Full Length ArticleRotatable magnetic anisotropy in Fe78Si9B13 thin films displaying stripe domains
Graphical abstract
Introduction
Stripe domains in thin films have been observed since many decades in Ni- and Co-based systems [1], [2], [3], [4], [5], and later in a variety of thin films, including amorphous or nanocrystalline alloys [6], [7], [8], [9], [10], [11], highly magnetostrictive alloys [12], [13], and multilayers [14], [15], and even in amorphous ribbons and bulk systems [16], [17], [18]. Their relatively weak perpendicular anisotropy is comparable to the shape anisotropy, and results in stripe domains whose magnetisation is tilted off the sample plane, therefore having both an in-plane and an out-of-plane component [11], [19], with the possible presence of closure domains [20]. Together with a characteristic in-plane hysteresis loop shape, often called “transcritical” [3], [7], these films typically display a “rotatable anisotropy” [2], [12], [13], [21], [22], i.e. the in-plane component of the magnetisation of the stripes (and therefore the stripes themselves) can align to any direction in the film plane, provided that a strong enough in-plane magnetic field is applied. Any in-plane direction being equivalent, the reason why a threshold value for the applied magnetic field must be overcome to induce the stripes rotation along its direction is still not clear, in spite of detailed investigations of the static and dynamic magnetisation processes in this kind of samples [6], [12], [13], [22], [23], [24]. Understanding this phenomenon is still an open question that needs to be addressed in order to be able to transfer magnetic thin films having a controlled weak perpendicular anisotropy [25], [26], [27] into applications, including high-frequency [15], [28] or biomedicine [29], [30].
In this paper, Fe78Si9B13 thin films displaying dense stripe domains have been prepared, an9d their perpendicular anisotropy controlled by means of suitable thermal treatments. The magnetisation processes have been studied in details both in “normal hysteresis loop” and in “stripes rotation” experiments, the former involving a field-induced magnetisation reversal, and the latter a rotation of the stripes orientation along a new direction at 90 with respect to the initial stripes alignment. Both kind of experiments have been performed with three techniques: vector vibrating sample magnetometry, that allows to simultaneously measure the two in-plane components of the magnetisation at the scale of the whole samples; field-dependent magnetic force microscopy [31], [32], that allows a direct observation of the stripe domains evolution as a function of the applied magnetic field at a local scale; and ferromagnetic resonance, that is sensitive to the perpendicular anisotropy variations and domains configurations [21], [23], [24]. The combined results of the three experimental techniques in the two kind of experiments allowed to establish a direct proportionality between the perpendicular anisotropy field value (measured with “normal hysteresis loops” experiments, and controlled by means of the thermal treatments) and the threshold field (measured with “stripes rotation” experiments) that must be overcome to induce the rotation of the stripes direction. This proportionality has been qualitatively ascribed to the peculiar mechanism involved in the stripes rotation, that forces the perpendicular component of the magnetisation in some regions to flip from downward to upward or viceversa.
Section snippets
Materials and methods
Fe78Si9B13 thin films have been prepared by rf sputtering on Si3N4 substrates from a target made by amorphous ribbons. Details on the preparation procedure are reported elsewhere, together with X-ray diffraction data showing that the as-deposited samples are in an amorphous phase [8]. The samples are 230 nm thick and are characterised by a dense stripe domain configuration, deriving from a weak perpendicular magnetic anisotropy originating from the magnetostrictive properties of the alloy and
Results and discussion
In-plane hysteresis loops of the studied samples are shown in Fig. 2. Fig. 2(a) shows the loops of the sample annealed at 200 °C measured along two arbitrary and different directions. The loops perfectly overlap, indicating that there is no preferred direction for the magnetisation in the film plane. The loops shown are representative of all directions and of all studied samples. This result is confirmed by the x and y components of the magnetisation measured on an hysteresis loop for the
Conclusions
Magnetisation reversal and rotation processes have been studied in Fe78Si9B13 thin films displaying dense stripe domain configuration by means of vibrating sample magnetometry, ferromagnetic resonance and an innovative application of field-dependent magnetic force microscopy. As a function of the annealing temperature, the perpendicular anisotropy responsible for the stripe domains is controlled, affecting the magnetic field at which the saturation is achieved. The samples do not display any
Acknowledgments
The authors would like to thank Maria Gloria Pini for inspiring this work and for the fruitful discussions.
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