Hikaru Nomura

Switching of perpendicular exchange bias in Pt/Co/Pt/α-Cr2O3/Pt layered structure using magneto-electric effect

Switching of the perpendicular exchange bias polarity using a magneto-electric (ME) effect of α-Cr2O3 was investigated. From the change in the exchange bias field with the electric field during the ME field cooling, i.e., the simultaneous application of both magnetic and electric fields during the cooling, we determined the threshold electric field to switch the perpendicular exchange bias polarity. It was found that the threshold electric field was inversely proportional to the magnetic field indicating that the EH product was constant. The high EH product was required to switch the exchange bias for the film possessing the high exchange anisotropy energy density, which suggests that the energy gain by the ME effect has to overcome the interfacial exchange coupling energy to reverse the interfacial antiferromagnetic spin.

Equilibrium surface magnetization of α-Cr2O3 studied through interfacial chromium magnetization in Co/α-Cr2O3 layered structures

We show experimental evidence of the equilibrium surface magnetization of α-Cr2O3(0001) by studying chromium magnetization at the Co(111)/α-Cr2O3(0001) interface. The soft X-ray magnetic circular dichroism intensity from uncompensated Cr spins was found to be almost independent of the interface roughness and exchange anisotropy energy. Moreover, no exchange bias training effect was found, except in the transition temperature regime, suggesting that the antiferromagnetic spin structure, including the interface spin arrangement, is in equilibrium. Furthermore, the exchange bias polarity was switched by magnetoelectric field cooling, which is also a salient feature of the surface magnetization of α-Cr2O3(0001).

High-Speed Phase-Modulation Atomic Force Microscopy in Constant-Amplitude Mode Capable of Simultaneous Measurement of Topography and Energy Dissipation

We have developed high-speed phase-modulation atomic force microscopy (PM-AFM) in a constant-amplitude (CA) mode. Using this imaging mode, we have theoretically demonstrated that energy dissipation due to tip–sample interaction can be obtained from the excitation amplitude of a cantilever. Moreover, we have found that the photothermal excitation method is better than the acoustic excitation method for cantilever oscillation in liquids. For the first time, we have demonstrated that a homebuilt high-speed PM-AFM in the CA mode has the capability to simultaneously measure the topography and energy dissipation with a material-specific contrast for a PS/PIB polymer-blend film.

NAND/NOR Logical Operation of a Magnetic Logic Gate with Canted Clock-Field

A magnetic logic gate (MLG), which is based on magnetic quantum dot cellular automata (MQCA), is capable of NAND/NOR logical operations. Here, we demonstrate that a single MLG can perform NAND/NOR logical operation by changing the binary state of the input dots in a single structure with a canted clock-field. To write digital information into the dots, we developed a magnetization manipulation method based on magnetic force microscopy. By using the canted clock-field from two general write wires in magnetoresistive random access memory (MRAM), we would locally activate the logical operation. MLG-MRAM will open the way to create high functional circuits based on MQCA.

Magnetic logic devices composed of permalloy dots

Magnetic logic devices have been investigated by micromagnetics simulation and experiment. The simulation shows that the magnetic logic devices composed of 4 elliptical permalloy dots perform both the NAND and NOR logic operations. The experiments indicate that the distance between adjacent dots must be shorter than 50 nm and 70 nm along the long axis and the short axis of the dots, respectively. The micro-fabricated test devices show the above logic operations.

Magnetic force microscopy using tip magnetization modulated by ferromagnetic resonance

In magnetic force microscopy (MFM), the tip-sample distance should be reduced to analyze the microscopic magnetic domain structure with high spatial resolution. However, achieving a small tip-sample distance has been difficult because of superimposition of interaction forces such as van der Waals and electrostatic forces induced by the sample surface. In this study, we propose a new method of MFM using ferromagnetic resonance (FMR) to extract only the magnetic field near the sample surface. In this method, the magnetization of a magnetic cantilever is modulated by FMR to separate the magnetic field and topographic structure. We demonstrate the modulation of the magnetization of the cantilever and the identification of the polarities of a perpendicular magnetic medium.

Effect of Surface Stress around the SA Step of Si(001) on the Dimer Structure Determined by Noncontact Atomic Force Microscopy at 5 K

Using noncontact atomic force microscopy (NC-AFM) at 5 K, we investigate the effect of surface stress around the S A step of Si(001) on the buckling of a surface dimer. Both topographic and dissipa...

Dissipative force modulation Kelvin probe force microscopy applying doubled frequency ac bias voltage

The authors propose a surface potential measurement technique using dissipative force modulation (DM) method with an ac bias voltage of doubled harmonic frequency (2ω) of the oscillating cantilever (2ωDM method). The effect of the stray capacitance between a cantilever and a sample on electrostatic force spectroscopy/Kelvin probe force microscopy measurement is almost completely removed in 2ωDM method, since the distance dependence of the modulated electrostatic force increases from 1∕z to 1∕z2. 2ωDM method has an advantage of high force sensitivity due to the high Q factor of the cantilever in vacuum. The authors demonstrate quantitative surface potential measurement by using dissipative tip-sample interactions.

Switching surface polarization of atomic force microscopy probe utilizing photoisomerization of photochromic molecules

An attempt to develop an atomic force microscopy (AFM) probe with optically switchable polarization is described. Modification with a single molecular layer of photochromic molecules was attempted onto a Si substrate that is a prototype for a probe surface. Polarization switching caused by alternate irradiation of UV and visible lights were detected using the electrostatic force?>spectroscopy (EFS) technique. Si substrates modified with spiropyran and azobenzene exhibited reversible polarization switching that caused changes in CPD of about 100 and 50 mV, respectively. Modification with spiropyran was also attempted onto a Si probe and resulted in a CPD change of about 100 mV. It was confirmed that modification of an AFM probe or substrate with a single molecular layer of photochromic molecules can generate surface polarization switching of a mechanically detectable level.

Force Microscopy Imaging of Rest Atom on Si(111)7×7 Surface under Strong Tip–Surface Interaction

We investigated NC-AFM imaging mechanism on a Si(111)7×7 surface under the strong interaction condition working between dangling bonds on the tip and the surface atoms. The surface adatom appeared in the image as a bright spot, which became larger with stronger interaction condition. Upon further strong interaction, we could detect the rest atoms in the unfaulted half unit of the 7×7 surface image as prominent sharp bright dots with 1 A width. Besides, the dot appeared with the lateral shift by 2 A as compared to the surface adatom image. We found the strong covalent interaction between the dangling bonds on the tip atom and the surface atoms influenced onto the image variation.