The spin reorientation and improvement of magnetocaloric effect in HoCr1-x Ga x O3 (0 ≤ x ≤ 0.5)

Highlights: • HoCr_1-xGa_xO₃ (0 ≤ x ≤ 0.5) compounds were prepared by solid state reaction method. • The crystal structure of Ga-doped HoCrO₃ was systematically studied. • Spin-phonon coupling can be mediated by Ga³⁺ doping. • The spin reorientation transition (Γ₂ → Γ₄) is triggered by Ga³⁺ doping in HoCrO₃. • The magneto-caloric effect is improved by Ga³⁺ doping. -- Abstract: The effect of non-magnetic Ga³⁺ ions doping at HoCrO₃ (HoCr_1-xGa_xO₃, 0 ≤ x ≤ 0.5) on the crystal structure, electronic structure, Raman spectra, and magnetic behaviours were systematically investigated. Most strikingly, a new spin-reorientation transition (Γ₂ → Γ₄ →Γ₂) is triggered by Ga³⁺ doping in HoCrO₃, and the spin configuration of Γ₄ is obviously displayed in ZFC and FC curves when x > 0.2. According to the XRD analysis, Ga³⁺ doping reduces the octahedral distortion and changes the lattice parameters, CrO and HoO bond lengths, and a turning point occurs at x = 0.2. Raman spectra further give evidence of the structural changes and suggest that spin-phonon coupling can be mediated by Ga³⁺ doping, illustrating the weakened effect of Ga³⁺ doping on the Ho and Cr interaction. The dilution of the competition between CrCr canted antiferromagnetic coupling and Ho-Cr antisymmetric coupling results in the appearance of spin configuration Γ₄. Moreover, the Griffiths phase can also be observed and gradually suppressed with doping. These complex magnetic transitions in HoCr_1-xGa_xO₃ are given in a magnetic phase diagram. From the application point of view, Ga³⁺ doping also improves the magneto-caloric effect (MCE) and refrigerant capacity (-ΔS_M = 10.74 J/KgK, and the RC is 416 J/K at 7 T and 10 K for HoCr_0.6Ga_0.4O₃), which sheds some light on how to optimize the MCE properties.