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Understanding how intrinsic composition gradients during sputter growth modify perpendicular magnetic anisotropy (PMA) of transition metal/rare earth ferrimagnetic alloys remains a key challenge for spintronic device applications. In this work, we intentionally engineer ferrimagnetic CoTb thin films with composition gradients by varying the sputter target power during growth, enabling a systematic investigation of the role of composition gradients on coercivity and magnetic anisotropy. We find that reducing the rare earth element deposition power at the top interface significantly enhances the anisotropy strength and coercivity compared to nominally uniform films, with coercivity enhancements exceeding 100 mT. These results are counter to atomistic spin simulations, suggesting reduced anisotropy in compositionally graded systems. This contradiction is resolved through x-ray photoelectron spectroscopy depth profiling, which reveals that this enhancement arises from the compensation of an intrinsic Cobalt deficiency at the capping interface, which creates an asymmetry between the top and bottom film boundaries. By controlling the sputter power during deposition, the intrinsic composition gradient can be compensated, enabling adjustments of PMA and improving the performance of the ferrimagnetic thin films for spintronic devices.
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Direct measurement of reduced exchange stiffness and its impact on magnetic vortex behavior in PyGd alloys
Thin films composed of sputtered transition metal/rare earth (TM/RE) ferrimagnets have emerged as promising building blocks for future spintronic devices, offering tunable magnetic properties critical for data storage, memory, and logic applications. However, understanding how the combination of TM and RE elements influences effective magnetic properties, such as exchange stiffness (Aex), remains challenging. Magnetic vortices provide a versatile tool for probing these properties in thin film systems. By combining magnetic imaging via soft x-ray microscopy and micromagnetic modeling, we quantify the effective exchange stiffness in PyGd ferrimagnetic disks with varying Gd concentrations. Our results indicate a reduction in Aex to below 3 pJ/m for a 20% Gd concentration when compared to reference Py, and values below 2 pJ/m for 30% Gd, reflecting weak Ni–Gd exchange coupling. These findings highlight the critical role of rare earth content in tuning the exchange stiffness. The reduced exchange stiffness facilitates a linear field response of the magnetization up to the edge of the disk, as well as significant deformations in the vortex core itself when compared to films with larger Aex. Our results are in line with, albeit lower than, recent measurements of the exchange stiffness in intermixed PyGd. This reduced exchange stiffness has implications for the development of spintronic devices based on ferrimagnetic skyrmions.
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- Award ID(s):
- 2138271
- PAR ID:
- 10584580
- Publisher / Repository:
- Applied Physics Letters
- Date Published:
- Journal Name:
- Applied Physics Letters
- Volume:
- 126
- Issue:
- 16
- ISSN:
- 0003-6951
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1063/5.0256674
