Isothermal Magnetic Entropy Change Research Articles

Pressure-induced changes in the magnetic and magnetocaloric properties ofRMn2Ge2(R=Sm,Gd)

We have studied the variation of magnetic and magnetocaloric properties of polycrystalline compounds ${\text{SmMn}}_{2}{\text{Ge}}_{2}$ and ${\text{GdMn}}_{2}{\text{Ge}}_{2}$ as a function of applied hydrostatic pressure. The magnetic transition temperatures are found to change considerably with pressure. The temperature regime of existence of antiferromagnetic (AFM) ordering is found to increase with pressure, in both the compounds. In ${\text{SmMn}}_{2}{\text{Ge}}_{2}$, the sign of the magnetocaloric effect at the low-temperature ferromagnetic (FM)-AFM transition changes with pressure. The isothermal magnetic entropy change in this compound is found to increase by about 20 times as the pressure is increased from the ambient value to 6.8 kbar. Effect of pressure in ${\text{GdMn}}_{2}{\text{Ge}}_{2}$ is less compared to that in ${\text{SmMn}}_{2}{\text{Ge}}_{2}$. The variations in the magnetic and magnetocaloric properties are attributed to the changes in the magnetic state of the Mn sublattice under pressure. The difference in $R\text{-Mn}$ coupling in Sm and Gd compounds is also found to play a role in determining the magnetic and magnetocaloric properties, both at ambient as well as under applied pressures.

Magnetism, heat capacity, magnetocaloric effect and magneto-transport in R2Al (R = Nd, Gd, Tb) compounds

Magnetic, magneto-thermal and magneto-transport properties investigated in R2Al compounds reveal interesting aspects regarding the magnetic state of these compounds. The coexistence of ferro and antiferro magnetic order is seen to determine the magnetic and other related properties. The crystal field effect is found to be significant in Nd and Tb compounds, as revealed by the magnetization and heat capacity data. The sign of the isothermal magnetic entropy change reflects the nature of magnetic transitions. The magnetic entropy change is found to be significant in all the three compounds, with Tb2Al showing the maximum value of 12 J kg−1 K−1. Like the entropy change, the magnetoresistance (MR) also shows different signs, reflecting the magnetic state. At low temperatures, all the compounds show very large MR.

Magnetic and magnetothermal properties of La1−xNdxFe11.5Al1.5 compounds

In this paper, we report the structural, magnetic, and magnetocaloric effect (MCE) of La1−xNdxFe11.5Al1.5 (x=0.1,0.2) compounds. Temperature dependence of magnetization data shows that with Nd substitution, the nature of magnetic transition changes from second order transition to multiple first order transition. This observation is confirmed by the thermodynamic analysis using the Landau theory of phase transitions. The MCE has been calculated in terms of the isothermal magnetic entropy change (ΔSM) using the magnetization isotherms obtained at temperatures close to the transition temperature. The maximum values of ΔSM are found to be 5.4 and 4.6Jkg−1K−1 of x=0.1 and x=0.2, respectively, for a field change of 50kOe, whereas the value for the undoped compound is about 3Jkg−1K−1. The refrigerant capacity has been calculated to be 544J∕kgK for x=0.1 and 470J∕kgK for x=0.2.

Magnetic properties and magnetocaloric effect of GdGa compound

Polycrystalline GdGa compound was prepared by arc-melting method. X-ray powder diffraction reveals that GdGa crystallizes in the CrB-type structure. The Curie temperature of the compound is 183 K. A distinct peak associated with spin reorientation transition was shown around 100 K on the magnetization versus temperature Curve measured in a field of 0.05T. Isothermal magnetic entropy change (vertical bar Delta S(M)vertical bar) was estimated based on the magnetization isotherms. The maximum value of vertical bar Delta S(M)vertical bar is 4.81 J kg(-1) K(-1) under the apphed field changing from 0T to 5.0 T. The relative cooling power (RCP(S)) of GdGa compound is about 576 J kg(-1) under an applied field change of 5 T. The specific heat of the compound has been estimated within the framework of Debye and mean field approximations to evaluate the adiabatic temperature change (Delta T(ad)). (C) 2008 Elsevier B.V. All rights reserved.

Magnetic and magnetocaloric properties of Tb 5Si 2Sb 2

The rare earth intermetallic compound, Tb 5Si 2Sb 2 (orthorhombic Tm 5Si 2Sb 2-type, space group Cmca), has been synthesized and characterized by means of magnetization measurements. This compound orders ferromagnetically at ∼181 K ( T C ) . Using magnetization vs. field data, the isothermal magnetic entropy change ( Δ S M ) has been computed in the vicinity of T C in applied fields up to 5 T. A maximum value of magnetic entropy change ∼4.4 J/kg/K has been achieved for a field change of 5 T close to the magnetic ordering temperature. The magnetocaloric effect in this compound is thus moderate and spread over a broad temperature range of ∼30 K near T C which is useful for Ericsson-cycle magnetic refrigeration.

Magnetic and magnetocaloric properties on the U1−yRyGa2 (R=Er and Dy) compound

The magnetic, calorimetric, and magnetocaloric properties of the pseudobinary U1−yRyGa2 (R=Er and Dy) series were studied to determine its potential as a candidate for use in cryogenic magnetic refrigeration. The partial substitution of Dy and Er for U provides a wide range of the ordering temperature and increases the saturation magnetic moment. The results for U1−yDyyGa2 with 0.6<y<0.9 show evidences of a spin-glass-like (SG) behavior, possibly as a consequence of competing anisotropy and exchange interactions within a frustrated hexagonal spin lattice. The isothermal magnetic entropy change (ΔSmag) observed for UGa2 shows a well defined peak centered on TC, which is gradually broadened and shifted to lower temperatures as the Er and Dy content increases. For low concentrations (0.2⩽y⩽0.4) a tablelike profile is observed in the ΔSmag curve.

Magnetic, magnetothermal, and magnetotransport properties in SmMn2Si2−xGex compounds

The effect of Ge substitution for Si in SmMn2Si2−xGex compounds has been studied. The Sm ordering temperature is found to be much larger in the compound with x=2, as compared to the compounds with x=0 and 1. The increase in the intralayer Mn–Mn distance is attributed to this variation. Among these three compounds, SmMn2Ge2 is found to show reentrant ferromagnetism at low temperatures. The magnetic contribution to the heat capacity has been calculated in all the three compounds. The splitting of the ground state multiplet has been estimated by fitting the magnetic part of the heat capacity data, using the Schottky formula. The isothermal magnetic entropy change is found to remain the same for x=0 and 1 but decreases in the compound with x=2, though the nature of magnetic transition changes from second order to first order, as x is increased from 0 to 2. The electrical resistivity increases with Ge concentration. The excess resistivity in the antiferromagnetic region has been calculated.

Reduction of Hysteresis Loss in Itinerant-Electron Metamagnetic Transition of 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> Magnetic Refrigerants at Low Temperatures

Since La 0.75 Ce 0.25 (Fe 0.85 Mn 0.03 Si 0.12 ) 13 itinerant-electron metamagnet exhibits large magnetocaloric effects at low temperatures because of the itinerant-electron metamagnetic (IEM) transition, the reduction of hysteresis loss E h estimated from the enclosed area between the ascendant and descendant magnetization has been investigated. We have found that E h associated with the IEM transition of La 0.75 Ce 0.25 (Fe 0.85 Mn 0.03 Si 0.12 ) 13 is remarkably reduced from 78 to 3 J/kg by a slight adjustment of compositions, that is, the increase of the Ce concentration up to z = 0.35 and the decrease of the Fe concentration down to x = 0.84. What is important is that such improvement is achieved without a striking decrease of the isothermal magnetic entropy change ΔS m at low temperatures.

Magnetocaloric Effect of Fe(Rh<SUB>1−<I>x</I></SUB>Pd<I><SUB>x</SUB></I>) Alloys

Magnetocaloric effects of Fe49(Rh1−xPdx)51 alloys with x=0.0, 0.01, 0.03, 0.06, 0.07, 0.08, and 0.09, were investigated using resistivity, magnetization, specific heat, and direct measurement of the adiabatic temperature change ΔT. Magnetization measurements yielded experimental values of the isothermal magnetic-entropy change ΔS for all the samples. The critical temperature of the anitiferromagnetic-ferromagnetic transition was found to depend largely on the substitution concentrations of Pd. A broad peak in ΔS, up to 1.7 J·mol−1 K−1 and some 90 K broad around room temperature, was observed with a field change of ΔB=0 to 7 T for Fe49(Rh0.97Pd0.03)51. The specific heat data of Fe49(Rh0.97Pd0.03)51 made it possible to estimate the ΔT values for various external fields. This estimated value of ΔT compares well with the value measured directly using a thermocouple.

An Application of the “Coloring Problem”: Structure−Composition−Bonding Relationships in the Magnetocaloric Materials LaFe13-xSix

The LaFe(13)-(x)Si(x) (1.0 < or = x < or = 5.0) series is studied experimentally and theoretically to gain possible understanding for the relationships among geometrical structure, chemical composition, magnetic behavior, and physical properties as related to the magnetocaloric effect in these compounds. As the Si concentration increases, LaFe(13)-(x)Si(x) exhibits a structural transformation from the cubic NaZn(13) structure type to a tetragonal derivative due primarily to preferential ordering of Fe and Si atoms. At room temperature, LaFe(13)-(x)Si(x) crystallize in the cubic structure for the range 1 < or = x < or = 2.6 and in the tetragonal for 3.2 < or = x < or = 5. In the range 2.6 < or = x < or = 3.2, it shows a two-phase mixture. Temperature-dependent single-crystal X-ray diffraction experiments near the corresponding Curie temperatures were performed on the room-temperature cubic phases to examine the origin of the large isothermal magnetic entropy changes. A thorough statistical and structural analysis of the data indicates that the noncentrosymmetric F43c space group provides a more adequate atomic arrangement than the centrosymmetric Fm3c space group. This change in space group leads to divergence for specific sets of Fe-Fe distances below the Curie temperature that arises from tilting of Fe-centered [Fe(12)-(x)Si(x)] icosahedra. The noncentrosymmetric space group also agrees with the predominance of icosahedral clusters lacking local inversion symmetry. From extended Hückel and tight-binding linear muffin-tin orbital (TB-LMTO) electronic structure calculations on various model structures, the F43c model is more energetically favorable than the Fm3c model. Extended Hückel calculations on various icosahedral [Fe(12)-(n)Si(n)] (n = 1-5) clusters and TB-LMTO calculations on "LaFe(13)," LaFe(11)Si(2), and LaFe(9)Si(4) have also been carried out to study the effects of a main group element (Si) on stabilizing the cubic NaZn(13)-type structure, influencing the transformation between cubic and tetragonal symmetries, and to study relationships among their chemical bonding and magnetic properties.

Magnetocaloric effect in Heusler alloysNi50Mn34In16 and Ni50Mn34Sn16

We present results of detailed ac susceptibility, magnetization and specific heat measurements in Heusleralloys Ni50Mn34In16 and Ni50Mn34Sn16. These alloys undergo a paramagnetic to ferromagnetic transition around305 K, which is followed by a martensitic transition in the temperature regimearound 220 K. Inside the martensite phase both the alloys show signatures offield-induced transition from martensite to austenite phase. Both field- andtemperature-induced martensite–austenite transitions are relatively sharp inNi50Mn34In16. We estimate the isothermal magnetic entropy change and adiabatic temperature changeacross the various phase transitions in these alloys and investigate the possible influence ofthese transitions on the estimated magnetocaloric effect. The sharp martensitic transition inNi50Mn34In16 gives rise to a comparatively large inverse magnetocaloric effect across this transition. Onthe other hand the magnitudes of the conventional magnetocaloric effect associatedwith the paramagnetic to ferromagnetic transition are quite comparable in thesealloys.

Structural, magnetic and magnetocaloric properties of La 0.8Gd 0.2Fe 11.4Si 1.6

We report the correlation between magnetism and magnetocaloric effect (MCE) in La 0.8Gd 0.2Fe 11.4Si 1.6. The temperature variation of magnetization shows that the magnetic transition is of second-order nature. This observation is also supported by the thermodynamic analysis using the Landau theory of magnetic phase transitions. The MCE has been measured in terms of the isothermal magnetic entropy change (Δ S M). The maximum value of Δ S M is found to be 7.9 J/kg K for a field change of 50 kOe. A comparative study of the magnetic and magnetocaloric properties of this compound with those of LaFe 11.4Si 1.6 is presented.

Enhancement of Magnetocaloric Effects in Single Phase La&lt;sub&gt;1-z&lt;/sub&gt;Nd&lt;sub&gt;z&lt;/sub&gt;(Fe&lt;sub&gt;0.88&lt;/sub&gt;S &lt;sub&gt;0.12&lt;/sub&gt;)&lt;sub&gt;13&lt;/sub&gt;

The single phase of a cubic NaZn13-type La1-zNdz(Fe0.88Si0.12)13 is obtained in the region z ≤ 0.2. The field-induced first-order transition from the paramagnetic to ferromagnetic state, that is, the itinerant-electron metamagnetic (IEM) transition is kept after the substitution of Nd. In addition, a discontinuous change of magnetization at the Cure temperature becomes larger with increasing z. As a result, the isothermal magnetic entropy change and the relative cooling power in the magnetic field change from 0 to 5 T increase to –27 J/kg K and 518 J/kg, respectively, by the partial substitution of z = 0.2.

Large Isothermal Magnetic Entropy Change after Hydrogen Absorption into La&lt;sub&gt;0.5&lt;/sub&gt;Pr&lt;sub&gt;0.5&lt;/sub&gt;(Fe&lt;sub&gt;0.88&lt;/sub&gt;Si&lt;sub&gt;0.12&lt;/sub&gt;)&lt;sub&gt;13&lt;/sub&gt;

The influences of hydrogen absorption on the Curie temperature TC and the isothermal magnetic entropy change for La0.5Pr0.5(Fe0.88Si0.12)13 have been investigated, because the magnetocaloric effects have been confirmed to be enhanced after a partial substitution of Pr for La in La(Fe0.88Si0.12)13. The value of TC for La0.5Pr0.5(Fe0.88Si0.12)13Hy increases from 185 to 324 K with increasing y from 0 to 1.6. The maximum value of the isothermal magnetic entropy change ,Sm MAX is slightly decreased by hydrogen absorption. However, ,Sm MAX = -26 J/kg K in a magnetic field change of 5 T for La0.5Pr0.5(Fe0.88Si0.12)13H1.6 is still larger than the value of -23 J / kg K for La(Fe0.88Si0.12)13H1.5 having almost the same value of TC. Consequently, ,Sm MAX of the La0.5Pr0.5(Fe0.88Si0.12)13Hy is larger than that of La(Fe0.88Si0.12)13Hy in a wide temperature range covering room temperature.

Magnetocaloric effect of Gd(Co 1− xMn x) 2 compounds

Polycrystalline samples of Gd(Co 1− x Mn x ) 2 have been prepared by arc-melting method. The compounds form a complete solid solution with the MgCu 2-type structure and the lattice constant varies monotonically as a function of the Mn content with a negative deviation from the Vegard’s law. The Curie temperatures of the compounds have been determined by thermomagnetic measurements and the thermogravimetric analysis. Isothermal magnetic entropy change ( | Δ S M | ) of the samples with x = 0.65 , 0.7, 0.8, and 0.9 have been calculated by the magnetization isotherms. In the four samples, the addition of Mn decreases the Curie temperature, while the | Δ S M | increases.

1,2,3-Triazolate-Bridged Tetradecametallic Transition Metal Clusters [M14(L)6O6(OMe)18X6] (M = FeIII, CrIII and VIII/IV) and Related Compounds: Ground-State Spins Ranging from S = 0 to S = 25 and Spin-Enhanced Magnetocaloric Effect

We report the synthesis, by solvothermal methods, of the tetradecametallic cluster complexes [M14(L)6O6(OMe)18Cl6] (M=FeIII, CrIII) and [V14(L)6O6(OMe)18Cl6-xOx] (L=anion of 1,2,3-triazole or derivative). Crystal structure data are reported for the {M14} complexes [Fe14(C2H2N3)6O6(OMe)18Cl6], [Cr14(bta)6O6(OMe)18Cl6] (btaH=benzotriazole), [V14O6(Me2bta)6(OMe)18Cl6-xOx] [Me2btaH=5,6-Me2-benzotriazole; eight metal sites are VIII, the remainder are disordered between {VIII-Cl}2+ and {VIV=O}2+] and for the distorted [FeIII14O9(OH)(OMe)8(bta)7(MeOH)5(H2O)Cl8] structure that results from non-solvothermal synthetic methods, highlighting the importance of temperature regime in cluster synthesis. Magnetic studies reveal the {Fe14} complexes to have ground state electronic spins of S<or=25, among the highest known, while in contrast the {Cr14} complex has an S=0 ground state despite having a very similar structure and all complexes being dominated by intramolecular antiferromagnetic exchange interactions. The {Fe14} complexes undergo a magnetic phase transition to long-range ordering at relatively high temperatures for molecular species, which are governed by the steric bulk of the triazole (TN=1.8 and 3.4 K for L=bta- and H2C2N3-, respectively). The huge spins of the {Fe14} complexes lead to very large magnetocaloric effects (MCE)-the largest known for any material below 10 K-which is further enhanced by spin frustration within the molecules due to the competing antiferromagnetic interactions. The largest MCE is found for [Fe14(C2H2N3)6O6(OMe)18Cl6] with an isothermal magnetic entropy change -DeltaSm of 20.3 J kg-1 K-1 at 6 K for an applied magnetic field change of 0-7 T.

Heat capacity and magnetocaloric effect in polycrystalline Gd 1−xSm xMn 2Si 2

We report the magnetocaloric effect in terms of isothermal magnetic entropy change as well as adiabatic temperature change, calculated using the heat capacity data. Using the zero-field heat capacity data, the magnetic contribution to the heat capacity has been estimated. The variations in the magnetocaloric behavior have been explained on the basis of the magnetic structure of these compounds. The refrigerant capacities have also been calculated for these compounds.

Effect of magnetic polarons on the magnetic, magnetocaloric, and magnetoresistance properties of the intermetallic compound HoNiAl

The magnetic, magnetocaloric, and magnetoresistive properties of the polycrystalline compound HoNiAl have been studied. The temperature variations of magnetization and heat capacity show that the compound undergoes two magnetic transitions, one at 14K and the other at 5K. The former is due to the paramagnetic-ferromagnetic transition, while the latter is attributed to the onset of an antiferromagnetic ordering, as the temperature is lowered. The M-H isotherm obtained at 2K shows a metamagnetic transition with a critical filed of about 13kOe. The maximum values of isothermal magnetic entropy change and adiabatic temperature change, for a field change of 50kOe, are estimated to be 23.6J∕kgK and 8.7K, respectively. The relative cooling power is found to be about 500J∕kg for a field change of 50kOe. A large magnetoresistance of about 16%, near the ordering temperature of 14K, is observed for a field of 50kOe. The magnetic, magnetocaloric, and magnetoresistance data seem to suggest the presence of magnetic polarons in this compound.

Anomalous magnetic and magnetocaloric properties of Er2Ni17

Intermetallic compound Er2Ni17 crystallizes in the hexagonal Th2Ni17-type structure with the space group P63/mmc and lattice parameters a = b = 8.282 Å and c = 8.033 Å. The magnetization measurements show that it is ferrimagnetic below its magnetic ordering temperature of 150 K. The temperature variation of zero-field-cooled magnetization in a field of 200 Oe shows the compensation point at about 63 K. On the other hand, the field-cooled magnetization in low fields is found to be negative at low temperatures. The compensation temperature is found to shift to low temperatures with an increase in the field. These variations are explained on the basis of the two sublattice model. The intersublattice molecular field coefficient (nRT) and the intersublattice exchange coupling constant (JRT) have been calculated using the high-field free-powder method. The nRT and JRT values are found to be 1.4 Tf.u./μB and 5.2 K, respectively. The magnetocaloric effect has been measured in terms of the isothermal magnetic entropy change, using the magnetization isotherms. The maximum entropy change is about 2 J kg−1 K−1 for a field of 50 kOe.

Magnetic and thermodynamic properties and MCE of intermetallic compounds PrNi 1−xPt x

We have studied the influence of Pt–Ni substitution on the crystal structure and magnetic behavior of the PrNi 1− x Pt x pseudobinary compounds. The orthorhombic CrB-type structure owned by PrNi was found to remain conserved when substituting Pt for Ni while the unit cell volume linearly increased. The polycrystalline PrNi 1− x Pt x samples were investigated by measuring the specific heat ( C P ) and magnetization ( M) as function of temperature and magnetic field. All studied compounds were found to order ferromagnetically; the Curie temperature T C is linearly decreasing with increasing Pt content. The magnetocaloric effect (the isothermal magnetic entropy change Δ S mag) was determined consistently from the specific heat and magnetization data. The maximum values of Δ S mag are the same for the boundary compounds PrNi and PrPt but become somewhat reduced in solid solutions.