Japanese Journal of Applied Physics Vol. 45, No. 2A, 2006, pp. 727–729
Brief Communication
#2006 The Japan Society of Applied Physics
Effect of Thermal Annealing on Structural, Electrical, and Magnetic Properties of Ag-doped La0:67 Ca0:33 MnO3 Thin Films Grown on LaAlO3 Substrates Peng-Xiang Z HANG, Shao-Jiang ZHU, Xiang LIU, Ju-Bo P ENG, Tae Whan K IM1 and H.-U. H ABERMEIER2 Institute of Advanced Material for Photoelectronics, Kunming University of Science and Technology, Kunming, Yunnan 650051, China 1 Advanced Semiconductor Research Center, Department of Electronics and Computer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea 2 Max-Planck-Institut fu¨r Festko¨rperforschung, D-70506 Stuttgart, Germany (Received October 25, 2005; accepted November 10, 2005; published online February 8, 2006)
Resistance as a function of temperature showed that the metal–insulator transition temperatures of as-grown and annealed Ag-doped La0:67 Ca0:33 MnO3 thin films were 235 and 300 K, respectively, and that the metal–insulator transition ranges of these films were relatively narrow, indicative of a sharp phase transition. The temperature coefficient of resistance was as high as 8% at 276 K. The magnetoresistance ratio of the annealed Ag-doped film under a magnetic field of 5 T was approximately 55% at 285 K. [DOI: 10.1143/JJAP.45.727] KEYWORDS: Ag-doped La0:67 Ga0:33 MnO3 , thermal annealing, electrical property, magnetic property, magnetoresistance ratio
RE1x Ax MnO3 (RE ¼ La, Pr, Nd; A ¼ Ca, Sr) materials are attractive because of the interest in investigating both their fundamental physical properties1–3) and potential applications in magnetic devices.4–6) The possible applications of magnetic devices using RE1x Ax MnO3 materials have led to extensive efforts to understand the structural and magnetic properties of such materials.6,7) Among the various kinds of RE1x Ax MnO3 materials, La1x Cax MnO3 materials, which exhibit colossal magnetoresistance, have been particularly interesting due to their promising applications to the fabrication of magnetic sensors, magnetic recorders, and bolometers operating at high temperatures.8–14) Even though many studies have been performed to increase the metal– insulator temperature (TMI ) of La1x Cax MnO3 materials,15–18) the highest TMI values obtained from La1x Cax MnO3 bulks and thin films have been 270 and 290 K, respectively.15,18) Recently, Ag-doped La1x Cax MnO3 films have been intensively investigated due to their enhanced TMI and magnetoresistance resulting from their increase in Mnþ concentration.17) Therefore, studies concerning the formation and physical properties of Ag-doped La1x Cax MnO3 thin films with a room-temperature TMI are indispensable, since these materials are potential candidates for use in magnetic devices operating at room temperature. In this communication, we report data on the structural, electrical, and magnetic properties of as-grown and annealed Ag-doped La1x Cax MnO3 thin films grown on LaAlO3 substrates using pulsed laser deposition (PLD) with the goal of producing a high TMI . X-ray diffraction (XRD) measurements were carried out characterize the structural properties of as-grown and annealed Ag-doped La1x Cax MnO3 thin films grown on LaAlO3 substrates, and resistance and magnetoresistance measurements were performed to determine TMI and the temperature coefficient of resistance (TCR) and to investigate the magnetic properties of these La1x Cax MnO3 thin films. The samples used in this study were Ag-doped La0:67 Ca0:33 MnO3 thin films grown on a LaAlO3 (100) substrate using PLD. The ceramic target used, with a nominal cation composition ratio of La : Ca : Mn ¼ 2 : 1 : 3 and a Ag dopant concentration of 4.0 wt %, was prepared using a
conventional co-precipitation technique by sintering at 1200 C for 36 h. An excimer laser was used to ablate the stoichiometric target, and the wavelength and pulse duration of the excimer laser utilizing a KrF gas mixture for producing ultraviolet radiation were 248 nm and 20 ns, respectively. The deposition of a Ag-doped La0:67 Ca0:33 MnO3 thin film was performed at a substrate temperature of 700 C and at a system pressure of approximately 40 Pa in an oxygen atmosphere. The distance between the target and the substrate was 4.0 cm, and the number of pulses and the thickness of the film were 1500 and 300 nm, respectively. After the Ag-doped La0:67 Ca0:33 MnO3 thin films had been deposited at 700 C, the thin films were cooled to room temperature at a cooling rate of 10 C/min and a system pressure of 3000 Pa in an oxygen atmosphere. Thermal annealing was performed at 900 C in an oxygen atmosphere at a pressure of 0.15 MPa for approximately 1 h. XRD measurement was performed using a conventional diffractometer equipped with an Euler cradle, and electrical and magnetotransport measurements were carried out using the conventional four-probe technique. Ohmic contacts to the samples were made by evaporating small amounts of silver, and the electrode wires were connected to the silver pads using silver epoxy. Resistance and magnetoresistance measurements were carried out at temperatures from 5 to 350 K at magnetic fields of 0, 0.1, 0.5, 1, and 5 T. The measurements were performed with a magnetic field applied parallel to the plane of the Ag-doped La0:67 Ca0:33 MnO3 thin film and in the direction of the electric current flow. Figure 1 shows the XRD curves of the (a) as-grown and (b) annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates. When Ag atoms are doped in the La0:67 Ca0:33 MnO3 thin films, they enter the Mn lattice, resulting in an increase in Mn4þ content19) and in a catalytic effect that promotes grain growth.17) The undoped and Agdoped La1x Cax MnO3 thin films have cubic perovskite structures with slightly different lattice parameters. A small decrease in the lattice parameter of the Ag-doped La1x Cax MnO3 film in comparison with the undoped film may be a result of better oxygenation. The (110) diffraction peak corresponding to the Ag-doped La0:67 Ca0:33 MnO3 thin films, together with the (100) and (100) K diffraction peaks corresponding to the LaAlO3 (100) substrate, are clearly
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XRD INTENSITY (arb. units)
LaAlO3 (100)
P.-X. ZHANG et al.
La0.67Ca0.33MnO3 / LaAlO3 (100)
300
LaAlO3 (200)
La0.67Ca0.33MnO3 (110) / LaAlO3 (100) 250
(b)
La0.67Ca 0.33MnO3 (110)
:0T : 0.1 T : 0.5 T :1T :5T
R (Ω)
200
150
100
(a)
50
0 20
25
30
35
2 θ (DEGREE)
40
45
0
50
La0.67Ca0.33MnO3 (110) / LaAlO3 (100)
600
R (Ω)
500 400
(a)
200
(b)
0 100
150
200
200
250
300
350
such films with a low Ca content.18) The marked increase in TMI for the annealed Ag-doped La0:67 Ca0:33 MnO3 thin film originates from an increase in Mn4þ /Mn3þ ratio owing to oxygen refilling caused by thermal treatment.20,21) The increase in Mn4þ /Mn3þ ratio is attributed to an enhancement of exchange interaction between neighboring Mn4þ and Mn3þ . The highest TCRs of the as-grown and annealed Ag-doped La0:67 Ca0:33 MnO3 (110) thin films are 3.1% at 211 K and 8% at 276 K, respectively. Figure 3 shows resistance and magnetoresistance as functions of the temperature under several magnetic fields for the annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates. Maximum resistance decreases with increasing magnetic field, and the peak position corresponding to the maximum resistance shifts to higher temperature. Magnetoresistance ratio, which is defined as ½Rð0Þ RðHÞ=Rð0Þ, where Rð0Þ is the resistance at a zero field and RðHÞ is the resistance in an applied magnetic field, as a function of temperature under several magnetic fields for the annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates is shown in Fig. 4. When the magnetic field is 5 T, the magnetoresistance ratio is 55% at 285 K, and this ratio decreases to 10% at 350 K. The magnetoresistance ratio of the annealed Ag-doped La0:67 Ca0:33 MnO3 thin films studied herein is significantly larger than those reported in the literature.22) In summary, the effects of postgrowth thermal annealing on the structural, electrical, and magnetic properties of Agdoped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates were investigated. The XRD curves showed that the crystallinity of the Ag-doped La0:67 Ca0:33 MnO3 (110) thin films was improved by thermal annealing. Resistance as a function of temperature showed that the TMI of the annealed Ag-doped La0:67 Ca0:33 MnO3 thin film was as high as 300 K and that the TCR of this film was approximately 8% at 276 K. The magnetoresistance ratio of the annealed Ag-doped La0:67 Ca0:33 MnO3 thin film under a magnetic field of 5 T was approximately 55% at 285 K. These results indicate that annealed Ag-doped La0:67 Ca0:33 MnO3 (110) thin films grown on LaAlO3 (100) substrates hold promise
800
100
150
Fig. 3. Magnetoresistance as function of temperature under several magnetic fields for annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates.
observed in the as-grown and annealed samples, as shown in Fig. 1. The as-grown and annealed Ag-doped La0:67 Ca0:33 MnO3 thin films have a single phase and show characteristics of a cubic perovskite structure. The (110) peak related to the annealed Ag-doped La0:67 Ca0:33 MnO3 thin film is slightly shifted to the lower-angle side in comparison with that corresponding to the as-grown thin film. This shift results from an increase in lattice constant owing to thermal annealing. The full width at half maximum of the (110) peak corresponding to the Ag-doped La0:67 Ca0:33 MnO3 thin film decreases by approximately 0.1 after annealing. This result indicates that the crystallinity of the Ag-doped La0:67 Ca0:33 MnO3 thin films is improved by thermal treatment. Plots of resistance as a functions of temperature for the asgrown and annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates are shown in Figs. 2(a) and 2(b), respectively. Figure 2 shows that the TMI values of the as-grown and annealed Ag-doped La0:67 Ca0:33 MnO3 thin films are 235 and 300 K, respectively. The TMI of the annealed Ag-doped La0:67 Ca0:33 MnO3 thin films studied in this work is the highest value reported in the literature for
300
100
T (K)
Fig. 1. XRD curves of (a) as-grown and (b) annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates.
700
50
250
300
T (K) Fig. 2. Resistance as function of temperature at 1 mA for (a) as-grown and (b) annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrates.
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60
(R(0)-R(H)) / R(0) (%)
La0.67Ca0.33MnO3 (110) / LaAlO3 (100)
40
20
0 150
5T 1T 0.5 T 0.1 T
200
250
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T (K) Fig. 4. Magnetoresistance ratio ½Rð0Þ RðHÞ=Rð0Þ as function of temperature under several magnetic fields for annealed Ag-doped La0:67 Ca0:33 MnO3 thin films grown on LaAlO3 (100) substrate.
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