L. F. Kiss

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.

Enhanced magnetocaloric response in Cr∕Mo containing Nanoperm-type amorphous alloys

The magnetocaloric effect of Fe76Cr8−xMoxCu1B15 (x=0,4) alloys is studied. Although the combined addition of Cr and Mo is more efficient in tuning the Curie temperature of the alloy, the Mo-free alloy presents a higher magnetocaloric response. The refrigerant capacity (RC) for the Mo-containing alloy is comparable to that of Gd5Ge1.9Si2Fe0.1 (for a field of 50kOe, RC=273Jkg−1 for the Mo alloy vs 240Jkg−1 for the Gd-based one), with a larger temperature span of the optimal refrigeration cycle (250K vs 90K, respectively). The restriction of the temperature span to 90K gives RC=187Jkg−1 for the Mo alloy. A master curve behavior for the magnetic entropy change is also evidenced.

Magnetocaloric response of FeCrB amorphous alloys: Predicting the magnetic entropy change from the Arrott–Noakes equation of state

The magnetic entropy change in Fe92−xCr8Bx (x=12,15) amorphous alloys has been studied. Increasing the B content, both the peak entropy change and the Curie temperature of the alloy increase. This is in agreement with an increase in the average magnetic moment per iron atom. The thermal and field dependences of the magnetic entropy change curves have been analyzed with the use of the Arrott–Noakes equation of state. It is shown that determining the parameters in this equation of state (through fitting the magnetization data) allows prediction of the field and temperature dependences of the magnetic entropy change curves in a broad temperature range around the Curie temperature.

Influence of the demagnetizing field on the determination of the magnetocaloric effect from magnetization curves

The influence of the demagnetizing factor (N) on the magnetic entropy change (ΔSM) curves is studied for materials with a second order phase transition. For this purpose, a soft magnetic amorphous ribbon is measured for different orientations of the magnetic field with respect to the plane of the sample. For temperatures below the Curie temperature (TC), the increase in N causes a decrease in ΔSM, while for temperatures above TC no change in the shape of the curves has been found for the different orientations, as expected. In order to eliminate this influence of N and compare the ΔSM(T) curves for samples with different shapes, the recently proposed universal curve for the magnetocaloric effect can be used.

Magnetocaloric response of Fe75Nb10B15 powders partially amorphized by ball milling

The magnetocaloric response of mechanically alloyed Fe75Nb10B15 powders was studied for samples with different amorphous and nanocrystal volume fractions. Thermomagnetic properties scale following a Ω3 law for different milling processes, where Ω is the milling frequency. Curie temperature of the amorphous phase increases as the amorphous fraction increases due to its progressive enrichment in B. The peak magnetic entropy change, |ΔSMpk|, as well as the refrigerant capacity increase with increasing amorphous fraction. The field dependence of |ΔSMpk| can be explained by the multiphase character of the studied samples.

Time and temperature dependence of nanocrystalline structure formation in a Finemet-type amorphous alloy

Abstract The magnetic softness and nanocrystalline structure formation of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 Finemet alloy as a function of annealing temperature (500–600°C) and annealing time (5 s - 3 h) were investigated by means of magnetic measurements, DSC and X-ray diffractometry. Annealed at 550°C the grain size, the solute silicon content of α-Fe grains and the improvement in magnetic softness saturates as a function of time. The deterioration of the magnetic softness after annealing at 575°C is attributed to the Fe 3 Si compound which segregates from the α-Fe solid solution, giving a third intermediate peak on the well-known double-peaked DSC diagram. The crystallization products appearing after annealing at 600°C destroy the intergranular magnetic coupling and give rise to a magnetic hardening.

Influence of Mn on the magnetocaloric effect of nanoperm-type alloys

In this paper, the influence of the Mn content on the magnetocaloric response of ribbon-shaped amorphous samples of Fe80−xMnxB20 (x=10, 15, 18, 20, and 24), has been studied. For this purpose, the temperature and field dependence of the magnetic entropy change (ΔSM) have been obtained from magnetization curves. The partial substitution of Fe by Mn leads to a monotonous change in the Curie temperature (TC) of the alloys from 438 K for x=10 to 162 K for x=24, in agreement with the coherent-potential approximation. These Curie temperatures could make them good candidates to be used for magnetic refrigeration at room temperature. For an applied field of 1.5 T, the maximum entropy change (ΔSMpk) passes from 1 J K−1 kg−1 (x=10) to 0.5 J K−1 kg−1 (x=24), and the refrigerant capacity varies between 117 J kg−1 (x=10) and 68 J kg−1 (x=24). A linear relationship between ΔSMpk and the average magnetic moment per transition metal atom (⟨μ⟩Fe,Mn) has been presented.

Soft magnetic properties of high-temperature nanocrystalline alloys: Permeability and magnetoimpedance

The technological applicability of FeCoNbBCu alloys is suggested in terms of measurements of room temperature magnetoimpedance and temperature dependence of magnetic permeability μr. Results for the Fe78-xCoxNb6B15Cu1 alloy series show that room temperature soft magnetic properties are enhanced in the lowest Co containing alloy (μr∼10 500 and magnetoimpedance ratio ∼60% at 1 MHz). However, permeability exhibits a smoother thermal dependence in the alloys with medium and high Co content. A tradeoff between magnetic softness and its thermal stability reveals the alloy with 39 at. % Co as the most suitable composition among those studied, characterized by a temperature coefficient of ∼ 0.02%/K from room temperature up to 900 K. This value is 1 order of magnitude smaller than those observed for FeSiBCuNb (FINEMET-type) alloys and Mn ferrites and extended over a much wider temperature range than in these materials.

Cluster spin-glass model for amorphous Fe-Zr alloys near the critical concentration: a magnetization study

Abstract The magnetic properties of melt-quenched amorphous Fe 100− x Zr x ( x = 7-12) alloys were studied in the temperature and magnetic field ranges 15 K T H ext 93 Zr 7 is an assembly of magnetic clusters with a blocking temperature distribution, the magnetization versus magnetic field curves were fitted using Neels theory for fine particles at several temperatures in order to obtain the evolution of cluster parameters with temperature. The same model is used for Fe 92 Zr 8 and Fe 88 Zr 12 above the Curie temperature ( T C ). When Neels theory is modified to account for the interaction between the cluster moments, the experimental data are properly described by this frame. This model can explain both the low-temperature anomalies of susceptibility observed for x = 7-10 and the high-temperature superparamagnetic behaviour which is shown to be a characteristic feature of the whole a-Fe 100− x Zr x alloy series. The evolution of magnetic cluster behaviour with temperature and concentration is discussed.

Superparamagnetic behaviour in an Fe76Cu1Nb3Si10.5B9.5 alloy

The superparamagnetic relaxation of a FINEMET-type alloy has been studied. It is demonstrated that this behaviour is a general feature of these nanocrystalline alloys. Since the practical requisite for observing superparamagnetic relaxation in these nanocrystalline alloys is the absence of interactions between the grains, the crystalline volume fractions approaching this optimal condition are those that produce a slight increase in coercivity measured at room temperature.