In a step towards generating switchable MRI cellular labels, we demonstrate in-situ field switching of micron scale metamagnetic Iron-Rhodium (FeRh) thin film particles. A thin-film (200 nm) FeRh sample was fabricated and patterned into an array of progressively smaller squares with sizes ranging from 500 μm down to 1 μm. The large first order phase change from antiferromagnetic to ferromagnetic state was characterized using vibrating sample magnetometry, magnetic force microscopy, and MRI. Room temperature MRI experiments sensitive to the local magnetic field surrounding the particles demonstrated the low moment state (OFF MRI contrast) at 4.7T and high moment state (ON MRI contrast) at 11.7T for the array where sizes down to 2-3 μm were observed in MRI at 50 μm resolution. The expected temperature dependent MRI contrast change was seen at 4.7T, where 10 μm particles could be observed at 150 μm resolution in the ON state. A shielded MRI insert, used to temporarily increase or decrease the magnetic field up to 0.77T amplitude, was used to reversibly switch the particle array at constant temperature and blink the particles ON and OFF at 4.7T. This work demonstrates the MRI contrast switching potential for FeRh particles with biological cell dimensions, and the use of magnetic field pulses for reversible MRI label contrast control.

• Publication type
• Journal Article MeSH

Vanadium dioxide (VO2) has received significant interest in the context of nanophotonic metamaterials and memories owing to its reversible insulator-metal transition associated with significant changes in its optical and electronic properties. The phase transition of VO2 has been extensively studied for several decades, and the ways how to control its hysteresis characteristics relevant for memory applications have significantly improved. However, the hysteresis dynamics and stability of coexisting phases during the transition have not been studied on the level of individual single-crystal VO2 nanoparticles (NPs), although they represent the fundamental component of ordinary polycrystalline films and can also act like nanoscale memory units on their own. Here, employing transmission electron microscopy techniques, we investigate phase transitions of single VO2 NPs in real time. Our analysis reveals the statistical distribution of the transition temperature and steepness and how they differ during forward (heating) and backward (cooling) transitions. We evaluate the stability of coexisting phases in individual NPs and prove the persistent multilevel memory at near room temperatures using only a few VO2 NPs. Our findings unveil the physical mechanisms that govern the hysteresis of VO2 at the nanoscale and establish VO2 NPs as a promising component of optoelectronic and memory devices with enhanced functionalities.

• Keywords
• coexisting phases, hysteresis, insulator−metal transition, nanophotonics, phase-change memory, transmission electron microscopy, vanadium dioxide,
• Publication type
• Journal Article MeSH