Giant Magnetocaloric Effect in Cr- and Fe-Substituted Frustrated Magnet Gadolinium Gallium Garnets, Gd3Ga5O12, for Magnetic Cooling at Cryogenic Temperatures.

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.

Giant anisotropic magnetocaloric effect in antiferromagnetic topological semimetal HoSb

Magnetic properties and magnetocaloric effect (MCE) of intermetallic HoNiSi compound have been investigated systematically. It is found that HoNiSi exhibits antiferromagnetic (AFM) state below the Néel temperature TN of 3.8 K, which is quite close to the liquid helium temperature (4 K). A giant MCE without hysteresis loss is observed in HoNiSi, which is related to the field-induced first-order metamagnetic transition from AFM to ferromagnetic states. For a magnetic field change of 2 T, the maximum values of magnetic entropy change (−ΔSM) and adiabatic temperature change (ΔTad) are 17.5 J/kg K and 4.5 K, respectively. In addition, HoNiSi presents both large values of positive and negative ΔSM for the low field changes, i.e., the maximum −ΔSM values are 9.2 J/kg K around TN and −7.2 J/kg K below TN for the field changes of 1 and 0.5 T, respectively. A universal curve of ΔSM is successfully constructed by using phenomenological procedure, proving the applicability of universal ΔSM curve for AFM materials. The giant reversible MCE for relatively low magnetic field change makes HoNiSi attractive candidate for magnetic refrigerant materials around liquid helium temperature.

Magnetic refrigeration (MR) is potentially a high efficiency, low cost, and greenhouse gas-free refrigeration technology, and with the looming phase out of HCFC and HFC fluorocarbons refrigerants is drawing more attention as an alternative to the existing vapor compression refrigeration. MR is based on the magnetocaloric effect (MCE), which occurs due to the coupling of a magnetic sublattice with an external magnetic field. With the magnetic spin system aligned by magnetic field, the magnetic entropy changes by SM as a result of isothermal magnetization of a material. On the other hand, the sum of the lattice and electronic entropies of a solid must be changed by SM as a result of adiabatically magnetizing the material, thus resulting in an increase of the lattice vibrations and the adiabatic temperature change, ∆Tad. Both the isothermal entropy change SM and adiabatic temperature change ∆Tad are important parameters in quantifying the MCE and performance of magnetocaloric materials (MCM). In general, SM and ∆Tad are obtained using magnetization and heat capacity data and the Maxwell equations. Although Maxwell equations can be used to calculate MCE for first order magnetic transition (FOMT) materials due to the fact that the transition is not truly discontinuous, there can be some errors depending on the numerical integration method used. Thus, direct measurements of ∆Tad are both useful and required to better understand the nature of the giant magnetocaloric effect (GMCE). Moreover, the direct measurements of ∆Tad allow investigation of dynamic performance of FOMT materials experiencing repeated magnetization/demagnetization cycles. This research utilized a special test facility to directly measure MCE of Gd5Si2Ge2, Gd5Si2.7Ge1.3, MnFePAs, LaFeSiH , Ni55.2M18.6Ga26.2, Dy, Tb, DyCo2 , (Hf0.83 Ta0.17)Fe1.98,

Magnetic entropy and adiabatic temperature changes in and above the room-temperature region have been measured for La0.7Sr0.3Mn1−xMx′O3 (M′=Al,Ti) by means of magnetization and heat capacity measurements in magnetic fields up to 6 T. The magnetocaloric effect becomes largest at the ferromagnetic ordering temperature Tc that is tuned (from 364.5 K for x=0) to ∼300 K by the substitution of Al or Ti for Mn. While the substitution of Al for Mn drastically reduces the maximum magnetic entropy (−ΔSm) and adiabatic temperature (ΔTa) changes, it extends considerably the working temperature span and therefore improves the relative cooling power (RCP). Under a magnetic field change ΔH=2 T, −ΔSm (or ΔTa) of La0.7Sr0.3Mn1−xAlxO3 decreases from 2.66 J/kg K (or 1.65 K) for x=0 to 1.18 J/kg K (or 0.69 K) for x=0.1, while RCP increases from 80.3 to 108.8 J/kg, respectively. While Tc is largely suppressed, the magnetocaloric effect is only lightly affected by the Ti substitution. With ΔH=2 T, the La0.7Sr0.3Mn0.95Ti0.05O3 sample exhibits −ΔSm (or ΔTa) =2.44 J/kg K (or 1.38 K) with RCP=89.9 J/kg. The decrease of magnetic moment is found to be one possible reason behind the suppression of the MCE. The magnetocaloric effect in manganite materials seems to be inhibited by the existence of short-range ferromagnetic correlations above Tc.

Magnetocaloric effect and applied refrigeration performance of La(Fe,Si)13-based compounds

Magnetic and lattice entropy change across martensite transition of Ni-Mn-Sn melt spun ribbons: Key factors in magnetic refrigeration

The magnetic properties and magnetocaloric effect (MCE) of antiferromagnetic HoCuSi compound have been studied. It is found that HoCuSi undergoes a field-induced first order metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) states below the Néel temperature (TN). A giant MCE without hysteresis loss is observed in HoCuSi around TN. The maximal magnetic entropy change (−ΔSM) and refrigerant capacity are 33.1 J/kgK and 385 J/kg, respectively, for a field change of 0–5 T. The excellent magnetocaloric properties can result from the field-induced AFM-FM transition below TN and the increase in magnetization change caused by the change in lattice volume at TN.

The Magnetocaloric effect (MCE) is a physical phenomenon that occurs in magnetic materials under the influence of a varying magnetic field. Is it usually expressed as the adiabatic temperature change or isothermal total entropy change of a material. For room temperature cooling, one utilizes that the magnetocaloric effect peaks near magnetic phase transitions and so the materials of interest all have a critical temperature within the range of 250−310 K. It is worth noting that from a practical point of view, the Suitable magnetocaloric material must exhibit a large MCE under relatively low magnetic fields (<2 T) that can be reached via permanent Magnets. Particularly, the Mn5Ge3 Based compounds offer key advantages for application in magnetic refrigeration when compared with the reference Gadolinium metal such as their low cost. In fact, for a large-scale commercialization of this emerging technology, the Gd is unfavourable because of rising prices of rare earths while the intermetallic Mn5Ge3 is completely composed of abundant and affordable elements. In this work, we investigated the magnetic properties as well as the electronic structure of the Mn5Ge3 compound using the density functional theory. The magnetocaloric properties in terms of both adiabatic temperature and magnetic entropy changes were determined using the Monte Carlo approach, while the thermodynamic performances were simulated according to the active magnetic refrigeration (AMR).KeywordsMagnetocaloric effectMonte Carlo simulationActive magnetic refrigerationDensity functional theoryAb initio calculation

We have investigated the magnetic and magnetocaloric properties of EuHo2O4 and EuDy2O4 by magnetization and heat capacity measurements down to 2 K. These compounds undergo a field-induced antiferromagnetic to ferromagnetic transition and exhibit a huge entropy change. For a field change of 0-8 T, the maximum magnetic entropy and adiabatic temperature changes are 30 (25) J kg−1 K−1 and 12.7 (16) K, respectively, and the corresponding value of refrigerant capacity is 540 (415) J kg−1 for EuHo2O4 (EuDy2O4). These magnetocaloric parameters also remain large down to lowest temperature measured and are even larger than that for some of the potential magnetic refrigerants reported in the same temperature range. Moreover, these materials are highly insulating and exhibit no thermal and field hysteresis, fulfilling the necessary conditions for a good magnetic refrigerant in the low-temperature region.

We have investigated the magnetocaloric effect in single and polycrystalline samples of quantum paraelectric EuTiO3 by magnetization and heat capacity measurements. Single crystalline EuTiO3 shows antiferromagnetic ordering due to Eu2+ magnetic moments below TN = 5.6 K. This compound shows a giant magnetocaloric effect around its Neel temperature. The isothermal magnetic entropy change is 49 Jkg-1K-1, the adiabatic temperature change is 21 K and the refrigeration capacity is 500 JKg-1 for a field change of 7 T at TN. The single crystal and polycrystalline samples show similar values of the magnetic entropy change and adiabatic temperature changes. The large magnetocaloric effect is due to suppression of the spin entropy associated with localized 4f moment of Eu2+ ions. The giant magnetocaloric effect together with negligible hysteresis, suggest that EuTiO3 could be a potential material for magnetic refrigeration below 20 K.

A team led by GE Global Research developed new magnetic refrigerant materials needed to enhance the commercialization potential of residential appliances such as refrigerators and air conditioners based on the magnetocaloric effect (a nonvapor compression cooling cycle). The new magnetic refrigerant materials have potentially better performance at lower cost than existing materials, increasing technology readiness level. The performance target of the new magnetocaloric material was to reduce the magnetic field needed to achieve 4 °C adiabatic temperature change from 1.5 Tesla to 0.75 Tesla. Such a reduction in field minimizes the cost of the magnet assembly needed for a magnetic refrigerator. Such a reduction in magnet assembly cost is crucial to achieving commercialization of magnetic refrigerator technology. This project was organized as an iterative alloy development effort with a parallel material modeling task being performed at George Washington University. Four families of novel magnetocaloric alloys were identified, screened, and assessed for their performance potential in a magnetic refrigeration cycle. Compositions from three of the alloy families were manufactured into regenerator components. At the beginning of the project a previously studied magnetocaloric alloy was selected for manufacturing into the first regenerator component. Each of the regenerators was tested in magnetic refrigerator prototypes at a subcontractor at at GE Appliances. The property targets for operating temperature range, operating temperature control, magnetic field sensitivity, and corrosion resistance were met. The targets for adiabatic temperature change and thermal hysteresis were not met. The high thermal hysteresis also prevented the regenerator components from displaying measurable cooling power when tested in prototype magnetic refrigerators. Magnetic refrigerant alloy compositions that were predicted to have low hysteresis were not attainable with conventional alloy processing methods. Preliminary experiments with rapid solidification methods showed a path towards attaining low hysteresis compositions should this alloy development effort be continued.

Giant magnetocaloric effect was observed in Mn1.1Fe0.9P1−xGex (x=0.2, 0.24) melt-spun ribbons. The maximum magnetic entropy change ∣ΔSM∣ of Mn1.1Fe0.9P0.76Ge0.24 reaches 35.4J∕kgK in a field change from 0 to 5 T at around 317 K. This value is superior to that reported for Mn1.1Fe0.9P0.76Ge0.24 synthesized by mechanical alloying (∼30J∕kgK at 306 K). The large magnetocaloric effect results from a more homogenous element distribution related to the very high cooling rate during melt spinning. The excellent magnetocaloric effect properties, the low material cost, and the accelerated aging regime make the melt-spun-type MnFePGe materials an excellent candidate for magnetic refrigerant applications.

Effect of partial substitution of Cr 3+ for Fe 3+ on magnetism, magnetocaloric effect and transport properties of Ba 2FeMoO 6 double perovskites

A giant magnetocaloric effect (ΔSmag) has been discovered in the Gd5(SixGe1−x)4 pseudobinary alloys, where x⩽0.5. For the temperature range between ∼50 and ∼280 K it exceeds the reversible (with respect to alternating magnetic field) ΔSmag for any known magnetic refrigerant material at the corresponding Curie temperature by a factor of 2–10. The two most striking features of this alloy system are: (1) the first order phase transformation, which brings about the large ΔSmag in Gd5(SixGe1−x)4, is reversible with respect to alternating magnetic field, i.e., the giant magnetocaloric effect can be utilized in an active magnetic regenerator magnetic refrigerator; and (2) the ordering temperature is tunable from ∼30 to ∼276 K by adjusting the Si:Ge ratio without losing the giant magnetic entropy change.