Frank Bridges, UCSC, DMR Mn La/Ca O Figure 1 Figure 2 Local Structure Studies of La 1-x Ca x MnO 3 : Evidence for Magnetic Dimers We have used X-ray absorption Fine Structure (XAFS) spectroscopy to investigate the atomic structure of La 1- x Ca x MnO 3 and can detect changes to less than a thousandth of a nanometer. Fig. 1 shows the distorted structure – a “kinked” cubic structure. The kinks are due to variations in the Mn-O bond lengths throughout the sample. From XAFS we have obtained a measure of the distortion of the Mn-O bonds – denoted as σ 2 – plotted in Fig. 2. We observe a large increase in the distortion at the ferromagnetic transition (T c ) and again at T *, defining three regions or phases. At low temperature, the system is magnetic with little distortion. At high temperature, the system is non- magnetic with a large distortion. Our recent high temperature data (Fig. 2) shows a new “intermediate” phase between T c and T *. The combined results of many experiments as a function of temperature, magnetic field and Ca concentration, x, indicate that magnetic pairs have formed above T c. These pairs, which we call a dimeron, consists of two Mn-sites [one initially distorted and one undistorted] with a reduced average distortion per site (see Figs. 3 and 4, next page); the new data suggest they form near T *. TcTc T*T* x = 0.3 La 0.7 Ca 0.3 MnO 3

Local Structure Studies of La 1-x Ca x MnO 3 : Evidence for Magnetic Dimers The EXAFS technique (Extended X-Ray Absorption Fine Structure: a technique that uses x-rays as an indirect nanoscope) has been used to probe the statistical distribution of Mn-O bond-lengths (the Mn are the white atoms in the figure and the O are the blue atoms) throughout the material. The kinks in the structure arise from variations in the Mn-O bond lengths throughout the sample. In CaMnO3, the six O bonds surrounding a Mn-site are nearly the same length (within nanometers); In LaMnO3, there are four shorter and two longer because, in LaMnO3, each Mn has an extra electron which reduces the attraction to the negatively charged oxygen ion. Therefore, in the doped structure, near a Ca site, the six Mn-O bonds will be the same length (no distortion) and near a La site, there will be a distortion of bond lengths. If two neighboring Mn-sites have parallel spins (i.e. are magnetized), an electron can hop rapidly back-and-forth. If on the other hand, the spins are not aligned, it takes some energy for the electron to align with the next spin and thus cannot hop as fast. If the electron hops rapidly between two sites the distortion is removed because the electron is not on any one site long enough for the oxygen atoms to move to produce the distortion. Our new results, presented here, in conjunction with other measurements as a function of temperature, magnetic field and calcium concentration, x, indicate there are three levels of distortions present: undistorted, partially distorted, and fully distorted. Our model suggest that the partially distorted phase arises from a paring of neighboring Mn sites – an undistorted site and a distorted site – acting as a single unit, which we call a Dimeron, with less overall distortion than the sum of the two independent sites (see Fig. 3). (Our group has recently proposed the concept of the Dimeron in a paper by Lisa Downward et. al. in Physical Review Letters vol. 95, (2005).) In the intermediate region (between 100K and 260K for the x=0.3 sample), these quasi-particle Mn-pairs further aggregate into spaghetti-like magnetic filaments (clusters) throughout the sample (see Fig. 4). The sample then becomes fully magnetized, and enters the undistorted phase at the lowest temperatures.

Dimeron Local Structure Studies of La 1-x Ca x MnO 3 : Evidence for Magnetic Dimers Figure 3 Figure 4 Broader implications Computer manufacturers are currently using giant magnetoresistors (GMR) in the read/write heads of magnetic hard-drives. The materials studied here (La 1-x Ca x MnO 3 ) are colossal magnetoresistors (CMR), for x ≥ 0.2 and x ≤ 0.5, and exhibit a much larger effect. They have the potential to improve the current technology, but first the properties must be better understood. Our new results indicate that the models of magnetoresistance in these systems must consider the possibility of magnetic pair formation above T c, in which the moments on two neighboring Mn- atoms are coupled such that the pair acts as a single tiny magnet. Educational An advanced graduate student, Lisa Downward, is working on the EXAFS studies of the manganites as part of her PhD thesis. Two new graduate students, Yu (Justin) Jiang and Jesse Guzman, have recently begun working on these materials, also. Lisa, Justin, and several undergraduate students have been trained to use the XAFS technique and to run experiments at a synchrotron source. They have learned to work together as a team, and have gained hands-on experience at state-of-the-art national facilities. They have also helped develop crystal structures and vibration models for our website.