Magnetic ordering in relation to room-temperature magnetoelectric effect of Z-type hexaferrite

Recent extensive studies on magnetoelectric multiferroics have revealed that a new class of multiferroics, that is, magneticallyinduced ferroelectrics in which ferroelectricity is induced by complex magnetic orders, such as spiral orders, exhibit giant magnetoelectric effects, remarkable changes in electric polarization in response to a magnetic field. Although these magnetically-induced ferroelectrics are attracting great interest in terms of both basic and technological points of view, none of the existing materials show magnetoelectric (ME) effect by applying a lowmagnetic field at ROOM TEMPERATURE. Recently, however, it is discovered that a Z-type hexaferrite Sr3Co2Fe2O41 (SCFO) shows the ME effect with a small magnetic field even at room temperature.[1] Although this discovery is important from an application standpoint, the origin of the ME effect of the Z-type hexaferrite is still unclear. To point out a route toward generalization of the promising room-temperature ME effect, it is necessary to clarify the underlying mechanism. For this purpose, we carried out powder neutron diffraction measurements on the Z-type hexaferrites (Sr,Ba)3Co2Fe24O41 over wide temperature and magnetic-field ranges. The first stepwas to search for T-dependent diffraction peaks in the absence of H. Figure 1 shows neutron powder diffraction (NPD) patterns of SCFO measured at T=150 K and 566 K. For the data at 566 K, which is below the magnetic ordering temperature (=670 K), a Rietveld analysis was carried out using the space group P6 3 mmc and the ferrimagnetic structure reported by Tachibana and coworkers[2,3]. As shown in Fig.1, the profile at 566 K is well explained by these chemical and magnetic structures. Compared with the NPD profile at 566 K, additional peaks exist evidently at 150 K: 00l o reflections (l o =odd), which are forbidden in the space group P6 3 mmc. In addition, intensities of 00l e Bragg reflections (l e =even) are enhanced in the profile at 150 K. The 00l o reflections appear below 400 K and increase in intensity with decreasing T. We conclude that they are the magnetic reflections characterized by the magnetic propagation vector of (0,0,1), indicating that 00l o corresponds with +(0,0,1) satellite points of 00l e . To elucidate the effect of H on the magnetic structure which develops below ̃400 K, the magnetic reflections of SCFO were also measured as a function of H at 10 K and 300 K. Significant changes in the neutron intensities were observed by applying H. The intensities of the 10L and 008 reflections increase with increasing H and are saturated above 3 T. By contrast, the intensity of the 009 reflection decreases and disappears above 3 T, indicating that the magnetic structure having the (0,0,1) propagation vector exists only below 3 T. Based on these results of the Tand H-dependence of the electric polarization P, we conclude that the magnetic structure characterized by the propagation vector (0,0,1) is the origin of the ME effect in the Z-type hexaferrite system. These magnetic reflections can be ascribed to a transverse conical spin structure which allows finite polarization in terms of the inverse Dzyaloshinskii-Moriya mechanism. Our results provide a natural explanation of the low-field magnetoelectric effect at room temperature in the Z-type hexaferrite.