Influence of Ti Doping on the Critical Behavior and Magnetocaloric Effect in Disordered Ferromagnets La_0.7Ba_0.3Mn_1−x Ti _x O₃

Abstract

The Ti-substitution influence on the magnetic and magnetocaloric properties of La_0.7Ba_0.3Mn_1−x Ti _x O₃ (x = 0.05 and 0.1) was investigated. Based on Banerjee’s criteria and Franco’s universal curves, we proved the existence of a second-order magnetic phase transition in the samples. Using the modified Arrott plot method, we determined the critical parameters T _C ≈ 245 K, β = 0.374 ± 0.013, γ = 1.228 ± 0.045, and δ = 4.26 ± 0.03 for x = 0.05, and T _C ≈ 169 K, β = 0.339 ± 0.001, γ = 1.307 ± 0.003, and δ = 4.78 ± 0.02 for x = 0.1. With these critical values, the predictable scaling behavior of the M(H) data above and below T _C proves that the calculated exponents are unambiguous and intrinsic. The values β = 0.374 for x = 0.05 and β = 0.339 for x = 0.1 suggest that the magnetic phase transition of the samples falls into the three-dimensional (3D) Heisenberg and 3D Ising universality classes, respectively, corresponding to short-range ferromagnetic (FM) order due to FM clusters in a wide temperature range even above T _C, as confirmed by electron spin resonance studies. In reference to the magnetocaloric effect around T _C, the magnetic entropy change reaches maximum values (|ΔS_max|) of about 4 and 3 J kg⁻¹ K⁻¹ for x = 0.05 and 0.1, respectively, for a magnetic field change 50 kOe. Magnetic field dependencies of |ΔS_max| obey a power function |ΔS_max(H)| ∝ H ⁿ, where exponent values n = 0.59 and 0.61 for x = 0.05 and 0.1, respectively, were determined from the relation n = 1 + (β-1)/(β + γ). The difference between the experimental n values and the theoretical value n = 2/3 of the mean field model is due to the presence of short-range FM order in the samples.