Magnetic ordering in the Ising antiferromagnetic pyrochlore Nd₂ScNbO₇

Abstract

The question of structural disorder and its effects on magnetism is relevant to a number of spin liquid candidate materials. Although commonly thought of as a route to spin glass behaviour, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd2ScNbO7 is shown to have a dispersionless gapped excitation observed in other neodymium pyrochlores below T N = 0.37 K through polarized and inelastic neutron scattering. However the dispersing spin waves are not observed. This excited mode is shown to occur in only 14(2)% of the neodymium ions through spectroscopy and is consistent with total scattering measurements as well as the magnitude of the dynamic moment 0.26(2) μ B . The remaining magnetic species order completely into the all-in all-out Ising antiferromagnetic structure. This can be seen as a result of local symmetry breaking due disordered Sc+3 and Nb+5 ions about the A-site. From this work, it has been established that B-site disorder restores the dipole-like behaviour of the Nd+3 ions compared to the Nd2B2O7 parent series.

Field-temperature phase diagram of the enigmatic Nd2(Zr1−xTix)2O7

“…In addition, the ordered magnetic moment is larger along the z direction, thus corresponding to a lower θ angle, which indicates that the disorder tends to tilt the pseudospins back toward the local 111 z axes. This picture is consistent with recent observations on Nd-based pyrochlores with a much stronger disorder on the B site [34][35][36], namely, Nd 2 NbScO 7 and Nd 2 GaSbO 7 .…”

“…The level of disorder introduced in the present study on the nonmagnetic site is not strong enough to destabilize thẽ z-AIAO ordering, and on the contrary seems to reinforce it. As previously mentioned, recent studies show that the AIAO state resists to an even stronger disorder [34][35][36]. Consequently, it may be relevant to directly make a substitution on the magnetic site to test the stability of the AIAO state on dilution and to possibly reach the predicted quantum spin liquid ground state when the ratioJz/Jx decreases [12,41].…”

Absence of moment fragmentation in the mixed B -site pyrochlore Nd2GaSbO7

“…Our fitted result yields a center-of-mass of 0.253(6) meV for the flat mode at 40 mK, as illustrated in Fig. 6(b), which corresponds to a significantly larger value than reported for the moment fragmentation system Nd 2 Zr 2 O 7 (0.07 meV) [17] and the mixed B-site pyrochlore Nd 2 ScNbO 7 (0.06 meV) [32]. The value for the center-of-mass shifts to lower energy transfers with increasing temperature and becomes gapless near T N , as shown in Figs.…”

“…While the data confirmed the presence of a thermally isolated ground state doublet, the Bsite mixing inherent to Nd 2 GaSbO 7 has broadened the crystal field excitations significantly, consistent with a distribution of local crystalline environments for the Nd 3+ ions. Such broadening has been previously observed in other mixed B-site pyrochlores including Tb 2 Sn 2−x Ti x O 7 [43], Nd 2 ScNbO 7 [32], and Yb 2 GaSbO 7 [31], transforming the determination of crystal field parameters, even average ones, into a significantly under-constrained problem for conventional INS data fitting approaches [33]. [40] using the scaling relation given by Eq.…”

“…In addition, the mixed B-site oxide pyrochlore Yb 2 GaSbO 7 [30] is a candidate for disorder-induced spin liquid or spin-singlet behavior [31]. Interestingly, Nd 2 ScNbO 7 shows the combination of AIAO long-range magnetic order and diffuse scattering expected for moment fragmentation, although open questions remain about the true nature of its magnetic ground state and the presence of this exotic phenomenon in this material [32,33]. More specifically, the diffuse scattering manifests as rods along the 111 directions, but no evidence for pinch points is observed, possibly due to the significant B-site disorder in the system.…”