Origin of the Magnetoelectric Coupling Effect inPb(Zr0.2Ti0.8)O3/La0.8

Abstract

The electronic valence state of Mn in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures is probed by near edge x-ray absorption spectroscopy as a function of the ferroelectric polarization. We observe a temperature independent shift in the absorption edge of Mn associated with a change in valency induced by charge carrier modulation in the La0.8Sr0.2MnO3, demonstrating the electronic origin of the magnetoelectric effect. Spectroscopic, magnetic, and electric characterization shows that the large magnetoelectric response originates from a modified interfacial spin configuration, opening a new pathway to the electronic control of spin in complex oxide materials.PACS numbers: 75.70. Cn,78.70.Dm,73.90.+f,75.60.Ej,85.30.Tv,75.30.Kz,85.70.Ay Understanding how to couple the electric and magnetic order parameters in the solid state is a long-standing scientific challenge that is intimately linked to the spatial and temporal symmetries associated with charge and spin. Coupling of the order parameters is observed in many different materials, but the effect is generally weak in magnitude, even in materials that are both ferroelectric and ferromagnetic (multiferroic) [1][2][3]. Increasing the magnitude of the coupling is a fundamental problem in condensed matter physics with important implications for applications. For example, strong magnetoelectric coupling allows for the ultra-sensitive measurement of weak magnetic fields, and at smaller length scales, enables spin-based technologies by allowing the control of the spin state at the atomic scale via electric fields.In single phase multiferroics, the magnetic and ferroelectric orders often occur largely independent of each other, and as a result the magnetoelectric coupling tends to be small [2,4]. In order to overcome this intrinsic limitation in the coupling between the order parameters, artificially structured materials with enhanced magnetoelectric couplings have been engineered, where a break in time reversal and spatial symmetry occurs naturally at the interface between the different phases [3,5,6]. Moreover, the coupling mechanism can be tailored to benefit from several phenomena, including elastic [7,8], magnetic exchange bias [9][10][11], and charge-based [12] couplings. In charge-based multiferroic composites, the sensitivity of the electronic and spin state of strongly correlated oxides to charge provides enhanced coupling between magnetic and ferroelectric order parameters [12]; it often relies on charge doping of a "colossal" magnetoresistive (CMR) manganite to modulate between high and low spin states, which compete for the ground state of the system. However, the microscopic origin of this effect is still not fully understood. In particular, the nature of the effect and how the interplay between charge, spin, and valency combines to yield the large magnetoelectric response in this system remain to be addressed. In this Letter, we explore the sensitivity of x-ray absorption near edge spectroscopy (XANES) to the atomic electronic state to demons...

Reversible Modulation of Orbital Occupations via an Interface-Induced Polar State in Metallic Manganites

“…However, the switchable polarization of ferroelectrics via external field yields a reversible orbital polarization at the ferroelectric/manganites interface, which is easier to detect, since by measuring the change in the signal of x-ray dichroism, any potential difficulties due to a bulk-like background are automatically eliminated. Therefore, we hope that our theoretical results stimulate further experiments that explore revserible control of orbital degree of freedom, in addition to the control of charge and spin in transition metal oxides, 20 and functional catalysis at ferroelectric interfaces and surfaces. 38,39 In conclusion, we have used ab initio calculations combined with experimental growth and characterization to describe the atomic-scale geometry and electronic structure of ferroelectric/manganite interfaces.…”

Ferroelectric Field Effect Control of Magnetism in Multiferroic Heterostructures

“…In the following, we provide an overview of the recent work demonstrating a large magnetoelectric coupling in PZT/LSMO heterostructures, as determined by probing directly the magnetic order parameter using local magnetooptic Kerr effect magnetometry Molegraaf et al (2009). By using advanced spectroscopy techniques, we show that the observed effect is electronic in origin, and that it results from a change in the valence state of the Mn cations with the change in the hole carrier density Vaz et al (2010b Fig. 1(b), inset).…”

“…The nature of the magnetoelectric effect due to charge screening can be distinguished between (i) enhanced spin imbalance at the Fermi level due to screening and the corresponding modification in the magnetic moment of the system as a function of the electric field Zhang (1999); (ii) changes in magnetic moment due to changes in electronic bonding at the polarized dielectric interface Duan et al (2006); (iii) changes in the magnetic order with the charge density Gerhard et al (2010); Kudasov & Korshunov (2007); Ovchinnikov & Wang (2008); Sun et al (2010); Vaz et al (2010b), whereby the magnetic state of the system is modified due to changes in the charge carrier density, either between magnetic and non-magnetic states, or between states with different magnetic spin configurations; and (iv) changes in the magnetic anisotropy that lead to different global magnetic states for different applied electric fields Maruyama et al (2009);Niranjan et al (2010).…”

“…One key advantage of x-ray absorption spectroscopy is its ability to probe buried layers, which is not possible with some other spectroscopic techniques, such as photoelectron spectroscopy. Details of the experimental set-up and measurement conditions are given in Vaz et al (2010b;. The room temperature variation of the x-ray light absorption as a function of the incident photon energy across the absorption K edge of Mn, for the depletion and accumulation states, is shown Fig.…”

“…The effect is indirect, but can be optimized to yield large magnetoelectric responses by a suitable choice of the material components and device geometry Nan et al (2008); Srinivasan (2010); Wang et al (2009). Another promising route involves a direct, charge-mediated magnetoelectric coupling in ferroelectric/ferromagnetic oxide composite multiferroic heterostructures, where the spin state of the magnetic oxide is controlled via charge doping induced by the electric polarization of a ferroelectric Molegraaf et al (2009);Vaz et al (2010b). For Pb(Zr 0.2 Ti 0.8 )O 3 /La 0.8 Sr 0.2 MnO 3 (PZT/LSMO) heterostructures, the effect is electronic in origin and results from a change in the magnetic spin configuration as a function of the ferroelectric polarization direction, demonstrating electric field control of magnetism in this system (Section 3).…”