Perpendicular Magnetic Anisotropy in Fe–N Thin Films: Threshold Field for Irreversible Magnetic Stripe Domain Rotation

Abstract

The magnetic properties of an iron nitride thin film obtained by ion implantation have been\ud investigated. N2+ ions were implanted in a pristine iron layer epitaxially grown on ZnSe/GaAs\ud (001). X-ray diffraction measurements revealed the formation of body-centered tetragonal\ud N-martensite whose c-axis is perpendicular to the thin film plane and c-parameter is close to that\ud of alpha' Fe8N. Magnetic measurements disclosed a weak perpendicular magnetic anisotropy (PMA)\ud whose energy density KPMA was assessed to about 105 J/m3. A sharp decline of the in-plane\ud magnetocrystalline anisotropy (MCA) was also observed, in comparison with the body-centered\ud cubic iron. The origin of the PMA is attributed to the MCA of N-martensite and/or stress-induced\ud anisotropy. As a result of the PMA, weak magnetic stripe domains with a period of about 130nm\ud aligned along the last saturating magnetic field direction were observed at remanence by magnetic force microscopy. The application of an increasing in-plane magnetic field transverse to the stripes\ud Htrans highlighted a threshold value above which these magnetic domains irreversibly\ud rotated. Interestingly, below this threshold, the stripes do not rotate, leading to a zero\ud remanent magnetization along the direction of the applied field. The interest of this system for\ud magnetization dynamics is discussed

Stripe domains reorientation in ferromagnetic films with perpendicular magnetic anisotropy

“…Each image was recorded at remanence. The dark/white dots in figure 5 highlight the presence of magnetic edge dislocations, namely topological defects spontaneously nucleated in the stripe domain structure [39]. We observed that the whole stripe structure remains unperturbed up to = H 800…”

“…Weak stripe domains were first predicted theoretically [32,33] and later discovered experimentally in NiFe films [19,34]. In the following years, the phenomenon has been found and deepened in different kinds of thin films with moderate PMA, such as FeGa [35][36][37][38], FeN [31,39,40], FeSiB [41,42], CoFeB [43], FeTaN [44], GdFe [45], NdCo [46], FeCoZr [23], La x Sr 1−x MnO 3 [47] and multilayers with moderate perpendicular anisotropy [48][49][50]. Very recently, it has been shown [51] that the formation of stripe domains can be induced in a Py film, even far below the critical thickness, by coupling the Py film to a NdCo one, characterized by a moderate PMA.…”

“…Then, the film was in situ protected against oxidization with an 8 nm thick gold capping layer. Four different samples have been prepared performing ion implantation of four pieces of this iron film, following the procedure detailed in our previous study [39]. Three samples were implanted with + N 2 ions accelerated to 26 keV with fluences of 3.0×10 16 + N 2 cm −2 and 3.5×10 16 + N 2 cm −2 , and 40 keV with a fluence of 5.3×10 16 + N 2 cm −2 , respectively.…”

“…along the direction of H trans was recorded. Following the procedure described in [39], this two-step measurement was repeated many times, each time increasing the maximum value of the transversal field, H trans max , As shown in figure 6, even if the four investigated samples exhibit a similar behavior, they are characterized by a different value of the threshold field H thr . In figure 7, the threshold field for stripe reorientation is plotted as a function of the in-plane saturation field, H s , the normalized in-plane remanence, M r /M s , and the perpendicular magnetic anisotropy energy density, K PMA , for the four investigated Fe-N thin film samples.…”

“…The calculations were performed by setting the material parameters equal to values obtained in previous investigations of the Fe-N films. The saturation magnetization is M s =1700 emu cm −3 [39]. The exchange interaction is A ex =1.8×10 −6 erg cm −1  [91].…”