Table-like Magnetocaloric Effect Research Articles

Magnetocaloric effect and evidence of superparamagnetism inGdAl2nanocrystallites: A magnetic-structural correlation

The correlation between structural and magnetic properties of $\mathrm{GdA}{\mathrm{l}}_{2}$, focusing on the role played by the disorder in magnetic ordering and how it influences the magnetocaloric effect (MCE) are discussed. Micrometric-sized particles, consisting of nanocrystallites embedded in an amorphous matrix, were prepared by a mechanical milling technique and characterized by means of x-ray diffraction, scanning and high-resolution transmission electron microscopy as well as magnetic measurements as a function of an applied external magnetic field and temperature. The results show that the average particle size is just slightly diminished (\ensuremath{\approx}7%) with the milling time (between 3 and 13 h), whereas the average crystallite size undergoes an expressive reduction (\ensuremath{\approx}43%). For long milling times, structural disorders mostly associated with crystallite size singularly affect the magnetic properties, leading to a large tablelike MCE in the temperature range between 30 and 165 K. Below 30 K, nanocrystallites with dimensions below a given critical size cause an enhancement in the magnetic entropy change related to superparamagnetic behavior. In contrast, for low milling times, relative cooling power values are improved. These striking features along with the small magnetic hysteresis observed make the milled $\mathrm{GdA}{\mathrm{l}}_{2}$ a promising material for application in the magnetic refrigeration technology. Finally, a discussion in an attempt to elucidate the origin of the spin-glass states previously reported in the literature for mechanically milled $\mathrm{GdA}{\mathrm{l}}_{2}$ samples for very long times (400 and 1000 h) is presented.

Large reversible magnetocaloric effect in RE2Cu2In (RE = Er and Tm) and enhanced refrigerant capacity in its composite materials

The magnetic properties and magnetocaloric effect (MCE) in RE2Cu2In (RE = Er and Tm) compounds and its composite materials have been investigated. A large reversible MCE is observed at around their own Curie temperature of TC ~ 39.4 and 31.6 K for Er2Cu2In and Tm2Cu2In, respectively. Under a magnetic field change of 0–7 T, the maximum values of magnetic entropy change () are estimated to be 18.4 and 18.2 J kg−1 K−1 for Er2Cu2In and Tm2Cu2In, respectively. A table-like MCE at a temperature of 20–50 K and enhanced refrigerant capacity (RC) are realized in the Er2Cu2In–Tm2Cu2In composite materials. For magnetic field changes of 0–5 and 0–7 T, the maximum improvements of RC reach 32% (12%) and 28% (14%), in comparison with that of the individual compound Er2Cu2In (Tm2Cu2In), respectively. The excellent MCE properties make the RE2Cu2In composite materials attractive for low-temperature magnetic refrigeration.

Room temperature table-like magnetocaloric effect in amorphous Gd50Co45Fe5 ribbon

Gd50Co45Fe5 amorphous alloy ribbon with a table-like magnetocaloric effect (MCE) suitable for the ideal Ericsson cycle at room temperature has been developed. In addition to a high magnetic transition temperature of 289 K very close to that of Gd (294 K), a relatively large value of refrigerant capacity (~521 J kg−1) has been achieved under a field change of 5 T. This value of refrigerant capacity (RC) is about 27% and 70% larger than those of Gd (~410 J kg−1) and Gd5Si2Ge2 (~306 J kg−1). More importantly, the peak value of magnetic entropy change () approaches a nearly constant value of ~3.8 J ⋅ kg−1 ⋅ K−1 under an applied field change of 0~5 T in a wide temperature span over 40 K around room temperature, which could be used as the candidate working material in the Ericsson-cycle magnetic regenerative refrigerator around room temperature.

Table-like magnetocaloric effect in Gd56Ni15Al27Zr2 alloy and its field independence feature

In order to obtain “table-like” magnetocaloric effect (MCE), multiple-phase Gd56Ni15Al27Zr2 alloy was prepared by arc-melting followed by suck-casting method. Powder x-ray diffraction and calorimetric measurements reveal that the sample contains both glassy and crystalline phases. The fraction of the glassy phase is about 62%, estimated from the heat enthalpy of the crystallization. The crystalline phases, Gd2Al and GdNiAl further broadened the relatively wider magnetic entropy change (−ΔSM) peak of the amorphous phase, which resulted in the table-like MCE over a maximum temperature range of 52.5 K to 77.5 K. The plateau feature of the MCE was found to be nearly independent of the applied magnetic field from 3 T to 5 T. The maximum −ΔSM value of the MCE platforms is 6.0 J/kg K under applied magnetic field change of 5 T. Below 3 T, the field independence of the table-like feature disappears. The relatively large constant values of −ΔSM for the respective applied magnetic fields have promising applications in magnetic refrigeration using regenerative Ericsson cycle.

Table-like magnetocaloric effect of Fe88−xNdxCr8B4 composite materials

The narrow working temperature range due to the sharp magnetic entropy change |ΔSM| peak and large thermal or magnetic hysteresis restricts the practical application of magnetocaloric materials. In this work, the table-like magnetocaloric effect (MCE) was obtained in the multilayer composite of Fe88−xNdxCr8B4 alloys with various Nd substitutions for Fe (x=5, 8, 10, 12, and 15), which were prepared by arc-melting followed by melt-spinning. The substation of Nd was found to enhance the glass-forming ability. For the alloys with Nd substitution from 5 at% to 15 at%, the Curie temperature (TC) ranged from 322K to 350K and the peak value of |ΔSM| remained almost constant, 3.4–3.5J/(kgK) under an applied field of 0–5T. The composite with various Nd contents was prepared by stocking the ribbons layer by layer. The |ΔSM| of the composite approached a nearly constant value of ∼3.2J/(kgK) in a field change of 0–5T in a wide temperature span over 40K, resulting in large refrigerant capacity value of >408J/kg. This |ΔSM| value was much larger than the previous reported Fe-based amorphous composite Fe78−xCexSi4Nb5B12Cu1. This composite can be used as the working material in the Ericsson-cycle magnetic regenerative refrigerator around room temperature.

Enhancement of refrigeration capacity and table-like magnetocaloric effect in La0.8Ca0.2MnO3/La0.8K0.2MnO3 nanocrystalline composite

In this paper we report on the magnetocaloric effect and refrigeration capacity (RC) of La0.8Ca0.2MnO3(LCMO)/La0.8K0.2MnO3(LKMO) composite with a mass fraction of 1:1. X-ray diffraction patterns indicate that the component manganites crystalize in the same rhombohedral structure. The LCMO and LKMO undergo second-order phase transitions at 247K and 281K, respectively. The maximum values of isothermal entropy change (−ΔST,max) are found to be 4.25J/kgK and 3.71J/kgK under a magnetic field change of 5T for LCMO and LKMO, respectively. The composite exhibits a table-like ΔST curve with slightly lowered peak value. An enhancement of 33%–35% for RC has been found in the composite when comparing with those of the components. It reveals that the combination of magnetocaloric materials with different Curie temperatures is a possible method for improving RC.

Achieving table-like magnetocaloric effect and large refrigerant capacity around room temperature in Fe78−xCexSi4Nb5B12Cu1 (x=0–10) composite materials

Fully amorphous Fe78−xCexSi4Nb5B12Cu1 (x=0, 1, 3, 5 and 10) alloy ribbons were prepared by melt-spinning. With partial substitution of Ce for Fe, the Curie temperature could be tuned in a large temperature range from 465 to 281K and the maximum magnetic entropy change (−ΔSM) for a field change from 0 to 5T decreased from 3.25 to 2.18J/kgK. Two types of composite materials with varied Ce contents were obtained by assembling the ribbons layer by layer or mixing the powders with various Ce contents. A table-like magnetocaloric effect (MCE) was observed in these composites. The −ΔSM of the composites approach a nearly constant value of ~2.0J/kgK for a field change of 0–5T in a wide temperature span over 80K, resulting in large refrigerant capacity values, >370J/kg. These composites can be used as the working material in an active magnetic regenerative refrigerator around room temperature.

A coupling of martensitic and metamagnetic transitions with collective magneto-volume and table-like magnetocaloric effects

A coupling of the first-order paramagnetic-to-induced-ferromagnetic martensitic and the second-order antiferromagnetic-to-ferromagnetic metamagnetic transitions was found in MnNi0.8Fe0.2Ge alloy. Based on the coupling, a magneto-volume effect driven by the martensitic transition and a table-like magnetocaloric effect generated by the successive magnetic phase transitions arise collectively. By using the magneto-volume effect, the internal stress in the volume-expansion martensitic transition was determined at 350 MPa. The magnetocaloric effect, with a wide working temperature range of 26 K around room temperature, shows a small hysteresis loss (5 J kg−1) and a large net refrigerant capacity (157 J kg−1).

Reversible table-like magnetocaloric effect in Eu4PdMg over a very large temperature span

The magnetic properties and magnetocaloric effect (MCE) of the ternary intermetallic compound of Eu4PdMg have been investigated. Eu4PdMg has a very magnetic field sensitive magnetic phase transition, resulting in a reversible, table-like MCE over a broad temperature range. For the magnetic field changes of 0–5 T and 0–7 T, the maximum values of magnetic entropy change (−ΔSMmax) are 5.5 J kg−1 K−1 and 7.2 J kg−1 K−1, respectively, the corresponding refrigeration capacity (RC) are 977 J kg−1 and 1346 J kg−1. The RC values are obviously larger than those of some potential magnetic refrigerant materials at similar temperature region, making Eu4PdMg attractive for magnetic refrigeration.

Table-like magnetocaloric effect in the Gd-Co-Al alloys with multi-phase structure

Two sorts of Gd-Co-Al ternary alloys of multi-phase structure with table-like magnetocaloric effect (MCE) suitable for the ideal Ericsson cycle have been developed. One is prepared by arc-melting Gd53Co19Al28 with the resulting alloy composed of Gd2Al, Gd2Co2Al, and GdCo0.74Al1.26 crystalline phases and the other is synthesized by suck-casting Gd52.5Co16Al31 which has a glassy structure and crystalline phases of Gd2Al and Gd2Co2Al. Metamagnetic transition of antiferromagnetic Gd2Al phase near 50 K and ferromagnetic transitions of Gd2Co2Al, GdCo0.74Al1.26, and the glassy phase near 80 K result in the occurring of table-like magnetocaloric effect with magnitude of ∼7 J/kg K over the temperature range from 47.5 K to 77.5 K for a magnetic field variation of 0 to 50 kOe. The fitting reproduces the temperature dependence of magnetic entropy change of samples and the results suggest that the magnetocaloric response of Gd2Al phase in the ternary alloys have been improved, which is crucial for the composite alloys to obtain table-like magnetocaloric effect.

Influence of magnetic interactions between phases on the magnetocaloric effect of composites

Magnetocaloric materials with coexisting magnetic phases appear either due to the phase coexistence in first order phase transitions, or due to the development of composites, which are known to enhance the refrigerant capacity and produce table-like magnetocaloric effect. However, interactions between phases are rarely considered. We have modeled the influence of interactions on the magnetocaloric effect of a biphasic composite by implementing a mean field model. Interactions shift the peak magnetic entropy change to higher temperatures than those of the pure phases and enhance the table-like character of the curves. Although there is no qualitative change of the magnetocaloric response of the composites due to interactions, the optimal fraction of phases which produces the largest enhancement of the refrigerant capacity is shifted to compositions richer in the low Curie temperature phase. This shift can be used to estimate the magnitude of the interactions in composites measured experimentally.

Low field giant magnetocaloric effect in RNiBC (R = Er and Gd) and enhanced refrigerant capacity in its composite materials

The magnetic properties and magnetocaloric effect (MCE) in RNiBC (R = Er and Gd) compounds and its composite materials were investigated. A giant low field reversible MCE was observed in RNiBC compounds at 5 and 13 K for R = Er and Gd, respectively. Under a relative low field change of 2 T, the maximum values of magnetic entropy change (−ΔSMmax) reach 17.1 and 9.3 J/kg K for R = Er and Gd, respectively. A table-like MCE in the temperature of 4-20 K and enhanced refrigerant capacity (RC) were found in the ErNiBC-GdNiBC composite materials. For the magnetic field changes of 1 T and 2 T, the maximum improvement of RC reach 31% (19%) and 30% (8%), in comparison with that of individual compound ErNiBC (GdNiBC), respectively. The excellent MCE properties make the RNiBC composite materials attractive for low temperature magnetic refrigeration.

Table-like magnetocaloric effect and enhanced refrigerant capacity in Eu8Ga16Ge30-EuO composite materials

A large reversible magnetocaloric effect (MCE) and enhanced refrigerant capacity (RC) were observed in multiphase composite materials composed of type-I clathrate Eu8Ga16Ge30 and EuO. Eu8Ga16Ge30 undergoes two successive ferromagnetic transitions at 10 K and 35 K, and EuO exhibits a ferromagnetic transition at 75 K. A large RC of 794 J/kg for a field change of 5 T over a temperature interval of 70 K was achieved in the Eu8Ga16Ge30–EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K-100 K. Adjusting the Eu8Ga16Ge30 to EuO ratio is shown to produce composites with table-like MCE, desirable for ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of these Eu8Ga16Ge30–EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.

Understanding the table-like magnetocaloric effect

In this work, we explain the table-like magnetocaloric effect as a consequence of multiple magnetic phase transitions for different temperatures. Using a Hamiltonian that takes into account the dipolar and quadrupolar interactions and the crystalline electric field, it was possible to study the magnetocaloric effect of the Γ 3–Γ 5 reduced magnetic system. We analyze the table-like magnetocaloric effect for different λ and K, dipolar and quadrupolar parameters, and for a suitable energy gap between Γ 3 and Γ 5 that permits successive magnetic orderings at different temperatures.

The correlation of the magnetic properties and the magnetocaloric effect in (Gd1−xErx)NiAl alloys

A study of the magnetic properties of several (Gd1−xErx)NiAl alloys (where x=0, 0.30, 0.40, 0.46, 0.50, 0.55, 0.60, 0.80, and 1.00) was undertaken using both ac and dc magnetic and heat capacity measurements in an attempt to understand the table-like magnetocaloric effect previously observed in (Gd0.54Er0.46)NiAl. Results indicate the presence of both antiferromagnetic and ferromagnetic ordering processes in all alloys containing Gd. For ErNiAl, a metamagnetic transition from an antiferromagnetic ground state was observed. Within each alloy, several magnetic transitions occur over a temperature range from 10 K [in (Gd0.20Er0.80)NiAl] up to 35 K (in GdNiAl), with all but the lowest temperature transition shifting to higher temperatures with increasing Gd content. The change in magnetic entropy (ΔSmag) induced by a change in field is observed to peak around the Néel temperature for ErNiAl while gradually broadening and shifting toward the Curie temperature as the Gd content is increased. For Gd-rich alloys, a significant contribution to ΔSmag is observed at both the low and high temperature transitions, resulting in a rounded, skewed caret-like temperature profile of the magnetocaloric effect. Factors, which are believed to contribute to this effect, include the presence and temperature spacing of multiple zero-field transitions, which most likely result from competing anisotropy and exchange interactions within a frustrated hexagonal spin lattice. This leads to broad peaks in the magnetic heat capacity that span several transition temperatures, providing for a substantial ΔSmag over an extended temperature range. This characteristic is desired for application to magnetic refrigeration, where certain thermodynamic cycles (e.g., Ericsson cycle) require specific temperature profiles of the magnetocaloric effect in refrigerant materials (e.g., a constant change in magnetic entropy as a function of temperature within the region of cooling). In general, the best materials are those which supply the maximum amount of cooling over the widest temperature range.