When a magnet is held near various metal objects, some cling firmly while others show no attraction. This common observation highlights a fundamental difference in how materials interact with magnetic fields. Understanding why some metals are magnetic and others are not involves delving into their atomic structure.
The Foundation of Magnetic Attraction
Magnetic attraction stems from ferromagnetism. In ferromagnetic materials, electrons possess tiny magnetic moments that align within specific regions called magnetic domains. When an external magnetic field is applied, these domains align, leading to a strong overall magnetic effect. This allows materials to become strongly magnetized and form permanent magnets.
Only a few elements exhibit ferromagnetism at room temperature: iron, nickel, and cobalt. Many alloys containing these elements, such as steel (which is primarily iron), also display strong magnetic properties.
Metals That Don’t Attract Magnets
Many common metals do not exhibit a noticeable attraction to a magnet. These include copper, aluminum, gold, silver, lead, zinc, and tin. Alloys such as brass and bronze, which are primarily copper-based, also fall into this non-magnetic category. While these metals do not “stick” to magnets like iron, they still interact with magnetic fields in much weaker, often unnoticeable ways.
Their lack of strong magnetic attraction is due to their classification as either diamagnetic or paramagnetic materials. Unlike ferromagnetic substances, the magnetic responses of diamagnetic and paramagnetic metals are significantly weaker.
Subtle Magnetic Interactions: Diamagnetism and Paramagnetism
Metals that do not attract magnets exhibit weak magnetic responses: diamagnetism and paramagnetism. Diamagnetic materials, such as copper, gold, and bismuth, are weakly repelled by magnetic fields. In these materials, all electrons are paired, meaning their individual magnetic moments cancel out, resulting in no permanent net magnetic moment. When an external magnetic field is applied, a weak opposing magnetic field is induced.
Paramagnetic materials, including aluminum, platinum, and magnesium, are weakly attracted to magnetic fields. They possess some unpaired electrons, which give their atoms small, permanent magnetic moments. While these moments tend to align with an external magnetic field, thermal energy causes a random orientation, preventing a strong, lasting magnetization. Neither diamagnetic nor paramagnetic interactions are strong enough for these materials to “stick” to a magnet.
Factors Influencing Magnetic Behavior
Several external factors influence a metal’s magnetic behavior. Temperature is one significant factor; ferromagnetic materials lose their strong magnetic properties when heated above a specific point called the Curie temperature. Above this temperature, increased thermal energy overcomes the forces that keep magnetic domains aligned, causing the material to become paramagnetic. For example, iron’s Curie temperature is around 770 °C, while nickel’s is approximately 358 °C.
The purity of a metal also plays a role in its magnetic characteristics. Impurities can disrupt the uniform crystal lattice, which can interfere with the movement of magnetic domains and alter magnetic efficiency.
Alloying, the process of mixing metals, can significantly change magnetic properties. Adding non-magnetic elements like chromium or nickel to iron, for instance, can reduce or even eliminate the magnetic properties of steel, depending on their concentration and the resulting microstructure.