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Transition metal compounds can feature different types of magnetic anisotropy. By studying a honeycomb-lattice material with disconnected transition-metal octahedra, the authors reveal the crucial role of the single-ion anisotropy term that arises despite the nominally quenched orbital moment of the ${d}^{3}$ magnetic ion. Their findings suggest that the effect of single-ion anisotropy should not be ignored when searching for signatures of Kitaev interactions in spin-3/2 honeycomb magnets.

Two $\mathrm{CaMn}B\mathrm{Sb}{\mathrm{O}}_{6}$ ($B=\mathrm{Cr}$ and Fe) high-pressure double double perovskites are reported. Spins in $\mathrm{Ca}\mathrm{Mn}\mathrm{Cr}\mathrm{Sb}{\mathrm{O}}_{6}$ order below ${T}_{\mathrm{C}}=49\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ into a collinear ferrimagnetic arrangement with spins in the $xy$ plane. $\mathrm{Ca}\mathrm{Mn}\mathrm{Fe}\mathrm{Sb}{\mathrm{O}}_{6}$ has the same ordered magnetic structure below ${T}_{\mathrm{C}1}=55$ K, but also shows a second magnetic transition at ${T}_{\mathrm{C}2}=21\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ where the spins reorient towards the $z$ axis. This may reflect a greater degree of magnetic disorder for $B=\mathrm{Fe}$ and A-B intersite charge transfer may also be significant.

Polycrystalline members of the ${\text{GaV}}_{4}{\text{S}}_{8\ensuremath{-}y}{\text{Se}}_{y}$ family of materials with small levels of substitution between $0\ensuremath{\le}y\ensuremath{\le}0.5$ and $7.5\ensuremath{\le}y\ensuremath{\le}8$ have been synthesized in order to investigate their magnetic and structural properties. Substitutions to the skyrmion hosting parent compounds ${\text{GaV}}_{4}{\text{S}}_{8}$ and ${\text{GaV}}_{4}{\text{Se}}_{8}$ are found to suppress the temperature of the cubic-to-rhombohedral structural phase transition that occurs in both end compounds and to create a temperature region around the transition where there is a coexistence of these two phases. Similarly, the magnitude of the magnetization and temperature of the magnetic transition are both suppressed in all substituted compounds until a glassy-like magnetic state is realized. There is evidence from the ac susceptibility data that skyrmion lattices with similar dynamics to those in ${\text{GaV}}_{4}{\text{S}}_{8}$ and ${\text{GaV}}_{4}{\text{Se}}_{8}$ are present in compounds with very low levels of substitution, $0lyl0.2$ and $7.8lyl8$, however, these states vanish at higher levels of substitution. The magnetic properties of these substituted materials are affected by the substitution altering exchange pathways and resulting in the creation of increasingly disordered magnetic states.

We report magnetic properties of a $3{d}^{9}$ $({\mathrm{Cu}}^{2+})$ magnetic insulator ${\mathrm{Cu}}_{2}{\mathrm{OSO}}_{4}$ measured on both powder and single crystal. The magnetic atoms of this compound form layers whose geometry can be described either as a system of chains coupled through dimers or as a kagome lattice where every third spin is replaced by a dimer. Specific heat and DC susceptibility show a magnetic transition at 20 K, which is also confirmed by neutron scattering. Magnetic entropy extracted from the specific heat data is consistent with an $S=1/2$ degree of freedom per ${\mathrm{Cu}}^{2+}$, and so is the effective moment extracted from DC susceptibility. The ground state has been identified by means of neutron diffraction on both powder and single crystal and corresponds to an $\ensuremath{\sim}{120}^{\ensuremath{\circ}}$ spin structure in which ferromagnetic intradimer alignment results in a net ferrimagnetic moment. No evidence is found for a change in lattice symmetry down to 2 K. Our results suggest that ${\mathrm{Cu}}_{2}{\mathrm{OSO}}_{4}$ represents a type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored.

Several previous studies reported that a one-dimensional Heisenberg chain model is inadequate in describing the magnetic properties of the low-dimensional quantum antiferromagnet $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$, but the origin for this observation has remained unclear. We have reinvestigated the magnetic properties of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ and found that our anisotropic magnetic susceptibility, neutron-powder diffraction, and electron paramagnetic spin-resonance measurements are in good agreement with extensive density-functional theory ($\mathrm{DFT}+U$) total energy calculations which indicate that the correct spin lattice model for $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ is rather a $S=1/2$ 2D-Heisenberg antiferromagnetic lattice. The magnetic susceptibility data are well described by a rectangular Heisenberg antiferromagnet with anisotropy ratio $\ensuremath{\alpha}\ensuremath{\sim}$ 0.7 consistent with the DFT results. Quantum Monte Carlo simulations of the magnetic susceptibilities for a rectangular lattice Heisenberg antiferromagnet were performed in the anisotropy range 0.5 $\ensuremath{\le}\ensuremath{\alpha}\ensuremath{\le}$ 1.0. The results of the Quantum Monte Carlo calculations were cast into a Pad\'e approximant which was used to fit the temperature-dependent magnetic susceptibility data. Neutron-powder-diffraction measurements were used to conclusively solve the collinear antiferromagnetic structure of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ below the N\'eel temperature of $\ensuremath{\sim}22.4$ K.

In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams, still to be explored in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $\sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.

We study magnetic behavior of the ${\mathrm{Yb}}^{3+}$ ions on a frustrated pyrochlore lattice in the spinel ${\mathrm{CdYb}}_{2}{\mathrm{Se}}_{4}$. The crystal-electric-field parameters deduced from high-energy inelastic neutron scattering reveal a well-isolated ytterbium ground-state doublet with a weakly Ising character. Magnetic order studied by powder neutron diffraction evolves from the XY-type antiferromagnetic ${\mathrm{\ensuremath{\Gamma}}}_{5}$ state to a splayed icelike ferromagnet (both with $\mathbf{k}=0$) in applied magnetic field with ${B}_{c}=3\phantom{\rule{0.28em}{0ex}}\mathrm{T}$. Low-energy inelastic neutron scattering identifies weakly dispersive magnetic bands around 0.72 meV starting at $|\mathbf{Q}|=1.1\phantom{\rule{4pt}{0ex}}{\AA{}}^{\ensuremath{-}1}$ at zero field, which diminish with field and vanish above 3 T. We explain the observed magnetic behavior in framework of the nearest-neighbor anisotropic exchange model for effective $S=\frac{1}{2}$ Kramers doublets on the pyrochlore lattice. The estimated exchanges position the ${\mathrm{CdYb}}_{2}{\mathrm{Se}}_{4}$ spinel close to the phase boundary between the ${\mathrm{\ensuremath{\Gamma}}}_{5}$ and splayed ferromagnet states, similar to the Yb pyrochlores, suggesting an important role of the competition between these phases.

Formation, crystal structure, and macroscopic and microscopic magnetism of the binary $\mathrm{T}{\mathrm{b}}_{3}\mathrm{N}{\mathrm{i}}_{2}$ and derivative pseudobinary $\mathrm{T}{\mathrm{b}}_{3}\mathrm{C}{\mathrm{o}}_{x}\mathrm{N}{\mathrm{i}}_{2\ensuremath{-}x}$ phases have been investigated using an array of experimental methods. While $\mathrm{T}{\mathrm{b}}_{3}\mathrm{N}{\mathrm{i}}_{2}$ crystallizes in the monoclinic $\mathrm{D}{\mathrm{y}}_{3}\mathrm{N}{\mathrm{i}}_{2}$ structure type ($\mathit{mS}20, C2$/m), the substitution of Co for Ni results in a structural transition into the rhombohedral $\mathrm{E}{\mathrm{r}}_{3}\mathrm{N}{\mathrm{i}}_{2}$ type ($\mathit{hR}\mathrm{45}, R\overline{3}h$) at $x(\mathrm{Co})\ensuremath{\approx}0.34$ and beyond in the $\mathrm{T}{\mathrm{b}}_{3}\mathrm{C}{\mathrm{o}}_{x}\mathrm{N}{\mathrm{i}}_{2\ensuremath{-}x}$ system. In both the monoclinic and rhombohedral phases, the addition of Co leads to an anisotropic change of lattice parameters and unexpected reduction of the cell volume. Measurements of bulk properties reveal that these compounds order ferrimagnetically or ferromagnetically at about 100 K. Complex noncollinear ferromagnetic ordering in the Tb sublattices is weakly dependent on composition. For $x$ g 0.34 the long-range magnetic ordering leads to a strong anisotropic magnetostriction accompanied by a symmetry reduction from rhombohedral to triclinic.

Barlowite, ${\mathrm{Cu}}_{4}{(\mathrm{OH})}_{6}\mathrm{FBr}$, has attracted much attention as the parent compound of a new series of quantum spin-liquid candidates, ${\mathrm{Zn}}_{x}{\mathrm{Cu}}_{4\ensuremath{-}x}{(\mathrm{OH})}_{6}\mathrm{FBr}$. While it is known to undergo a magnetic phase transition to a long-range ordered state at ${T}_{N}=15$ K, there is still no consensus over either its nuclear or magnetic structures. Here, we use comprehensive, high-flux powder neutron diffraction studies on deuterated samples of barlowite to demonstrate that the only space group consistent with the observed nuclear and magnetic diffraction at low temperatures is the orthorhombic $Pnma$ space group. We furthermore conclude that the magnetic intensity at $Tl{T}_{N}$ is correctly described by the $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}a$ magnetic space group, which crucially allows the ferromagnetic component observed in previous single-crystal and powder magnetization measurements. As such, the magnetic structure of barlowite resembles that of the related material clinoatacamite, ${\mathrm{Cu}}_{4}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$, the parent compound of the well-known quantum spin-liquid candidate hebertsmithite, ${\mathrm{ZnCu}}_{3}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$.

Magnetic behavior of BaMoP$_2$O$_8$ with the spatially anisotropic triangular arrangement of the $S=1$ Mo$^{4+}$ ions is explored using thermodynamic measurements, neutron diffraction, and density-functional band-structure calculations. A broad maximum in the magnetic susceptibility around 46\,K is followed by the stripe antiferromagnetic order with the propagation vector ${\mathbf k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ formed below $T_N\simeq 21$ K. This stripe phase is triggered by a pronounced one-dimensionality of the spin lattice, where one of the in-plane couplings, $J_2\simeq 4.6$ meV, is much stronger than its $J_1\simeq 0.4$ meV counterpart, and stabilized by the weak easy-axis anisotropy. The ordered moment of 1.42(9) $\mu_B$ at 1.5 K is significantly lower than the spin-only moment of 2 $\mu_B$ due to a combined effect of quantum fluctuations and spin-orbit coupling.

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr_{2}Se_{4} is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three orders of magnitude faster than the canonical spin ice Dy_{2}Ti_{2}O_{7}. In this Letter we use diffuse neutron scattering to show that both CdEr_{2}Se_{4} and CdEr_{2}S_{4} support a dipolar spin ice state-the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to those in Dy_{2}Ti_{2}O_{7}, i.e., dilute and uncorrelated, and so cannot provide three orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er^{3+} ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr_{2}X_{4} (X=Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr_{2}X_{4} offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.

We present the results of the investigation of magnetic properties of the Er 2 Ti 2 − x Sn x O 7 series. For small doping values, the ordering temperature decreases linearly with x , while the moment configuration remains the same as in the x = 0 parent compound. Around x = 1.7 doping level, we observe a change in the behavior, where the ordering temperature starts to increase and new magnetic Bragg peaks appear. For the first time, we present evidence of a long-range order (LRO) in Er 2 Sn 2 O 7 ( x = 2.0 ) below T N = 130 mK. It is revealed that the moment configuration corresponds to a Palmer-Chalker type with a value of the magnetic moment significantly renormalized compared to x = 0 . We discuss our results in the framework of a possible quantum phase transition occurring close to x = 1.7 .

Magnetic systems with spins sitting on a lattice of corner sharing regular tetrahedra have been particularly prolific for the discovery of new magnetic states for the last two decades. The pyrochlore compounds have offered the playground for these studies, while little attention has been comparatively devoted to other compounds where the rare earth $R$ occupies the same sublattice, e.g., the spinel chalcogenides $\mathrm{Cd}{R}_{2}{X}_{4}$ ($X=\mathrm{S}$ or $\mathrm{Se}$). Here, we report measurements performed on powder samples of this series with $R=\mathrm{Yb}$ using specific heat, magnetic susceptibility, neutron diffraction, and muon-spin-relaxation measurements. The two compounds are found to be magnetically similar. They long-range order into structures described by the ${\mathrm{\ensuremath{\Gamma}}}_{5}$ irreducible representation. The magnitude of the magnetic moment at low temperature is 0.77 (1) and 0.62 (1) ${\ensuremath{\mu}}_{\mathrm{B}}$ for $X=\mathrm{S}$ and $\mathrm{Se}$, respectively. Persistent spin dynamics is present in the ordered states. The spontaneous field at the muon site is anomalously small, suggesting magnetic moment fragmentation. A double spin-flip tunneling relaxation mechanism is suggested in the cooperative paramagnetic state up to 10 K. The magnetic space groups into which magnetic moments of systems of corner-sharing regular tetrahedra order are provided for a number of insulating compounds characterized by null propagation wave vectors.

The crystal structure, magnetic ground state, and the temperature-dependent microscopic spin-spin correlations of the frustrated honeycomb lattice antiferromagnet $\mathrm{N}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$ have been investigated by powder neutron diffraction. A long-range antiferromagnetic (AFM) ordering has been found below ${T}_{\mathrm{N}}\ensuremath{\sim}24.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The magnetic ground state, determined to be zigzag antiferromagnetic and characterized by a propagation vector $\mathbf{k}=(1/200)$, occurs due to the competing exchange interactions up to third-nearest neighbors within the honeycomb lattice. The exceptional existence of a limited magnetic correlation length along the $c$ axis (perpendicular to the honeycomb layers in the $ab$ planes) has been found even at 1.8 K, well below the ${T}_{\mathrm{N}}\ensuremath{\sim}24.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The observed limited correlation along the $c$ axis is explained by the disorder distribution of the Na ions within the intermediate layers between honeycomb planes. The reduced ordered moments ${m}_{\mathrm{Co}(1)}=2.77(3)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{C}{\mathrm{o}}^{2+}$ and ${m}_{\mathrm{Co}(2)}=2.45(2)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{C}{\mathrm{o}}^{2+}$ at 1.8 K reflect the persistence of spin fluctuations in the ordered state. Above ${T}_{\mathrm{N}}\ensuremath{\sim}24.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the presence of short-range magnetic correlations, manifested by broad diffuse magnetic peaks in the diffraction patterns, has been found. Reverse Monte Carlo analysis of the experimental diffuse magnetic scattering data reveals that the spin correlations are mainly confined within the two-dimensional honeycomb layers ($ab$ plane) with a correlation length of $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\AA{}$ at 25 K. The nature of the spin arrangements is found to be similar in both the short-range and long-range ordered magnetic states. This implies that the short-range correlation grows with decreasing temperature and leads to the zigzag AFM ordering at $T\ensuremath{\le}{T}_{\mathrm{N}}$. The present study provides a comprehensive picture of the magnetic correlations over the temperature range above and below the ${T}_{\mathrm{N}}$ and their relation to the crystal structure. The role of intermediate soft Na layers on the magnetic coupling between honeycomb planes is discussed.

Using a high-pressure reaction, we successfully synthesized $\mathrm{BaF}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{e}}_{2}\mathrm{O}$ with a uniform stack of $[\mathrm{F}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{e}}_{2}\mathrm{O}]$ layers. Magnetic susceptibility, heat capacity, M\"ossbauer spectroscopy, and neutron-diffraction measurements revealed that $\mathrm{BaF}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{e}}_{2}\mathrm{O}$ undergoes antiferromagnetic order at 106 K with a 2-$k$ spin structure where each Fe moment points to a neighboring oxide anion. The same spin structure has been observed in the related iron oxyselenides but with a staggered stack of $[\mathrm{F}{\mathrm{e}}_{2}\mathrm{C}{\mathrm{h}}_{2}\mathrm{O}]$ (where Ch represents chalcogen) layers $(\mathrm{Ch}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}\mathrm{S},\mathrm{Se})$. We propose that the strong uniaxial anisotropy inferred from the 2-$k$ structure originates from spin-orbit coupling (SOC) induced by a $\mathit{trans}\ensuremath{-}\mathrm{Fe}{\mathrm{O}}_{2}\mathrm{S}{\mathrm{e}}_{4}$ octahedron, which provides a quasilinear coordination environment as often found in single molecule magnetic complexes. A first-principles calculation with inclusion of SOC supports the stabilization of the 2-$k$ spin structure, giving an unquenched orbital momentum of $0.1\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$. The present paper provides an idea of how to design magnetic lattices of uniaxial anisotropy using oxychalcogenides and more generally mixed-anion compounds.

The finding of new multiferroic materials, where electric and magnetic orders coexist, is a challenging task currently. The double perovskite Y${}_{2}$CoMnO${}_{6}$ shows spontaneous magnetization and electrical polarization at low temperature. Previous investigations of this compound did not reach agreement about the type of magnetic structure present. This study demonstrates that this compound exhibits a collinear ferromagnetic ordering of Co${}^{2+}$ and Mn${}^{4+}$ moments in the $a\phantom{\rule{0}{0ex}}c$ plane with a small antiferromagnetic canting along the $b$ axis. A thorough characterization of the dielectric properties reveals the absence of any related anomaly in the dielectric permittivity and the lack of spontaneous electrical polarization ($P$) in the $P$($E$, electric field) loops. The pyroelectric current is strongly dependent on the number of antisite defects in the Co/Mn arrangement, the heating rate, and the poling field. Thus, the observed electric polarization is due to thermally stimulated depolarization currents ascribed to defect dipoles mainly placed at the antiphase boundaries. No ferroelectric transition occurs in this material, disproving the existence of intrinsic magnetoelectric multiferroicity.

We report ac susceptibility, specific heat, and neutron scattering measurements on a dipolar-coupled antiferromagnet LiYbF$_4$. For the thermal transition, the order-parameter critical exponent is found to be 0.20(1) and the specific-heat critical exponent -0.25(1). The exponents agree with the 2D XY/h$_4$ universality class despite the lack of apparent two-dimensionality in the structure. The order-parameter exponent for the quantum phase transitions is found to be 0.35(1) corresponding to (2+1)D. These results are in line with those found for LiErF$_4$ which has the same crystal structure, but largely different T$_N$, crystal field environment and hyperfine interactions. Our results therefore experimentally establish that the dimensional reduction is universal to quantum dipolar antiferromagnets on a distorted diamond lattice., 9 pages, 7 figures

The thermal evolution of structural and magnetic details of the orthovanadate $\mathrm{TmV}{\mathrm{O}}_{3}$, studied in detail by neutron and synchrotron powder diffraction, is reported. Crystallizing in space group Pnma at room temperature, $\mathrm{TmV}{\mathrm{O}}_{3}$ undergoes a first structural phase transition to $P{2}_{1}/a$ at ${T}_{\mathrm{OO}}=180\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where a $G$-type orbital ordered state develops. At ${T}_{\mathrm{S}}=75\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, a change back to Pnma occurs, and the establishment of $C$-type orbital order takes place. The ${\mathrm{V}}^{3+}$ ions order antiferromagnetically with a magnetic propagation vector $\mathbf{k}=0$ below ${T}_{\mathrm{N}1}=105\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, while the $\mathrm{T}{\mathrm{m}}^{3+}$ sublattice orders at ${T}_{\mathrm{N}2}=20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ following the same propagation vector. Between ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{S}}$, a coexistence of $G$-type ($P{2}_{1}/a$) and $C$-type (Pnma) orbital ordered states exists. The $P{2}_{1}/a$ phase is magnetically separated into two fractions, which adopt a ${C}_{x}{C}_{y}0$ and ${G}_{x}00$ coupling, respectively, while the Pnma volume fraction follows a $0{G}_{y}0$ magnetic structure. At ${T}_{\mathrm{N}2}$, the appearance of the Tm sublattice magnetization ($0{C}_{y}0$) leads to a spin flop transition of the V sublattice from $0{G}_{y}0$ to ${G}_{x}00$. The results are presented and analyzed in the general context of the series of $R\mathrm{V}{\mathrm{O}}_{3}$ compounds, and they are used to discuss recent magnetization results.

We have investigated the physical properties of a pyrochlore hafnate Nd2Hf2O7 using ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility \chi(T), isothermal magnetization M(H) and heat capacity C_p(T) measurements, and determined the magnetic ground state by neutron powder diffraction study. An upturn is observed below 6 K in C_p(T)/T, however both C_p(T) and \chi(T) do not show any clear anomaly down to 2 K. The \chi_ac(T) shows a well pronounced anomaly indicating an antiferromagnetic transition at T_N = 0.55 K. The long range antiferromagnetic ordering is confirmed by neutron diffraction. The refinement of neutron diffraction pattern reveals an all-in/all-out antiferromagnetic structure, where for successive tetrahedra, the four Nd3+ magnetic moments point alternatively all-into or all-out-of the tetrahedron, with an ordering wavevector k = (0, 0, 0) and an ordered state magnetic moment of m = 0.62(1) \mu_B/Nd at 0.1 K. The ordered moment is strongly reduced reflecting strong quantum fluctuations in ordered state.

We report measurements performed on a polycrystalline sample of the pyrochlore compound ${\mathrm{Nd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. It undergoes a second order magnetic phase transition at ${T}_{c}\ensuremath{\approx}0.91$ K to a noncoplanar all-in--all-out magnetic structure of the ${\mathrm{Nd}}^{3+}$ magnetic moments. The thermal behavior of the low temperature specific heat fingerprints excitations with linear dispersion in a three-dimensional lattice. The temperature independent spin-lattice relaxation rate measured below ${T}_{c}$ and the anomalously slow paramagnetic spin dynamics detected up to $\ensuremath{\approx}30{T}_{c}$ are suggested to be due to magnetic short-range correlations in unidimensional spin clusters, i.e., spin loops. The observation of a spontaneous field in muon spin relaxation measurements is associated with the absence of a divergence-free field for the ground state of an all-in--all-out pyrochlore magnet as predicted recently.

The magnetism of the A-site-ordered perovskite $\mathrm{LaM}{\mathrm{n}}_{3}{\mathrm{V}}_{4}{\mathrm{O}}_{12}$ is studied comprehensively by means of neutron powder diffraction experiments and theoretical calculations. Magnetic neutron diffraction results show that a rhombohedral 60\ifmmode^\circ\else\textdegree\fi{} spin structure emerges on the cubic lattice below a 44-K N\'eel transition. Ab initio electronic structure calculations confirm that high-spin ${\mathrm{Mn}}^{2+}$ moments are localized while V $3d$-band states are itinerant, and that the noncollinear 60\ifmmode^\circ\else\textdegree\fi{} spin structure is more stable than collinear ferromagnetic or G-type antiferromagnetic alternatives. Effective Heisenberg model calculations reveal that the appearance of such a nontrivial spin structure can be attributed to significant next-nearest-neighbor and third-nearest-neighbor magnetic interactions.

The crystal and magnetic structures of ${\mathrm{Tb}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ series have been studied by x-ray and neutron-diffraction techniques. Macroscopic magnetic properties have been characterized using ac magnetic susceptibility measurements. The relationships between structural and magnetic behaviors are discussed. Our study shows that the magnetism of manganese in this system is very complex due to the small size of the rare earth. ${\mathrm{TbMnO}}_{3}$ does not develop a simple A-type magnetic order, as does ${\mathrm{LaMnO}}_{3},$ but an incommensurate sinewave structure. As the Ca content increases up to $x=0.33,$ the magnetization increases without, however, reaching a truly ferromagnetic state. Instead, the magnetic ground state is composed of ferromagnetic clusters in a paramagnetic matrix. Charge ordering is observed at higher values of x and the magnetic ground state evolves, with increasing x values, from the CE type to a mixture of different magnetic structures including some noncollinear structures. For $x=0.85,$ we observe a phase segregation at low temperatures into an orthorhombic phase and a monoclinic phase.

We present direct evidence of the phase segregation and magnetic-field-induced structural transition in ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ as proposed earlier [R. Mahendiran et al., Phys. Rev. Lett. 82, 2191 (1999)] for the collapse of the charge-ordered insulating state. The spontaneous $(H=0)$ low-temperature phase at 125 K is found to be phase segregated into two different crystallographic structures and three magnetic phases: orthorhombic (Imma) ferromagnetic, orthorhombic (Imma) A-type antiferromagnetic, and monoclinic ${(P2}_{1}/m)$ charge-ordered CE-type antiferromagnetic phases. Under a field of $H=6$ T, the charge-ordered monoclinic phase collapses and completely transforms into the metallic ferromagnetic orthorhombic phase.

Jarosites are a family of minerals of general formula ${\mathrm{AFe}}_{3}{(\mathrm{OH})}_{6}{(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ (where ${A}^{+}$ is typically a univalent cation such as ${\mathrm{Na}}^{+},$ ${\mathrm{K}}^{+},$ ${\mathrm{Rb}}^{+},$ ${\mathrm{ND}}_{4}^{+},$ ${\mathrm{Ag}}^{+},$ ${\mathrm{Tl}}^{+},$ or ${\mathrm{D}}_{3}{\mathrm{O}}^{+}).$ They provide good model Heisenberg kagom\'e antiferromagnets with which to test suggestions that highly frustrated antiferromagnets have unconventional magnetic ground states and excitations. In all cases ${\mathrm{Fe}}^{3+}$ ions provide $S=\frac{5}{2}$ moments, arranged on the vertices of well-separated kagom\'e layers, and are coupled through strong antiferromagnetic exchange with values of the Weiss constants \ensuremath{\theta} of the order of -700 K. We report dc magnetic susceptibility $({\ensuremath{\chi}}_{\mathrm{dc}})$ and powder neutron diffraction studies of materials in which ${A}^{+}={\mathrm{Na}}^{+},$ ${\mathrm{Rb}}^{+},$ ${\mathrm{ND}}_{4}^{+},$ ${\mathrm{Ag}}^{+},$ or ${\mathrm{D}}_{3}{\mathrm{O}}^{+}$ and show that for all materials except the deuteronium ${(A}^{+}={\mathrm{D}}_{3}{\mathrm{O}}^{+})$ salt, long-range magnetic order with the in-plane $q=0$ spin structure sets in below a temperature ${T}_{f}$ of the order of 50 K. $({\mathrm{D}}_{3}{\mathrm{O})\mathrm{F}\mathrm{e}}_{3}{(\mathrm{S}\mathrm{O}}_{4}{)}_{2}{(\mathrm{OD})}_{6}$ shows only a spin-glass-like transition at ${T}_{f}\ensuremath{\cong}15\mathrm{K}.$ There is no obvious difference in the structures of the salts that show magnetic long-range order, and the deuteronium salt except for the coverage of the magnetic lattice, which is higher $(97\ifmmode\pm\else\textpm\fi{}3%)$ in the latter than that of the rest of the family (\ensuremath{\leqslant}95%, and most typically \ensuremath{\cong} 90%). It is proposed that reduction of the coverage of the magnetic lattice induces long-range order in the jarosites, and the material $({\mathrm{D}}_{3}{\mathrm{O})\mathrm{F}\mathrm{e}}_{3\ensuremath{-}x}{\mathrm{Al}}_{y}{(\mathrm{OD})}_{6}{(\mathrm{S}\mathrm{O}}_{4}{)}_{2},$ in which the coverage of the magnetic lattice is $89\ifmmode\pm\else\textpm\fi{}3%$ has been prepared and characterized to test this hypothesis. ${\ensuremath{\chi}}_{\mathrm{dc}}$ for this material has a cusp at 25.5 K, and powder neutron diffraction reveals long-range magnetic order at 1.4 K with the same ordering vector as that seen in the other materials.

Neutron scattering, ac susceptibility, and NMR measurements have been made on isotopically enriched ${}^{154}{\mathrm{SmMn}}_{2}{\mathrm{Ge}}_{2}.$ All the magnetic states are characterized from much clearer neutron-diffraction spectra than in previous works, in which natural abundance Sm was used. A temperature versus pressure magnetic phase diagram is proposed, consisting of six distinct magnetic states, including previously undetected incommensurate cone structures at low temperatures and pressures. The noncollinear magnetic structures of these magnetic phases are such that a larger antiferromagnetic component appears within (001) planes in the ferromagnetic states than in the antiferromagnetic states. This observation provides an explanation for the anisotropic changes in the lattice constant and the anomalous magnetoresistance of ${\mathrm{SmMn}}_{2}{\mathrm{Ge}}_{2}.$ Previously unexplained susceptibility and NMR measurements are interpreted in terms of changes to the cone structure. Changes in the Mn-Mn spacing and magnetic coupling are consistent with previous observations on related systems.

We have studied the oxygen isotope effect on ${(\mathrm{L}\mathrm{a}}_{0.5}{\mathrm{Nd}}_{0.5}{)}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}.$ Both isotope samples (${}^{16}\mathrm{O}$ and ${}^{18}\mathrm{O}$) segregate into a charge-ordered (CO) and a paramagnetic phase below 210 K, but the relative percentage of both phases strongly depends on the isotope mass. At low temperatures, the CO phase orders antiferromagnetically, whereas the remaining phase orders ferromagnetically. Our results can be explained by a scenario where the increase of the oxygen mass weakens the ferromagnetic interaction but has no influence on the electronic correlations that may be responsible for the CO state.

The highly ${\mathrm{Mn}}^{+4}$-doped compound ${\mathrm{La}}_{0.35}{\mathrm{Ca}}_{0.65}{\mathrm{MnO}}_{3}$ has been studied up to high temperature (700 K) by using thermal-expansion, magnetostriction, magnetoresistance, and neutron-diffraction techniques. From 700 K down to room temperature the electrical conduction takes place through thermally activated hopping of polarons with ${E}_{\mathrm{hop}}$=45 meV. At the charge-ordering (CO) transition temperature ${T}_{\mathrm{CO}}$=275 K, pronounced anomalies in the resistivity and the lattice are observed. The neutron thermodiffractogram clearly establishes that the CO state occurs in the paramagnetic (P) phase and is accompanied by a large anisotropic lattice distortion with a simultaneous large distortion of the ${\mathrm{MnO}}_{6}$ octahedra. The antiferromagnetic (AF) phase appears at ${T}_{N}$=160\ifmmode\pm\else\textpm\fi{}3 K. At this temperature no lattice effect is observed. The CO-P and the CO-AF ground states are stable under an applied magnetic field up to 12 T.

A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}}$ samples, with $\ensuremath{\delta}=0,$ 0.025, 0.07, 0.1, and 0.15. Measurements of the ac initial magnetic susceptibility, magnetization, magnetoresistance, and neutron diffraction, including small-angle neutron scattering (SANS), were performed in the temperature range 1--320 K using high magnetic fields up to 12 T. The antiferromagnetic order found in ${\mathrm{LaMnO}}_{3}$ evolves towards a ferromagnetic order as \ensuremath{\delta} increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the ${\mathrm{MnO}}_{6}$ octahedra. The ferromagnetic coupling weakens for $\ensuremath{\delta}g~0.1.$ The magnetic behavior is interpreted by taking into account two effects caused by the increase in \ensuremath{\delta}: cation vacancies and ${\mathrm{Mn}}^{4+}{/\mathrm{M}\mathrm{n}}^{3+}$ ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for $\ensuremath{\delta}g~0.1.$ The sample $\ensuremath{\delta}=0.1$ shows a structural transition from rhombohedral to orthorhombic below ${T}_{S}\ensuremath{\approx}300\mathrm{K}$ with a huge change in the cell volume. All the studied compounds were found to be insulating at low temperatures with no appreciable magnetoresistance, except for $\ensuremath{\delta}=0.15,$ in which we observed a large value for the magnetoresistance. The SANS results indicate that magnetic clustering effects are important below ${T}_{C}$ for $\ensuremath{\delta}g~0.07,$ which could explain the intriguing ferromagnetic insulator state. In the $\ensuremath{\delta}=0.07$ and $\ensuremath{\delta}=0.10$ samples we found at temperatures below ${T}_{C}$ magnetic and structural anomalies that are characteristic of charge ordering.

The various magnetic structures and magnetic phase transitions in the series Nd{sub x}Tb{sub 1{minus}x}Mn{sub 2}Ge{sub 2} have been thoroughly studied by means of macroscopic magnetic and thermal measurements (such as magnetization, ac initial magnetic susceptibility and linear thermal expansion) and microscopic neutron-diffraction experiments. As a result, the magnetic phase diagram has been determined over the whole temperature range. Large changes in the local Mn magnetic moments (e.g., {Delta}{mu}{sub Mn}/{mu}{sub Mn}{approx}16{percent} in TbMn{sub 2}Ge{sub 2}) have been detected at the magnetic phase transitions observed at low temperatures, {approx}100{endash}140 K, in the x=0{endash}0.4 alloys. This variation, together with the appearance of magnetic ordering in the rare-earth sublattice, has been related to the volume anomalies found (e.g., {Delta}V/V{approx}0.3{percent} in TbMn{sub 2}Ge{sub 2}). A new magnetic structure of the Mn sublattice in the RMn{sub 2}X{sub 2} (R=rare earth, X=Si, Ge) family has been found in Nd{sub 0.4}Tb{sub 0.6}Mn{sub 2}Ge{sub 2} (140K{lt}T{lt}350 K) where two antiferromagnetic commensurate components within the (001) plane coexist with a ferromagnetic component along the c axis. The peculiar layered structure of the RMn{sub 2}Ge{sub 2} compounds favors a cancellation of the molecular field at the rare-earth sites in the case of antiferromagnetic arrangements of the Mn sublattice, effectively isolating themore » R atoms and making a paramagnetic behavior of these possible despite the presence of long-range order. The existence of a ferromagnetic component in the Mn sublattice has been concluded to be indispensable to allow the ordering of the rare-earth magnetic moments. {copyright} {ital 1997} {ital The American Physical Society}« less

The effect of the substitution for Mn with Al in the magnetoresistive perovskite ${\mathrm{La}}_{2\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{MnO}}_{3}$ has been studied by preparing the series ${\mathrm{La}}_{2\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{Mn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (x\ensuremath{\leqslant}0.2). A careful study of the magnetic, structural, and transport properties has been carried out by means of electrical resistance, magnetoresistance, ac magnetic susceptibility, x-ray-diffraction, and neutron-diffraction techniques. Up to x=0.05 the Curie temperature (and the associated metal-insulator transition) decreases drastically with Al doping and the magnetoresistive properties do not change very much. For x\ensuremath{\geqslant}0.1 the lattice spontaneously begins to lose oxygen atoms and for x=0.2, 3% of oxygen vacancies are present. This fact along with the random distribution of the Al atoms makes these compounds rather disordered from a structural and magnetic point of view. However, the magnetoresistance is enhanced, reaching colossal values of ${10}^{7}$% at H=12 T at low temperatures for x=0.2.

The field-induced evolution of the magnetic ordering in (CuBr)Sr${}_{2}$Nb${}_{3}$O${}_{10}$ with a $⅓$ magnetization plateau has been investigated by neutron diffraction under magnetic fields up to 10 T. With an increasing magnetic field, the zero-field helical antiferromagnetic (AFM) phase, AF1, with \ensuremath{\kappa} $=$ [0 $⅜$ $\textonehalf{}$] is replaced by a simple ferromagnetic phase with \ensuremath{\kappa} $=$ [0 0 0], the formation of which is, however, retarded by the appearance of a second AFM, AF2, with \ensuremath{\kappa} $=$ [0 $⅓\ensuremath{\sim}0.46$]. Upon further increasing of the magnetic field, the AF2 phase disappears and only the ferromagnetic phase persists. The results clearly show that the magnetization plateau, induced by the competition between field-induced ferromagnetic, F, and AF2 phases, is coincidentally situated at $M$ $=$ $⅓$ ${M}_{S}$ of the dc magnetization curve. The AF1 and AF2 phases have strongly differing magnetic propagation vectors and are therefore not directly related.

We report on the synthesis and characterization of BiFe${}_{0.5}$Mn${}_{0.5}$O${}_{3}$, a potential type-I multiferroic compound displaying temperature-induced magnetization reversal. Bulk samples were obtained by means of solid-state reaction carried out under the application of hydrostatic pressure of 6 GPa at 1100 \ifmmode^\circ\else\textdegree\fi{}C. The crystal structure is a highly distorted perovskite with no cation order on the $B$ site, where, besides a complex scheme of tilt and rotations of the TM-O${}_{6}$ octahedra, large off-centering of the bismuth ions is detected. Below ${T}_{1}$ $=$ 420 K the compound undergoes a first weak ferromagnetic transition related to the ordering of iron-rich clusters. At lower temperatures (just below RT) a complex thermally activated mechanism induces at first an enhancement of the magnetization at ${T}_{2}$ $=$ 288 K, then a spontaneous reversal giving rise to a negative response. The complementary use of powder neutron diffraction, superconducting quantum interference device magnetometry, and M\"ossbauer spectroscopy allowed us to propose as a possible interpretation of the overall magnetic behavior the presence of an uncompensated competitive coupling between nonequivalent clusters of weakly ferromagnetic interactions characterized by different critical temperatures and resultant magnetizations.

The nature of triclinic to orthorhombic phase transition, at which colossal negative thermal expansion is observed, and the magnetic ordering of Bi${}_{1\ensuremath{-}x}$La${}_{x}$NiO${}_{3}$ have been investigated with neutron powder diffraction (NPD) and x-ray absorption spectroscopy techniques. The presence of a charge-transfer transition from (Bi/La)${}^{3+}$${}_{0.5}$Bi${}^{5+}$${}_{0.5}$Ni${}^{2+}$O${}_{3}$ to (Bi/La)${}^{3+}$Ni${}^{3+}$O${}_{3}$, accompanied by the simultaneous structural distortion, was confirmed. The NPD data also revealed that magnetic ordering is present only in the insulating triclinic phase. The metallic orthorhombic phase was found to be nonmagnetic down to 10 K.

Neutron-diffraction measurements have been carried out to determine the nature of the magnetic ordering in ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C. ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C exhibits superconductivity below 11 K and magnetic ordering below 1.5 K. The magnetic structure is incommensurate and consists of ferromagnetic (110) planes of Tm moments aligned along the c axis with the magnitude of the moments modulated sinusoidally along the (110) direction, the modulation wave vector having a magnitude of 0.241 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$. The magnetic structure as well as the modulation direction exhibited by ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C is different from that of other R${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C (R=Gd--Er) compounds. The modulated state formed by the Tm moments allows the superconductivity to coexist with the magnetic ordering below ${\mathit{T}}_{\mathit{N}}$. \textcopyright{} 1996 The American Physical Society.

The effects of magnetic field and pressure on the unusual spontaneous behavior of ${\mathrm{La}}_{2/3}$${\mathrm{Ca}}_{1/3}$${\mathrm{MnO}}_{3}$ have been thoroughly investigated. Resistivity and volume thermal expansion, both under magnetic field and pressure, ac susceptibility under pressure, magnetostriction, magnetoresistance, and neutron diffraction measurements, have allowed us to determine the relevant underlying mechanisms in this system. Above ${\mathit{T}}_{\mathit{c}}$ the neutron measurements reveal short-range ferromagnetic correlations and the anomalous volume thermal expansion indicates that local distortions are present. Both experiments support the formation of magnetic polarons above ${\mathit{T}}_{\mathit{c}}$. At ${\mathit{T}}_{\mathit{c}}$ the compound undergoes a paramagnetic-ferromagnetic transition accompanied by an insulator-metal-like transition with anomalies in the electrical and volume properties. Above ${\mathit{T}}_{\mathit{c}}$ the magnetic field and the pressure favor electrical conduction by enhancing the double-exchange interaction. Below ${\mathit{T}}_{\mathit{c}}$ the metallic state is favored by the magnetic field and the pressure in a different way. \textcopyright{} 1996 The American Physical Society.

A thorough study of the low-temperature magnetic state of polycrystalline $({\mathrm{Tb}}_{1/3}{\mathrm{La}}_{2/3}{)}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ has been carried out. This perovskitelike compound is an insulator at low temperatures and shows an insulator-metal transition under magnetic field. All the measurements of its magnetic state (magnetization, susceptibility, and neutron diffraction) are consistent with spin-glass behavior. The magnetic and electronic phase diagram for the series $({\mathrm{Tb}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{)}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ is depicted.

The effect of a small substitution of Mn with Al in TbMnO 3 has been studied. We report results of heat capacity, magnetization, and dielectric constant studies in TbMn 1-xAl xO 3 compounds (x ≤ 0.1). Al has the same valence as substituted Mn but is nonmagnetic and its small size gives rise to microstructural strain which affects the multiferroic properties of the parent compound. Long-range antiferromagnetic ordering is observed in all compounds but the transition temperature decreases as the Al content increases. TbMn 0.95Al 0.05O 3 exhibits a ferroelectric phase transition which is absent in TbMn 0.9Al 0.1O 3. The dielectric constant of the latter compound reveals a relaxor behavior suggesting the presence of nanosize polar domains for this compound. A neutron diffraction study on a single crystal of TbMn 0.9Al 0.1O 3 reveals that Mn shows a sinusoidal incommensurate ordering down to low temperature. Tb moments exhibit an incommensurate short-range ordering but the application of a magnetic field leads to metamagnetic transitions. In particular, a field parallel to the b axis induces a commensurate long-range ordering of Tb of type C xF y. The magnetic field also affects the magnetic structure of Mn3 + moments at low temperature which develop an incommensurate cycloid ordering in the ab plane. This result suggests that dilution of a magnetic multiferroic with a small nonmagnetic atom might yield materials with a relaxor to ferroelectric transition driven by a magnetic field., Financial support from Spanish MICINN (Projects No. FIS08-03951 and No. MAT2007-61621) and DGA (CAMRADS) is acknowledged. V.C. thanks the FPU research grant from MICINN.

We apply neutron diffraction, high-resolution synchrotron x-ray diffraction, magnetization measurements, electronic structure calculations, and quantum Monte-Carlo simulations to unravel the structure and magnetism of (CuCl)LaTa${}_{2}$O${}_{7}$. Despite the pseudo-tetragonal crystallographic unit cell, this compound features an orthorhombic superstructure, similar to the Nb-containing (Cu$X$)LaNb${}_{2}$O${}_{7}$ with $X$ $=$ Cl and Br. The spin lattice entails dimers formed by the antiferromagnetic fourth-neighbor coupling ${J}_{4}$, as well as a large number of nonequivalent interdimer couplings quantified by an effective exchange parameter ${J}_{\text{eff}}$. In (CuCl)LaTa${}_{2}$O${}_{7}$, the interdimer couplings are sufficiently strong to induce the long-range magnetic order with the N\'eel temperature ${T}_{N}\ensuremath{\simeq}7$ K and the ordered magnetic moment of 0.53${\ensuremath{\mu}}_{B}$, as measured with neutron diffraction. This magnetic behavior can be accounted for by ${J}_{\text{eff}}/{J}_{4}\ensuremath{\simeq}1.6$ and ${J}_{4}\ensuremath{\simeq}16$ K. We further propose a general magnetic phase diagram for the (CuCl)LaNb${}_{2}$O${}_{7}$-type compounds, and explain the transition from the gapped spin-singlet (dimer) ground state in (CuCl)LaNb${}_{2}$O${}_{7}$ to the long-range antiferromagnetic order in (CuCl)LaTa${}_{2}$O${}_{7}$ and (CuBr)LaNb${}_{2}$O${}_{7}$ by an increase in the magnitude of the interdimer couplings ${J}_{\text{eff}}/{J}_{4}$, with the (CuCl)La$M$${}_{2}$O${}_{7}$ ($M$ $=$ Nb, Ta) compounds lying on different sides of the quantum critical point that separates the singlet and long-range-ordered magnetic ground states.

We present a comprehensive study of the crystal structure, magnetic structure, and microscopic magnetic model of (CuBr)LaNb2O7, the Br analog of the spin-gap quantum magnet (CuCl) LaNb2O7. Despite similar crystal structures and spin lattices, the magnetic behavior and even peculiarities of the atomic arrangement in the Cl and Br compounds are very different. The high- resolution x-ray and neutron data reveal a split position of Br atoms in (CuBr) LaNb2O7. This splitting originates from two possible configurations developed by [CuBr] zigzag ribbons. While the Br atoms are locally ordered in the ab plane, their arrangement along the c direction remains partially disordered. The predominant and energetically more favorable configuration features an additional doubling of the c lattice parameter that was not observed in (CuCl) LaNb2O7. (CuBr) LaNb2O7 undergoes long-range antiferromagnetic ordering at T-N = 32 K, which is nearly 70% of the leading exchange coupling J4 similar or equal to 48 K. The Br compound does not show any experimental signatures of low-dimensional magnetism because the underlying spin lattice is three-dimensional. The coupling along the c direction is comparable to the couplings in the ab plane, even though the shortest Cu-Cu distance along c (11.69 angstrom) is three times larger than nearest-neighbor distances in the ab plane (3.55 angstrom). The stripe antiferromagnetic long-range order featuring columns of parallel spins in the ab plane and antiparallel spins along c is verified experimentally and confirmed by the microscopic analysis.

Magnetic correlation in the quantum $S=1$/2 square-lattice system (CuBr)Sr${}_{2}$Nb${}_{3}$O${}_{10}$ has been studied by neutron diffraction. A novel commensurate in-plane, helical antiferromagnetic (AFM) ordering, characterized by the propagation vector $k$ = (0 3/8 1/2), has been confirmed from the appearance of magnetic Bragg peaks below ${T}_{\mathrm{N}}$ \ensuremath{\sim} 7.5 K. The ordered moment at 2 K is found to be 0.79(7) ${\ensuremath{\mu}}_{\mathrm{B}}$/Cu${}^{2+}$-ion. The observed helical AFM structure differs from the ground state predicted theoretically from the ${J}_{1}$-${J}_{2}$ model as well as from experimentally reported states for other quantum $S=1$/2 square-lattice systems. However, the observed helical magnetic structure can be described in a ${J}_{1}$-${J}_{2}$-${J}_{3}$ model. Under a 4.5 T magnetic field, the spin-order changes drastically and is characterized by the propagation vector ${k}_{1}$ = (0 1/3 0.446) and a probable ${k}_{2}$ = (0 0 0) vector.