arXiv subjects

F. Millo

Publications and source records attributed to F. Millo.

6 recordsLinked to original sources

arXiv2026

Scalaron-driven Dark Matter during Warm Inflation via UV Freeze--in

We investigate the production of Dark Matter (DM) within the warm Higgs--Starobinsky (HS) inflation. By adopting the ultraviolet (UV) freeze--in mechanism -- where the DM number density is initially negligible but is populated over time through non-renormalizable interactions of defined mass dimensions -- we find that the DM production within the HS potential has a characteristic timing. For both DM masses, $m_\chi=1$ MeV and $m_\chi=100$ GeV, the yield of DM occurs close to the termination of inflation and its transition to the radiation dominated (RD) era. The present--day DM relic abundance is generated for both DM masses with non-renormalizable operators of mass dimension $D=\{8,9\}$ within strong dissipation regimes. The obtained results suggest that the form of the HS potential and the associated scalaron dynamics favor UV freeze--in DM production as the scalaron approaches the minimum of the potential, where energy transfer to the thermal bath becomes particularly efficient. This distinctive transition behavior differentiates the HS model from other inflationary models and advances our understanding of DM production.

hep-ph↗

arXiv2024

Experimental observation of vortex gyrotropic mode excited by surface acoustic waves

The traditional method for exciting spin-wave dynamics in magnetic materials involves microwave magnetic fields generated by current injection into inductive antennas. However, there is a growing interest in non-inductive excitation methods. Magneto-acoustic effects present a viable alternative, where strains produced by applying voltages to a piezoelectric substrate can couple to spin-waves in a magnetic film. Recently, it has been proposed that surface acoustic waves (SAWs) can excite the gyrotropic mode of the vortex state in a magnetic disk. Here we report on experiments utilizing a magnetic resonance force microscope to investigate magnetization dynamics in CoFeB sub-micrometer disks in the vortex state, grown on a Z-cut LiNbO$_3$ substrate. The device design enables excitation of the gyrotropic mode either inductively, using an antenna on top of the disks, or acoustically via SAWs launched from an interdigital transducer. Our modelling indicates that the lattice rotation {\omega}xz generates a localized magneto-acoustic field that displaces the vortex core from the disk center, initiating the gyration motion. Tuning of the magneto-acoustic torque acting on the vortex structure is achieved by a perpendicular magnetic field. These results demonstrate the clear excitation of the vortex gyrotropic mode by magneto-acoustic excitation.

cond-mat.mtrl-sci↗

arXiv2024

Symmetry of the coupling between surface acoustic waves and spin waves in synthetic antiferromagnets

Synthetic antiferromagnets host spin waves that are highly tunable. It is of practical interest to analyze the symmetry of their coupling to surface acoustic waves with the design of hybrid devices in view. For this we calculate the layer-resolved susceptibility tensor of a synthetic antiferromagnet, the effective magneto-elastic and magneto-rotation fields associated to a travelling elastic wave, and the power irreversibly transferred by the elastic wave to the magnetic layers. We consider Rayleigh-type surface acoustic waves: (a) that travel in an elastically isotropic, non-piezoelectric substrate, or (b) that propagate along the X direction at the surface of a Z-cut LiNbO$_3$ substrate, or (c) that are guided in a thin Z-cut LiNbO$_3$ film grown on a sapphire substrate. In particular, we show that the complementary angular dependencies of the acoustic and optical spin wave modes in synthetic antiferromagnets makes it possible to excite spin waves for any relative orientation of magnetization and acoustic wavevector. In addition, we discuss the symmetries of the driving fields and of the energy transferred to the magnetic degree of freedom. We evidence new interaction channels coupling the magnetization eigenmodes when elastic anisotropy and piezoelectricity of the substrate are considered.

cond-mat.mtrl-sci↗

arXiv2023

Unidirectionality of spin waves in Synthetic Antiferromagnets

We study the frequency non-reciprocity of the spin waves in symmetric CoFeB/Ru/CoFeB synthetic antiferromagnets stacks set in the scissors state by in-plane applied fields. Using a combination of Brillouin Light Scattering and propagating spin wave spectroscopy experiments, we show that the acoustical spin waves in synthetic antiferromagnets possess a unique feature if their wavevector is parallel to the applied field: the frequency non-reciprocity due to layer-to-layer dipolar interactions can be so large that the acoustical spin waves transfer energy in a unidirectional manner for a wide and bipolar interval of wavevectors. Analytical modeling and full micromagnetic calculations are conducted to account for the dispersion relations of the optical and acoustical spin waves for arbitrary field orientations. Our formalism provides a simple and direct method to understand and design devices harnessing unidirectional propagation of spin waves in synthetic antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2023

Inducing or suppressing the anisotropy in multilayers based on CoFeB

Controlling the uniaxial magnetic anisotropy is of practical interest to a wide variety of applications. We study Co$_{40}$Fe$_{40}$B$_{20}$ single films grown on various crystalline orientations of LiNbO$_3$ substrates and on oxidized silicon. We identify the annealing conditions that are appropriate to induce or suppress uniaxial anisotropy. Anisotropy fields can be increased by annealing up to 11 mT when using substrates with anisotropic surfaces. They can be decreased to below 1 mT when using isotropic surfaces. In the first case, the observed increase of the anisotropy originates from the biaxial strain in the film caused by the anisotropic thermal contraction of the substrate when back at room temperature after strain relaxation during annealing. In the second case, anisotropy is progressively removed by applying successive orthogonal fields that are assumed to progressively suppress any chemical ordering within the magnetic film. The method can be applied to CoFeB/Ru/CoFeB synthetic antiferromagnets but the tuning of the anisotropy comes with a decrease of the interlayer exchange coupling and a drastic change of the exchange stiffness.

cond-mat.mtrl-sci↗

arXiv2022

Exchange energies in CoFeB/Ru/CoFeB Synthetic Antiferromagnets

The interlayer exchange coupling confers specific properties to Synthetic Antiferromagnets that make them suitable for several applications of spintronics. The efficient use of this magnetic configuration requires an in-depth understanding of the magnetic properties and their correlation with the material structure. Here we establish a reliable procedure to quantify the interlayer exchange coupling and the intralayer exchange stiffness in synthetic antiferromagnets; we apply it to the ultrasmooth and amorphous Co$_{40}$Fe$_{40}$B$_{20}$ (5-40 nm)/Ru/ Co$_{40}$Fe$_{40}$B$_{20}$ material platform. The complex interplay between the two exchange interactions results in a gradient of the magnetization orientation across the thickness of the stack which alters the hysteresis and the spin wave eigenmodes of the stack in a non trivial way. We measured the field-dependence of the frequencies of the first four spin waves confined within the thickness of the stack. We modeled these frequencies and the corresponding thickness profiles of these spin waves using micromagnetic simulations. The comparison with the experimental results allows to deduce the magnetic parameters that best account for the sample behavior. The exchange stiffness is established to be 16 $\pm$ 2 pJ/m, independently of the Co$_{40}$Fe$_{40}$B$_{20}$ thickness. The interlayer exchange coupling starts from -1.7 mJ/m$^2$ for the thinnest layers and it can be maintained above -1.3 mJ/m$^2$ for CoFeB layers as thick as 40 nm. The comparison of our method with earlier characterizations using the sole saturation fields argues for a need to revisit the tabulated values of interlayer exchange coupling in thick synthetic antiferromagnets.

cond-mat.mtrl-sci↗