Wanfeng Li

Surface spin disorder and exchange-bias in hollow maghemite nanoparticles

We report a comparative study of the magnetic properties of polycrystalline hollow γ-Fe2O3 nanoparticles with two distinctly different average sizes of 9.2 ± 1.1 nm and 18.7 ± 1.5 nm. High-resolution transmission electron microscopy images reveal the presence of a shell with thickness of 2 nm and 4.5 nm for the 9.2 nm and 18.7 nm nanoparticles, respectively. The field-cooled hysteresis loops show interesting features of enhanced coercivity and horizontal and vertical shifts associated with the polarity of the cooling field for both types of nanoparticles. While the anomalously large horizontal shifts and open hysteresis loop in a field as high as 9 T observed for the 9.2 nm nanoparticles corresponds to a “minor loop” of the hysteresis loop, the loop shift observed for the 18.7 nm nanoparticles manifests an intrinsic “exchange bias” (EB). Relative to the 18.5 ± 3.2 nm solid nanoparticles, a much stronger EB effect is achieved in the 18.7 nm hollow nanoparticles. Our studies point to the importance of inner...

Mechanism and controlled growth of shape and size variant core/shell FeO/Fe3O4 nanoparticles

We report a novel synthesis approach for the growth of core/shell FeO/Fe3O4 nanoparticles with controlled shape and size. FeO particles were partially oxidized to form core/shell FeO/Fe3O4 structures, as evidenced from transmission electron microscopy, X-ray diffraction, and magnetometry analysis. We find that the molar ratios and concentrations of surfactants are the key parameters in controlling the particle size. The particles can grow in either isotropic or anisotropic shapes, depending upon a chemical reaction scheme that is controlled kinetically or thermodynamically. The competitive growth rates of {111} and {100} facets can be used to tune the final shape of nanoparticles to spherical, cubic, octahedral, octopod, and cuboctahedral geometries. FeO particles can also be oxidized chemically or thermally to form Fe3O4 nanoparticles. By following the same synthesis technique, it is possible to synthesize rods and triangles of Fe3O4 by introducing twinnings and defects into the crystal structure of the seed. The thermally activated first-order Verwey transition at ~120 K has been observed in all the synthesized FeO/Fe3O4 nanoparticles, indicating its independence from the particle shape. These core/shell nanoparticles exhibit a strong shift in field-cooled hysteresis loops accompanied by an increase in coercivity (the so-called exchange bias effect), but the low field-switching behavior appears to vary with the particle shape.

Synthesis and magnetic properties of core/shell FeO/Fe3O4 nano-octopods

We report the synthesis and magnetic properties of core/shell FeO/Fe3O4 nanoparticles with an average size of 30 nm in a complex quasi-octopod shape. FeO nanoparticles were synthesized by a wet chemical synthesis route followed by partial oxidation to form core/shell structured FeO/Fe3O4 octopods. X-ray diffraction and transmission electron microscopy confirmed the presence of iron oxide phases and the formed core/shell FeO/Fe3O4 morphology. Magnetic measurements revealed two distinct temperatures corresponding to the thermally activated Verwey transition (TV ∼ 120 K) of the ferrimagnetic Fe3O4 shell and the Neel temperature (TN ∼ 230 K) of the antiferromagnetic FeO core. The nanoparticles exhibited a strong horizontal shift in the field-cooled hysteresis loop (the so-called exchange bias (EB) effect) accompanied by enhanced coercivity. The Meiklejohn-Bean model has been implemented to quantify the amount of frozen spins that locate at the interface between FeO and Fe3O4 and are responsible for the observ...

Synthesis of air stable FeCo nanoparticles

Nanoparticulate FeCo alloys have been synthesized by Fe(CO)5 and Co2(CO)8 thermal decomposition in paraffin oil in the presence of oleic acid and oleyl amine. The crystal structure and morphology of the nanoparticles were confirmed by powder x-ray diffraction and transmission electron microscopy. The size and crystallinity of the particles was found to depend on the reaction conditions such as the precursors concentration, reaction time, and carbonyls injection temperature. Also the Fe/Co ratio can be easily controlled by controlling the Fe and Co carbonyls ratio. The as-made nanoparticles were annealed at 500 °C under CH4 stream in order to be protected from future oxidation. This treatment leads to the formation of a graphitic shell around the particles which also protects them from sintering. Additionally, these particles can be functionalized with 1-pyrenebutiric acid in order to be soluble in various solvents.

Anisotropic Nd2Fe14B nanoparticles and nanoflakes by surfactant-assisted ball milling

Anisotropic Nd2Fe14B nanoparticles and nanoflakes have been produced by surfactant-assisted high-energy ball milling (SA-HEBM) of precursor nanocrystalline alloys. A two-stage high-energy ball milling was performed to obtain the nanopowders and nanoparticles; first the coarse powders were subjected to a wet milling followed by a wet surfactant-assisted milling. Different shaped nanoparticles have been obtained by varying the time of the first stage of the milling process and then separated by sonication. For a surfactant-free wet milling of 4 h, followed by the SA-HEBM, the nanopowders consisted of a mixture of Nd2Fe14B flakes with a thickness below 200 nm and an aspect ratio as high as 102–103, and anisotropic square nanoparticles with a size of 10 nm. However, for a shorter wet milling, nearly spherical nanoparticles with a size of 2.7 nm were obtained. Low-temperature coercivities have been obtained with maximum values of 4 kOe for square nanoparticles and 2.5 kOe for the nearly spherical nanoparticles.

Influence of the type of surfactant and hot compaction on the magnetic properties of SmCo5 nanoflakes

In this study, we discuss the effects of a type of surfactant (oleylamine, oleic acid, and trioctylamine) and hot pressing on the hard magnetic properties of crystallographically anisotropic SmCo5 nanoflakes prepared by surfactant-assisted high energy ball milling. The phase, microstructure, and magnetic properties of the hot-pressed SmCo5 were investigated by using x-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry. The coercivities of the precursor flakes prepared using oleylamine, oleic acid, and trioctylamine were 14.9, 15.8, and 15 kOe, respectively. Hot-compacted SmCo5 magnets prepared from the nanoflakes milled with oleic acid had the lowest coercivity of 8.1 kOe. It is believed that even after repeated washing in an ultrasonic bath with different solvents, the remaining oleic acid in the SmCo5 nanoflakes led to oxidation of SmCo5 at the surface/interface of nanoflakes during the hot-pressing process. The compacted SmCo5 magnets prepared from the nanoflakes milled wi...

Chemically synthesized hollow nanostructures in iron oxides

In this work, we report a detailed study of the formation of hollow nanostructures in iron oxides. Core/shell Fe/Fe-oxide nanoparticles were synthesized by thermal decomposition of Fe(CO)(5) at high temperature. It was found that 8 nm is the critical size above which the particles have a core/shell morphology, whereas below this size the particles exhibit a hollow morphology. Annealing the core/shell particles under air also leads to the formation of hollow spheres with a significant increase in the average particle size. In the case of the thermally activated Kirkendall process, the particles do not fully transform into hollow structures but many irregular shaped voids exist inside each particle. The 8 nm hollow particles are superparamagnetic at room temperature with a blocking temperature of 70 K whereas the core/shell particles are ferromagnetic.

Mn-Bi Magnetic Powders With High Coercivity and Magnetization at Room Temperature

Mn₅₅Bi₄₅+x at.% Mn ( x=0, 7 and 11) magnetic powders were prepared by low energy ball milling technique. The x-ray diffraction pattern showed that the samples mainly contain over 90% of the low temperature Mn-Bi phase for x = 0 and 7 with a very small amount of Bi phase. Microstructural analysis showed that the amount of sub-micron particles increases with milling time. The milling time dependent magnetic properties were studied for all the compositions. An intrinsic coercivity (H_ci) of 12.2 kOe, remanent magnetization (4 πM_r) of 7.1 kG, and energy product (BH)_max of 11 MGOe have been obtained for the 8 hours milled Mn₅₅Bi₄₅+7 at.% Mn sample. These Mn-Bi powders have potential use as precursor for rare earth free permanent magnets.

Magnetic entropy change in core/shell and hollow nanoparticles

The development of positive magnetic entropy change in the case of ferromagnetic (FM) nanostructures is a rare occurrence. We observe positive magnetic entropy change in core/shell (Fe/γ-Fe2O3) and hollow (γ-Fe2O3) nanoparticles and its origin is attributed to a disordered state in the nanoparticles due to the random distribution of anisotropy axes which inhibits any long range FM ordering. The effect of the energy barrier distribution on the magnetic entropy change and its impact on the universal behavior based on rescaled entropy change curves for core/shell and hollow nanostructures is discussed. Our study emphasizes that the magnetic entropy change is an excellent parameter to study temperature and field dependent magnetic freezing in such complex nanostructures.

Size and composition control of core-shell structured iron/iron-oxide nanoparticles

Metallic iron nanoparticles with a crystalline iron oxide shell were synthesized by the thermal decomposition of iron pentacarbonyl [Fe(CO)5] in octadecene in the presence of oleic acid and oleylamine. The effect of different synthetic parameters was investigated in details including the refluxing time and temperature, the injection temperature of iron precursor and the surfactant concentrations. The particles size can be tuned by controlling the injection temperature of iron precursor. Particle composition was adjusted by controlling the refluxing time. Both XRD diffraction and magnetic measurements indicated that these particles are very stable against oxidation which was further evidenced by microstructure analysis.