Magnetic Properties of LSMO/STO Thin Films: Magnetocaloric, Spin Dynamics and Magnetic Viscosity Investigations

Abstract

While other films are discussed, this dissertation will focus on detailed studies of the dc and ac bulk magnetometry in a characteristic 7.6 nm thin film of La0.7Sr0.3MnO3 grown on SrTiO3 (001). The dc bulk magnetometry measurements show that the sample is magnetically inhomogeneous. Temperature variation of magnetization (M vs. T) was measured in zero-field-cooled and field-cooled protocols to determine the blocking temperature TB in different applied magnetic fields. The field variation of TB is interpreted as the presence of embedded spin clusters of 1.4 nm. Moreover, the M vs. T measurements show the presence of negative magnetization in low applied fields of H = 50 Oe and 100 Oe. The field variation of magnetization was also measured by performing hysteresis loop (HL) measurements. These measurements were performed from 5 K to 400 K and the HL parameters are calculated to detect the magnetic state of the sample in this temperature region. These measurements show that this sample has superparamagnetic spin clusters with TB = 240 K and a ferromagnetic state with an ordering temperature TC = 290 K. Within the temperature region of TB ≤ T ≤ TC, the HL is inverted whereas negative remanence magnetization (NRM) appears in the mixed SPM and FM phases leading to the anti-alignment spin of both magnetic phases with respect to each other. The presence of SPM and FM phases produces energy barriers that create different magnetic states. Therefore, to under the predominant magnetization processes in this magnetically inhomogeneous sample, the magnetic viscosity measurements were performed. Magnetic viscosity S measurements were performed by cooling the sample H = 50 Oe to the measured temperature and magnetization was measured as a function of time in H = 0. Magnetization has logarithmic decay which from the fit to the magnetization: M (t) =M (0) – S ln(t), with time t up to 2 h, shows a peak at 230 K above which M (2 h) switches to negative values for temperatures up to the TC of the sample. Here it is argued that this negative magnetization results from a magnetic interaction between the SPM and FM phases. In an effort to explore applications of this magnetic phenomenon, magnetocaloric studies are reported here to study the thermodynamic properties of the magnetic phases. This study proves the sample is magnetically inhomogeneous since the magnetic entropy change (-D ) vs. temperature data has two broad peaks one close