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Growth and Transport Properties of (110)-Oriented La2/3Sr1/3MnO3 Thin Films on
TiO2-Buffered Si (100) Substrates
View the table of contents for this issue, or go to the journal homepage for more 2003 Jpn. J. Appl. Phys. 42 L287
(http://iopscience.iop.org/1347-4065/42/3B/L287)
Growth and Transport Properties of (110)-Oriented La
2=3Sr
1=3MnO
3Thin Films
on TiO
2-Buffered Si (100) Substrates
Shiu-Jen LIU, Shyh-Feng CHEN, Jenh-Yih JUANG, Jiunn-Yuan LIN1, Kaung-Hsiung WU,
Tseng-Ming UEN and Yi-Shun GOU
Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan 300, R.O.C.
1Institution of Physics, National Chiao Tung University, Hsinchu, Taiwan 300, R.O.C.
(Received October 10, 2002; revised manuscript received January 14, 2003; accepted for publication January 20, 2003)
The growth and transport properties of (110)-oriented La2=3Sr1=3MnO3 thin films on TiO2-buffered silicon substrates by
pulsed-laser deposition are reported. An insulator-metal transition associated with a ferromagnetic transition was observed at about 360 K. Magnetic measurements showed that in-plane coercive fields are about 60 Oe and 300 Oe at 300 K and 5 K, respectively. A magnetoresistance with a =ðH ¼ 0Þ ratio of 20% in a magnetic field of 5 T was observed not only near the insulator-metal transition temperature but also below 30 K. Moreover, a T2- to T3-dependence crossover of resistivity was
observed around 60 K, indicating that an unconventional one-magnon scattering may have occurred. [DOI: 10.1143/JJAP.42.L287]
KEYWORDS: colossal magnetoresistance, buffer layer, hole-doped manganite, titanium dioxide, transport properties, pulsed-laser deposition, one-magnon scattering, insulator-metal transition
Hole-doped rare-earth manganites Ln1xAxMnO3, where
Ln and A are trivalent lanthanide and divalent alkaline-earth ions, respectively, have attracted much attention because of their unusual magnetic and electronic properties. The colossal magnetoresistance (CMR) observed near the insu-lator-metal transition temperature (TIM) is one of the most
important characteristics of these materials. Extensive studies revealed that the optimally doped La–Sr–Mn–O (LSMO) is half-metallic.1,2)That is, charge carriers respon-sible for the metallic behavior have the same spin, whereas carriers with the opposite spin are insulating. The half-metallic nature of the manganite materials exhibiting CMR characteristics has made them a candidate for spin-depen-dent electronic devices, which are expected to play a key role in the next-generation electronic industry.3,4)
From the application point of view, growing CMRfilms on technologically viable substrates is of vital importance. Several groups have attempted to grow manganite films on Si, the most prominent material used in semiconducting devices. However, the manganite films directly grown on Si were mainly (110) textured but mixed with many other orientations.5,6)To improve the epitaxy of manganite films on Si, several buffer layers have been introduced, such as yttria-stabilized ZrO2 (YSZ),7) Si3N4,8) SrTiO3/TiN9) and
SrTiO3/MgO/TiN.10)The results indicated that La–Ca–Mn–
O (LCMO) films can be epitaxially grown on buffered Si. However, the TIMof these films and, hence, the temperature
of the maximum magnetoresistance (MR) were reduced significantly. The attempt of using CeO2-buffered Si for
growing LCMO films has resulted in crystalline structures similar to the ones directly grown on Si.11)Perhaps one of the best results along this line was that demonstrated by
Trajanovic et al.12) who used a multi layered buffer
consisting of YSZ and Bi4Ti3O12.
TiO2 is one of the most extensively studied
transition-metal oxides. The preparation of TiO2 thin films has
received much attention during the last decade because of their remarkable optical and electronic properties. It is known that TiO2 exists in three different crystalline phases:
anatase and rutile (both tetragonal), and brookite (orthor-hombic). Only anatase and rutile have been observed in thin
films up to now. Recently, it was found that high-Tc
superconducting YBa2Cu3O7yfilms can be grown on TiO2
rutile epitaxially.14) In this article, we report the in situ deposition of pure (110)-oriented LSMO thin films on Si (100) substrates by using a TiO2 rutile buffer layer. The TIM
and ferromagnetic Curie temperature (TC) of the as-grown
films, in contrast to the previous results, remain very close to the reported bulk values. Moreover, the half-metallic nature of these films is clearly demonstrated.
The TiO2 rutile buffer layer was deposited by similar
procedures used for obtaining epitaxial CrO2 films on
buffered Si substrates.13) Briefly, the TiN layer was
deposited by pulsed-laser deposition (PLD) on the Si (100) substrate at 650C with a laser energy density of 4 J/cm2.
The PLD chamber was maintained under a base pressure of 4 105Torr during the deposition. When the deposition of TiN was completed, the substrate temperature was raised to 750C and the chamber was filled with pure oxygen up to
0.4 Torr. After maintaining these conditions for 10 min, the TiN layer was converted into TiO2rutile,14)and the LSMO
film was then deposited in situ on the TiO2-buffered Si
substrate. Upon completing the deposition of the LSMO
film, the chamber was filled with 700 Torr O2 and the
substrate was cooled to room temperature at a rate of 15C/
min. The thickness of both of the TiO2layer and LSMO film
was about 50 nm. The lattice structure and surface morphol-ogy of as-grown films were examined by X-ray diffraction (XRD) scans and scanning tunneling microscopic (STM) measurements, respectively. The resistivity and magnetiza-tion measurements were carried out using a quantum design physical property measurement system (PPMS). All MRand magnetization measurements in this work were performed with the application of magnetic field along the surface of the films.
Figure 1 shows the typical -2 XRD pattern of the LSMO/TiO2/Si multi layer structure. Except for the peaks of
the TiO2buffer layer and Si substrate, only the (ll0) peaks of
LSMO were observed. The XRD scans indicate that the TiO2 buffer layer and the LSMO film are both purely (110)-E-mail address: [email protected]
Jpn. J. Appl. Phys. Vol. 42 (2003) pp. Part 2, No. 3B, 15 March 2003
#2003 The Japan Society of Applied Physics
oriented. The full widths at half maximum of the (110) peaks of LSMO and TiO2 are about 0.25 and 0.28, respectively,
indicating excellent crystallinity. Moreover, the distances of the (110) lattice planes (dð110Þ), calculated from the XRD
data, of the as-grown TiO2 rutile and LSMO films are about
3.237 A and 2.721 A. The lattice constants of the bulk TiO2
rutile are a ¼ b 4:594 A, c 2:959 A giving dð110Þ of
3.248 A. The bulk LSMO is nearly cubic with a 3:876 A and dð110Þ2:741 A, respectively. The difference between
the dð110Þ of the as-grown LSMO films and bulk values
possibly results from the tensile strain induced by the film-substrate lattice mismatch. The surface image of the LSMO film is shown in Fig. 1(b). The grain size is about 25 nm. Moreover, it is clear that the grains orient in the same in-plane direction.
The temperature dependence of resistivity, ðTÞ, as plotted in Fig. 2, indicates a clear I-M transition at 360 K and 376 K for a zero field and 5 T applied field, respectively. The MR, as defined by ½ðH ¼ 5TÞ ðH ¼ 0Þ=ðH ¼ 0Þ, is about 23% around 350 K. It is worth noting that a large MRof about 20% can be sustained even below 30 K.
The temperature dependence of the in-plane magnetiza-tion, MðTÞ, indicates that the as-grown film is ferromagnetic below 350 K, as presented in Fig. 3. Due to the limitation of our PPMS system, the MðTÞ can only be measured up to 350 K. By extrapolating the MðTÞ curve, the ferromagnetic TC was estimated to be about 360 K. The inset of Fig. 3
shows the field-dependent magnetization, MðHÞ, measured at 5 K and 300 K. The in-plane coercive fields are 60 Oe and 300 Oe at 300 K and 5 K, respectively. The values are larger
than those of the c-axis-oriented films grown on LaAlO3
(LAO) substrates.2)
Figure 4(a) shows the normalized field-dependent resis-tance curves, RðHÞ=RðHcÞ, where RðHcÞ is the resistance
measured at the coercive field of the correspondent tem-perature. The RðHÞ=RðHcÞat 300 K decreases almost linearly
with increasing magnetic field up to 5 T. Nevertheless, the situation at 5 K is quite different. In this case, the RðHÞ=RðHcÞ drops precipitously at low fields (0–0.2 T). As
the applied magnetic field exceeds 0.5 T, the resistance becomes linear in the magnetic field again. The correlation between the MðHÞ and low-field RðHÞ=RðHcÞat 5 K is shown
in Fig. 4(b). It is worth noting that the peaks of the RðHÞ=RðHcÞcurves are closely associated with the coercive
field. The resistivity and coercive fields of films grown in this work are much higher than those of epitaxial films grown on single-crystalline LAO substrates.2,15) Moreover, the large MRat low temperatures observed in these (110)-oriented LSMO films is absent in high-quality epitaxial films 20 25 30 35 40 45 50 55 60 65 70 75 80 0 500 1000 1500 2000 2500 3000 (a) (b) Si (400) & LSMO (220) TiO 2 r utile (220) TiO 2 r utile (110) LSMO (110) 2 θ Intensity (CPS) (deg)
Fig. 1. XRD pattern (a) and STM image (b) of the (110)-oriented La2=3Sr1=3MnO3(LSMO) thin films grown on TiO2-buffered Si (100).
0 1 2 3 4 5 zero field H = 5 T ρ (m Ω cm) T (K) 0 50 100 150 200 250 300 350 400 -24 -22 -20 -18 -16 -14 -12 MR (%)
Fig. 2. Temperature dependence of the resistivity measured at magnetic fields of 0 and 5 T. The magnetoresistance (MR) defined as ½ðH ¼ 5TÞ ðH ¼ 0Þ=ðH ¼ 0Þ is also presented. 0 50 100 150 200 250 300 350 400 0 1 2 3 4 -1500 -1000 -500 0 500 1000 1500 -4 -3 -2 -1 0 1 2 3 4 H = 0.1 T M (10 - 4 em u) T (K) H (Oe) M (10 - 4 em u) T = 5 K T= 300 K
Fig. 3. Temperature dependence of magnetization measured in a magnetic field of 0.1 T applied along the plane of the film. The inset shows the magnetic hysteresis loop measured at 5 K and 300 K.
grown on LAO. It is noted that similar results have been found in the ‘‘engineered’’ polycrystalline LCMO films16,17) and attributed to the grain-boundary (GB) effects. The GB effects can result in the large MRat low temperatures and increase the resistivity and coercive field, without reducing the TIM. For these films grown in this work, the higher
resistivity can be ascribed to the spin-dependent scattering occurring in grain boundaries with magnetic inhomogeneity. The higher coercive field, on the other hand, may be due to the random orientation of magnetic domains possibly defined by the grains. As a result, the magnetic field needed to flip the magnetization of the whole domain is higher than that needed to propagate a domain.
Lastly, the large MRat low temperatures was attributed to the suppression of spin-dependent scattering by the applied fields. The coincidence of the RðHÞ=RðHcÞ peaks with the
coercive fields in the magnetization hysteresis loop indicates that spin-dependent scattering is dominant in the transport properties of as-grown films at low temperatures. The temperature dependence of the resistivity of manganite films also represents a much discussed issue of half-metallic material. In conventional ferromagnets, such as Fe, Co and Ni, a T2 dependence of resistivity, arising from the
one-magnon scattering, is usually observed at low temperatures. However, the one-magnon scattering, which is a spin-flip scattering process, is theoretically forbidden in a perfect half
metal due to the absence of minority spin states. Based on the rigid-band picture, Kubo and Ohata18) have proposed that two-magnon scattering, which gives a T4:5 dependence
of resistivity, is dominant at low temperatures. On the other hand, a T3 dependence has been proposed by Furukawa19) who considered an unconventional one-magnon scattering due to spin-fluctuation-induced non-rigid minority band at finite temperatures. The inset of Fig. 5 shows the T2plot of
the resistivity of the as-grown LSMO film. The resistivity
fits well with ðTÞ ¼ 0þAT2 between 144 K and 60 K.
However, it deviates from the T2 behavior significantly for T < 60 K. For comparison, we show in Fig. 5 the fitting of ðTÞ ¼ 0þAT with ¼ 2, 3 and 4.5 below 70 K. It is
clear that the resistivity is best fitted by ðTÞ ¼ 0þAT3
between 60 K and 20 K. The T2- to T3-dependence crossover at around 60 K, though slightly higher, is in good agreement with the theoretical prediction.19)The slightly higher cross-over temperature may have arisen from the enhanced incoherence of the minority band due to grain boundaries. The semiconductive behavior below 20 K can also be attributed to the GB resistance.
In summary, we have deposited La2=3Sr1=3MnO3 thin
films on Si substrates by means of a TiO2 buffer layer. The
as-grown films were pure (110)-oriented and exhibited both insulator-metal and ferromagnetic transitions at about 360 K, which is comparable to the bulk value. Moreover, a large MR(=ðH ¼ 0Þ 20%) in a magnetic field of 5 T was
observed not only around TIM but also below 30 K. The
coincidence of the peaks of field-dependent MRloop with the coercive field at 5 K indicates that spin-dependent scattering occurring in the grain boundaries may have played an important role in the low-temperature magneto-transport properties of these (110)-oriented LSMO films. The temperature dependence of resistivity displayed a T2- to
T3-dependence crossover at around 60 K. This temperature
is in fair agreement with the theoretically estimated value of 50 K based on the unconventional one-magnon scattering mechanism.
This work was supported by the National Science Council of Taiwan, R.O.C. under Grant Nos: NSC90-2112-M-009-025 and NSC90-2112-M-009-030. 0 4 -2000 -1000 0 1000 2000 0.90 0.95 1.00 -50000 -25000 25000 50000 0.80 0.85 0.90 0.95 1.00 R / R( H c ) H (Oe) -4 -2 0 2 T=5 K (b) M(10 -4 em u) (a) 5 K 300 K R / R( H c )
Fig. 4. (a) Normalized resistance, R=RðHcÞ, as a function of in-plane
magnetic fields measured at 300 K and 5 K. (b) The R=RðHcÞcurve and
the magnetization hysteresis loop measured at 5 K are plotted. The peaks of the R=RðHcÞcurves are associated with the coercive fields.
0 10 20 30 40 50 60 70 0.80 0.85 0.90 0.95 0 5000 10000 15000 20000 0.8 1.0 1.2 1.4 ρ (m Ω cm) T (K) zero-field data T3 T2 T4.5 zero-field data T2 (K2 ) ρ (m Ω cm)
Fig. 5. ðTÞ ¼ 0þAT( ¼ 2, 3 and 4.5) fitting of resistivity. The T2
dependence of resistivity is also shown in the inset. A T2- to T3
-dependence crossover is observed around 60 K. The semiconductive behavior below 20 K can be attributed to the grain-boundary resistance. Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 2, No. 3B S.-J. L et al.
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