Specific heat of diluted ferromagnets EuxSr1−xS

The specific heat C of diluted Eu_xSr_1?xS with Eu concentrations x = 0.95, 0.80, 0.70 and 0.60 was measured within the ferromagnetic regime. Both the temperature dependence of the magnetic part C_M and its behavior near the Curie temperature T_c show a variation with concentration which demonstrates clearly a change in magnetic behavior near x = 0.70. Eu_0.60Sr_0.40S exhibits properties which are more similar to spin glasses in spite of the long-range ferromagnetic order observed in the temperature range of measurement. This transient behavior is found to be consistent with previous observations.     

Structural and magnetic studies of the alloy system GdInxAg1-x

The structural and magnetic studies below room temperature of the alloy system GdIn_xAg_1-x(0?x?1) are reported. It has been found that alloys with x?0.5 crystallize in CsCl type cubic structure, but the distortion of the unit cell starts for alloys with x#62;0.5 and for these alloys the unit cell becomes tetragonal. No phase change is evident from the low temperature (295-8 K) structural studies. However, a break appears in the X^-1_m vs. T linear plot of each alloy of this system at a specific temperature (designated as break temperature T_B). The variation of T_B with x is similar to the variation of phase transition temperature with x reported for LaIn_xAg_1-x. Close agreement has been found in the values of effective magneton number (p), magnetic ordering [Néel (T_N) or Curie (T_C)] and paramagnetic Curie (θ_p) temperature for materials studied by us and earlier workers. The variation of magnetization with applied field strength (2.5-65)×10⁵ Am^-1) at 4.2 K has also been reported for ferromagnetic of this system. It has been concluded that alloys with 0.4?x?0.6 are simple ferromagnets with parallel alignment of magnetic moment in the ground state. The angular arrangement of the magnetic moment starts appearing in the ground state for alloys with x?0.4 for x#62;0.6 and continues till x becomes closure to 0.17 or 0.84. The alloys with x=0.17 or x=0.84 have θ_p and T_C equal to zero and appear paramagnetic. Angular arrangement in spins again appear for alloys with x?0.17 or x#62;0.84, however all materials with 0?x?0.17 or 0.84?x?1 remain antiferromagnetic.     

Magnetic ordering in EuxSr1−xSySe1−y

Specific heat and ac-susceptibility measurements are reported for Eu_xSr_1?xS_ySe_1?y samples. The critical concentrat ion for long-range magnetic ordering x_c increases with decreasing y, for y = 0.1, x_c = 1.0 holds. For samples with x < x_c a broad peak in the specific heat and a peak in the susceptibility at lower temperatures indicates a spin glass type of magnetic ordering.     

Ferromagnetic phase transitions in the magnetically heterogeneous solid solution system EuxLa1−xS

The ferromagnetic phase transitions of the solid solution system Eu_xLa_1-xS with Eu-concentrations x=0.85, 0.65 and 0.50 are analyzed by measurements of the initial permeability, the specific heat and the magnetization. For the sample with x=0.85 the spontaneous magnetization develops continuously between two temperatures T_c1 and T_c2. For the sample with x=0.65 a well defined magnetic ordering temperature exists. One observes strongly curved magnetization isotherms when plotting the magnetization data in form of modified Arrott plots. The M(H, T) data above the curved region show usual ferromagnetic scaling with the critical exponents β=0.5 and δ=4.7. These exponents fulfill the scaling relations with the exponents γ=2.1 and α≈-1 derived for the initial susceptibility and the magnetic specific heat. The sample with the concentration x=0.50 turns out to be not truely ferromagnetic. It is a spin glass with strong ferromagnetic short range order.     

Magnetic and transport property studies of nanocrystalline ZnxFe3−xO4

The magnetic and transport properties of nanocrystalline Zn_xFe_3−xO₄ with x=0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0, respectively, fabricated by the sol-gel method have been investigated. Large magnetoresistance (MR) was observed and found to be originated both from the tunneling of the spin-polarized electrons across the adjacent ferromagnetic grains and the scattering by the canted spins at the grain surface near the grain boundaries. It has been revealed that the MR for the Zn_xFe_3−xO₄ samples (x=0, 0.5 and 1.0) increases with the temperature decreasing from room temperature until a maximum is reached at around 55 K. Then a sharp drop occurs with the further decrease in temperature, regarded as a spin (cluster) glass transition. For the samples studied, a biggest low field (0.5 T) MR value of about 20% for x=0 at 55 K has been obtained. The mechanism of the MR behavior of the materials was discussed.     

Magnetic correlations in disordered MncZn1−cTe alloys

The spin correlations between Mn ions in Zn_1?cMn_cTe alloys have been measured as a function of temperature and concentration. The spin correlations follow the same sign sequence as for a type III antiferromagnet out to seven shells of neighbours. Complete long range magnetic order was not observed for any c or T.     

Limits of solubility,magnetic properties and electron concentration in Fe3-xTxSi system

The role of the outer electrons on the limits of solubility and bulk magnetic properties of the system Fe_3-xT_xSi (T - Ti, V, Cr, Mn, Co, Ni) is investigated. A correlation with the magnetic and structural behavior of Fe_1-cSi_c and Fe_1-cAl_c alloys is demonstrated.     

ψ1 of FexNi80−xB18Si2 alloys near the percolation threshold

Accurate measurements of the static magnetic susceptibility of amorphous Fe_xNi_80?xB₁₈Si₂ alloys with a concentration just above the critical one for the onset of ferromagnetism are reported. The effective critical exponent ψ¹ is found to vary non-monotonically with temperature. Shallow maxima in ψ¹ occur at increasing reduced temperatures (t=(T?T_c)/T_c) for decreasing Fe content (x). At elevated temperatures ψ¹ decreases with t but it does not reach the mean field value even for t=13. The results are consistent with the predictions of recent theoretical models.     

Upper critical fields of NbxGa1-x: A binary high temperature superconductor

The upper critical fields, H_C2, of several Nb_xGa_1-x alloys with values of T_c from 13.3 K to 20.2 K have been measured at temperatures from 4.2 to 20.4 K with dc and pulsed magnet fields. For the highest T_c material, H_C2(4.2 K) = 340 kG. The data are consistent with almost complete suppression of Pauli paramagnetic limiting. Comparisons with other high T_c materials, Nb₃Al and NbAlGe are presented.     

Magnetocaloric effect of GdCo2−xAlx compounds

The structure, magnetic property and magnetocaloric effect of GdCo_2−xAl_x (x=0, 0.06, 0.12, 0.18, 0.24, 0.4) compounds have been investigated by X-ray diffraction (XRD) and magnetic measurement techniques. The experimental results show that the GdCo_2−xAl_x (x≤0.4) compounds are single phase with a Laves-phase MgCu₂-type structure. The Curie temperature T_c initially increases, and then decreases with increasing Al content. The maximum value of T_c, 418 K, is reached for the compound with x=0.06. The magnetic entropy change, which is determined from the temperature and field dependence of the magnetization by the Maxwell relation, decreases almost linearly with increasing Al content.     

Magnetic properties of (NdxY1-x)Co2(1⩾ x ⩾ 0)

The concentration dependence of T_c and T_R (T_c magnetic ordering temperature, T_R spin reorientation temperature) of the pseudobinary system (Nd, Y)Co₂ is reported. Furthermore the influence of an external magnetic field on the spin reorientation and the magnetization is studied. The observed variation of the magnetization in the vicinity of the spin reorientation is compared with theoretical results. For the calculation a Hamiltonian with terms describing a molecular field, a cubic crystal field, and an external field is used.     

Weakferromagnetism in FexNi1−xS2 (x = 0−0.075)

The effects of nonmagnetic impurities (Fe²⁺ in a low spin state) on the weakferromagnetism in NiS₂ were investigated. The weak-ferromagnetic moment decreases rapidly with Fe content (x) and almost vanishes at about x = 0.05. On the other hand, T_c remains quite constant. Above T_c, two anomalies in the temperature dependence of the susceptibility were observed except for samples with x = 0 and 0.006. The effective magnetic determined from the susceptibility above T_c is about 3 μ_B/Ni, and is independent of x. Discussions are given on the origin of weakferromagnetism and on the hysteresis of the virgin magnetization curve.     

Structure and magnetic order of Cr-rich Rb2Cr1-xMnxCl4 crystals

The distorted K₂NiF₄-structure and the magnetic behaviour of the quasi two-dimensional magnetic system Rb₂Cr_1-xMn_xCl₄ (0 ? x ? 0.5) are studied by neutron diffraction and magnetic measurements. Strong correlations between ferromagnetic coupling and cooperative Jahn—Teller order are indicated by their occurence in the same concentration range 0 ? x ? 0.41.The percolation threshold x_F near x = 0.41 and the x, T-dependence of the para- to ferro-magnetic phase boundary are quite similar to that found in systems with non-magnetic impurities. Further we report the magnetic moment magnitudes in this percolative system.     

Analysis of magnetic field dependent mobility in ferromagnetic Ga1−xMnxAs layers

The magneto-transport properties of ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fractions in the range of x≈2.2-4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0-300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.     

Spin freezing properties in LaNi5-xFex

We report magnetic and transport measurements in the metallic pseudobinary compound LaNi_5-xFe_x, which show a Fe spin freezing. The magnetic phase diagram in the x-T plane is derived in the range 0 ⩽ x 1.2, 20 K ⩽ T ⩽ 300 K. Above a critical Fe concentration x_c ≈ 0.3, the system undergoes a superparamagnetic to long range ferromagnetic ordering at a finite temperature T. At lower concentrations, no long range spin ordering is observed, which suggests that x is the percolation threshold. Instead, a transition to a phase characterized by strong irreversibilities is observed at temperature T_f(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < x_c), which contrast with small values of T observed for x ∼ x_c. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice.     

Influence of Ce doping on electrical and thermal properties of La0.7−xCexCa0.3MnO3 (0.0≤x≤0.7) manganites

The effect of Ce-doping on structural, magnetic, electrical and thermal transport properties in hole-doped manganites La_0.7−xCe_xCa_0.3MnO₃ (0.0≤x≤0.7) is investigated. The structure of the compounds was found to be crystallized into orthorhombically distorted perovskite structure. dc Susceptibility versus temperature curves reveal various magnetic transitions. For x≤0.3, ferromagnetic regions (FM) were identified and the magnetic transition temperature (T_C) was found to be decreasing systematically with increasing Ce concentration. The electrical resistivity ρ(T) separates the well-define metal-semiconducting transition (T_MS) for low Ce doping concentrations (0.0≤x≤0.3) consistent with magnetic transitions. For the samples with 0.4≤x≤0.7, ρ(T) curves display a semiconducting behavior in both the high temperature paramagnetic (PM) phase and low temperature FM or antiferromagnetic phase. The electron–phonon and electron–electron scattering processes govern the low temperature metallic behavior, whereas small polaron hopping model is found to be operative in PM phases for all samples. These results were broadly corroborated by thermal transport measurements for metallic samples (x≤0.3) in entire temperature range we investigated. The complicated temperature dependence of Seebeck coefficient (S) is an indication of electron–magnon scattering in the low temperature magnetically ordered regime. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The observation of a significant difference between ρ(T) and S(T) activation energies and a positive slope in thermal conductivity κ(T) implying that the conduction of charge carriers were dominated by small polaron in PM state of these manganites.     

Quasi two-dimensional magnetic order of Tb3+ spins in Pb2Sr2Tb1−x Ca x Cu3O8 (x = 0 and 0.5)

Neutron diffraction experiments were performed to study the magnetic ordering of the Tb⁺³ sublattice in the high-T _c superconductor Pb₂Sr₂Tb_1?x Ca _x Cu₃O₈ (x = 0.5) and the undoped parent compound (x = 0). For the parent compound, a quasi two dimensional (2D) phase with a finite antiferromagnetic correlation along the c-direction and a three dimensional phase with ferromagnetic correlation along the c-direction were found. The coexistence of the two phases is likely to be related to structural imperfections such as stacking faults, strains, oxygen disorder or cation vacancies. The superconductor with a superconducting transition temperature of T _c = 71K exhibits a quasi 2D magnetic ordering of the Tb sublattice with a finite ferromagnetic correlation along the c-direction. The correlation lengths along the c-direction for the quasi 2D phases are 32 and 26Å for x = 0 and 0.5, respectively. The magnetic saturation moments are with 9.1 and 8.8 μ _B in excellent agreement with our mean-field crystal-field calculations. 2D short range correlation could be observed up to about 8T _N and is described by a Lorentzian distribution of magnetic intensity perpendicular to the 2D rod in reciprocal space.     

High field magnetization of Pr1-xGdxNi single crystals

Magnetization measurements were carried out on Pr_1−xGd_xNi single crystals for x=0.1, 0.2, 0.3 and 0.4 using pulsed magnetic fields up to 55 T and at 4.2 K. For the data observed along the c-axis there are clear indications of transitions, of the combined system of Pr and Gd moments, from a ferrimagnetic to a ferromagnetic state. The observed saturation magnetizations at 55 T and intermediate magnetizations at 1 T were well modeled assuming ferro- and ferri-magnetic structures, respectively.     

Metastable ferromagnetic phases with predictable Tcs in La2MnCo1−xNixO6

Metastable ferromagnetic phases, for different compositions in La₂MnCo_1−xNi_xO₆, are obtained for samples synthesized by a low-temperature method and annealed in air at different temperatures in the range 200-1350 °C. The T_cs of the ferromagnetic phases vary linearly between those of the phases of the end members. T_cs of the different phases of La₂MnCo_1−xNi_xO₆ can be predicted based on the T_cs and spin states of Mn, Co and Ni in the different phases of the end members, La₂MnCoO₆ and La₂MnNiO₆.