The influence of different compositional modifications on the magnetic entropy change and refrigerant capacity of Finemet, Nanoperm, HiTperm, and bulk amorphous alloys is presented. For all the studied alloys, the field dependence of the magnetic entropy change exhibits a quadratic dependence in the paramagnetic regime, a linear dependence in the ferromagnetic temperature range, and a potential law with a field exponent ϳ0.75 at the Curie temperature. This exponent can be explained using the critical exponents of the Curie transition. It is shown that for alloys of similar compositional series, the magnetic entropy change follows a master curve behavior. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2709409͔Magnetic refrigeration is a field of current interest due to the enhanced performance and reduced environmental impact of these systems when compared with those based on the gas compression-expansion cycle. 1-3 Although current prototypes are based on rare-earth-based materials, 4 soft magnetic amorphous alloys have been recently proposed as low cost candidates for high temperature magnetic refrigeration. [5][6][7][8][9][10][11][12][13] Albeit the maximum magnetic entropy change, ͉⌬S M pk ͉, for these alloys is modest when compared to that of rareearth-based materials, 14,15 the remarkable difference in material costs is an incentive for studying their suitability as magnetic refrigerants. Besides their reduced magnetic hysteresis ͑virtually negligible͒, higher electrical resistivity ͑which would decrease eddy current losses͒, tunable Curie temperature and, in the case of bulk amorphous alloys, outstanding mechanical properties are beneficial characteristics for a successful application of the material. The field dependence of the magnetic entropy change ⌬S M in materials with a second order phase transition has also been recently studied, demonstrating that a master curve behavior is fulfilled for ⌬S M curves measured up to different maximum fields. 16 The aim of this work is twofold: to present a comparison of the relevant characteristics of the magnetocaloric response of selected Finemet, Nanoperm, HiTperm, and bulk amorphous alloys ͑BAAs͒ prepared in ribbon shape and to demonstrate that ⌬S M ͑T͒ curves of different alloys from the same compositional series can also collapse in a master curve.Amorphous alloys from different compositional series have been prepared in ribbon shape ͑ϳ25 m thick͒ by melt spinning: Fe x Co
Low-temperature magnetic properties and the crystallization behavior of FINEMET alloy
A new approach is proposed for the study of crystallization processes under the hypothesis of isokinetic behavior. From a direct extension of the Avrami theory to non-isothermal regimes, an expression of the local Avrami exponent as a function of the crystalline volume fraction is obtained from a single DSC run performed at a constant heating rate and a raw estimation of the activation energy (a variation of the activation energy value between 3 and 5 eV produces a change on the Avrami exponent smaller than ± 0.1). This approximation was applied to the nanocrystallization process of a FeCoNbB(Cu) alloy series and the results found are in good agreement with previous isothermal analysis on the same alloys. The kinetics is characterized by an initial Avrami exponent close to unity, which decreases down to values below 0.5 as the crystalline volume fraction increases.
The magnetocaloric effect of Fe91−xMo8Cu1Bx (x=15,17,20) amorphous alloys has been studied. The temperature of the peak of magnetic entropy change can be tuned by altering the Fe∕B ratio in the alloy, without changing its magnitude, ∣ΔSMpk∣. The average contribution of the Fe atoms to ∣ΔSMpk∣ increases with increasing B content. This is correlated with the increase in the low temperature mean magnetic moment of Fe. A recently proposed master curve behavior for the magnetic entropy change is also followed by these alloys and is common for all of them.
The recrystallization process in FeCoNbBCu nanocrystallinealloys is evidenced from thermomagnetic results as a significant decrease in magnetization at the second crystallization stage. The lowering in the volume fraction of α-FeCo crystals indicates that some of these crystals contribute to the boride phases formed. Electron microscopy images reveal that the final microstructure consists of large crystals (∼500 nm) of a fcc (FeCo)23B6(FeCo)23B6 phase and small crystals (∼20 nm) of bcc α-FeCo and of some boride phases as such (FeCo)2B
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