Volume 93, Issue 10

The magnetization depth profiles of Pt have been investigated by X-ray resonant magnetic reflectivity measurements of sputtered Pt/Co bilayer systems with different chemical roughness values at the Pt/Co interface. The asymmetry ratio as a function of the angle of incidence has been measured at the Pt L 3 near-edge absorption region at the maximum of the X-ray magnetic circular dichroism effect. The observed asymmetry ratios in the order of 0.5% are described quantitatively on the basis of a magnetically modified optical approach. The curves of the extracted magnetization profiles in the Pt layer show a different behavior corresponding to chemical roughness, while the magnetic saturation moment of Pt at the Pt/Co interface remains unaltered.

A method to correct the anisotropy field distribution for granular recording media, measured by switching methods (isothermal remanent magnetization (IRM) and direct-current demagnetisation (DCD) processes were used as an example) for the effects of thermal relaxation, that supposes an a priori knowledge of the grain size distribution, is presented. For a hard-disk medium, it is shown that the anisotropy field distribution is much narrower (about one order of magnitude) with respect to the grain size distribution. The anisotropy field value estimated from the dependence of the remanence coercivity on the field application time is found in agreement (to within less than 10 %) with the most probable value of the anisotropy field distribution.

Zero-field magnetization states in Permalloy thin-film elements of rectangular shape are calculated by means of finite element micromagnetic modeling. The energies of several possible magnetization patterns are determined for different sizes and thicknesses of the specimen. Based on these data, a phase diagram of the lowest-energy configuration is set up for rectangles of edge lengths between 250 and 1000 nm. It is shown that a thickness- and size-dependent transition from quasi-homogeneous single-domain states to demagnetized flux-closure patterns occurs in that range. The regime of sizes and thickness of the phase diagram in which the lowest-energy configuration of the thin-film element is a single-domain state is particularly important, since in this case the sample is expected to show good stability with respect to thermal demagnetization, which is a required criterion to make it suitable for technological applications in magneto-electronic devices.

The selectivity problem in magnetic random access memory is caused by processing problems that are metallurgical in origin. In honor of the 70th birthday of Professor Kronmüller, who has made so many contributions to magnetism and metallurgy, a solution of the selectivity problem is presented in which the metallurgical problems are circumvented by the design of the patterned thin film element to make it insensitive to processing problems. The solution is theoretical, using micromagnetics, the limitations of which call for experiments to support the conclusions.

It has always been the endeavour of Professor Kronmüller to combine various methods in order to attain a comprehensive understanding of a physical phenomenon on all relevant scales. In this spirit, we combine the phenomenological nonlinear theory of magnetoelasticity with the relativistic density functional electron theory to describe the magnetoelastic behaviour of epitaxial films. We explain the two already performed cantilever bending beam experiments on the magnetoelasticity and underpin the assumption that second-order magnetoelastic effects are very important for epitaxial Fe films. A complete set of six cantilever experiments is introduced which allows to calculate the intrinsic first- and second-order magnetoelastic constants for cubic materials, and these constants are calculated ab initio for Fe, face-centred cubic Co, Ni, Ni 3 Fe and CsCl-type CoFe.

The magnetic relaxation of a noninteracting two-dimensional ensemble of magnetic nanoparticles is simulated as a function of temperature using a Monte Carlo technique. By properly fitting the decay of magnetization using real parameters it is possible to make, at any finite temperature, a clear correspondence between Monte Carlo steps and time measured in seconds. The results allow one to visualize the intrinsic problems related to the simulation of nonequilibrium systems, and to understand the limits and range of validity of a particular system.

We show that the damping of longitudinal mechanical oscillations in soft magnetic amorphous ribbons can be significantly reduced by domain refinement. The latter is realized by an out-of-plane anisotropy induced by magnetic field annealing. The phenomena is interpreted in terms of micro eddy currents associated with the magnetization process within a domain.

The character and role of grain boundaries in NdFeB alloys and magnets are described and discussed. The role of the grain boundaries in the hydrogen decrepitation process is considered and the influence of the character and mobility of grain boundaries on the properties of fully dense sintered magnets is described. Grain boundaries also play an important role in the hydrogenation disproportionation desorption and recombination process whereby the coarse grain, cast material is converted to very fine grain particles.

The domain structure was investigated in the spin reorientation region on the (100) and (001) planes of Nd 2 Fe 14 B and R(Fe, Co) 11 Ti (R = Tb, Dy) single crystals by means of the magnetooptical Kerr effect. The magnetic phase diagram of a tetragonal magnetic crystal was theoretically calculated and the effect of domain structures on phase transitions is investigated. On the basis of the observed domain configurations and domain wall density calculations, possible models for the volume magnetization distributions in tetragonal magnetic crystals with “easy cone” and “easy plane” anisotropy have been discussed.

Intermetallic compounds in the series TbFe 12 – x Mo x , with nominal Mo contents, x , ranging from 1.5 to 3.5, were synthesized and examined for their thermomagnetic properties. Magnetic measurements were performed in the temperature range 5 – 1130 K by using a SQUID magnetometer and a Faraday balance in combination. Synthesis of interstitially carburized TbFe 12 – x Mo x C powders by mechanical alloying with graphite and a subsequent heat treatment was attempted for the single-phase compositions between x = 2 and x = 3. The magnetic behavior of the materials (especially the coercivity) shows a number of peculiarities, which we try to explain in terms of crystal chemical and microstructural features.

A comprehensive and systematic study has been made on Sm(Co, Fe, M, L) z magnets (M = Cu or Ni, and L = Zr or Ti) to completely understand the effects of composition and processing on the microstructure and magnetic properties of magnets. Ti-containing magnets do not have a lamellar phase but exhibit only a cellular microstructure, resulting in a much lower coercivity (below 10 kOe). Ni-containing magnets exhibit a perfect cellular/lamellar microstructure, but without a large domain wall energy gradient at the interface of the 2 : 17 and 1 : 5 phases, leading to a low coercivity. Only in the magnets containing both Cu and Zr, a uniform and stable cellular/lamellar microstructure with a high domain wall energy gradient across the 1 : 5 phase is formed, resulting in high coercivity. These results indicate that the conditions for effective magnetic hardening are: (1) Formation of a cellular/lamellar microstructure, and (2) establishment of a domain wall energy gradient at the cell boundaries. Based on all of these experimental results, the magnetization reversal mechanism of 2 : 17 Sm–Co magnets can be explained by both the domain wall pinning and nucleation models. The nucleation mechanism holds at any temperature in the Cu-rich magnets, and only above the Curie temperature of the 1 : 5 phase in the alloys with the lower Cu content. In these cases, the 2 : 17 cells become magnetically decoupled.

The magnetic properties of hard magnetic materials are closely connected to their microstructure. The complexity of this correlation is demonstrated for the two most promising permanent magnet (pm) materials at present, namely nanocrystalline RE 2 Fe 14 B (RE = Nd,Pr) and nanostructured Sm 2 (Co, Cu, Fe, Zr) 17 . In both cases the detailed analysis of the microstructure – property interaction by means of different high-resolution electron microscopy techniques, hysteresis loop measurements and the theory of micromagnetism results in a quantitative interpretation of the occurring hardening mechanisms, enabling a quite specific tailoring of optimized magnetic properties up to elevated temperatures.

Using examples, old and recent, from the metallurgical literature, we examine the sources of metastable behavior in binary and ternary alloy systems that are subjected to cooling rates in the order of K/min to K/s. Because the metastability invariably involves enhanced unstable or suppressed stable crystalline phases, the key to this behavior must be sought in the crystallization kinetics.

In this brief review, an attempt has been made to bring out clearly the role of long-range interactions (such as dipole – dipole interactions or isotropic exchange interactions decaying with interspin distance ‘ r ’ as J ( r ) ~ r –(d+ σ ) (0 < σ < 2), where ‘ d ’ is the space dimensionality) (i) in determining the thermodynamic properties of ferromagnets (in which the electrons responsible for magnetism are either localized or itinerant ) in the asymptotic critical region, (ii) in causing a crossover from one critical regime to the other, and (iii) in stabilizing long-range magnetic order in site-diluted and/or bond-diluted ferromagnets with composition just above the percolation threshold. In the process, a number of long-standing/puzzling issues concerning the nature of magnetic order, static critical phenomena and crossover scenario in a wide variety of ferromagnetic systems have been resolved.

In several refractory body-centred cubic metals (α-Fe, V, Nb, Ta) the binding energy of positrons (e + ) trapped in vacancies is too small to permit accurate determinations of the enthalpy of formation of monovacancies, H1VF, $ H_{{\rm{1V}}}^{\rm{F}},$ by high-temperature positron annihilation. Owing to their larger mass, trapped positive muons (μ + ) and π-mesons (π + ) are much more firmly bound to vacancies. It is argued that the lattice steering (channelling or blocking) of their charged decay products (e + or μ + ) allows us to obtain accurate H1VF $ H_{{\rm{1V}}}^{\rm{F}}$ values of the refractory bcc metals. In ferromagnets with high Curie temperatures T C , such as α-Fe, Co, and FeCo alloys, H1VF $ H_{{\rm{1V}}}^{\rm{F}}$ may also be deduced from muon spin rotation (μ + SR) measurements. However, in Fe and Co this approach is limited by the strong sensitivity of the spontaneous magnetization against temperature fluctuations near T C . The reduction of this sensitivity in the so-called asymptotic critical regime by applying sufficiently strong external magnetic fields is investigated on the basis of the Arrott– Noakes equation. A method for determining the critical amplitudes occurring in this equation is proposed. In disordered Fe 1–x Co x alloys (0.2 ≤ x ≤ 0.75) the Curie temperatures are sufficiently high for the spontaneous magnetization in the bcc phase not to be critically affected by temperature fluctuations, hence these alloys are well suited for μ + SR investigations of thermal vacancies. From an analysis of the available positron-annihilation and self-diffusion data the vacancy migration enthalpy in disordered Fe 0.5 Co 0.5 is found to be (1.1 ± 0.2) eV, in good agreement with quenching data and with the value established for α-Fe.

Some properties of the flux-line lattice in conventional and high- T c superconductors are reviewed, with particular stress on phenomenological theories, nonlocal elasticity, irreversible magnetization curves, and influence of the specimen shape on the electromagnetic response.

A concept is introduced, using substrates for high-temperature superconductor (HTS) thin-film deposition beyond their usual application as chemical inert, lattice-matched support for the films. The substrates are applied as functional elements in order to controllably modify the growth mode of YBa 2 Cu 3 O 7– x thin films in the case of vicinal cut SrTiO 3 single crystals and, thus, generate regular arrays of pinning centers. Furthermore, the use of biaxial epitaxial strain, caused by lattice misfit of film and substrate, is briefly reviewed for HTS thin films and the application of this concept to tailor the properties of perovskite thin films in general is shown.

Superconducting permanent magnets form a completely new class of permanent magnets. Of course, they must be cooled to 77 K or below. At very low temperatures (24 K) their magnetization can be a factor of 10 higher than that of the best conventional magnets, providing magnetic forces and energies which are up to two orders of magnitude higher. These new supermagnets became only recently available by the extreme improvement of the quality of melt-textured massive YBa 2 Cu 3 O x samples. Besides having a high magnetization, these superconducting permanent magnets can freeze in any given magnetic field configuration allowing completely new applications like superconducting transport systems or superconducting magnetic bearings.

Quantitative magneto-optical imaging of magnetic flux distributions has developed in a powerful tool for the analysis of the local transport properties of superconductors. It allows a model-independent determination of the current density distribution of thin films and, thus, the local current density through individual defects. Also, local metastable properties are detectable, such as the local electric field distribution E with a high sensitivity down to 10 –12 V/m caused by thermally activated flux creep. Based on these tools, in this paper we present a systematic comparison of vortex pinning, vortex movement and current transfer of two kinds of planar defects which are typically present in high-temperature superconducting thin films: low-angle grain boundaries and antiphase boundaries. Special attention is drawn to the local magnetic field dependence of the critical current density and to the spatial distribution of E , giving insight into the collective behavior of vortices at planar defects.

A novel mechanism of atomic transport in amorphous metallic alloys, or metallic glasses, is proposed based upon the fluctuations in the local structure. The proposed mechanism is very different from those in crystalline solids and from the free volume model, and is characterized by the bond-exchange action triggered by the local topological instability of the atomic environment. The implications of this mechanism on the liquid fragility and bulk metallic glass formation are discussed.

After reminding the reader of the major techniques for measuring diffusion coefficients in solids, a detailed description of the conventional radiotracer techniques is presented. These allow to measure solid-state diffusivities in the regime 10 – 8 –10 – 24 m 2 s – 1 and therefore, and for other reasons to be discussed, are superior to all competing methods. However, the conventional radiotracer techniques suffer from the restriction that they are not applicable to radio-tracer atoms with half-lives t 1/2 ≲ 1 d. The main objective of this paper is to introduce a novel experimental set-up which enables us to perform radiotracer-type diffusivity measurements with short-lived radioisotopes (1 d ≳ t ≳ 10 min). First results on the diffusion of 31 Si ( t 1/2 = 2.6 h) and 11 C ( t 1/2 = 20.38 min) in Si–Ge alloys and Si –C–N-based ceramics measured in this set-up are presented.

A brief account is provided of the main results of a study of the tunneling states of H trapped by substitutional (S) impurities in Nb, mainly consisting in anelasticity experiments. The phenomenology is rather complex when the concentration of S atoms is higher than a few parts per thousand, and various and contrasting interpretations are possible. The complication arises from the destruction of the symmetry of the S–H pair by the elastic interactions among the defects. The situation becomes, however, clear for an S content around 0.1 %, when the anelastic spectra reveal the relaxation processes due to the now nearly undistorted S–H complexes. In this case H delocalizes over four tetrahedral sites of a face of the cube containing the S atom, giving rise to a four-level tunnel system (FLS). The parameters of tunneling and coupling to phonons and electron excitations are similar to those found for the two-level system of H near an interstitial impurity, but new effects are found, due to the symmetry of the additional eigenstates of a centrosymmetric FLS.

The magnetic relaxation spectrum of polycrystalline, unirradiated Co has been investigated from 4 to 700 K. Both polycrystalline and single-crystal hexagonal close-packed Co have also been investigated after 3 MeV electron irradiation at either 8 or 40 K to doses of 10 23 m –2 . For unirradiated Co, results are interpreted as due to interactions between dislocations and domain walls. After irradiation at 8 K, relaxation peaks occur in the range 10 –50 K due to the re-orientation of Frenkel pairs which stabilise at the start of the stage-I anneal; a relaxation at 75 K is attributed to an intrinsic interstitial trapped at interstitial carbon. On anneal in the range 220 – 400 K, low-temperature relaxations evolve in samples containing impurities; these are attributed to reorientation of the migrating stage-III defects trapped at impurities.

The effect of low-dose cation doping (0:005 < x < 0:08) of magnetite single crystals, Fe 3–x M x O 4 (M = Ni,Mg, Co, Al, Ti, Ga), has been studied by means of the magnetic after-effect (MAE) spectroscopy with respect to (i) the Verwey transition, (ii) the low-temperature (4 K < T < 125 K ≃ T V ) charge transport mechanisms and (iii) the zero-crossing of the crystal anisotropy. The observed low-temperature shifting of the transition ( T V ) is in fair agreement with previous conductivity measurements. Variations of the MAE spectra clearly indicate the low-temperature tunnelling (4 K < T < 35 K) to be far more affected by smallest impurity doping than variable long-range hopping (50 K < T < 125 K) – this outstanding sensibility of the tunnelling processes against impurities or any other defects is also true when compared with the corresponding T V shifting. All samples undergo a doping-induced temperature splitting, Δ T VC , between the Verwey transition (spontaneous jump of the susceptibility at T V ) and the zero-crossing of the crystal anisotropy (giving rise to a delayed susceptibility maximum) – in contrast to perfectly stoichiometric Fe 3 O 4 single crystals where both effects are coincident. This range of temperature-splitting Δ T VC , found to be extremely large in the case of Co 2+ doping, is characterized by destabilized magnetic domain structures due to locally disordered anisotropy distribution in the lattice.