Antiferromagnetic order and spin frustration in UNi4B

Abstract

Antiferromagnetic order and spin frustration in UNiB4Mentink, S.A.M.; Nieuwenhuys, G.J.; Menovsky, A.A.; Mydosh, J.A.; Drost, A.; Frikkee, E. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We present the novel magnetic ordering structure of the hexagonal antiferromagnet UNi4B (Tr~ = 20 K) as determined by single-crystal neutron diffraction. Uranium moments of 1.2(2)/x a are oriented perpendicular to c, forming ferromagnetic chains with antiferromagnetic in-plane coupling. Surprisingly, 1/3 of the U-spins remain paramagnetic. Large amplitude, onedimensional ferromagnetic spin-wave excitations in these free chains are proposed to cause the highly anomalous low-temperature behavior of the ordered state.

μ+SR in antiferromagnetic UNi4B

“…The magnetic ordering in UNi4B has clearly been observed by IX÷SR. Also no changes have been found to occur around a temperature of 10 K, where the magnetic susceptibility and the electronic specific heat attain a second maximum, in agreement with the results from neutron diffraction [2], where also no effects were found around 10 K. This supports the conclusion [2] that the second maximum in the specific heat and magnetic susceptibility in UNi4B is not due to an extra phase transition or reorientation of the spins, but that macroscopic properties of UNiaB should be considered as the summation of those of a three-dimensionally ordering antiferromagnet (twothirds) and those of a ferromagnetic linear chain (onethird).…”

“…Since there are two U layers in this structure, the U-U distance along the c-axis is much smaller (3.482 /~), than that in the basal plane (4.953 A). UNi4B orders antiferromagnetically for temperatures below T N = 20 K [1], where it attains a unique and new magnetic structure in the ordered phase [2] (see Fig. 1).…”

“…When the p~+ spin is directed along the a or b-axis, only a rapidly decaying signal is found (~(T--->0)= 38 ixs-1). We have plotted the normalized results for the spontaneous frequencies and relaxation rates together with the ordered moment as obtained from neutron diffraction [2] as a function of reduced temperature in Fig. 3.…”

“…Based on the deduced magnetic structure [2], the possible spontaneous frequencies for the 3f sites have been calculated. The 3f sites surrounding the 'free' U ion in the hexagon depicted in the lower left of Fig.…”

Zero-field current-induced Hall effect in ferrotoroidic metal

“…The possibility of the nonlinear Hall effect has been reported in two ferrotoroidic candidate metals UNi 4 B [22] and CuMnAs [23]. However, in the former case, the contact resistance is too high to suppress a local Joule heating, and hence Seebeck effect may reflect the ghost of a giant nonlinear Hall effect even at the disordered state [24,25]; this would also make it difficult to compare with the existing data on current-induced magnetization including its temperature dependence and amplitude [19]. By contrast, the latter one has already succeeded in observing and controlling the nonlinear Hall effect by deflecting Néel vector; this nonlinear response has been considered to come from the anisotropic magnetoresistance.…”