- Person
- Felix Bloch
- Lived
- 1905–1983
- Field
- Physics
- From
- Swiss-American
The story
After finishing his doctorate in Leipzig in 1928, Felix Bloch spent several years moving between the great centres of European physics, as young theorists did at the time. He was an assistant to Wolfgang Pauli in Zurich, a fellow with Hendrik Kramers in Utrecht and Adriaan Fokker in Haarlem, and then Werner Heisenberg's assistant back in Leipzig. A central puzzle of the day was ferromagnetism, the property that makes iron magnetic. Heisenberg had shown that it arises from the quantum exchange interaction between electrons, and Bloch set out to put that idea on a firmer footing.
In Utrecht, where he arrived as a Lorentz Foundation fellow in the autumn of 1929, Bloch came up with the idea of spin waves. He pictured flipped atomic spins travelling through the lattice as a wave, a very novel idea at the time. With it he worked out how the magnetisation of a ferromagnet falls as the temperature rises at low temperatures. John Slater had a similar idea but did not connect it to ferromagnetism. Robert Hofstadter's memoir for the National Academy of Sciences calls spin waves an important precursor of quasi-particle theories, which describe collective excitations in solids as if they were particles.
After the academic year 1930 to 1931 with Heisenberg, Bloch wrote his habilitation thesis in Leipzig, a monumental paper on exchange interactions and residual magnetisation in ferromagnets. Remarkably, he wrote it while in hospital after an injury suffered while climbing in the Alps with Egon Bretscher. One of its achievements concerned the walls between magnetic domains, the regions inside a magnet where atomic magnets all point one way. Experiments by R. Becker had shown that magnetisation proceeds as these domains change. Bloch worked out the thickness and structure of the boundaries, showing that the magnetisation turns gradually over a few hundred angstroms rather than flipping abruptly. These boundaries are now called Bloch walls.
Historians of solid state physics have described the paper as presenting a wealth of formalism that became part of the fabric of modern condensed matter theory, bridging the quantum theory of ferromagnetism in the 1930s and later theories of many-particle systems. The gradual turning Bloch predicted was later observed directly in experiments.
Both ideas remain basic vocabulary in magnetism. Spin waves are a standard way of describing how magnetic order is disturbed in a solid, and the Bloch wall is one of the two main types of domain wall that physicists use to explain how bulk magnetic materials, from transformer cores to permanent magnets, become magnetised.
Why it matters
Bloch walls and spin waves are basic concepts in magnetism, essential for magnetic materials used in data storage, motors, sensors and emerging spin-based electronics.
About Felix Bloch
Swiss-born physicist who laid the quantum foundations of solid-state physics, discovered nuclear magnetic resonance – the basis of MRI – and became CERN’s first director-general.
More about Felix Bloch
- Felix Bloch helped explain how particles slow in matterIn Copenhagen in the early 1930s, Felix Bloch refined Hans Bethe’s theory of how charged particles lose energy in matter, work remembered in the Bethe–Bloch formula.
- Felix Bloch's equations underpin MRIIn 1946 Felix Bloch wrote the Bloch equations for nuclear magnetisation, introducing the T1 and T2 relaxation times that later helped make MRI a powerful medical tool.
- Felix Bloch was CERN's first director-generalIn 1954 Felix Bloch became the first director-general of CERN, the new European laboratory for particle physics, as construction began on its site at Meyrin near Geneva.
- Felix Bloch measured the neutron's magnetismIn 1939, working with Luis Alvarez at the Berkeley cyclotron, Felix Bloch measured the neutron's magnetic moment to about one percent accuracy, a theorist turned experimenter.
- Felix Bloch's method became chemistry's key toolChemical shift experiments in Felix Bloch's own laboratory helped turn nuclear magnetic resonance into the most important spectroscopic tool in chemistry.
- Felix Bloch discovered how to listen to atomic nucleiIn 1946 Felix Bloch, William Hansen and Martin Packard at Stanford developed nuclear induction, one of the two founding methods of nuclear magnetic resonance.
- Felix Bloch explained how electrons move through metalsIn his 1928 thesis as Werner Heisenberg's first graduate student, Felix Bloch showed how electrons travel as waves through a crystal, founding the quantum theory of solids.
Sources
- Felix Bloch 1905–1983: A Biographical Memoir by Robert Hofstadter National Academy of Sciences
- Felix Bloch Wikipedia
- Domain wall (magnetism) Wikipedia
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