40\mathrm{}\left(5\right),$ in good agreement with results for the spin glasses. We discuss these results by assuming a frustration of the exchange in the disordered interfacial regions and by the presence of the weak magnetic Nb-rich shells around the nanocrystalline grains.