Colossal Magnetoresistive Materials

The Neutron and X-ray Scattering Group plays a key role in the division's program on layered manganites that exhibit colossal magnetoresistance (CMR).  Samples of new layered manganites compounds, including high quality single crystals are grown by J. F. Mitchell and J. Millburn (Emerging Materials Group).  Scattering measurements including determining crystal and magnetic structures, which change with composition and temperature, determining the structural properties that control the magnetism and transport and the structure responses to changes in magnetism and transport, and sophisticated studies of the polaron formation, magnetic-field-induced polaron melting, and the competition between charge ordering and magnetic ordering.  Additionally, studies of CMR materials with the perovskite structure are carried out in collaboration with Northern Illinois University.  These studies have focused on characterizing the structural, transport, and magnetic properties of key compounds as a function of chemical composition (including concentration of metal-site vacancy defects) and pressure.

Magnetic and Structural Phase Diagram of La2-2xSr1+2xMn2O7 (x=0.5-0.1). We have probed the magnetic and structural phase diagram of the layered manganites over the said region using neutron powder diffraction at IPNS and high resolution x-ray diffraction at the beam line 12 BM at APS. We find an extraordinarily rich phase diagram over this region. At the extremities of this region the expected type- A for x~0.5 and type - G for x~ 1 magnetic structures are found. While in the region of 0.88<x<0.75 we find evidence of an incommensurately modulated magnetic structure, with q = 0.4o(110)*. In addition we have observed a structural phase transition from tetragonal to orthorhombic symmetry in the region of 0.75<x<0.9. Although the orthorhombic splitting is very pronounced at x=0.8 it is much more subtle for values greater and less than x=0.8, and can be probed using only high resolution X-ray diffraction.

Structural Properties Associated with the Optimal TC of CMR Layered Manganites. Investigating La2-2xSr1+2xMn2O7, a series of layered manganites compounds we have determined the structural features that give rise to the highest magnetic ordering temperature and, thus, the highest temperature for the CMR effect.  We learned that coherent lattice anomalies (i.e., large magnetostrictive-like changes in the average crystal structure at the magnetic ordering temperature) reverse sign upon passing through the composition that gives the maximum TC with the structural anomalies being completely suppressed at the optimal composition.  At the same time, incoherent structural distortions, undergo and abrupt decrease below TC for all compositions.

Charge and Orbital Ordering in LaSr2Mn2O7. Dynamic charge ordering (CO) correlations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy in the temperature range 340-16 K.  However, they become static in a narrow temperature range below TCO=210 K. In the static regime, superlattice reflections are observed through neutron and x-ray diffraction with a propagation vector (h+1/4,k-1/4,l). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than the CO in three dimensional perovskite manganites. The weak charge-lattice coupling and its dynamic behavior are consistent with a weak charge density wave.

Magnetic Correlations in the Manganite La1.2Sr1.8Mn2O7. We have studied the development of magnetic correlations both above and below the ferromagnetic transition temperature TC of the 40% hole-doped bilayer manganite, La1.2Sr1.8Mn2O7 using inelastic neutron scattering.  On cooling within the paramagnetic state, we observe purely two-dimensional XY behavior with a crossover to three-dimensional scaling close to TC. Below TC, an effective finite size behavior is observed.  In contrast to other experimental groups, we do not believe that the very weak in-plane antiferromagnetic correlations are significant for CMR.  However, the correlations between the planes within each bilayer show evidence of a strong competition between ferromagnetic and antiferromagnetic interactions leading to a canting of ferromagnetic cluster spins in the paramagnetic phase.

Charge Correlations in the Manganite La1.2Sr1.8Mn2O7. X-ray scattering measurements taken on the SRI beamline at the Advanced Photon Source have provided direct evidence of the existence of polarons, i.e. charge defects and their surrounding lattice strain.  By combining these measurements with neutron scattering data, we have shown that diffuse scattering around various Bragg peaks arises from the long-range strain field caused by static defects.  This scattering grows in intensity with decreasing temperature but disappears abruptly at the ferromagnetic transition.  Furthermore, we have observed broadened superlattice reflections that show that these polarons have a tendency to order over a short range, although we have not yet established a reliable model of the ordered structure.  These superlattice peaks also collapse in intensity at TC.

Structure-Properties Phase Diagram for La1-xSrxMnO3.  We have investigated the structure-properties phase diagram for the heavily studied perovskite CMR compound La1-xSrxMnO3.  Although many researchers have looked in detail at specific features of this phase diagram, no one has previously presented comprehensive results for the structural, magnetic, and transport properties and how they are related.  Our data show seven distinct regions separated by intersecting phase lines that simultaneously manifest themselves in more than one property.  These regions are:  a paramagnetic insulating (PI) state with large coherent distortions (CD) and small incoherent distortions (ID), a PI state with small CD and large ID, a paramagnetic semiconducting (PS) state with no coherent distortions and large ID, a ferromagnetic insulating (FI) state with a charge density wave, a FI state with suppressed CD and large ID, a ferromagnetic metallic (FM) state with small CD and small ID, and a FM state CD are absent and ID are small.