Nonvolatile spin memory based on diluted magnetic semiconductor and hybrid semiconductor ferromagnetic nanostructures

The feasibility of two nonvolatile spin-based memory device concepts is explored. The first memory device concept utilizes the electrically controlled paramagnetic-ferromagnetic transition in a diluted magnetic semiconductor layer (quantum well or dot) when the ferromagnetism in the diluted magnetic semiconductor is mediated by itinerant holes. The specific structure under consideration consists of a diluted magnetic semiconductor quantum well (or quantum dot) that exchanges holes with a nonmagnetic quantum well, which acts as a hole reservoir. The quantitative analysis is done by calculating the free energy of the system. It takes into account the energy of holes confined in a nanostructure and the magnetic energy. Formation of two stable states at the same external conditions, i.e., bistability, is found feasible at temperatures below the Curie temperature with proper band engineering. The effect of scaling the magnetic quantum well to a quantum dot on bistability is analyzed. The bit retention time, i.e., lifetime, with respect to spontaneous leaps between the two stable states is calculated. The write/erase and read operations as well as the dissipation energy are discussed. Also, potential logic operations are proposed. In the second memory concept, the active region is a semiconductor quantum dot sharing an interface with a dielectric magnetic layer. The operating principle of the device is based on the spontaneous magnetic symmetry breaking due to exchange interaction between the magnetic ions in the magnetic layer and spins of the confined holes in the quantum dot. The quantitative analysis considers the holes thermal distribution over the energy spectrum in the quantum dot, hole-hole interaction, exchange interaction between the holes and the magnetic ions, and magnetic energy of the magnetic insulator. Room temperature operation is possible given the availability of insulating ferromagnetic or antiferromagnetic materials whose Curie or Neel temperature is above room temperature. The specific range of material parameters where bistability is achieved is found. Analysis is extended to different quantum dot and magnetic dielectric materials and designs. The influence of material choice and design on the memory robustness and lifetime is discussed.