Testing the radiation emanating from the molecular nanomagnetFe8during magnetization reversals

Abstract

In the molecular nanomagnet Fe8 tunneling can occur from a metastable state to an excited state followed by a transition to the ground state. This transition is accompanied by an energy release of 115.6 GHz. We constructed an experimental setup to measure whether this energy is released in the form of thermal or electromagnetic energy. Contrary to a previous publication we find no evidence for release of electromagnetic radiation. Our results for transitions between the first and second excited states to the ground state are consistent with a release of only thermal energy. This energy release extends for a longer time for the second excited state than for the first excited state.While investigating the Fe 8 mononuclear magnet Shafir and Keren 1 made a serendipitous observation; tunneling events where accompanied by a jump in the temperature of a thermometer placed far from the sample and attached directly to the mixing chamber of a dilution refrigerator (DR). When the line of site between the thermometer and sample was blocked, the tunneling signal remained, but the temperature jumps disappeared. This led to the conclusion that energy bursts accompany the tunneling event and arrive at the thermometer in the form of electromagnetic radiation. In order to block the line of site the DR had to warm up and cool down again. Therefore, the test experiment was not done simultaneously with main experiment. Here we revisit the same phenomena, but with an experimental setup designed to detect photons in the microwave range, and with a test experiment done simultaneously with the photon detection.At low temperature, the molecules are described by the Hamiltonian H = −DS 2 z + gµ B S z H + H where S = 10 is the spin, D = 0.292K is the anisotropy parameter, H is the applied magnetic field, µ B is the Bohr magneton, g ≈ 2 is the gyromagnetic factor, and H does not commute with S z and is responsible for tunneling between spin projection states m 2,3 . When the field is strong (∼ ∓1 T) only the m = ±S are populated. When the field is swept across zero and changes sign, the m = ±S state becomes metastable. At matching fields, which are separated by 0.225 T 4,5 , quantum tunneling of magnetization (QTM) can take place from m = ±S to an m = ∓(S − n) state, where n = 0, 1, 2 . . . is an excited state index. For n > 0, the excited m state decays spontaneously to the ground state ∓S and energy is emitted. For n = 1 this energy corresponds to a frequency of 115.6 GHz or wavelength of 2.6 mm and for n = 2 it corresponds to a frequency of 219 GHz. Interestingly, we found (see below) that the heat is released for a longer time from the second excited (n = 2) state than from the first one (n = 1).The experiment is preformed below 0.2 K in order to have temperature independent quantum tunneling 4,5 .

Field-dependent superradiant quantum phase transition of molecular magnets in microwave cavities

“…Superradiance, the relaxation of an ensemble of emitters at a rate proportional to the square of their number, is another manifestation of coherent coupling of emitters to the quantized cavity field. It was predicted to occur in the molecular magnets, but the experimental results so far remain inconclusive [24][25][26][27][28][29][30]. As opposed to superradiance, the superradiant phase is a ground state property of the coupled emitter-cavity system.…”

Temperature changes of the Fe8 molecular magnet during its spin reversal process

“…4 we present the sweep rates as a function of the normalized magnetization jumps for n = 1. The inset shows raw data taken from Leviant work 16 . Due to the limited number of data points the fit parameters are determined with large error bars.…”