Byeongki Kang

Spin dynamics of isolated donor electrons in phosphorus-doped silicon from high-frequency electron spin resonance

We present the spin dynamics of isolated donor electrons in phosphorus-doped silicon at low temperature and in a high magnetic field. We performed a steady-state electron spin resonance (ESR) on the sample with a dopant concentration of 6.5 × 10(16) cm(- 3) in a high field of 2.87 T (80 GHz) and at temperatures from 48 down to 1.8 K. As the temperature decreases below 16 K, the resonance spectral line changes from the usual derivative form characteristic of absorptions. Very long spin-lattice relaxation time T(1) at low temperature gives rise to rapid passage effects and results in a dramatic change in the line shape and intensity as a function of temperature. We show that the numerical analysis based on the passage effects well explains the observed spectral changes with temperature. The spin-lattice relaxation time T(1) is derived by numerical fit to the experimental data. We discuss the dynamic nuclear polarization of (31)P nuclear spins which shows up as asymmetric intensities of the hyperfine-split ESR resonance lines.

High field ESR of P-doped Si for Quantum Computing Application

We measured ESR of phosphorous-doped silicon with a low concentration of P, n, at high magnetic fields and low temperatures to investigate the states of nuclear spin. A sample with n = 6.52 × 1016 /cm3 was studied at 2.85 T (80 GHz) from 30 K to 2.3 K by field-modulating cw-ESR for a fixed 0 dB power. As the temperature was lowered, the out-of-phase signal appeared around 18 K, reached at a maximum intensity at 13 K, and disappeared around 6 K. The out-of-phase signal is referred to the field modulation. The in-phase signal started to change from the derivative of absorption spectrum at high temperatures to absorption-like shape around 15 K and asymmetry of intensity for two peaks of hyperfine-separated signals increased as temperatures was lowered. Below 10 K, the saturation of the in-phase signal started to appear. We speculate that the asymmetry is caused by saturation effect and dynamic nuclear polarization of 31P nuclear spin due to drastic change of electron T1.

Effects of Al substitution and thermal annealing on magnetoelectric Ba0.5Sr1.5Zn2Fe12O22 investigated by the enhancement factor of 57Fe nuclear magnetic resonance.

The magnetoelectric properties of hexaferrite Ba0.5Sr1.5Zn2Fe12O22 are significantly improved by Al substitution and thermal annealing. Measuring the enhancement factor of 57Fe NMR, we found direct microscopic evidence that the magnetic moments of the L and S blocks are rotated by a magnetic field in such a way as to increase the net magnetic moment of a magnetic unit, even after the field is removed. Al substitution makes magnetoelectric property arise easily by suppressing the easy-plane anisotropy. The effect of thermal annealing is to stabilize the multiferroic state by reducing the number of pinning sites and the electron spin fluctuation. The transverse conic structure gradually changes to the alternating longitudinal conic structure where spins fluctuate more severely.