arXiv subjects

E. J. G. Santos

Publications and source records attributed to E. J. G. Santos.

6 recordsLinked to original sources

arXiv2026

Integrating spin glass dynamics into nanomechanical resonators

Spin glasses are non-equilibrium magnetic systems with pronounced slow relaxation, ageing, and chaotic, synaptic-like properties, whose unique history-dependent dynamics have remained difficult to harness in functional devices. Two-dimensional van der Waals spin-glass magnets offer a route to overcome this limitation by allowing controlled coupling between magnetic and mechanical degrees of freedom. Here, we demonstrate the integration of a spin-glass functionality in nanoelectromechanical systems (NEMS) using thin exfoliated NbFeTe$_2$ films, which establishes a robust spin-glass state down to a few tens of nanometer-thick layers. Using cryogenic magnetic force microscopy, we reveal characteristic slow, hour-scale macroscopic magnetic relaxation, associated with glassy reorganisation of its magnetic domain clusters. Incorporation of thin NbFeTe$_2$ into membrane resonators allows the transduction of this glassy magnetic domain relaxation into strain, shifting their mechanical resonance frequency. This provides a functional nanomechanical readout of spin-glass dynamics and establishes a versatile platform for integrating frustrated magnetism into NEMS.

cond-mat.mes-hall↗

arXiv2024

Spin-glass states generated in a van der Waals magnet by alkali-ion intercalation

Tuning magnetic properties in layered van der Waals (vdW) materials has captured a significant attention due to the efficient control of ground-states by heterostructuring and external stimuli. Electron doping by electrostatic gating, interfacial charge transfer and intercalation is particularly effective in manipulating the exchange and spin-orbit properties, resulting in a control of Curie temperature ($T_{\text{C}}$) and magnetic anisotropy. Here, we discover an uncharted role of intercalation to generate magnetic frustration. As a model study, we intercalate Na atoms into the vdW gaps of pristine Cr$_2$Ge$_2$Te$_6$ (CGT) where generated magnetic frustration leads to emerging spin-glass states coexisting with a ferromagnetic order. A series of dynamic magnetic susceptibility measurements/analysis confirms the formation of magnetic clusters representing slow dynamics with a distribution of relaxation times. The intercalation also modifies other macroscopic physical parameters including the significant enhancement of $T_{\text{C}}$ from 66\,K to 240\,K and the switching of magnetic easy-hard axis direction. Our study identifies intercalation as a unique route to generate emerging frustrated spin states in simple vdW crystals.

cond-mat.mtrl-sci↗

arXiv2023

Coherent Spin-Phonon Coupling in the Layered Ferrimagnet Mn3Si2Te6

We utilize ultrafast photoexcitation to drive coherent lattice oscillations in the layered ferrimagnetic crystal Mn3Si2Te6, which significantly stiffen below the magnetic ordering temperature. We suggest that this is due to an exchange-mediated contraction of the lattice, stemming from strong magneto-structural coupling in this material. Additionally, simulations of the transient incoherent dynamics reveal the importance of spin relaxation channels mediated by optical and acoustic phonon scattering. Our findings highlight the importance of spin-lattice coupling in van der Waals magnets and a promising route for their dynamic optical control through their intertwined electronic, lattice, and spin degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2010

Spin-Strain Phase Diagram of Defective Graphene

Using calculations on defective graphene from first principles, we herein consider the dependence of the properties of the monovacancy of graphene under isotropic strain, with a particular focus on spin moments. At zero strain, the vacancy shows a spin moment of 1.5 $μ_B$ that increases to $\sim$2 $μ_B$ when the graphene is in tension. The changes are more dramatic under compression, in that the vacancy becomes non-magnetic when graphene is compressed more than 2%. This transition is linked to changes in the atomic structure that occurs around vacancies, and is associated with the formation of ripples. For compressions slightly greater than 3%, this rippling leads to the formation of a heavily reconstructed vacancy structure that consists of two deformed hexagons and pentagons. Our results suggest that any defect-induced magnetism that occurs in graphene can be controlled by applying a strain, or some other mechanical deformations.

cond-mat.mes-hall↗

arXiv2009

Magnetism of Substitutional Co Impurities in Graphene: Realization of Single $π$-Vacancies

We report {\it ab initio} calculations of the structural, electronic and magnetic properties of a graphene monolayer substitutionally doped with Co (Co$_{sub}$) atoms. We focus in Co because among traditional ferromagnetic elements (Fe, Co and Ni), only Co$_{sub}$ atoms induce spin-polarization in graphene. Our results show the complex magnetism of Co substitutional impurites in graphene, which is mapped into simple models such as the $π$-vacancy and Heisenberg model. The links established in our work can be used to bring into contact the engineering of nanostructures with the results of $π$-models in defective graphene. In principle, the structures considered here can be fabricated using electron irradiation or Ar$^+$ ion bombardment to create defects and depositing Co at the same time.

cond-mat.mes-hall↗

arXiv2008

Switching On Magnetism in Ni-doped Graphene

Magnetic properties of graphenic carbon nanostructures, relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni (Ni(sub)) impurities. Ni(sub) defects are non-magnetic in flat graphene and develop a non-zero magnetic moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Ni(sub). A similar magnetic/non-magnetic transition of Ni(sub) can be expected by applying anisotropic strain to a flat graphene layer.

cond-mat.mes-hall↗