Brownmillerite oxides with the composition Ca2Fe2-xMnxO5 had been investigated previously for the limited range x = 0.5 to 1.0. Mn3+ preferentially occupies the octahedral site. This substitution has a strong impact on the magnetic properties as the magnetic ordering temperature, TN, is suppressed from 730 K for x = 0.0 to 465 K for x = 0.5 and the magnetic structure changes from Gx (x = 0.0) to Gy (x = 0.5-1.00). In this study, the compositions x = 0.10 and 0.25 are investigated using single-crystal X-ray and neutron diffraction methods. It is found that TN decreases linearly from x = 0.0 to x = 0.5, with values of 584(2)K for x = 0.25 and 684(2)K for x = 0.1. This behavior is rationalized in terms of a competition between antiferromagnetic and ferromagnetic exchange pathways in the perovskite layers involving Fe3+-Fe3+ and Fe3+-Mn3+ interactions. Profound changes in the magnetic anisotropy also occur as for both x = 0.25 and x = 0.1, the magnetic structure is neither Gx or Gy, but the moments lie between the a and b axes over nearly the entire temperature range. It has been shown that enhanced dielectric parameters are associated with another brownmillerite oxide, Ca2FeCoO5, in which nonaxial magnetic anisotropy is found within a narrow temperature range, 120-250 K. In the materials prepared in this study, this behavior should be evident at ambient temperature and over a wide temperature range.

We report field -dependent high -resolution inelastic neutron scattering (INS) measurements on the honeycomb lattice magnet, CoTiO 3 , to study the evolution of its magnon excitations across a spin reorientation transition driven by an in -plane magnetic field. By carrying out elastic neutron scattering in a magnetic field, we show that the sample transitions from a collinear antiferromagnetic state with multiple magnetic domains at a low field to a monodomain state with a canted magnetic structure at a high field. Concurrent with this transition, we observed significant changes in both the energy and the width of the zone center magnon peak. The observed width change was argued to be consistent with an unusual zero -field state with extended domain walls. On the other hand, the magnon spectra near the K point of the Brillouin zone boundary were found to be largely insensitive to the changes in the ordered moment directions and the domain configuration. We argue that this observation is difficult to explain within the framework of the bond -dependent model proposed in a recent INS study [M. Elliot et al. , Nat. Commun. 12 , 3936 (2021)]. Our paper therefore calls for alternative explanations for the observed K -point gap in CoTiO 3 .

We report a resonant inelastic x-ray scattering investigation of ultrathin epitaxial films of Ba2IrO4, and compare their low-energy magnetic and spin-orbit excitations to those of their sister compound Sr2IrO4. Due to the 180 degrees Ir-O-Ir bond, the bandwidth of the magnon and spin orbiton is significantly larger in Ba2IrO4, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr2IrO4 is absent in Ba2IrO4, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba2IrO4 are substantially different from those of Sr2IrO4, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

The magnetism of the rocksalt fcc rare-earth monopnictide HoBi, a candidate topological material with extreme magnetoresistance, is investigated. From the Ho3+ non-Kramers J = 8 spin-orbital multiplet, the cubic crystal electric field yields six nearly degenerate low-energy levels. These constitute an anisotropic magnetic moment with a Jahn-Teller-like coupling to the lattice. In the cubic phase for T > TN = 5.72(1) K, the paramagnetic neutron scattering is centered at k = (1 1 2 ) and was fit to dominant antiferromagnetic interactions between 2 2 Ho spins separated by {100} and ferromagnetic interactions between spins displaced by { 1 20}. For T < TN, a type-II AFM long-range order with k = (1 1 2 ) develops along with a tetragonal lattice distortion. While neutron 2 2 diffraction from a multidomain sample cannot unambiguously determine the spin orientation within a domain, the bulk magnetization, structural distortion, and our measurements of the magnetic excitations all show the easy axis coincides with the tetragonal axis. The weakly dispersive excitons for T < TN can be accounted for by a spin Hamiltonian that includes the crystal electric field and exchange interactions within the random phase approximation.

We report inelastic neutron scattering (INS) measurements of the phonon dispersion relation in higher manganese silicides (HMSs). A large ingot of HMS is synthesized using a slow cooling method, which is found to have Mn15Si26 as the primary phase. The sample is composed of highly oriented crystallites as confirmed by a neutron pole-figure study and thermal conductivity data. Our INS results are mostly consistent with earlier experimental and theoretical phonon studies in HMS, including the presence of a low-lying twisting mode. However, some discrepancies are also observed. Most notably, a 5 meV gap at the zone center and the softer dispersion relation of the low-lying twisting mode. We discuss the potential origins of these observations and their implications for the thermal properties of HMS.

The pyrochlore magnet Ce2Zr2O7 has attracted much attention as a quantum spin ice candidate whose novelty derives in part from the dipolar-octupolar nature of the Ce3+ pseudospin-1/2 degrees of freedom it possesses. We report heat capacity measurements on single crystal samples of Ce2Zr2O7 down to T similar to 0.1K in a magnetic field along the [1, over bar 1,0] direction. These measurements show that the broad hump in the zero-field heat capacity moves higher in temperature with increasing field strength and is split into two separate humps by the [1, over bar 1,0] magnetic field at similar to 2T. These separate features are due to the decomposition of the pyrochlore lattice into effectively decoupled chains for fields in this direction: One set of chains (alpha chains) is polarized by the field while the other (beta chains) remains free. This situation is similar to that observed in the classical spin ices Ho2Ti2O7 and Dy2Ti2O7, but with the twist that here the strong transverse exchange interactions produce substantial quantum effects. Our theoretical modeling suggests that the beta chains are close to a critical state, with nearly-gapless excitations. We also report elastic and inelastic neutron scattering measurements on single crystal Ce2Zr2O7 in [1, over bar 1,0] and [0,0,1] magnetic fields at temperatures down to T=0.03K. The elastic scattering behaves consistently with the formation of independent chains for a [1, over bar 1,0] field, while the [0,0,1] field produces a single field-induced elastic magnetic Bragg peak at (0,2,0) and equivalent wavevectors, indicating a polarized spin ice state for fields above similar to 3T. For both [1, over bar 1,0] and [0,0,1] magnetic fields, our inelastic neutron scattering results show an approximately-dispersionless continuum of scattering that increases in both energy and intensity with increasing field strength. By modeling the complete set of experimental data using numerical linked cluster and semiclassical molecular dynamics calculations, we demonstrate the dominantly multipolar nature of the exchange interactions in Ce2Zr2O7 and the smallness of the parameter theta, which controls the mixing between dipolar and octupolar degrees of freedom. These results support previous estimates of the microscopic exchange parameters and place strong constraints on the theoretical description of this prominent spin ice candidate.

We report a resonant inelastic x-ray scattering (RIXS) investigation of ultra-thin epitaxial films of Ba 2 IrO 4 , and compare their low energy magnetic and spin-orbit excitations to those of their sister compound Sr 2 IrO 4 . Due to the 180 ◦ Ir-O-Ir bond, the bandwidth of the magnon and spin-orbiton is significantly larger in Ba 2 IrO 4 , making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr 2 IrO 4 is absent in Ba 2 IrO 4 , as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba 2 IrO 4 are substantially different from those of Sr 2 IrO 4 , suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

In his comment [arXiv:2209.03235], S. W. Lovesey argues that our analysis of neutron scattering experiments performed on Ce$_2$Zr$_2$O$_7$ is invalid. Lovesey argues that we have not properly accounted for the higher-order multipolar contributions to the magnetic scattering and that our use of pseudospin-$1/2$ operators to describe the scattering is inappropriate. In this reply, we show that the multipolar corrections discussed by Lovesey only become significant at scattering wavevectors exceeding those accessed in our experiments. This in no way contradicts or undermines our work, which never claimed a direct observation of scattering from higher-order multipoles. We further show that Lovesey's objections to our use of pseudospins are unfounded, and that the pseudospin operators are able to describe all magnetic scattering processes at the energy scale of our experiments, far below the crystal field gap. Finally, we comment on certain assumptions in Lovesey's calculations of the scattering amplitude which are inconsistent with experiment.

The Ce$^{3+}$ pseudospin-$\frac{1}{2}$ degrees of freedom in the pyrochlore magnet Ce$_2$Zr$_2$O$_7$ are known to possess dipole-octupole (DO) character, making it a candidate for novel quantum spin liquid (QSL) ground states at low temperatures. We report new polarized neutron diffraction at low temperatures, as well as heat capacity ($C_p$) measurements on single crystal Ce$_2$Zr$_2$O$_7$. The former bears both similarities and differences from that measured in the canonical dipolar spin ice compound Ho$_2$Ti$_2$O$_7$, while the latter rises sharply at low temperatures, initially plateauing near 0.08 K, before falling off towards a high temperature zero beyond 3 K. Above $\sim$0.5 K, the $C_p$ data set can be fit to the results of a quantum numerical linked cluster (NLC) calculation, carried out to 4$^{\mathrm{th}}$ order, that allows estimates for the terms in the near-neighbour XYZ Hamiltonian expected for such DO pyrochlore systems. Fits of the same theory to the temperature dependence of the magnetic susceptibility and unpolarized neutron scattering complement this analysis. A comparison between the resulting best fit NLC calculation and the polarized neutron diffraction shows both agreement and discrepancies, mostly in the form of zone-boundary diffuse scattering in the non-spin flip channel, which are attributed to interactions beyond near-neighbours. The lack of an observed thermodynamic anomaly and the constraints on the near-neighbour XYZ Hamiltonian suggest that Ce$_2$Zr$_2$O$_7$ realizes a U(1)$_\pi$ QSL state at low temperatures, and one that likely resides near the boundary between dipolar and octupolar character.

We present the synthesis and physical properties of a new breathing pyrochlore magnet CuAlCr$_4$S$_8$ with the help of synchrotron x-ray diffraction (XRD), magnetization under ambient and applied hydrostatic pressure, heat capacity, and muon spin relaxation/rotation ($\mu$SR) measurements. CuAlCr$_4$S$_8$ exhibits positive thermal expansion with concave upward temperature dependence. We observed a sharp antiferromagnetic ordering transition of a purely magnetic nature at 20 K, which shifts by as much as 3.2 K on the application of 600 MPa pressure. The breathing factor (B$_f$ = $J'/J$) in breathing pyrochlores can be an important parameter to tune the magnetic ground states of the pyrochlore lattice. The breathing factor can be modulated through breathing ratio, the ratio of sizes of the two tetrahedra, by using different elements at A and A' sites in the breathing pyrochlore structure. We find that CuAlCr$_4$S$_8$ has a breathing ratio of 1.0663(8), which is comparable to other sulfur breathing pyrochlores.

Iridates with the 5$d^4$ electronic configuration have attracted recent interest due to reports of magnetically-ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $J = 0$ electronic ground state for each Ir$^{5+}$ ion. The major focus of prior research has been on the double perovskite iridates Ba$_2$YIrO$_6$ and Sr$_2$YIrO$_6$, where the nature of the ground states (i.e. ordered vs non-magnetic) is still controversial. Here we present neutron powder diffraction, high energy resolution fluorescence detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates Ba$_2$LuIrO$_6$, Sr$_2$LuIrO$_6$, Ba$_2$ScIrO$_6$, and Sr$_2$ScIrO$_6$ that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $J = 0$ electronic ground states for the Ir$^{5+}$ ions in all cases, with similar values for Hund's coupling $J_{\rm H}$ and the spin-orbit coupling constant $\lambda_{\rm SOC}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do reveal weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large $\lambda_{\rm SOC}$ is the key driving force behind the electronic and magnetic ground states realized in the 5$d^4$ double perovskite iridates, which agrees well with conventional wisdom.

We report a resonant inelastic x-ray scattering (RIXS) investigation of ultra-thin epitaxial films of Ba_2IrO_4, and compare their low energy magnetic and spin-orbit excitations to those of their sister compound Sr_2IrO_4. Due to the 180^∘ Ir-O-Ir bond, the bandwidth of the magnon and spin-orbiton is significantly larger in Ba_2IrO_4, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr_2IrO_4 is absent in Ba_2IrO_4, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba_2IrO_4 are substantially different from those of Sr_2IrO_4, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

The phase transition from graphite to diamond is an appealing object of study because of many fundamental and also, practical reasons. The out-of-plane distortions required for the transition are a good tool to understand the collective behaviour of layered materials (graphene, graphite) and the van der Waals forces. As today, two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. They induce it by increasing either pressure or temperature on graphite. In this work, we report a third method consisting in the irradiation of graphite with ultraviolet photons of energies above 4.4 eV. We show high resolution electron microscopy images of pyrolytic carbon evidencing the dislocation of the superficial graphitic layers after irradiation and the formation of crystallite islands within them. Electron energy loss spectroscopy of the islands show that the sp2 to sp3 hybridation transition is a surface effect. High sensitivity X-ray diffraction experiments and Raman spectroscopy confirm the formation of diamond within the islands.

Topologically nontrivial spin textures host great promise for future spintronic applications. Skyrmions in particular are of burgeoning interest owing to their nanometric size, topological protection, and high mobility via ultra-low current densities. It has been previously reported through magnetic susceptibility, microscopy, and scattering techniques that Co8Zn8Mn4 forms an above room temperature triangular skyrmion lattice. Here, we report the synthesis procedure and characterization of a polycrystalline Co8Zn8Mn4 disordered bulk sample. We employ powder X-ray diffraction and backscatter Laue diffraction as characterization tools of the crystallinity of the samples, while magnetic susceptibility and Small Angle Neutron Scattering (SANS) measurements are performed to study the skyrmion phase. Magnetic susceptibility measurements show a dip anomaly in the magnetization curves, which persists over a range of approximately 305 K–315 K. SANS measurements reveal a rotationally disordered polydomain skyrmion lattice. Applying a symmetry-breaking magnetic field sequence, we were able to orient and order the previously jammed state to yield the prototypical hexagonal diffraction patterns with secondary diffraction rings. This emergence of the skyrmion order serves as a unique demonstration of the fundamental interplay of structural disorder and anisotropy in stabilizing the thermal equilibrium phase.

Using neutron spectroscopy, the authors map out the full magnon dispersion in the easy-plane antiferromagnet Bi${}_{2}$CuO${}_{4}$ and determine its complete spin Hamiltonian. In particular, they solve a long-standing puzzle in this material by determining its small anisotropic spin interactions. Equipped with the complete set of interaction parameters, the authors establish Bi${}_{2}$CuO${}_{4}$ as a rare example of quantum order by disorder, where quantum fluctuations break the in-plane spin degeneracy and are manifested as a spin-flop transition observed by neutron scattering.

A new compound, Mg3Ge1-δO4(1-δ)F2(1+2δ) was obtained by high temperature solid state reactions and flux method and its structure was characterized by single crystal X-ray diffraction. The structure relates to the silicate mineral norbergite, Mg3SiO4(OH)F, a member of the humite mineral group. The diffraction experiment revealed a deficiency of the Ge site in the new structure, which can be compensated by the substitution of either OH− groups or F− anions for O2− anions. The combination of the NMR, neutron scattering, and IR spectroscopy experiments suggest that the F− substitution is most likely; thus, the Mg3Ge1-δO4(1-δ)F2(1+2δ) formula is suggested. The F− substitution is present for the side product of the Mg3Ge1-δO4(1-δ)F2(1+2δ) synthesis, the compound known as Mg28Ge7.5O38F10. Refinement of the twined Mg28Ge7.5O38F10 crystal showed a slightly higher Ge content and the final composition was assigned as Mg28Ge8O40F8. The PbF2 flux method yielded a new compound, Mg2Pb2Ge2O7F2. The structure of this digermanate is related to the group of naturally occurring and synthetic silicates containing R3+ cations. High-temperature powder XRD experiments showed that the Mg3Ge1-δO4(1-δ)F2(1+2δ) phase cannot be synthesized from Mg2GeO4 and MgF2 in air due to the transformation of magnesium fluoride into MgO.

The electronic structure of LiNiO$_2$, a promising Li-ion battery cathode material, has remained a challenge to understand due to its highly covalent yet correlated nature. Here we elucidate the electronic structure in LiNiO$_2$ and the related compound NaNiO$_2$ using x-ray absorption spectra (XAS) and quantum many-body calculations. Notably, we use inverse partial fluorescence yield to correctly measure the Ni $L$-edge XAS, which is inaccurate using conventional methods. We show that the XAS are indicative of a strong Jahn-Teller effect in NaNiO$_2$ and a bond disproportionated state in LiNiO$_2$, supporting a theory of a high-entropy, glassy disproportionated state that stabilizes charging cycles in LiNiO$_2$.

Heavy transition metal magnets with J(eff) = 1/2 electronic ground states have attracted recent interest due to their penchant for hosting new classes of quantum spin liquids and superconductors. Unfortunately, model systems with ideal J(eff) = 1/2 states are scarce due to the importance of noncubic local distortions in most candidate materials. In this work, we identify a family of iridium halide systems [i.e., K2IrCl6, K2IrBr6, (NH4)(2)IrCl6, and Na2IrCl6.6(H2O)] with Ir-4(+) electronic ground states exhibiting extremely small deviations from the ideal J(eff) =1 limit. We also find ordered magnetic ground states for the three anhydrous systems, with single-crystal neutron diffraction on K2IrBr6 revealing type-I antiferromagnetism. This spin configuration is consistent with expectations for significant Kitaev exchange in a face-centered-cubic magnet. This work establishes that incorporating isolated IrX6 octahedra in materials, where X is a halogen ion with a low electronegativity, is an effective design principle for realizing unprecedented proximity to the pure J(eff) = state. At the same time, we highlight undeniable deviations from this ideal state, even in clean materials with ideal IrX6 octahedra as inferred from the global cubic crystal structures.

We carried out inelastic neutron scattering to study the spin-orbital (SO) exciton in a single crystal sample of CoTiO_3 as a function of temperature. CoTiO_3 is a honeycomb magnet with dominant XY-type magnetic interaction and an A-type antiferromagnetic order below T_N≈ 38 K. We found that the SO exciton becomes softer, but acquires a larger bandwidth in the paramagnetic phase, compared to that in the magnetically ordered phase. Moreover, an additional mode is only observed in the intermediate temperature range, as the sample is warmed up above the lowest accessible temperature below T_N. Such an unusual temperature dependence observed in this material suggests that its ground states (an S_eff=1/2 doublet) and excited states multiplets are strongly coupled, and therefore cannot be treated independently, as often done in a pseudo-spin model. Our observations can be explained by a multi-level theory within random phase approximation that explicitly takes into account both the ground and excited multiplets. The success of our theory, which is originally developed to explain temperature dependence of magnetic excitations in the rare-earth magnets, highlight the similarity between the magnetic excitations in rare-earth systems and those in transition metal systems with strong spin orbit coupling.