Dc Magnetic Susceptibility Research Articles

Low temperature magnetic properties of the quasicrystalline Al80Mn12Si4X4 (X = Cr, Y, Sm and Nd) alloys

Quasicrystalline Al80Mn12Si4X4 (X = Cr, Y, Sm and Nd) alloys were produced and their magnetic properties were investigated. X-ray diffraction and scanning electron microscopy revealed the presence of atomic arrangements with pentagonal symmetry. The nature of the coupling of the magnetic moments was investigated for temperatures (T) varying from 2.0 to 300.0 K and for applied magnetic fields in the range ± 85.0 kOe. The dc magnetic susceptibility indicated that the net magnetic interactions are antiferromagnetic for the samples with Cr and Y. However, the samples with Sm and Nd showed to be ferromagnetic at low-T. The coercivity and the remanence determined from hysteresis loops at 5 K for the sample with Sm (Nd) was, respectively, 0.54 kOe (0.40 kOe) and 0.047 emu/g (0.19 emu/g). A spin-glass-like phase was observed for the AlMnSiNd (AlMnSiSm) sample below 2.7 K (2.4 K). The results are accounted for by taking into consideration the coupling among the rare earth magnetic moments.

Spin Crossover in Bipyridine Derivative Bridged One-Dimensional Iron(III) Coordination Polymer

Herein, the syntheses, solid-state molecular structures, and characterization of two types of one-dimensional FeIII coordination polymers showing thermally induced spin crossover are reported. The reaction of [Fe(acen)Cl] (acen2− = N,N′-ethylenebis(acetylacetonylideneaminate) with 3,3′-bpy or 4,4′-bpy (bpy = bipyridine) produced zigzag and linear one-dimensional chain complexes, [Fe(acen)(3,3′-bpy)][BPh4] (1) or [NEt3H][Fe(acen)(4,4′-bpy)][BPh4]2·0.5(4,4′-bpy) (2), respectively, as confirmed by single crystal X-ray diffraction analysis. Variable-temperature single crystal X-ray diffraction measurements, continuous-wave X-band electron paramagnetic resonance (EPR) spectra, 57Fe Mössßauer spectra, and DC magnetic susceptibility data revealed that complex 1 exhibited a gradual and complete spin crossover at a transition temperature of 212 K, while complex 2 undergoes an incomplete spin crossover even at 400 K.

Long-Range Ferromagnetic Exchange Interactions Mediated by Mn-CeIV-Mn Superexchange Involving Empty 4f Orbitals.

Reactions involving reductive aggregation of MnO4- in methanol in the presence of CeIV and an excess of carboxylic acid have led to the synthesis of structurally related Ce/Mn clusters, [Ce3Mn5O8(OMe)(O2CBut)13(MeOH)] (1) and [Ce2Mn3O5(O2CPh)9(MeOH)3] (2), containing at least one {Mn2Ce2O4} cubane unit. The cores of both clusters contain Mnx units separated by three (1) or two (2) CeIV ions. Fits of variable-temperature, solid-state dc and ac magnetic susceptibility data reveal dominant ferromagnetic interactions within 1 and 2, resulting in the maximum S = 17/2 and S = 5 ground state spins, respectively, and thus suggesting significant ferromagnetic (F) interactions between the Mnx units that are ≥6 Å apart and separated by four intervening bonds through diamagnetic CeIV. Fits of magnetic susceptibility data also revealed unusual long-range F interactions, and this finding was further supported by high-field EPR measurements and simulations. Density functional theory calculations and a Wannier function analysis confirm long-range interactions and indicate a Mn-Ce-Mn superexchange pathway via Mn-d/Ce-f orbital overlap/hybridization.

Magnetic Properties of a New Hexahalorhenate(IV) Compound and Structural Comparison with Its Hexahaloplatinate(IV) Analog

Two mononuclear complexes of formula (Et3BzN)2[ReCl6] (1) and (Et3BzN)2[PtCl6] (2) (Et3BzN+ = triethylbenzylammonium cation) have been synthesized in good yield (70 and 75 %, respectively) and structurally characterized. From a structural point of view, they both crystallized in the monoclinic centrosymmetric P21/n space group and the two molecular structures are almost perfectly superimposable, with only minor differences in the M–Cl bond lengths. Solid‐state direct‐current (dc) magnetic susceptibility analysis in the temperature range 1.9–300 K for 1 reveal the occurrence of magnetically isolated rhenium(IV) ions with a significant zero‐field splitting (zfs), D = +6.7 cm–1 (2D being the energy gap between the MS = ±3/2 and MS = ±1/2 Kramers doublets). DFT, CASSCF and CASSCF/NEVPT2 theoretical calculations were carried out on the experimental geometry of the complex [ReCl6]2– in 1 to substantiate the parameters defining the zfs tensor. Alternating current (ac) magnetic susceptibility measurements of 1 in the temperature 2.0–10 K show incipient frequency–dependence of the out‐of‐phase signals below 3.0 K under non‐zero applied dc fields supporting a field–dependent Single Ion Single Molecule Magnet (SISMM) behaviour of this rhenium(IV) derivative.

Effect of Coordination Geometry on Magnetic Properties in a Series of Cobalt(II) Complexes and Structural Transformation in Mother Liquor.

The three Co(II) complexes [Co(bbp)2][Co(NCS)4]·4DMF (1), [Co(bbp)(NCS)2(DMF)]·2DMF (2), and [Co(bbp)(NCS)2] (3) have been synthesized and characterized by single-crystal X-ray diffraction, magnetic, and various spectroscopic techniques. Complexes 1 and 3 are obtained by the reaction of Co(NCS)2 with 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine (bbp), and complex 1 undergoes a structural transformation to form complex 2. A single-crystal X-ray study revealed that complex 1 is comprised of two Co(II) centers, a cationic octahedral Co(II) unit and an anionic tetrahedral Co(II) unit, while the Co(II) ion is in a distorted-octahedral environment in 2. Moreover, in complex 3, the Co(II) ion is in a distorted-square-pyramidal geometry. The effect of coordination geometry on the magnetic properties was studied by both static and dynamic magnetic measurements. Direct current (dc) magnetic susceptibility measurements showed that all of the Co(II) ions are in high-spin state in these complexes. Alternating current (ac) magnetic susceptibility measurements indicated that complexes 2 and 3 display slow relaxation of magnetization in an external dc magnetic field, while complex 1 displayed no such property. EPR experiments and theoretical calculations were consistent with the above findings.

Two new 4-coordinate Cu(II) chains with 1,2,3-triazole derivate as bridging ligand: Synthesis, structures and magnetic properties

Two one-dimensional Cu(II) coordination polymers, [Cu(tdp)X2]n (X = Cl(1), Br(2), tdp = 2,2’-(1H-1,2,3-triazole-1,5-diyl)dipyridine) have been synthesized and characterized, which are the first two examples of 1D CuII chains bridging by the rigid 1,2,3-triazole ligand. Compounds 1 and 2 are isomorphous. Single-crystal X-ray diffraction revealed that the ligands bridge adjacent CuII ions into an infinite one-dimensional chain with a zigzag motif in compound 1. Meanwhile, the presence of weak face-to-face π···π interactions between pyridine ring planes of two adjacent chains leads to the stabilization of the molecular packing. DC magnetic susceptibility measurements indicated the presence of weak ferromagnetic interactions (J = 1.91 cm−1 for 1, J = 0.48 cm−1 for 2) between adjacent CuII ions within the chains and antiferromagnetic interchain coupling (J′ = −0.20 cm−1 for 1, J′ = −0.10 cm−1 for 2) due to the π···π interactions.

Field-Induced Single-Ion Magnet Phenomenon in Hexabromo- and Hexaiodorhenate(IV) Complexes

Two mononuclear ReIV complexes of general formula (PPh4)2[ReX6] [PPh4+ = tetraphenylphosphonium cation, X = Br (1) and I (2)] have been prepared and structurally and magnetically characterised. Both compounds crystallise in the triclinic system with space group Pī. Their structures are made up of hexahalorhenate(IV), [ReX6]2−, anions, and bulky PPh4+ cations. Each ReIV ion in 1 and 2 is six-coordinate and bonded to six halide ions in a quasi regular octahedral geometry. In their crystal packing, the [ReX6]2− anions are well separated from each other through the organic cations, generating alternated anionic and cationic layers, and no intermolecular Re−X···X−Re interactions are present. Variable-temperature dc magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2 show a very similar magnetic behaviour, which is typical of noninteracting mononuclear ReIV complexes with S = 3/2. Ac magnetic susceptibility measurements reveal the slow relaxation of the magnetisation in the presence of external dc fields for 1 and 2, hence indicating the occurrence of the field-induced single-ion magnet (SIM) phenomenon in these hexabromo- and hexaiodorhenate(IV) complexes.

Optimized in situ crystal growth and disordered quasi-one-dimensional magnetism in Li2Mn2(MoO4)3

The quasi-one-dimensional structure of ${\mathrm{Li}}_{2}{\mathrm{Mn}}_{2}{({\mathrm{MoO}}_{4})}_{3}$ consists of three mutually distinct chains of ${\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$-centered polyhedra in which Mn ostensibly adopts a $J=5/2\phantom{\rule{4pt}{0ex}}{\mathrm{Mn}}^{2+}$ configuration. In situ x-ray scattering experiments carried out as crystallites emerge from a molten oxide solution facilitate the synthesis of large single crystals. Ex situ x-ray diffraction finds no evidence of long-range Li/Mn occupancy ordering, suggesting that the structure is effectively composed of finite chains of Mn moments of statistically varying lengths. UV/visible diffuse reflectance spectroscopy measurements establish a wide 3.43(12)-eV direct charge gap consistent with the local polyhedral coordination of the nominally ${\mathrm{Mn}}^{2+}$ species. The temperature $T$ dependence of the DC magnetic susceptibility $\ensuremath{\chi}$ reveals a fluctuating moment of only $2.74{\ensuremath{\mu}}_{B}\ifmmode\pm\else\textpm\fi{}0.01{\ensuremath{\mu}}_{B}$/Mn, dramatically reduced from the $5.9{\ensuremath{\mu}}_{B}$/Mn expected for ${\mathrm{Mn}}^{2+}$. Meanwhile, the Weiss temperature ${\mathrm{\ensuremath{\Theta}}}_{W}=\ensuremath{-}89\ifmmode\pm\else\textpm\fi{}1$ K reveals antiferromagnetic fluctuations that are stymied from reaching an ordered state apparently by the chemical disorder intrinsic to the polyhedral chains. Measurements of magnetization vs field $H$ at $T\ensuremath{\le}10$ K are far from saturation even at $H=5$ T and are strongly non-Brillouin-like, instead scaling as $H/{T}^{0.24(3)}$ and suggesting the presence of quantum fluctuations associated with an eventual quasi-one-dimensional, disordered magnetic phase.

Two hexanuclear [Co2III-Ln4III] clusters including a [Co2III-Dy4III] single molecule magnet

Structural determination and magnetic studies of two 3d-4f hexanuclear [CoIII2LnIII4(μ3-OH)2(NO3)4(OAc)4L4(DMF)2]·2C2H5OH [Ln = Dy(1), Gd(2); H2L = (E)-4-chloro-2-(((2-(hydroxymethyl)phenyl)imino)methyl)phenol] clusters are reported. Single-crystal X-ray diffraction studies reveal that the centrosymmetric hexanuclear metallic core is composed of a defect-dicubane Ln4 moiety and two CoIII ions. The Ln4 defect-dicubane central core is composed of four LnIIIions, which are supported by two oxygen atoms from the acetate groups, two alkoxo groups and two μ3-OH groups, which are displaced above and below the Ln4plane. And two CoIII ions (one positioned above and one below the Ln4 plane) are linked to the defect-dicubane unit through two alkoxo groups and two phenoxido groups. This structure gives a CIS chair-like topology. Variable temperature direct current (dc) magnetic susceptibility measurements reveal the LnIII ions in complex 1 are ferromagnetic coupling, while antiferromagnetic coupling in 2. Complex 1 also displays an out-of-phase signal in the alternating current susceptibility indicating slow relaxation of the magnetization with Ueff = 31.6 K and τ0 = 5.66 × 10−6 s.

The Synthesis of a Quasi-One-Dimensional Iron-Based Telluride with Antiferromagnetic Chains and a Spin Glass State.

The report on the superconductivity of the two-legged spin ladders BaFe2S3 and BaFe2Se3 has established 123-type iron chalcogenides as a novel subgroup in the iron-based superconductor family and has stimulated the continuous exploration of other iron-based materials with new structures and potentially novel properties. In this paper, we report the systematic study of a new quasi-one-dimensional (1D) iron-based compound, Ba9Fe3Te15, including its synthesis and magnetic properties. The high-pressure synthesized Ba9Fe3Te15 crystallized in a hexagonal structure that mainly consisted of face-sharing FeTe6 octahedral chains running along the c axis, with a lattice constant of a = 10.23668 Å; this led to weak interchain coupling and an enhanced one-dimensionality. The systematic static and dynamic magnetic properties were comprehensively studied experimentally. The dc magnetic susceptibility showed typical 1D antiferromagnetic characteristics, with a Tmax at 190 K followed by a spin glass (SG) state with freezing at Tf ≈ 6.0 K, which were also unambiguously proved by ac susceptibility measurements. Additionally, X-ray magnetic circular dichroism (XMCD) experiments revealed an unexpected orbital moment for Fe2+, i.e., 0.84 μB per Fe in Ba9Fe3Te15. The transport property is electrically insulating, with a thermal activation gap of 0.32 eV. These features mark Ba9Fe3Te15 as an alternative type of iron-based compound, providing a diverse candidate for high-pressure studies in order to pursue some emerging physics.

Strong anisotropy in the mixed antiferromagnetic system Mn1−xFexPSe3

We report the magnetic phase diagram of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PSe}}_{3}$, which represents a random magnet system of two antiferromagnetic systems with mixed spin, mixed spin anisotropies, mixed nearest-neighbor magnetic interactions, and mixed periodicities in their respective antiferromagnetic structure. Bulk samples of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PSe}}_{3}$ have been prepared and characterized phase pure by powder x-ray and neutron diffraction and x-ray fluorescence. Nature and extent of magnetically ordered state has been established using powder neutron diffraction, dc magnetic susceptibility, and heat capacity. Long-range magnetic ordering exists between $x=0.0$ and 0.25 (${\mathrm{MnPSe}}_{3}$ type) and between $x=0.875$ and 1 (${\mathrm{FePSe}}_{3}$ type). A short-range magnetic order with the existence of both ${\mathrm{MnPSe}}_{3}$- and ${\mathrm{FePSe}}_{3}$-type nanoclusters has been established between $x=0.25$ and 0.875. Irreversibility in dc magnetization measurements, also characterized by isothermal and thermoremanent magnetization measurements, suggest similarities to magnetic nanoparticles where uncompensated surface spins result in diverging thermoremanent and isothermal remanent magnetization responses, further reinforcing existence of magnetic nanoclusters or domains. A spin-glass state, observed in analogous ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PS}}_{3}$, has been ruled out, and formation of nanoclusters exhibiting both ordering types results from unusually high anisotropy values. The effect of ligand contributions to the spin-orbit interactions has been suggested as a possible explanation for high $D$ values in these compounds.

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe2 Complexes.

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged FeII2 complexes [(Me3TPyA)2Fe2(RL)]n+ (Me3TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: OMeLH2 = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, ClLH2 = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na2[NO2L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; n = 4: SMe2L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between FeII centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO2, SMe2) cm-1. In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm-1 for R = OMe, Cl, NO2, and SMe2, respectively. The minimal effects of substituents in the 3- and 6-positions of RLx-• on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of Ueff = 50(1), 41(1), 38(1), and 33(1) cm-1 for R = OMe, Cl, NO2, and SMe2, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.

Effect of Axial Ligands on Easy-Axis Anisotropy and Field-Induced Slow Magnetic Relaxation in Heptacoordinated FeII Complexes.

A rational approach to modulating easy-axis magnetic anisotropy by varying the axial donor ligand in heptacoordinated FeII complexes has been explored. In this series of complexes with formulae of [Fe(H4 L)(NCS)2 ]⋅3 DMF⋅0.5 H2 O (1), [Fe(H4 L)(NCSe)2 ]⋅3 DMF⋅0.5 H2 O (2), and [Fe(H4 L)(NCNCN)2 ]⋅DMF⋅H2 O (3) [H4 L=2,2'-{pyridine-2,6-diylbis(ethan-1-yl-1-ylidene)}bis(N-phenylhydrazinecarboxamide)], the axial positions are successively occupied by different nitrogen-based π-donor ligands. Detailed dc and ac magnetic susceptibility measurements reveal the existence of easy-axis magnetic anisotropy for all of the complexes, with 1 [Ueff =21 K, τ0 =1.72×10-6 s] and 2 [Ueff =25 K, τ0 =2.25×10-6 s] showing field-induced slow magnetic relaxation behavior. However, both experimental studies and theoretical calculations indicate the magnitude of the D value of complex 3 to be larger than those of complexes 1 and 2 due to the axial bond angle being smaller than that for an ideal geometry. Detailed analysis of the field and temperature dependences of relaxation time for 1 and 2 has revealed that multiple relaxation processes (quantum tunneling of magnetization, direct, and Raman) are involved in slow magnetic relaxation for both of these complexes. Magnetic dilution experiments support the role of intermolecular short contacts.

‘Metal Complexes as Ligands’ for the Synthesis of Coordination Polymers: A MnIII Monomer as a Building Block for the Preparation of an Unprecedented 1-D {MnIIMnIII}n Linear Chain

A relatively unexplored synthetic route in redox-active Mn(II/III) coordination chemistry has been employed toward the preparation of a new mixed-valence MnII/III 1-D linear chain from the reaction of [MnIII(sacb)2]− precursor with a MnII source, where sacbH2 is the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid. The mononuclear (Pr2NH2)[MnIII(sacb)2] (1) compound was obtained in excellent yields (>85%) from the 1:2:3 reaction of Mn(O2CMe)2∙4H2O, sacbH2 and Pr2NH, respectively. In 1, the two doubly deprotonated sacb2− ligands act as Ocarboxylate,Nimine,Ophenoxide-tridentate chelates, while the second carboxylate O atom of sacb2− is dangling and H-bonded to the Pr2NH2+ countercation. Complex 1 was subsequently used as a ‘ligand’ to react stoichiometrically with the ‘metal’ Mn(NO3)2∙4H2O, thus leading to the 1-D coordination polymer {[MnIIMnIII(sacb)2(H2O)2(MeOH)2](NO3)}n (2) in good yields (~50%). The removal of Pr2NH2+ from the vicinity of the [MnIII(sacb)2]− metalloligand has rendered possible (vide infra) the coordination of the second Ocarboxylate of sacb2− to neighboring {MnII(H2O)2(MeOH)2}2+ units, and consequently the formation of the 1-D polymer 2. Direct-current (dc) magnetic susceptibility studies revealed the presence of very weak antiferromagnetic exchange interactions between alternating MnIII and MnII atoms with a coupling constant of J = −0.08 cm−1 for g = 2.00. The combined results demonstrate the potential of the ‘metal complexes as ligands’ approach to yield new mixed-valence Mn(II/III) coordination polymers with interesting structural motifs and physicochemical properties.

Joint Inversion of DC Resistivity and Magnetic Data, Constrained by Cross Gradients, Compactness and Depth Weighting

In this paper we perform a 2-D joint inversion of DC resistivity and magnetic data, constrained by cross-gradients. Inspired by methods developed for potential fields, we introduce into both the separate and joint inversion algorithms also compactness and depth weighting functions, under the form of a model weighting-function. These constraints, usually not considered for DC resistivity inversion, reveal to be decisive for its joint inversion with magnetic data. A linear approximated forward problem of the resistivity is used for the joint inversion so that both the resistivity and magnetic problems are expressed as a linear integral equation under the form of a Fredholm integral of 1st kind. To examine the feasibility of the joint inversion algorithm, we first test the method with two synthetic cases: a thick dyke in a two-layered medium and a cavity located above a conductor. A third synthetic case involves a multisource model. The results are encouraging, revealing that the cross-gradient constraint is an effective tool to improve the separate inversions of DC resistivity and magnetic data. The joint inversion algorithm is also applied to data in the archeological area of the old Pompeii city, nearby Naples. Comparing the results of joint and separate inversions, we obtain a significant improvement in the interpretation of both kind of data in terms of buried walls of an ancient roman villa. In all the studied cases, the cross-gradient constraint appears to be a key-diagnostic tool to assess whether actual coherence is gained among DC resistivity and magnetic susceptibility models.

Synthesis, structure and physical properties of new intermetallic spin glass-like compounds RE2PdGe3 (RE = Tb and Dy)

New intermetallic compounds Tb2Pd1.25Ge2.75 and Dy2Pd1.25Ge2.75 have been synthesized using the arc-melting method. The crystallographic structure and magnetic, electronic transport, and thermal properties are reported. The crystal structure obtained from powder x-ray diffraction analysis suggests that these compounds crystallize in the AlB2-type structure (space group P6/mmm, no. 191) with lattice parameters a = 4.228 53(5)/4.230 54 (2) Å and c = 3.942 25(9)/3.945 52(5) Å for the compounds with Tb and Dy respectively. The ac and dc magnetic susceptibility studies reveal spin-glass like behavior, with freezing temperature Tf = 10.5 K for Tb2Pd1.25Ge2.75 and 4.5 K for Dy2Pd1.25Ge2.75. These data are in good agreement with the heat capacity measurements.

Two pentanuclear clusters: synthesis, crystal structures, luminescent and magnetic properties

Reactions of N,2-dihydroxy-3-methoxybenzamide (H3L) with transition metal acetates in the presence of pyridine and/or 2,2′-bipyridine under solvothermal conditions give two pentanuclear complexes, [Zn5L2(CH3COO−)4(py)3] (1) and [Co5L2(CH3COO−)4(py)(2,2′-bipy)2] (2). Photoluminescence studies indicate strong purple emission for 1 in the solid state at room temperature. Variable-temperature dc magnetic susceptibility studies of 2 reveal that antiferromagnetic CoII···CoII exchange interactions exist in 2.

High pressure synthesis and characterization of the pyrochlore Dy2Pt2O7: A new spin ice material**Project supported by the National Key R&D Program of China (Grant No. 2018YFA0305700), the National Natural Science Foundation of China (Grant Nos. 11834016, 11874400, and 11921004), the Beijing Natural Science Foundation, China (Grant No. Z190008), the Key Research Program of Frontier Sciences of the Chinese Academy of

The cubic pyrochlore Dy2Pt2O7 was synthesized under 4 GPa and 1000 °C and its magnetic and thermodynamic properties were characterized by DC and AC magnetic susceptibility and specific heat down to 0.1 K. We found that Dy2Pt2O7 does not form long-range magnetic order, but displays characteristics of canonical spin ice such as Dy2Ti2O7, including (1) a large effective moment 9.64 μB close to the theoretical value and a small positive Curie–Weiss temperature θCW = +0.77 K signaling a dominant ferromagnetic interaction among the Ising spins; (2) a saturation moment ∼4.5 μB being half of the total moment due to the local 〈111〉 Ising anisotropy; (3) thermally activated spin relaxation behaviors in the low (∼1 K) and high (∼20 K) temperature regions with different energy barriers and characteristic relaxation time; and most importantly, (4) the presence of a residual entropy close to Pauling’ estimation for water ice.

Structural and Magnetic Studies of ABO4-Type Ruthenium and Osmium Oxides.

Oxides of the form ABO4 with A = K, Rb, Cs and B = Ru and Os have been synthesized and characterized by diffraction and magnetic techniques. For A = K the oxides adopted the tetragonal (I41/a) scheelite structure. RbOsO4, which crystallizes as a scheelite at room temperature, underwent a continuous phase transition to I41/amd near 550 K. RbRuO4 and CsOsO4 were found to crystallize in the orthorhombic (Pnma) pseudoscheelite structure, and both displayed discontinuous phase transitions to I41/a at high temperatures. CsOsO4 was determined to undergo a phase transition to a P21/c structure below 140 K. CsRuO4 crystallizes with a baryte-type structure at room temperature. Upon heating CsRuO4 a first order phase transition to the scheelite structure in I41/a is observed at 400 K. A continuous phase transition is observed to P212121 below 140 K. DC magnetic susceptibility data is consistent with long-range antiferromagnetic ordering at low temperatures for all compounds except for CsOsO4, which is paramagnetic to 2 K. The effective magnetic moments are in agreement with the spin only values for an S = 1/2 quantum magnet. Effective magnetic moments calculated for Os compounds were lower than their Ru counterparts, reflective of an enhanced spin orbit coupling effect. A magnetic structure is proposed for RbRuO4 consisting of predominately antiferromagnetic (AFM) ordering along the 001 direction, with canting of spins in the 100 plane. A small ordered magnetic moment of 0.77 μB was determined.

Randomly Hopping Majorana Fermions in the Diluted Kitaev System α-Ru_{0.8}Ir_{0.2}Cl_{3}.

dc and ac magnetic susceptibility, magnetization, specific heat, and Raman scattering measurements are combined to probe low-lying spin excitations in α-Ru_{1-x}Ir_{x}Cl_{3} (x≈0.2), which realizes a disordered spin liquid. At intermediate energies (ℏω>3 meV), Raman spectroscopy evidences linearly ω-dependent Majorana-like excitations, obeying Fermi statistics. This points to robustness of a Kitaev paramagnetic state under spin vacancies. At low energies below 3meV, we observe power-law dependences and quantum-critical-like scalings of the thermodynamic quantities, implying the presence of a weakly divergent low-energy density of states. This scaling phenomenology is interpreted in terms of the random hoppings of Majorana fermions. Our results demonstrate an emergent hierarchy of spin excitations in a diluted Kitaev honeycomb system subject to spin vacancies and bond randomness.