Magnetic reversal in Mn5Ge3 thin films: an extensive study.

We present a comprehensive study of magnetization reversal process in thin films of Mn5Ge3. For this investigation, we have studied the magnetic anisotropy of Mn5Ge3 layers as a function of the film thickness using VSM and SQUID magnetometers. The samples grown by molecular beam epitaxy exhibit a reorientational transition of the easy axis of magnetization from in-plane to out-of-plane as the film thickness increases. We provide evidence that above a critical thickness estimated as 20 nm, the magnetic structure is most probably constituted of stripes with out-of-plane magnetization pointing alternately up and down. We have analyzed our results using different phenomenological models and all the calculations converge towards values for magnetocrystalline anisotropy constant and saturation magnetization that are in excellent agreement with the reported values for bulk Mn5Ge3. This study has also led to the first estimation in Mn5Ge3 of the exchange constant, the surface energy of domain walls as well as their width. These parameters are essential for determining whether this material can be used in the next generation of spintronic devices.

Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning.

We report a strategy to push the limits of solid-state NMR sensitivity far beyond its current state-of-the-art. The approach relies on the use of dynamic nuclear polarization and demonstrates unprecedented DNP enhancement factors for experiments performed at sample temperatures much lower than 100 K, and can translate into 6 orders of magnitude of experimental time-savings. This leap-forward was made possible thanks to the employment of cryogenic helium as the gas to power magic angle sample spinning (MAS) for dynamic nuclear polarization (DNP) enhanced NMR experiments. These experimental conditions far exceed what is currently possible and allows currently reaching sample temperatures down to 30 K while conducting experiments with improved resolution (thanks to faster spinning frequencies, up to 25 kHz) and highly polarized nuclear spins. The impressive associated gains were used to hyperpolarize the surface of an industrial catalyst as well as to hyperpolarize organic nano-assemblies (self-assembling peptides in our case), for whom structures cannot be solved using diffraction techniques. Sustainable cryogenic helium sample spinning significantly enlarges the realm and possibilities of the MAS-DNP technique and is the route to transform NMR into a versatile but also sensitive atomic-level characterization tool.

Magnetic field dependence of the magnetic susceptibility and the specific heat of the doped plasticized polyaniline (PANI-DB3EPSA)0.5.

Specific heat, magnetization and electron spin resonance (ESR) data obtained from a self-standing film of the doped plasticized polyaniline (PANI-DB3EPSA)(0.5) are shown. No long range magnetic order has been observed at zero magnetic field, above 2 K. For a magnetic field of 3.3 kOe applied perpendicular to the plane of the film, a clear signature of an induced ordered state can be seen in the specific heat data and ESR also reveals this antiferromagnetic order. An electronic contribution is detected from ESR, magnetization and specific heat; however, for T ≤ 5 K, the specific heat data show the existence of a gap. Magnetization data also show a low temperature dominant Curie behaviour which cannot be seen from ESR, probably due to a very large linewidth, suggesting short range correlations among spin 1/2 polarons.