Ferromagnetism in the Mott InsulatorBa2NaOsO6

Abstract

Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d1 ferromagnetic Mott insulator with an ordered moment of ∼ 0.2 µB per formula unit and TC = 6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet groundstate anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.PACS numbers: 75.50. Dd, 75.30.Cr, 71.70.Ej The interplay between spin, orbital and charge degrees of freedom in 3d transition metal oxides has proven to be a rich area of research in recent years. Despite the wide array of interesting physics found in these materials, much less is known about whether similar behavior can be found in related 4d and 5d systems, for which both the extent of the d-orbitals and larger spin-orbit coupling cause a different balance between the relevant energy scales. In this respect, oxides of osmium are of particular interest because the element can take formal valences from 4+ to 7+, corresponding to electron configurations 5d4 to 5d 1 . In this instance, we examine the simplest case of a 5d1 osmate for which the magnetic properties indicate that orbital ordering may indeed play a significant role.Simple oxides of osmium are typically Pauli paramagnets due to the large extent of the 5d orbitals. Examples include the binary oxide OsO 2 [1, 2] and the simple perovskites AOsO 3 (A= Sr, Ba) [3]. However, more complex oxides, including the double and triple perovskites La 2 NaOsO 6 [4], Ba 2 AOsO 6 (A = Li, Na) [5,6] and Ba 3 AOs 2 O 9 (A = Li, Na) [7], appear to exhibit local moment behavior. Presumably the large separation of Os ions in these more complex structures leads to a Mott insulating state, and indeed these and related materials are most often found to be antiferromagnetic. Of the above materials and their near relations containing no other magnetic ions, Ba 2 NaOsO 6 distinguishes itself as the only osmate with a substantial ferromagnetic moment (∼0.2 µ B ) in the ordered state [5].Weak ferromagnetism has been previously observed in other 5d transition metal oxides containing iridium. BaIrO 3 exhibits a saturated moment of 0.03 µ B , which has been attributed to small exchange splitting associated with charge density wave formation [8]. Sr 2 IrO 4 and Sr 3 Ir 2 O 7 exhibit similarly small saturated moments, attributed variously to either spin canting in an antiferromagnet due to the low crystal symmetry [9] or to a borderline metallic Stoner scenario [10,11]. The ferromagnetic moment in Ba 2 NaOsO 6 is substantially larger than in these materials. Furthermore, at room temperature the material has an undistorted double-perovskite structure, space group Fm3m (inset to Fig. 1) [5], in which OsO 6 octahedra are neither distorted nor rotated with respect to each other or the underlying lattice [12]. Such a high crystal symmetry, if preserve...

Magnetic moment suppression in Ba3CoRu2O9: Hybridization effect

“…4) and Ba 2 NaOsO 6 . 5 A strong reduction of the local magnetic moment was also found in Ba 3 CoRu 2 O 9 (Refs. 6 and 7) and was recently attributed to a special type of the orbital order, which leads to an unusual orbital filling.…”

Frustration by competing interactions in the highly distorted double perovskites La2NaB′O6(Aczel1, Bugaris2, Li3 et al. 2013Phys. Rev. B554512View full textAdd to dashboardCite

“…Recent band structure calculations have been performed and predict that La 2 NaRuO 6 is insulating [22]. La 2 NaOsO 6 is also expected to be an insulator based on the large spatial separation of the Os ions, and this behaviour has been confirmed in the related system Ba 2 NaOsO 6 [8].…”

“…Increased covalency causing delocalization of the magnetic moments, arising from greater orbital overlap between spatially-extended 4d and 5d orbitals and neighbouring anion p orbitals, is one possibility that should be taken into consideration. For example, the OsO 6 octahedra of the related double perovskite system Ba 2 NaOsO 6 likely form molecular orbitals due to the comparable energy scales of the Os 5d and O 2p orbitals [8]. Spin-orbit coupling plays an important role in the magnetism of 4d, and especially 5d, systems also [27], and is another mechanism that can lead to a reduction in the spin-only value of the ordered moment.…”

“…Since the double perovskite structure type is very versatile with respect to chemical substitution and the B sites can accommodate a variety of transition metals, systematic studies can be performed to investigate the effects of changing the spin quantum number S and increasing the relativistic spin-orbit coupling by considering materials with 4d and 5d electrons. A series of exotic magnetic ground states have been observed previously in S = 1/2 and 1 systems, including a collective singlet state coexisting with paramagnetism in Ba 2 YMoO 6 [1,2] described as a valence bond glass [3], a collective singlet state in La 2 LiReO 6 [4], spin freezing without long-range order in Ba 2 YReO 6 [4], Sr 2 MgReO 6 [5] and Sr 2 CaReO 6 [6], shortrange order in La 2 LiMoO 6 [1], and a ferromagnetic (FM) Mott insulating state in Ba 2 NaOsO 6 [7,8]. Theoretical studies have also indicated that a wealth of other magnetic ground states are possible in these 4d and 5d quantum spin systems [9,10].…”

Interplay between lattice distortion and spin-orbit coupling in double perovskites

“…1,2 One class of such frustrated antiferromagnets is found in the double perovskite oxides, which host a wide range of interesting behavior. [3][4][5][6][7][8] These compounds of chemical formula A 2 BB ′ O 6 feature ordered, interpenetrating facecentered cubic (FCC) lattices of the B and B ′ ions when the charge difference between these ions is large. 9 Both B and B ′ transition metal ions are octahedrally coordinated by oxygen.…”