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A microspectroscopic study of the electronic homogeneity of ordered and disordered Sr2FeMoO6

Besides a drastic reduction in saturation magnetization of disordered Sr2FeMoO6 compared to highly ordered samples, magnetizations as a function of the temperature for different disordered samples may also show qualitatively different behaviors. We investigate the origin of such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish that extensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples is responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with composition fluctuations of the type Sr2Fe1+xMo1-xO6 with Fe-rich (x>0) and Mo-rich (x<0) regions.


I. INTRODUCTION
A double perovskite system Sr 2 FeMoO 6 (SFMO) has come under intense investigations during the last few years, because of its spectacularly large magnetoresistance (MR) property, 1,2 as well as its unusual origin of magnetism. 3 Fully ordered Sr 2 FeMoO 6 is a half-metallic ferromagnetic system. The B site of the perovskite structure are occupied alternately with localized, nominal Fe 3+ (3d 5 , S=5/2) species and highly delocalized, nominal Mo 5+ (4d 1 , S=1/2) along all the three axes of the cube in the perfectly ordered double perovskite structure of Sr 2 FeMoO 6 . The localized Fe spin is coupled antiferromagnetically to the spin of the delocalized electron via a strong, kinetically-driven mechanism 3 leading to a high Curie temperature for the ferromagnetic ordering of the localized Fe spins with a total moment of 4 µ B per formula unit (f.u.). However, the saturation magnetization of most Sr 2 FeMoO 6 samples reported so far is always found 1,[4][5][6][7] to be lower than 4 µ B per formula unit. This lowering of saturation magnetization is attributed to the presence of mis-site disorders, where some Fe and Mo ions interchange their crystallographic positions, facilitated by their similar ionic radii. Disorder is hence introduced in the system, as the long range Fe-O-Mo-O-Fe ordering of the ideal structure is disrupted. Such disorder necessarily introduces Fe-O-Fe type bonds with antiferromagnetic coupling between them, arising from the strong superexchange interaction between the localized Fe moments. Theoretical calculations 8,9 do suggest that the magnetic moment should decrease due to the presence of disorder. However, the predicted decrease does not match the drastic reduction that is experimentally observed. 6,10 Beside the quantitative aspect of a decrease in the saturation magnetization with increasing disorder, we notice that the magnetization, M, as a function of the temperature, T for disordered samples shows qualitative differences between different samples. 10,11 Specifically, in certain cases the M(T) of disordered samples show 10 multiple steps, suggestive of magnetic phase separations that may or may not have an underlying chemical or electronic origin. In order to investigate these important issues, we prepared several highly disordered samples with high concentrations of mis-site defects. We study the spatially resolved electronic structure of such disordered samples using a highly focused beam of photons at a third generation synchrotron source and establish that certain samples with a higher concentration of mis-site defects indeed have signatures of extensive electronic inhomogeneities.

EXPERIMENTAL DETAILS
Disordered SFMO was synthesized by mixing high purity SrCO 3 , Fe 2 O 3 , MoO 3 and Mo in stoichiometric amounts. The obtained mixture was melt quenched 6 in an inert gas arc furnace. Ordered SFMO sample used for comparison was also made by melt quenching as described above, however, the molten ingot so obtained was further annealed at 1250 0 C for 10 hrs in a flow of Ar and H 2 (98:2) mixture, and then furnacecooled to room temperature under the same reducing atmosphere. The samples were characterized using x-ray diffraction and scanning electron microscopy (SEM). Most of the diffraction peaks are same for both ordered and disordered samples, arising from the common underlying perovskite structure. The only difference in the diffraction patterns is the existence of generally weak supercell reflections in case of the ordered sample, arising from the ordering of Fe and Mo sites. The intensities of such supercell reflections are strongly suppressed in the disordered sample, 6 establishing the absence of any long range Fe-Mo ordering. Magnetization measurements showed that the ordered sample has a saturation magnetic moment of 3.2 µ B per f.u. at 5 K, while that for disordered samples prepared by us was invariably close to 1 µ B per f.u. The grain sizes of all samples synthesized for this study were determined to be about 4-5 µm by SEM.
Photoelectron spectroscopy was performed on these samples using photon energy of 95 eV at the spectromicroscopy (3.2L) beam line at the Italian synchrotron light source, Elettra. The light from the synchrotron source is focused on to the sample using a Schwarzschild objective 12 providing a spatial resolution of 0.5 µm. The sample is moved relative to the photon beam using two perpendicular piezo drivers, allowing one to map out the photoemission spectra from the face of the sample with 0.5 µm resolution. The photoemission spectrum is collected using a commercial analyzer with a sixteen channel detector array. All experiments were performed at a base pressure of 1x10 -10 mbar at room temperature (~298 K). The sample was fractured in situ to get a clean surface for the surface sensitive photoemission study in each case.

RESULTS AND DISCUSSION
We generated intensity images by recording spectral intensities at two different energy regions of the valence band spectrum as shown in Figure 1a. 'Image 1' is formed by adding intensities from the near Fermi edge region, collected by all 16 channels, while 'image 2' is formed by adding intensities from central 8 channels (Channel 5-12) around 5.6 eV which is mainly contributed by O 2p states. In order to avoid any drift between the images, both the images were acquired simultaneously. The observed color contrasts in the images arise from the variation of spectral feature at different parts of the exposed surface. According to the convention used here, the red color denotes the lowest intensity and continuous color variations from red to yellow to green to blue show a gradual increase in intensity. It must be noted that the fracture of such polycrystalline samples to obtain a clean surface for photoemission studies in general does not produce a flat surface. Such uneven fracture produces a sample surface with pronounced topographic features that have strong influence on the spectral intensity due to the variations in the positioning and the orientation of the sample surface locally with respect to the focus point of the electron analyzer; as a consequence additional color contrasts are observed in the raw images ('image 1' and 'image 2'), which can very often be misleading.
Therefore, it is preferable to use a ratio image ('image 1'/'image 2'), with the assumption that sample topography affects both the images in a similar way and by division we can mostly get rid of the topographic effects. Hence the color contrast in the ratio images by and large represents the actual variation in the ratio of spectral intensities across the sample surface. Such a division essentially means a normalization of the intensity near the Fermi edge with respect to the corresponding intensity of the O 2p states. Ratio images obtained from disordered and ordered samples are shown in Figure 1b. We see a large variation in color contrast for the disordered sample while the ordered sample appears to be more or less homogeneous, when plotted on the same color scale. Such variations establishing a strong spatial dependence of the intensity of the spectral feature close to the Fermi energy relative to that at the O 2p feature at 5.6 eV binding energy, prove that there is an extensive electronic inhomogeneity in this disordered sample, in contrast to the ordered sample.
To understand the detailed nature of such electronic inhomogeneities observed in the images, we recorded a large number of spatially resolved high resolution spectra on freshly cleaved ordered and disordered samples at various points on the sample surface.
We show these spectra in Figure

CONCLUSION
In conclusion, we have investigated the nature of chemical inhomogeneity in the disor-

ACKNOWLEDGEMENT
We thank ICTP-Elettra Users Programme and Department of Science and Technology for financial support. S. R. K thanks ICTP for a TRIL Fellowship  Valence band spectra obtained from the disordered sample is classified into three types, Type-I spectra (blue solid circles) originating from Mo rich region, Type-II spectra (red solid circles) originating from Fe rich region, while Type III spectra (black solid lines) resembles the spectra obtained from ordered sample. Type-I and Type-II spectra are represented with examples of extreme cases, of the disordered sample.   Binding Energy (eV)