Exchange-spring behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite

In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe2O4 and ferrimagnetic oxide/ferromagnetic metal CoFe2O4/CoFe2 nanocomposites. The latter compound is a good system to study hard ferrimagnet/soft ferromagnet exchange coupling. Two steps were used to synthesize the bimagnetic CoFe2O4/CoFe2 nanocomposites: (i) first preparation of CoFe2O4 nanoparticles using the a simple hydrothermal method and (ii) second reduction reaction of cobalt ferrite nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures, particle sizes, morphology, and magnetic properties of CoFe2O4 nanoparticles have been investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) with applied field up to 3.0 kOe at room temperature and 50K. The mean diameter of CoFe2O4 particles is about 16 nm. Mossbauer spectra reveal two sites for Fe3+. One site is related to Fe in an octahedral coordination and the other one to the Fe3+ in a tetrahedral coordination, as expected for a spinel crystal structure of CoFe2O4. TEM measurements of nanocomposite show the formation of a thin shell of CoFe2 on the cobalt ferrite and indicate that the nanoparticles increase to about 100 nm. The magnetization of nanocomposite showed hysteresis loop that is characteristic of the exchange spring systems. A maximum energy product (BH)max of 1.22 MGOe was achieved at room temperature for CoFe2O4/CoFe2 nanocomposites, which is about 115% higher than the value obtained for CoFe2O4 precursor. The exchange-spring interaction and the enhancement of product (BH)max in nanocomposite CoFe2O4/CoFe2 have been discussed.Comment: 9 pages, 10 figure

Synthesis and characterization of nanometric magnetite coated by oleic acid and the surfactant CTAB

For the intermediate and largest particles a mixture of magnetite and maghemite phases were produced as the saturation magnetization values of MS ∼ 60 emu/g indicated; these values were measured for most samples, independently of the coating surfactant concentration, and according to the ZFC-FC curves the blocking temperatures were 225K and 275K for the smallest and largest magnetite nanoparticles, respectively. The synthesis method was highly reproducible.Nanometric magnetite (nm-Fe3O4) particles were prepared by the reverse co-precipitation synthesis method, obtaining particle sizes that ranged from 4 to 8.5 nm. In their synthesis, the concentration of iron salts of ferric nitrate, Fe(NO3)3⋅9H2O, and ferrous sulfate, FeSO4⋅7H2O, were varied relative to the chemical reaction volume and by using different surfactants such as oleic acid (OA) and hexadecyltrimethylammonium bromide (CTAB). The nm-Fe3O4 particles were characterized by transmission electron microscopy (TEM), Mössbauer spectroscopy (MS), magnetic and X-ray diffraction (XRD) measurements

[pt] ANTIFERROMAGNETISMO E PONTO CRÍTICO QUÂNTICO NO COMPOSTO CECOGE(2,1)SI(0,9) SOB PRESSÃO

Estudos no sistema pseudoternário CeCoGe(3-x)Six (com 0 = < x = < 3) mostraram que o sistema evolui continuamente de um estado antiferromagnético da rede de Kondo (CeCoGe(3) com ~21k) para um composto de valência intermediária (CeCoSi(3) com Tw ~230k). O sistema apresenta comportamento tipo não- líquido de Fermi (NLF) em torno do ponto crítico quântico (PCQ) na concentração crítica xC = 1,25. A substituição isoeletrônica dos átomos de por não aumenta o grau de desordem magnética, sendo ideal para o estudo de efeitos intrínsecos das variações das constantes de interação da rede Kondo. Estudamos este sistema em concentrações próximas à concentração crítica através de medidas de resistividade elétrica AC sob pressão (x=0,9) e campo magnético (x =1), em amostras policristalinas. Nossos resultados mostram que a ordem magnética de longo alcance presente na amostra CeCoGe(2,1)Si(0,9) é suprimida com o aumento da pressão e, para a pressão crítica PC ( aprox. 6,2 kbar) TN é aproximadamente zero. Para temperaturas inferiores a TN, as medidas de resistividade são bem descritas considerando um espalhamento de elétrons de condução por mágnons antiferromagnéticos anisotrópicos. Acima de PC observa-se o comportamento líquido de Fermi. Na região crítica o estado NLF foi observado. A análise do comportamento da linha crítica na proximidade do PCQ indica que as flutuações magnéticas relevantes são tipicamente bidimensionais. Por outro lado, no composto CeCoGe2Si, que apresenta ordem magnética de curto alcance com TN aproximadamente 2K, observa-se que a temperatura de ordenamento é reduzida com o aumento do campo magnético e, para campos acima de 3 T, surge o comportamento tipo líquido de Fermi.Studies on the pseudo ternary system CeCoGeSix (where 0 = < x = < ) have shown that the system evolves continuously from a Kondo lattice antiferromagnetic state ( CeCoGe(3) with ) towards a mixed valent compound ( CeCoSi(3)with Tw ~230k ). The system displays a non-Fermi-liquid-type behavior (NFL) in the vicinity of the quantum critical point (QCP) at the critical concentration . Isoelectronic substitution of atoms for does not enhance the degree of magnetic disorder, rendering it ideal for the study of the Kondo lattice¡ s interaction constants intrinsic effects. We have studied this system in polycrystalline samples at concentrations close to the critical one through AC electrical resistivity under pressure (x= 0,9) and magnetic field (x=1) measurements. Our results show that the long range magnetic order present in the CeCoGe(2,1)Si(0,9) sample is suppressed as pressure is increased, and that for the critical pressure PC (aprox. 6,2 kbar), TN (aprox. zero) . For temperatures below TN, the resisitivity data are well described considering conduction electron scattering by anisotropic antiferromagnetic magnons. Above PC we observe the Fermi liquid behavior. At the critical region, a NFL state with exponents close to 1 was found. The analysis of the behavior of the critical line in the neighborhood of the QCP indicates that the relevant magnetic fluctuations are typically two-dimensional. On the other hand, the CeCoGe(2)Si compound displays short range order (TN ~ 2k). The ordering temperature is reduced under an increase of an applied magnetic field, and for magnetic fields above 3 T a Fermi liquid behavior arises

Internal Structure and Magnetic Properties in Cobalt Ferrite Nanoparticles: Influence of the Synthesis Method

The design of novel nanostructured magnetic materials requires a good understanding of the variation in the magnetic properties due to different synthesis conditions. In this work, four different procedures for fabricating Co-ferrite nanoparticles with similar sizes between 7 and 10 nm are compared by studying their structural and magnetic properties. Non-aqueous methods based on the thermal decomposition of metal acetylacetonates at high temperatures, either with or without surfactants, provide highly crystalline nanoparticles with large saturation magnetization values and a coherent reversal of the magnetic moment. However, variations in the density of defects and in the shape of the nanocrystals determine the distribution of switching fields and the effective magnetic anisotropy, which reaches up to ≈1 × 107 erg cm−3 for oleic acid-capped 9 nm nanoparticles. It is shown that the saturation magnetization values for nanoparticles produced by different methods are in the range between 49 and 95 emu g−1 due to differences in the stoichiometry, in the cation occupancy, in the magnetic disorder and in the spin canting of the magnetic sub-lattices, the latter evaluated by in-field Mössbauer spectroscopyFil: Lavorato, Gabriel Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Alzamora, Mariella. Universidade Federal do Rio de Janeiro; BrasilFil: Contreras, Cynthia. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Burlandy, Gabriel. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Litterst, F. Jochen. Centro Brasileiro de Pesquisas Físicas; Brasil. Technische Universitat Carolo Wilhelmina Zu Braunschweig.; AlemaniaFil: Baggio Saitovitch, Elisa. Centro Brasileiro de Pesquisas Físicas; Brasi

Properties of manganese ferrite-paraffin composites

Manganese ferrite (MnFe2O4) nanoparticles were dispersed in paraffin and the physical properties of this composite were investigated by X-ray diffraction (XRD), ferromagnetic resonance (FMR), vibrating sample magnetometry (VSM), field cooled (FC) and zero field cooled (ZFC) curves and Mössbauer spectroscopy. The results showed that dispersion in moderate quantities of paraffin significantly increases the crystallite size, the absolute value of the anisotropy field, the saturation magnetization and the blocking temperature of the nanoparticles, a result that may be of interest for practical applications of the composite. Keywords: Manganese ferrite, Paraffin, Nanoparticles, Composite