10. Interlayer Exchange Coupling Magnetism

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Magnetism

10. Interlayer Exchange Coupling

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thin film growth: molecular beam epitaxy

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growth of ultrathin metallic layers under ultrahigh vacuum conditions

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Winter 08/09 Magnetism

homoepitaxy: Fe on Fe(001)

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growth depends on temperature

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lower temperature

causes higher roughness (smaller islands)

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layer-by-layer growth reveals perfect RHEED oscillations

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there is always a residual roughness – imperfect growth

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1986 – first step to Nobel Prize

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BLS Setup

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Phenomenology of Magnetic Interlayer Coupling

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Typical hysteresis loops for different types of

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Domain patterns in Fe/Cr/Fe

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Measurement of spin-wave or magnons by BLS

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Spin-waves (magnons) of a single layer

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Spin-waves in a coupled, parallel aligned trilayer

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Spin-waves in a coupled, antiparallel aligned trilayer

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Example: BLS data of Fe / Al / Fe(001) trilayers

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SEMPA

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SEMPA: Magnetic domain imaging

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direct observation of the coupling

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oscillation of coupling direction with film thickness

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quantized electronic states in the Cr film

J. Unguris et al., Phys. Rev. Lett. 67, 140 (1991)

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Phenomenological description

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Typical bilinear coupling strengths

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First simple explanation: RKKY-oscillations

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RKKY-model

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RKKY-model

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Quantum interference model for bilinear coupling

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What is the origin of spin-dependent reflectivity?

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QWS

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QWS

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Aliasing (or backfolding into first Brillouin zone)

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Which k are important?

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Example Fe / Au / Fe(001)

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Fe(001) surface states

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Photoelectron spectroscopy to study the electronic structure in the ferromagnet

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Quantum well states (inverse photoemission)

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Quantum well states in Co/Cu

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multiple quantum well states formed in the Cu band

structure

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Electronic structure Co/Cu

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Fermi surfaces of Cu and Co^↑ match closely, if lattice deformation in multilayers is taken into account

0 0,2 0,4

0,6 0,8

1

Co(001)

-5 -4 -3 -2 -1 0 1 2

k value

-5 -4 -3 -2 -1 0 1 2

0 0,2 0,4 0,6 0,8 1

Cu(001)

energy [eV]

k-value

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Transfer of magnetic moments

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XMCD measurement on Co/Cu multilayers and alloys

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Proximity of Co and Cu at the interface leads to transfer of magnetic moment

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First Cu monolayer is “magnetic”

with different contributions in sp- and d-states

spin density

Co (2)

Co (1)

Cu (2)

Cu (1)

Cu (C)

µ_Co^=1.50_µB µ_Cu^=0.02_µB µ_Co^{=1.85 µ}_B

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Co/Cu: A model system

0 1 2 3 4

t_Cu [nm]

0 50 100

1. afm-max.

2. afm-max.

3. afm-max.

T=4.2 K ο RT

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★perfect layer structure within grains

★{111} texturized grains

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Fluctuation mechanism

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Magnetic dipole mechanism

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Magnetic dipole mechanism

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Influence of interface roughness

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Influence of interfacial roughness: Fe/Cr/Fe

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combinatorial approach – domain imaging w/ SEMPA

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short coupling period

appears only for smooth interfaces

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growth of Cr on Fe(100) is critical for interfacial

roughness

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surface roughness kills short oscillation period

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accumulated roughness in the Cr wedge eventually destroys the coupling pattern

Influence of Interfacial Roughness: Fe/Cr/Fe

★ short coupling period appears only for smoot h int erfaces

★ growt h of Cr onFe(100) is crit ical for int erfacial roughness

★ surfaceroughness kills t heshort oscillat ionperiod

★ accumulat ed roughness int heCr wedgeevent ually dest roys t he coupling pat t ern

Fe(100) Cr growth

@30˚ C Cr growth

@350˚ C

Fe Cr Fe

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Fe/Cr multilayers

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asymptotic behavior ~1/tCr2

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reduction of the GMR with interlayer thickness can be understood as

shunting of the resistance by the nonmagnetic films

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Beyond Fe/Cr

[1] S. S. P. Parkin, Phys. Rev. Lett. 67, 3598 (1991).

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Co/TM multilayers

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also measured with Fe, Ni and Ni81Fe19

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