An implantable magnetic marker 6 includes a piece of a large Barkhausen jump material (LBJ) containing at least one loop. The coiled marker is deployed to mark a tissue site in the body for subsequent surgery, and a magnetic detection system with a handheld probe excites the marker above or below the switching field required for bistable switching of the marker causing a harmonic response to be generated in a bistable or sub-bistable mode that allows the marker to be detected and localised. Preferably an axial member 6c of LBJ material extends through the marker coil. The marker may comprise interwoven coils 6s, 6t which may be of different diameters. The coil may be made from an LBJ microwire in a housing or with a coating having a resistivity greater than 2x10-7 Ωm. The cross-section of the coil may be circular, square, triangular (figure 18) or otherwise.
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