Arabinda Haldar

Deterministic Control of Magnetization Dynamics in Reconfigurable Nanomagnetic Networks for Logic Applications

Information processing based on nanomagnetic networks is an emerging area of spintronics, as the energy consumption and integration density of the current semiconductor technology are reaching their fundamental limits. Nanomagnet-based devices rely on manipulating the magnetic ground states for device operations. While the static behavior of nanomagnets has been explored, little information is available on their dynamic behavior. Here, we demonstrate an additional functionality based on their collective dynamic response and explore the concept utilizing networks of bistable rhomboid nanomagnets. The control of the magnetic ground states of the networks was achieved by the geometrical design of the nanomagnets instead of the conventional interelement dipolar coupling. Dynamic responses of both the ferromagnetic and antiferromagnetic ground states were monitored using broadband ferromagnetic resonance spectroscopy, the Brillouin light scattering technique, and direct magnetic force microscopy. Micromagnetic simulations and numerical calculations validate our experimental observations. This method would have potential implications for low-power magnonic devices based on reconfigurable microwave properties.

Brillouin light scattering study of spin waves in NiFe/Co exchange spring bilayer films

Spin waves are investigated in Permalloy(Ni80Fe20)/Cobalt(Co) exchange spring bilayer thin films using Brillouin light scattering (BLS) experiment. The magnetic hysteresis loops measured by magneto-optical Kerr effect show a monotonic decrease in coercivity of the bilayer films with increasing Py thickness. BLS study shows two distinct modes, which are modelled as Damon-Eshbach and perpendicular standing wave modes. Linewidths of the frequency peaks are found to increase significantly with decreasing Py layer thickness. Interfacial roughness causes to fluctuate exchange coupling at the nanoscale regimes and the effect is stronger for thinner Py films. A quantitative analysis of the magnon linewidths shows the presence of strong local exchange coupling field which is much larger compared to macroscopic exchange field.

Artificial metamaterials for reprogrammable magnetic and microwave properties

We demonstrate a reliable method for realizing various antiferromagnetic states in lithographically defined, dipolar coupled rhomboid nanomagnets. We directly probe the remanent state using magnetic force microscopy and measured the microwave absorptions using broadband ferromagnetic resonance spectroscopy technique. Reprogrammable microwave absorption properties are shown by switching between ferromagnetic and antiferromagnetic remanent states using a simple field initialization. There is a direct correlation between the magnetic remanent states and the microwave responses. Experimental results were supported by micromagnetic simulations which show a good agreement. The results may find applications in low power magnonic devices based on reprogrammable magnetic metamaterials.

Vortex chirality control in circular disks using dipole-coupled nanomagnets

We report a systematic control of the chirality of circular magnetic disks where dipole field coupling drives the functionality. Our approach involves the use of two rhomboid nanomagnets with predetermined magnetization directions to bias the circular disk. Magnetostatic interaction was observed to initiate the vortex nucleation with selective chirality (clockwise or counterclockwise) when an appropriate in-plane magnetic field was applied and then removed. We have used in-field magnetic force microscopy technique to directly probe the direction of the vortex core displacements, which are sensitive to its chirality. Experimental results are in good agreement with micromagnetic simulations.

Magnetic antivortex formation in pound-key-like nanostructures

Magnetic antivortices have potential for applications but they are considerably more difficult to create than their topological counterpart, the vortex state. Here, we describe a method to generate isolated magnetic antivortex (AV) states reliably in pound-key-like patterned structures. Magnetic force microscopy images confirm that AV states are obtained after a simple two-step magnetic field procedure that involves first a saturating field along the structure diagonal followed by a smaller field applied in the opposing direction. Micromagnetic simulations show that the second field reverses areas of the structure that have lower shape anisotropy first, which facilitates the subsequent antivortex formation.

Unconventional spin distributions in thick Ni80Fe20 nanodisks

We study the spin distributions in permalloy (Py: Ni80Fe20) nanodisks as a function of diameter D (300 nm ≤ D ≤ 1 μm) and thickness L (30 nm ≤ L ≤ 100 nm). We observed that beyond a certain thickness, for a fixed disk diameter, an unconventional spin topology precipitates which is marked by the presence of a divergence field within the magnetic vortex curl. The strength of this divergence changes anti-symmetrically from negative to positive—depending on the core polarity—along the axis of the cylindrical nanodisk. This is also accompanied by a skyrmion-like out-of-plane bending of the spin vectors farther away from the disk center. Additionally, the vortex core dilates significantly when compared to its typical size. This has been directly observed using magnetic force microscopy. We determined from the ferromagnetic resonance spectroscopy measurements that the unconventional topology in the thicker nanodisks gyrated at a frequency, which is significantly lower than what is predicted by a magnetic vortex ...

Reconfigurable magnetic and microwave properties of a ferrimagnetic-type artificial crystal

We have demonstrated a ferrimagnetic-type anti-parallel remanent state of rhomboid shaped nanomagnets of two different widths alternated in a dipolar coupled nanomagnetic array. The anti-parallel state is reconfigured to a parallel remanent state by using a simple field initialization process. Magnetic states were directly visualized using the magnetic force microscopy technique. Due to a variation of the stray field distributions for the two different remanent states, distinct dynamic responses are found using the broadband ferromagnetic resonance spectroscopy technique without applying any external bias magnetic field. Experimental observations were supported by the micromagnetic simulations. The results are important in the context of bias-field-free and reprogrammable microwave magnetic devices and creation of artificial ferrimagnetic crystals.We have demonstrated a ferrimagnetic-type anti-parallel remanent state of rhomboid shaped nanomagnets of two different widths alternated in a dipolar coupled nanomagnetic array. The anti-parallel state is reconfigured to a parallel remanent state by using a simple field initialization process. Magnetic states were directly visualized using the magnetic force microscopy technique. Due to a variation of the stray field distributions for the two different remanent states, distinct dynamic responses are found using the broadband ferromagnetic resonance spectroscopy technique without applying any external bias magnetic field. Experimental observations were supported by the micromagnetic simulations. The results are important in the context of bias-field-free and reprogrammable microwave magnetic devices and creation of artificial ferrimagnetic crystals.

Isotropic transmission of magnon spin information without a magnetic field

A novel route for data processing is designed based on magnons where waves carry information unlike charges in electronics. Spin-wave devices (SWD), which use collective excitations of electronic spins as a carrier of information, are rapidly emerging as potential candidates for post-semiconductor non-charge-based technology. Isotropic in-plane propagating coherent spin waves (magnons), which require magnetization to be out of plane, is desirable in an SWD. However, because of lack of availability of low-damping perpendicular magnetic material, a usually well-known in-plane ferrimagnet yttrium iron garnet (YIG) is used with a large out-of-plane bias magnetic field, which tends to hinder the benefits of isotropic spin waves. We experimentally demonstrate an SWD that eliminates the requirement of external magnetic field to obtain perpendicular magnetization in an otherwise in-plane ferromagnet, Ni80Fe20 or permalloy (Py), a typical choice for spin-wave microconduits. Perpendicular anisotropy in Py, as established by magnetic hysteresis measurements, was induced by the exchange-coupled Co/Pd multilayer. Isotropic propagation of magnon spin information has been experimentally shown in microconduits with three channels patterned at arbitrary angles.

Bias field free tunability of microwave properties based on geometrically controlled isolated permalloy nanomagnets

We have investigated the static and dynamic properties of two lithographically patterned bi-stable nanomagnets. Different ground magnetic states were realized using a simple in-plane field initialization technique. These states were directly imaged with magnetic force microscopy. Using the broadband ferromagnetic spectroscopy, we show that different magnetic ground states are associated with distinct microwave absorption spectra due to the variation of the internal magnetic field leading to large shift between the absorption spectra. Our experimental observations are in good agreement with micromagnetic simulations which also indicate the possibility of sub-ns switching between magnetic states using a rectangular pulse field.