Ion beam induced magnetic transformation in layered structures: nonmagnetic to ferromagnetic and vice versa

A nonmagnetic to ferromagnetic transformation is observed when a Pt/C multilayer stack (total thickness ~ 60 nm, thickness of individual layers ~ 2 nm) with a small amount of Fe impurity, grown on a glass substrate, is irradiated with 2 MeV Au ions [1]. Using a subnanometer depth resolution technique it was found that ion beam induced preferential migration of Fe from C to Pt layers and the subsequent formation of FePt nanoparticles were responsible for the magnetic transformation [2]. Fe concentration in the C layers decreases exponentially with increasing ion fluence with simultaneous increase of the coercive field in the magnetic hysteresis loop. An example of ion beam induced opposite transformation, namely, ferromagnetic to nonmagnetic, is shown using a Si(5nm)/Ni(10 nm)/Si trilayer system, where ion-beam-induced atomic displacements across the Si/Ni interfaces form a mixed Ni1-xSix layer leading to a ferromagnetic to nonmagnetic transformation. In this case, with increasing ion (30 keV Ga) fluence the coercive field in the hysteresis loop decreases with increasing ion fluence and eventually magnetization is destroyed at a critical fluence. Having identified the critical fluence, a focused ion beam (FIB, 30 keV Ga) was used to fabricate a lateral multistrip pattern in this thin (10 nm) magnetic layer with alternate magnetic/nonmagnetic strips of nanometer dimensions. Magnetooptical Kerr effect microscopy reveals the anisotropy of the magnetic domain structure along and perpendicular to the FIB-patterned strips. In the light of the observed phenomena, future developments in the area of nanotechnology, such as ultrahigh density magnetic storage devices, single-electron spin-valve transistors etc. will be discussed. Some nanoscale structures that can be fabricated using FIB will offer the possibility of exploring new scientific aspects, such as magnons in ordered ferromagnetic spot arrays, in these structures.

[1] B. N. Dev, S. Bera, B. Satpati, D. K. Goswami, K. Bhattacharjee, P. V. Satyam, K. Yamashita, O. M. Liedke, K. Potzger, J. Fassbender, F. Eichhorn and R. Groetzschel,
Microelectronic Engineering 83, 172 (2006).

[2] S. Bera, K. Bhattacharjee, G. Kuri and B. N. Dev
Phys. Rev. Lett. 98, 196103 (2007).