Effect of chemical substitution on the Néel temperature of multiferroic ${\text{Bi}}_{1-x}{\text{Ca}}_x{\text{FeO}}_3$

Multiferroic ${\text{BiFeO}}_3$ ceramics have been doped with Ca and it is found that the magnetic Neel temperature $\left(T_{\text{Néel}}\right)$ increases as Ca concentration increases, at a rate of 0.66 K per 1% Ca (molar). The smaller ionic size of Ca compared with Bi results in a contraction of the lattice, suggesting that Ca doping can be regarded as a proxy for hydrostatic pressure, with an equivalence of $1\%\text{Ca}=0.3\text{GPa}$. Combining these results, we argue that hydrostatic pressure should increase the magnetic transition temperature of ${\text{BiFeO}}_3$ at a rate around $\partial T_N/\partial P\sim 2.2\text{K}/\text{GPa}$. Our results also suggest that pressure (chemical or hydrostatic) may be used to bring the ferroelectric critical temperature $T_c$ and the magnetic $T_{\text{Néel}}$ closer together, thereby enhancing magnetoelectric coupling, provided that electrical conductivity can be kept sufficiently low.

Figure 1

(Color online) Evolution of the perovskite pseudocubic unit cell as a function of Ca concentration. The line is a least-squares fit to the experimental results, yielding a compression rate of $-0.003\text{Å}$ per 1% mol of Ca.

Figure 2

(Color online) Differential scanning calorimetry of 7% Ca sample, showing a clear peak in specific heat at the Néel temperature. Inset: $T_{\text{Néel}}$ as determined from specific heat as a function of Ca concentration.

Figure 3

(Color online) Magnetic ac susceptibility measurement for BFO-3% Ca, showing the distinct peak at the Néel temperature. Inset: $T_N$ as a function of composition; the straight line is a least-squares fit.