Ferroelectricity Modulates Polaronic Coupling at Multiferroic Interfaces

Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework including screening of the polarization field and depolarizing charges. Largely unexplored still remains the band dependence of the interfacial charge modulation, as well as the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are explored using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, changing the band filling and orbital occupation in the interfacial region. Furthermore, these changes in the carrier density affect the coupling of the 2D holes and 3D electrons with the lattice which forms large Froelich polarons inherently reducing mobility of the charge carriers. We find that the fast dynamic response of electrons makes them much more efficient in screening of the electron-lattice interaction compared to the holes. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics.

Modulation of spin dynamics in Ni/Pb(Mg1/3}Nb2/3})O3-PbTiO3 multiferroic heterostructure

Motivated by the fast-developing spin dynamics in ferromagnetic/piezoelectric structures, this study attempts to manipulate magnons (spin-wave excitations) by the converse magnetoelectric (ME) coupling. Herein, electric field (E-field) tuning magnetism, especially the surface spin wave, is accomplished in Ni/0.7Pb(Mg1/3}Nb2/3})O3—0.3PbTiO3 (PMN—PT) multiferroic heterostructures. The Kerr signal (directly proportional to magnetization) changes of Ni film are observed when direct current (DC) or alternative current (AC) voltage is applied to PMN—PT substrate, where the signal can be modulated breezily even without extra magnetic field (H-field) in AC-mode measurement. Deserved to be mentioned, a surface spin wave switch of “1” (i.e., “on”) and “0” (i.e., “off”) has been created at room temperature upon applying an E-field. In addition, the magnetic anisotropy of heterostructures has been investigated by E-field-induced ferromagnetic resonance (FMR) shift, and a large 490 Oe shift of FMR is determined at the angle of 45° between H-field and heterostructure plane.

Reversal of the magnetoelectric effect at a ferromagnetic metal/ferroelectric interface induced by metal oxidation

Multiferroic materials composed of ferromagnetic and ferroelectric components are interesting for technological applications due to sizable magnetoelectric coupling allowing the control of magnetic properties by electric fields. Due to being compatible with the silicon-based technology, HfO2-based ferroelectrics could serve as a promising component in the composite multiferroics. Recently, a strong charge-mediated magnetoelectric coupling has been predicted for a Ni/HfO2 multiferroic heterostructure. Here, using density functional theory calculations, we systematically study the effects of the interfacial oxygen stoichiometry relevant to experiments on the magnetoelectric effect at the Ni/HfO2 interface. We demonstrate that the magnetoelectric effect is very sensitive to the interface stoichiometry and is reversed if an oxidized Ni monolayer is formed at the interface. The reversal of the magnetoelectric effect is driven by a strong Ni−O bonding producing exchange-split polarization-sensitive antibonding states at the Fermi energy. We argue that the predicted reversal of the magnetoelectric effect is typical for other 3d ferromagnetic metals, such as Co and Fe, where the metal-oxide antibonding states have an opposite spin polarization compared to that in the pristine ferromagnetic metals. Our results provide an important insight into the mechanism of the interfacial magnetoelectric coupling, which is essential for the physics and application of multiferroic heterostructures.

Electric Field Tuning of Magnetism in Fe3O4/Pt/PZN-PT Heterostructures Prepared by Atomic Layer Deposition

Tuning of magnetic properties by electric field (E-field) has received extensive attention because it is compact, fast, and energy efficient. Here, multiferroic heterostructures of Fe3O4/Pt/PZN-PT (011) (lead zinc niobate-lead titanate single piezoelectric substrate) were in-situ fabricated by atomic layer deposition (ALD) using C10H10Fe and O2 as precursors at a low temperature (400oC) without a subsequent annealing process in H2 atmosphere, which is beneficial in combining with traditional silicon-based semiconductor technology. The E-field dependence of the magnetic anisotropy was studied systematically by ferromagnetic resonance spectroscopy with the larger tunable in-plane magnetic anisotropy of 152 Oe and 318 Oe obtained along the [100] and [0-11] axes, corresponding to the largest magnetoelectric coupling coefficient of 31.8 Oe.cm/kV. Also, the tunable out-of-plane magnetic anisotropy of 35 Oe was obtained along the [011] axis. The outstanding E-field tuning magnetism in the Fe3O4/Pt/PZN-PT heterostructures offers significant possibilities for novel multiferroic devices.

Vector analysis of electric-field-induced antiparallel magnetic domain evolution in ferromagnetic/ferroelectric heterostructures

Electric field (E-field) control of magnetism based on magnetoelectric coupling is one of the promising approaches for manipulating the magnetization with low power consumption. The evolution of magnetic domains under in-situ E-fields is significant for the practical applications in integrated micro/nano devices. Here, we report the vector analysis of the E-field-driven antiparallel magnetic domain evolution in FeCoSiB/PMN-PT(011) multiferroic heterostructures via in-situ quantitative magneto-optical Kerr microscope. It is demonstrated that the magnetic domains can be switched to both the 0° and 180° easy directions at the same time by E-fields, resulting in antiparallel magnetization distribution in ferromagnetic/ferroelectric heterostructures. This antiparallel magnetic domain evolution is attributed to energy minimization with the uniaxial strains by E-fields which can induce the rotation of domains no more than 90°. Moreover, domains can be driven along only one or both easy axis directions by reasonably selecting the initial magnetic domain distribution. The vector analysis of magnetic domain evolution can provide visual insights into the strain-mediated magnetoelectric effect, and promote the fundamental understanding of electrical regulation of magnetism.

Magnetoelectric Memory Based on Ferromagnetic/Ferroelectric Multiferroic Heterostructure

Electric-field control of magnetism is significant for the next generation of large-capacity and low-power data storage technology. In this regard, the renaissance of a multiferroic compound provides an elegant platform owing to the coexistence and coupling of ferroelectric (FE) and magnetic orders. However, the scarcity of single-phase multiferroics at room temperature spurs zealous research in pursuit of composite systems combining a ferromagnet with FE or piezoelectric materials. So far, electric-field control of magnetism has been achieved in the exchange-mediated, charge-mediated, and strain-mediated ferromagnetic (FM)/FE multiferroic heterostructures. Concerning the giant, nonvolatile, and reversible electric-field control of magnetism at room temperature, we first review the theoretical and representative experiments on the electric-field control of magnetism via strain coupling in the FM/FE multiferroic heterostructures, especially the CoFeB/PMN–PT [where PMN–PT denotes the (PbMn1/3Nb2/3O3)1−x-(PbTiO3)x] heterostructure. Then, the application in the prototype spintronic devices, i.e., spin valves and magnetic tunnel junctions, is introduced. The nonvolatile and reversible electric-field control of tunneling magnetoresistance without assistant magnetic field in the magnetic tunnel junction (MTJ)/FE architecture shows great promise for the future of data storage technology. We close by providing the main challenges of this and the different perspectives for straintronics and spintronics.