A STUDY OF MAGNETIC ORDERING IN TRANSITION METAL DOPED TIN OXIDE THIN FILMS FOR SPINTRONIC
K. GOPINADHAN Department of Physics
in fulfillment of the requirements of the degree of Doctor of Philosophy to the
INDIAN INSTITUTUE OF TECHNOLOGY DELHI NEW DELHI-110016, INDIA
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This is to certify that the Thesis entitled "A STUDY OF MAGNETIC ORDERING IN TRANSITION METAL DOPED TIN OXIDE THIN FILMS FOR SPINTRONIC APPLICATIONS" being submitted by Mr. K. Gopinadhan to the Department of Physics, Indian Institute of Technology Delhi, for the award of the degree of 'Doctor of Philosophy' is a record of bonafide work carried by him. He has worked under our supervision and guidance and has fulfilled the requirement for the submission of this Thesis, which in our opinion has reached the requisite standard.
The results contained in this thesis have not been submitted, in part or full, to any other University or Institute for the award of any degree/diploma.
Dr. Dinesh K. Pandya Professor
Thin Film Laboratory Department of Physics
Indian Institute of Technology Delhi New Delhi-110016
Dr. Subhash C. Kashyap Professor
Thin Film Laboratory Department of Physics
Indian Institute of Technology Delhi New Delhi-110016
With folded hands and my head bent in humility, I thank Almighty, for providing me a chance to be a human being!
When I began, I thought it may be tough, but finally, I realized that it was a smooth sea, where knowledge, passion and intellectual freedom merge. This was possible only due to the complete freedom I enjoyed during my PhD work, even in selecting the research problem.
For this, I am so much thankful to my supervisors Prof. Dinesh K. Pandya and Subhash C.
Kashyap. They always persuaded me to do that extra needed for the research and gave the conscious call frequently. Even though Dr. Sujeet Chaudhary, the other spintronics group member, was not my supervisor officially, his immense experimental skills and timely advices really helped me in organizing my research work at a rapid pace. I will be really grateful to him for his continuous support.
I am really thankful to Prof. Kasturi L. Chopra for establishing Thin Film Laboratory (TFL) where I could start my research career.
It gives me immense pleasure to thank Mr. Nagendra Chaudhary, who helped me to advance the pace of my research activities by providing the necessary experimental designs, fault detections in spray pyrolysis system, renovating Kaufmann ion beam sputtering system and many other things, for which I may need many pages of this thesis. Also, I cherish in my life, the moments we spent together, with a fatherly touch, to induce values in my life. Also I thank Mr. Girija Bhooshan for his help in carrying out high temperature magnetic measurements.
It may not be enough to simply express my sincere thanks to my colleague Dr.
Kanwal Preet Bhatti, who helped me to learn the first lessons of my research work at IIT Delhi. Also the experiences we gained while creating many new experimental techniques,
computer interfaced systems and many other things, which took me a long way in simplifying my research. Also I sincerely thank my other two colleagues, Priyanka Gupta and Suchitra Rajput, who helped me in many ways during the initial stages of my research work.
I express my profound gratitude to my friend Mr. Mandeep Singh, whose timely help in critical reading of the manuscripts, discussions while having tea, really helped me to improve a lot. He was available whenever I needed his help.
I express my sincere thanks to my seniors Dr. Tarsame Singh Sian, Dr. Babu Dayal, Dr. Somnath C. Roy, Dr. Archana Gupta, Dr. Nirmalya Karar and Dr. Vamsi Krishna for their valuable and timely advices during my sojourn at IITD.
It may be highly inappropriate, if I don't mention the timely help I got from IDDC, especially from Rajaram Ji and Jaspal Singh Ji, starting from designs to the final product.
My sincere thanks to my lab mates Dr. Deepak Varandani, Dr. Vidhyanand Singh, Suneet Arora, Dr. Sanjay Srivastav, Gargi Agarwal, Ajay Mann, Sandeep Chokker, Ranga Rao, Shubhra Kala, Sangeeta Hinduja, Sanju Rani, Asha Bhardwaj, Mukesh Kumar, and Sudhesh Kumar who made my life at TFL a memorable and an unforgettable experience. The lessons I hade been taught, starting from Saturday cleaning to handling different instruments, all of them made me much more mature as a scientist.
My stay at Aravali hostel has been just wonderful. That has been due to my friend Brijesh Kumar, who made my boring nights fun filled. I vividly remember that dal, vada, bad sambar, rajamma etc, and the wait for every day's dinner (because it was excellent!).
I feel lucky and proud to be born to my parents since they gave me the opportunity to prove myself and showed me that hard work, sincerity and honesty can bring a difference to life.
I express my hearty thanks to my beloved wife Thulasi Krishna, who constantly encouraged me, and had been a source of relief and fun. Thanks are also to my in-laws for their constant encouragement and support.
Also there are many, who directly or indirectly helped me in carrying out certain measurements and bringing this thesis in its final form. I sincerely thank these people for their unprecedented support.
Inducing thermally stable room temperature ferromagnetism (RTFM) in non magnetic semiconductors with reproducible results is a challenge and an important step in realizing spintronic devices. In the present work, we have successfully attempted to induce the RTFM by doping the transparent wide band gap semiconductor Sn02, with three different transition metal (TM) dopants (i.e., Co, Mn and Ni). The undoped and TM doped SnO2 films have been made by spray pyrolysis technique on quartz substrates at 450°C followed by thermal annealing (in air) for 30min. at 500°C.
In the case of co-doped cobalt doped SnO2 thin films, a maximum ferromagnetic moment of 0.47µg/Co-ion has been observed in Sno 9Co0102_5 films at room temperature. The
films typically have high electrical conductivity -150 SZ cm and -70% transmittance in the visible region. A systematic variation of saturation magnetic moment, carrier concentration, electrical conductivity, and optical absorption edge in Sni,Co,02_,3 (0.05 0.15) films is correlated with the change in Co concentration, and a carrier mediated Ruderman-Kittel- Kasuya-Yoshida (RKKY) interaction has been proposed as the most probable mechanism for the ferromagnetic ordering. The observed blue-shift in the transmission edge of - 215 meV (maximum at x=0.10) is attributed to the extra carriers generated by Co-substitution and also to Coulomb interactions (to some extent).
In the case of sequentially deposited Sn1,Cox02_5 thin films, there exists a close relationship between the free carrier concentration and ferromagnetic saturation moment induced in Sni_xCox02_8 thin films (deposited at 450°C) with cobalt concentration upto x=0.10. The films with x>0.10 show paramagnetic behavior. All the films are highly conducting. Both, the carrier concentration and saturation magnetic moment, increase
monotonically with the concentration of Co, and attain maximum values of 2.2x1019 cm -3 and 0.26 ,u B /Co-ion respectively at x=0.10. Also an anomalous Hall effect (AHE) has been observed in the film with x=0.10, indicating the spin polarization of the charge carriers. The carrier mediated exchange interaction is, therefore, the most probable mechanism responsible for ferromagnetism in these films. F-doping in Sni,Cox02_8 films has resulted in even higher values of both the carrier concentration (2.07x1020cm-3) and magnetic moment (0.80 ,u B /Co- ion) thereby further, supporting the carrier mediated interaction. Also, the temperature dependence of the magnetization of these films predicts a Curie temperature exceeding 550K.
All the films are optically transparent (>75%) in the visible spectral region. A large blue-shift of —350meV of the transmittance edge has also been observed with increase in the cobalt- concentration in Sni,C0.02_8 thin films, which suggest that cobalt-ions alter the conduction and valence band edges of SnO2 and that there exists the Burstein-Moss effect.
It has been possible to induce room temperature ferromagnetism (RTFM), exhibiting high transition temperature but with weak magnetic moment in tin oxide thin films by introducing manganese in SnO2 lattice. The observed temperature dependence of the magnetization predicts a Curie temperature exceeding 550K. A maximum saturation magnetic moment of 0.18±0.041B per Mn-ion has been estimated for spray pyrolysed Sn1- xMnx02_8 thin films, with x = 0.10. For Mn-concentration (x) higher than 0.10, the films show linear behavior. The magnetization-versus-field studies indicate that the origin of ferromagnetism lies neither in the ferromagnetic metal clusters nor in the presence of metastable phases. The structure factor calculations reveal that Mn has been incorporated in the SnO2 lattice. Also, the electron transport investigation indicates that there is a change of Mn-occupancy from substitutional- to interstitial- sites of SnO2 lattice when the Mn-
concentration exceeds 7.5a/o. These films do not exhibit AHE at room temperature. The optical absorption study indicates that Sni,Mn,02_5 system behaves like a random alloy. The generation of additional free electrons by F-doping in Sn0.9oMn0.1o02_8 thin films does not cause any increase in the magnetic moment per Mn-ion, suggesting that the electrons do not play any significant role in bringing about the magnetic ordering.
The study on Sn02:Ni thick films identifies room temperature ferromagnetism with a maximum saturation magnetization at a Ni-concentration of 8 at.%. The orthorhombic phase, found along with the tetragonal phase in the undoped Sn02, disappears at 6 at.% of Ni and a secondary phase of NiO appears. Above 8 at.% Ni, the observed decrease in saturation magnetization well correlates with the presence of competing incorporation of Ni in anti-ferro NiO and in ferromagnetic Ni substituted SnO2 phases. The amount of FM fraction in the samples seems to be governed by the presence of ortho-SnO2 phase and NiO impurity phase, and is not carrier-mediated.
The study on Sn02:Ni thin films indicate that a weak RTFM exist at 5 at.% Ni concentration in Sn02, whereas the ferromagnetism disappears or very small for Ni- concentrations above 5at.%. The X-ray and optical analysis indicates that Ni has been incorporated in the SnO2 lattice.
Thus from the studies on various properties of transition metal (Co, Mn and Ni) doped SnO2 films it is concluded that nature (ferromagnetic, antiferromagnetic or paramagnetic) of the dopants play an important role in inducing the ferromagnetism. Out of the three dopants Co, Ni and Mn, Co has been found to be most efficient in inducing RTFM with a maximum magnetic moment of 0.47[1B/Co-ion observed in Sno Co0102_s films. Mn had been a weak magnetic dopant, may be because of its antiferromagnetic nature. In the case of Ni-dopant, it has been found a thickness dependency in observing the RTFM.
LIST OF FIGURES xi
LIST OF TABLES xx
CHAPTER 1: INTRODUCTION
1.1 What is Spintronics? 1
1.2 Branches of Spintronics 2
1.2.1 Metal-based Spintronics 3
1.2.2 Semiconductor Spintronics 4
1.2.3 DNA Spintronics 6
1.2.4 Dilute Magnetic Semiconductor 7
1.3 Different Materials Tried 9
1.3.1 II-VI Non-oxide Semiconductors 9
1.3.2 III-V Semiconductors 9
1.3.3 Oxide Semiconductors 11
1.4 Proposed Mechanisms of Ferromagnetism 13
1.4.1 Zener Mean Field Model(Carrier Mediated Exchange interaction) 13 1.4.2 Donor impurity band model (Magnetic polaron model) 14
1.5 Why Oxide Semiconductors? 16
1.6 Why Tin Oxide? 17
1.7 Applications of Ferromagnetic Semiconductors 19 1.8 Preparation Methods for Conducting Tin oxide- A Review 20
1.9 Spray Pyrolysis Technique 22
1.10 Objectives 23
1.11 Thesis Plan 23 CHAPTER 2: EXPERIMENTAL & CHARACTERIZATION
2.1 Introduction 25
2.1.1 Thin Film Deposition 25
2.1.2 Substrate Cleaning & Spray Pyrolysis 28 2.2 Glancing Angle X-ray Diffraction (GAXRD)Technique 29 2.3 Vibrating Sample Magnetometry (VSM) 32
2.4 UV-VIS-NIR Spectrophotometry 35
2.5 Electronic Transport Measurements 41
2.5.1 Electrical Resistivity 41
2.5.2 Hall effect measurement 43 2.5.3 Anomalous Hall Effect (AHE) 46
2.8 Thickness measurement 47
CHAPTER 3: CO-SPRAYED COBALT-DOPED SnO2 THIN FILMS
3.1 Introduction 51
3.2 Experimental Details 51
3.3 Results & Discussion 52
3.3.1 Structural Properties 52
3.3.2 Magnetic Properties 54
3.3.3 Electron-Transport Properties 55
3.3.4 Optical Properties 57
3.3.5 On the blue-shift in Sni_,Cox02_8 transparent ferromagnetic 59 semiconductor thin films
3.4 Conclusions 68
CHAPTER 4: SEQUENTIALLY SPRAYED COBALT-DOPED SnO2 THIN FILMS
4.1 Introduction 71
4.2 Experimental details 71
4.3 Results and Discussion 72
4.4 Conclusions 89
CHAPTER 5: MANGANESE-DOPED SnO2 THIN FILMS
5.1 Introduction 91
5.2 Experimental Details 91
5.3 Results and Discussion 92
5.4 Conclusions 108
CHAPTER 6: NICKEL-DOPED SnO2 THICK AND THIN FILMS
6.1 Introduction 111
6.2 Experimental Details 111
6.3 Results and Discussion 111
6.4 Conclusions 124
CHAPTER 7: SUMMARY AND SCOPE FOR FUTURE WORK
7.1 Summary 127
7.2 Scope for Future Work 130
LIST OF PUBLICATIONS 145