Magnetic Behavior of R2Ba2CuPtO8 Oxides (R=Ho, Er, Tm, Yb, Lu, and Y)

Magnetic properties of polycrystalline R₂Ba₂CuPtO₈ (R=Ho, Er, Tm, Yb, Lu and Y) oxides have been studied from magnetization and magnetic susceptibility measurements. The lutetium and yttrium oxides behave as antiferromagnets and the estimated Néel temperatures are 5.2 and 5.7 K, respectively. In the case of the remaining R₂Ba₂CuPtO₈ oxides, Cu²⁺ and R³⁺ become antiferromagnetically ordered simultaneously, with the exception of TmBa₂CuPtO₈, where the χ vs T plot exhibits two maxima at 8 and 5 K, which have been assigned to the Néel temperatures of Cu²⁺ and Tm³⁺ sublattices, respectively. Taking into account the structure, a superexchange mechanism of the type R-O-Cu-O-R has been proposed in which the Cu²⁺ sublattice plays an important role as promoter of the antiferromagnetic interactions of ferromagnetically R³⁺ coupled in the a-c plane of the structure. Field-induced metamagnetic transitions have been observed below the Néel temperature in all cases; however, different critical fields are achieved depending on the nature of R³⁺ ions.     

Synthesis, structures and magnetic properties of the new vanadates AgMnVO4 and RbMnVO4

The new compounds, AgMnVO₄ and RbMnVO₄ have been synthesized by solid state reaction route. Their crystal structures were determined from single-crystal X-ray diffraction data for RbMnVO₄ and powder X-ray diffraction data for AgMnVO₄. AgMnVO₄ crystallizes with the maricite-type structure in space group Pnma, a=9.5778(6) Å, b=6.8518(4) Å, c=5.3734(3) Å and Z=4. RbMnVO₄ crystallizes in space group P6₃ with a stuffed tridymite-type structure, a=11.2584(3), c=8.9893(13) Å and Z=8. A merohedral twinning was taken into account for its structural refinement. To our knowledge this is the first vanadate showing this structural type. AgMnVO₄ and RbMnVO₄ were characterized by magnetic susceptibility and specific heat measurements. AgMnVO₄ is antiferromagnetic with a Néel temperature of 12.3 K determined by specific heat measurements. RbMnVO₄ exhibits canted antiferromagnetism with a Néel temperature of 6.5 K.     

Crystal structures, physical properties and NMR experiments on the ternary rare-earth metal silicide boride compounds RE5Si2B8 (RE=Y, Sm, Gd, Tb, Dy, Ho)

The ternary rare-earth metal silicide borides RE₅Si₂B₈ (RE=Y, Sm, Gd, Tb, Dy Ho) were prepared by arc melting the elemental components and subsequent annealing up to . The crystal structure was determined for each term of the series from single-crystal X-ray data: tetragonal symmetry, space group P4/mbm, Z=2; unit cell parameters a=7.2616(3), and a=7.1830(2), for Sm₅Si₂B₈ and Ho₅Si₂B₈, respectively. The structure is a new type and can be structurally described as an intergrowth of ThB₄-like and U₃Si₂-like slabs of composition REB₄ and RE₃Si₂, respectively, alternating along the c direction. The boron and silicon substructures are wholly independent and well ordered. The magnetic properties are as follows: Y₅Si₂B₈ is a Pauli-type paramagnet above 1.8 K, Gd₅Si₂B₈ undergoes a weak (canted) ferromagnetic-like order at 70 K followed by a colinear antiferromagnetic spin alignment at 44 K. Tb₅Si₂B₈ and Dy₅Si₂B₈ order antiferromagnetically at a Néel temperature of T_N=45 and 28 K, respectively. In the paramagnetic regime, the effective moments are in good accord with the theoretical RE³⁺ free ion moments. The temperature dependence of the electrical resistivities for the Y, Gd, Tb, and Dy containing samples corroborates with the metallic state of the nonmagnetic (Y) and the magnetically ordered compounds. ¹¹B, ²⁹Si and ⁸⁹Y nuclear magnetic resonance (NMR) spectroscopy on nonmagnetic Y₅Si₂B₈ shows different signals, which correspond to the expected number of distinct crystallographic sites in the structure. ¹¹B NMR on Y₅Si₂B₈ indicates that the local magnetic susceptibilities are substantially different from the ones observed in the related compound YB₄.     

Structure and magnetic properties of RE2Cu2Cd

The ternary intermetallic compounds RE₂Cu₂Cd (RE=Y, Sm, Gd-Tm, Lu) were synthesized by induction-melting of the elements in sealed tantalum tubes. The samples were characterized by X-ray powder diffraction. The structure of Gd₂Cu₂Cd was refined from single crystal X-ray diffractometer data: Mo₂FeB₂ type, space group P4/mbm, a=756.2(3), c=380.2(3) pm, wR2=0.0455, 321 F² values, 12 variables. The structures are 1:1 intergrowth variants of slightly distorted CsCl and AlB₂ related slabs of compositions RECd and RECu₂. The copper and cadmium atoms build up two-dimensional [Cu₂Cd] networks (257 pm Cu-Cu and 301 pm Cu-Cd in Gd₂Cu₂Cd) which are bonded to the rare earth atoms via short RE-Cu contacts (290 pm in Gd₂Cu₂Cd). Temperature dependent susceptibility measurements of RE₂Cu₂Cd with RE=Gd, Tb, Dy, and Tm show experimental magnetic moments which are close to the free RE³⁺ ion values. The four compounds show ferromagnetic ordering at T_C=116.7(2), 86.2(3), 48.4(1), and 14.5(1) K, respectively, as confirmed by heat capacity measurements. Dy₂Cu₂Cd shows a spin reorientation at T_N=16.9(1) K.     

Magnetic structure of rare-earth dodecaborides

We have investigated the magnetic structure of HoB₁₂, ErB₁₂ and TmB₁₂ by neutron diffraction on isotopically enriched single-crystalline samples. Results in zero field as well as in magnetic field up to 5 T reveal modulated incommensurate magnetic structures in these compounds. The basic reflections can be indexed with q=(1/2±δ, 1/2±δ, 1/2±δ), where δ=0.035 both for HoB₁₂ and TmB₁₂ and with q=(3/2±δ, 1/2±δ, 1/2±δ), where δ=0.035, for ErB₁₂. In an applied magnetic field, new phases are observed. The complex magnetic structure of these materials seems to result from the interplay between the RKKY and dipole-dipole interaction. The role of frustration due to the fcc symmetry of dodecaborides and the crystalline electric field effect is also considered.     

Spin-glass-like behaviour in IrSr2RECu2O8 (RE=Sm and Eu)

We report the results of magnetic and specific heat measurements on the 1212-type compounds IrSr₂RECu₂O₈ with RE=Sm and Eu, prepared by high-pressure and high-temperature synthesis. The magnetic susceptibility of these compounds shows a large difference in the temperature dependence of the magnetization measured under zero-field-cooled and field-cooled conditions below 87 and 71 K, respectively, and upon further cooling below ∼10 K substantial maxima are observed too. Further AC susceptibility measurements support a glassy behaviour in lower magnetic transitions whereas the specific heat measurements do not show the typical long-range ordering commonly displayed in ferro, ferri or antiferromagnetic transitions. Hysteresis loops suggest the presence of magnetic clusters in the otherwise paramagnetic zone, indicating that these compounds probably display a reentrant spin-glass transition. Results are presented and discussed.     

Novel rare-earth-containing manganites Ba4REMn3O12 (RE=Ce, Pr) with 12R structure

Novel rare-earth-containing manganites, Ba₄REMn₃O₁₂ (RE=Ce, Pr), with 12R structure, have been prepared by solid-state reaction. Although the phases are formed at 950°C, to obtain single-phase samples high temperatures (up to 1300°C) and long synthesis periods are needed.Their structure is built up from chains of BO₆ face-sharing and corner-sharing octahedra running along the c-axis giving a quasi-one-dimensional oxide. Every polyhedral column consists of (Mn₃O₁₂) units of three face-sharing octahedra, both ends connected by the three terminal oxygen atoms to three different (REO₆) octahedra. Mixed occupation of the three octahedral positions in the structure, (Mn(1), Mn(2) and Re), was not found. Vacancies are not observed, neither in the cationic sublattice nor in the oxygen one. Thus, as in all the other 1-D manganites, the oxidation state of manganese ions seems to be four, as the rare-earth valence is. High-resolution electron microscopy suggests the eventual existence of ordered polytypes for different compositions, which could be stabilized by adjusting the thermodynamic conditions.     

Crystal and electronic structures of CaAl2Si2-type rare-earth copper zinc phosphides RECuZnP2 (RE=Pr, Nd, Gd-Tm, Lu)

The quaternary rare-earth phosphides RECuZnP₂ (RE=Pr, Nd, Gd-Tm, Lu) have been prepared by reaction of the elements at 900 °C, completing this versatile series which forms for nearly all RE metals. They adopt the trigonal CaAl₂Si₂-type structure (Pearson symbol hP5, space group P3?m1, Z=1), as confirmed by single-crystal X-ray diffraction analysis on ErCuZnP₂ and powder X-ray diffraction analysis on the remaining members. The Cu and Zn atoms are assumed to be disordered over the single transition-metal site. Band structure calculations on a hypothetically ordered YCuZnP₂ model suggest a semimetal, with a zero band gap between the valence and conduction bands. This electronic structure is supported by XPS valence band spectra for RECuZnP₂ (RE=Gd-Er), in which the intensity drops off smoothly at the Fermi edge. The absence of a band gap permits the electron count to deviate from the precise value of 16 e⁻ per formula unit, as demonstrated by the formation of a solid solution in GdCu_xZn_2−xP₂ (1.0≤x≤1.3), while still retaining the CaAl₂Si₂-type structure. Because the Cu 2p XPS spectra indicate that the Cu atoms are always monovalent, the substitution of Cu for Zn leads to a decrease in electron count and a lowering of the Fermi level in the valence band. The magnetic susceptibility of RECuZnP₂ (RE=Gd-Er), which obeys the Curie-Weiss law, confirms the presence of trivalent RE atoms.     

Antiferromagnetism of perovskite EuZrO3

Polycrystalline EuZrO₃ has been synthesized by the solid-state reaction between EuO and ZrO₂, and its structural and magnetic properties have been investigated. Rietveld analysis of the X-ray diffraction pattern indicates that EuZrO₃ crystallizes in an orthorhombic perovskite structure. ¹⁵¹Eu Mössbauer effect measurement reveals that almost all the europium ions are present as the divalent state and occupy distorted sites with non-axial electric field gradients, in agreement with the orthorhombic structure. In contrast to previous reports, an antiferromagnetic transition was observed around 4.1 K. The magnetic structure below the Néel temperature has been discussed.     

Magnetism and structural chemistry of ternary borides RE2MB6 (RE = rare earth, M = Ru,Os)

The magnetic behavior of the ternary borides RE₂RuB₆ and RE₂OsB₆ (RE = Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) was studied in the temperature range 1.5 K < T < 1100 K. All compounds crystallize with the Y₂ReB₆-type structure and are characterized by direct RE-RE contacts and the formation of planar infinite two-dimensional rigid boron nets. The magnetic properties reveal a typical Van Vleck paramagnetism of free RE³⁺-ions at temperatures higher than 200 K with ferromagnetic interaction in the low-temperature range T < 55 K. The ferromagnetic ordering temperatures vary with the De Gennes factor. There is no indication for a magnetic contribution from the Ru(Os)-sublattice. Above 1.8 K none of the samples were found to be superconducting.     

Synthesis, structures and magnetic properties of pseudo-hollandite chromium sulfides

The pseudo-hollandite chromium sulfides, ACr₅S₈ (A=K, Rb, Cs) and A′_0.5Cr₅S₈ (A′=Sr, Ba), have been synthesized and investigated in structural and magnetic properties. All the compounds crystallize in the isostructure with a monoclinic C2/m. Its crystal structure has triangular lattices and double chains made of Cr³⁺ (d³; S=3/2) triangles. The magnetic susceptibilities of all compounds behave as Curie-Weiss types in high temperature region. From magnetic susceptibility and specific heat measurements, all the compounds have antiferromagnetic ground states. The Néel temperatures are rather low compared to Weiss temperatures, reflecting magnetic frustration in the triangular lattices and double chains. The magnetic transitions in KCr₅S₈ and Ba_0.5Cr₅S₈ are a two-step transition around 50 and 60 K, respectively, while RbCr₅S₈ shows a sharp magnetic transition at 42 K, accompanied by magnetoelastic behavior. CsCr₅S₈ shows a structural transition around 100 K, followed by a magnetic transition at 10 K. In Sr_0.5Cr₅S₈, ferromagnetic interaction develops below 100 K and then a three-dimensional antiferromagnetic order takes place at 30 K. These magnetic properties are discussed from A-cation dependences of local structures and a model of magnetic structure for RbCr₅S₈ is proposed on the basis of the arguments of magnetic interactions and neutron diffraction data. It is different from the known magnetic structure of TlCr₅Se₈.     

Mn5Si3-type host-interstitial boron rare-earth metal silicide compounds RE5Si3: Crystal structures, physical properties and theoretical considerations

A series of binary rare-earth metal silicides RE₅Si₃ and ternary boron-interstitial phases RE₅Si₃B_x (RE=Gd, Dy, Ho, Lu, and Y) adopting the Mn₅Si₃-type structure, have been prepared from the elemental components by arc melting. Boron “stuffed” phases were subsequently heated at 1750 K within a high-frequency furnace. Crystal structures were determined for both binary and ternary series of compounds from single-crystal X-ray data: hexagonal symmetry, space group P6₃/mcm, Z=2. Boron insertion in the host binary silicides results in a very small decrease of the unit cell parameters with respect to those of the binaries. According to X-ray data, partial or nearly full boron occupancy of the interstitial octahedral sites in the range 0.6-1 is found. The magnetic properties of these compounds were characterized by the onset of magnetic ordering below 100 K. Boron insertion induces a modification of the transition temperature and θ_p values in most of the antiferromagnetic binary silicides, with the exception of the ternary phase Er₅Si₃B_x which was found to undergo a ferromagnetic transition at 14 K. The electrical resistivities for all binary silicides and ternary boron-interstitial phases resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states. Zintl-Klemm concept would predict a limiting composition RE₅Si₃B_0.6 for a valence compound and should then preclude the stoichiometric formula RE₅Si₃B. Density functional theory calculations carried out on some RE₅Si₃Z_x systems for different interstitial heteroatoms Z and different x contents from 0 to 1 give some support to this statement.     

Magnetic nanoparticles for power absorption: Optimizing size, shape and magnetic properties

We present a study on the magnetic properties of naked and silica-coated Fe₃O₄ nanoparticles with sizes between 5 and 110 nm. Their efficiency as heating agents was assessed through specific power absorption (SPA) measurements as a function of particle size and shape. The results show a strong dependence of the SPA with the particle size, with a maximum around 30 nm, as expected for a Néel relaxation mechanism in single-domain particles. The SiO₂ shell thickness was found to play an important role in the SPA mechanism by hindering the heat outflow, thus decreasing the heating efficiency. It is concluded that a compromise between good heating efficiency and surface functionality for biomedical purposes can be attained by making the SiO₂ functional coating as thin as possible.     

Field-Induced Magnetic Properties in RCrO4 Oxides (R=Pr, Gd, Tb, Tm, and Yb)

RCrO₄ oxides (R=Pr, Gd, Tb, Tm, and Yb) have been synthesized at 773 K using the corresponding nitrates as precursors. X-ray diffraction data reveal that these samples are single phases and crystallize with the zircon-type structure, showing tetragonal symmetry, space group I4₁/amd. All the compounds are antiferromagnetic and the Néel temperature, which depends on the R³⁺ ion, takes values lower than 30 K. The presence of a canting appears to be responsible for the negative values of the magnetic susceptibility found below the compensation temperature. This uncommon phenomenon is named reversal of magnetization. It is field-dependent, being suppressed at 500 Oe for the TmCrO₄ compound. The highest value of the compensation temperature (24 K) corresponds to the YbCrO₄ oxide. A metamagnetic transition has been observed in all cases at critical fields ranging from 225 Oe (GdCrO₄) to 1600 Oe (YbCrO₄).     

Magnetic specific heat and spin-glass-like ordering in the pyrochlores: RE2Mo2O7 (RE=Y,Sm, Gd,Ho)

The pyrochlores RE₂Mo₂O₇ (RE=Y, Sm, Gd, Ho) display, with the exception of Ho₂Mo₂O₇, magnetic transitions at 18 K, 68 K, 55 K, respectively. No long-range order occurs. The rare-earth atoms remain in the paramagnetic state down to 1.5 K. The magnetic specific heat behaviour is explained by spin-glass-like ordering of Mo⁴⁺ ions, imposing a molecular field of random character on the rare-earth ions, and by crystalline field-splitting effects.     

Synthesis, structure, and magnetism of Tb4PdGa12 and Tb4PtGa12

Single crystals of Tb₄MGa₁₂ (M=Pd, Pt) have been synthesized. The isostructural compounds crystallize in the cubic space group , with Z=2 and lattice parameters: a=8.5940(5) and 8.5850(3) Å for Tb₄PdGa₁₂ and Tb₄PtGa₁₂, respectively. The crystal structure consists of corner-sharing MGa₆ octahedra and TbGa₃ cuboctahedra. Magnetic measurements suggest that Tb₄PdGa₁₂ is an antiferromagnetic metamagnet with a Néel temperature of 16 K, while the Pt analog orders at T_N=12 K.     

Multivariate data analysis approach to understand magnetic properties of perovskite manganese oxides

Here we apply statistical multivariate data analysis techniques to obtain some insights into the complex structure-property relations in antiferromagnetic (AFM) and ferromagnetic (FM) manganese perovskite systems, AMnO₃. The 131 samples included in the present analyses are described by 21 crystal-structure or crystal-chemical (CS/CC) parameters. Principal component analysis (PCA), carried out separately for the AFM and FM compounds, is used to model and evaluate the various relationships among the magnetic properties and the various CS/CC parameters. Moreover, for the AFM compounds, PLS (partial least squares projections to latent structures) analysis is performed so as to predict the magnitude of the Néel temperature on the bases of the CS/CC parameters. Finally, so-called PLS-DA (PLS discriminant analysis) method is employed to find out the most influential/characteristic CS/CC parameters that differentiate the two classes of compounds from each other.     

Novel rare-earth borosilicide RE1−xB12Si3.3−δ (RE=Y, Gd-Lu) (0?x?0.5, δ≈0.3): synthesis, crystal growth, structure analysis and properties

New rare-earth boron-rich compounds with the formula of RE_1−xB₁₂Si_3.3−δ (RE=Y, Gd-Lu) (0?x?0.5,δ≈0.3) have been synthesized. They belong to a new type of rhombohedral structure with the space group of R-3m (No. 166) and z=9. The lattice constants were measured from powder XRD data. Crystal structure solved from powder XRD data for Tb_0.68B₁₂Si₃ as a representative has been compared with that of YB_17.6Si_4.6 (or Y_0.68B₁₂Si_3.01), whose structure was solved from single-crystal reflection data. The structure model is confirmed by high-resolution transmission microscope analysis. The vibrational modes of the new crystals were measured by Raman spectroscopy. Temperature dependence of magnetic susceptibility which was measured for RE_1−xB₁₂Si_3.3−δ single crystals by SQUID revealed that they are paramagnetic materials down to 2.0 K.     

Synthesis and structural studies of Sr2Co2−xGaxO5, 0.3?x?0.8

The novel oxide Sr₂Co_2−xGa_xO₅ with brownmillerite-type structure has been synthesized in the compositional range 0.3?x?0.8. Rietveld refinements using neutron powder diffraction data have been performed for the end compositions, x=0.3 and 0.8. The structure is best described in the space-group Icmm (no. 74) with unit cell parameters a=5.5678(6), 5.6126(7) Å, b=15.749(2), 15.733(2) Å and c=5.4599(6), 5.4559(7) Å for the x=0.3 and 0.8 compositions, respectively. The compounds were found to be G-type antiferromagnetic with the magnetic moments parallel to the c-axis. High-temperature magnetic susceptibility measurements confirmed the samples to be antiferromagnetic with Néel temperatures T_N=505, 468 and 423 K for the x=0.3, 0.5 and 0.8 samples, respectively. High-resolution transmission electron microscopy and electron diffraction studies confirmed the I-centred structure and revealed the presence of disorder.     

Physical properties of layered homologous RE-B-C(N) compounds

Physical properties of a series of homologous RE-B-C(N) B₁₂ cluster compounds REB₁₇CN, REB₂₂C₂N, and were investigated. The structures of the compounds are layer-like along the c-axis, with rare earth and B₆ octahedral layers separated by B₁₂ icosahedral and C-B-C chain layers whose number increases successively from two B₁₂ layers for the REB₁₇CN compound to four for the REB_28.5C₄ compound. The rare earth atoms are configured in two triangular flat layers which are stacked on top of one another in AB stacking where the nearest-neighbor rare earth directions are the three atoms forming a triangle in the adjacent layer. The series of homologous compounds exhibit a spin glass transition with T_f shifting in correspondence with variations of the basal plane lattice constants, consistent with the magnetic interaction being effective in the basal planes. The isothermal remanent magnetization shows a stretched exponential decay . Exponents determined for the different homologous compounds were scaled as a function of T_r=T/T_f and found to follow the empirical dependency determined for typical spin glasses. It is indicated that a mixture of disorder originating from the partial occupancy of the rare earth sites and frustration of interactions due to the unique configuration is responsible for the manifestation of spin glass transitions in these homologous systems.