Visible light effects on photostrictive/magnetostrictive PMN-PT/Ni heterostructure

Multiferroic (MF) heterostructures have been extensively explored in recent decades due to the possibility of controlling both ferromagnetic (FM) and ferroelectric (FE) orders across the interface via magnetoelectric coupling. The coupling is driven by three main mechanisms: interfacial charge accumulation or depletion (charge-driven), ion migration and strain-mediated effects. Despite being a relatively mature field of research and having already reached its technological implementation, the detailed energy landscape of both FE and FM layers needs to be evaluated with further investigations, in order to tailor and optimize functionality. In particular, the possibility of modifying the FM response of a MF by all-optical means, exploiting the combination of bulk photovoltaic effect and induced photostriction of the FE component, is an effective method to tune the interfacial properties.

When a FE material is illuminated by light with above-band gap energy, the superposition of bulk photovoltaic effect with inverse piezoelectric effect leads to non-thermal strain (i.e., expansion or contraction under light), known as the photostrictive effect. This light-induced FE strain modulation has proven to be an effective way for tailoring the FM interfacial properties in MF heterostructures. In this study, by combining electrical, structural, magnetic, and spectroscopic characterizations, we show how light illumination modifies significantly the properties of Pb(Mg1/3Nb2/3)O3-0.4PbTiO3 (PMN-PT) substrate, and how these changes affect the magnetic properties of interfacial nickel layer.

First, the magnetic properties of pristine PMN-PT/Ni heterostructure were analyzed by Magneto–Optic Kerr Effect (MOKE), both without and with illumination of continuous 405 nm laser of variable intensity. A schematic representation of the sample stack and experimental setup for MOKE measurements is shown in Fig. 1a. The effects of light illumination for the case of magnetic field H // [010] is shown in Fig. 1b. A large HC reduction was detected under illumination, with increasing effects as a function of irradiance, up to 45% under light illumination of 800 mW cm−2, while the changes in magnetic remanence were negligible. These magnetic changes are fully reversible and reproducible, with the magnetic signal fully restored after switching the laser off.

After MOKE characterization, x-ray magnetic circular dichroism (XMCD) measurements were carried out at the APE-HE beamline to determine the magnetic changes of the Ni film under illumination. The XMCD spectra at Ni L2,3 edges in absence and presence of laser light illumination are shown in Fig. 1c. The spectra shows an L3 edge XMCD intensity of (7.1 ± 0.1)% with no light and (7.9 ± 0.1)% under illumination, while the dichroic signal at L2 edge does not change. This is a signature of change of Ni orbital moment induced by PMN-PT photostriction under illumination. By applying the XMCD sum rules, we derived the spin (mspin) and orbital (morb) magnetic moments of Ni for both cases. It results that the ratio morb/mspin was found to be 0.0940 with no light and 0.1149 under illumination. The change of this ratio for Ni represents the interface induced effect driven by PMN-PT photostriction when under illumination. This is to our best knowledge the first reported spectroscopic evidence of this interfacial coupling in multiferroic heterostructures.

the figure accompanies the top story on changes in magnetism regulated by photostriction

Figure 1: a) Schematic representation of the PMN-0.4PT/Ni heterostructure and experimental setup. A He-Ne laser is used for the MOKE measurements, while a continuous 405 nm laser is illuminating the whole sample surface. b) Hysteresis loops measured with the magnetic field applied along the PMN-PT (010) axis under light illumination of variable power (0–800 mW cm−2). c) XMCD dichroic signal at Ni L2,3 edges in case of pristine PMN-PT/Ni, in absence and presence of light, applying the magnetic field along [010]. d) Photocurrent evolution of the PMN-PT/Ni heterostructure as a function of time for three different polarization states (Pristine, Pdown and Pup) under 1 V bias

Next, PMN-PT/Ni was electrically polarized and photocurrent measurements were taken to evaluate the effects of illumination on three polarization states (Pristine, Pup and Pdown). The resulting curves are shown in Fig. 1d. In the pristine case, as the light was turned on, an increase in sample current due to bulk photovoltaic effect was observed, with a value of around 0.7 nA. Once turned off the laser, the initial state was sharply restored. On the other hand, slow dynamics with opposite trends were measured in polarized state. As the light was switched on, a photocurrent peak of opposite sign (positive for Pdown, negative for Pup) appeared, exponentially decaying toward a steady value comparable to the pristine one. Once switched off the illumination, a similar transient effect, but opposite in sign, was observed for both cases, with a relaxation time quite comparable to that under illumination. These transient effects are signature of the presence of a net electric dipole after sample polarization. The steady intensity of the photocurrent under illumination remains almost identical in the three cases; this is important because it means that the amount of bulk photovoltaic effect is unmodified after polarization. Once polarized, the PMN-PT photostrictive effects on interfacial magnetostrictive Ni are reduced, with negligible changes for both MOKE and XMCD measurements under light. We attribute such difference between polarized and pristine substrate to its structural properties. After polarization, the FE domain population is mostly out-of-plane oriented, while the in-plane component observed in the pristine case is almost not present anymore. The detected magnetostrictive changes for the pristine case indicate that the in-plane domains play a dominant role in the photostrictive/magnetostrictive coupling. This stimulates further investigations aimed to maximize the interfacial effect.

This research was conducted by the following research team:

Deepak Dagur1,2, Vincent Polewzyk1, Aleksandr Yu. Petrov1, Pietro Carrara1,3, Marta Brioschi1,3, Sara Fiori1, Riccardo Cucini1, Giorgio Rossi1,3, Giancarlo Panaccione1, Piero Torelli1 and Giovanni Vinai1

1 Istituto Officina dei Materiali (IOM)-CNR, Trieste, Italy
2 Department of Physics, University of Trieste, Trieste, Italy
3 Department of Physics, University of Milan, Milan, Italy

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D. Dagur, V. Polewczyk, A. Yu. Petrov, P. Carrara, M. Brioschi, S. Fiori, R. Cucini, G. Rossi, G. Panaccione, P. Torelli and G. Vinai, "Visible Light Effects on Photostrictive/Magnetostrictive PMN-PT/Ni Heterostructure", Adv. Mater. Interfaces 2201337 (2022); DOI: 10.1002/admi.202201337

Last Updated on Wednesday, 14 December 2022 11:29