Measurements of electric conductivity and magnetic after-effect have been performed on single crystalline magnetite in order to obtain more information on the conductivity mechanisms below the Verwey temperature T v ≈124 K. Depending on the stoichiometry of the material, two or three temperature ranges were detected which are characterized by different magnetic relaxation processes and within which the conductivity follows Mott's T 1/2 -law as derived for the case of multi-range electron hopping. By combined measurements of the electrical conductivity and of the magnetic after-effects the activation energies for intra-atomic electronic excitations and inter-atomic hopping transitions of electrons were determined.

The rather complex correlation between the microstructure and the magnetic properties is demonstrated for two types of high-quality RE-TM permanent magnets (pms), namely nanocrystalline RE2Fe14B (RE = Nd, Pr) and nanostructured Sm2(Co, Cu, Fe, Zr)17 pms. The detailed analysis of this correlation for both pm materials leads to a quantitative comprehension of the hardening mechanism enabling the optimization of their magnetic properties and temperature dependences. In the case of RE2Fe14B, isotropic bonded pms are fabricated showing maximum energy products in the order of 90 kJ/m3. In the case of Sm2(Co, Cu, Fe, Zr)17, magnets with excellent high-temperature magnetic properties are tailored. Hereby, the investigations in addition provide important clues to the evolution of the characteristic microstructural and magnetic properties and to the role of the involved elements.

The hysteresis loops of nanocrystalline (nc) permanent magnets (pms) produced by the melt-spin technique have been investigated for compositions based on the intermetallic compounds R 2 Fe 14 B (R = Nd, Pr) and the carbides Sm 2 Fe 17−x Ga x C y . The following three types of pms have been studied: 1) High-coercivity pins with exchange decoupled grains. 2) High-remanence exchange-spring pms. 3) High-coercive-high-remanence composite pins with exchange coupled soft and hard magnetic grains. The temperature dependence of the coercive field μ 0 H c for all three types ofpms obeys a relation for a modified nucleation field, H c = (2 K 1 /J s ) α - N eff M s ( K 1 = first anisotropy constant, M s = spontaneous magnetization). For an analysis of the characteristic differences between the microstructural parameters α and N eff as obtained for the three types of pms, computational micromagnetism on the basis of the Finite Element Technique is applied. This powerful method allows a quantitative analysis of the role of grain size, grain boundaries (gbs), texture of easy directions and of soft magnetic phases in composite materials. In order to obtain satisfactory results, a self-adapting algorithm has been developed where the mesh size is adapted to the gradients of the direction cosines of the spontaneous magnetization. It turns out that excellent magnetic properties of composite pms can only be obtained if the gbs are as ideal as possible. Remanence and coercive field are found to decrease linearly with a corresponding reduction of both, the crystal anisotropy and the exchange constant within the gbs. In composite pins the diameters of the soft magnetic grains should be smaller than twice the domain wall width, δ hard B of the hard magnetic phase in order to obtain a remarkable remanence enhancement. From these model calculations general rules for the development of optimized nc pms with large remanences and large coercivities are derived.

A comprehensive and systematic study has been made on Sm(CobalFevCuyZrx)zmagnets to completely understand the effects of composition and processing on their magnetic properties. The homogenized Sm(Co, Fe, Cu, Zr)z magnets have a featureless microstructure. A cellular/lamellar microstructure develops after 2-3 hours of aging at 800-850°C, but the coercivity increases only after a subsequent slow cooling to 400°C. During cooling, diffusion takes place and Cu is concentrated in the 1:5 cell boundaries and Fe in the 2:17R cells. This dilutes the magnetic properties of the 1:5 phase and causes domain wall pinning/nucleation at the cell boundaries. Higher ratio z leads to larger cells as expected due to the larger amount of the 2:17 phase. For a fixed Cu content, this translates to a larger amount of Cu in the 1:5 cell boundaries, and therefore, to a higher coercivity. Magnets without Cu but with Zr have a lamellar and a cellular like microstructure. In Zr free samples, however, a larger amount of Cu is needed to form the cellular microstructure. This cellular microstructure is unstable with prolonged isothermal aging. A uniform and stable cellular/lamellar microstructure is only observed in alloys containing both Cu and Zr. A higher aging temperature Tag leads to larger cells and higher coercivity as explained above. The results of all these studies clearly show that the amount of Cu in the 1:5 cell boundaries controls both the coercivity and its temperature dependence leading to positive and negative temperature coefficients of coercivity in low and high Cu content alloys, respectively.

High-density magnetic recording in the Tbit/inch(2) range is based on single bit, single domain particles in the nanometer range. In order to fulfil the prerequisites for high-density recording the following magnetic properties must be guaranteed: Easy axis perpendicular to recording plane, thermal stability up to 200 degrees C, reversal fields of 1-2 T and switching times in the sub-ns range. Composite nanoparticles composed of a soft and a hard magnetic layer are suitable to fulfil the above conditions. The magnetic ground states of exchange coupled bilayers are determined by micromagnetism and the corresponding reversal fields determined as a function of the soft magnetic film thickness. It is shown that the magnetic ground states are characterized by three configurations: in-plane, oblique, and perpendicular magnetization depending on the thickness of the soft magnetic layer. These three ground states are characterized by reversal fields of different magnetization processes: Depinning of a charged Neel wall at the phase boundary for soft layer thicknesses L-s larger than the exchange length of the soft layer, spontaneous rotation of magnetization by a nucleation process for soft layer thicknesses smaller than the exchange length. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is concerned with the annealing kinetics of point defects giving rise – in the presence of Ti 4+ ‐induced internal stresses – to pronounced, thermally activated magnetic after‐effects (MAEs) near 450, 200 and 65 K in single‐crystalline titanomagnetites Fe 3– x – Δ Ti x O 4 of composition 0.1 < x < 0.3 and Δ < 0.005. These relaxation processes have been associated with reorientations of anisotropic point defect configurations of Ti 4+ ‐bound octahedral (B‐site) vacancies (450 K), Fe 2+ interstitials (200 K) and a specific mode of stress‐reducing electron hopping (65 K). In order to check the developed model conceptions, crystals have been systematically annealed and the induced variations of the MAE spectra carefully analysed with respect to the underlying defect kinetics. As an important result, the common recovery of the 450 K and 200 K MAEs, in one stage near T > 1000 K, yielded coincident kinetics of reaction order γ = 2 and recombination enthalpy Q = 2.45 ± 0.05 eV – as being compatible with bimolecular defect (vacancy) – anti‐defect (interstitial) recombination. The delayed recovery of the 65 K peak (at T ≥ 1100 K) points to a stress‐sensitive relaxation mechanism, being deactivated only with the final annihilation of stress‐producing anisotropic defects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

This article correlated Nd2Fe14B single-layer and [(Nd2Fe14B)x∕Nb]n multilayer film magnetization reversal processes to the observed microstructure image. Coercivity temperature dependence was fitted to modified nucleation and pinning models. The microstructure parameter (αK) describes the deteriorating effect of Nd2Fe14B grain anisotropy on coercivity in the nucleation model. For a single layer with 400 nm thickness, the αK values change from 0.18 to 0.43 when annealing temperature increases from 550 to 903 K. The highest values of single- and multilayer films are 0.46 and 0.70, respectively. Film magnetization reversals were predominant and fitted the reverse domain nucleation model well. The Nb spacer layer enriched the grain boundaries of the Nd2Fe14B phase, which in turn inhibited the grain growth.

In this paper we present a new formulation for the simulation of a Canonical Molecular Dynamics Ensemble. This approach is phenomenological and it is formulated in terms of the square and the power of the interacting forces. The model is designed to have a minimal effect on the natural evolution of the dynamical properties. The system evolves in such a way that it is not necessary to impose nonphysical constraints to achieve the prescribed temperature. As a bonus the model gives during the process of simulation some useful relations between physical quantities of the hydrodynamic theory and those which are directly measured in the laboratory. The model is tested on liquid argon. The comparison with the experimental data validates the model assumptions.

High-density magnetic recording requires large switching fields and short sub-ns switching times of single-domain particles. Switching times are shown to depend on sample size and geometry, the magnetic material parameters and type and strength of applied fields. Numerical solutions of the Landau–Lifshitz–Gilbert equation show a minimum of the switching times as a function of the damping parameter. With increasing magnetic field an increase of switching times is observed for small damping constants and a decrease for large ones.

The effect of dilute titanium (Ti4+)-doping on the magnetic after-effect (MAE) spectra of stoichiometric magnetite single crystals, Fe3-xTixO4 - with 0.0001 <= x <= 0.008 - is studied in the temperature range 4 K < T <= T-V similar or equal to 125 K and analysed in terms of our revised relaxation model. The effects of these relatively low doping rates comprise: (i) strong impact on low-temperature (4 K < T < 35 K) incoherent electron (e(-))-tunnelling and combined intra-ionic thermal excitation; (ii) minor, though highly instructive, modifications of thermally activated electron hopping (50 K < T < 125 K), and (iii) low-temperature shifting of the Verwey transition (T-V) relative to the temperature of zero-crossing of the crystal anisotropy (T-K1). Due to the high oxygen stoichiometry of the crystals (absence of B-site vacancies), no further MAEs appear in the high-temperature range (T > T-V).The recently accentuated discussion concerning the appropriate timescale of electron transport - deduced from modern x-ray resonant scattering to be about tau similar or equal to 10(-16) s, over the whole temperature range (T greater than or less than T-V), in contrast to up to thousands of seconds as determined from high-precision MAE experiments, in the low-temperature phase (T < T-V) - gives us the chance to sharpen our arguments in favour of a clarification of the electronic conductivity mechanisms in magnetite.

Recently, the so-far still open discussion concerning the appropriate time-scale of electron mobility in magnetite (Fe3O4), especially below the Verwey transition (T < Tv similar or equal to 125 K), has been accentuated again by considerations - based on the observed invariance of modern resonant X-ray scattering spectra on crossing the Verwey transition-in favour of extremely short fluctuation times of about tau <= 10(-16) s. These estimations, however, differ by about 20 orders of magnitude from the electronic relaxation times-amounting up to thousands of seconds-as determined from high-precision magnetic after-effect and electric conductivity experiments. The present note is concerned with the question as to what extent most recent electronic LSDA + U calculations on Fe3O4 may be helpful in bridging this outrageous discrepancy. (c) 2005 Elsevier B.V. All rights reserved.

In low Si‐charged silicon iron (Fe + 0.1%Si) the kinetics of carbon diffusion, precipitation and redissolution have been carefully studied by means of the magnetic after‐effect (MAE) within the temperature range 200 K < Ta < 1100 K. The activation parameters of respective processes have been determined by means of least squares fitting the experimental data. In the presence of substitutionally alloyed Si, the C‐Richter MAE gives rise to two Debye‐type relaxation peaks – resulting from elementary steps of carbon diffusion combined with reorientation in the (i) unperturbed and (ii) Si‐modified Fe matrix – situated near 265 K and 320 K, with activation enthalpies, Qi, of 0.84 eV and 1.08 eV. Two‐stage carbon precipitation, obeying first order kinetics, occurs, intensively, near 390 K with the elementary C diffusion enthalpy of 0.84 eV and, rather weakly, near 540 K with an enthalpy of 1.18 eV. The resulting precipitates are discussed in terms of partly intra‐grain deposited iron carbide phases (Fe3C) and, mainly, grain‐boundary determined C trapping. Decomposition of these precipitates occurring, again, in two stages – situated near 740 K and 950 K, with activation enthalpies of 1.72 and 2.02 eV – leads to a restitution of the state of maximum interstitially dissolved C in the matrix. Of practical importance for silicon steel fabrication is the observation that, after complete C precipitation (Ta . 580 K), the material can be kept in a state of minimum dissolved carbon content – and hence of minimum ac‐losses – by not allowing it to warm up above Ta ≤ 650 K. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The critical current density of YBa2Cu3O7−δ (YBCO) thin films strongly depends on the surface properties of the substrate. We modify the surface of SrTiO3 single crystalline substrates using a 30 keV focussed beam of gallium ions. After characterizing the created patterns on the substrate by atomic force microscopy, thin films of optimally doped YBCO are deposited by pulsed laser deposition. By means of quantitative magnetooptical measurements it is found that a proper irradiation process can enhance the critical current density of the deposited superconducting film. The film growth on the irradiated substrate can lead to a higher and controlled defect density and thus to a modified distribution of effective flux line pinning sites in the superconductor.

Single-domain, high-remanence particles are the prerequisite for their application in magnetic high-density recording systems. Analytical calculations for the transition between single and multidomain configurations remain unprecise because the calculation of the stray field requires strong simplifications. By means of the three-dimensional finite element method, zero-field magnetization structures of magnetically hard and soft thin film elements and cubic particles have been determined. Numerical results are compared with analytical calculations showing that the latter results depend sensitively on the energy determined for vortex structures.

Most interestingly, the low‐temperature (T < 45 K) magnetic aftereffect (MAE) spectra occurring in both the cubic C15 Laves phase GdAl2Dx, after deuterium (D) charging, and in stoichiometric, singlecrystalline magnetite (Fe3O4) are of striking similarity – being composed of a plateau‐like relaxation zone (5 K < T < 30 K) and a pronounced Debye‐peak near T ≥ 30 K. Whereas the well‐investigated spectra in magnetite are well understood as resulting from various modes of thermally assisted incoherent tunneling of small polarons, not so much is known, as yet, concerning the respective mechanisms in GdAl2Dx. Guided by the impressive similarity of their spectra, we attempt here an interpretation of the underlying mechanisms in both systems on the basis of well‐corroborated magnetite concepts – i.e., in terms of incoherent tunnelling of specific particles, such as small polarons in Fe3O4 and deuterons in GdAl2Dx. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Field-dependent AC susceptibility measurements on a chemically homogeneous but strongly disordered Cr3Fe single crystal establish long-range ferromagnetism with three-dimensional Heisenberg-like effective critical exponents βeff=0.305±0.015,γeff=1.33±0.06 and δeff=5.46±0.15.

In Sm2Co17 based magnets the coercive field is determined by the repulsive or attractive interaction of domain walls with the 1:5 cell walls. With increasing temperature the coercivity mechanism changes from repulsive to attractive pinning and above the Curie temperature of the cell walls a nucleation mechanism may be dominant.

In this paper the authors explained the magnetic domain structure of Sm(CoCuFeZr) alloys. The magnetic hysteresis loops were measured by a vibrating sample magnetometer. To understand the magnetic hardening behaviour, the magnetic domain structure was investigated after magnetisingifizing with different fields. Here, domain structure was exposed by TEM.