A magnetic particle (30, 70) has a layered structure (6, 56) between a top surface of the particle and an opposed bottom surface of the particle. Layers of the structure include one or more nonmagnetic layer(s) and one or more magnetized layer(s). The ratio of a lateral dimension of the one or more magnetized layers to the aggregate thickness of the magnetized layer or layers is greater than 500. A plurality of such magnetic particles (30, 70) can be functionalised and marked with readable codes (16, 66) corresponding to the functionalisation, for use for performing assays such as bioassays.