Growth and characterisation of cobalt doped zinc oxide 3

Chapter Experimental details CHAPTER EXPERIMENTAL DETAILS In this chapter, the details of thin film deposition and sample preparation for electrical transport measurement are described. An overview of the techniques that had been used to characterize the samples in this study were also discussed. 3.1 Sample preparation 3.1.1 Thin film deposition Zn1–xCoxO (x = 0.05 – 0.33) thin films were deposited on (0001) sapphire ( Al2O3) substrates with a size of × cm, in a high vacuum chamber with a base pressure < × 10–7 Torr, using a combination of radio-frequency (RF) and direct current (DC) magnetron sputtering. Sintered ZnO, Al2O3 and Co materials were used as the sputtering sources for ZnO, Al and Co, respectively. The Al2O3 target was used to dope the samples with Al for better conductivity. All the samples were sputtered in an atmosphere of pure Ar gas at a pressure of mTorr. Prior to deposition, the substrates were first degreased using acetone and then reversely sputtered by 20 mTorr Ar gas in the pre-cleaning chamber. A series of experiments had been carried out to optimize the substrate temperature (from room temperature to 600 oC) and sputtering powers for ZnO (50 W – 200 W) and Al2O3 (20 W – 50 W). Films of high structural quality and resistivity of 1.3 m .cm were obtained at a substrate temperature of 500oC, ZnO sputtering power of 150 W and Al2O3 sputtering power of 30W. Under these conditions, the deposition rate of ZnO:Al was ~ 4.8 nm/min. Subsequently, Co was 80 Chapter Experimental details doped into ZnO:Al by using the above-mentioned optimum deposition condition, while varying either the Co sputtering power from to 50 W for co-doped samples or the Co sputtering duration from 10 to 98 s for -doped samples at a sputtering power of 10 W. For the co-doped samples, Co was added to ZnO:Al by co-sputtering Co with ZnO and Al2O3. On the other hand, in -doped samples, Co was doped “digitally” into the ZnO:Al host matrix with a nominal thickness of 0.1 nm, 0.25 nm, 0.5 nm and nm, respectively. As the actual thicknesses might be different from the nominal ones because the latter were determined from the thickness of Co deposited at room temperature, hereafter the Co sputtering durations were used instead of nominal thicknesses in the discussion of the second group of samples which were 10 s, 25 s, 49 s and 98 s, respectively. The δ-doping was repeated for 60 cycles with each cycle consisting of a 2.38 nm ZnO:Al spacer and a δ-doped Co layer. The thickness of Co was chosen such that the average Co composition will be roughly the same as those of four co-doped samples obtained at a Co sputtering power of W, W, 16 W and 32 W, respectively. Table 3-1 summarizes the details of all the samples that have been investigated systematically in this work. The thickness of all the films prepared is around 200 nm. 81 Chapter Experimental details Table 3-1 Details of the samples under study. Sample Co doping Co sputtering Co composition or sample structure number technique power or duration A 3W Zn1-xCox:Al (x = 0.046) B 8W Zn1-xCox:Al (x = 0.137) C 10 W Zn1-xCox:Al (x = 0.159) D 15 W Zn1-xCox:Al (x = 0.198) E 20 W Zn1-xCox:Al (x = 0.235) 25 W Zn1-xCox:Al (x = 0.244) 30 W Zn1-xCox:Al (x = 0.273) H 32 W Zn1-xCox:Al (x = 0.287) I 35 W Zn1-xCox:Al (x = 0.300) J 40 W Zn1-xCox:Al (x = 0.304) K 45 W Zn1-xCox:Al (x = 0.296) L 50 W Zn1-xCox:Al (x = 0.324) M 10 s [ZnO:Al (2.38 nm) / Co(0.1 nm)]×60 N 25 s [ZnO:Al (2.38 nm) / Co(0.25 nm)]×60 49 s [ZnO:Al (2.38 nm) / Co(0.5 nm)]×60 98 s [ZnO:Al (2.38 nm) / Co(1.0 nm)]×60 F G O P co-doping δ-doping 82 Chapter Experimental details 3.1.2 Fabrication of Hall bars For electrical characterizations, Hall bars with a length of 324 m and a width of 80 m were fabricated for each sample using a direct laser writer (see Fig. 3-1). A 1-33-1 eight-contact Hall bar configuration was used to measure both the longitudinal and Hall voltages. The same sample had also been used to measure the MR and dynamic conductance curves. As shown in Fig.3-1, contacts and were current probes, and 3, 4, and were voltage probes (V3,4, V5,6: longitudinal voltage; V4,5: Hall voltage). The process flow of Hall bar fabrication is illustrated in Fig.3-2. A MicroTech laser writer was used to pattern both the Hall bar and electrode pads. An Al2O3 layer was used as a hard mask to pattern the Hall bar via ion-milling. Al was used as the electrode material to obtain an ohmic contact with ZnO. An additional layer of Au was used to prevent the Al electrodes from oxidation and also for ease of bonding the lead wires. p = 16 µm d = 76 µm W = 80 µm c = µm L = 324 µm Figure 3-1 Schematic of Hall bars. 83 Chapter Experimental details Resist ZnO : Co : Al ZnO : Co : Al Al2O3 Substrate Al2O3 Substrate a) Deposit ZnO:Co:Al b) Coat resist ZnO : Co : Al ZnO : Co : Al Al2O3 Substrate Al2O3 Substrate c) Write Hall bar pattern d) Develop Al2O3 ZnO : Co : Al Al2O3 Substrate Al2O3 Substrate e) Deposit Al2O3, Lift-off f) Ion-mill to form Hall bar Al2O3 Substrate Al2O3 Substrate g) Coat resist h) Write electrodes, develop Al2O3 Substrate Al2O3 Substrate j) Lift-off i) Al/Au electrode deposition Figure 3-2 Process flow of Hall bar fabrication. 3.2 Sample characterization techniques The samples had been characterized by XRD, TEM & EELS, XPS, AES & Ultra-violet photoemission spectroscopy (UPS), SQUID, optical transmission spectroscopy, MCD, MR and Hall effect. As most of them are standard techniques, in what follows those which were directly relevant to DMS or more specifically to ZnO:Co are described briefly. 84 Chapter Experimental details 3.2.1 Optical transmission spectroscopy As all the samples in this study were grown on transparent sapphire substrates, optical transmission spectroscopy is one of the most convenient yet powerful techniques to study ZnO:Co. Here, our interest was in two wavelength regimes, i.e., the near bandgap region of ZnO:Co and the visible region (1.8-2.4 eV) due to d-d transitions of Co2+ ions substituting Zn in ZnO. The former provides useful information on how the bandgap varies with Co composition at low doping and whether there was any new phases emerging at high doping levels. In the latter case, optical absorption of Co2+ in ZnO shows characteristic d-d transition peaks for tetrahedrally coordinated Co2+ in ZnO at 571, 618 and 665 nm for 4A2(F) → 2A1(G), A2(F) → 4T1(P), and 4A2(F) → 2E(G) transitions, respectively,1,2 as shown in Fig. 3- 3(a). Fig. 3.3(b) shows a typical optical absorption curve with the d-d transitions as marked. Thereafter, the observation of these dips in the optical absorption curves had been used frequently in literature to provide “evidence” that Co has substituted Zn in ZnO. However, as will be pointed out in this thesis, the observation of these characteristic peaks did not rule out the possibility of the formation of other phases; instead it only meant that, in some regions, Co substituted Zn in the host matrix. It should not be used to imply that the Co2+ observed was indeed responsible for the ferromagnetic behaviour observed in magnetic measurements. 85 Chapter Experimental details (b) (a) Figure 3-3 (a) Visible absorption bands of ZnO:Co2+ at 4.2 K[After P. Koidl, 1977, Ref. 1]; (b) Typical optical absorption curve in Co-doped ZnO with d-d transitions marked. [After I. Ozerov, 2005, Ref. 3] 3.2.2 Magnetic circular dichroism (MCD) Although optical transmission could be used to show the replacement of Zn by Co through d-d transitions, it could not be used to study magnetic properties of Codoped ZnO samples. The MCD method, on the other hand, is a useful technique to characterize this material as discussed below. The MCD is a magneto-optical technique which detects the difference in optical absorption between right and left circularly polarized light with the application of a magnetic field in the light propagation direction (see Fig.3-4).4 Its basic principle is as follows. Figure 3-4 Schematic diagram of incident, reflected and transmitted light of a MCD setup. 86 Chapter Experimental details In the absence of a magnetic field, the transmitted light intensity is given by, I = Io exp [-k(E).L] (3.1) where k (E) is the optical-absorption coefficient at energy E , Io the input light intensity and L the sample thickness. When a magnetic field is applied, it causes Zeeman splitting of the energy levels, leading to a difference in optical-absorption coefficient between light with two different polarizations (σ+ and σ-), resulting in the MCD effect. The magnitude of MCD, expressed as the angle change (θ) per unit light propagation distance is given by, θ= 180 ∆E (k − − k + ), where k ± ( E ) = k ( E ± ) 4π (3.2) where ∆E is the Zeeman splitting energy given by a first order perturbation treatment of the sp-d interaction, and k+ and k- are the optical absorption coefficient for the σ+ and σ- polarizations, respectively. As ∆E is small, Eq. (3.2) can be re-written as following: θ =− 180 dk ( E ) ∆E 4π dE ∆E = − S Z N o (α − β ) (3.3) (3.4) Where is the average spin per magnetic ion, No is the number of cation per unit volume, α and β are integrals of sp-d exchange interactions. Equation 3.3 shows that the MCD is proportional not only to Zeeman splitting energy, but also to the energy derivative of optical-absorption coefficient. As the latter is large near the band edge region, a strong MCD peak is expected in this region. In a pure semiconductor system, ∆E is small; but, in a DMS system, the ∆E is greatly enhanced due to the sp-d interactions. As the MCD signal is proportional to the average magnetization, one can easily observe the hysteresis, if any, in DMSs. 87 Chapter Experimental details In the MCD measurements out of ZnO:Co, discussion will be focused in two energy regions, i.e., the absorption edge (near 3.4 eV) and the d-d transition region (1.8 - 2.0 eV). Shown in Fig.3-5 are some typical MCD spectra for ZnO doped with different TMs. For ZnO without any TM doping, a weak MCD signal can be observed near the absorption edge at 3.4 eV, although not seen clearly in the figure. When Sc, Ti, V and Cr are introduced into the ZnO matrix, there is no observable change in the MCD spectra, indicating the absence of sp-d exchange interactions in these TM-doped ZnO. In comparison, for Mn, Co, Ni and Cu doped ZnO, a clear MCD signal can be observed near the absorption edge at 3.4 eV, indicating the presence of strong sp-d interaction. In addition to this pronounced MCD signal, small MCD structures have also been observed near eV in Co-doped ZnO, which correspond to the fingerprints of the d-d inter-ionic transitions of tetrahedrally coordinated Co2+ as determined from the optical transmission studies. Figure 3-5 MCD spectra of Zn1-xTMxO at 5K for various transition metal doping. [After K. Ando, 2001, Ref. 5] 88 Chapter Experimental details The biggest advantage of using MCD over other magnetometer-based techniques is that it is energy sensitive, thus enabling the measurement of magnetic hysteresis curves at different photon energies. The typical M-H curves determined by MCD are shown in Fig. 3-6. In this specific case, there is no hysteresis in pure ZnO (Fig. 3-6(a)); though a clear hysteresis is observed in Co-doped ZnO (Fig. 3-6(b)). However, as will be discussed in this thesis, the observation of a hysteresis by MCD does not necessarily mean that the material under study is ferromagnetic. It will be shown how the MCD taken at different photon energies can help establish the mechanism of ferromagnetic ordering in Co-doped ZnO, in combination with other techniques. (a) (b) Figure 3-6 MCD hysteresis loop obtained at 300 K with energy of 3.4 eV for (a) pure ZnO and (b) Codoped ZnO. [After J. R. Neal, 2006, Ref. 6] 3.2.3 Hall effect Next, turning the focus to the Hall effect, which is often used in semiconductor research to determine carrier concentration, mobility and resistivity. The fundamental principle of the Hall effect lies in the Lorentz force exerted on moving charges by an applied magnetic field. A magnetic field applied perpendicularly to a current flow would cause a deflection of charge carriers, leading to the buildup of a voltage, i.e., Hall voltage, across the sample, as shown in Fig. 3-7(a). The Hall voltage VH is 89 Chapter Experimental details proportional to the applied magnetic field B and the current I, i.e., VH = -IB/ned, where d is the sample thickness, e is the electron charge and n is the carrier concentration. The corresponding Hall resistance can be written as RHall= (RH/d) B, here RH is the Hall coefficient. The Hall effect in a magnetic materials is more complicated as compared to the non-magnetic counterpart. The Hall resistance in a ferromagnet is generally given by R Hall = Ro R B + s M⊥ d d (3.5) where Ro is the ordinary Hall coefficient (equivalent to RH in the non-magnetic case), Rs the anomalous Hall coefficient and M⊥ the magnetization of sample in the direction which is perpendicular to the current. Here, the first term is due to ordinary Hall effect (OHE), while the second term originates from the so-called AHE. The AHE has two proposed mechanisms: skew scattering and side-jump, as illustrated in Fig. 3-7 (b). In skew scattering, when electrons are scattered by a magnetic impurity, the angle of trajectory of the deflected electrons is spin-dependent. On the other hand, in the side jump, there is a lateral displacement of wave-packet after scattering. In magnetic materials, these cause a net spin current and a transverse component in the charge current, which leads to the AHE. (b) (a) Figure 3-7 (a) Schematic of Hall effect, (b) Schematic diagram of skew scattering and side jump contributing to the AHE. 90 Chapter Experimental details The presence of AHE is often considered as one of the strong evidences for intrinsic ferromagnetism in DMSs (see a typical result for Co-doped TiO2 shown in Fig. 3-8). - 10 However, considering the fact that the AHE has also been reported in ferromagnetic clusters, 11 granular materials 12 - 14 and inhomogeneous DMS in the hopping transport regime,15 the observation of AHE alone cannot support the claim that the DMS under study is a ferromagnet of intrinsic origin, unless it is correlated with ferromagnetism observed by other means and furthermore, secondary phases and precipitates must be absent in the sample. As will be discussed later in this thesis, in the present case, Zn1-xCoxO samples with x < 0.2 show only OHE. As Co concentration increases, the AHE appears in samples with x ≥ 0.25. The onset composition at which the AHE starts to appear also coincides with the composition at which the Co-rich phase becomes dominant and Co clusters start to appear. Therefore, the AHE in this case is due to extrinsic origin instead of intrinsic ones. Figure 3-8 Magnetic-field dependence of (a) magnetization and (b) Hall resistivity ( Co:TiO2– .at room temperatures. [After J. S. Higgins, 2004, Ref. 8] 91 xy) for rutile Chapter Experimental details There are two commonly used sample geometries in the Hall effect measurement: van der Pauw and Hall bar, with the commonly used geometries as shown in Fig. 3-9. In the van der Pauw geometry, four ohmic contacts are formed on an arbitrary shaped sample, as shown in Fig. 3-9 (a).16 The sample has to have uniform thickness, with square and circle being the most commonly used van der Pauw geometries. Also, in this geometry, the contacts are formed on the circumference of the sample and the contact pad area should be relatively small compared to the sample surface area. In order to measure Hall resistance or resistivity, magnetic field is applied perpendicular to the sample and two sets of voltages need to be taken to determine the sheet resistance of the sample. One resistance measurement is carried out along a vertical edge of the sample, while the other is taken along a horizontal edge. More measurements or a negative magnetic field can be applied in order to average the results. In the van der Pauw geometry, calculations not rely on contact spacing, but only on sample thickness, making sample preparation easy. However, there are two main disadvantages for the van der Pauw geometry. Firstly, error due to contact area size and their placement can be quite significant.17 And secondly, as a few voltage readings need to be taken before calculations can be carried out, it takes a slightly longer time to finish the measurements. In the Hall bar geometry, six to eight contacts are required and measurements are sensitive to sample geometry, as shown in Fig. 3-9 (b). Current flows through the sample along the long axis of the rectangular-shaped sample, with external magnetic field applied perpendicular to the sample. As current is passed through the sample in known dimensions, only one voltage reading is required to in calculations of carrier concentrations, mobility and resistivity. This enables readings to be completed in a much shorter time compared to the van der Pauw geometry; however, sample 92 Chapter Experimental details preparation is tedious. Also, although only four probes are used during measurements (two for current, two for Hall voltage), more probes are usually included to ensure the sample shape is symmetrical. These additional probes can also be used to take other voltage readings like longitudinal voltage and also used to check for sample homogeneity. The 1-3-3-1 eight-contact Hall bar configuration, used in this study, is the most ideally symmetrical Hall bar configuration. Also, with this Hall bar configuration, it is possible to carry out measurements for both Hall effect and MR, which is used in our project. (a) (b) Figure 3-9 Commonly used (a) van der Pauw and (b) Hall bar geometries, with sample thickness t, and contacts are black squares as indicated. [After Lakeshore Hall effect manual, Ref. 18] 93 Chapter Experimental details 3.2.4 Ferromagnetic-superconductor junctions At a normal metal (N) / SC interface, at a small applied bias, an electron is not able to enter the superconductor, due to a superconducting gap, ∆. For the electron to travel to the superconducting side, it will need to pair with an electron of opposite spin in the superconductor to form a Cooper pair. To conserve charge, a hole is then reflected back into the metal. This process described above is known as the Andreev reflection (AR) and will result in doubling of the normalized conductance, G NS , the G NN ratio of conductance between metal/superconductor interface, GNS, and conductance between metal/metal interface, GNN. When the applied bias is greater than ∆, electrons will start to enter the superconductor, as if it is a normal metal, leading to unity of the normalized conductance. For a metal/superconductor interface, the Blonder-Tinkham-Klapwijk (BTK) model is used in describing the electron transport, which accounts for interfacial scattering, single particle tunneling, normal and Andreev reflection.19 In this model, interfacial scattering is quantified by a dimensionless constant, Z. Z is zero in a ballistic contact with no scattering and tends to infinity for a tunnel junction. The main result from the BTK model is given as ( G NS = − 1+ Z G NN ) [1 + A( E ) − B( E )][ f ' (E − eV )]dE (3.6) where A is the Andreev reflection probability, B is the normal reflection probability and f’ is the derivative of the Fermi distribution function. However, when the normal metal is replaced by a ferromagnet, spin polarization of the current needs to be considered as it affects the Andreev reflection probability. Polarization, P, is dependent on the degree to which the s and d bands cross the Fermi level. When the orbital 94 Chapter Experimental details characteristic of the ferromagnet at the Fermi level is d-like (s-like), P is high (low). By considering spin polarization, Strijkers et. al. proposed a BTK modification, where current is given as a summation of polarized current and non-polarized current. 20 The main result of the BTK modification is given as [ ] G NS = − P(1 + Z ) + A( E ) − B( E ) [ f ' ( E − eV )]dE G NN − (1 − P)(1 + Z ) [1 + A( E ) − B( E )][ f ' ( E − eV )]dE (3.7) where and B are modified probabilities. By setting Z = 0, equation (3.7) can be simplified and the normalized conductance at zero bias would be determined by 2(1-P). Thus, when P = (fully spin polarized), the normalized conductance is zero and when P = (non-magnetic), the normalized conductance is 2. A schematic of a metal/superconductor interface, for P = and P = 1, is shown below in Fig. 3-10 below. (a) (b) Figure 3-10 (a) Schematic of metal/superconductor interface for spin polarization of (a) P = 0; (b) P = 100 %. The solid circles denote electrons and open circles denote holes. [After R. J. Soulen, 1998, Ref. 21] To determine spin polarization of a current, a ballistic contact, with a size smaller than the mean free path of electrons in bulk, is created between the superconductor and the ferromagnet. This method is also known as the PCAR 95 Chapter Experimental details spectroscopy. 21 , 22 In point contact measurements, a sharp superconducting tip is mechanically lowered onto the ferromagnetic sample. Polarization, as determined from conductance studies, is independent on the sign of polarization and only the magnitude is determined. Fig. 3-11 shows typical differential conductance curves of Ni0.8Fe0.2, Cu (P = 0), Co, NiMnSb, La0.7Sr0.3MnO3 and CrO2 (P = 1), as determined by forming a superconducting point contact.21 As shown in the figure, the differential conductance curves have very different shapes for materials of different polarizations. PCAR has also recently been carried out on DMS materials, InSb:Mn and GaAs:Mn. 23 Figure 3-11 The differential conductance curve for several spin polarized metals showing the suppression of Andreev reflection with increasing contact polarization. The vertical lines denote the bulk gap of Nb: ∆(T = 0) = 1.5 meV. [After R. J. Soulen, 1998, Ref. 21] In granular materials, when the distance between granules is smaller than superconductor coherence length, ξ, crossed Andreev reflections (CAR) will dominate over AR.24 In CAR, with two ferromagnetic electrodes, a spin-up electron from a spinup ferromagnetic electrode can go through AR and is reflected as a spin-down hole in the another spin-down ferromagnetic electrode. In another transport process, elastic cotunnelling, a spin-up electron from a spin-up ferromagnetic electrode is transferred as a spin-up electron into the another ferromagnetic spin-up electrode. Studies on granular 96 Chapter Experimental details metal have been carried out by Frydman et. al. using a disordered Ag film with superconducting Pb electrodes, as shown in Fig. 3-12 below. 25 As thickness of the films increases, disorder in the films decrease, and transport properties shift from CAR to the conventional BTK behaviour. In our work, ZnO:Co-Nb junctions will be formed and their differential conductance as a function of the bias voltage will be studied. Of particular interest, discussion will be concentrated how ZnO:Co with different Co compositions would affect the G-V curves of the junctions. Based on the work on CAR, it is expected that the conductance in the gap region will be enhanced for granular materials with optimized inter-granular distances. Figure 3-12 Dynamic resistance versus voltage curve for a mm Ag film at different evaporation stages at T=1.8 K. The corresponding Ag thicknesses are 58, 69, 86, and 118 Å, respectively. [After A. Frydman, 1999, Ref. 25] 97 Chapter Experimental details 3.3 Summary In this chapter, the preparation methods for Co-doped ZnO films and Hall bars used in the transport measurements have been described briefly. The following characterization techniques which are more relevant to this work were also discussed: optical transmission spectroscopy, MCD, Hall effect and ferromagnetic- superconductor junctions. Although all these techniques can be used either alone or in combination to “prove” the existence of ferromagnetism in a sample, it is extremely important that the results obtained by different techniques must be correlated with one another. This point will be discussed again when the objective is to resolve the origin of ferromagnetism in the next few chapters. References: P. Koidl, “Optical absorption of Co2+ in ZnO”, Phys. Rev. B 15, 2493 (1977). R. O. Kuzian, A. M. Daré, P. Sati, and R. Hayn, “Crystal-field theory of Co2+ in doped ZnO”, Phys. Rev. B 74, 155201 (2006). I. Ozerov, F. Chabre, and W. Marine, “Incorporation of cobalt into ZnO nanoclusters”, Mater. Sci. & Engineering C 25, 614 (2005). K. Ando, Magneto-optics, (Springer, Berlin, 2000), Vol 128, pp. 211. K. Ando, H. Saito, Z. 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Lett. 84, 4947 (2004); R. P. Panguluri, K. C. Ku, T. Wojtowicz, 100 Chapter Experimental details X. Liu, J. K. Furdyna, Y. B. Lyanda-Geller, N. Samarth, and B. Nadgorny, “Andreev reflection and pair-breaking effects at the superconductor/magnetic semiconductor interface”, Phys. Rev. B 72, 054510 (2005). 24 G. Deutscher and D. Feinberg, “Coupling superconducting-ferromagnetic point contacts by Andreev reflections”, Appl. Phys. Lett. 76, 487 (2000). 25 A. Frydman and R. C. Dynes, “Disorder-induced Andreev reflections in granular metals”, Phys. Rev. B 59, 8432 (1999). 101 [...]... Hayn, “Crystal-field theory of Co2+ in doped ZnO”, Phys Rev B 74, 155201 (2006) 3 I Ozerov, F Chabre, and W Marine, “Incorporation of cobalt into ZnO nanoclusters”, Mater Sci & Engineering C 25, 614 (2005) 4 K Ando, Magneto-optics, (Springer, Berlin, 2000), Vol 128, pp 211 5 K Ando, H Saito, Z Jin, T Fukumura, M Kawasaki, Y Matsumoto, and H Koinuma, ”Magneto-optical properties of semiconductors”, J Appl... (1999) 99 Chapter 3 Experimental details 15 A A Burkov and L Balents, ”Anomalous Hall effect in ferromagnetic semiconductors in the hopping transport regime”, Phys Rev Lett 91, 057202 (20 03) 16 L J van der Pauw, "A method of measuring specific resistivity and Hall effect of discs of arbitrary shapes," Philips Res Repts 13, 1 (1958); L J van der Pauw, "A method of measuring the resistivity and Hall coefficient... Mater 3, 221 (2004) 8 J S Higgins, S R Shinde, S B Ogale, T Venkatesan, and R L Greene, “Hall effect in cobalt- doped TiO2-δ”, Phy Rev B 69, 0 732 01 (2004) 9 N Manyala, Y Sidis, J F Ditusa G Aeppli, D P Young, and Z Fisk, “Large anomalous Hall effect in a silicon-based magnetic semiconductor”, Nat Mater 3, 255 (2004) 10 H S Kim, S H Ji, H Kim, S K Hong, D Kim, Y E Ihm, and W K Choo, “Observation of ferromagnetism... W K Choo, “Observation of ferromagnetism and anomalous Hall effect in laser-deposited chromium -doped indium tin oxide films”, Solid State Comm 137 , 41 (2006) 11 S R Shinde, S B Ogale, J S Higgins, H Zheng, A J Millis, V N Kulkarni, R Ramesh, R L Greene, and T Venkatesan, ”Co-occurrence of superparamagnetism and anomalous Hall effect in highly reduced cobalt- doped rutile TiO2–δ films”, Phys Rev Lett... which leads to the AHE (b) (a) Figure 3- 7 (a) Schematic of Hall effect, (b) Schematic diagram of skew scattering and side jump contributing to the AHE 90 Chapter 3 Experimental details The presence of AHE is often considered as one of the strong evidences for intrinsic ferromagnetism in DMSs (see a typical result for Co -doped TiO2 shown in Fig 3- 8) 7 - 10 However, considering the fact that the AHE has... Fig 3- 10 below (a) (b) Figure 3- 10 (a) Schematic of metal/superconductor interface for spin polarization of (a) P = 0; (b) P = 100 % The solid circles denote electrons and open circles denote holes [After R J Soulen, 1998, Ref 21] To determine spin polarization of a current, a ballistic contact, with a size smaller than the mean free path of electrons in bulk, is created between the superconductor and. .. 3 Experimental details spectroscopy 21 , 22 In point contact measurements, a sharp superconducting tip is mechanically lowered onto the ferromagnetic sample Polarization, as determined from conductance studies, is independent on the sign of polarization and only the magnitude is determined Fig 3- 11 shows typical differential conductance curves of Ni0.8Fe0.2, Cu (P = 0), Co, NiMnSb, La0.7Sr0.3MnO3 and. .. eV )]dE (3. 6) where A is the Andreev reflection probability, B is the normal reflection probability and f’ is the derivative of the Fermi distribution function However, when the normal metal is replaced by a ferromagnet, spin polarization of the current needs to be considered as it affects the Andreev reflection probability Polarization, P, is dependent on the degree to which the s and d bands cross... Coey, “Measuring the spin polarization of a metal with a superconducting point contact”, Science 282, 85 (1998) 22 S K Upadhyay, A Palanisami, R N Louie and R A Buhrman, “Probing ferromagnets with Andreev reflection”, Phys Rev Lett 81, 32 47 (1998) 23 R P Panguluri, B Nadgorny, T Wojtowicz, W L Lim, X Liu, and J K Furdyna, “Measurement of spin polarization by Andreev reflection in ferromagnetic In1-xMnxSb... Wojtowicz, 100 Chapter 3 Experimental details X Liu, J K Furdyna, Y B Lyanda-Geller, N Samarth, and B Nadgorny, “Andreev reflection and pair-breaking effects at the superconductor/magnetic semiconductor interface”, Phys Rev B 72, 054510 (2005) 24 G Deutscher and D Feinberg, “Coupling superconducting-ferromagnetic point contacts by Andreev reflections”, Appl Phys Lett 76, 487 (2000) 25 A Frydman and R C Dynes, . W). Films of high structural quality and resistivity of 1 .3 m.cm were obtained at a substrate temperature of 500 o C, ZnO sputtering power of 150 W and Al 2 O 3 sputtering power of 30 W. Under. be roughly the same as those of four co -doped samples obtained at a Co sputtering power of 3 W, 8 W, 16 W and 32 W, respectively. Table 3- 1 summarizes the details of all the samples that have. Co(1.0 nm)]×60 Chapter 3 Experimental details 83 3. 1.2 Fabrication of Hall bars For electrical characterizations, Hall bars with a length of 32 4 m and a width of 80 m were fabricated