Giant low-field cryogenic magnetocaloric effect in polycrystalline LiErF4 compound

Antiferromagnetic LiErF4has attracted extensive attention due to its dipolar interaction domination and quantum fluctuations action. In the present work, the crystal structure, cryogenic magnetic properties, and magnetocaloric effect(MCE) of polycrystalline LiErF4compound are investigated. Crystallographic study shows that the compound crystallizes in the tetragonal scheelite structure with I41/a space group. It exhibits an antiferromagnetic(AFM) phase transition around 0.4 K, accompanied by a giant cryogenic MCE. At 1.3 K, the maximum values of magnetic entropy changes are 24.3 J/kg·K,33.1 J/kg·K, and 49.0 J/kg·K under the low magnetic field change of 0–0.6 T, 0–1 T, and 0–2 T, respectively. The giant MCE observed above Néel temperature TNis probably due to the strong quantum fluctuations, which cause a large ratio of the unreleased magnetic entropy existing above the phase transition temperature. The outstanding low-field MCE below 2 K makes the LiErF4compound an attractive candidate for the magnetic refrigeration at the ultra-low temperature.

Large reversible cryogenic magnetocaloric effect in rare earth iron carbides of composition RE_(2)FeC_(4)(RE=Ho,Er,and Tm)

At cryogenic temperatures,the investigations of magnetic phase transition and magnetocaloric effect in RE_(2)FeC_(4)(RE=Ho,Er,and Tm) compounds were performed.Ho_(2)FeC_(4)and Er_(2)FeC_(4)compounds undergo two magnetic phase transitions with the temperature decreasing:from paramagnetic(PM) to ferromagnetic(FM) transition at their respective Curie temperature(Tc) and from FM to antiferromagnetic(AFM) or ferrimagnetic(FIM) transition below 2 K.Tm_(2)FeC_(4)compound exhibits only a second-order PM to FM phase transition at TC=K.Large reversible MCE without hysteresis loss is observed in RE_(2)FeC_(4)(RE=Ho,Er,and Tm) compounds.Particularly,the maximum value of magnetic entropy change(-ASM)is 21.62 J/(kg K) under the magnetic field change(Δ_(μ0)H) of 0-5 T for Er_(2)FeC_(4).The Er_(2)FeC_(4)compound presenting excellent magnetocaloric performance makes it a competitive cryogenic magnetic refrigeration material.

Exploration of the rare-earth cobalt nickel-based magnetocaloric materials for hydrogen liquefaction

Magnetic refrigeration based on the magnetocaloric effect(MCE)of magnetic solids has been considered as an emerging technology for hydrogen liquefaction.However,the lack of high-performance materials has slowed the development of any practical applications.Here,we present a family of rare-earth cobalt nickel-based magnetocaloric materials,namely Dy_(1-x)Ho_(x)CoNi and Ho_(1-x)Er_(x)CoNi compounds,and system-atically investigated their structural and magnetic properties as well as the MCE and magnetocaloric per-formance.All of these compounds crystallize in the C15-type Laves-phase structure and undergo typi-cal second-order magnetic phase transition(MPT).The change in magnetism and the MPT temperature for the Dy_(1-x)Ho_(x)CoNi and Ho_(1-x)Er_(x)CoNi compounds originate from the exchange interactions between nearest-neighbor RE 3+ion pairs.No hysteresis magnetocaloric effect was achieved,and the MPT tem-perature of these compounds could be tuned from the liquefaction temperature of nitrogen(∼77 K)to hydrogen(∼20 K)by adjusting the ratio of rare-earth elements.This study’s findings indicate that theDy_(1-x)Ho_(x)CoNi and Ho_(1-x)Er_(x)CoNi compounds are of potential for practical magnetic refrigeration applica-tions in the field of hydrogen liquefaction.

Enhanced low-field magnetocaloric effect in Dy-doped hexagonal GdBO_(3) compounds

Borates have attained increasing attention attributed to their excellent thermal stability,distinctive thermodynamic property,and high mechanical strength in recent years.A series of polycrystalline Dydoped GdBO_(3) compounds was prepared,their crystal structures,magnetic properties,and cryogenic magnetocaloric effects were comprehensively investigated.The compounds crystallize in hexagonal structure(space group P6_(3)/mmc),the lattice constant decreases with the increase of Dy content.Dydoping in GdBO_(3) significantly reduces critical magnetic field and enhances low-field magnetocaloric effect.The maximum magnetic entropy changes for the Gd_(1-x)Dy_(x)BO_(3)(x=0.6,0.8,and 1)compounds in a field change of 2 T surpass 17.3 J/(kg·K)at 2.5 K,enhanced by nearly 120%compared to GdBO_(3)(8.0 J/(kg·K)).Besides,the corresponding refrigeration capacity increases from 33.9 to 62.2,57.2,and 72.5 J/kg,respectively,with an enhancement of 70%-110%.The considerable maximum magnetic entropy change,refrigerating capacity,and temperature averaged entropy change make them competitive candidates for cryogenic magnetic refrigeration.

Enhanced low-field magnetocaloric effect in Nb and Al co-substituted EuTiO_(3)compounds

The magnetic ground state switching between antiferromagnetic(AFM)and ferromagnetic(FM)states in EuTiO_(3)provides the feasibility of regulating its magnetic properties and magnetocaloric effect.First-principles calculations demonstrate that the magnetic ground states for EuTi_(0.875)Nb_(0.0625)Al_(0.0625)O_(3),EuTi_(0.8125)Nb_(0.125)Al_(0.)0_(625)O_(3),and EuTi_(0.75)Nb_(0.125)Al_(0.125)O_(3)are FM coupling.Experimental results also exhibit the FM coupling domination in these compounds,accompanied by a significantly enhanced low magnetic field magnetocaloric effect.The maximum magnetic entropy change of all the samples surpasses15 J kg^(-1)K^(-1)with a field change of 1 T.which is 1.4 times as large as that of bulk EuTiO_(3).Especially,the maximum refrigerating capacity for EuTi_(0.8125)Nb_(0.125)Al_(0.0625)O_(3)compound is evaluated to be 88.1 J kg^(-1),more than three times of that of EuTiO_(3).The remarkable magnetocaloric performances prove Nb and Al co-substituted EuTiO_(3)compounds to be competitive candidates for magnetic refrigeration in the liquid helium temperature regime.