Cobalt and magnesium ferrite nanoparticles: preparation using liquid foams as templates and their magnetic characteristics

An easy and convenient method for the synthesis of cobalt and magnesium ferrite nanoparticles is demonstrated using liquid foams as templates. The foam is formed from an aqueous mixture of an anionic surfactant and the desired metal ions, where the metal ions are electrostatically entrapped by the surfactant at the thin borders between the foam bubbles and their junctions. The hydrolysis is carried out using alkali resulting in the formation of desired nanoparticles, with the foam playing the role of a template. However, in the formation of ferrites with the formula MFe(2)O(4), where the metal ion and iron possess oxidation states of +2 and +3, respectively, forming a foam from a 1:2 mixture of the desired ionic solutions would lead to a foam composition at variance with the original solution mixture because of greater electrostatic binding of ions possessing a greater charge with the surfactant. In our procedure, we circumvent this problem by preparing the foam from a 1:2 mixture of M(2+) and Fe(2+) ions and then utilizing the in situ conversion of Fe(2+) to Fe(3+) under basic conditions inside the foam matrix to get the desired composition of the metal ions with the required oxidation states. The fact that we could prepare both CoFe(2)O(4) and MgFe(2)O(4) particles shows the vast scope of this method for making even multicomponent oxides. The magnetic nanoparticles thus obtained exhibit a good crystalline nature and are characterized by superparamagnetic properties. The magnetic features observed for CoFe(2)O(4) and MgFe(2)O(4) nanoparticles are well in accordance with the expected behaviors, with CoFe(2)O(4) particles showing higher blocking temperatures and larger coercivities. These features can easily be explained by the contribution of Co(2+) sites to the magnetocrystalline anisotropy and the absence of the same from the Mg(2+) ions.     

铁酸钴纳米晶的低温合成、谱学表征及磁性

以十六烷基三甲基溴化铵（CTAB）为模板,Fe(NO3)3·9H2O和Co(NO3)2·6H2O为前躯体,NaOH为沉淀剂,低温回流合成了磁性铁酸钴纳米晶。利用X射线衍射、透射电子显微镜、红外光谱、拉曼光谱等测试技术对产品的结构进行了表征,借助振动样品磁强计测定了样品的室温磁性能。结果表明,铁酸钴纳米晶为单相立方尖晶石结构,纳米晶的平均粒径为15-20 nm。铁酸钴纳米晶在室温外加磁场下表现出明显的磁滞现象,饱和比磁化强度MS=36.5 A.m2/kg,矫顽力HC=5.89×104 A/m。     

Controlled synthesis and magnetic properties of bimagnetic spinel ferrite CoFe2O4 and MnFe2O4 nanocrystals with core-shell architecture

A combination of hard phase CoFe(2)O(4) and soft phase MnFe(2)O(4) as the bimagnetic nanocrystals in a core-shell architecture has been synthesized, and their magnetic properties have been systematically studied. Both HRTEM and EDS results confirmed the formation of bimagnetic core-shell structured nanocrystals. On the basis of the systematic and comparative studies of the magnetic properties of a mechanical mixture of pure CoFe(2)O(4) and MnFe(2)O(4) nanocrystals, chemically mixed Co(1-x)Mn(x)Fe(2)O(4) nanocrystals, and bimagnetic core-shell CoFe(2)O(4)@MnFe(2)O(4) and MnFe(2)O(4)@CoFe(2)O(4) nanocrystals, the bimagnetic core-shell nanocrystals show very unique magnetic properties, such as the blocking temperature and coercivity. Our results show that the coercivity correlates with the volume fraction of the soft phase as the theoretical hard-soft phase model has suggested. Furthermore, switching the hard phase CoFe(2)O(4) from the core to the shell shows great changes in the coercivity of the nanocrystals. The bimagnetic core-shell nanocrystals evidently demonstrate the rational design capability to separately control the blocking temperature and the coercivity in magnetic nanocrystals by varying the materials, their combination, and the volume ratio between the core and the shell and by switching hard or soft phase materials between the core and shell. Such controls via a bimagnetic core-shell architecture are highly desirable for magnetic nanocrystals in various applications.     

Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals

By combining nonhydrolytic reaction with seed-mediated growth, high-quality and monodisperse spinel cobalt ferrite, CoFe(2)O(4), nanocrystals can be synthesized with a highly controllable shape of nearly spherical or almost perfectly cubic. The shape of the nanocrystals can also be reversibly interchanged between spherical and cubic morphology through controlling nanocrystal growth rate. Furthermore, the magnetic studies show that the blocking temperature, saturation, and remanent magnetization of nanocrystals are solely determined by the size regardless the spherical or cubic shape. However, the shape of the nanocrystals is a dominating factor for the coercivity of nanocrystals due to the effect of surface anisotropy. Such magnetic nanocrystals with distinct shapes possess tremendous potentials in fundamental understanding of magnetism and in technological applications of magnetic nanocrystals for high-density information storage.     

One-pot synthesis of highly monodispersed ferrite nanocrystals: surface characterization and magnetic properties

In the present study, a facile one-pot synthetic route, utilizing a strong polar organic solvent, N-methyl 2-pyrrolidone (NMP), is demonstrated to obtain highly monodispersed ferrite nanocrystals. The equimolar mixture of oleic acid, C(17)H(33)COOH (R-COOH), and oleylamine, C(18)H(35)NH(2) (R'-NH(2)), was used to coat the magnetic nanocrystals. Structural and magnetic properties of the ferrite nanocrystals were studied by a multitechnique approach including X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), and M?ssbauer spectroscopy. FTIR spectral analysis indicates oleylamine helps in deprotonation of oleic acid, resulting in the formation of an acid-base complex, R-COOˉ:NH(3)(+)-R', which acts as binary capping agent. Structural and coordination differences of iron were studied by XPS and M?ssbauer spectral analysis. XPS analysis was carried out to examine the oxidation state of iron ions in iron oxide nanocrystals. The presence of a magnetically dead layer (～0.38 and ～0.67 nm) and a nonmagnetic organic coating (～2.3 and ～1.7 nm) may substantially reduce the saturation magnetization values for CoFe(2)O(4) and Fe(3)O(4) nanocrystals, respectively. The energy barrier distribution function of magnetic anisotropy was derived from the temperature dependent decay of magnetization. A very narrow energy barrier distribution elucidates that the ferrite nanocrystals obtained in this study are highly monodispersed.     

Polyol合成法制备生物医药用超小粒径Fe3O4磁性纳米晶体

采用一罐polyol合成法还原Fe（Ⅲ）乙酰丙酮化合物制备了粒径可调、单分散、直径5nm以下的磁性Fe3O4纳米晶体．其晶粒表面为所用聚合物表面活性剂PVP所包覆．运用透射电镜／高分辨透射电镜、X射线衍射、振动样品磁强计和超导量子干涉仪对其结构和性能进行了表征．结果表明所制得的Fe3O4磁性纳米晶体在室温下显示出优良的超顺磁性,且结晶度高、分散性好、化学性质稳定同时表面易修饰．磁滞回线的模型分析说明该Fe3O4纳米晶粒是磁性单畴．该法制得的超顺磁Fe3O4纳米晶粒在生物和医学领域具有重要的应用价值．     

Synthesis of amphiphilic superparamagnetic ferrite/block copolymer hollow submicrospheres

Self-assembly can be a powerful, but simple, synthetic method for the fabrication and surface modification of nanometer- to micrometer-sized hollow spheres. Here we report a facile route for preparation of submicrometer ferrite hollow spheres which are amphiphilic and superparamagnetic. This unique approach involves the formation of ferrite nanocrystals and the simultaneous self-assembly of nanocrystals and block copolymer PEO-PPO-PEO into hollow spheres. Furthermore, this approach is general for the preparation of a series of ferrite hollow spheres, including Fe3O4, Co1-xFe2+xO4, and Mn1-xFe2+xO4. Unlike conventional hollow spheres, which are either hydrophobic or hydrophilic, the products we obtained exhibit excellent dispersibility in both polar and nonpolar solvents.     

静电纺丝法制备铁酸锌中空纤维及其磁学性能

以锌盐、铁盐和聚乙烯吡咯烷酮(PVP)为原料,通过静电纺丝法先制备PVP/硝酸盐复合纤维,这些复合纤维以5℃·min-1的升温速率加热到500℃并保温3h,最终得到铁酸锌(ZnFe2O4)中空纤维.通过X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)以及振动样品磁强计(VSM)等分析手段对中空纤维的晶体结构、形貌和磁学性能进行了研究.结果显示,ZnFe2O4中空纤维属于尖晶石结构,高温处理后仍能保持一维结构,纤维直径在200-400nm之间,纤维壁由大小为25nm的颗粒堆积而成.室温磁化结果显示制备的ZnFe2O4中空纤维具有超顺磁性,在10kOe的磁化强度为2.03emu·g-1.     

铁酸钴纳米微粒的共沉淀法制备和磁性质(英)

The cobalt ferrite nanoparticles were prepared by coprecipitation in the presence of poly (N-vinylpyrrolidone) (PVP) and characterized by XRD, TEM, EDX and magnetometry. XRD results suggest the formation of pure cobalt ferrite. The mean particle sizes of CoFe₂O₄ samples annealed at 400 ℃ and 600 ℃ were ca. 6 and 25 nm, respectively as obtained by transmission electron microscopy (TEM). The magnetic measurements indicated that nano-particles obtained at 400 ℃ were superparamagnetic while that prepared at 600 ℃ were ferrimagnetic.     

低热固相反应制备一维棒状CoFe2O4及其电磁特性

以草酸、硝酸铁、硝酸钴为原料, 加入不同聚合度的PEG, 采用低热固相反应法制备出草酸铁和草酸钴的混合前驱物. 在不同的温度(25, 60, 70, 75 ℃), 混合前驱物分别恒温反应2 h, 然后经450 ℃灼烧. 对获得的粉体用XRD, SEM, 激光粒度仪表征后证明: 加入PEG400在75 ℃反应获得了长径比约为6～15范围, 具有立方晶系尖晶石结构的棒状CoFe2O4铁氧体粉末. 经测试电磁学性质发现, 与粒状的CoFe2O4相比, 棒状的CoFe2O4磁导率虚部在高频15～20 GHz范围增加了近1倍, 各向异性常数Ku则增加了近5倍. 预示一维棒状结构的CoFe2O4作为吸波材料具有良好应用前景.     

Spin-canting and magnetic anisotropy in ultrasmall CoFe2O4 nanoparticles

The magnetic properties of cobalt ferrite nanoparticles dispersed in a silica matrix in samples with different concentrations (5 and 10 wt% CoFe2O 4) and same particle size (3 nm) were studied by magnetization, DC and AC susceptibility, and Mossbauer spectroscopy measurements. The results indicate that the particles are very weakly interacting. The magnetic properties (saturation magnetization, anisotropy constant, and spin-canting) are discussed in relation to the cation distribution.     

Controlling phase formation in solids: rational synthesis of phase separated Co@Fe2O3 heteroparticles and CoFe2O4 nanoparticles

A wet chemical approach from organometallic reactants allowed the targeted synthesis of Co@Fe(2)O(3) heterodimer and CoFe(2)O(4) ferrite nanoparticles. They display magnetic properties that are useful for magnetic MRI detection.     

Size-controlled synthesis of magnetite nanoparticles

Monodisperse magnetite nanoparticles have been synthesized by high-temperature solution-phase reaction of Fe(acac)3 in phenyl ether with alcohol, oleic acid, and oleylamine. Seed-mediated growth is used to control Fe3O4 nanoparticle size, and variously sized nanoparticles from 3 to 20 nm have been produced. The as-synthesized Fe3O4 nanoparticles have inverse spinel structure, and their assemblies can be transformed into gamma-Fe2O3 or alpha-Fe nanoparticle assemblies, depending on the annealing conditions. The reported procedure can be used as a general approach to various ferrite nanoparticles and nanoparticle superlattices.     

CoFe2-xMnxO4纳米晶薄膜的结构、磁性及磁光效应研究

为了改善磁光材料CoFe2O4的磁学特性,本研究利用改进的溶胶-凝胶法在单晶硅衬底上制备了不同掺杂量的CoFe2-xMnxO4(x=0～2,0)纳米晶薄膜系列样品。并对其结构、磁性及磁光效应进行了研究。结果表明,Mn^3+的掺入可以有效地控制和降低CoFe2O4的居里温度,并可显著地增强薄膜的磁光效应。     

Nano-Architecture of Facetted NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/(Ni,Fe)O Particles Produced by Induction Plasma

Facetted nickel ferrite (NiFe₂O₄) and bunsenite [(Ni,Fe)O] nanocrystals were grown from the decomposition of iron and nickel nitrate precursors using an inductively coupled plasma reactor. The full range of the two-phase region of the Fe₂O₃–NiO pseudo-equilibrium phase diagram was investigated by producing nanopowders with bulk Ni/(Ni + Fe) ratios of 0.33, 0.4, 0.5, 0.75 and 1.0. A Ni-poor [Ni/(Ni + Fe) ≤ 0.5] precursor solution produced truncated octahedron nanocrystals, whereas nanocubes were obtained at higher ratios [Ni/(Ni + Fe) ≈ 1]. In both cases, it is shown that the nanocrystals adopt a morphology close to the Wulff shape of the crystalline system (spinel and NaCl, respectively). As the bulk Ni/(Ni + Fe) ratio increases from 0.33 (the stoechiometric composition of nickel ferrite), bunsenite is epitaxially segregated on the {110} and {111} facets of nickel ferrite, while leaving the NiFe₂O₄ {100} facets exposed. A precursor solution at a Ni/(Ni + Fe) ratio of 0.75 gave an (Ni,Fe)O-rich nanopowder with a random and irregular interconnected morphology. The structure of these nanocrystals can be understood in terms of their thermal history in the plasma reactor. These results highlights the possibility of producing organized multi-phased nanomaterials of binary systems having two phases stable at high temperatures, using a method known to be easily scalable.     

Preparation and characterization of thermosensitive polymers grafted onto silica-coated iron oxide nanoparticles

In this study, multifunctional nanoparticles containing thermosensitive polymers grafted onto the surfaces of 6-nm monodisperse Fe(3)O(4) magnetic nanoparticles coated by silica were synthesized using reverse microemulsions and free radical polymerization. The magnetic properties of SiO(2)/Fe(3)O(4) nanoparticles show superparamagnetic behavior. Thermosensitive PNIPAM (poly(N-isopropylacrylamide)) was then grafted onto the surfaces of SiO(2)/Fe(3)O(4) nanoparticles, generating thermosensitive and magnetic properties of nanocomposites. The sizes of fabricated nanoparticles with core-shell structure are controlled at about 30 nm and each nanoparticle contains only one monodisperse Fe(3)O(4) core. For thermosensitivity analysis, the phase transition temperatures of multifunctional nanoparticles measured using DSC was at around 34-36 degrees C. The magnetic characteristics of these multifunctional nanoparticles were also superparamagnetic.     

Monodisperse MFe2O4 (M = Fe, Co, Mn) nanoparticles

High-temperature solution phase reaction of iron(III) acetylacetonate, Fe(acac)(3), with 1,2-hexadecanediol in the presence of oleic acid and oleylamine leads to monodisperse magnetite (Fe(3)O(4)) nanoparticles. Similarly, reaction of Fe(acac)(3) and Co(acac)(2) or Mn(acac)(2) with the same diol results in monodisperse CoFe(2)O(4) or MnFe(2)O(4) nanoparticles. Particle diameter can be tuned from 3 to 20 nm by varying reaction conditions or by seed-mediated growth. The as-synthesized iron oxide nanoparticles have a cubic spinel structure as characterized by HRTEM, SAED, and XRD. Further, Fe(3)O(4) can be oxidized to Fe(2)O(3), as evidenced by XRD, NEXAFS spectroscopy, and SQUID magnetometry. The hydrophobic nanoparticles can be transformed into hydrophilic ones by adding bipolar surfactants, and aqueous nanoparticle dispersion is readily made. These iron oxide nanoparticles and their dispersions in various media have great potential in magnetic nanodevice and biomagnetic applications.     

磁电CoFe2O4/BaTiO3纳米管的溶胶-凝胶模板法合成和表征

用溶胶-凝胶模板法合成了 CoFe2O4)/BaTiO3(CFO/BTO)复合纳米管,管的直径约为 100、200 和 300 nm,其长度约为100 μm.x射线衍射(xRD)和选区电子衍射(sAED)都显示复合纳米管中同时存在尖晶石相的CoFe2O4(CFO)和钙钛矿相的 BaTiO3(BTO),进一步的透射电子显微镜(TEM)研究证实合成的纳米复合物具有明显的管状结构.磁、电研究表明,该复合纳米管的磁性与纯 CFO 纳米管的磁性相当;而铁电性与纯BTO纳米管的铁电性相当.     

First Principle Calculation of Electric and Magnetic Properties for New Half-metal Fe2ScO4

The new half-metals Fe2ScO4 and FeSc2O4 were designed and their spinel structures were optimized based on the first-principle pseudo-potential method. Their electric and magnetic properties including molecular magnetic moments and electronic structures were calculated and analyzed, and then were compared with those of Fe3O4. The calculation showed that Fe2ScO4 and FeSc2O4 were both new ferromagnetic II B-type half-metals, but Fe3O4 was ferrimagnetic. The molecular magnetic moment of Fe2ScO4 is about 7.28 1B, which is much larger than the 4.0 1B of Fe3O4 and 3.96 1B of Fe2ScO4. The molecular magnetic moment of Fe2ScO4 mainly came from the spin-polarization of Fe3d electrons. Also, the conductance of Fe2ScO4 was a little larger than that of Fe3O4. For Fe2ScO4, the average electronic structure of Sc on A-sites wasSc+3s23p43d2 and that of Fe on B-sites was Fe2+t2g3↑"tg2↑"t2g↓. It can be predicted that the new half-metal Fe2ScO4 has wider application ground in spin electronic instruments because of its larger magnetoresistance compared to Fe3O4 and FeSc2O4.     

Fe3O4超顺磁纳米晶的超声共沉淀法制备及表征

利用超声强化的共沉淀法结合阴离子表面活性剂十二烷基硫酸钠(SDS)修饰技术, 制备出Fe3O4超顺磁纳米晶, 采用X射线粉末衍射仪(XRD)、傅立叶转换红外线光谱仪(FT-IR)、高分辨透射电子显微镜(HRTEM)、N2吸附-脱附及热重-差示扫描同步热分析仪(TG-DSC)等方法对样品进行表征, 系统研究了样品的表面电性及磁学性质, 并探索了超顺磁纳米晶的生长机理. 结果表明: 所制备的Fe3O4超顺磁纳米晶结晶完整, 分散性良好, 平均粒径在10 nm左右; 其比表面积高达91.6 m2&#8226;g－1, 具有优异的热稳定性, 蒸馏水中等电点pHpzc＝5.7; 其饱和磁强度(Ms)可达65.0 emu&#8226;g－1, 属超顺磁性纳米材料; 超声强化及SDS表面修饰, 对Fe3O4超顺磁纳米晶的生长起着非常重要的作用. 这种Fe3O4超顺磁纳米材料可望被较好地应用于细胞或酶的固定化等生物和医药领域.