Magnetoelectric Effects in Complex Oxides with Competing Ground States

Abstract

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...

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

“…13 The energetic ordering of the e g orbitals on each Mn site as well as neighboring Mn sites profoundly affects the ground state magnetic properties. [14][15][16][17][18][19][20] Although structural distortions (e.g., Jahn-Teller or GdFeO 3 distortions) are common for bulk perovskite manganites, they only weakly remove orbital degeneracy. 13 With the development of epitaxial thin film growth techniques, it is possible to remove orbital degeneracy through straininduced Jahn-Teller-like distortions.…”

“…We realize this interface experimentally by using molecular beam epitaxy to deposit an integer number of unit cells of LSMO on a Nb-doped TiO 2 -terminated SrTiO 3 (001) substrate before depositing the ferroelectric BaTiO 3 thin film. 15 First principles calculations have shown that at a ferroelectric/LSMO interface, the termination of the ferroelectric polarization and presence of surface bound charge pull screening charges to the interface. [24][25][26] For out-of-plane ferroelectric polarizations, two interfacial states are possible: accumulation or depletion of holes, as illustrated in the left panels of 1.…”

Ferroelectric Field Effect Control of Magnetism in Multiferroic Heterostructures

“…Note that the difference in magnetic moment persists at zero applied electric field, which excludes electrostrictive or piezoelectric effects (strain) as being the cause of the observed magnetoelectric effect. The magnetoelectric effect in the PZT/LSMO system is robust and has been observed in structures where the LSMO film has been grown by off axis magnetron sputtering Molegraaf et al (2009) The variation of the magnetization as a function of temperature for the two states of the PZT polarization is shown in Fig. 2(a) Molegraaf et al (2009);Vaz et al (2010c).…”

“…Such fine control can be achieved with molecular beam epitaxy Vaz et al (2010d) or pulsed laser deposition Huijben et al (2008), where the film growth can be monitored layer-by-layer in real time by using the oscillations in the intensity of reflection high energy electron diffraction patterns. 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.…”

“…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).…”