Research paper Role of magnetic concentration in modulating the magnetic properties of ultra-small FePt nanoparticles Pius O. Adelani, Aaron N. Duke, Benjamin H. Zhou, Jeffrey D. Rinehart ⇑ Contribution from the Department of Chemistry and Biochemistry, University of California, San Diego, San Diego, CA 92093, United States article info Article history: Received 5 August 2016 Received in revised form 8 September 2016 Accepted 10 September 2016 Available online xxxx Keywords: FePt nanoparticles Superparamagnetism Magnetic blocking Polyol synthesis Surface modification abstract 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 (T B ), anisotropy energy barrier (U eff ), and coer- cive field (H c ) 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 tempera- ture and coercivity are relatively unaffected by particle dilution while the shorter timescale anisotropy energy barrier (U eff ) and attempt time (s 0 ) 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. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction The continuous demand for higher density magnetic and spin- tronic device architectures has pushed magnetic nanomaterials design to the edge of the molecular regime (1–10 nm) [1,2]. As in semiconductor nanoparticles, size distribution, morphology, and surface structure can play an outsized role in this size range. Importantly, however, the magnetic characteristics in this regime can be far more dependent on interparticle interactions, which can compete with or even outweigh the effects of intrinsic proper- ties [3]. This means that material characteristics can vary wildly based on individual preparations of a material – despite having the same crystal structure and composition. Properties such as coercive field (H c ), superparamagnetic blocking temperature (T B ), saturation magnetization (M s ) and remnant magnetization (M r ) provide important quality benchmarks for comparing and improv- ing nanoscale magnetic materials; yet, as these properties are the physical manifestation of intrinsic (innate crystal structure) and extrinsic (surface composition, ligands, morphology, and interpar- ticle spacing) properties, there are frequently large discrepancies between observations depending on sample preparation [4,5]. These discrepancies are not only a matter of magnitude but also of sign due to a complex interplay of dipolar interactions, exchange coupling, and particle clustering effects [6,7]. Certain investiga- tions have demonstrated enhanced coercivity (H c ) and remanent magnetization (M r ) with increased interparticle distance [8,9]. However, observations on other magnetic nanomaterial prepara- tions show no effect on coercivity and a significant drop in blocking temperature (T B ) accompanying diamagnetic dilution [10,11]. This dependence on sample preparation can severely limit the ability for synthetic chemists to participate in the optimization of materi- als and comparison of findings to those of other researchers. Most magnetic materials with sub-10 nm diameters are super- paramagnetic as their structures lack the intrinsic anisotropy to support magnetic order. One material capable of retaining single- domain ferromagnetism at very small sizes is the binary inter- metallic, FePt. Investigations of the magnetic properties of FePt nanoparticles have increased in recent years due to the ease of synthesis in nanoparticle form [2,12–15], and promise for applica- tions in high-density magnetic storage devices (e.g., hard disk drives), read-write technology, and high-performance permanent magnets [14–17]. The primary form of technological interest for FePt is the ordered face centered tetragonal (L1 0 ) phase (Fig. 1). http://dx.doi.org/10.1016/j.ica.2016.09.020 0020-1693/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: jrinehart@ucsd.edu (J.D. Rinehart). Inorganica Chimica Acta xxx (2016) xxx–xxx Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica Please cite this article in press as: P.O. Adelani et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.09.020