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The recent discovery of superconductivity in infinite layer thin films and bulk Ruddlesden-Popper nickelates has stimulated the investigation of other predicted properties of these materials. Among them, the existence of magnetism-driven ferroelectricity in the parent compounds RNiO3 (R = 4f lanthanide and Y) at the onset of the N\'eel order, TN, has remained particularly elusive. Using diffraction techniques, we reveal here the existence of magnetostriction at TN in bulk YNiO3 single crystals. Interestingly, the associated lattice anomalies are much more pronounced along the b crystal axis, which coincides with the electric polarization direction expected from symmetry arguments. This axis undergoes an abrupt contraction below TN that reaches deltab/b ~ - 0.01 %, a value comparable to those found in some magnetoresistive manganites and much larger than those reported for magnetism-driven multiferroics. This observation suggests a strong spin-lattice coupling in these materials, consistent with theoretical predictions. Using the symmetry-adapted distortion mode formalism we identify the main ionic displacements contributing to the lattice anomalies and discuss the most likely polar displacements below TN. Furthermore, our data support symmetric superexchange as the most likely mechanism responsible for the magnetoelastic coupling. These results, that may be common to the full RNiO3 family, provide new experimental evidence supporting the predicted existence of magnetism-driven ferroelectricity in RNiO3 perovskites, Comment: 36 pages, 6 figures

Layered perovskites of general formula AA'CuFeO$_5$ are one of the few examples of cycloidal spiral magnets where the ordering temperatures $T_{spiral}$ can be tuned far beyond room temperature by introducing modest amounts of Cu/Fe chemical disorder in the crystal structure. This rare property makes these materials prominent candidates to host multiferroicity and magnetoelectric coupling at room temperature. Moreover, it has been proposed that the highest $T_{spiral}$ value that can be reached in this structural family ($\sim$ 400 K) corresponds to a paramagnetic-collinear-spiral triple point with potential to show exotic physics. Since generating high amounts of Cu/Fe disorder is experimentally difficult, the phase diagram region beyond the triple point has been barely explored. To fill this gap we investigate here the YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ solid solutions ($0 \leq x \leq 1$), where we replace Ba with Sr with the aim of enhancing the impact of the experimentally available Cu/Fe disorder. Using a combination of bulk magnetization, synchrotron X-ray and neutron powder diffraction we show that the spiral state is destabilized beyond a critical degree of Cu/Fe disorder, being replaced by a non-frustrated, fully antiferromagnetic state with propagation vector k$_{c2}$ = $(\frac{1}{2}, \frac{1}{2}, 0)$ and ordering temperature $T_{coll2}$ $\geq$ $T_{spiral}$, which is progressively stabilized beyond the triple point. Interestingly, $T_{spiral}$ and $T_{coll2}$ increase with $x$ at the same rate. This suggests a common, disorder-driven origin, consistent with theoretical predictions.

We study the stability of the antiferromagnetic all-in--all-out state observed in dipolar-octupolar pyrochlores that have neodymium as the magnetic species. Different types of disorder are considered, either affecting the immediate environment of the Nd$^{3+}$ ion, or substituting it with a non-magnetic ion. Starting from the well studied Nd$_2$Zr$_2$O$_7$ compound, Ti substitution on the Zr site and dilution on the Nd magnetic site with La substitution are investigated. The recently discovered entropy stabilized compound NdMox, which exhibits a high degree of disorder on the non magnetic site is also studied. Using a range of experimental techniques, especially very low-temperature magnetization and neutron scattering, we show that the all-in--all-out state is very robust and withstands substitutional disorder up to large rates. From these measurements, we estimate the Hamiltonian parameters and discuss their evolution in the framework of the phase diagram of dipolar-octupolar pyrochlore magnets., Comment: 19 pages, 20 figures (main text: 12 pages, 13 figures)

Magnetic double perovskite compounds provide a fertile playground to explore interesting electronic and magnetic properties. By complementary macroscopic characterizations, neutron powder diffraction measurements and first-principles calculations, we have performed comprehensive studies on the magnetic ordering in the double perovskite compound Sr$_2$NiWO$_6$. It is found by neutron diffraction to order magnetically in a collinear type-II antiferromagnetic structure in a tetragonal lattice with $k$ = (0.5, 0, 0.5) below $T\rm_N$ = 56 K. In the ground state, the ordered moment of the spin-1 Ni$^{2+}$ ions is determined to be 1.9(2) $\mu\rm_{B}$, indicating a significant quenching of the orbital moment. The Ni$^{2+}$ moments in Sr$_2$NiWO$_6$ are revealed to cant off the $c$ axis by 29.2$^{\circ}$, which is well supported by the first-principles magnetic anisotropy energy calculations. Furthermore, the in-plane and out-of-plane next-nearest-neighbor superexchange couplings ($J\rm_2$ and $J\rm_{2c}$) are found to play a dominant role in the spin Hamiltonian of Sr$_2$NiWO$_6$, which accounts for the stabilization of the type-II AFM structure as its magnetic ground state., Comment: 10 Pages, 6 figures, accepted for publication in Physical Review B

Aliovalent doping has been adopted to optimize the electrical properties of semiconductors, while its impact on the phonon structure and propagation is seldom paid proper attention to. This work reveals that aliovalent doping can be much more effective in reducing the lattice thermal conductivity of thermoelectric semiconductors than the commonly employed isoelectronic alloying strategy. As demonstrated in the heavy-band NbFeSb system, a large reduction of 65% in the lattice thermal conductivity is achieved through only 10% aliovalent Hf-doping, compared to the 4 times higher isoelectronic Ta-alloying. It is elucidated that aliovalent doping introduces free charge carriers and enhances the screening, leading to the giant softening and deceleration of optical phonons. Moreover, the heavy dopant can induce the avoided-crossing of acoustic and optical phonon branches, further decelerating the acoustic phonons. These results highlight the significant role of aliovalent dopants in regulating the phonon structure and suppressing the phonon propagation of semiconductors.

We report muon spin rotation ($\mu$SR) and neutron diffraction on the rare-earth based magnets (Mo$_{2/3}$RE$_{1/3}$)$_2$AlC, also predicted as parent materials for 2D derivatives, where RE = Nd, Gd (only ($\mu$SR), Tb, Dy, Ho and Er. By crossing information between the two techniques, we determine the magnetic moment ($m$), structure, and dynamic properties of all compounds. We find that only for RE = Nd and Gd the moments are frozen on a microsecond time scale. Out of these two, the most promising compound for a potential 2D high ($m$) magnet is the Gd variant, since the parent crystals are pristine with $m = 6.5 \pm 0.5 \mu_B$, N\'eel temperature of $29 \pm 1$ K, and the magnetic anisotropy between in and out of plane coupling is smaller than $10^{-8}$. This result suggests that magnetic ordering in the Gd variant is dominated by in-plane magnetic interactions and should therefore remain stable if exfoliated into 2D sheets., Comment: Includes Supplementary Information

We investigated the influence of oxygen over-stoichiometry on apical oxygen disorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperature range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron single crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystalizes in a tetragonal commensurate structure with the P42/ncm space group with excess oxygen atoms occupy the 4b (3/4 1/4 1/4) interstitial sites, coordinated by four apical oxygen atoms. Large and anisotropic thermal displacement parameters are found for equatorial and apical oxygen atoms, which are strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase. Maximum Entropy analysis of the neutron single crystal diffraction data uncovered anharmonic contributions to the displacement parameters of the apical oxygen atoms, toward the nearest vacant 4b interstitial site, related to the phonon assisted oxygen diffusion mechanism. Macroscopic magnetization measurements and neutron powder diffraction studies reveal long-range antiferromagnetic ordering of the Ni-sublattice at TN ~ 53 K with a weak ferromagnetic component along the c-axis, while the long-range magnetic ordering of the Nd-sublattice occurs below 10 K. Temperature dependent neutron diffraction patterns show the appearance of a commensurate magnetic order at TN with the propagation vector k = (100) and the emergence of an additional incommensurate phase below 30 K, while both phases coexist at 2 K. The commensurate magnetic structure is best described by the P42/nc`m` Shubnikov space group. Refined magnetic moments of the Ni and Nd-sites at 2 K are 1.144(76) muB and 1.632(52) muB respectively. A possible origin of the incommensurate phase is discussed and a tentative magnetic phase diagram is proposed.

The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$ Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x=1, the compound crystallizes in the trigonal P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear antiferromagnetig structure with magnetic moments in the ab-plane. The compound with x=2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T$_C$ =47 K. For this compound a weakly non-collinear ferrimagnetic structure with two possible configurations in ab plane was derived from ND study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provide further information on the magnetic structure of the compounds., Comment: Published in Physical Review B, 8 pages, 8 figures

The quantum kagome antiferromagnets YCu3(OH)6OxCl3-x (x=0,1/3) are produced using a unified solid state synthesis route for polycrystalline samples. From structural refinements based on neutron diffraction data, we clarify the structure of the Y3Cu9(OH)18OCl8 (x=1/3) compound and provide a revised chemical formula. We use muon spin relaxation, as a local probe of magnetism, to investigate the exotic low temperature properties in the two compounds. In agreement with the low temperature neutron diffraction data, we find no evidence for long range ordering in both materials but they exhibit distinct ground states: while disordered static magnetism develops in the x=0 compound, we conclude on the stabilization of a quantum spin liquid in the x=1/3 one, since the local fields remain fully dynamical. Our findings are in contrast to previous reports based on thermodynamical measurements only. We then discuss their origin on the basis of structural details and specific heat measurements. In particular, the x=1/3 compound appears to realize an original spatially anisotropic kagome model., Comment: 9 pages, 8 figures

In insulating materials with ordered magnetic spiral phases, ferroelectricity can emerge due to the breaking of inversion symmetry. This property is of both fundamental and practical interest, in particular with a view to exploiting it in low-power electronic devices. Advances towards technological applications have been hindered, however, by the relatively low ordering temperatures $T_\mathrm{spiral}$ of most magnetic spiral phases, which rarely exceed 100 K. We have recently established that the ordering temperature of a magnetic spiral can be increased up to 310 K by the introduction of chemical disorder. Here we explore the design space opened up by this novel mechanism by combining it with a targeted lattice control of some magnetic interactions. In Cu-Fe layered perovskites we obtain $T_\mathrm{spiral}$ values close to 400 K, comfortably far from room temperature and almost 100 K higher than using chemical disorder alone. Moreover, we reveal a linear, universal relationship between the spiral's wave vector and the onset temperature of the spiral phase. This linear law ends at a paramagnetic-collinear-spiral triple point, which defines the highest spiral ordering temperature that can be achieved in this class of materials. Based on these findings, we propose a general set of rules for designing magnetic spirals in layered perovskites using external pressure, chemical substitutions and/or epitaxial strain, which should guide future efforts to engineer magnetic spiral phases with ordering temperatures suitable for technological applications., Comment: 5 figures, 35 pages, to be appeared on Science Advance

We present the crystal structure and magnetic properties of Y$_{3}$Cu$_{9}$(OH)$_{19}$Cl$_{8}$, a stoichiometric frustrated quantum spin system with slightly distorted kagome layers. Single crystals of Y$_{3}$Cu$_{9}$(OH)$_{19}$Cl$_{8}$ were grown under hydrothermal conditions. The structure was determined from single crystal X-ray diffraction and confirmed by neutron powder diffraction. The observed structure reveals two different Cu-positions leading to a slightly distored kagome layer in contrast to the closely related YCu$_{3}$(OH)$_{6}$Cl$_{3}$. Curie-Weiss behavior at high-temperatures with a Weiss-temperature $\theta_{W}$ of the order of $-100$ K, shows a large dominant antiferromagnetic coupling within the kagome planes. Specific-heat and magnetization measurements on single crystals reveal an antiferromagnetic transition at T$_{N}=2.2$ K indicating a pronounced frustration parameter of $\theta_{W}/T_{N}\approx50$. Optical transmission experiments on powder samples and single crystals confirm the structural findings. Specific-heat measurements on YCu$_{3}$(OH)$_{6}$Cl$_{3}$ down to 0.4 K confirm the proposed quantum spin-liquid state of that system. Therefore, the two Y-Cu-OH-Cl compounds present a unique setting to investigate closely related structures with a spin-liquid state and a strongly frustrated AFM ordered state, by slightly releasing the frustration in a kagome lattice.

In the past years, magnetism-driven ferroelectricity and gigantic magnetoelectric effects have been reported for a number of frustrated magnets with spiral magnetic orders. Such materials are of high current interest due to their potential for spintronics and low-power magnetoelectric devices. However, their low magnetic order temperatures (typically <100K) greatly restrict their fields of application. Here we demonstrate that the stability domain of the spiral phase in the perovskite YBaCuFeO5 can be enlarged by more than 150K through a controlled manipulation of the Fe/Cu chemical disorder. Moreover we show that this novel mechanism can stabilize the magnetic spiral state of YBaCuFeO5 above the symbolic value of 25{\deg}C at zero magnetic field. Our findings demonstrate that the properties of a magnetic spiral, including its wavelength and stability range, can be engineered through the control of chemical disorder, offering a great potential for the design of materials with magnetoelectric properties beyond room temperature., Comment: 4 Figures

We present a structural analysis of the substituted system (Ba$_{1-x}$Sr$_{x}$)CuSi$_{2}$O$_{6}$, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with lowtemperature neutron and synchrotron powder diffraction, room-temperature and cryogenic highresolution NMR, as well as magnetic- and specific-heat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi2O6, is absent for the x = 0.1 sample. Furthermore, synchrotron powder-diffraction patterns show a reduction of the unit cell for x = 0.1 and magnetic measurements prove that the Cu-dimers are preserved, yet with a slightly reduced intradimer coupling Jintra. Pulse-field magnetization measurements reveal the emergence of a field-induced ordered state, tantamount to Bose-Einsteincondensation (BEC) of triplons, within the tetragonal crystal structure of $I\,4_{1}/acd$. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.

Transition metal oxides with 4d or 5d metals are of great interest due to the competing interactions, of the Coulomb repulsion and the itineracy of the d-electrons, opening a possibility of building new quantum ground states. Particularly the 5d metal oxides containing Iridium have received significant attention within the last years, due to their unexpected physical properties, caused by a strong spin orbit coupling observed in Ir(IV). A prominent example is the Mott-insulator Sr2IrO4. Another member of this family, the honeycomb lattice compound Na2IrO3, also being a Mott-insulator having, most probably, a Kitaev spin liquid ground state. By deintercalating sodium from Na2IrO3, we were able to synthesize a new honeycomb lattice compound with more than 50% reduced sodium content. The reduction of the sodium content in this layered compound leads to a change of the oxidation state of iridium from +IV to +V/+VI and a symmetry change from C2/c to P-3. This goes along with significant changes of the physical properties. Besides the vanishing magnetic ordering at 15K, also the transport properties changes and instead insulating semiconducting properties are observed., Comment: 17 pages, 4 figures, 1 scheme, 3 tables

The spin dimer system $\mathrm{Ba}_{3-x}\mathrm{Sr}_x\mathrm{Cr_2O_8}$ is a solid solution of the triplon Bose-Einstein condensation candidates $\mathrm{Ba_3Cr_2O_8}$ and $\mathrm{Sr_3Cr_2O_8}$. The magnetic intradimer interaction constant $J_0$ in this spin system can be tuned by varying the Sr content $x$. Very interestingly, this variation of $J_0$ with $x$ is highly nonlinear. In the present study, we show that this peculiar behavior of $J_0$ can be only partly explained by the changes in the average crystal structure alone. We report on neutron powder diffraction experiments to probe the corresponding structural details. Performing extended H\"{u}ckel tight binding calculations based on those structural details obtained at liquid helium temperatures, we found that the change of the magnetic interaction constant can be well reproduced by taking into account the presence of a structural transition due to the Jahn-Teller active Cr$^{5+}$-ions. This transition, lifting the orbital degeneracy and thereby the magnetic frustration in the system, is heavily influenced by disorder in the system arising from partially exchanging Ba with Sr.

The crystal and magnetic structures and underlying magnetic interactions of Bi4Fe5O13F, a model system for studying the physics of the Cairo pentagonal spin lattice, are investigated by transmission electron microscopy, low-temperature synchrotron x-ray and neutron powder diffraction, thermodynamic measurements, and density functional band-structure calculations. The crystal structure of Bi4Fe5O13F contains infinite rutile-like chains of edge-sharing FeO6 octahedra interconnected by the Fe2O7 groups of two corner-sharing FeO4 tetrahedra. The cavities between the chains are filled with the fluorine-centered Bi4F tetrahedra. The Fe3+ cations form pentagonal units that give rise to an unusual topology of frustrated exchange couplings and underlie a sequence of the magnetic transitions at T1= 62 K, T2 = 71 K, and TN = 178 K. Below T1, Bi4Fe5O13F forms a fully ordered non-collinear antiferromagnetic structure, whereas the magnetic state between T1 and TN may be partially disordered according to the sizable increase in the magnetic entropy at T1 and T2. Therefore, Bi4Fe5O13F shows the evidence of intricate magnetic transitions that were never anticipated for the pentagonal Cairo spin lattice. Additionally, it manifests a sillimanite (Al2SiO5)-based homologous series of compounds that feature the pentagonal magnetic lattice spaced by a variable number of octahedral units along the rutile-type chains., Comment: 11 pages, 11 figures, 5 tables. Supplementary material included in the submission