Tuning magnetism in nanostructures by interface design

Marina Spasova, University of Duisburg-Essen, Duisburg, Germany
Sara Liébana-Viñas, University of Duisburg-Essen, Duisburg, Germany
Ruslan Salikhov, University of Duisburg-Essen, Duisburg, Germany
Ulf Wiedwald, University of Duisburg-Essen, Duisburg, Germany
Cristina Bran, Institute of Materials Science of Madrid, Madrid, Spain
Manuel Vazquez, Institute of Materials Science of Madrid , Madrid, Spain
Michael Farle, University of Duisburg-Essen, Duisburg, Germany

Magnetic nanorods and nanowires based on 3D transition metal alloys might be suitable candidates for alternative rare-earth free permanent magnets. Exploiting the alignment magnetocrystalline and shape anisotropy axes in conjunction with exchange bias is one strategy for maximized hysteretic energy products.

In this contribution, we report on Co80Ni20 nanorods (length 52.5 nm, diameter 6.5 nm) prepared by a polyol reduction [1]. Structural, morphological and chemical investigations revealed surface oxidized nanorods consisting of a metallic hcp CoNi core (single crystalline) and a Co-rich oxide shell with a thickness of about 2 nm (polycrystalline). The magnetic easy axis is the <0001> crystallographic direction and points parallel to the nanorods axis [2]. A Co-rich oxide shell has been formed by natural oxidation around the core with a defined crystallographic orientation, i.e. <0001>hcp || <001>fcc. The magnetic characterization shows strong unidirectional anisotropy at low temperatures after field cooling while the exchange bias vanishes above T = 175 K. Interestingly, the coercive field rises by almost a factor of 2 in the temperature range 175 K < T < 250 K before it starts decreasing again [2]. This effect is discussed within a three regime model considering superparamagnetic fluctuations of antiferromagnetic grains in the Co-rich oxide shell.

In a second example we focus on the magnetic hardening of Fe30Co70 nanowires by means of magnetic pinning at the tips of the nanowires. We observe that 3 nm FeCo oxide layers increase the coercive field by 20%, indicating that domain wall nucleation starts at the tips. Micromagnetic simulations support our experimental findings showing that the increase of the coercive field can be achieved by controlling domain wall nucleation using antiferromagnetic capping [3]. The results offer a guideline on tailoring exchange anisotropies in nanoscale systems for the enhancement of magnetic hardness.

Financial support from the Seventh Framework Programme under grant agreement no. 280670 (REFREEPERMAG) is acknowledged.

[1] Y. Soumare, J. Y. Piquemal, T. Maurer, et al., J. Mater. Chem. 18 (2008) 5696-5702.
[2] S. Liébana-Viñas, U. Wiedwald, A. Elsukova, et al., Chemistry of Materials 27 (2015) 4015–4022.
[3] S. Liébana-Viñas, R. Salikhov, C. Bran, et al., Nanotechnology 26 (2015) 415704-415712.

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