Microscopic Coexistence of Antiferromagnetic and Spin Glass States in Disordered Perovskites

Abstract

Disorder and frustration give rise to a multitude of exotic phenomena in magnetism. Spin glass, a randomly frozen state of spins, is one of the several magnetic ground states directly originating from the presence of these two ingredients. This ground state is experimentally observed in many magnetic materials including alloys, amorphous inter-metallics, and insulators. Long-range mean-field theory and numerical results also predict a spin glass ground state for systems containing disorder and frustration. These methods further predict another ground state where spin glass coexists with a long-range magnetic order. However, so far there is no unambiguous experimental data with which to study this ground state carefully. This thesis is devoted to the study of the magnetic ground states of disordered perovskites, particularly the coexistence of spin glass and long-range antiferromagnetic order. This work considers stoichiometric dilutions of magnetic ions in perovskite crystals. The major reason for doing this is to prevent any uncertainty due to chemical phase separation arising from non-stoichiometric dilution, which plagued all previous experiments. The compounds under study are: PbFe1/2Nb1/2O3 (PFN ), PbFe1/2Ta1/2O3 (PFT ), PbFe2/3W1/3O3 (PFW ), and PbCo1/3Nb2/3O3 (PCN ). PFN is a disordered antiferromagnet (TN = 143 K) in which magnetic Fe 3+ is diluted with 50% non-magnetic Nb ions. Below TSG = 12 K, the system undergoes a transition into spin glass state. In the present work, the exact magnetic ground state of PFN is probed using neutron scattering and Mossbauer spectroscopy. Neutron scattering confirms that the antiferromagnetic Bragg peak that sets in below TN persists into the spin glass state. However, intensity of the Bragg peak shows a slight reduction below ∼50 K. On a local scale, we observe that the magnetic hyperfine field seen in Mossbauer spectra rapidly increases below this temperature. Further, we find that the hyperfine field has dynamic fluctuations that slow down on cooling. These fluctuations vanish below the spin glass transition, and a homogenous ground state is established with a microscopic coexistence of antiferromagnetic and spin glass states (AFSG). This ground state can be described by the canting of spins in a conventional antiferromagnet where the angle of canting differs from site to site randomly. In this arrangement, the transverse components of the spins are randomly frozen giving rise to spin glass properties while the longitudinal components contribute to the mean antiferromagnetic structure. This is similar to the picture proposed by mean field theory in Heisenberg systems for a transition into a coexisting ferromagnetic and spin glass state. To better understand this ground state, the requirements for the AFSG state in these perovskites are probed by concentrating on the role of the non-magnetic ion and the concentration of the magnetic ion. PFT is a close relative of PFN in which the non-magnetic Nb is stoichiometri-