Décalage Doppler d'ondes de spin induit par un courant électrique

Nous présentons dans cette thèse une manière originale d'étudier le couplage par transfert de spin entre l'aimantation d'un métal ferromagnétique et un courant d'électrons polarisés en mesurant un décalage Doppler d'onde de spin induit par un courant électrique. Le passage d'un courant à travers une configuration non-uniforme de l'aimantation, telle qu'une onde de spin, est susceptible d'entraîner un transfert de spin entre le courant et l'aimantation locale. Dans le cadre de l'approximation adiabatique, où le courant de spin est en tout point colinéaire à l'aimantation locale, il en résulte simplement un décalage en fréquence des modes propres d'onde de spin. Ce décalage est proportionnel au produit vecteur d'onde - courant de spin, ce qui suggère l'analogie avec un décalage Doppler usuel. La mesure de ce décalage Doppler d'onde de spin induit donne directement le degré de polarisation en spin du courant électrique dans le volume du matériau.In this thesis, we propose a new way of studying the spin transfer, i.e. the coupling between a current of spin-polarized electrons and the magnetization of a ferromagnetic metal, by measuring a current-induced Doppler eect for the spin waves. A transfer of spin angular momentum is indeed expected when electrons ow through an inhomogeneous configuration of the magnetization, such as one provided by a spin wave. In the adiabatic approximation, for which the spin of the owing electrons aligns instantaneously with the magnetization, the spin wave modes are simply shifted in frequency. This frequency shift is proportional to the wave vector and the current density, which suggests an analogy with regular Doppler eects. The spin polarization of the electrical current in the ferromagnetic metal can be deduced directly from the measured current-induced spin-wave Doppler shift.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Nouveaux états du Si dans les multicouches Co/Si

Nous avons étudié les propriétés structurales et physiques des multicouches Co/Si déposées par pulvérisation cathodique à 90 K dans le but de limiter la diffusion aux interfaces. Nous avons obtenu des multicouches très bien cristallisées quoique polycristallines, formées de très grands grains > 300 nm et un mélange limité par rapport à celui des multicouches déposées à 300 K. Cela nous a permis d observer des phénomènes physiques nouveaux, originaux et spectaculaires dans ces multicouches : (1) Oscillation du couplage d échange magnétique en accord avec les calculs ab initio, (2) Oscillation de la résistance de faible à forte et (3) Oscillation de la rugosité de l interface, en fonction de l épaisseur de Si. La période des ces oscillations, bien corrélées entre elles, est courte de l ordre de 0.4 nm soit 2 ML. Cela nous a conduit de proposer une interprétation commune de ces résultats, basé sur le passage périodique au niveau de Fermi d un état de puits quantique dans la couche de Si.In this work we studied the structural and physical properties of the Co/Si multilayers deposited by sputtering at 90 K with the aim of limiting the interdiffusion at interfaces. We have obtained very well crystallized multilayers although polycristalline, formed of very big grains > 300 nm with limited interfacial mixing as compared to that of multilayers deposited at 300 K. It allowed us to observe new, original and spectacular physical phenomena in these multilayers: (1) Oscillation of the interlayer exchange coupling in agreement with ab-initio calculations, (2) Oscillation of the multilayer resistance from weak to strong and (3) Oscillation of the roughness of the interface, according to the thickness of Si. The period of these oscillations, which are well correlated between them, is short of the order of 0.4 nm (2 ML). This led to us to propose a common interpretation of these results, based on the periodic passage at the Fermi level of a quantum well state in the Si layer.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Quasi-2D XY Magnetic Properties and Slow Relaxation in a Body Centered Metal Organic Network of [Co-4] Clusters

Octahedral Co2+ centers have been connected by mu(3)-OH and mu(2)-OH2 units forming [Co-4] clusters which are linked by pyrazine forming a two-dimensional network. The two-dimensional layers are bridged by oxybisbenzoate (OBA) ligands giving rise to a three-dimensional structure. The [Co-4] clusters bond with the pyrazine and the OBA results in a body-centered arrangement of the clusters, which has been observed for the first time. Magnetic studies reveal a noncollinear frustrated spin structure of the bitriangular cluster, resulting in a net magnetic moment of 1.4 mu B per cluster. For T > 32 K, the correlation length of the cluster moments shows a stretched-exponential temperature dependence typical of a Berezinskii-Kosterlitz-Thouless model, which points to a quasi-2D XY behavior. At lower temperature and down to 14 K, the compound behaves as a soft ferromagnet and a slow relaxation is observed, with an energy barrier of ca. 500 K. Then, on further cooling, a hysteretic behavior takes place with a coercive field that reaches 5 Tat 4 K. The slow relaxation is assigned to the creation/annihilation of vortex-antivortex pairs, which are the elementary excitations of a 2D XY spin system

Synthesis, Structure, and Magnetic Properties of a New Eight-Connected Metal–Organic Framework (MOF) based on Co₄ Clusters

A hydrothermal reaction of cobalt nitrate, 4,4′-oxybis­(benzoic acid) (OBA), 1,2,4-triazole, and NaOH gave rise to a deep purple colored compound [Co₄(triazolate)₂(OBA)₃], <b>I</b>, possessing Co₄ clusters. The Co₄ clusters are connected together through the tirazolate moieties forming a two-dimensional layer that closely resembles the TiS₂ layer. The layers are pillared by the OBA units forming the three-dimensional structure. To the best of our knowledge, this is the first observation of a pillared TiS₂ layer in a metal–organic framework compound. Magnetic studies in the temperature range 1.8–300 K indicate strong antiferromagetic interactions for Co₄ clusters. The structure as well as the magnetic behavior of the present compound has been compared with the previously reported related compound [Co₂(μ₃-OH)­(μ₂-H₂O)­(pyrazine)­(OBA)­(OBAH)] prepared using pyrazine as the linker between the Co₄ clusters

Synthesis, Structure, and Magnetic Properties of a New Eight-Connected Metal–Organic Framework (MOF) based on Co₄ Clusters

A hydrothermal reaction of cobalt nitrate, 4,4′-oxybis­(benzoic acid) (OBA), 1,2,4-triazole, and NaOH gave rise to a deep purple colored compound [Co₄(triazolate)₂(OBA)₃], <b>I</b>, possessing Co₄ clusters. The Co₄ clusters are connected together through the tirazolate moieties forming a two-dimensional layer that closely resembles the TiS₂ layer. The layers are pillared by the OBA units forming the three-dimensional structure. To the best of our knowledge, this is the first observation of a pillared TiS₂ layer in a metal–organic framework compound. Magnetic studies in the temperature range 1.8–300 K indicate strong antiferromagetic interactions for Co₄ clusters. The structure as well as the magnetic behavior of the present compound has been compared with the previously reported related compound [Co₂(μ₃-OH)­(μ₂-H₂O)­(pyrazine)­(OBA)­(OBAH)] prepared using pyrazine as the linker between the Co₄ clusters

Spin-frustrated Complex, [Fe\u3csup\u3eII\u3c/sup\u3eFe\u3csup\u3eIII\u3c/sup\u3e(trans-1,4-cyclohexanedicarboxylate)₁.₅]\u3csub\u3e∞\u3c/sub\u3e: Interplay between Single-Chain Magnetic Behavior and Magnetic Ordering

A three-dimensional mixed-valent iron(II, III) trans-1,4- cyclohexanedicarboxylate, 1,4-chdc, coordination polymer, [FeIIFeIII-(µ4-O)(1,4-chdc)1.5]∞, 1, has been synthesized hydrothermally by mixing iron powder and 1,4-chdcH2 and investigated by X-ray diffraction, dc and ac magnetic susceptibility, and iron-57 Mössbauer spectroscopy over a wide range of temperatures. Single-crystal X-ray diffraction studies of 1 at 90(2), 293(2), and 473(2) K reveal a tetrahedral [Fell2(µ4-O)Felll2(µ4-O)]6+ mixed-spin-chain structure with no change in the P1- space group but with subtle changes in the Fe-0 and Fe Fe distances with increasing temperature. These changes are associated with the electron delocalization observed by Mossbauer spectroscopy above 225 K. Magnetic studies reveal three different magnetic regimes in 1 between 2 and 320 K. Above 36 K 1 is a one-dimensional ferrimagnetic-like complex with frustration arising from competing exchange interactions between the iron(ll) and iron(lll) ions in the chains. Between 36 and 25 K the interchain interactions are non-negligible and 1 undergoes three-dimensional ordering at 32.16 K but with some residual fluctuations. Below 25 K the residual fluctuations slow and eventually freeze below 15 K.; the small net moment of 0.22 µB per mole of 1 observed below 15 K may be attributed to a non-collinear or canted spin structure of the spins of the four iron ions in the [FeII2(µ4-O)Felll2-(µ4-O)]6+ chains. Below 32 K the Mössbauer spectra of 1 exhibit sharp sextets for both the iron(lll) and iron(ll) ions and are consistent with either a static long-range or a short-range magnetic ground state or a slow relaxation between two canted magnetic states that are indistinguishable at the observed spectral resolution. The 85 and 155 K spectra reveal no electron delocalization and correspond solely to fixed valence iron(ll) and iron(lll). Between 225 and 310 K the spectra reveal the onset of electron delocalization such that, at 295 to 310 K, 25, 25, and 50% of the iron in 1 is present as iron(ll), iron(lll), and iron(ll/lll) ions, respectively. The absence of any spectral line broadening associated with this electron delocalization and the coexistence of four doublets between 225 and 310 K indicate that the delocalization occurs through electron tunneling via vibronic coupling. The sudden increase in the tunneling rate beginning above about 260 K and the presence of a cusp in the magnetic susceptibility centered at about 275 K strongly suggest the existence of a charge order/disorder transition whose nature and order are discussed

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Size dependent dipolar interactions in iron oxide nanoparticle monolayer and multilayer Langmuir-Blodgett films

The dipolar interactions in monolayer and multilayer assemblies of iron oxide nanoparticles have been investigated as a function of the nanoparticle size. The magnetic properties of iron oxide nanocrystals of various sizes have been measured for particles as powders and assembled in mono-and multilayers by the Langmuir-Blodgett technique, and compared to the behavior of non-interacting nanoparticles. It is shown that increasing dipolar interactions lead to higher blocking temperatures and to deviation from the Neel-Brown law. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples. The effect of interactions increases strongly with the nanoparticle size in agreement with simulations, leading to an unusual behaviour for the larger particles assembled in monolayer, which could be a signature of a superferromagnetic state.Financial support was provided by the Agence Nationale pour la Recherche (ANR MAGARRAYS) and the Direction Générale de l’Armement (DGA). The authors thank Cedric Leuvrey for SEM pictures, Dris Ihiawakrim and Corinne Ulhaq for TEM pictures, Christophe Lefevre for XRD refinement, and Alain Derory for technical support with SQUID measurements

Spin-frustrated Complex, [FeIIFeIII(trans-1,4-cyclohexanedicarboxylate)1.5]∞: Interplay between Single-Chain Magnet Behavior and Magnetic Ordering

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