The structure and magnetotransport properties of La2/3Sr1/3MnO3/Ba(La)TiO3 composites

The effect of Ba(La)TiO₃ doping on the structure and magnetotransport properties of La_2/3Sr_1/3MnO₃(LSMO)/xBa(La)TiO₃ (x=0.0, 1.0, 5.0 mol%) have been investigated. The X-ray diffraction patterns and microstructural analysis show that BaTiO₃ and LSMO phases exist independently in BaTiO₃-doped composites. The metal-insulator transition temperature (T_MI) decreases whereas the maximum resistivity increases very quickly by the increase of BaTiO₃ doping level. The partial substitution of Ba by La(0.35 mol%) results in a decrease in resistivity of LSMO/xBa(La)TiO₃ composites. Magnetoresistance of BaTiO₃-doped composites decreases monotonously in the temperature range 200-400 K in a magnetic field of 5 T, which is completely different from that of LSMO compound. The value of MR decreases at low field (H<1 T) and increases at high fields (H>1 T) with increasing the BaTiO₃ doping level at low temperatures below 280 K. These investigations reveal that the magnetotransport properties of LSMO/xBa(La)TiO₃ composites are dominated by spin-dependent scattering and tunneling effect at the LSMO/BaTiO₃/LSMO magnetic tunnel junction.     

Structural, magnetic and magnetocaloric properties of La0.7−xEuxBa0.3MnO3 perovskites

Polycrystalline perovskite manganites La_0.7−xEu_xBa_0.3MnO₃(x=0.05, 0.1 and 0.15) were prepared by sol-gel method. The prepared samples remain single phase with a perovskite structure, revealed by X-ray diffraction. The structure refinement of La_0.7−xEu_xBa_0.3MnO₃(x=0.05, 0.1 and 0.15) samples was performed in the hexagonal setting of the R3¯c space group. The dependence of magnetization M on applied magnetic field H and temperature T was measured carefully near the Curie temperature T_C for all the samples. With the increasing Eu content, both the unit cell volume and Curie temperature T_C of 298 K has been detected with a maximum of magnetic entropy |ΔS_M^max| for the La_0.7−xEu_xBa_0.3MnO₃ with x=0.15, reaching a value of 2.3 J/kg K when a magnetic field of 10 kOe was applied and the relative cooling power (RCP) is 46 J/kg. These results suggest that the material may be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Magnetocaloric properties in the Cr-doped La0.7Sr0.3MnO3 manganites

Single-phase polycrystalline samples of La_0.7Sr_0.3Mn_1-xCr_xO₃ with nominal composition of x=0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature T_C decreases with increasing x and the maximum magnetic entropy change (−ΔS_M) for x=0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.     

Structure and magnetic properties of TbFe6−xGa6+x compounds

The TbFe_6−xGa_6+x compounds (x=0, 0.5, 1.0 and 1.45) have been prepared and studied by X-ray powder diffraction and magnetization measurements. The structure of the compound TbFe_6−xGa_6+x transfers from the orthorhombic ScFe₆Ga₆-type structure (space group Immm) (x=0 and 0.5) to the tetragonal ThMn₁₂-type structure (space group I4/mmm) (x=1.0 and 1.5) and the volume of the unit cell increases as Ga content increases. The lattice parameters are a=0.85551, b=0.8626 and c=0.50717 nm for TbFe₆Ga₆, and a=0.86938 and c=0.50918 nm for TbFe_4.55Ga_7.45.The magnetization measurements indicate all the TbFe_6−xGa_6+x compounds have magnetic ordering. The Curie temperatures decrease from 492 K for TbFe₆Ga₆ to 327 K for TbFe_4.55Ga_7.45. The magnetization of the TbFe_6−xGa_6+x decreases with temperature decreases below its magnetic ordering temperature due to the increasing Tb-sublattice magnetization which is antiparallel to the Fe-sublattice magnetization.     

Effect of Co or Mn addition on the soft magnetic properties of amorphous Fe89−xZr11Bx (x=5, 10) alloy ribbons

The temperature and field dependent magnetic properties of melt-spun amorphous Fe_89−x−yZr₁₁B_x(Co,Mn)_y (x=5, 10 and 0≤y≤10) alloys in the temperature range 5-1200 K are reported. The Curie temperature and saturation magnetization at room temperature increase (decrease) almost linearly with Co (Mn) addition. With increasing Co concentration, the room temperature coercivity increases at the rate of 2.26 (0.28) A/m per at% for the x=5 (10) samples. The high-field magnetic susceptibility and local magnetic anisotropy decrease (increases) rapidly with increasing Co (Mn) concentration. The thermomagnetic curves show a marked increase in magnetization above 850 K corresponding to the crystallization of α-FeCo (α-Fe) phase in samples containing Co (Mn). The Curie temperature of the crystalline phase increases (remains same) with increasing Co (Mn) concentration with the formation of α-FeCo (α-Fe). Addition of Co up to 10 at% in Fe-Zr-B improves the room temperature saturation magnetization from 0.56 to 1.2 T, and Curie temperature from 315 to 476 K. Also, the coercivity increases with Co addition from 1.27 to 23.88 A/m for x=5 and from 7.64 to 10.35 A/m for x=10 alloy. The non-collinear spin structures that characterize Fe rich Fe-Zr-B amorphous alloys have been used to describe the observed results.     

Spin-glass-like behavior in ZnxCryAlzSe4

The magnetic and electrical properties of the Al-doped polycrystalline spinels Zn_xCr_yAl_zSe₄ (0.13≤z≤0.55) with the antiferromagnetic (AFM) order and semiconducting behavior were investigated. A complex antiferromagnetic structure below a Néel temperature T_N≈23 K for the samples with z up to 0.4 contrasting with the strong ferromagnetic (FM) interactions evidenced by a large positive Curie-Weiss temperature θ_CW decreasing from 62.2 K for z=0.13 to 37.5 K for z=0.55 was observed. Detailed investigations revealed a divergence between the zero-field-cooling (ZFC) and field-cooling (FC) susceptibilities at temperature less than T_N suggesting bond frustration due to competing ferromagnetic and antiferromagnetic exchange interactions in the compositional range 0.13≤z≤0.4. Meanwhile, for z=0.55 a spin-glass-like behavior of cluster type with randomly oriented magnetic moments is observed as the ZFC-FC splitting goes up to the freezing temperature T_f=11.5 K and the critical fields connected both with a transformation of the antiferromagnetic spin spiral via conical magnetic structure into ferromagnetic phase disappear.     

The synthesis and the magnetic properties of Gd-doped FexCo1−x/CoyFe3−yO4 micro-octahedrons composites

Gd³⁺-substituted micro-octahedron composites (Fe_xCo_1−x/Co_yGd_zFe_3−y−zO₄) in which the Fe-Co alloy has either a bcc or fcc structure and the oxide is a spinel phase were fabricated by the hydrothermal method. The X-ray diffraction (XRD) patterns indicate that the as-synthesized Gd³⁺-substituted micro-octahedron composites are well crystallized. Scanning electron microscopy (SEM) images show that the final product consists of larger numbers of micro-octahedrons with the size ranging from 1.3 to 5 μm, and the size of products are increased with increasing the concentration of KOH. The effect of the Co²⁺/Fe²⁺ ratio (0?Co²⁺/Fe²⁺?1) and substitution Fe³⁺ ions by Gd³⁺ ions on structure, magnetic properties of the micro-octahedrons composites were investigated, and a possible growth mechanism is suggested to explain the formation of micro-octahedrons composites. The magnetic properties of the structure show the maximal saturation magnetization (107 emu/g) and the maximal coercivity (1192 Oe) detected by a vibrating sample magnetometer.     

Effect of Sn-doping on the structural,magnetic and magnetocaloric properties of La0.67Ba0.33Mn1−xSnxO3 compounds

Magnetocaloric effect (MCE) in fine-grained perovskite manganites of the type La_0.67Ba_0.33Mn_1−xSn_xO₃ (x=0.05, 0.1 and 0.15) were prepared by the solid-state method. The prepared samples remain single phase and exhibit paramagnetic to ferromagnetic phase transition (T_C) at 340, 325 and 288 K for x=0.05, 0.1 and 0.15, respectively. From the measured magnetization data of La_0.67Ba_0.33Mn_1−xSn_xO₃ compounds as a function of field (2 T), the associated magnetic entropy change close to their respective Curie temperatures and the relative cooling power (RCP) have been determined. Large MCE has been obtained in all samples and |ΔS_M|_max reached the highest value of 2.49 J/kg K at T_C (288 K) for the sample x=0.15, with H=2 T.     

Magnetocaloric properties of (La0.67−xGdx)Sr0.33MnO3 polycrystalline nanoparticles

In this paper, magnetic property and magnetocaloric effect (MCE) in nanoparticles perovskite manganites of the type (La_0.67−xGd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) synthesized by using an amorphous molecular alloy as precursor have been reported. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered that the Curie temperature (T_C) of the prepared samples is found to be strongly dependent on Gd content. The Curie temperature of samples is 358.4, 343.2, and 285.9 K for x=0.1, 0.15, and 0.2, respectively. A large magnetocaloric effect close to T_C has been observed with a maximum of magnetoentropy change in all the samples, ∣ΔS_M∣_max of 1.96 and 4.90 J/kg K at 2 and 5 T, respectively, for a substitution rate of 0.15. In addition, the maximum magnetic entropy change observed for samples with different concentration of Gd, exhibits a linear dependence with the applied high magnetic field. These results suggest that (La_0.67−x Gd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) compounds could be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

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)).     

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.     

Magnetocaloric properties of the La0.7Pr0.3Fe13−xSix compounds

Magnetic entropy change ΔS_m, hysteresis loss and refrigerant capacity of NaZn₁₃-type La_0.7Pr_0.3Fe_13−xSi_x (1.5?x?2.0) compounds have been investigated. The Curie temperature T_C increases linearly with the increase of silicon concentration. Although the maximum value of ΔS_m under a field change of 0−5 T decreases from 30.5 to 11.4 J/kg K as x increases from 1.5 to 2.0, the hysteresis loss at T_C reduces remarkably from 89.2 J/kg for x=1.5 to zero for x=2.0 because the increase of Si content can weaken the itinerant electron metamagnetic transition. The effective refrigerant capacity RC_eff is maintained at high values of 362−439 J/kg for a field change of 0−5 T. This implies that a large ΔS_m and a high RC_eff can be achieved simultaneously in the La_0.7Pr_0.3Fe_13−xSi_x compounds.     

EPR study of the diluted magnetic semiconductor CuGa1−xMnxTe2

Electron paramagnetic resonance (EPR) experiments were made in the diluted magnetic semiconductor CuGa_1−xMn_xTe₂, in the temperature range 70<T<300 K. The samples were synthesized by direct fusion of stoichiometric mixtures of the elements, with Mn composition from x=0.0 to 0.25. The EPR spectra were measured as function of temperature, Mn composition, and field orientation. The temperature variation of the resonance field shows a critical point at about 235 K, and is associated with a transition from the ferromagnetic to the superparamagnetic state. The resonance field was also measured as a function of the field angle, and displays a well-defined uniaxial symmetry. This uniaxial field depends on the Mn concentration and is due to tetragonal distortions induced by Mn²⁺ at Ga sites, and the demagnetizing effects due to formation of ferromagnetism (FM) Mn-clusters.     

Temperature dependence of magnetic anisotropy in nanoparticles of CoxFe(3−x)O4

In this paper we present a study of the magnetic anisotropy constant of nanocrystalline magnetic particles of Co_xFe_(3−x)O₄, with x ranging from 0.05 to 1.6, synthesized by a combustion reaction. The magnetic anisotropy constants were obtained by fitting the high-field part of the major hysteresis loops with the law of approach equation down to temperatures of 4 K and up to fields of 60 kOe. The anisotropy constant depends strongly on both temperature and cobalt content x, exhibiting a nonmonotic dome-shaped dependence on x with a maximum at x=1.0. We found that fits at lower temperatures, i.e., 4 and 72 K, give values of K₁ that are approximately one order of magnitude higher than those at higher temperatures, i.e., 272 and 340 K. For example, K₁ for specimens with x=0.8 and 1.0 are 4.21×10⁷ and 4.22×10⁷ ergs/cm³ at 4 K, and 7.64×10⁶ and 7.51×10⁶ ergs/cm³ at 340 K, respectively. Thus, our determination of temperature-dependence of the anisotropy constant represents an improvement over existing works.     

Structure,magnetic properties and magnetostriction of laves compounds Nd1−xPrx(Fe0.35Co0.55B0.1)2

The C15 Laves phases with composition Nd_1−xPr_x(Fe_0.35Co_0.55B_0.1)₂ (0?x?1) have been synthesized by arc melting and subsequent annealing. The Curie temperature T_c and the saturation magnetizations M_s at 5 and 295 K decrease with increasing Pr content. The linear anisotropic magnetostriction λ_a=λ_∥−λ_⊥ at room temperature for Nd_1−xPr_x(Fe_0.35Co_0.55B_0.1)₂ alloys with 0?x?0.4 initially reaches a negative minimum, then increases and changes its sign with increasing magnetic field H, and the λ_a for the alloys with x?0.6 is positive and increases as magnetic field H increases.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Synthesis and investigation of magnetic properties of substituted ferrite nanoparticles of spinel system Mn1−xZnx[Fe2−yLy]O4

Superparamagnetic nanoparticles of the spinel ferrite four-element system Mn_1−xZn_x[Fe_2−yL_y]O₄ (where L:Gd³⁺, La³⁺, Ce³⁺, Eu³⁺, Dy³⁺, Er³⁺,Yb³⁺) were synthesized by the co-precipitation method. The magnetic moments of the 10 nm diameter nanoparticles were comparable to the ones of Fe₃O₄ nanoparticles. A comparatively low T_C (∼52–72 °C) was observed for some of the compositions. The heating mechanism of the superparamagnetic particles in the AC magnetic field at radiofrequency range is discussed and especially the absence of the hysteresis loop in the M–H curve at room temperature. One possible explanation—spontaneous particle agglomeration—was experimentally verified.     

Entropy changes accompanying the magnetic phase transitions in low Si-doped Ce2Fe17−xSix Alloy

A summary of the results of ac susceptibility and isothermal magnetization measurements on polycrystalline samples of Ce₂Fe_17−xSi_x with nominal composition of x=0.0, 0.1, 0.2 and 1.0 is presented. These data reveal that the substitution of small amounts of Si for Fe produce a significant increase in temperature at which ferromagnetism appears, to the extent that, at x=1, characteristics of the anti ferromagnetic to paramagnetic transition (at temperature T_N) have disappeared completely. The nature of the various magnetic phase transitions — identified through the use of Arrott plots — and the accompanying magnetic entropy change, ΔSm, are both affected significantly by small amounts of Si substitution. In particular, while the peak entropy change is modest (occurring at x=0.1), the temperature interval over which a substantial entropy change occures is significant, approaching 150 K, an important criterion for improving the overall effectiveness of such materials for magnetic refrigeration.     

Monodispersed nanocrystalline Co1–xZnxFe2O4 particles by forced hydrolysis: Synthesis and characterization

Zinc-substituted cobalt ferrites, Co_1–xZn_xFe₂O₄, were for the first time successfully prepared by forced hydrolysis method. The obtained materials are single phase, monodispersed nanocrystalline with an average grain size of about 3 nm. These materials are superparamagnetic at room temperature and ferrimagnetic at temperature lower than the blocking temperature. When the zinc substitution increases from x=0 to 0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g. The high saturation magnetization of these samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications.