Spatially resolved strain-imprinted magnetic states in an artificial multiferroic

Artificial multiferroic systems, in which novel properties can emerge from elastic coupling between piezoelectric and magnetostrictive phases, are a promising route to obtain significant room-temperature magnetoelectric coupling at the nanoscale. In this work, we have used element-specific soft x-ray photoemission electron microscopy to spatially resolve the effects of ferroelectric BaTiO${}_3$ on the magnetic properties of ferromagnetic layers of CoFe${}_2$O${}_4$ and NiFe${}_2$O${}_4$. Resulting from the large magnetoelastic anisotropy present in these artificial multiferroic systems, a modification of the orientation and symmetry of the magnetic easy axes of the ferromagnetic film upon changing the underlying domain structure of the ferroelectric has been observed. This opens the possibility to strain-imprint magnetic states in these systems.

Figure 1

(a) Fe edge PEEM-XMCD image of CFO magnetic domain structure on MgO substrate. (b) CFO magnetic domains on BTO show a stripelike modulation of contrast whose borders are aligned along a [100] in-plane direction. Dashed lines are guides to the eye.

Figure 2

(a) X-ray absorption spectra with horizontal and vertical polarization at Fe L edge of the boxed area in Fig. 1, with (b) difference in spectra showing large dichroism at $L_3$ and $L_2$ peaks.

Figure 3

Magnetization projections along in-plane (a) [100] and (b) [010] directions and (c) out-of-plane [001] direction for a (001)-oriented CFO/BTO sample, with corresponding PEEM-XLD image in panel (d), illustrating that the ferromagnetic easy axis corresponds to the film strain state as measured by XLD at the Fe $L_2$ edge. The direction of x-ray incidence for panel (d) is indicated by the arrow.

Figure 4

Fe edge PEEM-XMCD image of NFO/BTO in a 6 $\mu$m $\times$ 6 $\mu$m area (a) in the demagnetized state and at remanence after (b) $-500$ Oe and (c) $+$500 Oe field pulses. The difference images in XMCD contrast before and after (d) the negative and (e) positive field pulses indicate little contrast change in stripes 1 and 3, and a large contrast change in stripe 2. The circled regions show NFO domains which align strongly along the applied field directions.

Figure 5

(a) Ti $L_{3a}$ edge linear dichroism image showing the different regions of $a$-$c$ and $a$1-$a$2 domain variants in BTO and (b) schematic of ferroelectric axis orientation for BTO surface domains. Spectra for the different domain variants are shown in panel (c), illustrating that domains with ferroelectric axis along the x-ray incidence direction (e.g., variants $a$1 and $a$3) yield little linear dichroism, while $a$2 and $c$ domain variants show large and opposing dichroism peaks.

Figure 6

(a) Angular dependence of linear dichroism in CFO at Fe $L_2$ edge for $a$1-$a$2 type BTO domains at 295 K, with XLD domains in panel (b) measured at $\theta ={180}^{\circ }$ and arrow indicating the in-plane [110] substrate direction. (c)–(e) XMCD domains in a 7 $\mu$m $\times$ 11 $\mu$m area for $a$1 (stripe with red box) or $a$2 (stripe with black circle) domain variants for various angles $\theta$ with the x-ray incidence direction shown with small arrows, showing uniaxial in-plane magnetic anisotropy.

Figure 7

Angular dependence of linear dichroism in CFO at Fe L${}_2$ edge for $a$-$c$ type domain variants, with inset PEEM-XLD image corresponding to the area of the PEEM-XMCD image shown in Fig. 1b. Clear differences in XLD symmetry due to the underlying BTO $a$-$c$ type domain structure are seen in the azimuthal plot.

Figure 8

Fe L edge PEEM-XMCD images of CFO domain structure on BTO in the same area above and below the tetragonal-orthorhombic phase transition at (a) 295 K and (b) 260 K. PEEM-XLD images also taken at the same area at (c) 295 K and (d) 260 K. Contrast levels are the same between panels (a)–(b) and panels (c)–(d), and red lines are added as guides to the eye indicating strain domain boundaries. (e) Angular dependence of three different domain variants at 260 K, with $\theta =0^{\circ }$ along the in-plane [100] direction at 295 K, illustrating a change in film strain symmetry due to the substrate phase transition when compared with Fig. 6a.

Figure 9

Fe edge PEEM-XLD image sequence on cooling from room temperature to 160 K and back to room temperature. Red lines are guides to the eye indicating XLD domain boundaries. The black scale bars are 5 $\mu$m long, and the black arrows indicate in-plane [100] and [010] directions.

Figure 10

Temperature dependence of (a) out-of-plane lattice parameter and contrast for CFO/BTO linear dichroism domains with in-plane ferroelectric axis (b) along or (c) orthogonal to the x-ray incidence direction at the Fe $L_{2b}$ edge upon warming (thick red lines) or cooling (thin blue lines) across the tetragonal-cubic BTO phase transition. Both sets of XLD domains converge towards a value of 0.005 above $T_{\mathrm{Curie}}$ indicating a uniform strain state in the film above 410 K.