Morphological stripe-bubble transition in remanent magnetic domain patterns of Co/Pt multilayer films and its dependence on Co thickness

Abstract

We report a morphological transition in the magnetic domain pattern exhibited by perpendicular anisotropy ferromagnetic [Co/Pt] 50 multilayer films at room temperature and remanence. We found that the remanent magnetic domain morphology and the associated domain density, defined as the number of domains of a given magnetization direction per area, strongly depend on the magnetic history. When the magnitude of the previously applied external field approaches a specific value, typically 75-95% of the saturation field, the magnetic pattern, which generally forms a maze of interconnected stripe domains, decays into a shorter stripe pattern, and the domain density increases. We mapped out this morphological transition as a function of the previously applied field magnitude as well as the Co thickness. We found that a Co thickness close to 30 Å yields the highest domain density with the formation of a pure bubble domain state. Three-dimensional micromagnetic simulations confirm the formation of a pure bubble state in that parameter region and allow an estimation of the perpendicular anisotropy (here 2 × 10 5 J/m 3 for an input magnetization of 1080 kA/m), as well as the interpretation of distinct features of the samples' hysteresis loop based on the corresponding domain pattern.

Control of domain structure and magnetization reversal in thick Co/Pt multilayers

“…11(a) suggests that the morphological stripe-bubble magnetic transition is accompanied by a significant enhancement in the domain density. In (radius of about 75 nm [49]) agrees well with the trend shown in Fig.11 (d). Moreover the sizes of bubble domains found in this work are close to values reported for skyrmions, which were measured in samples having similar magnetization curves [77].…”

“…Sec. III-A) and only N is varied, yielding particularly the same crystalline and interface anisotropy.Indeed, quantitative evaluations of the hysteresis loops, stripe domain periods, as well as micromagnetic simulations, indicate that our whole sample series can be approximately described by the same values of A, MS, and Ku = 2.58 ± 0.28 × 10 5 J/m 3 (see Supplemental Material[63]), yielding Q factors that are constantly smaller than 1 with an average value of Q ̅ = 0.31 ± 0.01.In summary, we see that the applied field orientation β as well as the balance of magnetic energy contributions have a significant impact on the morphology of the remanent domain states: a full demagnetization processes may result in either aligned or randomly distributed stripe domain patterns, by simply changing either β or N. In the next section, we explore the viability of modulating the remanent magnetic domain configuration via minor loop cycling, as suggested in prior studies[48][49][50].…”

“…Thus, we have grown our multilayer films on top of a thick Pt (111) buffer layer such that Co grows with the necessary texture to induce an out-of-plane anisotropy-axis orientation. By studying in detail the influence of the magnetic history on the remanent domain pattern, and after having determined an optimal Co thickness leading to the formation of a bubble state [49], we find an optimal N for which a dense lattice of bubble domains is favored and the bubble density is maximized.…”

“…The specifics of the resulting micromagnetic states are set by the relative strength of the competing interactions, whose ratio in multilayer structures can be tuned by changing for instance the individual layer thicknesses or the number of layer repetitions [17,[45][46][47]. Moreover, while the energy balance is fixed by the material parameters, it was shown by Gao et al [48] that the remanent magnetic domain state configuration can be efficiently manipulated by applying an appropriate magnetic field routine leading to a highly dense remanent bubble domain lattice [49,50].…”

“…Based on the prior knowledge of FM multilayer thin films and our observations of various magnetic domain morphologies depending on magnetic history [49,50], the purpose of this paper is to provide an extensive exploration and description of their magnetization reversal as a function of the energy balance between magnetostatic and anisotropy energies. More importantly, the aim is to determine the existence of a specific energy ratio able to enhance the domain densities at remanence [48,49] and in particular possibly stabilizing dense arrays of magnetic bubbles. This has been carried out by optimizing the total thickness of [Co( 3.0 nm)/Pt( 0.6 nm)]N multilayers by varying the Co/Pt bilayer repetitions N in a range unexplored heretofore, in combination with finely adjusting the magnitude of the previously applied magnetic field.…”