Isothermal Magnetic Entropy Change Research Articles

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.

Magnetic and magnetocaloric properties of DyCo2Cx alloys

The magnetic and magnetocaloric properties of DyCo2Cx (x = 0, 0.05, 0.1, and 0.15) alloys were investigated. The results show that the Curie temperature (TC) of the DyCo2Cx alloys increases with increasing C content, from 136 K (x = 0) to 152 K (x = 0.15), but the lattice parameter a of DyCo2Cx exhibits a maximum at x = 0.05. The suppression of the ac susceptibility of DyCo2Cx at low temperature indicates the enhancement of the domain wall pinning effect by carbon doping. The positive slops of the Arrott plots of the doped compounds indicate that the phase transition is second order for the carbon-doped alloys, and the maximum value of the isothermal magnetic entropy change (ΔSM) for the magnetic field change of 50 kOe decreases from −13.9 J/kg· K (x = 0) to −7.8 J/kg·K (x = 0.15). The relative cooling power (RCP) of DyCo2Cx is nearly the same in all studied alloys, while the temperature-averaged entropy change over 10 K temperature span, TEC(10), indicates decreasing magnetocaloric performance of carbon doped materials.

Numerical studies on the influence of sample inhomogeneity on the isothermal magnetic entropy change evaluated from magnetization data

Influence of sample inhomogeneity in the form of Curie temperature distribution on the isothermal magnetic entropy change and its field dependence is studied by numerical methods, using Landau theory based equation of state with gadolinium as the model system. Results show that the nature/shape of distribution influences both the isothermal magnetic entropy change (\(\Delta S_\mathrm{M}\)) and its field dependence near the critical temperature (\(T\approx T_\mathrm{C}\)) regime. From the present study, it is found that the nature/shape of \(T_\mathrm{C}\) distribution about its mean value significantly influences the values of both maximum isothermal magnetic entropy change (\(|\Delta S_\mathrm{M}|^{\max }\)) and the corresponding temperature (\(T^\mathrm{peak}\)).

Prediction of magnetocaloric effect and spontaneous magnetization in Cu-doped MnCoGe system

In the present work, the transition, magnetocaloric properties, and spontaneous magnetization in MnCo1−xCuxGe compounds were systematically studied. The maximum magnetic entropy change values of 22.4 J kg−1 K and 10.9 J kg−1 K estimated from Landau free-energy scheme under 5 T have been observed at 271 K and 260 K for x = 0.02 and x = 0.04, and the corresponding relative cooling power values are 211.1 and 231.4 J kg−1, respectively. The nature of the first-order transition for the optimized compositions has been verified using magnetic measurements, Landau coefficients, and phenomenological model. In addition, spontaneous magnetization values extracted from isothermal magnetic entropy change plots are in good agreement with those determined from Arrott plots construction, revealing the validity of this approach to estimate the spontaneous magnetization in the first-order ferromagnetic intermetallic system.

A study on the magnetocaloric effect in Ti doped manganites Gd0.7Sr0.3Mn1−xTixO3 (x = 0, 0.1 and 0.15)

Abstract This work focuses on Magnetic and magnetocaloric properties of doped Gd based manganites; Gd0.7Sr0.3Mn1−xTixO3 (x = 0, 0.1, 0.15) denoted by G, G0.1, G0.15 respectively. Isothermal magnetic entropy change (|ΔS(T)|) shows appreciably high values for the three compounds. For 10 T field, the peak values obtained for magnetic entropy change is 10.4 JKg−1 K−1, 9.6 JKg−1 K−1 and 22.2 JKg−1 K−1respectively for samples G, G0.1, G0.15 at 12.5 K. Interestingly, −ΔS(T) shows a universal behaviour near the transition region at the low temperature. Relative cooling power (RCP) is comparatively high for Gd0.7Sr0.3Mn0.85Ti0.15O3 at low fields, but for high fields Gd0.7Sr0.3MnO3 has comparatively high values for RCP.

Giant magnetocaloric effect in an exchange-frustrated GdCrTiO5 antiferromagnet

We report the effect of exchange frustration on the magnetocaloric properties of GdCrTiO$_5$ compound. Due to the highly exchange-frustrated nature of magnetic interaction, in GdCrTiO$_5$, the long-range antiferromagnetic ordering occurs at much lower temperature $T_N$=0.9 K and the magnetic cooling power enhances dramatically relative to that observed in several geometrically frustrated systems. Below 5 K, isothermal magnetic entropy change (-$\Delta S_{\rm m}$) is found to be 36 J kg$^{-1}$ K$^{-1}$, for a field change ($\Delta H$) of 7 T. Further, -$\Delta S_{\rm m}$ does not decrease from its maximum value with decreasing in $T$ down to very low temperatures and is reversible in nature. The adiabatic temperature change, $\Delta T_{\rm ad}$, is 15 K for $\Delta H$=7 T. These magnetocaloric parameters are significantly larger than that reported for several potential magnetic refrigerants, even for small and moderate field changes. The present study not only suggests that GdCrTiO$_5$ could be considered as a potential magnetic refrigerant at cryogenic temperatures but also promotes further studies on the role of exchange frustration on magnetocaloric effect. In contrast, only the role of geometrical frustration on magnetocaloric effect has been previously reported theoretically and experimentally investigated on very few systems.

Interplay of austenite and martensite phase inside martensite transition regime and its role on magnetocaloric effect and magnetoresistance in Ni-Mn-Sn based Heusler alloy

The first order martensite transition has been observed clearly through magnetic, electronic- and magneto-transport studies in Ni45Mn44Sn11 disordered ferromagnetic shape memory Heusler alloy. The alloy also shows second order ferromagnetic → paramagnetic phase transition, near the Curie temperature of austenite phase. From calorimetric measurement, the lattice entropy due to martensite transformation is found to be 38 Jkg−1K−1, both for heating and cooling cycle. Technologically important noticeably large positive magnetic entropy change has also been clearly noticed in this alloy near martensite phase transition. Maximum isothermal magnetic entropy changes at 275 K for the application of 5 T magnetic field are found to be 15 Jkg−1K−1 and 24 Jkg−1K−1, for field increasing and field decreasing protocol, respectively. A maximum temperature change −5.1 K is observed at 276 K under 5 T magnetic field in adiabatic condition. The noticeable maximum magnetoresistance of −38% for the field change of 5 T is observed at 266 K. The austenite and martensite phase fractions in the sample depend on the magnetic and thermal path history, which are the most possible reasons to observe different values of magnetic entropy change and magnetoresistance in Ni45Mn44Sn11 Heusler alloy at 275 K.

Magnetocaloric effect in single-crystal HoNi with a canted magnetic structure

We have studied the magnetocaloric effect (MCE) of single-crystal HoNi, which shows a ferromagnetic transition at TC = 35 K and a spin-reorientation transition at TSR = 13 K. Below TC, HoNi has ferromagnetic and antiferromagnetic components along the a- and c-axes, respectively. We find that the MCE in HoNi is highly anisotropic owing to the canted magnetic structure of this material. When a magnetic field is applied along the a- (B ∥ a) and b-axes (B ∥ b), the maximum isothermal magnetic-entropy change reaches 22.8 J K−1 kg−1 at 35 K and 18.8 J K−1 kg−1 at 13 K, respectively, when the magnetic field is increased from 0 to 5 T. On the other hand, for B ∥ c, is approximately three times smaller than those for B ∥ a and B ∥ b. In addition, the sign of depends on the temperature owing to the antiferromagnetic component along the c-axis and the existence of a metamagnetic transition.

Effect of composition changes on the structural, magnetic and thermodynamic properties in Tb1-xDyxNi2 intermetallic compounds

The crystal structure, magnetic and thermodynamic properties of polycrystalline Tb1-xDyxNi2 intermetallic compounds with x = 0.25, 0.5 and 0.75 have been investigated. Experimental data on thermodynamic properties, i.e. the isothermal entropy change ΔSmag, are compared with results of theoretical calculations which were performed considering a model Hamiltonian that includes the crystalline electric field effects and exchange magnetic interaction.High-field isothermal magnetic entropy change was calculated according to magnetic measurements using thermodynamic Maxwell's relation. A large entropy change has been observed for all Dy concentrations. The maximum relative cooling power (RCP) of studied compounds reaches fairly high values, which are close to those reported for other magnetic refrigerants operating at temperatures below 40 K. The obtained results show that studied compounds can be considered as cooling agents in magnetic refrigerators performing an Ericsson cycle in the temperature range 20–40 K.

Structure, magnetic and cryogenic magneto-caloric properties in intermetallic gallium compounds RE2Co2Ga (RE = Dy, Ho, Er, and Tm)

The intermetallic gallium compounds RE2Co2Ga with different rare earths (RE = Dy, Ho, Er, and Tm) were synthesized, and their crystal structure, microstructure, magnetic, and cryogenic magneto-caloric performances were investigated. The RE2Co2Ga compounds crystallize with an ordered orthorhombic W2B2Co-type structure (space group Immm). All the RE2Co2Ga compounds undergo ferromagnetic (FM) ordering (second-order) from a paramagnetic to FM transition which induces reversible magneto-caloric effects (MCE). The Curie temperatures (TC) in RE2Co2Ga compounds are determined to be 55.0, 38.5, 25.5, and 11.6 K for RE = Dy, Ho, Er, and Tm, respectively, which fulfill the de Gennes scaling. The RE2Co2Ga compounds show relatively small MCE (i.e., −ΔSM, isothermal magnetic entropy change) around their own phase transition temperatures, and the maximal values of −ΔSM are 7.5, 14.6, 11.2, and 13.2 J/kg K in the field change (ΔH) of 7 T for RE = Dy, Ho, Er, and Tm, respectively. Accordingly, the values of the relative cooling power are 212, 335, 304, and 239 J/kg, respectively.

High magnetocaloric efficiency of a NiFe/NiCu/CoFe/MnIr multilayer in a small magnetic field

The isothermal magnetic entropy changes are studied in Ni80Fe20/Ni67Cu33/Co90Cu10/Mn80Ir20 stacks at temperatures near the Curie point of the Ni67Cu33 spacer by applying magnetic fields in a few tens of Oersted. Such low fields were sufficient for toggling magnetic moments in the soft ferromagnetic (FM) layer (Ni80Fe20). It is found out that this switching provides the magnetic entropy change, which is up to 20 times larger than that achievable in a single Ni67Cu33 film subjected to such low fields. Our finding holds promise to be utilized in the magnetocaloric devices that would be based on FM/PM/FM heterostructures and would operate with moderate magnetic fields.

Iron for nickel substitution effects on magnetic and magnetocaloric properties of melt-spun Gd75(Ni1-xFex)25 alloys

The melt quenched Gd75(Ni1-xFex)25 alloys are found to exhibit a soft ferrimagnetic behavior in the whole concentration range (0 ≤ × ≤ 1). Substitution of Fe for Ni in the Gd75(Ni1-xFex)25 alloys results in a linear growth of the magnetic ordering temperature from 118 K to 244 K; although, an average value of the magnetic moment per transition metal atom, obtained from saturation magnetization measurements, does not change significantly with increasing Fe content. The variations of the Curie temperature and magnetization in Gd75M25-type amorphous alloys (M = Fe, Ni) are discussed bearing in mind a possible noncollinearity of magnetic moments of 3d metal atoms. The maximal values of the isothermal magnetic entropy change (ΔSm) are observed to exhibit the Fe-poor Gd75(Ni1-xFex)25 alloys. The change in the concentration of 3d-metal of different types in amorphous Gd75M25 alloys with a constant gadolinium content makes it possible to vary both the magnetocaloric effect and the operating temperature range.

Specific heat in magnetic field and magnetocaloric effects of α-R2S3 (R = Tb, Dy) single crystals

The magnetocaloric effects (MCE) of α-Tb2S3 and α-Dy2S3 single crystals exhibiting successive antiferromagnetic (AFM) transitions have been investigated by analyzing specific heat measured in magnetic field. The temperature dependence of specific heat in the vicinity of the successive transitions shows obvious distinction depending on the orientations of the applied magnetic field for both α-Tb2S3 and α-Dy2S3 that having orthorhombic crystal structures. When the magnetic field is increased, the specific heat is as follows: For α-Tb2S3 in H‖b, the peak around TN2 shifts to lower temperature but the other one peak around TN1 barely moves; In H⊥b, the peak around TN2 has no shift almost within 3 T but suddenly moves to lower temperature in 4 T and the other one peak around TN1 shifts to lower temperature in specific heat versus temperature. In the case of α-Dy2S3, the two peaks around TN2 and TN1 shift to lower temperatures in H‖b but move to higher temperatures when the magnetic field is increased up to 5 T by H⊥b in spite of antiferromagnetic transitions. Therefore, the maximum value and corresponding temperature of both isothermal magnetic entropy change (ΔSm) and adiabatic temperature change (ΔTad) in the magnetic field H⊥b are extremely different in low temperature range from that in the field of H‖b. The results propone that the MCE of α-Tb2S3 and α-Dy2S3 could be controlled at low temperature by the magnitude and orientation of magnetic field. It also indicates that the refrigerating capacity and thermal absorption capacity will be controlled by changing magnitude and orientation of magnetic field on the α-Tb2S3 and α-Dy2S3 single crystals.

Magnetic ordering and coercivity of {Y, Tb}Ni4Si, NdNi3TSi (T = Mn - Cu) and Sm1-xTbxNi3FeSi solid solutions

Magnetic ordering of the polycrystalline YNi4Si-type YNi4Si, disordered YNi4Si-type TbNi4Si and NdNi3CoSi (space group Cmmm, N 65, oC12), and CaCu5-type NdNi3TSi (T = Mn, Fe, Cu) and Sm1-xTbxNi3FeSi (x = 0.25 and 0.5) solid solutions (space group P6/mmm, N 191, hP6) have been studied by magnetization measurements. Down to 2 K, YNi4Si is paramagnetic, whereas TbNi4Si, NdNi3MnSi, NdNi3FeSi, NdNi3CoSi and NdNi3CuSi show ferromagnetic ordering (TC) at 30 K, 71 K, 120 K, 20 K and 5.6 K and field sensitive transitions (Tm) at 10 K, 43 K, 63 K, 16 K and 3.8 K, respectively. Sm.75Tb.25Ni3FeSi and Sm.5Tb.5Ni3FeSi show ferromagnetic-antiferromagnetic transformations below ferromagnetic ordering at 166 K and 120 K, respectively.Magnetocaloric effect (MCE) of TbNi4Si, NdNi3{Mn, Co, Cu}Si, Sm.75Tb.25Ni3FeSi and Sm.5Tb.5Ni3FeSi compounds were estimated in terms of isothermal magnetic entropy change (ΔSm). Among these compounds, TbNi4Si and NdNi3CuSi show maximal MCE with ΔSm of − 9.95 J/kg K (RCP50 kOe ~ 180 J/kg) at 26–30 K and − 6.29 J/kg K (RCP50 kOe ~ 31 J/K) at 6 K in a field change of 50 kOe, respectively. Below Tm, TbNi4Si, NdNi3MnSi, NdNi3FeSi, NdNi3CoSi, NdNi3CuSi and Sm.5Tb.5Ni3FeSi show hard magnetic properties with coercivity of 2.2 kOe, 4.8 kOe, 4.8 kOe, 9.5 kOe, 1 kOe and 15 kOe at 2 K, respectively. Sm.75Tb.25Ni3FeSi shows a coercivity of 39 kOe at 40 K.

Magnetic and magnetocaloric properties of CuMn2O4 and Mn3O4 composite system

Polycrystalline CuMn2O4 is synthesized by solid state reaction method. Structural and magnetization studies reveal that the sample is multiphase with CuMn2O4 as primary phase and Mn3O4 as secondary phase. Magnetocaloric properties such as isothermal magnetic entropy change (−ΔSM) and refrigerant capacity (RC) are evaluated from isothermal magnetization data. Value of isothermal magnetic entropy change () in the 40–80 K temperature range is 3.5–4.6 J kg−1K−1, for a field change of ΔH = 70 kOe. Value of refrigeration capacity (RC) evaluated for the same field change (ΔH = 70 kOe) is ∼190 J/kg for the Tcold and Thot pair of 40 and 90 K respectively. Also value of remains almost constant over a broad temperature range of 60–80 K.

Electrical resistivity and thermodynamic properties of the ferromagnet Nd2Pt2In

The Nd2Pt2In compound was investigated by means of electrical resistivity ρ(T), heat capacity Cp(T), magnetic susceptibility χ(T), magnetization M(μ0H) and magnetocaloric effect (MCE) measurements. The material orders ferromagnetically at TC = 16 K with a second - order phase transition. In the ordered state, ρ(T) can be represented in terms of ferromagnetic (FM) spin - wave dispersion with an energy gap ΔR = 13(1) K in zero field. In concert, the Cp(T) data in this region can be well described by the same model getting ΔC = 8(1) K in zero field. Above TC, the ρ(T) variation is characteristic of electron - phonon interaction in the presence of s−d scattering and crystalline-electric field (CEF). The 4f - electron specific heat shows a Schottky - type anomaly around 60 K associated with CEF. On the other hand, Cp(T) data of the non-magnetic homologue La2Pt2In can be described by the Debye - Einstein model, giving a Debye and Einstein temperature values of 190.3(5) K and 69.8(7) K respectively. At high temperatures, the χ(T) data follows the Curie - Weiss relation with an effective magnetic moment μeff = 3.61(2) μB and a Weiss temperature θp = 17(1) K. The magnitude of MCE was estimated from the isothermal magnetization data to be 6.25 J/(kg.K), 5.01 J/(kg.K), 3.18 J/(kg.K) and 0.47 J/(kg.K) for a field change of 7 T, 5 T, 3 T and 1 T, respectively. The characteristic behaviour of the isothermal magnetic entropy change points to a second - order character of the FM phase transition.

Magnetic and magnetocaloric properties of single crystal (Nd0.5Pr0.5)2Fe14B

In this work the magnetic and magnetocaloric properties of a (Nd0.5Pr0.5)2Fe14B single crystal have been investigated in a wide range of temperatures and magnetic fields. Magnetic phase transition temperatures (spin-reorientation transition (SRT) at TSR = 73 K and Curie point at Tc = 570 K) were determined together with the values of saturation magnetization Ms and magnetocrystalline anisotropy constants K1 and K2. In the vicinity of a spin-reorientation magnetic phase transition, the value of the magnetocaloric effect was determined as an isothermal magnetic entropy change (ΔSM). The universal curve of ΔS′(θ) around TSR under various magnetic field changes has been constructed by using a phenomenological procedure. It is found that this approach is applicable to materials with a second-order spin-reorientation phase transition.

Thickness as a control parameter for magnetocaloric effect in Cr75−xFe25+x (x = 0, 5) thin films

The results of a detailed investigation of the magnetocaloric effect (negative isothermal magnetic entropy change, -ΔSM(T,H)) in Cr70Fe30 and Cr75Fe25 thin films are presented. The generalized magnetic scaling equation of state in nonlinear scaling variables, which makes use of the previously reported critical exponents, not only reproduces the observed -ΔSM(T) at constant magnetic fields (1kOe⩽H⩽70kOe) over a wide temperature range around the ferromagnetic-paramagnetic phase transition temperature, T=Tc, but also describes correctly the functional dependence of -ΔSM on H at T=Tc. Reasonably large relative cooling power (RCP) and magnetic refrigerant capacity (RC), primarily due to the unusually large (130 K–180 K) full-width at half-maximum (FWHM) of the -ΔSM(T)|H curve, is observed for the magnetic field change ranging between 20 kOe and 70 kOe. This work clearly bears out that the film thickness can be used as a control parameter to tune both the peak value of -ΔSM as well as the FWHM, and hence RCP and RC. Another important result is the observation of giant isothermal magnetic entropy change at T = 2 K within the reentrant regime (where long-range ferromagnetic order coexists with cluster spin glass order) when the film thickness is reduced to ≃20nm.

Large magnetocaloric effect and critical behaviour analysis in Gd2Cu2In

The ternary intermetallic compound Gd2Cu2In crystallizes in the Mo2Fe2B-type structure with the space group . The compound undergoes a ferromagnetic-paramagnetic (FM-PM) phase transition at 94.1 K and large magnetocaloric effect has been observed over a wide range of temperature. The isothermal magnetic entropy change and adiabatic temperature change were observed to be 13.8 J/kg · K and 6.5 K for 7 T applied field. We have employed a modified Arrott plot (MAP) and Kouvel-Fisher (KF) procedures to estimate the critical exponents near the FM-PM phase transition. Critical exponents and are estimated through a non-linear fitting. The β value is close to the three-dimensional (3D) Ising value, whereas γ and δ values lie closer to the mean-field values. Scaled magnetic isotherms collapse into two branches below and above TC in accordance with a single scaling equation. Specific-heat measurements show a λ-type peak near 94 K indicating long-range magnetic ordering. The specific-heat exponent α was estimated by a non-linear fitting using the function . The fitting in the temperature range yields .