Isothermal Magnetic Entropy Change ΔSm Research Articles

First-principles, magnetothermal and magnetocaloric calculations on Gd1-xCexMn2Ge2 ([formula omitted])

In this research, we study the two compounds, Gd0.5Ce0.5Mn2Ge2 and Gd0.33Ce0.67Mn2Ge2, employing the molecular field theory and the first-principles calculations. The electronic and elastic characteristics are determined through the use of Density Functional Theory (DFT) calculations. Super-cell DFT computations are utilized to calculate the overall density of states (DOS), elastic stiffness constants Cij, and various elastic properties e.g. bulk and shear moduli. The primary objective is to investigate important physical properties such as magnetization, magnetic heat capacity, and key aspects of the magnetocaloric effect including both isothermal magnetic entropy change ΔSm and adiabatic temperature change ΔTad., relative cooling power (RCP), and magnetic phase transition. We investigate these characteristics in magnetic fields up to 6 T and at temperatures up to 400 K. The two compounds show a Curie temperature TC around 340 K. We also studied the effect of high magnetic fields on the nature of the phase transition.

Anisotropic magnetic critical behavior of van der Waals room-temperature ferromagnet Fe5GeTe2 identified by magnetocaloric measurements

Due to the room-temperature Curie temperature and large saturation moment, Fe5GeTe2 is considered a highly attractive van der Waals ferromagnet. Studying its magnetic critical behavior can provide valuable information about its magnetic phase transition. Notably, compared with the conventional methods for studying magnetic critical behavior, such as the modified Arrott plot, scaling analysis based on isothermal magnetic entropy change ΔSM(T,H) has advantages in dealing with the complex magnetic system Fe5GeTe2. However, studies on its magnetic critical behavior based on this method remain completely lacking. Here, we investigate the magnetic critical behavior of Fe5GeTe2 based on its ΔSM(T,H). Through scaling analysis of its ΔSM(T,H), two sets of reliable critical exponents β, δ, and γ are obtained, which are 0.320(8), 7.99(1), and 2.24(2) for H//ab and 0.494(2), 4.28(4), and 1.62(3) for H//c. The significant difference between H//ab and H//c indicates strong anisotropy in its magnetic critical behavior. Furthermore, the fact that the obtained critical exponents for both H//ab and H//c cannot be simply described by a single universality class reveals a crossover of magnetic interactions in Fe5GeTe2.

Large reversible magnetocaloric effect across the first-order magneto-structural transition in (Mn0.45Fe0.55) (Ni0.6Co0.4) Si alloy

Magnetic materials with reversible magnetocaloric effect across their respective first-order magneto-structural transitions (MST) have vital significance for further advancement towards the ultimate goal of practical magnetic cooling applications. In this present study, the reversibility of the MST is investigated by analyzing the isofield curves under different minor and major thermal hysteresis loops and the isothermal curves under different magnetic field cycles. The reversibility of the magnetocaloric effect (MCE) depends upon the width of the nucleation energy barrier existing across the MST. Minor hysteresis loops are found to be minimizing the energy loss by preventing the nucleation of the martensite phase and can be easily returned to the austenite phase immediately while switching from cooling to heating. A large reversible isothermal magnetic entropy change ΔSM of −14.4 J kg−1 K−1 nearly at 322 K is observed for the (Mn0.45Fe0.55) (Ni0.6 Co0.4) Si alloy under a low magnetic field change of 3 T. The observed large reversible MCE, very low magnetic field-induced hysteresis loss, and large reversible RCeffe ∼ 155 J kg−1 will be beneficial for reducing the energy loss during several magnetic field cycles in practical cooling applications which makes this alloy promising for the development of efficient magnetic refrigerants.

Magnetism, phase transition, and magnetocaloric effects of Co2Nb0.8Ga1.2 and Co2Nb1.2Ga0.8 Heusler alloys

Magnetism, phase transition, and magnetocaloric effects of Co2Nb0.8Ga1.2 and Co2Nb1.2Ga0.8 alloys were studied. The measured data show that the TC of Co2Nb1.2Ga0.8 and Co2Nb0.8Ga1.2 are 390 K and 250 K, respectively. Interestingly, calculated the high magnetic susceptibility and spontaneous magnetic moment special magnetic behavior was discovered. The spin fluctuation theory was used to prove the character of flowing electron ferromagnet characteristics in this series of alloys find the T0 and TA of the Co2Nb0.8Ga1.2 alloy are 0.268×103 K, and 0.984×103 K, respectively. Meanwhile, from the magnetocaloric effect, it can be found that the isothermal magnetic entropy change ΔSM was 1.8 J/kg⋅K of under a 5 T field while possessing a large refrigeration temperature region. The results show that changing the ratio of the Nb atom to the Ga atom can effectively reduce the Curie temperature and lay the foundation for the subsequent design of room temperature refrigerants.

Defect Engineering: Electron-Exchange Integral Manipulation to Generate a Large Magnetocaloric Effect in Ni41Mn43Co6Sn10 Alloys.

A promising magnetocaloric effect has been obtained in Ni-(Co)-Mn-X (X = Sn, In, Sb)-based Heusler alloys, but the low isothermal magnetic entropy change ΔSM restricts the further promotion of such materials. Defect engineering is a useful method to modulate magnetic performance and shows great potential in improving the magnetocaloric effect. In this work, dense Ni vacancies are introduced in Ni41Mn43Co6Sn10 alloys by employing high-energy electron irradiation to adjust the magnetic properties. These vacancies bring about intense lattice distortion to change the distance between adjacent magnetic atoms, leading to a significant enhancement of the average magnetic moment. As a result, the saturation magnetization of ferromagnetic austenite is accordingly improved to generate a high isothermal magnetic entropy change ΔSM of 20.0 J/(kg K) at a very low magnetic field of ∼2 T.

A molecular-field study on the magnetocaloric effect in Er2Fe17

A method, based on the molecular field theory of ferrimagnetism, and standard relations for the electronic and lattice heat capacities and entropies, is used to calculate the magnetocaloric effect (MCE) in Er2Fe17. This compound has a Curie temperature in the vicinity of room temperature and could be, therefore, of practical interest. The magnetization, magnetic, lattice, electronic and total entropies and specific heats, for different magnetic fields, have been calculated as function of temperature up to and beyond Tc. The magnetocaloric effect i.e. the isothermal magnetic entropy change ΔSM and the adiabatic temperature change, ΔTad for different magnetic fields, have been studied in the temperature range 0-400K. As an example of our results, the maximum isothermal magnetic entropy change ΔSM in Er2Fe17 is in the range 5-6J/kgK, for a magnetic field change of 80kOe. The adiabatic temperature change, ΔTad has a maximum value in the range 1.5-2.1K for ΔH=80kOe. For the purpose of comparison, a giant magnetocaloric bench-mark material, with first-order phase transition e.g. Gd5Si2Ge2 has a maximum ΔSm and ΔTad (for a field change of 50 kOe) in the range 20–36 J/kg. K and 11–17 K respectively. Our results are in fair agreement with available results of other studies on this compound, in which more involved Hamiltonian was used e.g. taking crystal electric field into account.

Multicaloric effect and spin-wave excitation in the YMn0.5Cr0.5O3 compound

Abstract We have studied magneto- and electro-caloric effects in multiferroic YMn0.5Cr0.5O3 compound, which has a ferrimagnetic transition at TN ∼73 K. Two peaks in the temperature dependence of isothermal magnetic entropy change ΔSM can be observed at TN and lower T0, respectively. While the first maximum of ΔSM corresponds to the ferrimagnetic transition as expected, the second one might be related to a metamagnetic transition. Using the Maxwell’s relation, an electric field driven entropy change (−ΔSE) has also been obtained at temperatures around TN, revealing the multicaloric effect in the YMCO sample. Temperature dependence of the specific-heat confirms the second-order feature for the ferrimagnetic transition in YMCO. By fitting the specific-heat and magnetization data at low temperatures according to the spin-wave excitation model, the nearest neighboring exchange integral J12 has been estimated to be around 10−4 eV, which is in agreement with the value gained from the molecular field theory.

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.

RNi8Si3 (R=Gd,Tb): Novel ternary ordered derivatives of the BaCd11 type

The title compounds have been synthesized and characterized both from the structural and magnetic point of view. Both crystallize in a new monoclinic structure strictly related to the tetragonal BaCd11 type. The structure was solved by means of X-ray single-crystal techniques for GdNi8Si3 and confirmed for TbNi8Si3 on powder data; the corresponding lattice parameters (obtained from Guinier powder patterns) are a=6.3259(2), b=13.7245(5), c=7.4949(3)Å, β=113.522(3)°, Vcell=596.64(3)Å3 and a=6.3200(2), b=13.6987(4), c=7.4923(2)Å, β=113.494(2)°, Vcell=594.88(2)Å3. The symmetry relationship between the tI48-I41/amd BaCd11 aristotype and the new ordered mS48-C2/c GdNi8Si3 derivative is described via the Bärnighausen formalism within the group theory.The large Gd–Gd (Tb–Tb) distances, mediated via Ni–Si network, likely lead to weak magnetic interactions. Low-field magnetization vs temperature measurements indicate weak and field-sensitive antiferromagnetic ground state, with ordering temperatures of 3K in GdNi8Si3 and about 2–3K in TbNi8Si3. On the other hand, the isothermal field-dependent magnetization data show the presence of competing interactions in both compounds, with a field-induced ferromagnetic behavior for GdNi8Si3 and a ferrimagnetic-like behavior in TbNi8Si3 at the ordering temperature TC/N of about (or slightly higher than) 3K. The magnetocaloric effect, quantified in terms of isothermal magnetic entropy change ΔSm, has the maximum values of –19.8J(kgK)−1 (at 4K for 140kOe field change) and −12.1J(kgK)−1 (at 12K for 140kOe field change) in GdNi8Si3 and TbNi8Si3, respectively.

Magnetic and magnetocaloric properties of itinerant-electron system Hf1−xTaxFe2 (x = 0.125 and 0.175)

Intrinsic magnetic properties and magnetocaloric effect (MCE) have been investigated for Hf1−xTaxFe2 (x=0.125 and 0.175) itinerant-electron compounds which exhibit a temperature-induced transition from the ferromagnetic (FM) to the antiferromagnetic (AFM) state. Upon increasing Ta concentration, the ferromagnetic ordering temperature strongly decreases with decreasing the lattice parameters and both the isothermal magnetic entropy change ΔSM and the adiabatic temperature change ΔTad are enhanced. An adiabatic temperature change of ΔTad=2 and 2.5K was observed in Hf0.875Ta0.125Fe2 and Hf0.825Ta0.175Fe2, respectively, for a magnetic field change of 3T. Therefore, the partial substitution of Ta for Hf is highly effective in the enhancement of the magnetocaloric effect in Hf1−xTaxFe2 systems. In addition substitution can be used to tune both the transition temperature but also the magnetic transition from first toward second order type.

Specific heat and magnetocaloric effect of the Mn5Ge3 ferromagnet

The specific heat Cp(T) of the Mn5Ge3 ferromagnet has been studied in a wide temperature range of 2–400 K. The ferromagnetic ordering has been confirmed at TC = 297.4 K, however the behaviour of Cp(T,H) in small magnetic fields suggests a presence of a weak antiparallel component in the magnetic structure. The combination of the Cp(T,H) and magnetization M(T,H) measurements enabled the determination of not only the isothermal magnetic entropy change ΔSM but the adiabatic temperature change ΔTad as well. It has been also found that the mechanical milling process leads only to a moderate drop of the magnetocaloric effect. The reduction of the grain size by 50% decreases the relative cooling power by 28%.

Magnetic, thermodynamic and transport properties at the first and second order magnetic phase transitions in Dy5Si3 compound

We present extended studies including the dc and ac magnetic susceptibility, magnetization, specific heat, electrical resistivity and magnetoresistivity measurements for the Dy5Si3 compound with the hexagonal Mn5Si3-type structure. The results indicate that this compound orders antiferromagnetically below TN=137K. The magnetic properties of Dy5Si3 are mainly governed by the presence of the magnetic moments of Dy3+ ions. In the paramagnetic range, the magnetic susceptibility follows the Curie–Weiss law with μeff=10.57μB/Dy, which is very close to the theoretical value of 10.6μB. From the magnetometric, specific heat and transport data it has been found that below 50K this compound reveals a non-collinear magnetic order, associated with a phase transition, probably of the first order type. On the basis of the thermodynamic approach, we report the magnetocaloric properties in the whole temperature range but concentrate mainly on the region around 50K. The magnetocaloric effect was calculated in terms of the isothermal magnetic entropy change ΔSM as well as the adiabatic temperature change ΔTad using the specific heat data. In spite of the only moderate ΔSM values a significant relative cooling power has been observed.

Magnetic and magnetocaloric properties of the high-temperature modification of TbTiGe

The high-temperature form (HT) of the ternary germanide TbTiGe was prepared by melting. The investigation of HT-TbTiGe by x-ray and neutron powder diffractions shows that the compound crystallizes in the tetragonal CeScSi-type structure (space group I4/mmm; a = 404.84(5) and c = 1530.10(9) pm as unit cell parameters). Magnetization and specific heat measurements as well as neutron powder diffraction performed on HT-TbTiGe reveal a ferromagnet having TC = 300(1) K as the Curie temperature; the Tb-moments are aligned along the c-axis. This magnetic ordering is associated with a modest magnetocaloric effect around room temperature. The isothermal magnetic entropy change ΔSm was determined from the magnetization data; ΔSm reaches, respectively, a maximum value of − 4.3 and − 2.0 J K−1 kg−1 for a magnetic field change of 5 and 2 T.

Structural chemistry and magnetic properties of R11M4In6 (R = Gd, Tb, Dy, Ho, Er, Y; M = Si, Ge) compounds

Crystal structure and magnetic properties of R11M4In6 compounds (R=Gd, Tb, Dy, Ho, Er, Y; M=Si, Ge) were investigated by means of X-ray diffraction and magnetometric measurements. The compounds crystallize in the tetragonal Sm11Ge4In6-type crystal structure (an ordered version of the Ho11Ge10-type; space group I4/mmm). Gd11Ge4In6, Tb11Ge4In6 and Tb11Si4In6 order antiferromagnetically at low temperatures. For the compounds with Dy, Ho and Er magnetic phase transitions to noncollinear magnetic structures with a ferromagnetic component were observed. The magnetocaloric effect, in terms of the isothermal magnetic entropy change ΔSm, was observed for Dy11Si4In6. Both yttrium compounds are Pauli paramagnets.

Influence of Homogenization of Microstructual Composition on Hydrogen Absorption into ${\rm La}{({\rm Fe}_{\rm x}{\rm Si}_{1-{\rm x}})}_{13}$ Magnetic Refrigerants

The Curie temperature TC of La(Fe0.89Si0.11)13 in inhomogeneous and homogeneous (single phase) states is controlled by heat treatment and hydrogen absorption. The magnetization change at TC around room temperature of the inhomogeneous specimens composed of α-Fe, LaFeSi, and La(FexSi1-x)13 phases becomes sluggish after hydrogen absorption, because TC in the specimens has a distribution accompanied by some inhomogeneity of the hydrogen content. As a result, the magnitude of the isothermal magnetic entropy change ΔSm due to the itinerant-electron metamagnetic transition of these specimens is decreased by hydrogen absorption. On the other hand, the magnitude of ΔSm after hydrogen absorption into the single-phase specimen is almost the same as that before hydrogen absorption. Consequently, the compositional homogenization in La(FexSi1-x)13 is necessary to obtain large magnetocaloric effects around room temperature.

Magnetic properties and magnetocaloric effect of (Mn1-xNix)3Sn2(x=0–0.5) compounds

The effects of Ni substitution on the magnetic properties and magnetocaloric effect (MCE) of (Mn1−xNix)3Sn2 compounds (x=0–0.5) have been investigated by x-ray diffraction and magnetization measurements. It was found that Ni substitution decreases the crystal cell volume and the magnetic transition temperatures compared with pure Mn3Sn2. The MCE for all samples has been calculated from the magnetization data in terms of the isothermal magnetic entropy change ΔSM. The maximum values of ΔSMmax at the magnetic phase transition temperatures resulting from a change in magnetic field of ΔH=5 T were found to be 28.2 mJ/cm3 K for Mn3Sn2 (TC1∼257 K), increasing to 31.2 mJ/cm3 K for (Mn0.9Ni0.1)3Sn2 (TC∼167 K).

Itinerant-electron metamagnetic transition and giant magnetocaloric effect in La0.8Ce0.2Fe11.4Si1.6 compound

The itinerant-electron metamagnetic (IEM) transition and magnetocaloric effects (MCE’s) in the La0.8Ce0.2Fe13−xSix (x=1.6,1.4,1.2,1.0) compounds have been investigated by x-ray powder diffraction and magnetic measurements. The La0.8Ce0.2Fe11.4Si1.6 compound exhibits giant values of the isothermal magnetic entropy change ΔSM around the Curie temperature TC∼186K in low magnetic fields. The maximum value ∣ΔSM∣max∼78.29J∕kgK is under a field of 3T. Such large MCE’s are attributed to the sharp change of the magnetization, the field-induced IEM transition and a strong temperature dependence of the critical field BC. The substitution of Ce for part of La in La0.8Ce0.2Fe11.4Si1.6 increases the value of a(T)c(T)∕b(T)2 and enhances the property of IEM.

Control of Working Temperature of Large Isothermal Magnetic Entropy Change in La(Fe<I><SUB>x</SUB></I>TM<I><SUB>y</SUB></I>Si<SUB>1−<I>x</I>−<I>y</I></SUB>)<SUB>13</SUB> (TM=Cr, Mn, Ni) and La<SUB>1−<I>z</I></SUB>Ce<I><SUB>z</SUB></I>(Fe<I><SUB>x</SUB></I>Mn<I><SUB>y</SUB></I>Si<SUB>1−<I>x</I>−<I>y</I></SUB>)<SUB>13</SUB>

The Curie temperature TC of La(FexSi1−x)13 is increased by a partial substitution of Ni for Fe. On the contrary, TC is decreased by a partial substitution of Cr or Mn. In addition, a partial substitution of Ce for La in La(FexMnySi1−x−y)13 causes the further decrease of TC. As a result, La0.65Ce0.35(Fe0.85Mn0.03Si0.12)13 exhibits a thermal-induced first-order transition at TC=60 K. This result means that TC of the La1−zCez(FexMnySi1−x−y)13 is tunable in the temperature range between 60 and 180 K by adjusting composition with keeping the itinerant-electron metamagnetic transition. In the magnetic field change from 0 to 4 T in the vicinity of TC=60 K, the La0.65Ce0.35(Fe0.85Mn0.03Si0.12)13 shows the isothermal magnetic entropy change ΔSm=−13 J kg−1 K−1 and the relative cooling power RCP=458 J kg−1. Consequently, the La1−zCez(FexMnySi1−x−y)13 compounds are useful for magnetic refrigerants working in a temperature range between 60 and 180 K.

Enhancements of Magnetocaloric Effects in La(Fe<sub>0.90</sub>Si<sub>0.10</sub>)<sub>13</sub> and Its Hydride by Partial Substitution of Ce for La

Magnetocaloric effects due to the itinerant-electron metamagnetic transition for La(Fe0.90Si0.10)13 compounds promising as refrigerants are enhanced by a partial substitution of Ce for La. The values of the isothermal magnetic entropy change ΔSm and the adiabatic temperature change ΔTad for La1−zCez(Fe0.90Si0.10)13 compound with z = 0.1 are −32 J/kg K and 12.8 K, respectively, just above the Curie temperature in the magnetic field change from 0 to 4 T. Such large magnetocaloric effects can also be obtained in the vicinity of room temperature by hydrogen absorption. Annealing at 1373 K for 20 days makes it possible to substitute up to z = 0.2.

Large magnetocaloric effect in La(FexSi1−x)13 itinerant-electron metamagnetic compounds

The magnetocaloric effect (MCE) originated from the itinerant-electron metamagnetic transition for La(FexSi1−x)13 compounds has been investigated. With increasing Fe concentration, the MCE is enhanced and both the isothermal magnetic entropy change ΔSm and the adiabatic temperature change ΔTad for the compound with x=0.90 are −28 J/kg K and 8.1 K, respectively, by changing the magnetic field from 0 to 2 T. Similar large MCE values are achieved around room temperature by controlling the Curie temperature by means of hydrogen absorption. Consequently, La(FexSi1−x)13 compounds are promising as magnetic refrigerant materials working in relatively low magnetic fields.