1. Which Metals Does a Magnet Grab, and Which Does It Ignore?
Only a few elements show the strong, everyday kind of magnetism. Iron, nickel and cobalt are the main ones, along with a couple of rare-earth metals. The name for this behavior is ferromagnetism, from ferrum, the Latin word for iron, because iron is the most common and most dramatic example. [1] Steel is mostly iron, so a steel nail or a steel paper clip counts too.
Aluminum and copper are not on that list. They respond to a magnetic field in other, much weaker ways, which section 4 comes back to. So the useful question is not "why do magnets stick to metal?" Most metals are not grabbed at all. The real question is "what is special about iron?"
2. Why Is an Ordinary Iron Nail Not a Magnet by Itself?
The first clue is inside the atoms. Every electron spins, and that spin makes it act like a very small magnet. In most materials these little magnets cancel each other or point every which way. In iron, nickel and cobalt, a quantum-mechanical interaction between neighboring atoms locks the unpaired electron spins into the same direction, even though the jiggling of heat tries to scramble them. [1]
That sounds like a recipe for a very strong magnet, so why do two iron nails not stick together? Because the alignment only reaches so far. It stretches across small patches called magnetic domains, roughly 0.1 millimeter to a few millimeters across. Inside one domain the magnetic field is strong. But in a lump of iron the many domains point in random directions, so they cancel and the lump as a whole is usually unmagnetized. [1]
The idea of domains goes back to the French physicist Pierre-Ernest Weiss, who suggested in 1906 that a ferromagnet contains many such regions. [3] Picture a crowd of compass needles split into hundreds of little teams. Each team agrees inside itself, but the teams disagree with one another.
3. How Does a Magnet Turn a Nail Into a Magnet for a Moment?
Bring a magnet close and its field pushes on the domains. Even a small outside field can make the domains line up with each other, and the iron is then said to be magnetized. [1] The domains that already pointed the right way grow at the expense of their neighbors. [1] The end of the nail nearest the magnet ends up with the opposite pole, and opposite poles attract. That is why a magnet pulls a plain iron nail toward it instead of pushing it away. The magnet does not only pull on the iron. It first makes the iron into a magnet, and then the two pull on each other. The sources describe the alignment; the step from alignment to attraction is my summary.
The effect is large. The same source says the magnetized iron raises the field by a large factor compared with the driving field alone, which is why an iron core makes an electromagnet far stronger than a bare coil of wire. [1]
4. Why Do Aluminum and Copper Not Stick to a Magnet?
A university physics text sorts materials into three groups by how they answer a magnetic field. [2]
- Ferromagnetic (iron, nickel, cobalt): the tiny magnets line up and become locked together, so the magnetization stays even after the field is turned off or reversed.
- Paramagnetic: the tiny magnets line up with the field only slightly, which adds a small extra field.
- Diamagnetic: the atoms have no net magnetic moment of their own, and the field creates a small one pointing against the applied field.
Both weak kinds exist, but the effect is so tiny that an ordinary magnet does not visibly tug a piece of foil. Textbooks usually list aluminum as paramagnetic and copper as diamagnetic. I did not see that specific listing in the pages I opened, so treat it as standard background rather than something I checked here.
5. Why Do Some Pieces of Iron Stay Magnetic and Others Do Not?
Ferromagnets tend to keep some magnetization after the outside field is removed, a kind of "memory of their magnetic history" called hysteresis. The fraction that remains is called remanence, and it matters a great deal for permanent magnets. [1] A plain nail keeps very little, because its domains drift back toward random directions. A permanent magnet is made from material and shapes that hold the aligned domains in place.
This is why "sticks to a magnet" and "is a magnet" are two different properties. A steel paper clip is attracted, but it does not attract other clips until a magnet is nearby. Stroking a nail in one direction with a magnet can leave it weakly magnetized, because some domains stay turned. That is the usual classroom explanation, and I did not find it stated in the sources I read.
6. Can Heat Switch Magnetism Off?
Yes. Every ferromagnet has a top temperature above which the ferromagnetic behavior disappears, because heat shakes the spins out of order. This is the Curie temperature. [1] A table of measured values lists iron at 1043 kelvin, nickel at 627 kelvin and cobalt at 1388 kelvin. [4] Converted to Celsius (subtract 273), that is about 770 °C for iron, 354 °C for nickel and 1115 °C for cobalt.
It works in the other direction too. Cooled below its Curie temperature, a ferromagnetic material spontaneously splits into domains. [3] So domains are not something a magnet puts into iron. They appear on their own as iron cools. Please do not test this at home: 770 °C is far hotter than a kitchen stove or a candle should ever be used for.
7. What Makes the Strongest Magnets So Strong?
The strongest permanent magnets on sale today are neodymium magnets, made from an alloy of neodymium, iron and boron. They were developed independently in 1984 by General Motors and Sumitomo Special Metals, partly because the samarium-cobalt magnets that came before used costly raw materials. [5] Pure neodymium is not a strong magnet at all. It orders magnetically only below 19 kelvin (about −254 °C), and it is the compound with iron that is ferromagnetic well above room temperature. [5]
Two ideas from the earlier sections explain the strength. The crystal prefers to magnetize along one particular axis and resists turning away from it, which makes the magnet very hard to demagnetize. And the grains are lined up in a strong field while the magnet is made, so their axes all point the same way. [5] In other words, a good permanent magnet is iron's domain story with the alignment firmly locked in. Because they are so strong, small neodymium magnets can pinch skin hard and are dangerous if swallowed, so keep them away from young children.
8. How Can You Test This With a Fridge Magnet and a Few Objects?
Collect a fridge magnet and a mix of things: a steel nail, a paper clip, aluminum foil, a copper wire or coin, and a plastic spoon. Guess first, then test which ones the magnet picks up. Some silver-looking things may surprise you, because many cans and utensils are steel and others are not.
Next, hang a paper clip from the magnet and see whether a second clip hangs from the first. The first clip has become a temporary magnet. Pull the first clip off the magnet and see whether the second one falls. If it drops at once, the clip kept very little of its magnetization. Use only weak magnets, keep them away from phones, bank cards and medical devices such as pacemakers, and keep small magnets away from little children.