Skyrmion states in multilayer exchange coupled ferromagnetic nanostructures with distinct anisotropy directions

Abstract

We report the experimental observation of magnetic skyrmion-like states in patterned ferromagnetic nanostructures consisting of perpendicular magnetized Co/Pt multilayer film exchange coupled with Co nanodisks in vortex state. The magnetic force microscopy and micromagnetic simulations show that depending on the magnitude of Co/Pt perpendicular anisotropy in these systems two different modes of skyrmion formation are realized.Magnetic skyrmion is a localized spin configuration demonstrating unusual topological and transport properties [1]. This state was predicted theoretically as the effect of Dzjaloshinskii-Moriya interaction in crystals without an inversion center [2][3][4]. Experimentally the skyrmion lattices were observed in some crystals (MnSi, FeGe and other) at low temperatures [5][6][7]. Now one of actual problem is to expand the class of magnetic materials suitable for realization of skyrmions stable at room temperature. Recently the formation of skyrmion-like states induced by magnetic vortex in artificial ferromagnetic nanostructures was considered theoretically in Ref. [8].In current letter we present the experimental realization of skyrmion-like states in Co/Pt multilayer films exchange coupled with Co nanodisks (Co/Pt-Co disk nanostructures).The initial thin film structures [Co (0.5 nm) / Pt (1 nm)] 5 (denoted further as Co/Pt) and [Co (0.5 nm) / Pt (1 nm)] 5 / Co (denoted further as Co/Pt-Co) was grown by DC magnetron sputtering on Si substrate with Ta (10 nm) / Pt (10 nm) buffer layer. The magnetic properties of Co/Pt and Co/Pt-Co thin film structures were investigated by magneto-optical Kerr effect (MOKE) and ferromagnetic resonance (FMR) methods. In experiments we used two Co/Pt structures differing by coercivity. First structure (structure I) had the coercive field H cI = 130 Oe and saturation field H sI = 210 Oe, while the second one (structure II) had coercive field H cII = 180 Oe and saturation field H sII = 300 Oe. The corresponding hysteresis curves are presented in Fig. 1a.Covering Co/Pt structures by Co layer with thickness Co t < 1.5 nm led to narrowing hysteresis loop and for Co t > 1.5 nm we registered the anhysteretic magnetization curves. For example, normalized MOKE remagnetization loop for the structure I covered by 20 nm Co layer is presented in Fig. 1b. The FMR measurements showed that Co/Pt-Co thin film structures consist of two coupled effective oscillators Co/Pt (easy axis anisotropy) and Co (easy plane anisotropy) with surface energy of exchange interaction J = 1.9 × 10 -3 J/m 2 and we believe that anhysteretic behavior of these structures with Co t >1.5 nm shows that easy plane anisotropy is dominant.Removing Co coating layer by ion etching with ion energy 200 eV does not destroy Co/Pt multilayer structure. The hysteresis loop for Co/Pt multilayer film after 20 nm Co coating removal is presented in Fig. 1c. The magnetic states and the magnetization reversal effects in these nanostructures were studied using a vacuum multimode magnetic force microscope (MFM)

Interlayer interaction in multilayer [Co/Pt]n/Pt/Co structures

“…1) were grown by dc magnetron sputtering on a Si substrate with Ta(10 nm) and Pt(10 nm) underlayers 20 . In addition, a multilayer [Co(0.9 nm)Pt(1.5 nm)] 5 structure and a Co(10 nm) film was grown separately on similar substrates.…”

“…Over the last few years certain attention has been focused on the properties of exchange coupled systems with distinct anisotropy directions [9][10][11][12][13][14][15][16][17][18][19][20] . In particular, CoPt/F structures that consist of CoPt multilayer film with "easy axis" anisotropy and ferromagnetic layer (F) with "easy plane" anisotropy have been proposed to control hysteresis loop shift (exchange bias) of magnetic system [12][13][14] .…”

“…In particular, CoPt/F structures that consist of CoPt multilayer film with "easy axis" anisotropy and ferromagnetic layer (F) with "easy plane" anisotropy have been proposed to control hysteresis loop shift (exchange bias) of magnetic system [12][13][14] . Besides, patterned CoPt/F structures were recently used to create artificial magnetic skyrmion states [15][16][17][18][19][20][21] . Note that the effects of exchange bias and skyrmion nucleation are based on the existence of strong exchange interaction both between Co layers in CoPt film and between CoPt film and ferromagnetic layer F. However, as far as we know, there are no direct experimental studies of exchange coupling in such structures.…”

Magnetic Force Microscopy of Nanostructured Co/Pt Multilayer Films with Perpendicular Magnetization

“…Additional possibilities are opened with the use of magnetic films with perpendicular (easy-axis) anisotropy. The fabrication of nanostructures based on multilayer systems consisting of films with easy-axis and easy-plane anisotropy enables the realization of structures with essentially noncollinear magnetization distribution [ 8 , 9 , 10 ] for magnetic devices with vertical architectures. Additionally, in recent years, new approaches to modifying the properties of magnetic films by focused ion beams have been developed [ 11 , 12 , 13 , 14 , 15 ].…”

Magnetic properties of ultrathin discontinuous Co/Pt multilayers: Comparison with short-range ordered and isotropicCoPt3films

“…Since the first reports on perpendicular magnetic anisotropy in (Co/Pt) multilayers 1,2 , this system has attracted an uninterrupted interest. The literature spans an extremely broad range of topics including interface effects 3,4 , perpendicular magnetic anisotropy [5][6][7][8][9] , domain pattern and magnetization reversal [10][11][12][13][14][15][16] , magnetooptical properties [17][18][19][20] , interlayer coupling 21,22 , magnetotransport properties [23][24][25] , exchange biasing 26,27 , spin injection 28 , spin-polarization 29,30 , spin-torque 31,32 , magnetic recording [33][34][35] , current-driven domain walls 36,37 , and skyrmions [38][39][40][41][42] .…”