Frequency domain magnetic resonance spectroscopy on the Mn 12 -acetate single molecule magnet

Abstract

In dieser Arbeit wurden allgemeine statische und dynamische zeitabhängige Eigenschaften des Mn12-Molekularmagneten durch magnetische Resonanzspektroskopie in der Frequenzdomäne (FDMRS) untersucht. Die statischen Materialeigenschaften wurden im Frequenzbereich 6-12 1/cm, Temperaturen zwischen 1,9-15 K und Magnetfeldern zwischen 0 und 6 T gemessen. Die Nullfeldaufspaltung und die Anisotropy der g-Faktoren und darüber hinaus der magneto-optische Effekt wurden untersucht und letzterer als Faraday-Effekt indentifiziert. Im dynamischen Fall wurden Relaxationsmessungen bei tiefen Temperaturen (1,75-3,30 K) und Magnetfeldern bis zu 6 T durchgeführt. Wir konnten sowohl thermisch aktivierte, als auch Quanten-Tunnel-Relaxationsmechanismen finden. Zusammenfassend kann gesagt werden, dass sich diese Methoden zur Untersuchung der magnetischen Resonanz und mesoskopischer Quantenphänomene im Bereich molekularer Magneten eignet. We present the studies of the general material property (time independent) and dynamic property (time dependent) of Mn12-acetate single-molecule magnet by using the frequency-domain magnetic resonance spectrsocopy (FDMRS) technique. For studies of the material property (time independent), the measurement was done within the frequency range 6-12 1\cm, temperature range 1.9-15 K, and magnetic field 0-6 T. We report the zero field splitting (ZFS) parameters and g-value anisotropy. We also report that the environmental effects, such as the effects of the internal field (e.g. dipolar field) and the inhomogeneous distribution of the main ZFS parameter, which both give contributions to the linebroadening mechanism in Mn12-acetate. We also studied the magneto-optical effects in the Mn12-acetate sample e.g. the orientation effect of the magnetic field and the wave propagation direction, the magnetic state of the sample (demagnetized and magnetized states,) and the polarization of the radiation (e.g. linear, and left hand circular, and right hand circular polarizations), on the magnetic transition lineshape. As a result, we found the Faraday effect in the Mn12-acetate system which suggests an application as the molecular rotator. For the dynamic studies (time dependent), we performed relaxation measurements at low temperature (1.75-3.3 K) and in magnetic fields up to 6 T. We studied the relaxation time as function of the temperature, applied magnetic field, and magneto-optical geometries. We observed both thermally activated and magnetic quantum tunneling relaxations. We qualitatively and quantitatively investigated the distribution of the relaxation time in the magnetic field domain via the phonon-assisted spin tunneling process. In conclusion, these results show that the FDMRS technique is a promising tool to study the magnetic resonance and mesoscopic quantum phenomena in the field of single-molecule magnets in the future.