for Physics 1970

The Royal Swedish Academy of has awarded the 1970 Nobel Prize jointly to two European : to H. Alfvén for his establishment of the fundamental principles of magnetohydrodynamics which decisively influenced the development of plasma physics, and to L. Néel for his research and funda­ mental discoveries regarding antiferromagnetism and ferrimagnetism, which have important applications in the physics of solids. An assessment of these men and their work is given below.

the predicted magnetic decomposition that there could exist, below the Curie of the solid into two sublattices. point, a compensation temperature at In 1936 Néel also showed theore­ which the magnetizations on the two tically that antiferromagnetism must sublattices are equal in magnitude be characterized by the existence of but opposite in sign, so that the net a critical external magnetic field. macroscopic moment becomes zero. Passing through this critical value the The moment of such a permanent magnetic susceptibility exhibits a magnet would change its sign at that discontinuity. Sixteen years later this temperature. A few years later, E.W. curious phenomenon was detected Gorter in Eindhoven found that a experimentally by C.J. Gorter and N.J. mixed ferrite of lithium and chromium Poulis in CUCI2.2 H2O. has this property. In 1948, Néel succeeded in inter­ The theory of ferrimagnetism en­ preting for the first time the magnetic abled Néel and Bertaut to elucidate properties of spinel structures of the the properties of pyrrhotine, Fe7S8, a Louis Néel ferrites, which had remained a mys­ mysterious substance on which Weiss Louis Néel received the award in tery to physicists for fifty years. He had already written important papers recognition of over forty years’ scien­ published his theory of ferrimagnetism, early this century. Here we encounter tific work in the domain of collective based essentially on the hypothesis of ferrimagnetic behaviour in which the order in magnetic solids. two magnetically unequal sublattices necessary inequality between the two A list of the novel and important of spins. On each sublattice the spins sublattices is caused by an inequality ideas introduced by him in the various are parallel. Spins on the two sub­ in vacancy distribution (1 atom of iron domains of magnetism would be very lattices are oriented in opposite out of 8 is missing). Above a certain long. Perhaps it matters more to note directions and differ in magnitude, temperature the inequality disappears that, on antiferromagnetism and ferri- so that a net moment results. Again, and the vacancies are distributed magnetism, his ideas have proved to this hypothesis was later confirmed equally between the sublattices ; con­ be of greatest originality and have by neutron diffraction experiments. sequently, pyrrhotine then becomes become classic since their formu­ The theory of ferrimagnetism at once antiferromagnetic. lation. Already in 1932, Néel showed explained many known experimental In 1956, the theory of ferrimagnetism that the concept of the Weiss molecu­ facts: the saturation value of the led to the discovery, by Bertaut and lar field had to be extended by magnetic moment, the variation with Pauthenet in Grenoble, of a new introducing local molecular fields, temperature of the spontaneous category of ferrimagnetic oxides, each affecting one sublattice in a magnetization below the Curie point, namely ferrites of the rare with magnetic solid. From this he deduced the hyperbolic temperature variation garnet structure. Their general formula that attractive interactions should of the susceptibility above the Curie is Fe5M3O12, where M is a rare- lead to temperature-independent point, the variation with concentration element. These substances immedi­ paramagnetism. In 1936 he con­ of the magnetic moment of mixed ately attracted great interest because cluded that solids exist in which ferrites, the variation with thermal of their remarkable properties at very the magnetic atoms differ in their spin treatment of the magnetic moment at high frequencies and the relative orientation: below a certain transition saturation of zinc- and magnesium- ease with which large crystals can be temperature Tc they are divided into ferrites by transfer of magnetic ions obtained; in general, they also possess two sublattices spontaneously spin- from one sublattice to the other, and a sharp compensation temperature. oriented in opposite directions with other phenomena. In the same funda­ Here we have three-sublattice ferri­ respect to each other. The net spin mental paper, Néel was the first to magnetism : two with iron and one is then zero; below Tc, the magnetic show that the predominant inter­ with the rare-earth element. Multiple susceptibility is independent of tempe­ actions in oxides and ferrites are not substitutions are possible and thus rature. At temperatures above Tc the direct exchange interactions, but that garnets are a choice material for solid obeys a Curie-Weiss law. At the the magnetic cations interact via studying magnetic interactions. time such solids were unknown; they the diamagnetic anions (indirect- More recently (1960-1961) an appli­ were discovered in 1938 by Bizette exchange, or ‘superexchange’ inter­ cation of the same ideas led Néel to and Tsai and called antiferromagnets actions). predict the existence of a superpara­ by Bitter in 1939. The transition tempe­ On the basis of the theory of ferri­ magnetism in antiferromagnetic crys­ rature Tc is now called the Néel magnetism many novel facts could be tals of 20 to 400 Å diameter. In such temperature Tn. About 1950 neutron predicted. In a study on the possible small crystals the magnetic moments diffraction experiments on MnO carried forms of thermal variation of spon­ on the two sublattices are no longer out by Shull and co-workers showed taneous magnetization, Néel showed completely compensated.

2 In closing, we note the contribution Alfvén’s Research Work of Louis Néel to international co­ The universal importance of plasma operation in physics, notably his physics was realized by Alfvén who activity in the International Union of had already treated the interaction Pure and Applied Physics (IUPAP) of between electromagnetic and me­ which he was President from 1963 to chanical phenomena in his earliest 1966. Néel also played a decisive role papers. This interaction leads to phe­ in the creation of the Ampère Group. nomena which cannot be predicted As President of the Electronics by applying previously known laws of Section of the CNRS he was res­ electricity and mechanics separately. ponsible for the 1st Ampère Collo­ Initially Alfvén’s interests concen­ quium in Paris (1952) and scientific trated on the electrical discharge phe­ director of the 2nd Ampère Collo­ nomena in the Northern Lights quium, held in Grenoble in 1953. (aurorae) and their interaction with G.J. Béné the earth’s magnetic field. In con­ nection with a theory on the aurora on solar phenomena. Alfvén pointed established in 1939, he developed a out that the motion of an electrically Hannes Alfvén perturbation method to deduce the conducting fluid situated in a mag­ netic field induces electric currents Hannes Alfvén’s pioneering work orbits of electrically charged particles has initiated and strongly influenced in inhomogeneous magnetic and elec­ which in their turn produce forces the development of plasma physics tric fields. Alfvén showed that the real reacting back on the fluid motion. This and its applications all over the world. particle orbit can very accurately be may be considered the result of two represented by a circular ‘gyro’ motion simultaneous interaction processes. The Plasma State directed perpendicularly to the mag­ The first is connected with the con­ The various states of matter are the netic field lines, with a superimposed cept of the magnetic field lines ‘frozen’ result of a balance between the bind­ drift of the centre of gyration. The in the fluid and ‘dragged’ along by its ing forces and the thermal motion of magnetic flux enclosed by the circular motion. The second is due to the its constituents. Transformation into motion is conserved and corresponds induced electric currents which another state usually occurs with to an adiabatic invariant, the ‘equi­ produce a restoring force on the mag­ increasing temperature, i.e. from a valent magnetic moment’. The pertur­ netic field lines, thus acting like solid body to a liquid and further to bation method greatly simplifies the elastic strings immersed in the fluid. a gas which, at sufficiently high study of the complicated individual These magnetohydrodynamic forces temperatures, is converted to the particle motion in inhomogeneous play an essential role in a wide range plasma state and disintegrates into its fields and has formed the basis of of plasma physical applications. They electrical constituents. These four innumerable investigations in plasma are also directly related to the funda­ states correspond to the ancient physics and its applications. Examples mental discovery by Alfvén of the Greek concept of the four elements: of this are a heating mechanism magnetohydrodynamic waves propa­ earth, water, air, and fire. Plasma proposed by Alfvén for the acceler­ gated along the magnetic field lines physics describes the fourth state of ation of charged particles in cosmic like waves along an elastic string. matter, covering the entire field of radiation, studies of the dynamics of Limited amounts and different forms classical and much of modern physics. ionized matter near the sun and in of energy are oscillating back and It is thus characterized by a multi­ interplanetary space, as well as inves­ forth in a wave motion. Under certain plicity of combinations and inter­ tigations of the confinement and conditions, however, a disturbance in actions between different physical stability of a plasma in magnetic a plasma may be able to extract an processes. The plasma state extends bottles for attempting to realize con­ increasing amount of energy from a over a temperature range from about trolled thermonuclear fusion. reservoir and will then develop into 104 to 108 degrees and is, therefore, In 1908 the American astronomer an instability. Alfvén realized this the most common state in the universe. Hale showed that strong magnetic possibility in his further studies of the The complexity of plasma physics is fields exist in the sunspots. For 30 magnetohydrodynamic forces. He also reflected in important applications years following this discovery, how­ found that magnetic energy could be in astrophysics, geophysics, controlled ever, interest in electromagnetic phe­ fed into a growing, screw-shaped, so- thermonuclear fusion, magnetohydro- nomena was very limited among called ‘kink’ instability. This mode has dynamic power generation, space cosmologists. A decisive change in since attracted considerable interest technique and aerodynamics, plasma this attitude was first brought about in connection with the experimental chemistry, and the development of by Alfvén in 1942 with a theory about work on gaseous discharges and electric power systems. the action of electromagnetic forces thermonuclear fusion. 3 In the history of physics, numerous attempts have been made to explain EPS Individual Ordinary Member the origin of the solar system and the universe. Many earlier theories on these problems were rejected after No. 1000 repeated analysis or comparison with later astronomical data. Alfvén’s theory on the origin of the solar system is one of the attempts that are still con­ sidered possible solutions of these problems. It is based on the ionization processes and plasma physical phe­ nomena occurring when a cosmic cloud of neutral gas falls through a In the autumn of last year, just when plasma cloud towards a magnetized the EPS was two years old, the 1000th central body which represents the sun. Individual Ordinary Member joined the As an important feature of the theory Society. He is Dr. Ing. Dipl. Phys. a violent ionization process is assumed Hubert Patalong of Munich, a long­ to take place when the motion of the standing member of the German two clouds through each other appro­ Physical Society. During a recent aches the velocity corresponding to interview Mr. Patalong kindly told us the ionization energy. This ‘critical about his background and outlined velocity’ has different values for what prompted him to join and what different elements. A separation of he hopes the EPS might help to achieve. matter is therefore expected to take Since leaving the University of place and should be evident in the Frankfurt in 1952 he has been working chemical composition of the . at research laboratories of Siemens Several laboratory experiments have A.G., engaged on what he terms the scientist often lacks the oppor­ confirmed the existence of the critical ‘development’ rather than ‘pure re­ velocity. Its significance extends far tunities of his academic colleagues. search’, and, while there, he obtained Attendance at scientific conferences beyond the problem of the origin of his doctorate under M. Kersten at the solar system. to make vital contacts with co-workers Aachen. He manages to keep in touch in the field is difficult, and time is An early cosmological theory by with the academic world by lecturing Klein has been further developed always too short to read even es­ at Erlangen University. sential journals. The EPS should help since 1962 byAlfvén and Klein. It is cha­ When he received the circular in racterized by the hypothesis that the to restore to the industrial , which the President K. Ganzhorn who often feels forgotten, the sense universe as a whole consists of equal appealed to all members of the amounts of matter and antimatter. The of belonging to the great community German Physical Society to support of physicists. main contribution by Alfvén is the the EPS by joining as Individual introduction of the concept ‘ambi- We hope that the EPS will be able Ordinary Members (see Europhysics to meet these expectations fully. plasma’ which comprises matter and News No. 11) his response was im­ antimatter. mediate. What impressed him in this Since 1940 the development in appeal was the idea that physicists plasma physics has resembled an are particularly qualified to contribute explosion and has strongly modified to a better understanding between Classified important parts of cosmical physics peoples because their scientific inter­ Advertisement and opened promising fields of appli­ ests transcend political and national Proceedings of the International cation, such as thermonuclear fusion. boundaries. The agreement on a Conference on Magnetism To a large extent this progress was common language (English) as a Grenoble, 14-19 September 1970 initiated by Hannes Alfvén and, in any working medium for the Society, he case, no other research worker in this feels, is a remarkable and gratifying These Proceedings are published field has had a comparable influ­ achievement. In helping to bring in the ‘Journal de Physique’ and ence on its development. Basic con­ together physicists of different nation­ comprise 48 invited papers by leading cepts and results due to Alfvén, ality but common interests, the EPS experts on the latest results concern­ already now to be regarded as can greatly promote the spiritual ing fundamental problems in magnet­ classical, are the equivalent magnetic unification of Europe. Having lived ism, as well as 475 contributed papers moment, the ‘frozen’ magnetic field through a period of violence and with recent original results regarding lines, the Alfvén velocity of magneto- destruction engendered by strife experiment and theory. The papers hydromagnetic waves and the critical between nations and witnessed the cover classical subjects, such as velocity for the interaction between a splitting of his country, Mr. Patalong anisotropy and the mechanism of plasma and a neutral gas. These welcomes particularly the increasing magnetization, and also new subjects achievements are a monument to contacts in between East and such as Kondo effect, metal-insulator Alfvén’s brilliance and working West encouraged by the EPS. He transition, etc. methods, so manysided and rich in strongly supports the Society’s ende­ About 1200 pages, 21 X 29.7 cm ideas, and characterized by the ability avours to improve the coordination of Publication date: February-March 1971 of co-ordinating complicated systems research, which incidentally could Price: US $ 30 and problems to form a logical and bring about greater freedom of move­ aesthetic entity. ment for physicists between their Journal de Physique B. Lehnert centres of activity. Working in industry 33, rue Croulebarbe, F-75 PARIS 13e

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