NEUTRON-SCATTERING STUDIES OF THE MAGNETIC ORDER IN RNI(2)B(2)C

Neutron diffraction has been used to investigate the magnetic order of RNi(2)B(2)C. For R=Er the system orders antiferromagnetically at T-N= 6.8 K, and this long range order coexists with superconductivity (T-c= 11 K). The magnetic structure is an incommensurate, transversely polar ized spin-density-wave state, with the modulation wave vector delta al ong the a axis and the moments along b. delta has a temperature-indepe ndent value of 0.5526 (2 pi/a), with the structure squaring up at low temperatures. For R=Ho the moments also prefer to reside in the a-b pl ane, but initially an incommensurate c-axis spiral state forms upon co oling, with T-N approximate to T-C approximate to 8 K. This c-axis spi ral consists of ferromagnetic sheets of holmium moments in the a-b pla ne, but with each sheet rotated by similar to 163 degrees as one proce eds along the c axis. Small a-axis peaks are also observed above the r eentrant superconducting transition over a narrow temperature range, b ut the c-axis peaks dominate. Just below the reentrant transition at s imilar to 5 K the magnetic system locks-in to a simple commensurate an tiferromagnetic structure, which permits superconductivity to be resto red. The c-axis spiral, the a-axis component, the commensurate antifer romagnetic structure, and the superconducting phase are all in a delic ate balance energetically, and this balance may be easily shifted by s ubtle changes in composition, magnetic field, and pressure. DyNi2B2C o rders antiferromagnetically at T-N=11 K, with the same commensurate an tiferromagnetic structure as found for the holmium material at low tem perature. The existence of superconductivity in some samples of DyNi2B 2C is consistent with the antiferromagnetic structure observed. (C) 19 96 American Institute of Physics.