Normal Magnetocaloric Effects Research Articles

Magnetocaloric effect in the first order antiferromagnetic phase transition compound CeMg

We report the magnetic and magnetocaloric properties of the antiferromagnetic intermetallic compound CeMg, focusing on its coupled magnetic and structural phase transition within first-order processes. Our study utilizes a model Hamiltonian incorporating interactions such as: crystalline electrical field, intra and inter sublattice exchange interactions, as well as quadrupolar and magnetoelastic to elucidate the behavior of CeMg. Through simulation, we predict and discuss both normal and inverse magnetocaloric effects, providing insights into the magnetocaloric behavior under different magnetic field directions. These findings contribute to a deeper understanding of the complex magnetism in CeMg and lay the groundwork for future experimental investigations.

Investigation of magnetocaloric effect in Holmium substituted Dysprosium iron garnet for magnetic refrigeration applications

The Dy1-xHox3Fe5O12(Ho-DyIG) with x=0.0 to 1.0 garnet samples are prepared by solid-state reaction route by sintering them at 1200 °C for 24 hours followed by calcination. The samples have simple cubic structure with Ia3¯d space group. The lattice constant is obtained from the Rietveld refinement technique and is found to be decreased from 12.4181Å to 12.3858Å with Ho doping. All the samples exhibit ferrimagnetic behavior below their transition temperature of 555K(x=0.0) to 545Kx=1.0. This drop in transition temperature is attributed to the dilution of Fea-O-Fed networks. All the samples show magnetic compensation because of the strong anisotropy induced at lower temperatures. In addition, we have also observed spin reorientation in all the samples of Ho-DyIG series. We have performed magnetocaloric measurements for x=0.4 and 0.8 samples and observed that the magnetic entropy is improved from 3.92 to 4.30J/kgK when measured at a maximum applied field of 5T around their respective Belov tranistion temperature i.e. TB. We have seen both normal and inverse magnetocaloric effects in these samples. To test the extent of magnetic refrigeration capacity, we have calculated the Relative Cooling Power, which comes out to be 364.6J/kg for x=0.4 and 313.0J/kg for x=0.8 samples at 5T magnetic field. The Arrott plots are also investigated and they have positive slope suggesting the presence of second-order phase transition in our samples.

Coexistence of normal and inverse magnetocaloric effect in inhomogeneous Ni95Cr5 layers

Understanding of the coexistence of normal and inverse magnetocaloric effects (MCE) makes it promising for use in magnetic cooling applications. This study discusses the selection of Ni95Cr5 layers and their expected magnetocaloric effects for an extended temperature range (200–800 K). These layers offer a wide range of suitable magnetic ordering temperatures as a function of thicknesses. A modified phenomenological model was used to estimate the entropy changes () and relative cooling power (RCP). Under adiabatic magnetic field variation, normal and inverse MCE are computed near the ferromagnetic-paramagnetic transition temperatures, T C and spin−glass transition temperatures, T G , respectively. Based on their values (= −0.06 to 0.11 J kg−1-K−1) and RCP( = 7.6 to 15.5 J kg−1) at the smallest magnetic field of 0.5 kOe, which are comparable to other MCE layer materials. We provide evidence of a strong correlation between the change in magnetic anisotropy (measured in terms of MR/MS and HC) below T G , which subsequently leads to the reversal of However, after annealing these effects somewhat vanish. The current study demonstrates that adjusting thickness, in addition to Cr doping, can externally regulate soft magnetic and magnetocaloric properties to create diverse magnetic ground states for future MCE and inverse MCE functionalities.

Residual entropy and magnetocaloric effect in a diluted sawtooth spin model of hole-doped CuO chains.

Ground-state and magnetocaloric properties of a site-diluted sawtooth magnetic chain in the presence of an external magnetic field are exactly investigated by using the transfer-matrix method. The model captures the main magnetic interactions along CuO chains present in some hole-doped cuprates. The ground-state diagram is exhibited and analytical expressions for the residual entropy within each ground state and along the transition lines are derived. We explicitly discuss the role of the underlying pairing correlations and the entropy maximization principle. The isothermal entropy change is determined as a function of interaction parameters, doping concentration, and magnetic-field amplitude. Normal and inverse magnetocaloric effects are reported. Adiabatic demagnetization curves are discussed in connection with configurational and spin contributions to the residual entropy.

Large inverse and normal magnetocaloric effects in HoBi compound with nonhysteretic first-order phase transition

HoBi single crystal and polycrystalline compounds with NaCl-type structure are successfully obtained, and their magnetic and magnetocaloric properties are studied in detail. With temperature increasing, HoBi compound undergoes two magnetic transitions at 3.7 K and 6 K, respectively. The transition temperature at 6 K is recognized as an antiferromagnetic-to-paramagnetic (AFM–PM) transition, which belongs to the first-order magnetic phase transition (FOMT). It is interesting that the HoBi compound with FOMT exhibits good thermal and magnetic reversibility. Furthermore, a large inverse and normal magnetocaloric effect (MCE) is found in HoBi single crystal in the H||[100] direction, and the positive ΔS M peak reaches 13.1 J/kg⋅K under a low field change of 2 T and the negative ΔS M peak arrives at –18 J/kg⋅K under a field change of 5 T. These excellent properties are expected to be applied to some magnetic refrigerators with special designs and functions.

Structure and magnetic properties of Ni–Mn–Ga shape memory alloys

In this paper, we present structure, structural phase transformation, magnetic phase transition, and magnetocaloric effect of Ni50Mn50-x Ga x (x = 17, 18, 19, 20, and 21) shape memory alloys. X-ray diffraction patterns display nano-crystalline phases in the alloys. The samples are soft magnetic material with very narrow magnetic hysteresis. The martensitic-austenite transformation temperature increases from 219 K (for x = 17) to 322 K (for x = 21) with increasing Ga-concentration. Ga also increases the Curie phase transition temperature and the saturation magnetisation of the alloy. The external magnetic field also clearly affects the structural phase transition of the alloy. The magnitude of the martensitic-austenitic phase transition decreases with the increase of the applied magnetic field. Both the normal and invert magnetocaloric effects coexist in the alloy. Under the magnetic field change of 10 kOe, the maximum magnetic entropy change, ∣ΔS m ∣max, for the Ni50Mn30Ga20 ribbon is 0.95 J.kg−1.K−1 for the normal magnetocaloric effect.

Successive inverse and normal magnetocaloric effects in the Mn-vacancy compound Mn0.95Co0.75Cu0.25Ge

Ferromagnetic-structural transformation has been studied widely in MnCoGe-based materials. However, the magnetostructural transition (MST) from antiferromagnetic (AFM) orthorhombic phase to ferromagnetic (FM) hexagonal phase, which may lead to a large inverse magnetocaloric effect (MCE), has rarely been reported. Here, the introduction of Mn vacancy lowers the structural transition temperature while retains the AFM state in the orthorhombic phase, thus successfully realizing the AFM-FM MST in Mn0.95Co0.75Cu0.25Ge. Moreover, successive inverse and normal MCEs are observed around the first-order AFM-FM MST and the second-order FM-paramagnetic (PM) transition, respectively. A thermostat is proposed based on this special feature, which could release heat above the critical temperature while absorb heat below the critical temperature by simply applying the same magnetization/demagnetization cycles. This thermostat can be very useful in many applications where a constant temperature is required, such as cryostats and incubators.

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.

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.

Magnetic transitions in delafossite CuFeO2: A magnetocaloric effect study

CuFeO2 was synthetized by a solid-state reaction and its low temperature magnetic properties were investigated using the magnetocaloric effect. Magnetic susceptibility measurements show that there are two magnetic transition temperatures at about 16 and 11 K. Measurement of isothermal magnetization curves for different applied magnetic fields near these temperatures show a reversal in the magnetization trend around 16 K, and Arrott plots indicate they are accompanied by second- and first-order magnetic phase transitions, respectively. Both normal and inverse magnetocaloric effects are observed, and the maximum magnetic entropy change is obtained at 11 K.

Impact of synthesis routes on normal and inverse magnetocaloric effects and critical behaviour in the charge-ordered Pr0.5Sr0.5MnO3 manganite

We investigated the effect of both synthesis routes on the physical properties of charge-ordered Pr0.5Sr0.5MnO3 (PSMO) manganites. The samples are prepared by the solid-state reaction (SSR) and the modified sol-gel (SG) method. X-ray diffraction (XRD), SEM and magnetic measurement are used to study the structural, morphological, magnetic, magnetocaloric effect and the critical behaviour of our manganites. XRD studies confirmed the single phase orthorhombic formation of PSMO. Both compounds undergo two successive magnetic phase transitions with the variation of temperature: a paramagnetic (PM)-to-ferromagnetic (FM) transition around $ T_{\rm C}=265$ and 243K followed by an FM-to-antiferromagnetic charge-ordered transition at $ T_{\rm CO}=85$ and 159K for the SG and SSR methods, respectively. Moreover, Banerjee's criteria, Landau analysis and universal curves of phase transitions are also studied to access the magnetic ordering of the PM-FM transition in the samples and confirmed the second-order character. Critical exponents associated with the ferromagnetic phase transition are analyzed and found to be inconsistent with any known universality class. Important magnetic entropy changes and the relative cooling power (RCP) were observed in the sample synthesized through the solid state method as compared to the sol-gel-synthesized sample. The PSMO compounds show both negative ( $ \Delta S_{\rm M}=-1.63813$ J/kgK for SSR and -1.1 for SG) as well as positive ( $ \Delta S_{\rm M}=+5.82$ J/kgK for SSR and +1.52 for SG) magnetocaloric effects under a 2T field at ferromagnetic and charge order transitions, respectively. This feature of successive inverse and normal MCEs in Pr0.5Sr0.5MnO3 are suggested to be applied in some magnetic refrigerators with special designs and functions.

Magnetocaloric Effect Caused by Paramagnetic Austenite–Ferromagnetic Martensite Phase Transformation

In the present work, the magnetization of Ni50Mn17.5Ga25Cu7.5 alloy undergoing the first-order phase transition from paramagnetic austenite to ferromagnetic martensite was measured to evaluate the magnetic-field-induced entropy change (MFIEC) and refrigerant capacity (RC) of the alloy. A standard method (SM) of evaluation of MFIEC is based on thermodynamic Maxwell relation. In view of the criticism of SM expressed by some scientists, the alternative method (AM), which is based on thermodynamic relationships for free energy, was proposed recently for the determination of MFIEC. We developed this method and computed MFIEC in two ways—by AM and SM. The values of MFIEC obtained for Ni50Mn17.5Ga25Cu7.5 alloy by these methods appeared to be large but very different from each other. Moreover, AM reveals the possibility of both normal and inverse magnetocaloric effects in the adjoining temperature ranges, while SM results only in the normal magnetocaloric effect.

Effects of external magnetic field and hydrostatic pressure on magnetic and structural phase transitions in Pr0.6Sr0.4MnO3

We investigate the effect of hydrostatic pressure on the temperature dependence of magnetization and also the influence of magnetic field on the linear thermal expansion in polycrystalline Pr0.6Sr0.4MnO3, which is ferromagnetic at room temperature (TC = 305 K) but its magnetization undergoes an abrupt decrease at TS = 89 K within the ferromagnetic state. Normal and inverse magnetocaloric effects around TC and TS, respectively, were reported earlier in this single phase compound [Repaka et al., J. Appl. Phys. 112, 123915 (2012)]. The thermal expansion shows an abrupt decrease at TS in zero magnetic field but transforms into an abrupt increase at the same temperature under 7 T, which we interpret as a consequence of magnetic field-induced structural transition from the low-temperature monoclinic (I2/a symmetry) to high-temperature orthorhombic (Pnma symmetry) phase in corroboration with a published neutron diffraction study in zero magnetic field. While the external magnetic field does not change TS, the application of a hydrostatic pressure of P = 1.16 GPa shifts the magnetic anomaly at TS towards high temperature. The pressure induced shift of the low-temperature magneto-structural anomaly (ΔTS = 27 K) is nine-times higher than that of the ferromagnetic Curie temperature (ΔTC = 3 K). Our results suggest that while the hydrostatic pressure stabilizes the low temperature monoclinic phase at the expense of the orthorhombic phase, the applied magnetic field does not affect the structural transition temperature.

Large reversible normal and inverse magneto-caloric effects in the RE2BaCuO5 (RE = Dy and Er) compounds

The magneto-caloric effect (MCE) in the polycrystalline rare-earth cuprates Dy2BaCuO5 and Er2BaCuO5 has been researched. An inverse MCE under low magnetic field changes (ΔH) and at low temperatures together with a large normal reversible MCE under high ΔH was observed for the both compounds. These behaviours are corresponding to the fact of AFM (antiferromagnetic) ground state and the field-induced metamagnetic transition (first order) from AFM to FM (ferromagnetic) state in Dy2BaCuO5 and Er2BaCuO5 compounds. For the ΔH of 0–7 T, the values of −ΔSMmax (maximum magnetic entropy change), RCP (relative cooling power) and RC (refrigerant capacity) are found to be 8.3 J/kg K, 204 J/kg and 153 J/kg for Dy2BaCuO5, and to be 9.6 J/kg K, 230 J/kg and 176 J/kg for Er2BaCuO5, respectively.

Magnetocaloric effects and transport properties of rare-earth (R = La, Pr, Sm) doped Ni50-xRxMn35Sn15 Heusler alloys

The structural, magnetic, magnetocaloric, and transport properties of Ni50Mn35Sn15 and Ni50-xRxMn35Sn15 (x = 1 and R = La, Pr, Sm) compounds has been studied through X-ray diffraction (XRD), differential scanning calorimetry, and magnetization measurements. Analysis of the XRD data reveals that the compounds crystallize in the cubic L21 austenite phase or mixture of L21 and B2 at room temperature. We have observed that the ground state magnetization (T = 5 K) and the martensitic transition temperature (TM) depended on the rare-earth ion. A drastic shift in TM by ∼62 K relative to the parent compound (TM = 190 K) was observed for the Ni49LaMn35Sn15 compound. The compounds exhibit both inverse and normal magnetocaloric effects. Magnetic entropy change of 5.2 J/kg K (Sm), 12 J/Kg K (Pr), and 6 J/kg K (La) were found at TM for ΔH = 5T. Relative cooling power values of 267, 336, and 230 J/Kg were found at the Curie temperatures of the compounds with Sm, Pr, and La substitution, respectively. Significant magnetoresistance values were observed in these compounds, with the largest being −30% for the La substituted compounds at TM and ΔH = 5T.

Normal and inverse magnetocaloric effects in structurally disordered Laves phase Y1-xGdxCo2 (0 ≤ x ≤ 1) compounds

Magnetic and magnetocaloric properties of Y1-xGdxCo2 compounds, where x = 0.2, 0.4, 0.6, 0.8 and 1.0, were investigated experimentally and theoretically. Crystal structures were characterized by X-ray diffraction (Rietveld analysis) and investigated samples possess the MgCu2-type single phase with Fd-3m space group. Melt-spinning process introduced a chemical and topological disorder, which directly affected the magnetic properties. Refrigerant capacity (RC), strictly connected to the full width at half maximum δTFWHM of the ΔSM(T) curve and the maximum of magnetic entropy changes ΔSMpk(T,ΔH), increases from 29 to 148 J/kg with replacement of Y by Gd atoms from x = 0.2 to x = 0.8. RC and δTFWHM indicate the presence of disorder. Temperature dependences of magnetic entropy change ΔSM(T,ΔH) and RC were measured in as-quenched and annealed state for Y0.4Gd0.6Co2. This particular composition was chosen for detailed investigation mainly due to its Curie point (TC = 282 K), which is close to the room temperature. After isothermal annealing (τa = 60 min, Ta = 700°C) RC decreased from 122 to 104 J/kg, which clearly indicates the homogenization of the heat treated sample. Furthermore, observed inverse magnetocaloric effect is associated with the presence of antiferromagnetically coupled Gd and Co magnetic moments. The phase transition temperature increases with increasing Gd content from 74 to 407 K for Y0.8Gd0.2Co2 and GdCo2, respectively. Within the FPLO-LDA DFT method the non-magnetic ground state for YCo2 and the magnetic ground state for GdCo2 are predicted in agreement with experiment. The dependence of calculated total and species-resolved magnetic moments on Gd concentration reasonably agrees with available experimental data.

Phase transformations, inverse magnetocaloric effect and critical behavior of Ni50Mn36Sn14−xSix Heusler alloys

The magnetic, magnetocaloric and critical behavior properties are systematically investigated for the Heusler alloy system Ni50Mn36Sn14−xSix (x=1, 2 and 3). For all the samples both the normal and inverse magnetocaloric effects (MCE) are observed. The magnetic entropy change and relative cooling power are evaluated near the structural phase transformation and magnetic transition regions under an applied magnetic field of 50 kOe. The maximum magnetic entropy change associated with the martensitic transition are 3.4 J kg−1 K−1, 2.5 J kg−1 K−1 and 1.3 J kg−1 K−1 for x=1, 2 and 3 respectively. The entropy change for x=1, 2 and 3 alloys near the Curie temperature of the austenite phase is found to be −2.8 J kg−1 K−1, −2.7 J kg−1 K−1 and −2.3 J kg−1 K−1 respectively. Relative cooling power is calculated in the vicinity of the Curie temperature of the austenite phase, as well as in the martensitic transition temperature regime. The critical parameters (β, γ and δ) of the samples were determined from the static magnetic data at the second-order ferromagnetic-paramagnetic transition region using the field dependence of magnetic entropy change.

Sign reversal of magnetization and exchange bias in Ni(Cr1−xAlx)2O4 (x=0–0.50)

Ni(Cr1-xAlx)2O4 (x=0–0.50) samples were prepared in single phase form by using sol-gel method and their structural and magnetic properties were studied. Al substitution transforms the crystal structure of NiCr2O4 from tetragonal cell with space group I41/amd to cubic cell of Fd3¯m space group. Magnetization measurements by varying the temperature and magnetic field were carried out to investigate the interesting magnetization reversal and exchange bias behaviors. Magnetization reversal is observed for x=0.10 sample with a magnetic compensation temperature of 40K and it is explained by considering different temperature dependences of magnetic moments of the two sublattices. Shifting of magnetic hysteresis loops towards the negative magnetic field axis and hence the presence of negative exchange bias field is observed for x=0.15 sample. The x=0.10 sample exhibits the tunable positive and negative exchange bias field. Exchange bias in these samples is explained considering the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic spins. However, the sign reversal of exchange bias field is due to the change in domination of one ferrimagnetic sublattice over the other with variation in temperature. Both normal and inverse magnetocaloric effects are observed for x=0.10 sample.

Evolution of magnetic properties and exchange interactions in Ru doped YbCrO3

Magnetic properties of YbCr1−xRuxO3 as a function of temperature and magnetic field have been investigated to explore the intriguing magnetic phenomena in rare-earth orthochromites. A quantitative analysis of x-ray photoelectron spectroscopy confirms the mixed valence state (Yb3+ and Yb2+) of Yb ions for the highest doped sample. Field-cooled magnetization reveals a broad peak around 75 K and then becomes zero at about 20–24 K, due to the antiparallel coupling between Cr3+ and Yb3+ moments. An increase of the Ru4+ ion concentration leads to a slight increase of compensation temperature Tcomp from 20 to 24 K, but the Néel temperature remains constant. A larger value of the magnetic moment of Yb ions gives rise to negative magnetization at low temperature. An external magnetic field significantly modifies the temperature dependent magnetization. Simulation of temperature dependent magnetization data, below TN, based on the three (two) magnetic sub-lattice model predicts stronger intra-sublattice exchange interaction than that of inter-sublattice. Thermal hysteresis and Arrot plots suggest first order magnetic phase transition. Random substitution of Ru4+ ion reduces the magnetic relaxation time. Weak ferromagnetic component in canted antiferromagnetic system and negative internal magnetic field cause zero-field-cooled exchange bias effect. Large magnetocrystalline anisotropy associated with Ru creates high coercivity in the Ru doped sample. A maximum value of magnetocaloric effect is found around the antiferromagnetic ordering of Yb3+ ions. Antiferromagnetic transition at about 120 K and temperature induced magnetization reversal lead to normal and inverse magnetocaloric effects in the same material.

Inverse and normal magnetocaloric effects in LaFe12B6

Intrinsic magnetic properties and magnetocaloric effect were studied for LaFe12B6 itinerant-electron system, which presents an antiferromagnetic ground state below 36 K. For certain magnetic fields values, LaFe12B6 exhibits a sequence of two successive magnetic transitions: an antiferromagnetic-ferromagnetic (AFM-FM) transition at low temperature followed by a ferromagnetic-paramagnetic transition, leading to normal and inverse magnetocaloric effects, respectively. At finite temperatures, both antiferromagnetic (AFM) and paramagnetic states can be transformed into a ferromagnetic (FM) state via a field-induced metamagnetic transition accompanied with a huge magnetic hysteresis. Moreover, we reveal that, at low temperatures, the magnetization displays abrupt jumps across the first-order AFM-FM transition, giving rise to an unusual and unique staircase-like behavior. LaFe12B6 exhibits both normal magnetocaloric effect around the Curie temperature and large inverse magnetocaloric effect around the AFM-FM transition temperature; for μ0ΔH = 7 T, ΔSM is found to be −6.8 and 19 J kg−1 K−1 around 38 and 8 K, respectively.