One material, two types of magnetism

Nov 22, 2012
BiTeI is formed from layers of bismuth, tellurium and iodine (left). The energy surface of this material is composed of two sheets (right), which is a manifestation of spin–orbit coupling in this material. Credit: 2012 Mohammad Saeed Bahramy, RIKEN Advanced Science Institute

When placed next to a bar magnet, an aluminum ball draws gently towards the magnet. In contrast, a ball made of silver moves out of the magnetic field. The mechanisms underlying these different behaviors are known as paramagnetism and diamagnetism, respectively. Surprisingly, the material called BiTeI—composed of layers of bismuth, tellurium and iodine atoms—can be either diamagnetic or paramagnetic, depending on how it is prepared.

The finding, by an international research team led by Naoto Nagaosa and Yoshinori Tokura from the RIKEN Advanced Science Institute in Wako, was unexpected because it requires an unusual mechanism to initially make the material magnetic. In BiTeI and related , the motion of electrons makes the most important contribution to their magnetism. Electrons move, roughly speaking, on orbital paths around atomic cores. " is usually associated with diamagnetism only," explains Nagaosa. "We were therefore surprised when our calculations [predicted] that there is [also] orbital paramagnetism in BiTeI." The experimental group headed by Tokura confirmed the prediction, as reported in the same paper.

Orbital diamagnetism is observed in a very wide variety of materials, including substances as common as water. Previously, the possibility of orbital paramagnetism, had been considered in only a few . One mechanism that can generate a paramagnetic response of electrons to a magnetic field is known as spin–orbit coupling. This coupling connects the charge of the electron with its spin—the intrinsic angular momentum of the electrons, which is also responsible for their . In BiTeI, spin–orbit coupling is particularly strong, making it an ideal material for exploring unusual effects of orbital magnetism.

To generate orbital paramagnetism in BiTeI, the trick is to control the number of moving through the crystal. Nagaosa and his colleagues found a specific range where the material is paramagnetic. They showed, however, that there is also a regime when orbital diamagnetism is strongly enhanced, making it possible to fundamentally change the way the material reacts to a magnetic field. 

Finding technological applications remains a task for the future, but Nagaosa expects that these unusual behaviors to appear in a broad class of materials. "Other bismuth compounds related to BiTeI should exhibit similar effects, but there might be entirely different solids with orbital paramagnetism," he explains. "Indeed, we have theoretical evidence that the effects we describe will also occur, for example, in various situations in graphene-like materials."

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More information: Schober, G.A.H., Murakawa, H., Bahramy, M.S., Arita, R., Kaneko, Y., Tokura, Y. & Nagaosa, N. Mechanisms of enhanced orbital dia- and paramagnetism: application to the Rashba semiconductor BiTeI. Physical Review Letters 108, 247208 (2012). prl.aps.org/abstract/PRL/v108/i24/e247208

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User comments : 7

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Tausch
1 / 5 (2) Nov 22, 2012
What do the 'field lines' formed from iron filings look like when nanofilings of silver or aluminum are used to show the 'lines of force'? Or that of 'Bitel'?
Benni
5 / 5 (4) Nov 22, 2012
To clarify what he is talking about:

Para-magnetism occurs when the loosely bound outermost shell of orbital electrons are unpaired, the more unpaired electrons the greater an atom's para-magnetism & the greater its attraction to magnetic fields.

Dia-magnetism is the opposite case. As the loosely bound outermost orbital shell fills up with "paired electrons" of opposite spin, the less that material is attracted to magnetic fields. When all the orbital positions are filled with oppositely spinning paired electrons, "magnetic moment" caused by unpaired electrons is sharply reduced, or eliminated at room temperature.

It is these two characteristics which determine how valence bonding occurs & that is what they are really trying to figure out here.

Tausch
1 / 5 (2) Nov 22, 2012
...the less that material is attracted to magnetic fields.-Beni

What state is the magnetic moment when...
"In contrast, a ball made of silver moves out of the magnetic field."

Benni
1 / 5 (1) Nov 22, 2012
...the less that material is attracted to magnetic fields.-Beni

What state is the magnetic moment when...
"In contrast, a ball made of silver moves out of the magnetic field."


If a substance has no unpaired loosely bound electrons, it will be repelled in a magnetic field. Silver is not repelled in a magnetic field.
Benni
3 / 5 (2) Nov 22, 2012
...the less that material is attracted to magnetic fields.-Beni

What state is the magnetic moment when...
"In contrast, a ball made of silver moves out of the magnetic field."


If a substance has no unpaired loosely bound electrons, it will be repelled in a magnetic field. Silver is not repelled in a magnetic field.


Whoops, I made a slight mistake above, I said "Silver is not repelled", I should have said "Silver is repelled" due to it's slightly diamagnetic structure of the electron shell.
ValeriaT
1 / 5 (2) Nov 23, 2012
Para-magnetism occurs when the loosely bound outermost shell of orbital electrons are unpaired, the more unpaired electrons the greater an atom's para-magnetism & the greater its attraction to magnetic fields
That's correct. But the reaction of electrons to external magnetic fields is rather weak even at the case, when the electrons are unpaired because most of electrons tends to revolve the atoms symmetrically around the equatorial plane. The different situation occurs, when the electrons are revolving atoms bellow or above the equatorial plane, which happens for some types f- and d-orbitals. The unpaired electrons have offset from rotation plane and they become strongly unbalanced in external magnetic field. We are calling such an atoms ferromagnetic. This effect is strong and depending on location of electrons within atomic bonds. So it can counterbalance the rather weak diamagnetism of another atoms depending on the state of atomic bonds.
Yenaldlooshi
1 / 5 (2) Nov 23, 2012
1st step to an interstellar magnetic engine.

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