Spin-transfer torque (STT) in hybrid magnetic nanostructures provides access to high frequency magnetization dynamics of a ferromagnetic element driven by a dc current. Here the term STT describes the transfer of angular (spin) momentum from a spin-polarized- or a pure spin current to the local magnetization of the magnetic element. Technologically, the STT may be used to conceptualize new types of magnetic memory devices (MRAM) that can be written using a direct current. Here we explore the possibility to create miniaturized and highly tunable microwave devices, such as emitters and filters, based on the STT in arrays of magnetic nanopillars (see Figure 1).

Figure 1: Illustration of a nanopillar-array STO and its integration in a coplanar microwave device.

Hybrid magnetic nanopillars with a radius 10nm to 50 nm and lengths up to several µm are grown electrochemically in a self-assembled alumina template. Their morphology, composition and stacking sequence can be controlled via the template as well as the processing and growth conditions. In contrast to earlier reports, we are focusing on arrays of nanopillars representing an ensemble of a large number of coupled spin-transfer oscillators (STOs). It has been shown that synchronization of few STOs coupled via propagating spin-wave modes is possible. Here we investigate whether a synchronized operation of many hundreds or thousands of nano-STOs is possible. Currently, we established that individual micro-resonators incorporating between 103 and 107 magnetic nanopillars with a diameter of approximately 30 nm can be grown and integrated in coplanar microwave circuits (Figure 2).

Figure 2: Micrographs of a nanopillar-array STO with an array size of 100 µm featuring in the order of 106 coupled oscillators.

The magnetic- and electronic transport properties as well as their magnetization dynamics are studied experimentally via magnetometry, ESR/FMR and transport spectroscopy and are correlated with the structural and morphological properties of the resonators. The immediate goal is to evaluate the effect of the coupling mechanism and the coupling strength among the nanopillars with respect to the synchronization of the STO modes, which is expected to result in significantly improved microwave emission power and lower linewidths in the frequency spectrum.


Contributors: Sergej Andreev, Roman Hartmann and Mengqi Fu
Former Contributors: Torsten Pietsch, Christoph Widmann (BSc student)
Period: Apr 2015 - June 2021