arXiv subjects

François Vernay

Publications and source records attributed to François Vernay.

4 recordsLinked to original sources

arXiv2026

Optimizing the magnetic hyperthermia efficiency from the structural dependence of dipolar interactions in magnetic nanoparticles assemblies

Magnetic hyperthermia, based on the conversion of the magnetic energy of magnetic nanoparticles (MNP) under an alternating external field into heat, presents a great interest, especially because of important applications such as promising advances in cancer treatment. The underlying frequency-dependent response of the MNP assembly under the excitation field depends on the first hand on the individual properties of the MNP such as their size, shape and magnetocrystalline anisotropy and on the other hand on collective properties generated by interactions between MNP. Due to the long-range dipolar component of the latter, the first consequence is the so-called demagnetizing effects, making the response dependent on the external sample shape. This is the reason for studying the properties of MNP in assemblies of anisotropic external shapes such as films, chains, or elongated aggregates. In the present work, using a recently developed Time Quantified Monte Carlo (TQMC) framework for dynamic simulations, we focus more precisely on the importance of the local structure in such anisotropic aggregates to the dipolar interactions contribution of the response to the external field. Here the local structure is understood as both the location of the nanoparticles and the distribution of their easy axes. We show that the latter must be determined in a self-consistent way starting with the equilibrium structure of a ferrofluid modeled as the dipolar hard sphere fluid.

cond-mat.mtrl-sci↗

arXiv2016

Ferromagnetic resonance of coupled shifted chains of nano-elements

We investigate the FMR characteristics of a magnetic dimer composed of two chains of iron nanoparticles coupled by dipolar interactions. The latter are treated beyond the point-dipole approximation taking into account the finite size and arbitrary shape of the nano-elements. The resonance frequency is calculated as a function of the amplitude of the applied magnetic field and the resonance field is computed as a function of the direction of the applied field, varied both in the plane of the two chains and perpendicular to it. We highlight a critical value of the magnetic field which marks a state transition that should be important in magnetic recording media.

cond-mat.mes-hall↗

arXiv2012

Antiferromagnetic spin-S chains with exactly dimerized ground states

We show that spin S Heisenberg spin chains with an additional three-body interaction of the form (S_{i-1}S_{i})(S_{i}S_{i+1})+h.c. possess fully dimerized ground states if the ratio of the three-body interaction to the bilinear one is equal to 1/(4S(S+1)-2). This result generalizes the Majumdar-Ghosh point of the J_1-J_2 chain, to which the present model reduces for S=1/2. For S=1, we use the density matrix renormalization group method (DMRG) to show that the transition between the Haldane and the dimerized phases is continuous with central charge c=3/2. Finally, we show that such a three-body interaction appears naturally in a strong-coupling expansion of the Hubbard model, and we discuss the consequences for the dimerization of actual antiferromagnetic chains.

cond-mat.str-el↗

arXiv2011

Theory of inelastic light scattering in spin-1 systems: resonant regimes and detection of quadrupolar order

Motivated by the lack of an obvious spectroscopic probe to investigate non-conventional order such as quadrupolar orders in spin S>1/2 systems, we present a theoretical approach to inelastic light scattering for spin-1 quantum magnets in the context of a two-band Hubbard model. In contrast to the S=1/2 case, where the only type of local excited state is a doubly occupied state of energy $U$, several local excited states with occupation up to 4 electrons are present. As a consequence, we show that two distinct resonating scattering regimes can be accessed depending on the incident photon energy. For $\hbarω_{in}\lesssim U$, the standard Loudon-Fleury operator remains the leading term of the expansion as in the spin-1/2 case. For $\hbarω_{in}\lesssim4U$, a second resonant regime is found with a leading term that takes the form of a biquadratic coupling $\sim({\bf S}_{i}\cdot{\bf S}_{j)^{2}$. Consequences for the Raman spectra of S=1 magnets with magnetic or quadrupolar order are discussed. Raman scattering appears to be a powerful probe of quadrupolar order.

cond-mat.str-el↗