H. Zijlstra

Domain‐Wall Processes in SmCo5 Powders

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.

Relation between coercive force and microstructure of sintered SmCo5 permanent magnets

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.

Critical Fields Determining Magnetic Coercivity in Microparticles of SmCo5 and LaCo5

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.

Evidence by Lorentz Microscopy for Magnetically Active Stacking Faults in MnAl Alloy

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...

Permanent Magnetic Properties of Iron‐Cobalt‐Phosphides

An investigation of the phase relations of the section Fe2P–Co2P in the system Fe–Co–P showed that at high temperature all (Fe, Co)2P alloys, including the final components Fe2P and Co2P, probably form a continuous range of solid solutions having the hexagonal Fe2P structure. At low temperatures Fe2P and Co2P form an almost complete range of solid solutions with the orthorhombic Co2P lattice. The Curie temperature of the hexagonal phase increases from about room temperature for Fe2P to a value of nearly 190°C for (Fe, Co)2P with an Fe:Co ratio of 70:30. The anisotropy field HA appeared to be a function of the Fe:Co ratio and possesses a maximum value of about 19 000 oe near an Fe:Co ratio of 90:10. Particles obtained by lixiviating an Fe–Co–Cu–P melt can possess a coercivity IHC up to 2000 oe. This value can be substantially increased by annealing. The temperature coefficient of IHC appears to be relatively large and seems to be linked to the change of K with temperature. The coercivity decreases apprecia...

Evidence by Lorentz Microscopy for Magnetically Active Stacking Faults

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...