  Good quality and fine grain Bi6Fe2Ti3O18 magnetic ferroelectric films with
single-phase layered perovskite structure have been successfully prepared via
metal organic decomposition (MOD) method. Results of low-temperature
magnetocapacitance measurements reveal that an ultra-low magnetic field of 10
Oe can produce a nontrivial magnetodielectric (MD) response in
zero-field-cooling condition, and the relative variation of dielectric
constants in magnetic field is positive, i.e., MD=0.05, when T<55K, but
negative with a maximum of MD=-0.14 when 55K<T<190K. The magnetodielectric
effect appears a sign change at 55K, which is due to transition from
antiferromagnetic to weak ferromagnetic; and vanishes abruptly around 190K,
which is thought to be associated with order-disorder transition of iron ion at
B site of perovskite structures. The ultra-low-field magnetodielectric
behaviour of Bi6Fe2Ti3O18 film has been discussed in the light of
quasi-two-dimension unique nature of local spin order in ferroelectric film.
Our results allow expectation on low-cost applications of detectors and
switches for extremely weak magnetic fields in a wide temperature range
55K-190K.
