The effect of doping on the spin-Peierls (SP) transition in single crystals of CuGehas been investigated using neutron scanering techniques and susceptibility measurements. When the Cuz+ site is partly substituted by either ZnZ+ (4%) or Nilt (1.5%) the SP is suppressed. and a long-range N&l state develops below TN = 2.8 K (a) and T,, = 2.2 K (Ni) with a wave vector of (0.1, I). This observation contrasts with earlier reports of a spin glass m s i d o n in the doped compounds in the same temperature range. Our results also indicate that the reported N&l state in the nominally pure compound results from impurities. Preliminary mea~urements of the Zn-doped sample show that the gap that opens in the magnetic excitations below Tsp collapses in the doped compound.

Single-crystal derivatives of the spin-Peierls (S-P) system, CuGeO 3 , doped with Zn (0.5 to 2.4%) and Ni (1.7 to 6%) have been studied using SQUID magnetometry and neutron scattering. Our study shows that the impurities act to suppress the S-P state and produce a 3D Néel state at low temperatures. A phase diagram is constructed which shows that doping with either Zn (S = 0) or Ni (S = 1) leads to qualitatively similar results: the temperature at which the transition to the S-P state occurs decreases linearly with dopant concentration, whereas the Néel temperature, T N , initially increases to a maximum at around 4% and then decreases for higher concentrations. These results are discussed with reference to models of defects in 1D spin chains and to earlier experimental work on this system. One difference between the Znand Ni-doped samples is that in the Néel state the Cu 2+ moments in the former point along the c-axis, while in the latter they are along the a-axis.

Neutron scattering measurements show UNi2A13 to be unique among heavy fermion superconductors in that belo~T&-5.2 K it displays long range magnetic order which is incommensurate with its chemical lattice. The ordering wave vector is ( -, ' 4 b, O, -, ' ) where 6=0.110+ 0.003. The measured superlaitice intensities are well described by the presence of a longitudinal spin density wave within the hexagonal basal plane, polarized along a*~T he magnitude of the maximum ordered moment is found to be very small; p,& = (0.24+ 0.10)ptt. PACS numbers: 74.70.Tx, 61.12.Ex, 75.25.+z, 75.30.Mb The interplay between magnetism and superconductivity is a central feature in the description of several classes of strongly correlated electron systems. The heavy fermion metals, a subset of which exhibits the apparent microscopic coexistence of f-electron magnetism and superconductivity at sufficiently low temperatures, form such a class of materials [1,2].At present, there exist four such uranium-based compounds which display antiferromagnetic order characterized by small ordered moments. UPt3 [3] forms a hexagonal-close-packed structure and orders magnetically below Tz-6 K. It undergoes at least one and likely two superconducting transitionsURu2Siz forms a body-centered-tetragonal crystal structure and undergoes magnetic and superconducting transitions at 17 and -1.2 K, respectively [5]. A remarkable feature of the magnetism in these two compounds is that the magnitude of the ordered moment is extremely small (p"d-0.02ptt for UPt3 [3], and p"d-0.04ptt for URu2Siz [6]) leading to speculation that these materials in fact display nontrivial ordering phenomena [7] distinct from simple antiferromagnetic ordering of the local magnetic dipoles.Recently, two new heavy fermion metals which display coexistence of superconductivity and antiferromagnetic order have been discovered. These are the simple hexagonal ternary metals UPdzA13 [8] and UNizA13 [9] which undergo magnetic phase transitions at T~-15 and -5 K, and superconducting transitions at T~-2 and -1 K, respectively. Considerably less is known about these new compounds, compared with the extensive literature which exists on UPt3 and URu2Si2. Neutron powder diffraction measurements indicate that UPd2A13 [10] orders into a simple, Q = (0,0, -, ' ), antiferromagnetic structure in which the moments lie ferromagnetically aligned within the basal plane, and the moment direction rotates by z from one basal plane to the next. Recent measurements [11,12] suggest a more complicated magnetic phase diagram. However, all measurements are consistent with an ordered magnetic moment of almost conventional size, chord0~5PB. In this Letter we report on the first observation of magnetic neutron scattering from UNi2A13 and determine the magnetic structure below T~. As we will discuss, the magnetic order is characterized by a very weak ordered moment and an incommensurate ordering wave vector. This combination made these experiments very difficult. They required detailed, systematic searches through rec...

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