Isothermal Magnetic Entropy Change Research Articles

Enhanced magnetocaloric effect in terbium-doped gadolinium oxide nanoparticles

A simple, solvothermal method has been used to synthesize gadolinium oxide nanoparticles with 3 wt% terbium doping (3% Tb-doped Gd2O3). The particles have cubic crystal structure (Space group Ia-3, No. 206). The mean particle size estimated from transmission electron microscopy is ∼55 nm and the crystallite size estimated from powder X-ray diffraction data is ∼28 nm. The sample, 3% Tb-doped Gd2O3, remains paramagnetic from 300 K down to 5 K. Magnetocaloric effect (MCE) is estimated in terms of isothermal magnetic entropy change (ΔSm) at low temperatures. The maximum value of ΔSm at 6 K for 70 kOe field change is found to be about −20.9 Jkg−1K−1. This value is quite large and is comparable with that of pristine nanosized Gd2O3. Therefore rare earth-doped Gd2O3 nanostructures could also be useful in low temperature magnetic refrigeration.

Machine learning accelerates design of single-molecule magnets for magnetocaloric applications

Machine learning models improve design process for new single-molecule magnets by predicting their isothermal magnetic entropy change.

Magnetic and magnetocaloric property studies on nanoparticles of electron-doped manganites R0.15Ca0.85MnO3 (R = Pr and Nd)

Magnetic and magnetocaloric properties of two electron-doped manganite nanoparticles namely Pr0.15Ca0.85MnO3 and Nd0.15Ca0.85MnO3 are studied. The samples were prepared by sol-gel method and characterized by powder x-ray diffraction and scanning and transmission electron microscopy. Both bulk and nanoparticles crystallize in orthorhombic structure at room temperature. The crystallite size varies between 30 to 85 nm. While the bulk Pr0.15Ca0.85MnO3 and Nd0.15Ca0.85MnO3 order antiferromagnetically at 147 K and 112 K (TN), their nano-counterparts of size 83 nm and 89 nm show antiferromagnetic cusp around 133 K and 130 K respectively, with larger magnetization values below TN in 5 kOe field. The field dependent magnetization data at 5 K also show this enhancement with decrease in particle size for Nd0.15Ca0.85MnO3 nanoparticles and this difference is attributed to the surface moments. The isothermal magnetic entropy change (ΔSm) shows a peak value of about +0.53 Jkg−1K−1 and −0.28 Jkg−1K−1 for 70 kOe field change near TN respectively, for the 83 nm size Pr0.15Ca0.85MnO3 and 32 nm size Nd0.15Ca0.85MnO3 samples. The occurrence of inverse magnetocaloric effect for larger size sample suggests dominant antiferromagnetic character while the normal magnetocaloric effect in small size particles is in accordance with dominant ferromagnetism. However, the isothermal magnetic entropy change values are about one order smaller in the nanoparticles underlining the role of long-range order in magnetocaloric effect of such materials.

Direct and indirect measurements of the magnetic and magnetocaloric properties of Ni0.895Cr0.105MnGe1.05 melt-spun ribbons in high magnetic fields

Abstract We report the magnetic and magnetocaloric properties of rapidly solidified Ni0.895Cr0.105MnGe1.05 melt-spun ribbons studied by both direct (adiabatic temperature change) and indirect (isothermal magnetic entropy change) methods in intermediate and high magnetic fields up to 10 T. The maximum values of the adiabatic temperature changes (ΔTad) and magnetic entropy changes ( Δ S M ) were found to be ∼2.6 K (µoH = 10 T) and 4.4 J kg−1 K−1 (µoΔH = 5 T), respectively, near the Curie temperature (TC). The ΔTad curves and magnetization isotherms were found to be completely reversible, which indicates the high degree of reversibility of the MCEs in this system. A large temperature span (of about 61 K) and a non-saturating behavior of ΔTad were observed at magnetic fields up to 10 T. The adiabatic temperature change was found to be a linear function of (µoH)2/3 near TC, in accordance with the Landau theory of phase transitions.

Magnetocaloric effect of Gd2O3 nanorods with 5% Eu-substitution

Abstract One dimensional Gd2O3 nanorods with 5% Eu substitution at Gd-site have been synthesized by a simple, inexpensive, hydrothermal process without using any catalyst or template. Structural, spectroscopic, magnetic and magnetocaloric properties of these nanorods are studied. Powder X-ray diffraction data confirm that these samples have cubic structure (Space group Ia-3) at 300 K. High resolution transmission electron microscopy images suggest the average diameter of nanorods to be ~14 nm. However, the rod length is found to vary from about 50 nm to 140 nm. Raman spectrum shows a peak at ~363 cm−1 as expected for the cubic phase of Gd2O3. The 5% Eu substituted Gd2O3 sample remains paramagnetic from 300 K down to 5 K. Using magnetization–field data obtained at various temperatures, isothermal magnetic entropy change (ΔSm) is calculated. The maximum ΔSm value at 6 K for 70 kOe field change is about −30.3 Jkg−1 K−1. This value is quite large compared to that of the pure Gd2O3 nanotube samples. Thus nanostructured Gd2O3 with small rare-earth substitution at Gd-site exhibit appreciable magnetocaloric effect and these nanomaterials may be suitable for low temperature magnetic refrigeration applications.

Pressure induced martensitic transition, magnetocaloric and magneto-transport properties in Mn-Ni-Sn Heusler alloy

Abstract In this work, we report the effect of hydrostatic pressure (P) on the martensitic transition in Mn50Ni40Sn10 Heusler alloy using the magnetization and electrical resistivity measurements. Martensitic transition temperature (TM) is found to shift significantly to higher temperatures with the application of pressure, which reflects the stabilization of the martensite phase. On the other hand, TM shifts to lower temperatures with magnetic field, which implies the stabilization of the austenite phase. The estimated rate of change of martensitic transition temperature with pressure (dTM/dP) for the present alloy is ∼4.6 K/kbar. The alloy shows a maximum negative magnetoresistance (MR) of 9.6% for P = 4 kbar at the martensitic transition. A large isothermal magnetic entropy change (ΔSM) of 16.6 J/kg.K and a refrigerant capacity (RC) of ∼146 J/kg are observed at TM under ambient pressure. Both quantities are found to decrease with the increase of pressure. The values of ΔSM and adiabatic temperature change (ΔTad), calculated from heat capacity measurements are ∼11.3 J/kg.K and −3.4 K respectively for 50 kOe field change. The observed pressure and field dependence results have been explained using the Clausius-Clapeyron equation. The combined effect of pressure and field on the martensitic transition is also discussed.

Thermal and magnetic effects in quasi-binary Tb1-xDyxNi2 (x = 0.25, 0.5, 0.75) intermetallics

Measurements of the low-temperature heat capacity performed for the polycrystalline Tb1-xDyxNi2 intermetallic compounds with x = 0.25, 0.5 and 0.75 enable us to determine their Debye temperatures and estimate the lattice, electron and magnetic contributions to the heat capacity. The measurements in magnetic fields of 1 and 2 T were performed to find the isothermal magnetic entropy and adiabatic temperature changes which characterize the magnetocaloric effect of these compounds. The experimental data obtained are paralleled by theoretical calculations performed in the frame of a microscopic model, which takes into account the exchange interaction and the crystal electric field anisotropy. For the Tb0.75Dy0.25Ni2 compound, direct measurements of the adiabatic temperature change in the temperature range of the magnetic transition were carried out in magnetic fields up to 14 T. The found regularities are discussed in terms of Landau's theory of second-order magnetic phase transitions. In addition, for the parent binary compound, TbNi2, the change of emitted (absorbed) quantity of heat ΔQ was determined using direct measurements.

Magnetic properties and magnetocaloric effects in (Ho1−xYx)5Pd2 compounds**Project supported by the National Basic Research Program of China (Grant No. 2014CB643703), the National Key Research and Development Program of China (Grant No. 2016YFB0700901), the National Natural Science Foundation of China (Grant Nos. 51261004 and 51761007), and Guangxi Natural Science Foundation, China (Grant No. 2018GXNSFAA294051).

The crystal structure, magnetic and magnetocaloric properties of (Ho1−xYx)5Pd2 (x = 0, 0.25, and 0.5) compounds are investigated. All the compounds crystallize in a cubic Dy5Pd2-type structure with the space group Fd3m and undergo a second order transition from spin glass (SG) state to paramagnetic (PM) state. The spin glass transition temperatures Tg decrease from 26 K for x = 0 to 13 K for x = 0.5. In the PM region, the reciprocal susceptibilities for all the compounds obey the Curie–Weiss law. The paramagnetic Curie temperatures () for Ho5Pd2, (Ho0.75Y0.25)5Pd2, and (Ho0.5Y0.5)5Pd2 are determined to be 32 K, 30 K, and 22 K, respectively, and the corresponding effective magnetic moments ( are 10.8 /Ho, 10.3 /RE, and 7.5 /RE, respectively. Magnetocaloric effect (MCE) is anticipated according to the Maxwell relation, based on the isothermal magnetization curves. For a magnetic field change of 0–5 T, the maximum values of the isothermal magnetic entropy change of the (Ho1−xYx)5Pd2 (x = 0, 0.25, and 0.5) compounds are determined to be , , and , with corresponding refrigerant capacity values of , , and , respectively.

Electronic, magnetic properties and magnetocaloric effect in La0.67Sr0.33MnO3 compound: Ab initio calculations and Monte Carlo simulation

We investigated the electronic structure, density of states and exchange couplings, magnetic properties and magnetocaloric effect of La0.67Sr0.33MnO3 composition by using a combination of ab initio 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, isothermal entropy changes, adiabatic temperature change and relative cooling power (RCP). The calculated critical temperature was Tc = 358 K and the isothermal magnetic entropy changes reaches a maximum value of 3.78 J kg−1 for a field change of 5 T. The adiabatic temperature reaches a maximum value equal to 3.55 K and the RCP maximum value is found to be 210 J kg−1 for a magnetic field of 5 T, which suggest La0.67Sr0.33MnO3 could be considered as a good candidate for magnetic refrigeration above room temperature at moderate magnetic fields.

Anisotropic magnetic properties and critical behaviour studies of trigonal Cr5Te8 single crystal

Magnetic and magnetocaloric properties of the single crystalline Cr5Te8 have been studied. Temperature dependent dc and ac magnetization of Cr5Te8, upon cooling, reveals a paramagnetic to ferromagnetic transition at ≈ 220 K (TC). Field dependent magnetization in the ordered state at 2 K reveals strong anisotropy with c-axis as the easy axis of magnetization. A weak itinerant behaviour of Cr-spins is also ascertained from the magnetization studies. Magnetocaloric effect in terms of isothermal magnetic entropy change has been estimated to be ≈-2 J/kg·K near TC for a field change of 70 kOe. The magnetic transition at TC is found to be second order. Critical behaviour analysis along the easy axis direction in Cr5Te8 suggests that this system does not belong to a particular universality class. However, the estimated critical exponent values β = 0.363, γ = 1.399 and δ = 4.83 from modified Arrott plots and scaling of magnetocaloric behaviour satisfactorily obeys the scaling hypothesis wherein the experimental isothermal magnetization curves collapse onto two branches of universal curves below and above TC. Our critical behaviour analyses confirm the quasi-2D weak itinerant ferromagnetic short range type interaction in Cr5Te8.

Simultaneous magnetic and structural transitions in Nd0.15Ca0.85MnO3 manganite: Magnetization and neutron diffraction studies

The manganite Nd0.15Ca0.85MnO3 crystallizes in GdFeO3-type, orthorhombic structure (space group Pnma) at 300 K and orders antiferromagnetically at 112 K (TN). Magnetization-field (M-H) isotherms indicate metamagnetic transition at temperatures below TN. However, metamagnetism does not lead to saturation in magnetization. The magnetization value at 2 K in 140 kOe field is only 0.4 μB/f.u. The isothermal magnetic entropy change (ΔSm) has been calculated using the M-H data and inverse magnetocaloric effect is observed around TN and the maximum value of ΔSm is ∼12.2 J kg−1K−1 at 107.5 K for 140 kOe field change. Powder neutron diffraction study in zero applied field reveals monoclinic distortion of Nd0.15Ca0.85MnO3 crystal lattice and antiferromagnetic ordering of Mn sublattice in ac-plane with wave vector K = [1/2, 0, 1/2] at 125 K. From the neutron data, the Mn magnetic moment value at 3 K is found to be 2.54 μB. Electrical resistivity increases tremendously below TN to an insulating state and shows thermal hysteresis near TN in zero field. Large magnetic fields drive insulator to metal-like transition and negative colossal magnetoresistance of about 70% is realized at 50 K in 70 kOe field and that could be due to the field-induced order.

Partially-disordered to frozen-state crossover induced magnetocaloric properties of the antiferromagnetic one-dimensional spin-chain Sr3CoIrO6

Low-dimensional and geometrically frustrated spin-chain Sr3(Co/Ni)IrO6 compounds exhibit interesting physical phenomena due to the exotic anisotropy associated with the spin–orbit entangled Ir4+ state. Temperature dependent magnetic susceptibility of polycrystalline Sr3CoIrO6 characterizes the partially-disordered antiferromagnetic with TN1 ∼ 90 K and the frozen state of this antiferromagnetic order below TN2 ∼ 30 K. The fitting of χT versus T (K) data to the Ising model reveal that the intra-chain exchange interactions are ferromagnetic, whereas the inter-chain exchange correlations are antiferromagnetic. Field-induced alteration of the partially-disordered antiferromagnetic state in the temperature regime of TN1 ≤ T ≤ TN2 is responsible for the anomalous magnetic hysteresis loop. Further, a large value of isothermal magnetic-entropy change (ΔSM ∼ 22 J kg−1K−1 for 7 T), absence of thermal hysteresis in magnetization and highly insulating nature consents Sr3CoIrO6 to be classified as a good magnetocaloric material for the low-temperature refrigeration.

Low temperature magnetocaloric properties of RE2/3Cu3Ti4O12 (RE = La, Ho and Dy) systems

Polycrystalline RE2/3Cu3Ti4O12 (RE = Ho, Dy and La) samples were synthesized by conventional solid-state reaction method. Isothermal magnetization measurements were performed to evaluate the corresponding isothermal magnetic entropy change (ΔSM) for the systems. Inverse magnetocaloric effect with negative values for −ΔSM(T, H) around the anti-ferromagnetic (AFM) ordering temperature is found for the La2/3Cu3Ti4O12 system. Normal magnetocaloric effect with positive values for −ΔSM(T, H) is observed for the Ho2/3Cu3Ti4O12 and Dy2/3Cu3Ti4O12 systems. Dy2/3Cu3Ti4O12 shows monotonous −ΔSM(T) behavior, while Ho2/3Cu3Ti4O12 shows non-monotonous −ΔSM(T) behavior with a peak around 10 K.

Isothermal magnetic entropy change in Gd/GdN superlattices

Superlattices of Gd/GdN were grown on silicon substrates with different thickness ratio between nanolayers of Gd and GdN. Films were characterized through XRD, XPS and magnetization measurements. Results indicate that different multilayer arrangements, with layers between 5 and 20 nm thickness, promote: (1) a broadening of the peak of isothermal magnetic entropy change around 250–270 K and (2) the emergence of a second peak around 70 K.

Effect of partial metamagnetic and magnetic transition coupling on the magnetocaloric effect of Ni-Mn-Sn-Fe alloy

For Ni-Mn-Z based (Z = Sn, In, Sb) alloys, the martensite transformation (MT) and magnetic transition in austenite state create opposite magnetic cooling effect, which offsets each other when the first-order MT (FOMT) coincides with the second-order magnetic transition (SOMT). In the present study, two successive magnetization peaks are introduced in a Ni48.4Fe1.1Mn38.5Sn12.0 alloy by creation of a partial overlap between reverse MT and magnetic transition in the austenite phase with 1.1 at.% Fe-doping. In addition, significant enhancement of the magnetization difference (ΔM) between martensite/austenite phases is achieved. Consequently, an inverse magnetocaloric effect (IMCE) associated with a positive isothermal magnetic entropy change (ΔSM) of 9.6 J/kg∙K and a working temperature interval (ΔTFWHM) of 306–314 K is found under an external magnetic field of 5.0 T in the reverse MT and TCA temperature range. This IMCE is followed by a conventional magnetocaloric effect (CMCE) with a negative ΔSM of −2.7 J/kg∙K and ΔTFWHM of 316–339 K under 5.0 T. By taking advantage of these two successive MCEs using a special designed device, large caloric cooling effect with enhanced working temperature interval may be of interesting for practical room-temperature refrigeration applications.

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.

Effect of preparation cooling rate on magnetocaloric effect of Fe80P13C7 amorphous alloy

Abstract In this work, the magnetocaloric effect (MCE) of Fe80P13C7 melt-spun amorphous ribbon and bulk glassy rod has been investigated and so the effect of the preparation cooling rate on the MCE of the present Fe-based amorphous alloy is studied. The value of peak of isothermal magnetic entropy change ( | Δ S M peak | ) and refrigerant capacity (RC) for the Fe80P13C7 amorphous ribbon is larger than those of the glassy rod under varies applied field. Hence, we consider that the demagnetization field effect may have a significant influence on the MCE measurement results of the Fe80P13C7 glassy rod. After the demagnetization field correction, the values of | Δ S M peak | and RCAREA for the Fe80P13C7 glassy rod are close or even superior to those of the amorphous ribbon. Further, it is indicated from the | Δ S M peak | ∝ H n relationship that the glassy rod has more homogeneous internal atomic structure compared with the amorphous ribbon. It has been proposed that more homogeneous structure could lead to greater magnetocaloric properties, and therefore we suggest that the intrinsic MCE performance of the Fe80P13C7 glassy rod prepared at a lower cooling rate should be better than that of the amorphous ribbon.

Normal and inverse magnetocaloric effect in colossal magnetoresistive electron-doped manganites R0.15Ca0.85MnO3 (R = Y, Gd and Dy)

Abstract Magnetic, magnetocaloric and electrical transport properties of polycrystalline R0.15Ca0.85MnO3 (R = Y, Gd and Dy) manganites (Orthorhombic, Pnma) have been studied. Magnetization measured in 5 kOe field reveals antiferromagnetic transition at 111 K, 119 K and 112 K (TN) respectively in R0.15Ca0.85MnO3 (R = Y, Gd and Dy). Magnetization in the ordered state undergoes field-induced transition and at 2 K, in 140 kOe field, the magnetic moment values are 1.5 μB/f.u., 2.8 μB/f.u. and 2.7 μB/f.u. respectively for R0.15Ca0.85MnO3 (R = Y, Gd and Dy) samples. While the electrical resistivity above TN shows non-adiabatic, small polaron hopping behavior, the resistivity value increases rapidly below TN. Hysteresis is observed around TN between field cooled cooling (FCC) and field cooled warming (FCW) magnetization data in 100 Oe and also between the cooling and warming data of zero-field electrical resistivity. Electrical resistivity exhibits a metal to insulator-like transition at TN in applied magnetic fields of 50 kOe and above. Colossal, negative magnetoresistance of ∼82%, 85% and 76% is observed at 50 K in 50 kOe field in R0.15Ca0.85MnO3 (R = Y, Gd and Dy) manganites. Magnetocaloric effect (MCE) is estimated in terms of isothermal magnetic entropy change (ΔSm) and inverse MCE (+ΔSm) is observed in the antiferromagnetically ordered state. The ΔSm changes sign for field changes greater than the metamagnetic critical magnetic field (Hc). Sign changing MCE signifies the evolution of underlying mixed magnetic interactions in these systems with variations in magnetic field and temperature.

Nanogranular, Melt-Spun Intermetallic Compound SmNi: Magnetocaloric Effect and Large Coercivity

Rare earth intermetallic compound SmNi has been synthesized by melt spinning under argon atmosphere. This sample crystallizes in orthorhombic structure (CrB-type, Space group Cmcm, No. 63) and exhibits texture and nanograin formation. The average crystallite size calculated from powder X-ray diffraction data is ~17 nm. Melt-spun SmNi orders ferromagnetically at ~47 K ( $T_{C}$ ) and this is close to $T_{C}$ of the arc-melted sample. The saturation magnetization value at 10 K is only ~0.24 $\mu _{B}$ /f.u., whereas a large coercivity of ~24 kOe is observed. Isothermal magnetic entropy change, $\Delta S_{m}$ , shows a maximum at $T_{C}$ , of ~−1 J/kg $\cdot$ K, for 50 kOe field change with a relative cooling power value of ~10 J/kg. Small moment of Sm, strong magnetocrystalline anisotropy, complex magnetic structure, and nanograin size all influence the magnetism of melt-spun SmNi.