Self-similar magnetic structures and magnetic flux “Giant” creep

The penetration of a magnetic flux into a type-II high-T _c superconductor occupying the half-space x > 0 is considered. At the superconductor surface, the magnetic field amplitude increases in accordance with the law b(0, t) = b ₀(1 + t)^m (in dimensionless coordinates), where m > 0. The velocity of penetration of vortices is determined in the regime of thermally activated magnetic flux flow: v = v ₀exp?ub;?(U _0/T )(1-b?b/?x)?ub;, where U ₀ is the effective pinning energy and T is the thermal energy of excited vortex filaments (or their bundles). magnetic flux “Giant” creep (for which U ₀/T? 1) is considered. The model Navier-Stokes equation is derived with nonlinear “viscosity” v ∝ U ₀/T and convection velocity v _f ∝ (1 ? U ₀/T). It is shown that motion of vortices is of the diffusion type for j → 0 (j is the current density). For finite current densities 0 < j < j _c, magnetic flux convection takes place, leading to an increase in the amplitude and depth of penetration of the magnetic field into the superconductor. It is shown that the solution to the model equation is finite at each instant (i.e., the magnetic flux penetrates to a finite depth). The penetration depth x _eff ^A (t) ∝ (1 + t)^{(1 + m/2)/2} of the magnetic field in the superconductor and the velocity of the wavefront, which increases linearly in exponent m, exponentially in temperature T, and decreases upon an increase in the effective pinning barrier, are determined. A distinguishing feature of the solutions is their self-similarity; i.e., dissipative magnetic structures emerging in the case of giant creep are invariant to transformations b(x, t) = β^m b(t/β, x/β^{(1 + m/2)/2}), where β > 0.     

Synthesis and magnetic properties of Co–Zn magnetic fluid

Co_1−xZn_xFe₂O₄ (with x varying from 0 to 0.7) nanoparticles to be used for ferrofluid preparation were prepared by chemical co-precipitation method. The fine particles were suitably dispersed in transformer oil using oleic acid as the surfactant. The magnetization (M_s) and the size of the particles were measured at room temperature. The magnetization (M_s) was found to decrease with the increase in zinc substitution. The magnetic particle size (D_m) of the fluid was found to vary from 11.19 to 4.25 nm decreasing with the increase in zinc substitution.     

Measurement of the magnetic field-dependent refractive index of magnetic fluids in bulk

An optical alignment-free and highly accurate method is employed to measure the magnetic field-dependent refractive index of magnetic fluid(MF)in bulk.The measured refractive index decreases significantly with the increasing magnetic strength and then tends to saturate in the high intensity range.By applying a tunable magnetic field ranging between 0 and 1661 Oe,the maximum shift of the refractive index of MF in bulk is found to be 0.0231.     

Structural and magnetic properties of nanostructured Mn–Al–C magnetic materials

Pre-alloyed Mn_50+x−yAl_50−xC_y   (x=0$x=0$, 2, 4, 6, 8; y=0$y=0$, 1.7, 3) powders were mechanically milled (MM), and the as-milled powders subsequently annealed at temperatures from 350 to 600 °C to produce the ferromagnetic metastable L1₀-structured τ-phase. Bulk Mn₅₄Al₄₆ specimens were also annealed under the same conditions for comparison. The effects of the Mn concentration and C additions on phase formation, microstructure, magnetic properties, as well as on the magnetization mechanism of the Mn–Al–C alloys were systematically investigated. It was found that the magnetic properties are strongly dependent both on the fraction of the τ-phase and its microstructure. There exists a strong influence of the microstructural refinement, due to the ball milling, on the rate of ε-phase to τ-phase transformation and on the stability of the τ-phase. The kinetics of formation and subsequent decomposition of the magnetic τ-phase were markedly different in the MM and bulk alloys. Both remanence curves and δM plots showed no exchange coupling among the τ-phase nanograins. The mechanism for the magnetization process was determined to be domain wall pinning.     

Response of self-assembly for magnetite nanocrystal in magnetic fluid under an applied magnetic field

The response time and transmittivity of the magnetic fluid(MF)for different concentrations at room temperature were investigated in this letter.The volume fraction of the investigated sample ranged from 0.44% to 6.47%.It was found that the transmittivity decreased with increasing concentration under a given magnetic field,and the evolution time was changed with different concentrations.Moreover,the light intensity decreased rapidly at the beginning and then became stable when the magnetic field was applied.     

Effect of magnetic pulse treatment on the magnetic characteristics of yttrium–iron garnets

The effect weak (10–100 kA m^–1) low-frequency (10–20 Hz) pulsed magnetic fields have on the surface structure and magnetic characteristics of yttrium–iron garnet Y₃Fe₅O₁₂ is studied by means of electron and Mössbauer spectroscopy. A mechanism is proposed for the variation of saturation magnetization in Y₃Fe₅O₁₂ after magnetic pulse treatment. The mechanism is associated with the change in the spin state of iron ions localized in the tetrahedral sublattice.     

Are cluster magnetic fields primordial?

We present results of a detailed and fully nonlinear numerical and analytical investigation of magnetic field evolution from the very earliest cosmic epochs to the present. We find that, under reasonable assumptions concerning the efficiency of a putative magnetogenesis era during cosmic phase transitions, surprisingly strong magnetic fields 10(-13)-10(-11) G on comparatively small scales 100 pc-10 kpc may survive to the present. Building on prior numerical work on the evolution of magnetic fields during the course of gravitational collapse of a cluster, which indicates that precollapse fields of approximately 4 x 10(-12) G extant on small scales may suffice to produce clusters with acceptable Faraday rotation measures, we argue that it seems possible for cluster magnetic fields to be entirely of primordial origin.     

Spin waves in magnetic Weyl semimetals

Intrinsic magnetic topological materials,namely the stoichiometric magnetic compounds possessing both inherent magnetic order and topological electronic states,have attracted tremendous interest in the research of condensed matter physics and materials science[1].Such materials not only bring new opportunities to realize many exotic topological phenomena under time-reversal symmetry breaking,but also show great potential in applications of quantum technology[2].     

The electronic and magnetic properties of （Mn,C）-codoped ZnO diluted magnetic semiconductor

The electronic and magnetic properties of (Mn,C)-codoped ZnO are studied in the Perdew-Burke-Ernzerhof form of generalized gradient approximation of the density functional theory. By investigating five geometrical configurations, we find that Mn doped ZnO exhibits anti-ferromagnetic or spin-glass behaviour, and there are no carriers to mediate the long range ferromagnetic (FM) interaction without acceptor co-doping. We observe that the FM interaction for (Mn,C)-codoped ZnO is due to the hybridization between C 2p and Mn 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. Meanwhile, we demonstrate that ZnO co-doped with Mn and C has a stable FM ground state and show that the (Mn,C)-codoped ZnO is FM semiconductor with super-high Curie temperature (T C = 5475 K). These results are conducive to the design of dilute magnetic semiconductors with codopants for spintronics applications.     

Investigation of the magnetic domain structure of （PtCoPt）／Si multilayers by magnetic force microscopy

The domain structure of (PtCoPt)/Si multilayers in the dc demagnetized state has been investigated by magnetic force microscopy.The domain structure is found to change dramatically as the thickness of the non-magnetic Si sublayer (tsi) increases.Together with the analysis of magnetic properties,the variation of the domain period indicates that the domain wall energy decreases.Using the model developed by Draaisma and de Jonge,the domain wall energy is obtained.     

Frustrated magnetic interactions,giant magneto–elastic coupling,and magnetic phonons in iron–pnictides

We present a detailed first-principles study of Fe-pnictides with particular emphasis on competing magnetic interactions, structural phase transition, giant magneto–elastic coupling and its effect on phonons. The exchange interactions J_i,j(R) are calculated up to ≈12 Å from two different approaches based on direct spin-flip and infinitesimal spin-rotation. We find that J_i,j(R) has an oscillatory character with an envelop decaying as 1/R³ along the stripe-direction while it is very short range along the diagonal direction and antiferromagnetic. A brief discussion of the neutron scattering determination of these exchange constants from a single crystal sample with orthorhombic-twinning is given. The lattice parameter dependence of the exchange constants, dJ_i,j/da are calculated for a simple spin-Peierls like model to explain the fine details of the tetragonal–orthorhombic phase transition. We then discuss giant magneto–elastic effects in these systems. We show that when the Fe-spin is turned off the optimized c-values are shorter than experimental values by 1.4 Å for CaFe₂As₂, by 0.4 Å for BaFe₂As₂, and by 0.13 Å for LaOFeAs. We explain this strange behavior by unraveling surprisingly strong interactions between arsenic ions, the strength of which is controlled by the Fe-spin state through Fe–As hybridization. Reducing the Fe-magnetic moment, weakens the Fe–As bonding, and in turn, increases As–As interactions, causing a giant reduction in the c-axis. These findings also explain why the Fe-moment is so tightly coupled to the As-z position. Finally, we show that Fe-spin is also required to obtain the right phonon energies, in particular As c-polarized and Fe–Fe in-plane modes that have been recently observed by inelastic X-ray and neutron scattering but cannot be explained based on non-magnetic phonon calculations. Since treating iron as magnetic ion always gives much better results than non-magnetic ones and since there is no large c-axis reduction during the normal to superconducting phase transition, the iron magnetic moment should be present in Fe-pnictides at all times. We discuss the implications of our results on the mechanism of superconductivity in these fascinating Fe-pnictide systems.     

Bose–Einstein condensates in magnetic waveguides

In this article, we describe an experimental system for generating Bose–Einstein condensates and controlling the shape and motion of a condensate by using miniaturised magnetic potentials. In particular, we describe the magnetic trap setup, the vacuum system, the use of dispenser sources for loading a high number of atoms into the magneto-optical trap, the magnetic transfer of atoms into the microtrap, and the experimental cycle for generating Bose–Einstein condensates. We present first results on outcoupling of condensates into a magnetic waveguide and discuss influences of the trap surface on the ultra-cold ensembles. Received: 21 August 2002 / Revised version: 10 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-7071/295-829, E-mail: fortagh@pit.uni-tuebingen.de     

Morphology evolution of SmCox permanent magnetic nanoparticles

Sm-Co nanoparticles (NPs) are promising candidates for preparing superstable magnets and exchange-coupled nanocomposite magnets with unprecedented magnetic properties.However,the morphology evolution of the NPs remains unclear.Here,single crystalline SmCox(x=4.07,4.79,6.94,and 8.61) NPs with dimensions below the critical size of a single magnetic domain were synthesized.These NPs consist of Sm₂Co₇,SmCo₅,and Sm2Co17phases with divergent typical morphologies.An evolution model for the different morphological characteristics was proposed based on phase-structure changes and surface-energy calculations using the density functional theory.The results show that these SmCo_4.79 NPs can be well aligned along the easy magnetization axis and exhibit an ultrahigh coercivity of 5.7 T,thus enabling to advance the control of NP morphology and to facilitate their use in superstable or nanocomposite magnets.     

Effect of the phonon and magnetic anharmonicity on the thermal and elastic properties of nearly magnetic δ-plutonium

The temperature dependences of the total heat capacity and the lattice components of the bulk modulus, the volume thermal expansion coefficient, and the mean-square deviation of atoms from the equilibrium positions of nearly magnetic δ-plutonium (using the Pu_0.96Ga_0.04 alloy as an example) have been calculated within the framework of the self-consistent thermodynamic model. The electronic heat capacity has been calculated using the results obtained in terms of the self-consistent spin-fluctuation theory based on the inclusion of the strong magnetic anharmonicity, which leads to a splitting of the electronic spectra by fluctuating exchange fields. On this basis, the effect of phonon anharmonicity not only on the lattice heat capacity but also on other thermal and elastic properties has been considered.     

The abnormal direct-current resistivity of Ni–Mn–Ni triple-layered magnetic thin-films and their magnetic wall observations

We have observed an abnormal electron transport characteristic from the Ni–Mn–Ni triple-layered magnetic thin-films. Due to the intercalated Mn ultra-thin interlayer, the magnetic domain structure and the electron transport characteristic differ a lot from their original magnetic (Fe, Co, and Ni) films. As inspected by a magnetic force microscopy (MFM), we observed the variation of the domain configuration with Mn interlayer thicknesses (for 1, 5, and 10 nm). Moreover, we also examined and found that the direct current (DC) resistivity have no significant change as the current conducts from the current-in-wall (CIW) to the current- perpendicular-to-wall (CPW), which are opposite to the results of single layered films.     

Simulation research on effect of magnetic nanoparticles on physical process of magneto–acoustic tomography with magnetic induction

Magneto–acoustic tomography with magnetic induction(MAT-MI) is a multiphysics coupled imaging technique that is combined with electrical impedance tomography and ultrasound imaging. In order to study the influence of adding magnetic nanoparticles as a contrast agent for MAT-MI on its physical process, firstly, we analyze and compare the electromagnetic and acoustical properties of MAT-MI theoretically before and after adding magnetic nanoparticles, and then construct a two-dimensional(2 D) planar model. Under the guidance of space-time separation theory, we determine the reasonable simulation conditions and solve the electromagnetic field and sound field physical processes in the two modes by using the finite element method. The magnetic flux density, sound pressure distribution, and related one-dimensional(1 D), 2 D, and three-dimensional(3 D) images are obtained. Finally, we make a qualitative and quantitative analysis based on the theoretical and simulation results. The research results show that the peak time of the time item separated from the sound source has a corresponding relationship with the peak time of the sound pressure signal. At this moment, MAMPTMI produces larger sound pressure signals, and the sound pressure distribution of the MAMPT-MI is more uniform, which facilitates the detection and completion of sound source reconstruction. The research results may lay the foundation for the MAT-MI of magnetically responsive nanoparticle in subsequent experiments and even clinical applications.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

“Nonmagnetic” (antiferromagnetic) magnetic polaron

It is shown that, within deep narrow quantum potential wells in systems based on semimagnetic semiconductors containing Mn²⁺ ions, both magnitude and sign of the exchange interaction between electrons in the conduction band and in the d-shell of a magnetic ion depend significantly on the spatial position of this ion. This situation allows magnetic polarons with zero magnetic moment to appear, for which the localized-electron-induced spatial distribution of polarization of the surrounding magnetic ions exhibits an antiferromagnetic character. The possibility of experimental detection of these “nonmagnetic” magnetic polarons is considered.     

Competing magnetic phases on a kagomé staircase

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T--one amplitude modulated, the other helical--plus a commensurate canted antiferromagnet for T-->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from subleading interactions in a highly frustrated system.