Interfacial coupling in multiferroic/ferromagnet heterostructures

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Interfacial coupling in multiferroic/ferromagnet heterostructures

M. Trassin, J. D. Clarkson, S. R. Bowden, Jian Liu, J. T. Heron, R. J. Paull, E. Arenholz, D. T. Pierce, and J. Unguris

Phys. Rev. B 87, 134426 – Published 30 April 2013

Abstract

We report local probe investigations of the magnetic interaction between BiFeO $_{3}$ films and a ferromagnetic Co $_{0.9}$ Fe $_{0.1}$ layer. Within the constraints of intralayer exchange coupling in the Co $_{0.9}$ Fe $_{0.1}$ , the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO $_{3}$ ferroelectric polarization. The magnetic anisotropy is uniaxial, and an in-plane effective coupling field of order 10 mT is derived. Measurements as a function of multiferroic layer thickness show that the influence of the multiferroic layer on the magnetic layer becomes negligible for 3 nm thick BiFeO $_{3}$ films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO $_{3}$ film based on our x-ray linear dichroism measurements. These observations are consistent with an interfacial exchange coupling between the CoFe moments and a canted antiferromagnetic moment in the BiFeO $_{3}$ .

Received 15 March 2013

DOI:https://doi.org/10.1103/PhysRevB.87.134426

Authors & Affiliations

M. Trassin^1,2,*, J. D. Clarkson³, S. R. Bowden^4,5, Jian Liu¹, J. T. Heron³, R. J. Paull³, E. Arenholz⁶, D. T. Pierce⁴, and J. Unguris^4,*

¹Department of Physics, University of California, Berkeley, California 94720, USA
²Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland
³Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
⁴Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6202, USA
⁵Maryland Nanocenter, University of Maryland, College Park, Maryland 20742, USA
⁶Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^*morgan.trassin@mat.ethz.ch, john.unguris@nist.gov

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Vol. 87, Iss. 13 — 1 April 2013

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Images

Figure 1
(a) Schematic of coupling model showing 71° ferroelectric domains with in-plane components of P and $M_{C}$ changing 90° from one domain to the next and the corresponding change in the magnetization in-plane in the CoFe layer. The cube on the right shows an example of the relative orienations of P, $M_{C}$ , and the L axis, [1,-1,-1], [1,-1,2], and [1,1,0], respectively, in the DM model. (b) PFM image of the BFO thin film grown on SRO/DSO. The inset shows the schematic of the polarization (open arrows) and the canted moment easy axes (double arrows) in each single domain. (c) MFM picture from the same sample after growth of the CoFe/Pt bilayer under a 20 mT growth field. In the inset the Fourier analysis shows a common domain width of 275 nm. [(d)–(f)] Magnetic-field-dependent MFM images as grown (d), under 0.1 T (e), and back to 0 T (f). The inset represents the corresponding ferroelectric polarization (open arrows) and magnetization (smaller red arrows) in each domain. (g) Profile line scan in the same area in the states (d), (e), and (f) labeled 1, 2, and 3, respectively.Reuse & Permissions
Figure 2
(a) SEMPA image of CoFe magnetization (direction indicated by the color wheel) and (b) simultaneously acquired BSE image of underlying ferroelectric domain structure. (c) Magnified region from the SEMPA image (a) shows measured in-plane magnetization directions with arrows. (d) Line scans from BSE (red curve) and SEMPA (filled black squares) and from an OOMMF calculation (open blue squares) with a 7 mT effective coupling field showing correlated structures and the magnitude of magnetic oscillations.Reuse & Permissions
Figure 3
(a) Magnetic in-plane hysteresis loop at room temperature measured on a Pt/CoFe/BFO (50 nm)/SRO/DSO heterostructure, perpendicular to the domain walls (open squares) and along the domain walls (filled triangles) when no growth field was applied. The inset shows the corresponding loop when a growth field was applied and the corresponding MFM image. (b) coercivity enhancement (open symbols, left axes) and exchange bias (closed symbols, right axes) as a function of the multiferroic thickness with (squares) and without (triangles) a growth field. The inset shows the PFM box in a box of the 3 nm thick BFO film. [(c) and (d)] Corresponding room temperature loops on a Pt/CoFe/BFO (50 nm)/SRO/STO heterostructure and MFM picture (c) and thickness dependence of the exchange bias field and enhanced coercivity (d).Reuse & Permissions
Figure 4
MFM (a) and SEMPA (b) pictures of the CoFe/Pt layer deposited with a growth field (axis labeled by the double arrow) on BFO (3 nm)/SRO/DSO, (c) x-ray linear dichroism as a function of temperature for a 150 nm (black filled squares), and a 3 nm thick (open squares) BFO film on STO and a 3 nm film on DSO (open circles). In the inset, schematics of the antiferromagnetic axis L, polarization vector P, and canted moment vector $M_{C}$ for the films grown on STO and DSO according to Refs. 4, 5. (d) Normalized change in $I_{XLD}$ from room temperature (see text). The inset shows the lattice parameter out-of-plane c (open squares) and in-plane a (filled squares), as well as the c/a ratio (triangles) as a function of thickness for BFO/SRO/DSO.Reuse & Permissions

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