in electromagnetism, the magnetic susceptibility (latin: susceptibilis, "receptive"; denoted χ) 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, χ > 0, called paramagnetism, or an alignment against the field, χ < 0, called diamagnetism.
this alignment has several effects. first, the magnetic susceptibility indicates whether a material is attracted into or repelled out of a magnetic field. paramagnetic materials align with the field, so are attracted to the magnetic field. diamagnetic materials are anti-aligned, so are pushed away from magnetic fields. second, on top of the applied field, the magnetic moment 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 paleomagnetic studies and structural geology.
fundamentally, the magnetic moment of materials comes from the magnetism of the particles they are made of. usually, this is dominated by the magnetic moments of electrons. electrons are present in all materials, but without any external magnetic field, the magnetic moments of the electrons are usually either paired up or random so that the overall magnetism is zero (the exception to this usual case is ferromagnetism). the fundamental reasons why the magnetic moments of the electrons line up or do not are very complex and cannot be explained by classical physics (see bohr–van leeuwen theorem). however, a useful simplification is to measure the magnetic susceptibility of a material and apply the macroscopic form of maxwell's equations. this allows classical physics to make useful predictions while avoiding the underlying quantum mechanical details.