obituaries

Clifford Glenwood Shull

Clifford Glenwood Shull

Néel was indeed a worthy successor to the French tradition of research in magnetism initiated by Pierre Curie, Paul Langevin, and Pierre Weiss. But he is better known in France as the one who developed Grenoble into a major international center of research after the war. With a large center for nuclear energy, a laboratory for semiconductor developments, two international instruments that use synchrotron radiation and neutrons, and the many university and CNRS (French National Center for Scientific Research) laboratories he developed, Néel profoundly changed, in only 20 years, the atmosphere of a provincial town with good industrial activities but poor research facilities before and during World War II.

Néel's successes in research and scientific administration were helped by circumstances; they came mostly from a clear awareness of his aims and possible limitations, and were stimulated by a great willpower. His sharp mind concentrated on scientific models of his own, simple enough to be developed on the back of an envelope, but powerful because they were well adapted to the problems at hand and general in nature. His greatest successes came against the stream of fashion. He published to the last in French; his work during the war was poorly published and thus hardly known.

In committees, he was a massive and impressive chairman, awake in daylong sessions to the last minute, when he often tried to realize his dearest wishes in the general tiredness. He liked lively discussions, however, and only respected determined opponents. Despite many honors and connections worldwide, Néel kept a quiet and happy family life with his wife, a teacher in philosophy, and their three children.

Néel's research centered on the arrangements of the magnetic moments in solids, at atomic and larger scales. In Weiss's laboratory in Strasbourg, Néel questioned the accepted Curie-Weiss law for susceptibility, which had been deduced from distance-independent interactions. Werner Heisenberg's work on exchange had just shown that short-range effects should be expected. Between 1931 and 1933, Néel observed those effects in the susceptibility of iron and alloys and in the specific heat of nickel. Then, assuming that short-range interactions could be antiparallel, Néel developed the concept of antiferromagnetism, in which two interpenetrating atomic lattices are treated in a molecular field approximation. Manganese and chromium showed the predicted susceptibility, with a peak at what became known as the Néel temperature. These proposals, made in 1936, were confirmed in 1938 on manganese oxide, an insulator with no possible contributions from metallic paramagnetism.

To describe antiferromagnetism, Lev Landau and Cornelis Gorter suggested quantum fluctuations to mix Néel's solution with that obtained by reversal of moments. But in a macroscopic crystal with magnetocrystalline anisotropy, the nucleation would involve a magnetic wall of high energy. Using neutron diffraction, Harry Shull confirmed (in 1950) Néel's model.

This model had been extended in 1947 to describe ferrimagnetism of spinel ferrites: Néel assumed that the atoms of the two lattices have different moments. In 1956, rare earth garnets were discovered in Grenoble, where the ferrimagnetism of Fe ions is coupled to the rare earths. This flexible family led to hard (noncubic) magnets and to soft and lossless (cubic) ones. At a dinner held to celebrate Néel's receipt of the Nobel Prize, his friend Hendrik Casimir stressed that these contributions were essential to Philips research laboratories' development of ferrite-based devices. Bell Laboratories and Japanese firms could have made similar statements.

Applications were at the root of Néel's work, which looked at larger scales. His interest in hysteresis dated from the war: Indeed, he personally supervised the magnetic protection of all main vessels of the French navy, during the spring of 1940, from magnetic mines by applying a short strong field, opposite in direction to Earth's, thus reducing the ships' magnetization due to Earth's field. Hysteresis in polycrystals or with diffusing impurities came to be of central interest in Grenoble, where Néel took refuge in 1940.

During the war, Néel developed reasonably hard magnets by compressing soft Fe powders. When smaller than the thickness of Bloch walls, each grain is a single domain; at low temperatures, its form factor blocks its magnetization along a specific axis. Cooling under an applied field produces a stable remanant magnetization. This model was later applied to hard cobalt-nickel steels and to ceramics and basalts cooled down under the influence of Earth's field. Néel was proud of this last work, which opened the possibility of relating the continental drift to the rate of reversal of Earth's field.

During or just after the war, Néel predicted the main features observed later in magnetic configurations near various surfaces, notably in thin sheets. In thin sheets that lack large magnetocrystalline effects, he pointed out that magnetization is parallel to the surfaces. In a "Néel wall" between domains in such sheets, magnetization rotates with an axis normal to the sheet; a singular line separates two parts of the wall that have opposite rotations, as indeed in a Bloch wall but with a different structure.

Néel was gifted with his hands and relaxed by making furniture. But, in Grenoble, his experiments were mostly done by collaborators, notably Louis Weil and Felix Lewy-Bertaut, two Jews protected during the war by Néel's industrial friends. Néel's difficulties with antiferromagnetism and inconclusive discussions in the Strasbourg international meeting of 1939 fostered his skepticism about the usefulness of quantum mechanics; this was one of the few limitations of this superior mind.