We study thermodynamic properties of an Ising model of a ferromagnetic nanoscopic pyramid deposited on a ferromagnetic bulk substrate. The influence of the interaction between the pyramid and the substrate is calculated in terms of the reduced-state (density) operator used for description of thermodynamic properties of nanoscopic systems. The spatial distribution of magnetization in the nanopyramid is obtained in the Gaussian fluctuations approximation.
We study thermodynamic properties of an Ising model of a ferromagnetic nanoscopic pyramid deposited onto a ferromagnetic bulk substrate. The influence of the interaction between the pyramid and the substrateis calculated in terms of the reduced-state (density) operator used for description of thermodynamic properties of nanoscopic systems. The spatial distribution of the magnetization in the nanoscopic pyramid is obtained in the molecular field approximation.
In the present paper the process of optimization of soft magnetic properties have been studied by applying different experimental techniques (magnetic measurements, electric measurements, X-ray analysis, and high-resolution electron microscopy observations). It has been shown that an increase in magnetic permeability after optimization annealing can be mainly attributed to annealing out of microvoids.
It was shown that magnetic reluctivity measured versus time after demagnetization for pre-annealed samples of the Fe_{72}Co_{10}Nb_6B_{12} amorphous alloy exhibits highly non-exponential behavior which can be described by the coupling model. The relaxation intensity and the coupling parameter describing correlation effects in free volume diffusion decrease with increasing 1 h annealing temperature indicating annealing out of free volume and formation of iron clusters in amorphous matrix.
Carbon coated cobalt-, nickel- and iron-nanoparticles were prepared by chemical method and their structural and magnetic properties were investigated. The samples were synthesized by carburization of the nanocrystalline form of the appropriate metal in CH_4, followed up by the reduction of the obtained carbon deposits. The X-ray diffraction and transmission electron microscopy study revealed carbon coated Co-, Ni- and Fe-nanocapsules of the mean size ca. 50 nm, and a small amount of carbon nanotubes. The measurements of magnetization and AC susceptibility were carried out for samples with different carbon content. A special attention was paid to the low temperature magnetic behavior. Decrease in the saturation moment at T=4.2 K due to the nanosize of the particles was stronger for the samples with higher carbon content, while the coercivity field (610 Oe, 330 Oe and 390 Oe, for Co-, Ni-, and Fe-particles, respectively) was independent of carbon content. From the temperature dependence of AC and DC susceptibility a wide size distribution of the particles and blocking temperature above room temperature could be expected.
A Taylor expansion of dipole-dipole interaction in 2D systems defines a Landau-like local dipolar interaction in spin derivative field. The lowest order of this interaction gives the dipolar anisotropy. The next non-zero order is responsible for the appearance of magnetic vortices and hyperbolic defects. The following non-zero orders indicate the occurrence of higher topological defects such as double circle and of modulations. The arrangement of self-screened topological defects is discussed in agreement with the Monte Carlo simulations and experimental observations. Excited localized modes associated with these defects are classified.
Structural relaxation, crystallisation and optimisation processes in soft magnetic amorphous alloys based on iron are examined by applying different experimental techniques: X-ray diffraction analysis, high-resolution electron microscopy, measurements of magnetic and electric properties (permeability, after-effect, resistivity). The presented results are discussed in terms of annealing out of microvoids, formation of a nanocrystalline phase and changes of effective magnetostriction constant.
It was shown that soft magnetic properties of Fe_{78}Nb_{2}B_{20} amorphous alloy can be significantly improved by applying 1-h annealing at temperature 623 K (permeability increases even about 8 times). The Mössbauer Spectroscopy technique indicated that the optimized microstructure (corresponding to the maximum magnetic permeability) is free of iron nanograins and should be attributed to annealing out of free volume and a reduction of internal stresses i.e. to the relaxed amorphous phase.
Magnetic and structural properties of EuS/Co multilayers were studied by magnetic optical Kerr effect and SQUID magnetometry techniques and by X-ray diffraction method. The multilayers containing monocrystalline, ferromagnetic EuS layer (thickness 35-55 Å) and metallic Co layer (thickness 40-250 Å), were grown on KCl (001) and BaF₂ (111) substrates using high vacuum deposition technique employing electron guns for Co and EuS. All investigated EuS/Co multilayers exhibit ferromagnetic properties at room temperature due to Co layer with the ferromagnetic transition in EuS layer clearly marked upon cooling below 16 K. In EuS/Co/EuS trilayers grown on KCl substrate the antiferromagnetic alignment of magnetization vectors of Co and EuS layers was experimentally observed as a characteristic low field plateau on magnetization hysteresis loops and a decrease in multilayer magnetization below 16 K. In Co/EuS bilayers the characteristic temperature dependent shift of magnetization loops was found due to exchange bias effect attributed to the CoO/Co interface formed by the oxidation of the top Co layer.
We investigated the magnetic properties of ultrathin magnetite films deposited directly on MgO(001) and on a Fe(001) buffer layer. In both cases the magnetite surface structure could be identified using low energy electron diffraction. The conversion electron MÖssbauer spectroscopy measurements proved that, for magnetite films deposited on the Fe buffer, superparamagnetic relaxation was strongly suppressed. The effect of a Fe overlayer on the magnetite film grown directly on MgO is considerably weaker. Longitudinal Kerr magnetometry indicated the presence of the ferromagnetic interfacial coupling between Fe and magnetite films. We conclude that the density of antiphase boundaries for films grown on the Fe buffer is lower than that of Fe_3O_4/MgO films.
Several recent experiments on micro- (or nano-) structured samples of ferromagnetic materials are introduced. Magnetization reversal phenomena are investigated on submicron wire samples of trilayer structure using the giant magnetoresistance effect. Domain wall movements are sensitively monitored by resistivity measurements and the velocity of propagation is determined. The contribution of domain wall to the resistivity is argued from the results on artificially designed samples of a spring-magnet system. In circular dots of permalloy, the existence of vortex magnetization is confirmed and the reversal of the vortex core magnetization is studied from magnetic force microscopy measurements.
Magnetic properties of MnAs nanocrystals embedded in GaAs are analyzed in the frame of phenomenological model proposed by Sasaki for ferritin superparamagnets. Our calculations explain qualitatively experimental data of magnetization versus temperature, obtained according to zero-field-cooled and field-cooled protocols. They show dynamics of magnetization of MnAs nanocrystals in range of temperature from 10 K to 320 K. There is transition from state in which very slow dynamics is observed (frozen state) to state in which dynamics is fast (quasi-superparamagnetic state).
Temperature dependences of magnetization of core-shell-type nanoparticles with non-magnetic core and ferromagnetic shell are obtained using Monte Carlo simulation. The influence of surface spin disorder of the ferromagnetic shell on overall shape of magnetization curve is analyzed. The magnetic state diagram (in shell thickness - surface anisotropy coordinates), separating collinear and non-collinear states, is determined.
We present a quasiclassical approach to few-electron quantum dots in strong magnetic fields based on the notion of a collectively rotating Wigner molecule. A quasiclassical many-particle wave function is derived and illustrated by its application to a two-electron quantum dot. In particular, we calculate the density-current correlation function (conditional current) and show that the Wigner crystal in high magnetic fields may be visualized as an ordered system of current vortices.
Optical and magnetic properties of ZnMnO films are discussed based on the results of cathodoluminescence, photoluminescence, and magneto-photoluminescence investigations. We show that photoluminescence/cathodoluminescence emissions are strongly quenched and become in-plane inhomogeneous in samples with increased Mn fractions. Strong polarization of photoluminescence is observed, even though excitonic lines do not shift and are not split at magnetic fields up to 6 T.
A systematic study of the magnetic properties of submicron equilateral Ni triangles is reported. The triangular shape is a result of the particular preparation method used, which is based on a new kind of nanosphere lithography technique. In this case, the magnetic material is deposited through a latex sphere mask, yielding a periodic hexagonal array of in-plane magnetised triangular shaped elements. The magnetic properties were investigated as a function of thickness by magnetic force microscopy, superconducting quantum interference device magnetometry and vibrating sample magnetometry.
We prepared Pt₃Ni and PtNi₃ nanoparticles of various sizes on conductive and atomically smooth highly oriented pyrolytic graphite surfaces using potentiostatic electrodeposition. We can control the size of electrodeposited nanoparticles and their density on the surface by changing the deposition time. The morphology of nanoparticles was determined by scanning electron microscopy. PtNi₃ particles have spherical shape, while Pt₃Ni particles have more irregular shape. Composition of particles was confirmed by energy dispersive spectroscopy. We have measured magnetic properties of both systems with 100 s preparation time, superparamagnetic behavior was observed in PtNi₃ nanoparticles with blocking temperature T_{B}=225 K.
Optical spectroscopy measurements of single InAs/GaAs self-assembled quantum dots have been performed. The multiexcitonic emission from the s-, p-, and d-shells of a single dot is observed and investigated in magnetic field up to 28 T. The Zeeman splitting of the s-shell excitons has been found to depend on the dot morphology. While the energy-splitting in flat (2 nm height) dots linearly increases with magnetic field, its significant non-linearity is observed for larger in size, tall (4 nm height) dots. The effect of magnetic field on the orbital motion of carriers in dots has also been investigated. It has been found that the modified Fock-Darwin pattern explains the observed evolution of the emission from highly-excited single tall quantum dot.
The basic properties of magnetism depend strongly on the spin and orbital components of magnetization. Information about the magnetic moments can be gained using new techniques, like X-ray magnetic circular dichroism or Compton scattering, developed at third-generation synchrotron sources. After a brief introduction to the basic principles of these new magnetic tools, examples of experiments on 5f-electron based systems are presented.
Assuming that quantum dots are treated as artificial impurities we consider the Ruderman-Kittel-Kasuya-Yosida interaction between their localized magnetic moments. We prove that due to the quantum confinement the carriers that mediate interactions can exhibit fractional spectral dimension. Basing on this result we discuss magnetic interactions in coupled system of quantum dots and leads.
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