Magnetism generated in non-magnetic metals
By subtly altering certain quantum interactions in matter, scientists from the University of Leeds have shown for the very first time how to generate magnetism in metals that aren’t normally magnetic. Synthetic magnets made using this technique may one day reduce our reliance on rare or toxic metals in such things as wind turbines, computer hard drives and magnetic field medical imaging devices.
"Being able to generate magnetism in materials that are not naturally magnetic opens new paths to devices that use abundant and hazardless elements, such as carbon and copper," says postgraduate student Fatma Al Ma’Mari, from the School of Physics & Astronomy at the University of Leeds.
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Ordinarily, there exist only three metals that are ferromagnetic – that is, able to be made magnetic in the presence of a magnetic field and remaining that way after the field is removed – at room temperature, these being iron, cobalt, and nickel. The reason behind this limited number of ferromagnetic materials is due to an atomic condition known as the Stoner criterion that determines the distribution of electrons in an atom and the strength of their interactions.
Specifically, the Stoner criterion dictates that when the number of different states that electrons are allowed to occupy in orbits around the nucleus of an atom are multiplied by the exchange interaction applicable (that is, the interaction between bound electrons inside an atom, and relative to the orientation of each electron’s spin as a quantum mechanical state ("up" or "down")), then the result must be greater than one for the metal to be ferromagnetic.
Put simply, if one thinks of an electron as having charge and a magnetic field, then in ordinary metals equal amounts of paired electrons (that is, where the Stoner criterion is equal to zero) effectively cancel out each other's magnetic fields, leaving the material magnetically neutral.
However, in ferromagnetic materials, where there are more "un-paired" electrons than paired ones (that is, where the Stoner criterion is greater than one), then the magnetic fields ordinarily canceled out by oppositely-oriented electrons are allowed to exist. As such, each un-paired electron can be said to act like a tiny magnet and, with many such electrons in a material, they provide an overall magnetic field to the metal in which they reside.
Unlike other types of magnetic materials that have had their properties altered by the addition of non-magnetic substances – such as in "non-Joulian" magnets – in the latest research the University of Leeds scientists have demonstrated how they altered the exchange interaction in non-magnetic metals by removing material. Or, more specifically, by removing electrons. They achieved this by using an interface coated with a thin layer of carbon C60 molecules (Buckminsterfullerene, aka buckyballs; spherical molecules of carbon in a fused-ring structure), to take out a number of electrons and effectively raise the un-paired number over the Stoner criterion to render the metals ferromagnetic.
"We and other researchers had noticed that creating a molecular interface changed how magnets behave," says Dr Oscar Cespedes, from the School of Physics & Astronomy at the University of Leeds. "For us, the next step was to test if molecules could also be used to bring magnetic ordering into non-magnetic metals."
As a result, the researchers were able to alter both copper and manganese to become ferromagnetic by adding C60 to remove a number of electrons. Though only small in magnetic strength compared to naturally-occurring ferromagnetic metals, the synthetic magnetic materials display these properties at room temperature. Unlike many strange material breakthroughs or weird properties that ordinarily stable compounds exhibit when their temperature is reduced to just above absolute zero, this means that such electron-reduced magnetic metals could find their way into all manner of applications in the real world.
"Future technologies, such as quantum computers, will require a new breed of magnets with additional properties to increase storage and processing capabilities," says Al Ma’Mari. "Our research is a step towards creating such ‘magnetic metamaterials’ that can fulfill this need."
It is, however, early days in the development of these materials, and a great deal more work will be required to increase their magnetic strength.
"Currently, you wouldn’t be able to stick one of these magnets to your fridge," says Dr Cespedes. "But we are confident that applying the technique to the right combination of elements will yield a new form of designer magnets for current and future technologies."
The results of this research were recently published in the journal Nature.
Source: University of Leeds