Multiferroic properties of orthorhombically distorted perovskite rare-earth chromites, such as HoCrO 3 , are being investigated extensively in recent years. In the present work, we report on the effect of Fe substitution on the magnetic properties in HoCrO 3 thin films. Thin films of nominal compositions with HoCrO 3 and HoCr 0.7 Fe 0.3 O 3 were fabricated via a solution route on platinized silicon substrates. Structural properties of the films were characterized by Xray diffraction and Raman spectroscopy. The surface morphology and cross-sections of the films were examined using scanning electron microscopy. Optical band gaps of these films are found to be 3.45 eV and 3.39 eV, respectively. The magnetization measurement shows that the Néel temperatures (where Cr 3+ orders) for the HoCrO 3 and HoCr 0.7 Fe 0.3 O 3 films are 134 and 148 K, respectively. In a magnetic field of 2 T, the maximum entropy change and relative cooling power, two parameters to evaluate the magnetocaloric properties of a material, were 0.813 J/kg K at 11 K and 21.1 J/kg for HoCrO 3 film, in comparison with 0.748 J/kg K at 15 K and 26.8 J/kg for HoCr 0.7 Fe 0.3 O 3 film. To our knowledge, this is the first work exploring the band gap and magnetocaloric properties of rare-earth chromite thin films. These findings should inspire the development of rare-earth chromite thin films for temperature control of nanoscale electronic devices and sensors in the low temperature region (<30 K).
Magnetic and magneto-caloric properties of polycrystalline powder samples of HoCrO3 with four different particle sizes are reported here. The samples were prepared by citrate method and were annealed at 700, 900, 1100, and 1300 °C to yield average particle sizes of 60 nm, 190 nm, 320 nm, and 425 nm, respectively, as determined by the analysis of X-ray diffraction patterns and images obtained by scanning electron microscopy. Additional structural characterization was done using Raman spectroscopy. Measurements of the magnetization of the samples were done from 5 K to 300 K in magnetic fields up to 70 kOe. Analysis of the temperature dependence of the paramagnetic susceptibility in terms of the modified Curie-Weiss law, including the Dzyaloshinsky-Moriya (DM) interaction, show small but systematic changes in the Néel temperature TNCr of Cr3+ ions, exchange constant J, and the DM interaction with variation in particle size. However, below TNCr the largest size-dependent effects are observed at 5 K, and the measured magnitudes of coercivity field HC are 1930, 2500, 4660, and 7790 Oe for the 60 nm, 190 nm, 320 nm, and 425 nm size particles, respectively, which can be interpreted by a single domain model. Enhancement of the particle size gives about a fourfold increase in the magnitude of the energy product, HC * MS, where MS is the saturation magnetization. However, as the particle size rises, an opposite trend is observed in the max magnetic entropy (ΔSM = 8.73, 7.22, 7.77, and 6.70 J/kg K) and the refrigerant capacity (RC = 388, 354, 330, and 310 J/kg) for the 60 nm, 190 nm, 320 nm, and 425 nm size particles, respectively. These results suggest ways to optimize the properties of HoCrO3 for applications in magnetic storage and magnetic refrigeration.
The structure, magnetic, and magnetocaloric (MC) properties of orthorhombic nanocrystalline GdCrO3 with six particle sizes: ⟨d⟩ = 87, 103, 145, 224, 318, and 352 nm are reported. The particle size was tailored by annealing under different temperatures and estimated by scanning electron microscopy. With increase in ⟨d⟩, Goldschmidt tolerance factor t, orthorhombic strain s, and out-of-plane Cr–O1–Cr bond angle first decrease, reaching minimum values for ⟨d⟩ = 224 nm, and then increase for sample with ⟨d⟩ = 318 nm and 352 nm, thus showing a V-shaped variation. Temperature dependence of the magnetization (M) reveals an antiferromagnetic transition at T N Cr ∼ 168 K for ⟨d⟩ ⩾ 224 nm and T N Cr ∼ 167 K for ⟨d⟩ < 224 nm and an essentially d-independent spin-reorientation at T SR = 9 K. M measured at 5 K and 7 T first increases with increase in ⟨d⟩, reaching maximum value for sample with ⟨d⟩ = 224 nm, and then decreases for samples with ⟨d⟩ = 318 nm and 352 nm, showing an inverted-V variation with ⟨d⟩. Similar ⟨d⟩-dependence is observed for the magnetic entropy change (MEC) and relative cooling power (RCP) showing a close relationship between the structural and magnetic properties of GdCrO3 nanoparticles investigated here. The 224 nm sample with the minimum values of t, s, and Cr–O1–Cr bond angle exhibits the maximum value of MEC (−ΔS) = 37.8 J kg−1 K−1 at 5 K under a field variation (ΔH) of 7 T and its large estimated RCP of 623.6 J Kg−1 is comparable with those of typical MC materials. Both (−ΔS) and RCP are shown to scale with the saturation magnetization M S, suggesting that M S is the crucial factor controlling their magnitudes. Assuming (−ΔS) ∼ (ΔH) n , the temperature dependence of n for the six samples are determined, n varying between 1.3 at 5 K to n = 2.2 at 130 K in line with its expected magnitudes based on mean-field theory. These results on structure-property correlations and scaling in GdCrO3 suggest that its MC properties are tunable for potential low-temperature magnetic refrigeration applications.
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