We study the high temperature magnetic order in SrRu2O6 by measuring magnetization and neutron powder diffraction with both polarized and unpolarized neutrons. SrRu2O6 crystallizes into the hexagonal lead antimonate (PbSb2O6, space group P 31m) structure with layers of edgesharing RuO6 octahedra separated by Sr 2+ ions. SrRu2O6 is found to order at TN =565 K with Ru moments coupled antiferromagnetically both in-plane and out-of-plane. The magnetic moment is 1.30(2) µB/Ru at room temperature and is along the crystallographic c-axis in the G-type magnetic structure. We perform density functional calculations with constrained RPA to obtain the electronic structure and effective intra-and inter-orbital interaction parameters. The projected density of states show strong hybridization between Ru 4d and O 2p. By downfolding to the target t2g bands we extract the effective magnetic Hamiltonian and perform Monte Carlo simulations to determine the transition temperature as a function of inter-and intra-plane couplings. We find a weak interplane coupling, 3% of the strong intra-plane coupling, permits three-dimensional magnetic order at the observed TN .
Transition metal compounds with the (t2g)4 electronic configuration are expected to be nonmagnetic atomic singlets both in the weakly interacting regime due to spin-orbit coupling, as well as in the Coulomb dominated regime with oppositely aligned L = 1 and S = 1 angular momenta. However, starting with the full multi-orbital electronic Hamiltonian, we show the low energy effective magnetic Hamiltonian contains isotropic superexchange spin interactions but anisotropic orbital interactions. By tuning the ratio of superexchange to spin-orbit coupling JSE/λ, we obtain a phase transition from nonmagnetic atomic singlets to novel magnetic phases depending on the strength of Hund's coupling, the crystal structure and the number of active orbitals. Spin-orbit coupling plays a non-trivial role in generating a triplon condensate of weakly interacting excitations at antiferromagnetic ordering vector k = π, regardless of whether the local spin interactions are ferromagnetic or antiferromagnetic. In the large JSE/λ regime, the localized spin and orbital moments produce anisotropic orbital interactions that are frustrated or constrained even in the absence of geometric frustration. Orbital frustration leads to frustration in the spin channel opening up the possibility of spin-orbital liquids with both spin and orbital entanglement.
The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr(1-x)Ca(x)RuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.
We systematically investigate the magnetic properties and local structure of Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this d 4 compound. We compare the magnetic properties and atomic imaging of (1) a slow cooled crystal, (2) a crystal quenched from 900• C after growth, and (3) a crystal grown using a faster cooling rate than the slow cooled one. Atomic imaging by scanning transmission electron microscopy (STEM) shows that quenching from 900• C introduces antiphase boundary to the crystals, and a faster cooling rate during crystal growth leads to clusters of Y and Ir antisite disorder. STEM study suggests the antiphase boundary region is Ir-rich with a composition of Ba2(Y1−xIrx)IrO6. The magnetic measurements show that Ba2YIrO6 crystals with clusters of antisite defects have a larger effective moment and a larger saturation moment than the slow-cooled crystals. Quenched crystals with Ir-rich antiphase boundary shows a slightly suppressed saturation moment than the slow cooled crystals, and this seems to suggest that antiphase boundary is detrimental to the moment formation. Our DFT calculations suggest magnetic condensation is unlikely as the energy to be gained from superexchange is small compared to the spin-orbit gap. However, once Y is replaced by Ir in the antisite disordered region, the picture of local non-magnetic singlets breaks down and magnetism can be induced. This is because of (a) enhanced interactions due to increased overlap of orbitals between sites, and, (b) increased number of orbitals mediating the interactions. Our work highlights the importance of lattice defects in understanding the experimentally observed magnetism in Ba2YIrO6 and other J = 0 systems.
PACS 05.30.Rt -Quantum phase transitions PACS 75.10.Lp -Band and itinerant models PACS 75.10.Nr -Spin-glass and other random models Abstract -We investigate the influence of spatial disorder correlations on smeared phase transitions, taking the magnetic quantum phase transition in an itinerant magnet as an example. We find that even short-range correlations can have a dramatic effect and qualitatively change the behavior of observable quantities compared to the uncorrelated case. This is in marked contrast to conventional critical points, at which short-range correlated disorder and uncorrelated disorder lead to the same critical behavior. We develop an optimal fluctuation theory of the quantum phase transition in the presence of correlated disorder, and we illustrate the results by computer simulations. As an experimental application, we discuss the ferromagnetic quantum phase transition in Sr1−xCaxRuO3.
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