Magnetic, electrical and structural properties of doped manganite, ferrite and nickelate perovskites
Multifunctional materials with novel magnetic and electric properties have attracted intense research interest due to prospects in technological applications as well as understanding of fundamental physics. Perovskite materials with ABO3 structure belong to one of the most interesting and vastly studied families by virtue of their rich magnetic and electrical properties. In the present thesis, efforts have been made to investigate the magnetic, electrical, and structural properties of A and B-site doped perovskites. In the beginning, a general introduction of perovskite materials is discussed. It is followed by some of important results of my thesis work on A and B site doped perovskite and double perovskites. Effects of size mismatch at B-site were investigated by doping Mn-site with Ni cations in Ho2NiMnO6 compound. This induces B-site ordering which leads to double perovskite structure and ferromagnetic ordering at TC = 86 K. Ideal Curie – Weiss law fails to provide reasonable results in paramagnetic region and inverse magnetic susceptibility fits reasonably well with a modified Curie-Weiss law by incorporating the paramagnetic response of the rare earth sublattice separately. Possible reasons for this deviation are addressed in which, presence of heavy rare earth element (Ho) plays an important role. Griffiths phase pertaining to the Ni/Mn subsystem was ascertained. Two dielectric relaxations due to phononic and Maxwell – Wagner mechanisms were observed. Magnetocaloric refrigeration efficiency of this material is further investigated and it has been shown to be a potential magnetocaloric refrigerant. A solid solution of RFeO3 and RMnO3 offer much prospects due to its vastly different magnetic properties. A single crystalline phase is essential for such studies, and we were successful in growing single crystals of ErFe0.55Mn0.45O3 which order antiferromagnetically at 365 K with spin canting-induced weak ferromagnetic moment along c axis. Upon cooling, magnetization along c axis passes through zero at 266.4 K and becomes negative below this temperature and a spin reorientation occurs from Γ 4(Gx, Ay, Fz) to Γ 1(Ax, Gy, Cz) configuration in the temperature window of 255 to 258 K. Magnetic behavior is explained with spin configuration and interplay between net magnetization of individual Er and Fe/Mn sublattices which are oppositely coupled and have different temperature evolution. DC electrical resistivity and activation energy corresponding to the conduction mechanism along different axes change distinctly around the spin reorientation which signifies that transport of charge carrier is affected by spin configuration. To observe the effect of A-site doping on RFeO3 perovskites, Ho0.5Dy0.5FeO3 single crystals were grown. Two spin reorientations of Fe magnetic sublattice were evident viz. Γ 4(Gx, Ay, Fz) → Γ 1(Ax, Gy, Cz) → Γ 2(Fx, Cy, Gz) at temperatures of 49 and 26 K. As magnetic field along c axis increases, the sample resumes Γ 4 state in place of Γ 1 state. Along c axis, field- induced transition from Γ 1 to Γ 4 is feasible which is triggered at a certain critical field and is completed at another higher critical field. Evolution of the lower and higher critical fields with temperature remains same. Studies on hybrid organic-inorganic perovskite compounds have also been carried out on heterometallic [(CH3)2NH2]Mn0.5Ni0.5(HCOO)3 and [(CH3)2 NH2]Co0.5Ni0.5(HCOO)3 which were found to crystallize in trigonal space group R-3c at room temperature and order antiferromagnetically with weak ferromagnetism induced by spin canting at 17 and 8 K, respectively. Hydrogen bond ordering leads to spontaneous polarization and structural transition occurs from R-3c to Cc through mixed phase. This is reflected in impedance data also. A general conclusion to this study is presented at the end.
Host: Prof. Je-Geun Park