Enhancement of Perpendicular Magnetic Anisotropy in Ultrathin TbFeCo Films via Interface Engineering with Ultrathin Platinum Buffers

Perpendicular magnetic anisotropy (PMA) in rare-earth transition-metal (RE-TM) alloys are important high-density data storage, spintronic devices and high-frequency electronics. Terbium iron cobalt (TbFeCo) alloys are especially noteworthy due to their strong PMA and high coercivity which make them prime candidates for these cutting-edge applications. However, to achieve and maintain strong PMA in ultrathin films of these alloys isn’t straightforward because their magnetic properties are highly sensitive to the film’s thickness and the nature of the materials surrounding them which pose a significant challenge. It is the balance between bulk anisotropy which is inherent to the material itself and surface or interface anisotropy that is influenced by the layers above or below is critical. Additionally, the phenomenon of magnetic compensation where the net magnetic moment cancels out due to the antiferromagnetic coupling between the RE and TM sublattices complicates things further. Indeed, this sensitivity to both the film’s thickness and its interfaces makes it tricky to achieve consistent and reliable magnetic performance in devices.Another difficulty is to ensure that the magnetic properties remain stable and robust under different operating conditions because interfaces within these films can sometimes introduce inconsistencies or reactions that degrade the material’s performance over time. To create reliable spintronic devices, it’s essential to understand these effects and figure out ways to counteract them.To find solutions for these challenges, researchers have been testing various methods to engineer the interfaces of RE-TM films. For example, one promising approach has been inserting ultrathin buffer layers made from platinum (Pt) which are known to influence the magnetic properties of the films. However, the precise impact of these buffer layers on the perpendicular magnetic properties of TbFeCo films remains an area that needs more research.

To this account, a new study published in Applied Surface Science and led by Professor Ke Wang from East China University of Technology and conducted by postgraduate students Zengli Guo, and Guanmei Chen along Dr. Zhihong Lu from Wuhan University of Science and Technology and Dr. Rui Xiong from Wuhan University, the researchers investigated how an ultrathin Pt buffer layer affects the perpendicular magnetic properties of Ta/TbFeCo/Ta layered structures. They studied in detail how Pt layer influences key magnetic parameters like magnetic anisotropy, saturation magnetization, and magnetic compensation in the hope uncover the mechanisms at play and find ways to optimize these materials for better performance in spintronic applications. First, the research team carefully designed a series of tests to study the effects of this ultrathin Pt buffer layer on the perpendicular magnetic properties of the layered structures. They fabricated film stacks using a magnetron sputtering system, with the TbFeCo films’ thickness varying from 4 nm to 48 nm, and the Pt buffer layer ranging from 0.5 nm to 4 nm. This careful control over the film thickness was important for their study because it allowed them to systematically observe how the Pt layer influenced the magnetic properties of the TbFeCo films. Moreover, they used vibrating sample magnetometry and extraordinary Hall effect to measure these properties which helped them map out the magnetic hysteresis loops in both in-plane and out-of-plane directions. The authors found the measurements to be interesting where without a Pt buffer layer, the TbFeCo films only exhibited perpendicular magnetic anisotropy when their thickness was greater than 6 nm. However, once they introduced the Pt buffer layer, PMA was observed even in films as thin as 4 nm which clearly indicated that the Pt layer had a significant effect in enhancing the perpendicular magnetic properties of the films. Furthermore, the authors found that the insertion of the Pt buffer layer also had a noticeable impact on the films’ saturation magnetization (Ms). As they increased the Pt layer’s thickness, Ms initially rose, peaking around 4 nm before starting to decline as the Pt layer got thicker. According to the authors, the pattern observed suggest that the Pt buffer enhanced PMA and also interacted with the TbFeCo layer in more complex ways possibly through spin-orbit coupling or chemical mixing at the interface. Additionally, the effective perpendicular anisotropy constant (Keff) was found to follow a similar trend with its value increasing alongside Ms and peaking at the same Pt thickness which also highlight the important role of the Pt layer in shaping the films’ magnetic properties. The researchers also reported the thermomagnetic measurements which added another layer of understanding by tracking how the magnetic properties change with temperature and showed the compensation temperature (Tcomp) which is a key indicator of the balance between the magnetic moments of Tb and FeCo—dropped sharply when they introduced Pt buffer. Initially, with just a thin 0.5 nm layer of Pt, Tcomp fell strongly whichmeans a significant weakening of the exchange interaction between the Tb and FeCo atoms at the interface,  however, as the Pt layer thickened to around 4 nm, Tcomp started to rise again, which suggest that the exchange interaction was recovering, possibly because of better magnetic ordering at the interface. Beyond this point, Tcomp began to decrease once more, indicating that there is a threshold beyond which the Pt layer no longer effectively supports the desired magnetic interaction.

Overall, the study of Professor Ke Wang and his colleagues provided compelling evidence that if you insert an ultrathin Pt buffer layer it will significantly alter the perpendicular magnetic properties of Ta/TbFeCo/Ta structures. The findings also showed that the Pt buffer enhances PMA especially in ultrathin films and that the magnetic properties are highly sensitive to the thickness of both the Pt buffer and the TbFeCo filmand such results emphasize the importance of precise interface engineering in developing spintronic devices and offer valuable insights into how buffer layers can be used to optimize the performance of RE-TM alloys. One of the key takeaways is the potential to create more efficient and reliable spintronic devices and the ability to maintain strong PMA in ultrathin films opens up new design possibilities for these devices which allows further miniaturization without compromising performance. In addition, the study also provides valuable data into how the Pt buffer layer modulates magnetic properties through interface engineering which provide a strategic approach to optimize the magnetic behavior of RE-TM alloys. “Our results show the interface plays an important role in tuning the perpendicular magnetic properties of ultrathin TbFeCo alloy films, which is critical for the application in spintronic devices,” Prof. Ke Wang says.