Louis Néel

French physicist

• Born: November 22, 1904; Lyon, France
• Died: November 17, 2000; Brive-la-Gaillarde, France

Louis Néel was awarded the 1970 Nobel Prize in Physics for his discovery of new forms of magnetism, including antiferromagnetism and ferrimagnetism. His work greatly strengthened magnetic theory and contributed fundamentally to the development of computer-memory technology.

Primary field: Physics

Specialty: Condensed-matter (solid-state) physics

Early Life

Louis Eugène Félix Néel was born in Lyon, France, on November 22, 1904. His father, trained as a lawyer, was a civil servant in the Ministry of Finance, and his family moved frequently to foreign postings, including Algeria, Tunisia, and Corsica. Néel was admitted to the École Normale Supérieure in Paris in 1924. In 1928, his last year at the school, he was appointed to a lectureship there, during which time he further developed his abilities in mathematics and physics and began formulating his ideas on magnetism. After graduating, Néel enrolled in the University of Strasbourg.

Working in the laboratory of French physicist Pierre Weiss at Strasbourg, Néel began research on basic problems of magnetism, a subject that would form the basis of his research for much of his career. While working on his doctoral thesis, he came into contact with the theory proposing that ferromagnetism is a result of a uniform molecular field created by interactions of individual atoms. No origin for this field had been discovered, so Néel abandoned the notion of a general field and proposed that ferromagnetism is caused by local molecular fields resulting from close-neighboring atoms. He found that an interaction causing an antiparallel alignment of the magnetic moments of closely spaced atoms would account for the change from constant paramagnetism (that is, attraction in the presence of an external magnetic field) to a temperature-dependent paramagnetism in elements such as manganese and chromium. He referred to this type of magnetism as antiferromagnetism.

In 1931, Néel married Hélène Hourticq, a philosophy and literature teacher, with whom he would have two daughters and a son. He earned his doctorate of science from the University of Strasbourg the following year.

Life’s Work

In 1937, Néel was appointed professor of physics at the faculty of science at Strasbourg, a position he held until 1945. His tenure was interrupted by World War II, during which time he served the government as an investigative scientist. His main topic of research was how to defend ships from floating German magnetized mines. To counteract the mines, he invented a new method of protection known as neutralization, which gave the ship a permanent magnetization in a direction opposite to the terrestrial magnetic field.

While developing his theory of antiferromagnetism, Néel also became interested in the properties of very fine-grained ferromagnets. In 1941, based on his understanding of how these minute particles must act in a magnetic field, he set out to explain the remanence (that is, the magnetization that remains in a substance after the external magnetizing force has ended) exhibited in diverse forms found in nature, including in solidified lava and clay. He found that material such as lava that has cooled while under the influence of the Earth’s magnetic field can yield information about that field. This type of “magnetic memory” allows geologists and archaeologists to retrace the history of paleomagnetism, or how the Earth’s nonconstant magnetic field has varied over the course of the planet’s development.

In the course of his research, Néel discovered a new class of magnets, the properties of which are a result of the very fine grains enclosed together. He called magnetic material of this sort ferrimagnetic. Such compounds exhibit a spontaneous magnetization; in other words, they develop a field by themselves, without any outside influence. Above the Curie temperature (the temperature at which materials lose their ability to hold a magnetic field), the magnetic susceptibility—that is, the degree to which a material is magnetized by an external magnetic field—of ferrimagnetic material does not obey the Curie-Weiss law, which states that the magnetic susceptibility of a substance is inversely proportional to its temperature. Néel resolved the problem by developing an unbalanced antiferromagnetic structure he called a ferrimagnet.

Néel’s concepts were used by others researchers to study garnet ferrites, which were found to be extremely useful in technological applications in high-frequency engineering. Equally important to the development of magnet-related technology was the discovery that ferromagnetic oxides and spinels (hard crystalline minerals composed of magnesium and aluminum oxides) can be tailored to suit practical applications based on the theoretical descriptions of the magnetic properties as proposed by Néel. Such materials became fundamental to core memories in computers and magnetic tapes, as well as the development of concepts related to domain memories.

In 1946, Néel became a professor at the University of Grenoble. During his tenure, the institution became internationally recognized for excellence in magnetism research. Néel founded the Centre d’Études Nucléaires de Grenoble in 1956 and became its director, overseeing the development of strong programs in a variety of areas. In 1967, he was one of the principals in the decision to install the Franco-German high-flux reactor in Grenoble. He was the director of the Polytechnic Institute of Grenoble, a director of the Centre National de la Recherche Scientifique, and chair of the Conseil Supérieur de Sureté Nucléaire. From 1960 to 1983, he served as the French representative on the North Atlantic Treaty Organization (NATO) Scientific Council.

In the 1950s and 1960s, Néel was concerned primarily with theoretical problems, and these formed the basis of his numerous publications. One series of papers concerned the importance of the role of the internal demagnetizing fields in the properties of ferromagnetic substances. He also investigated magnetic viscosity, internal dispersion fields, and superantiferromagnetism, among other topics. His findings on such subjects appeared in more than 150 publications.

For his work related to antiferromagnetism and ferrimagnetism, Néel was awarded the Nobel Prize in Physics in 1970, sharing the prize with Swedish physicist Hannes Alfvén. Néel retired to Meudon, near Paris, in 1975. During his retirement, he worked with the French Academy of Sciences and advised French political leaders on scientific matters. Néel died in Brive-la-Gaillarde, France, on November 17, 2000.

Impact

Néel’s contributions to physics were numerous and innovative; he virtually rewrote the understanding of how magnetic fields are generated, maintained, and modified in diverse natural materials. His discoveries during his studies of permanent magnetism in ferromagnets provided greater understanding of how antiferromagnetism and ferrimagnetism work and led to further work on more quantum-related topics such as superantiferromagnetism. In physics alone, his ideas made possible the development of a quantum-level understanding and theory of atomic magnetism, which in turn led to further advances in such diverse fields as aircraft and electronic high-frequency oscillatory controls, transference of information at exceedingly high rates, and the development of magnetic memory cores for computer technology. Outside the area of physics proper, he guided the development of successful laboratories in France, enhancing national prestige with developments in fields ranging from cryogenics to nuclear technology and the education of new generations of scientists.

For his many contributions, Néel was awarded numerous awards and honors in addition to the Nobel Prize, including the Robin Prize from the Société Française de Physique (1939), the Holweck Prize (1953), and the Gold Medal of the Centre National de la Recherche Scientifique (1965). He received honorary doctorates from Nottingham University, Oxford University, and Newcastle University, among others. Additionally, he was a member of numerous scientific organizations, including the French Academy of Sciences, the Royal Dutch Academy of Science, the German Academy of Sciences, the Royal Society of London, and the American Academy of Arts and Sciences.

Bibliography

Friedel, Jacques. “Louis Néel.” Physics Today 54.10 (2001): 88–89. Print. Comments on Néel’s character, achievements with magnetism, and efforts to build up France’s research facilities.

-?-?-, and Pierre Averbuch. “Louis Eugène Félix Néel.” Biographical Memoirs of Fellows of the Royal Society 49 (2003): 367–84. Print. Chronicles Néel’s life and career, providing a portrait of the man and a technical explanation of his scientific work.

Mattis, Daniel C. The Theory of Magnetism Made Simple. Hackensack: World Scientific, 2006. Print. Explains the mathematics pertinent to Néel’s discoveries and places his principal work in context.