Date of Graduation

2021

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mikel B. Holcomb

Committee Member

Wathiq Abdul-Razzaq

Committee Member

Tudor Stanescu

Committee Member

Matthew B. Johnson

Committee Member

Xueyan Song

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 to the TB and the other centered near T ~ TC. Detailed analysis of -D (T, H) vs. T data is used to determine the volume fraction of the magnetic phases which is in good agreement with the dead layer calculations by studying saturation magnetization of the sample. The relative cooling power of this film is less than that of nanoparticles and the reason why is discussed.

To understand the strength of interaction between the magnetic phases and spin dynamics of the sample, the ac magnetic susceptibilities and were performed. AC magnetic susceptibilities and were carried out to study the nature of magnetic phase transitions at TB and the TC of the sample in detail. The temperature dependence of and measured at ac frequencies in the range of 0.1-10 kHz shows a broad peak at TB associated with the SPM phase present in the 1.4 nm surface layer and the frequency-dependent peak near 270 associated with TC. The Cole-Cole analysis of temperature dependence of and is used to calculate the mean relaxation time which is shown to fit the Vogel-Fulcher law: t = with = 245 K close to the TB and = 270 K = TC, and = 1.2× s. The reported results in this dissertation shed light on the origin of magnetic dead layers and their magnetic properties that can provide an opportunity for new types of magnetic devices.

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