Session T9: Focus Session: Magnetic Oxide Thin Films And Heterostructures - Interactions at Interfaces and in Superlattices

Upper limit to magnetism in LaAlO$_{3}$/SrTiO$_{3}$ heterostructures

In 2004 Ohtomo and Hwang reported unusually high conductivity in LaAlO$_{3}$ and SrTiO$_{3}$ bilayer samples. Since then, metallic conduction, superconductivity, magnetism, and coexistence of superconductivity and ferromagnetism have been attributed to LaAlO$_{3}$/SrTiO$_{3}$ interfaces. Very recently, two studies have reported large magnetic moments attributed to interfaces from measurement techniques that are unable to distinguish between interfacial and bulk magnetism. Consequently, it is imperative to perform magnetic measurements that by being intrinsically sensitive to interface magnetism are impervious to experimental artifacts suffered by bulk measurements. Using polarized neutron reflectometry, we measured the neutron spin dependent reflectivity from four LaAlO$_{3}$/SrTiO$_{3}$ superlattices. Our results indicate the upper limit for the magnetization averaged over the lateral dimensions of the sample induced by an 11 T magnetic field at 1.7 K is less than 2 G. SQUID magnetometry of the neutron superlattice samples sporadically finds an enhanced moment (consistent with past reports), possibly due to experimental artifacts. These observations set important restrictions on theories which imply a strongly enhanced magnetism at the interface between LaAlO$_{3}$ and SrTiO$_{3}$. Work performed in collaboration with N.W. Hengartner, S. Singh, M. Zhernenkov (LANL), F.Y. Bruno, J. Santamaria (Universidad Complutense de Madrid), A. Brinkman, M.J.A. Huijben, H. Molegraaf (MESA+ Institute for Nanotechnology), J. de la Venta and Ivan K. Schuller (UCSD). \\[4pt] Work supported by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS and DMR under grant DE FG03-87ER-45332. Work at UCM is supported by Consolider Ingenio CSD2009-00013 (IMAGINE), CAM S2009-MAT 1756 (PHAMA) and work at Twente is supported by the Foundation for Fundamental Research on Matter (FOM).   

Magnetotransport behavior in (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ superlattices

Recent advances in the deposition of epitaxial complex oxides have enabled the fabrication of a wide range of materials structures with atomically abrupt interfaces, that are characterized by novel electronic and magnetic ground states. In particular, superlattices that combine the paramagnetic metal, LaNiO$_3$, with various band insulators, such as SrTiO$_3$ and LaAlO$_3$, have attracted considerable theoretical and experimental interest. In this work, (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ ($2 \leq n \leq 5$) superlattices that combine LaNiO$_3$ with an antiferromagnetic insulator are prepared on (001) SrTiO$_3$ substrates using ozone-assisted oxide molecular beam epitaxy. The total superlattice thickness is fixed at $\sim$30 nm. X-ray reflectivity and x-ray diffraction reveal single-crystalline growth with interfacial and surface roughnesses of $\sim$0.2 nm and $\sim$0.5 nm, respectively. Electrical transport measurements carried out on superlattices with $n \leq 3$ show insulating behavior between 5 K and 300 K, while samples with $n$ = 4,5 are metallic with resistivity minima at 90 K and 30 K, respectively, below which we observe negative magnetoresistance. We discuss the role of charge transfer between LaNiO$_3$ and LaMnO$_3$ in understanding these results.   

Confinement induced metal-to-insulator transition in strained LaNiO$_3$/LaAlO$_3$ superlattices

Using density functional theory calculations including a Hubbard $U$ term we explore the effect of strain and confinement on the electronic ground state of superlattices containing the band insulator LaAlO$_3$ and the correlated metal LaNiO$_3$. Besides a suppression of holes at the apical oxygen, a central feature is the asymmetric response to strain in single unit cell superlattices: For tensile strain a band gap opens due to charge disproportionation at the Ni sites with two distinct magnetic moments of 1.45$\mu_{\rm B}$ and 0.71$\mu_{\rm B}$. Under compressive stain, charge disproportionation is nearly quenched and the band gap collapses due to overlap of $d_{3z^2-r^2}$ bands through a semimetallic state. This asymmetry in the electronic behavior is associated with the difference in octahedral distortions and rotations under tensile and compressive strain. The ligand hole density and the metallic state are quickly restored with increasing thickness of the (LaAlO$_3$)$_n $/(LaNiO$_3$)$_n$ superlattice from $n=1$ to $n=3$.   

Electronic structure, charge modulation, and orbital polarization of LaNiO3/SrTiO3 superlattice

First-principles density functional theory calculations have been performed to understand the detailed electronic structure for the various (m, n) combinations of (LaNiO3)m/(SrTiO3)n superlattices. Due to the strong covalency of Ni-O bonds, the valence bands are dominated by Ni-d character and the electronic structure is mainly affected by the local environment rather than the ionic potential. Heterostructuring-induced quantum states and the interaction between them leads to the charge redistribution around the Fermi level, which may be responsible for the charge modulation and metal-insulator transition observed in the related systems.   

Electronic, magnetic, and structural coupling across oxide interfaces

Many complex oxide materials exhibit a strong interplay between spin, charge, and lattice effects. This coupling leads to a variety of novel electronic and magnetic properties, including charge ordered and magnetic states, ``colossal'' magnetoresistance (CMR), and a range of electron transport behavior. The possibility of integrating these different kinds behavior with other types of functionalities has motivated the development of new, artificially structured complex oxide-based materials systems, such as composite multiferroic heterostructures. In certain cases, the atomic-scale interface of these structures can dominate the observed behavior, with new physical properties emerging. For example, in epitaxial ferromagnetic/ferroelectric heterostructures, it is possible to achieve large magnetoelectric coupling that is controlled directly by the charge degrees of freedom. We have studied this coupling using a variety of techniques, including magnetization, magneto-optic Kerr effect magnetometry, and x-ray absorption spectroscopy. In addition, structural distortions that arise exclusively at the interface can influence simultaneously the interfacial electronic transport and magnetic properties. Using high resolution synchrotron scattering, we have determined the interplay between new interfacial structural motifs and the resulting electronic and magnetic function at the interface.   

Ferroelectric control of magnetocrystalline anisotropy and orbital magnetism in thin-film Fe/BaTiO$_{3}$ heterostructures

Correlations between magnetocrystalline anisotropy energy (MAE), ferroelectric (FE) polarization, and orbital magnetic moment are studied for ferroelectric/ferromagnetic heterostructures consisting of barium titanate (BaTiO$_{3})$ and thin-film iron (Fe). Using first-principles calculations we investigated different geometries of the BaTiO$_{3}$/Fe system, in particular with 1, 3, and 5 monolayers of Fe with either a free vacuum surface or Cu as a capping layer. We show that there is a large MAE change ($\sim $20{\%}) upon switching of the polarization sign in the case of a vacuum layer, while the presence of Cu effectively removes the difference in MAE for opposite FE polarization directions in BaTiO$_{3}$. This is explained by analyzing the correlation between MAE and orbital magnetic moments for different geometries and opposite polarization directions, as well as the film thickness. We show that the magnetoelectric coupling between MAE and FE polarization is directly linked to the degree of the magnetoelectric coupling between orbital moment and FE polarization.   

Electron-mediated ferromagnetic behavior in CoO/Al:ZnO multilayers

(111)-oriented epitaxial CoO/Al-doped ZnO (AZO) multilayers show a ferromagnetic behavior up to room tempareture. Their magnetization exhibits an oscillatory behavior as a function of $(i)$ the number of Co layers in the insulating antiferromagnetic CoO, and \textit{(ii)} the thickness of the AZO layers. The ferromagnetism vanishes if AZO is replaced by intrinsic ZnO. This behavior can be explained by the existence of an RKKY-coupling, mediated by the free electrons of the non-magnetic AZO layers, between the uncompensated (111) ferromagnetic planes of insulating CoO when there is an odd number of planes in the layer. The oscillation period of the spontaneous magnetization as a function of the AZO layer thickness matches the Fermi wavevector calculated from the carrier concentration that was deduced from Hall effect measurements. The spin-polarization of the carriers in the AZO layer is confirmed via anomalous Hall effect.   

Towards Room Temperature Spin Filtering in Oxide Tunnel Junctions

Spin filtering, in which the magnetic tunnel barrier preferentially filters spin-up and spin-down electrons from a nonmagnetic electrode, has been demonstrated in junction heterostructures. By incorporating two spin filtering barriers, double spin filter magnetic tunnel junctions (DSF-MTJs) were predicted to yield magnetoresistance (MR) values orders of magnitude larger than that of conventional magnetic tunnel junctions. Recently, DSF-MTJs have exhibited spin filtering with magnetic electrodes at room temperature and at low temperature with nonmagnetic electrodes in EuS-based devices [1,2]. We have fabricated DSF-MTJs with nonmagnetic SrRuO$_{3}$ electrodes and room temperature ferrimagnets, NiFe$_{2}$O$_{4}$ and CoFe$_{2}$O$_{4,}$ for spin filters in pursuit of room temperature functionality. Atomic force microscopy shows smooth films quantified by roughness values between 0.1--0.5nm. X-ray magnetic circular dichroism reveals ferromagnetic Ni$^{2+}$ and Co$^{2+}$, and element-specific hysteresis loops indicate the independent switching of the two spin filters. Transport data reveals junction MR and non-linear I-V characteristics consistent with tunneling. \\[4pt] [1] M.G. Chapline et al., PRB, 74, 014418 (2006).\\[0pt] [2] G.- X. Miao et al., PRL, 102, 076601 (2009).   

Magnetic Behavior of Complex Oxide Magnetic Tunnel Junctions

Half metallic manganese oxides have the potential of producing a large tunneling magnetoresistance (TMR) due to their high spin-polarization. To explore their applicability we investigated magnetic tunnel junctions (MTJs) with ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$(LCMO) electrodes and an insulating PrBa$_2$Cu$_3$O$_7$(PBCO) barrier. In these MTJs, with temperature, the TMR peaks rather than increasing with decreasing T [1]. Our Polarized Neutron Reflectivity studies reveal differences in the magnetization, reversal behavior, and anisotropy, between the bottom and top LCMO layers. As was observed in the YBa$_2$Cu$_3$O$_7$(YBCO)/LCMO system[2,3], with X-Ray Magnetic Circular Dichoism we have found a non-zero net moment on the Cu of PBCO at low temperature, originating at the interface. Unlike for YBCO, the Cu moment does not persist up to T$_C$ of LCMO. These combined results provide a possible origin of the anomalous TMR behavior.\\[0pt] [1] Z. Sefrioui et al., Appl. Phys. Lett. 88, 022512 (2006) [2] J. Chakhalian et al., Nature Phys. 2, 244 (2006); Science 318, 1114 (2007) [3] C. Visani et al., Phys. Rev. B 84, 060405(R) (2011)   

Ferroelectric control of orbital occupancy in manganites

Recent successful fabrication of epitaxial and coherent ferroelectric/manganite interfaces makes it possible to dynamically control charge and spin in manganites [1]. We demonstrate with \textit{ab initio} calculations that in this system, $d$-orbital occupancies of the interfacial Mn atom can also be modulated by flipping the ferroelectric polarization (i.e. flippable orbital polarization). The underlying mechanism is the structural distortions of the oxygen octahedron and the Mn atom inside induced by the ferroelectric polarization. The in-plane orbital $d_{x^2-y^2}$ is stablized by rumpling in MnO$_2$ layers, while the Jahn-Teller distortion ($c/a>1$) favors the out-of-plane orbital $d_{3z^2-r^2}$. This ferroelectric control of orbital occupancy serves as a new approach separate from strain for engineering orbital orderings in transition metal oxides. \\[4pt] [1] C.A.F.Vaz et al., Phys. Rev. Lett. 104, 127202 (2010)   

Giant tunneling electroresistance (up to $\sim $10,000{\%}) in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/BaTiO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ ferroelectric tunnel junctions

Tunnel junction with a Ferroelectric (FE) barrier (FTJ) presents an opportunity for nanoelectronics because of the bi-stable electric field control of the tunneling resistance. FTJs of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/BaTiO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ have been fabricated with pulsed-laser deposition. The special feature in the FTJ design is to insert an ultrathin (0.4 - 1.2 nm) La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ film between La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Ferromagnetic (FM) electrode and BaTiO$_{3}$ FE barrier. A giant and reproducible tunneling electroresistance effect ($\sim $10,000{\%}) was obtained with the reversal of FE polarization, about two orders of magnitude larger than the similar sized FTJ without the inserted La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ layer. This result is consistent with the theoretical prediction [PRL 106, 157203 (2011)] that at a BaTiO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ interface, an anti-FM insulating - FM metallic phase transition can occur in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ when the polarization of the BaTiO$_{3}$ is reversed due to the interfacial charge doping effect.