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Co/ZnO nanorod heterostructure: an innovative magneto-electric system

One of the major challenging topics of the last years is represented by the electric-field-induced reversible modulation of the magnetic properties. The ability to tune the magnetic properties through the application of a bias voltage and vice-versa paves the way towards the development of innovative, fast, compact and low-consumption memory devices, micro- and nanoelectromechanical systems, microwave systems and the emergence of the electric-field-controlled spintronics.
Because of the difficulties to simultaneously stabilize the ferroelectric and magnetic orders in the same compound, the attention is now addressed towards the development of heterostructures where an ultrathin ferromagnetic layer is coupled to a ferroelectric monocrystalline substrate.
In order to establish a good interface coupling, several parameters are reported in literature to come into play, such as (i) the quality of the contact between the layers, (ii) the thickness of the metal layer, (iii) the choice of a suitable substrate and (iv) epitaxy between different layers. The challenge is how to overcome all these requirements and limitations.
In this work, a new strategy is presented exploiting piezoelectric ZnO nanorods grown on top of a relatively thick and polycrystalline ferromagnetic Co layer.
An electric field of only 2 kV/m is able to change in a reversible way the magnetic coercivity of the system by a factor 5 with respect to the un-poled case (Fig. 1), thanks to a giant magneto-electric coupling which establishes at the ZnO/Co interface.

Figure 1 (a) MOKE hysteresis loops measured with different applied bias voltage and (b) Coercivity reduction in % for the heterostructures grown on two different substrates: Al2O3 (SAl) and naturally oxidized (100)-Si (SSi) substrates.

XANES and MOKE analysis performed at the APE and Magnedyn beamlines of Elettra, allow to shed light on the main mechanism at the base of this drastic and reversible change in the magnetic properties of Co. This has been interpreted as a giant strain-driven magneto-electric coupling. The application of a bias voltage results in the atomic displacement of Zn inside the ZnO host lattice due to the piezoelectric response of ZnO under bias (Fig. 2). The reduced dimensionality and the larger polarizability of the ZnO nanorods maximize the piezoelectric deformation. This deformation determines a strong and reversible reduction of Co coercivity, a change of the interface anisotropy and a variation of the net magnetic moment at the Co/ZnO interface. These results pave the way towards the development of a cost-effective efficient magnetoelectric device.

Figure 2. In-operando XANES spectra acquired at the (a) O K and (b) Zn L edges for the system before (b) and after (α) the nanorod removal.


 

This research was conducted by the following research team:

Giovanni Vinai 1, Piero Torelli 1, Regina Ciancio 1, Barbara Ressel 2, Federico Loi 3, Barbara Casarin 3, Fulvio Parmigiani 3, Benoit Gobaut 4, Antonio Caretta 4, Luca Capasso 4, Marco Malvestuto 4, Roberta Ciprian 4, Francesco Cugini 5and Massimo Solzi 5

CNR-Istituto Officina dei Materiali IOM, s.s. 14 km 163.5, 34149, Trieste, Italy
University of Nova Gorica, Slovenia
Universitá degli Studi di Trieste, Trieste, Italy.
Elettra - Sincrotrone Trieste S.C..pA., Trieste, Italy.
Universitá degli Studi di Parma, Parma, Italy


Contact persons:

Roberta Ciprian, email: roberta.ciprian@elettra.eu
Marco Malvestuto, email: marco.malvestuto@elettra.eu
 

Reference

Giovanni Vinai, Barbara Ressel, Piero Torelli, Federico Loi, Benoit Gobaut, Regina Ciancio, Barbara Casarin, Antonio Caretta, Luca Capasso, Fulvio Parmigiani, Francesco Cugini, Massimo Solzi, Marco Malvestuto and Roberta Ciprian, "Giant magneto-electric coupling in 100 nm thick Co capped by ZnO nanorods", Nanoscale (2017); DOI: 10.1039/C7NR09233D.

 
Last Updated on Friday, 19 January 2018 13:56