Inverse Magnetocaloric Effect Research Articles

Spin reorientation, normal and inverse magnetocaloric effects in heavy rare-earth iron garnets

Experiments for heavy rare-earth iron garnets Ho3Fe5O12 and Er3Fe5O12 show a compensation effect characterized by a near zero magnetization at 134 K and 80 K, respectively. The magnetic entropy change calculated according to the Maxwell's relation is shown to be positive and negative, indicating both normal and inverse magnetocaloric effects exist in the sample. The critical temperature (134 K for Ho3Fe5O12, 80 K for Er3Fe5O12) of the two types of magnetocaloric effect occurs at almost the same temperature as the compensation point. However, the compensation effect is attributed to the multiple exchange interactions among the octahedral sites Fe3+, the tetrahedral sites Fe3+ and the dodecahedral sites R3+, while the reversal of the magnetocaloric effect is originated from the spin reorientation. The maximum magnetic entropy change of the normal magnetocaloric effect is 4.72 J kg−1 K−1 for Ho3Fe5O12 at 34 K and 4.94 J kg−1 K−1 for Er3Fe5O12 at 24 K, respectively. Moreover, all the positive slopes of basic Arrott plots suggest only the second-order phase transition existing in Ho3Fe5O12 and Er3Fe5O12.

Monte Carlo study of magnetocaloric properties and hysteresis behavior of the double perovskite Sr2CrIrO6

The hysteresis behavior and the magnetocaloric properties of the double perovskite (DP) Sr2CrIrO6 near the first and second-order phase transitions have been studied using the Monte Carlo simulations based on the heath bath algorithm. We have investigated the magnetic behavior of the system as a function of the temperature. The amplitude of the total susceptibility and the specific heat peaks increase by increasing the values of the magnetic field h. The magnetic entropy change ΔSm, the adiabatic temperature change ΔTad are obtained for different external magnetic fields and temperatures. It is found that the system exhibits an inverse magnetocaloric effect when we have a ferrimagnetic interaction between the two sublattices. The relative cooling power (RCP) increases with increasing the external magnetic field and decreases with the temperature. The hysteresis loops of the system are examined as a function of the Hamiltonian parameters the result obtained show that the system may display various hysteresis behaviors such triple, double, single loop and step-like loops.

Magnetic and magnetocaloric properties of Ni47Mn40Sn13−xZnx alloys: Direct measurements and first-principles calculations

In the present study, the martensitic transformation (MT) and magnetic properties exhibited by the Ni-Mn-Sn Heusle-type magnetic shape memory alloys (MSMAs) doped with Zn have been investigated experimentally and theoretically. The inverse magnetocaloric effect (MCE) in ${\text{Ni}}_{47}{\text{Mn}}_{40}{\text{Sn}}_{13\ensuremath{-}x}{\text{Zn}}_{x}$ (x = 0, 1) was studied by direct measurements of the adiabatic temperature change, $\mathrm{\ensuremath{\Delta}}{T}_{\text{ad}}$, in pulsed magnetic fields of 5, 10, and 20 T. The Zn doping of the Ni-Mn-Sn alloy led to a striking enhancement of the value of $\mathrm{\ensuremath{\Delta}}{T}_{\text{ad}}$, e.g., from --2.5 for undoped to --11 K for Zn-doped alloys under a magnetic field amplitude of 20 T. The first-principles calculations were used to understand the origin of Zn-doping influence on MT, magnetic, and magnetocaloric properties. Particularly, the crystal structure and magnetic ordering influenced by the site occupancy in the undoped and Zn-doped alloys were analyzed. The results show that, whereas the usual transition metal elements with more valence electrons tend to enter the Ni sites, Zn atom prefers to occupy the Sn sublattice. The underlying physics of the drastic enhancement of MCE by Zn doping is discussed in terms of a partial disorder in the occupation sites of Zn atoms.

Unraveling magnetic interactions and the spin state in insulating Sr2−xLaxCoNbO6

We investigate the structural, magnetic, and spin-state transitions, and magnetocaloric properties of ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{CoNbO}}_{6}$ ($x=0$--1) double perovskites. The structural transition from tetragonal to monoclinic phase at $x\ensuremath{\geqslant}0.6$, and an evolution of (101)/(103) superlattice reflections and Raman active modes indicate the enhancement in the B-site ordering with $x$. The magnetic susceptibility data reveal the transition from weak ferromagnetic (FM) to antiferromagnetic (AFM) ordering for $x\ensuremath{\geqslant}0.6$ with ${T}_{\mathrm{N}}\ensuremath{\approx}9$--15 K. Interestingly, the La substitution drives towards a more insulating state due to an increase in high-spin ${\mathrm{Co}}^{2+}$, whereas a spin-state crossover is observed in ${\mathrm{Co}}^{3+}$ from high-spin to intermediate-/low-spin states with $x$. We discuss the correlation between complex magnetic interactions and the presence of various Co spin states in the system. Moreover, the emergence of metamagnetic nature due to the competition between FM and AFM interactions as well as crossover from conventional to inverse magnetocaloric effect have been demonstrated by a detailed analysis of temperature and field dependent change in magnetic entropy.

Studies on magnetocaloric effect of Tb2Ni0.90Si2.94 compound

A comparative study has been carried out on the magnetocaloric properties of as-cast and annealed Tb2Ni0.90Si2.94 intermetallic compound. While the as-cast material exhibits ferromagnetic cluster-glass behaviour below 9.9 K coexisting with antiferromagnetic (AFM) interaction, the annealed system shows AFM ordering below 13.5 K and spin freezing occurs below 4 K. The compound exhibits moderate magnetocaloric performance with maximum isothermal entropy changes (−ΔS M) 8.8 and 10.9 J kg−1 K−1, relative cooling power (RCP) 306 and 365 J kg−1, along with adiabatic temperature change (ΔT ad) 5.5 and 8.15 K for 70 kOe magnetic field change in as-cast and annealed forms, respectively. The estimated magnetic entropy change is found to be larger for annealed sample in comparison to that of as-cast analogue. However, the full width at half maxima (FWHM) of −ΔS M(T) behaviour is larger in as-cast compound due to the presence of inherent structural disorder which reduces with thermal annealing. A positive isothermal entropy change (−ΔS M) and adiabatic temperature change (ΔT ad) is observed for the as-cast compound in the measured field and temperature region. In contrast, the annealed system exhibits inverse magnetocaloric effect in the low field and temperature region where AFM interactions dominate. Magnetocaloric effect (MCE) is used as a tool to establish a subtle correlation between the observed magnetocaloric effect and the reported magnetic properties of the system.

Magnetocaloric effect in Ni–Fe–Mn–Sn microwires with nano-sized γ precipitates

Ni45.6Fe3.6Mn38.4Sn12.4 microwires, with nanoscale γ-phase precipitates that enhance the magnetocaloric effects (MCEs) and mechanical properties, were prepared by a melt-extraction technique and subsequent high temperature annealing. The atomic ordering degree significantly increased after annealing, leading to a considerable increment in the structural entropy change (ΔStr) from 4.5 J/kg·K in the as-extracted microwire to 26.6 J/kg·K in the annealed one, and the magnetization difference (ΔM) from 35 A·m2/kg to 51 A·m2/kg under a magnetic field of 5.0 T. Consequently, a positive magnetic entropy change (ΔSM) peak of 15.2 J/kg·K with working temperature span (ΔTFWHM) of 12 K for the first-order martensite transformation followed by a negative ΔSM peak of −4.3 J/kg·K with ΔTFWHM = 50 K for the second-order magnetic transition under μ0ΔH = 5.0 T was achieved. By employing both magnetizing and demagnetizing processes for magnetic cooling, the two successive inverse and conventional MCEs in Ni–Fe–Mn–Sn microwires may show potential applications for micro-devices and systems.

Abnormal field-dependence of magnetocaloric effect in ErB4 and TmB4

The coupled system of multi-degrees of freedom, such as charge, spin, orbital and lattice, has recently received much attention due to its potential to improve the magnetocaloric effect (MCE). The exotic inverse MCE was observed in rare-earth tetraborides of Ho1-xDyxB4 (x = 0.0, 0.5, and 1.0), associated with a strong coupling between magnetic dipoles and orbital quadrupoles in the strong spin-orbit coupling and geometric frustration. Here, the magnetism and magnetocaloric effects of ErB4 and TmB4 are investigated. It shows the maximum entropy changes of 11.4 J/kgK, and 12.6 J/kgK with the field of ΔH ≈ 40 kOe (H∥c) in ErB4 and TmB4, respectively. The field- and temperature-dependence of the entropy change is found to be quite different from those of the conventional MCE. And the entropy change is also found to have strong correlation with the field induced meta-magnetic transition. Because the field induced transition is due to magnetic moment reorientation, which is strongly coupled with quadrupole moment, the abnormal MCE of ErB4 and TmB4 is attributed to the dipole-quadrupole interaction and magnetic frustration. Thus, it supports the fact that the strong coupling between quadrupole and magnetic dipole moments plays important role in the exotic inverse MCE in rare-earth tetraboride system.

Large Rotating Magnetocaloric Effects in Polycrystalline Ni-Mn-Ga Alloys

Polycrystalline Ni50.4Mn27.3Ga22.3 alloy with texture was prepared by conventional casting. The alloy exhibited enhanced magnetocrystalline anisotropy after superelastic training. Decent conventional magnetocaloric effect (MCE) with magnetic-field-induced entropy change ΔS = 9.2 J/kg·K under a magnetic field 30 kOe and inverse MCE with ΔS = 2.5 J/kg·K under 5 kOe were demonstrated when the field was applied parallel and perpendicular to , respectively. As a result, large rotating MCE with ΔS = 4.2 J/kg·K under a low field of 20 kOe was achieved at room temperature. This shows that polycrystalline Ni-Mn-Ga alloys may act as promising magnetic refrigerants.

Quadrupolar ordering and exotic magnetocaloric effect in RB4 (R\u2009=\u2009Dy, Ho)

The interplay of charge, spin, orbital and lattice degrees of freedom has recently received great interest due to its potential to improve the magnetocaloric effect (MCE) for the purpose of magnetic cooling applications. Here, a new mechanism for a large entropy change with low magnetic fields in rare-earth tetraborides, especially for Ho1-xDyxB4 (x = 0.0, 0.5, and 1.0), is proposed. For x = 0.0, 0.5, and 1.0, the maximum entropy changes of the giant inverse MCE are found to be 22.7 J/kgK, 19.6 J/kgK, and 19.0 J/kgK with critical fields of 25 kOe, 40 kOe, and 50 kOe, respectively. For all compounds, systematic study on how the entropy changes as a function of the field and temperature is performed to investigate their correlation with consecutive double transitions, i.e., the magnetic dipolar order at T = TN and the quadrupolar order at T = TQ (TQ < TN). Based on Landau theory, it is found that this behaviour is attributed to the strong coupling between magnetic dipoles and quadrupoles in the presence of strong spin-orbit coupling and geometric frustration. Our work offers new insights into both academic and industrial interests in the discovery of giant MCE with various applications for magnetic cooling systems.

Metamagnetic transition, magnetocaloric effect and electronic structure of the rare-earth anti-perovskite SnOEu3

Rare-earth anti-perovskites with oxygen are an interesting magnetic materials family at the boundary between intermetallics and oxides, they however remain largely unexplored. Here, magnetic and heat capacity investigations, as well as density functional theory (DFT) calculations, were carried out on SnOEu3. At low magnetic field (B ≤ 0.5 T), a Néel temperature separates antiferromagnetic and paramagnetic phases at 31 K. When applying higher magnetic field below the Néel temperature, successive transformations toward a ferromagnetic state via a number of intermediate canted magnetic structures are observed and are associated with only modest latent heat and transition entropy. High-pressure magnetic measurements confirm the stable divalent state of Eu up to 1.05 GPa. A direct magnetocaloric effect progressively increases with applied magnetic field above the Néel temperature, reaching −16 J kg–1 K−1 for ΔB = 7 T. On the other hand, the inverse magnetocaloric effect of the field-induced transition below TN saturates at ~+5 J kg−1 K−1. DFT calculations support magnetic instabilities observed experimentally in SnOEu3 and reveal an unusual exchange mechanism and band topology near the Fermi level.

Magnetocaloric effect of the Gd3-xTbxCo system

Gd3-xTbxCo compounds with x = 0, 0.25, 0.5, 1, 1.5, 2, 2.5 and 3 were investigated using structural and magnetic methods. The lattice parameters, and the location of 4c and 8d Wyckoff sites occupied by Gd, Tb, and Co atoms, in an Fe3C–orthorhombic type of structure, were refined by the Rietveld method. The magnetic ordering temperature changes linearly with Tb content. The magnetocaloric effect (MCE) in Tb3Co at 77 K and a magnetic field of 7 T is equal to −28.8 J/kgK. An inverse MCE observed in GdTb2Co reaches its maximum value of 6.8 J/kgK at 47 K and a magnetic field of 6 T.

Effect of direct-current magnetic field on the specific absorption rate of metamagnetic CoMnSi: A potential approach to switchable hyperthermia therapy

Materials with 1st order antiferromagnetic (AFM) to high-magnetization (MM) phase transition known for their inverse magnetocaloric effect, abrupt rise in magnetization and magnetoelastic coupling, are promising for application in combined simultaneous diagnosis and targeted cancer therapy. A therapy that combines alternating-current (ac) and direct-current (dc) magnetic fields for simultaneous magnetic hyperthermia therapy (MHT) and magnetic resonance imaging (MRI), using same magnetic particles for heating and as contrast agents. We report a proof-of-concept study on the induction heating ability of 1st order metamagnetic material with moderate specific absorption rates (SAR) and no tendency for agglomeration, for potential MHT and MRI cancer therapy. CoMnSi, a metamagnetic antiferromagnet (MM) was used in this study because of its desirable ability to rapidly switch from a low to high magnetization state in an applied dc bias field condition without particle agglomeration on field removal. The results showed that the magnetization switched from &amp;lt; 20 Am2kg-1 at 0.75 T to about 53.31 Am2kg-1 at 1.0 T applied dc field, a field large enough for magnetic resonance imaging. An SAR value of 10.7 Wg-1 was obtained under an ac field of 31.0 kAm-1 at 212.0 kHz. When combined with a dc bias field of 1.0 T, SAR values of 9.83 Wg-1 and 6.65 Wg-1 were obtained in the directions 45° and 90° away from the dc bias field direction respectively. These SAR values obtained from CoMnSi particles in the presence of simultaneous ac and dc magnetic field bias are in comparison, at least 25 times greater than those obtained from 2nd order magnetic phase transition Fe3O4 suspension. It is observed that Fe3O4 particles showed large suppression of SAR, and agglomeration under the same experimental conditions. This study shows the great potential of 1st order phase transition metamagnets for simultaneous MHT and MRI cancer therapy using MRI equipment.

Магнитокалорический эффект и эффект памяти формы в сплаве Гейслера Mn-=SUB=-2-=/SUB=-NiGa

The work presents the investigations of the functional properties of the Heusler alloy Mn2NiGa, made by argon arc melting. The direct experimental methods were used to study: electrical resistance, magnetization, magnetocaloric effect, and shape memory effect in the wide temperature range of 100–400 K. The inverse magnetocaloric effect was found in both the martensitic and austenitic phases, which did not experience saturation in pulsed magnetic field up to 50 T. The values of reversible deformation in the alloy up to 0.35% were obtained with bending mechanical stress up to 247 MPa.

Negative magnetization, complex magnetic ordering and applications of Cr-doped Co2TiO4.

Polycrystalline Co2Ti1-xCrxO4 (0 ≤ x ≤ 0.2) inverse spinel ceramics have been synthesized via a sol-gel technique. The dc magnetization measurement in the field-cooled mode shows that negative magnetization could be observed until x reaches 0.2. The exchange constants are calculated using the ferrimagnetic Curie-Weiss fitting and the mean-field theory. This reveals that the strength of the inter sublattice magnetic interaction presents a non-monotonic trend with the increase in Cr content and reaches the minimum at x = 0.1, giving rise to the highest compensation temperature in the x = 0.1 sample. The applicability of the x = 0.1 sample is investigated in light of two prominent magnetic effects: (i) the stable magnetic switching effect indicates the potential applications in magnetic switching and data storage and (ii) the coexistence of normal and inverse magnetocaloric effects suggests a potential application in a constant temperature bath at 54 K.

Magnetocaloric Effect in Alloy Fe49Rh51 in Pulsed Magnetic Fields up to 50 T

Magnetocaloric effect (MCE) in pulsed magnetic fields up to 50 T was directly studied in alloy Fe49Rh51. An inverse MCE ΔT ≈ –8 K is observed at different initial temperatures around the metamagnetic phase transition upon field rising to 20 T; further growth of the field to 50 T leads to a decrease in the absolute adiabatic temperature change by nearly 1 K, which is due to the direct MCE and proves that the whole sample undergoes a transition into the ferromagnetic phase. Upon the field decrease, the maximal absolute value of the adiabatic temperature change of |ΔT| = 9.8 K was revealed at 6 T when the initial temperature is 310 K.

Magnetocaloric Effect in Anisotropic Mixed Spin –1 System: Pair Approximation Method

We use the Pair Approximation method to analyze the magnetic and magnetocaloric behaviors of diluted mixed spin SA = 1 and spin SB = 1/2 with the anisotropic Heisenberg model, on a cubic lattice with coordination number z=6. Our system is described in presence of an external magnetic field; the phase diagram and thermodynamic properties related to the concentration of magnetic atom (A or B) and the single ion anisotropy are constructed and discussed. Special attention is paid to magnetocaloric properties provided by isothermal entropy change as well as the cooling capacity. These cooling power keys are plotted and discussed as a function of interaction anisotropy and magnetic component concentration of two sublattices ions A and B. Numerical results show a double peak structure in the entropy change curve and the inverse magnetocaloric effect related to the presence of the negative single-ion anisotropy.

Ground-state ferrimagnetism and magneto-caloric effects in Nd2NiMnO6

Extending our earlier investigation of magnetic properties of Nd2NiMnO6, we show that it exhibits a magnetic transition below ∼6 K to a ferrimagnetic state. This behavior is interpreted as arising from a long-range ordering of Nd moments antiferromagnetically coupled to the ferromagnetic Ni–Mn ordered moments. Due to the richness of its multiple magnetic transitions and the easily influenced magnetic state by the application of an external magnetic field, established in our earlier study, it has a remarkable inverse magneto-caloric effect (IMCE) at low temperatures (T < 50 K) together with a significant conventional magneto-caloric effect (CMCE) at the ferromagnetic ordering temperature (Tc ∼ 200 K). IMCE and CMCE correspond to the antiferromagnetic arrangement of Nd and Ni–Mn sublattices and ferromagnetic ordering of Ni–Mn sublattices, respectively. Nd2NiMnO6 with its second order phase transition follows the universal behavior of ∆SM(T); it also shows a power law dependency on the magnetic field as

Magnetocaloric effect near room temperature in quintenary and sextenary Heusler alloys

An inverse magnetocaloric effect is studied in Ni2Mn1+xX1−x-type Heusler alloys. Principally known for their shape-memory properties, these alloys also exhibit significant entropy and temperature changes (ΔS and ΔTAd, respectively) under adiabatic conditions when a modest magnetic field is applied. We investigated the impact on magnetocaloric properties of introducing substantial chemical disorder on the X-site (X=Si,Ga,In), of replacing Ni with nonmagnetic Ag, and of replacing a small amount of Mn with Gd. While a reduction in ΔS is observed in the first two cases, we observe a significant enhancement of ΔS when a small amount of Gd is substituted for Mn. A thermodynamic analysis was conducted using magnetization and heat capacity data to estimate adiabatic temperature changes in the range of ΔTAd≃ −1 to −3 K for a 5 T magnetic field. Several alloys characterized in this study exhibit these respectable ΔTAd values near room temperature, making them potentially viable candidates for magnetic refrigeration applications.

Experimental evidence of magnetic training effect of electronic phase in Nd–Ca half-doped manganite

Cooling and heating modes are shown to be able to significantly modify the magnetic and electrical properties of the half-doped perovskite manganite. The present paper reports on a precise investigation of this phenomenon (the so-called training effect) carried out on Nd0.5Ca0.5MnO3 manganite, which allow a fine tuning of the magnetic ground state. Refinement of the X-ray diffraction pattern shows that the synthesized sample is single phase and crystallizes in the orthorhombic structure with Pnma space group. Using magnetometry measurements, we have found that the sample is the seat of interesting phenomena like charge ordering, magnetic phase separation, spin-glass and inverse magnetocaloric effect. Magnetic-field-driven magnetization (M(μ0H)) measurements evidence the metamagnetic transition which not only depends on field value, but also on the thermal process (cooling or heating). Metamagnetic irreversibility in the magnetic field range (±5 T) and memory effect are observed at low temperatures due to the kinetic arrest phenomenon. It is worthwhile to mention that the coercive field increases with decreasing temperature and reaches 955 Oe at 20 K, which is sufficiently large compared with that one in the soft magnetic and makes the material quite interesting for spintronic applications. The electrical resistivity in a zero field was measured on both cooling and warming modes, and the data obtained were fitted by using different theoretical models. At low temperatures, the resistivity shows the presence of insulator-metal transition and is found to be in conformity with the magnetization data. A magnetic training effect due to the thermo-magnetic history dependent behavior is observed, where the resistivity is consistently irreversible.

Preservation of large low temperature magnetocaloric effect in metamagnetic intermetallic compounds RCu2 (R = Gd, Tb, Dy, Ho and Er) upon rapid solidification

In this work, magnetic and magnetocaloric properties of melt-spun RCu2 (R = Gd, Tb, Dy, Ho and Er) compounds have been studied. The melt-spun samples have formed in the same orthorhombic crystal structure (space group Imma, no. 74) as that of the arc-melted samples but with a development of crystallographic texture and micron-size grains. The melt-spun ribbons of RCu2 (R = Gd, Tb, Dy, Ho and Er) order antiferromagnetically at 43 K, 51 K, 28 K, 10 K and 11 K (TN) respectively. The ordering temperatures are almost the same as that of the arc-melted samples. The RCu2 compounds show one or more field induced transitions below TN. Therefore, a normal magnetocaloric effect is observed in these compounds when the magnetic field change is larger than the metamagnetic critical field and inverse magnetocaloric effect is observed for smaller field changes. The microgranularity does not seem to smear out the metamagnetic transition. The maximum of isothermal magnetic entropy change, ΔSmmax, is about −2.1 Jkg−1K−1, -5.5 Jkg−1K−1, -5.2 Jkg−1K−1, -17.2 Jkg−1K−1 and -11.7 Jkg−1K−1 respectively for the melt-spun RCu2 (R = Gd, Tb, Dy, Ho and Er) samples for 50 kOe field change. The refrigeration capacity is found to be 40 Jkg-1, 160 Jkg-1, 166 Jkg-1, 230 Jkg-1 and 207 Jkg-1 respectively, for the above samples. These values are nearly the same as that of the arc-melted counterparts. Thus the large magnetocaloric effect in the temperature range of 10 K–70 K make these melt-spun RCu2 samples potential materials for low temperature refrigeration applications. The results also ascertain melt-spinning process as an alternate technique for the production of magnetocaloric alloys and intermetallics.