Spinpolarizing

Spinpolarizing is the process of inducing a preferential population of a given spin state in a collection of quantum particles, such as electrons, nuclei, or photons, resulting in a nonzero spin polarization. The degree of polarization is typically quantified by a polarization parameter P that ranges from -1 to +1, with P = 0 indicating an unpolarized ensemble and P = ±1 indicating full polarization. Spinpolarizing can be achieved by several mechanisms that transfer angular momentum to the target spins or select certain spin states during transport or interaction.

Common methods include optical orientation, where circularly polarized light transfers angular momentum to electronic or atomic spins via selection rules, often in semiconductors or atomic vapors. Spin injection uses spin-polarized currents from ferromagnetic materials into nonmagnetic conductors or semiconductors to create a polarized ensemble. Spin filtering relies on magnetic barriers or spin-dependent scattering to preferentially transmit or transmit-and-filter one spin orientation. Dynamic nuclear polarization transfers polarization from electron spins to nearby nuclei through hyperfine interactions, producing hyperpolarized nuclei used in imaging and spectroscopy. Spin pumping and related resonant techniques can also generate spin polarization in magnetic insulators and metals.

Measurement of spin polarization employs methods such as spin-resolved spectroscopy, magneto-optical effects like the Kerr or Faraday effect, and neutron or electron scattering. Key challenges include maintaining polarization against relaxation processes, characterized by spin-lattice (T1) and dephasing (T2) times, which depend on material properties and temperature.

Applications span spintronics devices (spin valves, magnetic tunnel junctions), polarized particle beams for high-energy experiments, and enhanced contrast in magnetic resonance imaging and spectroscopy through hyperpolarization techniques.