Magnets Made of QMG Crystals

Abstract

Magnetic flux density trapped in large QMG (Quench and Melt Growth) crystal was measured at 85K, 77K and 63K. QMG crystals which had no high-angle grain boundaries were made. A sample which size was 50mm in diameter 24mm thick was used. Distribution of magnetic flux density was measured on the suface of the QMG crystal using several Hall probes. Field cooling was performed to investigate trapping of the magnetic field. Temperature of coolant was varied by controlling the pressure over the nitrogen vessel. At 63K, flux density was roughly double of that at 77K. Maximum value of trapped field was 1.72T. Flux creep rates were 2.4x10−3, 2.2x10−3 and 0.9x10−3 at 84K, 77K and 63K respectively. Since flux creep is still not negligeable at these high temperatures, we tried to prevent it by cooling the QMG material down to 77K after trapping at 84K and also cooling down to 63K after trapping at 77K the flux allows to save a lot of time for the field stability to be achieved.