Magnetic Properties Of Co Nanoparticles Research Articles

Hydrothermal Synthesis of Co Nanoparticles: Structure, Morphology and Magnetic Properties

We have investigated the crystal structure, the microstructural and morphological characteristics, as well as the magnetic properties of Co nanoparticles (NPs) synthesized by a hydrothermal method. A series of samples has been elaborated for different concentrations of sodium hydroxide. The analysis of X-ray diffraction patterns, using two different wavelengths, has evidenced the coexistence of both [Formula: see text]-Co and [Formula: see text]-Co phases in the samples. The lattice parameter for both phases is in good agreement with those values expected for their bulk Co counterparts; the grain sizes of NPs were found to be dependent on the NaOH concentration. The scanning electron microscope micrographs show that Co NPs are agglomerated forming micrometer-sized entities whose shape evolves, indicating that the synthesis process affects the morphology of the powdered samples. Magnetic measurements indicate that the coercivity is slightly larger, [Formula: see text] Oe, for Co NPs with dendritic-like shape, probably due to an increase in the magnetocrystalline anisotropy.

Cobalt nanoparticles for biomedical applications: Facile synthesis, physiochemical characterization, cytotoxicity behavior and biocompatibility

Cobalt (Co) nanoparticles (NPs) were produced by a simple, one step hydrothermal method with the capping of oleic acid. Intrinsic structural, physiochemical and magnetic properties of Co NPs were investigated and demonstrated their applicability in biomedicine. X-ray diffraction, Raman spectroscopy and infrared (IR) spectroscopic studies confirm the single phase Co NPs with a high structural quality. The IR data revealed the capping of oleic acid via monodentate interaction. Small angle scattering studies suggest the existence of sticky hard sphere type of interaction among the Co NPs because of magnetic interaction which is further evidenced by electron microscopy imaging analyses. The Co NPs exhibit a ferromagnetic character over a wide range of temperature (20–300K). The temperature dependence of magnetic parameters namely, saturation magnetization, remanent magnetization, coercivity and reduced remanent magnetization were determined and correlated with structure of Co NPs. The Cytotoxicity studies demonstrate that these Co NPs exhibit the mild anti-proliferative character against the cancer cells (cisplatin resistant ovarian cancer (A2780/CP70)) and safe nature towards the normal cells. Haemolytic behavior of human red blood cells (RBC) revealed (<5%) haemolysis signifying the compatibility of Co NPs with human RBC which is an essential feature in vivo biomedical applications without creating any harmful effects in the human blood stream.

Control of the crystal habit and magnetic properties of Co nanoparticles through the stirring rate

Ferromagnetic hcp cobalt nanoparticles (NPs) are prepared by the polyol process using long-chain carboxylates as shape-directing agents, applying different stirring rates during the synthesis. Particles prepared with a stirring rate lower than ca. 150 rpm are very well-defined anisotropic crystals with smooth lateral facets and low stacking fault densities. The growth proceeds along the c axis of the hcp structure which corresponds to the long axis of the anisotropic particles. Increasing the stirring speed leads to a lowering of the mean aspect ratio, an increase in structural disorder as well as to a strong modification of the particle habit. The resulting Co NPs then display a marked surface roughness. We propose here a mechanism to explain the different morphologies observed. A high stacking fault density triggers the local disorganisation of the adsorbed capping agent layer which in turn favours the diffusion of Co monomers through the organic layer, resulting in the formation of excrescences perpendicular to the long axis of the particles. The magnetic properties are strongly related to the morphological and micro-structural properties of the particles. The stirring rate appears, so far, as the driving force to control the magnetic properties of the NPs from hard magnetic behaviour at low stirring rates to an almost soft magnetic one under high stirring conditions.

Shift of the blocking temperature of Co nanoparticles by Cr capping

We have studied the effect of Cr capping on the magnetic properties of Co nanoparticles (NPs). The NPs have an average diameter of 2.2 nm. The blocking temperature TB of the bare Co particles is 13.2 K. By capping with a thin Cr layer up to a thickness of tCr = 0.52 nm, we first observe a decrease of TB up to tCr = 0.14 nm, followed by an increase of TB for larger thicknesses 0.14 nm ≤ tCr ≤ 0.52 nm. X-ray magnetic circular dichroism measurements at the resonant Co and Cr L3 edges confirm a magnetic polarization of Cr which is opposite to the magnetization of Co. The antiparallel alignment of Co and Cr spins at the Co/Cr interface can explain the decrease at low capping layer thickness. However, for larger Cr capping layer thicknesses, the Cr film bridges the Co NPs, mediating interparticle exchange coupling and enhancing dipolar coupling that leads to an increase of the blocking temperature.

Matrix and interaction effects on the magnetic properties of Co nanoparticles embedded in gold and vanadium

The study of the magnetic properties of Co nanoparticles (with an average diameter of 10.3 nm) grown using a gas-phase aggregation source and embedded in Au and V matrices is presented. We investigate how the matrix, the number of embedded nanoparticles (counted by coverage percentage), the interparticle interactions and the complex nanoparticles/matrix interface structure define the magnetic properties of the studied systems. A threshold coverage of 3.5% of a monolayer was found in both studied systems: below this coverage, nanoparticles behave as an assembly of independent single-domain magnetic entities with uniaxial anisotropy. Above the threshold it is found that the magnetic behavior of the systems is more matrix dependent. While magnetic relaxation and Henkel plots measurements stress the importance of the dipolar interactions and the formation of coherent clusters in the case of the Au matrix, the magnetic behavior of cobalt clusters embedded in the vanadium matrix is explained through the formation of a spin glass-like state at the V-Co interface that screens the magnetic interactions between NPs.

Signatures of spin-glass freezing in Co/CoO nanospheres and nanodiscs

We present a study of the magnetic properties of Co nanoparticles having a combination of both spherical and disk shapes. The hcp Co nanospheres with an average diameter of 11nm and nanodiscs of dimensions ∼2.5×15nm2 were prepared by thermal decomposition of di-cobalt octacarbonyl in the presence of an amine surfactant. The as-synthesized nanoparticles were oxidized to grow an antiferromagnetic layer. High resolution transmission electron microscopy showed the presence of a ferromagnet/antiferromagnet (Co/CoO) interface with a 2.2-nm thick CoO shell on the spherical nanoparticles and 0.5nm thick on nanodiscs. We report the temperature and field dependent DC magnetization, frequency, field, and temperature dependent AC susceptibility, and the radio frequency transverse susceptibility. A low temperature paramagnetic behavior was observed in the DC magnetization at high fields and is assigned to defects in the CoO shell that are not coupled to the antiferromagnetic lattice. Our results support the existence of a low temperature frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass CoO shell and Co core.

Morphological, structural, and magnetic properties of Co nanoparticles in a silicon oxide matrix

We present a morphological, structural, and magnetic characterization of Co nanoparticles (mean diameter of 10.3 ± 1.8 nm) grown using a gas aggregation source and embedded in a silicon oxide matrix by sequential deposition of nanoparticles and silicon oxide. We show that the Co nanoparticles “soft-land” on the substrates and suffer a moderate oxidation in contact with the silicon oxide. Despite this moderate oxidation, it is found that, at room temperature, the magnetic volume of the resulting nanoparticles is below the superparamagnetic limit. The results presented in this article are compatible with the presence of an assembly of magnetically independent particles that also display a moderate exchange bias at low temperature.

Tuning the magnetic properties of Co nanoparticles by Pt capping

We show that by capping Co nanoparticles with small amounts of Pt strong changes of the magnetic properties can be induced. The Co nanoparticles have a mean diameter of 2.7 nm. From magnetometry measurements we find that for zero and for small amounts of Pt (nominal thickness t(Pt) < 0.7 nm) the nanoparticles behave superparamagnetic like. With increasing t(Pt) the blocking temperature is enhanced from 16 up to 108 K. Capping with Pd yields comparable results. However, for values t(Pt) > 1 nm a strongly coupled state is encountered resembling a ferromagnet with a T_c approx. 400 K

Study of the structure and magnetic properties of Co nanoparticles in the matrix of highly porous amorphous carbon

This paper presents the results of investigations into the structure and the magnetic properties of powders consisting of Co nanoparticles in the matrix of a high-porous amorphous carbon material. An analysis of X-ray diffraction and nuclear magnetic resonance spectra revealed the presence of hcp and fcc phases of cobalt in the particles of the powder and demonstrated the existence of a highly defective state, which could be related to a high concentration of defects of the “displaced-plane” type. The magnetometric measurements showed that the powder particles were in the ferromagnetic state and were characterized by a high field of the local magnetic anisotropy.

Magnetic Properties of Co Nanoparticles: Role of the Coexistence of Different Geometrical Phases

Following the experimental results of Respaud et al. [Phys. Rev. B 57, 2925 (1998)] we report self-consistent electronic structure calculations in order to analyze the magnetic properties of Co nanoparticles in which a coexistence of bcc and compact (fcc) phases are present within the particles. In all cases, the local spin moments S(i) are found to be saturated (approximately 1.7 microB) while, in contrast, the local orbital moments L(i) and the magnetic anisotropy energy (MAE) are found to be very sensitive to the size and structure of the systems. Interestingly, we obtain considerably enhanced values for L(i) at the internal bcc/fcc interfaces which can be even larger than at surfaces sites and, in addition, we found that by varying the fraction of bcc and fcc phases within the particles, several reorientations of the magnetization can be induced, a result that could open new possibilities to tune the MAE of magnetic nanoparticles.

Static and Dynamic Magnetic Properties of Co Nanoparticles

Co nanoparticles have been synthesized using wet-chemical methods. As-synthesized particles show a sharp low temperature peak in zero-field cooled (ZFC) magnetization well below the blocking transition temperature and this feature is associated with surface spin disorder. We have investigated the dynamic magnetic properties of Co using ac susceptibility and resonant RF transverse susceptibility (TS). We also studied the memory and relaxation effects in these nanoparticle systems. From these measurements we show a typical blocking behavior of an assembly of superparamagnetic nanoparticles with a wide distribution of blocking temperatures. The transverse susceptibility measurements on these particles show the presence of anisotropy even above the blocking temperature. The role of surface anisotropy and the size distribution of the particles on the observed memory and magnetic relaxation effects are discussed.

Morphological and magnetic properties of Co nanoparticle thin films grown on Si3N4

The morphological and magnetic properties of Co nanoparticles deposited by triode sputtering on Si3N4 at 550°C are reported. The nominal thickness of Co ranges from 2 up to 15nm, and two different capping layers, Au and Pt, are used. The nanoparticles were characterized by x-ray diffraction and atomic force microscopy. Morphological and structural studies show that the nanoparticles grow in a well-defined nanostructured pattern and adopt a hexagonal closed packed crystalline structure. Moreover, the average particle size and the particle size dispersion increase as the thickness increases, due to percolation. Experimental characterization of effective anisotropy field was carried out with transverse susceptibility. Transverse susceptibility measurements reveal an in-plane isotropic magnetic behavior. Both the effective anisotropy field and the coercive field increase as the particle size increases, following a D6 dependence, which is typical for three-dimensional structures in the framework of the random anisotropy model. The relationship between the particle size distribution and the anisotropy field distribution is shown, explaining the significant dependence of the magnetic behavior on the Co layer thickness. On the other hand, different capping layers give rise to a change in the magnetic response due to the modification of the interparticle interaction.

Orbital magnetism and magnetic anisotropy energy of Co nanoparticles: Role of polytetrahedral packing, polycrystallinity, and internal defects

We report extensive self-consistent electronic structure calculations in order to analyze the magnetic properties of Co nanoparticles with complex atomic configurations. Following recent experimental works, we consider polycrystalline structures in which a coexistence of bcc and compact (fcc) phases is present within the particles, polytetrahedral clusters, as well as fcc nanocrystals with ``stacking-fault-like'' defects. In all cases, the local spin moments ${S}_{\ensuremath{\delta}}(i)$ are found to be saturated $(\ensuremath{\sim}1.7{\ensuremath{\mu}}_{B})$, being almost independent of the direction of the magnetization $\ensuremath{\delta}$, the local atomic environment $i$, and the size of the particles, a behavior that is characteristic of magnetic materials in the large exchange regime. In contrast, the local-orbital moments ${L}_{\ensuremath{\delta}}(i)$ and the magnetic anisotropy energy (MAE) are found to be very sensitive to the size and structure of the systems. In our polycrystalline particles, we obtain considerably enhanced values for ${L}_{\ensuremath{\delta}}(i)$ at the internal bcc/fcc interfaces (even larger than at surfaces sites) and, interestingly, we obtain that by varying the fraction of bcc and fcc phases within the particles, several reorientations of the magnetization can be induced. Furthermore, our polytetrahedral structures are characterized by the existence of very isolated and low coordinated surface atoms, a fact that induces the formation of localized states with large local magnetic moments ${M}_{\ensuremath{\delta}}(i)$ $[{M}_{\ensuremath{\delta}}(i)=2{S}_{\ensuremath{\delta}}(i)+{L}_{\ensuremath{\delta}}(i)]$ of $3.7{\ensuremath{\mu}}_{B}$ and huge orbital contributions ${L}_{\ensuremath{\delta}}(i)$ as large as $2{\ensuremath{\mu}}_{B}$. Even if enhanced local MAE's are also found, negative and positive values are obtained in the different sites, resulting in nonadditive contributions that yield small energy barriers between our different considered $\ensuremath{\delta}$'s. Finally, we obtain that the existence of internal defects, created by locally interrupting the regular growth mode in fcc nanocrystals, considerably perturbs the electron spin-density distribution around the defect sites, leading to strong variations in ${L}_{\ensuremath{\delta}}(i)$ and to reorientations of the magnetization in the system.

Exchange bias in Co nanoparticles embedded in an Mn matrix

Magnetic properties of Co nanoparticles of 1.8 nm diameter embedded in Mn and Ag matrices have been studied as a function of the volume fraction (VFF). While the Co nanoparticles in the Ag matrix show superparamagnetic behavior with T B=9.5 K (1.5% VFF) and T B=18.5 K (8.9% VFF), the Co nanoparticles in the antiferromagnetic Mn matrix show a transition peak at ∼65 K in the ZFC/FC susceptibility measurements, and an increase of the coercive fields at low temperature with respect to the Ag matrix. Exchange bias due to the interface exchange coupling between Co particles and the antiferromagnetic Mn matrix has also been studied. The exchange bias field ( H eb), observed for all Co/Mn samples below 40 K, decreases with decreasing volume fraction and with increasing temperature and depends on the field of cooling ( H fc). Exchange bias is accompanied by an increase of coercivity.

Enhancement of the magnetic anisotropy of Co clusters by Au capping

We study the magnetic properties of Co nanoparticles, prepared by sputtering, with diameters ranging from 1to3.5nm. The effective anisotropy, which determines the activation energy for the magnetization reversal, increases with decreasing particle’s size, showing the dominant role played by surface atoms. We find that the superparamagnetic blocking temperature and the coercive field are significatively enhanced when the clusters are capped by a thin Au layer. This enhancement is probably caused by the bonding or hybridization of Co and Au atoms at the interface between the two metals. It provides thus a method to modify the magnetic anisotropy, which can be of interest for the applications of magnetic nanoparticles.