Dipolar-octupolar Ising antiferromagnetism in Sm2Ti2O7 : A moment fragmentation candidate

Abstract

Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm2Ti2O7. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm2Ti2O7. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm 3+ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm2Ti2O7 orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) µB below TN = 0.35 K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm 3+ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.Rare earth titanate pyrochlores of the form R 2 Ti 2 O 7 have long been a centerpiece in the study of geometricallyfrustrated magnetism [1]. In this family of materials, the magnetism is carried by the R 3+ rare earth ions, which decorate a network of corner-sharing tetrahedra.The study of this family has led to the discovery of a range of fascinating ground states such as the dipolar spin ice state, which was first observed in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 [2-4]. Here local Ising anisotropy combines with dominant dipolar interactions, which are ferromagnetic at the nearest neighbour level on the pyrochlore lattice [5]. The spin ice state is characterized by individual tetrahedra obeying two-in, two-out "ice rules", wherein two spins point directly towards the tetrahedron's center and the other two spins point outwards (left inset of Fig. 1). This configuration can be achieved in six equivalent ways for a single tetrahedron, giving rise to a macroscopic degeneracy for the lattice as a whole. In other titanates, where the rare earth moments are smaller than in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , dipolar interactions become less important and exchange interactions tend to dominate. This is exactly the case when R = Sm 3+ (∼ 1 µ B ), where the magnetic moment is reduced by a factor of ten from R = Ho 3+ and Dy 3+ (∼ 10 µ B ), corresponding to dipolar interactions that are weaker by two orders of magnitude.In this letter we show that anitferromagnetically coupled Ising spins with negligible dipolar interactions give rise to an all-in, all-out (AIAO) magnetic ground state in Sm 2 Ti 2 O 7 . The AIAO structure is characterized by adjacent tetrahedra alternating between all spins pointing inwards and all spins pointing outwards (right inset of Fig. 1). Unlike the ferromagnetic spin ice state, the antiferromagnetic AIAO state does not give rise to a macroscopic degenerac...

Intermultiplet transitions and magnetic long-range order in Sm-based pyrochlores

“…The anomaly of the singlecrystal sample of Ref. [11] has a smaller intensity than the anomaly of our powder, but its position and sharpness are very similar.…”

“…Clearly, the same behaviour of C p is expected also on the isotopically enriched 154 Sm 2 Sn 2 O 7 . Along with data of this work, the heat capacity of Mauws et al [11] is reproduced. The anomaly of the singlecrystal sample of Ref.…”

“…The former compound is well known for its photocatalytic [6], nuclear waste storage [7] and electronic applications [8]. Three key studies on magnetic frustration have been conducted on the titanate [9][10][11], and are going to be cited recurrently here. For the stannate, reported work is limited to static magnetic susceptibility results, which were presented by Bondah-Jagalu et al [12] Singh et al [9] shows results of DC-susceptibility (χ dc ), heat capacity (C p ) and Raman spectroscopy measurements conducted on a single-crystal sample of Sm 2 Ti 2 O 7 .…”

“…Recently, Mauws et al [11] presented a more complete set of experiments on another single crystal sample of Sm 2 Ti 2 O 7 , this time enriched with the isotope 154 Sm. It is shown that the titanate displays a heat capacity anomaly at T T i N = 350 mK, associated with the development of an antiferromagnetic all-in-all-out long-range order in the sample.…”

Recent developments of MCViNE and its applications at SNS

“…MCViNE has been improved in the last four years in software infrastructure and interfaces for users. Applications of the MCViNE package at SNS include assisting design of new instruments at the second target station, design of sample environments and collimators, as well as advanced data analysis [9,11,26,27]. MCViNE is an ongoing project.…”