Field dependence of μSR signals in a polycrystalline magnet

Studies of magnetically ordered materials using implanted muons usually focus on the behaviour of the muon polarization in zero applied magnetic field, where precession signals reflect the static internal fields that appear in the magnetically ordered state. In many cases the magnetic behaviour under applied magnetic field is also of interest. However, detailed analysis and interpretation of field-dependent mu SR signals requires a clear understanding of the different types of behaviour that may be expected following various possible responses of the spin distribution to the applied field. In order to aid such investigations, quantitative analytical expressions are provided here for polycrystalline systems describing the behaviour of the muon polarization components and the precession frequency distribution in an applied field, under the initial assumption that the magnetic sublattices are not significantly perturbed by the applied field. Given these expressions, departures from this simple unperturbed behaviour can then be identified and interpreted in terms of the actual magnetic response to the applied field. An example is given using this approach in the analysis of a metamagnetic transition in a layered molecular magnet.