Atomic scale structure of sputtered metal multilayers
Introduction
When ferromagnetic layers (such as CoFe) are separated by a non-magnetic spacer layer (such as Cu) of an appropriate thickness (for example, approximately 2 nm), their magnetic moments will align antiferromagnetically in a zero magnetic field [1]. Spin-dependent conduction electron scattering [2], [3] then results in a relatively high electrical resistance. If a sufficiently large magnetic field is applied to reverse the magnetic moment of one of the ferromagnetic layers, a ferromagnetically aligned sandwich can be created. This alignment reduces the spin-dependent conduction electron scattering, and causes a decrease in the electrical resistance of the film—an effect known as giant magnetoresistance (GMR) [2], [3]. The GMR effect was first observed in vapor deposited Fe/Cr/Fe sandwiches in 1988 [4]. Since then, many other material systems have been shown to exhibit the effect, including the Co/Cu/Co [5], [6], NiFe(Co)/Cu/NiFe(Co) [7], [8], and NiCo/Cu/NiCo systems [9]. The significant technological importance of GMR materials as magnetic field sensors arises from their very high sensitivity to an external magnetic field when fabricated as submicron devices. Because of this, GMR multilayers are now widely used as read-head sensors in hard disk drives, and are also being actively explored for a new form of magnetic random access memory [10].
GMR materials have been synthesized using numerous vapor deposition techniques including sputter-deposition [7], ion beam deposition [11], and molecular beam epitaxy [4]. These studies have shown that the magnetotransport properties are highly sensitive to the method and conditions of growth. Experiments have indicated that a significant decrease in the GMR ratio occurs when the average spacing between the ferromagnetic layers is changed, even by as little as one monolayer [5]. High-quality GMR multilayers must therefore have a precisely controlled layer thickness. In addition, if the layers are comparably rough in amplitude and wavelength to the spacer layer thickness, Néel coupling occurs and it becomes more difficult to reverse the magnetic moment of the layers. Furthermore, the alloying of one layer by the metal atoms of another causes an increase in spin independent scattering [12] and a loss of local magnetic alignment [13]. Relatively smooth, unmixed interfaces therefore appear desirable [7], [14]. The intrinsic properties of the material system are also important, together with other nanoscale structural features, including interfacial impurities [15], texture [16], any buffer layer material [17], the presence of surfactants during growth [18], the grain size/morphology of each layer, various lattice defects (vacancies, voids, dislocations and twins) [19], and perhaps residual stress. Of these, the atomic scale perfection across the multilayer interfaces appears to most critically affect the GMR properties [20].
Although the importance of these various effects is clear, the very small scale of the defects have made it difficult to make a direct link between a given effect and the change in magnetic properties. Results obtained from different investigations are often contradictory. In experimental studies of Fe/Cr/Fe multilayers, for instance, Petroff et al. found that increasing the interfacial roughness and the interfacial mixing both increased the GMR ratio [21], while Belien et al. indicated that a lower interfacial roughness and reduced interfacial mixing result in a higher GMR ratio [22]. The contradictory nature of these results most likely arises because the atomic scale structures of the deposited films are not well understood. In the present work, atomistic simulations based on the molecular dynamics (MD) approach have been used to investigate growth mechanisms and their control in order to create improved interfaces. This has been combined with direct atomic-scale characterization of sputter-deposited layers using the three-dimensional atom probe (3DAP), which has allowed experimental verification of the results of the simulation.
Section snippets
Atomistic simulation of atomic structure
Many factors contribute to the structure of a vapor-deposited multilayer film, including layer composition, growth temperature and deposition energy. To explore the effects of these factors within a computer model, a highly predictive atomistic simulation method must be used. The method must not only produce reliable results, but must also not require pre-existing knowledge of the film growth mechanisms. In our work, a MD approach is used to provide an accurate description of the way in which
Experimental measurement of atomic structure
Experimental characterization of the physical and chemical nature of interfaces in nanoscale multilayers is a challenging task, due to the very fine scale of any roughness or intermixing sufficient to generate changes in magnetic properties. What is required is a technique which has ultra-high spatial resolution in all three dimensions. Transmission electron microscopy (TEM) [28], traditionally used to study these materials, cannot in most cases chemically identify single atoms and can only
Mechanisms for formation of interface imperfections
Elementary thermodynamic arguments indicate that a material A cannot wet the surface of a material B if B wets A. When wetting does not occur, flat interfaces are difficult to form in nanoscale multilayers because of the tendency for the nonwetting layer to grow by an island mechanism. As a result, GMR multilayers with all interfaces flat cannot be obtained under kinetically unconstrained (thermal equilibrium) growth conditions. Previous MD simulations have indicated that the use of energetic
Growth process improvement
MD simulations provide compelling evidence that increasing the adatom energy flattens interfaces, but induces interlayer mixing at interfaces when a high tendency exists for the underlying material to segregate to the surface of the material being deposited. While lowering the adatom energy can sharpen the chemical boundary of the interface, it is accompanied by increased interfacial roughness. As a result, better GMR properties are obtained from materials grown with an intermediate adatom
Conclusions
An integrated approach combining atomistic simulations and three-dimensional atom probe experiments has been developed to study atomic scale structures of nanoscale metal multilayers. The application of this approach to the sputter deposition of nanoscale CoFe/Cu/CoFe multilayers revealed the following:
- 1.
Energetic adatom impact with the growth surface can cause exchange between the adatom and underlying atoms. The exchange probability is much higher when the underlying material is Cu rather than
Acknowledgements
The University of Virginia is grateful to the Defense Advanced Research Projects Agency for its support through the Virtual Integrated Prototyping program (program managers: A. Tsao, D. Healey and S. Wolf). This research was partially sponsored by the US National Science Foundation (RLM and TFK). We would also like to thank Dr B. D. Wisman and H. F. Erskine at Seagate for their assistance with this research and Prof. B. Cantor at Oxford University for provision of laboratory facilities.
References (35)
- et al.
J. Magn. Magn. Mater.
(1995) J. Magn. Magn. Mater.
(1995)- et al.
J. Magn. Magn. Mater.
(1993) - et al.
J. Magn. Magn. Mater.
(1995) - et al.
J. Magn. Magn. Mater.
(1997) - et al.
J. Magn. Magn. Mater.
(1995) - et al.
J. Magn. Magn. Mater.
(1993) - et al.
J. Magn. Magn. Mater.
(1995) - et al.
J. Magn. Magn. Mater.
(1991) - et al.
Acta mater.
(1997)