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Imaging Magnetic Microstructure Response to Substrate Strain

Traditional magnetic data-storage technology requires energy-intensive electromagnetic write heads to record information. A new class of composite multiferroic materials offer the possibility of directly controlling magnetic order with much more power-efficient electric fields.

For example, a ferromagnetic thin film grown on a piezoelectric substrate offers a way to control magnetization in ultralow-power devices via magnetoelastic coupling between the piezoelectric and ferromagnetic components. Accordingly, a direct measurement of both the electrically induced magnetic behavior and the piezo-strain driving such behavior is crucial for better understanding and further development of these systems.

Recently, a team of researchers at Berkeley Lab, UC Berkeley, and UCLA managed to measure the micron-scale strain and magnetic response to an applied electric field in a composite multiferroic system composed of Ni microstructures fabricated on a piezoelectric single-crystal substrate (PMN–PT).

To image both the strain and the magnetic state, x-ray microdiffraction at ALS Beamline 12.3.2 and photoemission electron microscopy (PEEM) with x-ray magnetic circular dichroism (XMCD) contrast at Beamline 11.0.1.1 were employed, respectively. These two complementary imaging experiments, focused on the same area of the sample, indicated the presence of a nonuniform strain distribution that directly linked to the observed nonuniform reorientation of the magnetic state in the Ni microstructures.

The study emphasized the importance of the interface in composite multiferroic structures. Surface and interface engineering at the micron- and nano-scale is critical to achieve uniform magnetic responses to electrically induced strains. This fusion of experimental techniques enables a robust assessment of future composite multiferroic devices.

Top left: Schematic of the composite multiferroic system, showing the strain map measured by x-ray microdiffraction (blue-green) superimposed with the magnetic microstructures imaged by XMCD-PEEM. Bottom: The electrically induced magnetic reorientation (left) and the piezo-strain (right). The colored frames relate locations in the strain map to individual magnetic microstructures in the Ni layer. Above the strain map are the electric-field values at which correspondingly colored magnets were reoriented. Higher-strain regions (green) corresponded to lower reorientation fields, while lower-strain regions (blue) corresponded to higher reorientation fields.

R. Lo Conte, Z. Xiao, C. Chen, C.V. Stan, J. Gorchon, A. El-Ghazaly, M.E. Nowakowski, H. Sohn, A. Pattabi, A. Scholl, N. Tamura, A. Sepulveda, G.P. Carman, R.N. Candler, and J. Bokor, “Influence of Nonuniform Micron-Scale Strain Distributions on the Electrical Reorientation of Magnetic Micro-Structures in a Composite Multiferroic Heterostructure,” Nano Lett. 18, 1952 (2018), doi:10.1021/acs.nanolett.7b05342.