Dynamic roughening and fluctuations of dipolar chains

Nonmagnetic particles in a carrier ferrofluid acquire an effective dipolar moment when placed in an external magnetic field. This fact leads them to form chains that will roughen due to Brownian motion when the magnetic field is decreased. We study this process through experiments, theory and simulations, three methods that agree on the scaling behavior over 5 orders of magnitude. The rms width goes initially as t(1/2), then as t(1/4) before it saturates. We show how these results complement existing results on polymer chains, and how the chain dynamics may be described by a recent non-Markovian formulation of anomalous diffusion.     

Spinpolarisierte Photoelektronen aus EuO

The measurement of the spinpolarization of photoelectrons from ferromagnetic semiconductors with a novel apparatus shows that the surface is magnetically not saturated even at temperatures far below the Curietemperature. In EuO the ionization threshold of the 4f ⁷-states depends on doping; furthermore two types of doping coexist, differing in their spinpolarization and the photoelectric yield spectrum. The surface magnetism is influenced by evaporation of a very thin metal film. EuO is an intense source of highly polarized electrons suitable for use with a pulsed high energy accelerator.     

Dipolar-dominated critical behavior of EuO

Through a careful analysis of specific heat data for EuO evidence has been found which supports the crossover from Heisenberg-like to dipolar critical behavior at a reduced temperature | t | ～ 2 × 10^?2. In the Heisenberg regime, the critical exponents are found to be α = α′ = -0.09 ± 0.01 while in the dipolar region α = α′ = -0.026 ± 0.005.     

Light induced molecular magnetic polaron in EuO

Stoechiometric EuO samples do not present insulator metal transition below Curie temperature (T_c = 70 K). Experimental data presented in this paper show that when these samples are illuminated, it appears: an insulator metal transition below T_c, and, at room temperature, a decrease of activation energy, as well as a small increase in conductivity. Oxygen deficient samples exhibit usually this insulator metal transition. When they are illuminated room temperature activation energy remains unchanged, and the insulator metal transition is more important. These results cannot be explained by means of a free electron model. Electrons are photoexcitated from 4? levels, or donor levels, to conduction band. Then, these electrons polarise Eu²⁺ spins and make magnetic polarons. These polarons cannot be “Bound Magnetic Polarons” because samples do not contain oxygen vacancies. In addition these polarons would be stable until room temperature. Thus, they are expected to be “Molecular Magnetic Polarons”. Our results confirm then this model proposed by Kusuya.     

Infrared reflectivity and coupled modes in Gd-doped EuO

The reflectivity of EuO single crystals, doped with various amounts of Gd, has been measured between 250μ and 12 eV photon energy. A Kramers-Kronig analysis of the data reveals LO and TO phonon modes, plasmons and LO phonon-plasmon coupled modes.     

Das kritische magnetische Verhalten von EuO

Mössbauer effect and magnetization measurements were employed in order to study the static and especially the dynamic magnetic properties of the nearly Heisenberg ferromagnet EuO near its Curie temperature,T _c=69.2 °K. The critical exponent β of the spontaneous magnetization was determined to be β=0.34±0.02. It was shown that critical slowing down of spin fluctuations takes place nearT _c with spin relaxation times between 7×10^?11 sec (T=1.01T _c) and 1.5×10^?1 sec (T=1.03T _c). The experimental values of the relaxation time were found to be in satisfactory agreement with theoretically computed ones. Just belowT _c the Mössbauer spectra exhibit relaxation effects, which are characteristic for the occurence of critical super-paramagnetism. Investigations of several samples indicated quantitatively, that critical superparamagnetism has its origin in the non ideal composition of the real crystal.     

Quadrupole coupling in EuO due to lattice imperfections

From ¹⁵³Eu nuclear spin-spin relaxation measurements it is concluded that quadrupole effects observed in ferromagnetic EuO are due to lattice imperfections.     

Magnetization dynamics below of EuO and EuS

Between 4.2 K and the Curie temperatures of the cubic Heisenberg ferromagnets EuS and EuO, their homogeneous dynamic susceptibilities have been investigated by means of a broad-band reflectometer operating from 0.1 GHz to 40 GHz. For internal magnetic fields larger than the anisotropy fields H A ( T ) of both materials, their static susceptibilities exhibit a -divergence, which reveals quantitatively the dominance of dipolar-anisotropic spin-wave fluctuations. displays a Lorentzian shape the damping frequency of which obeys scaling in terms of .The scaling function agrees quantitatively with work by Frey and Schwabl [#!FS88!#] for dipolar Heisenberg ferromagnets at temperatures above T_c. Building upon their approach, the resonance frequency of the Lorentzian can be related to a memory effect in the damping determined by the large value of the relaxation rate of the longitudinal magnetization fluctuations . For EuS, this relation is substantiated directly by inelastic neutron scattering. All these features reveal the hitherto uncovered importance of the dipolar anisotropic fluctuations below T_c of ferromagnets. Received: 4 March 1998 / Accepted: 12 May 1998     

Critical behavior of the optical absorption edge in EuO

Measurements of the temperature dependence of the optical absorption edge, E_g(T), in EuO, at the close vicinity of its critical temperature, T_c, are reported. In agreement with theoretical expectations the analysis of the results, with the constraints of continuous E_g and dE_g/dT, yielded the same critical exponents (α = α' ≈ -0.044) and amplitude ratio (|A/Á| ≈ 1.22) as the specific heat data analysis.     

Magnetic form factor of Eu2+ in EuO

Polarized neutron measurements of the magnetic form factor of Eu²⁺ in EuO were made, to check for possible solid state effects on the radial extent of the 4f wavefunctions in this ferromagnetic insulator. Previous studies show good agreement between the magnetic form factors observed for the heavy rare earth metals and those calculated for the free ions using relativistic Dirac-Fock wavefunctions. However, the same comparison in this case shows an approximate 9% expansion of the 4f wavefunctions in EuO relative to that calculated for the Eu²⁺ ion. This apparent solid state effect has the same sign but twice the magnitude of that recently calculated by Byrom, Ellis and Freeman for (EuO₆)^?12 clusters.     

A model for the insulator-metal transition in Eu-rich EuO

A simple model hamiltonian is proposed to explain the insulator-metal transition in Eu-rich EuO. This model is an extension of Oliver-kasuya's model so as to include the Coulomb repulsion between electrons in the localized states around O²-vacancies. Explicit calculations are performed as an illustration for the one-dimensional case, which can be solved exactly, and the results are summarized in a phase diagram. Qualitative behaviors of a three-dimensional system are deduced from the one-dimensional example.     

Dynamic simulations of the dipolar driven demagnetization process of magnetic multi-core nanoparticles

In this work, we investigate dynamically the dipolar driven demagnetization process of magnetic multi-core particles by solving the Landau-Lifshitz equation for single-domain particles distributed on a three-dimensional sphere. We analyze the relaxation time in respect to different geometry and material parameters. Further we show that the demagnetization times differ from the behaviour of a single magnetic sphere in the case of low damping. To explain these dynamics nanoparticular systems of different dimensions are investigated. We show that deviations can be attributed to a confinement of the relaxation dynamics to a lower dimensional submanifold of the k-space.     

Dynamic effects of dipolar interactions on the magnetic behavior of magnetite nanoparticles

Isothermal magnetization and initial dc susceptibility of spheroidal, nearly monodisperse magnetite nanoparticles (typical diameter: 8 nm) prepared by a standard thermo-chemical route have been measured between 10 and 300 K. The samples contained magnetite nanoparticles in the form of either a dried powder (each nanoparticle being surrounded by a stable oleic acid shell as a result of the preparation procedure) or a solid dispersion in PEGDA-600 polymer; different nanoparticle (NP) concentrations in the polymer were studied. In all samples the NPs were not tightly agglomerated nor their ferromagnetic cores were directly touching. The high-temperature inverse magnetic susceptibility is always found to follow a linear law as a function of T, crossing the horizontal axis at negative temperatures ranging from 175 to about 1,000 K. The deviation from the standard superparamagnetic behavior is related to dipolar interaction among NPs; however, a careful analysis makes it hard to conclude that such a behavior originates from a dominant antiferromagnetic character of the interaction. The results are well explained considering that the studied samples are in the interacting superparamagnetic (ISP) regime. The ISP model is basically a mean field theory which allows one to straightforwardly account for the role of magnetic dipolar interaction in a NP system. The model predicts the existence of specific scaling laws for the reduced magnetization which have been confirmed in all studied samples. The interaction of each magnetic dipole moment with the local, random dipolar field produced by the other dipoles results in the presence of a large fluctuating energy term whose magnitude is comparable to the static barrier for magnetization reversal/rotation related to magnetic anisotropy. On the basis of the existing theories on thermal crossing of a barrier whose height randomly fluctuates in time it is predicted that the rate of barrier crossing is substantially driven by the rate of barrier fluctuations, which is fast (10⁸–10⁹ Hz) and almost independent of temperature. As a consequence, the standard picture of superparamagnetic NPs which undergo single-particle blocking by a static barrier below the blocking temperature should be substantially revised, at least in the present materials. The ISP model is perfectly matching with the view of activated magnetization rotation whose kinetics is significantly modified by barrier height fluctuations.     

The characterization of EuO nanocrystals using synchrotron light

We have studied the physical structure, optical property, electronic band structure and atomic structure of europium oxide (EuO) nanocrystals. The physical structures of nanocrystals are confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The optical property and electronic band structure are studied by X-ray absorption near-edge structure (XANES) measurement. We have also investigated the L₃-edge energy of europium(II) ion by extended X-ray absorption fine structure (EXAFS) measurement, which clarified the atomic structure of europium oxide nanocrystals.     

Spin transistor in the EuO:Fe/GaAs contact

Spin-wave structures with magnetic-field-driven current-voltage characteristics at room temperature were produced using a spintronic europium-monoxide-based thin-film emitter and a single-crystal n-GaAs semiconductor collector. This manifests practical implementation of the spin current transport and creation of a high-temperature spin transistor with the magnetic semiconductor/nonmagnetic semiconductor contact.     

Spin polarized photoemission from La-doped EuO in the vacuum ultraviolet

The spin polarization of photoelectrons emitted form La doped EuO was measured as a function of light energy up to hv = 11 eV, together with the quantum yield. The spectrum of spin polarization can be understood in terms of the energy level diagram of bulk EuO.     

Volume magnetostruction in EuO and MnF2

Measurements of the volume magnetostriction in EuO and MnF₂ confirm predictions of a model based on the volume dependence of the dominant exchange constant. No adjustable parameters are used.     

Specificities of Spin Wave Focusing in EuO and EuS Crystals

Physics of the Solid State - Specificities of spin wave focusing in EuO and EuS crystals are studied. It is shown that spin wave focusing is absent in the long-wave approximation: it is observed...