Abstract
Vanadium tetracyanoethylene (V[TCNE]x) is an organic-based ferrimagnet that exhibits robust magnetic ordering (TC of over 600 K), high quality-factor (high-Q) microwave resonance (Q up to 3500), and compatibility with a wide variety of substrates and encapsulation technologies. Here, we substantially expand the potential scope and impact of this emerging material by demonstrating the ability to produce engineered nanostructures with tailored magnetic anisotropy that serve as a platform for the exploration of cavity magnonics, revealing strongly coupled quantum confined standing wave modes that can be tuned into and out of resonance with an applied magnetic field. Specifically, time-domain micromagnetic simulations of these nanostructures faithfully reproduce the experimentally measured spectra, including the quasiuniform mode and higher-order spin-wave (magnon) modes. Finally, when the two dominant magnon modes present in the spectra are brought into resonance by varying the orientation of the in-plane magnetic field, we observe anticrossing behavior, indicating strong coherent coupling between these two magnon modes at room temperature. These results position V[TCNE]x as a leading candidate for the development of coherent magnonics, with potential applications ranging from microwave electronics to quantum information.
Highlights
Manifestations of the potential of this material system can be found in the demonstration of control of magnetic properties via ligand-tuning[17–19] and metal-substitution,[20–25] optimized synthesis[26] (TC > 600 K), extremely sharp ferromagnetic resonance (FMR) features,[27,28] the demonstration of FMR-driven spin-pumping,[29] and encapsulation strategies that stabilize the magnetic properties for weeks to months under ambient conditions.[30]
This work demonstrates the ability to engineer the magnetic anisotropy in thin films deposited on patterned substrates and to engineer both the dispersion and anisotropy of confined spin wave modes in templated V[TCNE]x nanowires
When the trough spin-wave mode and quasiuniform mode are brought into resonance by varying the orientation of an in-plane magnetic field, we observe anticrossing behavior and the opening of a gap of 14 Oe, indicating strong coherent coupling between these two excitations at room temperature
Summary
The recent success of organic-based thin films in the areas of optoelectronics and electronics promises a new material basis for these applications that is mechanically flexible, facile to synthesize, and low cost when compared to traditional inorganic materials.[1,2,3] This success should in principle extend to magnetic and spintronic functionality, and to some extent, this promise has been realized in the observation of spin-dependent phenomenology including organic magnetoresistance[4,5,6,7] (OMAR), organic magnetoelectroluminescence[8,9] (OMEL), spin-pumping and spin transport,[10] and related phenomena.[11,12,13] this phenomenology is constrained by the fact that spins in these materials exhibit only diamagnetic, or at best paramagnetic, ordering and miss the rich phenomenology found in extended magnetic order (such as ferromagnetism and ferrimagnetism). It is likely that an anisotropic strain field is created in the nanowire structures due to the continuous contact with the SiO2 substrate along the nanowire axis and the ability for the nanowires to relax along the radial direction due to the presence of the grooves This phenomenology, along with successful fitting using Eq (3), suggests that the higher intensity set of peaks in Fig. 2 do result from the quasiuniform FMR mode of the nanowire and that the easy axis is surprisingly, perpendicular to the patterning axis. As with previous studies of uniform thin films, there is no straightforward way to disentangle this residual anisotropy from 4πMS, leading us to use the more general Heff ≡ 4πMeff in defining Eq (3) While this analysis resolves several long-standing mysteries in the nature of magnetic ordering and anisotropy in V[TCNE]x, it only describes the primary peak visible in Fig. 2 and does not describe either the additional resonances or the anticrossing behavior noted above. This gap is approximately seven times the peak-to-peak linewidth and 10% of the full field variation of the quasiuniform mode, indicating that these two excitations are in the strong coupling regime.[42–45] While there is a significant change in the intensity of these lines as they proceed through the crossing, it is difficult to disentangle effects due to the intrinsic strength of these resonances from the efficiency of their detection due to complicating factors such as the fact that these data are acquired by the physical rotation of the sample within the microwave cavity (which can perturb the cavity mode) and the varying spatial symmetries of the modes (which can affect their coupling efficiency to the microwave cavity and their detection)
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