Eckhard Nembach

Elastic properties (the stiffness constants, the shear modulus and the dislocation line energy and tension) of NiAl solid solutions and of the Nimonic alloy PE16

Abstract The stiffness constants Cik of the following nickel-base alloys were measured: NiAl solid solutions with aluminium contents up to 8 at.%, Nimonic alloy PE16 and an experimental alloy with the same composition as the matrix of fully precipitated PE16. The temperature range covered is 80–360 K. From the stiffness Cik a shear modulus and the dislocation line energy and tension are derived. Extrapolation formulae, which describe the temperature and concentration dependences of the elastic parameters, are given for the range 0–550 K.

Tunneling of domain walls in ferromagnetic materials

For picture‐frame‐type specimens of various ferromagnetic alloys, the dependence of the domain wall velocity v on the external magnetic field H and on temperature T has been experimentally investigated. T ranged from 1.4 to 5 K and v from 1 to 106 μm/s. For four alloys, v varied exponentially with H and T. A model has been developed which describes the function v(H,T) found experimentally. The basic assumptions of this model are: (i) v is governed by the interaction of the domain wall with small obstacles present in the specimen. (ii) These obstacles can be overcome by a combination of two processes: thermal activation and tunneling. The experimental data are in excellent agreement with this model. Below 2.0 K, v is nearly exclusively governed by tunneling. The obstacles, which pin the domain walls, are probably small clusters of atoms.

Low temperature mobility of domain walls in ferromagnetic nickel alloys

Abstract For various nickel-base alloys the dependence of the domain wall velocity on the applied magnetic field and on temperature has been experimentally investigated below 5K. The velocity is governed by the interaction of the domain wall with obstacles present in the specimen. The wall overcomes the obstacle by thermal activation and tunneling.

Characterization of periodic composites by laser‐beam diffraction

A method has been developed for deriving the characteristic lengths of periodic composites by diffracting a laser beam from their surface and analyzing the diffraction pattern. This method has been applied to two directionally solidified lamellar eutectics and to a fibrous composite produced by wire drawing. From the diffraction pattern the periodicity length, the width of the bright phase, and the effective fluctuation of these two lengths have been computed. The latter fluctuation characterizes the perfection of the composite.

Coercivity of single-domain nickel wires

Abstract The coercivity H c of compacts of thin (0.1−5 μ diameter) nickel wires embedded in a silver matrix has been measured as a function of the effective wire radius d eff . H c increases linearly with 1/ d eff 2 , in agreement with Aharonis theory of magnetization reversal by curling. From the slope, δH c/ gd ( d eff −2 ) , upper and lower bounds for the exchange constant A of Nickel have been derived (77K): 1.5 × 10 −11 Wsec/m ⩽ 3.0 × 10 −11 Wsec/m.

Tunneling and zero‐point vibrations of 180°‐domain walls in ferromagnetic materials

The experimentally established dependence of the velocity v of 180°‐domain walls in four ferromagnetic Ni‐base alloys on the applied magnetic field H and on temperature T is analyzed. T ranges from 1.3 to 5 K. The relation v(H,T) is governed by the interaction of the domain walls with obstacles. The temperature dependence of the walls’ release from these obstacles is derived from Gibbs’ formula. Below 2 K, quantum‐mechanical effects noticeably enhance the release of domain walls from obstacles. This is analyzed in two alternative models: (i) tunneling of the domain wall is allowed for; (ii) the wall is treated as a harmonic oscillator, whose eigenstates are calculated quantum‐mechanically. Zero‐point vibrations are included in this model. These models lead to two alternative functions v(H,T). Both of them represent the experimental data nearly equally well, though the agreement for model (i) (tunneling) is slightly better. Fitting the two functions to the data yields two sets of adjustable parameters. An ...

Recovery and recrystallization of cold-rolled nickel in magnetic fields

Abstract Cold-rolled nickel was annealed in a.c. and d.c. magnetic fields of 16 kA m−1. Measurements of the coercivity, the electrical resistance and the Vickers hardness showed that the effects of the magnetic fields on recovery and recrystallization are negligible. This is in contrast with the results published by Borisova and Starodubov, who observed that the fields had a marked effect.

Thermo-magnetic treatments of metglasses

Abstract Metglasses 2605, 2605A, and 2826 have been heat-treated in ac and dc magnetic fields. The application of these fields decreases the coercivity and the magnetic anisotropy of the specimens. The effects are probably due to local atomic rearrangements.

Domain wall mobility in ferromagnets and ferrites

Abstract Below 5 K, the relation H (ν, T) (H = applied magnetic field, ν=velocity of 180°-domain-walls, T = temperature) has been experimentally established for various ferromagnetic Ni-base alloys and one ferrite. ν is governed by the interaction of domain walls with pinning centers present in the specimen. They are overcome by thermal activation and tunneling. A model is presented which allows for these two processes. For the ferrite, H(ν, T = const.) has a maximum which is most pronounced at the lowest temperatures. It is probably caused by heating of the specimen by hysteresis losses.

The effect of thermal activation and quantum-mechanical tunneling on the AC-susceptibility

The ac-susceptibility χ of a ferromagnetic Ni-base alloy has been measured below 5K. The experimental data can be represented by a relation derived earlier for the unidirectional motion of 180°-domain walls. This relation is based on the assumption that the mobility of 180°-domain walls is governed by their interaction with obstacles present in the specimen. These pinning centers are overcome by combinations of thermal activation and quantum-mechanical tunneling.