J. Ćwik

Direct Measurement of Magnetocaloric Effect in Metamagnetic Ni 43 Mn 37.9 In 12.1 Co 7 Heusler Alloy

The magnetocaloric effect in the metamagnetic Ni43Mn37.9In12.1Co7 Heusler alloy is directly studied experimentally under the adiabatic and quasi-isothermal conditions in a magnetic field with induction of up to 14 T.

Magnetocaloric and thermomagnetic properties of Ni2.18Mn0.82Ga Heusler alloy in high magnetic fields up to 140 kOe

Measurements of the adiabatic temperature change (ΔT) and the specific heat transfer (ΔQ) of Ni2.18Mn0.82Ga Heusler alloy were taken in order to quantify the direct giant magnetocaloric effect of the alloy when it is in the vicinity of magneto-structural phase transition (PT) from paramagnetic austenite to ferromagnetic martensite, and their results are presented. A new vacuum calorimeter was used to simultaneously measure ΔT and ΔQ of magnetocaloric materials with a Bitter coil magnet in fields of up to H = 140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in H = 140 kOe were found to be ΔT = 8.4 K at initial temperature 340 K and ΔQ = 4900 J/kg at 343 K. Using this direct method, we show that the alloy indeed demonstrates the largest value of ΔQ as compared with previously published results for direct measurements of magnetocaloric materials, even though at 140 kOe the ...

Multifunctional Phenomena in Rare-Earth Intermetallic Compounds With a Laves Phase Structure: Giant Magnetostriction and Magnetocaloric Effect

We report on the magnetic and thermal properties [magnetization, specific heat, thermal expansion, magnetostriction, and magnetocaloric effect (MCE)] for the three multicomponent systems Tb_xDy_yGd_zCo₂, Tb_xDy_yHo_z(Co, Fe)₂, and Tb_xDy_yEr_z(Co, Fe)₂ (x + y + z = 1). We show that for Tb_xDy_yR_zCo₂, the Curie temperatures, TC (which ranged from 130 to 300 K), and order of the phase transition (first or second order) could be controlled by composition. The highest MCE values (the adiabatic temperature change ΔT_ad = 2.2-2.3 K at μ₀ΔH = 1.8 T) were observed for the compounds exhibiting the transitions of the first order. Giant volume magnetostriction of 1500-2000 and 500-600 ppm is demonstrated at a field of μ₀H = 10 and 1.2 T, respectively. Structural and magnetic entropy contributions to the total isothermal entropy change are estimated for Tb_xDy_yR_zCo₂. Regular recurrence and/or change of the physical properties across the varied composition of the compounds allows us to find the materials with desired magnetic characteristics, such as T_C, MCE, and magnetostriction, to use them in practice.

Magnetocaloric Effect of Gadolinium at Adiabatic and Quasi-Isothermal Conditions in High Magnetic Fields

High cooling power of magnetocaloric refrigeration can be achieved only at large amounts of heat, which can be transferred in one cycle from cold end to hot end at quasi-isothermal conditions. The simple experimental method for direct measurement of the transferred heat from material with magnetocaloric effect (MCE) to massive nonmagnetic block at quasi-isothermal conditions was proposed. The vacuum calorimeter was designed for the simultaneous measurements of MCE both at adiabatic conditions (∆T) and quasi-isothermal conditions (∆Q) in the magnetic fields of Bitter coil magnet. This calorimeter was tested on samples of pure polycrystalline Gd with direct MCE. The maximal obtained values were ∆T = 17.7 K and ∆Q = 5900 J/kg at initial temperature 20 °C in magnetic field 140 kOe.

Magnetic properties and transformation of crystal structure in the ErFe2-ErAl2 system

The modification of structural properties and magnetic behaviour of the ErFe2−xAlx Laves phase intermetallic compounds have been studied while changing the Al content in the range 0.36 ≤ x ≤ 1.5. Powder X-ray diffraction study at room temperature showed the formation of the cubic C15 structure, in the samples with Al content of 0.36 ≤ x < 0.75 and 1.4 ≤ x ≤ 2.0. For the intermediate compositions with 0.75 ≤ x ≤ 1.4, the hexagonal C14 structure is observed. Magnetic and magnetocaloric properties of polycrystalline ErFe2−xAlx intermetallic compounds were investigated experimentally using magnetic and heat capacity measurements. The Curie temperature TC decreases from 275 to 28 K as the Al content increases from x = 0.36 to x = 1.5, respectively. Magnetization measurements in strong magnetic fields showed the complex mechanism of magnetic saturation in the studied compounds. Under an external field change from 0 to 2 T, the maximum entropy change is 6 J/kg K at T = 28 K for the composition with x = 1.5. The ...

Magnetostriction in (Tb0.45Dy0.55)1−xErxCo2 (x = 0.1, 0.2): high-field investigation

Intermetallic compounds (Tb0.45Dy0.55)1−xErxCo2 (x = 0.1; 0.2) were synthesized using high purity rare-earth metals. Multicomponent compositions with compensated magnetic anisotropy (MA) were chosen with respect to theoretical predictions based upon the single-ion MA model. Magnetostriction was measured in magnetic fields up to 10 T within the temperature range of 1.5–200 K by means of strain gauges. In the vicinity of magnetic phase transitions (160–170 K), giant volume magnetostriction (~2×10−3 in 10 T) provided by the band magnetism of a Co sublattice was observed in the compounds studied.

Magnetostructural phase transitions and magnetocaloric effect in Tb-Dy-Ho-Co-Al alloys with a Laves phase structure

The influence of simultaneous substitution within the rare earth (R) and Co sublattices on the structural, magnetic, and magnetocaloric properties of the Laves phase RCo2-type compounds is studied. Main attention is devoted to the studies of the magnetostructural phase transitions and the transition types with respect to the alloy composition. Multicomponent alloys Tbx(Dy0.5Ho0.5)1−xCo2 and Tbx(Dy0.5Ho0.5)1−xCo1.75Al0.25 were prepared with the use of high purity metals. Majority of the Tbx(Dy0.5Ho0.5)1−xCo2 alloys exhibit magnetic transitions of the first-order type and a large magnetocaloric effect. The substitution of Al for Co in Tbx(Dy0.5Ho0.5)1−xCo2 increases the Curie temperature (TC) but changes the transition type from first-to the second-order. The discussion of the physical mechanisms behind the observed phenomena is given on the basis of the first principles electronic-structure calculations taking into account both the atomic disorder and the magnetic disorder effects at finite temperatures. T...

Properties of metamagnetic alloy Fe48Rh52 in high magnetic fields

A technique for differential scanning calorimetry (DSC) of materials in high magnetic fields is developed. Based on Peltier elements, a differential calorimeter is designed to work in Bitter coil magnet fields (up to 140 kOe). Calorimetric studies are conducted for Fe48Rh52 alloy with the reverse magnetocaloric effect (MCE) in the vicinity of the metamagnetic structural phase transformation (PT). It is shown that the transition latent heat falls during both forward and reverse transformation as the magnetic field grows.

Research of magnetocaloric effect of Ni-Mn-In-Co- based Heusler alloys by the direct method in magnetic fields up to 14 T

The magnetocaloric effect (MCE) is the adiabatic temperature change (dTad) or (and) the isothermal entropy change (dS) of magnet under the influence of a magnetic field.

General working characteristics of magnetocaloric materials in high magnetic fields

A big interest is attracted to the application of materials with a large magnetocaloric effect (MCE) at magnetic and magnetostructural phase transitions (PT) for creation of household refrigerators, operating near room temperature.