Polarization-dependent x-ray absorption spectroscopy of hexagonal and orthorhombic TbMnO3 thin films

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Polarization-dependent x-ray absorption

spectroscopy of hexagonal and orthorhombic

TbMnO

3

thin films

K H Wu1∗, I C Gou1, C W Luo1, T M Uen1, J-Y Lin2, J Y Juang1, T Kobayashi1

, C K Chen3

, J M Lee1,3_{, J M Chen}3 1

Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. 2

Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. 3

National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan, R.O.C. E-mail: [email protected]

Abstract. Pure phase TbMnO3 manganite thin films with hexagonal (h-TMO) and

orthorhombic (o- TMO) crystal structures were prepared by pulsed laser deposition. The

distinctive orientation alignments between film and substrate obtained here have allowed us to perform the x-ray absorption near edge spectroscopy (XANES) measurements with the electric field applied along the three major crystallographic directions. The XANES results, as expected, display significantly different spectral features for the h-TMO and o-TMO films. In addition, the XANES spectra also exhibit strong polarization dependence at O K and Mn L edges for both samples.

1. Introduction

Multiferroic rare earth manganites ReMnO3 (RMO, R = rare earth) have been investigated

extensively in materials science and condensed-matter physics recently. These complex adaptive materials exhibit the coexistence and mutual coupling of different order parameters, namely the ferroelastic strain, electric polarization and magnetization [1-4]. It is found that the ferroelectricity can be induced by either the complex lattice distortion in hexagonal RMO (h-RMO) or by the spin spiral structure in the orthorhombic RMO (o-(h-RMO). More importantly, the coupling between the electric polarization and magnetization enables the manipulation of the magnetic state of a material through the application of the electric field, or vice versa. In order to understand the physical mechanisms of the coexistence and coupling of magnetic and ferroelectric orders and to design the devices for accomplishing the potential applications of RMO manganites, it is essential to prepare the thin films with the two relevant hexagonal and orthorhombic crystalline structures and to investigate how the crystalline structure affects the manifestations of multiferroicity in the same compound.

It is possible to grow h-RMO or o-RMO thin films of the same compound on different substrates [5-7] by utilizing the technique of epitaxial strain stabilization. Recently, we have grown h-YMnO3 (h-HoMnO3, h-TbMnO3) and o- YMnO3 (o-HoMnO3, o-TbMnO3) thin

films with well-aligned in-plane orientation respectively and studied their anisotropic electrical, magnetic properties and ultrafast dynamics [5, 6]. In the present work, pure phase h-TMO

International Conference on Magnetism (ICM 2009) IOP Publishing Journal of Physics: Conference Series 200 (2010) 012227 doi:10.1088/1742-6596/200/1/012227

c

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and o-TMO manganite thin films were prepared and their anisotropic electronic structures were probed by the polarization-dependent x-ray absorption spectroscopy (XAS). Consequently, the polarization-dependent XAS measurement can elucidate distinctly the anisotropic property of Mn-O bonding and the factor responsible for the magnetoelectric anisotropy of these samples [8-10]. 10 20 30 40 50 60 70 80 1000 2000 3000 4000 5000 6000 2T(degree) (a) S S h -T M O (0 0 8 ) In te n s it y ( a rb . u n it ) h-TMO/YSZ(111) h -T M O (0 0 6 ) h -T M O (0 0 4 ) h -T M O (0 0 2 ) 10 20 30 40 50 60 70 80 0 1000 2000 3000 4000 5000 6000 In te n s it y ( a rb . u n it ) (b) 50100150200250300350400450 20000 40000 60000 80000 100000 I(deg.) TMO(022)/NGO(001) In te n sit y ( a. u .) S o -T M O (0 0 6 ) S o -T M O (0 0 4 ) o -T M O (0 0 2 ) o-TMO/NGO(001) 2T(degree) 20 40 60 80 100 120 140 160 2 4 6 8 10 12 14 M a g n e ti z a ti o n ( e m u *1 0 -5) Temperature (K) FC ZFC h-TMO/YSZ(111) H//ab= 100 Oe (c) 20 40 60 80 100 120 140 160 -4 -2 0 2 4 6 8 10 12 14 Temperature (K) M a g n e ti z a ti o n ( e m u *1 0 -6) FC ZFC o-TbMnO3/STO(001) H//ab= 100 Oe (d)

Figure 1. The θ-2θ XRD pat-tern for the as deposited (a) (00l) h-TMO/YSZ(111) films (b) (00l) o-TMO/NdGaO3(001) films. The

inset of figure 1(b) shows the φ scan of the o-TMO(022) and dis-plays a clear two-fold symmetry. Temperature dependent magnetiza-tion (M-T) curves for the (c) h-TMO/YSZ(111) films and (d) o-TMO/SrTiO3(001) with the

ap-plied magnetic field in the ab- plane (H kab) of the film, the inset shows the corresponding dM/dT curves.

2. Experiment

The pure phased h-TMO and o-TMO films used in this study were prepared by pulsed laser deposition. A KrF excimer laser operating at a repetition rate of 3-5 Hz with an energy density of 2-5 J/cm2

was used. The h-TMO films were deposited on YSZ(111) substrate, while the o-TMO films were deposited on SrTiO3(001) and NdGaO3(001) substrates. The thicknesses of

h-TMO and o-TMO films were around 80 nm and 120 nm, respectively. Figs. 1(a)-1(d) show the typical X-ray diffraction (XRD) patterns and magnetization (M-T) curves of h-TMO and o-TMO films. Fig. 1(a) shows the θ-2θ XRD scans for the as deposited h-TMO/YSZ(111) films. The diffraction peaks reveal pure (00l)-oriented h-TMO reflections without discernible impurity phase, indicating the formation of a pure c axis h-TMO thin film. Similar results for o-TMO/NdGaO3(001) films is shown in Fig. 1(b). Besides, as shown in the inset of Fig. 1(b),

the φ scan of the o-TMO(022) displays a clear two-fold symmetry, indicating that the films were indeed epitaxial and well aligned in the surface of the substrate. Figs. 1(c) and 1(d) show the M-T curves for the h-TMO and o-TMO films with the applied field in the ab-plane (H kab), respectively. The XAS measurements at the O K - and Mn L- edges were carried out using high- energy spherical grating monochromator (HSGM) beamline and Dragon beamline at the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan, operating at 1.5 GeV with a maximum stored current of 200 mA. The O K - edge XAS spectrum was recorded in the surface-sensitive total electron yield (TEY) mode and bulk-sensitive x-ray fluorescence yield mode. The Mn L- edge XAS spectrum was recorded in TEY mode.

3. Results and Discussion 3.1. XAS of o-TMO thin films

Figs. 2(a) and 2(b) show the O K - edge and Mn L- edge XAS spectra of an o-TMO/NGO(001) thin film, respectively. The polarizations Eka, Ekb, and Ekc could be measured individually in this sample. The results are similar to the XAS spectra of o-TMO single crystal that reported in

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640 645 650 655 660 (b)

Photon Energy (eV)

In te n s it y (a rb . u n it ) o-TMO/NGO(001) Mn L-edge E // a E // b E // c 525 530 535 540 545 550 eg e g In te n s it y (a rb . u n it ) o-TMO/NGO(001) (a)

E // a E // b E // c

t

2g + eg

Figure 2. (color online) Polarization-dependent (a) O K -edge and (b) Mn L-edge XAS of o-TbMnO3/NGO(001) thin film for the polarizations Eka, Ekb and Ekc. The assignments of

absorption features are described in the text.

[10]. The Mn 3d region consists of four pre-edge peaks, which appear at 529.7, 530.6, 531.2, and 532.2 eV. The broad structure located around 535 eV is attributed to the O 2p states hybridized with the Tb 5d states, whereas the peak near 542 eV is associated with transitions into the Mn 4sp and Tb 6s states. According to the LDA+U calculation [10] in o-TMO, the pre-edge peaks below 533 eV are identified respectively as the transitions into empty O 2p states hybridized with the majority-spin Mn eg(↑) bands, the minority-spin Mn t2g(↓) and Mn eg(↓) bands, and

the minority-spin Mn eg(↓) bands, as denoted in Fig. 2(a).

It is noted that the features of the O K - edge XAS and Mn L2,3- edge XAS spectra exhibit

significant anisotropy when measuring along the three principal crystallographic directions, which are in contrast to the conclusions drawn in [9]. In that, although the spectra of the hexagonal manganites exhibit strong polarization dependence at both Mn L2,3- and O K edges,

the orthorhombic manganites barely show polarization dependence in the similar absorption edges. However, since the in-plane orientation of the films they used was indistinguishable, therefore, only Ekab and Ekc spectra were measured. In fact, when we measured the spectra of the (001) o-TMO thin film deposited on the STO(001) substrate, in which the a- and b- axes are indistinguishable, or when we took the average of the Eka and Ekb distributions for the (001) o-TMO/NGO sample, the results (Ekab and Ekc spectra) were similar to that shown in Fig. 4 of Ref. [9]. Therefore, the present data are actually consistent with those in Ref. [9]. More importantly, we confirm that the XAS spectra for o-TMO are strongly polarization dependent both for the O K - edge and Mn L- edge along the three principal crystallographic directions, particularly in the region of the Mn eg(↑)-O 2p hybridization.

3.2. XAS of h-TMO thin films

Figs. 3(a) and 3(b) show the O K - edge and Mn L- edge XAS spectra (Ekc and Ekab) of the h-TMO thin film deposited on YSZ(111) substrate, respectively. The intensity distribution and the peak positions of the XAS spectra are distinctly different from those of o-TMO described above, which indicates that the local environment, geometrical arrangement and crystal field around Mn and O ions are different for both phases. The O K - edge XANES spectrum of the h-TMO thin film is very similar to that reported in [9] and can be divided into three parts, corresponding to Mn 3d, Y 4d, and Mn 4sp/Y 5sp regions. There are four pre-edge peaks in the Mn 3d region and are attributed to a1g(↑), e1g(↓), e2g(↓), and a1g(↓), as denoted in Fig. 3(a),

respectively. The spectrum also exhibits strong polarization dependence in the Mn 3d and Y 4d regions, indicating the existence of strong Mn 3d-O 2p and Y 4d- O 2p bond anisotropy. It is noted that the Mn L2,3- edge XAS of the h-TMO thin film (Fig. 3(b)) also shows the

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525 530 535 540 545 550 555 0.0 0.8 1.6 2.4 + e2g e1g a 1g a1g In te n s it y (a rb . u n it ) (a) h-TMO/YSZ(111) O K-edge E // ab E // c

640 645 650 655 660 0 2 4 6 8 10 12 In te n s it y (a rb . u n it ) h-TMO/YSZ(111) Mn L-edge

Photon Energy (eV) E // ab E // c (b)

Figure 3. (color online) Polarization-dependent (a) O K - edge (b) Mn L- edge XAS of h-TMO/YSZ(111) thin film for polarizations Ekab and Ekc.

strong polarization dependence and is very close to the spectral of h-YMnO3 sample reported

previously. As described in [9], these electronic and bond anisotropies of h-RMO are closely related to the ferroelectricity of these materials. Therefore, these results provide new insight into the microscopic understanding of the magnetoelectric anisotropy exhibited in many RMO manganites.

4. Summary

In summary, we have prepared pure hexagonal and orthorhombic TMO thin films for XAS measurements. It is evident that there are significant differences in the electronic structures of h-TMO and o-TMO thin films, indicating that the type and strength of hybridization between the Mn and O ions are different for both samples. Moreover, the XAS of Ekab and Ekc spectra for h-TMO and the Eka, Ekb, and Ekc spectra for o-TMO all exhibit strong polarization dependence at both O K and Mn L edges. These electronic and bond anisotropies are closely related to the magnetoelectric anisotropy and multiferroicity exhibited in these materials.

Acknowledgements

This project is financially sponsored by the National Science Council (grant no. NSC 95-2112-M-009-037-MY3) and the Ministry of Education (2007 MOE ATU program at NCTU) of Taiwan. References

[1] Fiebig M, 2005 J. Phys. D 38, R123

[2] Kimura T, Goto T, Shintani H, Ishizaka K, Arima T and Tokura Y, 2003 Nature 426, 55 [3] Khomskii D I, 2006 J. Magnetism and Magnetic Materials 306, 1

[4] Cruz C D, Yen F, Lorenz B, Wang Y Q, Sun Y Y, Gospodinov M M, and Chu C W, 2005 Phys. Rev. B 71, 060407

[5] Lin T H, Hsieh C C, Shih H C, Luo C W, Uen T M, Wu K H, Juang J Y, Lin J -Y, Hsu C H, Liu S J, 2008 Appl. Phys. Lett. 92, 132503

[6] Hsieh C C, Lin T H, Shih H C, Hsu C-H, Luo C W, Lin J Y, Wu K H, Uen T M, Juang J Y, 2008 J. Appl. Phys. 104, 103912

[7] Lee J H, Murugavel P, Ryu H, Lee D, Jo J Y, Kim J W, Kim H J, Kim K H, Jo Y, Jung M H, Oh Y H, Kim Y W, Yoon J G, Chung J S, Noh T W, 2006 Adv. Mater. 18, 3125

[8] Edwards D M, 2002 Adv. Phys. 51, 1259

[9] Chen J M, Lee J M, Chen C K, Chou T L, Lu K T, Haw S C, Liang K S, Chen C T, Jeng H T, Huang S W, Yang T J, Shen C C, Liu R S, Lin J Y, Hu Z, 2009 Appl. Phys. Lett. 94 044105

[10] Cho D Y, Kim J Y, Park B G, Rho K J, Park J H, Noh H J, Kim B J, Oh S J, Park H M, Ahn J S, Ishibashi H, Cheong S W, Lee J H, Murugavel P, Noh T W, Tanaka A, Jo T, 2008 Phys. Rev. Lett. 98, 217601