Why Magnets Have No Effects on Some Metals

Why Magnets Have No Effects on Some Metals
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Magnetism and electricity are connected so intimately that you might even consider them two sides of the same coin. The magnetic properties exhibited by some metals are a result of electrostatic field conditions in the atoms that compose the metal.

In fact, all elements have magnetic properties, but most don't manifest them in an obvious way. The metals that are attracted to magnets have one thing in common, and that's unpaired electrons in their outer shells. That's just one electrostatic recipe for magnetism, and it's the most important.

Diamagnetism, Paramagnetism and Ferromagnetism

Metals that you can permanently magnetize are known as ​ferromagnetic​ metals, and the list of these metals is small. The name comes from ​ferrum​, the Latin word for iron​.

There is a much longer list of materials which are ​paramagnetic​, which means they become temporarily magnetized when in the presence of a magnetic field. Paramagnetic materials aren't all metals. Some covalent compounds, such as oxygen (O2) exhibit paramagnetism, as do some ionic solids.

All materials which aren't ferromagnetic or paramagnetic are ​diamagnetic​, which means they exhibit a slight repulsion to magnetic fields, and an ordinary magnet does not attract them. Actually, all elements and compounds are diamagnetic to some degree.

To understand the differences between these three classes of magnetism, you have to look at what's going on at the atomic level.

Orbiting Electrons Create a Magnetic Field

In the currently accepted model of the atom, the nucleus consists of positively charged protons and electrically neutral neutrons held together by the strong force, one of the fundamental forces of nature. A cloud of negatively charged electrons occupying discrete energy levels, or shells, surrounds the nucleus, and these are what impart magnetic qualities.

An orbiting electron generates a changing electric field, and according to Maxwell's equations, that's the recipe for a magnetic field. ​The magnitude of the field is equal to the area inside the orbit multiplied by the current.​ An individual electron generates a tiny current, and the resulting magnetic field, which is measured in units called ​Bohr magnetons​, is also tiny. In a typical atom, the fields generated by all its orbiting electrons generally cancel each other out.

Electron Spin Affects Magnetic Properties

It isn't just the orbiting motion of an electron that creates charge, but also another property known as ​spin​. As it turns out, spin is much more important in determining magnetic properties than orbital motion, because overall spin in an atom is more likely to be asymmetric and capable of creating a magnetic moment.

You can think of spin as the direction of rotation of an electron, although this is just a rough approximation. Spin is an intrinsic property of electrons, not a state of motion. An electron that spins clockwise has ​positive spin​, or spin up, while one that rotates counterclockwise has ​negative spin​, or spin down.

Unpaired Electrons Confer Magnetic Properties

Electron spin is a quantum mechanical property without a classical analogy, and it determines the placement of electrons around the nucleus. Electrons arrange themselves in spin-up and spin-down pairs in each shell so as to create zero net ​magnetic moment​.

The electrons responsible for creating magnetic properties are the ones in the outermost, or ​valence​, shells of the atom. In general, the presence of an unpaired electron in an atom's outer shell creates a net magnetic moment and confers magnetic properties, whereas atoms with paired electrons in the outer shell have no net charge and are diamagnetic. This is an oversimplification, because valence electrons can occupy lower energy shells in some elements, particularly iron (Fe).

Everything is Diamagnetic, Including Some Metals

The current loops created by orbiting electrons make every material diamagnetic, because when a magnetic field is applied, the current loops all align in opposition to it and oppose the field. This is an application of ​Lenz's Law​, which states that an induced magnetic field opposes the field that creates it. If electron spin did not enter into the equation, that would be the end of the story, but spin does enter into it.

The total ​magnetic moment​ J​ of of an atom is the sum of its ​orbital angular momentum​ and its ​spin angular momentum​. When ​J​ = 0, the atom is non-magnetic, and when ​J​≠ 0, the atom is magnetic, which happens when there is at least one unpaired electron.

Consequently, any atom or compound with completely filled orbitals is diamagnetic. Helium and all the noble gases are obvious examples, but some metals are also diamagnetic. Here are a few examples:

  • Zinc
  • Mercury
  • Tin
  • Tellurium
  • Gold
  • Silver
  • Copper

Diamagnetism is not the net result of some atoms in a substance being pulled one way by a magnetic field and others being pulled in another direction. Every atom in a diamagnetic material is diamagnetic and experiences the same weak repulsion to an external magnetic field. This repulsion can create interesting effects. If you suspend a bar of a diamagnetic material, such as gold, in a strong magnetic field, it will align itself perpendicularly to the field.

Some Metals Are Paramagnetic

If at least one electron in an atom's outer shell is unpaired, the atom has a net magnetic moment, and it will align itself with an external magnetic field. In most cases, the alignment is lost when the field is removed. This is paramagnetic behavior, and compounds can exhibit it as well as elements.

Some of the more common paramagnetic metals are:

  • Magnesium
  • Aluminum
  • Tungsten
  • Platinum

Some metals are so weakly paramagnetic that their response to a magnetic field is hardly noticeable. The atoms align with a magnetic field, but the alignment is so weak that an ordinary magnet does not attract it.

You couldn't pick up the metal with a permanent magnet, no matter how hard you tried. However, you would be able to measure the magnetic field generated in the metal if you had a sensitive enough instrument. When placed in a magnetic field of sufficient strength, a bar of a a paramagnetic metal will align itself parallel to the field.

Oxygen Is Paramagnetic, and You Can Prove It

When you think of a substance having magnetic characteristics, you generally think of a metal, but a few non-metals, such as calcium and oxygen, are also paramagnetic. You can demonstrate oxygen's paramagnetic nature for yourself with a simple experiment.

Pour liquid oxygen between the poles of a powerful electromagnet, and the oxygen will collect on the poles and vaporize, producing a cloud of gas. Try the same experiment with liquid nitrogen, which isn't paramagnetic, and nothing will happen.

Ferromagnetic Elements Can Become Permanently Magnetized

Some magnetic elements are so susceptible to external fields that they become magnetized when exposed to one, and they maintain their magnetic characteristics when the field is removed. These ferromagnetic elements include:

  • Iron
  • Nickel
  • Cobalt
  • Gadolinium
  • Ruthenium

These elements are ferromagnetic because individual atoms have more than one unpaired electron in their orbital shells. but there's something else going on, too. The atoms of these elements form groups known as ​domains​, and when you introduce a magnetic field, the domains align themselves with the field and remain aligned, even after you remove the field. This delayed response is known as ​hysterisis,​ and it can last for years.

Some of the strongest permanent magnets are known as ​rare earth magnets​. Two of the most common are ​neodymium​ magnets, which consist of a combination of neodymium, iron and boron, and ​samarium cobalt​ magnets, which are a combination of those two elements. In each type of magnet, a ferromagnetic material (iron, cobalt) is fortified by a paramagnetic rare earth element.

Ferrite​ magnets, which are made of iron, and ​alnico​ magnets, which are made from a combination of aluminum, nickel and cobalt, are generally weaker than rare earth magnets. This makes them safer to use and more suitable for science experiments.

The Curie Point: a Limit to a Magnet's Permanence

Every magnetic material has a characteristic temperature above which it begins to lose its magnetic characteristics. This is known as the ​Curie point​, named after Pierre Curie, the French physicist who discovered the laws that relate magnetic ability to temperature. Above the Curie point, the atoms in a ferromagnetic material begin to lose their alignment, and the material becomes paramagnetic or, if the temperature is high enough, diamagnetic.

The Curie point for iron is 1418 F (770 C), and for cobalt it's 2,050 F (1,121 C), which is one of the highest Curie points. When the temperature falls below its Curie point, the material regains its ferromagnetic characteristics.

Magnetite is Ferrimagnetic, Not Ferromagnetic

Magnetite, also known as iron ore or iron oxide, is the gray-black mineral with the chemical formula Fe3O4 that is the raw material for steel. It behaves like a ferromagnetic material, becoming permanently magnetized when exposed to an external magnetic field. Until the mid-twentieth century, everyone assumed it to be ferromagnetic, but it's actually ​ferrimagnetic​, and there's a significant difference.

The ferrimagnetism of magnetite is not the sum of the magnetic moments of all the atoms in the material, which would be true if the mineral was ferromagnetic. It's a consequence of the crystal structure of the mineral itself.

Magnetite consists of two separate lattice structures, an octahedral one and a tetrahedral one. The two structures have opposing but unequal polarities, and the effect is to produce a net magnetic moment. Other known ferrimagnetic compounds include yttrium iron garnet and pyrrhotite.

Antiferromagnetism Is Another Type of Ordered Magnetism

Below a certain temperature, which is called the ​Néel temperature​ after French physicist Louis Néel, some metals, alloys and ionic solids lose their paramagnetic qualities and become unresponsive to external magnetic fields. They essentially become demagnetized. This happens because ions in the lattice structure of the material align themselves in antiparallel arrangements throughout the structure, creating opposing magnetic fields that cancel each other out.

Néel temperatures can be very low, in the order of -150 C (-240F), making the compounds paramagnetic for all practical purposes. However, some compounds have Néel temperatures in the range of room temperature or above.

At very low temperatures, antiferromagnetic materials exhibit no magnetic behavior. As the temperature rises, some of the atoms break free of the lattice structure and align themselves with the magnetic field, and the material become weakly magnetic. When the temperature reaches the Néel temperature, this paramagnetism reaches its peak, but as the temperature rises beyond this point, thermal agitation prevents the atoms from maintaining their alignment with the field, and the magnetism steadily drops off.

Not many elements are antiferromagnetic – only chromium and manganese. Antiferromagnetic compounds include manganese oxide (MnO), some forms of iron oxide (Fe2O3) and bismuth ferrite (BiFeO3).

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