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The coercive force of a sintered SmCo5 permanent magnet is found to depend reproducibly on the temperature of heat treatment. In order to find out the origin of this, the mechanism of the demagnetization was studied by magnetic‐domain observation, using the polar Kerr effect. It appears that the highest coercive forces which are obtained by heat treating at 850–950 °C and quenching are due to difficult nucleation of reverse domains and strong pinning of domain walls by the grain boundaries. After quenching from higher temperature (∼1100 °C) the coercive force decreases owing to weaker pinning of the domain walls. The drop of the coercive force which is found to occur upon heating at lower temperature (∼700 °C) comes from an easy nucleation of reverse domains. Indirect evidence is presented which supports the assumption that this easy reverse‐domain nucleation occurs on Sm2Co17 precipitates. The Sm2Co17 precipitation is governed by the Co‐rich SmCo5 phase boundary and its metastable extension below the eutectoid temperature of SmCo5. The state of the grain boundary which may be responsible for the strong pinning is discussed.

Measurements of minor loops of SmCo5 powders have revealed that wall motion is predominant in the magnetization process. A relation between the reversible susceptibility χ of the minor loops and the coercivity Hc has been found, showing that the coercivity of the investigated material is determined by the pinning of domain walls, rather than nucleation. The relation χHc2 = constant has been found in a wide range of coercivities obtained by aging of the material. The aging process is interpreted as a decrease of the density of pinning sites, each site retaining its pinning force. The influence of hydrogen on Hc, which is reported recently, is interpreted likewise. A hysteresis loop measured on a single particle of a few microns size confirms these interpretations and allows for a refined model of pinned‐wall coercivity of SmCo5 powders. A full account will be submitted as a regular article to this Journal.

Magnetic hysteresis curves measured by a specially designed magnetometer on a single particle taken from a ground SmCo5 powder show that the coercivity is determined by pinning of domain walls. Heat treatment at 100°C in air reduces the pinning force. Magnetizing the particle in a strong field increases the pinning force. A wall‐pinning model is proposed to explain this effect. The coercivity of a particle taken from the powder after electrolytic etching seems to be determined by nucleation of domain walls. Heat treatment at 100°C in air does not significantly influence the field at which nucleation occurs. The coercivity of an LaCo5 particle is determined by pinning. The pinning force is reduced by hydrogen absorption.

In the ordered MnAl crystal containing magnetic atoms which are coupled ferromagnetically, stacking faults are postulated through which an antiferromagnetic coupling exists between the atoms on either side of the fault. These account for the anomalous magnetic behavior of fine MnAl powder. Such faults give rise to a domain structure the walls of which are strongly pinned to these crystallographic defects. A direct evidence for the occurrence of these stacking faults is obtained by using Lorentz electron microscopy, whereby shadow patterns of opaque MnAl particles show a characteristic distortion of their contours when the microscope is used out of focus. The distortion is interpreted in terms of stray fields due to the domain structure.By using either the normal objective lens, which is an immersion lens, or the diffraction lens, the pattern can be studied with or without a strong applied field. Fields up to 9000 Oe could be applied. Domain structures were observed that showed a remarkable indifference fo...

The 4A2 ground state of tetrahedrally coordinated divalent cobalt ions in Cs3CoCl5 and Cs3CoBr5 is investigated with magnetic‐susceptibility measurements and paramagnetic resonance in pulsed magnetic fields up to 90 kOe at frequencies in the 70‐ and 120‐Gc/sec bands. The first excited state (4T2) which lies ∼3000 cm−1 above the ground state is studied optically on Cs3Zn0.9Co0.1Cl5. The sign of the splitting of the 4T2 state is opposite to that expected from the observed negative zero‐field splitting of the ground state.

Composites made up of MnBi needles embedded in bismuth single crystals are grown from eutectic melts by both Czochralski and zone‐melting techniques. The galvanomagnetic properties of the composite are determined mainly by the bismuth matrix. Magnetic measurements reveal a magnetic phase with an anisotropy field of more than 200 kOe at 77°K and a Curie point of 240°K. This phase is believed to be a modification of MnBi different from the two already known. The solubility of Mn in solid Bi is concluded to be less than 30 wt ppm.

A study was made of magnetic anisotropy and interparticle distance in “Ticonal” G (a dispersion-hardening permanent-magnet alloy) when subjected to heat treatment in a magnetic field. The experimental results agree with a theoretical model in which the precipitate particles are idealized to prolate spheriods which decrease their interfacial free energy by coarsening and their magnetostatic free energy by elongation. It is concluded that the response to thermomagnetic treatment increases with the ratio between the magnetostatic free energy and the interfacial free energy. The interface tension of “Ticonal” G is found to be extremely low, of the order of 1 erg cm−2at the annealing temperature.

An instrument for measuring magnetic anisotropy by recording magnetic torque curves is described. The special construction of the transducer which converts the torque exerted on the sample into an electric signal makes it possible to avoid bearings, so that the instrument is essentially free of friction. The instrument is operated with commercially available electronic apparatus and is suitable for routine measurements on large numbers of samples. The maximum sensitivity is 150 d cm per centimeter deflection of the recorder stylus.

In the ordered MnAl crystal containing magnetic atoms which are coupled ferromagnetically, stacking faults are postulated through which an antiferromagnetic coupling exists between the atoms on either side of the fault. These account for the anomalous magnetic behavior1 of fine MnAl powder. Such faults give rise to a domain structure the walls of which are strongly pinned to these crystallographic defects. A direct evidence for the occurrence of these stacking faults is obtained by using Lorentz electron microscopy, whereby shadow patterns of opaque MnAl particles show a characteristic distortion of their contours when the microscope is used out of focus. The distortion is interpreted in terms of stray fields due to the domain structure. Experiments were carried out in a Philips EM 200 electron microscope.By using the normal objective lens, the pattern can be studied with or without a strong applied field. Fields up to 9000 Oe could be applied. Domain structures were observed that showed a remarkable indi...