Thermoremanence anomaly in Fe-Zr(B,Cu) Invar metallic glasses: Volume expansion induced ferromagnetism

Abstract

We report the existence of a thermally induced sharp increase of thermoremanence around the Curie temperature of Invar-like Fe-Zr͑B,Cu͒ soft magnetic glasses. Neutron-diffraction measurements indicate that a true enhancement of the average local magnetic moment, rather than only a change in the domain structure, occurs. Such enhancement has been tentatively attributed to the increasing volume expansion that takes place beyond the Curie temperature and reinforces ferromagnetism in some low-density clusters.The origin of the complex magnetic properties of ironrich metallic glasses containing low concentrations of early transition metals ͑Zr,Sc,Hf,Y͒ have attracted considerable interest during more than two decades, giving rise to several controversial microscopic interpretations. [1][2][3][4] In particular Fe x Zr 100Ϫx amorphous alloys with x above 80 at.% rank among the most extensively studied spin system, 5-10 exhibiting weak itinerant ferromagnetism, low-temperature spinglass-like behavior and Invar character.Nowadays it is clear that Invar-like iron-rich Fe x Zr 100Ϫx , as well as Fe-Zr͑B,Cu͒, 12 soft magnetic glasses exhibit: ͑i͒ a sharp increase of the thermal expansion above the Curie temperature, T C , 2,3 ͑ii͒ a large high-field susceptibility, 5 ͑iii͒ a decrease in both the magnetic moment and T C with increasing x, 9 ͑iv͒ a large positive spontaneous volume magnetostriction, 11 ͑v͒ a positive shift in T C with the tensile stress, 12 and a large but negative shift with pressure. 2,3 Moreover, it is well established that, as a consequence of competitive positive and negative interactions, the spontaneous magnetization is low and the effect of the field is to induce local magnetic moment, as pointed out by the high volume magnetostriction at low fields. 11 In addition, due to the sensitivity of the local magnetic moment to the interatomic distances, the spontaneous magnetization is expected to fluctuate with density fluctuations which are always present in amorphous structures. The sign of the magnetic interactions depends on the interatomic distances giving rise to the coexistence of complex magnetic phases and in this way both cluster 13 and frustration 8 microscopic models have been proposed to account for the overall behavior. According to the positive volume magnetostriction experimentally observed, 11 it is accepted that those regions or clusters with lower local density should exhibit the higher local magnetic moment and thus stronger ferromagnetic character. On the other hand, spinglass-like regions with higher density will surround these ferromagnetic clusters so decreasing the average local magnetic moment.In this work we present the highlights of a detailed experimental study regarding the thermoremanence behavior in amorphous Fe-Zr͑B,Cu͒ which confirms the appearance of a reversible magnetic transition that occurs around their Curie points. 10 were produced in the form of ribbons, with approximate thicknesses of 20 m, using the melt spinning technique. The amorphous structure of all the samp...

Pressure effects on the magnetic properties of FeCuZr studied by x-ray magnetic circular dichroism: Evidence of weakening of ferromagnetism in FeCuZr alloys

“…Strong magnetovolume effects, such high volume magnetostriction pressure effects on magnetization, and Curie temperature as well as invar and anti-invar behavior have been reported not only in Fe alloys with FCC structure, such as FeNi, 4 FePt, 5 FeCu, [6][7][8] but also in Fe based amorphous alloys. 9,10 Such effects are likely caused by the dependence of the band width on the atomic distances, in consequence the magnetic structure changes upon modification of the lattice dimensions.…”

Direct measurements of the correlation between reentrant ferromagnetism and lattice expansion in FeCuZr alloys

“…12,13 It was shown that the TRM values spontaneously increased above Curie temperature. Such enhancement in TRM values could not be due to structural changes and was tentatively attributed to thermal-expansion effects.…”