A practical method for fabricating superparamagnetic films and the mechanism involved

Pei Cheng Jiang, Cheng Hsun Tony Chang, Chen Yuan Hsieh, Wei Bin Su, Jyh Shen Tsay

Research output: Contribution to journalArticle

Abstract

Due to the widespread applications of biosensors, such as in magnetic resonance imaging, cancer detection and drug delivery, the use of superparamagnetic materials for preparing biosensors has increased greatly. We report herein on a strategy toward fabrication of a nanoscale biosensor composed of superparamagnetic films. On increasing the film thickness of magnetic layers, a phase transition typically occurs from either a low-Curie-temperature state or a superparamagnetic state to a ferromagnetic state. A new finding is demonstrated wherein a phase transition of such a superparamagnetic phase can be induced by controlling the thickness of ultrathin ferromagnetic layers with perpendicular magnetic anisotropy. Both the M-H curve with zero coercive force at 300 K and deviations of the normalized hysteresis loop at 2 K confirm the superparamagnetic state of Co/Ir(111) at room temperature. An overstrained film transforming into clusters (OFTC) model based on the new finding and our experimental evidence is proposed for modeling this phenomenon. From the energetic point of view of the OFTC model, we propose a limited distortion mechanism that can be useful in determining the critical thickness for the phase transition. This mechanism considers the balance between interfacial strain energy and surface free energy. A method for producing superparamagnetic films by taking advantage of the accumulation of strain and relaxation is reported.

Original languageEnglish
Pages (from-to)14096-14105
Number of pages10
JournalNanoscale
Volume12
Issue number26
DOIs
Publication statusPublished - 2020 Jul 14

ASJC Scopus subject areas

  • Materials Science(all)

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