This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Magnetic nanoparticles: Synthesis, protection, functionalization, and application

5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

The last glacial cycle was characterized by substantial millennial-scale climate fluctuations1,2,3,4,5, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces6,7, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution8 or higher1. Records based on oxygen isotopes, however, contain uncertainties in the range of ±30 m, or ±1 °C in deep sea temperature9,10. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth—and hence global sea level—over the past 470,000 years (470 kyr). Our reconstruction is accurate to within ±12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr-1, occurred, coincident with abrupt changes in climate.

Sea-level fluctuations during the last glacial cycle

Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called 'superparamagnetic limit' in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.

Beating the superparamagnetic limit with exchange bias

Monodispersed Co/CoO cluster assemblies with the mean cluster sizes of 6 and 13 nm have been prepared by a plasma-gas condensation type cluster beam deposition apparatus. We measured the effects of the oxygen gas flow rate during deposition, temperature, and cluster size on the coercivity and hysteresis loop shift induced by field cooling. The large exchange bias field (10.2 kOe) and coercivity (5 kOe) were observed at 5 K for the monodispersed Co/CoO cluster assembly with $d=6\mathrm{nm}.$ The correlations between unidirectional anisotropy and uniaxial anisotropy, training effect and magnetic relaxation can be interpreted by the hypothesis of a spin disorder in the interfacial layer between the antiferromagnetic CoO shell and the ferromagnetic Co core.

Magnetic properties of monodispersed Co/CoO clusters

We have constructed a plasma-gas-condensation type cluster deposition apparatus and tried to find the optimum operation conditions for controlling the cluster size. Transmission electron microscope (TEM) observation has been done to evaluate sizes of Ni clusters produced when varying the volume of a cluster growth region, sputtering power, and inert gas pressure. The mean cluster size decreases by decreasing the volume of growth region and the sputtering power. The smallest cluster obtained in this work is about 2.3 nm in diameter. We have considered the following two models for the cluster growth: (1) a cluster–cluster collision growth and (2) an atomic vapor condensation growth. The cluster growth speed estimated from the former is too slow, while that from the latter is reasonable in comparison with the present experiments. When stable embryos are made from atom collisions, they grow up faster and the final cluster sizes estimated from the latter model are consistent with those observed by TEM.

Formation and size control of a Ni cluster by plasma gas condensation

This paper reports an in-depth study on nanocrystalline Gd-doped ZnO thin films synthesized using versatile pulsed spray pyrolysis method and exhibit room temperature ferromagnetism. The detailed structural and microstructural studies confirm that the doped Gd ions occupy Zn sites and the peak shift can be elucidated by charge neutrality. Optical investigation shows that the Gd doping in ZnO lattice leads to a decrease in the near band edge position due to the introduction of new unoccupied states by Gd 4f electrons. The electronic structure of the Zn1−xGdxO at the O K edge shows the evolution of pre-edge spectral features similar to cuprates and manganites, and also confirms the strong hybridization of O 2p–Gd 4f/5d states. Furthermore, the Gd M5 edge provides evidence that Gd ions are in the trivalent state. Hysteresis measurements demonstrate that the Gd-doped ZnO films are magnetically anisotropic and exhibit intrinsic ferromagnetic behavior at room temperature. Higher magnetization in 3 kOe values is...

Intrinsic ferromagnetism and magnetic anisotropy in Gd-doped ZnO thin films synthesized by pulsed spray pyrolysis method

We have fabricated CoO-coated monodispersive Co cluster assemblies with the mean cluster size of 13 nm at various oxygen gas-flow rate ${R}_{{\mathrm{O}}_{2}}$ by a plasma-gas-condensation-type cluster beam deposition technique, and studied their electrical conductivity, \ensuremath{\sigma}, and magnetoresistance. For ${R}_{{\mathrm{O}}_{2}}l0.24\mathrm{SCCM}$ (sccm denotes cubic centimeter per minute), the resistivity revealed a minimum and showed $\mathrm{ln}T$ dependence at lower temperatures, probably due to the weak localization of conduction electrons owing to presence of thin oxide shells covering Co cores. A small negative magnetoresistance was observed in this regime. For ${R}_{{\mathrm{O}}_{2}}g0.3\mathrm{SCCM},$ tunnel-type temperature dependence of \ensuremath{\sigma} in the form of ln \ensuremath{\sigma} vs $1/T$ was observed between 7 and 80 K. This differs from the well-known temperature dependence of ln \ensuremath{\sigma} vs ${1/T}^{1/2}$ for disordered granular materials. The magnetoresistance ratio, $({\ensuremath{\rho}}_{\mathrm{H}=30\mathrm{}\mathrm{kOe}}\ensuremath{-}{\ensuremath{\rho}}_{0})/{\ensuremath{\rho}}_{0},$ is negative and its absolute value increases sharply with decreasing temperature below 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This marked increase, by a factor of 6, is much larger than those observed for conventional metal-insulator granular systems. These results are ascribed to a prominent cotunneling effect in the Coulomb blockade regime, arising from the uniform Co core size and CoO shell thickness in the present monodispersed cluster assemblies.

Characteristic transport properties of CoO-coated monodispersive Co cluster assemblies

We systematically studied structural and magnetic characteristics of size- monodispersed Fe and oxide-coated Fe cluster assemblies with the mean cluster sizes of 7–16 nm. Transmission electron microscopy and scanning electron microscopy (SEM) observations show that the Fe clusters in the assemblies maintain their original size at room temperature. In the SEM images, a random stacking of the Fe clusters and a porous structure with a low cluster packing fraction of about 25% are observed. For the Fe cluster assemblies, magnetic coercivity (Hc) at room temperature increases from 4×101 to 4×102 Oe by increasing the mean cluster size from 7.3 to 16.3 nm. Using the experimental values of the coercivity at T⩾100 K and the fitting values of blocking temperature TB from Hc=Hc0[1−(T/TB)1/2], we estimated the values of magnetic anisotropy constant K of the order of 106 erg/cm3 from TB=KV/25kB, which is larger by an order of magnitude than the bulk Fe value (5×105 erg/cm3). Such a large effective anisotropy at T⩾100 K is ascribed to the large surface anisotropy effects of the small clusters and the low cluster-packing fraction of the Fe cluster assemblies. For the oxide-coated Fe cluster samples, the coercivity strongly depends on the oxygen gas flow rate during deposition, cluster size, and temperature. In the case of a high oxygen gas flow rate (namely high surface-oxidized clusters), the ferrimagnetic oxide shell crystallites also affect the coercivity at T>50 K: The hysteresis loop shift disappears, leading to a complex change in the coercivity and an enhancement of the effective anisotropy constant.

Takehiko Hihara

Papers

Magnetic properties of monodispersed Co/CoO clusters

Formation and size control of a Ni cluster by plasma gas condensation

Intrinsic ferromagnetism and magnetic anisotropy in Gd-doped ZnO thin films synthesized by pulsed spray pyrolysis method

Characteristic transport properties of CoO-coated monodispersive Co cluster assemblies

Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies