Magnetic thin films: from basic research to spintronics Christian Binek Physics 201H 11/18/2005 Why thin films Size matters Length (and time) scales determine the physics of a system Quantum mechanics tells us: Confinement of electrons by lowering dimensions affects the electronic states Electronic states 3D bulk 2D film 1D wire 0D quantum dots artificial atoms all macroscopic properties

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

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

How to grow magnetic heterostructures ? > 250 000

Molecular Beam Epitaxy Thin film growth @ low deposition rate Ultra high vacuum condition

Important growth modes in heteroepitaxy specific free energy Layer-by layer (Frank van der Merwe) substrate deposited material interface Monolayer followed by 3D islands (Stranski Krastanov) 3D islands (Volmer weber) Reflection High-Energy Electron Diffraction RHEED Electron gun up to 50 keV sample RHEED screen Eye camera

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: Quantum- information magnetic field sensors M-RAM Spin-transistor semiconductor

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

rotating sensor layer FM1 fixed layer FM2

Pinning of the ferromagnet by an antiferromagnet  How to pin FM2 while the sensor layer FM1 rotates? Exchange Bias! Pinning of the ferromagnet by an antiferromagnet field cooling: from T>TN to T<TN

AF/FM-interface coupling Meiklejohn Bean: uniform magnetization reversal of a pinned FM FM interface magnetization: SFM MFM tFM coupling constant: J AF interface magnetization: SAF KFM, H MFM :saturation magnetization of FM layer Exchange bias field: Stoner-Wohlfarth AF/FM-interface coupling

Electric control of the Exchange Bias Investigated multilayer system: Cr2O3(0001)/Pt0.67nm/(Co0.35nm/Pt1.2nm)3/Pt3.1nm tPt=1.20nm FM thin film with Pt tCo=0.35nm perpendicular magnetic anisotropy Co Pt Co Pt Co Cr2O3: Magnetoeletric AF, TN=308K U Magnetoeletric effect of Cr2O3 SQUID-magnetometry @ T=290K * electric field E=U/d Cr2O3 (0001) Magnetization M=m/V U Idea: Cr2O3 (0001) E M contributes to SAF *A. Hochstrat, Ch.Binek, Xi Chen, W.Kleemann, JMMM 272-276, 325 (2003)

Change of the exchange bias field as a function of the electric field at T = 150K Co Pt Cr2O3 (0001) U=Ed

Magnetoelectric Switching of Exchange Bias*: 2 Magnetoelectric Switching of Exchange Bias*: Control via field-cooling *P. Borisov, A. Hochstrat, Xi Chen, W. Kleemann and Ch. Binek, PRL 94 117203 (2005) Magneto-optical Kerr measurements @ T = 298 K after cooling from T>TN in 0Hfr = 0.6 T [T] Magnetic Field Cooling (MFC) cooling from T>TN in 0Hfr = +0.6 T and Efr=-500 kV/m (+,-) EfrHfr<0 (+,+) EfrHfr>0 M a g n e t o E l e c t r i F i e l d C o l i n g (+,-) cooling from T>TN in 0Hfr =+0.6 T and Efr=+500 kV/m M a g n e t o E l e c t r i F i e l d C o l i n g (+,+) The sign of the Exchange bias follows the sign of EfrHfr

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

V V FM2 FM2 NM NM FM1 FM1 ME ME R -He-Hi He-Hi H

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

Basic research with magnetic heterostructures generalized Meiklejohn Bean approach finite anisotropy KAF≠0 J :coupling constant SAF/FM :AF/FM interface magnetization tAF/FM :AF/FM layer thickness MFM :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

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

Relaxation towards equilibrium discretization of the LK- equation 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

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

experimental data recursive sequence min. e 0.015 (mT)-2 and 3.66 mT e

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

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

= 2 Here is a real fundamental question: Do dipolar systems still obey extensive thermodynamics What does this mean: Magnetic moment ,T,H Magnetic moment ,T,H = 2 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

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

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

Solution for: with

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

384,400 km Near earth outer space: