Observation of Griffiths Phase, Critical Exponent Analysis and High Magnetocaloric Effect Near Room Temperature at Low Magnetic Field in V doped La0.7Sr0.3MnO3

We report on observation of Griffiths phase, high magnetocaloric properties at low magnetic fields and temperature dependent critical exponents of La0.7Sr0.3VxMn1-xO3 (x=0, 0.05 & 0.1) perovskite bulk materials. The Curie temperature (TC) of pure La0.7Sr0.3MnO3 is seen to be 368.7 K and decreases towards room temperature (342.2 K) by 10 mol% vanadium doping at the Mn site. Vanadium doping leads to enhancement in magnetic entropy change (-SM) from 1 Jkg-1K-1to 1.41 Jkg-1K-1. Vanadium doping at Mn site leads to the formation of Griffiths phase, a magnetic disorder due to the co-existence of paramagnetic matrix and short range ferromagnetic clusters. X-ray photoelectron spectroscopy analysis confirm the presence of mixed valance V4+/V5+along with Mn3+/ Mn4+ ions contributing to various double exchange interactions. Nature of phase transitions and magnetic interactions are analyzed by evaluating critical exponents and. All the samples show second-order ferromagnetic (FM) to paramagnetic (PM) phase transition, confirmed from the modified Arrott’s plots and critical exponent analysis carried out using Kouvel-Fisher method. Enhancement in magnetic entropy change along with the decrease in Curie temperature towards room temperature by vanadium doping in the La0.7Sr0.3MnO3 oxides indicates the possible application of these materials for the magnetic refrigeration at low magnetic fields.

Thermomagnetic Correlation in La0.85-xBixNa0.15MnO3 Soft Ferromagnet due to Nonmagnetic Bi3+ Substitution

We report the structural, magnetic, and magnetocaloric properties of Bismuth (Bi)-substituted manganite La0.85-xBixNa0.15MnO3 (x=0, 0.1, 0.2, 0.25, and 0.3). X-ray diffraction data implicates the rhombohedral structure with $$ R\overline{3}c $$ R 3 ¯ c space group. Bi2O3 has helped in ensuring phase pure, densified compounds even at low sintering temperature and hence avoiding the evaporation of volatile sodium. The increase in grain size and decrease in magnetic transition temperature (TC) are due to the Bi chemical activity and electronic structure. The samples have shown indirect magnetic transformation from soft ferromagnet to canted ferromagnet/antiferromagnet with Bi. Griffiths phase-like behavior in the inverse magnetic susceptibility was observed for x=0.1; with further increase in Bi, the samples are found to develop the antiferromagnetic competing phase. The phenomenological model was used to model the thermomagnetic behavior of all the samples. The sample with x=0.1 shows an increase in magnetic entropy change upon Bi substitution and the maximum of magnetic entropy change is seen at 275K emphasizing its potential in room temperature magnetic refrigeration.