Magnetic Ordering and the Fluorescence of Concentrated Mn Systems

Abstract

The wavelength, lifetime, and relative intensity of the fluorescences of crystal samples of Mn${\mathrm{F}}_2$ and the alkali manganese trifluorides have been measured as a function of temperature above 21°K. In the vicinity of the Néel temperature, small changes in the fluorescent properties are observed in all samples. At approximately one-half the Néel temperature, larger and more strongly temperature-dependent changes are observed in the fluorescence of these materials. All the salts investigated showed Stokes shifts in their fluorescence spectra. It is believed that the magnetic ordering alters the Stokes shift through the magnetoelastic effect, causing the fluorescent changes observed near the Néel temperature. The low-temperature changes are believed to be caused by the same mechanism when the excited ${\mathrm{Mn}}^{2+}$ ion becomes aligned with respect to the surrounding lattice. The temperature at which this occurs is a measure of the exchange energy for the excited-ion electron configuration. The strong temperature dependence is attributed to a coupling of the lattice distortion with the magnetic interaction of the excited ${\mathrm{Mn}}^{2+}$ ion. This coupling changes the energy of the magnetic virtual local mode centered about the excited ion and causes rapid condensation of the local mode.