Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films upon Al-doping

Journal Pre-proof Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films upon Al-doping J.L. Izquierdo, A. Astudillo, J. Martínez, G. Bolaños, O. Morán PII:

S0022-3697(19)31262-4

DOI:

https://doi.org/10.1016/j.jpcs.2019.109302

Reference:

PCS 109302

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 2 June 2019 Revised Date:

2 December 2019

Accepted Date: 6 December 2019

Please cite this article as: J.L. Izquierdo, A. Astudillo, J. Martínez, G. Bolaños, O. Morán, Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films upon Al-doping, Journal of Physics and Chemistry of Solids (2020), doi: https://doi.org/10.1016/j.jpcs.2019.109302. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films upon Al-doping J. L. Izquierdo1, A. Astudillo2, J. Martínez2, G. Bolaños2, O. Morán3 1 Facultad de Ingeniería, Grupo de Investigación e Innovación en Energía GIIEN, Institución Universitaria Pascual Bravo, Medellín-Colombia 2 Universidad del Cauca, Departamento de Física, Laboratorio de bajas temperaturas, Popayán, Colombia 3 Universidad Nacional de Colombia, Medellín Campus, Facultad de Ciencias, Departamento de Física, Advanced Oxides Group, A.A. 568 Medellín, Colombia. Abstract The influence of Al3+ doping on the ferromagnetic response of epitaxial TbMn1-xAlxO3 (x=0, 0.1) thin films (~100 nm) was studied experimentally. TbMnO3 and TbMn0.9Al0.1O3 films (~100 nm thin) were epitaxially grown on (001)-SrTiO3 substrates by means of the highpressure oxygen DC magnetron sputtering technique. X-ray diffraction patterns clearly showed that both films were epitaxial, with the c-axis oriented in the (00ℓ) direction. X-ray photoelectron spectroscopy analysis confirmed that the nominal valence of the Mn ions in the TbMnO3 film is 3+. Although a small shake-up peak was observed in the Mn 2p edge of the TbMn0.9Al0.1O3 film, depth-profiling experiments showed that the presence of this peak was only a surface effect. Anomalous ferromagnetism was observed in the pristine TbMnO3 films, which seemed to be caused by coupling between the magnetization and the epitaxial strain, due to the lattice mismatch between the film and substrate. Interestingly, drastic changes in the ferromagnetic response of the TbMnO3 films were observed upon Al3+ substitution at the Mn3+ positions (chemical pressure). Although Al3+ and Mn3+ ions are isovalent, the smaller size of the Al3+ ions brings about further microstructural strain, which clearly influenced the ferromagnetism observed in the TbMnO3 films. The introduction of Al3+ ions into the TbMnO3 lattice resulted in a shift of the (00ℓ) reflections to lower angles as compared with those of the TbMnO3 films. The TbMnO3 films showed a well-defined transition at ~42 K, which corresponds to the magnetic ordering temperature from the paramagnetic phase to the

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sinusoidal antiferromagnetic structure of the Mn spins. The antiferromagnetic transition shifted to lower temperatures for the TbMn0.9Al0.1O3 thin films. The experimental results demonstrated that the magnetic response of the single-phase spin-driven multiferroic TbMnO3 can effectively be tuned by both epitaxial strain and chemical pressure at the Mn site. Corresponding author: E-mail: [email protected] Phone: 57-4-4309327. Fax: 57-4- 2604489

1. Introduction The verification of the presence of multiferroicity and giant magnetoelectric (ME) coupling in BiFeO3 (BFO) has spurred interest in exploring multiferroic materials with better ferroelectric and magnetic properties, with the aim of designing and realizing multifunctional devices [1]. The possibility of growing thin films on crystalline substrates is of pivotal importance in achieving this goal, because the multiferroicity can be controlled by variation of strain, dimensionality, and size effects without a significant effect on the chemical composition (low-level doping) of the substance. In particular, the physicochemical properties of epitaxial multiferroic thin films can be tuned by controlling the substratemediated strain [2]. In addition, these properties can also be tuned by the application of chemical pressure, i.e. through the replacement of the transition metal ions by other ions of different ionic radii. Orthorhombic TbMnO3 (TMO) is a multiferroic material, in which ferroelectricity emerges from the magnetic spiral order [3]. This feature is associated with competing magnetic interactions at low temperatures, which can be tuned by small lattice distortions [4]. In this regard, it has been established that strain-induced deformation of the TMO unit cell brings with it an unbalancing of magnetic interactions [3]. Thus intriguing ferromagnetic properties have been reported in TMO films, whereas their bulk counterpart

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remains an antiferromagnetic material [5]. In the well-studied multiferroic material BiFeO3, the antiferromagnetic spin order coexists with the ferroelectricity. Nevertheless, this antiferromagnetic spin order does not generate any macroscopic magnetic moment [6]. For potential multiferroic-based devices, it would be beneficial to improve the ferromagnetic response of these materials in order to achieve sizable values of spontaneous magnetization and coercive fields. Apart from the strain-mediated ferromagnetism reported in TMO thin films [7], a plausible approach to tuning the magnetism of TMO is the substitution of Mn3+ with Al3+ ions [8]. This substitution may provoke a local, small disturbance of the lattice around the Al3+ ions in order to relax the strain produced by a size mismatch [9]. Although the substitution is isovalent, still a 17% difference in the ionic sizes of Mn3+ and Al3+ (0.535 and 0.645 Å for Al3+ and Mn3+, respectively) exists [8]. Some efforts have been devoted to substituting Al3+ ions at the Tb3+ site in TMO, both in bulk and thin film form [10,11]. The results suggest that the large difference between the sizes of these ions (∼45%) can generate a large degree of disorder, which would in principle hamper the growth of Tb1-xAlxMnO3 films with high crystalline quality on SrTiO3 (STO) substrates. The bridge oxygen and the metal ions can form structural units of octahedra (CN6), tetrahedra (CN4), and cubes (CN8) [12]. In this context, the sizes of Al3+, Mn3+, and Tb3+ ions, measured in orienting structural units of octahedra (CN6), tetrahedra (CN4), and cubes (CN8), are 0.4607 Å, 0.3821 Å, and 0.8357 Å, respectively [13]. This indicates that substitution of ions forming structural units of coordination 4 with ions forming structural units of coordination 6 is more viable than substitution with ions forming structural units of coordination 8, as is the case with Tb3+. The relative strength of any bond in a structure can be determined by dividing the total charge of an ion by the number of close neighbors to which it is attached. So the strengths of 0.75 and 0.5 of Al3+ and Mn3+, which are significantly greater than the 0.375 of Tb3+, are more stable in a substitution. On considering the possibilities for substitution in the ABO3 perovskites, it

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is evident that Al3+ can occupy either the A or B position. Nevertheless, according to the previously-mentioned listing of ionic sizes, coordination numbers, and relative bond strengths, substitution at position B is more favorable. Study of the effect of a substitution of Mn3+ with Al3+ in TMO deserves special attention, because recent investigations have demonstrated that small substitutions (x≤0.1) of Mn3+ with nonmagnetic cations have no strong effect on the magnetoelectric properties of the Mn sublattice [8,9]. In general, the substitution of Mn3+ will lead to a variation of the exchange interactions between Mn ions, JMn–Mn, and consequently to a variation of the exchange interactions between Mn and Tb ions, JMn–Tb, keeping JTb–Tb fixed. Any variation in JMn–Tb will affect the Tb magnetic ordering, as reported in previous studies [14]. The effect of the substitution of Mn3+ with Al3+ on the magnetoelectrical properties of TMO both in bulk and single-crystal form has been previously reported [9]. However, the effect of a similar substitution on the physical properties of epitaxial TMO thin films is presently not part of the literature on this challenging multiferroic. Here it is also important to point out that most TMO films in which the presence of well-defined ferromagnetism is reported have been grown by means of pulsed-laser deposition (PLD) [15]. Nevertheless, DC magnetron sputtering technique can be a suitable alternative for growing high-quality TMO films on crystalline substrates. The low deposition rate of this technique allows having better control of the homogeneity and crystallinity of the films. In the present investigation, the successful growth of epitaxial TbMn1-xAlxO3 (x=0, 0.1) thin films on (001)-oriented STO substrates by means of the versatile and inexpensive DC magnetron sputtering technique, as well as the effect of the substitution of Mn3+ with Al3+ ions on their structural and magnetic properties, are reported.

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2. Experiment TbMn1-xAlxO3 (x=0, 0.1) films (~100 nm) were deposited on (001)-oriented STO substrates via DC magnetron sputtering. The films were deposited in an oxygen atmosphere at a pressure of 3 mbar, a power of 30 W, and a substrate temperature of 780° C [8]. Once the deposition was carried out, the substrate temperature was reduced to 450° C and the chamber was flooded with oxygen at a pressure of ~800 mbar [8]. The films were then annealed under these conditions for 1 h. X-ray diffraction patterns of the films were registered with a Bruker D8 Discover four-circle X-ray diffractometer using Cu Kα1 radiation (λ=0.15418 nm). X-ray reciprocal space mapping studies were carried out using high-resolution X-ray diffractometry (HRXRD). The elemental composition of the films and the depth profile with high resolution spectra were investigated by X-ray photoelectron spectroscopy (XPS). The source of the Xrays used was a monochromatic Al Kα lamp (300 W) at a photon energy of 1486.6 eV. Photoelectron detection was carried out by means of a concentric hemispherical analyzer (PHI 10360 Model) operating with an analytical aperture of 800 mm in diameter. The binding energy resolution of the instrument was 0.7 eV. The temperature and the magnetic field (H) dependence of the magnetization (M) for the films were measured under zero-field-cooling (ZFC) and field-cooling (FC) conditions using a SQUID magnetometer (Quantum Design). The isothermal M(H) curves were recorded at 40, 20, and 5 K after field cooling (7 T) from 300 K The applied magnetic field was always kept parallel to the film plane. 3. Results and discussion Figure 1(a) shows broader 2θ-ω scans of the TbMn1-xAlxO3 (x=0, 0.1) films. Only reflections stemming from the substrate and the TMO (00ℓ)-planes can be seen. This indicates that the films are c-textured. Secondary or impurity phases are not apparent in the analyzed samples. The absence of additional diffraction peaks in the XRD data of the TbMn0.9Al0.1O3 film

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suggests an adequate solid solution between Mn3+ and Al3+ cations [8]. Moreover, the successful incorporation of Al3+ ions into the TMO lattice is corroborated by the shift of the (00ℓ)-reflections to lower angles as compared with those of the pristine film. The full width at half maximum of the rocking curve of the (002)-reflection for the pristine and TbMn0.9Al0.1O3 films was 0.4° and 0.3°, respectively (inset of Fig. 1(a)). This result indicates that the crystalline quality of the TbMn1−xAlxO3 films (x≤0.1) deposited by means of the DC magnetron sputtering is high. On considering the position of the XRD peaks, the out-of-plane lattice parameter of the pristine and TbMn0.9Al0.1O3 films was evaluated. Values of 0.747(2) and 0.754(2) nm were determined for the pristine and TbMn0.9Al0.1O3 films, respectively. The in-plane measurements (not shown) showed that the values of the lattice parameters a and b of both films were lower than those of their bulk counterpart. The estimated values of parameters a and b of the TMO film were 0.529(1) and 0.577(2), and those of the TbMn0.9Al0.1O3 film were 0.527(1) and 0.574(2), respectively. A lower deviation of the a parameter from the bulk as compared to that of the b parameter is observed. These results confirmed the presence of an overall compressive in-plane strain. The fact that the lattice parameters of the films were larger than the c=0.740(2) nm reported for TMO in bulk form [16] further confirms that a small in-plane compressive strain is present in the films, resulting in the distorted orthorhombic phase. It is expected that the orthorhombic unit cell of TMO will grow along the diagonal 3.905√2x3.905√2 Å of a square STO lattice, which can better accommodate the in-plane film lattice [8,17]. Hence epitaxial strain arises from the coherent matching of TMO to the square-lattice STO substrate. The attained results are in good agreement with previous reports on TMO films deposited on STO substrates via pulsed-laser deposition [18]. As observed above, the most relevant effect arising from the substitution of Mn3+ with Al3+ ions is the increase in the value of the c-axis. Therefore, the orthorhombic distortion should be reduced in the ab plane, as expected from the ionic size difference 6

between Mn3+ and Al3+ cations [8,19]. Physically, a partial substitution of strongly distorted MnO6 octahedra (Jahn-Teller-like) with the regular AlO6 octahedra has been suggested as a possible cause of the reduction of the orthorhombic distortion in the ab plane [20]. Selected areas of the reciprocal space were mapped by means of the X-ray diffraction technique. A characteristic X-ray reciprocal space map (RSM) around the (103) STO Bragg reflection is shown in Fig. 1(b) for an epitaxially-grown TbMn0.9Al0.1O3 film. The position of the STO Bragg peak is determined by the central sharp spot. The other broader visible reflection around it stems from the film. It can be seen that the (116) diffraction peak of the film is not located near the vertical dashed line and is somewhat diffuse along the transverse scan direction [8]. The lack of coincidence of the two reflections and the shape of the peak are a result of the partial relaxation of strain [8,21]. The oxidation state of the Mn ions in the films was studied by means of X-ray photoemission spectroscopy (XPS). This information is of pivotal importance for the interpretation of the results obtained for the thin films. For instance, evidence of the formation of oxygen vacancies, generated by the transformation of some of the Mn3+ into Mn2+ to make up for the charge [22], can be seen in the XPS experiments. For the sake of verifying the valence state of Mn in the pristine and TbMn0.9Al0.1O3 films, narrow XPS scan spectra were recorded for the Mn 2p3/2 peaks. The study of the Mn 2p core level implies a certain degree of difficulty, due to the deconvolution of this core level into multiplets [23]. However, the Mn 2p level also shows distinctive features, according to the valence state of Mn. In this context, the expected values of the Mn 2p doublet of Mn2+ are 640.6 and 652.2 eV (MnO). For Mn3+ (MnO2), the peaks of the 2p3/2 and 2p1/2 states appear at 642.2 and 653.8 eV, respectively [24]. The shake-up satellites normally appear shifted from the main peaks of the Mn 2p doublet by ~5 eV towards higher binding energies [25]. Figure 2 shows the narrow XPS scan spectra around the Mn 2p core shell for TMO and TbMn0.9Al0.1O3 films. The low binding 7

energy peaks are found around 641.6 eV, which is consistent with a nominal Mn valence of +3. Moreover, a splitting of the Mn 2p core level of Eex=11.7(1) eV is found for both films, which certainly is expected for Mn3+ [26,27]. On the other hand, the Mn 2p core level of the pristine film features no shake-up satellites, thus ruling out the occurrence of Mn2+ (Fig. 2(a)). Hence the data for both peaks fit satisfactorily with a peak (no additional peaks were required in order to improve the fit of the experimental data) that correlates with a 3+ valence state of the Mn ions in this sample. This finding supports the existence of only Mn3+ for the TMO films grown via DC magnetron sputtering, and consequently oxygen vacancies are not anticipated in these films. For the TbMn0.9Al0.1O3 sample, a shake-up satellite peak can clearly be seen in the XPS spectrum at ~647 eV (Fig. 2(b)). It is evident that the Al3+ doping introduces sizeable variations in the surface configuration of the TMO film. In order to verify if the presence of the satellite peaks is a characteristic of the whole film, depth-profiling experiments were carried out. For this, an ion gun was used to etch the material for a period of time before being turned off while XPS spectra were acquired. Each ion gun etch cycle exposed a new surface, and the XPS spectra provided the means of analyzing the composition of these surfaces. The results (see supplement material) showed no shake-up peak in the different cycles, which suggested that the peak observed at ~647 eV was only a surface effect. The surface effects are complex, due to known factors such as the completion of the periodicity and the probable reduction of the octahedral symmetry around the Mn ions [28]. Moreover, at the surface, the concentration of defects such as oxygen vacancies can be higher [29]. The existence of a correspondence between oxygen vacancies and Mn2+ ion formation in novel oxides has been established through powerful experimental techniques such as Xray-absorption spectroscopy (XAS) and XPS [30]. The results of the analysis of XAS spectra showed that oxygen loss induces Mn2+ ions at the surface and that no ferromagnetic ordering of spins at the Mn2+ sites occurs [30]. A very important point concerning the presence of

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Mn2+ on the surface of the films is related to the fact that the size of the Mn2+ ion is 30% larger than that of Mn3+. Consequently, the chemical pressure should increase at the manganite surface, and hence the magnetic response stemming from the surface will be different from that of the interior. Another important aspect of the surface chemistry of the TbMn0.9Al0.1O3 film is the lack of an effect of the possible presence of oxygen vacancies on the observed splitting of the Mn 2p levels. Indeed, the Mn splitting remained the same (~11.7 eV) in the TMO and TbMn0.9Al0.1 samples. Figures 3(a) and 3(b) show the O 1s XPS spectrum of the TMO and TbMn0.9Al0.1O3 films. Both samples show pronounced peaks and clear shoulders, located at 530 and 532 eV, respectively. Such splitting indicates that the oxygen in both films exists in two kinds of chemical states. The binding energy at 530 eV is primarily due to the contribution of the crystal oxygen lattice (OL) [31]. The OL signal probably stems from the contribution of Tb–O and Mn–O in the TMO lattice. In turn, the shoulder at 532 eV can arise from chemisorbed oxygen species (OH) on the surface (e.g. chemisorbed water) [31]. The presence of the Al3+ ions in the films was clearly shown via the XPS technique. Figure 3(c) shows a typical XPS scan spectrum of the Al3+ core level. The experimental data fit well with only a peak, which suggests that no mixed valence is present for this doping element. The presence of the Al3+ ions in the core of the film was also verified by depth profiling measurements (not shown). The set of XPS spectra suggested a successful Al3+ substitution over the whole thickness of the film. The temperature dependence of the ZFC and the FC DC susceptibility,χ, for epitaxial TMO and TbMn0.9Al0.1O3 films deposited on STO substrates are shown in Fig. 4. Notable differences in the behavior of the χ(T)-dependence of both samples can be seen in Fig. 4. For instance, the value of the susceptibility at low temperatures is higher for the TbMn0.9Al0.1O3 film than for the pristine one. Moreover, the ZFC χ(T)-dependence of the TbMn0.9Al0.1O3 film shows a well-defined peak at ∼26 K, which is absent in the pristine film. The 9

dependence of the inverse susceptibility χ

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on the temperature for the pristine film shows a

transition at TN≈42 K. At this temperature, the Mn3+ magnetic moments feature a sinusoidal antiferromagnetic (AF) order [32]. The bifurcation of the FC and ZFC curves observed at low temperatures suggests the presence of ferromagnetic interactions in the films [7]. The Al3+ doping affects the long-range magnetic ordering of Mn moments at TN. As observed in the inset of Fig. 4(b), TN moves to lower temperatures (TN~35 K). This effect has also been verified for polycrystalline and single-crystal TbMn0.9Al0.1O3 samples [9]. The shift of TN toward lower temperatures could be linked to the expected attenuation of the superexchange Mn-O-Mn interaction as a result of the dilution of magnetic ions by non-magnetic Al3+ [14]. A very noticeable feature observed in the ZFC χ(T) dependence of the TbMn0.9Al0.1O3 films is the anomaly at ∼26 K. This significant anomaly certainly corresponds to the ferroelectric transition in the sample [2]. Probably the occurrence of a spontaneous polarization in the TbMn0.9Al0.1O3 films is associated with changes of the magnetic phase generated by the additional chemical pressure in the local environment of Mn3+ and Al3+. Figure 5 shows the magnetic hysteresis loops recorded at 40, 20, and 5 K for TMO and TbMn0.9Al0.1O3 films. No metamagnetic transitions, such as those observed in TMO single crystals [9], are in evidence for these films. The appearance of unexpected ferromagnetism in (001)-oriented TMO thin films deposited on (001)-oriented STO substrates has been already demonstrated [5,33]. Several physical approximations have been done in order to explain this phenomenon. First, analysis of polarized neutron reflectometry measurements has indicated that a possible surface layer of ferrimagnetic Mn3O4 (TC=42 K) is not the source of the observed ferromagnetism in strained orthorhombic TMO films [34]. Thus, the anomalous ferromagnetic behavior seems to be an intrinsic characteristic of strained orthorhombic TMO films [5]. Before examining in depth the issue of the ferromagnetism in TMO films, it is of value to mention that reports on the ferromagnetic response of TMO films grown by means 10

of DC magnetron sputtering on STO substrates are rather scarce. The anomalous ferromagnetism in epitaxial TMO films seems to be linked to the strain generated by the lattice mismatch between the film and the substrate, which induces lattice distortion [35]. Such distortion can modify the microstructure of the system and consequently its electronic structure [36]. For instance, the small in-plane compressive strain in TMO would reduce the distance between the two in-plane oxygen atoms ((O(2) and O(3)) of the crystal structure of the MnO2 plane [2], which mediates the in-plane next-nearest-neighbor (NNN) exchange interaction along the b-axis (Jb), as compared to that in the bulk or relaxed films. As a result, larger orbital overlap between these oxygen ions is expected, with a concomitant increase in the value of Jb. Increased values of the parameter Jb is believed to be essential in the triggering of the symmetric magnetostriction [37]. In turn, it has been theoretically demonstrated that the in-plane nearest neighbor (NN) interaction in strained TMO films is strongly ferromagnetic, whereas the in-plane NNN spin exchanges are antiferromagnetic [37,38]. As a consequence, the spins of the Mn3+ ions should align ferromagnetically in the (001) planes, and therefore the ground state of TMO films deposited on STO substrates should be ferromagnetic. In short, electron-lattice coupling tuning emerges as a plausible mechanism governing the ferromagnetic tendency in TMO thin films grown on crystalline substrates. On comparing the M(H) dependence for epitaxial TMO and TbMn0.9Al0.1O3 films (Figs. 5(a) and 5(b), respectively), it is evident that the substitution of Mn3+ with Al3+ ions decidedly modifies the ferromagnetic response of the pristine TMO films. The larger values of the remanence, magnetization saturation, and coercive field (inset) of the TbMn0.9Al0.1O3 film support this conclusion. Although the subject of the dilution of the Mn sublattice with small isovalent nonmagnetic cations is certainly complex [9], it is evident that this substitution generates sizeable structural effects in pristine TMO films in addition to those provoked by 11

the epitaxial strain, as previously discussed. Indeed, although the TMO and TbMn0.9Al0.1O3 compounds are structurally similar, sizeable modifications in the local surroundings of Mn3+ and Al3+ should be present, as verified using powerful experimental techniques [9]. On the one hand, the dilution of active Jahn-Teller cations (Mn3+) with closed shells cations (Al3+) leads to a reduction of the average Jahn-Teller distortion in the Mn(Al)O6 octahedron [8]. Also, it has been observed that the substitution of Mn3+ with Al3+ increased the M-O-M (M=Mn, Al) bond angles up to values that came close to those reported for a GdMnO3 sample [39]. This effect can easily be understood within the framework of the definition of the Goldschmidt tolerance factor [40]. Actually, when the Mn3+ ion is substituted by the smaller Al3+, the M-O bond length decreases, and therefore the tolerance factor t increases, and with it the M-O-M bond angle. The increase in the value of the M-O-M bond angle directly modifies the balancing between the NN and NNN interactions in a similar fashion, as in the case of epitaxial stress. As stated in the introduction, replacement of Mn3+ with nonmagnetic ions has mainly been performed with the purpose of testing the effect of the JMnTb

exchange interaction, which is of pivotal importance in the development of ferroelectricity

[41]. As for the magnetic properties, TbMn0.9Al0.1O3 single crystals have exhibited distinguishable magnetic loops and spontaneous magnetization along the b and c axes, which nevertheless are markedly different from those exhibited by the thin films (Fig. 5). There, it was assumed that the isothermal M(H) curves of the single crystals at low temperatures are mainly controlled by the Tb moments. In accordance with the previous discussion, it is apparent that the occurrence of a sizeable ferromagnetic order in epitaxially-strained TMO films is associated with the modification of the electronic structure as compared to bulk TMO. In this connection, the change of the ground state in TMO due to growth-induced stress is in many ways like that generated by chemical [3] or hydrostatic pressure [4]. In general, variation of bond distances and angles with increasing pressure will affect the

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exchange interactions [4]. In the case of multiferroic materials, in which strong coupling between (anti)ferromagnetism and ferroelectricity exists [42], it is to be expected that the pressure can modify the magnetic spin ordering or the symmetry. This in turn will affect the ferroelectric response and other physical properties of the material, as already observed in the present investigation. By increasing the pressure, both the in-plane NN interaction (FM) and the in-plane NNN interaction (AFM) will be enhanced [42]. As previously mentioned, the increase in the NN and NNN interactions with the pressure is related to the increased value of the Mn-O-Mn bond angles and structural changes in the Mn(1)–O(2)–O(3)–Mn(4) interaction paths, respectively. The effect of chemical pressure has also been verified in other multiferroic materials such as copper oxide compounds [43]. In these materials, the complete range of Cu-O-Cu bond angles (90°–180°) can be accessed through chemical pressure. Here, a variation of the angle of the Cu-O-Cu bond from 180° to 90° induced a change of the NN-Cu-Cu spin interaction from AFM to FM. The increase in the value of the chemical pressure led to increased FM coupling. However, the NNN interactions remained AFM [42]. Finally, it should be mentioned that the dilution with Al3+ does significantly affect the ordering of the Tb3+ ions and its coupling with the Mn3+ ordered sublattice [9]. Nevertheless, results of neutron diffraction experiments have revealed that the replacement of Mn3+ with Al3+ or other nonmagnetic ions is very counterproductive to the magnetic correlation between Mn3+ and Tb3+ sublattices and particularly detrimental to the long-range order of Tb3+ when the Al3+-doping level is 10%. [14]. By means of low-level Al3+ doping, the Tb magnetic ground state can be stabilized. Thus, neglecting the Tb magnetism, it is apparent that the chemical pressure generated by Al3+ doping leads to variations of the bond distances and angles, which in general will affect the exchange interactions and hence the magnetic response of the pristine multiferroic material. 13

4. Summary and conclusions Epitaxially strained TMO and TbM0.9Al0.1O3 thin films were successfully deposited on STO substrates by means of the DC sputtering technique. The XRD patterns indicated that the films were single-phase and (00ℓ)-oriented. The diffraction map showed that the ~100 nm thick TbMn0.9Al0.1O3 films were partially relaxed. XPS analysis confirmed the presence of only Mn3+ in the pristine TMO film, ruling out the presence of oxygen vacancies in these samples. Nevertheless, the Mn 2p core level of the TbM0.9Al0.1O3 film exhibited satellite shake-up peaks, which is typical for the presence of Mn2+ in the films. A sizeable ferromagnetic order was detected in the pristine TMO films, which seemed to be related to the strain-mediated lattice distortion. Larger values of the remanence, magnetization saturation, and coercive field were observed for the Al3+-doped films, which suggested that the substitution of Mn3+ with Al3+ ions decidedly improves the magnetic performance of TMO. It is to be expected that the enhanced ferromagnetism in these films is linked to the additional modification of bond distances and MnO-Mn bond angles by the chemical pressure in the Mn3+, Al3+ environment, which directly affects the super-exchange interactions. A well-defined anomaly was observed in the ZFC χ(T) dependence of the TbMn0.9Al0.1O3 film at 26 K, which could be associated with the occurrence of a ferroelectric transition in these samples. The achieved results showed that the magnetic response of multiferroic TMO can be tuned by epitaxial strain and/or chemical pressure. This finding is relevant for the field of magnetoelectric multiferroics, which in general possess low magnetic moments because of the predominantly antiferromagnetic order. Acknowledgments This paper was supported by the Universidad Nacional de Colombia, Sede Medellín. Three of the authors (A.A, J.M, and G.B) acknowledge the financial support of the Universidad del

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Cauca. O.M acknowledges the collaboration of Dr. S. Geprägs and Dr. M. Althammer at the Walther Meissner Institute (Federal Republic of Germany) for the magnetic measurements. The authors also acknowledge the collaboration of Dr. C. Ostos at the Universidad de Antioquia for the XPS measurements. References [1] S.W. Cheong, and M. Mostovoy, Nat. Mater. 6 (2007)13. [2] K. Shimamoto, S. Mukherjee, S. Manz, J.S. White, M. Trassin, M. Kenzelmann, L. Chapon, T. Lippert, M. Fiebig, C.W. Schneider, C. Niedermayer, Tuning the multiferroic mechanisms of TbMnO3 by epitaxial strain, Scientific Reports 7 (2017) 44753. [3] T. Kimura, G. Lawes, T. Goto, Y. Tokura, A. P. Ramirez, Phys. Rev. B 71 (2005) 224425. [4] N. Terada et al. Magnetic ordering in pressure-induced phases with giant spin-driven ferroelectricity in multiferroic TbMnO3, Physical Review B 93 (2016) 081104. [5] B. J. Kirby, D. Kan, A. Luykx, M. Murakami, D. Kundaliya, I. Takeuchi, J. Appl. Phys. 105 (2009) 07D917. [6] D.-C. Jia, J.-H. Xu, H. Ke, W. Wang, Y. Zhou, J. European Ceramic Society 29 (2009)3099. [7] D. Rubi, C. D. Graaf, C. J. M. Daumont, D. Mannix, R. Broer, B. Noheda, Phys. Rev. B 79 (2009) 014416. [8] J.L. Izquierdo, Role of the Al-doping and epitaxial strain in the multiferroic behavior of TbMnO3 bulk and thin films, Ph.D Theis, Universidad Nacional de Colombia (2017). [9] V. Cuartero, J. Blasco, J.A. Rodríguez-Velamazán, J. García, G. Subías, C. Ritter, J. Stankiewicz, L. Canadillas-Delgado, Phys. Rev. B 86 (2012) 104413. [10] F. Pérez, J. Heiras, R. Escudero, Physica Status Solidi (c) 4 (2007) 4049. [11] F. Perez-Osuna, J. M. Siqueiros, A. Duran, M. P. Cruz, L. Salamanca-Riba, J. Heiras, J. Appl. Phys. 115 (2014) 17D909. [12] W.K. Li, G.D. Zhou, H.C.W. Mak, Advanced StructuralInorganic Chemistry, Oxford University Press, Oxford New York (2008). [13] L.E. Smart, E.a. Moore, Solid State Chemistry, Taylor & Francis Group, New York (2012)

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S.W. Cheong, O.P. Vajk, J.W. Lynn, Magnetic Inversion Symmetry Breaking and Ferroelectricity in TbMnO3. Phys. Rev. Lett. 95 (2005) 087206 [38] E.E. Gordon, S. Derakhshan, C.M. Thompson, M-H. Whangbo, Spin-Density Wave as a Superposition of Two Magnetic States of Opposite Chirality and Its Implications, Inorg. Chem. 57 (2018) 9782. [39] K. Noda, S. Nakamura, H. Kuwahara, Control of ferroelectric phase by chemical pressure in (Gd,Tb)MnO3 crystals, IEEE Trans. Magn. 41 (2005) 2814. [40] V.M. Goldschmidt, Geochemistry, Oxford University Press, Oxford, 1958. [41] Y. Tokura, S. Seki, Adv. Mater. 22 (2010) 1554. [42] T. Aoyama, K. Yamauchi, A. Iyama, S. Picozzi, K. Shimizu & T. Kimura, Giant spindriven ferroelectric polarization in TbMnO3 under high pressure, Nature Comm. 5 (2014) 4927. [43] X. Rocquefelte, K. Schwarz, P. Blaha, S. Kumar, J. van den Brink, Room-temperature spin-spiral multiferroicity in high-pressure cupric oxide. Nat. Commun. 4 (2013) 2511. [42] X. Rocquefelte1, K. Schwarz, P. Blaha, Theoretical Investigation of the Magnetic Exchange Interactions in Copper(II) Oxides under Chemical and Physical Pressures, Scientific Reports | 2 (2012) 759. Figure captions Figure 1. 2θ-ω scans of ~100 nm TMO and TbMn0.9Al0.1O3 thin films grown on a (001)STO substrates. Inset: rocking curves measured around the (002) peaks. (b) RSM around the (103) STO Bragg reflection for a TbMn0.9Al0.1O3 (~100 nm) film. 17

Figure 2. Narrow XPS scan spectra around the Mn 2p for a TMO film (a) and TbMn0.9Al0.1O3 film (b) deposited on STO. The symbols and the solid lines correspond to the experimental data and fits respectively. Figure 3. XPS spectra around the O 1S edges for a TMO film (a) and TbMn0.9Al0.1O3 film (b). The symbols and the solid lines represent the experimental data and the corresponding fits respectively. (c) XPS spectrum around the Al3+-edge for the TbMn0.9Al0.1O3 film. The solid line is the fit to the experimental data. Figure 4. Temperature dependence of the ZFC and FC DC susceptibility (at 1000 Oe) for TMO (a) and TbMn0.9Al0.1O3 films (b). The insets show the temperature variation of inverse magnetic susceptibility for the respective films. Figure 5. Magnetic-field-dependent magnetization for TMO (a) and TbMn0.9Al0.1O3 (b) films at three different temperatures. Inset: Temperature dependence of the coercive field strengths and remanent magnetization for the TMO and TbMn0.9Al0.1O3 films.

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10

1

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Figure 1. Morán

0 23

24 ω [°]

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Figure 2. Morán

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Figure 3. Morán

528

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TbMn0.9Al0.1O3

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Figure 4. Morán

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Figure 5. Morán

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5

Highlights Title: Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films upon Al-doping •

Epitaxial TbMnO3 and TbMn0.9Al0.1O3 thin films are grown via DC magnetron sputtering.

•

Nominal valence of the Mn ions in the TbMnO3 film is 3+.

•

Anomalous ferromagnetism is observed in pristine TbMnO3 films

•

Drastic changes in the ferromagnetic response of TbMnO3 films are observed upon Al3+ substitution at the Mn3+ positions.

Author statement

Title: Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films under chemical pressure. In good faith, I state that the data reported on the aforementioned paper “by O. Morán (corresponding author), J. Izquierdo, J. Martínez, A. Astudillo, and G. Bolaños are original. The data have been measured by at least one of us on samples produced in our laboratory. The achieved results are novel and have not published or are been considered for publication elsewhere.

Oswaldo Morán (corresponding author) E-mail: [email protected]

Conflict of interest statement, in good faith, the authors of the manuscript Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films under chemical pressure (PCS_2019_1213) declare that there is no conflict of interest arising from conflicting financial or other interests.

Yours sincerely Dr. Oswaldo Morán, corresponding author Physics Department, National University of Colombia, Medellín Campus A. A. 568, Medellín, Colombia e-mail: [email protected] Phone: 57-4-4309327 Fax: 57-4- 2604489