We consider a small metallic particle (quantum dot) where ferromagnetism arises as a consequence of Stoner instability. When the particle is connected to electrodes, exchange of electrons between the particle and the electrodes leads to a temperature- and bias-driven Brownian motion of the direction of the particle magnetization. Under certain conditions this Brownian motion is described by the stochastic Landau-Lifshitz-Gilbert equation. As an example of its application, we calculate the frequency-dependent magnetic susceptibility of the particle in a constant external magnetic field, which is relevant for ferromagnetic resonance measurements.
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.
Magnetization of nanoparticles of NiO are measured and analyzed taking into account a distribution in particle magnetic moment. We find that disregarding this distribution in the analysis is the reason for the many anomalous observations reported on this system in the literature.
We investigate slow dynamics of collection of a few antiferromagnetic NiO nanoparticles. The zero-field cooled magnetization exhibits size dependent fluctuations. We find memory effects in field cooled magnetization, as well as aging effects in thermoremenant magnetization of antiferromagnetic nanoparticles. The antiferromagnetic nanoparticles show a stronger memory effect than the corresponding effect in the ferromagnetic particles, when the distribution of particles include very small sizes. The situation reverses for bigger sizes. The relaxation of the magnetization after a sudden cooling, …
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…
Remote sensing of magnetic nanoparticles has exciting applications for magnetic nanoparticle hyperthermia and molecular detection. We introduce, simulate, and experimentally demonstrate an innovation---a sensing coil that is geometrically decoupled from the excitation field---for magnetic nanoparticle spectroscopy that increases the flexibility and capabilities of remote detection. The decoupling enhances the sensitivity absolutely; to small amounts of nanoparticles, and relatively; to small changes in the nanoparticle dynamics. We adapt a previous spectroscopic method that measures the relaxa…
We develop an analytical model for describing the magnetization dynamics in ferromagnetic metal nanoparticles, which is based on the coupled system of the Landau-Lifshitz-Gilbert (LLG) and Maxwell equations. By solving Maxwell's equations in the quasi-static approximation and finding the magnetic field of eddy currents, we derive the closed LLG equation for the magnetization that fully accounts for the effects of conductivity. We analyze the difference between the LLG equations in metallic and dielectric nanoparticles and show that these effects can strongly influence the magnetization dynamic…
We demonstrate the use of the magnetic-field-dependence of highly spatially confined, GHz-frequency ferromagnetic resonances in a ferromagnetic nanostructure for the detection of adsorbed magnetic nanoparticles. This is achieved in a large area magnonic crystal consisting of a thin ferromagnetic film containing a periodic array of closely spaced, nano-scale anti-dots. Stray fields from nanoparticles within the anti-dots modify resonant dynamic magnetisation modes in the surrounding magnonic crystal, generating easily measurable resonance peak shifts. The shifts are comparable to the resonance …
We address the theory of the coupled lattice and magnetization dynamics of freely suspended single-domain nanoparticles. Magnetic anisotropy generates low-frequency satellite peaks in the microwave absorption spectrum and a blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency resonances are very sharp with maxima exceeding that of the FMR, because their magnetic and mechanical precessions are locked, thereby suppressing Gilbert damping. Magnetic nanoparticles can operate as nearly ideal motors that convert electromagnetic into mechanical energy. The Barnett/Einstein-de H…
We present a theory of the low-energy excitations of a ferromagnetic metal nanoparticle. In addition to the particle-hole excitations, which occur in a paramagnetic metal nanoparticle, we predict a branch of excitations involving the magnetization-orientation collective coordinate. Tunneling matrix elements are in general sizable for several different collective states associated with the same band configuration. We point out that the average change in ground state spin per added electron differs from non-interacting quasiparticle expectations, and that the change in the spin-polarization, due…
Manipulation of magnetization with ultrashort laser pulses is promising for information storage device applications. The dynamic of the magnetization response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage is FePt nanoparticles , on which optical writing with optical angular momentum was demonstrated recently by Lambert et al., although the mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magneti…
We present a study by ferromagnetic resonance at microwave Q band of two sheets of cobalt nanoparticles obtained by annealing SiO2 layers implanted with cobalt ions. This ex- perimental study is performed as a function of the applied magnetic field orientation, tempera- ture, and dose of implanted cobalt ions. We demonstrate that each of those magnetic sheet of cobalt nanoparticles can be well modelled by a nearly two dimensional ferromagnetic sheet hav- ing a reduced effective saturation magnetization, compared to a regular thin film of cobalt. The nanoparticles are found superparamagnetic ab…
We represent the results of micromagnetic modeling and experimental magnetic force microscopy investigations of domain wall (DW) pinning-depinning effects in the hybrid systems consisting of ferromagnetic nanowire (NW) and two ferromagnetic nanoparticles (NPs). It was demonstrated that the special NW-NPs configuration including the NW with DW nucleating bulb at one end and two-NP gate enables the realization of controlled DW pinning-depinning based on switching of magnetization in NPs subsystem. The algorithm of external driving field manipulation and independent NPs switching, which allow one…
We introduce a highly sensitive and relatively simple technique to observe magnetization motion in single Ni nanoparticles, based on charge sensing by electron tunneling at millikelvin temperature. Sequential electron tunneling via the nanoparticle drives nonequilibrium magnetization dynamics, which induces an effective charge noise that we measure in real time. In the free spin diffusion regime, where the electrons and magnetization are in detailed balance, we observe that magnetic damping time exhibits a peak with the magnetic field, with a record long damping time of $\simeq 10$~ms.
We study the deterministic and stochastic rotational dynamics of ferromagnetic nanoparticles in a precessing magnetic field. Our approach is based on the system of effective Langevin equations and on the corresponding Fokker-Planck equation. Two key characteristics of the rotational dynamics, the average angular frequency of precession of nanoparticles and their average magnetization, are of our interest. Using the Langevin and Fokker-Planck equations, we calculate both analytically and numerically these characteristics in the deterministic and stochastic cases, determine their dependence on t…
We report the results of experimental investigations of controlled domain wall (DW) pinning in a ferromagnetic nanowire (NW) by stray fields of two uniformly magnetized bistable ferromagnetic nanoparticles (NPs) placed on either side of the NW and elongated parallel to the NW axis. We show by magnetic force microscopy measurements that DW pinning strength essentially depends on the orientation of the NP magnetic moments relative to the NW magnetization and can reach 20 mT. We also performed micromagnetic simulations confirming the influence of the magnetostatic interaction of the DW with the N…
We report studies on magnetization dynamics in NiO nanoparticles of average size 5 nm. Temperature and time dependence of dc magnetization, wait time dependence of magnetic relaxation (aging) and memory phenomena in the dc magnetization are studied with various temperature and field protocols. We observe that the system shows memory and aging in field cooled and zero field cooled magnetization measurements. These experiments show that the magnetic behavior of NiO nanoparticles is similar to spin glasses. We argue that the spin glass behavior originates from the freezing of spins at the surface…
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…
We present a theory of the elementary spin excitations in transition metal ferromagnet nanoparticles which achieves a unified and consistent quantum description of both collective and quasiparticle physics. The theory starts by recognizing the essential role played by spin-orbit interactions in determining the energies of ferromagnetic resonances in the collective excitation spectrum and the strength of their coupling to low-energy particle-hole excitations. We argue that a crossover between Landau-damped ferromagnetic resonance and pure-state collective magnetic excitations occurs as the numb…
We investigate the magnetization dynamics of a conducting magnetic nanoparticle weakly coupled to source and drain electrodes, under the assumption that all relaxation comes from exchange of electrons with the electrodes. The magnetization dynamics is characterized by a relaxation time $t_1$, which strongly depends on temperature, bias voltage, and gate voltage. While a direct measure of a nanoparticle magnetization might be difficult, we find that $t_1$ can be determined through a time resolved transport measurement. For a suitable choice of gate voltage and bias voltage, the magnetization pe…
We report studies on temperature, field and time dependence of magnetization on cupric oxide nanoparticles of sizes 9 nm, 13 nm and 16 nm. The nanoparticles show unusual features in comparison to other antiferromagnetic nanoparticle systems. The field cooled (FC) and zero field cooled (ZFC) magnetization curves bifurcate well above the Néel temperature and the usual peak in the ZFC magnetization curve is absent. The system does not show any memory effects which is in sharp contrast to the usual behavior shown by other antiferromagnetic nanoparticles. It turns out that the non-equilibrium behav…
We calculate the mesoscopic fluctuations of the magnetic anisotropy of ferromagnetic nanoparticles. A microscopic spin-orbit Hamiltonian considered as a perturbation of the much stronger exchange interaction first yields an explicit expression for the anisotropy tensor. Then, assuming a simple random matrix model for the spin-orbit coupling allows us to describe the fluctuation of such a tensor. In the case of uniaxial anisotropy, we calculate the distribution of the anisotropy constant for a given number of electrons, and its variation upon increasing this number by one. The magnitude of the …
We report studies on temperature, field and time dependence of magnetization on cupric oxide nanoparticles of sizes 9 nm, 13 nm and 16 nm. The nanoparticles show unusual features in comparison to other antiferromagnetic nanoparticle systems. The field cooled (FC) and zero field cooled (ZFC) magnetization curves bifurcate well above the Neel temperature and the usual peak in the ZFC magnetization curve is absent. The system does not show any memory effects which is in sharp contrast to the usual behavior shown by other antiferromagnetic nanoparticles. It turns out that the nature of CuO nanopar…
The complex permittivity and permeability of superparamagnetic iron-oxide nanoparticles has been quantified using a circular waveguide assembly with a static magnetic field to align the nanoparticle's magnetization. The high sensitivity of the measurement provides the precise resonant feature of nanoparticles. The complex permeability in the vicinity of ferromagnetic resonance (FMR) is in agreement with the nanoparticle's measured magnetization via conventional magnetometry. A rigorous and self-consistent measure of complex permittivities and permeabilities of nanoparticles is crucial to ascer…
Single domain magnetic nanoparticles (MNP) interacting through dipolar interactions (DDI) in addition to the magnetocrystalline energy may present a low temperature ferromagnetic (SFM) or spin glass (SSG) phase according to the underlying structure and the degree of order of the assembly. We study, from Monte Carlo simulations in the framework of the effective one-spin or macrospin models, the case of a monodisperse assembly of single domain MNP fixed on the sites of a perfect lattice with fcc symmetry and randomly distributed easy axes. We limit ourselves to the case of a low anisotropy, name…
The autoresonant approach to excitation and control of large amplitude uniformly precessing magnetization structures in finite length easy axis ferromagnetic nanoparticles is suggested and analyzed within the Landau-Lifshitz-Gilbert model. These structures are excited by using a spatially uniform, oscillating, chirped frequency magnetic field, while the localization is imposed via boundary conditions. The excitation requires the amplitude of the driving oscillations to exceed a threshold. The dissipation effect on the threshold is also discussed. The autoresonant driving effectively compensate…
Size-dependent spatial magnetization profile of manganese-zinc ferrite Mn0.2Zn0.2Fe2.6O4 nanoparticles
University of Luxembourg（卢森堡大学）；Università degli Studi di Firenze（佛罗伦萨大学）；ICCOM-CNR（国家研究委员会分子化学与催化研究所）；Technische Universität München（慕尼黑工业大学）；Heinz Maier-Leibnitz Zentrum (MLZ)（海因茨·迈尔-莱布尼茨中心）；General Numerics Research Lab（通用数值计算实验室）
We measure electron tunneling via discrete energy levels in ferromagnetic cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due to magnetic anisotropy, the energy of each tunneling resonance shifts as an applied magnetic field rotates the particle's magnetic moment. We see both spin-increasing and decreasing tunneling transitions, but we do not observe the spin degeneracy at small magnetic fields seen previously in non-magnetic materials. The tunneling spectrum is denser than predicted for independent electrons, possibly due to spin-wave excitations.
We study the effect of an elliptically polarized magnetic field on a system of non-interacting, single-domain ferromagnetic nanoparticles characterized by a uniform distribution of easy axis directions. Our main goal is to determine the average magnetization of this system and the power loss in it. In order to calculate these quantities analytically, we develop a general perturbation theory for the Landau-Lifshitz-Gilbert (LLG) equation and find its steady-state solution for small magnetic field amplitudes. On this basis, we derive the second-order expressions for the average magnetization and…
The ability to control the magnetization switching in nanoscale devices is a crucial step for the development of fast and reliable techniques to store and process information. Here we show that the switching dynamics can be controlled efficiently using a microwave field with slowly varying frequency (autoresonance). This technique allowed us to reduce the applied field by more than $30%$ compared to competing approaches, with no need to fine-tune the field parameters. For a linear chain of nanoparticles the effect is even more dramatic, as the dipolar interactions tend to cancel out the effect…
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…
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…
In this investigation, Gd and Mn co-doped Bi$_{0.85}$Gd$_{0.15}$Fe$_{1-x}$Mn$_x$O$_3$ (x=0.0-0.15) nanoparticles have been prepared to report the influence of co-substitution on their structural, optical, magnetic and electrical properties. Due to simultaneous substitution of Gd and Mn in BiFeO$_3$, the crystal structure has been modified from rhombohedral (R3c) to orthorhombic (Pn21a) and the Fe-O-Fe bond angle and Fe-O bond length have been changed. For Mn doping up to 10 % in Bi$_{0.85}$Gd$_{0.15}$Fe$_{1-x}$Mn$_x$O$_3$ nanoparticles, the saturation magnetization (Ms) has been enhanced signi…
The performance characteristics of magnetic nanoparticles towards application, e.g. in medicine, imaging, or as sensors, is directly determined by their magnetization relaxation and total magnetic moment. In the commonly assumed picture, nanoparticles have a constant overall magnetic moment originating from the magnetization of the single-domain particle core surrounded by a surface region hosting spin disorder. In contrast, this work demonstrates the significant increase of the magnetic moment of ferrite nanoparticles with applied magnetic field. At low magnetic field, the homogeneously magne…
We review studies on spin-dependent tunneling phenomena in systems containing ferromagnetic nanoparticles. We discuss preparation methods of assembling nanoparticles as well as the mechanisms and results of spin-dependent transport properties. The emphasis of this review is on characteristic spin-dependent tunneling phenomena such as enhanced tunnel magnetoresistance (TMR) due to co-tunneling in the Coulomb blockade regime and sign changes of the TMR due to spin accumulation in nanoparticles.
We report detailed studies on magnetic properties of Co3O4 nanoparticles of average size 12.5 nm. Temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the particles show some features characteristic of nanoparticle magnetism such as bifurcation of field cooled (FC) and zero field cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which ZF…
We consider the influence of interparticle interaction on the magnetization reversal in the oriented Stoner-Wohlfarth nanoparticles ensemble. To do so, we solve a kinetic equation for the relaxation of the overall ensemble magnetization to its equilibrium value in some effective mean field. Latter field consists of external magnetic field and interaction mean field proportional to the instantaneous value of above magnetization. We show that the interparticle interaction influences the temperature dependence of a coercive field. This influence manifests itself in the noticeable coercivity at $T…
Both spin and orbital degrees of freedom contribute to the magnetic moment of isolated atoms. However, when inserted in crystals, atomic orbital moments are quenched because of the lack of rotational symmetry that protects them when isolated. Thus, the dominant contribution to the magnetization of magnetic materials comes from electronic spin. Here we show that nanoislands of quantum spin Hall insulators can host robust orbital edge magnetism whenever their highest occupied Kramers doublet is singly occupied, upgrading the spin edge current into a charge current. The resulting orbital magnetiz…
Measurements of magnetic hysteresis loops in single Co nanoparticles at dilution refrigerator temperatures are presented. The nanoparticles are in electric contact with bulk Al leads via tunnel junctions. The tunnel current versus magnetic field displays a magnetic hysteresis loop. The magnetic switching field is reduced by current, and the magnetization of the nanoparticle can be switched by applying a voltage pulse, demonstrating that the magnetic stability of the nanoparticle is diminished by electron transport. Additional transitions and random switches of magnetization can be driven by la…
We study, both analytically and numerically, the phenomenon of energy dissipation in single-domain ferromagnetic nanoparticles driven by an alternating magnetic field. Our interest is focused on the power loss resulting from the Landau-Lifshitz-Gilbert equation, which describes the precessional motion of the nanoparticle magnetic moment. We determine the power loss as a function of the field amplitude and frequency and analyze its dependence on different regimes of forced precession induced by circularly and linearly polarized magnetic fields. The conditions to maximize the nanoparticle heatin…
ZnO nanoparticles with Wurtzite structure were prepared by chemical methods at low temperature in aqueous solution. Nanoparticles are in the range from about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room temperature and above. Magnetization vs temperature, M(T) and isothermal measurements M(H) were determined. The coercive field clearly shows ferromagnetism above room temperature. The chemical synthesis, structural defects in the bulk related to oxygen vacancies are the main factors for the observed magnetic behavior. PACS numbers: 61.46.Hk Nanocrystals, 75.50.Pp Magneti…
The theoretical analysis of the effect of uniaxial stress on the magnetization of the system of noninteracting nanoparticles is done by an example of heterophase particles of maghemite, epitaxially coated with cobalt ferrite. It is shown that stretching leads to a decrease in the coercive force $H_c$, and compression leads to its growth. The residual saturation magnetization $I_{rs}$ of nanoparticles does not change. With increasing of interfacial exchange interaction, coercive force varies nonmonotonically
Magnetic nanoparticles are useful biological probes as well as therapeutic agents. There have been several approaches used to model nanoparticle magnetization dynamics for both Brownian as well as Néel rotation. The magnetizations are often of interest and can be compared with experimental results. Here we summarize these approaches including the Stoner-Wohlfarth approach, and stochastic approaches including thermal fluctuations. Non-equilibrium related temperature effects can be described by a distribution function approach (Fokker-Planck equation) or a stochastic differential equation (Lange…
The present report is dedicated to show that ferromagnetic La0.67Ca0.33MnO3 (LCMN) particles can be better described in the framework of ferrimagnetic model. To confirm the ferrimagnetic signature in ferromagnetic LCMN particles, the temperature dependence of the inverse of magnetic susceptibility in the paramagnetic state of the samples was taken as a tool of data analysis. The observed ferrimagnetism is understood as an effect of of the core-shell spin structure in LCMN particles.
We report the synthesis of cobalt nanoparticles with different shapes and sizes by rapid pyrolysis of cobalt carbonyl in the presence of various surfactants. The size and shape of the nanoparticles were influenced by reaction conditions, such as type of the surfactant, molar ratio of surfactant to precursor, reflux temperature and reaction time. The shapes that we have achieved include spherical, nearly spherical, disc and cube. The presence of linear amine yielded nanodiscs and they spontaneously self-assembled into long ribbons. The effect of shape anisotropy on magnetic nanoparticles has be…
The internal energy of magnetic anisotropy for some nanoparticles dominates over the thermal energy even at room temperature. Strong magnetic anisotropy of nanoparticles can significantly affect the process of magnetization of the magnetic fluid. This influence is substantial if the system of nanoparticles is in a state of mechanical anisotropy in which the anisotropy axes of the particles have the same direction. In this work, it is shown that the magnetization curve of the magnetic fluid in a state of mechanical anisotropy is significantly different from that of Langevin. It is located betwe…
We report the discovery of an effect where two ferromagnetic materials, one semiconductor ((Ga,Mn)As) and one metal (permalloy), can be directly deposited on each other and still switch their magnetization independently. We use this independent magnetization behavior to create various resistance states dependent on the magnetization direction of the individual layers. At zero magnetic field a two layer device can reach up to four non-volatile resistance states.
We have developed a model for the interacting ore/shell nanoparticles, which we used to analyze the dependence of the coercive field Hc, the remanent saturation magnetization Mrs and the satu- ration magnetization Ms on the interfacial exchange interaction between the core and the shell, the size of the nanoparticles and their interaction for Fe/Fe3O4 nanoparticles have been carried out. It has been shown that the hysteresis characteristics increase together with the size of nanoparticles. Hc and Mrs are changing nonmonotonic when the constant interfacial exchange interaction changes from nega…
We report magnetic studies on gelatin coated NiO nanoparticles of average size 7 nm. Temperature and time dependence of dc magnetization, wait time dependence of magnetic relaxation (aging), memory effects in the dc magnetization and frequency dependence of ac susceptibility have been investigated. We observe that the magnetic behavior of coated NiO nanoparticles differs substantially from that of bare nanoparticles. The magnetic moment of the particles is highly enhanced and the ZFC magnetization data displays a sharp peak (Tp1 = 15 K) at a low temperature in addition to a usual high temperat…
Using finite element micromagnetic simulations, we study how resonant magnetisation dynamics in thin magnetic discs with perpendicular anisotropy are influenced by magnetostatic coupling to a magnetic nanoparticle. We identify resonant modes within the disc using direct magnetic eigenmode calculations and study how their frequencies and profiles are changed by the nanoparticle's stray magnetic field. We demonstrate that particles can generate shifts in the resonant frequency of the disc's fundamental mode which exceed resonance linewidths in recently studied spin torque oscillator devices. Imp…
We report detailed studies on non-equilibrium magnetic behavior of antiferromagnetic Co_3O_4 nanoparticles. Temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features characteristic of nanoparticle magnetism such as bifurcation of field cooled (FC) and zero field cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at …
In recent measurements of tunneling transport through individual ferromagnetic Co nanograins, Deshmukh, Guéron, Ralph et al. \cite{mandar,gueron} (DGR) observed a tunneling spectrum with discrete resonances, whose spacing was much smaller than what one would expect from naive independent-electron estimates. In a previous publication, \cite{prl_kleff} we had suggested that this was a consequence of nonequilibrium excitations, and had proposed a ``minimal model'' for ferromagnetism in nanograins with a discrete excitation spectrum as a framework for analyzing the experimental data. In the presen…
During the last years great attention has been given to the encapsulation of magnetic nanoparticles. In this work we investigated the stability of small magnetic particles inside magnetic nanotubes. Multisegmented nanotubes were tested in order to optimize the stability of the particle inside the nanotubes. Our results evidenced that multisegmented nanotubes are more efficient to entrap the particles at temperatures up to hundreds of kelvins.
Ensembles of gold nanoparticles present a magnetic behavior which is at odds with the weakly diamagnetic response of bulk gold. In particular, an unusual ferromagnetic order has been unveiled by several experiments. Here we investigate if the combined effect of orbital magnetism of conduction electrons and interparticle dipolar interaction can lead to magnetic ordering. Using different model systems of interacting mesoscopic magnetic dipoles, together with a microscopic description of the electron dynamics within the nanoparticles, we find that a spontaneous magnetic moment may arise in dimers…
A theoretical model of the coherent precession of magnetization excited by a picosecond acoustic pulse in a ferromagnetic semiconductor layer of (Ga,Mn)As is developed. The short strain pulse injected into the ferromagnetic layer modifies the magnetocrystalline anisotropy resulting in a tilt of the equilibrium orientation of magnetization and subsequent magnetization precession. We derive a quantitative model of this effect using the Landau-Lifshitz equation for the magnetization that is precessing in the time-dependent effective magnetic field. After developing the general formalism, we then …
Novel hybrid superconductor/ferromagnetic particles structures are presented. Arrays of Co submicron particles were fabricated on overlap, edge type Josephson junctions and on a narrow Nb microbridge by means of high-resolution e-beam lithography.We observed a strong dependence between the magnetic state of the particles and the field-dependent critical $I_c(H)$ of these structures.
We have observed conventional signature of exchange bias (EB), in form of shift in field-cooled (FC) hysteresis loop, and training effect, in BiFe0.8Mn0.2O3 nanoparticles. From neutron diffraction, thermoremanent magnetization and isothermoremanent magnetization measurements, the nanoparticles are found to be core-shell in nature, consisting of an antiferromagnetic (AFM) core, and a 2-dimensional diluted AFM (DAFF) shell with a net magnetization under a field. The analysis of the training effect data using the Binek's model shows that the observed loop shift arises entirely due to an interface…
The magnetism of nanoparticles and thin films of wide-bandgap oxides that include no magnetic cations is an unsolved puzzle. Progress has been hampered both by the irreproducibility of much of the experimental data, and the lack of any generally-accepted theoretical explanation. The characteristic signature is a virtually anhysteretic, temperature-independent magnetization curve which saturates in an applied field that is several orders of magnitude greater than the magnetization. It appears as if a tiny volume fraction, < 0.1%, of the samples is magnetic and that the energy scale of the probl…
