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

Abstract

The soft nanocrystalline alloy Fe 80.5 Nb 7 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 ЈϩiЉ between 30 mHz and 100 kHz. The shifts of the Љ maxima towards low temperatures with decreasing frequency reveal a critical slowing down of the characteristic time, characterized by a critical exponent zϭ6.9(2), and a collective freezing temperature of T g ϭ7.5 K. The collective nature of the freezing is supported by a dynamical scaling analysis of Љ(T,), yielding for the order parameter exponent ␤ ϭ0.40(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.

Nanocrystalline soft magnetic materials: intergrain coupling and spin freezing effects

“…2a) is closely related with spin freezing effects in these alloys [18,19]. Within this magnetic hardening regime, the strongest changes of the coercive field with a decrease of temperature are again observed for the sample that exhibits medium degree of crystallinity.…”

“…Below T c (am) the exchange averaging of the particle anisotropy is further reinforced by the presence of the long-range ordered domains. Along these lines, the low-temperature hardening may be attributed to the decrease of the ferromagnetic correlation length ξ f (T) to the order of the shell thickness as a consequence of the collective magnetic freezing (for more details see [19]). Then these inhomogeneties can effectively pin the magnetization reversal, which is of collective nature, if the reversal is mediated by extended domain walls.…”

Inverted near-surface hysteresis loops in annealed Fe80.5Nb7B12.5 ribbons

“…The iron-based FeMB alloys with M: Zr, Ta, Mo or Nb (NaNOPERM), are especially interesting because of their superior magnetic properties [2,3] as compared with the conventional FINEMET alloys and the simplicity of the alloy system. In the temperature range 300-1050 K, all FeNbB nanocrystalline samples were found to show behavior typical for the materials containing amorphous and nanocrystalline phases [4][5][6][7][8][9][10]. The aim of this work is the comparative study of the near-surface and volume magnetic properties of as-cast and annealed Fe 80.5 Nb 7 B 12.5 ribbons to understand the influence of structural changes of the examined samples on their magnetic-field behavior.…”

Magnetic transition in nanocrystalline soft magnetic alloys analyzed via ac inductive techniques

“…1 shows, the temperature dependence of the real ͑ r ͒ and imaginary ͑ i ͒ permeability components are characterized by a sharp decrease and a maximum value, respectively, around a certain transition temperature. This maximum in i is a characteristic feature of different magnetic transitions (i.e., freezing temperature in spin glasses, 19 blocking temperature of superparamagnetic particles 20 ). In the present case, the occurrence of such a maximum could be ascribed to the magnetic decoupling between ferromagnetic crystallites for temperatures close to the Curie temperature of the residual amorphous phase, T C2 (T C2 Ϸ 340°C estimated from previous thermogravimetry studies, that is, the temperature dependence of dc saturation magnetization, M S ͑T͒.)…”