SEMIMAGNETIC SEMICONDUCTORS
Abstract
Semimagnetic semiconductors (SMS) are a group of solids at the interface between semiconductors and magnetic materials. Semimagnetic semiconductors (also referred to as diluted magnetic semiconductors) are semiconductor-based solid solutions where a part of the cations are replaced by transition metals or rare earth elements. The crystallographic structure of the semiconductor is conserved; the lattice constant is a function of composition. From a magnetic point of view SMS is a disordered magnetic material, since magnetic atoms are randomly distributed in cation sublattice of the semiconductor compound. Table 1 presents the most complete list of bulk ternary SMS. Quaternary SMS are also investigated (e.g., PbSnMnTe or CdMnSeTe). The table does not contain III-V SMS obtained by MBE technique, although these broadly investigated materials can be treated as 3D bulk crystals. The table contains only crystals obtained under thermodynamic equilibrium conditions, whilst III-V SMS and some other SMS obtained by MBE are rather metastable structures.
Generally speaking, in SMS there coexist two interrelated and interacting subsystems: mobile delocalized charge carriers and localized magnetic moments connected with paramagnetic ions. Electronic properties of SMS have been the subject of intensive studies since early seventies. Due to the strong spin exchange interaction between mobile carriers and localized magnetic moments (the exchange constant of the sp-d interaction is of the order of 1 eV for II-VI SMS), significant changes of the band structure and behavior of carriers were observed. A number of new physical phenomena were discovered, such as giant Faraday rotation, the magnetic field induced metal-insulator transition, the bound magnetic polaron, the quantum thermomagnetic oscillations. Magnetic properties of SMS are the subject of studies since about 1980. Investigations of the influence of electron subsystem on magnetic properties of SMS are connected with the observation of a ferromagnetic phase in PbSnMnTe induced by carrier concentration – 1986, and other more subtle effects as magnetic polaron.
About 10 years later, molecular beam epitaxy (MBE) grown GaAs with Mn opened a new chapter of III-V SMS showing ferromagnetic behavior with the highest transition temperature TC = 140 K. In 1987, the first paper devoted to layered structure and magnetic properties of low dimensional (LD) SMS was published starting intensive studies of superlattices and other LD structures made of SMS.
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