The Einstein-de Haas effect is a phenomenon observed in magnetism that demonstrates the relationship between the angular momentum of a spinning object and its magnetization. It was first described by Albert Einstein and Wander Johannes de Haas in 1915. When a ferromagnetic material is magnetized, the spins of the electrons in the material align, leading to a net magnetic moment. When this magnetized material is rotated or twisted, the alignment of the spins changes, resulting in a change in the magnetization.
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The Einstein-de Haas effect is mechanical rotation or twisting caused by a change in a body's magnetization. The microscopic carriers of magnetic dipole moment also carry angular momentum: changing their net orientation can transfer angular momentum to the body. A freely suspended ferromagnet can therefore turn when its magnetization reverses. A suspension instead stores part of the response as an elastic twist. This makes a microscopic property accessible through a macroscopic angle.
The central measurement compares a change of magnetic dipole moment with transferred angular momentum, giving a gyromagnetic ratio. It complements an ordinary magnetic dipole torque measurement: even with the applied magnetic field parallel to the suspension axis, so that an ideal axial magnetic dipole moment has , changing magnetization can drive an axial twist. The external drive supplies energy; conservation of angular momentum still requires accounting for the suspension and any external axial torque.