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.     

Magnetic properties in BaFe12O19 nanoparticles prepared under a magnetic field

It was observed that the nanocrystallites of BaFe₁₂O₁₉ formed at 140°C under a 0.25 T magnetic field exhibited a higher saturation magnetization (6.1 emu/g at room temperature) than that of the sample (1.1 emu/g) obtained under zero magnetic field. Both of the two approaches yielded plain-like particles with an average particle size of 12 nm. However, the Curie temperature (T_c), a direct measuring of the strength of superexchange interaction of Fe³⁺–O²⁻–Fe³⁺, increased from 410°C for the nanoparticles prepared without an external field applied to 452°C for the particles formed under a 0.25 T magnetic field, which indicates that external magnetic fields can improve the occupancy of magnetic ions and then increase the superexchange interaction. This was confirmed by electron paramagnetic resonance and Mössbauer spectrum analysis. The results present in this paper suggest that in addition to oxygen defects, surface non-magnetic layer and a fraction of finer particles in the superparamagnetic range, cation vacancies should be responsible for the decreasing of saturation magnetization in magnetic nanoparticles.     

Nd含量对Bi_(6-x)Nd_xFe_(1.4)Ni_(0.6)Ti_3O_(18)多晶材料多铁性的影响

采用柠檬酸-硝酸盐法制备了Bi_(6-x)Nd_xFe_(1.4)Ni_(0.6)Ti_3O_(18)(BNFNT-x,x=0.00,0.10,0.20,0.25和0.30)前驱液,再经过干燥、烧结过程制备了单相多晶材料.研究发现,少量Nd掺杂有助于提高样品的铁电性能,BNFNT-0.25样品的铁电性能(2Pr)最大,约达到19.7μC/cm~2.室温下BNFNT-0.20样品磁性能(2Ms)最大约达到4.132 emu/g(1 emu/g=10–3 A·m~2/g).变温介电损耗结果表明Nd掺杂降低了Fe~(3+)和Fe~(2+)间的电子转移或跃迁的激活能.X射线光电子能谱结果表明小量Nd掺杂有助于增强Bi离子稳定性,对改善样品的铁电性能有积极意义.     

Magnetic properties of xFe2O3(1−x)[B2O3 · PbO] glasses

The magnetic measurements performed on xFe₂O₃(1-x)[B₂O₃ · PbO] glasses show that for x 5 mol.% Fe₂O₃ the thermal variation of reciprocal susceptibility obeys a Curie behaviour. For higher iron content, at T 50 K, a nonlinear variation, typical for systems with random distribution of exchange interactions is observed. At greater temperatures than 50 K a Curie-Weiss behaviour is shown. The composition dependence of the Curie constants is analysed in correlation with the number of Fe³⁺ and Fe²⁺ ions as determined from Mössbauer effect measurements. A comparison with the data obtained in case of xFe₂O₃(1-x)[3B₂O₃ · PbO] glasses is made.     

Magnetic and Mössbauer data of Fe in K2ZnF4, analyzed in the spin Hamiltonian formalism

Magnetic measurements on single crystals and Mössbauer spectra of powders under magnetic field, recorded on Fe²⁺ doped K₂ZnF₄, have been analyzed in the spin Hamiltonian formalism. The data slightly depart from the expectations of a spin 2 in D_4h symmetry. The occurrence of vibronic effects is discussed.     

Studies of anisotropy mechanisms in polyphosphate-treated magnetic iron oxide particles

The coercive force of small, acicular ( 2000 Å length, 8:1 length-to-diameter ratio) spinel-type iron oxide particles increases substantially following surface treatment with sodium polyphosphate. Regardless of Fe²⁺/Fe³⁺ cation ratio, H_c always attains a peak value when the polyphosphate/iron oxide weight ratio P/Fe is in the range 0.5–0.6, denoted (P/Fe)_max. The maximum change in H_c is observed when Fe²⁺/Fe³⁺ ≈ 0.10–0.15. When P/Fe0.5–0.6, however, both magnitude and sign of the change in coercive force show strong dependence on the Fe²⁺ content of the oxide, suggesting that the H_c changes are caused by a magnetostrictive mechanism at these high treatment ratios. Calculated anisotropy field distributions of treated specimens show that both the mean anisotropy field δH_kδ_G and predicted H_c reach a peak when particles are treated at (P/Fe)_max, where the distribution becomes very broad. At high treatment ratios both δH_kδ_G and predicted H_c decrease to values below that of the untreated oxide. Mössbauer studies of treated and untreated particles show no significant change in the environment of surface iron ions following treatement at P/FE = 0.5, but indicate a small increase in the Fe³⁺ concentration of the particle core. When P/Fe = 2.5, however, the Fe²⁺ concentration of the core increases markedly.     

Effects of pH and citric acid contents on characteristics of ester-derived BaFe12O19 powder

The single domain size of BaFe₁₂O₁₉ powder with crystallite sizes less than 200 nm was produced using a citric acid precursor method. Fe³⁺ and Ba²⁺, in a molar ratio of 12, were chelated by COOH⁻ in an aqueous solution. After ethylene glycol additions, esterification, dehydration, and calcination led to the formation of ester-derived BaFe₁₂O₁₉ powder. High pH and/or high citric acid contents in the starting solution are required to complete chelate metallic ions in the solution and to form pure barium ferrite powder at 1073 K. Pure single magnetic domain BaFe₁₂O₁₉ particles of M(30 kOe)≈54 emu/g, M_r≈28 emu/g, and H_c≈3.7 kOe were produced using [citric acid]/[metallic ions]=1.5 and pH7.     

Surface and interface properties of epitaxial Fe3O4 films studied by Mössbauer spectroscopy

Stoichiometric Fe₃O₄ films have formed epitaxially on -Al₂O₃ and MgO single-crystal substrates by a reactive vapor deposition method. In order to apply conversion electron Mössbauer spectroscopy depth-selectively, a 5–7 Åthick probe layer containing ⁵⁷Fe was formed at various depths in inactive ⁵⁶Fe₃O₄ matrix films. At the topmost surfaces and also at the interfaces, the essential electronic features of bulk Fe₃O₄ are retained, including a rapid electron hopping between the Fe²⁺ and Fe³⁺ ions at B sites. Minor depth-dependent changes are confined to a few outermost atomic layers, and the changes depend on the orientation and the lattice mismatch with the substrate. For (111) growth on -Al₂O₃, the surface layer seems to be strongly relaxed to reduced the electric polarization, while a high density of defects seems to be concentrated at the interface with -Al₂O₃. For (001) growth on MgO, the surface retains the spinel lattice though slightly oxidized, while the interface with MgO has good crystallinity and stoichiometry. An enhanced thermal fluctuation of the Fe³⁺-spins in contact with the MgO substrate and in the topmost surface layer can be seen in their reduced magnetic hyperfine field at 300 K.     

Coexisting antiferromagnetism and ferromagnetism in mechanically alloyed Fe-rich Fe–Ni alloys: implications regarding the Fe–Ni phase diagram below 400°C

Fe–Ni alloys below the Invar region with compositions Fe_100−xNi_x (x=21, 24, and 27 at%) were prepared by high-energy ball milling technique (mechanical alloying). The as-milled samples, characterized by X-ray diffraction and Mössbauer spectroscopy, contain a mixture of (BCC) and γ (FCC) phases, whereas the samples annealed at 650°C for 0.5 h show a single γ (FCC) phase displaying a single line Mössbauer spectrum at room temperature (RT). At low temperature, the Mössbauer spectra of annealed Fe₇₆Ni₂₄ and Fe₇₃Ni₂₇ alloys show the existence of a magnetically split pattern together with a broad singlet, which are ascribed to a high-moment ferromagnetic Ni-rich phase and a low-moment Fe-rich phase, respectively. The Fe-rich phase in annealed Fe₇₆Ni₂₄ alloy, which is paramagnetic at RT, undergoes antiferromagnetic ordering at 40 K, estimated from the dramatic line broadening of its spectrum, giving rise to a small hyperfine field (e.g. 2 T at 6 K). The coexistence of these phases is attributed to phase segregation occurring in these alloys as a result of enhanced atomic diffusion. The stability of these alloys towards martensitic (FCC→BCC) transformation at low temperatures is discussed in connection with the Fe–Ni phase diagram below 400°C.     

Quadrupole splitting and ground state in FeS and NiS (Fe)

From a Mössbauer study of an oriented single crystal of Ni_0.996S (2% Fe⁵⁷) at 298 K (above the metal-insulator transition at 230 K) the sign of the quadrupole splitting in Fe⁵⁷ is determined to be positive; this contrasts with a negative sign found for FeS and Fe_0.93S. A tentative model is presented that is consistent with these results and with magnetic anisotropy data.     

Crystallization kinetics of the Fe40Ni38Mo4B18 amorphous alloy

X-ray diffraction (XRD) and Mössbauer spectroscopy were used to study the annealing of the Fe₄₀Ni₃₈Mo₄B₁₈ amorphous alloy. The samples were isothermally annealed in the 858–878 K temperature range several times. Two crystalline phases were observed in the annealed samples: FeNi₃ and (Fe, Ni, Mo)₂₃B₆. Preliminary results indicate that assuming a linear relationship between the area under the main XRD peak associated with the FeNi₃ phase and its volume fraction, this can be fitted to a Johnson–Mehl–Avrami equation with an exponent n close to 1.0. Mössbauer results show a broad magnetic hyperfine field distribution in as-received samples and, consistent with XRD results, a sextet attributed to precipitates of FeNi₃ (B_hf=29.5 T) for long annealing times.     

Electrical and magnetic properties of polyaniline/Fe3O4 nanostructures

We report on electrical and magnetic properties of polyaniline (PANI) nanotubes (150 nm in diameter) and PANI/Fe₃O₄ nanowires (140 nm in diameter) containing Fe₃O₄ nanoparticles with a typical size of 12 nm. These systems were prepared by a template-free method. The conductivity of the nanostructures is 10⁻¹–10⁻² S/cm; and the temperature dependent resistivity follows a ln ρT^−1/2 law. The composites (6 and 20 wt% of Fe₃O₄) show a large negative magnetoresistance compared with that of pure PANI nanotubes and a considerably lower saturated magnetization (M_s=3.45 emu/g at 300 K and 4.21 emu/g at 4 K) compared with the values measured from bulk magnetite (M_s=84 emu/g) and pure Fe₃O₄ nanoparticles (M_s=65 emu/g). AC magnetic susceptibility was also measured. It is found that the peak position of the AC susceptibility of the nanocomposites shifts to a higher temperature (>245 K) compared with that of pure Fe₃O₄ nanoparticles (190–200 K). These results suggest that interactions between the polymer matrix and nanoparticles take place in these nanocomposites.     

Mössbauer investigations on Np3S5

The 59.5 keV Mössbauer resonance of ²³⁷Np has been measured in Np₃S₅ at 77 and 4.2 K. At 77 K, two different quadrupolar splittings are observed. The first one (δ₁ = −5(1) mm sec⁻¹/NpAl₂) is assigned to the Np⁴⁺ ions and the second one (δ₂ = 28(1) mm sec⁻¹/NpAl₂) is assigned to the Np³⁺ ions. At 4.2 K the spectrum gives a ratio of areas S(Np³⁺)/S(Np⁴⁺) of 2.0. These results clearly confirm the crystallographic data of this mixed-valence material.     

掺杂晶体材料ZnGa_2O_4:Fe~(3+)局域结构畸变及其微观自旋哈密顿参量研究

基于Newman的晶场叠模型与微观自旋哈密顿理论,建立了ZnGa2O4:Fe3+晶体材料中磁性离子Fe3+局域结构与其自旋哈密顿(spin-Hamiltonian,SH)参量(包括二阶零场分裂(zero-field splitting,ZFS)参量D,四阶ZFS参量(a-F),Zeeman g因子:g//,g⊥,△g(=g//-g⊥))之间的定量关系.采用以全组态完全对角化方法为理论背景的CFA/MSH(Crystal Filed Analysis/Microscopic Spin Hamiltonian)研究软件,研究了ZnGa2O4:Fe3+材料中磁性离子Fe3+的SH参量与其局域结构的依赖关系.研究表明:对于ZnGa2O4:Fe3+晶体材料,当磁性离子Fe3+的局域结构畸变参数△R=0.0487 nm,△θ=0.192°时,其基态SH参量理论计算结果与实验测量符合很好,进一步表明Fe3+掺入晶体材料后将引起磁性Fe3+离子局域结构的微小畸变,但其仍然保持D3d点群对称局域结构.在此基础上研究分析了SH参量的微观起源,结果表明:ZnGa2O4:Fe3+晶体材料的SH参量主要来源于SO(spin-orbit)磁相互作用机理,来自其他磁相互作用机理(包括SS(spin-spin),SOO(spin-other-orbit),OO(orbit-orbit),SO-SS-SOO-OO)的贡献比较小.     

Magnetic behavior of 3d dopants in superconducting REBa2Cu3−xFexO7+δ (RE=Dy, Er)

Mössbauer studies on ⁵⁷Fe-doped superconducting REBa₂Cu₃O_7+δ (RE=Er, Dy) were made as a function of temperature for x=0.15 and 0.30. The magnetic behavior of the 3d dopants, which mainly occupy Cu(1) sites, undergoes antiferromagnetic ordering which is coexistent with superconductivity at low temperature. The dimensionality of the magnetic interaction changes from 2D to 3D when the rare earth changes from Er to Dy. the line-widths of the Mössbauer subspectra are characteristic of magnetic fluctuation behavior in the vicinity of a phase transition. Combining these results with those of Fe-doped Y-123 (pseudo 1D) and Gd (3D), the magnitude of the rare earth moments appears to be strongly correlated with the dimensionality of the magnetic interaction of Fe dopants in these compounds. However, the Mössbauer spectrum for ¹⁵⁵Gd in GdBa₂Cu_2.85Fe_0.15O_7+δ (T_{N(Fe) 14 K}) shows no magnetic order at 4.9 K.     

Magnetic properties of UFe5Sn single crystals

Millimetre-size UFe₅Sn single crystals were grown by the top seed solution growth method and characterized by magnetization, ⁵⁷Fe Mössbauer spectroscopy and specific heat measurements in order to study the magnetic transitions detected in powder samples at 248 and 178 K. The magnetization measurements show different behaviour along the three crystallographic directions but with similar values of spontaneous magnetization along a and c. The transition at 248 K is associated with ferromagnetic ordering of iron moments along the c-axis, while the transition at lower temperature is associated with a reorientation towards b. Mössbauer data show that this reorientation is concomitant to the ordering of the Fe2 sites, which in a large proportion remain paramagnetic between the two transition temperatures. Specific heat measurements are consistent with the establishment of magnetic ordering at 248 K, followed by a spin reorientation at 178 K, yielding γ(0 K)140 mJ/(mol K²) and θ290 K for UFe₅Sn.     

Superconductivity in non-stoichiometric and tin-substituted La2CuO4: preparation, characterization, Mössbauer and microwave-absorption studies

Two samples of non-stoichiometric La₂CuO₄ were synthesized, one with La/Cu<2, and the other with 10% Sn substituting Cu. They were investigated by X-ray diffraction, Mössbauer spectroscopy, and microwave-absorption techniques. The microwave-absorption data indicated that they were both superconducting, with the transition temperatures T_c of 40.5 and 41.5 K, the one doped with Sn possessing the higher T_c. The Mössbauer spectra revealed that there exist two kinds of Sn(IV) atoms disordered with Cu. Their isomer shift, δ=−0.244(4) mm/s, is in agreement with Sn(IV) coordinated by oxygen. One site was characterized by a single Mössbauer line, being associated with a weakly distorted environment, wherein the Sn, coordinated more symmetrically, is surrounded by four Cu²⁺ ions. On the other hand, the other site, characterized by a Mössbauer doublet exhibited a quadrupole splitting Δ=1.07(2) mm/s, being associated with a highly distorted coordination, explained to be due to Sn occupying two adjacent cationic sites. To our knowledge, such a substitution for copper ions not resulting in a decrease of T_c has not been reported previously.     

LiZn铁氧体磁结构的穆斯堡尔研究

本文报道Li_(0.5(1-x))Zn_xFe_(2.5-0.5x)O₄体系在77K,195K和室温的条件下,加上与γ射线方向平行的磁场,可观察到六线磁分裂谱中2,5线的强度随x增加和温度降低变得明显,可确认LiZn铁氧体在B亚点阵的Fe³⁺磁矩存在倾角。以此计算得到的磁化强度与磁测量得到的数据符合较好。 关键词：     

Magnetic properties of the ternary carbides Tm2Fe17Cx

We have investigated the crystallographic and magnetic properties of the ternary carbides Tm₂Fe₁₇C_x by means of X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, ¹⁶⁹Tm Mössbauer spectroscopy and magnetic measurements. It is shown that small amounts of carbon raise the Curie temperature in Tm₂Fe₁₇C_x from below room temperature to about 500 K, at the same time increasing the average Fe moment. Important conclusions regarding the rare-earth sublattice anisotropy were derived from the quadrupole splitting of the ¹⁶⁹Tm Mössbauer spectra and from the strong concentration dependence of the spin reorientation temperature in Tm₂Fe₁₇C_x.     

Ca~(2+)掺杂对SmFeO_3的介电、铁磁特性及磁相变温度的影响

采用固相反应法制备Sm_(1-x)Ca_xFeO_3(x=0,0.1,0.2,0.3)样品,研究Ca~(2+)掺杂对SmFeO_3介电性能、铁磁性及磁相变温度的影响.X射线衍射图谱分析表明:所有样品的主衍射峰与SmFe03相符合且具有良好的晶体结构.随着x的增加,SmFeO_3样品的晶粒尺寸由原来的0.5μm逐渐增大到2μm.当f=1 kHz时,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的ε_r分别是SmFe03的5倍、3倍和2.6倍,而tgσ增大一个数量级.在3T磁场作用下,SmFe03样品的M-H呈线性,随着x的增加,M-H逐渐趋向饱和,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的M_r分别是SmFeO_3的20倍、31倍和68倍.X射线光电子能谱分析表明:Fe~(2+)和Fe3+共存于Sm_(1-x)Ca_xFeO_3样品中,Fe~(2+)/Fe~(3+)比例随着x的增加而增大,证明Ca~(2+)掺杂增加了Fe~(2+)的含量,形成Fe~(2+)—O~(2-)—Fe~(3+)超交换作用,增强SmFe03的铁磁特性.测量了Sm_(1-x)Ca_xFeO_3样品在外加磁场为1000 Oe(1 Oe=79.5775 A/m)的M-T变化关系,观测到其自旋重组温度(T_(SR))和尼尔温度(T_N)分别为438 K和687 K,发现SmFe03样品的T_(SR)和T_N均随着x的增加向低温方向移动,当x=0.3时,自旋重组现象消失.这主要是SmFeO_3样品磁结构的稳定性和Fe~(3+)—O~(2-)—Fe~(3+)及Sm~(3+)—O~(2-)—Fe~(3+)超交换三者共同作用的结果.