Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.     

Unusual magnetic hysteresis behavior of oxide spinel MnCo2O4

Magnetic hysteresis behavior of the oxide spinel MnCo₂O₄ has been studied at different temperatures below its T_c≈184 K. Normal hysteresis behavior is observed down to 130 K whereas below this temperature the initial magnetization curve, at higher magnetic fields, lies outside the main loop. No related anomaly is observed in the temperature variation of magnetization or coercivity. However, the anisotropy field overcomes the coercivity below 130 K. The unusual magnetic hysteresis behavior of MnCo₂O₄, at low temperatures, may be associated with irreversible domain wall movements due to the rearrangement of the valence electrons.     

Relation of structure and magnetic properties in nickel substituted MgFe2O4

This study presents the structure-related magnetic properties of nickel substituted MgFe_2−xNi_xO₄ with x=0.01-0.20, synthesized using thermal decomposition of metal organic starting materials. The samples were annealed at temperatures ranging from 900 to 1100 °C in air. Magnetic and electric properties were characterized using vibrating sample magnetometer and an impedance analyzer, respectively. The fraction of inverse spinel structure was found to decrease with increase in nickel concentration. The samples exhibited soft magnetic behavior. The magnetization was found to depend on the substitution of Ni atoms on the octahedral sites and on the average number of valence electrons. The dielectric properties were found to have higher values at low frequency and decreased with increase in frequency. An enhancement of the dielectric constant was observed upon increase in the annealing temperature.     

Superconducting and magnetic properties of La1-xGdxRu2 and La1−xPrxRu2 mixed crystals

LaRu₂ in contrast to CeRu₂ shows the normal T_c-depression by magnetic rare earth ions. This depression is much larger for praseodymium than for gadolinium.     

Silane treatment of Fe3O4 and its effect on the magnetic and wear properties of Fe3O4/epoxy nanocomposites

In this study, the effect of silane treatment of Fe₃O₄ on the magnetic and wear properties of Fe₃O₄/epoxy nanocomposites was investigated. Fe₃O₄ nanopowders were prepared by coprecipitation of iron(II) chloride tetrahydrate with iron(III) chloride hexahydrate, and the surfaces of Fe₃O₄ were modified with 3-aminopropyltriethoxysilane. The magnetic properties of the powders were measured on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites using SQUID magnetometer. Wear tests were performed on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites under the same conditions (sliding speed: 0.18 m/s, load: 20 N).The results showed that the saturation magnetization (M_s) of surface-modified Fe₃O₄/epoxy nanocomposites was approximately 110% greater than that of unmodified Fe₃O₄/epoxy nanocomposites. This showed that the specific wear rate of surface-modified Fe₃O₄/epoxy nanocomposites was lower than that of unmodified Fe₃O₄/epoxy nanocomposites. The decrease in wear rate and the increase in magnetic properties of surface-modified Fe₃O₄/epoxy nanocomposites occurred due to the improved dispersion of Fe₃O₄ into the epoxy matrix.     

Comparative study of structural and magnetic properties of nano-crystalline Li0.5Fe2.5O4 prepared by various methods

Lithium ferrite has been considered as one of the highly strategic magnetic material. Nano-crystalline Li_0.5Fe_2.5O₄ was prepared by four different techniques and characterized by X-ray diffraction, vibrating sample magnetometer (VSM), transmission electron microscope (TEM) and Fourier transform infrareds (FTIR). The effect of annealing temperature (700, 900 and 1050 °C) on microstructure has been correlated to the magnetic properties. From X-ray diffraction patterns, it is confirmed that the pure phase of lithium ferrite began to form at 900 °C annealing. The particle size of as-prepared lithium ferrite was observed around 40, 31, 22 and 93 nm prepared by flash combustion, sol-gel, citrate precursor and standard ceramic technique, respectively. Lithium ferrite prepared by citrate precursor method shows a maximum saturation magnetization 67.6 emu/g at 5 KOe.     

The effect of urea-to-nitrates ratio on the morphology and magnetic properties of Mn0.8Mg0.2Fe2O4

Mn_0.8Mg_0.2Fe₂O₄ ferrite was synthesized using flash auto-combustion technique using urea as fuel. The effect of the urea-to-nitrates ratio was examined and found to affect the samples characteristics. The as-burnt powder was crystallized in single-phase spinel structure of cubic symmetry. The lattice parameter was decreased with increase in the urea-to-nitrates ratio (n) while the crystal size increased from 21 to 42 nm with n changing from 6.67 to 10. The coercivity increases while the saturation and remanence magnetization decreases with increase in n. This was attributed to the disturbance of the spin order as a result of the surface effects.     

Structural, microstructural and magnetic properties of NiFe2O4, CoFe2O4 and MnFe2O4 nanoferrite thin films

The structural, microstructural and magnetic properties of nanoferrite NiFe₂O₄ (NF), CoFe₂O₄ (CF) and MnFe₂O₄ (MF) thin films have been studied. The coating solution of these ferrite films was prepared by a chemical synthesis route called sol-gel combined metallo-organic decomposition method. The solution was coated on Si substrate by spin coating and annealed at 700 °C for 3 h. X-ray diffraction pattern has been used to analyze the phase structure and lattice parameters. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been used to show the nanostructural behavior of these ferrites. The values of average grain's size from SEM are 44, 60 and 74 nm, and from AFM are 46, 61 and 75 nm, respectively, measured for NF, CF and MF ferrites. At room temperature, the values of saturation magnetization, M_s∼50.60, 33.52 and 5.40 emu/cc, and remanent magnetization, M_r∼14.33, 15.50 and 1.10 emu/cc, respectively, are observed for NF, CF and MF. At low temperature measurements of 10 K, the anisotropy of ferromagnetism is observed in these ferrite films. The superparamagnetic/paramagnetic behavior is also confirmed by χ′(T) curves of AC susceptibility by applying DC magnetizing field of 3 Oe. The temperature dependent magnetization measurements show the magnetic phase transition temperature.     

Size-induced effect on nano-crystalline CoFe2O4

Cobalt ferrite (CoFe₂O₄) nano-particles have been synthesized successfully and we studied the effect of temperature on them. The particles have been annealed at different temperatures ranging from 373 to 1173 K. Significant effect on the physical parameters like crystalline phase, crystallite size, particle size, lattice strain and magnetic properties of the nano-particles has been investigated. The studies have been carried out using a powder X-ray diffractometer (XRD), a transmission electron microscope (TEM) and a vibrating sample magnetometer (VSM). A thorough study of the variation of specific surface area and particle size with annealing is presented here, with their effects on saturation magnetization.     

Structural, magnetic and magnetocaloric properties of the Gd5Si4−xSbx (x=0.5-3.5) phases

Substitution of Sb for Si in the Gd₅Si₄ phase results in the formation of ternary Gd₅Si_4−xSb_x phases with three different structure types. For x≤0.38 the Gd₅Si₄-type structure (Pnma space group) with all interslab T-T dimers intact exists (T is Si or Si/Sb mixture), in the x=1.21-1.79 region the Sm₅Ge₄-type structure (Pnma) with all interslab dimers broken is found. A further increase in the Sb concentration (x=2.27-3.1) leads to the appearance of the Eu₅As₄-type structure (Cmca) with broken but equivalent interslab T?T bonds. The Gd₅Si_4−xSb_x phases with 1.21≤x≤3.1 undergo ferromagnetic transitions in the temperature range from 164 to 295 K. Magnetocaloric effect in terms of the isothermal entropy change, ΔS, reaches the maximum values of -3.7(4), -4.2(6) and -4.6(7) J/kg K for the Gd₅Si₂Sb₂, Gd₅Si_1.5Sb_2.5 and Gd₅SiSb₃ samples, respectively.     

Magnetic pair making and breaking effect of Ru in La0.7Sr0.3Mn0.9Ru0.1O3 and La0.5Sr0.5Co0.9Ru0.1O3

The substitutional effect of Ru on the magnetic and transport properties of double exchange ferromagnets, La_0.7Sr_0.3MnO₃ and La_0.5Sr_0.5CoO₃ has been investigated. It is found that substitution of 10% Ru at the Mn site of La_0.7Sr_0.3MnO₃ decreases the Curie temperature by 20 K than that of the parent compound. However, a large decrease in the Curie temperature, ΔT_c80 K and the system undergoes a transition from metallic state to insulating state is observed when 10% Ru is doped in La_0.5Sr_0.5CoO₃. The marginal effect of Ru in the Mn–O–Mn sublattice in comparison to the Co–O–Co sublattice could be due to the magnetic exchange interaction between Mn and Ru by virtue of the fact that Ru exhibits variable valence states, Ru⁺⁴/Ru⁺⁵. The e_g and t_2g parentage of Ru⁺⁵ is similar to Mn⁺⁴ and therefore, Ru⁺⁵ ion appears to participate in the double exchange mediated ferromagnetic (FM) interaction. On the other hand, Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co⁺³: t_2g⁵e_g¹), forcing a double exchange FM state to anti-FM state.     

Magnetic properties of nanocrystalline CoFe2O4 synthesized by modified citrate-gel method

Nanocrystalline CoFe₂O₄ with an average grain size of about 40 nm was successfully prepared by a modified citrate-gel method. At temperatures of 3 and 300 K, the measured coercive fields are 0.43 and 0.07 T and the magnetizations at 7 T are 89 and 83 emu/g, respectively. At room temperature, the longitudinal and transversal magnetostriction values are −130 and 70 ppm, respectively. The contribution of a disordered magnetic phase was detected by the occurrence of a peak in the ac-susceptibilities curves at around 250 K. The temperature dependence of the field-cooled and zero field-cooled low-field magnetization showed a larger irreversibility below this temperature. This disordered phase behaves like a spin-glass, which is coexisting with the ferrimagnetically ordered main phase     

Investigation of the static and dynamic magnetic properties of CoFe2O4 nanoparticles

We have investigated the magnetic behavior of cobalt ferrite nanoparticles with a mean diameter of 7.2 nm. AC susceptibility of colloidal cobalt ferrite nanoparticles was measured as a function of temperature T from 2 to 300 K under zero external DC field for frequencies ranging from f=10 to 10,000 Hz. A prominent peak appears in both χ′ and χ″ as a function of T. The peak temperature T₂ of χ″ depends on f following the Vogel–Fulcher law. The particles show superparamagnetic behavior at room temperature, with transition to a blocked state at T_B^m94 K in ZFC and 119 K in AC susceptibility measurements, respectively, which depends on the applied field. The saturation magnetization and the coercivity measured at 4.2 K are 27.3 emu/g and 14.7 kOe, respectively. The particle size distribution was determined by fitting a magnetization curve obtained at 295 K assuming a log-normal size distribution. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy, K=6×10⁶ erg/cm³. Mössbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamagnetic relaxation. At 4.2 K, the Mössbauer spectrum was fitted with two magnetic subspectra with internal fields H_int of 490, 470 and 515 kOe, corresponding to Fe³⁺ ions in A and B sites.     

Unusual magnetization suppression induced by higher magnetic field in (La0.83Bi0.17)0.67Ca0.33MnO3

Magnetization behavior of (La_0.83Bi_0.17)_0.67Ca_0.33MnO₃ has been investigated in the temperature range from 100 to 180 K. A metamagnetic transition was observed in the temperature region, where the magnetization was measured after a zero-field-cooling from room temperature to a selected temperature. Experimental results show that, after a higher magnetization route, the field-increasing branches of the magnetization curves shows an unusual training effect: below a magnetic field H₀, the applied magnetic field enhances the value of magnetization; however, above H₀ the magnetic field suppresses the value, and the behavior cannot be totally attributed to the enhancement effect of the applied magnetic field on ferromagnetic phase fraction. It is proposed that, in the two-phase coexistence region, the higher magnetic field promotes the phase separation and leads to both the fraction of ferromagnetic domain and the stabilization of antiferromagnetic domain increase.     

Magnetic order and crystal field in Dy2Ru2O7 and Yb2Ru2O7

The magnetic properties of R₂Ru₂O₇ pyrochlore compounds (R=Yb, Dy) were studied using specific heat down to 0.4 K and bulk magnetic measurements. These two rare-earth elements were chosen to demonstrate the effect of Ru-R exchange interaction on R magnetic sublattice, in two cases of anisotropy: axial in Dy and planar in Yb. Dy₂Ru₂O₇ undergoes a second order transition to a fully ordered state at 1.85 K with no signs of the spin-ice state. In Yb₂Ru₂O₇ the Yb sublattice orders gradually around 8 K due to the Ru molecular field and no further transition is observed down to 0.4 K. Including the Ru molecular field at the R site in calculations based on crystal field parameters known from titanates R₂Ti₂O₇, allowed us to interpret experimental data.     

低温固相反应法制备的NiFe2O4纳米颗粒的结构与磁性

采用低温固相反应法制备了晶粒尺寸在8—47nm之间的NiFe₂O₄纳米颗粒系列样品，用X射线衍射仪(XRD)、高分辨中子粉末衍射谱仪、振动样品磁强计和超导量子干涉仪等对样品的晶体结构、宏观磁性和纳米颗粒的表面各向异性进行了分析研究.XRD和中子衍射测量结果显示纳米颗粒的晶格常数略高于块体材料，样品的氧参量表明纳米颗粒的晶格畸变程度没有块体材料严重.相对块体材料，纳米颗粒具有较小的磁化强度、较大的矫顽力和各向异性能密度.纳米颗粒从多畴转变为单畴的临界尺寸约为40nm，超顺磁性临界尺寸约为16nm.     

Spectroscopic and magnetic properties of Fe3Fe4(AsO4)6

The infrared (IR) and ⁵⁷Fe-Mössbauer spectra of Fe₃^IIFe₄^III(AsO₄)₆ were recorded and analyzed on the basis of its structural characteristics. The IR spectrum presents a high complexity, showing an important number of bands and splittings, as a consequence of the presence of three structurally independent AsO₄³⁻ groups. The analysis of the four quadrupole signals shown by the Mössbauer spectrum allowed to attain a detailed insight into the cation distribution over the available crystallographic sites. The alternating current susceptibility measurements indicate a paramagnetic to ferrimagnetic transition in the material at about 59 K.     

Magnetic and transport properties of single-crystal Yb3Cu4Ge4

The magnetic and transport properties of single-crystal Yb₃Cu₄Ge₄ with the Gd₃Cu₄Ge₄-type orthorhombic structure are presented. Magnetization along the b-axis at 2 K saturates to 2.8_μB/Yb$2.8{\mu }_{\mathrm{B}}/\mathrm{Yb}$ at 3 kOe, while that along the a- and c-axes at 2 K are gradually increasing to the value of 1.5_μB/Yb$1.5{\mu }_{\mathrm{B}}/\mathrm{Yb}$ and 0.39_μB/Yb$0.39{\mu }_{\mathrm{B}}/\mathrm{Yb}$ at 50 kOe, respectively. The electrical resistivity within the ab-plane shows a metallic behavior in contrast to a broad maximum at around 30 K for that along the c-axis. Each resistivity for the principal axis suddenly decreases below 8 K. The specific heat shows a λ-type$\lambda \mathrm{-}\mathrm{type}$ sharp peak at 7.8 K. The electronic specific heat coefficient is estimated to be 29.4 mJ/mol Yb K² by fitting the magnetic part of the specific heat below 3 K. The magnetic entropy released up to T_C is 68% of that of R ln 2, expected for the doublet ground state. It is revealed that Yb₃Cu₄Ge₄ is categorized to a weak heavy-fermion system showing a ferromagnetic transition at 7.8 K with uniaxial anisotropy along the b-axis.     

Structural, magnetic and electric properties of HoMnO3 films on SrTiO3(001)

HoMnO₃ films were grown on pure and Nb-doped SrTiO₃ (001) substrates by pulsed laser deposition. The films grew epitaxially with the c-axis along the substrate normal. Varying the deposition temperature between 650 and 850 °C did not significantly affect the structural and magnetic properties of the films, whereas growth in oxygen partial pressures below 0.01 mbar lead to a degradation of the structural properties. Some of the films had a ferromagnetic-like magnetic phase transition at about 45 K, probably related to Mn₃O₄ precipitates; this magnetic response was isotropic. The Ho sublattice was found to be paramagnetic down to 5 K, but showing a pronounced anisotropy with the c-axis being the hard axis. The films showed a distinct dielectric anomaly at 16 K that depended on voltage and slightly on frequency in the range between 1 kHz and 1 MHz. The magnetoelectric effect was large with an in-plane field of 8 T suppressing the dielectric anomaly completely.