Unit of magnetic susceptibilty is parts per thousand(ppt).
Magnetic susceptibility is a measure of how much a material will become magnetized in an applied magnetic field. Mathematically, it is the ratio of magnetization (M) (magnetic moment per unit volume) to the applied magnetizing field intensity (H). This allows a simple classification of most materials' response to an applied magnetic field into two categories, an alignment with the magnetic field called paramagnetism, or an alignment against the field called diamagnetism.
Paramagnetic materials align with the applied field and are attracted to regions of greater magnetic field. Diamagnetic materials are anti-aligned and are pushed away, toward regions of lower magnetic fields. On top of the applied field, the magnetization of the material adds its own magnetic field, causing the field lines to concentrate in paramagnetism, or be excluded in diamagnetism.
Quantitative measures of the magnetic susceptibility also provide insights into the structure of materials, providing insight into bonding and energy levels. Furthermore, it is widely used in geology for paleo-magnetic studies and structural geology.