Inverse Magnetocaloric Effect Research Articles

Analysis of the magnetic field dependence of the isothermal entropy change of inverse magnetocaloric materials

In this work, the magnetic field dependence of the inverse magnetocaloric (MC) effect is analyzed using a mean field approach for describing antiferromagnetic to ferromagnetic magnetoelastic transitions. The model is able to describe both second- and first-order transition through the introduction of a magnetovolume energy term. The power law dependence for the field dependence of the isothermal magnetic entropy change (ΔSiso∝ΔHn), has an exponent n with an overshoot above 2 for first-order transitions, while it is not present for the second-order case. This is in excellent agreement with previous phenomenological observations, supporting the validity of recently proposed criterion to distinguish between first- and second-order thermomagnetic transitions. A main difference with respect to direct MC effect is that negative values of the exponent n are obtained at temperatures close to the transition. This is ascribed to the reduction of the inverse MC response due to the influence of the unavoidable ferromagnetic to paramagnetic transition at higher temperatures. The obtained features are qualitatively compared to those of GdBaCo2O6 (antiferromagnetic to ferromagnetic magnetoelastic transition), showing a good agreement between both experiments and the model. The obtained information is extrapolated to understand the behavior of the exponent n for a Ni49Mn36In15 sample (magnetostructural transition).

The antiferromagnetic to ferrimagnetic phase transition in Mn2Sb1-xBix compounds

The influence of partial substitution of Bi for Sb on the structure, magnetic properties and magnetocaloric effect of Mn2Sb1-xBix (x = 0, 0.02, 0.04, 0.05, 0.07, 0.09, 0.15, 0.20) compounds has been investigated. The transition temperature of the antiferro-to-ferrimagnetic (AFM-FIM) transition initially increases with increasing Bi and decreases above 9%. Density functional theory calculations indicate that the Bi atoms prefer to occupy only the Sb site, which accounts for the large magnetization jump in Mn2Sb0.93Bi0.07. As large lattice parameters are found for Bi substituted Mn2Sb, the origin of the AFM-FIM transition in Mn2Sb(1-x)Bix compounds is ascribed to an enhanced coefficient of thermal expansion along the c axis, resulting from the Bi substitution. The moderate entropy change of 1.17 J/kg K under 2 T originating from the inverse magnetocaloric effect and the strong magnetic field dependence of the transition temperature of dTt/dµ0H = −5.4 K/T in Mn2Sb0.95Bi0.05 indicate that this alloy is a promising candidate material for magnetocaloric applications.

Magnetothermal properties and magnetocaloric effect in [formula omitted] compound

Magneto-thermal properties and magnetocaloric effect of ErFe2 have been studied. Molecular field theory has been applied on ErFe2 to calculate the magnetization and the magnetic contribution to the heat capacity and entropy. Our calculation shows that ErFe2 is a ferrimagnetic compound with a compensation temperature of around 475 K and a Curie temperature of 596 K. Applying magnetic field to ErFe2 induces canting in moments of the Er sublattice. Density functional theory, as implemented in Wien2k code, was used to evaluate the density of states at Fermi energy to calculate the electronic contribution of heat capacity and entropy. We used available values of the shear and bulk moduli to evaluate the Debye temperature in order to calculate the lattice contribution to the heat capacity and entropy. We calculated the magnetocaloric quantities, namely isothermal magnetic entropy change ΔSmiso and adiabatic temperature change ΔTad, for magnetic fields up to 6 T and temperatures up to 800 K. For an applied field change of 6 T, ΔSmiso was found to be ≈0.23 J/mole K, while the maximum inverse magnetocaloric effect was found to be ≈0.13 J/mole K. The maximum adiabatic change |ΔTad|maxis ≈ 0.5 K per Tesla.

In situ TEM-исследование фазовых превращений в нестехиометрическом сплаве Гейслера Ni-=SUB=-46-=/SUB=-Mn-=SUB=-41-=/SUB=-In-=SUB=-13-=/SUB=-

This paper presents a study of the metamagnetostructural transition of the martensitic type in the Ni46Mn41In13 alloy with magnetic shape memory and inverse magnetocaloric effect. The characteristic temperatures of the direct transition starting Ms = 253 K and its finishing Mf = 164 K, as well as the reverse temperatures As = 203 K and Af = 236 K, respectively, were determined by differential scanning calorimetry. The characteristic peculiarities of the transition: a decrease of the Ms and the presence of a residual austenite phase, as well as pre-martensitic states, were studied using transmission electron microscopy. The estimated the thickness of alloy was 50 nm, when martensitic transition suppressed.

Магнитоструктурные особенности фазовых переходов в системе Mn-=SUB=-1-x-=/SUB=-Co-=SUB=-x-=/SUB=-NiGe Часть 2. Анализ

Within the framework of the model of interacting parameters of the magnetic and structural orders, taking into account the internal periodic magnetic field orthogonal to the exchange field, we analyzed the features of magnetostructural transitions in the Mn_{1-x}Co_{x}NiGe system. A qualitative description of changes in the nature of magnetic phase transitions from magnetostructural transitions of the 1st order paramagnetism-antiferromagnetism (x = 0.05-0.1) to isostructural transitions of the 2nd order paramagnetism-ferromagnetism (x = 0.15-0.8) with a change in the concentration of Co is presented. An explanation is given for the onset of irreversible magnetic-field-induced transitions at temperatures on the order of 5 K in strong magnetic fields, accompanied by a change in the saturation magnetization for samples x = 0.15-0.8. The low-temperature inverse magnetocaloric effect at liquid helium temperatures is predicted for these samples.

Influence of the martensitic transformation kinetics on the magnetocaloric effect in Ni-Mn-In

The inverse magnetocaloric effect (MCE) in Ni-Mn-based Heusler compounds occurs during the magnetostructural transition between low-temperature, low-magnetization martensite and high-temperature, high-magnetization austenite. In this study, we analyze the metamagnetic transformation of a $Ni_{49.8}Mn_{35}In_{15.2}$ compound by simultaneous adiabatic temperature change and strain measurements in pulsed magnetic fields up to 10 T. We observe an adiabatic temperature change of -10 K and a strain of -0.22 % when the reverse martensitic transition is fully induced at a starting temperature of 285 K. By a variation of the magnetic field-sweep rates between 316 Ts$^{-1}$, 865 Ts$^{-1}$ and 1850 Ts$^{-1}$, the transitional dynamics of the reverse martensitic transformation have been investigated. Our experiments reveal an apparent delay upon the end of the reverse martensitic transformation at field rates exceeding 865 Ts$^{-1}$ which is related to the annihilation of retained martensite. As a consequence, the field hysteresis increases and higher fields are required to saturate the transition. In contrast, no time-dependent effects on the onset of the reverse martensitic transformation were observed in the studied field-sweep range. Our results demonstrate that kinetic effects in Heusler compounds strongly affect the magnetic cooling cycle, especially when utilising a multicaloric "exploiting-hysteresis cycle" where high magnetic field-sweep rates are employed.

Oxygen vacancy modulating inverse and conventional magnetocaloric effects coexisting in double perovskite Bi2NiMnO6-δ films

The oxygen vacancies (VO) modulating inverse and conventional magnetocaloric effects in double perovskite Bi2NiMnO6-δ polycrystalline films are claimed. The vacuum annealing processor showed large regulated artificial VO concentration from 5.1% to 17.6%, which has been verified by X-photoelectron spectrum results. The role of VO concentration in magnetocaloric effect is clarified by the charge transfer mechanism. The band length and angle (O-Ni-O, O-Mn-O) are sensitive to the valence confusion of elements (Mn and Ni), which is disturbed by VO concentration. This scenario contributes to the spin orientation and antiferromagnetic/ferromagnetic (AFM/FM) competition near the phase transition point by coulomb exchange interaction. The temperature dependent transport behavior, meanwhile, presents metallic-FM state in Bi2NiMnO6-δ (high VO concentration). While stoichiometric Bi2NiMnO6 (low VO concentration) films show dielectric traits. The structure, magnetization and optical absorption properties were characterized systematically. The present work explores a route to design magnetocaloric materials via controllable artificial VO defects.

Magnetocaloric Properties of Ni-Rich Ni50−xCoxMn38Sn12B3 Shape Memory Ribbons

The present study presents the magnetocaloric effect (MCE) properties of Ni-rich Ni50−xCoxMn38Sn12B3 (x = 0, 1, 3, 5) ribbons during both heating and cooling processes. The Co substitution caused an increase of magnetization difference ΔM and hence an improvement of the MCE. An inverse giant magnetocaloric effect (IMCE) and a high effective refrigerant capacity RCeff were observed in the x = 3 ribbons. A high amount of Co (x = 5) content led to an inverse magnetic entropy ΔSM peak with a wide temperature range. Magnetostructural coupling over a wide temperature range is of great importance for technological purposes. On the other hand, the MCE properties were thermal hysteresis dependent, which has to be considered in the technological applications.

The resistivity upturn in Ni–Mn–In alloys exhibiting magnetic field induced shift in martensitic transition with and without Co doping

The temperature dependent resistivity behavior of Ni2Mn1+xIn1−x (0.32 ≤x≤ 0.48), Ni1.81Co0.22Mn1.46In0.51 and Ni1.81Co0.22Mn1.45In0.52 is studied with and without applied magnetic field. These alloys exhibit inverse magnetocaloric effect and giant magnetoresistance that are important for applications like magnetic refrigeration and magnetic sensors. The properties arise due to shift in martensitic transition to lower temperature with applied magnetic field. The doping of Co at Ni site and Mn at In site affects the disorder in the system. The disorder is an important factor that decides the requirement of magnetic field for applicability. In present manuscript, the effect of disorder is studied through resistivity behaviour. The resistivity upturn is observed at low temperature. The upturn is fitted by least square method. The fitting reveals nature of disorder, origin of low temperature resistivity behavior with magnetic field, and consequently reveals the cause of difference in magnetic field induced large shift in martensitic transition to lower temperature with composition. The results reveal that long-range antiferromagnetic coupling and balance of Co and Mn doping amount is desired in new material design for technical applications.

Low-Temperature Magnetocaloric Properties of V12 Polyoxovanadate Molecular Magnet: A Theoretical Study.

The paper presents a computational study of the magnetocaloric properties of the V12 polyoxovanadate molecular magnet. The description is restricted to low-temperature range (below approximately 100 K), where the magnetic properties of the system in question can be sufficiently modelled by considering a tetramer that consists of four vanadium ions with spins . The discussion is focused on the magnetocaloric effect in the cryogenic range. The exact and numerical diagonalization of the corresponding Hamiltonian is used in order to construct the thermodynamic description within a version of the canonical ensemble. The thermodynamic quantities of interest, such as magnetic entropy, specific heat, entropy change under isothermal magnetization/demagnetization, temperature change under adiabatic magnetization/demagnetization, refrigerant capacity, and magnetic Grüneisen ratio, are calculated and discussed extensively. The importance of two quantum level crossings for the described properties is emphasized. The significant ranges of direct and inverse magnetocaloric effect are predicted. In particular, the maximized inverse magnetocaloric response is found for cryogenic temperatures.

Antiferromagnetism, spin-glass state, H–T phase diagram, and inverse magnetocaloric effect in Co2RuO4

Static and dynamic magnetic properties of normal spinel Co2RuO4 = (Co2+) are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature range T = 2 K–400 K and H up to 90 kOe. These investigations show that Co2RuO4 exhibits an antiferromagnetic (AFM) transition at TN ∼ 15.2 K, along with a spin-glass state at slightly lower temperature (TSG) near 14.2 K. It is argued that TN is mainly governed by the ordering of the spins of Co2+ ions occupying the A-site, whereas the exchange interaction between the Co2+ ions on the A-site and randomly distributed Ru3+ on the B-site triggers the spin-glass phase, Co3+ ions on the B-site being in the low-spin non-magnetic state. Analysis of measurements of M (H, T) for T < TN are used to construct the H–T phase diagram showing that TSG shifts to lower T varying as H2/3.2 expected for spin-glass state whereas TN is nearly H-independent. For T > TN, analysis of the paramagnetic susceptibility (χ) vs. T data are fit to the modified Curie–Weiss law, χ = χ0 + C/(T + θ), with χ0 = 0.0015 emu mol−1Oe−1 yielding θ = 53 K and C = 2.16 emu-K mol−1Oe−1, the later yielding an effective magnetic moment μeff = 4.16 μB comparable to the expected value of μeff = 4.24 μB per Co2RuO4. Using TN, θ and high temperature series for χ, dominant exchange constant J1/kB ∼ 6 K between the Co2+ on the A-sites is estimated. Analysis of the ac magnetic susceptibilities near TSG yields the dynamical critical exponent zν = 5.2 and microscopic spin relaxation time τ0 ∼ 1.16 × 10−10 sec characteristic of cluster spin-glasses and the observed time-dependence of M(t) is supportive of the spin-glass state. Large M–H loop asymmetry at low temperatures with giant exchange bias effect (HEB ∼ 1.8 kOe) and coercivity (HC ∼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed below TN is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) for T > TN to inverse MCE for T < TN observed in Co2RuO4 (and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs. T data.

Structure and magnetism of a new hexagonal polymorph of Ba3Tb(BO3)3 with a quasi-2D triangular lattice

This article reports the structural and magnetic characterisation of a new hexagonal (P63cm) low-temperature phase of Ba3Tb(BO3)3, isostructural with the heavier lanthanide borates with formula Ba3Ln(BO3)3 (Ln ​= ​Dy–Lu). The crystal structure contains a quasi-two-dimensional (2D) triangular lattice of Tb3+ ions and is predicted to display unusual magnetic behaviour as a result of the low dimensionality. Magnetic susceptibility χ(T) shows antiferromagnetic interactions (θCW=−7.15 K) and no sharp ordering transition at T≥2 K, but analysis of χ’(T), isothermal magnetisation and heat capacity suggests the possibility of short-range ordering at T≈10 K. The material exhibits an inverse magnetocaloric effect at T<4 K.

Onset of inverse magnetocaloric effect in multiferroic FeVO4 below the antiferromagnetic transition temperature TN1

We investigated the evolution of magnetic entropy in a triclinic FeVO4 multiferroic using temperature and magnetic field dependent magnetic measurements at magnetic transitions. The observed antiferromagnetic transitions are at TN1 = 22 K and TN2 = 15 K, together with the paramagnetic phase above TN1. A magnetic field induced maximum magnetic entropy change (ΔSM = 0.37 J kg−1K−1 at H = 50 kOe) at multiferroic transition, 15 K is observed in conjunction with a crossover from the negative to positive in ΔSM(T) data at TN1 towards TN2. The crossover of magnetic entropy substantiates the onset of inverse magnetocaloric effect below the antiferromagnetic transition temperature at TN1 together with maximum ΔSM at TN2.

Inverse magnetocaloric effect in the solids undergoing ferromagnetic – antiferromagnetic phase transition: Landau theory applied to Fe-Rh alloys

In the present article a magnetic solid exhibiting the phase transition from the high-temperature ferromagnetic (FM) to the low-temperature antiferromagnetic (AFM) phase is considered. The Landau theory of phase transitions is used for the analysis of inverse magnetocaloric effect (MCE) observed below the phase transition temperature. The magnetic-field-induced shift of the phase transition temperature has been calculated and the dependence of the phase transition field on the temperature of solid has been computed; the quantitative agreement between the theoretical results and the experimental data obtained in the recent experiments with Fe49Rh51 alloy has been achieved. The magnetic-field-induced entropy change and refrigerant capacity of Fe49Rh51 alloy have been calculated. Reasonable agreement between theoretical results and experimental data has been demonstrated. The dependence of characteristics of MCE on the spin-exchange parameter has been shown. The MCE in the artificial AFM structures has been discussed.

Theory of giant magnetocaloric effect in the shape memory alloy undergoing magnetostuctural phase transition

The theory of the giant conventional and inverse magnetocaloric effects (MCEs) is advanced on the basis of the theory of magnetism developed by A. I. Akhiezer, V. G. Bar’yakhtar, and S. V. Peletminsky in the classic monograph “Spin Waves”. The MCE in the shape memory alloy undergoing the first-order phase transition from the cubic ferromagnetic (FM) phase to the tetragonal antiferromagnetic (AFM) phase is considered. The agreement between the theoretical results and existing experimental data is demonstrated. It is argued that the alloys undergoing the phase transition to AFM phases with weak exchange interaction between the magnetic sublattices are promising for use in the magnetic refrigeration technique. The possibility of observation of the giant MCE in the layered magnetic structure with weak spin-exchange coupling between the FM layers is discussed.

Large refrigeration capacity in a Ni48Co1Mn37In14 polycrystalline alloy with low thermal hysteresis

For Ni–Mn–In based alloys, simultaneously achieving large magnetic field-induced transformation temperature shift and low thermal hysteresis is of vital significance to obtain significant magnetocaloric properties. In this work, we employ low content Co doping to tune the magnetostructural transformation parameters and demonstrate large inverse magnetocaloric effect in a Ni48Co1Mn37In14 alloy. It is shown that 1% Co doping confers a strong magnetic field dependence of transformation temperature, i.e., 7.2 KT−1, giving rise to a wide refrigeration temperature window of 34 K under the magnetic field change of 5 T. Together with relatively low thermal hysteresis, i.e., ~8 K, giant effective refrigeration capacity up to 284 Jkg−1 is achieved under the magnetic field change of 5T. The present work is expected to offer some instructions for the design of Ni–Mn-based alloys with excellent magnetocaloric properties.

Magnetocaloric properties and unconventional critical behavior in (Gd,Tb)6(Fe,Mn)Bi2 intermetallics

The magnetic and magnetocaloric properties of the intermetallic family (Gd,Tb)6(Fe,Mn)Bi2 have been studied from 2 K to temperatures above the respective Curie temperatures TC. The substitution of Gd by Tb (Gd6FeBi2, Gd3Tb3FeBi2, Tb6FeBi2) tunes TC in the range 350-250 K and favors the apparition of a metamagnetic transition at very low temperature (below 10 K) from a complex magnetic state to a ferromagnetic one, as well as a spin reorientation transition below Tm = 72 K. As a consequence, an important inverse magnetocaloric effect (IMCE) appears below 20 K and an interesting direct magnetocaloric effect (DMCE) appears over a wide temperature span between TC and Tm with maxima at those temperatures. The partial substitution of Fe by Mn in Tb6Fe0.5Mn0.5Bi2 shifts these effects upwards in temperature while expanding the region of the direct magnetocaloric effect between 70 and 400 K. The combination of adjoint IMCE and DMCE as well as the wide span of the latter shows that tuning this family allows to locate the magnetocaloric effect in different regions of interest. The critical behavior of the PM-FM transitions has been studied obtaining the critical exponents α, β, γ, δ and checking that the respective magnetocaloric effects also scale with the critical parameters n and δ. The transition in Gd6FeBi2 belongs to the Heisenberg universality class with deviations due to magnetocrystalline anisotropies; the critical exponents for Gd3Tb3FeBi2 (in agreement with the Mean Field model) suggest the presence of long range order magnetic interactions, while Tb6FeBi2 and Tb6Fe0.5Mn0.5Bi2 present an unconventional critical behavior aligned with long range order interactions.

Simultaneous direct measurements of conventional and inverse magnetocaloric effects in Ni–Mn-based Heusler alloys using lock-in thermography technique

The second-order conventional and first-order inverse magnetocaloric effects (MCEs) in Ni–Mn-based quaternary Heusler alloys have been systematically investigated by means of the lock-in thermography technique, which enables the direct measurement of the MCE-induced temperature change in a periodic magnetic field. Through systematic measurements of the temperature dependence of the MCE signals, the tuning of the conventional and inverse MCEs with temperature for the same Heusler alloys has been demonstrated, where the phase transitions responsible for the MCEs are clearly distinguished. The lock-in thermography measurements show that some Ni–Mn-based Heusler alloys exhibit much smaller temperature changes due to the inverse MCEs in the periodic field as compared to the conventional MCEs, even though they exhibit a larger magnetic entropy change across the first-order transition responsible for the inverse MCEs. We discuss the origin of this behavior in terms of the field-induced entropy change and thermal hysteresis of the alloys. These findings will be useful not only in accelerating the optimization of inverse MCE materials but also in understanding the mechanism of the MCEs.

Single-Crystal Growth and Room-Temperature Magnetocaloric Effect of X-Type Hexaferrite Sr2Co2Fe28O46.

X-type hexaferrites have been receiving considerable attention due to their promising applications in many magnetic-electronic fields. However, the growth of single-crystal X-type hexaferrite is still a challenge. Herein we reported, for the first time, the preparation of single crystal X-type hexaferrite Sr2Co2Fe28O46 (Sr2Co2X) with high-quality and large size using floating-zone method with laser as the heating source. The crystals show rhombohedral symmetry with space group of R-3m (No. 166, a = 5.8935(1) Å and c = 83.7438(17) Å). Co2+ and Fe3+ oxidation states were confirmed by the X-ray absorption near-edge spectroscopy. The prepared Sr2Co2X exhibits a spin reorientation transition from easy-cone to easy-axis at T2 of 343 K and a ferrimagnetism-paramagnetism transition at Curie temperature (TC) of ∼743 K. The spin reorientation transition was accompanied by magnetocaloric effect (MCE). Both conventional and inverse MCEs were observed near T2 with a magnetic field applied along the c-axis. The maximum value of the magnetic entropy change along the c-axis was evaluated to be 1.1 J/kg·K for a magnetic field change of 5 T.

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.