Magnetic correlations in dipolarly coupled iron oxide monolayer and multilayer films

(1)

Magnetic correlations in dipolarly coupled iron oxide monolayer and multilayer films

D. Mishra

¹

, D. Greving

¹

, O. Petracic

^{1, 2}

, G. A. Badini-Confalonieri

^{1, 3}

, A. Devishvili

^{1, 4}

, K. Theis-Bröhl

⁵

, B. P. Toperverg

^{1, 6}

and H. Zabel

¹

1Condensed Matter Physics, Ruhr University Bochum, D-44780 Bochum, Germany

2 Jülich Centre for Neutron Science JCNS-2 and Peter Grünberg Institute PGI-4, Forschungszentrum Jülich, 52425 Jülich, Germany

3Instituto de Ciencia de Materiales, E-28049 CSIC Madrid, Spain

4Institute Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France

5University of Applied Sciences Bremerhaven, D-27568 Bremerhaven, Germany

6Petersburg Nuclear Physics Institute RAS, Gatchina 188350, St Petersburg, Russia, E-Mail: o.petracic@fz-juelich.de

Self-assembled magnetic nanoparticles (NPs) have promising applications in future electronic and magnetic nano-devices [1]. The collective properties of self-assembled NPs strongly differ from those of the individual building blocks. This is due to the structural and magnetic ordering inside self-assembled systems. Particularly magnetic dipolar coupling can suppress thermal fluctuations and can lead to long-range ordering of the NPs. Polarized neutron reflectivity (PNR) provides a unique method to investigate both the structural and magnetic ordering [2]. We prepared iron oxide NP (diameter = 20 ± 1.4 nm) monolayer and multilayer films on a silicon substrate by spin-coating method. The NPs are arranged in a hexagonal close-packed geometry. Annealing at 230° C in vacuum yields magnetite (Fe3O4) as the major phase as confirmed from x-ray diffraction and magnetometry measurements [3]. PNR measurements were performed using the Super ADAM reflectometer stationed at ILL, Grenoble. In case of the multilayer the PNR shows a multilayer Bragg peak indicating a high degree of out-of-plane ordering. The magnetic correlation was deduced from the fitting of the PNR curves. In addition, the magnetic correlation in a monolayer was deduced in a special approach, where neutron standing waves were formed by creating a neutron potential well.

The well consists of a vanadium film (50 nm) sandwiched between iron oxide NPs and an Al2O3 substrate. In remanence, the NP films (both monolayer and multilayer) show that dipolar coupling among the NPs lead to formation of quasi-domains resembling a superferromagnet [4].

References

[1] S. A. Majetich, T. Wen and R. A. Booth; ACS Nano 5, 6081 (2011)

[2] H. Zabel, K. Theis-Bröhl, B. P. Toperverg; Handbook of Magnetism and Advanced Magnetic Materials, vol. 3, Wiley (2007)

[3] M. J. Benitez et al; J. Phys: Condens. Matter 23, 126003 (2011)

[4] D. Mishra et al; Nanotechnology 23, 055707 (2012)