Magnetic Field-induced Entropy Change Research Articles

Magnetocaloric effect at the reorientation of the magnetization in ferromagnetic multilayers with perpendicular anisotropy

We investigate the magnetocaloric effect obtained by the rotation of a magnetic field applied to an exchange-coupled multilayer system composed of two different ferromagnetic (FM) materials. We specifically consider a system in which the two FMs have perpendicular uniaxial anisotropy axes and utilize conditions which yield a reorientation of the total magnetization when compensation between the anisotropies of the two layers occurs. We calculate the consequent entropy change associated with the “artificial” reorientation. By using known parameters from MnBi and Co we predict an entropy change of Δs=0.34 J kg−1 K−1 for perfect coupling. Lastly, we study the behavior of the multilayer under a rotating magnetic field via a micromagnetic model. When the layer thicknesses are of the order of the local domain wall width, the magnetic field-induced entropy change can be obtained with magnetic fields one order of magnitude lower than in the uncoupled case.

Borderline first-order phase transition and large cryogenic magnetocaloric effect in PrNdIn

We report a large cryogenic magnetocaloric effect stemming from an unconventional borderline first-order magnetic phase transition with negligibly small thermomagnetic hysteresis in a rare-earth-based intermetallic compound PrNdIn. The sample exhibits maximum magnetic field-induced entropy change as large as −10 J/Kg K (for 20 kOe magnetic field change) near the boiling point of oxygen. Magnetocaloric properties of PrNdIn are comparable to those of other known potential magnetocaloric materials with operating temperatures ranging between 50 and 125 K. The magnetic properties of the present sample are qualitatively reminiscent of those of the binary Pr2In and Nd2In, including the emergence of a second low-temperature anomaly in the temperature dependence of magnetization.

Magnetostructural transition and magnetocaloric effect in thermally annealed Mn0.5Fe0.5NiSi0.945Al0.055 melt-spun ribbons

Were fabricated melt-spun ribbons of the quinary alloy Mn0.5Fe0.5NiSi0.945Al0.055. After an annealing time of 4 h at temperatures between 1073 K and 1223 K, samples underwent a martensitic-like magnetostructural transformation around room temperature from a paramagnetic Ni2In-type hexagonal structure to a ferromagnetic TiNiSi-type one. The phase transformation practically remained unchanged with the annealing temperature, whereas the first cooling process induced a massive cracking in the samples. In comparison to the reported in literature for bulk alloys, the melt-spun ribbons showed a lower maximum magnetic field-induced entropy change but a well-broader working temperature range. A relatively low magnetic hysteresis loss was obtained across the martensitic transformation.

Mastering a 1.2 K hysteresis for martensitic para-ferromagnetic partial transformation in Ni-Mn(Cu)-Ga magnetocaloric material via binder jet 3D printing

Magnetocaloric (MC) materials have gained traction in the research and industry communities for their prospects in solid state magnetic refrigeration. Important to the commercialization of MC materials are: (1) establishment of a fabrication method that can combine high surface area for heat transfer and geometric freedom for designing an efficient heat exchanger which has low pressure drop for the coolant and (2) advancement of low cost alloys with appropriate MC properties. In this regard, additive manufacturing may provide the geometric freedom necessary for adapting designs to solid state cooling, and the Ni-Mn(Cu)-Ga Heusler ferromagnetic shape memory alloys (FSMAs), exhibiting a martensitic para-ferromagnetic transformation at Tms=304 K, can provide a low-cost MC material, very promising for magnetic cooling. In this study, a Ni49.5Mn19.1Cu6.6Ga24.8 (at.%) alloy is additively manufactured using powder bed binder jet 3D printing with subsequent sintering. This printed and sintered material enabled a large change of magnetization during partial transformation cycles with the smallest temperature hysteresis recorded for FSMAs, equal to about 1.2 K, regardless of the value of magnetic field applied. Under 2 T and at 304 K it exhibits an adiabatic temperature change (ΔTad) of 2 K and a stable cycling behavior of ΔTad = │1.7│ K for 100 cycles. The maximum of magnetic field-induced entropy change |ΔSm, 2T| ≈ 12.0 J/kg·K was estimated at 304 K. These results demonstrate the viability of powder bed binder jet 3D printing as an effective fabrication method for functional magnetocalorics, as well as the outstanding MC characteristics of a low-cost Ni-Mn(Cu)-Ga Heusler-type FSMA.

Managing hysteresis of Gd5Si2Ge2 by magnetic field cycling

The influence of magnetic field cycling through the first-order magnetostructural transformation on the magnetic and magnetocaloric properties, as well as hysteresis of polycrystalline Gd5Si2Ge2, has been studied using magnetometry. The cycling has a minimal effect on the magnetic field-induced entropy change and the phase transformation temperature of the material. On the other hand, magnetic hysteresis decreases by 30% after approximately ten cycles and remains low unless the sample is moved far into the paramagnetic regime. Factors playing a role in the history dependence of hysteresis have been discussed.

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.

Best practices in evaluation of the magnetocaloric effect from bulk magnetization measurements

Conventional magnetometry is irreplaceable in evaluating bulk magnetization of materials over broad temperature and field ranges. The technique is also effective in quantifying hysteresis that may be associated with magnetic and structural phase transitions that occur during the magnetizing/demagnetizing cycling, and the derived magnetic field-induced isothermal entropy change – one of the most important properties in the field of magnetocalorics. Both systematic and random errors present during the measurements of magnetization, however, may lead to erroneous conclusions. Using two well-known materials – elemental Gd and intermetallic Gd5Si2Ge2 as examples, we consider best practices in performing reliable and rapid magnetization measurements for proper characterization of magnetocaloric properties.

The influence of crystalline electrical field on magnetic and magnetocaloric properties in Er1−yTbyAl2 compounds

We report the anisotropy of magnetic field-induced entropy change in rare earth Er1-yTbyAl2 compounds (y=0.00, 0.25, 0.50, 0.75 and 1.00). In the present work, we use a model Hamiltonian that includes the crystalline electrical field anisotropy in both Er and Tb magnetic sublattices, chemical disorder in exchange interactions among Er-Er, Tb-Tb and Er-Tb magnetic ions and the Zeeman effect. We investigated the isothermal magnetic entropy change ΔST for a magnetic field of 1T rotating from a hard 〈001〉 to the easy 〈111〉 direction. We also performed a systematic analysis of the reorientation temperature as a function of the magnetic field intensity. The anisotropic magnetocaloric effect highlights the applicability of this effect on the rotating magnetic refrigeration.

Enhanced magnetocaloric effect tuning efficiency in Ni-Mn-Sn alloy ribbons

The present work was undertaken to investigate the effect of microstructure on the magnetic entropy change of Ni50Mn37Sn13 ribbon alloys. Unchanged sample composition and cell parameter of austenite allowed us to study strictly the correlation between the average grain size and the total magnetic field induced entropy change (ΔST). We found that a size-dependent martensitic transformation tuning results in a wide temperature range tailoring (>40K) of the magnetic entropy change with a reasonably small variation on the peak value of the total field induced entropy change. The peak values varied from 6.0Jkg−1K−1 to 7.7Jkg−1K−1 for applied fields up to 2T. Different tuning efficiencies obtained by diverse MCE tailoring approaches are compared to highlight the advantages of the herein proposed mechanism.

On the S(T) diagram of magnetocaloric materials with first-order transition: Kinetic and cyclic effects of Heusler alloys

For magnetocaloric materials the total entropy diagram S(T) in different magnetic fields is of central importance. From such a plot the isothermal entropy change ΔST and the adiabatic temperature change ΔTad can be well-defined. Aim of this work is to investigate the reversible magnetocaloric effect of Heusler alloys under cycling in terms of the S(T) diagram. For this purpose we selected three different Heusler alloys of Ni–Mn–In with transition temperatures between 200 k and room temperature. The S(T) diagrams of the three materials cover all conceivable shapes and are therefore representative for inverse magnetocaloric Heusler alloys. First we comprehensively analyzed the magnetocaloric properties of this model system using calorimetry, magnetometry and direct measurements of the adiabatic temperature change ΔTad and subsequently combined those results into the S(T) diagram. It turns out that under certain measurement conditions significant discrepancies can appear. With the help of in-situ optical microscopy, this effect could be attributed to kinetic relaxation effects in the pure martensite phase before the actual ΔTad experiment. Furthermore we found that the S(T) diagram obtained under continuous heating and cooling fails to describe the reversible magnetocaloric properties of minor loops under cycling. Utilizing cyclic measurements of the adiabatic temperature change together with calorimetric data it is however possible to determine the reversible magnetic field induced entropy change ΔST in fast operation comparable to real device conditions which can be applied to every magnetocaloric material with a first-order transition.

Low-field magnetocaloric effect in antiperovskite Mn3Ga1−xGexC compounds

A series of Mn3Ga1−xGexC (x=0, 0.1, 0.2) compounds were prepared by solid state reaction, and their magnetic properties, phase transitions, and magnetocaloric effect were investigated. All samples, after becoming ferromagnetic below Curie temperature Tc, undergo a first-order ferromagnetic–antiferromagnetic transition at Tt at low temperature, giving rise to conventional and inverse magnetocaloric effect. Tt increases while Tc shows a decrease with the increase of Ge content. Isothermal magnetization curves around Tt are strongly history-dependent, which may result in spurious peak in the magnetic field induced entropy change (ΔSM). The magnetothermal properties near Tt, compared with those near Tc, are of larger ΔSM, but have larger magnetic hysteresis and smaller refrigerant capacity.

Entropy change at magnetic phase transitions of the first order and second order

Magnetocaloric properties close to first order and second order phase transitions are presented and discussed. The specific heat capacity as a function of the magnetic field and the magnetic field induced entropy change has been measured by direct heat flux calorimetry in Gd, hydrogenated La(Fe–Mn–Si)13, La(Fe–Co–Si)13 and the Heusler alloy Ni–Mn–Co–Sn. The resulting s(Ha,T) entropy constitutive relation is compared with a model including the ferromagnetic, electronic and structural contributions to the free energy.

Magnetocaloric effect in inhomogeneous ferromagnets

The article is devoted to the theoretical study of magnetocaloric effect in imhomogeneous ferromagnets in vicinity of magnetic phase transitions of first and second orders as well as in vicinity of tricritical and critical points. In the frame of Landau theory the formulae for the magnetic field induced entropy change are derived. The theory is compared with the experimental data obtained on single crystals of La0.7Ba0.3MnO3 (second order transition) and La0.7Ca0.3MnO3 (first order transition).

Magnetoelastic coupling and competing entropy changes in substituted CoMnSi metamagnets

We use neutron diffraction, magnetometry and low temperature heat capacity to probe giant magneto-elastic coupling in CoMnSi-based antiferromagnets and to establish the origin of the entropy change that occurs at the metamagnetic transition in such compounds. We find a large difference between the electronic density of states of the antiferromagnetic and high magnetisation states. The magnetic field-induced entropy change is composed of this contribution and a significant counteracting lattice component, deduced from the presence of negative magnetostriction. In calculating the electronic entropy change, we note the importance of using an accurate model of the electronic density of states, which here varies rapidly close to the Fermi energy.

Inconvenient magnetocaloric effect in ferromagnetic shape memory alloys

Abstract Critical analysis available in the literature experimental results on magnetocaloric effect in ferromagnetic shape memory alloys Ni–Mn–X (X = Ga, In, Sn, Sb) is given. Based on a model developed by Pecharsky et al. [22] , it is shown that the isothermal magnetic field-induced entropy change in the Ni–Mn–X alloys should not greatly exceed 30 J/kg K. Considering thermodynamics of temperature- and magnetic field-induced martensitic transformations, it is demonstrated that a contribution of the structural subsystem to the magnetocaloric effect in the Ni–Mn–X alloys studied so far is irreversible in magnetic fields below 5 T. This makes ferromagnetic shape memory alloys an inconvenient system for the practical application in modern magnetic refrigeration technology.

Influence of sample geometry on determination of magnetocaloric effect for Gd60Co30Al10 glassy ribbons using direct and indirect methods

Rare-earth based metallic glasses with high saturation magnetization show a sizeable magnetocaloric effect (MCE) and are subject of extensive research concerning magnetic refrigeration materials. In this work, the magnetic phase transition from paramagnetic to ferromagnetic of Gd60Co30Al10 metallic glass has been characterized and three different methods were applied for the determination of its magnetocaloric specific parameters: (a) direct measurement of the adiabatic temperature change by exposing the material to an adiabatically applied magnetic field; (b) determination of the magnetization M(H,T) and calculation of the temperature dependent magnetic field induced entropy change ΔSm by application of the Maxwell relation and (c) measuring the total heat capacity Cp(H,T) in zero and non-zero magnetic field. Gd60Co30Al10 glassy ribbons were prepared by melt spinning, a technique that offers very high cooling rates due to the low dimensionality of the sample. Depending on the particular method of measurement, pieces of these glassy ribbons form samples with different appropriate total volume and dimensions. We show that the combination of the pronounced two-dimensionality of the ribbon pieces (aspect ratio ∼100) together with the very high magnetic permeability principally can cause strong internal demagnetizing fields that cannot be neglected when evaluating the intrinsic MCE parameters obtained from different methods.

Spin reorientation transition: phase diagrams and entropy change

ABSTRACTIn this paper we review the phase diagram and derive the entropy change for spin reorientation transitions by considering first order magnetization process theory with temperature dependent magneto-crystalline anisotropy constants. We derive the magnetic field-induced entropy change Δs for a transition between easy axis and easy plane, showing that for alternating magnetic field, Δs has a change of sign at the reorientation temperature, while for rotating magnetic field its sign is definite. We apply the model to CoZn W-type barium ferrite.

Field dependence of magnetic entropy change: Whence comes an intercept?

The magnetic field-induced entropy change in a ferromagnet undergoing a second-order phase transition at the Curie point is maximum near that point. The maximum magnetic entropy change, ( - Δ S ) max , is demonstrated to contain, besides the usual term in H 2 / 3 , a small negative term independent of H. Spatial inhomogeneity of real ferromagnetic materials is shown to be the origin of the off-set term, whose magnitude proves proportional to the width of the distribution of local Curie points.

Magnetic field-induced entropy change in phase-separated manganites

Magnetic field-induced entropy change has been experimentally studied for phase-separated manganites Eu0.55Sr0.45MnO3 and La0.27Nd0.40Ca0.33 MnO3. The entropy derived from the measured heat capacity exhibits a significant decrease under applied field in a broad temperature range below Curie temperature, and the maximum change is ∼8J∕kgK for Eu0.55Sr0.45MnO3 and is ∼4.2J∕kgK for La0.27Nd0.40Ca0.33MnO3 for a field change of 0–5T. In comparison with the calorimetric technique, Maxwell relation fails to give a proper description for the entropy change. It underestimates the entropy reduction in the low temperature range and even anticipates an entropy increase for Eu0.55Sr0.45MnO3. Failures of the Maxwell relation could be ascribed to the coexistence of two phases in the compound and to the variation of the proportion of each phase with applied field.

Magnetic field induced entropy change and magnetoelasticity in Ni–Mn–Ga alloys

Abstract A study on the magnetocaloric effect in Ni–Mn–Ga shape-memory alloys has been carried out. It is shown that in addition to the (negative) contribution arising from microscopic spin–lattice coupling, a positive contribution originating from magnetostructural coupling takes place at the length scale of martensitic variants and magnetic domains. The latter contribution is dominant for low applied fields.