Radiation- and phonon-bottleneck–induced tunneling in the Fe8 single-molecule magnet

We measure magnetization changes in a single crystal of the single-molecule magnet Fe8 when exposed to intense, short ( 20μs) pulses of microwave radiation resonant with the m = 10 to 9 transition. We find that radiation induces a phonon bottleneck in the system with a time scale of ∼5μs. The phonon bottleneck, in turn, drives the spin dynamics, allowing observation of thermally assisted resonant tunneling between spin states at the 100 ns time scale. Detailed numerical simulations quantitatively reproduce the data and yield a spin-phonon relaxation time T1 ∼ 40 ns. Copyright c © EPLA, 2008 Since the discovery of resonant tunneling between spin states more than a decade ago [1,2], single-molecule magnets (SMMs) have been intensively studied. In the past few years, much effort has focused on the behavior of SMMs in the presence of microwave/millimeter-wave radiation as a way to understand the fundamental spin dynamics and the coupling of the spin to its environment [3–12]. This work has been motivated, in part, by the possibility that these systems could serve as qubits [13]. At low temperatures ( 10K), the Fe8 SMM behaves as a spin-10 object with uniaxial anisotropy. The spin dynamics can be well described by the Hamiltonian H=−DS2 z +E(S x −S2 y)+C(S + +S −)− gμB S · H, (1) where the anisotropy constants D, E, and C are 0.292K, 0.046K, and −2.9× 10−5 K, respectively, and g= 2 [14–16]. The first term produces a double-well potential for the spin’s orientation, making the “up” and “down” directions (relative to the z-axis) lowest in energy and producing a ∼ 25K barrier between the two orientations [17]. A magnetic field H along the z-axis makes one well lower in energy than the other, as illustrated in the (a)E-mail: [email protected] insets to fig. 4. There are 2S+1= 21 energy levels for this S = 10 system. The second and third terms in eq. (1) break its cylindrical rotational symmetry and result in tunneling between levels. Resonant tunneling occurs when the magnetic field causes levels in opposite wells to align. Recent studies of the magnetization dynamics in a radiation field have attempted to observe radiation-induced dynamics (such as Rabi oscillations) that would allow a direct determination of the lifetimes of excited spin states and the dephasing time T2. In previous work on Fe8, we showed that such efforts are complicated by the fact that resonant radiation heats the sample and drives the spins and lattice out of equilibrium on the millisecond time scale [6], a process that can be quantitatively described [8]. To circumvent this heating phenomenon, we have done experiments at a much shorter time scale in which an Fe8 sample is subjected to intense, short microwave pulses. Such a time-domain technique allows us to investigate the magnetization dynamics as a temporal sequence of transitions between spin levels. We observe the development of a phonon bottleneck that limits the spin-phonon relaxation and plays an essential role in the magnetization dynamics. The phonon bottleneck drives subsequent relaxation and allows us to directly observe the thermally assisted resonant tunneling process at time scales of 100 ns.