Date of Graduation
Temperature dependence of all the optical absorption bands of MnF(,2) in the range 2000-6000 (ANGSTROM) has been studied using Cary 14 spectrophotometer in conjunction with Air Products Displex, a closed cycle helium refrigerator. Peak position, width and oscillator strength of the bands, namely A through I, were measured as a function of temperature in the region between 10 K-300 K, with a particular attention to the region near the Neel temperature (T(,N) = 67.3 K). On lowering the temperature through T(,N), all bands undergo blue-shifts, the intensity and line-width of main bands decreased except band G and I whose intensity increased, the splitting of the bands became clearly defined and fine structures appeared. The temperature dependence of the oscillator strength of the bands can be classified into two groups; Group I whose intensity decreases on cooling through T(,N) and Group II whose intensity increases below T(,N). The temperature dependence of the oscillator strength of group I agreed semiquantitatively with the theory of Shinagawa and Tanabe. Based on this, these bands have been identified as exciton-magnon or exciton-magnon-phonon bands. The temperature dependence of group II show a semiquantitative agreement with the theory of Fujiwara et al. and these bands have been identified as double exciton bands. The blue-shift of peak position of band C and F below T(,N) (whose line-positions are nearly temperature independent above T(,N)) has been explained in terms of molecular field theory of Yen, Imbush and Huber. Both bands are blue-shifted by (DELTA)E ((TURNEQ)60 cm('-1) for C and 80 cm('-1) for F between T(,N) and 10 K). It is found that (DELTA)E varies as the sublattice magnetization below T(,N) and nearly as the magnetic energy above T(,N). Some of the bands in the fine structure can be associated with magnons or phonons. However complete identification of all the fine structure bands still needs to be carried out.
ABUMANSOOR, SABRI ASAAD, "ANTIFERROMAGNETIC ORDERING EFFECT ON THE OPTICAL TRANSITIONS IN MANGANESE-DIFLUORIDE." (1985). Graduate Theses, Dissertations, and Problem Reports. 8354.