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Development of High-Speed Imaging System in Pulsed-Magnetic Fields

Tokunaga Group

The coupling between spin and lattice degrees of freedom realizes a variety of magnetic field-induced structural transitions. Recent development of diffraction measurements in pulsed-fields enabled us to obtain direct evidences of these phenomena for some materials in high-fields [1,2]. The accuracy, however, remains insufficient to detect faint changes caused by superstructures. Here, we developed an alternative technique to study these transitions: high-speed polarizing microscope imaging in pulsed-magnetic fields. Some kinds of structural transitions can be identified through observation of creation or annihilation of twin boundaries in crystals. With using a high-speed camera, we can perform this imaging in high magnetic fields achieved by pulse magnets. To this end, we introduced a high-speed camera (6,000 frames/s for VGA resolution) on a commercial polarizing microscope. Using this microscope system and an objective lens with long working distance, we obtain real images of samples located on the center of a small pulse magnet, which can generate magnetic fields up to 37 T in a duration time of about 5 ms. Using a low-vibration cryocooler system, the sample and the magnet can be cooled down to 8 K and 80 K, respectively.

Fig. 1. Schematic drawing of the high-speed imaging system combined with a pulse magnet. The magnet and sample are cooled by the first and second stage of a cryocooler, respectively. The sample stage was mechanically isolated from the vibration of the compressor of the cryocooler and of the magnet. A high-speed camera is mounted on a commercial polarizing microscope to trace fast changes in images induced by pulsed-magnetic fields.

Fig. 2. Polarizing microscope images in the ab-plane of La1/2Sr3/2MnO4 at 200 K at (a) zero filed and (b) 28 T. The blight areas represent the domains of the COO, which disappear by application of a magnetic field of 28 T along the c-axis.

As the first target of the present imaging system, we chose a crystal of La1/2Sr3/2MnO4. This layered manganite exhibits charge/orbital ordering (COO) below about 220 K. Since the orbital ordering restricts the direction for hopping of mobile eg electrons in the Mn3+ ions, optical anisotropy emerges within the ab-plane in this state. Consequently, the COO state shows up brightly in polarizing microscope images at the crossed-Nicols configuration [3]. Figures 2 show polarizing microscope images on the cleaved ab-plane of La1/2Sr3/2MnO4 at 200 K. Corresponding to the field-induced melting of the COO, bright domains in zero field (Fig. 2(a)) become dark in 28 T (Fig. 2(b)).

The utilized camera has 12-bit resolution for intensity in each pixel, so that we can quantitatively analyze the obtained images in pulsed-fields. In Fig. 3, we show the intensity in a part of the images as a function of applied field at several temperatures. These clear changes in the optical intensity manifest the field-induced melting of the COO in contrast to rather moderate changes in magnetization and magnetoresistance in this temperature region [4]. The present result provides a novel opportunity for investigation of some kinds of field-induced transitions even in tiny crystals.


References
  • Y. H. Matsuda et al., Physica B 346-347, 519-523 (2004).
  • Y. Narumi et al., J. Synchrotron Radiat. 13, 271-274 (2006).
  • T. Ogasawara et al., Phys. Rev. B 63, 113105-1-4 (2001).
  • M. Tokunaga et al., Phys. Rev. B 59, 11151-11154 (1999).
Authors
  • M. Tokunaga, I. Katakura, A. Matsuo, K. Kawaguchi, and K. Kindo