Bonds between atoms are electrical in character: atoms share electrons or mutually ionize, creating an attractive force binding them together. However, researchers are now suggesting that it may be possible to generate magnetic bonds, resulting in stable molecules of different types than exist on Earth. While these molecules can't be produced with even our strongest laboratory magnets, they could form in the extreme magnetic fields near white dwarfs and neutron stars, and their unique spectral signatures may make them visible through observations.

As described in a new Science paper, Kai K. Lange, E. I. Tellgren, M. R. Hoffmann, and T. Helgaker performed detailed quantum mechanical calculations for two atoms in exceedingly strong magnetic fields. While previous work had shown that a relatively weak bond could form when the molecule is parallel to the magnetic field, Lange and colleagues discovered an additional stronger bond might result when the molecule is perpendicular. Their calculation relied on very few assumptions, so it is useful for determining the properties of the molecules formed. Intriguingly, their model also described a magnetic molecule could be made from helium, which is famously inert and doesn't form stable electric bonds.

Why are magnetic fields so extreme?



White dwarfs are the dense cores of stars similar to our Sun that shed their outer layers after exhausting their nuclear fuel. Neutron stars are the even denser remains of stars at least 8 times more massive than the Sun; they form when the star's core collapses as the star exploded in a supernova. In both cases, the small size of the stellar remnant and the high density combine to intensify the magnetic field near the surface.

The strongest laboratory magnets can produced magnetic field strengths of about 40 Teslas (40T). However, fields surrounding white dwarfs can be a thousand times greater, and neutron star field strengths are even stronger. (For comparison, magnetic resonance imaging (MRI) machines may run as high as 7T, and Earth's magnetic field ranges from 25 to 65 microteslas.) In other words, the magnetic environment near extreme stellar remnants is substantially different than anything we can produce on Earth, so it's unsurprising that at least some new phenomena could arise in such a setting.

The authors used a common method in molecular chemistry and physics known as an full configuration-interaction (FCI) calculation, in which atoms are modeled directly with a minimum of assumptions. In this way, they were able to obtain all the possible molecular binding configurations. They focused on hydrogen, which has the twin advantages of being simple (one electron per atom) and common. At low temperatures and negligible electric or magnetic fields, hydrogen forms the two-atom molecule H 2 through covalent bonding, where the electrons are shared equally between the two atoms. However, the environment around white dwarfs and neutron stars is too hot for this bond to survive, and the molecules dissociate.

Intense magnetic fields could change that, based on the FCI analysis. As the magnetic field strength increased, the researchers found the electron orbitals (the patterns of the electron cloud of an atom) distorted, making the atoms themselves magnetic. This effect, known as paramagnetism, is seen in many materials: they are magnetic only in the presence of an external field (as opposed to ferromagnets—"permanent magnets"—which don't require an external field). In the case of hydrogen atoms in extreme magnetic fields, the result of the paramagnetism was the formation of an H 2 molecule that's held together through magnetic bonding.

While previous calculations had found magnetic bonding when the two atoms were oriented perpendicular to a magnetic field, they didn't show bonding in other orientations. The new results revealed the bonds persist when the atoms are rotated by any angle relative to the field, though the perpendicular orientation was still preferred. Additionally, in the earlier results, bonding was due to motion of the electrons, not a paramagnetic effect. The differences arise because of the approximations used in the earlier work used, which are not present in the current one.

The researchers also performed FCI calculations for helium, which only forms molecules under extreme conditions—and even then the results are highly unstable. They found magnetic bonds were possible, meaning quasi-stable paramagnetic He 2 could exist. As with H 2 , however, the molecules were found to break apart when the external field was turned off.

Because of the fundamentally different character of magnetic H 2 , its spectrum—the wavelengths of light absorbed and emitted—will be different than the spectrum of covalent H 2 . Similarly, magnetic He 2 has a unique spectrum. If magnetic molecules exist in the atmospheres of white dwarfs or neutron stars, they might be detectable, assuming they are produced in sufficient quantities.

While current laboratory magnetic fields aren't strong enough to create magnetic molecules, new pulsed magnetic fields are able to achieve higher strengths for brief periods of time. While the molecules would only persist as long as the field was switched on, future experiments should be able to hunt for their predicted spectra.

Science, 2012. DOI: 10.1126/science.1219703 and 10.1126/science.1224869 (About DOIs).