J Supercond Nov Magn DOI 10.1007/s10948-014-2838-z
Stronger photo-induced effect in epitaxial thin films of small-bandwidth manganite Pr1-xCaxMnO3 compared to the polycrystalline bulk Sayani Majumdar1,2, Hannu Huhtinen1, Tomi Elovaara1 and Petriina Paturi1 1
Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
2
Nanospin, Department of Applied Physics, Aalto University School of Science, P.O. Box 15100, FI-00076 Aalto, Finland
Abstract Photo-induced magnetization have been studied in small-bandwidth manganite Pr1xCaxMnO3
(x= 0.1) (PCMO) bulk and thin films. It is observed that the persistent change in
magnetization under optical excitation is much stronger in epitaxial thin films of PCMO compared to the bulk. This change is manifested in change of both coercive field and Curie temperature in the thin films. Stronger photo-induced effect in the thin films might have their origin in the substrate-induced strain effect on the magnetic properties.
Corresponding author, e-mail:
[email protected], Phone: +358503633627
1. Introduction Photo-voltaic (PV) energy conversion has become one of the major research topics since the 1990s and large effort has been devoted to explore materials that are capable of producing improved PV conversion efficiency. In conventional semiconductor p-n junctions, one major constraint for PV energy conversion arises due to the presence of an energy band gap Eg which leads to the splitting of the quasi-Fermi energies of photoexcited electrons and holes [1, 2]. This energy band gap leads to loss of photo-excited carriers. Hence, a highly desirable material for efficient energy conversion could be one without any optical band gap in the solar spectrum. In this respect, materials with strong electron correlation i.e. electron-electron or electron-phonon interactions could play an important role where modification of the correlation interactions can lead to exciting opportunities for the study of fundamental mechanisms of photo-excited carrier generation [3, 4]. Perovskite manganites with the general formula R1-xAxMnO3 (where R = La, Pr, Nd etc. and A= Ca, Sr, Ba etc.) have been widely investigated since the 1990s because of their fascinating magnetic and electrical properties like colossal magnetoresistance (CMR), coexistence of different phases, large resistive switching under different external stimuli etc. [5-7]. In hole doped manganites, Mn-valence state is mixed i.e. Mn3+ (3d4, t2g3 eg1 ) and Mn4+ (3d3, t2g3) and the double-exchange (DE) interaction between these Mn ions via the intermediate ligand O(2p6) ion [8] governs the ferromagnetic (FM) and transport properties in these materials. Antiferromagnetic (AF) ordering in manganites is closely associated with superexchange interaction between Mn3+–Mn3+ and Mn4+–Mn4+ ions and hence the ground state magnetic properties can be controlled by the ratio of Mn3+ and Mn4+ ions, i.e., by varying the amount of hole doping (x). For different R and A 2
cations, choice of x etc., the average A-site ionic radius of the compound, , changes significantly and the perovskite lattice starts to get distorted. For smaller like Pr1xCaxMnO3,
the Mn and O ions try to bend towards the cube center modifying the MnO6
octahedra and the Pr1-xCaxMnO3 is insulating in the entire hole doping range. Earlier photo-induced carrier generation has been reported in manganites p-n junctions [4]. In small bandwidth manganite Pr1-xCaxMnO3, large change in resistivity has been reported under optical excitation [9]. Earlier we have also observed a large change in magnetization under light in small bandwidth compound Pr1-xCaxMnO3 (x=0.1) (PCMO) [10, 11] and intermediate bandwidth compound La1-xCaxMnO3 (x=0.1) (LCMO) thin films [12, 13]. However, the photo-induced change in magnetization in these two classes of materials is completely different, i.e. intermediate bandwidth compound LCMO shows increased ferromagnetic (FM) interaction both in low and saturation field regime, but small bandwidth compound PCMO showed increased FM interaction at low fields and decreased FM behavior near saturation fields. Also relaxation of photo-induced magnetization exhibits stretched exponential behavior in LCMO compared to the much faster relaxation in PCMO, beyond detection limit of the SQUID magnetometer. Analysis of the experimental data in light of the charge transfer theory in manganites lead to the conclusion that due to large crystalline distortion, the incident light energy is unable to produce significant charge transfer transition in low bandwidth PCMO and consequently significant improvement in FM ordering, while in intermediate bandwidth LCMO significant improvement in FM ordering is possible under similar incident photon-energy [14]. In the present article we have studied the photo-induced magnetization effect in PCMO polycrystalline bulk and epitaxial thin films. Our results show that photo-induced 3
magnetization is much stronger in the thin films compared to the bulk material which indicate that photo-induced charge transfer is not very significant in PCMO and the observed effect in the films might have its origin in the destabilization of the substrateinduced strain state. 2. Experimental Details PCMO bulk sample was prepared by a solid state method by drying of high-purity Pr6O11, Mn2O3 and CaCO3 salts for 3 h at 500 °C in air and preparing a stoichiometric mixture in the needed metal ion molar ratio. After grinding, the mixed powder is pressed into a pellet and final bulk (target) is prepared by a three-step grinding-pressing-sintering process using 14, 36 and 36 h sintering sequences at 1200 °C in air. Details of the preparation of the bulk as well as their structural analysis have been reported elsewhere [15]. PCMO films with a thickness of 100 nm were prepared by pulsed laser deposition (PLD) on (100) SrTiO3 (STO) substrates using an excimer XeCl 308 nm laser with a pulse duration of 25 ns and a repetition rate of 5 Hz with a laser fluence of 2 J/cm2. The flowing oxygen pressure in the chamber was p = 0.2 torr and the substrate temperature during the deposition was 500 °C, and after introducing the normal pressure of O2 in the chamber, the temperature was increased up to 700 °C, then the film was annealed for 10 min, followed by a cooling process down to room temperature at a rate of 25 °C/min. The post-annealing treatment was made in order to reinforce the optimized single phase FM transition, optimal crystallization and oxygen content in the films. Details of optimization of the film deposition parameters and structural properties of the films have been reported in our earlier communication [16]. The detailed structural characterization was made by x-ray diffraction (XRD) analysis at room temperature using Philips X’Pert Pro diffractometer in the Bragg-Brentano 4
configuration (bulk) and with a Schulz texture goniometer using CuK
radiation, an
incident Ni-filter, 0.04 rad Soller slit and an 0.18° thin film collimator. The -2 data of the bulks was analyzed using Maud-Rietveld refinement and the films with detailed 2D ,2 ) scans of (112)/(031) peaks using 2D Levenberg-Marquardt fitting of Gaussian peaks, in order to calculate the lattice parameters a, b and c. The temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization was measured between temperatures 10 and 300 K with a Quantum Design superconducting quantum interference device (SQUID) magnetometer with external magnetic field of 50 mT. Virgin magnetization as a function of B and the magnetic hysteresis curves were recorded in a field of B = ±500 mT at different temperatures 5, 10 and 30 K. For the films, the external field was always oriented along the film plane, i.e. along the PCMO [101] axis. All the photo-induced measurements were made for the bulk and thin film samples with a special fibre-optic sample holder (different for bulk and film setups) where the laser spot covered the whole sample. The measurements in the dark or with photo-excitation through an optical fibre were made using an AlGaInP laser diode with a line of = 658 nm (1.88 eV). The temperature of both bulk and film samples is controlled during illumination with two independent thermometers located in the gas flow before and after the sample. Depending on the position and the alignment of the optical fibre and the sample holder in bulk of film samples, the magnetic background signal vary slightly, but between the each measurement pair in dark and under illumination, the sample and the optical fibre alignment is not disturbed and therefore the measured differences are always reliable.
3. Results and Discussion 5
3.1.
Structural Properties
From the Rietveld fitting of the XRD data of the bulk sample it was confirmed that the prepared sample is single-phase without any detectable impurity. From the calculation of the lattice structure and the lattice parameters [15] it was found that the data fits best with orthorhombic crystal structure with space group pbmn. The calculated values of the lattice parameters are the following a = 5.451(5), b = 7.661(6), c = 5.572(4). For the thin film, XRD texture measurements confirmed full texturing of the films and 45 orientation of the PCMO in-plane lattice with respect to the STO lattice, thus minimizing the lattice mismatch. Film lattice parameters were calculated to be the following a = 5.449(5), b = 7.638(2), c = 5.563(3). From the calculation of the unit cell volume V, it becomes clear that the PCMO unit cells in the films are more close to the STO substrate unit cell values, which is expected due to close lattice matching between the PCMO and STO. The PCMO lattice is under tensile strain on the STO substrate and even up to 100 nm thick films, the strain is not fully released. The bulk PCMO unit cell volume is found to be 232.687 Å3 whereas the unit cell volume of the film is 231.529 Å3. 3.2.
Magnetic Properties
The virgin curves at 5 K for PCMO bulk in dark shows initial slow change in magnetization (M) with increasing magnetic field B followed by a sharp increase up to 200 mT and eventually a slow increase again up to 500 mT field (Fig. 1(a)). No saturation was observed until 500 mT field. Under illumination almost no change in M-B curve was observed. However, upon closer inspection it is found that magnetic moment increases slightly under light with a peak close to 80 mT (Inset of Fig. 1(a)). This small effect decreased even further with increasing temperature and at 30 K, the change in magnetization was negative i.e. ferromagnetic (FM) interaction decreased under light. 6
This point has been discussed in detail later in this article while discussing the temperature dependent magnetization. For the film, however, the change in magnetization under light was quite substantial. The magnetization of the film in dark indicates metamagnetic transition which upon application of optical excitation vanishes completely and the FM interaction gets improved at the lower B region (
