Local and average structures of the spin-glass pyrochloreY2Mo2O7from neutron diffraction and neutron pair distribution function analysis

Abstract

The observation of canonical spin-glass behavior in the pyrochlore oxide Y 2 Mo 2 O 7 has been a subject of considerable interest as the original structural studies were interpreted in terms of a well-ordered crystallographic model. It is widely held that the stabilization of the spin-glass state requires some level of positional disorder along with frustration. Recent reports from local probe measurements, extended x-ray-absorption fine structure ͑EXAFS͒ and 89 Y NMR, have been interpreted in terms of disorder involving the Mo-Mo distances ͑EXAFS͒ and multiple Y sites ͑NMR͒. This work reports results from temperature-dependent ͑15-300 K͒ neutron diffraction ͑ND͒ and neutron pair distribution function studies which can provide from the same data set information on both the average and local structures. The principal findings are that: ͑1͒ there is no crystallographic phase transition over the temperature region studied within the resolution of the ND data; ͑2͒ the diffraction data are well fitted using a fully ordered model but with large and anisotropic displacement parameters for three of the four atomic sites; ͑3͒ the pairwise real-space correlation function G͑r͒ shows clear evidence that the principal source of disorder is associated with the Y-O1 atom pairs rather than the Mo-Mo pairs, in disagreement with the interpretation of the EXAFS results; ͑4͒ fits to the G͑r͒ improve significantly when anisotropic displacements for all sites are included; ͑5͒ inclusion of a split-site position parameter for O1 improves, slightly, both the G͑r͒ fits and the Rietveld fits to the ND data; and ͑6͒ for all models the fits become worse as the temperature decreases and as the fitting range decreases. These results are qualitatively consistent with the 89 Y NMR observations and perhaps recent muon-spin-relaxation studies. The issue of static versus dynamic disorder is not resolved, definitively. An estimate of the distribution of exchange constants due to the disorder is made using spin-dimer analysis and compared with the Saunders-Chalker model for the generation of spin-glass behavior from "weak" disorder on geometrically frustrated lattices.

Orbital Dimer Model for the Spin-Glass State inY2Mo2O7Thygesen1, Paddison2, Zhang3 et al. 2017Phys. Rev. Lett.421510View full textAdd to dashboardCite

“…Instead, it showed pronounced variation in Y-O1 distances consistent with a local splitting of the O1 site-a result interpreted as directly contradicting the EXAFS study [25,28].…”

“…Each pair of Mo neighbours is bridged by a single O1, forming the main magnetic superexchange pathway. The degree of site-mixing and O non-stoichiometry is too small to be measured [24,25] and is calculated to be minimal (∼1%) [26].A plausible mechanism for the spin-glass transition invokes local Mo 4+ displacements to generate variation in magnetic interactions [27]. Arguably the clearest experimental signature of anomalies on the Mo site is its large and anisotropic atomic displacement parameter (ADP) obtained from Rietveld refinement to powder neutron diffraction data [25] [ Fig.…”

“…There is a problem with this proposal, however: a state-of-the-art study using neutron pairdistribution function (PDF) analysis found no evidence for a local splitting of the Mo site [25]. Instead, it showed pronounced variation in Y-O1 distances consistent with a local splitting of the O1 site-a result interpreted as directly contradicting the EXAFS study [25,28].In this Letter, we argue that a disorder model that explains these apparently contradictory results holds the key to understanding the mechanism of spin-glass formation in Y 2 Mo 2 O 7 . We critically assess the validity of the key assumption [25] that the absence of obvious Mo-Mo splitting in the PDF implies that Mo off-centring does not occur.…”

Amorphous ferromagnetism and re-entrant magnetic glassiness in single-crystallineSm2Mo2O7

“…23 From a local point of view, bond-randomness is accepted to be the origin of glassiness in Y 2 Mo 2 O 7 , 24 the principal source of disorder arising from Y-Mo rather than Mo-Mo pairs. 25 However, more recent works 22,[26][27][28][29] suggest that a complicated interplay between spin and orbital degrees of freedom 30 on the basis of a Kugel-Khomskii mechanism 31 may be crucial in determining the overall magnetic properties of pyrochlore molybdates. In particular, on the basis of a multi-orbital Hubbard model, Shinaoka et al 29 conclude that Y 2 Mo 2 O 7 should be considered as the realization of a "spin-orbital frustrated Mott insulator " rather than a conventional frustrated magnet.…”

Emergent order in the spin-frustrated systemDyxTb2−xTi2O7Xing1, He2, Feng3 et al. 2010Phys. Rev. B13060View full textAdd to dashboardCite

“…The pyrochlores 4 provide a variety of geometrically frustrated magnetic materials, including spin liquid, 5 spin ice, 6,7 and spin glass, [8][9][10] where magnetic exchange and dipolar interactions of the nearest-neighbor spins dominate the lowtemperature magnetic property. Among these, two pyrochlores are of particular interests, spin ice Dy 2 Ti 2 O 7 (DTO), Ho 2 Ti 2 O 7 and spin liquid Tb 2 Ti 2 O 7 (TTO), which have completely different ground states while sharing essential similarities.…”