Einstein–de Haas effect
A change in the magnetic moment of a free body causes that body to rotate. This physical phenomenon is a direct consequence of the conservation of angular momentum. It proves strong enough to be observable within ferromagnetic materials like iron. The experimental observation and accurate measurement demonstrated that magnetization results from the alignment of electron angular momenta along an axis. These measurements allow scientists to separate two contributions to magnetization. One contribution comes from electron spin while the other stems from orbital motion. The effect also demonstrates the close relation between classical and quantum physics notions of angular momentum.
Experiments involve a cylinder of ferromagnetic material suspended with a thin string inside a cylindrical coil. This coil provides an axial magnetic field that magnetizes the cylinder along its axis. A change in electric current alters the magnetic field produced by the coil. This change modifies the magnetization of the ferromagnetic cylinder and consequently its angular momentum. A change in angular momentum causes a change in rotational speed monitored using optical devices. Ambient magnetic fields such as Earth's field can provide 10^7 to 10^8 times larger mechanical impact on the magnetized cylinder. Later accurate experiments were done in specially constructed demagnetized environments with active compensation for ambient fields. Measurement methods typically use torsion pendulum properties providing periodic current at frequencies near resonance.
Owen Willans Richardson described the expected effect and possible experimental approach in a paper published in 1908. The electron spin was discovered in 1925 so only orbital motion of electrons was considered before that date. Richardson derived the expected relation of g equals one point zero. The paper mentioned ongoing attempts to observe the effect at Princeton University. Samuel Jackson Barnett realized that opposite effects should happen after reading Richardson's work. He believed a change in rotation should cause magnetization known as the Barnett effect. Barnett published this idea in 1909 and pursued experimental studies of the phenomenon. Einstein and de Haas published two papers in April 1915 containing descriptions of expected results and measurements.
Albert Einstein and Wander Johannes de Haas claimed the first experimental observation of the effect in their 1915 publications. Their result for the ratio of angular momentum to magnetic moment was very close within three percent to the expected value of one point zero. It was later realized that their quoted uncertainty of ten percent was not consistent with the correct value near two. The authors underestimated the experimental uncertainties involved in their work. Einstein wrote three papers with de Haas on experimental work regarding Ampère's molecular currents. He immediately wrote a correction when Dutch physicist H. A. Lorentz pointed out an error. Einstein announced the work in a report to the German Physical Society on the 7th of May 1915 stating experiments firmly established existence of molecular currents. Einstein referred to Wander de Haas who was married to Geertruida de Haas-Lorentz daughter of Hendrik Lorentz.
Later measurements demonstrated that the gyromagnetic ratio for iron is indeed close to two rather than one. This phenomenon dubbed gyromagnetic anomaly was finally explained after discovery of spin and introduction of Dirac equation in 1928. For pure iron the measured value is g equals two point zero and s equals nine six percent. Therefore ninety-six percent of magnetization in pure iron comes from polarization of electron spins. The remaining four percent is provided by polarization of orbital angular momenta. The experimental equipment was donated by Geertruida de Haas-Lorentz to the Ampère Museum in Lyon France in 1961. It went lost and was later rediscovered in 2023.
The effect was used to measure properties of various ferromagnetic elements and alloys. Key to more accurate measurements was better magnetic shielding while methods remained essentially similar to first experiments. Experiments measure value of g-factor using projections of pseudovectors onto magnetization axis. Magnetization and angular momentum consist of contributions from both spin and orbital angular momentum. Using known relations scientists can derive relative spin contribution to magnetization as a percentage. For pure iron calculations show ninety-six percent of magnetization arises from spin polarization. Remaining four percent stems from orbital angular momentum polarization. These findings allow separation of spin from orbital contributions in modern physics research.
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Common questions
What is the Einstein, de Haas effect?
The Einstein, de Haas effect describes how a change in the magnetic moment of a free body causes that body to rotate. This physical phenomenon serves as a direct consequence of the conservation of angular momentum.
When did Albert Einstein and Wander Johannes de Haas publish their first experimental observation of the effect?
Albert Einstein and Wander Johannes de Haas published two papers containing descriptions of expected results and measurements in April 1915. Einstein announced the work in a report to the German Physical Society on the 7th of May 1915 stating experiments firmly established existence of molecular currents.
How does the gyromagnetic ratio for iron differ from early predictions by Richardson?
Owen Willans Richardson derived an expected relation of g equals one point zero before electron spin was discovered in 1925. Later measurements demonstrated that the gyromagnetic ratio for iron is indeed close to two rather than one, with pure iron showing a value of g equals two point zero.
Why do modern calculations show ninety-six percent of magnetization in pure iron comes from electron spins?
Later accurate experiments revealed that ninety-six percent of magnetization in pure iron arises from polarization of electron spins while only four percent stems from orbital angular momentum polarization. These findings allow scientists to separate contributions from electron spin and orbital motion within ferromagnetic materials like iron.
What happened to the experimental equipment donated by Geertruida de Haas-Lorentz to the Ampère Museum?
Geertruida de Haas-Lorentz donated the experimental equipment to the Ampère Museum in Lyon France in 1961. The equipment went lost and was later rediscovered in 2023 after being missing for over six decades.