Tuning ferromagnetism and spin state in La(1−x)AxCoO3 (A= Sr, Ca) nanoparticles

We investigate the structural, magnetic susceptibility, isothermal magnetization, and electronic properties of La(1−x)AxCoO3 (A= Sr, Ca; x= 0-0.2) nanoparticles. The Rietveld refinement of room temperature powder x-ray diffraction data shows that all the samples crystallize in the rhombohedral structure (space group R3¯c). We observe a significant increase in the ferromagnetic transition temperature as well as in the spontaneous magnetic moment with Sr/Ca substitution at the La site. Interestingly, the coercive field HC decreases/increases with Sr/Ca substitution [4.4 and 13.2 kOe for x= 0.2 (Sr) and 0.2 (Ca) samples, respectively] as compared to the x= 0 sample (7 kOe). X-ray photoemission spectra of core-levels confirm the oxidation state of the constituent elements. Moreover, we demonstrate that the Sr/Ca substitution increases the population of intermediate-spin (IS) and low-spin (LS) states of Co3+ and Co4+ ions, respectively; which tune the ferromagnetism in nanoparticles via double-exchange interaction. More importantly, the contribution of IS states of Co3+ ions and induced strain are significantly larger in Sr substituted samples than in Ca samples, which favours the interpretation of a Jahn-Teller distortion in the Sr samples. This is consistent as in contrast to Ca samples, the magnetic behavior of La(1−x)SrxCoO3 nanoparticles is significantly different than the counterpart bulk samples having same substitution level. Our results suggest the important role of hole carriers, induced strain and nano-size effect in tuning the spin-state and magnetism in La(1−x)AxCoO3 nanoparticles.