Magnetic multilayers with perpendicular magnetic anisotropy (PMA) have garnered significant attention in the field of spintronics due to their potential applications in devices such as magnetic random access memory and logic memory. Strong PMA enhances thermal stability and reduces switching current, making these materials highly desirable for next-generation spintronic devices. Traditional magnetic materials have limitations in terms of spin polarization, prompting researchers to explore novel materials with higher spin polarization and robust PMA. One such candidate for spintronic applications is the Co2MnSi (CMS) alloy, which exhibits high spin polarization, a large bandgap, and low damping. However, individual CMS alloy films do not exhibit PMA on their own. To induce PMA in CMS films, they need to be sandwiched between oxide and heavy metal layers, such as Pt or Pd. This interfacial PMA arises from the orbital hybridization at the interfaces of CMS/oxide and CMS/heavy metal layers. In a recent study led by Prof. Ke Wang from the East China University of Technology together with Prof. Zhihong Lu from Wuhan University of Science and Technology, and Prof. Rui Xiong from Wuhan University, developed a novel approach to engineer perpendicular magnetic properties in Pt/CMS multilayered structures. Specifically, high-K dielectric Ta2O5 was used as a capping layer in these multilayers. Ta2O5 is known for its excellent chemical stability at high temperatures and has applications in microelectronics, thin-film capacitors, optical coatings, and corrosion-resistant protective coatings.
The researchers fabricated [Pt/CMS]n/Ta2O5 stacks on Si (100) substrates. Ta layers were pre-sputtered as a buffer, followed by the deposition of Pt/CMS multilayer structures with varying thicknesses and repetition numbers. The sputtering process allowed precise control over the stack parameters. X-ray diffractometry was used to characterize the stack’s structure, while magnetization properties were measured using a vibrating sample magnetometer. The extraordinary Hall effect was employed to study the magnetic behavior of the samples.
The research team observed that strong PMA was achieved in the Pt/CMS multilayered structures with ultrathin CMS thickness in the range of 0.6~0.8 nm. Furthermore, when the repetition number (n) of Pt/CMS bilayers exceeded two, strong PMA was demonstrated, which was attributed to enhanced exchange coupling between CMS layers. They also systematically studied the influence of Ta2O5 capping layer thickness. As the thickness of Ta2O5 increased, reduced coercivity and saturation magnetization were observed. This effect was primarily attributed to oxidation at the top interface between Ta2O5 and the multilayer stack. The researchers also calculated an interfacial perpendicular anisotropy energy density (Ks) of approximately 0.52 erg/cm2, which indicated the presence of strong PMA. The contribution of the Pt/CMS interface was found to be dominant, with the CMS/Ta2O5 interface contributing weakly. Intriguingly, the study revealed that for stacks with n ≥ 6 repetitions, a transition occurred, resulting in a multi-domain structure at remanence. This transition from single domain to multi-domain structure was attributed to the increasing thickness of the stack, favoring a maze-like remnant configuration.
In conclusion, this study offers valuable insights into the engineering of perpendicular magnetic properties in Pt/CMS multilayered structures with high-K dielectric Ta2O5 capping. Strong PMA was observed with ultrathin CMS thickness and for repetition numbers greater than two. The research also highlighted the impact of Ta2O5 capping layer thickness on the magnetic properties, with reduced coercivity and saturation magnetization due to oxidation effects. The findings of Professor Ke Wang and colleagues contribute to the development of materials for perpendicular spintronic devices with low energy consumption. The use of high-K dielectrics like Ta2O5 in multilayered structures opens up new possibilities for enhancing spintronic applications, ultimately driving advances in memory and logic devices. “This is the first report of perpendicular magnetic properties in Pt/CMS multilayered structures with Ta2O5 dielectric. Our findings show perpendicular magnetic properties of [Pt/CMS]n/Ta2O5 stacks may be well engineered for perpendicular spintronic devices with low consumption. “, Prof. Ke Wang comments.
Ke Wang, Zhenxiao Nie, Yalin Wang, Zhihong Lu, Rui Xiong, Engineering perpendicular magnetic properties of Pt/Co2MnSi multilayered structures capped with dielectric Ta2O5, Surfaces and Interfaces, Volume 37, 2023, 102690,