Specific Heat and Magnetocaloric Effect in Ho-Er-Co Solid Solutions
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Specific heat measurements have been performed on polycrystalline HoCo2, ErCo2 and their solid solutions Ho1−xErxCo2 (0.1≤x≤0.5). These compounds were synthesized using high-purity rare-earth metals and cobalt. X-ray diffraction patterns taken at room temperature reveal that all compounds have the C15 cubic Laves phase structure. Heat capacity measurements have been performed in the temperature range of 2–300 K without magnetic field and in a magnetic field of 1 and 2 T. The magnetocaloric effect has been estimated in terms of isothermal magnetic entropy change for all solid solutions in magnetic fields up to 2 T. The effect of increasing Er amount in Ho1−xErxCo2 on the magnetic and magnetocaloric properties will be discussed.
KeywordsMagnetic properties of magnetically ordered materials (75.30.-m) Specific heat of magnetic materials (75.40.-s) Magnetoelectric effects (75.85.+t)
In recent years, the intermetallic compounds RCo2 (R = rare earth) belonging to the C15-type cubic Laves-phase crystal structure were the subject of extensive investigations because of their interesting intrinsic magnetic properties. A large magnetocaloric effect (MCE) observed in some of these compounds makes them suitable candidates for magnetic refrigeration technology [1, 2]. Attractive materials for magnetic refrigeration applications are compounds that display first-order magnetic phase transitions . In this context, RCo2 intermetallic compounds are one of the promising candidates. It has been found that in the case of RCo2 with heavy rare-earth elements (R = Ho, Dy and Er) the ferrimagnetic–paramagnetic phase transitions are of first order and these compounds exhibit a large magnetic entropy change [4, 5]. In the RCo2 compounds the magnetic moment on the Co sites is induced by an exchange interaction with the rare-earth moments [6, 7]. The Co moments range from 0.5 to 0.8 μB in the light rare-earth compounds, while they are almost constant (∼1.0 μB) in the heavy rare-earth compounds. The induced moments are aligned parallel to the 4f moments in the light rare-earth compounds and antiparallel in the heavy rare-earth compounds . From a large number of intermetallic compounds, HoCo2 and ErCo2 were selected and their Curie temperature is 75 and 32 K, respectively . It should be noted that MCE properties of HoCo2 and ErCo2 were investigated. According to Tishin et al.  for HoCo2 the maximum ΔTad at 84 K reaches 10 K for a field change μ0H of 10 T. Giguere et al.  reported the maximum ΔTad=14.4 K for ErCo2 at 33 K for μ0H=14 T. In the case of Ho1−xErxCo2 some theoretical calculation was performed by Oliveira et al. . They have shown that the introduction of Er ions decreases the magnetic ordering temperature of the pseudobinary compounds and the nature of the magnetic phase transitions continues to be first-order for all Er concentrations.
In the present work, the influence of the partial replacement of Ho by Er in Ho1−xErxCo2 solid solutions on the phase structure and MCE has been studied experimentally.
2 Experimental Details
Polycrystalline samples of Ho1−xErxCo2 (0.1≤x≤0.5) solid solutions were prepared by arc-melting stoichiometric proportions of starting materials (of at least 99.9 % purity) on a water-cooled copper crucible under a high-purity argon atmosphere. The alloys were remelted four times to ensure good homogeneity. The mass losses after the melting were less than 1 wt%. The buttons obtained were wrapped in a tantalum foil, sealed in evacuated quartz ampules, and annealed at 700 °C for two weeks.
Specific heat measurements were performed in the temperature range of 2–300 K in applied magnetic fields of 1 and 2 T using Quantum Design PPMS 14 Heat Capacity System. X ray powder diffraction revealed that Ho1−xErxCo2 samples are single phase and have the cubic C15 structure (space group Fd-3m). The lattice parameters decrease with increasing the Er content from 0.7171 for x=0.1 to 0.7164 for x=0.5.
3 Results and Discussion
The best fit for the wide temperature range could be obtained by fixing the parameters γ=30 mJ/mol K2 for all the measured samples, while the Debye temperature increases from 240 to 280 K with the increase of Er content.
The effect of the partial replacement of Ho by Er in Ho1−xErxCo2 solid solutions on the structure and physical properties has been studied. It was confirmed that the main phase in all solid solutions is cubic C15 structure. The substitution of Er in Ho1−xErxCo2 results in the increase in the temperatures TSR and decrease in TC, as observed in the specific heat measurements. The TSR temperature increases from 24 K to 45 K, and TC decreases from 73 K to 50 K. The maximum MCE estimated in terms of isothermal magnetic entropy change reaches 7 K (magnetic field of 2 T) for x=0.1 and is highly attractive for magnetic cooling applications.
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