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.