F. Lucari

Magnetic behaviour of γ-Fe2O3 nanoparticles by mössbauer spectroscopy and magnetic measurements

Abstractγ-Fe2O3 nanoparticles with varying state of dispersion in a polymer have been investigated by Mössbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10−2–104 Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mössbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.

Collective magnetic state in nanoparticles systems

Studies of static and dynamic properties of γ-Fe2O3 nanoparticles (4.7 nm) with interparticle interactions of increasing strength reveal a progressive change of regime from pure superparamagnetic, for very weak interactions, to collective for strong interactions. The collective regime presents a glass low-temperature state similar to the spin-glass state, but the glass transition shows distinct features.

Magnetic and electronic transport percolation in epitaxial Ge 1 – x Mn x films

Electronic transport and magnetic properties of Ge{sub 1-x}Mn{sub x}/Ge(100) films are investigated as a function of Mn dilution. Depending on x, characteristic temperatures separate different regimes in both properties. Resistivity exhibits an insulatorlike behavior in the whole temperature range and, below about 80 K, two distinct activation energies are observed. At a higher temperature value, T{sub R}, resistivity experiences a sudden reduction. The Hall coefficient shows a strong contribution from the anomalous Hall effect and, at T{sub R}, a sign inversion, from positive to negative, is recorded. The magnetic properties, inferred from magneto-optical Kerr effect, evidence a progressive decrease of the ferromagnetic long range order as the temperature is raised, with a Curie temperature T{sub C} not far from T{sub R}. The transport and magnetic results are qualitatively consistent with a percolation mechanism due to bound magnetic polarons in a GeMn diluted magnetic semiconductor, with localized holes [A. Kaminski and S. Das Sarma, Phys. Rev. B 68, 235210 (2003)].

Magneto-optical study of Mn ions implanted in Ge

Summary form only given. The synthesis of optimal materials for spin dependent electronics is the challenging goal of recent studies. Magnetic multilayers are the most popular solution, but a promising alternative is constituted by magnetic semiconductors. One of these is Mn:Ge which shows ferromagnetic properties and magnetoresistance phenomena below room temperature, attributed to Mn/sub x/Ge/sub 1-x/ clusters. In the present work, we investigate the properties of samples obtained by implanting Mn/sup +/ ions on Ge(100) at energy of 100 keV, with a dose of 2 x 10/sup 16/ at./cm/sup 2/. The temperature of the substrate was 300/spl deg/C. We report the results obtained in the as-prepared material, in the material annealed for 1 hour at 400/spl deg/C, and 600/spl deg/C.

Near infrared magnetooptical effects in Sb doped YIG

Abstract Measurements were made on Sb doped YIG in order to understand the dopant influence on the magnetooptical effects in iron garnets. The results are analyzed to determine the character of the symmetry of active centers, which are responsible for these effects.

Optical and magneto-optical properties of ferromagnetic full-Heusler films: Experiments and first-principles calculations

We report a joint theoretical and experimental study focused on understanding the optical and magneto-optical properties of Co-based full-Heusler compounds. We show that magneto-optical spectra calculated within ab initio density functional theory are able to uniquely identify the features of the experimental spectra in terms of spin resolved electronic transitions. As expected for

Magneto‐optical Kerr rotation on Fe‐Al2O3 granular thin films

3d

Magnetization measurements by magneto‐optical methods on films with Fe particles in an Al2O3 amorphous matrix

-based magnets, we find that the largest Kerr rotation for these alloys is of the order of 0.3\ifmmode^\circ\else\textdegree\fi{} in polar geometry. In addition, we demonstrate that (i) multilayered structures have to be carefully handled in the theoretical calculations in order to improve the agreement with experiments, and (ii) combined theoretical and experimental investigations constitute a powerful approach in designing new materials for magneto-optical and spin-related applications.

Annealing effects on YIG:Sn, YIG:Sb and YIG:Zr probed by measurements of magnetic linear dichroism

We made measurements of polar Kerr rotation (KR) on granular thin films, composed of iron particles, with different average diameter (50<Φ<100 A), dispersed in an amorphous Al2O3 matrix. The measurements were performed at 300 and 4 K in the wavelength interval 0.5–2.5 μm. The spectra observed show a decreasing tail in the visible region and then oscillations in the near infrared due to composition of the light beams reflected by the two surfaces of the sample. Values as high as 2.8° of KR in the visible are obtained when a magnetic field of 4 KOe is applied perpendicular to the surface of the sample. The KR signal was also measured as a function of the temperature at the specific values of wavelength corresponding to some maxima and minima of the spectrum. The KR intensity increases as the temperature decreases down to a characteristic temperature T*1, below which it remains constant. T*1 increases with increasing iron concentration and particle dimensions. This behavior is in agreement with previous resu...

Blocking temperature distribution in implanted Co–Ni nanoparticles obtained by magneto-optical measurements

We made magneto‐optical measurements (Faraday Rotation) on four different films with an approximate thickness of 0.5 μm obtained by cosputtering. The films are composed by Fe particles, with different average diameter in the range 50<Φ<100 A, dispersed in amorphous Al2O3. At the surface of each particle the presence of Fe2+ and a smaller quantity of Fe3+ ions were previously detected. Fe3+ ions were also found in large quantities at the surface of the whole sample. Our measurements are able to distinguish signals coming from the two different ions. We followed these signals by varying the temperature down to 4 K. We observed that the magneto‐optical response of Fe3+ ions remains constant, while that of Fe2+ increases with decreasing temperature (T) for T above a characteristic temperature Tc. Below Tc the Fe2+ response remains constant. Measured Tc values were 23, 29, 36, and 250 K for the four samples containing 45, 55, 60, and 70–85 wt. % iron, respectively. These results are comparable with those obtai...