Ultrafast magnetism and spin-orbit transport

6 problems

• Mechanism of helicity-dependent all-optical switching in ferromagnets

  Partially resolved

  A single laser pulse can flip the magnetization of some alloys with two opposed magnetic sublattices, and repeated circularly polarized pulses can flip ordinary ferromagnetic films. The first case is understood as fast heating of the two coupled sublattices; what drives switching that depends on the light's handedness is not settled.

• Where spin angular momentum goes during sub-picosecond demagnetization

  Partially resolved

  When a laser pulse demagnetizes nickel in under a trillionth of a second, the lost spin angular momentum must go somewhere, since angular momentum is conserved. Experiments show the crystal lattice eventually takes it, but how it gets there, and how much leaves as spin currents or electron orbital motion, is unclear.

• Magnitude of the orbital Hall effect and orbital torques

  Partially resolved

  Theory predicts that currents in light metals such as titanium push electron orbital angular momentum sideways, called the orbital Hall effect, and that this can twist nearby magnets. Experiments have detected it, but measured sizes and theoretical values disagree and are hard to separate from spin effects.

• Bulk versus interface origin of spin-orbit torques

  Contested

  A current in a heavy metal like platinum or tungsten can flip an adjacent magnetic layer through spin-orbit torque. Whether the torque comes mostly from spin currents generated inside the metal or from effects at the interface is disputed.

• Microscopic origin of the spin Seebeck effect

  Open

  A temperature difference across a magnetic insulator like yttrium iron garnet drives a spin current into an attached platinum strip, where it appears as a voltage. Which spin waves carry this current, and how lattice vibrations help, is not settled.

• Has long-distance spin-superfluid transport been demonstrated?

  Contested

  In magnets whose spins lie in a plane, a steady twist of the spin direction should carry spin current with little loss, decaying only slowly with distance, like a superfluid. Some experiments claim this, and magnon condensation in yttrium iron garnet is established, but other explanations of the signals remain.