Einstein–de Haas effect

The experimental observation and accurate measurement of the effect demonstrated that the phenomenon of magnetization is caused by the alignment (polarization) of the angular momenta of the electrons in the material along the axis of magnetization.

These measurements also allow the separation of the two contributions to the magnetization: that which is associated with the spin and with the orbital motion of the electrons.

The effect also demonstrated the close relation between the notions of angular momentum in classical and in quantum physics.

It is named after Albert Einstein and Wander Johannes de Haas, who published two papers[2][3] in 1915 claiming the first experimental observation of the effect.

If a number of electrons in a unit volume of the material have a total orbital angular momentum of

The experiments involve a cylinder of a ferromagnetic material suspended with the aid of a thin string inside a cylindrical coil which is used to provide an axial magnetic field that magnetizes the cylinder along its axis.

In these experiments the magnetization happens along the direction of the field produced by the magnetizing coil, therefore, in absence of other external fields, the angular momentum along this axis must be conserved.

The magnetization can be measured accurately with the help of a pickup coil around the cylinder, but the associated change in the angular momentum is small.

The later accurate experiments were done in a specially constructed demagnetized environment with active compensation of the ambient fields.

The expected effect and a possible experimental approach was first described by Owen Willans Richardson in a paper[1] published in 1908.

The paper mentioned the ongoing attempts to observe the effect at Princeton University.

In that historical context the idea of the orbital motion of electrons in atoms contradicted classical physics.

This contradiction was addressed in the Bohr model in 1913, and later was removed with the development of quantum mechanics.

Samuel Jackson Barnett, motivated by the Richardson's paper realized that the opposite effect should also happen – a change in rotation should cause a magnetization (the Barnett effect).

Einstein and de Haas published two papers[2][3] in April 1915 containing a description of the expected effect and the experimental results.

Their result for the ratio of the angular momentum of the sample to its magnetic moment (the authors called it

It was realized later that their result with the quoted uncertainty of 10% was not consistent with the correct value which is close to

In October 1915 he published the first observation of the Barnett effect in a paper[6] titled "Magnetization by Rotation".

This phenomenon, dubbed "gyromagnetic anomaly" was finally explained after the discovery of the spin and introduction of the Dirac equation in 1928.

The experimental equipment was later donated by Geertruida de Haas-Lorentz, wife of de Haas and daughter of Lorentz, to the Ampère Museum in Lyon France in 1961.

"Experimental Proof of Ampère's Molecular Currents" (Experimenteller Nachweis der Ampereschen Molekularströme) (with Wander J. de Hass).

Considering [André-Marie] Ampère's hypothesis that magnetism is caused by the microscopic circular motions of electric charges, the authors proposed a design to test [Hendrik] Lorentz's theory that the rotating particles are electrons.

The aim of the experiment was to measure the torque generated by a reversal of the magnetisation of an iron cylinder.

"Experimental Proof of the Existence of Ampère's Molecular Currents" (with Wander J. de Haas) (in English).

Koninklijke Akademie van Wetenschappen te Amsterdam, Proceedings 18 (1915–16).

He immediately wrote a correction to paper 52 (above) when Dutch physicist H. A. Lorentz pointed out an error.

This topic was only indirectly related to Einstein's interest in physics, but, as he wrote to his friend Michele Besso, "In my old age I am developing a passion for experimentation."

The second paper by Einstein and de Haas[3] was communicated to the "Proceedings of the Royal Netherlands Academy of Arts and Sciences" by Hendrik Lorentz who was the father-in-law of de Haas.

According to Viktor Frenkel,[10] Einstein wrote in a report to the German Physical Society: "In the past three months I have performed experiments jointly with de Haas–Lorentz in the Imperial Physicotechnical Institute that have firmly established the existence of Ampère molecular currents."

[4] The key to more accurate measurements was better magnetic shielding, while the methods were essentially similar to those of the first experiments.