Magnetocaloric effect in the binary intermetallic compound DySi

Polycrystalline binary rare earth intermetallic compound DySi is found to be dimorphic at room temperature (orthorhombic FeB type, space group Pnma, No. 62 and CrB type, space group Cmcm, No. 63). This compound exhibits interesting magnetic properties including an antiferromagnetic transition at ∼38 K (T_N) and a low-temperature field-induced transition in a critical field of 65 kOe, at 5 K. The values of magnetic entropy change and adiabatic temperature change near the magnetic transition in DySi have been estimated using the heat capacity data obtained in different applied fields. Negative magnetocaloric effect is observed at temperatures close to and below T_N, in fields up to 50 kOe.     

Magnetocaloric effect in antiferromagnetic Dy3Co compound

Magnetic properties and magnetocaloric effects (MCEs) of the Dy₃Co compound are studied. Two successive magnetic transitions: the antiferromagnetic (AFM)-to-AFM transition at T _AF =29 K and the AFM-to-paramagnetic (PM) transition with increasing temperature at the Néel temperature T _N =44 K are observed. Dy₃Co undergoes a field-induced metamagnetic transition from the AFM to the ferromagnetic (FM) state below T _N , giving rise to a large MCE. The maximal value of magnetic entropy change ΔS _m is −13.9 J/kg K with a refrigerant capacity (RC) of 498 J/kg around T _N for a field change of 0–5 T. A sign change of MCE in Dy₃Co with magnetic field and temperature is observed near the critical field where the metamagnetic transition occurs.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

Metastable states in phase separated electron doped Sm0.15Ca0.85MnO3

In order to study the mechanism behind the phase separation scenario in the Sm_0.15Ca_0.85MnO₃ compound, magnetization and resistivity measurements have been carried out in pulsed magnetic fields up to 50 T at temperatures 4.2 K<T<200 K. It is found that external magnetic field causes a collapse of a C-type AFM (P2₁/m) phase resulting in field-induced insulator-metal transition, which is irreversible below T₁=75 K. In zero field the content of a G-type phase in the mixed C-G state can vary from 10 to 17% at T=10 K. A set of metastable states with different volume ratios of G-type to C-type phases is observed below T₁ depending on the history of the sample. The obtained results indicate that the phase separation plays a dominant role for the electric and the magnetic properties of this material.     

Magnetic phase transitions in Nd7Rh3 single crystal

Magnetic phase transitions in rare earth intermetallic compound Nd₇Rh₃ have been investigated using a single crystal. Measurement results of magnetization, magnetic susceptibility, specific heat, and electrical resistivity reveal that Nd₇Rh₃ has two magnetic phase transitions at T_N=34 K, T_t2=9.1 K and a change of the magnetic feature at T_t1=6.8 K in the absence of an external magnetic field. Antiferromagnetic orderings exist in all the three magnetic states; a large magnetic anisotropy between the c-axis and the c-plane is observed. In the magnetic phase below T_t2, an irreversible field-induced magnetic phase transition takes place in the c-plane; after removing external magnetic field, a coexistence state of ferro- and antiferromagnetic ordering or a ferrimagnetic state having a remanent magnetization M_R is stabilized. The M_R decays to a certain value for several hours after the first process; a magnetic field cooling effect was also observed in the c-plane below T_t2. In the antiferromagentic state above T_t2, the irreversibility disappears and an ordinary antiferromagnetic state takes place. As the origin of this phenomenon, a kind of martensitic structural transition that is observed in Gd₅Ge₄ can be considered.     

Field-induced structural transition and the related magnetic entropy change in Ni43Mn43Co3Sn11 alloy

Magnetic properties and magnetic entropy change ΔS were investigated in Heusler alloy Ni₄₃Mn₄₃Co₃Sn₁₁. With decreasing temperature this alloy undergoes a martensitic structural transition at T_M=188 K. The incorporation of Co atoms enhances ferromagnetic exchange for parent phases. Austenitic phase with cubic structure shows strong ferromagnetic behaviors with Curie temperature T_C^A at 346 K, while martensitic phase shows weak ferromagnetic properties. An external magnetic field can shift T_M to a lower temperature at a rate of 4.4 K/T, and a field-induced structural transition from martensitic to austenitic state takes place at temperatures near but below T_M. As a result, a great magnetic entropy change with positive sign appears. The size of ΔS reaches 33 J/kg K under 5 T magnetic field. More important is that the ΔS displays a table-like peak under 5 T, which is favorable for Ericsson-type refrigerators.     

Field induced sign reversal of magnetocaloric effect in Gd2In

We report the magnetocaloric effect in the metamagnetic compound Gd₂In obtained from magnetization measurement. Gd₂In was previously reported to have two magnetic transitions: (i) a paramagnetic to ferromagnetic transition below 190 K and (ii) a ferromagnetic to an antiferromagnetic state below 105 K. The low temperature antiferromagnetic state is unstable under an applied magnetic field and undergoes metamagnetic transition to a ferromagnetic like state. We observe conventional positive magnetocaloric effect (the magnetic entropy change, ΔS_M<0) around 190 K at all applied fields. The magnetocaloric effect is found to be inverse (negative) at low fields around 105 K (ΔS_M>0), however it turns positive at higher fields (ΔS_M<0). The observed anomaly is found to be related to the field induced transition which drives the system from an antiferromagnetic to a ferromagnetic state.     

Successive magnetic transitions and low temperature magnetocaloric effect in RE2Ni7 (RE=Dy, Ho)

Magnetic properties of rare-earth intermetallics RE₂Ni₇ (RE=Dy, Ho) are reported. Both the samples undergo two successive magnetic transitions at T_h (paramagnetic to ferromagnetic) and T_l (spin reorientation) below 100 K. The transitions are found to be second order in nature as evident from the Arrot plot analysis. Large reversible magnetocaloric effect (MCE) was observed at low temperature in the studied samples. The maximum value of the magnetic entropy change in Ho₂Ni₇ is found to be −12.5 J/kg K (for 0 to 50 kOe of field change) around 25 K with a high relative cooling power (RCP) of 534 J/kg. The Dy counterpart also shows moderately large values of MCE (−7.3 J/kg K) and RCP (475 J/kg) around the magnetic transition region for similar change in the magnetic field. RE₂Ni₇ compounds can be promising materials for magnetic refrigeration in the temperature range of helium and hydrogen liquefaction.     

Influence of Dy addition on the magnetocaloric effect of La0.67Ca0.33Mn0.9V0.1O3 ceramics

The influence of partial substitution of La by Dy on the magnetocaloric response of (La_1−xDy_x)_0.67Ca_0.33Mn_0.9V_0.1O₃, where x=0.03, 0.15 and 0.25 is studied. Rietveld refinement of X-ray diffraction pattern using GSAS method shows that the compounds adopt the orthorhombic structure with Pnma space group. The systematic change in lattice parameters and magnetic phase transition indicates the substitution effect of Dy. From the magnetization isotherms at different temperatures, magnetic entropy change close to their respective transition temperatures (T_C) has been evaluated. The maximum value of entropy change near T_C is found to be about 4.8 J/kg K at 187.5 K for LCMVDy_0.03, 2.45 J/kg K at 107.5 K for LCMVDy_0.15 and 2.15 J/kg K at 92.5 K for LCMVDy_0.25 at 4 T. Dy addition produces a reduction in T_C and in magnitude of the magnetic entropy change. Even though the entropy change decreases with increasing Dy substitution the refrigerant temperature range, ΔT, is found to be 10 K for LCMVDy_0.03, 31 K for LCMVDy_0.15 and 35 K for LCMVDy_0.25 compounds [90%] at 4 T. The field dependence of the magnetic entropy change is also analyzed showing the power law dependence, ΔS_M∞Hⁿ where n=0.75(2) for LCMVDy_0.03, n=0.80(4) for LCMVDy_0.15 and n=0.92(8) for LCMVDy_0.25 compounds at their respective transition temperatures. The relative cooling power and its field dependance are also analyzed.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

Structure, magneto-structural transitions and magnetocaloric properties in Ni50−xMn37+xIn13 melt spun ribbons

Melt spun Ni_50−xMn_37+xIn₁₃ (2≤x≤5) ribbons were investigated for the structure, microstructure, magneto-structural transitions and inverse magnetocaloric effect (IMCE) associated with the first-order martensitic phase transition. The influence of excess Mn in Ni site (or Ni/Mn content) on the martensite transition and the associated magnetic and magnetocaloric properties are discussed. It was found that with the increase in Mn content, the martensitic transition shifted from 325 to 240 K as x is varied from 2 to 4, and the austenite phase was stabilized at room temperature. The x=5 ribbon did not show the martensitic transition. For the x=3 ribbon, the structural and magnetic transitions are close together unlike in the x=4 ribbon in which they are far (∼60 K) apart. The zero field cooled and field cooled curves support the presence of exchange bias blocking temperature due to antiferromagnetic interactions in the ribbons. A large change in the magnetization between the martensite and austenite phases was observed for a small variation in the Ni/Mn content, which resulted in large IMCE. A large positive magnetic entropy change (ΔS_M) of 32 J/kg K at room temperature (∼ 300 K) for a field change of 5 T with a net refrigeration capacity of 64 J/kg was obtained in the Ni₄₇Mn₄₀In₁₃ ribbon.     

Effect of monovalent metal substitution on the magnetocaloric effect of perovskite manganites Pr0.5Sr0.3M0.2MnO3 (M=Na,Li, K and Ag)

The influence of monovalent doping on the magnetocaloric effect (MCE) and refrigerant capacity or relative cooling power (RCP) of Pr_0.5Sr_0.3M_0.2MnO₃ (M=Na, Li, K and Ag) materials has been investigated. A large magnetocaloric effect was inferred over a wide range of temperature around the second order paramagnetic–ferromagnetic transition. The maximum magnetic entropy changes (ΔS_M) reached 1.8, 2.2, 1.6 and 2.1 J/kg K and the relative cooling power (RCP) approached 58.9, 59.3, 69.6 and 54.6 J/kg for Na, Li, K and Ag doped materials in the magnetic change of 15 kOe, respectively. According to the results determined by the Maxwell relation, the magnetic entropy change fits well with the Landau theory of phase transition above T_C for Pr_0.5Sr_0.3Li_0.2MnO₃. The large magnetic entropy change induced by low magnetic field suggested that these materials are beneficial for practical applications.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Effect of K doping on the physical properties of La0.65Ca0.35−xKxMnO3 (0?x?0.2) perovskite manganites

The effects of K doping in the A-site on the structural, magnetic and magnetocaloric properties in La_0.65Ca_0.35−xK_xMnO₃ (0?x?0.2) powder samples have been investigated. Our samples have been synthesized using the solid-state reaction method at high temperature. The parent compound La_0.65Ca_0.35MnO₃ is an orthorhombic (Pbnm space group) ferromagnet with a Curie temperature T_C of 248 K. X-ray diffraction analysis using the Rietveld refinement show that all our synthesized samples are single phase and crystallize in the orthorhombic structure with Pbnm space group for x?0.1 and in the rhombohedral system with R3¯c space group for x=0.2 while La_0.65Ca_0.2K_0.15MnO₃ sample exhibits both phases with different proportions. Magnetization measurements versus temperature in a magnetic applied field of 50 mT indicate that all our investigated samples display a paramagnetic-ferromagnetic transition with decreasing temperature. Potassium doping leads to an enhancement in the strength of the ferromagnetic double-exchange interaction between Mn ions, and makes the system ferromagnetic at room temperature. Arrott plots show that all our samples exhibit a second-order magnetic-phase transition. The value of the critical exponent, associated with the spontaneous magnetization, decreases from 0.37 for x=0.05 to 0.3 for x=0.2. A large magnetocaloric effect (MCE) has been observed in all samples, the value of the maximum entropy change, |ΔS_m|_max, increases from 1.8 J/kg K for x=0.05 to 3.18 J/kg K for x=0.2 under a magnetic field change of 2 T. For x=0.15, the temperature dependence of |ΔS_m| presents two maxima which may arise from structural inhomogeneity.     

Magnetocaloric effect in the intermetallic compound DyNi

Magnetic and heat capacity measurements have been carried out on the polycrystalline sample of DyNi, which crystallizes in the orthorhombic FeB structure (space group Pnma). This compound is ferromagnetic with a Curie temperature of 59 K. Magnetization-field isotherms at low temperatures show a multi-step behavior characteristic of metamagnetic transitions. The magnetocaloric effect has been measured both in terms of isothermal magnetic entropy change and adiabatic temperature change for various applied magnetic fields. The maximum values of the entropy change and the temperature change are found to be 19 J kg⁻¹ K⁻¹ and 4.5 K, respectively, for a field of 60 kOe. The large magnetocaloric effect is attributed to the field-induced spin-flop metamagnetism occurring in this compound, which has a noncollinear magnetic structure at low fields.     

Effect of interstitial nitrogen on magnetism and entropy change of LaFe11.7Si1.3 compound

Crystal structure, magnetism and magnetocaloric properties of LaFe_11.7Si_1.3N_y (y=0, 1.3) compounds have been studied by X-ray diffraction and magnetic measurements. The LaFe_11.7Si_1.3N_y compounds present a cubic NaZn₁₃-type structure. Insertion of 1.3 nitrogen atoms per LaFe_11.7Si_1.3 formula increases the lattice parameter and Curie temperature from 11.467 to 11.733 Å and from 190 to ∼230 K, respectively. Besides, the absorption of nitrogen drives drastically the magnetic transition from first to second order and accordingly strongly decreases the magnetocaloric effect compared to the parent alloy. Under an external field change of 5 T, the value of isothermal entropy change −ΔS is about 28 and 3.5 J/kg K for LaFe_11.7Si_1.3 and LaFe_11.7Si_1.3N_1.3, respectively, close to their Curie temperature. However, the relative cooling power RCP(S) of the nitride is about half that of the parent alloy.     

Magnetoelastic properties of ErMn6Sn6 intermetallic compound

The thermal expansion and magnetostriction of polycrystalline sample of the ErMn₆Sn₆ intermetallic compound with hexagonal HfFe₆Ge₆-type structure are investigated in the temperature range of 77 K to above 400 K. The thermal expansion measurement of the sample shows anomalous behavior around its T_N=340 K. The isofield curves of volume magnetostriction also reveal anomalies at paramagnetic-antiferromagnetic and antiferromagnetic-ferrimagnetic phase transitions. In the antiferromagnetic state, the transition to ferrimagnetism can be induced by an applied magnetic field. The threshold field for the metamagnetic transition H_th increases from 0.18 T at 84 K to about 1 T around 220 K, and then decreases monotonously to T_N. This behavior is well consistent with that observed earlier on magnetization curves attributed to exchange-related metamagnetic transition rather than the anisotropy-related one. Furthermore, the low H_th values suggest that the Mn-Mn coupling in ErMn₆Sn₆ is not so strong. The experimental results obtained are discussed in the framework of two-magnetic sublattice by bearing in mind the lattice parameter dependence of the interlayer Mn-Mn exchange interaction in this layered compound. From the temperature dependence of magnetostriction values and considering the magnetostriction relation of a hexagonal structure, we attempt to determine the signs of some of the magnetostriction constants for this compound.     

Magnetocaloric effect in R2Fe17 (R=Sm,Gd, Tb,Dy, Er)

A series of R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) have been synthesized. The magnetocaloric effect (MCE) of these compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. The Curie temperature of Er₂Fe₁₇ is 294 K. The maximum magnetic entropy change of Er₂Fe₁₇ under 5 T magnetic field is ∼3.68 J/kg K. In the R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) system, the maximum magnetic entropy change under 1.5 T magnetic field is 1.72, 0.89, 1.32, 1.59, 1.68 J/kg K corresponding to their Curie temperature (400, 472, 415, 364, 294 K), respectively.     

Systematic investigations on the magnetic properties of Gd3Ni2In4 compound

We have investigated the structural, magnetic, magnetocaloric, thermodynamic and transport properties of polycrystalline Gd₃Ni₂In₄ compound. X-ray powder diffraction pattern shows that Gd₃Ni₂In₄ crystallizes in hexagonal Lu₃Co₂In₄ – type structure. Magnetization studies reveal the presence of two magnetic transitions, T_N and T_C at 21 K and 55.5 K, respectively. The maximal value of magnetic entropy change, -ΔS_M, computed from isothermal magnetization data in a magnetic field of 9 T is 4.57 J/kg K, which is spread over a wide temperature (ΔT = 61.5 K) and hence yields a relative cooling power (RCP) of 281 J/kg. In addition, the compound shows a significant positive magnetoresistance, MR (T = 2 K) = 44% in B = 9 T. These results on Gd₃Ni₂In₄ compound signify that such materials exhibiting successive reversible magnetic transitions may comprise a distinct class of magnetocaloric materials as they work in a wider temperature range than conventional refrigerant materials.