Low Temperature Microwave Emission from Molecular Clusters

We investigate the experimental detection of the electromagnetic radiation generated in the fast magnetization reversal in Mn12–acetate at low temperatures. In our experiments we used large single crystals and assemblies of several small single crystals of Mn12–acetate placed inside a cylindrical stainless steel waveguide in which an InSb hot electron device was also placed to detect the radiation. All this was set inside a SQUID magnetometer that allowed to change the magnetic field and measure the magnetic moment and the temperature of the sample as the InSb detected simultaneously the radiation emitted from the molecular magnets. Our data show a sequential process in which the fast inversion of the magnetic moment first occurs, then the radiation is detected by the InSb device, and finally the temperature of the sample increases during 15 ms to subsequently recover its original value in several hundreds of milliseconds. Molecular clusters are nanomagnets showing important phenomena associated to their magnetic anisotropy and the possibility to tune their quantum mechanical properties by applying a magnetic field. The discovery of the resonant spin tunneling between the degenerate spin levels at both sides of the anisotropy energy barrier [1, 2] was the first sign that quantum mechanics can reveal itself in these magnetic units made of several hundreds of atoms. Since then, near thousand papers on molecular magnets have been published [3]. Very recently a new field combining molecular magnets, magnetization studies, cavities and electromagnetic radiation has attracted the attention of different groups. The most recent works on this particular topic deal with the absorption and emission of microwaves [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. In this letter we report low temperature experimental studies of the detection of microwave emission of Mn12 single crystals inside a cylindrical waveguide. This work follows that published very recently [13], focusing now on the time sequence of the reversal of the magnetic moment, the emission of radiation and the heat release in the sample on the millisecond scale. In Figure 1 we show the details of our experimental setup. Two kinds of samples were investigated. In some experiments, the sample was made by assembling together several tens of oriented single crystals of Mn12–acetate with a total volume around 20 mm. In some others, we used only one large single crystal having a magnetic moment M close to one third of that of the assemblies of crystals (M ∼ 1 emu). An InSb hot electron device, with a spectral bandwidth of 60 GHz – 3 THz, was used to detect the electromagnetic radiation possibly emitted by the sample. Both the sample and the detector were placed inside a stainless steel cylindrical