Magnetic hardening and spin-glass phenomena in nanocrystalline FeNbB at low temperatures -: art. no. 184437

The soft nanocrystalline alloy Fe80.5Nb7B12.5, 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'+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 zv = 6.9(2), and a collective freezing temperature of T-g = 7.5 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(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.