Disorder and mixed valence properties of Sr2FeMoO6 studied by photoelectron diffraction and x-ray absorption spectroscopy
Graphical abstract
Introduction
Sr2FeMoO6 (SFMO) has been extensively studied over a past decade because of its rich and puzzling properties [1], [2], [3], [4]. It shows a pronounced negative magnetoresistance at high temperature and low magnetic field. The findings of room temperature tunneling magnetoresistance [1] and half-metallic behavior [5], [6] have opened renewed interest and expectation for promising application to the spintronics devices. Another interesting feature of SFMO is a high Curie temperature for a ferromagnet, which is well above room temperature.
SFMO is a member of the homologous series A2MM′O6 known as the double perovskites, which is composed of a large cation (A) and two alternate M′O6 and MO6 octahedral units bounded by oxygen bridges as depicted in Fig. 1. In the ideal picture, it is believed that Fe 3d5 up-spin electrons can be considered to be localized while the Mo 4d1 down-spin electrons are delocalized and contribute to the itinerant electron transport. Its electronic property is regarded as a half metallic where Fe (S=5/2) and Mo (S=1/2) are opposite direction with each other. Its net magnetization is 4 μB in the theory and it is likely ferrimagnetism. Despite the extensive amount of works performed on the double perovskites, there is still an open debate about the magnetic properties. Although the simple antiferromagnetic coupling model predicted a magnetic moment of 4 μB, the experimental value is always less than the theoretical values around 3 ∼ 3.5 μB in magnetization [1], [7], [8], [9], [10]. This is a key controversial issue in this material. The puzzle involves distinguishing between ”mixed valence” or ”disorder (mismatch)” scenario to explain the small saturation magnetization of SFMO. The mixed valence scenario [5], [11], [12], [13], [14] involves formation of Fe- Mo and Fe- Mo states. While several works conclude that a Fe- Mo configuration is present in SFMO [8], [15], other reports also suggested a Fe- Mo configuration or mixed Fe and Mo valence states [11], [16], resulting the decrease net magnetization. In an alternative picture, the disordering of Fe/Mo heavily influences the magnetic properties [17], [18], [19], [20], [21], [22], [23]. Using x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) [23], it was shown that a very high degree of short range order is preserved even in samples with highly reduced long range chemical order.
Two-dimensional photoelectron diffraction (2DPED) is an element-specific probe of the local structure and electronic states around a certain element. Unlike conventional angle-integrated photoelectron spectroscopy, 2DPED has shown forward focusing peaks (FFPs) in the photoelectron intensity angular distribution (PIAD) originating from the interference of the diffracted outgoing photoelectron waves. It allows us to obtain a direct information on the directions of surrounding atoms seen from the photoelectron emitter atom [24], [25]. By using this measurement, we can reveal the difference between disorder and order states clearly and and select each element in the sample, especially double perovskite.
In this paper, we critically examined the nature of local structure and addressed the fundamental issue mentioned above. By employing 2DPED and XAS, we showed that Fe/Mo order is almost random (i.e. positional disorder) in SFMO and the mixed valence state existed at the surface but it is suppressed in the bulk.
Section snippets
PED
The electron diffraction pattern is most efficiently measured by using the display-type analyzer (DIANA) [26], which enables acquisition of a two-dimensional angular distribution snapshot of a specific kinetic energy electron up to ± 60° without distortion. The 2DPED experiments were performed at the circularly polarized soft x-ray beam line BL25SU of SPring-8, Japan. A light was incident from the direction 45° inclined from the center of the analyzer. The diameter of the beam was 400 µm.
Conclusion
In conclusion, we have investigated the atomic mis-arrangement and valency of SFMO by using two different x-ray spectroscopy and addressed the controversial issue on the reduction of saturation magnetization of this material. From Fe 3p and Mo 3d 2DPED patterns, the evidence of disorder of Fe/Mo in SFMO was confirmed. On the other hands, thanks to the surface sensitivity of XAS with different detection mode, the Fe components at the surface was found, but this mixed valence state was
Acknowledgments
SK would like to thank professor Koji Yoshida. Authors express deepest gratitude to Dr. Tomohiro Matsusita, Dr. Tetsuya Nakamura and Dr. Takayuki Muro for their illuminating comments and experimental supports. This work was performed with the approval of the Japan Synchrotron Radiation Institute, Proposal No. 2010A1468 and 2013A1627 and partly supported by Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research on Innovative Areas: Grant Number 26105001.
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