Beschreibung
The exchange coupling in bimagnetic core-shell nanoparticles is a promising pathway to permanent magnetic materials [1]. For iron oxide core-shell nanoparticles, consisting of a wuestite-like particle core and a spinel-type shell, transition metal doping was recently shown to significantly enhance the magnetic anisotropy and exchange coupling [2]. Native iron oxide core-shell nanoparticles synthesized by thermal decomposition of iron oleate typically form as an intermediate through topotaxial oxidation of an initial wuestite phase towards highly defective maghemite [3]. We have recently reported how the combination of such native core-shell nanoparticles (with their alignment of core and shell phases) and cobalt doping leads to a significant enhancement of the exchange pinning between both phases, which is promising for a rational synthesis of nanoparticles with strong coercivity and exchange field. Using magnetic SANS [4,5], we have unambiguously revealed a significant net magnetization even in the wuestite-type nanoparticle core that is commonly presumed antiferromagnetic or paramagnetic at room temperature [6].
In this contribution, we will present the systematic influence of a subtle variation in particle size on the exchange coupling within such native core-shell, Co-doped iron oxide nanoparticles. For freshly synthesized samples with a particle diameter ranging from 8.5 to 9.6 nm, a clear transition from exchange spring to exchange bias behavior is evident. We employ magnetic SANS to elucidate the intraparticle magnetization individually for the wuestite-like particle core and the spinel-type shell and to follow their coupling mechanism.
References:
[1] A. López-Ortega et. al., Phys. Rep. 553, 1−32 (2015).
[2] B. Muzzi et. al., Small 18, 2107426 (2022).
[3] E. Wetterskog et al., ACS Nano 7, 7132–7144 (2013).
[4] S. Mühlbauer, S. Disch, A. Michels et al., Rev. Mod. Phys. 91, 015004 (2019).
[5] D. Zákutná, S. Disch et al., Phys. Rev. X 10, 031019 (2020).
[6] D. Zákutná, N. Rouzbeh, S. Disch et al., Chem. Mater. 35, 2302–2311 (2023).
