On the problem of the “Colossal” magnetoelectric effect

In connection with the recent experiments on manganites, it is shown in the framework of the Landau theory that the magnetoelectric effect appears under a critical magnetic field. The signs of the spinorbit interaction constant and the helix wave number are interrelated. A helical structure and weak ferromagnetism occur simultaneously due to the first-order phase transition at a critical temperature, which depends on the magnetic field. The temperature range of the effect increases with the field. In this range, the magnetic moment increases and the correction to the electric polarization, being negative, also increases in magnitude with the magnetic field above its threshold. These changes are directly related to the increase in the helix wave number with a decrease in temperature below the phase transition point.