We present the first ferromagnetically-coupled Er3+ complex with linked, high-anisotropy Er–COT (COT2− = cyclooctatetraene dianion) subunits. The dinuclear complex, [Er(μ2-Cl)(COT)(THF)]2, demonstrates single-molecule magnetism with a single, zero-field magnetization relaxation barrier of Ueff = 113 cm−1. This system offers evidence that anisotropy can be preserved in the presence of ferromagnetic linking of the Er–COT subunits, providing a rational means to build strong molecular magnets of tunable dimensionality.

•Influence of interparticle distance on fcc FePt magnetic properties is analyzed.•Superparamagnetic blocking temperature and coercivity are relatively unaffected by particle dilution.•Indicators of fast-time-scale dynamics are strongly affected by interparticle distance.Magnetic characterization of nanoscale materials is often hindered by the role that sample preparation techniques play in the determination of interparticle interaction strength. Well-dispersed d = 2.6(4) nm FePt nanoparticles synthesized by a slight modification of a known polyol synthesis were employed to study this effect at the extreme lower limit of the nanoscale regime. Suspension in a diamagnetic matrix material at varying concentrations was used to characterize the relationships between average particle distance and representative magnetic properties (zero-field cooled/field-cooled (ZFC/FC) magnetization curves, hysteresis M(H) loops at 5 K, and ac-susceptibility). By increasing the interparticle distance through diamagnetic dilution, the blocking temperature (TB), anisotropy energy barrier (Ueff), and coercive field (Hc) drop continuously until reaching a dilution ratio where the magnetic signals were limited by the competing diamagnetic contribution from the matrix material. Long-timescale blocking temperature and coercivity are relatively unaffected by particle dilution while the shorter timescale anisotropy energy barrier (Ueff) and attempt time (τ0) are strongly affected. These results demonstrate the need for well-defined sample preparation conditions when comparing materials properties, especially those with applications dependent on superparamagnetism.The effects of interparticle distance on a battery of magnetically important properties were studied by dilution of d = 2.6(4) nm FePt nanoparticles in an n-eicosane matrix at different concentrations. Zero-field cooled/field-cooled (ZFC/FC) magnetization curves, magnetization vs. magnetic field, and ac-susceptibility indicate that while the long-timescale blocking temperature and coercivity are relatively unaffected by particle dilution, the anisotropy energy barrier (Ueff) and attempt time (τ0) are strongly affected. These results demonstrate how long-range interparticle interactions can interfere with estimations of important superparamagnetic properties.Download high-res image (203KB)Download full-size image

We describe a synthetic method for increasing and controlling the iron loading of synthetic melanin nanoparticles and use the resulting materials to perform a systematic quantitative investigation on their structure–property relationship. A comprehensive analysis by magnetometry, electron paramagnetic resonance, and nuclear magnetic relaxation dispersion reveals the complexities of their magnetic behavior and how these intraparticle magnetic interactions manifest in useful material properties such as their performance as MRI contrast agents. This analysis allows predictions of the optimal iron loading through a quantitative modeling of antiferromagnetic coupling that arises from proximal iron ions. This study provides a detailed understanding of this complex class of synthetic biomaterials and gives insight into interactions and structures prevalent in naturally occurring melanins.Keywords: antiferromagnetic coupling; magnetometry; MRI; polymerization; synthetic melanin

We present the first ferromagnetically-coupled Er3+ complex with linked, high-anisotropy Er–COT (COT2− = cyclooctatetraene dianion) subunits. The dinuclear complex, [Er(μ2-Cl)(COT)(THF)]2, demonstrates single-molecule magnetism with a single, zero-field magnetization relaxation barrier of Ueff = 113 cm−1. This system offers evidence that anisotropy can be preserved in the presence of ferromagnetic linking of the Er–COT subunits, providing a rational means to build strong molecular magnets of tunable dimensionality.