A very-high-sensitivity magnetometer is described in detail. The sensitivity of the apparatus is typically of the order of 10−9 A cm2/(Hz)1/2, so that changes in magnetic moment of 10−10 A cm2 can be detected. The magnetic moment can be calculated by simulation of the measurement system. An example of a hysteresis loop is shown. The size of the sample is determined by electron microscopy. The physical limits of the measurement system are worked out and discussed.