First- and second-order phase transitions in RE6Co2Ga (RE = Ho, Dy or Gd) cryogenic magnetocaloric materials

Abstract

Rare-earth (RE) rich intermetallics crystallizing in orthorhombic Ho ₆Co ₂Ga-type crystal structure exhibit peculiar magnetic properties that are not widely reported for their magnetic ordering, order of magnetic phase transition, and related magnetocaloric behavior. By tuning the type of RE element in RE ₆Co ₂Ga (RE = Ho, Dy or Gd) compounds, metamagnetic anti-to-paramagnetic (AF to PM) phase transitions could be tuned to ferro-to-paramagnetic (FM to PM) phase transitions. Furthermore, the FM ground state for Gd ₆Co ₂Ga is confirmed by density functional theory calculations in addition to experimental observations. The field dependence magnetocaloric and Banerjee’s criteria demonstrate that Ho ₆Co ₂Ga and Dy ₆Co ₂Ga undergo a first-order phase transition in addition to a second-order phase transition, whereas only the latter is observed for Gd ₆Co ₂Ga. The two extreme alloys of the series, Ho ₆Co ₂Ga and Gd ₆Co ₂Ga, show maximum isothermal entropy change (∣Δ S _iso ^max (5 T)∣) of 10.1 and 9.1 J kg ⁻¹K ⁻¹ at 26 and 75 K, close to H ₂ and N ₂ liquefaction, respectively. This outstanding magnetocaloric effect performance makes the RE ₆Co ₂Ga series of potential for cryogenic magnetic refrigeration applications.

摘要

具有正交Ho ₆Co ₂Ga型晶体结构的富稀土金属间化合物因其具备独特的磁性能而被关注, 但它们的磁有序、 磁相变类型以及磁热性能尚未见系统报道. 本文通过对RE ₆Co ₂Ga化合物中的稀土元素类型的调控, 实现了材料相变类型从反铁磁(AF)到顺磁(PM)的变磁调控为铁磁(FM)到顺磁(PM)的转变. 此外, 通过实验观察结合密度泛函理论(DFT)计算证明了Gd ₆Co ₂Ga的基态为铁磁态. 利用磁热效应的场依赖性关系结合Banerjee准则的判断标准研究表明, Ho ₆Co ₂Ga和Dy ₆Co ₂Ga同时具备二级相变和一级相变特征, 而在Gd ₆Co ₂Ga中只存在二级相变. 在0–5 T的磁场变化下, Ho ₆Co ₂Ga和Gd ₆Co ₂Ga的磁熵变分别在26和75 K附近达到最大值10.1和9.1 J kg ⁻¹ K ⁻¹, 而这两个温度分别接近于H ₂液化和N ₂液化温度. 优异的磁热性能使得RE ₆Co ₂Ga体系在低温磁制冷领域具有潜在的应用前景.