Recent experimental investigations of individual magnetic nanoparticles reveal a diverse range of magnetic relaxation times which cannot be explained by considering their size, shape, and surface anisotropy, suggesting other factors associated with the internal microstructure of the particles are at play. In this letter, we apply Langer's theory of thermal activation to single magnetic domain fcc Co nanoparticles, whose experimental microstructures are characterized by planar defects, and derive an analytic expression for the relaxation time. The obtained Arrhenius exponential and its prefactor, which is often assumed to be a constant, are here found to both depend exponentially on system size and the number of defects. Together they provide a quantitative prediction of the experimental findings, and more generally highlight the importance of structural defects when considering magnetic stability.
Computational and experimental results on the thermally-induced magnetization reversal in single-domain magnetic nanoparticles are reported. The simulations are based on the direct integration of the Fokker-Planck equation that governs the dynamics of the magnetic moment associated with the nanoparticles. A mean field approximation is used to account for the influence of the dipolar interaction between nanoparticles. It is shown that the interactions can either speed up or slow down the reversal process, depending on the angle between the external magnetic field and the axis of easy magnetizat…
We investigate the magnetization reversal of single-domain magnetic nanoparticle driven by linear down-chirp microwave magnetic field pulse. Numerical simulations based on the Landau-Lifshitz-Gilbert equation reveal that solely down-chirp pulse is capable of inducing subnanosecond magnetization reversal. With a certain range of initial frequency and chirp rate, the required field amplitude is much smaller than that of constant-frequency microwave field. The fast reversal is because the down-chirp microwave field acts as an energy source and sink for the magnetic particle before and after cross…
It has been shown that a single-domain magnetic nanoparticle can be effectively switched by a linear down-chirp microwave field pulse (DCMWP) in zero temperature limit. However, finite temperature is ubiquitous in practice. Here, we study the effect of finite temperature on the DCMWP-induced magnetization reversal based on the stochastic Landau-Lifshitz-Gilbert equation. It is found that any one of the three controlling parameters of a DCMWP, i.e. the amplitude, chirp rate, or initial frequency, decreases with increasing temperature while the other two are fixed. The maximal temperature at whi…
We investigate the magnetization reversal of single-domain magnetic nanoparticle driven by the circularly polarized cosine chirp microwave pulse (CCMP). The numerical findings, based on the Landau-Lifshitz-Gilbert equation, reveal that the CCMP is by itself capable of driving fast and energy-efficient magnetization reversal. The microwave field amplitude and initial frequency required by a CCMP are much smaller than that of the linear down-chirp microwave pulse. This is achieved as the frequency change of the CCMP closely matches the frequency change of the magnetization precession which leads…
We develop a dipole model describing the thermal far-field radiation of a nanoparticle in close vicinity to a substrate. By including in our description the contribution of eddy currents and the possibility to choose different temperatures for the nanoparticle, the substrate, and the background, we generalize the existing models. We discuss the impact of the different temperatures, particle size, emission angle, and the distance dependence for all four combinations of gold and SiC nanoparticles or substrates.
Visualization and manipulation of nanoscale matter is one of the main and current challenges in nanosciences. To this aim, different techniques have been recently developed to non-invasively trap and manipulate nano-specimens, like nanoparticles or molecules. However, operating in air or vacuum still remains very challenging since most approaches are limited to a liquid environment. In this letter, we design and characterize a planar Paul trap optimized to trap and manipulate individual charged nanoparticles. This configuration offers competitive capabilities to manipulate nano-specimens in ai…
Recent progress in experiment on quantum tunnelling of the magnetic moment in mesoscopic systems will be reviewed. The emphasis will be made on measurements of individual nanoparticles. These nanomagnets allow one to test the border between classical and quantum behaviour. Using the micro-SQUID magnetometer, waiting time, switching field and telegraph noise measurements show unambiguously that the magnetisation reversal of small enough single crystalline nanoparticles is described by a model of thermal activation over a single-energy barrier. Results on insulating BaFeO nanoparticles show stro…
The combination of high sensitive superconducting quantum interference device (SQUID) with high quality nanoparticles allowed to check the simplest classical model describing the magnetisation reversal by uniform rotation which were proposed more than 50 years ago by Neel, Stoner and Wohlfarth. The micrometer sized SQUIDs were elaborated by electron beam lithography and the nanoparticles were synthesised by arc-discharge. The measured angular dependence of switching fields of nearly all f.c.c. Co nanoparticles revealed a dominating uniaxial magnetic anisotropy. This result suggests that twin b…
When two objects made of a material which supports surface modes are brought in close proximity to each other such that the vacuum gap between them is less than the thermal wavelength of radiation, then the coupling between the surface modes provides an important channel for the heat transfer to occur which is different from that mediated by long range propagating electromagnetic waves. Indeed, the heat transfer then exceeds Planck's blackbody limit by several orders of magnitude, and consequently has been used for several energy applications such as near-field thermophotovoltaic systems. This…
The recently obtained spin-crossover nanoparticles are possible candidates for applications in the recording media industry as materials for data storage, or as pressure and temperature sensors. For these applications the intermolecular interactions and interactions between spin-crossover nanoparticles are extremely important, as they may be essential factors in triggering the transition between the two stable phases: the high-spin and low-spin ones. In order to find correlations between the distributions in size and interactions and the transition temperatures distribution, we apply the FORC …
We report evidence of the magnetization reversal in nanoparticles by surface acoustic waves (SAWs). The experimental system consists of isolated magnetite nanoparticles dispersed on a piezoelectric substrate. Magnetic relaxation from a saturated state becomes significantly enhanced in the presence of the SAW at a constant temperature of the substrate. The dependence of the relaxation on SAW power and frequency has been investigated. The effect is explained by the effective ac magnetic field generated by the SAW in the nanoparticles.
We present a theoretical investigation of magnetostatic interaction effects in geometrically frustrated arrays of anisotropic multilayer ferromagnetic nanoparticles arranged in different spatially configured systems with triangular symmetry. We show that the interlayer magnetostatic interaction significantly expands the opportunities to create magnetically frustrated systems. The effects of the magnetostatic interaction in magnetization reversal processes and the possibility to control the ferromagnetic resonance spectrum in such systems are discussed.
We consider a Josephson bijunction consisting of a thin $SIS$ $π$-Josephson junction sandwiched between two-dimensional semiconducting layers with strong Rashba spin-orbit interaction. Each of these layers forms an $SNS$ junction due to proximity-induced superconductivity. The $SIS$ junction is assumed to be thin enough such that the two Rashba layers are tunnel-coupled. We show that, by tuning external gates, this system can be controllably brought into a time-reversal invariant topological superconducting phase with a Kramers pair of Majorana bound states being localized at the end of the no…
Various single particle measuring techniques are briefly reviewed and the basic concepts of a new micro-SQUID technique are discussed. It allows measurements of the magnetization reversal of single nanometer-sized particles at low temperature. The influence of the measuring technique on the system of interest is discussed.
We develop an analytical framework to study the influence of a weakly intercoupled inplane spherical metal nanoparticle (MNP) assembly on a coherently illuminated quantum emitter (QE). We reduce the analytical expressions derived for the aforementioned generic planar setup into simple and concise expressions representing a QE mediated by a symmetric MNP constellation, by exploiting the symmetry. We use the recently introduced generalized nonlocal optical response (GNOR) theory that has successfully explained plasmonic experiments to model the MNPs in our system. Due to the use of GNOR theory, …
We report that the irreversible magnetization switching process in ferromagnetic nanoparticles is governed by the formation and dynamics of topological point-defects in the form of hedgehog-antihedgehog pairs. After nucleation, these pairs rapidly separate with speeds exceeding domain wall velocities, and they generate an emergent electric field of solenoidal character and substantial magnitude.
We investigate the influence of shape anisotropy on the magnetization reversal of a single-domain magnetic nanoparticle driven by a circularly polarized linear down-chirp microwave field pulse (DCMP). Based on the Landau-Lifshitz-Gilbert equation, numerical results show that the three controlling parameters of DCMP, namely, microwave amplitude, initial frequency and chirp rate, decrease with the increase of shape anisotropy. For certain shape anisotropy, the reversal time significantly reduces. These findings are related to the competition of shape anisotropy and uniaxial magnetocrystalline an…
We fabricated YBa$_2$Cu$_3$O$_7$ (YBCO) direct current (dc) nano superconducting quantum interference devices (nanoSQUIDs) based on grain boundary Josephson junctions by focused ion beam patterning. Characterization of electric transport and noise properties at 4.2$\,$K in magnetically shielded environment yields a very small inductance $L$ of a few pH for an optimized device geometry. This in turn results in very low values of flux noise $<50\,{\rm n}Φ_0/{\rm Hz}^{1/2}$ in the thermal white noise limit, which yields spin sensitivities of a few $μ_{\rm B}/{\rm Hz}^{1/2}$ ($Φ_0$ is the magnetic…
We present measurements of mesoscopic resistance fluctuations in cobalt nanoparticles and study how the fluctuations with bias voltage, bias fingerprints, respond to magnetization reversal processes. Bias fingerprints rearrange when domains are nucleated or annihilated. The domain-wall causes an electron wavefunction phase-shift of $\approx 5π$. The phase-shift is not caused by the Aharonov-Bohm effect; we explain how it arises from the mistracking effect, where electron spins lag in orientation with respect to the moments inside the domain-wall. Dephasing time in Co at $0.03K$ is short, $τ_ϕ\…
The effect of noise on the process of high-speed remagnetization of vortex state of a pentagonal array of five circular magnetic nanoparticles is studied by means of computer simulation of Landau-Lifshits model. The mean switching time and its standard deviation of the reversal between the counterclockwise and clockwise vorticities have been computed. It has been demonstrated that with the reversal by the pulse with sinusoidal shape, the optimal pulse duration exists, which minimizes both the mean switching time (MST) and the standard deviation (SD). Besides, both MST and SD significantly depe…
Curvature effects are important for a proper description of the properties of magnetic systems. In this paper the exchange and dipolar energy of vortices on a paraboloidal shell is studied. Using analytical calculations it is shown that the in-plane component of vortices has larger energy on a paraboloidal shell than in a planar disk with same thickness. On the other hand, the dipolar energy associated to the vortex core diminishes if the vortex core is on a paraboloidal surface. This reduction in the dipolar energy may cause a vortex pinning mechanism by a paraboloidal shaped defect in a plan…
Magnetic vortices existing in soft magnetic nanoparticles with sizes larger than the single-domain diameter can be efficient nano-heaters in biomedical applications. Using micromagnetic numerical simulation we prove that in the optimal range of particle diameters the magnetization reversal of the vortices in spherical iron and magnetite nanoparticles is possible for moderate amplitudes of external alternating magnetic field. In contrast to the case of superparamagnetic nanoparticles, for the vortex configuration the hysteresis loop area increases as a function of frequency. So that high values…
In this work, and by means of micromagnetic simulations, we study the magnetic properties of toroidal nanomagnets. The magnetization ground state for different values of the aspect ratio between the toroidal and polar radii of the nanotorus has been obtained. The hysteresis curves are also obtained, evidencing the existence of two reversal modes depending on the geometry: a vortex mode and a coherent rotation. A comparison between toroidal and cylindrical nanoparticles has been performed evidencing that nanotori can accommodate a vortex as the ground state for smaller volume than cylindrical n…
Magnetic skyrmions formed at temperatures around room temperature in square-based parallelepiped magnetic FePt nanoparticles with perpendicular magnetocrystalline anisotropy (MCA) were studied during the magnetization reversal using micromagnetic simulations. Finite Differences (FD) method were used for the solution of the Landau-Lifshitz-Gilbert equation. Magnetic configurations exhibiting Néel skyrmionic formations were detected. The magnetic skyrmions can be created in different systems generated by the variation of external field, side length and width of the squared-based parallelepiped m…
The specific absorption rate (SAR) of a dilute assembly of spherical iron nanoparticles with cubic anisotropy distributed in a viscous liquid is calculated using the solution of stochastic Landau - Lifshitz equation for unit magnetization vector and stochastic equations for multiple particle directors that specify the spatial orientation of the nanoparticle in a liquid. The viscous and magnetic magnetization reversal modes of particles are revealed at low and sufficiently high amplitudes of alternating magnetic field, respectively. The SAR of iron nanoparticle assembly is shown to exceed signi…
An improved micro-SQUID technique is presented allowing us to measure the temperature dependence of the magnetisation switching fields of single nanoparticles well above the critical superconducting temperature of the SQUID. Our first measurements on 3 nm cobalt nanoparticle embedded in a niobium matrix are compared to the Neel Brown model describing the magnetisation reversal by thermal activation over a single anisotropy barrier.
Nanomagnetism has recently attracted explosive attention, in particular, because of the enormous potential applications in information industry, e.g. new harddisk technology, race-track memory[1], and logic devices[2]. Recent technological advances[3] allow for the fabrication of single-domain magnetic nanoparticles (Stoner particles), whose magnetization dynamics have been extensively studied, both experimentally and theoretically, involving magnetic fields[4-9] and/or by spin-polarized currents[10-20]. From an industrial point of view, important issues include lowering the critical switching…
We report unambiguous experimental evidence of broken time reversal symmetry for the interaction of light with an artificial non-magnetic material. Polarized colour images of planar chiral gold-on-silicon nanostructures consisting of arrays of gammadions show intriguing and unusual symmetry: structures, which are geometrically mirror images, loose their mirror symmetry in polarized light. The symmetry of images can only be described in terms of anti-symmetry (black-and-white symmetry) appropriate to a time-odd process. The effect results from a transverse chiral non-local electromagnetic respo…
The magnetic properties of Fe$_3$O$_4$ nanoparticle assemblies have been investigated in detail through a combination of vibrating sample magnetometry and muon spin relaxation ($μ$SR) techniques. Two samples with average particle sizes of 5 nm and 20 nm, respectively, were studied. For both samples, the magnetometry and $μ$SR results exhibit clear signatures of the superparmagnetic state at high temperature and the magnetically blocked state at low temperature. The $μ$SR data demonstrate that the transition from the superparamagnetic to the blocked state occurs gradually throughout the sample …
We demonstrate the operation of low-noise nano superconducting quantum interference devices (SQUIDs) based on the high critical field and high critical temperature superconductor YBa$_2$Cu$_3$O$_7$ (YBCO) as ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have been patterned by focused ion beam milling. This allows to precisely define the lateral dimensions of the SQUIDs so as to achieve large magnetic coupling between the nanoloop and individual MNPs. By means of focused electron beam induced de…
Laser-induced manipulation of magnetism at the nanoscale is a rapidly growing research topic with potential for applications in spintronics. In this work, we address the role of the scattering cross section, thermal effects, and laser fluence on the magnetic, structural, and chemical stability of individual magnetic nanoparticles excited by single femtosecond laser pulses. We find that the energy transfer from the fs laser pulse to the nanoparticles is limited by the Rayleigh scattering cross section, which in combination with the light absorption of the supporting substrate and protective lay…
The magnetic relaxation characteristics are investigated in the two-dimensional ($l^{}_x\times l^{}_y$) assembly of nanoparticles as a function of out-of-plane positional disorder strength $Δ(\%)$ using numerical simulations. Such defects are redundantly observed in experimentally fabricated nanostructures, resulting in unusual magnetization dynamics. The magnetization decays exponentially for small and negligible dipolar interaction strength $h^{}_d\leq0.2$. In such a case, the magnetization relaxation does not depend on $Δ$ and aspect ratio $A^{}_r=l^{}_y/l^{}_x$, as expected. In square-like…
We propose a mechanism to generate dissipationless current in time reversal symmetry breaking Weyl semimetals through the anomalous chiral magnetic effect (ACME). The ACME current is induced by chiral imbalance and flows along the direction of the Weyl nodes separation in momentum space. In contrast to the chiral magnetic effect, the ACME is not related to the chiral anomaly and does not require external magnetic field. In the presence of parallel electric and magnetic fields, the ACME gives rise to an antisymmetric linear magnetoresistance and a planar Hall conductivity, which we estimate to …
We investigate the spatial distribution of spin orientation in magnetic nanoparticles consisting of hard and soft magnetic layers. The nanoparticles are synthesized in a core / shell spherical morphology where the magnetically hard, high anisotropy layer is CoFe$_2$O$_4$ (CFO) while the lower anisotropy material is Fe$_3$O$_4$ (FO). The nanoparticles have a mean diameter of $\sim$9.2 - 9.6 nm and are synthesized as two variants: a conventional hard / soft core / shell structure with a CFO core / FO shell (CFO@FO) and the inverted structure FO core / CFO shell (FO@CFO). High resolution electron…
We show that the magnetic moment of a nanoparticle embedded in the surface of a solid can be switched by surface acoustic waves (SAW) in the GHz frequency range via a universal mechanism that does not depend on the structure of the particle and the structure of the substrate. It is based upon generation of the effective ac magnetic field in the coordinate frame of the nanoparticle by the shear deformation of the surface due to SAW. The magnetization reversal occurs via a consecutive absorption of surface phonons of the controlled variable frequency. We derive analytical equations governing thi…
Magnetic nanoparticles constitute potential nanomedicine tools based on the possibility to obtain different responses triggered by safe remote stimulus. However, such richness can be detrimental if the different performances are not accurately differentiated (and controlled). An example of this is the reorientation of magnetic nanoparticles under the influence of AC fields, which can be exploited for either magneto-mechanical actuation (MMA) at low frequencies (tens of Hz); or heat release at large ones (MHz range). While it is clear that Brownian rotation is responsible for MMA, its heating r…
We propose theoretically a new effect, i.e. nonlinear planar Nernst effect (NPNE), in nonmagnetic topological insulator (TI) Bi2Te3 in the presence of an in-plane magnetic field. We find that the Nernst current scales quadratically with temperature gradient but linearly with magnetic field and exhibits a cosine dependence of the orientation of the magnetic field with respect to the direction of the temperature gradient. The NPNE has a quantum origin arising from the conversion of a nonlinear transverse spin current to a charge current due to a joint result of hexagonal warping effect, spin-mom…
We developed a technique to determine suitable spin models for small embedded clusters of arbitrary geometry by combining the spin-cluster expansion with the relativistic disordered local moment scheme. We present results for uncovered and covered hexagonal Co clusters on Au(111) surface, and use classical Monte Carlo simulations to study the temperature dependent properties of the systems. To test the new method we compare the calculated spin-model parameters of the uncovered clusters with those of a Co monolayer deposited on Au(111). In general, the isotropic and DM interactions are larger b…
We study the quantum nonlinear planar Hall effect in bilayer graphene under a steady in-plane magnetic field. When time-reversal symmetry is broken by the magnetic field, a charge current occurs in the second-order response to an external electric field, as a result of the Berry curvature dipole in momentum space. We have shown that a nonlinear planar Hall effect originating from the anomalous velocity is deduced by an orbital effect of an in-plane magnetic field on electrons in bilayer graphene in the complete absence of spin-orbit coupling. Taking into account the symmetry analysis, we deriv…
The deterministic Landau-Lifshitz-Gilbert equation has been used to investigate the nonlinear dynamics of magnetization and the specific loss power in magnetic nanoparticles with uniaxial anisotropy driven by a rotating magnetic field, generalizing the results obtained for the isotropic case found in [P. F. de Chatel, I. Nandori, J. Hakl, S. Meszaros and K. Vad, J. Phys.: Condens. Matter 21, 124202 (2009)]. As opposed to many applications of magnetization reversal in single-domain ferromagnetic particles where losses must be minimized, in this paper, we study the mechanisms of dissipation used…
The Casimir-Lifshitz torque between two biaxially polarizable anisotropic planar slabs is shown to exhibit a non-trivial sign-reversal in its rotational sense. The critical distance $a_c$ between the slabs that marks this reversal is characterized by the frequency $ω_c\!\sim \!c/2a_c$ at which the in-planar polarizabilities along the two principal axes are equal. The two materials seek to align their principal axes of polarizabilities in one direction below $a_c$, while above $a_c$ their axes try to align rotated perpendicular relative to their previous minimum energy orientation. The sign-rev…
We use the methods of topological quantum chemistry to explore the topology of phonons on time-reversal symmetric crystals with the structure of the planar honeycomb (layer group $p6/mmm$). This approach is not tied to a particular model of atomic vibrations, but is applied to the most general dynamical matrix constrained only by the symmetries of the system. We show that four distinct fragile topological phonon phases are generically possible. Truncating the dynamical matrix to third nearest neighbors yields a model that realizes the different phonon topologies, characterized by the existence…
We discuss the topology of the wavefunctions of two-dimensional time-reversal symmetric superconductors. We consider (a) the planar state, (b) a system with broken up-down reflection symmetry, and (c) a system with general spin-orbit interaction. We show explicitly how the relative sign of the order parameter on the two Fermi surfaces affects this topology, and clarify the meaning of the $Z_2$ classification for these topological states.
Interplay between Exchange Interaction and Magnetic Shape Anisotropy of ferromagnetic nanoparticles in a non-magnetic matrix for rare-earth-free permanent magnets
Absorption of electromagnetic energy by a dissipative material is one of the most fundamental electromagnetic processes that underlies a plethora of applied problems, including sensing and molecular detection, radar detection, wireless power transfer, and photovoltaics. Perfect absorption is a particular regime when all of the incoming electromagnetic energy is absorbed by the system without scattering. Commonly, the incident energy is delivered to the absorbing system by a plane wave, hence perfect absorption of this wave requires an infinitely extended planar structure. Here, we demonstrate …
