Investigations on the Magnetic and Magnetocaloric Properties of Rare Earth Chromites Bulk Powders and Thin Films

As a safe, efficient, and environmentally friendly technology, magnetic refrigeration is an alternative solution to conventional vapor compression technology and requires materials with large magnetocaloric effect (MCE), such as rare-earth chromites (RCrO3). In this dissertation, the structural, magnetic, and magnetocaloric properties of bulk HoCrO3 has been investigated. In order to improve MCE, HoCrO3 samples with different particle sizes were synthesized and sample with smaller particle size shows larger MCE. Furthermore, Ho1-xGdxCrO3 (x=0, 0.33, 0.67, and 1) solid solutions were explored, and their structural, magnetic, and magnetocaloric properties show systematic change versus the substitution ratio x. Gd substitution considerably improves MCE of HoCrO3 and pure GdCrO3 bulk sample shows larger MCE than any other reported RMnO3, RCrO3, and RFeO3 bulk powder samples. Besides, a clear understanding of change with ionic radius, structural distortion on the magnetic and magnetocaloric properties of HoCrO3 were achieved by Tm (smaller ionic radii) and Gd (larger ionic radii) doping on the A-site. Gd doping increases the magnetic transition temperature of Cr3+ () and Tm doping decreases . The application of external hydrostatic pressure was found to enhance of HoCrO3, similar to the effect of Gd substitution. In addition, HoCrO3 thin films were fabricated via a solution route on platinized silicon substrates and characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. By doping Fe at the B-site, the HoCr0.7Fe0.3O3 films show larger than the HoCrO3 films. The large MCE in HoCrO3 bulk system, along with its tunability by particle size or chemical doping, make it suitable for magnetic refrigeration in low temperature region (below 30 K)