H. Pascard
École Polytechnique
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Featured researches published by H. Pascard.
Journal of Applied Physics | 1997
Wolfgang Wernsdorfer; E. Bonet Orozco; B. Barbara; K. Hasselbach; A. Benoit; D. Mailly; Bernard Doudin; J. Meier; J.-E. Wegrowe; J.-Ph. Ansermet; N. Demoncy; H. Pascard; A. Loiseau; L. Francois; N. Duxin; M.P. Pileni
Low temperatures magnetization measurements of individual ferromagnetic particles and wires are presented (0.1<T(K)<6). The detector was a Nb micro-bridge-dc-SQUID, fabricated using electron-beam lithography. The angular dependence of the magnetization reversal could be explained approximately by simple classical micromagnetic concepts. However, our measurement evidenced nucleation and propagation of domain walls except for the smallest particles of about 20 nm. The switching field distributions as a function of temperature and field sweeping rate and the probabilities of switching showed that the magnetization reversal was thermally activated. These measurements allowed us to estimate the “activation volume” which triggered the magnetization reversal. Our measurements showed for the first time that the magnetization reversal of a ferromagnetic nanoparticle of good quality can be described by thermal activation over a single-energy barrier as originally proposed by Neel and Brown.
IEEE Transactions on Magnetics | 1981
M. Le Floc'h; J. Loaec; H. Pascard; A. Globus
After a review of previous work, the authors show their results obtained in the study of the effect of pressure on the magnetization mechanisms of polycrystalline soft ferrites up to 1.5 kbar and in a temperature range from 263°K to 573°K. Three kinds of pressures were applied : hydrostatic pressure, lateral pressure inducing stresses along the director circles of toroids and pressure perpendicular to the plane surfaces of toroids. All the results concerning reversible and irreversible magnetization mechanisms under hydrostatic pressure are explained from an unique idea: the change in the topography of domain walls in the grains due to random stresses from the hydrostatic pressure because of compressibility anisotropy. That was confirmed by the effect of directional stresses and by a pressure induced hysteresis. The origin of the compressibility anisotropy was found in the closed porosity.
Journal of Magnetism and Magnetic Materials | 1995
J.M. Greneche; J. Teillet; H. Pascard
Abstract Out-field and in-field Mossbauer experiments were performed at low temperature on several mixed nickel-cadmium ferrites. We report on a first analysis of the in-field Mossbauer spectra, which involve two independent discrete distributions of effective fields: the relative area of octahedral and tetrahedral ferric contributions suggests the existence of reversed magnetic moments located in octahedral sites. In addition, the line intensities of the in-field spectra give clear evidence for a collinear ferrimagnetic structure up to x = 0.30 and a canted spin structure for x > 0.30.
IEEE Transactions on Magnetics | 1990
M. Le Floc'h; A.M. Konn-Martin; H. Pascard
The transverse susceptibility was measured in polycrystalline soft ferrimagnetic ring samples. It is shown that the very peculiar shape of the transverse susceptibility curve comes form the combination of two magnetization processes: the 180 degrees domain-wall motions which give rise to the initial susceptibility, and the sudden rotation of domains magnetized perpendicular to the applied field when a certain threshold of the biasing field is reached. The temperature dependence of this threshold has been investigated and has shown a close connection with the effective anisotropy field. >
Journal of Magnetism and Magnetic Materials | 1995
J.M. Greneche; H. Pascard
Abstract Substituted iron garnets exhibiting very high magnetic permeability properties were investigated by 57Fe Mossbauer spectrometry. The temperature dependences of the hyperfine field and the quadrupole shift at the different iron sites suggest the existence of a distribution of spin reorientation transitions, distribution attributed to the cationic substitution. This phenomenon which is in qualitative agreement with permeability measurements, is attributed to the introduction of scandium and terbium cations in the iron and yttrium sublattices, respectively. The hyperfine data are consistent with a gradual change of the easy magnetization axis from the [100] to [111] directions with increasing temperatures. The cationic distribution resulting from the octahedral and tetrahedral iron site occupancies differs from that encountered in pure iron garnets.
IEEE Transactions on Magnetics | 1981
M. Le Floc'h; J. Loaec; A. Globus; H. Pascard
The effect of hydrostatic pressure (up to 1200 bar)on the reversible and irreversible magnetization mechanisms has been investigated in YIG samples. The relations between the reversible and irreversible parts of the magnetization, previously established for samples without defect inside the grains and no applied pressure, are shown to remain valid under pressure. The product of field parameters such as coercive force H c by magnetization parameters such as susceptibility remains constant also in samples under pressure; this fact permits a single curve, pressure independent, to be obtained. The interpretation is based on a hypothesis of a variation with the applied pressure of the domain wall topography.
IEEE Transactions on Magnetics | 1998
Wolfgang Wernsdorfer; E. Bonet Orozco; B. Barbara; A. Benoit; D. Mailly; N. Demoncy; H. Pascard; O. Kubo; H. Nakano
The combination of highly sensitive Superconducting Quantum Interference Device (SQUID) with high quality nanoparticles allowed us to check for the first time simple classical models proposed nearly 50 years ago by Neel, Stoner and Wohlfarth. For nanoparticles containing about 10/sup 5/ to 10/sup 6/ /spl mu/B the quantitative agreement with the Neel-Brown theory of thermal activated magnetization reversal allowed us to identify unambiguously the magnetization reversal mechanism as uniform rotation. In the case of insulating barium ferrite nanoparticles containing about 10/sup 5/ /spl mu//sub B/ and below 0.4 K, strong deviations from this model are found which are quantitatively in agreement with the predictions of the theory of macroscopic quantum tunneling of magnetization in the low dissipation regime.
Journal of Magnetism and Magnetic Materials | 1995
M. Le Floc'h; H. Pascard; E. du Tremolet de Lacheisserie
Abstract Effects of compressive stresses applied parallel to the magnetization direction have been investigated on ring-shaped high susceptibility garnets in which the sign of the saturation magnetostriction has been changed by rare earth ion substitutions. The results are consistent and agree with the expression E σ = − 3 2 λ s σ cos 2 θ .
Journal of Magnetism and Magnetic Materials | 1993
H. Pascard; J.M. Greneche
Abstract We have discovered a new combination in chemical composition in garnets showing high values of initial magnetic permeability. These garnets were found by using two magnetic phase transition phenomena in the same crystallographic structure: a spin reorientation transition and a ferrimagnetic transition. The critical temperatures are fixed by the introduction in the garnet Y 3 Fe 5 O 12 of terbium ions x in dodecahedral sites and scandium ions y in octahedral sites. Values of permeability from 10 3 to 10 4 have been measured in a board range of temperature.
Physical Review Letters | 1997
Wolfgang Wernsdorfer; E.B. Orozco; K. Hasselbach; A. Benoit; B. Barbara; N. Demoncy; A. Loiseau; H. Pascard; D. Mailly