Spherical sample holders to improve the susceptibility measurement of   superparamagnetic materials
  The design of two custom sample holders with a spherical cavity for commercial vibrating sample magnetometer systems is described. For such cavities the magnetization M and the internal magnetic field Hi of a sample are both homogeneous. Consequently, the material parameter M(Hi) of a sample can be determined even for liquids and powders with a high magnetic susceptibility. 
Thomas Friedrich, Tobias Lang, Ingo Rehberg, Reinhard Richter
physics.ins-det cond-mat.mtrl-sci