Kyongha Kang

Recoil hysteresis of Sm–Co∕Fe exchange-spring bilayers

The exchange-spring behavior found in Sm–Co (20nm)∕Fe epitaxial bilayer films was investigated by analyzing major hysteresis and recoil curves as a function of anneal conditions. The hard layer consists of nanocrystalline intermetallic Sm–Co hexagonal phases (majority phase Sm2Co7 with SmCo3 and SmCo5). Recoil curves, obtained from the successive removal to remanence and reapplication of an increasingly negative field from the major demagnetization curve, reveal the reversible and irreversible components of the magnetization. The Sm–Co thickness was fixed at 20nm while the Fe thicknesses of 10 and 20nm were studied, with ex situ annealing carried out in evacuated, sealed silica tubes at different temperatures. The peak in the recoil curve area is associated with the coercivity of the hard phase. The development of the soft component magnetization is revealed by the departure of the recoil area from zero with application of a reverse field. These two features together confirm that annealing stabilizes the ...

Alignment and analyses of MnBi∕Bi nanostructures

A Mn5Bi95 alloy was rapidly solidified into a mixture of nanocrystalline Bi and metastable Bi(Mn). Heating the ribbons to temperature T=525K in a dc magnetic field causes formation and c-axis alignment of low-temperature phase (LTP) MnBi nanorods along the applied field direction. Nanorod alignment increases with increased magnetic field, with a calculated alignment half-angle of 47° for a sample heated to 520K at 50kOe. In situ magnetization changes suggest that nanorod alignment is achieved by rotation of MnBi particles. Particle alignment enables the measurement of the MnBi nanorod spin reorientation temperature of 100K, the same as its bulk counterpart.

MnBi nanostructures: Size dependence of magnetostructural transition and matrix templating

Rapidly solidified MnxBi(1−x) (x=0.05 Mn5, x=0.10 Mn10) alloys form as NiAs-type low-temperature phase MnBi nanoparticles in a Bi matrix. Microstructural differences underlie diverse magnetic behavior: Mn5 has well-separated nanorods along the basal plane axial directions of Bi, while Mn10 consists of MnBi nanorods and submicron-sized MnBi grains. The Curie transition of Mn10 is TC=630K with a second-order thermodynamic character, while Mn5 nanorods exhibit a reduced and hysteretic TC=520K. Magnetic field annealing fosters (00n) alignment of the MnBi phase. Mn10 also shows alignment of the Bi matrix by a proposed templating mechanism.

Magnetism and metastability of melt-spun Pd40(Fe,Ni)40P20 metallic glass

The magnetic character of Pd40FexNi40−xP20, a metallic glass where x=17.5, is experimentally studied over a temperature range of 10–375K. Hysteresis and temperature-dependent magnetization measurements derived from superconducting quantum interference device magnetometry on the amorphous alloy provide insight into the role of applied magnetic field on magnetic phase formation during the nanocrystallization process. Specifically, the microstructure of the melt-spun form of Pd40(Fe,Ni)40P20 is discussed in correlation to the observed superparamagnetic and spin-glass behavior. Despite the extremely high quenching rate used to synthesize the alloy, this composition shows evidence of chemical segregation on the nanoscale in the as-solidified state.

Crystal structure and magnetic properties of MnBi–Bi nanocomposite

Isolated NiAs-type MnBi nanoparticles of 10 nm diameter, embedded in a Bi matrix, were fabricated by melt-spinning and then annealing at 528 K for 1 h. A large coercivity, over 16 kOe at 300 K, with a positive temperature coefficient is observed. In agreement with bulk results, ac susceptibility measurements show a spin reorientation around 90 K coincident with an abrupt decrease in the coercivity with decreasing temperature. The high coercivity at 300 K is attributed to a mixture of nucleation and pinning mechanisms for magnetic reversal.

Formation of MgB2 at low temperatures by reaction of Mg with B6Si

Formation of MgB2 by reactions of Mg with B6Si and Mg with B were compared, the former also producing Mg2Si as a major product. Compared to the binary system, the ternary reactions for identical time and temperature were more complete at 750??C and below, as indicated by higher diamagnetic shielding and larger x-ray diffraction peak intensities relative to those of Mg. MgB2 could be produced at temperatures as low as 450??C by the ternary reaction. Analyses by electron microscopy, x-ray diffraction and of the upper critical field show that Si does not enter the MgB2 phase.

Magnetic and transport properties of MnBi∕Bi nanocomposites

The magnetic and transport properties of a nanostructured Mn-Bi eutectic composition (∼Mn5Bi95) produced by melt spinning and low-temperature/short time vacuum annealing were studied. A hysteretic magnetostructural transformation from low-temperature phase to high-temperature phase MnBi is confirmed at 520K. The fact that the transition temperature is lower than that reported for bulk MnBi (633K), is tentatively attributed to interfacial strain between MnBi and the Bi matrix. A positive temperature coefficient of coercivity is confirmed in the nanocomposites, with a maximum coercivity value of 36kOe at 500K. Magnetic field annealing the as-spun composites (525K at 10kOe) produces nanoparticle alignment. Annealed MnBi∕Bi composites have a very large ordinary magnetoresistance (MR) ratio normal to the ribbon at 5T, 275% at room temperature and 10 000% at 5K.

Large magnetoresistance in rapidly solidified bismuth

Rapidly solidified, annealed ribbons (20μm thick) of elemental Bi show a room-temperature ordinary magnetoresistive effect of 250% at 5T with the field applied perpendicular to the ribbon surface. The effect increases to 10000% at 5K and 5T. These values are intermediate to those obtained for single-crystal Bi films and sputtered or evaporated polycrystalline Bi films of comparable thicknesses. The large magnetoresistance of the ribbons is attributed to a very good crystallinity and partial c-axis texture of the ribbon achieved during solidification. Rapid solidification by melt spinning is a promising technique for synthesis of Bi with potential application in magnetoelectric devices.