Mn 3d bands and Y–O hybridization of hexagonal and orthorhombic YMnO₃ thin films

Ferroic materials present a spontaneous ordering of a physical parameter below a critical temperature T_C. This parameter is, for instance, the magnetization in a ferromagnet, the electric polarization in a ferroelectric, and the strain in a ferroelastic compound. On the other hand, multiferroic materials exhibit the ordering of at least two different physical parameters [1–3]. Multiferroic compounds with a simultaneous ferromagnetic and ferroelectric ordering are extremely rare [4]. For this reason, the multiferroic definition was relaxed to admit antiferromagnetic and/or antiferroelectric ordering.

Multiferroic materials may present interactions between the magnetic, electrical and elastic degrees of freedom. This means that a given type of ordering could affect, or even induce, a second type of ordering. Importantly, a magnetic (electric) moment could be generated, or modulated, by an external electrical (magnetic) field. This cross magnetic/electric coupling is related to the microscopic origin of the magnetoelectric effect, which differs among different families of multiferroic materials [3, 5]. This behavior opens the possibility of producing memory devices based on this kind of materials. In this context, the production and determination of the properties of epitaxial thin films is crucial for practical applications [6].

Figure 1 shows the schematic diagram of the crystalline structure of hexagonal (top) and orthorhombic (bottom) YMnO₃. The hexagonal structure (h-YMnO₃) corresponds to the stable bulk phase, whereas the orthorhombic structure (o-YMnO₃) can be epitaxially stabilized in thin films using suitable substrates [7]. The hexagonal structure presents a collection of triangular MnO₅ bipyramids, where the local symmetry of the Mn ions is approximately D_3h. On the other hand, the orthorhombic structure exhibits corner sharing MnO₆ octahedra, where the local symmetry of the Mn ions is close to O_h.

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The h-YMnO₃ phase is antiferromagnetic with an ordering temperature T_N of around 70 K [8]. The ferroelectric order in bulk h-YMnO₃ develops at much higher temperatures (570–990 K), and results from steric effects [9]. Below T_N, this material exhibits coupling between the magnetic, electrical, and elastic degrees of freedom [10, 11]. It has been shown that h-YMnO₃ thin films (or the closely related h-LuMnO₃ multiferroic) can be used to modulate the magnetic exchange bias across h-YMnO₃/NiFe interfaces, and subsequently controlling the magnetization of the soft ferromagnetic layer by using an external electric field [12, 13]. On the other hand, due to its robust room-temperature ferroelectricity, h-YMnO₃ has been considered a promising candidate for the production of non-volatile ferroelectric memories [14].

On the other hand, the o-YMnO₃ phase develops an antiferromagnetic order at about 40 K. Upon further lowering of temperature, it transforms to a cycloidal spin ordering, which triggers the appearance of ferroelectricity [15]. The polar axis can be switched by applying a suitable magnetic field, as found in other orthorhombic manganites [15]. Interestingly enough, this responsivity survives in thin films [16, 17]. To be noticed is that thin films of both h-YMnO₃ and o-YMnO₃ phases can be epitaxially stabilized using different substrates [7].

The h-YMnO₃ and o-YMnO₃ thin films present different dielectric properties and magnetic orderings [7]. These differences are directly related to their electronic structures, which reflect the different metal-oxygen bonding properties. We present here a comparative study of hexagonal and orthorhombic YMnO₃ thin films by using O K-edge x-ray absorption (XAS), aiming at elucidating the role of the M-O hybridization on the development of ferroelectricity in these compounds.

It has been recently proposed that in h-YMnO₃ and h-DyMnO₃, there is a large Y–O hybridization that allows off-centering and thus ferroelectricity [18]. It thus follows that in o-YMnO₃ where ferroelectricity is related to the magnetic ordering of the Mn³⁺ magnetic moments, the Y–O hybridization should display distinctive differences compared to h-YMnO₃. XAS data have been reported for h-YMnO₃ single crystals [18] and for hexagonal and orthorhombic thin films of DyMnO₃ [18] and TbMnO₃ [19]. However, to the best of our knowledge a comparative study of the h-YMnO₃ and o-YMnO₃ thin films has never been communicated. Our results, analyzed on the basis of first principles DFT band structure calculations, show significant differences in the Mn 3d bands, mainly reflecting the distinct crystalline field and Mn-O hybridization. On the other hand, the observed Y–O hybridization is relatively large, but of comparable strength, in both the hexagonal and orthorhombic YMnO₃ phases, which has important implications for the origin of ferroelectricity in this compound.

The YMnO₃ (YMO) thin films were grown using pulsed laser deposition. The substrates were yttria-stabilized zirconia (YSZ) and SrTiO₃ (STO). The repetition rate of the KrF excimer laser was 5 Hz; the wavelength was 248 nm, and the pulse duration was 34 ns. The fluence of the laser on the YMO ceramic target was 1.5 J cm⁻². The substrate was placed in front of the target at a distance of 5 cm. The temperature of the substrate was 800 °C and the dynamic oxygen pressure was 0.2 mbar. The films were obtained at a rate of 0.1 Å/pulse and the thickness was 50 nm. The samples were cooled down from 500 °C to room temperature in 1 atm of oxygen.

The thickness of the films was determined using x-ray reflectivity (XRR) measurements. The crystal structure was investigated by x-ray diffraction (XRD). In particular, the structure was determined by θ/2θ scans of the symmetrical reflections, and the epitaxial growth by ϕ scans around selected asymmetrical reflections.

The XAS experiments were conducted at the Laboratrio Nacional de Luz Síncrotron (Brazil). The O K-edge x-ray absorption spectra were measured at the SGM beamline. The base pressure in the experimental chamber was about $1\times {{10}^{-9}}$ mbar. The energy resolution at the O K-edge (about 530 eV) was set to approximately 0.5 eV. The photon energy scale was calibrated using the peak position of reference samples. The angle of incidence was set to about 56 degree, which averages any dependence of the XAS on the polarization of light. The spectra were acquired at room temperature using the total electron yield (TEY) method. Finally, the spectra were normalized to the maximum after a constant background subtraction.

The band structure calculation was performed using the DFT program WIEN2k [20]. This package is based on the full-potential linearized augmented plane wave method (FP-LAPW). The calculations were carried out using the generalized gradient approximation (GGA). The exchange and correlation potential was calculated using the PBEsol functional [21]. The convergence criterion was that the difference in the total energy $\Delta$E was less than 10⁻⁶ eV.

The space group for YMnO₃ in the hexagonal structure was P6₃cm. This structure contains 3 chemical formulas per unit cell. The lattice parameters were $a=6.139~{\mathring{\rm A}}$, $b=6.139~{\mathring{\rm A}}$, and $c=11.397~{\mathring{\rm A}}$. The mesh of $\mathbf{k}$-points in reciprocal space for this structure was set to 7  ×  7  ×  3. The space group for YMnO₃ in the orthorhombic structure was Pbnm. This structure has 4 chemical formulas per unit cell. The lattice parameters were $a=5.259~{\mathring{\rm A}}$, $b=5.842~{\mathring{\rm A}}$, and $c=7.355~{\mathring{\rm A}}$. The mesh of $\mathbf{k}$-points in reciprocal space in this case was set to 6  ×  6  ×  4. The structures of both h-YMnO₃ and o-YMnO₃ were doubled to account for the antiferromagnetic ordering.

4.1. X-ray diffraction

Figure 2 summarizes the XRD results of the hexagonal and orthorhombic YMnO₃ thin films. The top panel exhibits the θ/2θ scans of the symmetrical reflections, whereas the bottom panel displays the ϕ scans around selected asymmetrical reflections.

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The θ/2θ scan of the YMO deposited on the YSZ(1 1 1) presents, besides the YSZ substrate reflections, the (0 0 0 1) reflections of the YMO hexagonal phase. This indicates that the out-of-plane orientation of the thin film is YMO(0 0 0 1)/YSZ(1 1 1). The estimated out-of-plane c-axis of the film is 11.39 Å, which is in agreement with the bulk value of about 11.40 Å. The ϕ scans around the YMO(1 1 2 4) and YSZ(2 2 0) reflections confirm the epitaxial growth with a two crystal twin domains. The corresponding in-plane epitaxial relationships are [1 $\bar{1}$ 0 0]YMO//[1 $\bar{1}$ 0]YSZ(1 1 1) and [1 $\bar{1}$ 0 0]YMO//[0 $\bar{1}$ 1]YSZ(1 1 1).

The θ/2θ scan of YMO deposited on STO(1 1 1) shows, besides the STO substrate reflections, the (2 0 2) and (4 0 4) reflections of the YMO orthorhombic phase. This indicates that the out-of-plane orientation of the thin film is YMO(1 0 1)/STO(1 1 1). The out-of-plane lattice parameter calculated from the peaks positions is $d\left(1\,0\,1\right)=4.30~{\mathring{\rm A}}$, which is close to the bulk value of around 4.28 Å. The ϕ scans around the YMO(1 1 1) and STO(1 1 0) reflections indicate the presence of three crystal domains. The domains are rotated in-plane by 120° due to the three-fold symmetry of the STO(1 1 1) substrate. The corresponding in-plane epitaxial relationships are: [1 0 $\bar{1}$]YMO//[1 1 $\bar{2}$] STO and STO[1 2 1], [1 0 $\bar{1}$]YMO//[1 $\bar{2}$ 1]STO and [1 0 $\bar{1}$]YMO//[$\bar{2}$ 1 1]STO [22, 23].

We have earlier reported on the impact of changing the oxygen pressure during the growth of o-YMnO₃; not surprisingly, it was found that reducing oxygen pressure promotes the appearance of a minor non-collinear spin ordering and an associated net magnetic moment, below the corresponding Néel temperature (about 70 K). This observation is directly connected to the cell parameter variation [24, 25] but with negligible effect on the Mn 3s XPS spectra, where only Mn³⁺ states are visible [26]. Although to the best of our knowledge, no similar systematic studies have been yet reported for h-YMnO₃, under the conditions used here to grow the h-YMnO₃ films, the corresponding cell parameters are almost coincident with the corresponding bulk one, thus suggesting a negligible residual strain due to either epitaxial strain and/or non stoichiometry [27].

4.2. O K-edge x-ray absorption

Figure 3 shows the experimental O K-edge x-ray absorption spectra (dots) of hexagonal (top) and orthorhombic (bottom) YMnO₃. These spectra correspond to transitions from the O 1s core level to empty O 2p states in the conduction band. These states are hybridized with the available metal states at neighboring cations [28, 29], namely: Mn 3d, Y 4d and Mn 4sp/Y 5sp. The region from 528 to 532 eV reveals the Mn 3d bands, from 532 to 538 eV exposes the Y 4d bands, and from 538 to 546 eV discloses the Mn 4sp bands.

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The O K-edge XAS spectrum of h-YMnO₃ is in good agreement with previous reports obtained with h-YMnO₃ single crystals [18, 30] or ceramic samples [31]. The XAS spectrum is also in good agreement with that reported for hexagonal DyMnO₃ [18] and TbMnO₃ [19] films. The O K-edge XAS spectrum of o-YMnO₃ is also similar to that reported for orthorhombic DyMnO₃ and TbMnO₃ films as well as other orthorhombic perovskites (i.e. LaMnO₃) [18, 19, 32].

When comparing the O K-edge XAS spectrum of h-YMnO₃ and o-YMnO₃ films, the most apparent differences appear at the low energy side. The Mn 3d region is clearly different in these samples. This reflects the distinct symmetry of the oxygen coordination of Mn³⁺ in these structures: trigonal bipyramid MnO₅ (D_3h) in h-YMnO₃ and distorted octahedra MnO₆ (O_h) in o-YMnO₃. In turn, this affects the Mn 3d–O 2p hybridization, and consequently, the Mn 3d band region.

A more interesting insight is obtained by inspecting the XAS signal at the Y 4d–O 2p region. First, one notices that in h-YMnO₃ the signal is relatively strong, even overpassing that of the Mn 3d–O 2p structure. A similar observation was made by Cho et al [18], who claimed that this is a fingerprint of the YMnO₃ hexagonal phase. Further, they suggested that this Y 4d⁰ hybridization would be a key ingredient of the ferroelectricity in the hexagonal phase, somehow reminiscent of the Ti 3d⁰ hybridization as a source of electrical polarization in ferroelectric BaTiO₃.

We turn now to the XAS of o-YMnO₃ films. It is clear that the Y 4d–O 2p region in o-YMnO₃ is also strong. A similarly intense Y 4d–O 2p XAS signal can be appreciated in the reported O K-edge XAS data of o-LaMnO₃ and o-DyMnO₃ films [18]. This would challenge the proposed unique key role of the Y 4d–O 2p hybridization on the development of ferroelectricity in h-YMnO₃. Otherwise, the orthorhombic phase should also exhibit a robust ferroelectricity as in the hexagonal phase. We recall here that ferroelectricity in h-YMnO₃ is of structural origin, and thus potentially linked to the Y–O hybridization, whereas in o-YMnO₃ ferroelectricity is linked to the magnetic order.

Figure 3 also compares the spectra to the O 2p states (solid lines) obtained from band structure calculations. These states were calculated in the observed structures with a h-YMnO₃ (layer-by-layer) and o-YMnO₃ (3D chessboard) magnetic ordering. These orderings are within a narrow energy range of 5 meV of alternative magnetic structures, both collinear and non-collinear [33–35]. The calculated spectra were broadened with a 0.5 eV Gaussian to account for the experimental resolution in XAS. The calculated spectra reproduce reasonably well the main features experimentally observed in h-YMnO₃ and o-YMnO₃. The differences are attributed to the core-hole potential and many body effects beyond the GGA approach.

4.3. Density of states calculations

Figure 4 presents the calculated density of states (DOS) of YMnO₃ in the hexagonal and orthorhombic structures. The results for h-YMnO₃, assuming a layer-by-layer antiferromagnetic structure, are in good agreement with previous studies [36, 37]. The DOS of o-YMnO₃, assuming a 3D chessboard YMnO₃ antiferromagnetic structure, is similar to that reported for the hypothetical A-type ordering of the o-TbMnO₃ compound [32], although some fine details differ. The O 2p band is located mainly from  −7 to  −2 eV, the Mn 3d states appears mostly from  −2 to  +3 eV, and the Y 4d levels emerge from  +3 to  +9 eV. We note that the O 2p states are hybridized with both the Mn 3d and Y 4d metal states, and that the Y–O hybridization is similar in both the h-YMnO₃ and o-YMnO₃ structures. The Mn 3d states are split by the corresponding crystal field effects, as well as by the intra-atomic exchange interactions.

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It can be appreciated that the landscape of the Mn 3d band across the Fermi level is completely different for the hexagonal and orthorhombic structures. This was already established by the observed differences in the Mn 3d region of the O K-edge XAS spectra above. These differences are important because they determine the Mn 3d orbital hierarchy, which is of fundamental relevance to rationalize the distinct electrical, magnetic and optical properties of YMnO₃ in these structures.

In summary, we studied the electronic structure of hexagonal and orthorhombic YMnO₃ thin films. The hexagonal film was produced on the YSZ(1 1 1) substrate, and the orthorhombic film was obtained on a SrTiO₃(1 1 1) substrate. The O K-edge XAS spectra show differences between the hexagonal and orthorhombic films. The changes in the Mn 3d bands are mostly related to differences in the crystal field environment in these structures. Most remarkable, is that the Y 4d–O 2p hybridization is relatively large in both structures. This suggests that this hybridization may not be a key ingredient for the origin of ferroelectricity in the hexagonal phase.

We would like to thank the technical and administrative staff of the LNLS for their support. This work was partially supported by the Brazilian funding agencies CNPq and CAPES. This work was also supported by grants from the Spanish government (MAT2014-56063-C2-1-R, Severo Ochoa SEV-2015-0496) and the Generalitat de Catalunya (2014 SGR 734).