Optical and magneto-optical studies of (La(0.7)A(0.3))MnO(3) (A = Sr, Ca, and Ce) thin films

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Journal of Magnetism and Magnetic Materials 304 (2006) e303–e305

Optical and magneto-optical studies of (La

_0.7

A

_0.3

)MnO

₃

(A ¼ Sr, Ca, and Ce) thin ﬁlms

H.L. Liu

a,

, Kenneth Y.-J. Zhang

a

, L.C. Wu

a

, L. Uba

b

, S. Uba

b

,

W.J. Chang

c

, J.-Y. Lin

c

, L.M. Wang

d

a_{Department of Physics, National Taiwan Normal University, Taipei, Taiwan} b_{Institute of Experimental Physics, University of Bialystok, Bialystok, Poland}

c_{Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan} d_{Department of Electrical Engineering, Da-Yeh University, Changhua, Taiwan}

Available online 6 March 2006

Abstract

The optical and magneto-optical (MO) properties of (La0.7A0.3)MnO3(A ¼ Sr, Ca, and Ce) thin ﬁlms have been investigated. It is

found that the value shape of spectra is strongly dependent on different cation dopings. The Sr-doped thin ﬁlm exhibits a strong Drude response and maximum Kerr rotation angle of 0.321 at 3.4 eV. Interestingly, the replacement of Sr ions with the smaller Ca ions and the tetravalent Ce ions all lead to a remarkable suppression of the carrier density and the MO response. Such differences are interpreted as due to their different electronic band structures.

Keywords: Colossal magneto-resistance; Optical property; Magneto-optical effect

1. Introduction

The subject of colossal magneto-resistance (CMR) effect in the manganite thin ﬁlms R1xAxMnO3(R and A being

trivalent rare-earth and divalent alkaline-earth ions, respectively) continues to attract much attention [1,2]. Interest in these systems is exempliﬁed by their potential technological applications related to magnetism and spintronics. The ﬁlms often have different CMR effects and Curie temperatures (TC) compared to bulk materials

due to lattice mismatch between the ﬁlm and the substrate. In this study, we further show that the optical and magneto-optical (MO) properties of the manganite thin ﬁlms are critically dependent on different cation dopings such as Sr, Ca, and Ce. To the best of our knowledge, few studies of the MO properties of the manganites have been reported in the past[3–5]. Our investigations are therefore

very important to understand the optical and MO Kerr effects of devices based on the manganite thin ﬁlms. 2. Results and discussion

Thin ﬁlms of La0.7Sr0.3MnO3(LSrMO), La0.7Ca0.3MnO3

(LCaMO), and La0.7Ce0.3MnO3(LCeMO) were grown on

(1 0 0) SrTiO3substrates using the pulsed-laser deposition

or the off-axis magneton sputtering technique described previously [6,7]. The thickness of the films is around 300 nm. The strain effect due to lattice mismatch between the film and the substrate is fully relaxed[8]. Characteriza-tions of X-ray diffraction, DC resistivity, and magnetiza-tion were also preformed on these samples. Fig. 1 shows the temperature dependence of in-plane magnetization under 100 G for the three films, clearly revealing that they have a ferromagnetic transition. The TC for LSrMO is

350 K, whereas that of LCaMO and LCeMO is 275 K. The near-normal optical reﬂectance spectra of the three ﬁlms were taken over a wide frequency range (50–55000 cm1) and at temperatures between 20 and

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340 K [9]. The polar Kerr rotation yk and ellipticity Zk

spectra were measured at room temperature with an MO spectrometer based on the polarization modulation tech-nique[10] over the photon energy range of 0.74–5.8 eV.

Fig. 2shows the measured optical reflectance of LSrMO, LCaMO, and LCeMO thin films over the entire spectral range at 20 K. Note the logarithmic scale. There are several important features to these spectra. First, the far-infrared reflectance in LSrMO displays a metallic character, with a line shape in accord with a Drude theory. As the frequency increases, the reflectance drops steadily throughout the infrared, with a sort of plasmon minimum around 15,000 cm1. Several electronic features are observed for frequencies above 20,000 cm1. Second, for the LCaMO sample with a reduction in TC, the reflectance in the whole

infrared region is decreased, indicating the decreased carrier concentrations. Third, although both LCaMO and LCeMO thin ﬁlms have approximately the same TC, the

far-infrared spectrum of LCeMO can be described by a very weak, overdamped Drude contribution typical of a poor conductor. As a consequence, the effect of multiple reflection of light in the film on the substrate is clearly seen in the mid-infrared frequency region. Finally, the Drude– Lorentz fits[9]to the reflectance yield an estimation of the Drude plasma frequency values opD: LSrMO (5600 cm1)

4LCaMO (4800 cm1)4LCeMO (2500 cm1), and the Drude scattering rate 1=tD: LSrMO (120 cm1)oLCaMO

(270 cm1)oLCeMO (870 cm1). These are consistent with the results by the transport measurements [6], suggesting the replacement of Sr ions with the smaller Ca ions and the tetravalent Ce ions all modify the electronic structure near the Fermi level in the manganite thin ﬁlms[6].

The room-temperature polar Kerr rotation yk and

ellipticity Z_k spectra of the three ﬁlms measured under saturation conditions are shown in Figs. 3(a) and (b),

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0.0 0.4 0.8 1.2 1.6 0 200 400 Magnetization (emu /cm 3 ) Temperature (K) LSrMnO LCaMnO LCeMnO 100 300

Fig. 1. (Color online) Temperature-dependent field-cooled magnetization of LSrMO (red, solid line), LCaMO (blue, dashed line), and LCeMO (green, dashed–dotted line) thin films measured with a field of 100 G in the plane of the films. 0.0 0.2 0.4 0.6 0.8 1.0 100 10000 Reflectance Frequency (cm-1) LSrMnO LCaMnO LCeMnO

Fig. 2. (Color online) The reﬂectance spectra of LSrMO (red, solid line), LCaMO (blue, dashed line), and LCeMO (green, dashed–dotted line) thin ﬁlms at 20 K. -0.4 -0.2 0.0 0.2 0

Polar Kerr Rotation (degree)

Photon Energy (eV) LSrMnO LCaMnO (x 10) LCeMnO (x 10) -0.4 -0.2 0.0 0.2 0.4

Polar Kerr Ellipticity (degree)

Photon Energy (eV) LSrMnO LCaMnO (x 10) LCeMnO (x 10) (b) (a) 2 4 6 0 2 4 6

Fig. 3. (Color online) Experimental polar Kerr (a) rotation and (b) ellipticity of LSrMO (red, solid line), LCaMO (blue, dashed line), and LCeMO (green, dashed–dotted line) thin ﬁlms at 300 K.

H.L. Liu et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e303–e305 e304

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respectively. One ﬁrst observes that the Kramers–Kronig relation seems to be fulﬁlled between ykand Zk, which were

independently deduced. Second, the Kerr spectra of the three ﬁlms exhibit similar overall structure, but the magnitude of LCaMO and LCeMO is smaller than that of LSrMO by a factor of 10. This difference can be due to different magnetization related with a reduction of the ferromagnetic ordering temperature. Third, it was found that the maximum values of ykand Zkfor the LSrMO thin

ﬁlm are 0.321 at 3.4 eV and 0.241 at 3.4 eV, respectively. The characteristic behavior in our LSrMO spectra is qualitatively similar with two earlier studies [3,5], but appears to disagree with those reported on corresponding single crystals[4].

3. Summary

In summary, we report the optical and MO properties of the LSrMO, LCaMO, and LCeMO thin ﬁlms. The value shape of the measured spectra is strongly dependent on different cation dopings. Such differences are interpreted as due to their different electronic band structures. Our results are important to those attempting to understand the material properties and improve device qualities based on the manganite thin ﬁlms.

Acknowledgments

This work was supported by the National Science Council of Republic of China under Grant no. NSC94-2112-M-003-002, 94-2120-M-007-013, and the National Taiwan Normal University under Grant no. ORD93-B. References

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