Professor Stephen Blundell

Abstract:

We present the results of a muon-spin relaxation study of the high-T-c analog material Cs2AgF4. We find unambiguous evidence for magnetic order, intrinsic to the material, below T-C=13.95(3) K. The ratio of interplane to intraplane coupling is estimated to be vertical bar J(’)/J vertical bar=1.9x10(-2), while fits of the temperature dependence of the order parameter reveal a critical exponent beta=0.292(3), implying an intermediate character between pure two- and three-dimensional magnetism in the critical regime. Above T-C we observe a signal characteristic of dipolar interactions due to linear F-mu(+)-F bonds, allowing the muon stopping sites in this compound to be characterized.

Abstract:

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi$_2$O$_5$, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferromagnetic with propagation vector ${\bf k}=(\pm \frac{1}{2}, \frac{1}{2}, 0)$ and moment of 2.72(1)$\mu_{\rm B}$ per Co$^{2+}$ ion. This long range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be associated with a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi$_2$O$_5$ is a highly unusual low symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.

Abstract:

We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around T ≈ 40 K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.