Abdel-Fatah D. Lehlooh

Mössbauer and X-ray diffraction studies of heat-treated Fe3O4 fine particles

Abstract Samples of Fe3O4 fine particles heated at different temperatures have been studied using Mossbauer spectroscopy and X-ray diffraction. The fresh sample and that heated at 160°C both show superparamagnetic behavior. However, the Mossbauer spectra of samples heated at higher temperatures are all magnetically split. The Mossbauer data with and without the application of ∼ 0.4 T magnetic field perpendicular to the direction of propagation of the γ-rays indicate a gradual conversion of Fe3O4 to γ-Fe2O3 and then to α-Fe2O3 with increasing heating temperature, and a complete conversion to α-Fe2O3 in the sample heated at 550°C. The observed increase in the Bhf with heating temperature is attributed to particle growth. Also, X-ray data indicate particle growth with increasing heating temperature, and partial transformation to α-Fe2O3 in the sample heated at 350°C, and a total conversion in the sample heated at 550°C.

Mössbauer spectroscopy of Al substituted Fe in holmium iron garnet

A Mössbauer spectroscopy study was made on Ho3Fe5-xAlxO12 (x=0.0, 0.05, 0.7). X‐ray diffraction patterns indicate that the samples with x=0.0 and 0.05 have the garnet structure, while the sample with x=0.7 has an additional noncubic structural phase. The room temperature spectrum for samples with x=0.0 and 0.05 consists of two magnetic components corresponding to the octahedral and tetrahedral sites with hyperfine magnetic fields (Bhf) of 50 T and 40 T, respectively. For x=0.7 we observe a new magnetic component with Bhf= 45 T, a reduction in the intensity and broadening of the tetrahedral component, and the evolution of a nonmagnetic central component. These variations are evidently due to the addition of aluminium to the system. At liquid nitrogen temperature the samples with x=0.0 and 0.05 are nearly identical. It was also observed that the increase in Bhf for the octahedral site is smaller than that for the tetrahedral site as the temperature is lowered to 80 K.

Mössbauer spectroscopy of Fe3O4 ultrafine particles

Abstract Mossbauer spectroscopy is used to study a system of Fe3O4 ultra-fine particles at temperatures down to 5 K. The effective magnetic anisotropy constant, K, for the system is calculated using the method based on the superparamagnetic relaxation phenomenon, and that based on collective magnetic excitation model, and the results are 2.6 × 105 and 2.0 × 105 J/m3, respectively. These values are larger than those obtained by others for systems with larger mean diameters, implying a significant size dependence of K.

Mossbauer spectroscopy study of iron nickel alloys

Three samples of Fe100−x Ni x (with x = 30, 35 and 40) were prepared by arc melting technique. The Mossbauer spectra of the three samples were collected and analyzed. The spectrum of the sample with x = 30 consists of a singlet and a sextet. The singlet component which has isomer shift (IS = −0.08 mm/s) is attributed to a superparamagnetic phase. The value of the magnetic hyperfine field associated with the sextet component, 34.0 T, is consistent with that of α-Fe-Ni alloy. In the spectra of the other samples the central line disappears. The magnetic component, used in fitting the spectrum of the sample with x = 40 has a hyperfine magnetic field B hf = 30.0 T. This component is assigned to the high-spin γ-FCC Fe-Ni phase. Two magnetic components of 16.3 T and 27.3 T are used to fit the spectrum of the sample with x = 35. The 27.3 T component is associated with the typical high-spin γ-FCC Fe-Ni phase while the 16.3 T component is associated with a γ-FCC Fe-Ni phase with magnetic relaxation.

Mössbauer Spectroscopy Study of Fe-Si Solid Solution Prepared by Mechanical Milling

A specimen of Fe-Si solid solution is prepared by ball milling of proper amounts of the pure elements (3Fe:Si) for different milling times. X-ray diffraction and Mossbauer spectroscopy have been used to characterize the solid solution. The Mossbauer spectra show four different sites corresponding to Fe atoms in a bcc structure having 0, 1, 2 and 3 Si atoms in the 1st nearest-neighbor (nn) shell. The hyperfine magnetic field decreases by 31 kOe for each Si atom in the 1st nn shell. A magnetic component with hyperfine field around (180 kOe) characterized by a broadened sextet was observed which could be due to iron sites having more than 3 Si atoms in the 1st nn. A theoretical model based on the binomial distribution was adopted to analyze the data. Good agreement between the experimental and the theoretical hyperfine field distribution in the high hyperfine field region was found, and the silicon content in the disordered A2 phase is deduced from the parameters which give the best agreement.

Mössbauer study of magnetic phase transition in FeAl1−xCox

Mossbauer spectroscopic study of the alloy system FeAl1−xCox (0.25 < x < 0.45) at room temperature shows the onset of magnetic splitting at x = 0.31. The hyperfine field values for each alloy were used to obtain an average magnetic moment on the Fe atom. These values are discussed and compared with the calculated values using a simple model based on random atomic distribution.

Mössbauer spectroscopy study of GdFe6−xCoxGe6 alloy

Abstract Room temperature Mossbauer spectroscopy and X-ray diffractometry have been used to study GdFe6−xCoxGe6 alloys (with x=0, 1, 2 and 4) and the effect of partially substituting the cobalt for iron in the alloy. The X-ray diffraction study of the alloy indicates that its crystal structure is a single phase and does not change with this substitution, apart from a reduction in the lattice constant. The Mossbauer study shows that substituting cobalt for iron in the system significantly changes its magnetic properties. As cobalt is introduced, it occupies the iron sites randomly in accordance with the binomial distribution, leading to new magnetic components of the spectra with appreciably lower magnetic fields. The reduction in the hyperfine field with cobalt concentration is due to the fact that Co in this structural phase has zero moment. At higher cobalt concentrations the hyperfine field vanishes as the material becomes magnetically disordered at room temperature.