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1. Title Huanlong Liu Advisor: Andrew Kent Department of Physics New York University

2. Outline

3. Motivation How will the spin of electrons interact with the background magnetization? What will happen?

4. Motivation • Electrons scatter with lattice – not only exchange their energy, but also their angular momentum. • Spin diffusion length can be much longer than electrons’ mean free path. Spin diffusion length Motivation

5. Background magnetization influences the spin of electrons. • Spins current can also influence the background magnetization. 0 Spin torque effect Spin torque effect GMR effect Conservation of angular momentum Motivation

6. GMR effects • What will happen if we add another ferromagnetic layer? Giant Magneto-Resistance (GMR) Discovered in 1988 Nobel Prize in 2007 Motivation

7. Introduction How to describe the motion of magnetization ? Ferro magnet Temperature Spin current ? Magnetic field Motivation

8. Introduction • Exchange energy • Dipole energy 1 2 Coulomb interaction + Pauli exclusion principle for ferromagnetic materials 2 1 Motivation

9. Introduction • Zeeman energy • Uniaxial anisotropy energy • Total energy for one spin is the sum of all the four kinds: Motivation

10. Introduction • For a ferromagnetic system with many spins: • The exchange energy dominates within the length scale of d. • is a constant. d a Macro spin Motivation

11. Introduction • No exchange energy between “big spins”. • Sum of dipole energy demagnetization energy • Zeeman energy and uniaxial anisotropy energy take the similar form after summation. is the demagnetization factor depended on the shape of the FM material Motivation

12. Introduction • Spin torque – is the amount of angular momentum change in unit time. e mp electron m magnetization Motivation

13. Introduction • Landau-Lifshitz-Gilbert (LLG) equation + Spin Torque Normalize with the magnitude of magnetization Motivation

14. Introduction • Thermal effects Langevin random field • The LLG equation will be: Motivation

15. Introduction • This equation describes the dynamics of a spin sitting inside a potential well and being activated by thermal fluctuation. • The finite life time of being inside the potential well obeys Boltzmann distribution. Motivation

16. Experimental Techniques Motivation

17. Experimental Techniques • Sample structure Need at least two FM layers to detect magnetization reversal by GMR effect Add another polarizing layer to maximize spin torque effect Motivation

18. Experimental Techniques • Probe station I V Arbitrary Waveform Generator AWG7120 Digital Phosphor Oscilloscope DPO72004 Signal Generator MG3692B Projected field electromagnet GMW5201 S N Bipolar Operational Power Supply BOP20_20 Bias Tee Bias Tee Lock-in Amplifier SR830 Source Meter Keithley2400 Motivation

19. Experimental Techniques • Experiment types: • DC measurements: • Hysteresis measurements • Current sweep measurements • High speed measurements: • Pulse measurements • Frequency sweep measurements Motivation

20. Initial Results DC measurements Motivation

21. Initial Results and Analysis • Short time pulse measurements: Raw data Interpolation Motivation

22. Initial Results and Analysis Switching boundary from LLG equation : A is the dynamic parameter Motivation

23. Initial Results and Analysis • Long time pulse measurements Switching boundary: Motivation

24. Initial Results and Analysis Motivation

25. Initial Results and Analysis Dynamic Regime: I ~ 1 / duration Thermal Regime: I ~ log(duration) We can tell the different regimes by comparing when does the theoretically predicted value derivate from the experimental data Dynamic Intermediate Thermal Motivation

26. Summary • What do we know: • There are three distinguishable regimes of the switching boundary due to whether the thermal effect influences the switching process or not • In short time (dynamics) regime, the switching boundary goes as , which satisfies LLG equation without thermal effect. • In long time (thermal) regime, the switching current goes as , which indicates that the switching process is just like a particle escaping a potential barrier from thermal fluctuations. Motivation

27. Summary • What do we want to know: • How do the dynamics parameter A and critical current Ic0depend on material parameters? • How can we get the correct energy barrier from material parameters? Motivation

28. Further Plans • Analyze data • Probability distribution for short time – to fit data • Energy barrier for long time – is there any domain wall motion • Do the same measurements on smaller samples –macro spin model may work better Motivation

29. Further Plans • Low Temperature measurements • Compare the extrapolated zero temperature switching current from short time pulse with real zero temperature switching current – how much does temperature anticipate the short time switching process • New Structure with non-collinear magnetization configurations. • Maximize spin torque and reduce thermal dependence of initial conditions – deterministic switching, shorter switching time and lower switching current. Motivation