Recent progress in the field of permanent magnets K. Hoselitz To cite this version: K. Hoselitz. Recent progress in the field of permanent magnets. J. Phys. Radium, 1951, 12(3), pp.448-458. 10.1051/jphysrad:01951001203044800. jpa-00234404 HAL Id: jpa-00234404 https://hal.archives-ouvertes.fr/jpa-00234404 Submitted on 1 Jan 1951 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. LE JOURNAL DE PHYSIQUE ET LE RADIUM. TOME 12, MARS 1951~ PAGE 448. RECENT PROGRESS IN THE FIELD OF PERMANENT MAGNETS By K. HOSELITZ. Sommaire. - Après un bref rappel des conditions techniques exigées pour la construction des aimants permanents, le rapport classe les alliages magnétiques en quatre types principaux : les aciers à aimants, les alliages durcissant par précipitation, les alliages durcissant par diffusion et les aimants en poudres comprimées. Les aciers ne sont pas étudiés et quelques alliages à durcissement par précipitation seulement sont mentionnés : le « vicalloy » et les alliages fer-nickel-cuivre. On étudie, de façon plus détaillée, les alliages durcissant par diffusion et, après un bref historique de leur développement, on décrit les plus récents et les plus utiles d’entre eux, comme l’alcomax et l’alnico V. On tente d’expli- quer le champ coercitif élevé et l’anisotropie de ces matériaux. Leur champ coercitif élevé est attribué à l’hétérogénéité magnétique des alliages et les résultats expérimentaux sont en accord quantitatif raisonnable avec la théorie. L’origine de l’anisotropie réside probablement dans la forme des aggrégats hétérogènes, due à l’action du champ magnétique pendant le refroidissement. On indique les derniers progrès réalisés avec la construction des éléments à cristaux orientés parallèlement, qui ont des propriétés magnétiques supérieures à celles des aimants à cristaux orientés au hasard. Technical Requirements. - In the majority assumption that the major hysteresis loop in the of applications of permanent magnet materials second quadrant obeys an algebraical equation, the salient requirement is that a high magnetic usually of the second order and whilst such design field should be maintained in an air gap of the principles are possibly of considerable practical magnet system. The system must often be stable importance they are our not thought to be of funda- against external influences of changing temperature, mental scientific interest. Useful ,contributions to mechanical vibration or shock or electromagnetic the subject have been made by Hornfeck and fields and this should be achieved preferably with Edgar [I940], Desmond [1945, 1949] and San- a minimum of magnet alloy. For this purpose ford it is necessary that the magnet system is correctly Even greater complications arise in the case of designed. The basic principles governing the design external magnetising forces, arising for instance, of permanent magnets working under static condi- from armature reaction in a generator or magneto. i.e. in where there are no considerable some . tions, These also to been dealt . systems have, extent, external demagnetising fields, have been adequa- with by the above authors. In the present account tely described by Evershed [1920]. From these design considerations are not included because considerations it is clear that the most important a superficial description would be inadequate and a characteristic of a permanent magnet material detailed treatment would make this report into- is its product and the flux density and lerably long. demagnetising field strenght corresponding to the > In principle it is considered that the most impor- The of the point. significance tant characteristics are those of the major hyste- has been described and point extensively variously resis loop, probably with the addition of the rever- in the literature and I do not to existing propose sible permeability or recoil permeability. Consi- enter any further into this question. derable ambiguity is found in the values quoted If the magnet is applied in an instrument or for recoil permeability of permanent magnet mate- which involves either a external apparatus varying rials by various authors and hence no figures are reluctance or a varying external magnetising force, given in the present report. the of the economic utilisation of the question In an efficient permanent magnet material in material suffers some modification and the slight general one desires a high coercive force, a high minor have to be considered, as hysteresis loops remanence and a high energy product. Further- has been out Evershed. For the already pointed by more the manufacturing process will govern the case of external reluctance the varying only, working external shape of the magnet and in some cases point of the permanent magnet should preferably the mechanical properties will influence the appli- be chosen on that minor hysteresis loop on which cation. the useful recoil is a maximum. A number energy There exists four main of of workers have to find types permanent attempted algebraical materials : solutions to the problem of determining the optimum magnet magnet dimensions for a given application from the Firstly magnet steels, which are essentially carbon demagnetisation curve of the permanent magnet steels with additional elements. These are by now material. Such solutions usually rely on the standardised and cannot be claimed in any way Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01951001203044800 449 to be classed as recent developments. They are, for the tabulation of their main properties in therefore, not dealt with in this account, except Table I. ° TABLE I. Composition, magnetic, physical, mechanical properties anelheat treatlnent of magnet steels. Nominal composition percent (1) Unless otherwise specified; steels are available in Great Britain. (2) Equivalent to British steel, available under the name stiown in country inrlicated by letter G. : Germany : U. S. A. : United States of America. TABLE II. Jfagnetic properties of precipitation Hardening klagnetic aLloJ"s. Secondly there are various types of precipitation Thirdly there are the diffusion hardening magnet hardening permanent magnet alloys which can be alloys (Table III) based on the iron-nickel-alu- worked by hot or cold working methods. Here minium system with additions of one or more of we have the conventional alloys such as iron-cobalt- the elements cobalt, copper, titanium and niobium. molybdenum, iron-nickel-copper, and iron-cobalt- By far the greatest number of modern permanent vanadium. Many of the precipitation hardening magnets are made from one of the many good alloys alloys can be cold worked and after severe cold of this group. Some of these alloys exhibit a pro- reduction a pronounced anisotropy in the rolling nounced magnetic anisotropy when heat treated in a or wire drawing direction is observed. The manu- magnetic field and these anisotropic alloys are the most facture of these materials is usually expensive and efficient modern permanent magnet alloys. The they are used only to a small extent. Their charac- iron-nickel-aluminium alloys are all brittle and hard teristics are quoted in Table II. and at present there is no way of making them workable. 450 MARLE III. Magnetic properties of diilusion hardening Magnet alloys. TABLE IV. Magnetic properties of permanent magnet materials made from Consequently these alloys have to be cast approxi- of powder particles each of which are smaller than mately to shape and final accurate dimensions have a given critical size exhibit large coercive forces to be achieved by grinding. and this fact is used in manufacturing permanent Finally there is a large number of various per- magnets out of chemically produced colloidal powders manent magnet materials made by processes of of pure iron or iron-cobalt. powder metallurgy, but it does not appear at present as though their practical importance would justify Precipitation Hardening Permanent Magnet a very extensive discussion. Some of these materials Alloys. ---- It was suggested by Seljesater and are included in Table IV. Rogers [1931] and K6ster [I932 a, b] that preci- The most notable discoveries along newer lines, pitation of a second phase from a supersaturated are based on the magnetic properties of ferro- solution could be utilised in producing materials of magnetic materials in a fine state of sub-division high magnetic hardness for permanent magnets. (N6el, Weil and Aubry; 1942). Magnets composed The alloys investigated where those which form 451 extended solid solutions with iron at high temperatures for 3 hours at 65oo C. The material is now magne- with decreasing solubility at low temperatures, i.e. tically hard and is further cold rolled to the final iron-molybdenum, iron-tungsten, iron-beryllium. and dimensions where the reduction in area should iron-titanium. In order to obtain a supersaturated be about 80 to go per cent. The magnetic properties solid solution these alloys have to be quenched are somewhat spoilt during cold rolling, but further