Masafumi Kobune

© 2020 The Japan Society of Applied Physics. Microrod-type CoFe2O4(CFO)/Bi3.25Nd0.65Eu0.10Ti3O12(00ℓ) (BNEuT) composite thin films were fabricated by a combination of high-temperature sputtering, reactive ion etching, and metal organic chemical vapor deposition (MOCVD) on Pt(100)/MgO(100) substrates. The substrate temperature for MOCVD was varied from 450 °C to 600 °C to examine its effect on the structural, magnetic, and ferroelectric properties. The substrate temperature affects the compressive stress at the interface between the CFO and BNEuT. The surface morphology changed drastically above 550 °C. The room temperature magnetization-magnetic field hysteresis loops for the films showed clear ferromagnetic hysteresis loop and magnetic shape anisotropy. The room temperature polarization-electric field (P-E) hysteresis loops for the films showed a clear ferroelectric hysteresis loop, and slightly leaky P-E hysteresis loop. The coercive field increased slightly with increasing substrate temperature. Judging from the structural, ferromagnetic, and ferroelectric properties, the film deposited at 550 °C has potential as an excellent multiferroic material.

Using a a-axis-oriented epitaxial (Bi3.25Nd0.65Eu0.10)Ti3O12 (BNEuT) thin films with a microplate-like shape as a ferroelectric pillar material, micropillar-type CoFe2O4 (CFO)/BNEuT(h00)/Nb:TiO2(101) composite multiferroic films were fabricated by metal organic chemical vapor deposition. The deposition time was varied from 90-150 min to examine its effect on the structural, ferroelectric, and ferromagnetic characteristics of the films. All the films exhibit single-phase cobalt ferrite with a cubic inverse-spinel structure. The deposited CFO films have a similar columnar structure with a comparatively good step coverage of 37%-66%. The room-temperature magnetization-magnetic field hysteresis loop of the films showed a clear ferromagnetic hysteresis loop, and coercivity decreased from 1.1 to 0.9 kOe with increasing the deposition time. The room-temperature polarization-electric field hysteresis loop of the films showed a ferroelectric hysteresis loop, and the effect of the leakage current was the smallest for a deposition time of 120 min.