You probably know that steel or iron are considered magnetic materials. They can be picked up by a magnet. But did you know that all substances are affected by magnetism to some degree? Some are affected a lot, and some very little. Let’s look at different types of materials, classified according to how they are affected by a magnetic field.
These are the metals that are strongly attracted to magnets. They include iron, nickel, cobalt, and steel. The atoms of these substances contain electrons which are all spinning in the same direction. This gives the atoms strong polarity, as if they were little magnets themselves. They are strongly influenced by a magnetic field, and the atoms can be turned to point their poles in the same direction. These metals can become strong magnets themselves.
These are metals that are weakly attracted to magnets. They include aluminum, gold, and copper. The atoms of these substances contain electrons most of which spin in the same direction … but not all . This gives the atoms some polarity. They are only weakly influenced by a magnetic field, (you couldn’t pick up even a tiny piece of gold with a magnet) and since some of the atoms can be turned to point their poles in the same direction, these metals can become very weak magnets. Their attractive force can only be measured with sensitive instruments.
The main ferrimagnetic material is magnetite, a crystal which occurs naturally in rocks called lodestones, which were the first magnetic materials discovered by man. The crystal structure of the mineral allows only some of the atoms to line up when a magnetic field is present, so it is only weakly attracted to a magnet. The crystal itself is only a weak magnet.
However, if magnetite is ground up into a powder, its magnetic properties, although weak, are very useful. Magnetite powder is what coats the tape in a cassette, allowing you to record sounds and music. Magnetite powder mixed with plastic and pressed into rectangles makes the stick-on magnets used for fridge magnets.
These materials are everything else … plants, water, soil, wood, your skin … all other substances. Diamagnetic substances show no visible reaction to the presence of a magnetic field, because the electrons in their atoms are spinning in equal numbers in opposite directions. These materials are magnetically neutral.
However, the molecules of these substances do have a very small polarity, because they are usually not symmetric; one end may have more electrons moving around it than another. When they are exposed to a strong magnetic field, they are repelled a tiny amount (rather than attracted). This has useful consequences. When human tissue, for example, is exposed to a strong magnetic field, some of the atoms move away; analysis of the movement is called magnetic resonance imaging, or MRI, and is a useful diagnostic tool in medicine.
Temperature can affect the magnetic properties of a material. Paramagnetic materials like aluminum and copper become more magnetic when they are very cold.
Alternately, strongly magnetic ferromagnetic materials like nickel or steel lose all their magnetic properties if they are heated to a high enough temperature. The atoms become too excited by the heat to remain pointing in one direction for long.
The temperature at which a metal loses its magnetism is called the Curie temperature, and it is different for every metal. The Curie temperature for nickel, for example, is about 350°C.