Pulsed Laser Deposited Ferromagnetic Chromium Dioxide thin Films ...

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ScienceDirect Physics Procedia 54 (2014) 62 – 69

International Conference on Magnetic Materials and Applications, MagMA 2013

Pulsed laser deposited ferromagnetic chromium dioxide thin films for applications in spintronics S. Dwivedia, J. Jadhava, H. Sharmab, S. Biswasa* a

Department of Physics, The LNM Institute of Information Technology, Jaipur-302031, India b Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India

Abstract Stable rutile type tetragonal chromium dioxide (CrO2) thin films have been deposited on lattice-matched layers of TiO2 by KrF excimer laser based pulsed laser deposition (PLD) technique using Cr2O3 target. The TiO2 seed layer was deposited on oxidized Si substrates by the same PLD process followed by annealing at 1100°C for 4 h. The lattice-matched interfacial layer is required for the stabilization of Cr (IV) phase in CrO2, since CrO2 behaves as a metastable compound under ambient conditions and readily converts into its stable phase of Cr (III) oxide, Cr2O3. Analyses with X-ray diffraction (XRD), Glancing-angle XRD (GIXRD), Raman spectroscopy and grazing-angle Fourier transform infra-red (FTIR) spectroscopy confirm the presence of tetragonal CrO2 phase in the as-deposited films. Microstructure and surface morphology in the films were studied with field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). Electrical and magnetic characterizations of the films were performed at room temperature. Such type of stable half-metallic CrO2 thin films with low field magnetoresistive switching behaviour are in demand for applications as diverse as spin-FETs, magnetic sensors, and magnetooptical devices. © by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license © 2014 2014The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati. Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati Keywords : Pulsed laser deposition (PLD); Chromium dioxide (CrO2); Half-metals; Spintronics

1. Introduction Half-metallic CrO2 exhibits low-field magnetoresistive switching behaviour that makes it apposite for pertinent applications in diverse fields including spintronics, magnetic sensors and magneto-optics [1-8]. The half-metallic

* Corresponding author. Tel.:+91-7597171575; fax: +91-141-2689014. E-mail address: [email protected]

1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati doi:10.1016/j.phpro.2014.10.037

S. Dwivedi et al. / Physics Procedia 54 (2014) 62 – 69

character of CrO2 arises because of its peculiar distribution of spin density of states at the Fermi energy. Selfconsistent spin polarized band structure calculations describe that for electrons of one spin it is a metal with a Fermi surface while for the opposite spin there is a gap in spin polarized density of states, like a semiconductor or insulator [1,5,7]. This band gap originates from the separation of filled O2- 2p4-levels from the Cr4+ 3d2-levels with only spinup electrons at the Fermi level, and hence CrO2 is a class I type half-metallic compound as classified by Coey et al. [7]. It shows ~100% spin-polarization behaviour at extremely low temperatures and has an advantageously high Curie temperature of 395 K [5,7]. Being a type I half-metal, CrO2 has a spin moment that is precisely an integral number of Bohr magnetons (2ȝB) per formula unit at 0 K. Spin-orbit coupling is detrimental to this ideal halfmetallicity. Spintronics devices exclusively exploit the spin property of electrons in addition to the charge. CrO2 is a promising candidate material for applications in spintronics because of its electrical conduction being dominated by charge carriers of spin-polarized (spin-up) electrons [7]. The magnetoresistance effect in CrO2 has been observed in simple device structures with ferromagnetic regions of CrO2 separated by a suitable barrier (insulator and/or nonmagnetic) layer [2,3,6,9]. There is enough evidence of low-field magnetoresistive switching behaviour in CrO2, which makes it an efficient spin-injecting material for spin-FETs and spin-dependent tunnelling devices. With the use of a suitable spin-carrying medium in the channel region, a practical spin-FET with CrO2 can be designed [10]. Additionally, magneto-optical properties of CrO2 add an interesting phase to these unusual spin-dependent transport properties and applications based on the coupling of the two phenomena can also be realized [11]. However, despite its advantages, the major drawback of CrO2 is its natural conversion to Cr2O3, the most stable oxide phase of Cr, at atmospheric pressure in air [12,13]. When exposed to ambient air, the pristine CrO2 particles develop a 2-3 nm thick topotactic layer of Cr2O3. A complete CrO2ĺCr2O3 conversion occurs at temperatures above 400°C according to the microstructure. A lot of research efforts have been devoted to obtain high quality stable CrO2 in different shape and morphologies. Gupta et al. have reported studies on magneto-resistance properties of CrO2 thin films developed by chemical vapour deposition technique [14]. Rabe et al. applied molecular beam epitaxy process to obtain textured CrO2 films [15]. Controlled thermal and hydrothermal methods have been followed to obtain CrO2 in the form of powders [16,17]. Few reports are also available on the deposition of CrO2 thin films using rather unconventional laser induced techniques. Guinneton et al. used PLD technique for the deposition of CrO2 thin films using CrO3 and Cr8O21 targets [18]. Heinig et al. reported the fabrication of epitaxial nanostructures of CrO2 on MgO (100) substrates by PLD [19]. Madi et al. reported remote plasma assisted PLD method for depositing chromium oxide thin films using Cr2O3 target [20]. Shima et al. also used PLD for deposition of CrO2 (200) phase on Si (111) substrates [21]. In general, PLD is considered to be a useful method for the deposition of multi-component thin films at low temperatures and for non-stoichiometric compositions [22,23]. It could be a versatile technique for the formation of otherwise metastable phases like CrO2. Here, we report part of our work on the development of spin-FET devices with CrO2 electrodes. In this regard, a major work is to develop the experimental conditions with the thin film deposition techniques to fabricate the devices on a Si-wafer. We have followed PLD technique for the deposition of stable rutile type tetragonal CrO2 thin films using Cr2O3 target on lattice-matched layers of TiO2. The TiO2 seed layer was deposited on oxidized Si substrates by the same PLD process. Structural, magnetic and electrical properties of the deposited films were studied in detail. 2. Experimental details Standard RCA method was followed to clean the Si (100) wafers (p-type, resistivity 4-7 -cm). After cleaning, the Si wafers were subjected to dry thermal oxidation at a temperature of 1100°C for 2 h. PLD system of Coherent Compex Pro 201 with a KrF excimer laser of wavelength 248 nm was used to deposit the TiO2 and CrO2 films. The pulse duration and repetition rate was 30 ns and 10 Hz, respectively. The target-to-substrate distance was maintained at 5.8 cm in both cases. The substrate temperature during the depositions was maintained at 700ºC for TiO2 and 350ºC for CrO2. The deposition chamber was pumped down to pressures of the order of 10-5 mbar with the help of a rotary and a turbo-molecular pump. During both depositions, oxygen was introduced in the chamber at a controlled rate with the help of a gate valve and flow meter. The oxygen pressure was maintained at 10-2 mbar for TiO2 and 7 x10-1 mbar for CrO2. The energy density of the laser was maintained at 2 J/cm2 and 3.3 J/cm2 for TiO2 and CrO2,

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respectively. A total of 14,000 and 4,000 laser shots were exposed to TiO2 and Cr2O3 targets, respectively, with 1000 shots exposed before opening the shutter in both depositions. The crystalline phase in the deposited TiO2 and CrO2 thin films was analyzed with an X`Pert-Pro X-ray diffractometer using CuKĮ radiation of 0.1540 nm wavelength. For CrO2 thin films, GIXRD was performed at an incident angle of 1° from the film surface. The microstructure in the films was studied with a FESEM (RAITH150 model), which could image down to 10 nm sized structures. Surface topography was analyzed using an AFM of Molecular Imaging, PicoScanTM 2100. Raman spectroscopy measurements were performed with a Horiba Jobin Yvon HR800 instrument employing a continuous wave air-cooled Ar-ion laser source (Spectra Phys, supplier Morrison) of wavelength 514.5 nm. FTIR spectrometer from Perkin Elmer of model Spectrum 100 Optica was used to determine the various bonds present in the thin films. Magnetic characterization of the thin films was performed with a MPMS SQUID VSM system of Quantum Design, with a maximum applied field of 1 Tesla. Current-voltage characteristics were studied with a Keithley 4200 SCS semiconductor characterization system at room temperature. Contact pads of Cr(20 nm)/Au(100 nm) were made with a thermal evaporator of Hind HiVac Vacuum Coating Unit (Model 12A4D) for the electrical analysis. 3. Results and discussions Figure 1 shows the XRD pattern of the TiO2 thin films deposited by PLD followed by annealing at 1100°C for 4 h. After annealing, (110) crystal orientation of rutile type appears to be the dominant phase in the films [24]. Minor peaks of (101) and (200) crystal orientations of the rutile phase can also be observed in the annealed films. As outlined before, this rutile type tetragonal TiO2 film provides a platform for the stabilization of the CrO2 phase.

Fig. 1. Typical XRD pattern of the TiO2 thin films annealed at 1100°C for 4 h.

Figure 2 shows the GIXRD pattern of the CrO2 thin films deposited by PLD on lattice-matched seed layer of rutile type TiO2. As can be observed, the tetragonal CrO2 phase appears in the as-deposited polycrystalline thin film with crystal orientations of (110), (200), (210) and (211) [25]. Rietveld analysis revealed the lattice parameter values, a = 0.4469 nm and c = 0.2739 nm with a lattice volume of V0 Ł a2c = 0.0547 nm3 in CrO2. However, nonferromagnetic phases such as CrO3 and Cr2O3 also appear in considerable amount in the as-deposited films. Cr2O3 is the most stable Cr oxide and is anti-ferromagnetic in nature. The presence of the lattice-matched rutile type tetragonal seed layer of TiO2 prevents the complete conversion of

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Fig. 2. Typical GIXRD pattern of the CrO2 thin films deposited on the interfacial TiO2 seed layer.

CrO2 into Cr2O3. It has been noticed that the presence of oxygen (above a certain level) during depositions and high laser fluence (imparting more energy per unit area to the target) favors the formation of t-CrO2. In this typical example, the PLD process using Cr2O3 targets serves as a unique technique for the retention of the metastable CrO2 phase. The high laser fluence used in the present work forms an extremely energetic plume ablating the highly stable target and enhancing the mobility of the ablated materials with favorable thermodynamic conditions for the formation of the metastable CrO2 phase. Figure 3 shows the typical (a) FESEM and (b) AFM images of the as-deposited CrO2 thin films on latticematched TiO2 layer. The average grain size in this highly polycrystalline film was estimated to be 260 nm from the FESEM image. The AFM image was acquired in a scanning area of 2 μm2. The simulated surface morphology corresponding to this image is shown in Figure 3c. The average grain size from the AFM image was determined to be 265 nm with a RMS roughness of 19 nm.

(b)

(c)

Fig. 3. (a) Typical FESEM and (b) AFM images of the CrO2 thin films. (c) Simulated surface morphology corresponding to the AFM image.

The Raman and FTIR spectra of the as-deposited CrO2 thin films are shown in Figure 4(a) & (b). In the Raman spectrum, the regions extending from 400 cm-1 to 480 cm-1 and 540 cm-1 to 640 cm-1 have been deconvoluted by multiple peak fit method and the details are given in Table 1. The deconvoluted bands at 457 cm-1 and 571 cm-1 and another band at 691 cm-1 correspond to t-CrO2 [26,27]. The bands of Cr2O3 appear at 306 cm-1, 348 cm-1 and 559 cm-

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S. Dwivedi et al. / Physics Procedia 54 (2014) 62 – 69 1

as can be seen in the full range spectrum shown in the inset [28]. The deconvoluted bands at 413 cm-1 and 584 cmalso belong to Cr2O3 [28]. The band at 430 cm-1 is ascribed to the bending motions of oxygen atoms in the random network of amorphous silica [29]. Raman bands arising from the interfacial TiO2 seed layer have also been observed at 450 cm-1, 631 cm-1 and 828 cm-1, respectively [26]. The bands at 247 cm-1, 442 cm-1, 469 cm-1, 524 cm-1 and 611 cm-1 arise from the Si substrate [30]. One of the defect bands of SiO2 is observed at 510 cm-1, while the other defect band is observed at 596 cm-1 [29]. The weak band at 872 cm-1 is attributed to the Cr-O-Cr bending mode [18]. The other bands in the range 800-1000 cm-1 correspond to CrO3, SiO2 and those arising from Si substrates [18, 30]. 1

Fig. 4. (a) Deconvoluted Raman peaks and (b) grazing-angle FTIR spectrum of the CrO2 thin films. Inset in figure (a) shows the full range Raman spectrum. Table 1 : Deconvoluted Raman bands of the CrO2 thin films. Sr. No.

Bands

[Ref.]

1.

Deconvoluted Region (400-480 cm-1) Cr2O3 - 413 cm-1

[28]

2.

Bending motions of oxygen atoms in SiO2 - 430 cm-1

[29]

3.

Si - 442 cm

-1

[30]

4.

TiO2 - 450 cm

[26]

5.

CrO2 - 457 cm-1

[26,27]

6.

Si - 469 cm-1

[30]

-1

Deconvoluted Region (540-640 cm-1) 1.

CrO2 -571 cm-1

[26,27]

2.

Cr2O3 -584 cm-1

[28]

3.

SiO2 - 596 cm-1

[29]

4.

Si - 611 cm-1

[30]

5.

-1

TiO2 - 631 cm

[26]

The grazing angle FTIR spectrum of the as-deposited film is shown in Figure 4b and the band identities are mentioned in Table 2. The bands at 561 cm-1, 630 cm-1, 715 cm-1, 829 cm-1 and 930 cm-1 clearly show the presence of t-CrO2 in the films [8]. The bands at 455 cm-1 and 610 cm-1 are due to the presence of Cr2O3 [31]. The sharp band at 489 cm-1 belongs to the rutile Ti-O-Ti vibrations [32].

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S. Dwivedi et al. / Physics Procedia 54 (2014) 62 – 69 Table 2 : FTIR bands of the CrO2 thin films. Sr. No.

Bands

Observed position

[Ref.]

1.

Cr2O3 (Eu)

455 cm-1

[31]

2.

Rutile Ti-O-Ti vibrations

489 cm-1

[32]

-1

3.

Cr2O3 (Eu)

532 cm

[31]

4.

CrO2 angle bending (v5)

561 cm-1

[8]

5.

Cr2O3 (Eu)

610 cm-1

[31]

6.

CrO2 deformation (v4)

630 cm-1

[8]

-1

7.

CrO2 bending (v2)

715 cm

[8]

8.

CrO2 symmetric stretching (v1)

829 cm-1

[8]

-1

[8]

9.

CrO2 (v3)

930 cm

The typical room temperature M-H curve shown in Figure 5, confirms the ferromagnetic nature of the asdeposited films. The saturation magnetization is estimated to be 116 emu/cm3. As can be clearly observed, the presence of antiferromagnetic Cr2O3 phase causes a shift in the curve. The coercivity values of the film are found to be 23 Oe during increasing field and 28 Oe during decreasing field. Figure 6 shows typical current-voltage characteristics of the as-deposited CrO2 thin films. A sudden increase in the current levels is observed above an applied voltage of ~ 6.5 V. At high voltages, the curve shows typical ohmic behaviour. Since the as-deposited films contain mixed phases of ferromagnetic CrO2, antiferromagnetic Cr2O3 and other non-magnetic phases of chromium oxides, therefore the electrical transport properties in these films are extremely complex and depends on several parameters, which primarily include, (i) the crystal structure and local symmetry in different phases, (ii) the artificial interstitial impurities and defects, (iii) particle size and morphology

Fig. 5. Typical M-H plot of the CrO2 thin films at room temperature. Enlarged portion near the coercive field is shown in the inset.

Fig. 6. Typical current-voltage plot of the deposited CrO2 thin films at room temperature.

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or topology, (iv) the surface interface or surface layer, if any, (v) the distribution of CrO2 of ordered lattice in a specific microstructure, i.e., homogeneous or heterogeneous {in combination to the factors (iii) and (iv)}, (vi) the exchange coupling between CrO2 particles, (vii) the intergranular tunnelling of charge carriers (both spin-dependent and spin-independent), and (viii) the α-β macroscopic interactions in the basic CrO2 particles or crystallites (α) and the matrix (β) of Cr2O3 and other non-magnetic phases of chromium oxides. The last four-five factors become extremely important in the case of thin films, involving especially small particles, or more precisely nanoparticles and multi-domain complex phases. They influence very effectively the equilibrium electronic band structure as well as the spin structure (or the spin entropy) and their dynamics. A thorough investigation of the electrical transport properties of the as-deposited films is presently undergoing. 4. Conclusions Stable CrO2 thin films were deposited on lattice matched layers of TiO2 by PLD technique. The TiO2 layer plays a vital role in stabilizing and retaining the metastable Cr (IV) phase under ambient conditions. The laser fluence and oxygen pressure plays an important role in controlling the crystallinity and microstructure in the deposited films. Studies with GIXRD, FTIR and Raman spectroscopy confirmed the presence of CrO2 in the thin films. SQUID analysis revealed the ferromagnetic nature of the films. Development of a convenient deposition process of stable half-metallic CrO2 is an important step towards fabrication of next generation spintronics devices. Acknowledgements The authors sincerely thank the Centre of Excellence in Nanoelectronics, Indian Institute of Technology Bombay, Mumbai for providing us the instrumental facilities under the Indian Nanoelectronics Users’ Program (INUP) of the Department of Information Technology, Government of India. References [1]. [2]. [3]. [4].

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