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Electric field driven Magnetization switching in multiferroic Nanoislands

Final Report Summary - ELMAGNANO (Electric field driven Magnetization switching in multiferroic Nanoislands)

The aim of ELMAGNANO has been to implement structures in which switching of magnetization using an electric field is achievable at room temperature, locally, and in an addressable, selectable and reliable way.
Microelectronic-based magnetic device development requires several issues to be addressed, among which heat management and power consumption have particular significance. Ideally, purely electric field-controlled devices should be employed: without the currents needed to generate magnetic fields, less Joule heating would occur and less power would be needed. Such technological requirements have revived interest in multiferroic magnetoelectrics, materials possessing simultaneously ferromagnetic (FM) and ferroelectric (FE) ordering and in which magnetism can be controlled via electric field and vice versa.
Scarcity of single phase room temperature multiferroic materials has resulted in the development of multiferroic composites, in which there is a juxtaposition of a ferroelectric and a ferromagnetic material. Magnetoelectric coupling is achieved via interfacial transfer mechanisms. Such composites have been successfully employed to demonstrate electric field control of magnetization. The main issue is the control of in-plane polarization of the ferroelectric constituent by use of a largely out-of plane electric field, which is a necessary step in order to achieve magnetization switching in the magnetic constituent in a planar multilayer structure.
So far, the principle of coupled magnetoelectric heterostructures has been demonstrated in mesoscopic devices; however, for most applications the concept should be demonstrated in a geometry with strongly reduced lateral dimensions. Lateral patterning could have an advantage in allowing better control over functional properties.
The objectives of this project have been the following:
1. To establish deterministic switching in ferroelectric nanoislands of bismuth ferrite (BFO) and lead zirconium titanate - lead iron tantalate solid solution (PZTFT): this means control over both out-of plane and in-plane components of polarization using largely out-of plane electric fields;
2. To demonstrate electrically driven selectable magnetization switching in multiferroic nanoislands, to be investigated in composite BFO-based multiferroic heterostructures and/or in PZTFT nanoislands.

Thin film BFO samples were obtained through external collaborations. Ferroelectric characterization allowed the selection of the best samples for the research: those with minimal ferroelectric imprint, which would allow stable deterministic switching experiments.
A nanofabrication process was developed by patterned sacrificial layer evaporation followed by focused ion beam (FIB) milling. The use of low acceleration voltages and high currents during FIB milling demonstrated that switching properties of the material deteriorate under high milling rates. Low voltage and low current milling of samples, covered with patterned sacrificial layers, followed by wet-etching, yielded a set of nanoislands with diameters of ~400nm and heights of ~25nm. Switching properties of these nanoislands were investigated via Piezoresponse Force Microscopy (PFM). Point switching hysteresis loops demonstrated retention of ferroelectric properties with minimal variation in the imprint induced by the fabrication process. Switching experiments of the full island by applying DC bias via the PFM probe, while raster scanning, were performed in order to investigate the degree of control achievable over in-plane polarization using out-of plane electric fields: the aim was to see if we were able to establish deterministic selection of each of the 8 polarisation variants available in (001)pseudocubic BFO (2 out-of plane possibilities, 4 in-plane equivalent states).

It was demonstrated that the polarization in BFO nanoislands can be effectively toggled between any of the four variants pointing towards the bottom electrode; this feat has not been achieved in previous research. Each polar orientation can be selected, after a ‘reset’ operation, via the raster scanning direction of the Scanning Probe Microscopy (SPM) probe. By aid of electric field simulations, it was demonstrated that the operation is possible due to the inhomogeneous electric field generated by a SPM probe, giving rise to a transverse field confined to a volume at the edges of the contact area, by which the in-plane component of the polarization can be controlled during the out-of-plane switching procedure. It was demonstrated that the SPM scanning direction is critical (see figure 1 below). Slow raster axis scanning along the desired in-plane component of polarization (while applying bias) yields a quasi-monodomain configuration in the nanoislands, while scanning at an angle of 45° away from it leads to a multidomain configuration.

The results represent a significant step towards fully deterministic switching in multiferroic nanoislands. Such knowledge will be of relevant use for future fabrication and investigation of electric field switching of magnetization in magnetoelectric nanoislands, which will have an impact in the future development of magnetoelectric heterostructured devices, not only for those based on exchange bias coupling, but even those based on strain coupling.
The research provides a framework for better control of switching in rhombohedral ferroelectrics and for a deeper understanding of exchange coupling in multiferroic nanoscale heterostructures.

Figure 1: Vector PFM images resulting after switching experiments (typically -6V) with the slow raster axis for each image indicated in the centre of the diagram. The obtained variant is unequivocally determined by the slow raster axis direction. Planar color coding (left) for polar orientations as viewed from the BFO surface, and a diagram of the BFO unit cell with its <111> polarization variants (right). Image from article submitted to Nanoletters.