Study of magnetic properties of Mn1−xCuxCr2S4 by: High-temperature series expansions

The high-temperature series expansions method applied in the systems Mn_1−xCu_xCr₂S₄ in the range 0?x?1. The exchange interactions and the magnetic exchange energies are calculated by using the probability law. The high-temperature series expansions have been applied in the spinel Mn_1−xCu_xCr₂S₄ systems, combined with the Padé approximants method, to determine the magnetic phase diagram, i.e. T_C versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced. The obtained value of γ is insensitive to the dilution ratio x and may be compared with other theoretical results based on 3D Heisenberg model.     

Magnetic phase diagram of diluted spinel Zn1−xCuxCr2Se4 system

By using mean field theory, we have evaluated the nearest-neighbour and the next-neighbour super-exchange J₁(x) and J₂(x), respectively, for Zn_1−xCu_xCr₂Se₄ in the range 0?x?1. The intraplanar and the interplanar interactions are deduced. High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation. The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The values are comparable to those of the 3D Heisenberg model, and are insensitive to the dilution x.     

Possible anisotropy gap in La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 detected through specific heat and magnetization measurements

Magnetization and specific heat measurements, as a function of temperature, were performed on single crystals of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄, under different applied magnetic fields (H). The specific heat in La_1.35Sr_1.65Mn₂O₇ was decreased for H=9 T parallel to the crystal c axis, compared with H=0, possibly due to a suppression of spin-wave excitations (magnons) in that ferromagnetic bilayer structure. On the other hand, the applied magnetic field had no effect in the specific heat of the antiferromagnetic La_1.5Sr_0.5NiO₄. For H=9 T and below the temperature of 4 K the specific heat data, for each crystal, was well fitted by an exponential decay law. This allowed the calculation of energy gaps around 1 meV for both compounds, in close agreement with Δ=2μ_BH for an expected energy gap in the magnon spectrum. Detailed magnetization measurements showed monotonic variations below 4 K and a steep increase close to 2 K. Both magnetization and specific heat measurements suggest the existence of an anisotropy gap in the energy spectrum of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄.     

Magnetic phase transitions and structures of Co3V 2O8

Co₃V ₂O₈ is a spin- 3/2 system on a Kagomé staircase and is known to undergo two magnetic phase transitions between 6 and 11 K. The H-T phase diagram of Co₃V ₂O₈ derived by magnetization measurements on a single crystal is presented. Additionally both ordered magnetic structures were investigated by neutron powder diffraction experiments and solved using Bertaut’s macroscopic theory. For the ferromagnetic phase the magnetic moments of the Co²⁺ ions were found to be 1.5(3)μ_B and 2.7(1)μ_B at 3.5 K along the crystallographic a axis for the (4a) and (8e) sites, respectively. The antiferromagnetic phase exhibits a magnetic cell with a doubled b axis with respect to the nuclear one. The magnetic moments point along the a axis being 1.8(2)μ_B (4a) and 1.8(1)μ_B (8e) at 8 K.     

Magnetic structure of TbNiAl4

Magnetisation and specific heat measurements, in the range 2 K to room temperature, demonstrate that three magnetic phases exist for the intermetallic compound TbNiAl₄. Powder neutron diffraction, also carried out over a wide temperature range, establishes that the intermediate magnetic phase is incommensurate, and confirms that the lowest temperature phase has a linear antiferromagnetic structure with a (0 1 0) propagation vector. The respective transition (Néel) temperatures, in zero applied magnetic field are 34.0 and 28.0 K.     

Magnetic dimensionality of Y(2−x)DyxBaCuO5

In this communication, we report the effect that doping Y₂BaCuO₅ with Dy has on its two-dimensional (2D) magnetic structure. Pure samples at both ends of the series, as well as samples doped with 1, 5, 10 and 25% dysprosium, have been characterised using X-ray diffraction, and AC susceptibility together with neutron diffraction studies on the 1 and 5% samples, which were used to measure the magnetic ordering at low temperatures. The results show that 1% Dy is enough to disrupt the 2D magnetic ordering turning it into a 3D array. The low dysprosium concentration indicates that the 3D ordering is achieved without the existence of a rare earth magnetic sublattice. The change in the ordering temperature from 27 K for the pure Y₂BaCuO₅ to 16 K for the 1 and 5% Dy compounds, together with the parameters from the AC susceptibility fittings, reveal that the effect of the Dy doping affects the electronic structure of the Cu ions that become involved in the superexchange pathways. The discrepancy between the parameters obtained by the Curie-Weiss fittings of the real part of the AC susceptibility and the neutron diffraction results, shows that the exchange mechanism deviates from the mean field model for all dysprosium concentrations.     

Magnetic anisotropy and geometrical frustration in the Ising spin-chain system Sr5Rh4O12

A structural and thermodynamic study of the newly synthesized single crystal Sr₅Rh₄O₁₂ is reported. Sr₅Rh₄O₁₂ consists of a triangular lattice of spin chains running along the c-axis. It is antiferromagnetically ordered below 23 K with the intrachain and interchain coupling being ferromagnetic (FM) and antiferromagnetic (AFM), respectively. There is strong evidence for an Ising character in the interaction and geometrical frustration that causes incomplete long-range AFM order. The isothermal magnetization exhibits two step-like transitions leading to a ferrimagnetic state at 2.4 T and a FM state at 4.8 T, respectively. Sr₅Rh₄O₁₂ is a unique frustrated spin-chain system ever found in 4d and 5d based materials without a presence of an incomplete 3d-electron shell.     

Magnetic properties of the Pr1−xGdxCo4B compounds

In this work, we have investigated the effect of the substitution of Gd for Pr on the crystal structure and magnetic properties of the Pr_1−xGd_xCo₄B compounds for 0?x?1 using X-ray powder diffraction, magnetic measurements, and differential scanning calorimetry (DSC). These compounds have hexagonal CeCo₄B-type structure with the space group P6/mmm. The substitution of Gd for Pr leads to a decrease of the unit-cell parameters a and the unit-cell volume V, while the unit-cell parameter c increases slightly. Magnetic measurements indicate that all samples are ordered magnetically below room temperature. The Curie temperatures determined by DSC technique increase as Pr is substituted by Gd. The saturation magnetization at 5 K decreases upon Gd substitution up to x=0.6, and then increases again.     

The magnetic state of diamagnetically diluted antiferromagnetic cobalt and nickel monoxide

The nearest neighbour J₁(x) and the next-neighbour super-exchange J₂(x) interactions are evaluated by using the mean field theory for Mg1−xBxO (B=Co and Ni) systems. The magnetic energy E(x) is obtained. A magnetic phase diagram of the Mg1−xBxO (B=Co and Ni) solid solutions with 0?x?1 is drawn by high-temperature series expansions (HTSE) combined with the Padé approximants method (PA). The critical exponents associated with the magnetic susceptibility (γ) and with the correlation length (ν) are deduced in order phase.     

Hydrogen bonds mediated magnetism in Cu(bmen)2Pd(CN)4

Results of synthesis, X-ray structure analysis, electron spin resonance, susceptibility, magnetization and specific heat measurements of powdered Cu(bmen)₂Pd(CN)₄ (,N^′-dimethyl-1,2-diaminoethane) are reported. Its structure is formed of quasi-linear chains of the [-Cu(bmen)₂-NC-Ni(CN)₂-CN-]_n composition; these are interlinked by hydrogen bonds (HBs) leading to two-dimensional patterns. Upon magnetic, spectral and thermodynamic measurements the compound was identified as an S=1/2 Heisenberg antiferromagnet on a square lattice with due to the dominant role of HBs in creating a square network. The long-range ordering observed at 0.24 K is proposed to be of a Néel type. The possibility of tuning the exchange interactions in various directions is considered.     

Magnetocaloric effect in DyCu2

The magnetocaloric effect of the rare-earth intermetallic compound DyCu₂ is explored through magnetization measurements. DyCu₂ is paramagnetic at the room temperature but becomes antiferromagnetic below 27 K (Neel temperature). Strong temperature and field dependence of magnetization in DyCu₂ at and around the Neel temperature lead to a large magnetocaloric effect. An appreciable magnetocaloric effect persists well above the Neel temperature probably because of the presence of short-range ferromagnetic correlations in the paramagnetic state of DyCu₂. This along with the absence of magnetic hysteresis lead to a large effective refrigerant capacity of 194 J/kg below 44 K, which makes the material important as a potential magnetic refrigerant for the cryogenic liquefier cycles.     

Enhanced superconducting properties of Eu2O3-doped MgB2

Bulk polycrystalline samples of Eu₂O₃-doped MgB₂ have been synthesized by a standard solid state reaction route and their structural and superconducting properties have been investigated. As a function of Eu₂O₃ content we have found a significant increase in the critical current density (J_c) and the irreversibility field (H_irr) in the magnetic field range 0–6 T. The XRD results reveal the presence of MgO and EuB₆ secondary phases along with the main hexagonal phase of MgB₂. The strain values and the lattice distortions have been found to increase almost linearly with the nominal Eu₂O₃ content. The observed significant improvement in J_c(H) and H_irr in the Eu₂O₃-doped MgB₂ samples, thus is mainly attributed to the lattice distortions introduced by Eu₂O₃ doping.     

Magnetic structure and phase diagram of the frustrated spinel Cd1−xZnxCr2Se4

In attempt to characterise the magnetic ordering in the whole composition range of the Cd_1−xZn_xCr₂Se₄ system, various magnetic measurements were performed on both crystalline and polycrystalline samples with 0?x?1. The magnetic properties of the system are typical of a ferromagnet below x=0.4 and of a complex antiferromagnet one above x=0.6. In this work the intermediate region was carefully studied. The variations of both M(T) and χ_ac at low fields suggest that transitions from ferromagnetic to Gabay–Toulouse ferromagnetic-spin-glass mixed phase at low temperature occur in the range 0.41?x?0.58. The high-temperature susceptibility measurements show that for the whole concentration range the system obeys Curie–Weiss laws. The results can be explained by the coexistence of competing interactions (ferromagnetic between nearest neighbours and antiferromagnetic between higher order neighbours) and disorder due to the random substitution between zinc and cadmium ions in the tetrahedral sites of the spinel lattice. An experimental magnetic phase diagram of the system is established.     

Magnetic properties of the quasi-one-dimensional system BaMn2V 2O8

Magnetic properties of BaMn₂V ₂O₈ are investigated by means of susceptibility, magnetization, and heat capacity measurements. Our experimental results show that BaMn₂V ₂O₈ is a one-dimensional canted antiferromagnet. The antiferromagnetic transition at a relatively high T_N of 37 K may be due to an enhancement of interchain interaction in the system, since Mn²⁺ ions have large spin moment of S=5/2. Weak ferromagnetism in BaMn₂V ₂O₈ may be due to Dzyaloshinskii-Moriya interactions arising from its noncentrosymmetric crystal structure.     

Effect of interchain coupling on spin correlation functions of spin-1 Heisenberg antiferromagnet CsNiCl3

The inelastic neutron-scattering experiment on CsNiCl₃ gives much bigger total intensity of multi-particle continuum than the theoretical calculation by the (1+1$1+1$)-dimensional O(3) non-linear σ-model. Three-dimensional effect has to be considered. A scenario was proposed where the interchain interaction is treated as an effective staggered magnetic field. We propose another model to approximately include the effect of the interchain coupling, and use the large-N expansion to do the calculations. In this circumstance we find that the Néel temperature is about 5 K, and also that for a range of wave-vectors the single magnon is unstable and will decay into three magnons.     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.     

Magnetic and photomagnetic studies in Nd0.7Sr0.3CoO3

We report magnetic and photomagnetic studies on polycrystalline Nd_0.7Sr_0.3CoO₃. Strong irreversibility in field cooled and zero field cooled data is observed. The hysteresis plot exhibits a very high coercivity at low temperatures. On photon irradiation, a decrease in the coercivity and an irreversible magnetization enhancement are observed. The analysis of all these data suggest that microscopic phase separation of competing ferromagnetism and antiferromagnetism and/or light-induced spin conversion processes among low, intermediate and high spin states of cobalt ions are responsible for all the properties.     

Dopant distribution, oxygen stoichiometry and magnetism of nanoscale Sn0.99Co0.01O2

In recent work, we have shown that chemically synthesized Sn_1−xCo_xO₂ nanoscale powders with x≤0.01 are ferromagnetic at room temperature when prepared by annealing the reaction precipitate in the narrow temperature window of 350-600 ^°C. Combined high resolution x-ray photoelectron spectroscopy (on as-prepared and Ar⁺ ion sputtered samples), x-ray diffraction and magnetometry measurements showed that the Co distribution is more uniform throughout the individual Sn_0.99Co_0.01O₂ particles when prepared at lower annealing temperatures of 350-600 ^°C and this uniform dopant distribution is essential to produce stable high temperature ferromagnetism. However, surface segregation of the dopant atoms in samples annealed at >600 ^°C destroys the room-temperature ferromagnetic behavior and reduces the Curie temperature to <300 K.     

Unusual magnetic and transport properties above the Curie temperature in La2/3Ca1/3MnO3

Measurements of susceptibility, electron paramagnetic resonance and resistivity for La_2/3Ca_1/3MnO₃ reveal a common temperature range between ∼266 and ∼300 K, where unusual aspects of physical properties are observed. The experimental results are discussed from the Griffiths theory that predicts the formation of ferromagnetic clusters before the Curie temperature is reached. It is emphasized that this theory incorporating the double-exchange mechanism could provide a good physical basis for the understanding of unusual transport properties.