The intrinsic magnetic material parameters of the 2: 17 type matrix phase and the 1: 5 type precipitation phase in sintered multiphase Sm(Co0.68Cu0.08Fe0.22Zr0.02)7.6 permanent magnets are determined in the whole ferromagnetic temperature regime between 10 K and Tc = 1070 K. Magnetization curves measured parallel and perpendicular to the alignment axis are analysed taking into account the volume fractions of ferromagnetic phases, their magnetostatic interactions, and the angular distribution of grain axes. The influence of annealing treatments on the intrinsic magnetic properties of the relevant ferromagnetic phases are investigated. Die intrinsischen magnetischen Materialparameter der 2: 17 Matrixphase und der 1: 5 Ausscheidungsphase in gesinterten Mehrphasen-Sm (Co0,68Cu0,08Fe0,22Zr0,02)7,6-Permanentmagneten werden im gesamten ferromagnetischen Temperaturbereich zwischen 10 K und Tc = 1070 K bestimmt. Die parallel und senkrecht zur Ausrichtungsachse gemessenen Magnetisierungskurven werden unter Berücksichtigung der ferromagnetischen Volumenanteile, ihrer magnetostatischen Wechselwirkungen und der Winkelverteilung der Kornachsen analysiert. Der Einfluß von Temperatugen auf die intrinsischen magnetischen Eigenschaften der relevanten ferromagnetischen Phasen wird untersucht.

Determination des champs de nucleation d'aimants frittes sur la base du micromagnetisme en prenant en compte la deterioration due aux champs parasites locaux, aux grains desalignes et aux constantes d'anisotropie reduites aux surfaces de grains

The magnetic hysteresis loops for sintered multiphase Sm(Co0.68Cu0.08–Fe0.22Zr0.02)7.6 permanent magnets are investigated in the whole ferromagnetic temperature range between 10 K and Tc=1070 K. The measured temperature and field dependence of the coercive field is analysed within the framework of theoretical results for pinning fields of different magnetic phases. The influence of annealing treatments on the coercive field is compared with the intrinsic magnetic properties of the appearing ferromagnetic phases of 2:17 and 1:5 type. It is concluded that in the whole ferromagnetic temperature range nucleation of reversed domains and pinning of domain walls simultaneously occurs in the 1:5 type precipitation structure. For a quantitative interpretation of the coercive field a consideration of gradients of the magnetic material parameters inside coherent phase boundaries is necessary. A phenomenological model for phase boundaries is presented which qualitatively explains the measured hysteresis loops and coercive fields. Magnetische Hystereseschleifen werden für gesinterte Mehrphasen-Sm(Co0.68Cu0.08–Fe0.22Zr0.02)7.6 Permanentmagnete im gesamten ferromagnetischen Temperaturbereich zwischen 10 K und Tc = 1070 K untersucht. Die gemessene Temperatur- und Feldabhängigkeit des Koerzitivfeldes wird im Rahmen der theoretischen Ergebnisse für Pinningfelder verschiedener magnetischer Phasen analysiert. Der Einfluß einer Anlaßbehandlung auf das Koerzitivfeld wird mit den intrinsischen magnetischen Eigenschaften der real auftretenden ferromagnetischen Phasen vom 2:17- und 1:5-Typ verglichen. Es wird angenommen, daß im gesamten ferromagnetischen Temperaturbereich Keimbildung von inversen Domänen und Pinning von Domänenwänden simultan in der 1:5-Typ-Ausscheidungsstruktur auftritt. Für eine quantitative Interpretation des Koerzitivfeldes ist eine Betrachtung der Gradienten der Materialparameter innerhalb der kohärenten Phasengrenzen notwendig. Ein phänomenologisches Modoll der Phasengrenzen wird angegeben, das die gemessenen Hystereseschleifen und Koerzitivfelder qualitativ erklärt.

For the interpretation of extremely high coercive fields of RE-NdB permanent magnets in general either models of pinning or nucleation mechanisms are considered. In both cases the magnetic properties of the phase boundaries between the grains are suggested to be responsible for the actual value of Hc. In this paper a detailed analysis of the temperature dependence of Hc is given on the basis of the predictions of micromagnetic theories for pinning and nucleation mechanisms. It is shown that the nucleation theory leads to a far more coherent interpretation of all relevant properties of Hc than the pinning theory if the effects of misaligned grains, local stray fields and reduced anisotropies in grain boundaries are taken into account.

The intrinsic nucleation and instability fields of uniaxial single domain particles are determined including the effects of the second anistropy constant and of oblique applied magnetic fields. The theoretical results for the angular dependences of the nucleation and pinning fields are compared with the angular dependence of the coercive field as measured for oriented sinter magnets of composition Fe77Nd15B8. The experimental results are compatible with the assumption that at room temperature the coercive field in Fe77Nd15B8 is determined by a nucleation mechanism.

The magnetic domain structures of sintered Fe77Nd15B8 permanent magnets are observed under applied fields. The increase of the coercive field with increasing “saturation field” is shown to be due to the sequential removal of residual reverse domains. For magnetically saturated samples the demagnetization process starts with nucleation of reversed domains in grain boundary regions between Fe77Nd2B bulk grains which are adjacent to nonmagnetic Fe4NdB4 grains, pores, or oxides. Further magnetization reversal processes occur in channels nearly parallel to the alignment axis by expansion of reversed domains into adjoining Fe14Nd2B grains. It can be shown that strongly misoriented Fe14Nd2B grains possess considerably lower nucleation fields for reversed domains than well aligned Fe14Nd2B grains. Die magnetischen Domänenstrukturen von gesinterten Fe77Nd15B8-Permanentmagneten werden unter angelegten Feldern beobachtet. Es wird gezeigt, daß das Anwachsen des Koerzitivfeldes mit steigendem „Sättigungsfeld”︁ auf die nachfolgende Beseitigung der restlichen inversen Domänen zurückzuführen ist. Für magnetisch gesättigte Proben beginnt der Entmagnetisierungsprozeß mit der Keimbildung von reversen Domänen in Korngrenzenbereichen zwischen Fe14Nd2B-Volumenkörnern, die zu nichtmagnetischen Fe4NdB4-Körnern, Poren oder Oxiden benachbart sind. Weitere Magnetisierungsumkehrprozesse treten in Kanälen nahezu parallel zu der Ausrichtungsachse durch Expansion der inversen Domänen in benachbarte Fe14Nd2B-Körner auf. Es kann gezeigt werden, daß stark fehlorientierte Fe14Nd2B-Körner beträchtlich niedrigere Keimbildungsfelder für inverse Domänen als gut ausgerichtete Fe14Nd2B-Körner besitzen.

The magnetic hysteresis loops have been investigated in the temperature range between 4.2 and 575 K for aligned sintered permanent magnets of nominal composition Nd15Fe77B8 and for isotropic melt-spun ribbons of composition Nd15Fe76B9. The measured temperature and field dependence of the coercive field is analysed within the framework of theoretical results for nucleation fields of the ideal Nd2Fe14B matrix and, of disturbed surface regions of Nd2Fe14B grains. Furthermore the pinning of domain walls at thin soft magnetic grain boundary phases is considered for the high temperature range. It is concluded that for both types of NdFeB magnets the relevant magnetic hardening mechanisms at lower temperatures are nucleation processes in magnetically inhomogeneous regions whereas at higher temperatures the pinning of domain walls at grain boundaries predominates. The critical temperature where the change of nucleation hardening to pinning hardening occurs depends sensitivity on the crystal anisotropy, the grain boundary microstructure and the macroscopic grain- and multi phase arrangements.

The magnetic anisotropy constants K1 and K2 of Nd2Fe14B have been determined in the temperature range between 4.2 and 575 K using aligned sintered Nd15Fe77B8 magnets. Magnetization curves measured perpendicular to the aligning direction were analysed taking into account the volume fraction of the Nd2Fe14B phase and the angular distribution of grain axes. The grain alignment was determined by means of magnetic domain observation and X-ray analysis. It is shown that for sintered magnets an analysis under the assumption of the validity of the so-called Sucksmith-Thompson plot leads to significant errors especially of K2. As a further method for the determination of the anisotropy constant K1 measurements of the magnetic initial susceptibility parallel to the alignment direction are proposed. The experimental results of both methods are compared with each other.

The spontaneous magnetization and the coercive field of sintered Sm(Co, Fe, Cu, Zr)7.6 alloys of cellular microstructure which were annealed at 775°C (sample A1) and also at 400°C (sample A2), were measured in the temperature range between 4.2and 1200 K. Materials A1 show an intrinsic type magnetic hardness with exponential type temperature dependence at cryogenic temperatures. Samples given subsequent heat treatments at lower temperatures (A2) show a more or less linear temperature dependence of the coercive field resulting in μ0HC = 7.6 T at 4.2 K. Implications of these findings for an understanding of the magnetic hardening process in cellular magnets are discussed. Die spontane Magnetisierung und die Koerzitivfeldstärke von gesinterten Sm(Co, Fe, Cu, Zr)7.6−Legierungen mit zellularer Mikrostruktur wurden, nach einer Wärmebehandlung bei 775°C (Probe A1) und bei 400°C (Probe A2), im Temperaturbereich zwischen 4,2 und 1200 K unter-sucht. Die Probe A1 zeigtbei tiefen Temperaturen einen intrinsischen magnetischen Härtungs-mechanismus mit einer exponentiellen Temperaturabhängigkeit. Bei tieferen Temperaturen zeigen wärmebehandelte Proben (A2) näherungsweise eine lineare Temperaturabhängigkeit der Koerzitivfeldstärke mit μ0Hc = 7,6 T bei 4,2K. Die Bedeutung dieser Ergebnisse fur das Verständnis der magnetischen Härtungsprozesse in Zellularmagneten werden diskutiert.

In multiphase Sm(Co,Fe,Cu,Zr)7.6alloys the relations between the microstructure and magnetization processes have been analysed after different annealing treatments by investigations of the domain structure and of the characteristic properties of the hysteresis loop. It is shown that large coercive fields may be attributed to concentration gradients of Cu in the phase boundary between the 5:1 and the 17:2 phases.