Spin amplification, reading, and writing in transport through anisotropic magnetic molecules

Inelastic transport through a single magnetic molecule weakly coupled to metallic leads is studied theoretically. We consider dynamical processes that are relevant for writing, storing, and reading spin information in molecular memory devices. Magnetic anisotropy is found to be crucial for slow spin relaxation. In the presence of anisotropy we find giant spin amplification: The spin accumulated in the leads if a bias voltage is applied to a molecule prepared in a spin-polarized state can be made exponentially large in a characteristic energy divided by temperature. For one ferromagnetic and one paramagnetic lead the molecular spin can be reversed by applying a bias voltage even in the absence of a magnetic field. We propose schemes for reading and writing spin information based on our findings.Comment: 5+ pages with 5 figure

Collective effects in spin-crossover chains with exchange interaction

The collective properties of spin-crossover chains are studied. Spin-crossover compounds contain ions with a low-spin ground state and low lying high-spin excited states and are of interest for molecular memory applications. Some of them naturally form one-dimensional chains. Elastic interaction and Ising exchange interaction are taken into account. The transfer-matrix approach is used to calculate the partition function, the fraction of ions in the high-spin state, the magnetization, susceptibility, etc., exactly. The high-spin-low-spin degree of freedom leads to collective effects not present in simple spin chains. The ground-state phase diagram is mapped out and compared to the case with Heisenberg exchange interaction. The various phases give rise to characteristic behavior at nonzero temperatures, including sharp crossovers between low- and high-temperature regimes. A Curie-Weiss law for the susceptibility is derived and the paramagnetic Curie temperature is calculated. Possible experiments to determine the exchange coupling are discussed.Comment: 9 pages, 13 color figures, published versio

Changement de phase structural dans RbCdf3 et TlCdf3

We have shown that a tetragonal distortion with c/a > 1 occurs at 124 K in RbCdF3 and at 191 K in TlCdF3. The anomalous behaviour of the elastic constant C44 is consistent with a slight rotation of the CdF6 octahedra. The study of both X-ray superstructure reflexions and Raman scattering spectrum allows us to retain the space group D184h for the low temperature phase. Two low frequency modes show the behaviour of soft modes.Nous avons mis en évidence dans les pérovskites fluorées RbCdF3 et TlCdF3 un changement de phase structural., La symétrie du cristal devient quadratique (c/a > 1) pour T < Tc (Tc = 124 K pour RbCdF3 et Tc = 191 K pour TlCdF3). Les anomalies d'élasticité observées sur C44 sont compatibles avec un mécanisme de distorsion par rotations des octaèdres. L'indexation des raies de surstructure du spectre de rayons X ainsi que l'analyse du spectre de diffusion Raman nous permettent de retenir le groupe D184h pour la phase basse température. Deux modes de basse fréquence ont dans ces composés un comportement de modes mous

INTRODUCTION TO THE STUDY OF DAMPING EFFECTS DUE TO MAGNETIC, ELECTRONIC PROPERTIES AND TO PHONONS INTERACTIONS

This introduction to theme V will be devided into two parts. The first part will be devoted to magnetic damping (7 papers) ; in the second part the interest will be focussed on electronic properties and phonons interactions (7 papers)

ATTÉNUATION HYPERSONORE ET ULTRASONORE, RELAXATION DU PARAMÈTRE D'ORDRE AU VOISINAGE D'UN CHANGEMENT DE PHASE INCOMMENSURABLE

Le comportement élastique en fonction de la température et de la pression (vitesse et atténuation) de 3 composés monocristallins présentant une transition de phase de type incommensurable permet d'illustrer les différences de comportement des fluctuations du paramètre d'ordre au voisinage de Ti. Dans le cas de la thiourée et du sélénate de potassium un mécanisme de relaxation de type Debye permet d'interpréter les résultats expérimentaux. Pour le tétramethyl ammonium zinc-chlorure la relaxation se produit à plus basse fréquence et les mouvements des domaines doivent jouer un rôle important.The elastic behaviour with temperature and pressure near the incommensurate phase transition of thiourea, potassium selenate and tetramethylammonium zinc-chlorure, is a good illustration of different relaxational mechanisms of order parameter. For thiourea and potassium selenate a Debye like relaxational mechanism give a good fit with experimental results. In the case of tetramethylammonium zinc-chlorure the relaxation process is probably related to domains motion

FREQUENCY DEPENDENCE OF SOUND ABSORPTION OF VITREOUS SILICA BETWEEN 10 MHz AND 35 GHz

We present new measurements of the attenuation of ultranonic and hypersonic waves for temperatures ranging from 10 to 300 K in silica glass. The results show that the relaxation process which is predominant for the absorption of ultrasonic waves is not sufficient for describing hypersonic attenuation. Anharmonic three-phonon interactions are proposed to explain the excess damping, and are seen to be in qualitative agreement with the observed temperature dependence above 300 K

FIRST COORDINATION SPHERE AND SPIN-STATE IN SPIN CROSS-OVER IRON AND COBALT COMPLEXES

Nous étudions par spectroscopie d'absorption des rayons X les transitions de spin de complexes du cobalt(II), du fer (II) et du fer(III). La contraction de la première sphère de coordination lors du passage haut-spin → bas-spin est mesurée par EXAFS pour tous les complexes. Le XANES des complexes du fer met en évidence les variations de structure électronique lors de la transition. La variation d'ordre local est confrontée aux résultats d'autres techniques pour discuter le caractère coopératif du phenomène.We study by X-Ray absorption spectroscopy the spin crossover for Co(II), Fe(II) and Fe(III) complexes. EXAFS allows to measure the first coordination sphere contraction through the thermally induced high-spin → low-spin transition for all the complexes. The XANES of the iron spectra is modified with the changes in the electronic structure during the transition. The modification of the short range order is compared to the results of other techniques to discuss the cooperative character of the phenomenon