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Magnetic thin films: from basic research to spintronics Christian Binek 11/18/2005 Physics 201H Why thin films Length (and time) scales determine the physics.

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Presentation on theme: "Magnetic thin films: from basic research to spintronics Christian Binek 11/18/2005 Physics 201H Why thin films Length (and time) scales determine the physics."— Presentation transcript:

1 Magnetic thin films: from basic research to spintronics Christian Binek 11/18/2005 Physics 201H Why thin films Length (and time) scales determine the physics of a system all macroscopic properties Electronic states Quantum mechanics tells us: Confinement of electrons by lowering dimensions affects the electronic states 3D bulk2D film1D wire0D quantum dots artificial atoms Size matters

2 11/18/2005 Physics 201H When can films considered to be thin or thin with respect to what Thin in comparison with the characteristic length scale Examples: -Superconducting thin film thickness correlation length -optical thin film like dielectric mirrors Length scale /4 500nm/4 d characteristic length d

3 d 11/18/2005 Physics 201H -Magnetic thin films approach the ultimate extreme ferromagnet spacer nonmagnetic Spacer thickness d in # of atomic layers thickness quantum mechanical exchange interaction length a few atomic layers d=8 monolayer J(d=8)>0 Ferromagnetic coupling d=10 monolayer J(d=10)<0 Antiferromagnetic coupling Exchange J(d)

4 How to grow magnetic heterostructures ? >

5 M olecular B eam E pitaxy Thin film low deposition rate Ultra high vacuum condition

6 deposited material Important growth modes in heteroepitaxy R eflection H igh- E nergy E lectron D iffraction RHEED Layer-by layer (Frank van der Merwe) 3D islands (Volmer weber) Monolayer followed by 3D islands (Stranski Krastanov) Electron gun up to 50 keV sample RHEED screen Eye camera specific free energy substrate interface

7 What are the magnetic heterolayers good for Basic components of modern spintronic devices Conventional electronics has ignored the spin of the electron Advantages using spin degree of freedom: magnetic field sensorsM-RAM ? Spin-transistor semiconductor Quantum- information

8 Superparamagnetic effect inductive read head Magnetoresistive heads GMR Evolution of magnetic data storage on hard disc drives Impact of GMR based field sensors on magnetic data storage

9 rotating sensor layer FM 1 fixed layer FM 2

10 How to pin FM 2 while the sensor layer FM 1 rotates? Exchange Bias! Pinning of the ferromagnet by an antiferromagnet field cooling: from T>T N to T { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/1430221/4/slides/slide_9.jpg", "name": "How to pin FM 2 while the sensor layer FM 1 rotates.", "description": "Exchange Bias. Pinning of the ferromagnet by an antiferromagnet field cooling: from T>T N to T

11 Meiklejohn Bean: uniform magnetization reversal of a pinned FM coupling constant: J FM interface magnetization: S FM M FM :saturation magnetization of FM layer t FM K FM, H M FM Exchange bias field: AF interface magnetization: S AF Stoner-WohlfarthAF/FM-interface coupling

12 Cr 2 O 3 (0001)/Pt0.67nm/(Co0.35nm/Pt1.2nm) 3 /Pt3.1nm Electric control of the Exchange Bias Investigated multilayer system: perpendicular magnetic anisotropy t Pt =1.20nm t Co =0.35nm FM thin film with Magnetoeletric effect of Cr 2 O 3 Magnetization M=m/V electric field E=U/d Cr 2 O 3 (0001) U Co Pt Cr 2 O 3 (0001) Cr 2 O 3 : Magnetoeletric AF, T N =308K U E M contributes to S AF Idea: T=290K * * A. Hochstrat, Ch.Binek, Xi Chen, W.Kleemann, JMMM , 325 (2003)

13 Co Pt Cr 2 O 3 (0001) U=Ed Change of the exchange bias field as a function of the electric field at T = 150K

14 Magnetoelectric Switching of Exchange Bias * : 2 Control via field-cooling * P. Borisov, A. Hochstrat, Xi Chen, W. Kleemann and Ch. Binek, PRL (2005) M agnetic F ield C ooling (MFC) cooling from T>T N in 0 H fr = T and E fr =-500 kV/m M a g n e t o F i e l d E l e c t r i c C o o l i n g (+,-)(+,-) cooling from T>T N in 0 H fr = T and E fr =+500 kV/m M a g n e t o F i e l d E l e c t r i c C o o l i n g (+,+)(+,+) Magneto-optical Kerr T = 298 K after cooling from T>T N in 0 H fr = 0.6 T The sign of the Exchange bias follows the sign of E fr H fr [T] (+,-) E fr H fr <0 (+,+) E fr H fr >0

15 H R Spintronic applications * ME FM 1 FM 2 V V ME FM 1 FM 2 * Ch. Binek and B. Doudin, J. Phys.: Condens. Matter 17 (2005) L39–L44

16 ME V FM 1 FM 2 NM V FM 1 FM 2 NM H R -H e -H i He-HiHe-Hi

17 H R Voltage Input X:= 0 1 +H -H Exclusive Or magn. field Y:= -V +V 1 0 Output R high R low 0 1 Example: 0 +V -H 0 x | y | xORy 0 | 0 | 0 0 | 1 | 1 1 | 0 | 1 1 | 1 | 0

18 finite anisotropy K AF 0 Basic research with magnetic heterostructures generalized Meiklejohn Bean approach J : coupling constant S AF/FM : AF/FM interface magnetization t AF/FM : AF/FM layer thickness M FM : saturation magnetization of FM layer Experimental check of advanced models understanding the basic microscopic mechanism of exchange bias Exchange bias is a non-equilibrium phenomenon new approach to relaxation phenomena in non-equilibrium thermodynamics

19 reduction of the EB shift upon subsequent magnetization reversal of the FM layer Training effect: - origin of training effect - simple expression for The training effect: a novel approach to study relaxation physics

20 Relaxation towards equilibrium Landau-Khalatnikov : phenomenological damping constant Training not continuous process in time, but triggered by FM loop discretization of the LK- equation Discretization: LK- differential equation difference equation

21 1st& 9th hysteresis of NiO(001)/Fe Comparison with experimental results on NiO-Fe NiO 12nm Fe (001) compensated

22 experimental data recursive sequence min. and (mT) -2 e 3.66 mT e

23 Magnetic Nanoparticles Collaborations self-assembled Co clusters ~5nm Transmission electron microscopic image I thermally decompose metal carbonyls in the presence of appropriate surfactants You want to know what I am doing?

24 Fundamental questions Which magnetic interactions dominate the system What kind of magnetic order can we observe For large particle distances the dipolar interaction will dominate

25 Here is a real fundamental question: Do dipolar systems still obey extensive thermodynamics What does this mean: Magnetic moment,T,H = 2 Magnetic moment,T,H Simulations suggest: Yes: for a 2 dimensional array of dipolar interacting particles but No: for a 3 dimensional array of dipolar interacting particles Modifications of conventional thermodynamics required

26 Summary MBE is a technology at the forefront of modern material science magnetic heterolayers are basic ingredients for spintronic applications magnetism of thin films and nanoparticles provides experimental access to fundamental questions in statistical physics

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29 V(X) x equilibrium Mechanical analogy equilibrium F( ) eq - eq x eq Damped harmonic oscillator:

30 Solution for: with

31 also derived from integration of: where Temporal evolution of X with increasing damping: 0 m

32 Near earth outer space: 384,400 km


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