Sample Length Dependence of Magnetic Moment Detected by a SQUID Magnetometer Using Nickel Cylinders

We report here on sample length dependence measurements in a second-order gradiometer superconducting quantum interference device (SQUID) magnetometer of pure nickel (Ni) cylinders, having a diameter of 1 mm and a length of 1-7 mm. The measured magnetic moment per unit mass significantly decreases with the sample length of the Ni cylinder, when a magnetic field of 7 T was applied parallel to the long axis of the cylinder. As an example, the magnetic moment per unit mass for the 5 mm Ni cylinder is between 97% and 94% of that for the 1 mm Ni cylinder, the number depending on the specific type of magnetometer used. The magnitude of decrease also depends on the length of the second-order gradiometer superconducting coil which differs with different models of SQUID magnetometers.