Seokwon Yoon

Magnetic properties of ZnO-based diluted magnetic semiconductors

We report a study of the magnetic properties of transition-metal doped Zn1−xTMxO (TM=Mn, Co, Fe). Polycrystalline powder samples were synthesized by both solid-state and liquid-phase reactions. From the Curie–Weiss behavior of susceptibility at high temperatures, it was found that the TM–TM interaction is dominated by antiferromagnetic coupling with effective nearest-neighbor exchange constants J=−90 to −30 K. The magnetization data measured at low temperature as a function field H are fit to a parameterized Brillouin function to obtain the effective concentration xeff of magnetically active TM2+ ions. As x increases, the fraction of magnetically active ions, xeff/x, decreases. This is ascribed to an increase in average AF interaction between doped magnetic spins as the average distance between them decreases with an increase in x.

Ferromagnetic GaN:MnAlSi nanowires

The fabrication of crystalline Al-codoped GaN:Mn nanowires with a Mn doping rate of approximately 7at.% is reported. The magnetism measurements show that the Curie temperature is above 350K. X-ray and electron diffractions do not show the presence of any secondary magnetic phases. The electrical transport measurement indicates that the nanowires are of n-type semiconductor.The fabrication of crystalline Al-codoped GaN:Mn nanowires with a Mn doping rate of approximately 7at.% is reported. The magnetism measurements show that the Curie temperature is above 350K. X-ray and electron diffractions do not show the presence of any secondary magnetic phases. The electrical transport measurement indicates that the nanowires are of n-type semiconductor.

Atomic Force Microscopy Study of Mn12O12(O2CC4H3S)16(H2O)4 Single-Molecule Magnet Adsorbed on Au Surface

A film of Mn12O12(O2CC4H3S)16(H2O)4 molecules was chemically deposited onto an atomically flat Au(111) surface. The surface morphology of the film and the elastic property of the molecules were investigated using tapping-mode atomic force microscopy. The molecules were determined to be randomly arranged with an abnormal height distribution due to the anisotropic shape and bonding probability of the molecules. We also measured the amplitude-distance curves for the molecules and determined that the mechanical stiffness ks of the molecules adsorbed on the Au(111) surface is about 0.2 N/m.

Magnetic properties of artificially synthesized ferritins

Human ferritin homopolymers with H or L subunits (rHF and rLF) were genetically engineered in E coli. Apoferritins were then reconstituted with 2000 Fe atoms. A big difference was observed in the rates of iron uptake, whereas the mean core size was similar in rHF and rLF. Magnetization of the recombinant human ferritins were measured as functions of temperature and field. The blocking temperature TB(H) at low fields is considerably higher in rLF than in rHF. From the fit of M(H) data to a modified Langevin function: M(H)=M0L(μpH∕kBT)+χaH, the effective magnetic moment μp is found to be much larger in rLF than in rHF. Experimental data demonstrate that the magnetic properties, in particular, the uncompensated spins of ferritin core are related to the biomineralization process in ferritins.