thin films

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Investigation of Jahn–Teller splitting with O 1s x-ray absorption spectroscopy in strained Nd

_1−x

Ca

_x

MnO

₃

thin films

Daniel Hsu,¹Y. S. Chen,^1,2M. Y. Song,^1,3C. H. Chuang,^3,4Minn-Tsong Lin,^3,5W. F. Wu,² and J. G. Lin^1,a兲

1Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan

2Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan

3Department of Physics, National Taiwan University, Taipei 106, Taiwan

4National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan

5Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 115, Taiwan

共Received 24 November 2009; accepted 6 January 2010; published online 29 January 2010兲 Electronic structures of strained Nd_1−xCaxMnO₃共NCMO兲 thin films with x=0 to 0.8 are investigated via x-ray absorption spectroscopy 共XAS兲. The obtained O 1s spectra within the photon energy 529–535 eV can be decomposed into eg↑¹, eg↑², t_2g↓, and eg↓ bands. Based on the assigned energy levels of these band states, the energies of magnetic exchange, crystal field and Jahn–Teller共JT兲 splitting are determined. Particularly, the JT splitting is around 0.8 eV, which is observed with O 1s XAS for the first time in NCMO thin films. © 2010 American Institute of Physics.

关doi:10.1063/1.3299023兴

Perovskite manganites with chemical compositions Re_1−xA_xMnO₃, where Re is a rare-earth共Re=La, Pr, and Nd兲 and A is an alkaline-earth metal 共A=Sr, Ba, and Ca兲, have activated a great amount of research because of their extraor- dinary physical properties such as the colossal magnetoresistance 共CMR兲 and the strong correlations of spin, charge, and orbital ordering.^1–8 In this mixed-valence system of Mn³⁺/Mn⁴⁺, the strong coupling between the magnetic ordering and the electrical conductivity is explained by the double-exchange model, in which the holes in e_gorbitals are the electrical carriers that move on a background of t_2g ions.^3–5 The phase separation exists in these materials and was suggested to be the origin of the CMR effect.^9–11One of the important features of these perovskite oxides is the Jahn–

Teller 共JT兲 distortion, which is related to the local oxygen octahedra and can be controlled with the effective radius of 共R,A兲-ion. For example, the O octahedron sustains a stronger static JT distortion in ReMnO₃ than AMnO₃ due to the high crystal anisotropy.¹² Overall, the electronic properties of Re_1−xA_xMnO₃are mainly determined by the state energies of the Mn 共3d兲 bands under the combined effects of the crystal field splitting, magnetic exchange and the JT distortion.^13,14 Based on the theoretical prediction for LaMnO₃, the splitting energy should about 3.0 eV due to the magnetic exchange, 2.0 eV from the crystal field, and 1.5 eV from the JT distortion.¹⁵However, the JT splitting energy was never been confirmed with any experimental evidence.

XAS is a particularly sensitive probe for the electronic structures and thus can provide the direct information about the valence, the unoccupied electronic states, and the effective charge of the absorber atom in a solid.^16–18Despite the numerous XAS studies and the related work of electron- energy-loss spectroscopy in CMR materials,^18–24there is still lack of information about the splitting energy of JT distortion. Therefore, it is our speculation that the actual JT splitting energy in CMR system may be smaller than the theoret-

ical value, which makes it hard to be detected.

In this work, we report a systematic XAS study on the strained Nd_1−xCaxMnO₃ 共NCMO-x兲 thin films with x=0 to 0.8. The reason for choosing the system of NCMO-x is that the tolerance factor, defined as t =共rNd/Ca+ r_O兲/ 冑2共rMn+ r_O兲, is between 0.900 and 0.915, which is smaller than that in 共La,Sr兲MnO3 共t⬃0.930兲, thus generating a relatively stron- ger lattice distortion.²⁵In addition, the Ca-doping could raise the population of egunoccupied state to enhance the intensity of the corresponding XAS spectra. The goal of this work is to provide an experimental evidence on the strain/doping effects on the JT splitting in the CMR films.

The NCMO-x thin films with x = 0 to 0.8 are deposited on LaAlO₃ 共LAO兲 single-crystal substrate by using the pulsed laser deposition with a KrF 共248 nm兲 laser in the flowing O₂ atmosphere of 50 mTorr at the temperature of 800 ° C. Sintered targets of stoichiometric single-phase oxides of Nd₂O₃, CaCO₃, and MnO₂are used for ablation at an energy density of ⬃2 J/cm². The thickness of film is controlled by the deposition time and around 100 nm for all samples. The phase purity of all the films is analyzed with x-ray diffraction共XRD兲 using Bruker D8 system. The XAS experiments on NCMO-x thin films are performed at the beamline 05B2-EPU at the National Synchrotron Radiation Research Center of Taiwan using the end station photoemis- sion electron microscopy.

X-ray diffraction patterns exhibit only 共00l兲 lines for both the NCMO-x thin films and the LAO substrate as shown in Fig. 1共a兲, indicating good crystalline structures of these films with a preferential growth along the c-axis. The c lat- tice parameters of all thin films are determined from the 共002兲 d-spacing and plotted against x as open symbols in Fig.

1共b兲, with the dashed line marking the c-parameter of LAO and the solid symbols representing the data of bulk NCMO-x.²⁶ Since the b-parameter of the bulk sample with x⬍0.3 is much larger than that of LAO substrate and it decreases with increasing x, the compressive strain for x = 0 and 0.1 is relatively greater than the highly doped ones.

Therefore, the c-parameter of NCMO-x film first keeps as a

a兲Author to whom correspondence should be addressed. Electronic mail:

jglin@ntu.edu.tw.

APPLIED PHYSICS LETTERS 96, 041914共2010兲

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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constant value of 7.799 Å for x⬍0.4, and starts to decreases linearly at x = 0.4 to 7.477 Å at x = 0.8. In addition, the varia- tion of c-parameter with increasing x is more profound in films than the bulks, reflecting the dominant role of interface strain over the doping effect on the modulation of crystal structures. It is worthy to note that the interfacial strain changes from compressive to tensile at x = 0.8 due to the doping-induced reduction of unit-cell of NCMO-0.8.

The changes of XAS spectra with Ca doping are ana- lyzed on the leading features of O 1s as shown in Fig.2for 共a兲 NCMO-0.1 and 共b兲 NCMO-0.6. Following the standard analysis by using software IFEFFIT with FEFF theoretical functions,²⁷the curve is fitted with five peaks for x = 0.1 and six peaks for x = 0.6, including the first structure of Mn 3d 共from P1 to P4兲 and the second structure of Nd 5d/Ca 4d 共P5 and P6兲. Three features of Mn 3d structure are attributed to the hybridization of O 2p with Mn t_2gand e_g, in which the crystal field of the MnO₆ octahedron splits the Mn 3d band into the low-energy t_2g and the high-energy e_g sub- bands. In addition to the effect of crystal field, magnetic exchange field splits each energy levels into two different polarizations of spin up↑ and spin-down ↓. The correspond- ing electronic structure of Mn 3d with spin polarizations and crystal-field effect is sketched in Fig.2共c兲. It should be noted that e_g↑ level could further split into eg↑¹ and e_g↑² in the presence of JT effect. Based on this sketch, the relative en- ergy levels of Mn 3d states can be obtained from the peak position in XAS spectra. For example, the first feature of NCMO-0.1 from 529 to 531.5 eV is assigned to a superpo- sition of majority eg↑² and minority t_2g↓ bands, which cor- respond to P2 at 529.6 eV and P3 at 530.7 eV, respectively.

The second feature centered at 532.3 eV共P4兲 is attributed to the band of minority e_g↓. On the other hand, the first pre- edge of NCMO-0.6 can be well fitted with three peaks: P1 at 528.7 eV for e_g↑¹, P2 at 529.5 eV for e_g↑², and P3 at 530.7 eV for t_2g↓. It is interesting to note that P1 becomes observ- able in the samples of xⱖ0.5, implying that the absorption

of eg↑¹ gradually enhances with x and becomes strong enough only for xⱖ0.5.

In Fig. 3共a兲 the position of peak i 共PEi with i=1–4兲 versus the Ca content 共x兲 is plotted, indicating that the PE- value of P3 and P4 slightly increases with increasing x, while

20 30 40 50 60

0.0 0.2 0.4 0.6 0.8

7.5 7.7 7.9

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Intensity(a.u.)

2_θ(degrees)

x=0.0

(002)

x=0.1 x=0.4 x=0.6 x=0.8

s s

(a)

(b)

Nd1-xCa

xMnO

3

cparameter(angs.)

x(Ca content) Film Bulk

LAO

FIG. 1. 共Color online兲共a兲 The XRD patterns of NCMO-x films on LAO substrate with all lines being identified as共00l兲. 共b兲 The c-parameter vs Ca content for NCMO-x films and bulks with dashed line being the value for the LAO substrate.

528 531 534 528 531 534

Intensity(a.u.)

PE(eV) x=0.1

(a) (b)

exp. data fitting data p1 p2 p3 p4 p5 p6

x=0.6

FIG. 2. 共Color online兲 Leading feature of O 1s from 525 to 540 eV in 共a兲 NCMO-0.1 and共b兲 NCMO-0.6 film; the curves are fitted with five to six peaks. 共c兲 Schematic energy diagram of Mn 3d levels in NCMO-x films with⌬EME,⌬ECF, and⌬EJTdenoted as the magnetic exchange interaction, crystal field splitting, and JT distortion, respectively.

0.0 0.2 0.4 0.6 0.8

0.5 1.0 1.5 2.0 2.5 3.0 528 529 530 531 532

∆E_ME

∆E_JTe

g↑

∆E_CF

∆E(eV)

x(Ca content)

p4

p3 PE(eV) p2

p1 (a)

(b)

FIG. 3.共Color online兲共a兲 Plot of all positions of peaks in the first structure with various Ca content. P1, P2, P3, and P4 represent the eg↑¹, eg↑², t_2g↓, and e_g↓ bands, respectively. 共b兲 Plot of energy parameters ⌬EME,⌬ECF, and

⌬EJTin the first structure of O 1s spectra for various Ca content.

041914-2 Hsu et al. Appl. Phys. Lett. 96, 041914共2010兲

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that of P2 is near a constant with increasing x. It suggests that the combined effects of strain and doping may cause a gentle modulation in the energy level for eg↑²and eg↓ /t2g↓.

Based on the differences between various energy levels, sev- eral important parameters can be derived, including the magnetic exchange energy⌬EME= PE4 −共PE2+PE1兲/2, the crystal field energy ⌬ECF= PE4 − PE3 and the JT distortion energy ⌬EJT= PE2 − PE1. The results of various energy parameters versus Ca-content are plotted in Fig. 3共b兲. With increasing the content of Ca from zero to 0.1,⌬E_MEfirst jumps from 2.32 to 2.92 eV with x from 0 to 0.1 and then near linearly increases to 3.02 eV at x = 0.8. Meanwhile, ⌬ECF

increases greatly from 0.76 to 1.20 eV for x changing from 0 to 0.1 and then keeps as 1.45⫾0.05 eV for xⱖ0.1.

Most interestingly, the value of ⌬EJT is estimated as

⬃0.8⫾0.1 eV for the samples with xⱖ0.5, which is about half of the value predicted for LaMnO₃.¹⁴The appearance of P1 for xⱖ0.5 关as seen in Fig. 3共a兲兴 and the small value of

⌬EJT suggests that JT splitting can be detected with XAS only when the e_g↑¹ band has significant amount of unoccupied states.

In conclusion, the electronic structures of strained Nd_1−xCaxMnO₃films with x = 0 – 0.8 are systematically inves- tigated via XAS. The O 1s spectrum is used to identify the energy states and is decomposed into e_g↑¹, e_g↑², t_2g↓, and eg↓ bands. The energies of magnetic exchange, crystal field splitting and JT distortion are obtained as 2.3–3.0, 1.45, and 0.8 eV, respectively. It is worthy to mention that this XAS study demonstrates that the JT splitting of CMR system can be detected only in the sample with significant amount of holes.

This work is supported by the National Science Counsel of R. O. C. 共Grant No. NSC-98-2811-M-002-061兲, and the top-project of National Taiwan University.

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