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Uniaxial magnetic anisotropy of cobalt thin films on different substrates using CW-MOKE technique
Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Uniaxial magnetic anisotropy of cobalt thin films on different substrates using CW-MOKE technique Vijay Shukla a,n, C. Mukherjee b, R. Chari a, S. Rai c, K.S. Bindra d, A. Banerjee e a
Laser Physics Applications Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India Mechanical and Optical Support Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India c Indus Synchrotron Utilization Division, Raja Ramnna Centre for Advanced Technology, Indore 452013, India d Solid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India e BARC training school at RRCAT and Homi Bhabha National Institute, Raja Ramanna Centre for Advanced Technology, Indore 452013, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 March 2014 Received in revised form 29 May 2014 Available online 3 July 2014
Cobalt thin films were deposited on GaAs, Si and Glass substrates by RF-magnetron sputtering. The structure was studied using atomic force microscopy, X-ray reflectivity and grazing incidence X-ray diffraction. Magnetic properties were determined with the magneto-optic Kerr effect. The deposited films have in-plane uniaxial anisotropy and after annealing the anisotropy reduces. The reduction in anisotropy may be due to release of stress and the remaining anisotropy after annealing may be due to shape anisotropy of the particulates. & 2014 Elsevier B.V. All rights reserved.
Keywords: Cobalt thin film Magnetic anisotropy AFM Magneto-optic Kerr effect
1. Introduction Magnetic properties of thin films are inherently related to the structure and morphology of the films. From the application viewpoint, magnetic anisotropy is one of the most important properties of the magnetic materials [1]. Depending upon the type of application e.g. permanent magnets, storage media or magnetic cores in transformers and magnetic recording heads, materials with high, medium or low magnetic anisotropy are required. In addition, due to extensive development in semiconductor technology, the properties of magnetic metal films especially of nanometer scale structures on semiconductor substrates have gained more importance. The magnetic and interfacial properties of thin films of cobalt (Co) on different substrates (e.g. Pt, Ge, SiO2, GaAs, glass, MgO, Si, Cu and Pd) have been reported in the literature, but the emphasis have been on to Cu, Pt and Si substrates because of the possibility of growth of FCC Co in the case of Cu, perpendicular anisotropy in the case of Pt or Pd substrates and studies of growth on semiconductors for spintronics devices [2–13]. The effect of surface roughness, thickness dependence, oblique deposition etc. has been extensively studied [8,10,11–14]. Co thin films have been deposited by metal organic chemical vapor deposition (MOCVD), DC and RF magnetron E-mail address: [email protected] (V. Shukla). Corresponding author at: Laser Physics Applications Section, R&D Block “A”, Raja Ramanna Centre for Advanced Technology, Indore 452013 (Madhya Pradesh), India. Tel.: þ 91 731 2488378; mobile: þ 91 9575832401. n
http://dx.doi.org/10.1016/j.jmmm.2014.06.061 0304-8853/& 2014 Elsevier B.V. All rights reserved.
sputtering, thermal evaporation, e-beam evaporation and molecular beam epitaxy [4–14]. Uniaxial magnetic anisotropy in Co thin films on glass, Si, MgO, etc. has also been observed [8,9,11–14]. Effect of surface roughness on the magnetization reversal in Co thin films on Si has been studied and it has been reported that increasing the roughness changes the magnetization reversal process from the magnetization rotation to a combination of magnetization rotation and domain wall motion [11]. Similarly, for studies of Co films on rippled Si substrates, the strong uniaxial anisotropy has been reported with its easy axis along a direction normal to the ripple wave vector [7]. Uniaxial magnetic anisotropy has been studied in Co films on glass using MOKE technique and the shape of the substrate is reported as the probable cause for the rotation of the easy axis of the magnetization for amorphous or polycrystalline films [4]. The correlation between micro-structure and magnetic properties has been studied and roughness is found to lower the uniaxial anisotropy and raise the coercivity [12]. However, in spite of the significant research effort, much of the observed magnetic behavior remains unexplained, including a detailed understanding of the magnetic anisotropy and the change from perpendicular to in plane magnetization. The reason for this surely arises from the large sensitivity of the magnetic anisotropy to structural, morphological, and chemical details at the interfaces, which is at the origin of a variety of experimental results due to a lack of complete and accurate control of the fabrication parameters. Therefore, it becomes important to study the effect of substrate on the magnetic properties of the sample. To study the effect of substrate on the magnetic properties of the thin film,
V. Shukla et al. / Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
Uniaxial magnetic anisotropy was studied in cobalt thin films using continuous wave (CW) magneto-optic Kerr effect (MOKE) technique. The MOKE setup consists of a monochromatic light source (intensity stabilized He-Ne laser) followed by a polarizer to select the incident polarization, a photo-elastic modulator (PEM), the sample in a magnetic field between the poles of an electromagnet, an analyzer and the photo detector. The photodiode signal is fed to the Lock-in amplifier (SR830). The MOKE signal is measured in longitudinal geometry i.e. the applied magnetic field is in the plane of the film. Azimuthal angle dependent hysteresis loops were recorded by rotating the sample in the plane of the applied magnetic field. All MOKE measurements were carried out using marked axis as 01. Cobalt thin films (thickness of 45 nm) were deposited on different substrates i.e. BK7glass, GaAs and silicon by RF magnetron sputtering of Co target (99.99%) in Ar (99.9995%) environment. The GaAs substrate has 〈111〉 orientation and Si substrate has 〈100〉 orientation. Both the substrates have rms roughness less than 5 Å while the glass substrates are optically polished and have a surface roughness 7 Å. Ultrasonic cleaning was done in a class 10,000 clean bench and immediately loaded in the deposition system kept in a clean room. Films were deposited in RF sputtering system (V0301, Elettrorava spa., Italy), with in-situ physical thickness monitoring. Before deposition the system was pumped down to 5 10 7 mbar at 150 1C. Deposition rate was 0.1 Å/s at 50 W RF power and substrate temperature was 75 1C. Pre-sputtering was done for 5 min to remove the top surface of the target, to avoid contamination. All the substrates were loaded simultaneously and films were deposited at the same time. Axis was marked on each substrate prior to deposition. One sample on each substrate was annealed in Argon atmosphere for 1 h at 300 1C. The structural characterization was done by grazing incidence X-ray diffraction (GIXRD) and Atomic Force Microscopy (AFM). Ex-situ X-ray reflectivity (XRR) measurements were carried out to find out exact thickness of the deposited films. Surface topographies of samples were imaged using a multimode scanning probe microscope (NT-MDT, SOLVER-PRO, Russia). AFM measurements were carried out in a non-contact mode using silicon cantilever tips having radius of curvature 20 nm and a spring constant 5.5 N/m with a resonant frequency of 190 kHz under ambient conditions. The top surface was electrically grounded during measurements. Analysis was carried out at various places on the surface of samples.
Fig. 1 shows the XRR data of as deposited Co film grown on GaAs wafer. The reflectivity profile was fitted using Parratt's formalism [15]. The best fitting was obtained by considering three layer structure consisting (i) native oxide layer of 2.5 nm thickness on GaAs wafer, (ii) 45 nm thick Co layer and (iii) 4.7 nm top oxide layer. The topmost layer shows somewhat lower electron density as compared to thick Co underlayer. This topmost layer can be due to formation of CoO because of exposure to atmosphere. The presence of a 5 nm thick CoO surface layer has also been reported by Paranjape et al. [13], which was confirmed by secondary ion mass spectroscopy (SIMS). All the samples grown in a single deposition run show the same average film thickness of 45 nm as determined by XRR. The film adjacent to wafer can be attributed to formation of native oxide. The top layer surface roughness obtained from the XRR fitting is 2.9 nm which is comparable to the surface roughness obtained from AFM results (4.5 nm). Figs. 2 and 3 show GIXRD results for as deposited Co films on Si and GaAs respectively. GIXRD peak corresponding to Co (002) phase is observed at 44.51 for both the substrates. However, it is to be noted here that we do not observe any peak for Co, deposited on glass substrate. Although, we do not observe any peak in GIXRD data for Co/glass sample, we believe that the film deposited on glass is not amorphous. The presence of a broad hump consisting of multiple peaks corresponding to (100), (002) and (101) phases of Co has been reported for approximately same thickness of Co film deposited on glass substrate [6,8]. For study of magnetic properties of these thin films, azimuthal angle dependent hysteresis loops were measured in longitudinal 10 CoO
1
Normalized Intensity
2. Experimental
3. Results and discussion
Co
0.1
Native oxide GaAs Wafer
0.01 1E-3 1E-4 1E-5 1E-6 1E-7 0.00
0.05
0.10
0.15
q
0.20
0.25
(A-1)
Fig. 1. XRR data and fitting (solid curve) for Co/GaAs: inset shows the assumed layered structure for film.
40
Co/Si
Intensity(Arb. Units)
we deposited thin films on different substrates under same deposition conditions. The earlier studies are independent studies mostly with one substrate at a time and there are very few reports devoted to the magnetic properties of thin films on two or more substrates. The magneto-optic Kerr effect (MOKE) has become a standard tool for the study of magnetic ultrathin films and magnetic multi-layers due to its simple experimental setup, ability for making in-situ measurements and high sensitivity down to monolayer resolution [2,4,6,8,14]. In the present work, we have studied the in-plane magnetic anisotropy of cobalt thin film on different substrates by MOKE technique. We have deposited Co thin films of 50 nm thickness on different substrates simultaneously and have shown that anisotropy is due to stress during deposition and with annealing, the anisotropy decreases which may be due to release of stress and the remanant anisotropy is due to shape anisotropy of the particulates.
101
30
20
10
0 40
50
60
70
80
Angle(2 ) Fig. 2. GIXRD data for Co/Si; circle shows the data points and the continuous line shows the fitting.
102
V. Shukla et al. / Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
Intensity(Arb. Units)
50
Co/GaAs
40
30
20
10
0 40
50
60
70
80
Angle( 2 ) Fig. 3. GIXRD data for Co/GaAs; circle shows the data points and the continuous line shows the fitting.
geometry using MOKE technique. Fig. 4 shows the representative CW-MOKE pattern for Co/glass sample for three different azimuthal angles. The angle is taken with respect to the marked axis. When the applied field is in the direction of the easy axis of magnetization, the almost square loop shows that the magnetization process takes place by the domain wall motion. Curving of the hysteresis loop e.g. for azimuthal angle 401 indicates the increasing contribution of the rotation of the domain magnetization in the magnetization process. The coercivities were obtained from the hysteresis loop at different angles and plotted as a function of angle of rotation of the samples, as shown in Fig. 5(a) and (b). The inherent two fold anisotropy is present in all the samples. We also tried to measure the out of plane component using the polar MOKE configuration, but it was not possible to saturate the magnetization in this geometry with maximum field of 1200 Gauss, maximum magnetic field that can be obtained with this magnet. It indicates that the in-plane magnetization component is dominant in these samples.
Magnetization (a.u.)
0.15 0.10
Table 1 Fitting parameters: anisotropy field (Ha) and domain wall pinning field (Hcw) for the as-grown and annealed samples.
0.05 0.00
Substrates Ha as-grown (Oe)
HCW as-grown (Oe)
Ha annealed (Oe)
HCW annealed (Oe)
Si GaAs Glass
112 120 90
21 27 46
183 178 80
-0.05 -0.10 -0.15 -200 -150 -100 -50
0
50
57 46 74
100 150 200
Magnetic Field (Oe) Fig. 4. Representative hysteresis loop for Co/glass at different azimuthal angles; square for azimuthal angle 601, circle for azimuthal angle 1501 and triangle for azimuthal angle 1001.
180
Coercivity(Oe)
160 140 120 100 80 0
60
120
180
240
300
360
Angle( )
220
Coercivity (Oe)
200 180 160 140 120 100
0
60
120
180 240 Angle( )
300
360
Fig. 5. (a) Azimuthal angle dependent coercivity for Co/glass as grown (open square) and annealed films (open triangle). Fittings are shown by solid lines. (b) Azimuthal angle dependent coercivity for Co/GaAs as grown (open square) and annealed films(open triangle). Fittings are shown by solid lines.
Fig. 6. AFM figure for Co/glass as-grown.
V. Shukla et al. / Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
103
As discussed in Ref. 11, in case of uniaxial anisotropy for the magnetization reversal process to happen, the applied field H must overcome not only the domain wall pinning coercivity, Hcw, but also the component of the magnetic anisotropy field in the applied field direction, Ha Cos θ. Hence, the azimuthal angle (θ) dependence of the coercivity can be written as follows [11]: H C ¼ H a Cos 2 θ þ ðH 2cw H 2a Cos 2 θ Sin θÞ1=2 2
where, Ha is the magnetic anisotropic field and θ is the angle between the applied field and the easy axis of magnetization. Fig. 5(a) and (b) shows the representative fitting graphs for the coercive field dependence on the azimuthal angle for as-grown and annealed Co on glass and Co on GaAs thin films. Table 1 shows the fitted parameters (Ha and Hcw) for different substrates. As shown in Table 1, the oriented substrates i.e. GaAs and Si have smaller Ha as compared to glass for both as grown and annealed films. Figs. 6 and 7 show the 2D and 3D surface morphology for as grown and annealed Co/Glass samples respectively. Similarly, Figs. 8–11 show the 2D and 3D surface morphology for as grown and annealed Co/Si and Co/GaAs samples respectively. Figures show that the films are particulate in nature. The formation of particulate structure on the crystalline substrate (Si, GaAs) may be due to some oxide formation on the substrates prior to the deposition. This is consistent with the XRR results which show the presence of a layer of very small thickness over the substrate. We have estimated particle sizes over the entire AFM image using mage analysis software of Solver Pro version 2.2 [16]. Standard procedure for estimating particle sizes was adopted, in which, an imaginary horizontal plane was considered and moved in the
Fig. 8. AFM figure for Co/Si as-grown.
Fig. 7. AFM figure for Co/glass annealed 300 1C.
z-direction such that the number of particles is maximum on that imaginary plane and the statistical data was obtained. This analysis gives mean diameter, mean size in x and y-directions, their standard deviations, number of particles etc. For instance, an imaginary plane at the height of 16.8 nm was used for Co/GaAs as grown sample having maximum number of particles. The mean particle size before annealing is 47 nm for glass, 71 nm for Si and 63 nm for GaAs and their standard deviations (SD) are 16 nm, 24 nm and 20 nm respectively. This shows that there is large size distribution. The particles are found to be oval in shape with longer axis along y direction with aspect ratio of 1.17 for glass, 1.44 for Si, and 1.36 for GaAs respectively. The average particle size is found to increase after annealing. It is 71 nm, 102 nm and 71 nm for glass, Si and GaAs respectively and their standard deviations (SD) are 16 nm, 25 nm and 18 nm respectively. Fig. 12(a) and (b) shows the mean particle sizes in x and y directions for as-grown and annealed films. The large standard deviation in the particle size distribution can be attributed to low substrate temperature (75 1C). The root mean square (rms) roughness was also estimated by taking the average of at least five regions of area 2 μm 2 μm. The rms roughness value is found to increase with annealing for all the substrates except GaAs. The reason for this is not known and is required to be explored further. Fig. 13 shows the variation of roughness for different substrates after annealing. The increase in rms roughness is also observed in Co/glass films, the reason is attributed to grain growth and structural transformation from HCP to FCC phase [5]. The high values of domain wall pinning field for all the samples except Co/glass can be understood from the structure of the films as shown in AFM figures. As discussed earlier from AFM data, the large average particle size and aspect ratio of Co film on Si and
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V. Shukla et al. / Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
Fig. 9. AFM figure for Co/Si annealed 300 1C. Fig. 10. AFM figure for Co/GaAs as-grown.
GaAs may be the reason for the large HCW. The particulate nature of the film and the connectivity between the domains can be the one of the reason for high coercivity. The low HCW value for Co/glass may be due to more compact film. After annealing the anisotropy field Ha is found to decrease for all the samples. It may be due to release of stress by annealing. This corroborates AFM results where particle size increases after annealing resulting in a more isotropic and homogeneous film. We tried to correlate the reason for high coercivity to roughness of the deposited thin films. There seems to be no apparent correlation between the anisotropy fields and coercivity with the roughness of the deposited films. The reason may be that the roughness of the deposited film (2–5 nm) is much smaller as compared to the thickness of the deposited film (45 nm). Because of particulate structure of the film, magneto-crystalline anisotropy cannot be the reason for the anisotropic behavior of the films. The different shapes (slightly oval in case of GaAs and glass and spherical in case of Si) also indicate towards the growth mechanism and stress developed during the deposition. The roughness of the films also increase after annealing indicating the larger size particles with worse connectivity as compared to as grown film leading to high domain wall pinning field. The change in anisotropy field after annealing also indicates towards the release of stress because the change in anisotropy value is in the order Si oGaAso Glass and the thermal conductivities of the substrates are also in the same order. A strong two-fold uniaxial magnetic anisotropy has also been reported in MBE deposited Co/glass film, but there is no correlation between the magnetic anisotropy and the film morphology for films of different thicknesses [4,5]. As discussed in Ref. 4, the external magnetic field of RF magnetron sputtering cannot be the reason
for the anisotropy of these films as the sample holder is rotating with respect to the deposition direction and there is no correlation between the GIXRD data and the magnetic anisotropy. They have also observed magnetic anisotropy in polycrystalline as well as amorphous films. The magnetic anisotropy has been attributed to the long range order stress during the deposition in samples and anisotropic ejection of the sputtered atoms relative to the direction of the ion beam [6,9]. The decrease in anisotropy on annealing above 3001 C is also attributed to relaxation of internal stress [5]. We emphasize here that influence of stress is more as compared to shape anisotropy as can be seen from the change in magnetic anisotropy field values before and after the annealing and not a significant change in structure of the film. The remaining anisotropy in these films even after annealing may be due to the slightly oval shape of particles and hence can be attributed to shape anisotropy.
4. Conclusion Cobalt thin films were deposited on glass, silicon and GaAs by RF magnetron sputtering. In plane uniaxial magnetic anisotropy was observed in cobalt thin films using MOKE technique. AFM studies show that the deposited films are of particulate nature. The particulate nature and the stress induced during deposition may be the reason for the anisotropic magnetic behavior of these films. On annealing the decrease in magnetic anisotropy confirms that the contribution of the stress is dominant cause for the magnetic anisotropy of these films. The remaining anisotropy after annealing can be attributed to shape anisotropy of the particulates.
V. Shukla et al. / Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
105
RMS Roughness (nm)
10 As-grown Annealed
8 6 4 2 Glass
Si
GaAs
Substrates Fig. 13. RMS roughness for as-grown and annealed films for different substrates; filled square for as-grown films and empty squares for annealed films.
Acknowledgments The authors thank Mr. Ayukt Pathak and Mrs. Shradha Tiwari, LSED, RRCAT for the Lab view based MOKE software and Dr. (Mrs.) Archna Sagdeo, ISUD, RRCAT for their critical comments on the manuscript and Dr. H.S. Rawat, LPAS, RRCAT for his encouragement and support in this work. VS is thankful to Mr. S. Khan for his help in data analysis. References
Fig. 11. AFM figure for Co/GaAs annealed 300 1C.
Particle Size(nm)
250 X-direction As-grown Annealed
200 150 100 50 0
Glass
Si
GaAs
Substrates
Particle Size(nm)
250 Y-direction As-grown Annealed
200 150 100 50 0
Glass
Si
GaAs
Substrates Fig. 12. (a) Particle size in x direction for different substrates, filled square for asgrown samples and empty square for annealed samples and (b) particle size in y direction for different substrates, filled square for as-grown samples and empty square for annealed samples.
[1] W.B. Zeper, F.J.A.M. Greidanus, P.F. Carcia, C.R. Fincher, J. Appl. Phys 65 (1989) 4971 http://dx.doi.org/10.1063/1.343189. [2] T. Hirahara, Yuki Saisyu, R. Hobara, S. Hasegawa, J. Appl. Phys 110 (2011) 053902 http://dx.doi.org/10.1063/1.3624662. [3] K. Tobari, M. Ohtake, K. Nagano, M. Futamoto, J. Magn. Magn. Mater. 324 (2012) 1059, http://dx.doi.org/10.1016/j.jmmm.2011.10.026. [4] T. Kushel, T. Becker, D. Bruns, M. Suendorf, F. Bertram, P. Fumagalli, J. Wollschlager, J. Appl. Phys. 109 (2011) 093907 http://dx.doi.org/10.1063/1. 3576135. [5] D. Kumar, A. Gupta, J. Magn. Magn. Mater. 308 (2007) 318, http://dx.doi.org/ 10.1016/j.jmmm.2006.06.008. [6] R. Gupta, A. Khnadelwal, D.K. Avasthi, K.G.M. Nair, A. Gupta, J. Appl. Phys. 107 (2010) 033902 http://dx.doi.org/10.1063/1.3294609. [7] K.V. Sarathlal, Dileep Kumar, V. Ganesan, A. Gupta, Appl. Surf. Sci. 258 (2012) 4116, http://dx.doi.org/10.1016/j.apsusc.2011.07.105. [8] A. Jaiswal, S. Rai, M.K. Tiwari, V.R. Reddy, G.S. Lodha, R.V. Nandedkar, J. Phys.: Condens. Matter 19 (2007) 016001, http://dx.doi.org/10.1088/0953-8984/19/1/ 016001. [9] O. Benamara, E. Snoeck, T. Blon, M. Respaud, J. Cryst. Growth 312 (2010) 1636, http://dx.doi.org/10.1016/j.jcrysgro.2010.02.001. [10] M.T. Umlor, Appl. Phys. Lett. 87 (2005) 082505 http://dx.doi.org/10.1063/1. 2032592. [11] M. Li, Y.-P. Zhao, G.-C. Wang, H.G. Min, J. Appl. Phys. 83 (1998) 6287 http://dx. doi.org/10.1063/1.367718. [12] Chung-Hee Chang, Mark H. Kryder, J. Appl. Phys. 75 (1994) 6864 http://dx.doi. org/10.1063/1.356810. [13] M.A. Paranjape, A.U. Mae, A.K. Raychaudhari, K. Shalini, S.A. Shivshankar, B. R. Chakravarty, Thin Solid Films 413 (2002) 8. [14] K. Chen, R. Fromter, S. Rossler, N. Mikuszeit, Oepen Hans Peter, Phys. Rev. B 86 (2012) 064432 〈http://link.aps.org/doi/10.1103/PhysRevB.86.064432〉. [15] L.G. Parratt, Phys. Rev. 95 (1954) 359 〈http://link.aps.org/doi/10.1103/PhysRev. 95.359〉. [16] Image analysis software of Solver Pro (NDT-MT) Version 2.2.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Uniaxial magnetic anisotropy of cobalt thin films on different substrates using CW-MOKE technique Vijay Shukla a,n, C. Mukherjee b, R. Chari a, S. Rai c, K.S. Bindra d, A. Banerjee e a
Laser Physics Applications Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India Mechanical and Optical Support Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India c Indus Synchrotron Utilization Division, Raja Ramnna Centre for Advanced Technology, Indore 452013, India d Solid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India e BARC training school at RRCAT and Homi Bhabha National Institute, Raja Ramanna Centre for Advanced Technology, Indore 452013, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 March 2014 Received in revised form 29 May 2014 Available online 3 July 2014
Cobalt thin films were deposited on GaAs, Si and Glass substrates by RF-magnetron sputtering. The structure was studied using atomic force microscopy, X-ray reflectivity and grazing incidence X-ray diffraction. Magnetic properties were determined with the magneto-optic Kerr effect. The deposited films have in-plane uniaxial anisotropy and after annealing the anisotropy reduces. The reduction in anisotropy may be due to release of stress and the remaining anisotropy after annealing may be due to shape anisotropy of the particulates. & 2014 Elsevier B.V. All rights reserved.
Keywords: Cobalt thin film Magnetic anisotropy AFM Magneto-optic Kerr effect
1. Introduction Magnetic properties of thin films are inherently related to the structure and morphology of the films. From the application viewpoint, magnetic anisotropy is one of the most important properties of the magnetic materials [1]. Depending upon the type of application e.g. permanent magnets, storage media or magnetic cores in transformers and magnetic recording heads, materials with high, medium or low magnetic anisotropy are required. In addition, due to extensive development in semiconductor technology, the properties of magnetic metal films especially of nanometer scale structures on semiconductor substrates have gained more importance. The magnetic and interfacial properties of thin films of cobalt (Co) on different substrates (e.g. Pt, Ge, SiO2, GaAs, glass, MgO, Si, Cu and Pd) have been reported in the literature, but the emphasis have been on to Cu, Pt and Si substrates because of the possibility of growth of FCC Co in the case of Cu, perpendicular anisotropy in the case of Pt or Pd substrates and studies of growth on semiconductors for spintronics devices [2–13]. The effect of surface roughness, thickness dependence, oblique deposition etc. has been extensively studied [8,10,11–14]. Co thin films have been deposited by metal organic chemical vapor deposition (MOCVD), DC and RF magnetron E-mail address: [email protected] (V. Shukla). Corresponding author at: Laser Physics Applications Section, R&D Block “A”, Raja Ramanna Centre for Advanced Technology, Indore 452013 (Madhya Pradesh), India. Tel.: þ 91 731 2488378; mobile: þ 91 9575832401. n
http://dx.doi.org/10.1016/j.jmmm.2014.06.061 0304-8853/& 2014 Elsevier B.V. All rights reserved.
sputtering, thermal evaporation, e-beam evaporation and molecular beam epitaxy [4–14]. Uniaxial magnetic anisotropy in Co thin films on glass, Si, MgO, etc. has also been observed [8,9,11–14]. Effect of surface roughness on the magnetization reversal in Co thin films on Si has been studied and it has been reported that increasing the roughness changes the magnetization reversal process from the magnetization rotation to a combination of magnetization rotation and domain wall motion [11]. Similarly, for studies of Co films on rippled Si substrates, the strong uniaxial anisotropy has been reported with its easy axis along a direction normal to the ripple wave vector [7]. Uniaxial magnetic anisotropy has been studied in Co films on glass using MOKE technique and the shape of the substrate is reported as the probable cause for the rotation of the easy axis of the magnetization for amorphous or polycrystalline films [4]. The correlation between micro-structure and magnetic properties has been studied and roughness is found to lower the uniaxial anisotropy and raise the coercivity [12]. However, in spite of the significant research effort, much of the observed magnetic behavior remains unexplained, including a detailed understanding of the magnetic anisotropy and the change from perpendicular to in plane magnetization. The reason for this surely arises from the large sensitivity of the magnetic anisotropy to structural, morphological, and chemical details at the interfaces, which is at the origin of a variety of experimental results due to a lack of complete and accurate control of the fabrication parameters. Therefore, it becomes important to study the effect of substrate on the magnetic properties of the sample. To study the effect of substrate on the magnetic properties of the thin film,
V. Shukla et al. / Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
Uniaxial magnetic anisotropy was studied in cobalt thin films using continuous wave (CW) magneto-optic Kerr effect (MOKE) technique. The MOKE setup consists of a monochromatic light source (intensity stabilized He-Ne laser) followed by a polarizer to select the incident polarization, a photo-elastic modulator (PEM), the sample in a magnetic field between the poles of an electromagnet, an analyzer and the photo detector. The photodiode signal is fed to the Lock-in amplifier (SR830). The MOKE signal is measured in longitudinal geometry i.e. the applied magnetic field is in the plane of the film. Azimuthal angle dependent hysteresis loops were recorded by rotating the sample in the plane of the applied magnetic field. All MOKE measurements were carried out using marked axis as 01. Cobalt thin films (thickness of 45 nm) were deposited on different substrates i.e. BK7glass, GaAs and silicon by RF magnetron sputtering of Co target (99.99%) in Ar (99.9995%) environment. The GaAs substrate has 〈111〉 orientation and Si substrate has 〈100〉 orientation. Both the substrates have rms roughness less than 5 Å while the glass substrates are optically polished and have a surface roughness 7 Å. Ultrasonic cleaning was done in a class 10,000 clean bench and immediately loaded in the deposition system kept in a clean room. Films were deposited in RF sputtering system (V0301, Elettrorava spa., Italy), with in-situ physical thickness monitoring. Before deposition the system was pumped down to 5 10 7 mbar at 150 1C. Deposition rate was 0.1 Å/s at 50 W RF power and substrate temperature was 75 1C. Pre-sputtering was done for 5 min to remove the top surface of the target, to avoid contamination. All the substrates were loaded simultaneously and films were deposited at the same time. Axis was marked on each substrate prior to deposition. One sample on each substrate was annealed in Argon atmosphere for 1 h at 300 1C. The structural characterization was done by grazing incidence X-ray diffraction (GIXRD) and Atomic Force Microscopy (AFM). Ex-situ X-ray reflectivity (XRR) measurements were carried out to find out exact thickness of the deposited films. Surface topographies of samples were imaged using a multimode scanning probe microscope (NT-MDT, SOLVER-PRO, Russia). AFM measurements were carried out in a non-contact mode using silicon cantilever tips having radius of curvature 20 nm and a spring constant 5.5 N/m with a resonant frequency of 190 kHz under ambient conditions. The top surface was electrically grounded during measurements. Analysis was carried out at various places on the surface of samples.
Fig. 1 shows the XRR data of as deposited Co film grown on GaAs wafer. The reflectivity profile was fitted using Parratt's formalism [15]. The best fitting was obtained by considering three layer structure consisting (i) native oxide layer of 2.5 nm thickness on GaAs wafer, (ii) 45 nm thick Co layer and (iii) 4.7 nm top oxide layer. The topmost layer shows somewhat lower electron density as compared to thick Co underlayer. This topmost layer can be due to formation of CoO because of exposure to atmosphere. The presence of a 5 nm thick CoO surface layer has also been reported by Paranjape et al. [13], which was confirmed by secondary ion mass spectroscopy (SIMS). All the samples grown in a single deposition run show the same average film thickness of 45 nm as determined by XRR. The film adjacent to wafer can be attributed to formation of native oxide. The top layer surface roughness obtained from the XRR fitting is 2.9 nm which is comparable to the surface roughness obtained from AFM results (4.5 nm). Figs. 2 and 3 show GIXRD results for as deposited Co films on Si and GaAs respectively. GIXRD peak corresponding to Co (002) phase is observed at 44.51 for both the substrates. However, it is to be noted here that we do not observe any peak for Co, deposited on glass substrate. Although, we do not observe any peak in GIXRD data for Co/glass sample, we believe that the film deposited on glass is not amorphous. The presence of a broad hump consisting of multiple peaks corresponding to (100), (002) and (101) phases of Co has been reported for approximately same thickness of Co film deposited on glass substrate [6,8]. For study of magnetic properties of these thin films, azimuthal angle dependent hysteresis loops were measured in longitudinal 10 CoO
1
Normalized Intensity
2. Experimental
3. Results and discussion
Co
0.1
Native oxide GaAs Wafer
0.01 1E-3 1E-4 1E-5 1E-6 1E-7 0.00
0.05
0.10
0.15
q
0.20
0.25
(A-1)
Fig. 1. XRR data and fitting (solid curve) for Co/GaAs: inset shows the assumed layered structure for film.
40
Co/Si
Intensity(Arb. Units)
we deposited thin films on different substrates under same deposition conditions. The earlier studies are independent studies mostly with one substrate at a time and there are very few reports devoted to the magnetic properties of thin films on two or more substrates. The magneto-optic Kerr effect (MOKE) has become a standard tool for the study of magnetic ultrathin films and magnetic multi-layers due to its simple experimental setup, ability for making in-situ measurements and high sensitivity down to monolayer resolution [2,4,6,8,14]. In the present work, we have studied the in-plane magnetic anisotropy of cobalt thin film on different substrates by MOKE technique. We have deposited Co thin films of 50 nm thickness on different substrates simultaneously and have shown that anisotropy is due to stress during deposition and with annealing, the anisotropy decreases which may be due to release of stress and the remanant anisotropy is due to shape anisotropy of the particulates.
101
30
20
10
0 40
50
60
70
80
Angle(2 ) Fig. 2. GIXRD data for Co/Si; circle shows the data points and the continuous line shows the fitting.
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Intensity(Arb. Units)
50
Co/GaAs
40
30
20
10
0 40
50
60
70
80
Angle( 2 ) Fig. 3. GIXRD data for Co/GaAs; circle shows the data points and the continuous line shows the fitting.
geometry using MOKE technique. Fig. 4 shows the representative CW-MOKE pattern for Co/glass sample for three different azimuthal angles. The angle is taken with respect to the marked axis. When the applied field is in the direction of the easy axis of magnetization, the almost square loop shows that the magnetization process takes place by the domain wall motion. Curving of the hysteresis loop e.g. for azimuthal angle 401 indicates the increasing contribution of the rotation of the domain magnetization in the magnetization process. The coercivities were obtained from the hysteresis loop at different angles and plotted as a function of angle of rotation of the samples, as shown in Fig. 5(a) and (b). The inherent two fold anisotropy is present in all the samples. We also tried to measure the out of plane component using the polar MOKE configuration, but it was not possible to saturate the magnetization in this geometry with maximum field of 1200 Gauss, maximum magnetic field that can be obtained with this magnet. It indicates that the in-plane magnetization component is dominant in these samples.
Magnetization (a.u.)
0.15 0.10
Table 1 Fitting parameters: anisotropy field (Ha) and domain wall pinning field (Hcw) for the as-grown and annealed samples.
0.05 0.00
Substrates Ha as-grown (Oe)
HCW as-grown (Oe)
Ha annealed (Oe)
HCW annealed (Oe)
Si GaAs Glass
112 120 90
21 27 46
183 178 80
-0.05 -0.10 -0.15 -200 -150 -100 -50
0
50
57 46 74
100 150 200
Magnetic Field (Oe) Fig. 4. Representative hysteresis loop for Co/glass at different azimuthal angles; square for azimuthal angle 601, circle for azimuthal angle 1501 and triangle for azimuthal angle 1001.
180
Coercivity(Oe)
160 140 120 100 80 0
60
120
180
240
300
360
Angle( )
220
Coercivity (Oe)
200 180 160 140 120 100
0
60
120
180 240 Angle( )
300
360
Fig. 5. (a) Azimuthal angle dependent coercivity for Co/glass as grown (open square) and annealed films (open triangle). Fittings are shown by solid lines. (b) Azimuthal angle dependent coercivity for Co/GaAs as grown (open square) and annealed films(open triangle). Fittings are shown by solid lines.
Fig. 6. AFM figure for Co/glass as-grown.
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As discussed in Ref. 11, in case of uniaxial anisotropy for the magnetization reversal process to happen, the applied field H must overcome not only the domain wall pinning coercivity, Hcw, but also the component of the magnetic anisotropy field in the applied field direction, Ha Cos θ. Hence, the azimuthal angle (θ) dependence of the coercivity can be written as follows [11]: H C ¼ H a Cos 2 θ þ ðH 2cw H 2a Cos 2 θ Sin θÞ1=2 2
where, Ha is the magnetic anisotropic field and θ is the angle between the applied field and the easy axis of magnetization. Fig. 5(a) and (b) shows the representative fitting graphs for the coercive field dependence on the azimuthal angle for as-grown and annealed Co on glass and Co on GaAs thin films. Table 1 shows the fitted parameters (Ha and Hcw) for different substrates. As shown in Table 1, the oriented substrates i.e. GaAs and Si have smaller Ha as compared to glass for both as grown and annealed films. Figs. 6 and 7 show the 2D and 3D surface morphology for as grown and annealed Co/Glass samples respectively. Similarly, Figs. 8–11 show the 2D and 3D surface morphology for as grown and annealed Co/Si and Co/GaAs samples respectively. Figures show that the films are particulate in nature. The formation of particulate structure on the crystalline substrate (Si, GaAs) may be due to some oxide formation on the substrates prior to the deposition. This is consistent with the XRR results which show the presence of a layer of very small thickness over the substrate. We have estimated particle sizes over the entire AFM image using mage analysis software of Solver Pro version 2.2 [16]. Standard procedure for estimating particle sizes was adopted, in which, an imaginary horizontal plane was considered and moved in the
Fig. 8. AFM figure for Co/Si as-grown.
Fig. 7. AFM figure for Co/glass annealed 300 1C.
z-direction such that the number of particles is maximum on that imaginary plane and the statistical data was obtained. This analysis gives mean diameter, mean size in x and y-directions, their standard deviations, number of particles etc. For instance, an imaginary plane at the height of 16.8 nm was used for Co/GaAs as grown sample having maximum number of particles. The mean particle size before annealing is 47 nm for glass, 71 nm for Si and 63 nm for GaAs and their standard deviations (SD) are 16 nm, 24 nm and 20 nm respectively. This shows that there is large size distribution. The particles are found to be oval in shape with longer axis along y direction with aspect ratio of 1.17 for glass, 1.44 for Si, and 1.36 for GaAs respectively. The average particle size is found to increase after annealing. It is 71 nm, 102 nm and 71 nm for glass, Si and GaAs respectively and their standard deviations (SD) are 16 nm, 25 nm and 18 nm respectively. Fig. 12(a) and (b) shows the mean particle sizes in x and y directions for as-grown and annealed films. The large standard deviation in the particle size distribution can be attributed to low substrate temperature (75 1C). The root mean square (rms) roughness was also estimated by taking the average of at least five regions of area 2 μm 2 μm. The rms roughness value is found to increase with annealing for all the substrates except GaAs. The reason for this is not known and is required to be explored further. Fig. 13 shows the variation of roughness for different substrates after annealing. The increase in rms roughness is also observed in Co/glass films, the reason is attributed to grain growth and structural transformation from HCP to FCC phase [5]. The high values of domain wall pinning field for all the samples except Co/glass can be understood from the structure of the films as shown in AFM figures. As discussed earlier from AFM data, the large average particle size and aspect ratio of Co film on Si and
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Fig. 9. AFM figure for Co/Si annealed 300 1C. Fig. 10. AFM figure for Co/GaAs as-grown.
GaAs may be the reason for the large HCW. The particulate nature of the film and the connectivity between the domains can be the one of the reason for high coercivity. The low HCW value for Co/glass may be due to more compact film. After annealing the anisotropy field Ha is found to decrease for all the samples. It may be due to release of stress by annealing. This corroborates AFM results where particle size increases after annealing resulting in a more isotropic and homogeneous film. We tried to correlate the reason for high coercivity to roughness of the deposited thin films. There seems to be no apparent correlation between the anisotropy fields and coercivity with the roughness of the deposited films. The reason may be that the roughness of the deposited film (2–5 nm) is much smaller as compared to the thickness of the deposited film (45 nm). Because of particulate structure of the film, magneto-crystalline anisotropy cannot be the reason for the anisotropic behavior of the films. The different shapes (slightly oval in case of GaAs and glass and spherical in case of Si) also indicate towards the growth mechanism and stress developed during the deposition. The roughness of the films also increase after annealing indicating the larger size particles with worse connectivity as compared to as grown film leading to high domain wall pinning field. The change in anisotropy field after annealing also indicates towards the release of stress because the change in anisotropy value is in the order Si oGaAso Glass and the thermal conductivities of the substrates are also in the same order. A strong two-fold uniaxial magnetic anisotropy has also been reported in MBE deposited Co/glass film, but there is no correlation between the magnetic anisotropy and the film morphology for films of different thicknesses [4,5]. As discussed in Ref. 4, the external magnetic field of RF magnetron sputtering cannot be the reason
for the anisotropy of these films as the sample holder is rotating with respect to the deposition direction and there is no correlation between the GIXRD data and the magnetic anisotropy. They have also observed magnetic anisotropy in polycrystalline as well as amorphous films. The magnetic anisotropy has been attributed to the long range order stress during the deposition in samples and anisotropic ejection of the sputtered atoms relative to the direction of the ion beam [6,9]. The decrease in anisotropy on annealing above 3001 C is also attributed to relaxation of internal stress [5]. We emphasize here that influence of stress is more as compared to shape anisotropy as can be seen from the change in magnetic anisotropy field values before and after the annealing and not a significant change in structure of the film. The remaining anisotropy in these films even after annealing may be due to the slightly oval shape of particles and hence can be attributed to shape anisotropy.
4. Conclusion Cobalt thin films were deposited on glass, silicon and GaAs by RF magnetron sputtering. In plane uniaxial magnetic anisotropy was observed in cobalt thin films using MOKE technique. AFM studies show that the deposited films are of particulate nature. The particulate nature and the stress induced during deposition may be the reason for the anisotropic magnetic behavior of these films. On annealing the decrease in magnetic anisotropy confirms that the contribution of the stress is dominant cause for the magnetic anisotropy of these films. The remaining anisotropy after annealing can be attributed to shape anisotropy of the particulates.
V. Shukla et al. / Journal of Magnetism and Magnetic Materials 370 (2014) 100–105
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RMS Roughness (nm)
10 As-grown Annealed
8 6 4 2 Glass
Si
GaAs
Substrates Fig. 13. RMS roughness for as-grown and annealed films for different substrates; filled square for as-grown films and empty squares for annealed films.
Acknowledgments The authors thank Mr. Ayukt Pathak and Mrs. Shradha Tiwari, LSED, RRCAT for the Lab view based MOKE software and Dr. (Mrs.) Archna Sagdeo, ISUD, RRCAT for their critical comments on the manuscript and Dr. H.S. Rawat, LPAS, RRCAT for his encouragement and support in this work. VS is thankful to Mr. S. Khan for his help in data analysis. References
Fig. 11. AFM figure for Co/GaAs annealed 300 1C.
Particle Size(nm)
250 X-direction As-grown Annealed
200 150 100 50 0
Glass
Si
GaAs
Substrates
Particle Size(nm)
250 Y-direction As-grown Annealed
200 150 100 50 0
Glass
Si
GaAs
Substrates Fig. 12. (a) Particle size in x direction for different substrates, filled square for asgrown samples and empty square for annealed samples and (b) particle size in y direction for different substrates, filled square for as-grown samples and empty square for annealed samples.
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