Incommensurate magnetic order in the heavy fermion superconductorUNi2Al3

Abstract

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...

Fluctuation-Driven Selection at Criticality in a Frustrated Magnetic System: The Case of Multiple-kPartial Order on the Pyrochlore Lattice

“…In the most interesting case, the leading degeneracy-lifting perturbations, which may be relevant or irrelevant in the renormalization group sense, are thermal or quantum fluctuations -a phenomenon called order-by-disorder (ObD) [2][3][4][5]. The competition among diverse degeneracy-lifting effects can result in a modulated long-range ordered state at nonzero wave vector k, which may or may not be commensurate with the lattice [6][7][8][9][10][11][12]. In some cases, a number of superposed symmetryrelated k modes within the first Brillouin zone form a socalled multiple-k order [6,7,13,14].…”

Magnetoresistance and Phase Diagram of Thin-Film UNi2Al3

“…The critical temperatures T N for antiferromagnetism and T c for superconductivity differ by a factor of 3 and 2, respectively (T N ≈ 4 K and T c = 1 K for UNi 2 Al 3 ; T N = 14 K and T c = 2 K for UPd 2 Al 3 ), and also the effective magnetic moment per uranium atom in the antiferromagnetic phase differs considerably: 0.85µ B for UPd 2 Al 3 , 4) and approximately 0.2µ B for UNi 2 Al 3 . 5,6) Most remarkable is the difference of the antiferromagnetic order: UPd 2 Al 3 has a simple, commensurate ordering wave vector, 4) whereas the wave vector of UNi 2 Al 3 is incommensurate. 5,6) According to detailed neutron studies in zero magnetic field, the antiferromagnetic order of UNi 2 Al 3 is an amplitude modulated magnetization wave with the magnetic moments in the basal plane.…”

“…5,6) Most remarkable is the difference of the antiferromagnetic order: UPd 2 Al 3 has a simple, commensurate ordering wave vector, 4) whereas the wave vector of UNi 2 Al 3 is incommensurate. 5,6) According to detailed neutron studies in zero magnetic field, the antiferromagnetic order of UNi 2 Al 3 is an amplitude modulated magnetization wave with the magnetic moments in the basal plane.…”

Novel Coexistence of Superconductivity with Two Distinct Magnetic Orders

“…1(b) as circles. A large staggered magnetic moment of 0.85µ B /U [15] coexisting with superconductivity in UPd 2 Al 3 has been considered a puzzling anomaly [8], while UPt 3 [20] has a tiny moment of 0.02 µ B /U, and the moment in UNi 2 Al 3 , 0.24 µ B /U [17], is also quite small. Here in the coexistence composition region of Ce(Rh,Ir)In 5 , M ranges from 0.27 to 0.78 µ B /Ce, bridging the gap between UNi 2 Al 3 and UPd 2 Al 3 .…”

“…Some of the U-based heavy fermion compounds form an interesting subset of these superconductors where superconductivity develops out of a magnetically ordered state. For UPd 2 Al 3 [14,15] the magnetic state is a commensurate antiferromagnet; for UNi 2 Al 3 [16,17] an incommensurate antiferromagnet; for UGe 2 [18] and URhGe [19] a ferromagnet. For UPt 3 [20], three distinct superconducting phases [21,22,23] coexist with a commensurate short-range antiferromagnetic order.…”