Magnetic Ordering in Relation to the Room-Temperature Magnetoelectric Effect of ${\mathrm{Sr}}_3{\mathrm{Co}}_2{\mathrm{Fe}}_{24}{\mathrm{O}}_{41}$

The origin of a room-temperature magnetoelectric (ME) effect has been examined by means of neutron powder diffraction measurements for a $Z$-type hexaferrite ${\mathrm{Sr}}_3{\mathrm{Co}}_2{\mathrm{Fe}}_{24}{\mathrm{O}}_{41}$. The temperature and magnetic-field dependence of the electric polarization $P$ and several magnetic Bragg reflections show that a commensurate magnetic order with a (0,0,1) propagation vector has an intimate connection with the ME effect. The room-temperature ME effect can be understood in terms of the appearance of $P$ which is induced by a transverse conical spin structure through the inverse Dzyaloshinskii-Moriya mechanism in analogy with $Y$-type hexaferrites.

Figure 1

Schematic crystal and proposed magnetic structures of (a) $Y$-type and (b) $Z$-type hexaferrites. In the $Y$-type hexaferrite, the proper-screw or longitudinal conical magnetic structure transforms into the transverse conical one which allows finite $P$ by applying $H$ along the $c$ plane [9, 10]. When the additional block is inserted into the $Y$-type hexaferrite, the $Z$-type structure is obtained.

Figure 2

Neutron powder diffraction (NPD) patterns of (a) SCFO and (b) BCFO measured at two temperatures. Major Bragg positions are indicated by arrows and indices of the space group $P{6}_3/mmc$. In the data taken at 566 K of SCFO, the calculated curve obtained by the Rietveld fitting with the space group $P{6}_3/mmc$ and the ferrimagnetic structure [15, 16] is shown by the solid curve. At the bottom of (a), the Bragg positions are indicated by the vertical lines and the difference between the observed and calculated data is also shown. The insets show NPD patterns around the 009 reflection at various temperatures.

Figure 3

(a) $T$ dependence of the peak intensity of the $10L$ peak and the integrated intensities of the 008 and 009 reflections estimated from the neutron profiles of SCFO and BCFO. The nuclear components of the $10L$ peak ($n_{10L}$) and the 008 reflection ($n_{008}$) are shown by gray lines. (b) $T$ dependence of $M$ and $P_{\max }$ for the polycrystalline samples of SCFO and BCFO. The $P_{\max }$ values of BCFO are multiplied by 5 for presentation. Inset: The integrated intensities of the $00l_o$ magnetic reflections observed at 25 K in SCFO are compared with those calculated using the transverse conical structure shown in Fig. 1b.

Figure 4

(a) $H$ dependence of the peak intensity of the $10L$ peak and the integrated intensities of the 008 and 009 reflections estimated from the neutron profiles of SCFO. The inset shows the NPD profiles around the 008 and 009 reflections at several magnetic fields. (b) $H$ dependence of $M$ and $P$ for the polycrystalline samples of SCFO and BCFO at several temperatures. The $P$ values of BCFO are multiplied by 5 for presentation.