153 research outputs found
Particle orientation distribution in Y-Fe2O3magnetic tapes by Mössbauer and hysteresis loop measurements
The particle orientation in several Y-Fe2O3magnetic tapes has been quantitatively evaluated by using the data of both Mössbauer and hysteresis loop measurements performed in the three orthogonal directions. A texture function has been obtained as a development of real harmonics. The profile of the texture function gives the quality of the different magnetic tapes. A different degree of particle orientation at the surface of the tape is evidenced by means of conversion electron Mössbauer spectra
Modelling exchange bias in core/shell nanoparticles
We present an atomistic model of a single nanoparticle with core/shell
structure that takes into account its lattice strucutre and spherical geometry,
and in which the values of microscopic parameters such as anisotropy and
exchange constants can be tuned in the core, shell and interfacial regions. By
means of Monte Carlo simulations of the hysteresis loops based on this model,
we have determined the range of microscopic parameters for which loop shifts
after field cooling can be observed. The study of the magnetic order of the
interfacial spins for different particles sizes and values of the interfacial
exchange coupling have allowed us to correlate the appearance of loop
asymmetries and vertical displacements to the existence of a fraction of
uncompensated spins at the shell interface that remain pinned during field
cycling, offering new insight on the microscopic origin of the experimental
phenomenology.Comment: 7 pages, 3 figures. Contribution presented at HMM 2007 held at Napoli
4-6 June 2007. To be published in J. Phys. Condens. Matte
Monte Carlo simulation study of exchange biased hysteresis loops in nanoparticles
We present the results of Monte Carlo simulations of the magnetic properties
of a model for a single nanoparticle consisting in a ferromagnetic core
surrounded by an antiferromagnetic shell. The simulations of hysteresis loops
after cooling in a magnetic field display exchange bias effects. In order to
understand the origin of the loop shifts, we have studied the thermal
dependence of the shell and interface magnetizations under field cooling. These
results, together with inspection of the snapshots of the configurations
attained at low temperature, show the existence of a net magnetization at the
interface which is responsible for the bias of the hysteresis loops.Comment: 9 pages, 3 figures embedded. To be published in Physica
Interaction effects and energy barrier distribution on the magnetic relaxation of nanocrystalline hexagonal ferrites
The static and dynamic magnetic properties of nanocrystalline BaFe10.4Co0.8Ti0.8O19 M-type doped barium
ferrite were studied in detail to clarify the effect of interactions on the magnetic relaxation of an assembly of
small particles. The logarithmic approximation was unable to account for the magnetic relaxation of the
sample. Interaction effects were analyzed from the low-field susceptibility, DM plots and the time dependence
of thermoremanence, indicating that demagnetizing interactions led to an enhancement of both the relaxation
rate at low temperatures and the amount of the lowest energy barriers. It is thus suggested that care should be
taken when analyzing thermoremanent data at low temperature, in order not to confuse these experimental
findings with the signature of macroscopic quantum tunneling
Aggregation state and magnetic properties of magnetite nanoparticles controlled by an optimized silica coating
The control of magnetic interactions is becoming essential to expand/improve the applicability of magnetic nanoparticles (NPs). Here, we show that an optimized microemulsion method can be used to obtain homogenous silica coatings on even single magnetic nuclei of highly crystalline Fe3-xO4 NPs (7 and 16nm) derived from a high-temperature method. We show that the thickness of this coating is controlled almost at will allowing much higher average separation among particles as compared to the oleic acid coating present on pristine NPs. Magnetic susceptibility studies show that the thickness of the silica coating allows the control of magnetic interactions. Specifically, as this effect is better displayed for the smallest particles, we show that dipole-dipole interparticle interactions can be tuned progressively for the 7 nm NPs, from almost non-interacting to strongly interacting particles at room temperature. The quantitative analysis of the magnetic properties unambiguously suggests that dipolar interactions significantly broaden the effective distribution of energy barriers by spreading the distribution of activation magnetic volumes. Published by AIP Publishing
Mossbauer and magnetization studies of amorphous NdFeB compositionally modulated thin films
Several NdFeB compositionally modulated thin films are studied by using both conversion electron Mossbauer spectra and SQUID (superconducting quantum-interference-device) magnetometry. Both the hyperfine fields and the easy magnetization magnitude are not correlated with the modulation characteristic length (lambda) while the magnetization perpendicular to the thin-film plane decreases as lambda increases. The spectra were recorded at room temperature being the gamma rays perpendicular to the substrate plane. The magnetization measurements were recorded by using a SHE SQUID magnetometer in applied magnetic fields up to 5.5 T and in the temperature range between 1.8 and 30 K
Temperature dependence of the magnetization processes in Co/Al oxide/Permalloy trilayers
The magnetization process of Co/Al oxide/Py trilayers and its evolution with the temperature have been analyzed. The particular behavior of the Co layers, including the shift of the hysteresis loops and a coercivity increase with the decrease of temperature, is related with the apparition of a CoO layer at the Co/Al-oxide interface
Acoustic emission across the magnetostructural transition of the giant magnetocaloric Gd5Si2Ge2 compound
We report on the existence of acoustic emission during the
paramagnetic-monoclinic to ferromagnetic-orthorhombic magnetostructural phase
transition in the giant magnetocaloric Gd5Si2Ge2 compound. The transition
kinetics have been analyzed from the detected acoustic signals. It is shown
that this transition proceeds by avalanches between metastable states.Comment: 5 pages, 4 figure
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