# # arev:/usr/local/unicat/unireq/beamreq/req00099.txt # UNICAT Member Beam Time Request #99 # created Wed Dec 08 10:17:06 CST 1999 # collaboration: No collaborator+Paul: on days: 5 description: The interplay between magnetism and superconductivity has emerged as one of the key problems in condensed matter physics. Nowhere is this competition more apparent than in the series of rare-earth nickel borocarbides having the chemical formula RNi_2B_2C. When R is a non- magnetic rare-earth element such as Lu, the superconducting transition is 16 K. Even with magnetic elements, Tc remains rather high: 10.5 K for Er, 8 K for Ho, and 6.1 K for Dy. Surprisingly, the Tb analog, which has the highest Neel temperature of the series T_N = 15 K, shows no superconducting transition. Of these compounds, only Dy (excluding Tb, of course) has T_N = 10.6 K > Tc. Recently, mixed rare-earth substitutions have been studied [1], and their structure is the subject of this proposal. We focus here on Dy1-xTbxNi2B2C. For x = 0, neutron powder diffraction [2] has shown that the Dy moments order as an A-type antiferromagnet, with ferromagnetic sheets of spins in the a-b plane oriented along <110> directions and alternating along c. At x =1, the Tb spins form a longitudinal spin-density wave (SDW) with q = 0.55a*, with antiferromagnetic stacking along the c-axis. It is tempting to assign the absence of superconductivity to the existence of the SDW, except that the Er analog has a transverse SDW with essentially the same wavevector, but exhibits a robust superconducting transition temperature. As Tb is substituted for Dy, the superconduct ing transition decreases rapidly, but TN remains essentially constant, in defiance of the increasing deGennes factor. The superconducting transition temperature extrapolates to zero at x = 0.6, and at that point the Neel temperature begins to rise toward 15 K at x = 1. It has been argued that each Ni atom in the Dy analog is surrounded by nearest neighbors whose net spin adds to zero, while no such cancellation occurs for the SDW state of the Tb version. For this to hold in the mixed crystal, there must be some coexistence of SDW and AF ordering, or at least short-range SDW correlations. We propose here to use resonant x-ray scattering to examine the ordering characteristics of mixed Dy-Tb crystals in the low concentration range (near x = 0.2) where there is significant suppression of Tc, and near x = 0.6, where the rising Neel temperature suggests the emergence of SDW order. Magnetic x-rays scattering offers the opportunity to examine single crystal samples (typically of millimeter dimensions) without the need to use ^11B substitution as required for neutron scattering. We have the added advantage of examining the degree of ordering of Dy and Tb moments separately. Magnetic x-ray scattering has been applied to the Nd, Sm and Gd analogs of these materials previously with great success.[3] Samples have been prepared at Pohang University, and were fully characterized there. Samples of several concentrations will be available for this initial run, with other concentrations to be supplied for later experiments. The experiment will require the use of the Displex%96and eventually the Heliplex%96 and the UIUC Polarimeter. Appropriate analyzer crystals for Dy and Tb are available for polarization analysis, if required to separate dipole and quadrupole contributions. 1. B.K. Cho, P.C. Canfield, and D.C. Johnson, Phys. Rev. Lett. 77, 163 (1996); H. Doh, M. Sigrist, B.K.Cho, and S-I. Lee, Phys. Rev. Lett. (in press. 2. S. K. Skanthakumar and J. W. Lynn, Physica B 259-261, 576 (1999). 3. C. Detlefs, et al., Phys. Rev. B 53, 6355 (1996); ibid. 55, R680 (1997). equipment+required: Displex (to 6 K); UIUC Polarimeter experiment: Evolution of magnetic order in Dy_(1-x)Tb_xNi_2B_2C hazards: No known safety hazards name: Myron Salamon new+request: on nonmembers: Sung-Ik Lee, Pohang University (Professor) Jae-Hyuk Choi, Pahang University (graduate student) station: 33ID-D unacceptable+dates: Before January 10 #QUERY_STRING: #REMOTE_HOST: salamon.mrl.uiuc.edu #REMOTE_ADDR: 130.126.103.14 #CONTENT_LENGTH: 4452 #HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm #HTTP_USER_AGENT: Mozilla/4.7 [en] (WinNT; I)