Muons and magnets

Abstract:

We have combined polarised neutron reflection (PNR), X-ray reflection, the extraordinary Hall effect (EHE) and magnetoresistance (MR) in a detailed multi-technique study of the magnetic properties of a 200 Å polycrystalline Co film supported by a GaAs(001) substrate. X-ray reflection and PNR data are found to be well fitted by the same values of layer thickness so providing an additional check on the structural parameters yielded by the fits to the PNR data. Measurements of the reflected neutron polarisation for the same 200 Å Co film are consistent with a layer averaged moment of 1.6μB and a magnetisation profile in which the moment per Co atom reaches 1.7μB in the interior of the film and falls to 0.8μB in a 20 Å thick layer at the GaAs interface. A perpendicular saturation field of 1.39 T is obtained from EHE measurements extended to 20 T on the same sample, from which we estimate a small perpendicular anisotropy field which opposes the demagnetising field. This result indicates that the magnetisation profile is not associated with a large interface anisotropy energy. © 1992.

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.