Ferromagnetic Single Crystals Research Articles

Die Magnetisierungskurven von Fe - Si-Einkristallen in [010]-, [O11]- und [111]-Richtung unter Berücksichtigung mechanischer Spannungen und der Probenform

The spatial distribution of the total energy is used to determine the magnetic behaviour of ferromagnetic single crystals. For a single ferromagnetic domain there exists only one homogeneous magnetization, the direction of which must correspond to a minimum energy direction. The influence of the geometric dimension of a specimen is discussed for a Fe-3.5% Si steel disc with Goss-texture. Magnetization curves are calculated by observating the stable position of the magnetization under application of strong magnetic fields

The effect of mechanical stress on the magnetization curves of Ni- and FeSi-single crystals at strong fields

The spatial distribution of the total energy is used to determine the magnetic behavior of ferromagnetic single crystals. For a single ferromagnetic domain there exists only a homogeneous magnetization the direction of which must correspond to a minimum energy direction. Magnetization curves are calculated by observing the stable positions of the magnetization under the application of strong magnetic fields. These magnetization curves show significant variations under different states of mechanical stress for FeSi and Ni crystals. >

Brillouin Scattering in Itinerant Ferromagnet MnP

Thermal magnons within the optical skin layer of ferromagnetic MnP single crystal ( T c ∼290 K) have been studied by Brillouin light scattering in a temperature range between 80 K and 310 K under zero magnetic field. For temperatures below ∼ T c /2, the magnon energy gaps obtained from Brillouin scattering show good agreement with the bulk magnon gaps obtained from neutron scattering and torque method. Well below the Curie temperature, Brillouin scattering has been established as a convenient technique to determine the energy gaps in itinerant ferromagnets. The spins near the surface seem to undergo a surface magnetic phase transition at much lower temperatures than the bulk T c . A complete set of the elastic constants at room temperature has been determined.

Growth, structure and energetics of ultrathin ferromagnetic single crystal films on Mo(110)

The growth mode, structure and thermal stability of ultrathin Fe, Co and Ni films on Mo(110) surfaces is studied by Auger electron spectroscopy, low energy electron diffraction, work function change measurements at various temperatures at and above room temperature and by thermal desorption spectroscopy. In all cases pseudo-Frank-van der Merwe growth is found near room temperature, Stranski-Krastanov growth at intermediate temperatures and alloying at higher temperatures. In Co and Ni films structural rearrangements during growth occur already in the submonolayer range, in Fe films only towards the end of the second monolayer. Implications for the study of magnetic properties are discussed.

160TbTb(sc): A New Absolute Nuclear Orientation Thermometer for Use in High Magnetic Fields

We have studied the rare earth system consisting of radioactive 160Tb in ferromagnetic terbium single crystal in order to develop it as an absolute nuclear orientation thermometer for use at low temperatures and in high magnetic fields.

X-Ray Resonance Magnetic Scattering

A resonance-type X-ray magnetic scattering at the K -absorption edge was first observed in 220 Bragg reflection of ferromagnetic Ni single crystal. Further a theoretical discussion on the resonance magnetic scattering curve observed was carried out; A difference of the oscillator-strength density for each direction of the spins was also derived from this curve. The oscillator-strength density for minority spin excesses that for the majority spin between the Fermi level and 25 eV above the level. The difference of oscillator-strength density amounts to about 1×10 -4 /Ry at the maximum.

X-Ray Resonance Magnetic Scattering

A resonance-type X-ray magnetic scattering at the K -absorption edge was first observed in 220 Bragg reflection of ferromagnetic Ni single crystal. Further a theoretical discussion on the resonance magnetic scattering curve observed was carried out; A difference of the oscillator-strength density for each direction of the spins was also derived from this curve. The oscillator-strength density for minority spin excesses that for the majority spin between the Fermi level and 25 eV above the level. The difference of oscillator-strength density amounts to about 1×10 -4 /Ry at the maximum.

Field and Stress Dependence of the Magnetization in a Ferromagnetic Cubic System

The effect of uniaxial stress on the magnetization of a ferromagnetic single crystals of cubic structure is calculated by minimizing the free energy. Calculations are made for the case of a specimen in the shape of an oblate spheroid parallel to a (100)-plane, with field and stress applied in arbitrary directions in the plane of the specimen. The magnetization curves obtained show three separate regions. Analytical expressions are derived for the first part of the magnetization curve, the two other parts are calculated by means of numerical methods.

Temperature dependence and critical behaviour of the thermoelectric power in ferromagnetic TbZn compound

High-accuracy data on the thermoelectric power (S) are obtained for ferromagnetic single crystals of TbZn (CsCl structure), over a wide range of temperatures (80-300K). The general temperature dependence S(T) is analysed in the context of s-f scattering models. Information is obtained on such parameters as the s-f exchange constant G, the Fermi level EF, and the effect of simultaneous spin disorder and phonon scattering on the thermopower. Experiment suggests the possible existence of extra S contributions in TbZn, besides that arising from s-f scattering. Finally, the critical behaviour of dS/dT near the Curie point (Tc=199.6K) is investigated in detail, and related to that previously observed in the temperature derivative of the electrical resistivity (d rho /dT) in TbZn.

Thermally activated domain-wall motion in ferromagnetic single crystals

For picture-frame-type single-crystal specimens of the ferromagnetic nickel-base alloy PE 16, the dependence of the domain wall velocity v on the applied magnetic field H and on temperature T has been experimentally investigated for 1.7<T<5 K. The results are interpreted by assuming that the domain wall can overcome the interaction with obstacles present in the specimen by thermal activation. Interaction potentials, which have been in common use for such problems, e.g., triangular ones, failed to give agreement between theory and experiment. Therefore a formalism has been developed, by which information on the interaction force, F (x), between the obstacle and the domain wall can directly be obtained from the experimental data. By using F (x), a relation H (v,T) can be reconstructed which is in perfect agreement with the experimental results. The obstalces, which interact with the domain wall, are probably single atoms or small clusters.

Elastic properties of ferromagnetic single crystal with cubic symmetry

A group theoretical description is given of the first and second order magnetoelastic coupling for cubic ferromagnets. First, this coupling is expanded as a series of the Lagrangian tensor componentsηij, which definesN coefficients. An alternative expansion is given in terms of the symmetrical (eij) and antisymmetrical (ωij) components of the homogeneous strainsuij, which definesN(>N) coefficients. The conditions for rotational invariance of the energy are then written: they provide relations which reduce fromN toN the number of independent coefficients in the 2nd description. These relations were previously derived in the framework of the localised model and some features of this model are briefly discussed here: in particular, the exchange contributions to the elastic constants are corrected for volume effects and differ significantly from Fuch's and Sato's formulae. In the next step, the magnetostriction is taken into account to derive the internal energy of a distorted crystal. It is then possible to analyse the ultrasonic waves propagation under high magnetic fields in terms of three main magnetic contributions, namely an isotropic exchange effect, an anisotropic morphic effect and a field dependent Simon effect.

Hardening of ferromagnets by non-magnetic inclusions

Earlier measurements [1] of the coercivity H c of cylindrical ferromagnetic single crystals, hardened by spherical, non-magnetic particles, yielded agreement with current theories concerning the relation H c ( r). r particles' radius, but disagreed with the relation H c ( ν), ν their volume fraction. Since it seemed possible that a dependence of the domain configuration on ν, for which the theories did not allow, might affect H c ( ν), the measurements were repeated for picture frame single crystals, whose domain configuration is fixed. The result is: the discrepancy between H c ( ν) predicted theoretically and found experimentally is smaller for picture frames, but still considerable.

A capacitance torquemeter suitable for use in a superconducting split-pair magnet from 4 to 300K

An instrument is described which is capable of accurate, absolute measurements of the torque curves arising from the magnetocrystalline anisotropy of ferromagnetic single crystals in an applied magnetic field. A 'null' method is used in which the rotation against a stiff flat-spring suspension is measured by a capacitance technique. The torquemeter is suitable for use in a wide temperature range (4-300K) and in high magnetic fields (up to 4T). A detailed experimental investigation of the system is made and some preliminary results on a basal plane terbium sample and a (111) plane nickel sample are reported.

Magnetic and magnetoelastic properties of a ferromagnetic single crystal with cubic symmetry

A crystal hamiltonian is written from symmetry considerations, including magnetic and elastic contributions, first-order magnetoelastic coupling (magnetostriction) and second-order magnetoelastic coupling (isotropic and morphic effects). This phenomenological expression is then explicitly derived from microscopic interactions based on a generalized pair model. This work develops partial results concerning rotational and magnetoelastic effects, but the methods proposed here are simpler than the « rotational invariance theory » so widely used in the recent literature. The number of independent coefficients in this hamiltonian is strongly reduced by two sets of relations. The first one illustrates the rotational invariance theorem: all the coefficients associated with rotational terms are linear combinations of the anisotropy and (usual) magnetoelastic coefficients. The second one is established only for a localized model. In the classical case of isotropic magnetization modulus, an analytical expression of the free energy can be written: magnetic anisotropy, magnetoelastic effects and elastic properties are then discussed. Last, the quantum case with anisotropic magnetization is treated and illustrated with some recent experimental results. For instance, the tetragonal strains observed in the cubic rare earth compounds with antimony (R — Sb) and zinc (R — Zn) are analyzed through each series. The corresponding magnetoelastic coefficient is checked by determination from ultrasonic data: quadrupolar exchange effects are then taken into account.

Production of high circular polarization by ion surface scattering of ferromagnets

From a theoretical analysis we predict large fractions of circular polarization of light emitted by ions scattered from surfaces of ferromagnetic single crystals. Maximum enhancement of circular polarization as compared to surface ion scattering from non-magnetic materials is again expected at grazing incidence. A circular polarization of 89% was estimated for the case of Ar+ ions scattered from the (110) plane of a Ni single crystal.

Coercivity of picture-frame single crystals

The coercivity H c of ferromagnetic picture-frame single crystals has been investigated experimentally. The material was the commercial alloy PE 16, which could be hardened by non-magnetic, spherical inclusions, which do not strain the matrix. A picture-frame single crystal has been used, since only one 180°-Bloch wall surrounds the whole specimen. With the aid of an electronic circuit this Bloch wall could be moved with constant velocity. The observed dependence of H c on the inclusion's volume fraction is found to be in rather poor agreement with current theories. For cylindrical specimens, the domain configuration seems to depend on the particles' volume fraction.

Magnetic excitations on ideal and atomically rough surfaces

The surface density of states for the magnetic excitations of ferromagnetic single crystals is calculated within the Heisenberg model by the continued fraction method. In particular, the influence of adsorption is studied. Large resonance effects are observed at the adsorbed magnetic atoms and their nearest neighbours.

The magnetization process in hexagonal ferromagnetic and ferrimagnetic single crystals

Neel's 'theory of phases' is used to study the magnetization process in hexagonal ferromagnetic and ferrimagnetic single-crystal ellipsoids for which the six fold contribution to the magnetocrystalline anisotropy cannot be neglected. Simultaneous equations are presented, the solution of which yields the intensity and orientation of the mean sample magnetization and the various orientations of the saturation magnetization in the domains, for all values and orientations of the magnetic field. In some cases, the fractional contents of different types of domain can also be determined. Detailed consideration is given to the three special cases of oblate spheroids the minor ellipsoidal axes of which are parallel to the principal hexagonal axes when the magnetic field is in the plane of the sample. In addition, it is shown that the magnetization process is simplified for a number of special cases in which the magnetocrystalline anisotropy assumes particularly good simple forms. A comparison of the results of the theory with the limited experimental data available is made and, as an example of the application of the theory, the magnetization of gadolinium and dysprosium is calculated as a function of the intensity and direction of the magnetic field.

Magnetic-field-induced recrystallization in MnBi

A magnetic annealing effect has been observed in a ferromagnetic MnBi single crystal. It was found that by heating an MnBi single crystal in a magnetic field perpendicular to the easy axis of magnetization of the crystal, the easy axis of the magnetization and the hexagonal c axis of the crystal would change into the direction parallel to the field through a recrystallization process induced by the magnetic annealing. This recrystallization phenomenon is similar to that observed by Boothby et al. on a polycrystalline sample. Based on the microstructure investigation and transformation kinetic analysis, it was found that the mechanism of the recrystallization in a single crystal is a thermally activated nucleation and interface controlled growth process. It was established that the magnetocrystalline anisotropy energy of the crystal is responsible for the recrystallization.

Positron Annihilation in Ferromagnetic Nickel

The angular correlation of annihilation radiation in ferromagnetic nickel single crystals has been measured by using standard parallel-slit system with a newly developed 90 Nb positron source. The crystals were oriented in the [100] and [110] directions. The observed angular distribution curves were anisotropic with appreciable structures. Interpretation of the angular distribution in terms of the shapes of the Fermi surfaces alone were not successful, indicating that consideration of the Bloch character of the electrons in 3d bands is important.