Tohoku University · Materials Science
Professor Hiroshi Naganuma's research lab specializes in the development and characterization of multiferroic thin films, with a primary focus on bismuth ferrite (BiFeO3) systems. The lab investigates the enhancement of ferroelectric and magnetic properties through elemental doping—particularly cobalt and copper—aiming to achieve room-temperature multiferroicity with low leakage current and high coercivity. Key research directions include optimizing film deposition techniques such as chemical solution deposition, understanding conduction mechanisms in leaky ferroelectrics, and exploring spintronic applications such as spin oscillators and magnetic tunnel junctions. The lab also addresses challenges in achieving low-power, high-frequency spin dynamics for next-generation nanoscale electronic devices.
Figures are computed from collected data and may differ slightly.
Cr, Mn, Co, Ni, and Cu were added to polycrystalline BiFeO3 films, and their influence on the ferroelectric, electrical, and magnetic properties was investigated. All the additives except Ni reduced the leakage current density in the high electric field region. The addition of Cu and Co decreased the coercive field without reducing remanent polarization. The addition of Co caused spontaneous magnetization at room temperature, which exhibited a large coercive field of 16kOe at 10K. It was reveale
Polycrystalline BiFeO3 film has been fabricated by a chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) substrates. A ferroelectric hysteresis loop showed a high remanent polarization of 47μC∕cm2 at room temperature. Leakage current density was on the order of 10−1A∕cm2 at 100kV∕cm, indicating the high leakage current density in the present BiFeO3 film. The leakage current mechanism could be considered as follows: Ohmic conduction at low electric field and Poole-Frenkel trap-assisted conduction
The electrical properties of leaky ferroelectric BiFeO3 thin films were evaluated by using a high-speed positive-up-negative-down (PUND) measurement technique. The leakage current density was estimated from the gradient of the pulse response when a constant electric field was applied. The relative permittivity was estimated from an abrupt decrement when the applied field was removed. The twofold remanent polarization without the influence of the leakage current was estimated by subtracting the c
Spin dynamics excited by spin-polarized current in magnetic tunnel junctions (MTJs) is potentially useful in nanoscale electrical oscillation sources and detection devices. A spin oscillator/detector should work at a high frequency, such as that of a millimeter-wave, where the quality of a semiconductor device is restricted by carrier mobility, the CR time constant, and so on. Developers of spin systems for practical use need to find out how to excite spin dynamics (i) in the millimeter-wave reg
Both the ferroelectric and magnetic properties of polycrystalline BiFeO3 films fabricated by chemical solution deposition were enhanced by adding small amounts of cobalt. Addition of 3at.% cobalt to BiFeO3 films increased the remanent polarization from 49to72μC∕cm2 and decreased the electric coercive field from 0.54to0.44MV∕cm. The ferroelectricity degraded when the cobalt concentration exceeded 9at.% due to the formation of the secondary phases of Bi2Pt. The saturation magnetization was drastic
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