Stable and Tunable Current-Induced Phase Transition in Epitaxial Thin Films of Ca2RuO4

Owing to the recent discovery of the current-induced metal-insulator transition and unprecedented electronic properties of the concomitant phases of calcium ruthenate Ca2RuO4, it is emerging as an important material. To further explore the properties, the growth of epitaxial thin films of Ca2RuO4 is receiving more attention, as high current densities can be applied to thin-film samples and the amount can be precisely controlled in an experimental environment. However, it is difficult to grow high-quality thin films of Ca2RuO4 due to the easy formation of the crystal defects originating from the sublimation of RuO4; therefore, the metal-insulator transition of Ca2RuO4 is typically not observed in the thin films. Herein, a stable current-induced metal-insulator transition is achieved in the high-quality thin films of Ca2RuO4 grown by solid-phase epitaxy under high growth temperatures and pressures. In the Ca2RuO4 thin films grown by ex situ annealing at >1200 degrees C and 1.0 atm, continuous changes in the resistance of over 2 orders of magnitude are induced by currents with a precise dependence of the resistance on the current amplitude. A hysteretic, abrupt resistive transition is also observed in the thin films from the resistance-temperature measurements conducted under constant-voltage (variable-current) conditions with controllability of the transition temperature. A clear resistive switching by the current-induced transition is demonstrated in the current-electric-field characteristics, and the switching currents and fields are shown to be very stable. These results represent a significant step toward understanding the high-current-density properties of Ca2RuO4 and the future development of Mott-electronic devices based on electricity-driven transitions

Dynamics of an Electrically Driven Phase Transition in Ca2RuO4 Thin Films: Nonequilibrium High‐Speed Resistive Switching in the Absence of an Abrupt Thermal Transition

Abstract In Mott‐type resistive switching phenomena, which are based on the metal–insulator transition in strongly correlated materials, the presence of an abrupt temperature‐driven transition in the material is considered essential for achieving high‐speed and large‐resistance‐ratio switching. However, this means that the freedom of material/device design in applications is significantly reduced for this type of switching by the strict requirement of transition abruptness. Here, high‐speed, abrupt resistive switching with a switching time of 140 ns is demonstrated in epitaxial films of Ca2RuO4/LaAlO3 (001), which is a material with a nonthermal metal–insulator transition driven by current, despite the complete absence of an abrupt thermal transition in the resistivity–temperature characteristics. Highly smooth negative‐differential‐resistance behavior, very high cycling stability, and an endurance over 106 cycles are also demonstrated in the current–voltage and current–time characteristics, which confirm the nonstochastic nature of the abrupt switching. These results suggest that strict control of the resistivity–temperature characteristics is not necessarily required in a material with a nonthermal‐type metal–insulator transition to obtain high‐speed resistive switching because of the independence of the dynamics from those of the thermal transition, and this phenomenon potentially has important advantages in resistive switching applications

Magnetic Particle Imaging for Biomedical Applications

An imaging technique for the detection of magnetic nanoparticles (MNPs) has been studied for biomedical applications. First, a detection system, which measures the second harmonic signal from the MNPs, was designed and developed based on nonlinear propertied of the MNPs under strong excitation field. By measuring the second harmonic signal, interference of the excitation field can be significantly reduced, when compared to the case of measuring the fundamental signal. Using this system, we could detect 100 μg of MNPs which were located at z=30mm under the detection coil. The detected signal field was 128 pT at the measurement frequency of 38.6 kHz, and the signal to noise ratio was as high as 10. Next, a field map from the MNPs was measured, and a clear contour map was obtained. The obtained contour map agreed well with numerical simulations. Using the contour map, we could directly estimate the position of markers. Finally, a field map from two markers, which were separated with a distance Δx, was measured in order to study the ability of position identification of two markers. It was shown that we could distinguish two markers located at z=20mm if they were separated by Δx=20mm. This simple detection system could be applied for the detection of sentinel lymph node during the biopsy