Magnetism - fz-juelich.de

Magnetism10. Interlayer Exchange Coupling

Winter 08/09 Magnetism

thin film growth: molecular beam epitaxy

• growth of ultrathin metallic layers under ultrahigh vacuum conditions


homoepitaxy: Fe on Fe(001)

• growth depends on temperature

• lower temperature causes higher roughness (smaller islands)

• layer-by-layer growth reveals perfect RHEED oscillations

• there is always a residual roughness – imperfect growth


1986 – first step to Nobel Prize


BLS Setup


Phenomenology of Magnetic Interlayer Coupling


Typical hysteresis loops for different types of


Domain patterns in Fe/Cr/Fe


Measurement of spin-wave or magnons by BLS


Spin-waves (magnons) of a single layer


Spin-waves in a coupled, parallel aligned trilayer


Spin-waves in a coupled, antiparallel aligned trilayer


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


SEMPA

• SEMPA: Magnetic domain imaging

• direct observation of the coupling

• oscillation of coupling direction with film thickness

• quantized electronic states in the Cr film

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


Phenomenological description


Typical bilinear coupling strengths


First simple explanation: RKKY-oscillations


RKKY-model


Quantum interference model for bilinear coupling


What is the origin of spin-dependent reflectivity?


QWS


Aliasing (or backfolding into first Brillouin zone)


Which k are important?


Example Fe / Au / Fe(001)


Fe(001) surface states

• Photoelectron spectroscopy to study the electronic structure in the ferromagnet


Quantum well states (inverse photoemission)

C. Carbone et al., PRL 71, 2805-2808 (1993).


Quantum well states in Co/Cu

• multiple quantum well states formed in the Cu band structure


Electronic structure Co/Cu

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

00,20,40,60,81

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)

ener

gy [e

V]k-value


Transfer of magnetic moments

• XMCD measurement on Co/Cu multilayers and alloys

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

• 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)

µCo=1.50µB

µCu=0.02µB

µCo=1.85µB


Co/Cu: A model system

0 1 2 3 4tCu [nm]

0

50

100

1. afm-max.

2. afm-max.

3. afm-max.

T=4.2 Kο RT•

★ perfect layer structure within grains★ {111} texturized grains★ structural quality improves with number of periods


Fluctuation mechanism


Magnetic dipole mechanism


Influence of interface roughness


Influence of interfacial roughness: Fe/Cr/Fe

• combinatorial approach – domain imaging w/ SEMPA

• short coupling period appears only for smooth interfaces

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

• surface roughness kills short oscillation period

• accumulated roughness in the Cr wedge eventually destroys the coupling pattern

Influence of Interfacial Roughness: Fe/Cr/Fe

★ short couplingperiodappears onlyfor smoot hint erfaces

★ growt hof Cr onFe(100) is crit icalfor int erfacial roughness

★ surfaceroughness kills t heshortoscillat ionperiod

★ accumulat ed roughness int heCrwedgeevent ually dest roys t hecouplingpat t ern

Fe(100)

Cr growth@30˚C

Cr growth@350˚C

FeCrFe


Fe/Cr multilayers

• asymptotic behavior ~1/tCr2

• reduction of the GMR with interlayer thickness can be understood as shunting of the resistance by the nonmagnetic films


Beyond Fe/Cr

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

• Co/TM multilayers

• also measured with Fe, Ni and Ni81Fe19

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