Magnetization and magnetoelectric effect in ${\text{Bi}}_{1-x}{\text{La}}_x{\text{FeO}}_3$ $\left(0\le x\le 0.15\right)$

We report on magnetization and magnetocapacitance measurements in the ${\text{Bi}}_{1-x}{\text{La}}_x{\text{FeO}}_3$ series for $0\le x\le 0.15$. We confirm that doping with La reduces the threshold magnetic field $H_{\text{c}}$ for cancelling the magnetic spiral phase, and we show that $H_{\text{c}}$ decreases as the La content increases up to $x=0.15$, which is the highest concentration for maintaining the noncentrosymmetric rhombohedral structure of ${\text{BiFeO}}_3$. Measurements of the dielectric constant as a function of magnetic field in the series also show a maximum magnetocapacitance for $x=0.15$.

Figure 1

Powder x-ray diffraction pattern in ${\text{Bi}}_{1-x}{\text{La}}_x{\text{FeO}}_3$ $\left(0\le x\le 0.25\right)$ at 300 K using $\text{Cu}K\alpha$ radiation. The asterisks correspond to the lines of the ${\text{Bi}}_{25}{\text{FeO}}_{39}$ impurity, especially visible for $x=0$. Insets: enlarged view near (a) $2\theta =39°$ and (b) $50°$ showing the progressive disappearance of the rhombohedral splitting (arrows) with increasing La content.

Figure 2

Schematic representation of the magnetic and electric polarization vectors in ${\text{BiFeO}}_3$. The magnetic spiral is taken to lie in the $zOy$ plane, the angle $\theta$ determining the current position of the vector $\mathit{L}\cdot {\mathit{P}}^{\mathbf{0}}$ is the spontaneous electric polarization along $Oz$ and ${\mathit{m}}_{\mathit{s}}$ the local net moment.

Figure 3

(Color online) (a) Magnetization curves at 20 K in ${\text{Bi}}_{1-x}{\text{La}}_x{\text{FeO}}_3$ ceramic samples for $0\le x\le 0.15$. The lines are guide for the eyes; (b) calculated $m\left(H\right)$ curves in a polycrystalline sample for different values of the anisotropy energy density $K$.

Figure 4

(Color online) In the ${\text{Bi}}_{1-x}{\text{La}}_x{\text{FeO}}_3$ series: (a) magnetic field dependence of the normalized dielectric constant at 100 K; (b) threshold fields for deviation of $m\left(H\right)$ from a linear law (full squares) and for deviation of $\epsilon \left(H\right)$ from a constant (open circles).