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 properties of ferromagnetic diluted Zn x Cd1–x Cr2Se4 spinels are studied by Green's functions,mean field theory and high temperature series expansions theories

The field induced reorientation of the magnetization of ferromagnetic (or antiferromagnetic) structure is treated within the framework of many-body Green's function theory by considering all components of the magnetization. The mean field theory is used to calculate the nearest neighbour and the next-neighbour super-exchange J₁(Cr–Cr) and J₂(Cr–(Zn(Cd)–Se)–Cr), respectively, for the Zn_1–x Cd _x Cr₂Se₄ in the range 0 < x < 1. The intraplanar and the interplanar interactions are deduced. The high temperature series expansions (HTSEs) are derived for the magnetic susceptibility and the 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 (RPA). The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The obtained 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 obtained values are comparable to those of 3D Heisenberg model.     

Magnetic phase diagram of the Ca1−xMnxO systems

The magnetic properties of the Ca_1−xMn_xO systems in the range 0?x?1 have been studied by mean field theory and high-temperature series expansions (HTSEs). By using the first theory, we have evaluated the nearest neighbour and the next-neighbour super-exchange interaction J₁(x) and J₂(x) respectively, in the range 0.45?x?1. The corresponding classical exchange energy for magnetic structure is obtained for the Ca_1−xMn_xO systems. The HTSEs combined with the Padé approximants (PA) method is applied to the Ca_1−xMn_xO systems; we have obtained the magnetic phase diagrams (TN or TSG versus dilution x) in the range 0?x?1. The obtained theoretical results are in agreement with experimental ones obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) are deduced in the range 0?x?1.     

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.     

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.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

Selected properties of EuCo2(Si1−xGex)2 alloys

EuCo₂(Si_1−xGe_x)₂, x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 samples were synthesised by induction melting followed by annealing at 900 °C and rapid quenching. X-ray powder diffraction and Auger electron spectroscopy studies revealed that solid solutions are formed only for x?0.2 and x?0.7. Magnetic susceptibility investigations for the solid solutions revealed a dominant divalent europium valence state in the germanium-rich samples and a dominant trivalent europium component in the silicon-rich samples. In the germanium-rich samples, a long-range antiferromagnetic ordering was observed. In all samples studied, additional magnetic transitions at various temperatures were detected, which could be attributed to small clusters containing different europium chemical surrounding from that in the predominant phase.     

Magnetic properties of MnCr2O4 nanoparticle

The exchange interactions and the magnetic exchange energies are calculated by using the mean field theory and the probability law of Zn_1−xMn_xCr₂O₄ nanoparticles. The high-temperature series expansions have been applied in the spinels Zn_1−xMn_xCr₂O₄ 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 the 3D Heisenberg model.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

Magnetic and transport properties in Sr1−xLaxFe1−xMnxO3

The magnetic and transport properties in the perovskite Sr_1−xLa_xFe_1−xMn_xO₃ have been explored. As x rises, the systemic ferromagnetism increases gradually and cluster-spin-glass state occurs in the low-temperature region. For 0.3?x?0.7, the ferromagnetic phase separation from the paramagnetic phase was observed from the results of electron-spin-resonance measurement. Although all samples show a semiconducting behavior, their transport properties are dominated by two different mechanisms, namely, the electronic transport of x?0.5 samples is realized by thermal activation but the variable-range hopping is applied in x?0.7 ones. The different transport mechanism can be understood from the Mn/Fe ions interaction.     

The role of iron in the formation of the magnetic structure and related properties of La0.8Sr0.2Co1−xFexO3 (x=0.15, 0.2, 0.3)

La_0.8Sr_0.2Co_1−xFe_xO₃ (x=0.15, 0.2, 0.3) samples were studied by means of AC magnetic susceptibility, magnetization, magnetoresistance and ⁵⁷Fe Mössbauer spectrometry. Iron was found to take on a high spin 3d^5−α electronic state in each of the samples, where α refers to a partly delocalized 3d electron. The compounds were found to exhibit a spin-cluster glass transition with a common transition temperature of ∼53 K. The spin-cluster glass transition is visualized in the ⁵⁷Fe Mössbauer spectra as the slowing down of magnetic relaxation below ∼70 K, thereby showing that iron takes part in the formation of the glassy magnetic phase. The paramagnetic-like phase found at higher temperatures is identified below T_c≈195 K as being composed of weakly interacting, magnetically ordered nanosized clusters of magnetic ions in part with a magnetic moment oriented opposite to the net magnetic moment of the cluster. For each of the samples a considerable low-temperature negative magnetoresistance was found, whose magnitude in the studied range decreases with increasing iron concentration. The observed results obtained on the present compounds are qualitatively explained assuming that the absolute strengths of magnetic exchange interactions are subject to the relation ∣J_Co–Co∣<∣J_Fe–Co∣<∣J_Fe–Fe∣.     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

Characterization of magnetic phases in the FexMn0.65−xAl0.35 disordered alloys

Melted alloys of the Fe_xMn_0.65−xAl_0.35 disordered system, 0.25?x?0.65, were experimentally studied by Mössbauer spectrometry, vibrating sample magnetometry and AC magnetic susceptibility. All the alloys exhibit the BCC structure with a nearly constant lattice parameter (2.92 Å). Mössbauer studies at room temperature (RT) show that in the 0.25 ?x?0.45 range the alloys are paramagnetic (P) while in the 0.50?x?0.65 range, they are ferromagnetic. At 77 K, Mössbauer studies show that the alloy with x=0.25$x=0.25$ presents weak magnetic character that is consistent with an antiferromagnetic (AF) behavior due to the high Mn content, while those with 0.30?x?0.40 are paramagnetic, and those in the 0.45?x  ?0.65 range are ferromagnetic (F) with a mean field increasing with the Fe content. Hysteresis cycles at RT prove the paramagnetic character of the alloys between x=0.25$x=0.25$ and 0.40 and the ferromagnetic character for x?0.45$x?0.45$. Complementary measurements using AC magnetic susceptibility permit a magnetic phase diagram to be proposed, with the P phase for high temperature and all the compositions, the AF phase for low Fe content and at low temperature, the F phase for high Fe content above RT and the spin glass phase for all the compositions and at temperatures lower than 46 K. In addition, the mean field renormalization group (MFRG) method, applied to a random competitive and site dilute Ising model with nearest-neighbor, gives rise to magnetic phase diagram, which fairly agrees with previous experimental one.     

Magnetic anisotropy of Tb1−xGdxMn6Sn6 compounds

Magnetization curves of Tb_1−xGd_xMn₆Sn₆ compounds (0?x?1) have been measured for aligned powder samples in the temperature range 4.2–300 K in pulsed magnetic fields up to 30 T. Temperature and concentration dependences of the magnetocrystalline anisotropy constants K₁ and K₂ and concentration dependence of the temperature of spontaneous spin-reorientation transition have been determined. Using these data, we estimated the contribution of the manganese and terbium atoms to the magnetic anisotropy of Tb_1−xGd_xMn₆Sn₆ and analyzed the origin of the appearance of field-induced first-order magnetic phase transition in these compounds.     

The effect of annealing process on the physical properties of La1−xNaxMnOy

The effect of Na doping and annealing time on the structure, electrical properties, magnetoresistance and thermopower properties has been investigated in perovskite La_1−xNa_xMnO_y (x=0.025, 0.075 and 0.1) systems. La_1−xNa_xMnO_y crystallizes in a single-phase rhombohedral structure. It is observed a simultaneous occurrence of the ferromagnetic to paramagnetic state and metallic to insulating state. In the meanwhile, a large negative magnetoresistance with low applied magnetic field is observed. In addition, ρ(T) curves for Na-doped samples exhibit another broad transition T_ms2 below T_ms. Such double peak behavior in the ρ(T) curve interpreted by the electronic inhomogeneity in the samples. The sign of S changes from positive to negative depending on composition. The values of Seebeck coefficient are small (in the microvolt range).     

Structural, electronic and optical calculations of CaxZn1−xO alloys: A first principles study

First principles density functional calculations, using full potential linearized augmented plane wave (FP-LAPW) method, have been performed in order to investigate the structural, electronic and optical properties of Ca_xZn_1−xO alloy in B1 (NaCl) phase. Dependence of structural parameters as well as the band gap values on the composition x have been analyzed in the range 0?x?1. Calculated electronic structure and the density of states of these alloys are discussed in terms of the contribution of Zn d, O p and Ca p and d states. Furthermore, optical properties such as complex dielectric constants ε(ω), refractive index including extinction coefficient k(ω), normal-incidence reflectivity R(ω), absorption coefficient α(ω) and optical conductivity σ(ω) are calculated and discussed in the incident photon energy range 0-45 eV.     

Oxygen vacancy doping effect on the electrical and magnetic behavior of Ba5−xLaxNb4−xTixO15

We report electric and magnetic properties of oxygen deficient Ba_5−xLa_xNb_4−xTi_xO_15−δ phases, which have been prepared by solid-state reaction method followed by a controlled reduction process under hydrogen atmosphere. The extra electrons added by the formation of the oxygen vacancies (δ) introduce localized spins and the magnetic susceptibility can be described by a temperature-independent contribution and a Curie-Weiss term associated to the Ti³⁺ ion formation. Besides, the experimental resistivity (ρ) data of these four reduced compounds are well described in a wide temperature range with the equation , which suggests the presence of small polarons in the system. Although, all samples present electrical insulating behavior, the electrical resistivity decreases four orders of magnitude for intermediate x values. We interpreted this fact as a consequence of the mix between the localized bands of the Nb and Ti ions, which favors the promotion of carriers due to reduction of the band gap.     

Compensation effect in an exchange enhanced superconductor: [(La.993Lu.007)1−xGdx]Ru2

We have measured the upper critical field, H_c2, of alloys of the form [(La_.993Lu_.007)_1?xGd_x]Ru₂. For large $\text{(x?0.02)}$ concentrations of Gd, the critical fields are re-entrant. For small (x?0.01) concentrations of Gd, the critical fields cross the pure sample critical fields and rise above them due to compensation of the externally applied magnetic field by the negative effective internal field of the magnetic impurity. Compensation occurs at relatively low fields (25 KG) in this system as a result of substantial exchange enhancement. From the concentration and critical field of the most compensated sample, we find a value for the exchange integral, J(0) = ?(15+?2) meV, and a susceptibility enhancement factor in excellent agreement with EPR, specific heat and static susceptibility measurements.     

Hard magnetic properties of ErCo7−xCux [0.3⩽x⩽1] system

We report the observation of excellent hard magnetic properties on purely single phase ErCo_7−xCu_x compounds with x=0.3, 0.5, 0.8 and 1. Cu substitution leads to a decrease in the saturation magnetization, but enhances the uniaxial anisotropy in this system. The large anisotropy field (∼100 kOe) is attributed to the Er and the Co sublattices. Domain wall pinning effect seems to play a crucial role in determining the temperature and field dependences of magnetization in these compounds. The hard magnetic properties obtained at room temperature (RT) are comparable to the best results obtained in other RCo₇ based materials.     

Investigations of Sm6(Mn1-xFex)23 system

The new Sm₆(Mn_1-xFe_x)₂₃(0?x?1.0) system hasbeen synthesized and investigated in a wide temperature range by the X-ray, magnetometric and Mössbauer effect methods. The X-ray studies show that the system forms solid solutions which are isostructural with the Th₆Mn₂₃ type crystal structure throughout the entire compositional range. Both Fe-rich and Mn-rich regions of the system are magnetically ordered and are separated from each other by the non-magnetically ordered 0.22?x?0.33 region. The substitution of Fe atoms for Mn atoms in the Mn-rivh region and similarly of Mn atoms for Fe atoms in the Fe-rich region decreases both the Curie temperature and the value of the magnetic moment per molecule. The temperature dependence of the reciprocal susceptibility obeys the Néel law. The Mössbauer absorption spectra reflect wide distributions of the ⁵⁷Fe hyperfine interaction parameters, and disappearance of long range magnetic coupling of Fe atoms in the magnetically ordered x=0 to 0.22 composition range.