Ion Irradiation Induced Cobalt/Cobalt Oxide Heterostructures: From Materials to Devices

Selective removal of oxygen in CoO and Co3O4 films by ion irradiation offers an efficient alternative to the fabrication of spintronic devices. With low energy ion irradiation, these paramagnetic and insulating cobalt oxides can be reduced to form ferromagnetic and metallic cobalt. Within this project , we study the effect of ion irradiation on cobalt oxide films in order to determine the physics behind the oxygen removal.

Spintronic devices are often patterned from continuous films into micro- or nanostructures. Fabrication of such nano-devices is self-limited and depends on the lateral resolution of the chosen fabrication method. Ion irradiation offers an alternative route to introduce smaller magnetic patterns limited by the resolution of electron beam lithography. It has been shown that the irradiation of oxide materials can cause a reduction of oxygen which leads to the local formation of metallic species. In a study involving the reduction of oxygen in Co3O4/Pd multilayers, it was demonstrated that oxygen reduction is possible with the use of low energy protons [1]. Figure 1 (a) shows the Co3O4/Pd multilayer system in which ferromagnetic and metallic regions are formed inside the surrounding, paramagnetic and insulating oxide matrix as a result of proton irradiation. The reduction is confirmed by SQUID magnetometry measurements as shown in figure 1 (b). A magnetic hysteresis curve of a H+ irradiated [Co3O4/Pd]10 system shows saturation magnetization and perpendicular magnetic anisotropy comparable to that of a [Co/Pd]10 reference.

No sufficient information has been provided so far to fully explain the physical mechanism behind the ion induced oxygen removal. Therefore, our studies focus on H+, He+, Ne+, and O+ irradiation of CoO and Co3O4 systems in an effort to understand the underlying process. We irradiate extended films as well as implementing striped irradiation masks onto extended films to geometrically confine the irradiated regions (figure 2). In doing so we can investigate the effect the dimensions of such a confinement has on the resulting magnetic properties of the material. Once optimized, this method may be used to tune exchange bias direction and prepare nano-contacts for synchronized spin torque nano-oscillators.

Figure 1: (a) [Co3O4/Pd]10 stack irradiated with 0.3 keV protons forming ferromagnetic and conducting Co volumes within the matrix. (b) SQUID hysteresis curves with both easy (red) and hard (blue) axis magnetization [1].

Figure 2: SEM image of an irradiation mask prepared by electron beam lithography consisting of 0.5 µm stripes with a 1 µm pitch.

The support of the following facilities is gratefully acknowledged:

Nanofabrication Facilities Rossendorf at IBC in Helmholtz-Zentrum Dresden-Rossendorf (Dr. Artur Erbe, Bernd Scheumann, Heik Hilliges).
Ion Beam Centre in Helmholtz-Zentrum Dresden-Rossendorf (Dr. Roman Böttger, Dr. Gregor Hlawacek).
Structural Characterization Facilities Rossendorf at in in Helmholtz-Zentrum Dresden-Rossendorf (Andrea Scholz, Dr. Rene Hübner).
This work is supported by the Helmholtz Young Investigator Initiative Grant No. VH-N6-1048.

Reference
[1] S. Kim, S. Lee, J. Ko, J. Son, M. Kim, S. Kang, and J. Hong. Nature Nanotechnology,
7:567, 2012.