Measuring the depolarization rate of a $^3$He hyperpolarized gas is a sensitive method to probe hypothetical short-range spin-dependent forces. A dedicated experiment is being set up at the Institute Laue Langevin in Grenoble to improve the sensitivi
ty. We presented the status of the experiment at the 10th PATRAS Workshop on Axions, WIMPs and WISPs.
A bounded random walk exhibits strong correlations between collisions with a boundary. For an one-dimensional walk, we obtain the full statistical distribution of the number of such collisions in a time t. In the large t limit, the fluctuations in th
e number of collisions are found to be size-independent (independent of the distance between boundaries). This occurs for any inter-boundary distance, including less and greater than the mean-free-path, and means that this boundary effect does not decay with increasing system-size. As an application, we consider spin-polarized gases, such as 3-Helium, in the three-dimensional diffusive regime. The above results mean that the depolarizing effect of rare magnetic-impurities in the container walls is orders of magnitude larger than a Smoluchowski assumption (to neglect correlations) would imply. This could explain why depolarization is so sensitive to the containers treatment with magnetic fields prior to its use.
We have studied the relaxation of a spin-polarized gas in a magnetic field, in the presence of short-range spin-dependent interactions. As a main result we have established a link between the specific properties of the interaction and the dependence
of the spin-relaxation rate on the magnitude of the holding magnetic field. This allows us to formulate a new, extremely sensitive method to study (pseudo-) magnetic properties at the sub-millimeter scale, which are difficult to access by other means. The method has been used as a probe for nucleon-nucleon axion-like P,T violating interactions which yields a two-order-of-magnitude improved constraint on the coupling strength ($g_s g_p$) as a function of the force range ($lambda$): $g_s g_p lambda^2 < 3 times 10^{-27}$ m$^2$.
We present pump-probe measurements on the single-molecule magnet Fe_8 with microwave pulses having a length of several nanoseconds. The microwave radiation in the experiments is located in the frequency range between 104 GHz and 118 GHz. The dynamics
of the magnetization of the single Fe_8 crystal is measured using micrometer-sized Hall sensors. This technique allows us to determine the level lifetimes of excited spin states, that are found to be in good agreement with theoretical calculations. The theory, to which we compare our experimental results, is based on a general spin-phonon coupling formalism, which involves spin transitions between nearest and next-nearest energy levels. We show that good agreement between theory and experiments is only obtained when using both the Delta m_S = +-1 transition as well as Delta m_S = +-2, where Delta m_S designates a change in the spin quantum number m_S. Temperature dependent studies of the level lifetimes of several spin states allow us finally to determine experimentally the spin-phonon coupling constants.
We present magnetization measurements on the single molecule magnet Fe8 in the presence of pulsed microwave radiation. A pump-probe technique is used with two microwave pulses with frequencies of 107 GHz and 118 GHz and pulse lengths of several nanos
econds to study the spin dynamics via time-resolved magnetization measurements using a Hall probe magnetometer. We find evidence for short spin-phonon relaxation times of the order of one microsecond. The temperature dependence of the spin-phonon relaxation time in our experiments is in good agreement with previously published theoretical results. We also established the presence of very short energy diffusion times, that act on a timescale of about 70 ns.
