Magnetism is an invisible force present in certain materials due to the ordered motion and alignment of their internal structures. When a metal object is magnetized, these magnetic moments, grouped into microscopic regions called domains, are coaxed into pointing predominantly in the same direction. This article will explore the practical methods you can use to achieve this alignment and turn an ordinary piece of metal into a magnet.
Prerequisites for Magnetization
Achieving a magnetic state requires a starting material whose internal structure can be influenced by an external magnetic field. The fundamental requirement is the presence of magnetic domains, small, localized areas within the metal where magnetic moments are already aligned. In an unmagnetized piece of metal, these domains point randomly, causing their magnetic effects to cancel each other out. Only a few elements possess the necessary internal structure to become strongly magnetized, a property known as ferromagnetism. The most common ferromagnetic metals are iron, nickel, and cobalt, and their alloys like steel, which are suitable because they can retain domain alignment after the external field is removed.
Mechanical Magnetization Techniques
The simplest way to magnetize a suitable piece of metal is by using an existing strong magnet to physically align its internal domains. This method, often called the stroking method, requires only a ferromagnetic object and a permanent magnet, such as a neodymium magnet.
To perform this, hold the metal object—such as a steel nail or paperclip—firmly and stroke it repeatedly with one pole of the permanent magnet. It is crucial to stroke the object in one direction only, moving from one end to the other, and then lifting the magnet high above the object before beginning the next stroke from the original starting point. Repeating this process approximately 30 to 50 times applies a continuous directional magnetic field that forces the domains within the metal to pivot and align themselves parallel to the stroke direction.
The end of the metal where the stroking finishes will acquire a magnetic pole opposite to the pole of the magnet used for stroking. This mechanical process typically only produces a weak magnet with a relatively short magnetic retention time, especially when compared to electrical methods.
Electrical Magnetization Techniques
A more powerful and controllable method involves using electricity to generate a magnetic field, a process known as electromagnetism. While often used to create temporary electromagnets, this technique can also permanently magnetize a ferromagnetic core.
The process involves constructing a device called a solenoid, which is a cylindrical coil created by wrapping insulated copper wire tightly around the metal object, such as an iron nail. This wire coil focuses the magnetic field.
Connecting the ends of the coil to a direct current (DC) power source, like a battery, generates a strong, uniform magnetic field that permeates the metal core. The strength of the resulting magnetic field is directly related to the current flowing through the wire and the density of the coil windings. A greater number of turns packed into a shorter length, combined with a higher current, will result in a much stronger magnetic influence, which more effectively aligns the metal’s internal domains. After maintaining the current for a short period, the metal core can be removed from the coil, and its newly aligned domains will grant it permanent magnetism. This method is considered the most effective for creating strong magnets.
Demagnetization and Magnetic Retention
The magnetism induced in a metal object can be intentionally or accidentally removed through various physical or thermal processes. One common method of demagnetization is applying sharp physical impact, such as repeatedly dropping or hammering the magnetized object. The shock and vibration jostle the aligned magnetic domains, causing them to return to a more random, disorganized state.
Heat is another effective way to demagnetize metal; exposing the object to high temperatures causes the atoms to vibrate intensely, disrupting the magnetic domain alignment. When a ferromagnetic material is heated above its Curie temperature—which is approximately 770 degrees Celsius for pure iron—it permanently loses its ferromagnetic properties and becomes non-magnetic. The object will not regain its full magnetic strength even after cooling down unless it is re-magnetized.
The object’s ability to hold onto its magnetism is determined by whether it is a magnetically “hard” or “soft” ferromagnetic material. Hard materials, such as specific steel alloys, have high coercivity, meaning their domains are stable and difficult to demagnetize, making them suitable for permanent magnets. Conversely, magnetically soft materials like pure iron have low coercivity, meaning they are easily magnetized but also quickly lose their alignment and are better suited for electromagnets where the magnetism must be temporary.