J.M. Borrego

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 Co addition on the magnetocaloric effect of FeCoSiAlGaPCB amorphous alloys

The FeCoSiAlGaPCB alloys can be prepared as bulk amorphous materials, with outstanding mechanical properties and increased electrical resistivity. These features can be beneficial for their application as a magnetic refrigerant. The influence of Co addition on the magnetic entropy change of the alloy has been studied. This compositional modification displaces the temperature of the peak entropy change closer to room temperature, but reduces the refrigerant capacity of the material. For the Co-free alloy, the peak entropy change is increased with respect to a Finemet alloy containing Mo, but its refrigerant capacity is not enhanced.

Refrigerant capacity of FeCrMoCuGaPCB amorphous alloys

The magnetocaloric effect of the FeCrMoCuGaPCB alloy series, suitable for being prepared as bulk amorphous alloys, has been studied. Optimal refrigeration cycles have a cold reservoir close to room temperature. The refrigerant capacity of these alloys is comparable to that of a Mo-containing Finemet-type alloy and is ∼40% bigger than that of other bulk amorphous alloys with comparable working temperatures. Analysis of the field dependence of the magnetic entropy change evidences a power law for all the magnetic regimes.

Glass-forming ability and soft magnetic properties of FeCoSiAlGaPCB amorphous alloys

The glass-forming ability of (FexCoyBzCu)80Si3Al5Ga2P10 with x=5–70, y=0–63, z=5–12, and u=0–5 amorphous alloys has been analyzed in terms of the width of the supercooled liquid region, the reduced glass transition temperature, and the Vogel–Fulcher–Tammann parameters. Substitution of Fe by Co slightly decreases the glass-forming ability of the studied alloys. The value of the fragility parameter m is discussed in the frame of the general classification scheme of glass-forming liquids. The crystalline phases formed during the first crystallization step are identified. Magnetic moment at low and room temperature, Curie temperature, room temperature magnetostriction, and coercivity decrease with increasing Co content.

On the effects of partial substitution of Co for Fe in FINEMET and Nb-containing HITPERM alloys

A comparative study of the effects of partial substitution of Co for Fe on thermal stability, crystallization and magnetic properties of Co-containing FINEMET and HITPERM alloys series is presented. The difference in metalloid and Nb content between the two alloy series and the presence of Si in the nanocrystals in the case of FINEMET alloys appear as key parameters. A recrystallization process involving the α-Fe type phase in nanocrystalline alloys of both series is evident from thermomagnetic results as a significant decrease in magnetization at the second crystallization stage.

Glass-forming ability and crystallization behavior of Co62−xFexNb6Zr2B30 (x=0,16) amorphous alloys with large supercooled liquid region

The kinetics of the glass transition and the crystallization process of Co62−xFexNb6Zr2B30 (x=0,16) amorphous alloys with large supercooled liquid region was investigated by differential scanning calorimetry. The dependence of the glass transition on the heating rate was analyzed in terms of the Vogel–Fulcher–Tamman equation. The glass-forming ability of these alloys is discussed with respect to the width of the supercooled liquid region, the reduced glass transition temperature, and the fragility parameter. The studied alloys are found to be rather strong glass formers. The crystalline phases formed after the devitrification are identified by means of x-ray diffraction and thermomagnetic measurements. The complicated crystallization process requiring long-range atomic rearrangements seems to contribute to the high stability of the supercooled liquid against crystallization.

Thermomagnetic study of devitrification in Fe-Si-B-Cu-Nb(-X) alloys

Thermomagnetic measurements have been used to study the magnetic and structural changes occurring at the two steps of the crystallization process of Fe73.5Si13.5B9Cu1Nb1X2 (X = Zr, Nb, Mo and V) alloys. Alloying raises the thermal stability of the amorphous phase against nanocrystallization in the order V < Mo < Nb < Zr and some differences in the final crystalline phases are found. The Curie temperature of the amorphous phase increases (about 15K) during structural relaxation. In the course of nanocrystallization a further increase of about 30K in the Curie temperature of the amorphous intergranular is observed for samples with X = Zr, Nb and Mo, but only of about 15K for samples containing V. The observed increase in the Curie temperature of the Fe-Si phase between the end of the first crystallization process and the end of the second crystallization process is associated with a reduction in the Si content, in agreement with X-ray diffraction results.

Magnetocaloric effect of Co62Nb6Zr2B30 amorphous alloys obtained by mechanical alloying or rapid quenching

Amorphous samples of nominal composition Co62Nb6Zr2B30 have been prepared using mechanical alloying (MA) and rapid quenching (RQ) techniques. Differences appear in Curie temperature and the phases developed after crystallization. Refrigerant capacity is enhanced 20% in the MA-sample with respect to that of RQ-sample. Neglecting the demagnetizing factor of powder samples significantly affects the exponent n characterizing the field dependence of the maximum magnetic entropy change.

Structural relaxation processes in FeSiB-Cu(Nb, X), X=Mo, V, Zr, Nb glassy alloys

Abstract Structural relaxation in FeSiB-Cu(NbX) alloys is evidenced by enthalpy release exothermic DSC maxima and by changes in the Curie temperature of the amorphous phase upon annealing. A monotonic increase in T C with annealing temperature and time is found and the equilibrium T C values show a linear dependence with the inverse of the annealing temperature. An uniformly linear log (time) kinetics results and the relaxation time decreases exponentially as the annealing temperature increases.

Nanocrystallite compositions for Al- and Mo-containing Finemet-type alloys

New composite soft magnetic alloys in the compositional series Fe 73.5 x Al x Si 13.5 B 9 Cu 1 Mo 3 were recently obtained by partial thermal devitrification of amorphous ribbons. We report the results of Mossbauer and X-ray diffraction (XRD) investigations of nanocrystalline alloys containing 0, 2, 4, 6 and 8 at.% Al, following annealing at 560°C for 1 h. The results indicate that most of the Si and Al are located in the cubic-DO 3 Fe(Si, Al) crystallites, after the primary crystallization is achieved.