Voltage-controlled magnetization switching
In magnetic sandwiches with a nonmagnetic semiconducting interlayer, for example, Fe/Si/Fe structures, a strong antiferromagnetic interlayer exchange coupling is observed, which is stronger than for most metallic spacer layers. At the same time, the transport through the interlayer is dominated by tunneling, i.e., the resitivity is high. This combination of high resitivity and strong interlayer coupling allows one for the first time to study the effect of a bias voltage applied across the spacer on its interlayer coupling behavior.
Dr. Daniel E. Bürgler
Prof. Peter Grünberg
Spin-current induced magnetization switching
Spin-current induced magnetization is a kind of an inverse GMR effect. The angular momentum carried by a spin-current passing through a thin magnetic film exerts by spin momentum transfer a torque on the film magnetization. If the torque is strong enough, the magnetization switches. Spin-current induced magnetization switching is observed in FM1/M/FM2 structures, where the ferromagnetic layer FM1 acts as polarizer, the nonmagnetic metal M as a spacer to avoid direct exchange between the magnetic layers, and FM2 is the layer being switched.
Dr. Daniel E. Bürgler
Prof. Peter Grünberg
Spintronic devices employ the spin degree of freedom of the electrons to store or process information. Examples are the magnetic random access memory (MRAM) or the envisaged spin-transistor. The spintronic functionality relies on the ability to remagnetize nanoscale magnetic objects, e.g. the storage layer of an MRAM cell or the electrode of a spin-transistor. In order to avoid crosstalk between
the so-called neighboring nanoscale magnetic elements in a device, the remagnetization process –
magnetization switching– must be triggered locally. Therefore, one aims at avoiding external magnetic fields, which act via dipolar interaction over fairly large distances. One rather prefers a switching concept based on electric means.
Figure 1: Phenomenon of current-induced magnetization switching due to a DC current flowing through a so-called magnetic nanopillar.
If the external magnetic field exceeds the coercivity, only the parallel alignment of both the free and fixed magnetization pointing along the external field is a stable configuration. In this situation, the torque due to the current-induced spin transfer (for the proper current polarity) drives the free magnetization into oscillatory magnetic modes, which are not attainable with magnetic fields alone (e.g. by FMR). Due to the GMR effect any periodic motion of the free magnetization gives rise to a periodic variation of the resistance, and thus the voltage drop across the nanomagnet. Typical oscillations frequencies lie in the GHz range. Therefore, the spin transfer effect due to a DC current allows generating tunable microwaves in a nanoscale object.
We have developed a structuring process involving optical and e-beam lithography to fabricate nanopillar structures from epitaxially grown multilayers with a pillar diameter of about 150 nm. The process is successfully tested with epitaxial Fe/Ag/Fe/Cr/Fe(001) structures (see Figure 2), for which we found hysteretic switching of the Fe/Ag/Fe and Fe/Cr/Fe subsystems at opposite current polarities as well as microwave signals in the frequency range between 3 and 10 GHz. Ongoing research deals with the influence of the magnetocrystalline anisotropy and interlayer exchange coupling on the spin torque effects (e.g. critical current densities and microwave frequencies). Additionally, the up to now rather narrow material basis (mostly Co-based systems) will be extended.
Figure 2: Epitaxial Fe/Ag/Fe/Cr/Fe(001) layer sequence grown on a Ag(001)-buffered GaAs(001) wafer and photograph of the final structure.
The spin-dependent transport concepts such as spin accumulation, spin-polarized tunnelling, or spin injection may be extended to nanostructures or even molecular systems. This is the field of nano-spintronics, representing the spin-polarized counterpart to conventional molecular electronics and posing a large experimental challenge. The current interest includes the spin transport through single-walled and multi-walled carbon nanotubes, and nano-contacts as well as spin effects in quantum transport through single-walled carbon nanotubes and peapods with paramagnetic fullerenes.
Dr. Carola Meyer
Prof. Dr. Claus M. Schneider