Patterning of magnetic structures on austenitic stainless steel by ion beam nitriding

Nitriding of austenitic stainless steel (ASS) at moderate temperatures (~400ºC) leads to the formation of the supersaturated nitrogen solid solution often called in the literature “expanded austenite” or γN phase [1,2]. This causes an enhancement of the microhardness and the wear resistance without loss of the corrosion resistance. Moreover, this phase shows ferromagnetic behavior, whose origin is linked to the expansion of the austenite (γ) lattice due to the incorporation of nitrogen atoms into interstitial positions [3,4]. Actually, since there is a nitrogen depth profile and the onset of ferromagnetism is connected with nitrogen concentrations of ~15 at.%, the nitrided layer consists of two magnetically different parts (paramagnetic and ferromagnetic) determined by the obtained nitrogen concentration.

In this study, we report the influence of the nitriding temperature and time on the ASS ferromagnetic properties. AISI 304L ASS polycrystalline samples (discs of 10 mm diameter and 2 mm thickness) have been ion beam nitrided in the temperature range of 300-400ºC. The ion energy and the ion current density were ~1 keV and 0.5 mA/cm2 (the corresponding ion flux of ~5•1015 ions•cm-2•s-1), respectively. The processing times were 5 and/or 30 minutes [2]. Periodic arrays of ferromagnetic structures in the micrometer range have been prepared at the surface of the samples using a 2000 mesh Cu transmission electron microscopy grid as a shadow mask (mesh size of 7.5 x 7.5 µm2, 12.5 µm pitch, 20 µm thickness and 3.05 mm diameter). The structure was characterized by X-ray diffraction (XRD) and nuclear reaction analysis (NRA). The magnetic properties were determined by magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM).
The XRD patterns of the nitrided ASS samples are presented in Figure 1. The XRD pattern consistent with the FCC lattice structure can be identified for the virgin ASS sample (not shown). For the nitrided samples, each austenite peak exhibits a satellite peak, located at lower diffraction angles which is related with the formation of the “expanded austenite”. The amount of this “expanded” phase increases with the processing temperature, as it is evidenced by the increase of intensity of the γN XRD peaks in detriment to the γ ones.
This is consistent with NRA observations. For instance, a nitrided layer of around 1 µm depth is obtained in the sample nitrided at 400ºC for 30 min, whereas ~15 at.% of nitrogen is obtained around 0.5 µm of depth.
MOKE measurement results of the virgin and the nitrided sample at 400°C for 5 min are compared in Figures 2(a) and 2(b) (patterned area).
It can be seen that the virgin sample does not show any hysteretic behavior, i.e. it is non-ferromagnetic. Conversely, the nitrided samples show clear hysteresis loops indicating the existence of ferromagnetic constituents in the nitrided layer. Figure 2(c) shows the AFM image of a patterned area of the nitrided sample which shows that a moderate sputtering process of the surface has taken place during nitriding resulting in the formation of the periodic array of squared pits. Figure 2 (d) is the corresponding MFM image in an applied magnetic field of 70 mT, where a magnetic dipolar contrast can be clearly seen in each entity, confirming the feasibility of the production of periodic arrays of isolated ferromagnetic structures. It is worth noting that the hysteresis behavior of the continuously nitrided areas and the patterned ones are quite similar due to the fact that the induced ferromagnetic structures are relatively large (micrometer range), leading to entities with magnetic multi-domain configurations.

[1] M.P. Fewell et al., Surf. Coat. Technol. 131, 300 (2000)
[2] G. Abrasonis, et al., J. Appl. Phys. 97, 083531 (2005)
[3] O. Öztürk and D.L. Williamson, J. Appl. Phys. 77, 3839 (1995)
[4] J. Baranowska, Vacuum 81, 1216 (2007)