Bimagnetic core/shell Fe58Pt42/Fe3O4 nanoparticles are synthesized from high-temperature solution phase coating of 4 nm Fe58Pt42 core with Fe3O4 shell. The shell is tunable from 0.5 to 3 nm. Magnetic properties of the as-synthesized core/shell particles are dependent on shell thickness due to the exchange coupling between core and shell. Upon reductive annealing, an assembly of the core/shell nanoparticles is transformed into a hard magnetic nanocomposite with enhanced energy product.

FePt nanoparticles have great application potential in advanced magnetic materials such as ultrahigh-density recording media and high-performance permanent magnets. [1][2][3] The key for applications is the very high uniaxial magnetocrystalline anisotropy of the L1 0 -FePt phase, which is based on crystalline ordering of the face-centered tetragonal (fct) structure, described by the chemical-ordering parameter S.[4] Higher chemical ordering results in higher magnetocrystalline anisotropy. Unfortunately, as-synthesized FePt nanoparticles take a disordered face-centered cubic (fcc) structure that has low magnetocrystalline anisotropy. Heat-treatment is necessary to convert the fcc structure to the ordered fct structure. Several previous theoretical and experimental investigations have been reported on the size-dependent chemical ordering of FePt nanoparticles. [5][6][7][8][9] It has been observed that the degree of ordering decreases with decreasing particle size of the sputtered FePt nanoparticles. [5,6] Theoretical simulation predicted that the ordering would not take place when the particle size is below a critical value. [8,9] However, there have not been systematic experimental studies on quantitative size dependence of chemical ordering of FePt nanoparticles due to the lack of monodisperse L1 0 -FePt nanoparticles with controllable sizes.There are also few studies reported to date on the quantitative particle size dependence of magnetic properties, including the Curie temperature, coercivity, and magnetization of the L1 0 -FePt phase, although it has been well accepted that there is a size effect on the ferromagnetism of any low-dimensional magnets. [10,11] Additionally, the magnetic properties of FePt ferromagnets, as observed in thin-film samples, [4,12] are affected by the degree of chemical ordering, which is in turn size dependent. It is therefore highly desirable to understand the size and chemical-ordering effects, and their influence on the magnetic properties of the nanoparticles. A major hurdle in obtaining the particle size dependence of structural and magnetic properties of the L1 0 phase is particle sintering during heat-treatments that convert the fcc phase to the fct phase. [13,14] This long-pending problem has been solved recently by adopting the salt-matrix annealing technique. [15,16] With this technique, particle aggregation during the phase transformation has been avoided so that the true size-dependent properties of the fct phase can be measured. In this paper, we report results on quantitative particle size dependence of the chemical-ordering parameter S and selected magnetic properties, including the Curie temperature, T c , magnetization, M s , and coercivity, H c , with the particle size varying from 2 to 15 nm. Figure 1 shows the transmission electron microscopy (TEM) images of the FePt nanoparticles with different sizes before and after annealing in a salt matrix at 973 K for 4 h. The images, from left to right, show nanoparticles with nominal diameters of 2, 4, 6, 8, and 15 nm, res...

Monodisperse FePt nanoparticles with particle size of about 2 nm have been prepared by 1,2-hexadecanediol reduction of iron acetylacetonate and platinum acetylacetonate in dioctyl ether. The as-synthesized particles have a chemically disordered fcc structure and can be transformed into chemically ordered fct structure after thermal processing at temperatures above 500°C. The ordered fct FePt phase has high magnetic anisotropy and thus large coercivity up to 1.8 T.

Bimagnetic FePt/ MFe 2 O 4 ͑M=Fe,Co͒ core/shell nanoparticles are synthesized via high-temperature solution phase coating of 3.5 nm FePt core with MFe 2 O 4 shell. The thickness of the shell is controlled from 0.5 to 3 nm. An assembly of the core/shell nanoparticles shows a smooth magnetization transition under an external field, indicating effective exchange coupling between the FePt core and the oxide shell. The coercivity of the FePt/ Fe 3 O 4 particles depends on the volume ratio of the hard and soft phases, consistent with previous theoretical predictions. These bimagnetic core/shell nanoparticles represent a class of nanostructured magnetic materials with their properties tunable by varying the chemical composition and thickness of the coating materials.

SmCo5 magnets are synthesized by the facile Ca reduction of core/shell‐structured Co/Sm2O3 nanoparticles, as schematically illustrated in the figure. The magnets exhibit coercivities reaching 24 kOe at 100 K and 8 kOe at room temperature. The synthesis represents an important first step towards the fabrication of SmCo‐based exchange‐spring nanocomposites for high‐performance permanent magnet applications.

A methodology for the determination of the subsurface line direction of dislocations using scanning tunneling microscopy (STM) images is presented. The depth of the dislocation core is derived from an analysis of the displacement field measured by STM. The methodology is illustrated for dislocations at GaN(10 10) cleavage surfaces. It is found that the dislocation line bends toward the surface, changing from predominantly edge-type to more screw-type character, when approaching the intersection point. Simultaneously, the total displacement detectable at the surface increases due to a preferred relaxation towards the surface. V C 2015 AIP Publishing LLC.

In Sm-Co/ Fe exchange-spring magnet films, the magnetization reversal processes of constituent elements and layers were studied with an emphasis on the role of diffused Co atoms. Enhanced coupling effectiveness was observed in a film with a graded interface where significant Co diffusion into the Fe layer was observed by means of electron microscopy. Comprehensive insight into the magnetization reversal processes was obtained by combining micromagnetic simulation with element-and depth-resolved x-ray resonant magnetic scattering. The approach unambiguously identifies distinctive composition profiles across the graded interface and provides the magnetization behavior of the diffused Co.

The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) with different thicknesses of low temperature grown GaN cap layers are investigated. It is found that the MQW emission energy red-shifts and the peak intensity decreases with increasing GaN cap layer thickness, which may be partly caused by increased floating indium atoms accumulated at quantum well (QW) surface. They will result in the increased interface roughness, higher defect density, and even lead to a thermal degradation of QW layers. An extra growth interruption introduced before the growth of GaN cap layer can help with evaporating the floating indium atoms, and therefore is an effective method to improve the optical properties of high indium content InGaN/GaN MQWs.