Ilkyu Yang

Universal Ti-rich termination of atomically flat SrTiO3 (001), (110), and (111) surfaces

We have studied the surface termination of atomically flat SrTiO3 surfaces treated by chemical etching and subsequent thermal annealing, for all commercially available orientations (001), (110), and (111). Atomic force microscopy confirms that our treatment processes produce unit cell steps with flat terrace structures. We have also determined the topmost atomic layer of SrTiO3 surfaces through time-of-flight mass spectroscopy. We found that all three orientations exhibit a Ti-rich surface. Our observation opens doors for interface engineering along the [110] and [111] directions in addition to a well known [100] case, which widens the range of functional heterostructures and interfaces.

High density array of multiferroic nanoislands in a large area

We report the innovative fabrication of a high density array of multiferroic BiFeO3 (BFO) nanoislands on a conductive substrate in a large area. The anodic aluminum oxide (AAO) template has widely been used to fabricate highly arranged nanostructures, because of easy control of pore size and perfect hexagonal pore packing. The existing AAO mask-assisted pulsed laser deposition (PLD) method is limited to fabricating a nanoisland array in a small area. To supplement the shortcoming of this method, a thick AAO membrane in a large area was electrochemically detached and floated on polystyrene (PS) film without crack. Then, a nanoporous polystyrene (PS) template was prepared by dry etching with the thick AAO membrane mask, followed by spin coating of the BFO precursor on the PS template. After removing the PS template at high temperature, we prepared a high density array of multiferroic BFO nanoislands in a large area epitaxially grown on an STO:Nb (100) substrate. A high density array of BFO nanoislands in a large area showed both ferroelectricity of individual nanoislands obtained by piezoresponse force microscopy (PFM) and macroscopic magnetism measured by a superconducting quantum interference device (SQUID) based magnetic property measurement system (MPMS). A high density array of BFO nanoislands could be employed as a next-generation memory device capable of electric writing and magnetic reading (or vice versa).

Charge ordering, ferroelectric, and magnetic domains in LuFe2O4 observed by scanning probe microscopy

LuFe2O4 is a multiferroic system which exhibits charge order, ferroelectricity, and ferrimagnetism simultaneously below ∼230 K. The ferroelectric/charge order domains of LuFe2O4 are imaged with both piezoresponse force microscopy (PFM) and electrostatic force microscopy (EFM), while the magnetic domains are characterized by magnetic force microscopy (MFM). Comparison of PFM and EFM results suggests that the proposed ferroelectricity in LuFe2O4 is not of usual displacive type but of electronic origin. Simultaneous characterization of ferroelectric/charge order and magnetic domains by EFM and MFM, respectively, on the same surface of LuFe2O4 reveals that both domains have irregular patterns of similar shape, but the length scales are quite different. The domain size is approximately 100 nm for the ferroelectric domains, while the magnetic domain size is much larger and gets as large as 1 μm. We also demonstrate that the origin of the formation of irregular domains in LuFe2O4 is not extrinsic but intrinsic.

Direct observation of the M2 phase with its Mott transition in a VO2 film

In VO2, the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Here, we study a granular VO2 film using conductive atomic force microscopy and Raman scattering. Upon the structural transition from monoclinic to rutile, we observe directly an intermediate state showing the coexistence of monoclinic M1 and M2 phases. The conductivity near the grain boundary in this regime is six times larger than that of the grain core, producing a donut-like landscape. Our results reveal an intra-grain percolation process, indicating that VO2 with the M2 phase is a Mott insulator.

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.

Characterization of the insulator barrier and the superconducting transition temperature in GdBa2Cu3O7−δ/BaTiO3 bilayers for application in tunnel junctions

The optimization of the superconducting properties in a bottom electrode and the quality of an insulator barrier are the first steps in the development of superconductor/insulator/superconductor tunnel junctions. Here, we study the quality of a BaTiO3 tunnel barrier deposited on a 16 nm thick GdBa2Cu3O7−δ thin film by using conductive atomic force microscopy. We find that the tunnel current is systematically reduced (for equal applied voltage) by increasing the BaTiO3 barrier thickness between 1.6 and 4 nm. The BaTiO3 layers present an energy barrier of ≈1.2 eV and an attenuation length of 0.35–0.5 nm (depending on the applied voltage). The GdBa2Cu3O7−δ electrode is totally covered by a BaTiO3 thickness above 3 nm. The presence of ferroelectricity was verified by piezoresponse force microscopy for a 4 nm thick BaTiO3 top layer. The superconducting transition temperature of the bilayers is systematically suppressed by increasing the BaTiO3 thickness. This fact can be associated with stress at the interface...

Domain engineering of the metastable domains in the 4f-uniaxial-ferromagnet CeRu2Ga2B

In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu2Ga2B. We demonstrate a feasible control both globally and locally of domain shapes and sizes by the external field as well as a smooth transition from single stripe to bubble domains, which opens the door to future applications based on magnetic domain tailoring.

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.