Materials for nanoscale spatial control of ferromagnetic phase transitions*

Multilayer structures have revealed an impressive variety of collective properties, and they represent an established approach to materials design with widespread applications [1]. One example are exchange spring systems, in which low-moment magnetically hard and high-moment magnetically soft layers are strongly coupled [2], a concept that was further advanced by utilizing a magnetic anisotropy gradient along the thickness [3]. Subsequent experimental efforts have led to the discovery that compositional grading can also be used to fabricate all-ferromagnetic films that exhibit a magnetic behavior dominated by local Curie temperatures representing the local exchange coupling strength only [4]. Such materials were demonstrated to exhibit a quasi-phase-boundary between strongly and weakly magnetized regions that can be continuously and reversibly moved along the depth of the films, which in turn allows a temperature-dependent (de-)correlation of the magnetic state and reversal behavior [5]. Correspondingly, phase transitions in such graded compositional materials may be more complex than in usual ferromagnets. Due to the exchange strength profile, the resulting magnetic state along the thickness may exhibit a temperature dependent confinement in contrast to bulk-like conventional magnetic systems, and thus lead to significantly altered critical behavior if compared to conventional scaling and universality [6].
[1] O. Hellwig, et al., Nat. Mat. 2, 112 (2003).
[2] E. Kneller, et al., IEEE Trans. Magn. 27, 3588 (1991).
[3] D. Suess, et al., Appl. Phys. Lett. 87, 012504 (2005).
[4] B. J. Kirby, et al., Phys. Rev. Lett. 116, 047203 (2016).
[5] L. Fallarino, et al., Materials 11, 251 (2018).
[6] L. Fallarino, et al., Nano-scale design of critical exponents in prep.