Surface Valence States of Mn Ions and Magnetic Properties of La0.67Sr0.33MnO3 Films
Ruikun Pan*, Yang Li, Fan Fang, Wanqiang Cao
School of Materials Science & Engineering, Hubei University, Wuhan, China
To cite this article:
Ruikun Pan, Yang Li, Fan Fang, Wanqiang Cao. Surface Valence States of Mn Ions and Magnetic Properties of La0.67Sr0.33MnO3 Films. International Journal of Materials Science and Applications. Vol. 5, No. 5, 2016, pp. 222-227. doi: 10.11648/j.ijmsa.20160505.17
Received: September 10, 2016; Accepted: October 7, 2016; Published: October 13, 2016
Abstract: La0.67Sr0.33MnO3 (LSMO) films were prepared on SrTiO3 single-crystal substrates by the pulsed laser deposition method. X-ray photoelectron spectra (XPS) were measured for the LSMO films as-prepared, annealed in oxygen and in vacuum, respectively. Multiple peaks fitting for Mn 2p3/2 XPS spectra shows that Mn2+ appears and the ratio of Mn3+/Mn4+ increases on the surface of LSMO film annealed in vacuum comparing with the as-prepared film, though the saturation magnetization (Ms) changes little. Mn6+ and Mn7+ can be detected while Mn4+ decreases much on the surface of LSMO film annealed in oxygen. The change of Mn3+/Mn4+ results in the obviously reduce of Ms. O1s spectra show the gradual changes of Mn-O, Sr-O and La-O. Analysis indicates that Mn2+forms at the surface defects of LSMO films. Annealing in vacuum hardly affects the magnetic property of LSMO films. But annealing in oxygen greatly changes the valence states of Mn, which weakens the magnetic property of LSMO films.
Keywords: La0.67Sr0.33MnO3 Film, XPS, Annealing, Magnetic Property
Perovskite-like doped lanthanum manganite La0.67Sr0.33MnO3 (LSMO) is a half-metallic material as its Curie temperature is higher than room temperature, which is very attractive for spintronics applications, e.g. ferroelectric tunnel junctions and all-oxide magneto-electric tunnel junction with LSMO films used as ferromagnetic electrodes [1-5]. Recently many reports focus on the interface structural and magnetic properties of LSMO films deposited on SrTiO3 (STO) substrates [6-8].
After depositing films, annealing process always needs to improve the crystallization and remove some defects. Different annealing temperature, process time and atmosphere can lead to different results for the annealed films. D.R. Sahu et al. reported that La0.7Sr0.3MnO3 thin film were annealed ex-situ at 1173 K and 1273 K temperatures in oxygen flow for 1 h, which improves the magnetic homogeneity of the grain and grain boundaries . High annealing temperature even causes composition segregation. H.P. Xie et al. reported LaMnOx is formed due to strontium segregation on the surface at a temperature higher than 1023 K annealed in air for LSMO thin films . But few studies have focused on the annealing process and the magnetic properties.
X-ray photoemission spectra (XPS) measurements of the core levels and valence electronic structure of Mn 2p in LSMO and other metal oxides have been reported in recent years [11-13]. Mn 2p XPS spectra of LSMO films are more complicated than those of bulk and free ions since Mn2+, Mn3+ and Mn4+ can coexist on the surface or in the films [14, 15]. Multiple-peak fitting is always used to analyze the multiple splitting of Mn 2p spectra of Mn2+, Mn3+ and Mn4+ ions. It should be mentioned that for each Mn ion, multiple peaks with various intensities can be got during the spectral fitting . The intensity ratios of these multiple peaks of each ion in film materials may be changed from those of bulks due to the complexity and metastability of the film materials.
Here we prepared LSMO films by the pulsed laser deposition (PLD) method and fitted Mn 2p3/2 spectra. The intensity ratios of Mn2+, Mn3+ and Mn4+ were changed with different annealing treatments, which have different effects on the magnetic properties of the LSMO films. Highly oxidized states Mn6+ and Mn7+ can even be seen on the surface of LSMO film annealed in oxygen. Mn valence states on the films surface are discussed with the magnetic properties of LSMO films.
LSMO films were prepared on STO (001) single-crystal substrates at 973 K by PLD. A KrF excimer laser (the wavelength is 248 nm) operating at about 2 J/cm2 was focused on the surfaces of a rotating LSMO target. STO substrates were placed on a heater, which was 50 mm away from the target. The working pressure of O2 flow during the deposition was maintained at 30 Pa. As-prepared LSMO samples with same deposition conditions were annealed at 973 K for 1 hours in flowing oxygen at atmospheric pressure (in a tube furnace) and in high vacuum (~1.0×10-5 Pa in the PLD chamber), respectively.
The crystalline structure of LSMO films was analyzed by an X-ray diffractometer (Bruker, D8 Discover) with Cu Kα radiation (λ = 0.15405 nm). The magnetization vs magnetic filed (M-H) curves were measured by using a commercial physical property measurement system (Quantum Design, PPMS). The thickness of LSMO films was indirectly tested by comparing with the corresponding thicker films (above 500 nm) measured by a surface profiler (Taylor Honson, S4C-3D). XPS experiments were carried out on an X-ray photoelectron spectrometer (Thermo Scientific, VG Multilab 2000) with Al Kα radiation (hv=1486.6 eV) under UHV (1.33×10-8 Pa). The energy resolution was set to 0.5 eV for wide scans and 0.05 eV for narrow scans, respectively. The analyzer pass energy was set to 40 eV for narrow scans. All peaks are calibrated by the carbon deposit C1s binding energy at 284.8 eV.
3. Results and Discussion
XRD pattern in Fig. 1 shows that only (00l) diffraction peaks of the as-prepared LSMO film are observed. No other peaks can be detected, which indicates that high quality LSMO films are orientated on STO substrates. After annealing, there are no detectable changes in XRD patterns (not shown). XPS wide scans also indicate that the stoichiometric ratios of the LSMO films did not changed much after annealing (see Fig. 2). The thicknesses of LSMO films are all controlled at about 100 nm by the laser pulses during the preparation.
The inset in Fig. 1 shows that the coercive forces are about 35 Oe, which is attributed to the high quality and low defect density of epitaxial LSMO films on STO substrates . H. Boschker reported the coercive forces (25~100 Oe) of LSMO thin films with different film thickness and substrate surface .
The inset in Fig. 2 shows the Mn 2p photoelectron spectra of the LSMO films. For the as-prepared LSMO film, a broad doublet emission line between 635 eV and 658 eV can be seen, which is agreed with the reports [14, 15, 18]. Two maxima (around 641.4 eV and 653.1 eV) can be assigned to 2p3/2 and 2p1/2 states, respectively. There are also broad doublet emission lines in the annealed films, which shift to lower energy side and higher energy side, respectively. The values of the Mn 2p splitting in these LSMO films are all about 11.7 eV.
Mn 2p3/2 spectra were fitted as shown in Fig. 3. Spectra were analyzed using the PEAKFIT 4.0 software. A standard Shirley background is used for all spectra. The FWHM values of all peaks are set as 2.5~2.7 eV. The peak shapes are all kept as 70% Gaussian-30% Lorentzian. Since there could be more complex valence states in film materials than that of quality bulk samples, spectra fittings can not do as well as that of bulk samples. The peak centers of Mn2+, Mn3+, Mn4+, Mn6+ and Mn7+ are around 639.4, 640.8, 642.4, 644, and 645.2 eV, respectively [13,14,18].
The percentages of total area of peaks after spectra fitting are calculated. The ratio of Mn3+/Mn4+ is about 1.86 on the surface of the as-prepared LSMO film, which is nearly equal to Mn3+/Mn4+ (about 2) in the stoichiometric La0.67Sr0.33MnO3 material. After the film annealed in vacuum, the ratio of Mn3+/Mn4+on the film surface increases to about 3. Since XPS measurements only confirm the surface condition and considering the nearly unchanged M-H curves (see inset in Fig. 1), it can deduce that Mn3+/Mn4+remains at 2 in the film annealed in vacuum.
Jong et al. reported an increase of Mn2+ in La0.7Sr0.3MnO3 films from the resonant photoemission (RPE) measurements after deoxygenating treatments in vacuum, which was connected with Mn2+ ions close to oxygen vacancies mainly formed at surface defects . Here Mn2+on the surface of the films annealed in vacuum might due to the deoxygenating process. Films deposited by PLD usually have low defects. In practice, nearly oxygen stoichiometric LSMO films can be obtained for oxygen pressures above 25 Pa during the deposition [19,20]. The magnetic property of LSMO is originated from Mn3+-O-Mn4+ double exchange interaction . As nonferromagnetic ions , Mn2+ has little effect on Ms of LSMO films.
Fig. 3(c) shows that Mn4+ nearly disappeared on the surface of LSMO film annealed in oxygen. Considering the decreased magnetic property (see inset in Fig. 1), we can deduce the contents of Mn3+ and Mn4+ also changed in the interior of LSMO film after annealing in oxygen, though it can not be seen from the XRD pattern. Because the double exchange interaction between pairs of Mn3+ and Mn4+ ions is responsible for the ferromagnetic properties in LSMO [15, 16, 21].
For the film annealed in oxygen, Mn6+ and Mn7+ states can be seen. Although annealing can improve crystallization, epitaxial LSMO films as-deposited by PLD have well-crystallized [12, 16, 22]. So no improvements in the magnetic properties can be seen from M-H curves after annealing. On the other hand, high oxygen partial pressure can sweep off oxygen vacancies and Mn2+, and then even oxidize Mn ions into higher valence states. The disproportionate Mn3+/Mn4+ in the film annealed in oxygen can be related to the lower saturation magnetization of LSMO films.
Fig. 4 shows O 1s spectra, in which every spectrum was deconvoluted into three peaks. The positions of the three peaks are around 529, 530.2, and 531.3 eV, which are associated with Mn-O, La-O and Sr-O, respectively . It can be seen that the ration of Mn-O is biggest on the surface of LSMO film annealed in vacuum. La-O and Sr-O increase after the film annealed in oxygen, which indicate that the somewhat segregation of Sr might occur on the surface .
Orientated LSMO films prepared by PLD were annealed in oxygen and vacuum, respectively, which show apparently peak-shifting Mn 2p XPS spectra and changed magnetic properties. Multiple peaks fitting for Mn 2p3/2 shows that Mn3+/Mn4+ increase and Mn2+ appears on the surface after the as-prepared film annealed in high vacuum. The saturation magnetization remains unchanged indicates Mn3+/Mn4+ in the film changed little. For the LSMO film annealed in oxygen, high oxygen partial pressure can sweep off Mn2+ and even oxidize Mn ions into higher valence states, which changes Mn3+/Mn4+ anddecreases the saturation magnetization.
We are grateful to the National Natural Science Foundations of China (nos. 51272072).