Sol–gel derived nanocrystalline multiferroic BiFeO3 and R3+ (R=Er and Tm) doped therein: Magnetic phase transitions and enhancement of magnetic properties

Nanocrystalline BiFeO3 and rare earth ion doped BiFeO3 (Bi0.9R0.1FeO3, R=Er and Tm) were prepared by sol–gel method. Rietveld analysis of the X-ray diffractograms of the samples revealed that small amount of impurity phase of Bi2Fe4O9 was formed together with the desired phase. In the thermal variation of magnetic mass susceptibility (χm) of the samples, one sharp transition below TM (TM∼100 K, 50 K and 30 K for BiFeO3, Bi0.9Er0.1FeO3 and Bi0.9Tm0.1FeO3, respectively) was observed, which clearly hint the change of the domination of the ferromagnetic exchange interaction over the usual antiferromagnetic exchange interaction. Also, static magnetization (M) and susceptibility of each doped sample have been drastically enhanced compared to that of BiFeO3. The values of χm and M measured at different temperatures confirmed that the magnetic behavior of the doped systems has been dominated by the paramagnetic/ferromagnetic clusters below ∼TM. Another phase transition were observed in the χm vs. T curve of the samples at relatively higher temperature TB (∼260 K for BiFeO3, ∼220 K for Bi0.9Er0.1FeO3 and ∼180 K for Bi0.9Tm0.1FeO3), which may be attributed to the charge ordering transition. Ferroelectric hysteresis loops of the samples observed at 100 Hz confirmed the presence of ferroelectric ordering of the samples. Measured values of dielectric constants at 1 kHz of each sample in presence and absence of magnetic field confirmed a substantial magnetoelectric coupling of all the samples.

► Nanocrystalline BiFeO3 and Er3+ and Tm3+ ions doped BiFeO3 were prepared by sol–gel method.
► Magnetizations of the doped samples were enhanced.
► Different magnetic phase transitions were observed.
► Magnetocapacitance coefficient value indicates that the samples are type-II multiferroics.