A.M. Tishin

Alloys of the FeRh system as a new class of working material for magnetic refrigerators

Abstract The temperature dependences of initial magnetic permeability, specific heat capacity and magnetocaloric effect in annealed and quenched samples of Fe 48 Rh 51 alloys near the antiferromagnetic-ferromagnetic (AF-F) first-order phase transition have been investigated. The application of a magnetic field of about 2 T to a quenched sample of this alloy at 308.2 K causes a temperature drop of 12.9 K under adiabatic conditions. The magnetocaloric temperature changes were combined with zero-field specific heat data to construct T - S and T - Δ ( S m = S e ) diagrams for various heat-treated samples of Fe 49 Rh 51 alloys and on the basis of these diagrams the refrigerant capacity of alloys was evaluated. The value of the refrigerant capacity of a quenched sample of the alloy at a field of 1.95 T is 135.22 J kg −1 T −1 . This value is significantly greater than the refrigerant capacities of well known magnetocaloric materials. The possibility of using the AF-F transition in FeRh alloys for magnetocaloric refrigeration is assessed.

The magnetocaloric effect in Fe49Rh51 compound

Abstract The magnetocaloric effect and magnetic permeability in annealed and quenched samples of Fe 49 Rh 51 alloys have been investigated. Giant negative temperature changes about 13 K by applying a magnetic field at B =2 T to the quenched sample of Fe 49 Rh 51 alloy have been observed.

Direct measurements of magnetocaloric effect in the first-order system LaFe11.7Si1.3

The magnetocaloric effect was investigated in LaFe11.7Si1.3, which undergoes a first-order transition at ∼188 K from the ferromagnetic to paramagnetic state. The magnetic entropy change upon a field increase from 0 to 5 T is as large as 29 J/kg K (212 mJ/cm3 K). The adiabatic temperature change obtained via direct measurements reaches 4 K under a field change from 0 to 1.4 T. The large values of entropy change and adiabatic temperature change confirmed the large potential of present compound LaFe11.7Si1.3 as a magnetic refrigerant in the corresponding temperature range.

Magnetocaloric effect in itinerant electron metamagnetic systems La(Fe1-xCox)11.9Si1.1

The NaZn13-type compounds La(Fe1−xCox)11.9Si1.1 (x=0.04, 0.06, 0.08) were successfully synthesized, in which the Si content is the limit that can be reached by arc-melting technique. TC is tunable from 243 to 301 K with Co doping from x=0.04 to 0.08. Great magnetic entropy change ΔS in a wide temperature range from ∼230 to ∼320K has been observed. The adiabatic temperature change ΔTad upon changing magnetic field was also directly measured. ΔTad of sample x=0.06 reaches ∼2.4K upon a field change from 0 to 1.1 T. The temperature hysteresis upon phase transition is small, ∼1K, for all samples. The influence of Co doping on itinerant electron metamagnetic transition and magnetic entropy change is discussed.

Experimental device for studying the magnetocaloric effect in pulse magnetic fields

A device for studying the magnetocaloric effect in pulsed magnetic fields up to 8 T is described. Data on magnetocaloric effect in gadolinium metal obtained by this technique was compared to that obtained by other experimental methods.

The maximum possible magnetocaloric ΔT effect

The current boom of research activity in magnetocaloric materials science is fuelled by the expectation that new advanced refrigerants may be found whose ΔT will significantly surpass that of gadolinium (Gd) metal (2.6–2.9 K/T). Because of this expectation, the main effort in the field has been diverted from the important issues of refrigerator design to the routine characterization of magnetic materials. Estimating the maximum adiabatic temperature change that can be achieved in principle by applying a certain magnetic field, say 1 T, is a matter of priority. In this work the problem of maximum ΔT is approached from general principles. According to the most optimistic estimates, ΔT can never exceed ∼18 K/T, the more realistic upper limit lying somewhere in high single figures. We therefore deem it most unlikely that a refrigerant much better than Gd, in respect of the ΔT value, will ever be found.

Low temperature electron paramagnetic resonance anomalies in Fe-based nanoparticles

A study of the electron paramagnetic resonance of Fe-based nanoparticles embedded in polyethylene matrix was performed as a function of temperature ranging from 3.5 to 500 K. Nanoparticles with a narrow size distribution were prepared by the high-velocity thermodestruction of iron-containing compounds. A temperature-driven transition from superparamagnetic to ferromagnetic resonance was observed for samples with different Fe content. The unusual behavior of the spectra at about 25 K is considered evidence of a spin-glass state in iron oxide nanoparticles.

Magnetocaloric effect in strong magnetic fields

Abstract Calculations of magnetic entropy change, ΔSM, and magnetocaloric effect, ΔT, in 3d and 4f magnetics have been carried out, based on the molecular field theory. ΔSM and ΔT have been studied as a function of Debye temperature, θD, Lande factor, gj, quantum number of total mechanical momentum, J, and also of magnetic phase transition temperatures. Limiting values of ΔSM and ΔT have been determined in extremely strong magnetic fields. The results obtained are compared with experimental data. It is shown that the use of ferromagnetic alloys Tbx Gd1-x as operating devices of magnetic refrigerating machines in the room temperature range is more efficient than the use of pure Gd. These alloys have been found to have high specific refrigerant capacity over a wide range of fields from 0.1 to 6 T, which enables one to develop highly economic refrigeration devices in which weak fields are applied.

Electron paramagnetic resonance of ferrite nanoparticles

Three types of iron-based oxide nanoparticles (weight compositions Fe2O3, BaFe2O4, and BaFe12O19) embedded in a polyethylene matrix are studied using the electron paramagnetic resonance technique. All nanoparticles are found to be multiphase. Thermal variations of electron paramagnetic resonance spectra reveal the presence of two phases in the Fe2O3 nanoparticles. One such phase undergoes an antiferromagnetic-like transition near 6 K. Nanoparticles of BaFe2O4 demonstrate a resonance anomaly near 125 K that could indicate the presence of a magnetic phase. Reduced magnetic anisotropy in BaFe12O19 nanoparticles may be related to either structural imperfection or particle smallness (effective diameter of less than 10 nm). Our data clearly show that low temperature experiments are desirable for the correct identification of nanoparticles by means of the electron paramagnetic resonance technique.

Field dependence of the adiabatic temperature change in second order phase transition materials: Application to Gd

The field dependence of the adiabatic temperature change ΔTad of second order phase transition materials is studied, both theoretically and experimentally. Using scaling laws, it is demonstrated that, at the Curie temperature, the field dependence of ΔTad is characterized by H1/Δ. Therefore, as the magnetic entropy change ΔSM follows a H(1−α)/Δ power law, these two dependencies coincide only in the case of a mean field model. A phenomenological construction of a universal curve for ΔTad is presented, and its theoretical justification is also given. This universal curve can be used to predict the response of materials in different conditions not available in the laboratory (extrapolations in field or temperature), for enhancing the resolution of the data and as a simple screening procedure for the characterization of materials.