MAGNETIC-FIELD-INDUCED FORMATION OF EXCITON MAGNETIC POLARONS IN ZNSEZN1-XMNXSE QUANTUM-WELL STRUCTURES/

cw and time-resolved photoluminescence spectroscopy is used to study Z nSe/Zn1-xMnxSe quantum wells with semimagnetic barriers in an external magnetic field. The data demonstrate a change of the dominant energy relaxation mechanism from disorder localization of light-hole excitons at zero-field to heavy-hole exciton interface magnetic polaron format ion at intermediate fields and, again, disorder localization of heavy- hole excitons at large magnetic fields. The formation of the interface magnetic polaron is promoted by a magnetic-field-induced type-I-type- II transition for heavy-hole excitons. Despite the transition, neither the exciton lifetime nor its optical oscillator strength is dramatica lly altered. This is, as we confirm by numerical solution of the two-p article Schrodinger equation, a result of the electron-hole Coulomb in teraction. The polaron formation time is initially 110 ps, but decreas es with growing magnetic held down to 70 ps (B = 5 T). A theoretical i nvestigation of the polaron formation dynamics shows that the associat ed change of the exciton wave function is smaller, the closer the Mn2 spin system is driven into saturation by the external field. As a con sequence. the polaron formation time approaches the characteristic Mn2 + spin response time. Our measurement uncovers a fast primary localiza tion prior to the polaron process--but also of magnetic origin--that w e believe to be necessary to start the polaron formation.