Total angular momentum conservation in laser-induced femtosecond magnetism

Total angular momentum conservation is a fundamental law in classical and quantum mechanics, but since spin momentum is not a classical quantity, it is far from obvious how the law affects spin momentum change in laser-induced femtosecond magnetization. Here it is shown that if a system has full rotational symmetry, the law requires that the spin and orbital momenta are coupled, but there is no genuine magnetization change for linearly polarized light. To induce such a change, this very symmetry has to be broken. In solids, the rotational symmetry is lifted by the translational symmetry, and the spin and orbital momenta components of different total angular momenta mix to some extent. This mixing is the origin of the time-dependent total angular momentum as observed by Bartelt et al. [Appl. Phys. Lett. 90, 162503 (2007)]. The remaining unmixed portion accounts for an extra spin change in three independent circularly polarized laser experiments.