Reconfigurable and self-biased magnonic metamaterials

In magnonics, magnetic waves and oscillations are exploited for signal and information processing at microwave frequencies. A magnonic metamaterial is employed to configure different microwave bands by spatial engineering of magnetizations using different magnetic states or magnetic couplings. Magnetic field hysteretic variation of microwave responses has conventionally been used for tunable microwave operations. The use of such bias magnetic fields hinders the device integration of microwave magnonic devices. Here, we discuss a route to eliminating the requirement of bias magnetic field and simple initialization process for reconfigurable microwave operations. The distinct microwave responses are associated with different remanent magnetic states which are engineered by shape induced magnetic anisotropy rather than the conventional dipolar coupling driven magnetic states. However, the origin of the shift in the microwave spectra is associated with the variation of dipolar coupling for nanomagnetic networks, multilayer nanomagnets, and their arrays. This perspective provides an outlook on current challenges and potential future scopes of magnonic devices. We discuss some of our recent demonstrations toward the realizations of reconfigurable magnonic devices without any external bias magnetic field. Self-biased nanomagnets are also shown to have applications in designing a waveguide for spin wave transport and spin wave gating which operates without any bias magnetic field

Effect of quenching rate on the structural and hard magnetic properties of Nd-Fe-B melt-spun ribbons

The phase structure, microstructure, magnetic and thermomagnetic properties of nanostructured Nd-Fe-B melt-spun ribbons were investigated. The melt-spun ribbons have been prepared at different wheel speeds varying from 17 to 25 m/s. The hard magnetic Nd2Fe14B phase with (00l) texture, indicating preferred crystallographic orientation, was observed in all the ribbons with some α-Fe(Co) as the minor phase. Nd2Fe14B grains are uniformly distributed with grain sizes ranging from 50 to 150 nm. A decrease in the average grain size of Nd2Fe14B and fading away of texture formation in the ribbons were found with the increase in the wheel speeds. The best combination of magnetic properties with a coercivity of 14.5 kOe, the saturation magnetization of 132 emu/g, and the energy product of 16 MGOe was achieved at 23 m/s and these ribbons are suitable for the fabrication of hot deformation Nd-Fe-B magnets

Magnetism in Gallium doped CeFe_2: The martensitic scenario

Ce(Fe_{1-x}Ga_x)_2 compounds with x = 0, 0.01, 0.025 and 0.05 have been investigated to unravel the effect of Ga on the magnetic state of CeFe_2. For the first time, we find that the dynamic antiferromagnetic phase present in CeFe_2 gets stabilized with Ga substitution. The hysteresis loops show that while the compounds with x = 0 and 0.01 show normal behavior, the other two show multiple magnetization steps across the antiferromagnetic-ferromagnetic transition region. The virgin curve is found to lie outside the envelope curve in these two compounds, similar to the observations made in Ru and Re substituted CeFe_2 compounds. Temperature, sweep rate and time dependences of the magnetization show that the compounds with x >=0.025 possess glassy behavior at low temperatures. Various results obtained reveal that these two compounds belong to the martensite family.Comment: 23 pages, 12 Figure

Investigation of magnetization dynamics in trilayer width-modulated nanowires

We have investigated the magnetization reversal processes and dynamic behavior of trilayered Py(50 nm)/Pd(tPd)/Py(20 nm) nanowires with periodic width modulation as a function of spacer layer thickness tPd in the range from 0 to 10 nm and compared them with single-layer nanowires. The ferromagnetic resonance spectra show more than three modes that result from a non-uniform demagnetizing field in width-modulated nanowires. We observe that the spacer layer thickness influenced the ferromagnetic resonance spectra, which showed different numbers and values of modes and frequencies due to the different magnetization configurations for different spacer layer thicknesses. We also found that the two ferromagnetic layers are exchange-coupled for tPd = 2 nm nanowire arrays, showing the sharp switching of magnetization from the static measurements and sharp frequency jump from 13.6 to 14.7 GHz around −18 mT from the dynamic measurements. However, for tPd = 10 nm, the two layers switch at different fields, indicating a gradual decrease in magnetization as the reversal is mediated through dipolar coupling. The origin of modes is well explained from the spatial mode profiles of top and bottom magnetic layers. The dynamic responses in this spin-valve-type structure are useful for designing microwave-based spintronic devices

Magnetic and the magnetocaloric properties of Ce1-xRxFe2 and Ce(Fe1-xMx)2 compounds

We have studied selected rare earth doped and transition metal doped CeFe2 compounds by examining their structural, magnetic and magneto-thermal properties. With substitution of Ce by 5 and 10% Gd and 10% Ho, the Curie temperature can be tuned to the range of 267-318 K. Localization of Ce 4f electronic state with rare earth substitutions is attributed for the enhancement of Curie temperature. On the other hand, with Ga and Al substitution at the Fe site, system undergoes paramagnetic to ferromagnetic transition and then to an antiferromagnetic phase on cooling. The magnetocaloric effect across the transitions has been studied from both magnetization isotherms and heat capacity data. It is shown that by choosing the appropriate dopant and its concentration, the magnetocaloric effect around room temperature can be tuned.Comment: 13 pages, 6 figures, 2 table

Spin Textures in High-Aspect-Ratio Ni80Fe20 Nanodisk Arrays: Implications for Next-Generation Spintronic Devices

Recent progress in nanomagnetism has generated significant enthusiasm for the creation of high-aspect-ratio nanostructures. Nonetheless, fabricating large-area thick nanostructures encounters substantial hurdles due to inherent lithographic constraints. In this study, we showcase the fabrication of magnetic nanodisks patterned with deep UV, reaching thicknesses of up to 200 nm, accomplished through the creation of nanotrenches in the Si substrate. Subsequently, the evolution of spin texture and spin dynamics as a function of thickness (20−200 nm) has been presented. The magnetization reversal studies reveal that the disks have a vortex as their ground state configuration; the nucleation and annihilation fields associated with the vortex increase with increasing thickness. We observe an increase in the vortex core diameter as the disk thickness is increased. Micromagnetic simulations suggest that the presence of an out-of-plane magnetization component is observed along the circumference, in addition to the into-the-plane magnetization at the center for disks of higher thicknesses. The magnetization dynamics studies reveal that the center mode frequency decreases with increasing thickness, and there is a mirror symmetry in the excitation amplitude between the top and bottom layers for nanodisks with thicknesses greater than 50 nm. The results are substantiated with micromagnetic simulations. Our results open horizons in the utilization of the third dimension for emerging spin textures and their potential applications in future spintronic devices

Fluorescence resonance energy transfer from tryptophan in human serum albumin to a bioactive indoloquinolizine system

The interaction between a bioactive molecule, 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ), with human serum albumin (HSA) has been studied using steady-state absorption and fluorescence techniques. A 1:1 complex formation has been established and the binding constant (K) and free energy change for the process have been reported. The AODIQ-HSA complex results in fluorescence resonance energy transfer (FRET) from the tryptophan moiety of HSA to the probe. The critical energy-transfer distance (R0) for FRET and the Stern-Volmer constant (Ksv) for the fluorescence quenching of the donor in the presence of the acceptor have been determined. Importantly, KSV has been shown to be equal to the binding constant itself, implying that the fluorescence quenching arises only from the FRET process. The study suggests that the donor and the acceptor are bound to the same protein at different locations but within the quenching distance