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We use a novel solution-based disassociation synthesis scheme of the ionic complex Fe(LiBH4)2 to form Fe nanoparticles. The complex was formed initially using a gentle mechanochemical process, and the Fe nanoparticles emerged after 4 h of ball milling in an air-free environment. Rietveld refinement of x-ray diffraction measurements in an air-free sample holder identified a Im3m α-Fe phase. A room temperature Mössbauer spectrum of the sample presented a six-line spectrum unique to Fe0 metal, and the Fe nanoparticles were extremely well crystallized. Magnetometry results presented a reduced saturation magnetization (e.g., Ms ∼ 85 emu/g at 50 K) that had a Bloch-like T2 temperature dependence, consistent with a gap in the magnon fluctuation spectrum due to finitesize effects. The Fe nanoparticles were magnetically soft, with a coercivity ranging from ∼10 to 20mT with decreasing temperature from 350 K. [ABSTRACT FROM AUTHOR]

A series of core/shell Fe-oxide/SiO2 nanoparticles were produced to form Fe-oxide (1:1 Fe3O4 and c-Fe2O3) cores with diameters of approximately 4.6 nm and shell thicknesses ranging from 3.6 to 5.4 nm. Hot press sintering of core/shell nanoparticles created a nanocomposite of Fe-oxide nanoparticles in a SiO2 matrix. The presence of an iron-orthosilicate at the core-shell interface defines the intrinsic magnetic properties of the nanoparticle systems, resulting in an increase in magnetic anisotropy with thicker SiO2 shell. We find that dipole-dipole interactions are mediated by the overall SiO2 shell, and that these interactions are coupling neighbouring particles' magnetization with increasing correlation lengths. When the nanoparticles from a composite material, packing significantly increases the interaction strengths, altering the overall magnetization of the system so that the iron-oxide cores present an approximate 25% increase in (saturation) magnetization to a bulk-like (∼80 emu/g) value. [ABSTRACT FROM AUTHOR]

We have investigated the role of spontaneously formed interfacial metal silicates on the magnetism of FeCo/SiO2 and Fe49%Co49%V2%/SiO2 core/shell nanoparticles. Element specific x-ray absorption and photoelectron spectroscopy experiments have identified the characteristic spectral features of metallic iron and cobalt from within the nanoparticle core. In addition, metal silicates of iron, cobalt, and vanadium were found to have formed spontaneously at the interface between the nanoparticle core and silica shell. X-ray magnetic circular dichroism experiments indicated that the elemental magnetism was a result of metallic iron and cobalt with small components from the iron, cobalt, and vanadium silicates. Magnetometry experiments have shown that there was no exchange bias loop shift in the FeCo nanoparticles; however, exchange bias from antiferromagnetic vanadium oxide was measured in the V-doped nanoparticles. These results showed clearly that the interfacial metal silicates played a significant role in the magnetism of these core/shell nanoparticles, and that the vanadium percolated from the FeCo-cores into the SiO2-based interfacial shell. [ABSTRACT FROM AUTHOR]

We have fabricated thin films of iron nanocrystallites embedded in a copper matrix using a dual ion beam assisted deposition technique. A secondary End-Hall ion beam bombarded the iron atoms during deposition altering significantly the morphology of the films and allowing for control of the intermixing between iron and copper components. Cross-sectional transmission electron microscopy and x-ray reflectometry experiments indicated that the morphology of the films was that of iron nanocrystallites embedded in a copper matrix. Rietveld refinements of the diffraction pattern identified fcc-copper and amorphous iron. An increased amount of disorder was observed with a reduction in the amount of deposited iron from a 1:1 Fe:Cu ratio to 0.25:0.75 Fe:Cu ratio. Interfacial copper-iron alloys were identified by DC susceptibility experiments through their reduced TC,Alloy (370, 310, and 280 K) compared with that of bulk iron (~1000 K). Element specific x-ray absorption and x-ray magnetic circular dichroism experiments were performed to identify the contributions to the magnetism from the iron and the copper-iron alloy. [ABSTRACT FROM AUTHOR]

Ni3Fe/(Ni, Fe)O thin films with bilayer and nanocrystallite dispersion morphologies are prepared with a dual ion beam deposition technique permitting precise control of nanocrystallite growth, composition, and admixtures. A bilayer morphology provides a Ni3Fe-to-NiO interface, while the dispersion films have different mixtures of Ni3Fe, NiO, and FeO nanocrystallites. Using detailed analyses of high resolution transmission electron microscopy images with Multislice simulations, the nanocrystallites' structures and phases are determined, and the intermixing between the Ni3Fe, NiO, and FeO interfaces is quantified. From field-cooled hysteresis loops, the exchange bias loop shift from spin interactions at the interfaces are determined. With similar interfacial molar ratios of FM-to-AF, we find the exchange bias field essentially unchanged. However, when the interfacial ratio of FM to AF was FM rich, the exchange bias field increases. Since the FM/AF interface 'contact' areas in the nanocrystallite dispersion films are larger than that of the bilayer film, and the nanocrystallite dispersions exhibit larger FM-to-AF interfacial contributions to the magnetism, we attribute the changes in the exchange bias to be from increases in the interfacial segments that suffer defects (such as vacancies and bond distortions), that also affects the coercive fields. [ABSTRACT FROM AUTHOR]

Ni3Fe/(Ni,Fe)O thin films having a nanocrystallite dispersion morphology were prepared by a reactive ion beam-assisted deposition technique. The crystallite sizes of these dispersion-based films were observed to decrease from 8.4±0.3 nm to 3.4±0.3 nm as the deposition flow-rate increased from 2.78% to 7.89% O2/Ar. Thin film composition was determined using selective area electron diffraction images and Multislice simulations. Through a detailed analysis of high resolution transmission electron microscopy images, the nanocrystallites were determined to be Ni3Fe (a ferromagnet), NiO, and FeO (both antiferromagnets). It was determined that the interfacial molar Ni3Fe ratio in the nanocrystallite dispersions increased slightly at first, then decreased as the oxygen content was increased; at 7.89% O2/Ar, the interfacial molar ratio was essentially zero (only NiO and FeO remained). For nanocrystallite dispersion films grown with O2/Ar flow-rate greater than 7.89%, no interfacial (intermixed) Ni3Fe phase was detected, which resulted in no measurable exchange bias. Comparing the exchange bias field between the nanocrystallite dispersion films at 5 K, we observed a decrease in the magnitude of the exchange bias field as the nanocrystallite size decreased. The exchange bias coupling for all samples measured set in at essentially the same temperature (i.e., the exchange bias blocking temperature). Since the ferromagnetic/anti-ferromagnetic (FM/AFM) contact area in the nanocrystallite dispersion films increased as the nanocrystallite size decreased, the increase in the magnitude of the exchange bias could be attributed to larger regions of defects (vacancies and bond distortions) which occupied a significant portion of the FM/AFM interfaces in the nanocrystallite dispersion films. [ABSTRACT FROM AUTHOR]

We have used a protein crystallization technique to fabricate three-dimensional (3D) ordered arrays of magnetoferritin nanoparticles up to a few hundreds of micrometers in size in the form of free standing perfectly faceted crystals. We compare the ac and dc susceptibilities of the nanoparticles in an uncrystallized system (amorphous assembly) and in a macroscopic 3D fcc crystal of nanoparticles. Completely different magnetism is measured due to the different interparticle interactions in the two systems. For example, at temperatures above 50 K the uncrystallized nanoparticle system is superparamagnetic while the 3D crystal displays no superparamagnetism up to 400 K. [ABSTRACT FROM AUTHOR]

A single precursor technique was used to synthesize γ-Fe2O3 nanoparticles with diameters from 5 to 7 nm. The blocking temperature of the nanoparticles essentially increases with Fe precursor concentration. Transmission Mössbauer spectroscopy of the different γ-Fe2O3 nanoparticle systems reveal that the local Fe environments in the nanoparticle vary considerably with Fe precursor concentration. Not only does the nanoparticle size change, but the magnitude of the hyperfine field (a measure of the nanoparticle moment), the amount of chemical disorder in the nanoparticle, and the number of Fe moments in the octahedral and tetrahedral sites of the γ-Fe2O3 spinel structure vary with Fe concentration as well. [ABSTRACT FROM AUTHOR]

The bulk and local magnetisms of ∼10 nm diameter CoFe2O4 nanoparticles have been characterized from 5 to 425 K. Magnetometry results indicate that the nanoparticles have a blocking temperature around 380 K and they exhibit a temperature dependent cubic magnetocrystalline anisotropy similar to that of bulk CoFe2O4 with K(0)∼1.2×106 ergs/cm3. However, the local, atomic-level spin dynamics examined by Mössbauer spectroscopy indicates strongly that rather than exhibiting a typical static, blocked magnetism to superparamagnetism behavior, the CoFe2O4 nanoparticle moments become paramagnetic, with this transition percolating from the surface to the core of the nanoparticle with increasing temperature. [ABSTRACT FROM AUTHOR]

We have examined the local magnetism of 7 nm γ-Fe2O3 nanoparticles as a function of temperature using transmission Mössbauer spectroscopy. Spectra indicate that the nanoparticles exhibit two different spin populations: one is due to the core Fe moments, while the other is from the surface moments. We find that the uniaxial anisotropy of the core increases with temperature from 5 to 75 K, whereupon it remains constant. Simultaneously, the surface moment fluctuations increase in frequency with warming. [ABSTRACT FROM AUTHOR]

Neutron diffraction and Mössbauer spectroscopy have been used to examine the connections between long- and short-range magnetic order in the rare-earth iron ferroborate YFe3(BO3)4. We establish the presence of short-range magnetic order in the pseudo-one-dimensional Fe-spin chains of the FeO6 octahedra while the ferroborates are in their (long-range) paramagnetic phase. The short-range order exists at temperatures several times the Néel temperature. In addition, we observe thermal hysteresis of the short-range ordered Fe spins. The observed Fe-spin dynamics are suggestive of the presence of solitons within the domains that form in the Fe-spin chains. [ABSTRACT FROM AUTHOR]