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...

We have investigated the effect of site dilution with substitution of nonmagnetic element in SrRu Ti O (x  ⩽  0.7). The nature of ferromagnetic state in SrRuO is believed to be of itinerant type with transition temperature [Formula: see text] K. Crystallographically, SrRuO has a distorted orthorhombic structure. Substitution of [Formula: see text] (3d ) for Ru (4d ), however, does not introduce significant structural modification due to their matching ionic radii. This substitution, on the other hand, is expected to tune the electronic correlation effect and the d electron density in the system. With Ti substitution, we find that magnetic moment and Curie temperature decreases but T remains unchanged which has been attributed to opposite tuning of electron correlation effect and density of states within the framework of itinerant ferromagnetism. The estimated critical exponent (β) related to magnetization implies a mean-field type of magnetic nature in SrRuO. The value of β further increases with x which is understood from the dilution effect of magnetic lattice. The system evolves to exhibit Griffiths phase like behavior above T which is usually realized in diluted ferromagnet following local moment model of magnetism. Our detail analysis of magnetization data indicates that magnetic state in SrRuO has contribution from both itinerant and local moment model of magnetism.

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Here, we report an evolution of structural, magnetic and transport behavior in doped SrRu 1−x Ga x O 3 (x 0.2). The nonmagnetic dopant Ga 3+ (3d 10 ) not only acts for magnetic site dilution in SrRuO 3 but also modifies the Ru charge state and electronic density. Our studies show that Ga 3+ substitution does not affect the original orthorhombic-Pbnm structure of SrRuO 3 which is due to its matching ionic radii with Ru 4+ . However, Ga 3+ has a substantial effect on the magnetic behavior of SrRuO 3 where it decreases both magnetic moment as well as magnetic transition temperature T c . Further, this dilution induces Griffiths phase behavior across T c and cluster-glass behavior at low temperature with higher concentration of doping. The magnetic critical exponent β increases with x due to this site dilution effect. The Ga 3+ induces an insulating state in SrRuO 3 with x > 0.05. The charge transport in paramagnetic as well as in insulating state of samples can be well described with Mott's modified variablerange-hopping model. The metallic charge transport just below T c in SrRuO 3 obeys Fermi liquid behavior which breaks down at low temperature. We further find a correlation between field dependent magnetoresistance and magnetization through power-law behavior over the series.

SrRuO 3 presents a rare example of ferromagnetism among the 4d based oxides. While the nature of magnetic state in SrRuO 3 is mostly believed to be of itinerant type, recent studies suggest a coexistence of both itinerant and localized model of magnetism in this material. Here, we have investigated the evolution of magnetic state in doped SrRu 1−x Ti x O 3 through studying the critical behavior using standard techniques such as, modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis across the magnetic transition temperature T c . The substitution of nonmagnetic Ti 4+ (3d 0 ) for Ru 4+ (4d 4 ) would simply dilute the magnetic system apart from modifying the electron correlation effect and the density of states at Fermi level. Surprisingly, T c does not change with x. Moreover, our analysis show the exponent β related to spontaneous magnetization increases while the exponents γ and δ related to initial inverse susceptibility and critical magnetization, respectively decrease with Ti substitution. The estimated exponents do not match with any established theoretical models for universality classes, however, the exponent obey the Widom relation and the scaling behavior. Interestingly, this particular evolution of exponents in present series has similarity with that in isoelectronic doped Sr 1−x Ca x RuO 3 . We believe that site dilution by Ti leads to formation magnetic clusters which causes this specific changes in critical exponents.