"With CMR manganites, the application of a magnetic field can cause the material's electrical resistance to change by as much as 1,000 percent," says Charles Fadley, a physicist affiliated with Berkeley Lab's Materials Sciences Division and a professor of physics at the University of California at Davis. "Today's best data storage devices are based on the GMR effect" — gigantic magnetoresistance — "in which the field application reduces the electrical resistance only 20 to 50 percent."

Fadley, working with his student Norman Mannella, now at Stanford University, used the exceptionally bright, soft x-ray beams (lower energy x-rays) and sophisticated experimental facilities of Berkeley Lab's Advanced Light Source (ALS) to shed new light on what could prove to be a crucial element of the CMR effect in manganites and other metal oxides. This is the surprising formation of a type of polaron, an electron that is somehow bound to a local distortion of the atoms in the crystal.

The distortion creates a sort of energy "well" that traps the electron, like a divot on a fairway that traps a golf ball. The type of polaron that Fadley and Mannella detected is called a Jahn-Teller polaron, after Hans Jahn and Edward Teller (the same Teller who later led development of the hydrogen bomb), who predicted such polaron distortions in a famous 1937 paper. Fadley and Mannella found that Jahn-Teller polaron formation took place in the CMR manganite only after it was heated past its Curie temperature, the point at which a material ceases to be magnetic.

"We've shown for the first time, using a combination of all of the primary ALS spectroscopies, that one of the most studied of the CMR manganites, a mixture of lanthanum, strontium, manganese, and oxygen, exhibits the formation of polarons above its Curie temperature" of about 350 degrees Kelvin, Fadley says. "The combination of ALS techniques we used showed much more directly than any previous measurements that one electron is localized to the magnetic manganese atoms, thus altering the electrical resistance of the entire material."

Manganites typically contain four or more constituents; manganese is often the only magnetic atom present. According to the findings of Fadley and Mannella, as the CMR manganites cool to below their Curie temperature, the Jahn-Teller polarons disappear, releasing the trapped electrons. The ability of the CMR manganites to conduct electricity is very different depending on whether or not a polaron is present.

Adds Fadley, "It seems clear that to understand CMR in manganites, you will have to take into account the effects of polarons. As for the temperature dependence of the polaron formation, it was much bigger than we ever imagined it would be."

The CMR effect in manganites is an important subject of study for reasons other than its potential impact on high-density data storage devices. Some CMR materials conduct electricity via electrons with only one direction of spin (spin is a quantum-mechanical property, considered to be up or down), rather than equal numbers of electrons with either direction of spin, as in all of today's typical electronic devices. This means that CMR manganites can have nearly 100 percent spin polarization, making them excellent candidates for lightning-fast new logic devices, such as spin transistors and magnetic tunneling transistors.

"Beyond spintronic applications, our results could also have implications for the magnetic states of atoms under high pressure, as in the earth's core," Mannella says.

Geophysical studies have shown that iron-containing perovskites, metal-oxide materials with the same general crystal structure as the CMR manganites, can exhibit a marked reduction in the electron spin state of their iron atoms as they move through the earth's mantle toward conditions of extreme pressure at the earth's core. From an initial high spin state, the iron atoms drop to a low spin state as pressure increases. This leads to a gradual loss of magnetic moment, which has significant influence on the magnetic, thermoelastic and transport properties of the deep mantle. It could also affect the partitioning of iron between the upper and lower mantle, and between the lower mantle and core, with a possibility of iron enrichment in the deep mantle.

Contact: Lynn Yarris, lcyarris@lbl.gov