"Interlayer exchange coupling in metallic and all-semiconductor multilayered structures". OUTLINE Why are interlayer coupling phenomena interesting and Important? The explanation will be in the form of a longer story about magnetoresistance and GMR, a Nobel Prize effect.
Let me explain why. My plan is to post this Power Point
Presentation on the Web. For some people it will perhaps
be a useful tutorial. And, I hope, after doing some more
work on it, it may also serve as sort of “propaganda
movie” for informing people – e.g., prospective students --
about research conducted in our Department (we will
then need a whole “package” of such slide shows,
In 1857 Scottish scientist William Thomson,
who later becomes Lord Kelvin, discovers
that the application of external magnetic
field to a nickel (Ni) wire increases its
electric resistance. The term “magneto-
resistance” is introduced for this new
The picture shows Lord and Lady Kelvin
chairing the ceremony of coronation of King
Edward II in 1902. Scientist at that time were
given all respect they deserved – in sharp
contrast with the present situation!
...physicists rushed to study other metals. Essentially, it was found that MR effects occur in any metal. For the non-magnetic ones, those findings can be summarized as a simlpe „rule of thumb”: the worse conductor the metal is, the stronger the MR effects are manifested.
Bismuth (which is not even classified as a metal, but a “semimetal”) was found to be the “record-holder” – in strong magnetic fields its resistance could increase by as much as 50%. But in copper or gold the resistance changed only by a small fraction of 1%, even in very strong fields. Not surprisingly, the MR phenomena did not find too many practical applications…
…that magnetoresistance is not an effect “standing by itself”, but it belongs to a larger class of phenomena, called “galvanomagnetic effects”, or “magnetotransport effects”, which can be all described in the framework of the same theory. Another member of this class is the well-known Hall Effect.
By taking the equation
of motion for electrons:
Standard Hall Effect
One obtains a solution in a matrix
form, where the diagonal elements
represent magnetoresistance, and
the off-diagonal – the Hall effect:
In ferromagnets(FMs), B is a
non-linear function of applied
field an T, showinghysteresis.
However, this function can be
readily determined from experi-
It was therefore expected that if experimental B values were used, the same theory would work well for FMs.
But it did not work!! Both Hall Effect and magneto-resistance in FMs were found to behave in a highly
unpredictable way. New terms were coined for them:
Anomalous Hall Effect (AHE) and
Anomalous MagnetoResistance (AMR).
The first successful theory of AHE and AMR was created by another British scientist-aristocrat ☺,
the famous Sir Nevil Mott (Nobel 1977). He asked himself: why certain transition metals – Ni, Pd, Pt – are much poorer conductors than their immediate neighbors in the Periodic Table, Cu, Ag and Au?
Electron in the d-bands
are more tightly bound
and less mobile.
But the s-band electrons
may be scattered by de-
fects (always present) or
by phonons, and may
end up in the d-band,
losing mobility and incre-
asing the resistance.
Schematic representation of the bands in
a transition metal with a partially filled
d-band (the bands for spin-up and spin-
down electrons are shown separately).
However, in nickel, copper’s next-door neighbor, the situation is different
The d-band is not completely filled, so that s→d
scattering may occur, making Ni a poorer conductor
There is one more important aspect: in the FM state, the
situation is no longer symmetric – the 3d sub-band for
only one of the spin states is now incompletely filled.
This fact, it turns out, has far-reaching consequences!
From the Mott’s picure, it follows that there are two currents:
For “spin-up” current the resistance is low (no scattering).
For “spin-down” current the resistance is high because such
electrons may be scattered into the 3d sub-band
By applying an external magnetic field, one can re-orient
the domains, and thus change the specimen resistance –
as had been originally observed by Lord Kelvin.
In bulk specimens the effect is not particularly strong, though, which makes practical applications difficult ☹
The result was a beautiful confimantion of the Mott
model – yet, whiskers are “technologically unfriendly”
Everything grows giant these days:
Pumpkins, pandas, schnauzers….
Magnetoresistance is NOT an
The credit for introducing
the term Giant Magneto-
resistanceshould be given
to Dr. S. von Molnar, who
used it in a 1967 paper
reporting unusually strong
seen in EuSe crystals doped
with Gadolinium (Gd).
The application of such sensors in the reading heads
of hard-drives made it possible to increase their
capacity by nearly two orders of magnitude…
Since 1997, about 5 billions
of such reading heads have
Recently, they have been “dethroned” by even more
efficient sensors utilizing another magnetoresistance
effect – namely, Tunnel MagnetoResistance (TMR)
Outwardly, a TMR system is similar to a GMR one – but now the two FM conducting layers are separated by a thin (~ 1 nm)insulating layer (e.g., MgO)
High tunneling probability
Low tunneling probability
Zero magnetic field
↑↑↑↑↑ Applied field ↑↑↑↑
In the initial state, the magne-
tization vectors in the two FM
layers must be antiparallel…
...because only then the applied
field will change their mutual
…then the applied field would
not change their mutual orien-
tation, and such system would
not be sensitive to the field.
If the magnetization vectors
were initially parallel…
…in all types of thin film magnetoresistance
sensors there has to be an interaction that
couples the FM films antiferromagnetically
acros the intervening non-magnetic spacer:
This interaction also assures that the system returns
to its initial configuration after the field is removed.
But how can one obtain a coupling of a desired sign between two FM films?
Well, the whole “GMR saga”
started when one day in
1986 Peter Grunberg prepa-
red a “trilayer” consisting
of two iron films, with a
tic chromium metal layer
in between. He observed
that a domain pattern with
directions formed in the
top layer, meaning that the
sign of the interaction be-
tween the Fe layers was an
oscillating function of the
Cr layer thickness. So,
Grunberg’s discovery sho-
wed that the desired con-
figuration can be obtained
by choosing an approp-
riate spacer thickness.
There is still no consensus among researchers ragarding this issue. Some argue that it is simply the “old” RKKY
interaction (known since 1950s). It couples magnetic at- oms embedded in non-magnetic metals, and its sign osc- illates with distance r . It is mediated by Fermi electrons
In this model, the non-magnetic spacer is though of as a quantum well, in which electrons are confined between two “walls”, with the magnetized
layers playing such a role. There are discrete E levels in such a well (recall “particle in a box”). When the well expands, these energies decrease.
Each time a consecutive E level cuts through the Fermi level, the sign of the
But no matter who is right, there is no doubt
about one point: namely, it is the conduction
electrons that play a crucial role in interlayer
coupling effects seen in multilayered metal-
lic GMR systems.
In semiconductors, in contrast, the concent-
tration of conduction electrons is orders of
magnitude lower than in metals. Some of
them are nearly-insulating. So, the above
may imply that in analogous systems made
of semiconductors there is no chance of
seeing interlayer coupling effects.
We have been conducting neutron scat-
tering studies on all-semiconductor
multilayered systems consisting of
alternating magnetic and nonmagnetic
layers, and in many of them we observed
pronounced interlayer magnetic coupling
The existing all-metal GMR sensors are the
first generation of spintronics systems. But in
the opinion of many experts the future belongs
to semiconductor spintronics. Such devices
can be more easily integrated with existing
electronics. Also, semiconductors have many
highly interesting optical properties. Semicon-
ductor spintronics may become an ideal
partner for photonics!
For building practical spintronics devices
one would need semicondutors that are
ferromagnetic at room temperature. And
God did not make them. Rather, God left
it as a challenge for us to create such
materials synthetically. Material techno-
gists in many labs worldwide continue
to work hard on this problem…
The “record-holder” now is epitaxially prepared
Ga(Mn)As alloy, with about 10% of Mn. It stays
FM up to 175 K – still more than 100K below
the “target value”.
What can be done in such situation? Well, there
are some fundamental problems that need to be
studied. For instance – what is the mechanism
giving rise to interlayer coupling effects in sys-
tems with low concentration of mobile electrons?
We decided to do such studies on multilayers
containing EuS, a well-known “prototypical” FM
semiconductor (with Curie T of only 16 K, though).
EuS – Heisenberg ferromagnet TC = 16.6 K (bulk), Eg=1.5 eV
PbS – narrow-gap (Eg=0.3 eV) semiconductor (n ≈ 1017 cm-3)
YbSe – wide-gap (Eg=1.6 eV) semiconductor (semiinsulator)
all NaCl-type structure with lattice constants:
(lattice mismatch ≈ 0.5%)
number of repetitions
Neutron reflectivity experiments
onthe EuS/PbS system
NIST Center for Neutron Research)
Situation corresponding tored data points:
Situat. corresponding togreen data points:
Situation corresponding toblue data points
Unpolarized neutron reflectivity experiments on
the EuS/PbS system
(NG-1 reflectometer, NIST Center for Neutron Research)
Interlayer exchange coupling mediated by valence band electrons
J.Blinowski & P.Kacman, Phys. Rev. B 64 (2001) 045302.
P.Sankowski & P.Kacman, Acta Phys. Polon. A 103 (2003) 621