Magnetic structure in relation to the magnetic field induced ferroelectricity in Y-type hexaferrite Ba2−xSrxZn2Fe12O22

This thesis consists of in depth study of spin structure of magnetoelectric Y-type hexaferrite Ba2−xSrxZn2Fe12O22 system. Magnetoelectric and multiferroic materials are of high interest from information technology point of view, but traditional mechanism of magnetism and ferroelectricity makes it rare to observe both in single phase materials. One of the most widely studied non conventional mechanism where cross coupling of magnetism and ferroelectricity takes place in non-collinear spin systems is spin driven ferroelectricity by in-verse Dzyaloshinskii-Moriya (IDM) mechanism or spin currect mechanism (SC). The spin frustration leading to non-collinear spin arrangements also leads to low magnetic ordering temperature hinders room temperature realization of magnetoelectric coupling effect. Hexaferrite systems are an answer to high temperature magnetoelectric behavior in single phase materials. Y-type hexaferrite with composition Ba2−xSrxZn2Fe12O22 was the first hexaferrite to show magnetoelectric behavior, where a 2-fan planar spin structure was proposed, notcompatible with the magnetic field (H) induced ferroelectricity by IDM/SC mechanism. In depth investigation of the spin structure and its relationship with macroscopic field driven ferroelectricity was the central point of this thesis. The widely accepted spin structure of Y-type hexaferrite is a spin block model with large (L) and small (S) spin blocks, collinear in spin arrangements inside the blocks but between the blocks arrangements changes from collinear to spiral depending upon the chemical composition. With change in the Sr content in the composition Ba2−xSrxZn2Fe12O22 spin structure was reported to change from collinear to in plane spiral in earlier studies. The change in surements in small horizontal applied field perpendicular to (0 0 1) in horizontal scattering geometry. The zero and very low field a helical incommensurate phase with circular dichroism in the diffraction conditions in the magnetic satellite was found. First metamagnetic transition took place from incommensurate to commensurate spin structure when a small magnetic field (< 100 mT) was applied, characterized by (0 0 3/2) and (0 0 3/4) propagation. Equal intensity for incoming σ and π polarization for (0 0 3/2) reflection and absence of circular dichroism in both the satellite reflections leads to a non-chiral 4-fan structure as reported earlier. By fitting our calculated intensity ratios for linear and circular polarized x-ray shows coexistence of the 4-fan structure coexisting with the incommensurate low field phase and high field phase in some field ranges. The high field phase, i.e.the ferroelectric phase was a commensurate phase characterized by presence of only (0 0 3/2) magnetic satellite. Difference in intensity for (0 0 3/2) reflection with incoming σ and π polarization singles out this phase from the low field 4-fan phase and circular dichroism in the same satellite reveals vector spin chirality in thescattering plane. Maps to circular dichroism produces some spin chiral domains with both chirality, which get inverted upon application of magnetic field in reverse direction. In ver- tical field perpendicular to scattering plane in horizontal scattering geometry the circular dichroism disappears establishing the direction of vector spin chirality. In addition a new phenomena, position dependent linear dichroism appears, which gives same domain config-uration as that was observed in circular dichroism. For the high field phase already proposed 2-fan model is non chiral and can neither give ferroelectric polarization by IDM/SC mechanism nor circular dichroism and new model is required. Numerical calculations on various probable models to fit our experimental data established the spin structure is a transverse conical structure with both in-plane and out of plane components for both the L and S spin blocks. This transverse conical spin structure with spin chirality in the direction of H can give ferroelectric polarization by IDM/SC mechanism in the direction perpendicular to c and H. Unpolarized neutron diffraction experiments in our sample in vertical magnetic field are also consistent with the proposed model. The ferroelectric chiral phase is separated from the chiral zero field phase by a non chiral 4-fan phase in the magnetization process. The inversion of chirality by reverse magnetic field can be explained on the basis of absence of non chiral phase in the demagnetization process, facilitating the linkage between the chiral helical phase and the transverse conical phases. Inversion of chirality implies the flip of sign of polarization upon reversing magnetic field direction. Macroscopically no sign flipping was observed in previous reports which we attribute to the difference in composition of our samples. Spin structures were easily visible in magnetization curves, when magnetic field was applied perpendicular to c-axis. In three samples with x = 1.4,1.18 and 0.72 grown by flux methods, our refinement of single crystal x-ray diffraction data shows that Sr substitution causes local distortion and change in occupancies of Zn in the tetrahedral site which happens to be in the spin block boundary, accompanying a change in the Zn/Fe-O bond length. We speculate that the cumulative effect of local distortion, change in occupations of Zn and bond length change changes the superexchange interaction near the block boundary changing the spin structure. The chiral transverse conical spin structure was found from low field close to 80 mT at 100 K predicting H induced ferroelectricity from this field, considerably lower than that reported by macroscopic measurements. At 300 K similar shape of spin chiral domains with circular dichroism as well as with linear dichroism also found, suggesting transverse conical spin structure at room temperature (RT). Presence of (0 0 3/2) propagation was also verified by in field unpolarized neutron diffraction at 300 K. With input from all the scattering experiments and macroscopic magnetization measurements a new phase diagram was constructed which suggested field induced ferroelectricity in this transverse conical spin structure can be realized at very low H and RT in highly resistive samples. As a whole this thesis concludes that magnetic field induced ferroelectricity in y-typehexaferrite of composition Ba2−xSrxZn2Fe12 O22 is by IDM/SC mechanism in a transverse conical spin structure available at very low H and up to RT