Session J9: Focus Session: Magnetic Oxide Thin Films And Heterostructures - Multiferroic Thin Films

Electric-field induced magnetization reversal using multiferroics

Controlling magnetism using solely electric fields is interesting not only from a fundamental standpoint, but presents great potential for ultimately low energy consumption logic and memory. The evidence of the electrically controllable antiferromagnetic ordering in the multiferroic magnetoelectric bismuth ferrite (BiFeO3) drew an increasing interest in the pursuit for new emerging devices. To use such functionality for device applications, deterministic control not only of antiferromagnetism, but also ferromagnetism is essential. To achieve this goal, a ferromagnet/multiferroic heterostructure has been proposed based on the combination of magnetoelectric coupling in BiFeO3 and exchange coupling between magnetic materials and offers a new pathway for the electrical control of magnetism. By combination of a piezoresponse force microscopy, photoemission electron microscopy and anisotropic magnetoresistance measurements, we demonstrated the non-volatile reversal of a CoFe layer magnetization induced solely by the application of an electric field at room temperature. This 180 degree rotation of the magnetization of the ferromagnetic layer is mediated by a strong interfacial coupling. The correlation between the ferroelectric state in the multiferroic layer and the CoFe ferromagnetic domain architecture is evidenced. The projection of this strong magnetoelectric coupling in an out-of-plane configuration, allowing the reduction by an order of magnitude of voltage required, will be discussed. Our results show the high potential of magnetoelectric-based heterostructures for future low energy consumption data storage devices.   

Properties of LuFe$_{2}$O$_{4}$ Films Grown by Molecular-Beam Epitaxy

LuFe$_{2}$O$_{4}$ is an exotic material with a simultaneous existence of ferroelectricity and ferrimagnetism at the highest temperature (240 K) of any known material [1]. 25 nm thick films of this unusual multiferroic were grown by MBE on MgAl$_{2}$O$_{4}$, MgO, and SiC substrates. XRD shows that the LuFe$_{2}$O$_{4}$ films are single-phase and epitaxial. Film stoichiometry was regulated using an adsorption controlled growth process by depositing LuFe$_{2}$O$_{4}$ in an iron rich environment at pressures and temperatures where the excess iron desorbs from the film surface during growth. STEM images reveal the layered structure of LuFe$_{2}$O$_{4}$ and a clean substrate-film interface free of second phases. The magnetization data exhibits a rapid increase in magnetization below 240 K consistent with the bulk paramagnetic to ferrimagnetic phase transition. On further cooling, the zero field cooled (ZFC) branch of the magnetization displays a peak at 205 K that is suggestive of a glassy transition, which is also seen in bulk samples. At 100 K and 70 kOe, we observe a saturation magnetization of 2.4 $\mu _{B}$/ f. u. (theoretical value of 3 $\mu _{B}$/ f. u.) \\[4pt] [1] Ikeda \textit{et. al}., Nature \textbf{436} (2005) 1136--1138.   

Growth and Characterization of Magnetoelectric Fe$_{2}$TeO$_{6}$ Thin Films

Promising spintronic concepts such as Cr$_{2}$O$_{3}$ based voltage-controlled exchange bias systems [1] employ electric controlled boundary magnetization. Symmetry arguments reveal that equilibrium boundary magnetization is a generic property of magnetoelectric antiferromagnets [2]. However, experimental evidence of boundary magnetization is scarce and microscopic evidence has only been provided for the Cr$_{2}$O$_{3}$ (0001) surface [3]. In order to bring the concept of boundary magnetization into a broader experimental context we prepare the magnetoelectric antiferromagnet Fe$_{2}$TeO$_{6 }$with tri-rutile structure. We use two distinct approaches for the thin film growth, RF sputtering and pulsed laser deposition (PLD). Both methodologies start from targets which we prepare from sintered powder of Fe$_{2}$TeO$_{6}$ produced in a solid-state reaction. We characterize the magnetoelectric thin film of Fe$_{2}$TeO$_{6}$ structurally, magnetically and magnetoelectrically using XRD, SQUID, RHEED, LEED and MOKE. Our investigation aims on an experimental test of the predicted generality of the equilibrium boundary magnetization in magnetoelectric antiferromagnets. \\[4pt] [1] He, Xi \textit{et al.}, Nature Materials 9, 579 - 585 (2010)\\[0pt] [2] Belashchenko, K.D., Phys. Rev. Lett. 105, 147204 (2010)\\[0pt] [3] Wu N. \textit{et al.}, Phys. Rev. Lett. 106, 087202 (2011)   

Probing of ferroelectric and antiferromagnetic orders of multiferroic YMnO$_{3}$ via second harmonic generation

The ferroelectric and antiferromagnetic properties of epitaxial, hexagonal (0001) YMnO$_{3}$ thin films grown on GaN/Al$_{2}$O$_{3}$ substrates were studied using second harmonic generation. A Ti:sapphire laser with a 15 W Nd:YVO$_{4}$ pump was used to generate the second harmonic signal. Above the N\'{e}el temperature, ferroelectric ordering was clearly observed as deduced from angular plots of the incoming and outgoing polarization of the second harmonic generation (SHG) signals. Additional antiferromagnetic order was identified below the N\'{e}el temperature. The ferroelectric-magnetic coupling studied via SHG will be discussed.   

Investigation of Electric Field Control of Antiferromagnetic Domains in Epitaxial BiFeO3 Thin Films Using Neutron Diffraction

BiFeO3 (BFO) is a multiferroic which displays both ferroelectric and magnetic order at room temperatures.~~ Thin films possess a simple G-type antiferromagnetic order.~ In the bulk, this ordering takes on an additional long-wavelength modulation in the form of a spiral [1].~ Recent neutron diffraction results have revealed that it is possible to recover a modulated magnetic structure in thin films which is strongly dependent on the orientation of the substrate on which the film is grown [1].~ Based on this, BFO thin films were deposited on a vicinal SrTiO3 substrate. PFM measurements demonstrate that a ferroelectric monodomain was achieved.~ New neutron diffraction results show that it is possible to change the population of magnetic domains in such films through the application of an electric field (which also switches the ferroelectric domain state).~~ Based on these results, magneto optic Kerr effect (MOKE) measurements were performed on patterned pads of exchanges coupled Co film deposited on top of the BFO films.~~ The results of these measurements will be discussed in the context of device applications.~~[1]~Lee Seongsu; Choi Taekjib; Ratcliff W. II; et al.,~ Phys. Rev. B 78, 100101 (2008); Ratcliff William II; Kan Daisuke; Chen Wangchun; et al., Advanced Functional Materials 21, 1567 (2011).   

Antiferromagnetic and structural phase transitions in tetragonal-like BiFeO$_3$

The recent observation that strain stabilizes a tetragonal-like (``T-like'') polymorph of BiFeO$_3$ has illustrated how epitaxial constraints can fundamentally alter the properties of this multiferroic material. We performed detailed temperature-dependent neutron and x-ray scattering experiments on epitaxial BiFeO$_3$ films on different substrates (SrTiO$_3$, LaAlO$_3$, YAlO$_3$) to study the nature of the monoclinic crystal structure and of the antiferromagnetic (AFM) order. In agreement with Monte Carlo simulations for a classical Heisenberg model, we observe a much lower N\'eel temperature in the T-like morph (T$_N$ $\simeq$ 325K) than in less-strained (``R-like") films (T$_N$ $\simeq$ 645K), and additionally a low-temperature coexistence of C-type and G-type AFM. Independent of the antiferromagnetic transition, at T $\simeq$ 375K we also observe a structural phase transition from one type of monoclinic distortion to a different one. In combination, these measurements shed light on the complexity of the phase diagram of BFO, and provide routes to explore how the material's properties can be tuned by external parameters. Research supported by the U.S. DOE, BES, MSED (H.M.C., W.S., S.L., E.D., H.M.C) and SUFD (M.D.B., G.J.M, J.L.Z, S.E.N.).   

Reversible switching of magnetic easy axis in Co/BiFeO$_{3}$ thin film heterostructures

We are investigating the magnetic properties of a thin Co layer deposited on top of and exchange coupled to (001) BiFeO3 (BFO) thin films. 5 nm Co layer is evaporated and patterned into 100 micron x 100 micron pads. We measure angular dependent magnetic hysteresis loop of the Co layer using the magneto-optical Kerr effect and by sweeping the direction of the external magnetic field. We find that exchange-coupled Co layer always develops a magnetic easy axis along the [100] or [010] direction of BFO. As electric field pulses are applied between the Co pad and the bottom electrode (SrRuO3), non-volatile changes in the magnetic properties are observed in the Co layer. For a particular Co/BFO configuration, application of an electric field corresponding to the ferroelectric coercive field of the BFO film switches the magnetic easy axis by 45\r{ }. Upon application of the opposite electric field pulse, the easy axis switches back to the original direction. Subsequent applications of the alternating electric field pulses result in repeating reversible switching between the initial easy axis direction and the 45\r{ } rotated direction.   

Probing interface reconstructions in multiferroic BiFeO$_{3 }$ and charge ordered La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ heterostructures

Spurred by the potential for device structures with multiple tuning parameters, recent explorations of carefully engineered oxide interfaces have highlighted intriguing possibilities. A famous example includes the LaAlO$_{3}$/SrTiO$_{3}$ heterostructure where the individual layers are insulating but an electron gas appears at the interface. In similar fashion, the atomically engineered interface between antiferromagnetic BiFeO$_{3}$ and ferromagnetic manganite [La,Sr]MnO$_{3}$ results in the formation of a ferromagnetic state in BiFeO$_{3}$ at the interface. Here we explore the interface between BiFeO$_{3}$ and the charge ordered manganite, La$_{0.5}$Ca$_{0.5}$MnO$_{3}$. The insulating nature of La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and BiFeO$_{3}$ allows us to directly probe the electronic properties of the interface via transport measurements. A combination of capacitance and field effect measurements combined with structural probes shed new light on the charge ordered manganite and multiferroic interface. We explore the effects of cross-plane ferroelectric switching in BiFeO$_{3}$ on charge ordering in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$, and hence the electronic and magnetic properties of La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ near the interface. We discuss our results and implications.   

Promise of new multiferroics: Synthesis and characterization of epitaxial NiTiO$_{3}$ films

In a search for new multiferroic materials where the direction of magnetization can be switched by an applied electric field, we have looked for materials in which polarization and magnetization are strongly coupled. Recent theory calculations predicted that the family of compounds MTiO$_{3}$ (M = Mn, Fe, Ni), in a certain polymorphic structure (acentric $R3c)$, are promising candidates where a polar lattice distortion can induce weak ferromagnetism. Guided by these insights, a rhombohedral phase of NiTiO$_{3}$ has been prepared in epitaxial thin film form, whose structure is very close to that predicted to be a multiferroic. The synthesis of such new epitaxial films, their full structural characterization and physical property measurements along with our first-principles DFT calculations to predict the desired NiTiO$_{3}$ structure and its stability are reported.   

Structural Control of Magnetic Anisotropy in a Multiferroic EuTiO$_3$ Thin Film

Strain control of EuTiO$_3$ has been shown under tensile strain the system converts to a multiferroic groundstate with ferromagnetic and ferroelectric order[1]. Here we present a study of the magnetic order in thin films of EuTiO$_3$ grown on DyScO$_3$(110) substrates by reactive molecular-beam epitaxy (MBE). Neutron scattering and magnetic measurements show the magnetic moment orders with an easy axis along only one of the (110) pseudocubic axis of the unit cell. Such an easy axis is connected to the uniaxial crystal structure that evolves from cubic to tetragonal with octahedral tilting, which agrees well with the strain dependent structure predicted under biaxial tensile strain. The magnetic anisotropy for Eu is attributed to an asymmetric crystal field due to the uniaxial symmetry of the Eu-O coordination. Work at Argonne, including the Advanced Photon, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [1] J.-H. Lee et al. Nature {\bf 466}, 954 (2010).   

Growth and Optical Properties of Multiferroic LuFe2O4 Thin Films

We present surface, structural, electronic, and optical properties of multiferroic LuFe$_{2}$O$_{4}$ thin films grown on single crystal (0001) sapphire, (111) YSZ, and (001) LiNbO$_{3}$ substrates using electron-beam deposition. LFO thin films have been deposited on substrates at temperature 750 $^{\circ}$C in an oxygen environment and post-deposition annealed at temperatures ranging from 600 to 800 $^{\circ}$C to improve the stoichiometry and the crystal quality. We have used AFM and XRD for surface and structural characterization of the LFO thin films. To investigate the charge order phenomenon and the electronic properties, we carried out variable temperature (78 -- 450 K) optical and resistivity measurements on the LFO thin films. The absorption spectra of LFO thin films show strong electronic excitations with the energy gap of $\sim $2.18 eV at 300 K. Further, the energy gap of LFO displays strong temperature dependence, exhibiting a ferrimagnetic transition at $\sim $240 K and a charge-order transition at $\sim $350 K, respectively. We will also discuss the electronic excitations of the LFO thin film in the energy range 0.5 -- 5.0 eV and their correlations with different magnetic and charge-ordered states.   

Exchange bias in BiFeO3/CoFe2O4 heterostructure

Room temperature multiferroics BiFeO3 (BFO), both ferroelectric and antiferromagnetic, has been extensively investigated as a part of exchange bias structures since it promises the electrical field control over the exchange bias. This work focuses on the exchange interaction between ferromagnet CoFe2O4 (CFO) and BFO. Bilayer films CFO and BFO were first grown on SrTiO3 (STO) by Pulsed Electron Deposition (PED) and then field cooled in magnetic field. XRD showed single phase CFO and BFO were grown epitaxially on STO single crystal substrates, and the typical thickness was 21nm for BFO and 3.6$\sim $18nm for CFO. In the bilayer structure, we observed that the coercive field increased 100{\%} at 300K and 30 {\%} at 50K, comparing to these of CFO single layer. Also in the bilayer structure, a noticeable exchange bias field (Hex) increased from $\sim $30 Oe at 300 K to $\sim $ 60 Oe at 50 K. We will discuss the impact of the film thickness and the roughness of interface on exchange bias.   

Magnetic and ferroelectric properties of patterned multiferroic CoFe$_{2}$O4-BiFeO$_{3}$ nanocomposites

CoFe$_{2}$O$_{4}$ (CFO) offers unique properties as a magnetoelectric material due to its large magnetoelastic response when strained. Previous work has shown that when CFO is co-deposited with BiFeO$_{3}$ (BFO) nanostructured phase segregation occurs with CFO pillars forming in a BFO matrix and electrical control of the magnetic anisotropy is possible.[1] Such a system offers unique possibilities for an electrically-controlled spintronic logic scheme.[2] We have recently demonstrated the ability to control the location of the CFO pillars in CFO-BFO nanocomposites using e-beam lithography patterning of uniform CFO films grown on Nb-doped SrTiO$_{3}$. Square arrays of pillars with spacings as small as 100 nm have been grown in nanocomposites using pulsed electron deposition. Piezoresponse force microscopy (PFM) measurements show clear ferroelectric response in the BFO matrix. The out-of-plane piezoelectric response, $d_{33}$, has been measured via PFM within the BFO matrix and is in good agreement with published results for BFO. Magnetic force microscopy (MFM) shows in-plane magnetic anisotropy in the pillars due to compressive in-plane strain. [1] F. Zavaliche, et al. \textit{Nano Lett.} \textbf{7} (2007). [2] S.A. Wolf, et al. \textit{Proc. IEEE} \textbf{98} (2010).