Muons and magnets

Abstract:

Structural characterization by neutron diffraction, supported by magnetic, SHG, and μ(+)SR data, reveals that the n = 2 Ruddlesden-Popper phase La2SrCr2O7 adopts a highly unusual structural configuration in which the cooperative rotations of the CrO6 octahedra are out of phase in all three Cartesian directions (ΦΦΦz/ΦΦΦz; a(-)a(-)c(-)/a(-)a(-)c(-)) as described in space group A2/a. First-principles DFT calculations indicate that this unusual structural arrangement can be attributed to coupling between the La/Sr A-site distribution and the rotations of the CrO6 units, which combine to relieve the local deformations of the chromium-oxygen octahedra. This coupling suggests new chemical "handles" by which the rotational distortions or A-site cation order of Ruddlesden-Popper phases can be directed to optimize physical behavior. Low-temperature neutron diffraction data and μ(+)SR data indicate La2SrCr2O7 adopts a G-type antiferromagnetically ordered state below TN ∼ 260 K.

Abstract:

The low energy muon (LEM) technique has been used to probe local changes in the dynamical spectrum of thin film polymer samples taking place as a function of the temperature and the implantation depth below the free surface. The studies have been made on samples of polydimethylsiloxane (PDMS) and polybutadiene (PB) using the transverse magnetic field (TF) configuration and diamagnetic probe muons. In PDMS evidence is found for suppression of the glass transition temperature near the surface, along with significantly modified dynamics in the near-surface region as well as at depths significantly below the surface. For PB the LEM technique reveals well-defined layers of dynamical and spatial inhomogeneity at depths of order 0.1–0.2 μm below the free surface. These inhomogeneous regions may be assigned to nanopores produced by solvent streaming during preparation of spin-cast films. A thermal annealing procedure is shown to significantly reduce the thickness of these inhomogeneous layers. These results demonstrate that using LEM in the TF configuration provides a promising new method for studying surface-modified local dynamics of polymers that is also able to reveal nanostructured buried layers in polymer films.