Ordered arrays of multiferroic epitaxial nanostructures

Abstract

Epitaxial heterostructures combining ferroelectric (FE) and ferromagnetic (FiM) oxides are a possible route to explore coupling mechanisms between the two independent order parameters, polarization and magnetization of the component phases. We report on the fabrication and properties of arrays of hybrid epitaxial nanostructures of FiM NiFe(2)O(4) (NFO) and FE PbZr(0.52)Ti(0.48)O(3) or PbZr(0.2)Ti(0.8)O(3), with large range order and lateral dimensions from 200 nm to 1 micron.

Methods: The structures were fabricated by pulsed-laser deposition. High resolution transmission electron microscopy and high angle annular dark-field scanning transmission electron microscopy were employed to investigate the microstructure and the epitaxial growth of the structures. Room temperature ferroelectric and ferrimagnetic domains of the heterostructures were imaged by piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM), respectively.

Results: PFM and MFM investigations proved that the hybrid epitaxial nanostructures show ferroelectric and magnetic order at room temperature. Dielectric effects occurring after repeated switching of the polarization in large planar capacitors, comprising ferrimagnetic NiFe2O4 dots embedded in ferroelectric PbZr0.52Ti0.48O3 matrix, were studied.

Conclusion: These hybrid multiferroic structures with clean and well defined epitaxial interfaces hold promise for reliable investigations of magnetoelectric coupling between the ferrimagnetic / magnetostrictive and ferroelectric / piezoelectric phases.

Keywords: epitaxial nanostructures; magnetic force microscopy; multiferroic composites; piezoresponse force microscopy; pulsed-laser deposition.

Fig. 1

Atomic force microscopy height-images of arrays of ordered epitaxial (a) NiFe₂O₄ dots made through a stencil mask with 200 nm diameter pores (12 × 12 µm large area) and of (b) NiFe₂O₄ dots made with a stencil with 800 nm diameter pores and embedded in an epitaxial PbZr_0.52Ti_0.48O₃ film (10 µm×10 µm large area).

Fig. 2

Magnetic force microscopy phase image of (a) NiFe₂O₄ dots (stencil with 800 nm pores) and of (b) NiFe₂O₄ dots (stencil with 400 nm pores) embedded in a ferroelectric 20 nm thick PZT20/80 film. In (c) a piezoresponse force microscopy phase image of the same sample as in (b) is shown, in which the polarization of the PZT20/80 film in central area was switched (uniform dark contrast) by applying a suitable dc voltage during scanning. The piezoresponse phase hysteresis loop measured on top of a PZT20/80/NiFe₂O₄ dot is shown in (d). All images are 10 µm×10 µm large.

Fig. 3

(a) Cross-section TEM and (b) Z-contrast STEM micrographs of an epitaxial NiFe₂O₄ dot embedded in a PbZr_0.52Ti_0.48O₃ film grown on a SrRuO₃-coated SrTiO₃ (100) substrate. (c) High resolution TEM image taken close to an edge of a NiFe₂O₄ dot and (d) the corresponding dark-field reconstructed image, using the selected reflections of the spinel (red) and perovskite (green) structures, as marked in the fast Fourier transform pattern shown in (e).

Fig. 4

Polarization hysteresis loops measured on top of NiFe₂O₄ dots (stencil with 400 nm diameter pores), embedded in a PZT52/48 film. The inset shows the polarization loops measured on the bare PZT52/48 film, away from the area with NiFe₂O₄ dots.

Fig. 5

Polarization hysteresis loops measured at 1 kHz and RT through Pt electrodes on top of NiFe₂O₄ dots (stencil with 800 nm diameter pores) embedded in a PZT52/48 film: (a) on the pristine capacitor and (b) of the same capacitor after a fatigue experiment. (c) Capacitance versus bias voltage measurements performed before (black solid circles) and after two consecutive polarization fatigue experiments (red open squares and blue open triangles).