magnetic susceptibility
/ mag-NET-ik suh-sep-tuh-BIL-uh-tee /
Push two people on swings with the same gentle nudge: one barely moves, the other swings high. They have different 'responsiveness' to the same push. Magnetic susceptibility is exactly that, for materials and magnetism: it measures how strongly a material magnetizes when you apply a given magnetic field — a lot, a little, or in the wrong direction.
Concretely, it is the ratio of how much magnetization you get out to how much field you put in. The sign and size tell the whole story. A small negative susceptibility means the material magnetizes faintly against the field — that is a diamagnet. A small positive value means it magnetizes faintly with the field — a paramagnet. A huge value means a tiny field produces enormous magnetization — the hallmark of a material on the verge of, or already in, ferromagnetic order.
Susceptibility matters because it is one of the most direct things you can actually measure, and its behavior reveals what is happening inside. Watching how susceptibility changes with temperature is a primary way physicists detect magnetic phase transitions and identify which kind of order a material has. The common confusion is treating susceptibility as a fixed number; for most magnetic materials it changes strongly with temperature, and that very change is what carries the interesting information.
Cool a paramagnet and measure its susceptibility: it grows steadily as the temperature drops, because colder atoms are jostled less and align more readily with the field. Plotting susceptibility against temperature is a routine first step in sizing up any new magnetic material.
Susceptibility rises as a paramagnet cools — a basic fingerprint of its magnetic behavior.
Susceptibility (response to a field) should not be mixed up with magnetization (the actual magnetic strength a material has). A permanent magnet keeps strong magnetization even at zero field, where the simple notion of susceptibility no longer captures its behavior.