Magnetocaloric Properties and Microstructures of HoB2 and Nb-Substituted HoB2

TL;DR

This paper studies the effects of niobium substitution on the magnetic and structural properties of HoB2, a material with potential for magnetic cooling.

Contribution

The novel contribution is the synthesis and characterization of Nb-substituted HoB2 with detailed magnetocaloric and microstructural analysis.

Findings

Nb substitution leads to a smaller grain size and higher Curie temperature in HoB2.

Ho0.93Nb0.07B2 has a magnetic entropy change of 38.2 Jkg−1K−1 at 21 K under 10 T.

Despite minor phases, both samples show high relative cooling powers exceeding 600 Jkg−1.

Abstract

We report on the arc melt syntheses of HoB2 and Nb-substituted HoB2 polycrystalline ingots and their magnetocaloric and microstructural properties. XRD data and microstructural analysis reveal that a nominal 10% Nb addition during synthesis results in changes to unit cell parameters and grain morphology. Interpretation of the refined cell parameters using Vegard’s law shows that Nb substitutes into HoB2 with stoichiometry Ho0.93Nb0.07B2. Arc-melted products are polycrystalline bulk samples containing minor phases such as Ho2O3, Ho, and HoB4. Nb substitution results in a smaller grain size (~sub-micron) and a higher Curie temperature, TC, compared to HoB2. With a 10 T applied field, the maximum magnetic entropy, ΔSM, for HoB2 and for Ho0.93Nb0.07B2, is 46.8 Jkg−1K−1 and 38.2 Jkg−1K−1 at 18 K and 21 K, respectively. Both samples show second-order phase transitions. Despite high totals of minor phases (e.g., ~10 wt.% and ~25 wt.%), the calculated relative cooling powers are greater than 1300 Jkg−1 and 600 Jkg−1 at 10 T and 5 T, respectively. The magnetocaloric properties of both samples are consistent with Holmium boride compounds prepared via alternative methods.

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