The study of magnetoelectric materials has recently received renewed interest, in large part stimulated by breakthroughs in the controlled growth of complex materials and by the search for novel materials with functionalities suitable for next generation electronic devices. In this Progress Report, we present an overview of recent developments in the field, with emphasis on magnetoelectric coupling effects in complex oxide multiferroic composite materials.

The drive to develop materials with new multifunctional capabilities has rekindled interest in multiferroics-systems which are characterized by the simultaneous presence of, and coupling between, magnetic and electric order parameters. In naturally occurring multiferroics the magnetoelectric coupling is often weak, and new classes of artificially structured composite materials that combine dissimilar magnetic and ferroelectric systems are being developed to optimize order parameter coupling.[1-6] Here, we describe direct, charge-mediated magnetoelectric coupling in a heterogeneous multiferroic that takes advantage of the sensitivity of a strongly correlated magnetic system to competing electronic ground states. Using magneto-optic Kerr effect magnetometry, we observe large magnetoelectric coupling in ferroelectric/lanthanum manganite heterostructures, including electric field-controlled on/off switching of magnetism. These results open a new vista for the development of novel magnetoelectric devices with large charge coupling between electric and magnetic degrees of freedom.Doped lanthanum manganites are complex oxides characterized by a strong interplay between electron transport, magnetism, and crystal lattice distortions, leading to a rich variety of electronic behavior, including magnetic and charge-ordered states, colossal magnetoresistance (CMR), and a diversity of electron transport behavior. Underlying the competition between these ground states is the prominent role of charge in double exchange, hopping, and orbital overlap. [7,8] To date, controlling charge as a parameter has most often been achieved using chemical doping, which is robust, and permanent. An alternative approach to modulate carrier density is to use an electrostatic field, [9][10][11][12][13][14][15] which has been used successfully to modulate charge-dependent phenomena, including superconductivity [16] and dilute magnetic semiconducting behavior. [12,13,17] In these systems, the nature of the electron correlations results in a strong sensitivity of the material properties to the charge-carrier concentration.Magnetism has also been controlled at interfaces using field effects. Magnetotransport measurements (planar and anomalous Hall effect, magnetoresistance, resistance) indicate large changes in critical temperature, [12,13,[18][19][20] while changes in coercivity have also been observed. [14] Moreover, magnetoelectric effects at interfaces have been predicted to arise from spin density accumulation in metallic ferromagnet/ferroelectric structures, induced by charge screening of the electric field.[5] These experimental and theoretical results point to the potential of these types of structures for nanostructured multiferroics.Here, we demonstrate a large charge-driven magnetoelectric coupling effect in a Sr-doped lanthanum manganite/ferroelectric composite structure resulting from direct control of magnetism via charge carrier density. This approach has the advantage that its physical mechanism is transparent and the size of the effec...

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...

We have fabricated n-layer graphene field effect transistors on epitaxial ferroelectric Pb(Zr 0.2 Ti 0.8 )O 3 (PZT) thin films. At low gate voltages, PZT behaves as a high- dielectric with  up to 100. An unusual resistance hysteresis occurs in gate sweeps at high voltages, with its direction opposite to that expected from the polarization switching of PZT. The relaxation of the metastable state is thermally activated, with an activation barrier of 50-110 meV and a time constant of 6 hours at 300 K. We attribute its origin to the slow dissociation/recombination dynamics of water molecules adsorbed at the graphene-PZT interface. This robust hysteresis can potentially be used to construct graphene-ferroelectric hybrid memory devices.