7.2 A₁C₆₀

The first observation of Mu in a metallic system other than the A₃C₆₀ materials is reported here for the first time. The structure of the Mu centre is different than the endohedral centre in A₃C₆₀, with a smaller temperature independent hyperfine parameter. To clarify the details of the structure of Mu in this system, more measurements are required. A better understanding of the hyperfine structure may aid in explanation of the T₁ relaxation of Mu in this system, which is remarkably slow compared to the A₃C₆₀ systems.

In the magnetic state of A₁C₆₀ (A = Rb, Cs), the static nature of the internal fields has been confirmed. A rapidly damped oscillation in the ZF spectra at the lowest temperatures in Cs₁C₆₀ is the first evidence from $\mu {\cal SR}$ that there is long-range order in the magnetic state. It is, however, not possible from this data to make any conclusion as to the form of the magnetic structure. A phenomenological form was used to parametrize the magnetic relaxation. The features apparent in the data are:

1.

The two component nature of the relaxation at low temperature;

2.

The amplitudes and relaxation rates of the two components change with temperature;

3.

In Rb₁C₆₀, the amplitudes haven't saturated by 2.5K in ZF, but they seem to have in high TF as well as in ZF in Cs₁C₆₀;

4.

The saturation of the low temperature amplitudes is to roughly equal amplitudes for the fast and slow components;

5.

The transition is quite broad and the temperature dependence is quite gradual.

This list of features of the relaxation in the magnetic state may be due to microscopic inhomogeneity in the magnetic state. An alternative interpretation has been suggested to explain this behaviour: the inhomogeneity may be a characteristic of a homogeneous magnetic state in conjunction with the manner in which the muons sample the internal fields. The similarity of the $\mu {\cal SR}$ relaxation and the ¹3C NMR spectrum in the magnetic state may be due to a similar sampling of the field distribution. Confirming the potential validity of such a model requires more sophisticated modelling of the field distribution than that presented here. The presence of a spin-flop transition in Rb₁C₆₀, may be related to the change in the relative magnitudes of the fast and slow components which occurs at fields less than 1T.