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...

The temperature and energy dependence of the magnetic scattering in the ferromagnetic spiral phase of Tb has been studied between 220 and 235 K by high-resolution x-ray-scattering techniques using synchrotron radiation. In order to see the magnetic scattering it was necessary to exploit the resonant enhancement of the magnetic x-ray cross section by tuning the incident photon energy to the Tb Lyly edge. A lower bound for this enhancement is of order 20. The magnetic modulation wave vector r decreases continuously with decreasing temperature over most of the range between the Neel and Curie points, in agreement with prior neutron-diffraction measurements. However, no evidence was found for any lock-in behavior above T"as has been observed in other rare earths. A very strong hysteresis eKect is present in Tb below T~, with the value of the modulation wave vector apparently depending on the rate of cooling from the paramagnetic regime. In addition, the radial linewidth of the magnetic satellite at (002)+ is not resolution-limited and increases as T~T + while that of the charge peak at (002) remains constant. No conclusive evidence was found for scattering at higher harmonics.

The magnetic ordering and phase transitions have been studied in UNi2Si, single crystals by neutron diffraction. Three distinct ordered phases are observed, all with magnetic moments along the c axis. The high-temperature (103 & T& 124 K) phase is an incommensurate longitudinal spin-density wave with temperature-dependent wave vector. At 110 K, q=(0,0,0.745+0.002). The intermediate temperature (53 & T& 103 K) phase is a simple body-centered antiferromagnet with a moment of (1.6+0.3)pz. The low-temperature phase shows a similar scattering pattern to that of the hightemperature phase with q=(0, 0, 3 ), and with extra scattering at the nuclear Bragg peaks corresponding to a ferromagnetic moment p, of (1.0 0.3)p& per uranium atom. This pattern can be interpreted either as a longitudinal spin-density wave with maximum moment po=(2.7+0.3)p~and a ferromagnetic component or as a squared-up wave with uranium moments of 2.22p&, two thirds of which are along +z and one third alongz. Hysteresis is observed in the low-temperature phase transition. The phase transitions at 53 and 103 K are both first order. At 124+1 K there is a critical phase transition to the paramagnetic state with f3=0.35+0.03. Above 124 K conventional critical scattering is observed centered on the wave vector q.

We present results of specific heat, electrical resistance, and magnetic susceptibility measurements on the heavy fermion system CePtSi and compare it to CeRuSi. Both compounds show a low-temperature maximum in cp/T, usually associated with the formation of a coherent heavy fermion ground state. An additional feature in cp/T is observed for CePtSi and is attributed to cooperative magnetism, an interpretation confirmed by magnetic measurements. A comparison with other heavy fermion systems, especially stoichiometric and nonstoichiometric CeCu2Si2, is performed.

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