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

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 ${\ensuremath{\chi}}^{\ensuremath{'}}+i{\ensuremath{\chi}}^{\ensuremath{''}}$ between 30 mHz and 100 kHz. The shifts of the ${\ensuremath{\chi}}^{\ensuremath{''}}$ maxima towards low temperatures with decreasing frequency reveal a critical slowing down of the characteristic time, characterized by a critical exponent $z\ensuremath{\nu}=6.9(2),$ and a collective freezing temperature of ${T}_{g}=7.5\mathrm{K}.$ The collective nature of the freezing is supported by a dynamical scaling analysis of ${\ensuremath{\chi}}^{\ensuremath{''}}(T,\ensuremath{\omega}),$ yielding for the order parameter exponent $\ensuremath{\beta}=0.40(5),$ 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.