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${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ is a double perovskite material where the ${\mathrm{Re}}^{5+}$ (${d}^{2}$) ions occupy a frustrated face-centered cubic lattice. Despite strong antiferromagnetic interactions between the Re ions, as indicated by a large negative Weiss constant $\ensuremath{\theta}=\ensuremath{-}616$ K, spin freezing in ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ only occurs at 45 K. Since no long-range order of magnetic dipoles has previously been found in either muon spin rotation or neutron diffraction experiments below this temperature, it has been assumed that the low-temperature state is a spin glass. In stark contrast with these findings, we here show that ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ does in fact order with a strongly reduced dipole moment ${\ensuremath{\mu}}_{0}=0.29--0.42\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ via a polarized neutron diffraction experiment on a powder sample. Using the symmetries of the two likeliest magnetic structures and the properties of the $5{d}^{2}$ configuration in the presence of strong spin-orbit coupling and strong crystal fields, we use a recently derived single-ion wave function for ${\mathrm{Re}}^{5+}$ to predict a large quadrupolar moment based on the experimental ${\ensuremath{\mu}}_{o}$.

$\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ represents a borderline case in the range of solid solutions formed in the pseudobinary system $\mathrm{LaC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}\text{\ensuremath{-}}\mathrm{CeC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$. The material undergoes ferromagnetic ordering at \ensuremath{\sim}225 K followed by a structural collapse at \ensuremath{\sim}190 K, which leads to a strong suppression of magnetization. The structural phase transition manifests itself in a gradual decrease in the parameter $c$ and a relatively smaller increase of the parameter $a$ of the tetragonal lattice. Interestingly, a combination of magnetic measurements and nonpolarized and polarized neutron scattering experiments suggests that the structural collapse does not lead to an antiferromagnetically ordered state, observed in samples with the higher Ce content. On the contrary, $\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ appears to enter a disordered, spin glass state, with gradual dissipation of the ferromagnetic ordering taking place simultaneously with the structural collapse, as evidenced by temperature-dependent measurements of the depolarization factor for a polarized neutron beam passing through the sample. The observed behavior is analogous to that reported for so-called reentrant spin glasses. In the present case, however, the appearance of the reentrant spin glass regime is caused not by tuning the chemical composition but by the structural phase transition. Electronic structure calculations confirm that the loss of magnetic ordering is caused by the subtle change to the density of states at the Fermi level due to the variation of the crystal structure of the material.

Neutron diffraction and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) studies are presented for the newly characterized polymorph of ${\mathrm{NiNb}}_{2}{\mathrm{O}}_{6} (\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6})$ with space group ${P4}_{2}/n$ and $\ensuremath{\mu}\mathrm{SR}$ data only for the previously known columbite structure polymorph with space group $Pbcn$. The magnetic structure of the ${P4}_{2}/n$ form was determined from neutron diffraction using both powder and single-crystal data. Powder neutron diffraction determined an ordering wave vector $\stackrel{P\vec}{k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$. Single-crystal data confirmed the same $\stackrel{P\vec}{k}$ vector and showed that the correct magnetic structure consists of antiferromagnetically coupled chains running along the $a$ or $b$ axis in adjacent ${\mathrm{Ni}}^{2+}$ layers perpendicular to the $c$ axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related trirutile phases, ${\mathrm{NiSb}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{NiTa}}_{2}{\mathrm{O}}_{6}$. $\ensuremath{\mu}\mathrm{SR}$ data finds a transition temperature of ${T}_{N}\ensuremath{\sim}15\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for this system, while the columbite polymorph exhibits a lower ${T}_{N}=5.7(3)$ K. Our $\ensuremath{\mu}\mathrm{SR}$ measurements also allowed us to estimate the critical exponent of the order parameter $\ensuremath{\beta}$ for each polymorph. We found $\ensuremath{\beta} =0.25(3)$ and 0.16(2) for the $\ensuremath{\beta}$ and columbite polymorphs, respectively. The single-crystal neutron scattering data give a value for the critical exponent $\ensuremath{\beta} =0.28(3)$ for $\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6}$, in agreement with the $\ensuremath{\mu}\mathrm{SR}$ value. While both systems have $\ensuremath{\beta}$ values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to be well described by $S=1$ spin chains, the interchain interactions in the $\ensuremath{\beta}$ polymorph are likely much larger.

We report resonant inelastic X-ray scattering (RIXS) measurements on ordered double perovskite samples containing Re$^{5+}$ and Ir$^{5+}$ with $5d^2$ and $5d^4$ electronic configurations respectively. In particular, the observed RIXS spectra of Ba$_2$YReO$_6$ and Sr$_2$MIrO$_6$ (M=Y, Gd) show sharp intra-t$_{2g}$ transitions, which can be quantitatively understood using a minimal `atomic' Hamiltonian incorporating spin-orbit coupling ($\lambda$) and Hund's coupling ($J_H$). Our analysis yields $\lambda=0.38(2)$eV with $J_H=0.26(2)$eV for Re$^{5+}$, and $\lambda=0.42(2)$eV with $J_H=0.25(4)$eV for Ir$^{5+}$. Our results provide the first sharp estimates for the Hund's coupling in $5d$ oxides, and suggest that it should be treated on equal footing with spin-orbit interaction in multi-orbital $5d$ transition metal compounds., Comment: Added theoretical RIXS spectrum + expanded. To appear in PRB

$\mathrm{L}{\mathrm{a}}_{0.25}\mathrm{P}{\mathrm{r}}_{0.75}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ crystallizes in the tetragonal $\mathrm{ThC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{i}}_{2}$ structure type and shows multiple magnetic phase transitions driven by changes in temperature and magnetic field. The nature of these transitions was investigated by a combination of magnetic and magnetoresistance measurements and both single crystal and powder neutron diffraction. The Co magnetic moments order ferromagnetically (FM) parallel to the $c$ axis at 282 K, followed by antiferromagnetic (AFM) ordering at 225 K. In the AFM structure, the Co magnetic moments align along the $c$ axis with FM $[\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}]$ layers arranged in an alternating sequence, $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$, which leads to the doubling of the $c$ axis in the magnetic unit cell. Another AFM transition is observed at 27 K, due to the ordering of a half of Pr moments in the $ab$ plane. The other half of Pr moments undergoes AFM ordering along the $c$ axis at 11 K, causing simultaneous reorientation of the previously ordered Pr moments into an AFM structure with the moments being canted with respect to the $c$ axis. This AFM transition causes an abrupt decrease in electrical resistivity at 11 K. Under applied magnetic field, two metamagnetic transitions are observed in the Pr sublattice at 0.8 and 5.4 T. They correlate with two anomalies in magnetoresistance measurements at the same critical fields. A comparison of the temperature- and field-dependent magnetic properties of $\mathrm{L}{\mathrm{a}}_{0.25}\mathrm{P}{\mathrm{r}}_{0.75}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ to the magnetic behavior of $\mathrm{PrC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ is provided.

Three closely related double perovskites (DP) based on the $5{d}^{2}$ ion, $\mathrm{O}{\mathrm{s}}^{6+},\mathrm{B}{\mathrm{a}}_{2}M\mathrm{Os}{\mathrm{O}}_{6}$, with $M=\mathrm{Mg}$, Zn, and Cd have been prepared and characterized using x-ray (XRD) and neutron diffraction (ND), dc magnetization, heat capacity, and muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ techniques. All three are cubic, Fm-$3m$, at ambient temperature from XRD with $\mathrm{\ensuremath{\Delta}}d/d\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ resolution. For both $M=\mathrm{Mg}$ and Zn, ND data at 3.5 K and lower, $\mathrm{\ensuremath{\Delta}}d/d=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, resolution show no signs of a distortion. The results are compared with the known DP material, $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, which shows antiferromagnetic (AF) order below ${T}_{\mathrm{N}}=49\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a moderate frustration index, $f\ensuremath{\sim}4$, $(f=|{\ensuremath{\theta}}_{\mathrm{CW}}|/{T}_{\mathrm{N}})$, where ${\ensuremath{\theta}}_{\mathrm{CW}}$ is the Curie-Weiss temperature. $\mathrm{B}{\mathrm{a}}_{2}\mathrm{MgOs}{\mathrm{O}}_{6}$ with a unit cell constant ${a}_{0}=8.0757(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, 3% smaller than for $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, also shows N\'eel order below ${T}_{\mathrm{N}}=51\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with $f\ensuremath{\sim}2$. However, $\mathrm{B}{\mathrm{a}}_{2}\mathrm{ZnOs}{\mathrm{O}}_{6},{a}_{0}=8.0975(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, 0.27% larger than $\mathrm{B}{\mathrm{a}}_{2}\mathrm{MgOs}{\mathrm{O}}_{6}$, does not show N\'eel order from either heat capacity or $\ensuremath{\mu}\mathrm{SR}$ data. A zero field cooled/field cooled (ZFC/FC) irreversibility occurs near 30 K and a broad heat capacity anomaly is detected at a similar temperature. The $\ensuremath{\mu}\mathrm{SR}$ data are consistent with a weak spin ordering with an onset below 28 K but with a coexisting dynamic component. $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CdOs}{\mathrm{O}}_{6}$ with ${a}_{0}=8.3190(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, 0.5% smaller than $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, shows no evidence for any type of order/spin freezing to 0.47 K from any of the measurement techniques applied. The results for $M=\mathrm{Zn}$ and Cd appear to lie outside of the mean field theory of Chen and Balents [Phys. Rev. B 84, 094420 (2011)] for cubic ${d}^{2}$ DP with strong spin orbit coupling, but $\mathrm{B}{\mathrm{a}}_{2}\mathrm{MgOs}{\mathrm{O}}_{6}$, along with $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, is likely one of the three predicted AF phases. The remarkable contrast between the doppelg\"anger pairs $M=\mathrm{Mg}$/Zn and $M=\mathrm{Ca}$/Cd may be traceable to differences in electronic structure of the diamagnetic $M$ ions. All of the super-super exchange pathways in these DP materials involve Os--O--$M$--O--Os linkages.

The frustrated double perovskite $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, based on $\mathrm{O}{\mathrm{s}}^{5+}\phantom{\rule{0.16em}{0ex}}(5{d}^{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{t}}_{2}^{3})$ is studied using magnetization, elastic neutron scattering, heat capacity, and muon spin relaxation (\ensuremath{\mu}SR) techniques and compared with isostructural $(P{2}_{1}/n)\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6},\phantom{\rule{0.16em}{0ex}}\mathrm{R}{\mathrm{u}}^{5+}(4{d}^{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{t}}_{2}^{3})$. While previous studies of $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$ showed a broad susceptibility maximum $({\ensuremath{\chi}}_{\mathrm{max}})$ near 40 K, heat capacity data indicate a sharp peak at 30 K, similar to $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$ with ${\ensuremath{\chi}}_{\mathrm{max}}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a heat capacity peak at 24 K. Significant differences between the two materials are seen in powder neutron diffraction where the magnetic structure is described by $\mathbit{k}=(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{4pt}{0ex}}0)$ for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, while $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$ has been reported with $\mathbit{k}=(000)$, structure for face centered lattices. For the $\mathbit{k}=(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{4pt}{0ex}}0)$ structure, one has antiferromagnetic layers stacked antiferromagnetically, while for $\mathbit{k}=(0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}0)$ structure, ferromagnetic layers are stacked antiferromagnetically. In spite of these differences, both can be considered as type I fcc antiferromagnetic structures. For $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, the magnetic structure is best described in terms of linear combinations of basis vectors belonging to irreducible representations ${\mathrm{\ensuremath{\Gamma}}}_{2}$ and ${\mathrm{\ensuremath{\Gamma}}}_{4}$. The combinations ${\mathrm{\ensuremath{\Gamma}}}_{2}--{\mathrm{\ensuremath{\Gamma}}}_{4}$ and ${\mathrm{\ensuremath{\Gamma}}}_{2}+{\mathrm{\ensuremath{\Gamma}}}_{4}$ could not be distinguished from refinement of the data. In all cases, the $\mathrm{O}{\mathrm{s}}^{5+}$ moments lie in the $yz$ plane with the largest component along $y$. The total moment is 1.81(4) ${\ensuremath{\mu}}_{\mathrm{B}}$. For $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$, the $\mathrm{R}{\mathrm{u}}^{5+}$ moments are reported to lie in the $xz$ plane. In addition, while neutron diffraction, \ensuremath{\mu}SR and NMR data indicate a unique ${T}_{\mathrm{N}}=24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$, the situation for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$ is more complex, with heat capacity, neutron diffraction, and \ensuremath{\mu}SR indicating two ordering events at 30 and 37 K, similar to the cases of cubic $\mathrm{B}{\mathrm{a}}_{2}\mathrm{YRu}{\mathrm{O}}_{6}$ and monoclinic $\mathrm{S}{\mathrm{r}}_{2}\mathrm{YRu}{\mathrm{O}}_{6}$.

We report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite ${\mathrm{Ba}}_{2}{\mathrm{YOsO}}_{6}$. The $Fm\overline{3}m$ space group is found both at 290 K and 3.5 K with cell constants ${a}_{0}=8.3541(4)$ \AA{} and $8.3435(4)$ \AA{}, respectively. ${\mathrm{Os}}^{5+} (5{d}^{3})$ ions occupy a nondistorted, geometrically frustrated face-centered-cubic (fcc) lattice. A Curie-Weiss temperature $\ensuremath{\theta}\ensuremath{\sim}\ensuremath{-}700$ K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long-range antiferromagnetic order, consistent with a type-I fcc state below ${T}_{\mathrm{N}}\ensuremath{\sim}69$ K, is revealed by magnetization, Fisher heat capacity, and elastic neutron scattering, with an ordered moment of 1.65(6) ${\ensuremath{\mu}}_{B}$ on ${\mathrm{Os}}^{5+}$. The ordered moment is much reduced from either the expected spin-only value of $\ensuremath{\sim}3 {\ensuremath{\mu}}_{B}$ or the value appropriate to $4{d}^{3} {\mathrm{Ru}}^{5+}$ in isostructural ${\mathrm{Ba}}_{2}{\mathrm{YRuO}}_{6}$ of 2.2(1) ${\ensuremath{\mu}}_{B}$, suggesting a role for spin-orbit coupling (SOC). Triple-axis neutron scattering measurements of the order parameter suggest an additional first-order transition at $T=67.45$ K, and the existence of a second-ordered state. Time-of-flight inelastic neutron results reveal a large spin gap $\ensuremath{\Delta}\ensuremath{\sim}17$ meV, unexpected for an orbitally quenched, ${d}^{3}$ electronic configuration. We discuss this in the context of the $\ensuremath{\sim}5$ meV spin gap observed in the related ${\mathrm{Ru}}^{5+},\phantom{\rule{0.16em}{0ex}}4{d}^{3}$ cubic double perovskite ${\mathrm{Ba}}_{2}{\mathrm{YRuO}}_{6}$, and attribute the $\ensuremath{\sim}3$ times larger gap to stronger SOC present in this heavier, $5d$, osmate system.

Here we investigated diffuse magnetic scattering in the highly frustrated double perovskite Ba2YRuO6 using polarized neutrons. Consistent with previous reports, the material shows two apparent transitions at 47 and 36 K to an eventual type I face-centered-cubic magnetic ground state. The (100) magnetic reflection shows different behavior from the five other observed reflections upon heating from 1.8 K, with the former broadening well beyond the resolution limit near 36 K. Closer examination of the latter group reveals a small, but clear, increase in peak widths between 36 and 47 K, indicating that this regime is dominated by short-range spin correlations. Diffuse magnetic scattering persists above 47 K near the position of (100) to at least 200 K, consistent with strong frustration. Reverse Monte Carlo (RMC) modeling of the diffuse scattering from 45 to 200 K finds that the spin-spin correlations between nearest and next-nearest neighbors are antiferromagnetic and ferromagnetic, respectively, at temperatures near the upper ordering temperature, but both become antiferromagnetic and of similar magnitude above 100 K. The significance of this unusual crossover is discussed in light of the super-superexchange interactions between nearest and next-nearest neighbors in this material and the demands of type I order. The dimensionality ofmore » the correlations is addressed by reconstructing the scattering in the (hk0) plane using the RMC spin configurations. This indicates that one-dimensional spin correlations dominate at temperatures close to the first transition. In addition, a comparison between mean-field calculations and (hk0) scattering implies that further neighbor couplings play a significant role in the selection of the ground state. Finally, the results and interpretation are compared with those recently published for monoclinic Sr2YRuO6, and similarities and differences are emphasized.« less

Magnetic behavior of La${}_{0.75}$Pr${}_{0.25}$Co${}_{2}$P${}_{2}$ was investigated by a combination of magnetic measurements, magneto-optical imaging, neutron diffraction, and x-ray absorption spectroscopy, including x-ray magnetic circular dichroism. The material crystallizes in the ThCr${}_{2}$Si${}_{2}$ structure type and exhibits three consecutive magnetic phase transitions. At 167 K, the Co magnetic moments order ferromagnetically in the $ab$ plane of the tetragonal crystal structure. At 66 K, a ferromagnetic ordering of Pr(4$f$) moments parallel to the $c$ axis causes a rotation of the Co(3$d$) moments towards the $c$ axis in the direction opposite to the Pr moments, thus forming a noncollinear ferrimagnetically ordered structure and switching the direction of the total magnetization from the $ab$ plane to the $c$ axis. The third magnetic transition observed at 35 K is likely associated with the establishment of the collinear ferrimagnetic order along the $c$ axis.