SPIN

Magnetic nanodot arrays are interesting systems for future applications in nanotechnology including patterned magnetic media, magnonic crystals, magnetic logic, sensors, STNOs and biomedical applications. All applications require the knowledge base of magnetization processes of magnetic nanodot arrays at various time and length scales. Here, we review the present status of experimental studies of picosecond precessional magnetization dynamics in magnetic nanodot arrays. We discuss the fabrication methods of magnetic nanodot arrays and excitation and detection methods of precessional dynamics by optical means. We further discuss the all-optical excitation and detection of precessional dynamics in Ni₈₀Fe₂₀ (permalloy) nanodot arrays with width between 200 nm and 50 nm and with interdot separation between 50 nm and 400 nm. A transition from strongly collective dynamics to completely isolated dynamics through various weakly collective regimes, variation of precession frequency and damping with the interdot separation, effects of dipolar and quadrupolar interdot interaction, effects of the variation of dot size on the dynamics of single elements and arrays, and anisotropy of collective dynamics have been thoroughly studied by experimental and micromagnetic simulation results. Finally, we discuss the future directions in the research on the dynamics of magnetic nanodot arrays.

We report that the current induced translational motion of transverse domain wall and domain wall switching in ferromagnetic nanowire is primarily governed by the configuration and polarization of the domain wall. The out-of-plane torque due to nonadiabacity and spin relaxation is found to be the dominant mechanism in the configuration and polarization dependent translational motion of the domain wall. The transverse domain wall undergoes a damped periodic back and forth translational motion along with a periodic switching from +ve polarization to -ve polarization and vice versa under the application of a pulsed current. The domain wall switching occurs at the start of each period of translation.

Thermal spin-transfer torque describes the manipulation of the magnetization by the application of a heat flow. The effect has been calculated theoretically by Jia et al. in 2011. It is found to require large temperature gradients in the order of Kelvins across an ultra thin MgO barrier. In this paper, we present results on the fabrication and the characterization of magnetic tunnel junctions with three monolayer thin MgO barriers. The quality of the interfaces at different growth conditions is studied quantitatively via high-resolution transmission electron microscopy imaging. We demonstrate tunneling magnetoresistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness. Magnetic tunnel junctions with perpendicular magnetization anisotropy show spin-transfer torque switching with a critical current of 0.2 MA/cm². The thermally generated torque is calculated ab initio using the Korringa–Kohn–Rostoker and nonequilibrium Green's function method. Temperature gradients generated from femtosecond laser pulses were simulated using COMSOL, revealing gradients of 20 K enabling thermal spin-transfer-torque switching.

We report magnetoresistance and spin-filtering efficiency of DNA-sandwiched ferromagnetic nanostructures. Commercially available FeCo target was sputtered over a gold-coated silicon wafer coated with mercaptosuccinic acid and DNA. This was made to contact with another FeCo sputtered gold-coated silicon wafer coated with mercaptosuccinic acid forming FM/DNA/FM based magnetic junction. DNA forms a covalent bonding with FeCo nanoparticles through mercaptosuccinic acid. Magnetoresistance for this was measured in the room temperature and was found to be 18%. Spin-filtering efficiency was calculated as 30%, which is half the value of experimental observation of spin-filtering of electron on pure gold surface coated with DNA.

We have studied Mn₁₂O₁₂(C₆H₅COO)₁₆(H₂O)₄(Mn₁₂-Ph) single-molecule magnets on highly ordered pyrolytic graphite (HOPG) using low-temperature scanning tunneling microscopy (LT-STM) experiments. We report Mn₁₂-Ph in isolation, resembling single molecules with metallic core atoms and organic outer ligands. The local tunneling current observed within the molecular structure shows a strong bias voltage dependency, which is distinct from that of the HOPG surface. Furthermore, evidence of internal inhomogeneity in the local density of states has been observed with high spatial resolution, and this inhomogeneity appears to be due to localized metallic behavior. These results facilitate magneto-metric studies of single-molecule magnets in isolation. As compared to bulk crystal studies, our experiments allow the specific investigation of atomic sites in the molecule.

We systematically investigate the room-temperature antiferromagnetic tunneling anisotropic magnetoresistance (TAMR) effect in [Pt/Co]/IrMn/AlO_x/Pt vertical tunnel junctions as a function of IrMn thicknesses and field directions. The experimental results indicate that a partial rotation of IrMn moments is triggered by ferromagnetic Co/Pt with the formation of exchange-spring in vertical fields, whereas two distinct modes of positive and negative spin-valve-like signals are detected with different in-plane fields due to the unidirectional anisotropy of IrMn. Electrical transport and magnetization reversal measurements confirm the crucial role of stable antiferromagnetic IrMn moments in the observed TAMR effect, which is profoundly affected by the operation temperature, IrMn thicknesses, magnetic fields and field directions.

We have fabricated Nb/magnetic tunnel junction (MTJ) structure to investigate superconductivity-induced impact on magnetization states of the proximal ferromagnet. By sweeping temperature in a parallel or antiparallel magnetization state, a thermal hysteresis in MTJ resistance was observed at temperature below the Nb superconducting critical temperature (T_c). In the parallel configuration, a Barkhausen-like jump in junction resistance arises when the magnetization of free layer is close to switching from parallel to antiparallel configuration. We attribute the hysteresis and Barkhausen-like jump to the pinning effect of Nb on the proximal magnetic domains and estimate the magnitude of the moment change of the free layer responsible for the resistance change resulting from Barkhausen-like jump.