Study of Morin transition in hematite (a- Fe₂O₃) nanoparticles by Mössbauer spectroscopy
AdvisorHamdeh, Hussein H.
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The change in magnetic properties of hematite (a- Fe₂O₃) nanoparticles were investigated by observing the change of Morin transition fraction at different temperatures. X-ray diffraction, transimission electron microscope (TEM), vibrating sample magnetometer (VSM), and Mössbauer spectroscopy techniques applied to study the magnetic and structural properties of the sample. According to the XRD results the sample has a double phases with dominant hematite (a- Fe₂O₃) phase. The TEM analysis confirmed the prepared nanoparticles have a large size distribution with average diameter less than 20 nm. The Morin transition temperature of the sample was measured by both the magnetization measurements and Mössbauer study. The zero-field cooled (ZFC) and field-cooled (FC) magnetization measurements show the particles undergo a magnetic phase transition from a weakly ferromagnetic to antiferromagnetic phase at 250 K. Room temperature Mössbauer spectra show the superparamagnetic behavior of the materials where the magnetic moment fluctuates in all directions. Morin transition started just below (285 K) the room temperature. The Morin transition was found strongly temperature dependent and increased constantly with decreasing temperature. The magnetic and structural changes with changing the Morin transition fraction revealed important characterisitics about the sample. The Mössbauer spectra recorded at 140 K and below showed complete Morin transition, which indicates the materials are strongly antiferromagnetic. The average hyperfine magnetic field and isomer shift have similar relationship and both continuously increased with decreasing temperature. The weighted average Morin transition temperature for the sample obtained from Mössbauer studies is 265 15:8 K which is higher than the Morin transition found from the magnetization measurements (250 K) measured by VSM.
Thesis (M.S.)-- Wichita State University, College of Liberal Arts and Sciences, Dept. of Mathematics, Statistics, and Physics