L. Yiping

Size effects on the magnetic properties of FeCoB powders

Abstract The effect of particle size has been studied on the magnetic properties of FeCoB fine particles prepared by chemical reduction. The initially reduced powders were amorphous with a particle size in the range of 15–55 nm and had an approximate composition Fe 42 Co 34 B 24 . XPS composition analysis indicated the presence of Fe-oxides on the surface of the particles. The temperature coefficient of coercivity varied with particle size from positive to negative. This anomalous behavior in coercivity may due to the large surface and interface anisotropy contributions to coercivity.

Structural and magnetic properties of ultrafine Fe‐Pd particles

The structural and magnetic properties of evaporated ultrafine Fe‐Pd particles have been studied near the equiatomic composition. Particles with an average size in the range of 65–360 A were obtained with the argon pressure varied between 0.5–40 Torr. All of the as‐made particles had a face centered cubic (fcc) structure and they were magnetically soft. A fcc to face centered tetragonal (fct) phase transformation was observed after annealing the powders at 500 °C resulting in room‐temperature coercivities up to 3.6 kOe and a magnetization (at 55 kOe) around 95 emu/g. An enhanced magnetic moment of about 3.0μB per atom was found for Fe.

Magnetic properties of fine Fe‐Co‐B particles prepared by chemical reduction

The effect of chemical composition and particle size on the magnetic properties of Fe‐Co‐B fine particles has been studied. The powders were produced by chemical reduction of aqueous solutions of ferric chloride and cobalt chloride with sodium borohydride. The structure of the initially reduced powders varied from amorphous to crystalline bcc depending on the boron concentration. Chemical composition analysis showed a boron content in the range 5–30 at. %. The particle size was found to be 25 nm. The magnetization and coercivity of as‐made powders were relatively low. After annealing, the maximum values of magnetization and coercivity obtained were about 160 emu/g and 1940 Oe, respectively. The temperature coefficient of coercivity varied from negative to positive with sample composition and packing fraction.

Heat treatment effects on structural and magnetic properties of fine FeB particles

Abstract The structural and magnetic properties of fine Fe B particles with a size of about 50 nm and B concentrations of 5, 14, and 17 at.% were studied in as-prepared samples and after annealing in vacuum and in H 2 atmosphere. The samples annealed in vacuum were found to be dominated by α-Fe in the high B concentrations. Oxide components appear in the particles with low B content. Boron is found to prevent oxidation of the as-made samples and promote the formation of mixed valence oxides in the crystallized samples.

Co-Pt-B particles prepared by chemical reduction

The magnetic and structural properties of ultrafine Co-Pt-B particles are studied in samples near the equiatomic CoPt composition. The particles are prepared by the chemical reduction of CoCl/sub 2/:6H/sub 2/O and PtCl/sub 4/ with NaBH/sub 4/. The as-made powders have an fcc structure, and they are magnetically soft. An fcc to fct phase transformation is observed after annealing at 700 degrees C, resulting in coercivities up to 17.8 kOe at room temperature. The variation of coercivity with particle morphology and structure is investigated. >

Ultrafine Magnetic Particles

Fine magnetic particles are scientifically and technologically very important. The particles are of great scientific interest in developing a better understanding of magnetic phenomena. From the technological point of view, they find wide applications in many types of materials including magnetic tapes, ferrofluids, catalysts, medical diagnostics, drug delivery systems, and pigments in paints and ceramics.1,2 Magnetic particles are also found in the cells of some animals and in some bacteria which help them to navigate in geomagnetic fields.2

Giant magnetoresistance in melt-spun CuMnAl ribbons

Abstract Giant negative magnetoresistance (GMR) has been observed in melt-spun Cu x Mn y Al z ( x =50−65, y =5−25, z =25−30) ribbons, with a value of ( R ( H ) − R (0))/ R (0) ≈ 15% at 30 K. The GMR was found not to saturate even in a field of 50 kOe, whereas the magnetization is saturated at low fields. X-ray diffraction patterns showed a mixture of Cu 2 MnAl (2:1:1) and Cu 9 Al 4 (9:4) phases in the Cu-rich samples and a single 2:1:1 phase in the samples with relatively higher Mn content. Microstructure studies show a fine mixture of Cu 2 MnAl and Cu 9 Al 4 phases which we believe to be the main cause of the GMR.

Preparation of magnetic fine particles by borohydride reduction

Abstract Binary (CoB) and ternary (FeCoB) particles were prepared by the chemical reduction of corresponding metal salts with sodium borohydride in aqueous solutions. The reduction of Co2+ (aq) by NaBH4 has been studied with mechanistic and stoichiometric factors as the focus. The primary product of this reduction is nanoscale Co2B particles, which are formed possibly through the intermediacy of [(H2O)5CoOH(BH2)HOCo(H2O)5]3+, and this complexion is reduced by electrons provided by three more moles of NaBH4. However, a side reaction where NaBH4 reacts directly with H2O becomes important if sufficient time is available before Co2+ addition or if Co/Co2B particles are allowed to catalyze it. In this way, NaBO2 can be formed that can react with Co2+ to give Co(BO2)2 as a product. The secondary product is metallic Co particles, which are formed by heat treatment of Co2B/Co(BO2)2 mixtures, or by sacrificial oxidation of Co2B. A tertiary product is Co3B formed by heating a mixture of Co2B and Co. The FeCoB particles were prepared with varying composition and particle size. The as-made samples are amorphous for lower Fe/Co ratios and crystalline for higher Fe/Co ratios.

Magnetism of Small Particles

The magnetic and structural properties of fine Fe particles and Fe-B particles prepared by vapor deposition and NaBH4 reduction respectively, have been studied. Fe particles in the range of 50–300 were obtained by varying the argon pressure during evaporation in the range of 0.5–30 torr. The Fe-B powders were prepared by reducing FeCl3 with NaBH4 in aqueous solution. Particle sizes in the range of 300–600 \(\dot A\) were obtained. The X-ray patterns indicate that powders with B content higher than 20% are amorphous and those with lower than 20% are crystalline. The dependence of magnetization and coercivity on particle size and temperature have been studied, using SQUID magnetometry and Mossbauer spectroscopy. The magnetic and structural data suggest a core/shell type of structure, where the core consists of metallic Fe (and/or Fe-B) and the shell is composed of Fe-oxides. The values of coercivity obtained (1200 Oe) are two orders of magnitude higher than those in bulk Fe and Fe-B metallic glasses.

Size Effects on the Structural and Magnetic Properties of Ultrafine Fe-Pd Particles

The structural and magnetic properties of ultrafine Fe-Pd particles have been studied near the equiatomic composition. Particles with an average size in the range of 65–360 A were prepared by gas evaporation under argon atmosphere with the Ar pressure varied between 0.5–40 torr. All of the as-made particles had a face centered cubic (fcc) structure. A fcc to face centered tetragonal (fct) phase transformation was observed in particles with a size in the range 120–360 A after annealing the powders at 500°C; smaller particles with equiatomic composition did not show this transformation. X-ray diffraction and Mossbauer data showed the presence of a higher percentage of Fe-O in the surface of the smaller particles which leads to an inhomogeneous Fe-Pd composition distribution. An enhanced magnetic moment of about 3.0 μ B per atom was found for Fe, and this is attributed to the polarization of Pd atoms.