Tuning the core–shell morphology of bimagnetic nanoparticles and its associated exchange bias behavior is a promising way to overcome the superparamagnetic limit and stabilize the particle moment in extended time and temperature ranges. The intraparticle magnetization distribution and magnetic coupling between the two phases, however, is still unclear. We report a significant nonzero magnetization in the Co x Fe(1–x)O core of native core–shell bimagnetic nanoparticles that is typically considered antiferro- or paramagnetic. Co0.14Fe0.86O@Co0.4Fe2.4O4 (6 nm@2 nm) and Co0.08Fe0.92O@Co0.58Fe2.28O4 (12 nm@2 nm) core–shell nanoparticles have been synthesized by thermal decomposition of a mixed cobalt–iron oleate with a similar Fe/Co distribution throughout the nanoparticle. We determine the exact phase composition and the magnetization distribution in the core and shell using a combination of X-ray and neutron small-angle scattering. Core and shell magnetization are traced separately with a varying magnetic field. Our results reveal that the magnetization of the core and the spinel-type shell phases are coupled at room temperature, i.e., rotating coherently with the magnetic field. This is a mandatory condition to observe a significant exchange bias effect at low temperatures. These findings highlight the enormous potential of finite size and exchange coupling in bimagnetic nanoparticles to control the magnetic properties via interface-induced magnetization.

Two isomeric 2-bromomethylpyridine Cu(II) complexes [Cu(C6H9NBr)2(NO3)2] with 2-bromo-5-methylpyridine (L1) and 2-bromo-4-methylpyridine (L2) were synthesized as air-stable blue materials in good yields. The crystal structures were different with [Cu(L1)2(NO3)2] (CuL1) crystallizing in the monoclinic space group P21/c, while the 4-methyl derivative CuL2 was solved and refined in triclinic P1¯. The orientation of the Br substituents in the molecular structure (anti (CuL1) vs. syn (CuL2) conformations) and the geometry around Cu(II) in an overall 4 + 2 distorted coordination was very different with two secondary (axially elongated) Cu–O bonds on each side of the CuN2O2 basal plane in CuL1 or both on one side in CuL2. The two Br substituents in CuL2 come quite close to the Cu(II) centers and to each other (Br⋯Br ~3.7 Å). Regardless of these differences, the thermal behavior (TG/DTA) of both materials is very similar with decomposition starting at around 160 °C and CuO as the final product. In contrast to this, FT-IR and Raman frequencies are markedly different for the two isomers and the UV–vis absorption spectra in solution show marked differences in the π–π* absorptions at 263 (CuL2) or 270 (CuL1) nm and in the ligand-to-metal charge transfer bands at around 320 nm which are pronounced for CuL1 with the higher symmetry at the Cu(II) center, but very weak for CuL2. The T-dependent susceptibility measurements also show very similar results (µeff = 1.98 µB for CuL1 and 2.00 µB for CuL2 and very small Curie–Weiss constants of about −1. The EPR spectra of both complexes show axial symmetry, very similar averaged g values of 2.123 and 2.125, respectively, and no hyper-fine splitting.

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