J.S. Blázquez

Field dependence of the magnetocaloric effect in materials with a second order phase transition: A master curve for the magnetic entropy change

The field dependence of the magnetic entropy change can be expressed as ΔSM∝Hn. For soft magnetic amorphous alloys n=1 well below the Curie temperature (TC), n=2 in the paramagnetic range, and n≈0.75 for T=TC. The first value can be explained with simple arguments, n=2 is a consequence of the Curie-Weiss law, but n(TC) deviates from mean field predictions. From the Arrott-Noakes equation of state, a relation between n(TC) and the critical exponents has been obtained, showing remarkable agreement with experimental data (for an example alloy, predicted n=0.72 versus experimental n=0.73). A master curve behavior for the temperature dependence of ΔSM measured for different maximum fields is proposed.

A universal curve for the magnetocaloric effect: an analysis based on scaling relations

The universal character of the recent experimentally found master curve for the magnetic entropy change, ΔSM, in studies of the magnetocaloric response of materials is analytically justified by using scaling arguments. The validity of the obtained scaling relations is checked against experimental data as well as the mean field and Heisenberg models. The curves are unique for each universality class. It is shown that the universal curve can be practically constructed in two different ways, reducing the number of required parameters with respect to the previous phenomenological derivation. This opens the possibility of an inexpensive screening of the performance of magnetocaloric materials, as it allows extrapolations to magnetic fields or temperatures not available in some laboratories.

A Finemet-type alloy as a low-cost candidate for high-temperature magnetic refrigeration.

The refrigerant capacity (RC) of Fe68.5Mo5Si13.5B9Cu1Nb3 alloy is studied. For the amorphous sample, RC=63Jkg−1 for an optimal reversible cycle with cold and hot ends at 328K and 520K, respectively, for a maximum applied field H=15kOe. Nanocrystallization diminishes both the peak entropy change and RC of the material. Although the measured RC is smaller than for Gd5Ge1.9Si2Fe0.1 (240Jkg−1 for H=50kOe), the Mo-Finemet alloy is more than 20 times cheaper, the applied field employed is smaller, and the temperature span of the optimal cycle is increased. This makes this alloy a promising material for high-temperature refrigeration.

The influence of Co addition on the magnetocaloric effect of Nanoperm-type amorphous alloys

The effect of Co addition on the magnetocaloric effect of amorphous alloys with Nanoperm-type composition has been studied for temperatures above room temperature. Co addition produces an increase in the maximum magnetic entropy change and a shift of its associated temperature to higher temperatures. The maximum refrigerant capacity (RC) value obtained in this study is 82Jkg−1 for a maximum applied field H=15kOe. This value is ∼30% larger than that of a Mo-containing Finemet-type alloy measured under the same experimental conditions. However, the RC of the alloys, when calculated from temperatures corresponding to the half-maximum entropy change value, deteriorates with the presence of Co in the alloy. The field dependence of the magnetic entropy change has also been analyzed, showing a power dependence for all the magnetic regimes of the samples. This field dependence at the Curie temperature deviates from mean field predictions.

A constant magnetocaloric response in FeMoCuB amorphous alloys with different Fe∕B ratios

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 magnetocaloric effect in soft magnetic amorphous alloys

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 ∼0.75 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.

Influence of Ge addition on the magnetocaloric effect of a Co-containing Nanoperm-type alloy

The influence of the partial substitution of B by Ge on the magnetocaloric response of Fe78Co5Zr6B10Cu1 is studied. Ge addition produces a reduction in the temperature at which the peak entropy change takes place, as well as a slight decrease in the magnitude of the peak, ∣ΔSMpk∣. The refrigerant capacity, RC, and its field dependence is also analyzed: although Ge addition increases RC of the Co-containing alloy, the largest RC value corresponds to the Co- and Ge-free alloy. This will be discussed on the basis of the recently proposed universal curve for the magnetic entropy change, which is also followed by the FeZrBCu(Co,Ge) alloy series.

Crystallisation process in (FeCo)78Nb6(BCu)16 alloys

Crystallisations of Fe 78-x Co x Nb 6 B 16-y Cu y (x = 18, 39, 60; y = 0, 1) alloys at different heating rates have been studied by differential scanning calorimetry (DSC), thermomagnetic gravimetry (TMG), and X-ray diffraction techniques. α-Fe,Co nanometer sized crystals dispersed in a residual amorphous matrix are obtained in a first stage and the crystallisation is completed after a second stage with the formation of various boride phases. Co alloying decreases the temperature for the crystallisation onset and increases the Curie temperature of the amorphous phase. The volume fraction and the lattice parameters of the α-Fe,Co crystals decrease as the Co content in the alloy increases. The Avrami exponents for the first crystallisation stage are approximately 1.

The influence of Cu addition on the crystallization and magnetic properties of FeCoNbB alloys

The influence of Cu addition on the crystallization process, the nanocrystalline microstructure and the magnetic properties has been studied in the Fe78−x CoxNb6B16−yCuy (x = 18, 39, 60; y = 0, 1) alloy series. Cu addition clearly refines the nanocrystalline microstructure for the alloys with the lowest Co content, but the effect is reduced as the Co content increases. Refinement of the microstructure stabilizes the nanocrystalline microstructure by shifting the second crystallization process to higher temperatures. Cu addition mainly affects the magnetic properties for the lowest Co containing alloys. A fitting procedure of coercivity versus crystalline volume fraction is performed for these alloys in the frame of the random anisotropy model extended to twophase systems and assuming constant magnetoelastic anisotropy.

Partitioning of Co during crystallisation of Fe-Co-Nb-B(-Cu) amorphous alloys

Partitioning behaviour of alloying elements during the primary crystallisation of both Fe 18 Co 60 Nb 6 B 16 and Fe 19 Co 39 Nb 6 B 15 Cu 1 alloys has been studied using three-dimensional atom probe (3DAP). Cu rich clusters were found in the Cu-containing alloy, and they provided nucleation sites for the primary α-Fe(Co) particles. In the Cu clusters, concentration of both Fe and Co was greatly decreased. In both Cu-containing and Cu-free alloys Nb and B were depleted in the α-Fe(Co) particles and enriched in the remaining amorphous phase. However, Co concentration in the α-Fe(Co) was the same as the remaining amorphous phase. Based on the 3DAP analysis, the phase volume fraction has been estimated.