Isothermal Magnetic Entropy Change Research Articles

Magnetocaloric effect at low temperature in robust charge ordered [formula omitted] compounds

We have presented the magnetic properties and magnetocaloric effect of Sm1-xCaxMnO3 compounds (x = 0.50, 0.55, 0.60 and 0.70) in this work. In association to the experimental verification regarding the formation of the ferromagnetic droplets in robust charge ordered materials through exchange bias and training effect measurements, utilization of the magnetocaloric effect for treating the same problem have been addressed here. Our present study indicates that magnetocaloric effect (isothermal magnetic entropy change) at low temperature regime gets drastically modified according to the influence of the stability of the charge ordering in the host matrix. Additionally, the modifications of the ground states was also addressed accordingly.

Magnetocaloric Effect in RFe10X2( X=Mo, V)

We present a mean-field study on the magnetocaloric effect (MCE) in RFe10X2, where X=Mo, V, and R=Gd, Tb, Ho, Tm, Dy, Er, Nd for X=V. For X=Mo, R=Dy, Gd, and Nd. The two-sublattice model, involving the 4f (rare earth) and 3d(Fe) sublattices, is used. For both systems, magnetization, magnetic heat capacity, magnetic entropy and isothermal entropy change ∆Sm are calculated for different magnetic fields in the 0-5T range and the temperature range from 0 to 700K. Direct and inverse MCEs are shown to take place in these ferromagnetic/ferrimagnetic compounds. For a field change ∆H=5T, the maximum isothermal magnetic entropy change has been calculated for ferromagnetic NdFe10Mo2 compound to be 6.6 J/K mol at Tc=441 K. Both direct, and inverse MCEs have been found in ferrimagnetic compounds, e.g., for TmFe10V2, with maximum -∆Sm= J/K mol at Tc=521K and ∆Sm= J/K mol at TN=127 K. Mean-field analysis is suitable for handling the systems we report on. Further study on the lattice and electronic contribution to entropy is planned.

High-Nuclearity Lanthanide-Containing Clusters as Potential Molecular Magnetic Coolers.

High-nuclearity cluster-type metal complexes are a unique class of compounds, many of which have aesthetically pleasing molecular structures. Their interesting physical and chemical properties arise primarily from the electronic and/or magnetic interplay between the component metal ions. Among the extensive studies in the past two decades, those on lanthanide-containing clusters, lanthanide-exclusive or heterometallic with transition metal elements, are most notable. The research was driven by both the synthetic challenges for these generally elusive species and their intriguing magnetic properties, which are useful for the development of energy-efficient and environmentally friendly magnetic cooling technologies. Our efforts in this vein have been concentrated on developing rational synthetic methods for high-nuclearity lanthanide-containing clusters. By means of the now widely adopted approach of "ligand-controlled hydrolysis" of lanthanide ions, a great variety of cluster-type lanthanide hydroxide complexes had been prepared in the first half of this developing period (1999-2006). In this Account, our efforts since 2007 are summarized. These include (1) further development of synthetic strategies in order to expand the ligand scope and/or to increase the nuclearity (>25) of the cluster species and (2) magnetic studies pertinent to the pursuit of materials with a large magnetocaloric effect (MCE). Specifically, with the hope of expanding the family of ligands and producing clusters of previously unknown structures, we tested under hydrothermal or solvothermal conditions the use of readily available yet not commonly used ligands for controlling lanthanide hydrolysis; such ligands, carboxylates as mundane examples, tend to form insoluble complexes prior to any possible hydrolysis. We have also validated the use of preformed transition metal complexes as metalloligands for subsequent control of lanthanide hydrolysis toward heterometallic 3d-4f clusters. Furthermore, we demonstrated using ample examples that the presence of small anions as templates is essential to the assembly of high-nuclearity lanthanide-containing clusters and that maintaining a low concentration of the anion template(s) is a key to such success. It has been found that slow production/release of such anion templates by in situ ligand decomposition or absorption of atmospheric CO2 is effective in preventing precipitation of their lanthanide salts, allowing not only controllable lanthanide hydrolysis but also gradual and modular assembly of the giant cluster species. Magnetic studies targeting potential applications of such clusters as molecular magnetic coolers have also been conducted. The results are summarized in the second portion of this Account in an effort to establish a certain magneto-structure relationship. Of particular relevance is the possible correlation between MCE (evaluated using the isothermal magnetic entropy change, -ΔSM) and magnetic density, and the intracluster antiferromagnetic exchange coupling. We have also made some preliminary attempts at preparing processable and practically useful materials in the form of a monodisperse core-shell nanostructure. We succeeded in encapsulating a single nanosized heterometallic molecular cluster in a nanoshell of silica. It was found that such passivation not only helped stabilize the cluster but also reduced the magnetic interactions between individual clusters. These effects are reflected in the slightly enhanced value of -ΔSM for the core-shell composite over the parent unprotected cluster.

Mo2NiB2-type Sm2Co2Al and Sm2Co2Ga compounds: Magnetic properties and giant low-temperature coercivity

The magnetic ordering of Mo2NiB2-type Sm2Co2Al and Sm2Co2Ga (Immm, No. 71, oI10) compounds has been established using bulk magnetic measurements. Polycrystalline Sm2Co2Al and Sm2Co2Ga undergo ferromagnetic transitions (TC) at 50 K and 62 K, respectively, and low-temperature field induced transitions (Tm) around 14 K and 16 K (in a field of 10 kOe), respectively. Between TC and Tm Sm2Co2Al and Sm2Co2Ga are soft ferromagnets. Below Tm Sm2Co2Al and Sm2Co2Ga exhibit permanent magnet properties with a residual magnetization per samarium of 0.38 μB and 0.36 μB, respectively, and a large coercive field of 69 kOe and 72 kOe, respectively, at 5 K.The magnetocaloric effects of Sm2Co2Al and Sm2Co2Ga were calculated in terms of isothermal magnetic entropy change and they reach maximum values of −1.62 J/kg K and −1.31 J/kg K for a field change of 50 kOe at 50 K and 58 K, respectively. Low temperature magnetic ordering with enhanced anisotropy in Sm2Co2Al and Sm2Co2Ga is accompanied by a positive magnetocaloric effect with isothermal magnetic entropy changes of +1.63 J/kg K and +1.06 J/kg K for a field change of 50 kOe at 10 K and 8 K, respectively.

Magnetocaloric effect in textured rare earth intermetallic compound ErNi

Melt-spun ErNi crystallizes in orthorhombic FeB-type structure (Space group Pnma, no. 62) similar to the arc-melted ErNi compound. Room temperature X-ray diffraction (XRD) experiments reveal the presence of texture and preferred crystal orientation in the melt-spun ErNi. The XRD data obtained from the free surface of the melt-spun ErNi show large intensity enhancement for (1 0 2) Bragg reflection. The scanning electron microscopy image of the free surface depicts a granular microstructure with grains of ∼1 μm size. The arc-melted and the melt-spun ErNi compounds order ferromagnetically at 11 K and 10 K (TC) respectively. Field dependent magnetization (M-H) at 2 K shows saturation behaviour and the saturation magnetization value is 7.2 μB/f.u. for the arc-melted ErNi and 7.4 μB/f.u. for the melt-spun ErNi. The isothermal magnetic entropy change (ΔSm) close to TC has been calculated from the M-H data. The maximum isothermal magnetic entropy change, -ΔSmmax, is ∼27 Jkg-1K-1 and ∼24 Jkg-1K-1 for the arc-melted and melt-spun ErNi for 50 kOe field change, near TC. The corresponding relative cooling power values are ∼440 J/kg and ∼432 J/kg respectively. Although a part of ΔSm is lost to crystalline electric field (CEF) effects, the magnetocaloric effect is substantially large at 10 K, thus rendering melt-spun ErNi to be useful in low temperature magnetic refrigeration applications such as helium gas liquefaction.

Metamagnetism, sign reversal and low temperature magnetocaloric effect in single-crystalline EuV2Al20

The Frank-Kasper cage compound EuV2Al20 crystallizes in the cubic structure with Fd3‾m space group and exhibits unusual magnetic and transport properties. The system undergoes an antiferromagnetic transition below 5.6 K wherein the Eu2+ moments are aligned anti-parallel along 〈1 1 1〉 direction and the system exhibits a weak metamagetic transition at the field of 1 T. Arrott plots (M2 vs H/M) show a “S” shaped variation in the low fields below TN and the plausible reason for the occurrence of negative slope is discussed. Isothermal magnetic entropy change is estimated from both magnetization and heat capacity measurements invoking the Maxwell’s thermodynamic relations. Temperature variation of ΔSm showed a weak negative minimum and a sign reversal at the field value of 1 T due to field induced metamagnetic transition. Universal master curve is constructed by rescaling the ΔSm vs T curves in the context of analysing the nature of the magnetic transition.

The study of entropy change and magnetocaloric response in magnetic nanoparticles via heat capacity measurements

In our work, we have investigated the magnetocaloric effect (MCE) of core@shell Co@Au nanoparticles by heat capacity Cp measurement in temperature range 1.9–55 K under external magnetic field. After the subtraction of lattice heat capacity contribution, the maximum of isothermal magnetic entropy change ΔSm = 3.11 J K−1 kg−1 was obtained at field change from 0 to 9 T. Adiabatic temperature change ΔTad = 3.31 K, refrigerant capacity power RCP = 88.9 J kg−1 and efficiency of adiabatic temperature change η = 0.37 K/T at the same external magnetic field change were established for the investigated system. The comparison of the results to the independent magnetic measurements data revealed the accordance of fundamental magnetocaloric characteristics obtained by both methods. Thus, this work is also the first experimental confirmation of relation between magnetocaloric response of nanoparticles and the low temperature magnetic transition from superparamagnetic to blocking/freezing state.

Structural, magnetic and magnetocaloric properties of HoNi2 and ErNi2 compounds ordered at low temperatures

In this work we report on the structural, magnetic and magnetocaloric properties of polycrystalline HoNi2 and ErNi2 Laves-phase compounds. Powder X-ray diffraction studies at room temperature show that the compounds crystallize in the cubic C15 structure (MgCu2 type). The magnetic measurements reveal that the second-order magnetic phase transition from ferromagnetic to paramagnetic state appears at TC equal 13.5 K for HoNi2 and 6.5 K for ErNi2. Using direct measurements over a wide field range (up to 11 T), along with indirect methods, we have found low and high field regularities of the magnetocaloric effect characterized by the adiabatic temperature change, ΔTad, and the isothermal magnetic entropy change, ΔSmag. The magnetocaloric properties near the phase transition are discussed in the framework of the Landau theory for the second-order phase transitions.

Tunable Magnetocaloric Performance of Cluster Microfibers Induced by Magnetization Direction

Herein, the work describes a comparative study of the magnetocaloric performance along different magnetization directions of cluster Gd‐based fibers for potential applications. Experimental results indicate that there exists the relatively obvious difference of magnetocaloric effect (MCE) in both longitudinal and transverse magnetization directions. Moreover, the isothermal magnetic entropy changes −ΔSm of 10.62 J kg−1 K−1 and refrigerant capacity (RC) of 756.37 J kg−1 of cluster fibers in transverse direction exhibit a higher value than those in longitudinal direction at relatively high field. This distinction depends on magnetic coupling (magnetic charges induced by electron spin‐orbit polarization) among cluster fibers and magnetic anisotropy (namely magnetic moment rotation) by the action of high magnetic field energy. Therefore, the cluster fibers by changing magnetization direction can provide an effective MCE modulation or to be a candidate for practical magnetic refrigeration applications.

Enhanced magnetocaloric effect of Sr1.8Eu0.13Ba0.07FeMoO6 perovskite compound

Double perovskite (Sr1.8Eu0.13Ba0.07)FeMoO6 (SEBFMO) as well as Sr2FeMoO6 (SFMO) were prepared by the traditional solid reaction method. X-ray diffraction results show that all compounds are single phase, which belongs to I4/m space group. The magnetic hysteretic loops (M-H) and field-cooled magnetization (FC) curves indicate that all compounds have good ferromagnetic properties. The FC curves show a phase transition temperature at around 370K. The Arrot plots (H/M-M2) of samples show that a second-order magnetic transition has occurred. An enhanced magnetocaloric effect was observed in SEBFMO. The maximum isothermal magnetic entropy change of SEBFMO reaches to 0.76Jkg−1K−1 for ∆H = 3T, but that of SFMO only reaches to 0.50Jkg−1K−1.

Electronic Properties and Magnetocaloric Effect of GdNi4Si: Ab Initio Calculations, Mean Field Approximation, and Monte Carlo Simulation

We investigated the magnetic properties and magnetocaloric effect of GdNi4Si by using a combination of first-principle density functional theory (DFT) calculations and Monte Carlo Simulation. Ab initio calculations show the stability of the ferromagnetic phase in this material. The Monte Carlo simulations combined with ab initio exchange coupling constants as input parameters lead to discuss the transition temperature, critical exponents, and isothermal entropy changes. The measured critical temperature and the isothermal magnetic entropy changes for a field change of 14T in good agreement with the experimental data.

Structural and magnetic properties of Fe2P-type R6TTe2 compounds (R = Tb, Dy, Ho, Er, T = Fe, Co, Ru): Magnetic properties and specific features of magnetic entropy change

The magnetic properties and magnetic entropy changes of the Fe2P-type Ho6FeTe2, Ho6CoTe2, Er6CoTe2, Tb6RuTe2, Dy6RuTe2 and Ho6RuTe2 (space group P-62m, N 189, hP9) have been studied by means of bulk magnetization and heat capacity measurements. Also, the tentative magnetic entropy change ΔSm* = (∂M / ∂T) × H was used for characterization of magnetic ordering.Ho6FeTe2, Ho6CoTe2, Er6CoTe2, Tb6RuTe2, Dy6RuTe2 and Ho6RuTe2 exhibit complex field sensitive ferro-antiferromagnetic ordering. The correlation between the isothermal magnetic entropy change and the magnetic ordering of these compounds have been studied.

Experimental study of magnetocaloric effect in the two-level quantum system KTm(MoO4)2

KTm(MoO4)2 belongs to the family of binary alkaline rare-earth molybdates. This compound can be considered to be an almost ideal quantum two-level system at low temperatures. Magnetocaloric properties of KTm(MoO4)2 single crystals were investigated using specific heat and magnetization measurement in the magnetic field applied along the easy axis. Large conventional magnetocaloric effect (–ΔSM ≈ 10.3J/(kg K)) was observed in the magnetic field of 5T in a relatively wide temperature interval. The isothermal magnetic entropy change of about 8J/(kgK) has been achieved already for the magnetic field of 2T. Temperature dependence of the isothermal entropy change under different magnetic fields is in good agreement with theoretical predictions for a quantum two-level system with Δ ≈ 2.82cm−1. Investigation of magnetocaloric properties of KTm(MoO4)2 suggests that the studied system can be considered as a good material for magnetic cooling at low temperatures.

Effect of demagnetization factor on the magnetocaloric property of ErMnO3

Isothermal magnetization measurements were performed on a square cuboid shaped polycrystalline sample with a field applied along different directions, having different demagnetization factor values. The magnetocaloric property, namely the isothermal magnetic entropy change (), is evaluated from the magnetization data with and without demagnetization factor correction. The results indicate that without demagnetization factor correction the values are slightly underestimated. The difference between values with and without demagnetization factor correction increases with an increase in demagnetization factor. However, as the field increases, the variation/error in values due to uncertainties related to magnetization (M), field (H) and temperature (T) values becomes higher due to the demagnetization factor correction.

The structural, magnetic and magnetic entropy changes on CoFe2O4/CoFe2 composites for magnetic refrigeration application

In this article, the magnetocaloric effect on thermally reduced CoFe2O4/CoFe2 composite was reported. Co-precipitated CoFe2O4 nanoparticles were thermally reduced at 500 and 800°C for 2h under H2 atmosphere. The reduction process resulted in different fractions of soft and hard (CoFe2O4/CoFe2) magnetic composites and the products were characterized through various procedures. The structure of CoFe2O4/CoFe2 composites was found to be cubic with an average crystallite size of 31 and 30nm for the samples reduced at 500 and 800°C, respectively. The stoichiometry of the composites confirmed that the percentage of soft CoFe2 magnetic phase was increased to 87% with temperature up to 800°C for 2h, besides showed accumulation of nanoparticles. The room temperature saturation magnetization of the CoFe2O4/CoFe2 composites increases from 184 to 190emu/g as a consequence of high concentration of CoFe2 phase. Conversely, the sample reduced at 800°C attained the maximum magnetization of 200 and 221emu/g at 300 and 5K, respectively. Field cooled (FC) and zero field cooled magnetization (ZFC) and the isothermal magnetic entropy change (ΔSm) on the CoFe2O4/CoFe2 composite (with 87% of soft CoFe2 magnetic phase) were estimated. The composite showed the interesting positive and negative magnetic entropy change (ΔSm) of 0.923 and −0.923J/kg.K−1 near room temperature at around 310 and 290K, respectively.

Magnetic frustration induced large magnetocaloric effect in the absence of long range magnetic order

We have synthesized a new intermetallic compound Ho2Ni0.95Si2.95 in a single phase with a defect crystal structure. The magnetic ground state of this material found to be highly frustrated without any long range order or glassy feature as investigated through magnetic, heat capacity and neutron diffraction measurements. The interest in this material stems from the fact that despite the absence of true long range order, large magnetocaloric effect (isothermal magnetic entropy change, −ΔSM ~ 28.65 J/Kg K (~205.78 mJ/cm3 K), relative cooling power, RCP ~ 696 J/Kg (~5 J/cm3) and adiabatic temperature change, ΔTad ~ 9.32 K for a field change of 70 kOe) has been observed which is rather hard to find in nature.

Magnetocaloric effect and other low-temperature properties of [formula omitted

We report on X-ray diffraction, electrical transport, heat capacity and magnetocaloric effect measurements of a polycrystalline sample of Pr2Pt2In. The compound forms in the tetragonal Mo2FeB2 type structure and orders ferromagnetically at TC=9K. In the ordered state, its thermodynamic and electrical transport properties are dominated by magnon contributions with an energy gap of about 8K in the spin-wave spectrum. The magnitude of magnetocaloric effect is similar to the values reported for most rare-earth based intermetallics. Characteristic behavior of the isothermal magnetic entropy change maximum points to a second-order character of the ferromagnetic phase transition in the compound studied.

Magnetocaloric effect in melt-spun Laves phase intermetallic compound HoCo2

Magnetic and magnetocaloric properties of the melt-spun HoCo2 compound have been investigated. The sample orders ferrimagnetically at ∼94K (TC). This is about 14K larger than that of the TC reported for the arc-melted HoCo2 sample. This is found to be a second order transition whereas the arc-melted analogue is known to undergo a first order transition. Melt-spinning synthesis leads to micrograin formation in the sample. The maximum isothermal magnetic entropy change, ΔSmmax, near TC of the melt-spun HoCo2 is about −4.6Jkg−1K−1 and −9.5Jkg−1K−1 for 20kOe and 50kOe field changes, respectively. The corresponding refrigeration capacity and relative cooling power values are 115J/kg and 148J/kg for 20kOe field change and 308J/kg and 398J/kg for 50kOe field change. The relative cooling power is nearly equal to that of the arc-melted sample because the change of order of transition leads to broadening of ΔSm vs T curve. The increase in TC could be due to the enhanced ferromagnetic interactions present in the transition metal sublattice of the melt-spun HoCo2.

The influence of crystalline electrical field on magnetic and magnetocaloric properties in Er1−yTbyAl2 compounds

We report the anisotropy of magnetic field-induced entropy change in rare earth Er1-yTbyAl2 compounds (y=0.00, 0.25, 0.50, 0.75 and 1.00). In the present work, we use a model Hamiltonian that includes the crystalline electrical field anisotropy in both Er and Tb magnetic sublattices, chemical disorder in exchange interactions among Er-Er, Tb-Tb and Er-Tb magnetic ions and the Zeeman effect. We investigated the isothermal magnetic entropy change ΔST for a magnetic field of 1T rotating from a hard 〈001〉 to the easy 〈111〉 direction. We also performed a systematic analysis of the reorientation temperature as a function of the magnetic field intensity. The anisotropic magnetocaloric effect highlights the applicability of this effect on the rotating magnetic refrigeration.

Magnetization and isothermal magnetic entropy change of a mixed spin-1 and spin-2 Heisenberg superlattice

A superlattice composed of spin-1 and spin-2 with ABAB… structure was described with Heisenberg model. The magnetizations and magnetic entropy changes under different magnetic fields were calculated by the Green's function method. The magnetization compensation phenomenon could be observed by altering the intralayer exchange interactions and the single-ion anisotropies of spins. Along with the temperature increasing, the system in the absence of magnetization compensation shows normal magnetic entropy change and displays a peak near the critical temperature, and yet the system with magnetization compensation shows normal magnetic entropy change near the compensation temperature but inverse magnetic entropy change near the critical temperature. Finally, we illustrated the reasons of different behaviors of magnetic entropy change by analyzing the contributions of two sublattices to the total magnetic entropy change.