Some Properties of Supported Smallα−Fe2O3Particles Determined with the Mössbauer Effect

Abstract

Mossbauer spectra of Fe 67 in a-Fe 2 0 3 have been measured as a function of particle size and temperature. Bulk a-Fe 2 03 shows a change in the sign of the quadrupole interaction in going through the Morin transition temperature, 263°K. Analyses of the spectra show that the magnetization vector is in the с plane above the transition temperature and parallel to the с axis below it. In contrast, finely divided a-FtzOz particles, sup ported on a high-area silica, do not undergo a Morin transition. The spectra show that the magnetization vector remains perpendicular to the с axis at least down to 10°K. It is evident that the spins are pinned by the surface in small particles. When the particle size is less than 135 A, the room-temperature spectrum con sists only of a quadrupole-split center line corresponding to superparamagnetic a-Fe 2 03. As the particle size is gradually increased, a 6-line hyperfine spectrum of increasing intensity appears. This transition from superparamagnetic to ferromagnetic behavior, coupled with a measurement of the average particle size by x-ray line broadening, leads to a calculated value of the crystalline anisotropy constant in the с plane of (4.7db 1.1) X Ю 4 erg per cm 3 . A sample with an average particle size just below that required for ferromagnetic behavior at room temperature, when cooled, displays an increasing fraction of ferromagnetic material as the temperature is decreased. From these data the particle size distribution of the a-FezOz is determined. In addition, these data are used to calculate an independent value of the anisotropy constant of (4.1±1.1)X10 4 erg per cm 3 . Careful measurements on the series of samples with varying particle sizes show that the quad rupole splitting increases from the bulk value of 0.42 mm sec -1 to a value of 0.98 mm sec -1 for particles with an undetermined average size of less than 100 A.

Mössbauer Effect Study of Natural Egyptian Chalcopyrite

“…To confirms that pyrite was decomposed to a-Fe203 at 450, the ME spectrum of a pure natural pyrite sample was measured after heating the sample to 450 O C and the same magnetic pattern was obtained. The ME spectrum of the same sample at liquid nitrogen temperature showed change in the sign of the quadrupole splitting due to Morin transition [15]. This insure that the magnetic pattern which was concluded that Egyptian chalcopyrite ore contains the different phases a, B, and y-CuFeS,, pyrite, sphalerite and sellite.…”

Magnetic dynamics of weakly and strongly interacting hematite nanoparticles

“…The quadrupole interaction strength and the intrinsic linewidths were fixed to be identical for all spectra in the temperature series and the change of the isomer shift with temperature due to the second order Doppler shift was accounted for using the Debye approximation with a Debye temperature of 500 K. 12,49 Simultaneous fits of this model to the whole temperature series of Mössbauer spectra were performed for both fixed values of 0 between 1ϫ10 Ϫ12 s and 5ϫ10 Ϫ10 s and for 0 as a free parameter. The variation of the quality of the fit, 2 , as a function of ln( 0 ) had a parabola-like minimum. This variation was used for an estimation of the uncertainties on the parameters from the fit.…”

“…It is convenient to express the interaction strength in terms of 2 . This is easily carried out in Eqs.…”

“…Within the last few decades the magnetic properties of nanocrystalline hematite have therefore attracted considerable attention. [2][3][4][5][6][7][8][9][10][11][12] Recently a sample of hematite nanoparticles with a size of about 16 nm, prepared by heating of Fe(NO 3 ) 3 •9H 2 O, was studied by magnetization measurements, x-ray and neutron diffraction, and Mössbauer spectroscopy. 12 The nanocrystalline hematite had a spontaneous magnetization of about 0.3-0.4 J T Ϫ1 kg Ϫ1 which is only slightly enhanced compared to the value for polycrystalline bulk WF hematite ͑0.3 J T Ϫ1 kg Ϫ1 ).…”

Oriented attachment and exchange coupling ofα−Fe2O3nanoparticles

“…22,24 In ␣-Fe 2 O 3 nanoparticles the spins are confined to lie in the low anisotropy ͑001͒ plane. 25,26 Within this plane, the sublattice magnetization directions can easily rotate while rotation out of the plane requires much more energy. Therefore, superparamagnetic relaxation mainly takes place within the ͑001͒ plane.…”