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 Ho6Co2Ga-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 RE6Co2Ga (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 Gd6Co2Ga is confirmed by density functional theory calculations in addition to experimental observations. The field dependence magnetocaloric and Banerjee’s criteria demonstrate that Ho6Co2Ga and Dy6Co2Ga undergo a first-order phase transition in addition to a second-order phase transition, whereas only the latter is observed for Gd6Co2Ga. The two extreme alloys of the series, Ho6Co2Ga and Gd6Co2Ga, show maximum isothermal entropy change (∣ΔS iso max (5 T)∣) of 10.1 and 9.1 J kg−1K−1 at 26 and 75 K, close to H2 and N2 liquefaction, respectively. This outstanding magnetocaloric effect performance makes the RE6Co2Ga series of potential for cryogenic magnetic refrigeration applications.

Gadolinium-oxide nanoparticles for cryogenic magnetocaloric applications

“…It has been theoreticaly justified and experimentaly evidenced in the series of compound families 32 that overshooting of n = 2 value is one of the hallmarks unequivocally corresponding to the first order magneto-structural phase transitiuon (FOPT). Employing the criterion, FOPT has been recognized in RE 6 Co 2 Ga compounds when RE = Ho, Dy, wherease only SOPT has been observed in the case of introducing Gd 33 . Accurate determination of the phase transition nature is important both from fundamental and application point of view.…”

Magnetic properties and promising magnetocaloric performances in the antiferromagnetic GdFe2Si2 compound

“…The order of MPT can be checked by the novel MC criterion proposed by Law et al [24,53,54], i.e., based on the field dependence exponent n of the ΔS M (T) curves, which can be estimated using the following formula:…”

“…Xu et al [20] recently reported an MC effect in ferromagnetic Sr 2 GdNbO 6 double-perovskite oxide with the −ΔS M max value of 29.7 J kg −1 K −1 under ΔH of 0-7 T at around 2 K. In recent years, we have fabricated several series of RE-based materials and checked their magnetic properties, MPT, and MC effects in detail; several of these materials exhibited promising MC performances. For instance, large MC effects in a wide temperature range from the liquefaction temperature of H 2 to N 2 have been noted in RE 6 Co 2 Ga compounds [24]. A table-like MC effect has been achieved in the highentropy Er 20 Ho 20 Gd 20 Ni 20 Co 20 amorphous alloys [25].…”

“…The magnitudes of the MC effect have a strong correlation with the corresponding magnetic phase transition (MPT); therefore, the investigation of the MC effects of magnetic materials can provide considerable valuable information for the better understanding of MPT [5][6][7][8][9]. Therefore, many magnetic materials have been synthesized and systematically determined with regard to the magnetic proper-ties, MPT, and MC performances not only to search for suitable candidates for active MR application at cryogenic and near room temperature but also to better understand the MPT of magnetic materials [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. However, a large gap still exists at the present stage between the practical MR application requirements and the performances of known MC materials.…”

“…In the last several decades, investigations have been performed on the MPT and MC effects in the rare-earth (RE)-based materials in amorphous and crystallized states owing to the large magnetic moments of RE ions, which result in considerable MC effects [18][19][20][21][22][23][24][25]. Law and Franco [13] reviewed the MC performances in RE-based high-entropy alloys.…”