Finding Fine Fields Could Mean Major Magnetic Manipulation

The ability of metal oxides to change electrical resistance in the presence of a magnetic field—a process called magnetoresistance—has long fascinated researchers. One group of metal oxides, manganese oxides, is of particular interest because its members exhibit colossal magnetoresistance—a dramatic increase in electrical conductivity under a magnetic field.

Understanding why and finding ways to make them more sensitive to such fields is the work of a group of Argonne scientists interested in laying the groundwork for such technological applications as increasing the information storage capacity of computers. Hard-disk drive technology is based on magnetoresistance.

Argonne researchers are interested in the metal oxide compounds called manganites, which contain manganese and oxygen as active ingredients and other elements that can be used to “tune” the properties of the material. One goal is to use combinations of these ingredients to tune the sensitivity to magnetic fields.

The “naturally layered manganites” have crystal structures that grow in alternating layers of atoms. While none of these compounds occurs naturally, some are quite stable and can be easily made. Others are “metastable” and must be trapped by special synthetic techniques before they decompose. Argonne investigators have succeeded in making the full range of naturally layered manganites, both stable and metastable.

They used X-rays and neutrons generated, respectively, by Argonne’s Advanced Photon Source and Intense Pulsed Neutron Source to study each crystal’s structure, both in and out of magnetic fields. They found that in the absence of a magnetic field electrons are bound tightly to the crystal’s lattice by a distortion of the oxygen atoms surrounding each manganese atom. A magnetic field aligns the spins of the manganese atoms, removing the distortion in the lattice, and freeing the electrons. This allows electricity to move as it would in a metal. When the field is removed, the electrons return to the lattice and the material becomes an insulator.

“The fascinating thing about the manganites is that the energy scales of the electronic, magnetic and lattice states are so delicately balanced that small changes in any one has a dramatic impact on the others,” John Mitchell, research team leader, said. “It makes the manganites a spectacular laboratory for looking at important fundamental physics.”

The research is providing insight into the basic interactions among electrons, magnetism and crystal lattice structures that are crucial to the physics and chemistry of metallic oxides.

In addition to the manganese-oxide research, Argonne scientists are also examining the physics of oxides of copper and ruthenium. Just as manganese oxides become better electrical conductors in a magnetic field, the copper and ruthenium compounds become superconductors when cooled.

For more information please contact Richard Greb at 630-252-5565

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