Magnetic anisotropy, interlayer coupling and Dzyaloshinskii-Moriya interaction in epitaxial W/Co/Pt multilayers

Abstract

Systematic studies of magnetic properties observed in epitaxial W/Co/Pt layered structures grown by molecular beam epitaxy are discussed. In this system contributions to interfacial Dzyaloshinskii-Moriya interaction (DMI) from the asymmetric interfaces between ferromagnet (Co) and non-magnetic heavy metals with high spin-orbit coupling (W or Pt) are of additive nature. DMI can stabilize the chiral magnetic texture, vortex and skyrmions, being promising in future data storage technology.Presented progressive investigations encompass layered structures containing: a single Co layer (magnetic anisotropy), two Co layers (interlayer coupling) and multilayers (dipolar interactions, complex domain structure).Layered structures containing a single Co layer exhibit spin reorientation transition (SRT) with increasing thickness, dCo,from perpendicular (perpendicular magnetic anisotropy, PMA) to in-plane aligned magnetisation. The range of dCo with PMA and the critical thickness dSRT depend on the type and sequence of adjacent covers. In the Pt/W/Co/Pt system dCo range with PMA shifts towards higher values (from 0.6-0.9 nm to 2.1-2.4 nm) with increasing bottom W layer thickness, dW, from 2.4 nm to 8.0 nm. This behaviour differs from the similar system with Mo component leyer, studied earlier [1]. Hysteresis loops measured by polar magnetooptical Kerr effect (PMOKE) magnetometry exhibit shapes typical of fine domain structure with perpendicular magnetisation.In the W/Co(1.0 nm)/Pt(dPt)/W(dW)/Co(1.0 nm)/Pt interlayer antiferromagnetic coupling of perpendicular magnetization with the strength as high as 0.6 kOe is very distinct for the component spacers of equal thickness, dPt = dW, in the range between 0.7 and 1.1 nm. For dW ≠ dPt (keeping the same total thickness dW + dPt) the coupling strength decreases. Particularly, the single spacer systems (Pt or W; dPt or dW kept in the corresponding range from 1.4 nm to 2.0 nm) do not exhibit such feature, indicating a crucial role of the complex spacer structure affecting both coupling and PMA (Figure 1).The multilayer structures W/Co(0.6 < dCo < 1.0 nm)/Pt (10-20 repetitions) are magnetised in perpendicular direction to the film plane and display at the remanent state typical labyrinth domain structure with tendency towards bubble domains formation with diameter of around 75 nm (Figure 2). DMI strength determined form (aligned) domain structure reaches the value of D = 2.49 mJ/m2 [2]. Such high value of D is a result of high quality crystalline structure characteristic for epitaxial systems and additive contribution to DMI from asymmetric well-defined interfaces.The above discussed structures are compared to those with different component layer thickness and opposite sequence (i.e. Pt/Co/W).This work is supported by IEEE educational seed funding 2020 (beneficiary:-Sukanta Kumar Jena), Foundation for Polish Science (FNP) under the European Regional Development Fund – Program REINTEGRATION 2017 OPIE 14-20 and by the National Science Centre in Poland, projects no: 2016/23/G/ST3/04196 and 2020/37/B/ST5/02299. **