Jinho Yang

Spatially resolved penetration depth measurements and vortex manipulation in the ferromagnetic superconductor ErNi2B2C

We present a local probe study of the magnetic superconductor ErNi2B2C, using magnetic force microscopy at sub-Kelvin temperatures. ErNi2B2C is an ideal system to explore the effects of concomitant superconductivity and ferromagnetism. At 500 mK, far below the transition to a weakly ferromagnetic state, we directly observe a structured magnetic background on the micrometer scale. We determine spatially resolved absolute values of the magnetic penetration depth λ and study its temperature dependence as the system undergoes magnetic phase transitions from paramagnetic to antiferromagnetic, and to weak ferromagnetic, all within the superconducting regime. We estimate the absolute pinning force of Abrikosov vortices, which shows a position dependence and temperature dependence as well, and discuss the possibility of the purported spontaneous vortex formation.

Magnetic domain tuning and the emergence of bubble domains in the bilayer manganite La2-2xSr1+2xMn2O7 (x=0.32)

Here, we report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite La2–2xSr1+2xMn2O7 (with x = 0.32). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripelike domains, followed by the formation of magnetic bubbles due to a field- and temperature-dependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.

Proximity to a commensurate charge modulation in IrTe2−xSex (x = 0 and 0.45) revealed by Raman spectroscopy

Optical phonons are employed to probe a structural phase transition of IrTeSex at K for x = 0 and K for x = 0.45, which is dictated by Ir dimerization with charge modulations. Phonon Raman spectra show a discontinuous change at TC and thermal hysteresis. A mode involving out-of-plane motions of Te atoms is mostly affected by Se doping. This highlights the role of interlayer Te?Te interactions in inducing the structural instability. For temperatures below TC both x = 0 and x = 0.45 compounds exhibit commonly five zone-folded peaks for the in-plane mode and six zone-folded peaks for the mode. This is taken as a signature of a respective quintupling and sextupling of the unit cell. Our results, on the one hand, are consistent with a 1/5 charge modulation reported by previous studies and, on the other hand, indicate that the ground state of IrTeSex lies in proximity to a 1/6 commensurate charge modulation phase.

Construction of a 3He magnetic force microscope with a vector magnet

We constructed a (3)He magnetic force microscope operating at the base temperature of 300 mK under a vector magnetic field of 2-2-9 T in the x-y-z direction. Fiber optic interferometry as a detection scheme is employed in which two home-built fiber walkers are used for the alignment between the cantilever and the optical fiber. The noise level of the laser interferometer is close to its thermodynamic limit. The capabilities of the sub-Kelvin and vector field are demonstrated by imaging the coexistence of magnetism and superconductivity in a ferromagnetic superconductor (ErNi2B2C) at T = 500 mK and by probing a dipole shape of a single Abrikosov vortex with an in-plane tip magnetization.

Ultra-low-temperature Vector-magnet Magnetic-force Microscope

We have built a He magnetic force microscope (MFM), operating at a base temperature of 300 mK with a vector field of Z  9 T, X  2 T, Y  2 T for the three axes, respectively. Our microscope has a capability of scanning multiple samples in a single cooldown. Together with the comparative Meissner technique, this capability allows measurements of the magnetic penetration depth as a function of the doping level, which is important for understanding the nature of the purported quantum critical point within a superconductor dome in high-Tc, pnictides and heavy-fermion superconductors. Here, we show applications of our novel microscope to investigate unconventional magnetic materials and magnetic superconductors after it had been built at IBS-POSTECH in 2014.