Effects of Pd- and Ta-buffer layer on magnetic and interfacial perpendicular properties of sputtered Co2FeSi/MgO heterostructures

Perpendicular magnetic anisotropy (PMA) is a crucial property of magnetic films. Strong PMA has been demonstrated in ultra-thin multilayers, rare-earth transition metal alloy films as well as ordered Co(Fe)-based alloys. To achieve high spin polarization and low damping desired for various practical applications, Co-based Heusler alloys have been identified as promising ferromagnetic electrode materials for magnetoelectronic devices. Among them, Co₂FeSi (CFS) has particularly attracted significant research attention owing to its low damping, high Curie temperature,and large saturation magnetization properties. Additionally, CFS is easy to fabricate by sputtering and pulsed laser deposition techniques. On the other hand, recent research revealed that PMA could be built by sandwiching most of the Co-base Heusler alloys, such as Co₂Fe(Mn.Al)Si and Co₂(Fe.Mn)Al, by MgO and heavy metals. Despite the potential applications of the CFS/MgO heterostructures, very little research has been conducted to investigate the effects of heavy metal buffers on their magnetic properties.

To this note, a research group led by Professor Ke Wang at the East China University of Science and Technology investigated the effects of Pd and Ta buffer layers on the magnetic and interfacial perpendicular magnetic properties of the CFS/MgO heterostructures. First, they fabricated a series of Co₂FeSi/MgO heterostructures with Pd and Ta buffers using the sputtering method at room temperature. The thickness of the stack layers was varied by controlling the deposition time. The magnetic properties of the annealed samples were characterized by vibrating sample magnetometer along in-plane and out-of-plane field directions. Their research work is currently published in the journal, Surface Engineering.

The research team found that the Pd and Ta buffers exhibited considerable influence on the magnetic properties of the fabricated heterostructures. Both magnetic dead layer (MDL) and saturation magnetization (Ms) thickness were observed to increase with the annealing temperature due to the improved structural order of the alloy and the interdiffusion at the interfaces, respectively. For instance, PMA was more pronounced in the heterostructure with CFS thickness in the range of 2.5 – 5 nm. However, the Ta-buffered heterostructure reported a thicker magnetic dead layer and lower saturation magnetization compared to that buffered with Pd because Ta easily mixed with the CFS at the interface. In contrast, large saturation magnetization demonstrated in the Pd-buffered heterostructure was attributed to the alloying effect at the bottom interface. Furthermore, it was worth noting that PMA was identified to originate from the heterostructure interface. ” Heavy metal/Co₂FeSi/MgO structure is of great importance for practical applications in spin transfer torque magnetic random access memory (STT-MRAM) and magnetic sensors. Dedicated experiments were committed on understanding the effects of heavy metal buffers of Pd and Ta on magnetic properties of the Co₂FeSi/MgO heterostructures. We aim to provide guidance for design and integration of next-generation CFS/MgO-heterostructure-based spintronic devices,” Prof. Ke Wang explains.

In summary, the study investigated the magnetic properties of CFS/MgO heterostructures annealed with Pd and Ta buffers. Results showed that both of the two metal buffers demonstrated a perpendicular magnetic anisotropy with an energy density of about. Moreover, the perpendicular magnetic anisotropy was identified to originate from the heterostructure interface, as revealed in the literature. Altogether, the authors are confident that their study will pave the way for the design and development of next-generation spintronic devices based on CFS/MgO heterostructures.