  The phase diagram of a quantum paraelectric antiferromagnet EuTiO3 under an
external electric field was calculated using Landau-Ginzburg-Devonshire theory.
It was shown that the application of an external electric field E leads to the
appearance of a ferromagnetic phase due to the magnetoelectric coupling. In
particular, electric field application decreases the transition temperature
TAFM to antiferromagnetic (AFM) phase and induces ferromagnetic (FM) phase, so
that at some E field larger than the critical field (Ecr), TFM becomes higher
than TAFM and the FM phase appears. Note that Ecr increases and magnetization
decreases as the temperature increases. The value of the critical field Ecr =
0.40*10^6 V/cm we calculated appeared close to the value Ecr = 0.5*10^6 V/cm
obtained recently by Ryan et al. with the help of density functional theory for
EuTiO3 film under a compressive strain produced by substrate. At the fields E
>0.83*10^6 V/cm, AFM disappears for all considered temperatures and so FM
becomes the only stable magnetic phase. We find that ferromagnetic phase can be
induced by an E-field in other paraelectric antiferromagnet oxides with a
positive AFM-type magnetoelectric (ME) coupling coefficient and negative
FM-type ME coupling coefficient. In particular, the critical E-field was
estimated for another paraelectric antiferromagnet Sr0.7Ba0.3MnO3 as
0.2x10^5V/cm at 0 K. Analysis of the dependence of magnetization and
antimagnetization on the external electric field and the polarization induced
by the field, which yields the magnetoelectric coupling, is reported. The
results show the possibility to control multiferroicity, including the FM and
AFM phases, with help of an electric field application.
