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Electric control of magnetism at the Fe/BaTiO3 interface

Controlling the magnetism of materials by means of electric fields is the key for the development of fast and low power-consumption electronics. Recent years have witnessed many attempts to achieve such goal and significant progresses have been made in the electric control of both the magnetic anisotropy and the Curie temperature of magnetic films and interfaces. However, the demonstration of a full and reversible switching of the magnetization of a film via an electric pulse is still lacking. The most promising way to obtain such kind of control is to couple a ferroelectric (FE) with a ferromagnetic (FM) layer in order to artificially create a multiferroic material with high magnetoelectric coupling. The prototypical FE/FM bilayer is the Fe/BaTiO3 (Fe/BTO) interface. In this work we have investigated the magnetoelectric coupling of few atomic planes at the Fe/BTO interface.
To this scope we fabricated the following in-situ epitaxially grown capacitors: Au(4)/Co(1)/Fe(0.3)/BTO(150)/LSMO(50)//SrTiO3(001)), where the limited thickness of the Fe layer allows to focus on the Fe interfacial properties and Co is used to stabilize ferromagnetism at room temperature. Subsequently, the capacitors, with A1 = 0.02 mm2 and A2 = 1 mm2 areas, were fabricated using optical lithography and ion milling. A SiO2 layer was deposited by sputtering to allow the fabrication of the final Au/Ti contacts.

The capacitors were characterized by an electrical point of view. In the top-top configuration two BTO capacitors are connected in series through the conducting LSMO electrode. Current (I-E) and polarization (P-E) loops were measured at RT and 80 K. The  I-E characteristics measured at RT and at 100 Hz on capacitors with A1 = 0.02 mm2 area, allow the determination of the coercive field ( ∼120 kV cm-1), while the P-E loop indicates remanence and saturation polarization, Pr = 15 μC cm-2 and Ps = 30 μC cm-2, which are characteristic of a good quality BTO.
XMCD experiments were performed at the APE beamline where a new experimental set-up devoted to the in-operando experiment has been developed: the system allows measuring XAS and XMCD spectra while applying a voltage bias to a capacitor structure (as depicted in fig 1). In this way we were able to acquire XMCD spectra of the FM layer (the Fe layer) while switching the electric state of the FE layer (BTO). 

Figure 1.  Schematic configuration of the in-operando XMCD experiment on the Fe/BTO interface. The electrical polarization was induced on micrometer sized pillars (prepared by lithography) with electrical pulses between top and bottom contacts. Reversing the synchrotron radiation helicity and/or the magnetization direction, Fe L2,3 XMCD spectra were collected  at interface with BTO for the two different electric polarization of the BTO film (indicated by the white arrows).




Figure 2: Details of the interface for the Antiferromagnetic (left) and Ferromagnetic (right) configurations of 2 ML system. The FE polarization points in the direction of c-axis, downwards and upwards for the (left) and (right) configurations, respectively.

From the analysis of the XAS spectra of the Fe L3 edge, it is immediately evident that a fraction of the Fe layer has reacted with BTO, giving rise to the formation of a Fe oxide layer at the interface between Fe and BTO. More surprisingly, the XMCD spectra show that the Fe oxide component (indicated with a grey stripe in insets of Fig. 1) displays a net magnetization when the BTO is in the UP state, while no signal is present when the BTO is in DOWN state (i.e. with the opposite polarization state).
To gain insights on the interface structure, we have performed a TEM analysis of the stack and found that, in addition to the confirmation of epitaxial growth of the different layers, the BTO layer termination (i.e. the atomic plane in contact with Fe) is (mostly) a TiO2 plane and that the Fe layer reacts in contact with the TiO2 layer forming an atomic plane of Fe oxide.

The effect that can lead to the appearance of a net magnetization in the Fe oxide layer upon the change of the electric state of the BTO layer is clearly a magnetic state transition of the Fe oxide layer. To verify this hypothesis, we performed first-principles density functional calculations. We simulated a supercell built by aligning the [100] axis of the TiO2-terminated BTO and the [110] axis of bct cobalt sandwiching (i) a single interfacial FeO layer and (ii) 1 FeO ML in contact with BTO, covered by 1 ML of “metallic” Fe in contact with Co. The total energy difference of the FeO interface with ferromagnetic (FM) and antiferromagnetic (AF) spin order has been calculated,  highlighting the tendency of the system towards a change in the magnetic order depending to the sign of the BTO polarization. This effect results from the polarization-induced atomic-position shifts and subsequent modifications of the Fe-O bond topology when interfaced with the TiO2 surface of BTO (as shown in Fig. 2).

In summary, we observed a magnetic phase transition of an iron oxide layer, which is induced by the switch of electrical polarization of the BTO substrate and is therefore electrically controllable.
 

This research was conducted by the following research team:

  • Greta Radaelli, Daniela Petti, Matteo Cantoni, Christian Rinaldi, Riccardo Bertacco, LNESS – Dipartimento di Fisica – Politecnico di Milano, Como, Italy

  • Evgeny Plekhanov, Silvia Picozzi, Consiglio Nazionale delle Ricerche, CNR-SPIN, L’Aquila, Italy 
  • Ignasi Fina, Diego Gutiérrez, Josep Fontcuberta, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Catalonia, Spain 
  • Piero Torelli, Benjamin R. Salles, Giancarlo Panaccione, Consiglio Nazionale delle Ricerche, CNR - IOM, Laboratorio TASC, Trieste, Italy
  • Maria Varela, Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, USA /Dpto. Fisica Aplicada III, Universidad Complutense de Madrid, Madrid, Spain

Contact person:

Riccardo Bertacco : riccardo.bertacco@polimi.it

Reference

G. Radaelli, D. Petti, E. Plekhanov, I. Fina, P. Torelli, S.R. Salles, M. Cantoni, C. Rinaldi, D. Gutiérrez, G. Panaccione, M. Varela, S. Picozzi, J.  Fontcuberta, R. and Bertacco, “Electric control of magnetism at the Fe/BaTiO3 interface Nat. Commun. 5, 3404 (2014)    DOI:10.1038/ncomms4404
Last Updated on Friday, 04 April 2014 16:48