Ion beam induced modification of exchange interaction and spin-orbit coupling in the Co${}_2$FeSi Heusler compound

Abstract

A Co${}_2$FeSi (CFS) film with L2${}_1$ structure was irradiated with different fluences of 30 keV Ga${}^{+}$ ions. Structural modifications were subsequently studied using the longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical Kerr effect. Both the coercivity and the LMOKE amplitude were found to show a similar behavior upon irradiation: they are nearly constant up to ion fluences of $\approx 6\times 10^{15}$ ion/cm${}^2$, while they decrease with further increasing fluences and finally vanish at a fluence of $\approx 9\times 10^{16}$ ion/cm${}^2$, when the sample becomes paramagnetic. However, contrary to this behavior, the QMOKE signal nearly vanishes even for the smallest applied fluence of $3\times 10^{14}$ ion/cm${}^2$. We attribute this reduction of the QMOKE signal to an irradiation-induced degeneration of second or higher order spin-orbit coupling, which already happens at small fluences of 30 keV Ga${}^{+}$ ions. On the other hand, the reduction of coercivity and LMOKE signal with high ion fluences is probably caused by a reduction of the exchange interaction within the film material.