Shear-strain-mediated magnetoelectric effects revealed by imaging

Abstract

Large changes in the magnetization of ferromagnetic films can be electrically driven by non-180 ferroelectric domain switching in underlying substrates, but the shear components of the strains that mediate these magnetoelectric effects have not been considered so far. Here we reveal the presence of these shear strains in a polycrystalline film of Ni on a 0.68Pb(Mg 1/3 Nb 2/3)O 3-0.32PbTiO 3 substrate in the pseudo-cubic (011) pc orientation. Although vibrating sample magnetometry records giant magnetoelectric effects that are consistent with the hitherto expected 90 rotations of a global magnetic easy axis, high-resolution vector maps of magnetization (constructed from photoemission electron microscopy data, with contrast from x-ray magnetic circular dichroism) reveal that the local magnetization typically rotates through smaller angles of 62-84. This shortfall with respect to 90 is a consequence of the shear strain associated with ferroelectric domain switching. The non-orthogonality represents both a challenge and an opportunity for the development and miniaturization of magnetoelectric devices. Wisconsin-Madison (J.-M. H.). D. P. acknowledges funding from the Agència de Gestió d'Ajuts Universitaris i de Recercaa-Generalitat de Catalunya (Grant 2014 BP-A 00079). We thank Diamond Light Source for time on beamline I06 (proposal SI-8876), and we thank Sen Zhang for discussions. Author contributions M.G. initiated the study. M.G. and N.D.M. led the project with S.S.D. R.M., R.P.C. and C.H.W.B. were responsible for the growth of thin film Ni. The collection and preliminary analysis of PEEM data were performed by M.G., with assistance from X.M., L.C.P and W.Y. All other experimental work was performed by M.G. F.M. and S.S.D. were responsible for constructing PEEM vector maps, and the subsequent pixel by pixel analysis. D.P. performed image and data processing. N.D.M. proposed the pixel-by-pixel analysis of PEEM vector maps that led to the key finding of sub 90° magnetization rotation. J. M.H. identified and calculated the shear strain that accompanies ferroelectric domain switching in PMN-PT. M.G. identified the resulting principal axes of strain and hence magnetic easy axes. M.G. and N.D.M. interpreted the observed magnetoelectric effects.

Interplay between morphology and magnetoelectric coupling in Fe/PMN-PT multiferroic heterostructures studied by microscopy techniques

“…These data show that by simply reversing the PMN-PT polarization, the average magnetization direction of the Fe layer is drastically changed in absence of any magnetic fields [32]. It is clear that in the P up case M strongly prefers a different orientation compared to the P down case.…”

“…After probing magnetoelectric coupling at the macroscopic 176 and mesoscopic scale, we employed XMCD-PEEM to reach 177 submicrometer resolution [11,32,39,40] and to study the evo-178 lution of magnetic domains as a function of out-of-plane 179 ferroelectric polarization of the substrate. With a field of view of 50 μm, the imaged area is comparable with the one probed by XMCD measurements.…”

“…This can be used to modify the 67 magnetic state of a film (or nanostructures) deposited on top 68 of it [29][30][31]. Considering shear strain, it has been shown that 69 a rotation of magnetization M by an angle of about 62°is 70 expected when the polarization P switches from out of plane 71 to in plane with a rotation of 71°or 109° [32]. In the (001) cut 72 instead, the in-plane strain at remanence is modified only if the 73 (c) Two-terminal current vs electric field characteristic measured during the out-of-plane poling of PMN-PT substrates.…”

Large magnetoelectric coupling in multiferroic oxide heterostructures assembled via epitaxial lift-off

“…5f). Although the clockwise and anticlockwise nature of these rotations could be explained in terms of the ambipolar shear strains associated with rhombohedral ferroelectric domain switching 54 , the very different rotation magnitudes imply the presence of an additional factor. By assuming this additional factor to be a spatially varying uniaxial magnetic anisotropy, due to spatially varying stress associated with the formation of the observed cracks (Supplementary Note 5), a magnetic free energy model was able to approximately reproduce both the local and macroscopic magnetoelectric effects (Supplementary Note 12).…”

Giant non-volatile magnetoelectric effects via growth anisotropy in Co40Fe40B20 films on PMN-PT substrates

“…28,29 Large voltage-driven switches of net magnetization have only been achieved in the latter systems, [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] for example, via strain from ferroelectric substrates of BaTiO 3 (BTO) 30,31 and [24][25][26] However, the vast majority of strain-mediated CMEs are volatile because the ferroelectric substrates show voltage-strain "butterfly" characteristics that are single-valued at zero electric field. 15,[30][31][32][33][34][35] Nonvolatile CMEs include 90 rotations [24][25][26] and reversals 28,29 but suffer the need for magnetic-field assistance, 26,28 the need for setting an inplane polarization before deposition, 24,25 or inhomogeneity. 29 Here, we use voltage-driven anisotropic strain from ferroelectric substrates of PMN-PT (011) to control the magnetization in ferromagnetic films of Co 40 Fe 40 B 20 (CFB).…”