The low temperature magnetostructural transition in Pr 0 . 50 Sr 0 . 50 CoO 3 : Bulk versus thin film behavior

Bulk versus thin film behavior J. Padilla-Pantoja, J. Herrero-Mart ın, X. Torrelles, B. Bozzo, J. Blasco, C. Ritter, and J. L. Garc ıa-Mu~ noz Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus Univ. de Bellaterra, E-08193 Bellaterra, Spain ALBA Synchrotron Light Source, 08290 Cerdanyola del Vallès, Barcelona, Spain Instituto de Ciencia de Materiales de Arag on, CSIC-Univ. de Zaragoza, 50009 Zaragoza, Spain Institute Laue Langevin, BP 156, 38042 Grenoble Cedex 9, France


I. INTRODUCTION
The properties of Pr 0.50 Ca 0.50 CoO 3 (PCCO) and Pr 0.50 Sr 0.50 CoO 3 (PSCO) perovskites are a very good example of the interesting phenomena in structurally simple cobalt oxides with spin-charge-lattice coupling.2][3] This exceptional electronic mechanism in PCCO and other (Pr,Ln) 1Àx Ca x CoO 3 cobaltites (Ln: lanthanide) with Pnma symmetry is based on the contraction of selected Pr-O bonds 1 that generate a first-order Pr 3þ to Pr 4þ valence transition at T mi .5][6] It becomes ferromagnetic (FM) below T C ¼ 230 K, but a second transition at T a $ 120 K produces an unusual step-like behavior of the magnetization and presumably a change of the magnetic easy axis. 3This unexpected second magnetic transition in FM/metallic PSCO has been ascribed to a coupling of structural and magnetocrystalline anisotropy instabilities. 6owever, a convincing detailed explanation, which must account for some intriguing observations, is still pending.Among them, (i) the hysteretic sharp decrease in the magnetization at low fields at T a ; (ii) the anomalous H dependence of M(T), which increases at the transition on cooling at higher fields; (iii) the absence of the second transition in other half-doped cobaltites without Pr ions, which suggests the importance of this ion.So, the transition was shown to progressively disappear as Pr is replaced by La in (La 1-y Pr y ) 0.5 Sr 0.5 CoO 3 and does not occur in Nd 0.5 Sr 0.5 CoO 3 ; 6 and (iv) The evidence of sudden structural changes and the observation of new structural reflections at T < T a opened the possibility to the occurrence of an orbital or charge order, and seems to be at the origin of changes in the magnetocrystalline anisotropy of this perovskite.Troyanchuk et al. 5 and later Leighton et al., 6 using different structural descriptions for the low temperature phase, underlined the possible importance of the Pr 4f -O 2 p hybridization for this transition.

II. EXPERIMENT
Polycrystalline samples of Pr 0.50 Sr 0.50 CoO 3 were prepared following standard solid-state reaction methods, from the intimate mixture of high purity precursor oxides (Co 3 O 4 and Pr 6 O 11 and SrCO 3 ).The final sintering temperature was 1170 C, under oxygen atmosphere.Samples were then slowly cooled down to room temperature in the presence of oxygen.In addition, thin Pr 0.50 Sr 0.50 CoO 3 films (70 nm) were grown on (100)-oriented LaAlO 3 (LAO) single-crystal substrates (3 Â 5 Â 1 mm 3 ) by RHEED assisted Pulsed Laser Deposition technique by using a KrF excimer laser at energies of 65 mJ and a fluence of 1.4 J/cm 2 , and stoichiometric PSCO ceramic dense pellet as a target.After exploring different growth and annealing conditions, strained epitaxial PSCO (010) film was grown at 700 C under 0.40 mbar O 2 partial pressure.An annealing was done at 500 C for 1 h under 400 mbar O 2 pressure, followed by a slow cooling from 500 C to room temperature (RT).Sample characterization included conventional dc magnetometry and transport measurements, and x-ray and neutron diffraction.
Polycrystalline samples were tested with x-ray powder diffraction using a Siemens D-5000 diffractometer, and found to be single phase.Neutron powder diffraction (NPD) experiments at the Institute Laue-Langevin (Grenoble) were performed using D20 diffractometer (k ¼ 1.87 A ˚) in the temperature range between 15 and 250 K.The structure of the film was studied at the surface diffraction beamline BM25B at the ESRF (Grenoble), using a one dimensional detector (NaI scintillator).

III. RESULTS AND DISCUSSION
The x-ray diffraction (XRD) scans confirmed an epitaxial growth of the PSCO film, with the longest axis (2a c ) of the perovskite structure perpendicular to the surface.Four-fold symmetry was shown by azimuthal (Phi) scans.In addition, the PSCO film was mounted in a cryogenically adapted UHV baby chamber coupled to a six-circle diffractometer in BM25B.The incident beam energy was set to 15.0 keV.The (H,K,L) indices of the measured reflections are defined in the cubic basis of the LAO substrate (a LAO ¼ 3.79 A ˚).The PSCO film grows with their in-plane lattice parameters identical and rotated 45 respect to the LaO (100) substrate (a f ¼ c f ¼ ͱ2*a LAO ¼ 5.35 A ˚) with an out-of plane lattice parameter close to that of the long PSCO bulk lattice value, b f ¼ 7.71 A ˚(bulk value 7.60 A ˚). Varying the incident beam angles, diffraction patterns showed uniform lattice parameters in the z direction, suggesting a strained 2D-like growth.Selected synchrotron x-ray diffraction scans of the PSCO epitaxial film at RT are shown in Figure 1.In Ref. 7, the reflections labelled by letters are described in the basis of the film.Reflections in Fig. 1(a) (hþkþl ¼ 2 n) are compatible with both Pnma and Imma symmetries.Reflections of "d" and "e" type (hþkþl ¼ 2 nþ1), permitted by Pnma symmetry and forbidden by Imma, are absent in the scans shown in Figs.1(b) and 1(c).Thus, Bragg reflections conditions in the film exclude the Pnma symmetry, and agree with the Imma space group, with the glide plane perpendicular to the surface.The mismatch of the LAO substrate induces an in-plane compressive strain into the PSCO film with respect to the bulk (À1.1%); and a tensile strain (þ1.3%) along the out-of-plane direction (b f ).
Figure 2 shows the magnetic susceptibility of the PSCO film as a function of temperature.The resistivity curve is shown in the inset.They are compared with the behaviour of the ceramic sample.Similarly, as for bulk PSCO, the film is metallic down to the lowest temperature.Both samples are ferromagnetic below T c $ 235 K (ceramic) and $225 K (film) (measured under H ¼ 100 Oe).Despite the Curie temperature is rather similar and both specimens are metallic, their low temperature behaviour is markedly different.On cooling, the ceramic PSCO ceramic shows a notable reduction of the FM moment (step in the magnetization) at about 120 K. NPD data confirm a structural transition from orthorhombic to monoclinic symmetry (Imma to I2/a) at this temperature.However, this second transition has been suppressed in the epitaxial PSCO film.No anomalies were detected at low temperatures in the magnetization (black filled dots in Fig. 2) or the crystal symmetry of the film by synchrotron x-ray surface diffraction.
NPD patterns from polycrystalline PSCO were satisfactorily refined using Imma (v 2 ¼ 1.40) and I2/a (v 2 ¼ 1.66) symmetries, respectively, above and below the magnetostructural transition at T A $ 120 K.Besides a reduction of the FM ordered moment (from 1.6 to 0.9 l B /Co), the observed cell transformation agrees with the evolution of cell parameters reported in Ref. 6.A full description of the low temperature phase and a more detailed analysis of the transformation will be published elsewhere.Here, we wish to draw the attention on the different role of Pr-O and Co-O bonds (Fig. 3).From Rietveld analysis of NPD data collected as a function of temperature, we have extracted the temperature evolution of the average R-O and Co-O bond-lengths across the transition at T a .Their evolution across T a , plotted in Figure 3, is rather similar to Pr 0.50 Ca 0.50 CoO 3 across T MI 1 : the Co-O distance hardly changes across the second magnetic transition; however, the average h(Pr,Sr)-Oi bond length exhibits a sudden contraction at T a on cooling (h(Pr,Sr)-Oi XII $ À0.5%, or h(Pr,Sr)-Oi VIII $ À4% if only the first eight oxygens in the nearest neighbor shell around R-site is considered). 8n the monoclinic phase of PSCO, the four shortest R-O bondlengths are (Pr,Sr)-O2 ¼ 2.494(5) and 2.546(5) A ˚(both of them double), indicating a probable strong hybridization of Pr 4f and O 2 p orbitals.Completing the first coordination shell, the following four shortest R-O distances below T a are 2.65, 2.67 and two bondlengths of 2.70 A ˚(all four bonds of (Pr,Sr)-O1 type).As usually, we used labels O1 and O2 referred to, respectively, apical and basal oxygens in CoO 6 octahedra.Therefore, the orthorhombic to monoclinic structural transition stabilizes four short Pr-O2 bonds to basal oxygens in CoO 6 octahedra, giving strong support to the active participation of 4f electrons of Pr in the atypical magnetostructural changes of Pr 0.50 Sr 0.50 CoO 3 .

IV. CONCLUSION
In summary, the formation of four short Pr-O2 bonds to oxygens in the basal plane of the CoO 6 octahedra below T a strongly suggests an active participation of Pr 4f electrons in the anomalous transition of Pr 0.50 Sr 0.50 CoO 3 .These results emphasize the need for further experiments (like x-ray absorption studies at Pr edges) to confirm and clarify the role of Pr 4f-O 2p orbital hybridization effects on this transition.The suppression of the transition in metallic PSCO (010) films epitaxially grown on LAO (100), with very similar Curie temperature, evidences the relevance of the lattice degrees of freedom for the magnetostructural changes in bulk PSCO samples.

FIG. 1 .
FIG.1.Selected synchrotron x-ray diffraction reciprocal space scans at RT from PSCO epitaxial film (see explanation in the text and Ref. 7).