Magnetic characteristics of LaMnO_3+δ thin films deposited by RF magnetron sputtering in an O₂/Ar mixture gas

The x-ray diffraction (XRD) patterns of LMO films grown on quartz glass substrates are displayed in figure 1. Obviously, the LMO films without post annealing exhibits amorphous and non-crystal characteristics, which can be ascribed to low substrate temperature during deposition process [19]. Except for as-grown LMO film, the sharp and intense peaks indicate that the post-annealed LMO films are highly crystallized and no impurity phases are founded. Those post-annealed LMO films have peaks of (100), (110), (111), (200) and (210), which can be well indexed by a single-phase perovskite-type rhombohedral structure with R3 C space group [14]. Notably, comparing with (111) and (210) peaks, the relative intensities of (100) and (200) peaks in the samples S4 are much strong than other samples, which demonstrates that the LMO film has a preferential (100) orientation and a large exposed (100) surface when the O₂/Ar flux ratios reach 3 sccm/20 sccm. These observations reveal that the O₂/Ar flux ratios can also affect the anisotropic growth of the crystal. In general, the crystal structure of LMO depends on stoichiometry, varying from orthorhombic (Pbnm) to rhombohedral (R3C) as the oxygen content increases [11, 21]. Therefore, their rhombohedral structures imply that the as-prepared LMO films are non-stoichiometric of LaMnO_3+δ with excessive oxygen in lattices. Furthermore, the crystallite size of S1, S2, S3, S4 and S5 are estimated to be 22, 30, 32, 32 and 31 nm respectively by using FWHM of the highest (110) diffraction peak.

Zoom In Zoom Out Reset image size

The Raman spectroscopy is a powerful technique to further study structure distortion in perovskite manganite oxides. Figure 2 shows Raman spectra for all films in the frequency range of 100 cm⁻¹ to 1200 cm⁻¹. Three distinct vibrational bands at around 220 cm⁻¹, 528 cm⁻¹ and 620 cm⁻¹ are observed, which is in consistent with those previously reported rhombohedral LMO thin films [14]. The peak centered at ∼528 cm⁻¹ is attributed to an antisymmetric stretching (S) model of MnO₆ octahedron, at ∼620 cm⁻¹ are related to the bending (B) model of MnO₆ octahedron and both are associated with JT distortion of MnO₆ octahedron in LMO structure [22]. From the sample S1 to S5, the phonon frequency of peak at ∼620 cm⁻¹ seems unchanged, while the peak located at 528 cm⁻¹ move to the low frequency side (∼511 cm⁻¹ in the sample S5), which suggests a strong spin-phonon coupling and a reduction of polaron-activated JT distortion [23]. The reduction of JT interaction has been proven to influence the magnetic behavior in LMO film. Moreover, the movement of frequency to lower wave numbers generally implies the increasing of oxygen content in LaMnO_3+δ structure induced by increasing O₂/Ar flux ratios during deposition [11]. Our XRD and Raman results demonstrate that high-quality LMO films have been synthesized by RF magnetron sputtering and give evidences that the oxygen content in films increases with increasing O₂/Ar flux ratios.

Zoom In Zoom Out Reset image size

The surface morphologies images of representative LMO films (S1 and S5) are displayed in figure 3. The surface of the sample S1 (figure 3(a)) is relatively smooth with regular grains when deposited in pure argon atmosphere and cracks can be observed distributing randomly on its surface. After O₂/Ar flux ratios increase to 4 sccm/20 sccm (figure 3(b)), cracks and gaps between grains are also easily found on the surface. On the one hand, those cracks induced by the coarsening process of crystallization are almost inevitable in perovskite-type manganite thin films after post-annealing [24]. On the other hand, residual stress exists in the internal of LMO films during RF magnetron sputtering process will cause defects on surfaces of LMO films [25]. Moreover, while deposited at a higher oxygen partial pressure, the film will exhibit more cracks and gaps due to the fragmentation of oxide particulates, as evidenced by surface morphology of the sample S5 [19]. Furthermore, the atomic chemical compositions of oxygen in LMO films are detected by EDX. The oxygen composition in the S1 and S5 are 58.99% and 64.26% respectively. The increase of oxygen content in LMO films with increasing O₂/Ar flux ratios will promote the conversion of Mn³⁺ from Mn⁴⁺. In addition, the thickness for LMO films deposited at different O₂/Ar flux ratios of 0 sccm/20 sccm to 4 sccm/20 sccm are determined to be about 684 nm, 632 nm, 574 nm, 458 nm and 431 nm, respectively. As is seen, the corresponding deposition rate increases with decreasing O₂/Ar flux ratios, which is in coincident with the deposition rate trend of other oxide films grown by RF magnetron sputtering [20]. These results indicate that the O₂/Ar flux ratios have a significant effect on the surface morphologies and the oxygen content in LMO films.

Zoom In Zoom Out Reset image size

To investigate the surface elemental compositions and the valence state of manganese for as-obtained samples, XPS measurements are employed. In the XPS survey spectra for the representative LMO film (the sample S1, figure 4(a)), characteristic La, Mn, O and C elements are found, of which the presence of C elements is due to the hydrocarbon contaminants. As described in figures 4(b) to (f), the classic signals of core level Mn 2p of all films are given via the high-resolution XPS spectrum. The Mn 2p spectrum for LMO films display two distinct peaks at around 641.6 eV and 642.6 eV, corresponding to Mn 2p_1/2 and Mn 2p_3/2 [26]. The peak values of Mn 2p_3/2 can be deconvoluted into two peaks at 641.6 eV and 642.6 eV, which are assigned to Mn₂O₃ (Mn³⁺) and MnO₂ (Mn⁴⁺) respectively [24]. That is, the coexistence of Mn³⁺ and Mn⁴⁺ valence state appears in all samples, even in the LMO film deposited in pure argon atmosphere. As shown in figures 4(b) to (f), the percentage of Mn⁴⁺ in the films increases with increasing O₂/Ar flux ratios, which is calculated to be about 17% (figure 4(b)), 21% (figure 4(c)), 30% (figure 4(d)), 33% (figure 4(e)) and 42% (figure 4(f)) for the sample S1, S2, S3, S4 and S5 respectively by peak fitting analysis. Correspondingly, the percentage of Mn³⁺ in the films gradually decreases from about 83% (figure 4(b)), 79% (figure 4(c)), 70% (figure 4(d)), 67% (figure 4(e)) to 58% (figure 4(f)) with increasing O₂/Ar flux ratios. In general, the Mn³⁺/Mn⁴⁺ ratio is sensitive to oxygen content in manganite films, which is strongly affected by deposition condition [10]. The increase of oxygen content in non-stoichiometric LaMnO_3+δ lattice (seen from EDX data) will favor the presence of higher valence of Mn⁴⁺ and accelerate the transition from Mn³⁺ to Mn⁴⁺ [9, 24]. As we know, the double exchanges of Mn³⁺–O–Mn⁴⁺ are ferromagnetic interactions and the superexchanges of Mn³⁺–O–Mn³⁺ are antiferromagnetic interactions [27]. Consequently, the ferromagnetic interactions and antiferromagnetic interactions will coexist in LMO films, which conjointly determine their overall magnetic. Moreover, the double exchange interactions are strongest when the Mn³⁺/Mn⁴⁺ ratio is about 2:1 [13]. It is clear that the sample S4 has the strongest ferromagnetic interaction with the optimum Mn³⁺/Mn⁴⁺ ratio, thereby has the highest Curie temperature and negligible competition between ferromagnetic and antiferromagnetic interactions. The variation of Mn³⁺/Mn⁴⁺ ratios tuned by O₂/Ar flux ratios in LMO films are responsible for their magnetic characteristics, which will be discussed below.

Zoom In Zoom Out Reset image size

In order to clarify effects of O₂/Ar flux ratios on magnetic properties of LMO films, the temperature-dependent magnetization (M-T, 5–300 K) curves under a magnetic field of 0.1 T for all films are recorded in both ZFC and FC modes, as shown in figure 5. The corresponding magnetic field-dependent magnetization (M-H) loops in a range of ±1 T at 10 k are also measured, and the data are displayed in figure 6. Furthermore, we also test M-H loops at 300 K of all samples (Those figures not presented), which exhibit paramagnetic characteristics. However, LMO films show ferromagnetic behavior (as seen in figure 6) below the transition temperature.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

It is observed that all M–T curves (except for S1) for LMO films increase steeply with decreasing temperature below their Curie temperature (T_C), exhibiting clear paramagnetic-ferromagnetic transition, which is typical characteristics for FM behavior. By observing the inflection points in M–T curves, the T_C values of the sample S2, S3, S4 and S5 are identified to be about 110, 135, 224 and 140 K respectively. The T_C values of LMO films first increase and then decrease as O₂/Ar flux ratios increase. And the highest T_C value (∼224 K) can be obtained in the sample S4, which suggest that the film has the strongest strength of ferromagnetic interaction among all samples. On the contrary, the M–T curve of the S1 sample varies litter or even fluctuates as the temperature decrease and exhibits almost negligible FM signals, leading to an inconspicuous T_C [28]. Please note that the prominent discrepancies between ZFC and FC curves at irreversibility temperature (T_irr) can be observed in S2, S3 and S5, which confirm that a cluster glass state (CGS) occur in those films [13, 29]. The T_irr values of films are determined to be about 24, 65 and 100 K for S2, S3 and S5 respectively. A similar behavior of bifurcation has been also observed in LaMnO_3+δ films deposited on LaAlO₃ substrate, indicating the ferromagnetic nanoclusters are embedded in the antiferromagnetic matrix [22, 30]. Moreover, the corresponding ZFC curves show obvious freezing temperatures (T_f), where magnetizations reach the maximum, implying that ferromagnetic property originate from ferromagnetic nanoclusters [7, 9]. The separation of T_irr and T_f is also another critical indicator of CGS phenomena [29]. While for the sample S4, both ZFC and FC magnetization changes almost simultaneously with the temperature, which is the typical characteristic of the FM state. That is to say, the magnetic characteristics of the S1 and S4 sample are mainly robust AFM and FM state respectively; while for the S2, S3 and S5 samples, the coexistence and competition between ferromagnetic and antiferromagnetic interactions conjointly dominate their magnetic behavior. In detail, the variations of T_C and T_irr and can be evaluated by different strength of ferromagnetic interaction and the degree of competition between ferromagnetic and antiferromagnetic interactions in LMO films originated from the transition from Mn³⁺ to Mn⁴⁺, which are confirmed by Raman and XPS observations. When LMO films deposited in a pure argon atmosphere (S1), most of Mn ions are trivalent (83%), which results in the negligible FM state. In the S2 and S3 films, the percentage of Mn³⁺ ions decreases, while the percentage of Mn⁴⁺ ions increases. Thus, the double exchange ferromagnetic interactions between Mn³⁺ and Mn⁴⁺ and the competition between ferromagnetic and antiferromagnetic interactions are gradually enhanced, as evidenced by the increase of T_C and T_irr from S2 to S3. As discussed above, the S4 film has the strongest ferromagnetic interaction due to the optimum Mn³⁺/Mn⁴⁺, while the competition between competition between ferromagnetic and antiferromagnetic interactions become weakest correspondingly, as is featured by the highest T_C and the illegible T_irr. In our oxygen-sufficient film (S5), a lower Mn³⁺/Mn⁴⁺ ratio leads to a weaker ferromagnetic interactions between Mn³⁺ and Mn⁴⁺ due to the deviation of Mn³⁺/Mn⁴⁺ ratio from 2:1 [13]. As a consequence, the competition between ferromagnetic and antiferromagnetic interactions increases intensively in the S5 film, as evidenced by the reduction of T_C and the enhancement of T_irr. Similar FM/AFM competition and CGS behavior have also been observed in LMO_3+δ films [30], LaMnO₃/SrMnO₃ superlattices [28] and lightly doped manganite nanostructures [31].

More magnetic information can be acquired from hysteresis loops, as shown in figure 6. With O₂/Ar flux ratio increases from 0 sccm/20 sccm to 4 sccm/20 sccm, the saturated magnetization (M_s) is enhanced by almost seven times in magnitude, which is mainly due to the enhancement of Mn³⁺–O–Mn⁴⁺ double exchange interaction. In addition, the reduction JT interaction means the suppression of AFM, as is evidenced by Raman results. It is evident from figure 5 that the sample S4 reaches a high saturated magnetization (M_s) of ∼130 emu cm⁻³ and miniscule coercivity (H_c) of ∼185 Oe, further suggesting a typical ferromagnetic behavior. In comparison with the S4 sample, the sample S5 exhibits a remarkable increase of H_c (∼700 Oe). The enhanced H_c mainly originates from the CSG, which pins or disturbs the ferromagnetic part below T_f [13]. Furthermore, the exchange bias (EB) effect (H_EB = 115 Oe) is only found in the S5 film by a shift of loop along the magnetic field axis, as clearly shown in the inset of figure 6. Zhou et al observes a similar EB phenomenon in LMO films and contributes the EB to the pinning of FM part by the AFM region [5]. The strongest degree of competition between ferromagnetic and antiferromagnetic interactions in the S4 sample among all films will lead to the obvious CGS, thereby forming EB. Moreover, surface morphology affected by growth atmosphere also plays an influence on the magnetic properties of films [32]. The grain boundary and cracks can also restrain the carrier scattering and contribute to the increase of carrier mobility, which may promote the double exchange interaction and enhance magnetization [20]. Accordingly, the sample S5 has a relatively high magnetization. However, the residual strain in LMO films which bring about racks and gaps on its surfaces can decrease the Mn–O–Mn bond length and bond angle, which may weaken the double exchange interaction [33]. Therefore, the saturation magnetization of all as-prepared LMO films is still much lower than that of bulk LMO materials [19]. All these magnetic characteristics in LMO films can be well explained by the conversion from Mn³⁺ to Mn⁴⁺, which is strongly dependent on O₂/Ar flux ratios during deposition.