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Polaron Behavior in CMR Manganites Print

Spintronic devices manipulate electron spin to sense magnetic fields, store information, or perform logical operations. Colossal magnetoresistive (CMR) manganites are a class of materials under study for future spintronic applications such as nonvolatile magnetic computer memory (MRAM). Researchers have recently used several soft x-ray spectroscopies at the ALS to study a prototypical CMR manganite as it was heated past its Curie temperature—the point at which the material ceases to be magnetic. They were able to observe the formation of polarons: electrons whose interaction with the lattice creates a deformation (energy well) that traps the electron, as a pocket on a pool table traps a billiard ball. For the first time, this provided a direct look inside polaron formation in a CMR material, indicating that electron localization as polarons is a defining characteristic of all CMR materials.

A New Spin on Electrons

Spintronics, or spin-based electronics, is a new technology that manipulates an electron's spin (its orientation up or down), in addition to its charge, to store and transmit information. In 1988, "giant magnetoresistance" (GMR) was discovered, a spintronic effect that used a weak magnetic field to align the spin of electrons, inducing up to a 20% change in a magnetic material's electrical resistance. GMR technology is used in modern computer hard drives for dense data storage that can be read quickly. In the mid-1990s, materials were discovered that allowed magnetic resistance changes several orders of magnitude greater than GMR, and this became known as "colossal magnetoresistance" (CMR).

Computers with spintronic memory (MRAM—magnetic random-access memory) will be able to store more data in a smaller area, access that data faster, and consume less power than today's semiconductor RAM chips. After the electron spins have been aligned, they remain that way until changed by a magnetic field, even if the computer is shut off. This will create an "instant-on" PC that won't require booting up the computer to move hard-drive data into memory.

CMR manganites are important to spintronics for two reasons. First, they exhibit an extremely large drop in electrical resistance (the CMR effect) when a magnetic field is applied. Second, for some cases, CMR materials conduct electricity via electrons of only one spin (half-metallic ferromagnetism). However, a complete understanding of the charge and spin state of the manganese atoms, crucial to determining and engineering the properties of these materials, is still lacking.

The Mn 3d levels are split into two subsets: t2g (threefold degenerate) and eg (twofold degenerate). The t2g electrons, lower in energy, are more localized; the outermost eg electrons are more delocalized, capable of hopping from site to site provided that the t2g spins on adjacent manganese atoms are parallel. According to the Jahn-Teller effect, as soon as an electron hops into an empty eg orbital, a distortion of the octahedral cage of O atoms lowers the symmetry and further splits the eg and t2g levels. The electron is now more tightly bound, forming a so-called lattice polaron.

To explain the magnetoconductive properties of these manganites, in which the manganese is present in at least two different valence states (Mn3+ and Mn4+), Zener proposed the mechanism of double exchange (DE). According to DE, the alignment of adjacent localized t2g spins on manganese atoms rules the dynamics of itinerant eg carriers, which hop from one atom to the next to yield electrical conductivity. If adjacent t2g spins are parallel (the ferromagnetic state), conduction is favored; if they are randomly aligned (the paramagnetic high-temperature state), conductivity drops dramatically.

While Zener's DE model provides a qualitatively correct picture of the CMR effect, theoretical calculations have shown that DE alone is insufficient to account for the observed CMR resistance. An additional consideration is the localization of itinerant eg electrons by Jahn-Teller distortions of the octahedral cage of oxygen atoms surrounding each manganese atom.

A Jahn-Teller distortion can take place when an electron hops into an empty eg orbital so that, while hopping from site to site, the electron "drags" the lattice distortions after itself. The electron with its accompanying lattice distortion forms a so-called "lattice polaron." Since the surrounding oxygen atoms are much more massive than the bare electron, the polaron behaves as a negatively charged particle with a larger mass and lower mobility than an isolated electron.

In this study, the temperature-dependent evolution of the electronic and crystal structure of a prototypical CMR compound, La0.7Sr0.3MnO3 (LSMO), was investigated. Spectroscopic experiments were performed using the multitechnique spectrometer/diffractometer at ALS Beamline 4.0.2. As the LSMO compound was heated through its Curie temperature (TC), core photoelectron spectroscopy data provided direct evidence for charge localization onto the manganese atom via a change in the manganese 3s multiplet splitting and for chemical shifts in the core levels of the other atoms in the sample. Valence photoemission spectra also showed parallel changes with temperature. Additional hard-x-ray EXAFS measurements detected the presence of Jahn-Teller lattice distortions in the oxygen octahedra surrounding manganese atoms as the temperature rose above TC.

Mn 3s and O 1s photoelectron spectra. (a) The Mn 3s spectrum exhibits a doublet due to multiplet splitting. The energy separation of this doublet depends on the net spin of the Mn atom, providing a direct and element-specific measure of its magnetic moment. The Mn 3s splitting changes markedly as the temperature increases from TC to a saturation temperature TMAX, indicating an increase in the magnetic moment corresponding to the transfer of about one electron to Mn. (b) The "bulk" O 1s binding energy shows a concomitant increase, consistent with charge transfer to Mn. Similar shifts are also found for La and Sr.

These experiments permitted the detection of polaron formation via its effects on the manganese and other atoms in the LSMO. This challenges the long-standing belief that LSMO is a simple DE system that can be described without the formation of polarons. Therefore, the presence of polarons above TC is a general defining characteristic of all CMR materials, bringing unity to their theoretical description. Beyond spintronic applications, these results could also have implications for the magnetic states of atoms under high pressure, as in the Earth's core.

Research conducted by N. Mannella, B.S. Mun, and C.S. Fadley (University of California, Davis, and Berkeley Lab); A. Rosenhahn, C.H. Booth, S. Marchesini, and S.-H. Yang (Berkeley Lab); K. Ibrahim (Beijing Synchrotron Radiation Laboratory, China, and Berkeley Lab); and Y. Tomioka (Correlated Electron Research Center, Japan).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Operation of the ALS is supported by BES.

Publication about this research: N. Mannella, A. Rosenhahn, C.H. Booth, S. Marchesini, B.S. Mun, S.-H. Yang, K. Ibrahim, Y. Tomioka, and C.S. Fadley, "Direct Observation of High-Temperature Polaronic Behavior in Colossal Magnetoresistive Manganites," Phys. Rev. Lett. 92, 166401 (2004).