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Formation of strong L1₀-FePd/α-Fe nanocomposite magnets by visualizing efficient exchange coupling.

ABSTRACT: Conceptual nanocomposite magnets (NCMs) composed of exchange-coupled hard/soft magnetic phases have been expected to show excellent magnetic performance based on simultaneous high coercivity (H _c) and high saturation magnetization (M _s). In our previous works, however, the H _c was considerably lower than its theoretical value (H _a), which prevented us from improving the performance of NCMs. Here, we show that the H _c of isolated particulate L1₀-FePd/α-Fe NCMs is dominated by their phase segregation into core/shell-like structures versus Janus-like structures. Using first-order reversal curve (FORC) analysis, we clearly distinguished a microscopically undetectable difference in the phase-segregation structure in the NCMs, finding both efficient and inefficient exchange coupling. The nanostructurally controlled NCMs dominated by core/shell-like structure with efficient exchange coupling showed the largest energy product ((BH)_max = 17.5 MGOe) in the Fe-Pd system and the highest H _c/H _a value (26.5%) among all NCM powders.

SUBMITTER: Matsumoto K

PROVIDER: S-EPMC9419099 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature

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Formation of strong L10-FePd/α-Fe nanocomposite magnets by visualizing efficient exchange coupling.

Matsumoto Kenshi K Sato Ryota R Trinh Thang Thuy TT Sakuma Noritsugu N Shoji Tetsuya T Haruta Mitsutaka M Kurata Hiroki H Teranishi Toshiharu T

Nanoscale advances 20190520 7

Conceptual nanocomposite magnets (NCMs) composed of exchange-coupled hard/soft magnetic phases have been expected to show excellent magnetic performance based on simultaneous high coercivity (H c) and high saturation magnetization (M s). In our previous works, however, the H c was considerably lower than its theoretical value (H a), which prevented us from improving the performance of NCMs. Here, we show that the H <s ...[more]

PMID: 36132735

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Project description:A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of L10-ordering, and the outermost Fe bonds to the carbon of Gr at the interface. When Gr is grown by CVD, the crystal orientation of hexagonal Gr toward tetragonal L10-FePd selects an energetically stable structure based on the van der Waals (vdW) force. The atomic relationship of Gr/L10-FePd, which is an energetically stable interface, was unveiled theoretically and experimentally. The Gr armchair axis was parallel to FePd [100]L10, where Gr was under a small strain by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the distance was theoretically and experimentally determined to be approximately 0.2 nm. This shorter distance (≈0.2 nm) can be explained by the chemisorption-type vdW force of strong orbital hybridization, rather than the longer distance (≈0.38 nm) of the physisorption-type vdW force. Notably, depth-resolved X-ray magnetic circular dichroism analyses revealed that the orbital magnetic moment (Ml) of Fe in FePd emerged at the Gr/FePd interface (@inner FePd: Ml = 0.16 μB → @Gr/FePd interface: Ml = 0.32 μB). This interfacially enhanced Ml showed obvious anisotropy in the perpendicular direction, which contributed to interfacial perpendicular magnetic anisotropy (IPMA). Moreover, the interfacially enhanced Ml and interfacially enhanced electron density exhibited robustness. It is considered that the shortening of the interatomic distance produces a robust high electron density at the interface, resulting in a chemisorption-type vdW force and orbital hybridization. Eventually, the robust interfacial anisotropic Ml emerged at the crystallographically heterogeneous Gr/L10-FePd interface. From a practical viewpoint, IPMA is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. A micromagnetic simulation assuming both PMA and IPMA predicted that perpendicularly magnetized magnetic tunnel junctions (p-MTJs) using Gr/L10-FePd could realize 10-year data retention in a small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively. We unveiled the energetically stable atomic structure in the crystallographically heterogeneous interface, discovered the emergence of the robust IPMA, and predicted that the Gr/L10-FePd p-MTJ is significant for high-density X nm generation magnetic random-access memory (MRAM) applications.

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Formation of strong L1₀-FePd/α-Fe nanocomposite magnets by visualizing efficient exchange coupling.

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Formation of strong <i>L</i>1<sub>0</sub>-FePd/α-Fe nanocomposite magnets by visualizing efficient exchange coupling.

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