GMR was first discovered in 1988 when Baibich et al. reported 50% resistance change at low temperatures with an applied field in (Fe/Cr)n multilayer thin films. The Fe/Cr superlattices
were antiferromagnetically coupled in zero field and grown by molecular beam epitaxy [1]. With the application of a sufficiently high field, the exchange interaction could be overcome and the magnetisation of all the layers aligned parallel. The resistance of the structure depends upon the magnetic arrangement of the layers, and was observed to be higher when the moments were antiparallel to one another. When the GMR effect was first discovered in the (Fe/Cr)n system, there was an assumption for some time that the Cr spacer layer, with its
unusual antiferromagnetic properties, was necessary for the interlayer antiferromagnetic exchange to exist. Work published by Parkin et al. in 1990 [2] showed that similar results could be obtained in polycrystalline Fe/Cr structures grown by the much simpler technique of magnetron sputtering. These experiments also revealed that GMR could be observed in a wide variety of transition-metal magnetic multilayers. One of these systems, namely ferromagnetic cobalt layers separated by thin copper layers, was found to exhibit very large GMR effects in excess of 110% at room temperature [3]. While the largest GMR values require magnetic fields exceeding ∼20kOe, magnetoresistance values of 50-60% are obtained in fields of several hundred oersteds and values of ∼20% or more in fields of a few tens of oersteds. These lower values are obtained by using thicker Cu layers, for which the interlayer exchange coupling is weaker. In addition, the experiments revealed that the magnitude of the GMR oscillated as the thickness of the non-ferromagnetic spacer layers between the ferromagnetic layers was increased [4,5]. This oscillation was shown to be caused by an oscillation in the sign of the interlayer exchange coupling between the ferromagnetic layers. The coupling oscillated between antiferromagnetic and ferromagnetic coupling such that the magnetic moments of successive ferromagnetic layers were either parallel (ferromagnetic) or antiparallel (antiferromagnetic) in small magnetic fields. Fig. 3.1 shows the room temperature saturation magnetoresistance versus Cu spacer thickness for a series of polycrystalline sputter-deposited Co/Cu multilayers. The magnetic state of the multilayer is shown schematically for various Cu layer thicknesses (only two magnetic layers are shown). Oscillations of the GMR as a function of Cu thickness occur because the magnetoresistance effect is measurable only for those thicknesses of Cu for which the interlayer exchange coupling aligns the magnetic moments of the Co layers antiparallel. According to the reviews, it is the induced spin density wave in the spacer layer material that mediates the magnetic coupling of the magnetic layers in magnetic multilayers and sandwiches [6]. Oscillatory coupling has been shown to be a general property of almost all transition-metal magnetic multilayered systems in which the non-ferromagnetic layer comprises one of the 3d, 4d, or 5d
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transition metals or one of the noble metals. The oscillation period was found to be just a few atomic layers, typically about 10Å, but varying up to ∼20Å. Only those multilayers for which the interlayer coupling is antiferromagnetic display significant giant magnetoresistance effects. Various models have been proposed to account for the long range of this oscillatory exchange coupling. Theoretical approaches based on quantum confinement [7,8] and on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction [9,10] have been able to describe the origin of the coupling.
Fig. 3.1 Room temperature saturation magnetoresistance versus Cu spacer layer thickness
for a series of polycrystalline sputter-deposited Co/Cu (After Ref. [5]). The magnetic state of the multilayer is shown schematically for various Cu layer thicknesses (only two magnetic layers are shown).
The decay in GMR with increasing Cu thickness can be described approximately by Eqn. (3.1), where tCu is the Cu thickness and λCu describes the scattering within the Cu layer
interior [6].
−
≈
∆
Cu Cu Cut
t
R
R
λ
exp
1
(3.1)
This functional dependence can be understood by considering two effects. Firstly increasing the Cu layer thickness dilutes the interfacial scattering regions because the measuring current,
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which is parallel to the layers, is shunted through the bulk of the Cu layers away from these regions. Secondly, scattering within the interior of the Cu layers will diminish the flow of electrons between the Co layers. This scattering can be described by the scattering length, λCu.
3.2 The Physical Origin of GMR.
The detailed origin of GMR has provoked considerable interest. Many theoretical models have been developed but most of them are based on a model of the electrical conduction in ferromagnetic metals due to Mott [11]. According to Mott, the electrical current in ferromagnetic metals is carried independently in two conduction channels corresponding predominantly to the spin-up and spin-down s-p electrons. These electrons are in broad energy bands with low effective masses. This assumption is believed to be good at temperatures significantly below the magnetic ordering temperature of the magnetic material so that there is little spin-mixing between the two conduction channels. Mott theorised that the conductivity can be significantly different in the two spin channels because the conduction electron scattering rates in these two channels will be related to the corresponding spin-up or spin-down density of empty states at the Fermi level.
Fig. 3.2 Density of states of copper, cobalt and iron. Broken line denotes the position of the