Magnetic Reversal Time in Open Long Range Systems

Topological phase space disconnection has been recently found to be a general phenomenon in isolated anisotropic spin systems. It sets a general framework to understand the emergence of ferromagnetism in finite magnetic systems starting from microscopic models without phenomenological on-site barriers. Here we study its relevance for finite systems with long range interacting potential in contact with a thermal bath. We show that, even in this case, the induced magnetic reversal time is exponentially large in the number of spins, thus determining {\it stable} (to any experimental observation time) ferromagnetic behavior. Moreover, the explicit temperature dependence of the magnetic reversal time obtained from the microcanonical results, is found to be in good agreement with numerical simulations. Also, a simple and suggestive expression, indicating the Topological Energy Threshold at which the disconnection occurs, as a real energy barrier for many body systems, is obtained analytically for low temperature

Magnetoelastic effects in Jahn-Teller distorted CrF2_2 and CuF2_2 studied by neutron powder diffraction

We have studied the temperature dependence of crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF$_2$ and CuF$_2$ by neutron powder diffraction in the temperature range 2-280 K. The lattice parameters and the unit cell volume show magnetoelastic effects below the N\'eel temperature. The lattice strain due to the magnetostriction effect couples with the square of the order parameter of the antiferromagnetic phase transition. We also investigated the temperature dependence of the Jahn-Teller distortion which does not show any significant effect at the antiferromagnetic phase transition but increases linearly with increasing temperature for CrF$_2$ and remains almost independent of temperature in CuF$_2$. The magnitude of magnetovolume effect seems to increase with the low temperature saturated magnetic moment of the transition metal ions but the correlation is not at all perfect

Magnétostriction, anomalie de dilatation et magnétostriction forcée du yig pur et dopé au ruthénium

The magnetoelastic coupling of a séries of pure and ruthenium-doped YIG single crystals has been studied by means of a capacitance dilatometer. The volume anomaly and forced magnetostriction near the Curie point are qualitatively interpreted in terms of a molecular field model. The thermal variations of the magnetostriction coefficients for doped crystals differ markedly from previous resonance measurements, and are well interpreted within the framework of a simple statistical model. The one ion model of magnetostriction fails in interpreting the values of the magnetostriction coefficients at 0K, for Ru3+ at octahedral sites.Le comportement magnétoélastique d'une série de cristaux de YIG pur et dopés au ruthénium a été étudié expérimentalement à l'aide d'un dilatomètre capacitif. L'anomalie de volume et la magnétostriction forcée autour du point de Curie ont été qualitativement interprétées dans le modèle du champ moléculaire. Les variations thermiques des coefficients de magnétostriction diffèrent notablement, pour les cristaux dopés, des résultats obtenus antérieurement par résonance, et s'interprètent très bien dans un modèle statistique simple. Il apparait que le modèle à un ion de la magnétostriction, basé sur l'approximation d'un champ moléculaire isotrope, ne s'applique pas au cas du Ru3+ en site octaédrique

Magnetostriction of an amorphous (Fe82B18)0.9La 5Tb5 alloy

The thermal variations of the spontaneous and forced magnetostriction of an amorphous (Fe82B18)0.9La5Tb 5 alloy are given. The room temperature magnetostriction is one order of magnitude smaller than the predicted value, which disproves the potential interest of this alloy as a magnetostrictive transducer. The thermal variation of λs is well described by the one-ion model for ferrimagnets.On donne les variations thermiques des magnétostrictions spontanée et forcée d'un alliage amorphe (Fe82B18)0,9La 5Tb5. A la température ambiante, la magnétostriction est un ordre de grandeur plus faible que la valeur prévue, ce qui ôte à cet alliage tout intérêt en tant que transducteur magnétostrictif. La variation thermique de λs s'interprète assez bien dans le modèle à un ion des ferrimagnétiques

On the free energy of a ferromagnet : anisotropy and rotation

Using the property of rotational invariance of the free energy, F can be expanded as the sum of Fa the magnetocrystalline energy, F me its derivative with respect to the symmetrical strains and F r its derivative with respect to the rotations. This latter term, always ignored, provides a simple analytical expression of the magnetic torque for any given direction of the magnetization.Mettant à profit la propriété d'invariance rotationnelle de l'énergie libre, on peut développer celle-ci, F, comme la somme de l'énergie magnétocristalline Fa, de sa dérivée par rapport aux déformations symétriques F me, et de sa dérivée par rapport aux rotations, Fr. Ce dernier terme, toujours négligé, fournit une expression analytique simple du couple magnétique pour n'importe quelle direction de l'aimantation

MAGNETOSTRICTION OF AMORPHOUS Co80-xMnxB20 RIBBONS

Thermal variation of magnetization and magnetostriction of CoMnB alloys with x = 10 and 20 is studied. λs/Î5/2 [[MATH]-1 (m)] is plotted vs. m2/Î5/2 [[MATH]-1 (m)] where m is the reduced magnetization : the origin of λs, is discussed ; for x = 10, annealing 1 hour at 580 K the as-quenched ribbon changes dramatically λs, from 3 to 27 x 10-8

Quadrupole interactions in cubic rare earth intermetallics

Cette vieille science qu'est la magnétoélasticité trouve actuellement un plein renouveau à propos de l'étude des composés cubiques de terres rares. Il est en effet nécessaire de développer un formalisme quantique, les hypothèses classiques tombant du fait de l'anisotropie de l'aimantation. La variation du coefficient magnéto-élastique est suivie à travers les séries RZn et RSb. Une meilleure compréhension des mécanismes d'aimantation est permise en considérant la magnétostriction et l'échange quadrupolaire.Magnetoelasticity in presence of crystalline electric field has recently received a great development. Indeed, cubic rare earth intermetallics exhibit large lattice distortions appearing through either a magnetostrictive process or a cooperative Jahn-Teller mechanism. Due to the anisotropic reduction of the moment, the classical theory fails and one has to develop a quantum treatment. The variation of the magnetoelastic coefficient is analyzed throughout the RSb and RZn series. Taking into account magnetostriction and quadrupolar exchange allows then a good description of the magnetization processes