A. P. Kamantsev

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 Hu2009=u2009140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in Hu2009=u2009140 kOe were found to be ΔTu2009=u20098.4u2009K at initial temperature 340u2009K and ΔQu2009=u20094900u2009J/kg at 343u2009K. 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 ...

Thermodynamic and Relaxation Processes near Curie Point in Gadolinium

An experimental method is suggested for the determination of the rate of magnetic phase transitions. The method is based on the measurement of the change of magnetic susceptibility of a ferromagnetic sample in the vicinity of the phase transition in response to an abrupt change of the sample temperature. This paper describes the measurement of the change of the magnetic susceptibility of a thin gadolinium plates, cooled by water-flow at a temperature below the Curie point (TC=292 K). It was found that the relaxation time of the magnetic susceptibility of gadolinium in the temperature range from 289.9 to 291.3 K can be approximated using the Landau-Khalatnikov equation with a kinetic coefficient value γ = 3.9×10-8 cm3/(erg×s). The linear approximation does not fit well in the range from 291.3 to 293.2 K. The fundamental restriction of specific power of the magnetocaloric refrigerator (made by gadolinium plates) was estimated.

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.

Field Dependence of the Magnetocaloric Effect in MnFe(P,Si) Materials

The field dependence of the magnetocaloric effect (MCE) in Mn_1.22Fe_0.73P_0.47Si_0.53 is studied in terms of the entropy change (ΔS) and the temperature change (ΔT) for applied magnetic fields up to 5 and 14 T, respectively. The magnetic fields required to saturate the MCE in this system are ~1.7 and 4-5 T for ΔS and ΔT, respectively. The MCE field dependence is compared with the two approaches of the literature: 1) latent heat model and 2) the power law evolution predicted from the universal analysis of the MCE. It turns out that both of these methods are unsuitable to describe the MCE field evolution in MnFe(P,Si) materials.

Annealing Influence on the Exchange-Bias and Magnetostructural Properties in the Ni50.0Mn36.5Sn13.5 Ribbon-Shape Alloy

We report on the influence of short annealing treatments at 923 K and 1073 K during 10min on both martensitic transformation and exchange bias effect for the Ni50.0Mn36.5Sn13.5 Heusler alloy ribbon by means of magnetic measurements. We have observed that the martensitic transformation is shifted towards higher temperatures with increasing annealing temperature. Furthermore, isothermal M(H) hysteresis loops performed under field-cooling protocol show an exchange bias effect for as-quenched and two annealed ribbons, which indicates the existence of ferromagnetic-antiferromagnetic interactions at low temperatures. In particular, we observe that HC diminishes with the increasing of the annealing temperature, but HE is not affected by the heat treatment.

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.

Experimental simulation of a magnetic refrigeration cycle in high magnetic fields

The complete magnetic refrigeration cycle has been simulated on a sample of gadolinium in magnetic fields of a Bitter coil magnet up to 12 T. The total change of temperature of the sample during the cycle is a consequence of magnetic refrigeration, and the dependence of the magnetization of the sample on the magnetic field exhibits a hysteretic behavior. This makes it possible to determine the work done by the magnetic field on the sample during the magnetic refrigeration cycle and to calculate the coefficient of performance of the process. In a magnetic field of 2 T near the Curie temperature of gadolinium, the coefficient of performance of the magnetic refrigeration is found to be 92. With an increase in the magnetic field, the coefficient of performance of the process decreases sharply down to 15 in a magnetic field of 12 T. The reasons, for which the coefficient of performance of the magnetic refrigeration is significantly below the fundamental limitations imposed by the reversed Carnot theorem, have been discussed.

Direct and Inverse Magnetocaloric Effect in Ni1.81Mn1.64In0.55 Multifunctional Heusler Alloy

The magnetocaloric effect (MCE) in Ni1.81Mn1.64In0.55 Heusler alloy has been measured by the direct method. The field dependences of the magnetization were obtained. The phase transition temperatures were determined. The maximal adiabatic temperature change ΔTad near the Curie temperature is 1.8 K under the magnetic field change ∆H = 18 kOe. The inverse MCE (∆Tad = -3.72 K) in the same field change takes place near the temperature of martensitic transformation.

Rapidly quenched ferromagnetic ribbons with shape memory for magnetically controlled micromechanic devices

One-way shape-memory effect (SME) controlled by temperature and magnetic field in rapidly melt-quenched (RMQ) Heusler-alloy (Ni53Mn24Ga23) ribbons is experimentally studied. Two-way SME that results from training is demonstrated for submicron Ni53Mn24Ga23 samples. Reversible thermally and magnetically controlled bending of no less than 1.5% and deflection of no less than 2 μm are reached for composite Ni53Mn24Ga23/Pt microactuators with sizes of 25 × 2.3 × 1.7 μm3 in the presence of magnetic field of μ0Н = 8 T at an initial temperature of 63°С.