You probably know that magnets don't stick to nonmetal materials such as wood, plastic, glass, cotton and wool, but you may not know that magnets don't stick to all metals. In fact, most metals are not magnetic.
TL;DR (Too Long; Didn't Read)
Magnets stick to metals that have strong magnetic properties themselves, such as iron and nickel. Metals with weak magnetic properties include aluminum, brass, copper and lead.
How Magnets Work
A magnet is a piece of metal with the ability to attract other metals. The Earth itself is a huge magnet. It has two ends called poles, a north-seeking pole and a south-seeking pole, and an invisible area of magnetism surrounding it called a magnetic field.
Billions of positively charged atoms produce negatively charged electrons, which spin around the core of an atom and create a magnetic force, transforming the atom into a minuscule magnet. In most materials, the atoms are organized in a way that the magnetic forces point in haphazard directions. However, in some materials, the atoms are organized in a way in which most of the magnetic forces point in the same direction. These forces merge, and the object acts like a magnet. The north pole of one magnet attracts the south pole but repels the north pole of another magnet – unlike poles attract and like poles repel. A metal is a magnet if it repels a known magnet.
Types of Magnetic Materials
When we discuss materials under the guise of magnetism, it can be helpful to separate out the classes of magnetic materials. The primary types of magnets we will discuss are diamagnetic, paramagnetic and ferromagnetic materials.
Even though materials like wood and glass do not appear to exhibit any reaction to magnetic fields, all matter has a fundamental feature encoded in its composition of atoms and magnetic moments.
Magnetic moments are essentially a property of an object that determines how it might react to an external magnetic field. Diamagnetic metals and materials have atoms without consistent magnetic moments. As a result them do not experience a strong reaction to an external magnetic field in the same way classical magnets might. Some diamagnetic metals and materials include copper, wood, water, and bismuth.
These materials are very similar to diamagnetic materials because they only exhibit a response to external magnetic fields, and they do not inherently posses their own magnetic moment. Paramagnetic metals and materials include aluminum, magnesium, oxygen, iron oxide, and titanium.
Metals That Attract Magnets
Metals that attract magnets are known as ferromagnetic metals. These metals are made up of billions of individual atoms that have magnetic properties, meaning magnets stick to them firmly. Some examples are iron, cobalt, nickel, steel (because it is mostly iron), manganese, gadolinium and lodestone. Some metals, such as iron, are referred to as magnetically soft because they become strong temporary magnets when you hold a magnet near them but lose some or all their magnetism when you remove the magnet. Alloys of iron and rare-earth metals such as samarium and neodymium maintain most of their magnetism even when they are not within a magnetic field, so they are called magnetically hard and make good permanent magnets.
AlNiCo elements describe a group of iron alloys that use aluminum (Al), nickel (Ni), and cobalt (Co) to create permanent magnets. Permanent magnets using materials like these are formed using an external magnetic fields to align all of their magnetic domains together. Because of their crystal structure and composition, they then maintain this magnetic alignment. Other external factors like heat and corrosion can impact this acquired property.
Often metals that contain iron are ferromagnetic, but adding other kinds of metal into a magnetic alloy can dramatically alter structure and the availability of unpaired electrons. The varying types of stainless steel (as discussed in the following section) are a great example of this effect in action. Many common metals can be magnetic, but many other metals on the periodic table range in their magnetic properties; when we mix them together, it can result in many surprising effects.
Metals That Don't Attract Magnets
In their natural states, metals such as aluminum, brass, copper, gold, lead and silver don't attract magnets because they are weak or non-magnetic metals. However, you can add properties such as iron or steel to the weak metals to make them stronger. Adding even a small quantity of iron to a metal such as silver makes it magnetic. You may think this would make stainless steel magnetic because it contains iron, a magnetic metal, but when nickel is added during the manufacturing process, the physical structure is changed, creating a nonmagnetic form of stainless steel called austenitic stainless steel. On the other hand, adding chromium to stainless steel creates ferritic stainless steel which does exhibit magnetic properties.
Electromagnets and Induced Magnetic Fields
When scientists and engineers want to produce a very strong magnetic field, they may need to go beyond static magnetics made of iron and neodymium. In these cases, we can use electromagnets to create incredibly strong magnetic fields. Electromagnets depend less on the type of metal, and they rely more on the capacity to carry an electric current.
Electric current is run through coils of wire in something called an inductor. The inductor creates a magnetic dipole through the laws of electromagnetism and the effect of moving charges. Because we can vary and intensify the current and flow of charge so greatly, it can create very strong magnets.
- University of Illinois at Urbana-Champaign: Q & A: How Do Magnets Work?
- Boston University Physics: Magnetic Fields and How to Make Them
- Northeastern University College of Engineering: Magnetism Basics
- National High Magnetic Field Laboratory: Magnets From Mini to Mighty
- University of Maryland Division of Information Technology: Is Stainless Steel Magnetic?
- NDT Resource Center: Diamagnetic, Paramagnetic, and Ferromagnetic Materials
About the Author
Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.