E. Kojima

A copper-lined magnet coil with maximum field of 700?T for electromagnetic flux compression

A copper-lined (CL) primary coil, which is a composite of steel and copper, was devised for the electromagnetic flux compression technique to generate ultrahigh magnetic fields. The newly developed coil was found to be highly efficient for electromagnetic energy transfer and provided stabilization of the liner implosive motion with less influence from the current feeding gap. Dynamical current density distribution of the materials used in a primary coil was evaluated and applied to the design of the CL coil. Fields of up to 730 T were achieved by employing the CL coil with an energy injected from a 4 MJ condenser bank. This value is the highest achieved thus far in an indoor setting. The peak magnetic fields were found to depend significantly on the initial seed magnetic field. The optimum seed fields for obtaining the highest peak magnetic field were determined.

THE MEGA-GAUSS FACILITY AT KASHIWA: RECENT MAGNETO-OPTICAL STUDIES

We review recent activities at the mega-gauss facility at Kashiwa mega-gauss facility. The plan of introducing a giant DC generator is now under process for aiming at 100 T nondestructive magnetic field achievement in near future. Our current progress in the destructive mega-gauss magnetic fields and some of the selected topics focusing on the magneto-optical subjects are described.

Newly Designed Destructive Magnetic Coils for Mega-Gauss Fields at ISSP

Electro-magnetic flux compression (EMFC) is one of the most powerful methods to generate mega-gauss magnetic fields. The EMFC facility at ISSP has attained the world record for indoor generation of magnetic fields. We have recently implemented successful improvements to generate a higher field with less energy injection and with more simplified coil preparation processes. We have changed the structure and the materials of the primary coil. We have tested the new coil system and succeeded in generating fields up to 350 T with 1 MJ, 470 T with 2 MJ and 520 T with 3 MJ. Hence, electro-magnetic energy transfer efficiency has been increased a great deal. The liner implosive motion was much improved with good cylindrical symmetry. A simple calculation has given insight into the relations between the current density distribution of a primary coil and the shape of the liner during the process of the implosion.

Magnetization process of geometrically frustrated spinel oxide up to an ultra-high magnetic field

We conducted precise magneto-optical Faraday rotation measurements in CdCr2O4and ZnCr2O4up to ultra-high magnetic fields using methods of the single turn coil(up to 190 T) and also the electro-magnetic flux compression(up to 360 T). In CdCr2O4, we have succeeded in observation of the full magnetization process. In ZnCr2O4, we observed magnetization process up to 2.7 ?B(the full moment is 3.0 ?B). The magnetization processes of both materials were well described by a 4 sub-lattice Heisenberg-spin model including spin-lattice interactions.

Magnetization of highly frustrated Zn-chromium spinel oxides up to a megagauss magnetic field

Precise magnetization processes of ZnCr2O4 were obtained by means of the Faraday rotation method subjected to ultra high magnetic fields up to 190 T generated by a single turn coil technique. In magnetization processes of ZnCr2O4, there observed a coplanar 2:1:1 canted state (the cant 2:1:1 phase) which is predicted by the theoretical model including a spin-lattice coupling. A magnetic-field width of the 1/2 plateau widens as the temperature increases.

Charged Excitons and Biexcitons in CdZnTe/(Cd,Zn,Mn)Te Quantum Wells in Pulse Magnetic Fields

We have studied exciton complexes such as charged excitons and biexcitons in dilute magnetic semiconductor Cd 1- x Zn x Te/Cd 1- x - y Zn x Mn y Te ( x =0.07, y =0.43) quantum wells (well widths 4 and 9 nm). Laser excitation power dependence and time-resolved photoluminescence (PL) showed unique features related to excitons, charged excitons, and biexcitons. Four-wave-mixing spectroscopy with different choices of polarized incident light exhibited beating signals arising from exciton-biexciton interaction. Circular polarized PL was measured in magnetic fields of up to 42 T using a pulse magnet. The observed Zeeman splitting and the degree of circular polarization in magnetic fields revealed that the formation of biexcitons is quenched upon applying a magnetic field, and PL from the charged excitons was preserved up to the highest magnetic field.

HIGH MAGNETIC FIELD STUDY OF EXCITON COMPLEXES IN DILUTED MAGNETIC CdZnTe/(Cd,Zn,Mn)Te QUANTUM WELLS

We measured the polarization selective photo-luminescence (PL) and the reflection spectra in the diluted magnetic semiconductor CdZnTe/(Cd,Zn,Mn)Te quantum well under the high magnetic field of up to 40 T. A double peak structure was observed in PL spectra. The higher energy peak could be assigned to the exciton level from the behavior of Zeeman shift or polarization degree. The lower energy peak could not be explained by J=0 biexciton state. We could determined that this peak was due to the charged exciton from the results of the polarization selective reflection measurement.

Photo-induced demagnetization observed by time-resolved mid-infrared transmittance spectroscopy in Ga0.94Mn0.06As

The spin response of a dilute magnetic Ga0.94Mn0.06As semiconductor sample subject to femtosecond photoexcitation is observed by mid-infrared differential transmittance. A slow rise-time in the hundreds of picoseconds timescale is found.