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Superparamagnetic iron oxide nanoparticles (SPIONs) can align in polymer-stabilized aggregates, changing their properties. 

We demonstrate indirect electric-field control of ferromagnetic resonance (FMR) in devices that integrate the low-loss, molecule-based, room-temperature ferrimagnet vanadium tetracyanoethylene (V[TCNE]x∼2) mechanically coupled to PMN-PT piezoelectric transducers. Upon straining the V[TCNE]x films, the FMR frequency is tuned by more than 6 times the resonant linewidth with no change in Gilbert damping for samples with α = 6.5 × 10−5. We show this tuning effect is due to a strain-dependent magnetic anisotropy in the films and find the magnetoelastic coefficient |λs| ∼ (1–4.4) ppm, backed by theoretical predictions from density-functional theory calculations and magnetoelastic theory. Noting the rapidly expanding application space for strain-tuned FMR, we define a new metric for magnetostrictive materials, magnetostrictive agility, given by the ratio of the magnetoelastic coefficient to the FMR linewidth. This agility allows for a direct comparison between magnetostrictive materials in terms of their comparative efficacy for magnetoelectric applications requiring ultra-low loss magnetic resonance modulated by strain. With this metric, we show V[TCNE]x is competitive with other magnetostrictive materials, including YIG and Terfenol-D. This combination of ultra-narrow linewidth and magnetostriction, in a system that can be directly integrated into functional devices without requiring heterogeneous integration in a thin film geometry, promises unprecedented functionality for electric-field tuned microwave devices ranging from low-power, compact filters and circulators to emerging applications in quantum information science and technology. 

The molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene (V[TCNE]x, x ≈ 2) has garnered interest from the quantum information community due to its excellent coherent magnonic properties and ease of on-chip integration. Despite these attractive properties, a detailed understanding of the electronic structure and mechanism for long-range magnetic ordering have remained elusive due to a lack of detailed atomic and electronic structural information. Previous studies via x-ray absorption near edge spectroscopy and the extended x-ray absorption fine structure have led to various proposed structures, and in general, V[TCNE]x is believed to be a three-dimensional network of octahedrally coordinated V2+, each bonded to six TCNE molecules. Here, we elucidate the electronic structure, structural ordering, and degradation pathways of V[TCNE]x films by correlating calculations of density functional theory (DFT) with scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) of V[TCNE]x films. Low-loss EELS measurements reveal a bandgap and an excited state structure that agree quantitatively with DFT modeling, including an energy splitting between apical and equatorial TCNE ligands within the structure, providing experimental results directly backed by theoretical descriptions of the electronic structure driving the robust magnetic ordering in these films. Core-loss EELS confirms the presence of octahedrally coordinated V+2 atoms. Upon oxidation, changes in the C1s-π* peak indicate that C=C of TCNE is preferentially attacked. Furthermore, we identify a relaxation of the structural ordering as the films age. These results lay the foundation for a more comprehensive and fundamental understanding of magnetic ordering and dynamics in these classes of metal–ligand compounds. 

Abstract A cavity‐magnonic system composed of a superconducting microwave resonator coupled to a magnon mode hosted by the organic‐based ferrimagnet vanadium tetracyanoethylene (V[TCNE]_x) is demonstrated. This work is motivated by the challenge of scalably integrating a low‐damping magnetic system with planar superconducting circuits. V[TCNE]_xhas ultra‐low intrinsic damping, can be grown at low processing temperatures on arbitrary substrates, and can be patterned via electron beam lithography. The devices operate in the strong coupling regime, with a cooperativity exceeding 1000 for coupling between the Kittel mode and the resonator mode at T≈0.4 K, suitable for scalable quantum circuit integration. Higher‐order magnon modes are also observed with much narrower linewidths than the Kittel mode. This work paves the way for high‐cooperativity hybrid quantum devices in which magnonic circuits can be designed and fabricated as easily as electrical wires. 

Integrating patterned, low-loss magnetic materials into microwave devices and circuits presents many challenges due to the specific conditions that are required to grow ferrite materials, driving the need for flip-chip and other indirect fabrication techniques. The low-loss (α = (3.98 ± 0.22) × 10−5), room-temperature ferrimagnetic coordination compound vanadium tetracyanoethylene (V[TCNE]x) is a promising new material for these applications that is potentially compatible with semiconductor processing. Here, we present the deposition, patterning, and characterization of V[TCNE]x thin films with lateral dimensions ranging from 1 μm to several millimeters. We employ electron-beam lithography and liftoff using an aluminum encapsulated poly(methyl methacrylate), poly(methyl methacrylate-methacrylic acid) copolymer bilayer [PMMA/P(MMA-MAA)] on sapphire and silicon. This process can be trivially extended to other common semiconductor substrates. Films patterned via this method maintain low-loss characteristics down to 25 μm with only a factor of 2 increase down to 5 μm. A rich structure of thickness and radially confined spin-wave modes reveals the quality of the patterned films. Further fitting, simulation, and analytic analysis provide an exchange stiffness, Aex = (2.2 ± 0.5) × 10−10erg/cm, as well as insights into the mode character and surface-spin pinning. Below a micron, the deposition is nonconformal, which leads to interesting and potentially useful changes in morphology. This work establishes the versatility of V[TCNE]x for applications requiring highly coherent magnetic excitations ranging from microwave communication to quantum information. 

Here we present the synthesis and characterization of a hybrid vanadium-organic coordination polymer with robust magnetic order, a Curie temperature T C of ∼110 K, a coercive field of ∼5 Oe at 5 K, and a maximum mass magnetization of about half that of the benchmark ferrimagnetic vanadium(tetracyanoethylene) ∼2 (V·(TCNE) ∼2 ). This material was prepared using a new tetracyano-substituted quinoidal organic small molecule 7 based on a tricyclic heterocycle 4-hexyl-4 H -pyrrolo[2,3- d :5,4- d ′]bis(thiazole) ( C6-PBTz ). Single crystal X-ray diffraction of the 2,6-diiodo derivative of the parent C6-PBTz , showed a disordered hexyl chain and a nearly linear arrangement of the substituents in positions 2 and 6 of the tricyclic core. Density functional theory (DFT) calculations indicate that C6-PBTz -based ligand 7 is a strong acceptor with an electron affinity larger than that of TCNE and several other ligands previously used in molecular magnets. This effect is due in part to the electron-deficient thiazole rings and extended delocalization of the frontier molecular orbitals. The ligand detailed in this study, a representative example of fused heterocycle aromatic cores with extended π conjugation, introduces new opportunities for structure–magnetic-property correlation studies where the chemistry of the tricyclic heterocycles can modulate the electronic properties and the substituent at the central N -position can vary the spatial characteristics of the magnetic polymer.