Interaction d'échange | EPFL Graph Search

En physique, l'interaction d'échange (avec une énergie et un terme d'échange) est un effet en mécanique quantique qui se produit seulement entre particules identiques. L'interaction d'échange modifie la valeur moyenne de la distance entre deux ou plusieurs particules indistinguables lorsque leur fonctions d'onde se chevauchent. A cause de l’interaction d’échange cette distance va augmenter (pour les fermions) ou diminuer (pour les bosons) par rapport au cas où les particules seraient distinguables. En dépit d'être parfois appelée force d'échange par analogie avec la force classique, ce n’est pas une véritable force car il manque le vecteur de force. L'interaction d'échange est aussi appelée force d'échange, mais est à distinguer des forces d’échange produites par l'échange de porteurs de forces, comme la force électromagnétique produite entre deux électrons par l'échange d'un photon, ou la force forte entre deux quarks produite par l'échange d'un gluon. L'interaction d'échange est un

Rare Earth and Bimetallic Transition Metal Islands at Surfaces

Dimitrios Mousadakos

This thesis reports on the growth and on the magnetic characterization of nanostructures made of 3d-4d transition metals and rare earths. In all the experiments the nanostructures have been grown by atomic beam epitaxy (ABE) in ultra high vacuum conditions (UHV) by performing growth steps with the precise selection of external parameters as: the deposition flux F, the sample deposition temperature Tdep and the sample annealing temperature Tann . The morphological and magnetic properties of the nanostructures have been characterized by performing two in situ measurement techniques in our experimental setup, namely: scanning tunneling microscopy (STM) and magneto-optical Kerr effect (MOKE). The first aims of the thesis consisted in the formation of a supperlattice of rare earth on the graphene moiré pattern induced by the lattice mismatch of a single graphene layer grown on Ir(111) crystal surface. We report the first cluster supperlattice made of rare earths, namely Sm, where clusters nucleate in registry with the moiré pattern, forming a superlattice for deposition temperatures between 80 K and 110 K. Within this temperature range, the Sm supperlattice shows long-range order extending over several tens of nanometers and a cluster size distribution competing with the finest superlattices grown by ABE. Sm cluster spatial order is preserved, up to a coverage of 0.5 ML yielding a mean cluster size of 50 atoms, while for higher coverages coalescence starts. Moreover, Sm clusters with a mean size of 9 atoms are thermally stable up to Tann = 130 K from where on the order is lost and the density reduces progressively by cluster coalescence. Similar experiments carried out for Dy in the same deposition temperature range show the absence of cluster arrays. The second part focused on the enhancement of the MAE of bimetallic nanostructures, grown on Pt(111), having a Co-core by forming atomically sharp interlines and interfaces with three 4d elements, namely Ag, Pd and Rh. Our approach is based on the experimental measurement of the island magnetic susceptibility x(T) and morphology by means of MOKE and STM, respectively, combined with magnetic simulation analysis in order to quantify the different contributions to the island magnetic anisotropy. The monolayer capping for all studied elements contributes positively to the out-of-plane MAE of Co islands, with Pd giving the largest increase in the magnetic hardness. In addition, the capping with two Pd monolayers of pure Co islands maximizes the blocking temperature Tb . However, the lateral decoration of Co islands with all elements contributes negatively to MAE, with the Co/Pd shell giving the smallest effect and Rh the largest. Morphologically, Ag was found to produce a partial decoration of the Co island edges while Rh completely decorates laterally Co islands forming a rim of irregular shape without nucleating on top of Co islands for the used deposition parameters. Simulations of the magnetization reversal process by using coherent rotation (CR) and/or domain wall nucleation and propagation (DW) reversal models including different contributions to the MAE for atoms located at the interface (Ks) and at interline (Kp) and an exchange interaction between Co and 4d transition metals have reproduced the experimental x'(T) and x''(T) curves.

EPFL2017

SrCu2(BO3)(2) under pressure: A first-principles study

Frédéric Mila

Using density-functional theory (DFT) band-structure calculations, we study the crystal structure, lattice dynamics, and magnetic interactions in the Shastry-Sutherland magnet SrCu2(BO3)(2) under pressure, concentrating on experimentally relevant pressures up to 4 GPa. We first check that a ferromagnetic spin alignment shortens the nearest-neighbor Cu-Cu distance and reduces the Cu-O-Cu angle compared to the state with the antiferromagnetic spin alignment in the dimers, in qualitative agreement with the structural changes observed at ambient pressure as a function of temperature and applied field. Next, we determine the optimal crystal structures corresponding to the magnetic structures consistent with, respectively, the dimer phase realized at ambient pressure, the Neel ordered phase realized at high pressure, and two candidates for the intermediate phase with two types of dimers and different stackings. For each phase, we performed a systematic study as a function of pressure, and we determined the exchange interactions and the frequencies of several experimentally relevant phonon modes. In the dimer phase, the ratio of the inter- to intradimer couplings is found to increase with pressure, in qualitative agreement with various experiments. This increase is mostly due to the decrease of the intradimer coupling due to the reduction of the Cu-O-Cu angle under pressure. The phonon frequency of the pantograph mode is also found to increase with pressure, in qualitative agreement with recent Raman experiments. In the Neel phase, the frequency of the pantograph mode is larger than the extrapolated value from the dimer phase, again in agreement with the experimental results, and accordingly the intradimer coupling is smaller than the extrapolated value from the dimer phase. Finally, all tendencies inside the candidate intermediate phases are thoroughly worked out, including specific predictions for some Raman active phonon modes that could be used to pin down the nature of the intermediate phase.

AMER PHYSICAL SOC2020

Quantum dynamics in individual surface spins

Tobias Bilgeri

This thesis presents a combined experimental and theoretical study of the classical and quantum magnetization dynamics in single magnetic adatoms and molecules, and on the classical and quantum coherent control thereof. First, a detailed description of the methods and in particular electron spin resonance scanning tunneling spectroscopy is given. Next, we give a detailed account of how to experimentally update a standard scanning tunneling microscope for such experiments, which ultimately lead us to set up the world's second microscope with such capabilities. Subsequently, we show how we iteratively improved the setup, and how we were able to achieve a transmission of radiofrequency voltages to the tunneling junction with almost no additional losses beyond the intrinsic limitations of the materials used, a great improvement compared to most setups presently in use.Afterwards, we apply this technique to the metal-organic complex iron phthalocyanine. On MgO, we find it to form a spin-1/2 system, and we demonstrate quantum coherent control thereof, and investigate the magnetic interaction between surface-adsorbed molecules. Furthermore, we use this technique to experimentally demonstrate the magnetic stability of Dy adatoms on MgO. We find that this system, by several metrics, represents the worlds most stable single atom magnet discovered thus far. Using the capability of scanning tunneling microscopes to perform atomic manipulation, we ensemble atomically precise Dy nanostructures. By classically controlling the magnetic orientation of the Dy atoms, we are thus able to store information long-term in single adatoms and nanostructures even in absence of an external field, and create nanomagnets and local magnetic fields with unprecedented precision and stability.Finally, we investigate the magnetic properties of Ho/MgO. Depending on the precise adsorption site and charge configuration, we find either long lifetimes in the case of top-site 4f10 Ho, which was recently discovered as world's first single atom magnet, or fast sub-barrier relaxation due to strongly mixed quantum states. Using a novel theoretical approach, we are able to disentangle the different contributions of the individual species measured in ensemble-averaging X-ray absorption studies, and lift the discrepancies between previous X-ray and scanning tunneling microscopy based studies on Ho/MgO. By theoretically describing the perturbing effects of the experimental probes driving the spin degree of freedom out of thermal equilibrium, we are able to explain the measured dynamics in good agreement with the experimental data.

EPFL2021