Iron exchange-field penetration into the amorphous interphase of nanocrystalline materials

Abstract

In this work an enhancement of the Curie temperature of the intergranular amorphous region in nanocrystalline alloys with respect to amorphous ribbons of the same composition is shown. The Curie temperature reaches a value of 125 C. The experimental results are discussed in terms of both an inhomogeneous distribution of Nb atoms in the interphase and the magnetic interactions between the Fe ferromagnetic grains and the matrix. Finally, it is proposed that the existence of a molecular field of about 80 T originated by the penetration of the exchange field of n-Fe nanocrystals into the amorphous paramagnetic intergranular region is the main cause of this Curie temperature enhancement. Moreover, the compositional dependence of the Curie temperature in Fe-8-Nb-Cu amorphous ribbons has been reported.

Stress-induced large Curie temperature enhancement inFe64Ni36Invar alloy

“…2͒ probably due to frustration of Fe magnetic moments at the disordered grain boundaries. 42,43 The results plotted in Fig. 2 obviously suggest the need to carry out a detailed study of possible correlations between the crystal structures, the large increase in T C , and the role played by magnetovolume effects.…”

“…These findings can be understood on the basis of the magnetic coupling after milling between the MS crystals and the grain boundaries that can induce variations in the intrinsic properties such as Curie temperature and/or magnetic anisotropy. 42,43 Further heating-cooling cycles overlap the M͑T͒ curve of the treated sample even when the sample is measured again with time intervals of several years, indicating that the magnetic behavior of the milled powders is maintained and is reversible after a first heating up to 1073 K is done. Moreover, a Curie-Weiss behavior has been found, for both alloys, in the temperature dependence of the reciprocal magnetic susceptibility near T C .…”

High-temperature induced ferromagnetism onγ−Feprecipitates in FeCu solid solutions

“…5͒, the fact that during heating from RT, the isolated ␥-Fe precipitates are surrounded by ferromagnetic ␣-Fe grains which are producing a high local magnetic field. 36 Hence, this field acts as a "driving force" which contributes to the appearance of ferromagnetism in the ␥-Fe. On the other hand, and during cooling from high temperature, the whole system is nonmagnetic, thus the combined effect of a different thermal expansion evolution of ␥-Fe together with the absence of a high local magnetic field coming from ␣-Fe phase leads to such a magnetic irreversibility.…”

The magnetocaloric effect of partially crystalline Fe-B-Cr-Gd alloys

“…The increase in crystalline volume fraction in the partially crystalline alloys monotonically shifted the T C(am) to higher temperatures, this behavior is also observed in other Febased nanocrystalline MCM. 20,[30][31][32] The increase in T C(am) with higher annealing temperatures has been attributed to the exchange field from the nanocrystalline precipitates penetrating into the residual amorphous matrix 20,30,32 and/or to the lowered Fe content in the amorphous matrix. 30 The compositional results obtained from EPMA measurements revealed good agreement with the nominal composition with oxygen content of $0.71 wt.…”