Magnetic phase diagram of frustrated spin-chain compounds

The magnetic ordering in frustrated triangular lattices of Ising chains is discussed. It is shown that the differences in the low-temperature CsCoCl₃, Ca₃Co₂O₆, and Sr₅Rh₄O₁₂ phases are related to the interaction between the next-to-nearest pairs of chains, which partially lifts the degeneracy of the ground state of the 2D antiferromagnet Ising model. In zero magnetic field and at low temperature, a stripe structure arises in the new compound Sr₅Rh₄O₁₂.     

Dynamic susceptibility of a spin ice near the critical point

We consider spin ice magnets (primarily, Dy₂Ti₂O₇ in the vicinity of their critical point on the (H, T) plane. We find that the longitudinal susceptibility diverges at the critical point, leading to the behavior qualitatively similar to the one which would result from non-zero conductance of magnetic charges. We show that dynamics of critical fluctuations belongs to the universality class of easy-axis ferroelectric and calculate logarithmic corrections (within two-loop approximation) to the mean-field critical behavior.     

Specific heat of Dy2Ti2O7 in magnetic fields: comparison between single-crystalline and polycrystalline data

We report the specific heat of single crystals of the spin ice compound Dy₂Ti₂O₇ under magnetic fields down to 1.8 K. Our polycrystalline results down to 0.4 K are fully consistent with the previous report by Ramirez et al. For the magnetic field along the [100] direction, C_spin/T is interpreted as one Schottky peak characterized by the Zeemann splitting of the Ising levels of all four equivalent spins for each tetrahedron. However, for the magnetic field along the [111] direction, C_spin/T is interpreted as two-component Schottky peak reflecting the fact that there are two kinds of spin components. Along the [110] direction, it is also interpreted as two-component Schottky peak because of experimental misalignment. We found that the features of the polycrystalline data in magnetic fields cannot be reconstructed by simply taking appropriate averages of the present anisotropic single-crystalline data.     

Magnetic Properties of Dipolar Magnets with Strong Tetragonal Distortion

The paper overviews the behaviour of dipolar magnets with tetragonal lattice characterized by the ratio r = c/a reflecting the measure of tetragonal distortion of the crystal structure. It is shown that in zero magnetic field the dipolar systems with sufficiently large r despite the long-range character of the magnetic correlations can be mapped to two-dimensional Ising model with the nearest-neighbour interaction only. The magnetic properties of dipolar magnets with r 2 are described by reviewing the results obtained primarily on rare earth (R) dipolar magnets from the RBa₂Cu₃O_7- and AR(MoO₄)₂ series (A = Cs,K).     

Magnetic properties of defects in spin-gap magnets

The magnetic properties of defects were studied in spin-gap magnets such as spin-Peierls magnet CuGeO₃, Haldane magnet PbNi₂V₂O₈, and charge-ordered ladder magnet NaV₂O₅. Doping of these systems with nonmagnetic impurities leads to additional magnetic degrees of freedom, which manifest themselves at low temperatures, where the intrinsic magnetic susceptibility of a spin-gap system is close to zero. Magnetic susceptibility appears due to the local destruction of the singlet ground state as a result of impurity-induced breakage of spin chains. Antiferromagnetically correlated areas arise near the spin-chain breaks. The sizes of these areas and the effective spin of these specific spin clusters are estimated. The order parameter and its spatially modulated depth are determined for impurity-induced magnetically ordered phases. The magnetic properties of defects for the NaV₂O₅ ladder structure are explained in the model of electrons “hopping” near the chain break. The hopping degree of freedom effectively influences the total spin of a spin-chain fragment and magnetization of the system.     

A new quasi-Ising magnet

We have performed measurements of the magnetization and differential magnetic susceptibility of Dy_0.62Y_2.38Fe₅O₁₂ single crystals in pulsed magnetic fields up to 45 T at liquid-helium temperature for three orientations of the external field: H‖[100], H‖[110], and H‖[111]. It was found that the magnetization reversal in the rare-earth magnetic subsystem occurs via several phase transitions, whose number depends on the direction of the external field, as is characteristic for Ising magnets. The anomalies in the field dependences of the magnetization are interpreted on the assumption of quasi-Ising ordering of the rare-earth ions. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 552–556 (25 April 1998)     

Thermodynamics of spin ice in staggered and direct (along the [111] axis) fields in the cluster approximation

We have analyzed the low-temperature thermodynamic properties of spin ice in the staggered and direct (acting along the [111] axis) fields for rare-earth oxides with the chalcolamprite structure and general formula Re₂ ³⁺Me₂ ⁴⁺O₇ ^2-. Calculations have been performed in the cluster approximation. The results have been compared with experimental temperature dependences of heat capacity and entropy for Dy₂Ti₂O₇ compound for different values of the external field in the [111] direction. The experimental data and calculated results have also been compared for the Pr₂Ru₂O₇ compound with the antiferromagnetic ordering of magnetic moments of ruthenium ions, which gives rise to the staggered field acting on the system of rare-earth ions. The calculated temperature dependences of heat capacity and entropy are in good agreement with experimental data.     

Magnetic ordering of two dimensional antiferromagnets on the triangular lattice: Mössbauer study

Mössbauer study has been performed on⁵⁷Fe in a single crystal of YbFe₂O₄ in order to obtain the information about the magnetic ordering. The essential difference between zerofield-cooling- and field-cooling- spectra has been decisive for settling the problem. A model of Ising spin system in 2d-antiferromagnet on the triangular lattice is proposed to interpret the spin behavior of Fe³⁺ in this system. It is very different from ordinary antiferromagnets that the microscopic change of 3d-state is induced by the external magnetic field even at low enough temperatures.     

Low-temperature specific heat of Er2Ba4Cu7O15-δ

The magnetic ordering of Er ions in Er₂Ba₄Cu₇O_15-δ (δ = 0:7 and δ = 0:08) has been studied by low temperature heat capacity measurements. The Néel temperature T _N of Er₂Ba₄Cu₇O_15-δ is almost independent of the oxygen concentration (TN = 0:54K for δ = 0:08 and T _N = 0:50K for δ = 0:7). While an anisotropic two-dimensional (2D) Ising model describes the experimental data very well for Er₂Ba₄Cu₇O_14:92, this model cannot be applied to the oxygen reduced sample Er₂Ba₄Cu₇O_14:3. By using a model consisting of 1D-Ising chains and of 2D-Ising clusters of Er³⁺ ions we could fit the specific heat data of Er₂Ba₄Cu₇O_14:3 accurately. Our measurements show clearly that with the reduction of the oxygen content the in-plane anisotropy of the magnetic exchange coupling increases.     

Temperature-dependent Mössbauer and dielectric studies of Mg0.95Mn0.05Fe1.0Ti1.0O4

The effect of tetravalent Ti⁺⁴ substitution in Mg_0.95Mn_0.05Fe₂O₄ on its magnetic and electrical properties has been studied using X-ray diffraction, Mössbauer spectroscopy, isothermal dc magnetization and dielectric measurements. X-ray diffraction studies have shown the structural transformation from cubic to tetragonal with the Ti⁺⁴ substitution. The Mössbauer spectra of Mg_0.95Mn_0.05Fe_1.0Ti_1.0O₄ recorded in the temperature range 20-300 K shows the presence of the magnetic as well as quadrupole interactions. The isothermal hysteresis loop infers that the system exhibits a ferrimagnetic ordering at room temperature. The Zero-field-cooled (ZFC) and field-cooled (FC) magnetization studies support ferrimagnetic ordering of Mg_0.95Mn_0.05Fe_1.0Ti_1.0O₄ at room temperature. Signatures of ferroelectric transition have been observed in the temperature range 200-300 K from dielectric measurements. The observed magnetic and dielectric behaviour indicate that this material exhibits multiferroic behaviour.     

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho_2−xY_xTi₂O₇ (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1 −1 0) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6 K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30 mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi₂O₇ and compare our results with pure spin ice Ho₂Ti₂O₇. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.     

Magnetic structure and phase diagram of Sr<Subscript>5</Subscript>Rh<Subscript>4</Subscript>O<Subscript>12</Subscript> frustrated Ising magnet

The magnetic structure of Sr₅Rh₄O₁₂ is based on Ising chains of rhodium ions with a variable valence, Rh³⁺-Rh⁴⁺. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr₅Rh₄O₁₂ are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.     

Relationship between short- and long-range orders in nonstoichiometric titanium monoxide (TiO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>)

The relationship between the short- and long-range orders in various phases of nonstoichiometric titanium monoxide (TiO _y ) has been analyzed for the first time. The types of the local environment of lattice sites in the metal and nonmetal (oxygen) sublattices of Ti₅O₅, Ti₃O₂, Ti₂O₃, and Ti₄O₅ superstructures are described. It is established that, in phases where ordering takes place simultaneously in both sublattices, all parameters of the superstructural short-range order determining the positions of atoms and vacancies in the first three coordination spheres can be uniquely expressed via the long-range order parameters. If the ordering takes place only in one sublattice, then five of the six short-range order parameters vanish. It is shown that, using data on the maximum absolute values of six short-range order parameters and on the fractions of occupied atomic positions in titanium and oxygen sublattices, it is possible to predict the type of ordered phase expected to form in the nonstoichiometric titanium monoxide TiO _y .     

强耦合磁失措自旋冰系统Dy2Ti2O7单晶生长和基本磁性质测量

利用红外光源浮区法生长出大尺寸、高质量的磁失措自旋冰化合物Dy₂Ti₂O₇单晶体.X射线衍射实验证实晶体具有面心立方结构，空间群为Fd3m，晶胞参数a＝1.0112(2) nm，［111］和［400］方向X射线衍射摇摆曲线半高宽分别仅为0.07°和0.05°.直流磁化率与温度关系测量给出晶体的Van Vleck顺磁因子为2.46×10^-5 m³/mol，有效磁矩μ_eff＝10.24(4)μ_B，Cure-Weiss温度Θ_CW＝1.1 K，揭示Dy₂Ti₂O₇具有弱的铁磁性.对磁性起源的综合分析表明，该自旋冰晶体磁性质主要来源于磁偶极相互作用，且相关最近邻长程偶极相互作用能量标度D_nn＝3.00 K. 关键词： 2Ti₂O₇')" href="#">Dy₂Ti₂O₇ 浮区法晶体生长 关联电子系统 自旋冰     

Estimation of single-ion anisotropies, crystal-field and exchange interactions in Gd-based frustrated pyrochlore anti-ferromagnets Gd2M2O7 (M=Ti, Sn, Hf, Zr)

The temperature dependence of the experimental results of dc (macroscopic) magnetic susceptibility and nuclear hyperfine properties of frustrated magnetic Gd-based pyrochlore compounds, Gd₂Ti₂O₇, Gd₂Sn₂O₇, Gd₂Hf₂O₇ and Gd₂Zr₂O₇, are analyzed within the frame work of appropriate crystal-field theory and a mean field approximation by introducing effective anisotropic molecular field tensors, and formulating an exact relation between single-ion susceptibility tensors and site susceptibility tensors. Components of the calculated susceptibility along and perpendicular to the local 〈1 1 1〉 axis of the tetrahedral sublattice of pyrochlore structure show that these pyrochlores are easy-planar anisotropic magnetic systems. The crystal-field parameters and anisotropic exchange coupling have been determined and their systematic variations over the Gd-based pyrochlores studied here are discussed.     

Ion-impact chemistry in the system titanium-oxygen (studies on bombardment-enhanced conductivity—III)

The bombardment of TiO₂, whether poly- or single crystalline, with Kr ions leads to an altered surface layer having the following characteristics. It exhibits a high electrical conductivity, has the diffraction pattern of finely polycrystalline Ti₂O₃, is on the average 110 ± 20 Å thick (for 30 keV Kr), and is indefinitely stable in air at room temperature. The formation of the layer is favored by increasing the target temperature. Formation is half complete at (6 ± 2) × 10¹⁶ ions/cm², hence at a dose substantially greater than that for the half completion of sputter equilibrium ([7 ± 2] × 10¹⁵ ions/cm²). One model which could lead to Ti₂O₃ can be excluded fairly readily: this is thermal-spike stimulated vaporization, as the relevant vapor pressures are too low. More satisfactory is a model in which, due to either preferential oxygen sputtering or internal precipitation of oxygen, Ti₂O₃ nuclei are formed and grow. The reason that the stoichiometry is precisely Ti₂O₃ can be rationalized by an argument based on surface binding energies (E_b), in the sense that E_b for TiO₂ to sputter congruently is 6.4 eV, to yield nuclei of Ti₃O₅ is 5.7, to yield nuclei of Ti₂O₃ is 5.1, and to yield TiO is 6.4. A similar rationalization holds also for impact-induced chemical changes observed or inferred with AgBr, CuO, Fe₂O₃, MoO₃, U₃O₈ and V₂O₅, except that here thermal-spike stimulated vaporization cannot be excluded.     

Mössbauer studies of YBa2Cu3O7−δ-type high-T c superconductors

The local magnetic, electronic, and structural properties of (RE)Ba₂Cu₃O_7?δ supercondcutors (RE=Gd, Dy, and Eu) were studied by Mössbauer spectroscopy using the resonances of¹⁵⁵Gd,¹⁶¹Dy,¹⁵¹Eu, and⁵⁷Eu. In GdBa₂Cu₃O_7?δ, different magnetic ordering behaviors of the Gd sublattice are found for the orthorhombic (superconducting) and tetragonal (non-superconducting) phases. In DyBa₂Cu₃O_7?δ, the magnetic moments of the respective CEF ground states in the orthorhombic and tetragonal phases are derived from paramagnetic hyperfine splittings at 1.4 K. In both DyBa₂Cu₃O_7?δ and EuBa₂Cu₃O_7?δ, anomalies connected with the superconducting transitions in isomer shift, recoil-free fraction, and relaxation behavior were looked for, but not found. The electric-quadrupole splittings observed for both systems are discussed in connection with the lattice EFGs derived for the Gd system. In GdBa₂ (Cu_0.995Fe_0.005)₃O_7?δ, the local properties of the various Fe sites are investigated over a wide temperature range in both the orthorhombic and tetragonal phase. The magnetic ordering of the Gd sublattice in the orthorhombic phase and of the Cu(2) sublattice in the tetragonal phase, respectively, is monitored via the magnetic splittings at the various Fe sites. Possible assignments of Cu(1) and Cu(2) sites as well as different oxygen configurations around the substituted Fe ions are discussed.     

An investigation of the magnetic anisotropy change in BaFe12−2xTixCoxO19 single crystals

This paper presents a study of BaFe_12−2xTi_xCo_xO₁₉, where the substitution-induced magnetic anisotropy change has been investigated through magnetic measurements and single-crystal neutron diffraction. The study shows that the magnetic structures of BaFe_12−2xTi_xCo_xO₁₉ change drastically when the temperature and the substitution rate are altered. It is proposed that an antiferromagnetic mode in doped Ba-hexaferrites is one of the key elements to understand the origin of their promising microwave properties.     

L10 Stacked Binaries as Candidates for Hard‐Magnets: FePt,MnAl and MnGa

We present a novel approach for designing new hard magnets by forming stacks of existing binary magnets to enhance the magneto crystalline anisotropy. This is followed by an attempt at reducing the amount of expensive metal in these stacks by replacing it with cheaper metal with similar ionic radius. This strategy is explored using examples of FePt, MnAl and MnGa. In this study a few promising materials are suggested as good candidates for hard magnets: stacked binary FePt₂MnGa₂ in structure where each magnetic layer is separated by two non‐magnetic layers, FePtMnGa and FePtMnAl in hexagonally distorted Heusler structures and FePt_0.5Ti_0.5MnAl.