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Coupled Lattice Polarization and Ferromagnetism in Multiferroic NiTiO3 Thin Films Tamas Varga, Timothy C. Droubay, Libor Kovarik, Manjula I Nandasiri, Vaithiyalingam Shutthanandan, Dehong Hu, Bumsoo Kim, Seokwoo Jeon, Seungbum Hong, Yulan Li, and Scott A. Chambers ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04481 • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017

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ACS Applied Materials & Interfaces

Coupled Lattice Polarization and Ferromagnetism in Multiferroic NiTiO3 Thin Films

Tamas Varga,1* Timothy C. Droubay,2 Libor Kovarik,1 Manjula I. Nandasiri,3 Vaithiyalingam Shutthanandan,1 Dehong Hu,1 Bumsoo Kim,4,5 Seokwoo Jeon,5 Seungbum Hong,4,5 Yulan Li,6 and Scott A. Chambers2

1 2

Environmental Molecular Sciences Laboratory, Richland, WA

Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 3

Imaging and Chemical Analysis Laboratory, Montana State University, Bozeman, MT 4

5

Materials Science Division, Argonne National Laboratory, Argonne, IL

Department of Materials Science and Engineering, KAIST, Daejeon 34141, Republic of Korea 6

Physical and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, WA

*Corresponding author: [email protected]

Keywords: multiferroic, nickel titanate, epitaxial film, epitaxial strain, ferroic properties

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ABSTRACT: Polarization-induced weak ferromagnetism has been predicted a few years back in LiNbO3-type compounds MTiO3 (M = Fe, Mn, Ni). While coexisting ferroelectric polarization and ferromagnetism have been demonstrated in this rare multiferroic family before, first in bulk FeTiO3, then in thin-film NiTiO3, the coupling of the two order parameters has not been confirmed. Here we report the stabilization of polar, ferromagnetic NiTiO3 by oxide epitaxy on LiNbO3 substrate utilizing tensile strain, and demonstrate the theory-predicted coupling between its polarization and ferromagnetism by x-ray magnetic circular dichroism under applied fields. The experimentally observed direction of ferroic ordering in the film is supported by simulations using phase-field approach. Our work validates symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and weak ferromagnetism in MTiO3 transition metal titanates crystallizing in the LiNbO3 structure. It also demonstrates the applicability of epitaxial strain as a viable alternative to high-pressure crystal growth to stabilize metastable materials, and a valuable tuning parameter to simultaneously control two ferroic order parameters to create a multiferroic. Multiferroic NiTiO3 has the potential in spintronics applications where ferroic switching is employed, such as new four-stage memories and electromagnetic switches.


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INTRODUCTION MTiO3 (M = Fe, Mn, Ni) compounds have seen a recent spike of attention following a theoretical prediction that, when crystallizing in the LiNbO3 (LNO)-type structure, they exhibit multiferroic properties.1 These properties originate from a rare case of polarization-induced ferromagnetism stemming from the antisymmetric Dzyaloshinskii-Moriya interaction,2 and may enable electricfield switching of the magnetization. This 180o-switching of magnetization holds the promise for new technologies that include electric-field controlled low-power spintronics3 and four-state memories for data storage.4-5 There are early reports on the successful recovery of the LNO-type phase (space group R3c) for the MnTiO3 and FeTiO3 compounds in the bulk from hightemperature, high-pressure synthesis.6-9 Following the above theoretical prediction, there have been new efforts to make LNO-type MTiO3 for their multiferroic properties; specifically bulk FeTiO3,9 thin-film NiTiO3,10 and bulk MnTiO3.11 However, to date, the multiferroic property in the strict sense, that is, the coupling of ferroelectric polarization and ferromagnetism has not been experimentally demonstrated in this group of compounds. Demonstration of magnetoelectric coupling requires bulk single crystals or epitaxial thin films where the polar and magnetic domains aligned in a certain direction in the magnetic and electric fields can be switched in concert by switching the direction of one of the fields.12-13 Building on our previous work showing that epitaxial strain can be used to stabilize LNO-type NiTiO3 and tune its ferroic properties,10, 14 we relied on epitaxial strain in synthesis to prepare multiferroic epitaxial NiTiO3 films deposited on LNO substrates using pulsed laser deposition (PLD). Substrate-exerted strain has been successfully employed to tune physical properties in many thin-film materials.12, 15-22 Insulators that were neither ferroelectric nor ferromagnetic could be transformed into ferroelectric ferromagnets using a single control parameter, strain.23 Biaxial strain was used to 3 ACS Paragon Plus Environment


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increase the transition temperature in high-TC superconductors,15-16 and ferroelectric materials.1719

Regarding multiferroics, the resurgence of the field was triggered by the growth of strained

BiFeO3 thin films.12 In a similar fashion to the above-mentioned MTiO3 (M = Fe, Mn, Ni) phases, EuTiO3 was first predicted to exhibit strong ferromagnetism (ferromagnetic moment: 7 µB/Eu atom) and strong ferroelectricity (polarization: 10 µC/cm2) simultaneously under strain,23 and was later successfully grown as multiferroic thin film under tensile strain.24 Recently, we have demonstrated that epitaxial NiTiO3 films with the LNO-type structure can be grown on (001)-cut sapphire (Al2O3) or LNO substrates by PLD.10, 14 Bulk unstrained NiTiO3 displays the one-atmosphere, equilibrium, ilmenite (ILM) structure with space group R3̅.25 However, a significant strain (either compressive or tensile) can be employed to stabilize the R3c LNO-type structure in NiTiO3, and introduce a polar lattice distortion that may induce ferromagnetism in the compound. We have recently found that from substrates exerting different strain states for NiTiO3; sapphire Al2O3 – very large compressive strain (5.7%), hematite Fe2O3 – small tensile (0.1%), and LNO – large tensile (2.3%), the NiTiO3/LNO films displayed the best structural and ferroic properties.14 In this paper we show, based on synchrotron x-ray magnetic circular dichroism (XMCD) results, that NiTiO3 films grown on LNO substrates exhibit the theory-predicted1 magnetoelectric coupling, and therefore they are multiferroic. Theoretical calculations using phase-field modeling that predicted the direction of the expected ferroic ordering in NiTiO3 thin films are also presented.


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RESULTS AND DISCUSSION The structural phase transition from the ILM to the LNO-type phase is accompanied by an increased distortion of the coordination octahedra and a reduction of the unit cell size.9, 26 In the bulk, this change had been brought about by compression; in FeTiO3 it resulted in a volume reduction of 1.1 % (the unit cell volume of the ilmenite phase is V = 315.93 Å3.27 while for the high-pressure LNO phase V = 312.53 Å3).9 This suggested that compressive strain was needed to stabilize multiferroic NiTiO3. However, due to the lack of appropriate substrates that provide compression to NiTiO3 (only sapphire Al2O3 seemed appropriate), and the very large strain that sapphire exerts, we had to explore tensile strain in our synthesis effort as well. The in-plane mismatch strain, ε, can be estimated by ε = (asubstrate – afilm)/asubstrate, where asubstrate and afilm are the in-plane lattice parameters of the substrate and film, respectively, in Å. Strain associated with the LNO substrate was estimated to be: εLNO = 2.3% (tensile strain). The lattice parameters used for the calculation were a = 5.030 Å for NiTiO328 and a = 5.151 Å for LNO.29 The predicted effect of strain imparted by the substrate and the expected polar (P) (and ferroelectric; FE) as well as ferromagnetic (FM) properties are shown in the schematic strain/ferroic phase diagram in Figure 1 along with the schematic structures of unstrained bulk NiTiO3 and a strained NiTiO3/LNO film. The point of the schematic is simply to convey that polarization is due to structural distortion brought about by strain, and, in turn, polarization leads to ferromagnetism. According to the theory, this happens by enabling canting of the antiferromagnetic spins on Ni in the slightly distorted R3c structure.1 The crystal structures of the films grown on the three different substrates mentioned above have been reported earlier in detail.14 Here we highlight the properties most important to our pick of NiTiO3 on LNO substrate for the magnetoelectric coupling experiment. Here we simply note



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that the NiTiO3/LNO films proved to be of superior structural quality to, and having more desired ferroic properties to create a multiferroic material than the NiTiO3/Al2O3 and NiTiO3/Fe2O3 films. In the remainder of this paper, we focus solely on the NiTiO3/LNO films and discuss their microscopic structure, ferroic properties, leading to their magnetoelectric coupling. Figure 2a shows a θ-2θ XRD scan for NiTiO3 on LNO substrate in the vicinity of the (006) and (0012) out-of-plane reflections. The pattern shows that only (001) reflections are present with no evidence for secondary phase formation. The (003) and (009) peaks that would indicate the presence of an ordered R͞3 phase, but are extinguished in the R3c symmetry, are not present for the film. The film is epitaxial, and consists of a single phase, of the nominal NiTiO3 composition based on the XRD and the microscopy (see further below). The inset in Figure 2a shows a typical rocking curve around the (116) in-plane reflection of NiTiO3 deposited on a (001)-cut LNO substrate. The narrow peak full width at half maximum (FWHM = 0.024o) suggest good NiTiO3 crystallinity with the growth of >100 nm NiTiO3 crystallites with a high degree of in-plane alignment. XRD φ scans in the (104) plane for both film and substrate are shown in Figure 2b. Three reflections are expected 120o apart due to the threefold symmetry. The equal number and the overlap of the Bragg peaks for the film and the substrate confirm the expected rhombohedral-onrhombohedral structure with the epitaxial relationship NiTiO3 (100) [001] // LNO (100) [001]. The out-of-plane lattice parameter (c) for the NiTiO3 film was determined by averaging the values obtained from the positions of the (006) and (0012) Bragg reflections. The in-plane lattice parameters (a and b) were determined by averaging the inter-planar spacing values for Bragg reflections (104), (116), and (1010). As reported before by our group,14 and as reflected in the crystallographic properties from the x-ray diffraction (XRD) results, different strain states


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resulted in different lattice parameters and crystalline quality in the films. More detail on the structure and physical properties of NiTiO3/Al2O3 and NiTiO3/Fe2O3 samples in relation to strain have been published elsewhere.14 The measured lattice parameters for the NiTiO3/LNO sample were 5.039(1) Å for the in-plane a (and b), and 13.836(1) Å for the out-of-plane parameter c. While these parameters are close to those reported for bulk and thin-film NiTiO3 with the ILMtype structure,28, 30 they are consistent with the tensile strain exerted by the LNO substrate with its larger lattice parameters of a = b = 5.151 Å, c = 13.865 Å.29 The lattice parameters for the ILM- and LNO-type polymorphs of other MTiO3 were also similar, e.g. for FeTiO3, the parameters are a = b = 5.09 Å; c = 14.09 Å in the ILM-type,27 and a = 5.123 Å; c = 13.760 Å in the LNO-type phase.7 Scanning transmission electron microscopy (STEM) analysis confirms that the films are well crystallized and have a high degree of epitaxy (Figure 3), in agreement with the XRD. Atomic resolution images (such as Figure 3b) of the NiTiO3/LNO interface reveal that there is a > 1 nm thick, structurally distinct transition layer between the substrate and the well-oriented film. STEM images evidence a number of defects along this interface, consistent with the sizeable lattice mismatch and the apparent columnar growth. No evidence of Ni/Ti segregation was found by energy dispersive x-ray spectroscopy (EDX) analysis. We expect that at over 2 % in-plane mismatch, there should be a strong tendency for the structure to release strain by at least some partial relaxation. Some of the strain is released by the columnar nature of growth (see Figure S1 in Supplementary showing the presence of columnar grains in the sample). These columnar grains are on the order of 5-20 nm, which should significantly relax the epitaxial strain. In addition, some of the flawed regions near the interface may be due to the formation of strained islands to release strain energy as observed in other large or moderate mismatch systems.31



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However, the structural relaxation is incomplete and significant strain exists to distort the structure inducing lattice polarization. Our high-resolution high angle annular dark field (HAADF)-STEM imaging shown in Figure 3c suggests that NiTiO3 exhibits a rhombohedral arrangement that is more consistent with the R3c structure. It is inconsistent with the R͞3 structure, where Ni2+-Ni2+ and Ti4+-Ti4+ cation pairs alternate as shown for a reference sample (NiTiO3 grown on Al2O3(001) with a Ni0.7TiO2.7 actual composition) stabilized in the ILM structure in Figure 3d. It is noted that the contrast between the Ni2+ and Ti4+ ions in Figure 3c is not as well defined as in 3d due to both reduced HAADF intensity and some cation disorder in the sample. X-ray photoelectron spectroscopy (XPS) revealed that Ni was present as Ni2+, with the presence of some Ni3+ on the surface, while Ti was in the +4 oxidation state (see Figure S2 in Supporting Information). The elemental compositions by RBS suggest the Ni:Ti ratio to be 0.83 in the sample measured, suggesting that the films are not perfectly stoichiometric, they are actually of a Ni1−xTi1−yO3-z general composition (see Figure S3 in Supplementary). Film orientation, magnetic easy axes and domain morphologies were modeled by a phase field model32-33 considering a film with a rhombohedral crystal structure (space group R3c). An orthogonal coordinate x = (x1,x2,x3) is set with x3 axis being normal to the film plane and parallel to the [0001] axis of the hexagonal crystal structure. The [0001] axis coincides with one of the rhombohedral [111]rh direction (see Figure 4c), where the subscript rh refers to the rhombohedral crystal structure. The x1 axis is set to be parallel to one edge of the hexagonal crystal structure. The magnetic anisotropic energy density for a rhombohedral crystal was generally described by: ݂௔௡௜௦ = ‫ܭ‬ଵ ሺ1 − ݉ଷଶ ሻ + ‫ܭ‬ଶ ሺ1 − ݉ଷଶ ሻଶ


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where K1 and K2 are magnetic anisotropy parameters, and m3 is the component of the unit magnetic vector m = (m1,m2,m3) along the x3-axis. Our simulation considered periodic boundary conditions along the x1 and x2 directions in the film plane. The schematic of the film model, the visualization of the directions of ferroelectric polarization vector P and magnetization vector M, and possible domain morphologies are shown in Figure 4 and additional details are provided in the Supporting Information (Figure S4). We predict two possible ferromagnetic domain morphologies depending on the values of the exchange parameters. When K10 and K2=0, the magnetic easy axis is along the [001]x. In this case, the magnetization in the film is either along the [001]x direction (indicated as red in Figure 4b), or the [00-1]x direction (blue). As predicted before, the ferroelectric easy-axis lies in the material is the [111]rh direction1 or [001]x, which is perpendicular to the film plane. This model with K1

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