Ion beam induced modifications of Pt/Co/Pt magnetic films for control of perpendicular magnetic anisotropy

Modifications of magnetic and magnetooptical properties of Pt/Co/Pt tri-layers upon irradiation with ion beam are investigated in this work. They are studied in detail as a function of the Co layer thickness, dCo (up to 5 nm), ion species (Ne+, Ar+, Ga+) the energy of ions, E (up to 30 keV) and ion fluence, F (range from 1013 to 3∙1016 ions/cm2). Modified magnetic properties are correlated with structural features of irradiated samples. Numerical simulations of in-depth atomic concentration profile and surface etching carried out by TRIDYN software complements explanation of the observed effects.
MBE grown Pt/Co/Pt tri-layers display a spin reorientation transition (SRT) at a well-defined Co thickness, dSRT from out-of-plane to in-plane magnetization alignment as dCo increases. Upon ion irradiation various processes undergo: degradation of chemically sharp interfaces, formation of Co-Pt alloys and lattice strain development [1, 2]. Spread interfaces reduce perpendicular anisotropy whereas formation of the ordered alloy may strength this property. Therefore a resulting magnetic state of the sample is a product of interplay of these two opposing trends in anisotropy evolution and the sample surface etching, particularly effective for highest fluences. It is very sensitive to dCo, ion species, their energy and the ion fluence.
Discussion is focused on results obtained with Ar+ ions irradiation, which is commonly used for technological processes especially for nanostructures sputtering. Set of (dCo, log F) diagrams of remanence, mr (Figure. 1), (normalized magnetization perpendicular component) measured by means of magnetooptical Kerr effect magnetometry in polar configuration (PMOKE) upon irradiation with Ar+ ions with different energies. Irradiation of the samples modifies magnetic properties in various manners depending on dCo, F and ion energy. The suppression of the dSRT with F in the range from 1013 to 1014 ions/cm2 and formation of branch(es) corresponding to strong perpendicular anisotropy (higher value of mr) are the most distinguished differences depending on the ion energy. As the ion energy increases a single branch with high mr (for 1.2 keV) begins to split into two parts for 5.0 keV, which are clearly separated for 30.0 keV. Qualitatively this evolution resembles that one already reported for Ga ions [1]. However, the mr modifications strength are weaker due to the lower mass of Ar atoms. Nevertheless, up to 5 transitions between various magnetic states are observed with F increase, e. g. for dCo = 1.6 nm and E = 30 keV. It is worth to notice that even the lowest applied energy of Ar+ ions induces visible modifications of magnetic properties. In the case of the lightest studied Ne ions modification of magnetic anisotropy is weaker than for Ar+ ions and the branches in a similar diagram are non-continuous.
TRIDYN software determined in-depth chemical profile (Figure 2) is helpful in identification of possible alloy composition being responsible for enhanced perpendicular magnetic anisotropy. Also the surface etching extent allows to estimate an amount of sputtered (also magnetic) material that may result in superparamagnetic state observed for the highest ion doses.
Structural properties of the irradiated samples are investigated by Rutherford backscattering (RBS). Evolution of the acquired chemical profiles is correlated with that simulated with TRIDYN software. Moreover, the etching efficiency is estimated from topography step profile between irradiated and non-irradiated areas of the sample investigated by means of profilometer and atomic force microscopy.
Discussed above abundance of magnetic investigated states in combination with controlled structural features developing upon ion irradiation enable a precise tuning of desired magnetic properties. In particular, such approach, exploiting a focused ion beam, might be of great importance in fabrication of magnonic crystals – a new type of metamaterial with reprogrammable band structure.