HIGH PRESSURE NEUTRON DIFFRACTION STUDIES OF RCo_2Si_2 (R : RAREEARTHS) WITH ISING-TYPE LONG PERIOD MODULATED STRUCTURES

在京都大学反应堆B2束流孔安装了高压中子衍射系统。它可以很容易地达到10k和3gpa的条件，将夹紧式高压电池与闭式循环冰箱相结合。利用该体系建立了具有ising型长周期调制结构的PrCo_2Si_2的（P，T）磁相图，温度在10K至2.25 Gpa之间。此外，在低温高压下研究了铒的磁性结构，在常温下由圆锥形结构转变为Q=2/7 （2pi/c）调制的摆线结构，其c轴分量的奇次谐波高达7阶。设计并制作了一种适用于高压电池的新型低温恒温器。该低温恒温器可提供低至4.2K的温度，此外，通过将液氦泵入附在高压电池上的副罐中，预计温度可达2K左右。

A high pressure neutron diffraction system has been installed at the B2 beam hole at Kyoto University Reactor (KUR) . It can easily reach a condition of-10K and 3 GPa, combining a clamp-type high pressure cell with a closed cycle refrigerator. Using this system a (P,T) magnetic phase diagram of PrCo_2Si_2 with Ising-type long period modulated structures has established down to 10K up to 2.25 Gpa. Furthermore, the magnetic structures of erbium have been studied under high pressure at low temperatures, where a conical structure at ambient pressure has changed to a cycloidal structure with a modulation of Q=2/7 (2pi/c) having odd harmonics of the c-axis component up to the 7th order.A new cryostat available for the high pressure cell has been designed and fabricated. This cryostat can give a temperature down to 4.2K and moreover, be expected about 2K by pumping liquid helium in a subsidiary tank which is attached to the high pressure cell.