Sublattice Orders Research Articles

Spin reorientation and the interplay of magnetic sublattices in Er2CuMnMn4O12.

Through a combination of magnetic susceptibility, specific heat, and neutron powder diffraction measurements we have revealed a sequence of four magnetic phase transitions in the columnar quadruple perovskite Er2CuMnMn4O12. A key feature of the quadruple perovskite structural framework is the complex interplay of multiple magnetic sublattices via frustrated exchange topologies and competing magnetic anisotropies. It is shown that in Er2CuMnMn4O12, this phenomenology gives rise to multiple spin-reorientation transitions driven by the competition of easy-axis single ion anisotropy and the Dzyaloshinskii-Moriya interaction; both within the manganese B-site sublattice. At low temperature, one Er sublattice orders due to a finite f-d exchange field aligned parallel to its Ising axis, while the other Er sublattice remains non-magnetic until a final, symmetry-breaking phase transition into the ground state. This non-trivial low-temperature interplay of transition metal and rare-earth sublattices, as well as an observed k = (0, 0, ½) periodicity in both manganese spin canting and Er ordering, raises future challenges to develop a complete understanding of the R2CuMnMn4O12 family.

Interplay of two magnetic sublattices in related compounds Sm2Mn1-xGa6-yGey (x = 0.1-0.3, y = 0.6-1.0) and Sm4MnGa12-yGey (y = 3.0-3.5) with different ordering of empty and filled (Ga,Ge)6 octahedra.

Single crystals of two new intermetallic phases Sm2Mn1-xGa6-yGey (x = 0.1-0.3, y = 0.6-1.0) and Sm4MnGa12-yGey (y = 3.0-3.5) were grown using a self-flux technique. According to single crystal X-ray diffraction data, Sm4MnGa12-yGey is characterised by the Y4PdGa12 structure type (a ∼ 8.65 Å; Im3̄m), while Sm2Mn1-xGa6-yGey formally adopts the K2PtCl6 structure type (a ∼ 8.71 Å; Fm3̄m). The general features of both compounds with rather similar crystal structures are represented by the alternation of empty and Mn-filled p-element octahedra, the order of which is determined by the Mn concentration. The diffraction data for Sm2Mn1-xGa6-yGey reveal a large concentration of Mn vacancies (x ∼ 0.3), which affects adjacent Ga/Ge atoms leading to their shift towards the vacancy. Both compounds demonstrate two ferromagnetic-like transitions and the presence of two interacting Mn and Sm magnetic sublattices. The Mn sublattice orders at TC1 of 143 K and 318 K, while the Sm one orders at lower temperatures at TC2 of 50 K and 280 K for Sm4MnGa8.6Ge3.4 and Sm2Mn0.74Ga5.1Ge0.9, respectively. The increase in Mn content not only increases the ordering temperatures, but also dramatically decreases the coercivity μ0HC from 230 mT to just 6.5 mT at 2 K. Despite the presence of two magnetically active sublattices in Sm2Mn0.74Ga5.1Ge0.9, the magnetic entropy change is quite low and only reaches 0.3 J kg-1 K-1 at T = 300 K and μ0H = 5 T, while the estimated relative cooling power (RCP) is about 36 J kg-1 at 5 T.

A New Sodium Thioborate Fast Ion Conductor: Na3 B5 S9.

We report a new sodium fast-ion conductor, Na3 B5 S9 , that exhibits a high Na ion total conductivity of 0.80 mS cm-1 (sintered pellet; cold-pressed pellet=0.21 mS cm-1 ). The structure consists of corner-sharing B10 S20 supertetrahedral clusters, which create a framework that supports 3D Na ion diffusion channels. The Na ions are well-distributed in the channels and form a disordered sublattice spanning five Na crystallographic sites. The combination of structural elucidation via single crystal X-ray diffraction and powder synchrotron X-ray diffraction at variable temperatures, solid-state nuclear magnetic resonance spectra and ab initio molecular dynamics simulations reveal high Na-ion mobility (predicted conductivity: 0.96 mS cm-1 ) and the nature of the 3D diffusion pathways. Notably, the Na ion sublattice orders at low temperatures, resulting in isolated Na polyhedra and thus much lower ionic conductivity. This highlights the importance of a disordered Na ion sublattice-and existence of well-connected Na ion migration pathways formed via face-sharing polyhedra-in dictating Na ion diffusion.

A New Sodium Thioborate Fast Ion Conductor: Na3B5S9

AbstractWe report a new sodium fast‐ion conductor, Na3B5S9, that exhibits a high Na ion total conductivity of 0.80 mS cm−1 (sintered pellet; cold‐pressed pellet=0.21 mS cm−1). The structure consists of corner‐sharing B10S20 supertetrahedral clusters, which create a framework that supports 3D Na ion diffusion channels. The Na ions are well‐distributed in the channels and form a disordered sublattice spanning five Na crystallographic sites. The combination of structural elucidation via single crystal X‐ray diffraction and powder synchrotron X‐ray diffraction at variable temperatures, solid‐state nuclear magnetic resonance spectra and ab initio molecular dynamics simulations reveal high Na‐ion mobility (predicted conductivity: 0.96 mS cm−1) and the nature of the 3D diffusion pathways. Notably, the Na ion sublattice orders at low temperatures, resulting in isolated Na polyhedra and thus much lower ionic conductivity. This highlights the importance of a disordered Na ion sublattice—and existence of well‐connected Na ion migration pathways formed via face‐sharing polyhedra—in dictating Na ion diffusion.

Magnetic ordering induced magnetodielectric effect in Ho2Cu2O5 and Yb2Cu2O5

Materials with strongly coupled magnetic and electronic degrees of freedom provide new possibilities for practical applications. In this paper, we have investigated the structure, magnetic property, and magnetodielectric (MD) effect in Ho2Cu2O5 and Yb2Cu2O5 polycrystalline samples, which possess a non-centrosymmetric polar structure with space group Pna21. In Ho2Cu2O5, Ho3+ and Cu2+ sublattices order simultaneously, exhibiting a typical paramagnetic to antiferromagnetic transition at 13.1 K. While for Yb2Cu2O5, two magnetic transitions which originate from the orderings of Yb3+ (7.8 K) and Cu2+ (13.5 K) sublattices are observed. A magnetic field induced metamagnetic transition is obtained in these two cuprates below Néel temperature (T N). By means of dielectric measurement, distinct MD effect is demonstrated by the dielectric anomaly at T N. Meanwhile, the MD effect is found to be directly related to the metamagnetic transition. Due to the specific spin configuration and different spin evolution in the magnetic field, a positive MD effect is formed in Ho2Cu2O5, and a negative one is observed in Yb2Cu2O5. The spontaneous dielectric anomaly at T N is regarded as arising from the shifts in optical phonon frequencies, and the magnetoelectric coupling is used to interpret the magnetic field induced MD effect. Moreover, an H–T phase diagram is constructed for Ho2Cu2O5 and Yb2Cu2O5 based on the results of isothermal magnetic and dielectric hysteresis loops.

Canted antiferromagnetic phases in the candidate layered Weyl material EuMnSb2

${\mathrm{EuMnSb}}_{2}$ is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. Here we present single-crystal neutron diffraction, magnetization, and heat-capacity data as well as polycrystalline $^{151}\mathrm{Eu}$ M\"ossbauer data which show that three AFM phases exist as a function of temperature, and we present a detailed analysis of the magnetic structure in each phase. The Mn magnetic sublattice orders into a C-type AFM structure below ${T}_{{\text{N}}_{\text{Mn}}}=323(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with the ordered Mn magnetic moment ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below ${T}_{{\text{N}}_{\text{Eu1}}}=23(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with the ordered Eu magnetic moment ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ canted away from the layer normal and ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ retaining its higher temperature order. ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}{a}^{\ensuremath{'}}$ with the chemical and magnetic unit cells having the same dimensions. Cooling below ${T}_{{\text{N}}_{\text{Eu2}}}=9(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ reveals a third AFM phase where ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ remains unchanged but ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ develops an additional substantial in-plane canting. This phase has MSG $P11\frac{{2}_{1}}{{a}^{\ensuremath{'}}}$. We also find some evidence of short-range magnetic correlations associated with the Eu between $12\phantom{\rule{4pt}{0ex}}\text{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}30\phantom{\rule{4pt}{0ex}}\text{K}$. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg model. We then discuss implications of the various AFM states in ${\mathrm{EuMnSb}}_{2}$ and their potential for tuning topological properties.

Crystal growth and magnetic properties of LaMn0.91Sb2 and NdMn0.88Sb2

In this work, single crystals of ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_{2}$ and ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_{2}$ with deficiencies of Mn have been successively grown by using the Bi-flux method. They all crystallize in the ${\mathrm{HfCuSi}}_{2}$-type tetragonal structure with space group $P4$/nmm (No.129). ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_{2}$ displays signs of successive antiferromagnetic transitions around 133 and 108 K for $H||c$. In the case of $H\ensuremath{\perp}c$, ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_{2}$ shows signs of weak ferromagnetic transition around 138 K and antiferromagnetic transition at 56 K. However, there is no anomaly observed in the temperature-dependent specific heat curve. For ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_{2}$ measured with $H||c$, the Mn sublattice orders antiferromagnetically below 170 K and the Nd sublattice shows successive antiferromagnetic orders at 34 and 26 K. The antiferromagnetic state of Nd sublattice can be induced to be a ferromagnetic state through a spin-flop transition at 2 K. When $H\ensuremath{\perp}\mathrm{c}$, the Mn sublattice displays antiferromagnetic orders at 171, 156, and 74 K. The Nd sublattice orders antiferromagnetically below 32 K. Ferromagnetic state for the Nd sublattice is induced by magnetic fields through a spin-flop transition at 2 K. Magnetic phase diagrams for ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_{2}$ are depicted for cases of $H||c$ and $H\ensuremath{\perp}c$, respectively. ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_{2}$ and ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_{2}$ show metallic behavior. Negative magnetoresistance in ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_{2}$ and ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_{2}$ are possibly related to canted-antiferromagnetic orders of Mn sublattices.

Magnetic structures of Mn-rich and of Fe-rich TmMn6−xFexSn6 stannides with (Mn, Fe) kagome networks and related Sn119 hyperfine magnetic fields

Stannides ${\mathrm{TmMn}}_{6\text{\ensuremath{-}}x}{\mathrm{Fe}}_{x}{\mathrm{Sn}}_{6}$, with hexagonal ${\mathrm{HfFe}}_{6}{\mathrm{Ge}}_{6}$-type structures, were studied by x-ray and neutron diffraction, magnetic measurements, and by $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy. Transition metal atoms form kagome networks in (001) planes with an intraplanar ferromagnetic coupling of their moments. At temperatures >\ensuremath{\sim}20 to 50 K, Mn-rich stannides ($x=0.4$, 0.6, 1.2) have an AFII easy-plane antiferromagnetic (AFM) structure, with interplanar couplings + + \ensuremath{-} \ensuremath{-} along the $c$ axis. The thulium sublattice orders magnetically at low temperature. The complexity of the resultant neutron diffraction patterns arises from a mixture of several magnetic phases, some being incommensurate. The Tm sublattice does not order >1.6 K in Fe-rich stannides ($x=4.25$, 4.5, 5.0). Fe-rich stannides have an AFI AFM structure, this time with interplanar couplings + \ensuremath{-} + \ensuremath{-}. The $M=\mathrm{Mn}/\mathrm{Fe}$ magnetic moments are directed along the $c$ axis for $x=5$ at any temperature. Further, neutron diffraction shows that moments rotate from the $c$ axis toward the basal plane >4.2 K with maximum rotation angles of \ensuremath{\sim}70\ifmmode^\circ\else\textdegree\fi{} and \ensuremath{\sim}55\ifmmode^\circ\else\textdegree\fi{} reached at \ensuremath{\sim}50 and \ensuremath{\sim}80 K for $x=4.25$ and 4.50, respectively. Transferred hyperfine fields at tin sites of ${\mathrm{TmMn}}_{6\text{\ensuremath{-}}x}{\mathrm{Fe}}_{x}{\mathrm{Sn}}_{6}$ stannides are the moduli of $^{119}\mathrm{Sn}$ vectorial hyperfine magnetic fields that are modeled for the AFI and AFII magnetic structures, assuming a random substitution of Mn with Fe. Models involve sums of dipolar-type and of isotropic vector components with simple assumptions about the model parameters. The transferred hyperfine magnetic fields at $^{119}\mathrm{Sn}$ nuclei are measured by M\"ossbauer spectroscopy. The hyperfine magnetic fields of Sn atoms, whose six transition metal nearest neighbors are ferromagnetically coupled, are predicted and observed to vary linearly with their number of Fe first nearest neighbors. The hyperfine magnetic fields on Sn atoms sandwiched between two AFM coupled kagome planes are expected proportional to $|{p}_{2}\ensuremath{-}{p}_{1}|$, where one of the two kagome planes contains ${p}_{1}$ Fe first nearest neighbors and the other ${p}_{2}$. Tin atoms with a ferromagnetic first nearest neighbor shell experience negative hyperfine fields in Mn-rich stannides, whereas they experience positive hyperfine fields in the Fe-rich stannides. This change is explained by a change of sign of the isotropic hyperfine magnetic fields which occurs when going from Mn- to Fe-rich stannides.

Magnetic structure of magnetoelectric multiferroic HoFeWO6

The polar magnetic oxide, HoFeWO6, is synthesized, and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoFeWO6 forms the polar crystal structure (space group Pna21 (#33)) due to the cation ordering of W6+ and Fe3+. An antiferromagnetic transition at TN = 17 K is accompanied by a significant change in magnetic entropy with a value of ≈ 5 J kg−1 K−1 in a 70 kOe applied field. Temperature dependent neutron diffraction and magnetization data indicate that the Fe sublattice orders in a strongly non-collinear and non-coplanar arrangement below TN. The Fe ordering initially leads to induced ordering of the Ho spins such that the Ho spins also show behavior of long-range ordering that is evident from the neutron diffraction measurements. Below T ≈ 4 K, the Ho spins order independently and pull the Fe spins toward the direction of Ho spins. A comparison with the magnetic structures and corresponding ferroelectric properties of other members of RMWO6 (R = Y, Sm-Tm, M = Cr, Fe, V) family indicate that the spontaneous polarization is due to the magnetic structure specific to the Fe sublattice through magnetoelectric coupling whereas the polarization is independent of the Ho sublattice.

Crystal and magnetic structure of polar oxide HoCrWO6

Polar magnetic oxide HoCrWO6 is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO6 forms the polar crystal structure (space group Pna21 (#33)) due to the cation ordering of W6+ and Cr3+. There is an antiferromagnetic transition at TN = 24.5 K along with the magnetic entropy change (~5 J.Kg.−1K−1 at 70 kOe). Neutron diffraction measurement indicates that both Cr and Ho sublattices are ordered with the moment of 2.32(5)μB and 8.7(4)μB at 2 K, respectively. While Cr forms A-type collinear antiferromagnetic (AFM) structure with magnetic moment along the b axis, Ho sublattice orders in a non-coplanar AFM arrangement. A comparison with isostructural DyFeWO6 and DyCrWO6 indicates that the magnetic structure of this family of compounds is controlled by the presence or absence of eg electrons in the transition metal sublattice.

Magnetic properties of EuFeAs2 and the 14 K superconductor EuFe0.97Ni0.03As2

The findings of a 57Fe and 151Eu Mössbauer spectroscopy investigation between 1.8 and 299.5 K of EuFeAs2 and the 14 K superconductor EuFe0.97Ni0.03As2 are reported. In both compounds, the Fe sublattice orders in the antiferromagnetic spin-density-wave fashion with the Néel temperatures and Fe saturation magnetic moments of 106.2(1.9) K, 0.78(1)μB and 56.6(2.2) K, 0.47(1)μB, respectively. The Néel temperatures and the saturation hyperfine magnetic fields of the antiferromagnetically ordered Eu sublattice in both compounds are 44.4(5) K, 294.2(7) kOe and 43.5(1) K, 290.5(1) kOe, respectively. The 3% substitution of Fe by Ni in EuFeAs2 has a dramatic effect on the magnetism of the Fe sublattice and virtually no effect on the magnetism of the Eu sublattice. The presence of Fe and Eu magnetic order in EuFe0.97Ni0.03As2 is direct proof of the coexistence of superconductivity and magnetism. The increase of the magnitude of the main component of the electric field gradient tensor, at both Fe and Eu sites, with decreasing temperature is explained by a T3/2 power-law relation. The Debye temperatures of EuFeAs2, EuFe0.97Ni0.03As2, and the FeAs2 impurity phase are 355(18), 428(14), and 594(25) K, respectively.

Magnetism in NdMn0.1Fe0.9O3 compound

The low temperature crystal and magnetic structures of NdMn0.1Fe0.9O3 were determined on the basis of X-ray and neutron powder diffraction experiments as well as from specific heat and magnetization measurements. The compound crystallizes in the orthorhombic crystal structure, space group Pnma, in the entire temperature interval 20 mK < T < 300 K. The lattice parameters exhibit standard thermal expansion effects for T > 20 K and at lower temperatures the anomalies due to magnetostriction effect were observed. The iron sublattice orders magnetically into 3 different magnetic structures, namely into Γ5 = (Ax, Fy, Gz) for 130 K < T < TN (Néel temperature); Γ1 = (Gx, Cy, Az) for 15 K < T < 130 K and Γ3 = (Cx, Gy, Fz) for 20 mK < T < 15 K. The Nd ions order at temperatures below 1.6–1.75 K into the same Γ3 phase as the iron sublattice at these temperatures. The obtained magnetic structures fit perfectly in between the magnetic phases of closely related NdFeO3 and NdMn0.5Fe0.5O3 compounds. Our study, together with all previously published data, completes the entire magnetic phase diagram of NdMn1−xFexO3 (0 ≤ x ≤ 1) solid solution substitutional system.

Contrasting exchange interactions in the new R3Pd5 (R = Tb, Dy, Ho, Er) compounds. Multiple magnetic transitions, spin-reorientation and frustration, as revealed by temperature dependent neutron diffraction studies: A question of sublattices?

In a recent work we have reported the formation and the crystal structure of the new family of compounds R3Pd5 (R = Sc, Y, Gd–Lu; Pu3Pd5-type, oS32, spacegroup 63, Cmcm). DFT calculations for Gd3Pd5 and heat capacity and magnetic measurements for Tb3Pd5, Dy3Pd5, Ho3Pd5 and Er3Pd5 were able to discriminate at least two distinct antiferromagnetic transitions at low temperatures; at the same time these data suggested these separate magnetic orderings to be likely associated to the two structurally different rare earth sublattices (4c and 8e, respectively). The magnetic structures of the R3Pd5 compounds, with R = Tb, Dy, Ho, Er, have been now investigated in this work. High-resolution and high-intensity neutron powder diffraction data were recorded for the four compounds of the series. The results reveal that the magnetic structures adopted are significantly diverse and their temperature dependence more complex than originally deduced from magnetic and heat capacity measurements. Tb3Pd5 is in fact the only of the four compounds where the rare earth sublattices order independently with two different magnetic propagation vectors on the two sites (k1 = [1 0 ½] for Tb2 and k2 = [kx 0 0] or k3 = [1 0 0] for Tb1). In Dy3Pd5 both sublattices follow the propagation vector k = [kx 0 0], with the second transition corresponding to a spin reorientation. In Ho3Pd5 ordering of both sublattices follow the k = [1 0 0] propagation vector; the second transition seen in the magnetic and heat capacity data corresponds to an accelerated increase in the magnetic moment of the Ho2 sublattice. The case of Er3Pd5 is special as it is the sole case where one of the rare earth sublattices does not order down to T = 1.5 K; here the second transition determined from the magnetic data corresponds to a change of the magnetic propagation vector from k1 = [kx 0 kz] to k2 = [1 0 0], both only acting on the Er2 sublattice. Irrespectively of the very similar structures not even two of the four studied R3Pd5 compounds show identical magnetic structures on either of the two sublattices. These strong variations have to be connected to changes in the RKKY interactions induced by the progressive filling of the f-orbitals of the heavy rare earth ions and the concomitant changes of the crystal electric fields.

Effects of oxygen annealing on magnetic properties of epitaxial PrNi0.5Mn0.5O3−δ thin films

A series of epitaxial thin films of PrNi0.5Mn0.5O3 (∼12 nm) were grown on single crystal LSAT [(LaAlO3)0.3(Sr2AlTaO6)0.7] substrate by pulsed laser deposition method. With a purpose to vary oxygen content in the films, the in situ oxygen annealing time was varied (0 to 5 min) during the thin film formation. One film was not annealed after the deposition in order to create high oxygen deficiency. This oxygen-deficient film grew with a different tensile strain. In spite of oxygen variation, all the films of this series show epitaxial growth [00l] on LSAT. Temperature-dependent resistivity and magnetization measurement were performed over a temperature range of 300 K to10 K. All the films show insulating behavior. The temperature-dependent magnetization shows two transitions in the system finally leading to a magnetically frustrated state at low temperatures. These results indicate that Ni and Mn sub-lattices order in antiparallel spin-states in these thin films. A comparative study of magnetization in polycrystalline bulk PrMn0.5Ni0.5O3 and these thin films shows that the epitaxial strain and oxygen content strongly influences the overall magnetic behavior of this system.

Exotic Magnetic Field-Induced Spin-Superstructures in a Mixed Honeycomb-Triangular Lattice System

The temperature-magnetic-field phase diagram of the mixed honeycomb triangular lattice system K$_{2}$Mn$_{3}$(VO$_{4}$)$_{2}$CO$_{3}$ is investigated by means of magnetization, heat capacity and neutron scattering measurements. The results indicate that triangular and honeycomb magnetic layers undergo sequential magnetic orderings and act as nearly independent magnetic sublattices. The honeycomb sublattice orders at about 85 K in a Ne\'{e}l-type antiferromagnetic structure, while the triangular sublattice displays two consecutive ordered states at much lower temperatures, 3 K and 2.2 K. The ground state of the triangular sublattice consists of a planar `Y' magnetic structure that emerges from an intermediate collinear `up-up-down' state. Applied magnetic fields parallel or perpendicular to the $c$-axis induce exotic ordered phases characterized by various spin-stacking sequences of triangular layers that yield bilayer, three-layer or four-layer magnetic superstructures. The observed superstructures cannot be explained in the framework of quasi-classical theory based only on nearest-neighbor interlayer coupling and point towards the presence of effective second-nearest-neighbor interactions mediated by fluctuations of the magnetic moments in the honeycomb sublattice.

Deciphering structural and magnetic disorder in the chiral skyrmion host materials Co x Zn y Mn z (x + y+ z = 20).

Co x Zn y Mn z (x + y + z = 20) compounds crystallizing in the chiral β-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, Co x Zn y Mn z compounds have also been found to host metastable nonequlibrium skyrmion lattices in a broad temperature and field range, including down to zero field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and dc and ac magnetic measurements to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site preference for the diamondoid 8c site in the crystal structure, while Mn tends to share the geometrically frustrated 12d site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich 8c sublattice and the Mn on the 12d sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 and 470 K) with a moment around 1μ B /atom and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about 3μ B ) which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

Incompressible even denominator fractional quantum Hall states in the zeroth Landau level of monolayer graphene

Incompressible even denominator fractional quantum Hall states at fillings $\nu = \pm \frac{1}{2}$ and $\nu = \pm \frac{1}{4}$ have been recently observed in monolayer graphene. We use a Chern-Simons description of multi-component fractional quantum Hall states in graphene to investigate the properties of these states and suggest variational wavefunctions that may describe them. We find that the experimentally observed even denominator fractions and standard odd fractions (such as $\nu=1/3, 2/5$, etc.) can be accommodated within the same flux attachment scheme and argue that they may arise from sublattice or chiral symmetry breaking orders (such as charge-density-wave and antiferromagnetism) of composite Dirac fermions, a phenomenon unifying integer and fractional quantum Hall physics for relativistic fermions. We also discuss possible experimental probes that can narrow down the candidate broken symmetry phases for the fractional quantum Hall states in the zeroth Landau level of monolayer graphene.

Crystal and magnetic properties of YbMn6−yFeySn6 (y ≤ 1)

We have synthesized the new YbMn6−yFeySn6 alloys (y = 0.25, 0.50, 0.75, and 1.00) and investigated their crystal and magnetic properties using X-ray diffraction and magnetization measurements. All alloys crystallize in the orthorhombic HoFe6Sn6 type of structure (Immm) with cell parameters shrinking upon increasing the iron content. In these alloys, the T = Mn/Fe sublattice orders ferromagnetically near TC ∼ 270 K. Yb does not magnetically order and its valency is estimated to be υ ∼ 2.6–2.7 from lattice constants. Guidelines are proposed to go through the Yb magnetic instability and the quantum phase transition associated with. The YbMn5.75Fe0.25Sn6 compound is a pure ferromagnet over the whole ordered temperature range, as the ternary YbMn6Sn6 parent compound, while the richer Fe alloys evolve upon cooling towards a low-magnetization state through a first-order transition. A field-induced metamagnetic transition allows recovering the ferromagnetic alignment and the alloys present maximal magnetizations of close magnitude (∼12.5 µB/f.u. at 5 K).

Magnetic structure of Tb2Ir2O7 determined by powder neutron diffraction

We report on the determination of the full magnetic structure of ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ including the Ir sublattice by means of powder neutron diffraction. Below ${T}_{N}\ensuremath{\sim}125\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, both the Tb and the Ir sublattices order in a so-called all-in/all-out magnetic structure. However, additional magnetic intensity appears below 10 K on the (111) reflection indicating a change in the magnetic structure. We were able to describe the low-temperature magnetic structure with an additional $XY$ component on the Tb sublattices. The moment sizes at 1.5 K amount to $0.55(3){\ensuremath{\mu}}_{B}/\mathrm{Ir}$ and $5.22(3){\ensuremath{\mu}}_{B}/\mathrm{Tb}$. The ${\mathrm{Tb}}^{3+}$ moments are titled about ${6}^{\ensuremath{\circ}}$ off from the [111] direction. Moreover, it was found that the Tb moments couple antiferromagnetically with their six nearest-neighboring Ir moments below ${T}_{N}$. This is in contrast to the case of ${\mathrm{Nd}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. The ${\mathrm{Ir}}^{4+}$ moment sizes of ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ are larger than that of ${\mathrm{Nd}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ indicating that hybridization effects between the Ir $5d$ and the O $2p$ orbitals are smaller in ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$.

Induced quadrupolar singlet ground state of praseodymium in a modulated pyrochlore

The complex structure and magnetism of Pr$_{2-x}$Bi$_x$Ru$_2$O$_7$ was investigated by neutron scattering and EXAFS. Pr has an approximate doublet ground-state and the first excited state is a singlet. This overall crystal field level scheme is similar to metallic Pr$_2$Ir$_2$O$_7$, which is also reported here. While the B-site (Ru) is well ordered throughout, this is not the case for the A-site (Pr/Bi). A distribution of the Pr-O2 bond length indicates the Pr environment is not uniform even for $x=0$. The Bi environment is highly disordered ostensibly due to the 6s lone pairs on Bi$^{3+}$. Correspondingly we find the non-Kramers doublet ground state degeneracy otherwise anticipated for Pr in the pyrochlore structure is lifted so as to produce a quadrupolar singlet ground state with a spatially varying energy gap. For $x=0$, below T$_N$, the Ru sublattice orders antiferromagnetically, with propagation vector \textbf{k}= (0,0,0), as for Y$_2$Ru$_2$O$_7$. No ordering associated with the Pr sublattice is observed down to 100 mK. The low energy magnetic response of Pr$_{2-x}$Bi$_x$Ru$_2$O$_7$ features a broad spectrum of magnetic excitations associated with inhomogeneous splitting of the Pr quasi-doublet ground state. For $x=0$ ($x=0.97$) the spectrum is temperature dependent (independent). It appears disorder associated with Bi alloying enhances the inhomogeneous Pr crystal field level splitting so that inter-site interactions become irrelevant for $x=0.97$. The structural complexity for the A-site may be reflected in the hysteretic uniform magnetization of B-site ruthenium in the N\'{e}el phase.