Monica Sorescu

The crucial role of particle morphology in the magnetic properties of haematite

Haematite particles of four different morphologies (polyhedral, platelike, needlelike and disk shaped) were synthesized by the hydrothermal method. The morphology and average particle diameter (1.4, 7.4, 0.2, and 0.12 μm, respectively) were determined by transmission electron microscopy combined with electron diffraction. The haematite samples were studied by transmission Mossbauer spectroscopy in the temperature range 4.2–300 K. In all cases, a weak ferromagnetic (WF) phase was present above the Morin temperature of 230 K and found to coexist with an antiferromagnetic (AF) phase below this temperature. However, the populations of the two phases at 230 K were demonstrated to depend on the morphology of the particles. Moreover, the WF and AF phases exhibit a different dependence of the magnetic texture on temperature and particle morphology.

Effect of light on the magnetic properties of cobalt ferrite nanoparticles

We report variations in the coercivity of CoFe/sub 2/O/sub 4/ nanoparticles as a function of particle size, temperature and light intensity. For 30 nm particles, this change in was 2300 Oe at 10 K, 120 Oe at 170 K, for a light intensity of under 2 milliwatts. The remanent magnetization was nearly unchanged by illumination. A simple model of optical absorption followed by electron transfer between Co/sup 2+/ and Fe/sup 3+/ ions is proposed.

MOSSBAUER AND MAGNETIC STUDY OF SUBSTITUTED MAGNETITES

Co, Ni, Mn, Cr, and Cu substituted magnetites were prepared by the hydrothermal method at 300 °C, with concentrations ranging from 8.2% to 12.5%. Transmission electron microscopy determined the average particle diameter 〈Φ〉 to be in the hundred of nm range and the morphological modifications induced by the various substitutions employed. Hysteresis loop measurements were performed to determine the coercive field Hc, and saturation magnetic moment ms. While Hc decreased with increasing 〈Φ〉, the particle shape was found to play an important role in explaining the dependence of ms on 〈Φ〉. Transmission Mossbauer spectroscopy was used to determine the site preference of the substitutions and their effect on the hyperfine magnetic fields. The room-temperature Mossbauer spectra were analyzed assuming a random distribution of substitutents using the binomial distribution from the ionic crystal point of view. Superparamagnetic particles were observed at room temperature in the case of Cu and Cr substituted magnetites.

A new method for direct determination of the recoilless fraction using a single room-temperature Mössbauer measurement of a two-foil absorber

Abstract A new method is proposed for the measurement of the recoilless fraction from a Mossbauer experiment performed at room temperature. The method relies on the use of a two-foil absorber, providing simultaneous information on two independent materials. In the first part of the experiment, the method was tested for the absence of thickness effects by recording spectra with the foils interchanged. The absence of topological effects was also examined using the same geometry with a split-area absorber. The method was then checked by deriving the ratio of resonant nuclei in natural iron and stainless steel AISI 304 from the relative areas and chemical arguments. Finally, the method was applied to derive the value of the recoilless fraction in Metglas 2605 SC at room temperature. The proposed method is dramatically simpler and more precise than the conventional method based on the determination of the Debye temperature, using the temperature variation of the recoilless fraction and complicated equation plots.

Structural and magnetic properties of Fe50Co50 system

Abstract The Fe 50 Co 50 alloys were prepared by mechanical alloying of the Fe and Co powders using a high-energy ball mill. The Fe 50 Co 50 system was also obtained as a thin film by the pulsed laser deposition method. Both powder and thin film samples were characterized by X-ray diffraction and Mossbauer spectroscopy. The formation of the bcc-FeCo solid solution was investigated as a function of milling time. Both Fe 50 Co 50 systems reveal similar magnetic hyperfine fields of about 35 T. Typically, the powders have random spin orientation, while the thin film shows an out-of-plane magnetic texture.

From Magnetite to Cobalt Ferrite

We synthesized Fe3−xCoxO4 (x = 0−1) using the hydrothermal method in order to demonstrate the compositional modulation of magnetite to cobalt ferrite. Our Mössbauer spectroscopy results provided direct evidence for the presence of the Co substitution in the B sublattice, which was found to be accompanied by a systematic increase of the hyperfine magnetic field at these sites. The mechanism we propose relies on the substitution of Fe2+ by Co2+ in the B sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites on content x of cobalt substitution. The X-ray diffraction (XRD) determinations demonstrated a linear increase in the lattice parameter when going from magnetite to cobalt ferrite. For the particular value x = 0.1, we report that the two sublattices of magnetite become equally populated with Fe. For this particular value of the cobalt content, we obtained a thin film sample by laser ablation deposition and characterized its properties by XRD and conversion electron Mössbauer spectroscopy (CEMS).

Structural and magnetic properties of NiZn and Zn ferrite thin films obtained by laser ablation deposition

Laser ablation deposition has been used to synthesize nanoscale ferrite structures. Our investigations were performed on NiZn and Zn ferrite films deposited on silicon(100) substrates. Films produced by laser ablation at room temperature were annealed at 550°C for 1h. Other films were deposited directly at a 550°C substrate temperature without subsequent annealing. Complementary x-ray diffraction and superconducting quantum interference device magnetometry measurements helped identify the optimum laser ablation deposition conditions for obtaining the desired nanoferrite structures. From the hysteresis loops at 300 and 10K we identified the paramagnetic or ferromagnetic behavior of the films. The zero field cooled–field cooled (ZFC–FC) magnetization, M(T), curves yielded the value of the blocking temperature in both NiZn and Zn ferrite systems.

Influence of cobalt and nickel substitutions on populations, hyperfine fields, and hysteresis phenomenon in magnetite

In this work the magnetic properties of magnetite powders doped with Co and Ni are investigated as a function of dopant concentration. Two sets of Fe3−xTxO4 powders with T=Co and Ni, x=0–1 were prepared using a hydrothermal method, with particle sizes of about 1 μm. The Mossbauer measurements revealed that both Co2+ and Ni2+ ions are located mostly on the octahedral sites, affecting the hyperfine fields and relative populations of both sites. In the case of Co-doped magnetite, the hyperfine magnetic fields increase almost linearly with increasing cobalt content. In the case of Ni-doped magnetite, the influence of annealing temperature during preparation was studied. For both subcritical and critical temperatures, the hyperfine fields of the tetrahedral and octahedral sites are larger than those corresponding to the magnetite powder. Bulk magnetic properties of these powders were studied by means of hysteresis loops recorded at 4.2 K in an applied field of 1.5 T. The results are compared with the pure magn...

The role of magnetostriction in pulsed laser irradiation of amorphous alloys

Abstract Samples of Fe 66 Co 18 B 15 Si 1 , Fe 40 Ni 38 Mo 4 B 18 and Fe 72.6 Cr 22 Al 4.8 Si 0.3 Y 0.3 metallic glasses were irradiated with a pulsed alexandrite laser ( λ =750 nm, τ =60 μs) using different laser fluences. Irradiation-induced structural and property modifications were characterized using Mossbauer spectroscopy. Complementary information was obtained from hysteresis loop measurements and scanning electron microscopy (SEM). The experimental data obtained demonstrate the key role played by the magnetostriction constant in explaining the mechanism of irradiation-induced phase transformations in amorphous magnets.

Determination of the recoilless fraction in iron oxide nanoparticles using the two-lattice method

We propose a two-lattice method for direct determination of the recoilless fraction using a single room-temperature transmission Mössbauer measurement. The method is first demonstrated for the case of iron and metallic glass two-foil system and is next generalized for the case of physical mixtures of two powders. We further apply this method to determine the recoilless fraction of hematite and magnetite particles. Finally, we provide direct measurement of the recoilless fraction in nanohematite and nanomagnetite with an average particle size of 19 nm. A list of values obtained for the recoilless fraction in various materials using the two-lattice method is given.