Design of Bistable Gold@Spin‐Crossover Core–Shell Nanoparticles Showing Large Electrical Responses for the Spin Switching

A simple chemical protocol to prepare core–shell gold@spin‐crossover (Au@SCO) nanoparticles (NPs) based on the 1D spin‐crossover [Fe(Htrz)₂(trz)](BF₄) coordination polymer is reported. The synthesis relies on a two‐step approach consisting of a partial surface ligand substitution of the citrate‐stabilized Au NPs followed by the controlled growth of a very thin layer of the SCO polymer. As a result, colloidally stable core@shell spherical NPs with a Au core of ca. 12 nm and a thin SCO shell 4 nm thick, are obtained, exhibiting a narrow distribution in sizes. Differential scanning calorimetry proves that a cooperative spin transition in the range 340–360 K is maintained in these Au@SCO NPs, in full agreement with the values reported for pristine 4 nm SCO NPs. Temperature‐dependent charge‐transport measurements of an electrical device based on assemblies of these Au@SCO NPs also support this spin transition. Thus, a large change in conductance upon spin state switching, as compared with other memory devices based on the pristine SCO NPs, is detected. This results in a large improvement in the sensitivity of the device to the spin transition, with values for the ON/OFF ratio which are an order of magnitude better than the best ones obtained in previous SCO devices.     

Magnetic Susceptibility Study of Sub‐Pico‐emu Sample Using a Micromagnetometer: An Investigation through Bistable Spin‐Crossover Materials

A promising and original method to study the spin‐transition in bistable spin‐crossover (SCO) materials using a magnetoresistive multiring sensor and its self‐generated magnetic field is reported. Qualitative and quantitative studies are carried out combining theoretical and experimental approaches. The results show that only a small part of matter dropped on the sensor surface is probed by the device. At a low bias‐current range, the number of detected nanoparticles depends on the amplitude of the current. However, in agreement with the theoretical model, the stray voltage from the particles is proportional to the current squared. By changing both the bias current and the concentration of particle droplet, the thermal hysteresis of an ultrasmall volume, 1 × 10^?4 mm³, of SCO particles is measured. The local probe of the experimental setup allows a highest resolution of 4 × 10^?14 emu to be reached, which is never achieved by experimental methods at room temperature.     

Interpenetrating‐Lattice‐Structured Magnets Exhibiting Anomalous Magnetic Properties

Cubic [Ru₂(O₂CMe)₄]₃[Cr(CN)₆] prepared from water has two interpenetrating lattices and exhibits anomalous magnetic hysteresis, saturation magnetization, out‐of‐phase alternating current (AC) susceptibility, χ″(T), and zero‐field‐cooled/field‐cooled temperature‐dependent magnetization data with respect to either the material prepared from acetonitrile or the layered two‐dimensional (2D) [Ru₂(O₂CBu^t)₄]₃[Cr(CN)₆] · 2 H₂O prepared from water. These unexpected properties are attributed to the presence of the interpenetrating lattices and reveal that, albeit uncommon, interpenetrating‐lattice‐structured materials can stabilize new cooperative magnetic behavior.     

Inside Front Cover: A Combined Top‐Down/Bottom‐Up Approach for the Nanoscale Patterning of Spin‐Crossover Coordination Polymers (Adv. Mater. 16/2007)

The background scanning electron microscopy image shows nanometric patterns of the 3D spin crossover coordination polymer Fe(pyrazine)[Pt(CN)₄] (see schematic structure in the circle), which have been fabricated using a combination of lift‐off and multilayer sequential assembly methods. These patterns, reported by Gábor Molnár, Azzedine Bousseksou, and co‐workers on p. 2163, exhibit a bistability of their electronic states (¹A₁ ? ⁵T₂), and thus represent a novel platform for a wide array of potential applications.