. An implantable magnetic marker comprising at least one 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. (Figure 6A)
