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Magnetic nanostructures have potential applications in ultrahigh-density magnetic storage. In this paper, the dynamics of the dual magnetic vortices in elliptical Permalloy nanodisk are investigated using micromagnetic modeling. Under the stimulation of the short-axis ( $y$ -axis) Gaussian field pulse and long-axis ( $x$ -axis) constant field, the equilibrium positions of dual-vortex cores can be driven away from the initial positions. The displacement in the $y$ -direction is proportional to the magnitude of the $x$ -axis constant field. The displacement in the $x$ -direction exhibit periodic reliance on the pulsewidth of the $y$ -axis Gaussian field pulse. The threshold value of pulse strength for dual-vortex motion increases with the magnitude of the $x$ -axis constant field. The mechanism of polarity and chirality switching in dual-vortex cores are studied through analyzing the trajectories of vortex cores in the annihilation and nucleation processes. The switching of polarity exhibits an oscillatory dependence on the pulsewidth and strength. The minimum pulse strength required for chirality switching is 120 mT, while the corresponding pulsewidth is influenced by the $x$ -axis constant field. This paper provides insights on the magnetization dynamics of nanostructures containing dual vortices subject to external excitations. [ABSTRACT FROM AUTHOR]

We report a novel spin-torque diode (STD) radio-frequency detector by utilizing tilted fixed-layer magnetization and in-plane free-layer magnetization. Through macrospin modeling and numerical simulation, we study both the direct current (dc) dependence and the perpendicular external field dependence of the output voltage of the detector. We systematically investigate both the fixed-layer magnetization tilt angle dependence and the dc dependence of the resonance detection regime of the detector. It is found that an ultrawide resonance detection regime 6.5 GHz can be achieved, which is 1200% higher than the traditional STD magnetic tunnel junction detector with in-plane fixed-layer magnetization. [ABSTRACT FROM AUTHOR]

Methods to modify the magnetic coercivity and exchange bias field of nanocrystalline antiferromagnetic/ferromagnetic \NiO/\Ni80{\Fe}20 thin films were investigated for bilayers grown using ion-assisted deposition onto different single crystalline substrates. An enhanced coercivity was found at 298 K for the films deposited on single crystalline MgO (100) and \Al_2 \O_3 (11-20) substrates. After field cooling the films to 50 K, the NiO/NiFe bilayer grown on \Al_2 \O_3 (11-20) exhibited the largest exchange bias (\- 25~\Oe). The second part of the study investigated ion-beam modification of the ferromagnetic surface prior to the deposition of the NiO layer. A range of ion-beam bombardment energies (VEH) were used to modify in situ the NiFe surface during the deposition of \NiO/\NiFe/\SiO_2 films. Cross-sectional transmission electron microscopy showed a systematic reduction in the thickness of the NiFe layers with increasing \Ar^ + bombardment energies attributed to etching of the surface. In addition, the bombardment procedure modified the magnetic exchange bias of the composite structure in both the as-prepared and field-cooled state. [ABSTRACT FROM PUBLISHER]

As the dimension of magnetic devices drastically decreases to nanometer range, thermally excited mag-noise gradually becomes the dominant noise source. Thermally excited mag-noise plays an important role in ferromagnetic ring structures. By conducting micromagnetic simulation, the saturated state, triangle state, half triangle state, onion state, and vortex state are explored and studied, respectively. The mag-noise calculation shows that triangle state is the main reason for the mag-noise exhibiting 1/f tendency in both the low-frequency range and high-frequency range in relaxed state, while the onion state explains why a noise peak appears in high-frequency range in relaxed state. Meanwhile, it is proved that the area of the ferromagnetic rings is not the determining factor for the mag-noise distribution in saturated state. These results offer the theoretical framework for explaining the relation between domain structure and mag-noise, which is conducive to the future application of ferromagnetic ring structures as magnetic random access memory elements. [ABSTRACT FROM PUBLISHER]

The relation between the spatial distribution of thermally excited mag-noise and the geometry of magnetic tunnel junction (MTJ) sensors was studied by micromagnetic simulation. The thermally excited mag-noise in MTJ sensors with rectangular cross section is found to exhibit a spatial distribution in which the tendency could be described as edge effect. The level of this edge effect changes with the aspect ratio of the free layer. As the aspect ratio increases from 0.6 to 1.4, the edge effect is suppressed linearly by nearly two times. This is ascribed to the change of the stiffness field in the free layer. On the other hand, the edge effect is not applicable for MTJ sensors with circular cross section. The central regions of the circular MTJ sensors are the main contributor of the mag-noise. These findings further our understanding on mag-noise behavior in MTJ sensors, and they are useful for suppressing noise in sensor design. [ABSTRACT FROM AUTHOR]

The effect of TaOx capping layer on microstructures and magnetic properties of FePt thin films via annealing was studied. The structural ordering of FePt from face-centered cubic to face-centered tetragonal phases depends strongly on the oxygen contents in the capping TaOx layer. The role of the TaOx layer is used to separate the FePt grains, as revealed by TEM. The annealed FePt/TaOx (15%O2/Ar) exhibited the largest out-of-plane coercivity (Hc\sim 4.2 kOe) amongst all samples, compared to that (\sim2.4 kOe) in the reference FePt layer. At low oxygen content in FePt/TaOx bilayers, the Ta atoms may act as defects to obstruct the FePt ordering whereas at high oxygen contents, the excess oxygen atoms diffuse into the FePt layer and react with Fe to form iron oxides which give rise to the low coercivities, as characterized by the XPS depth profile and binding energy analysis. [ABSTRACT FROM AUTHOR]

Thermally excited magnetic noise (mag-noise) has gradually become a major concern in magnetic tunnel junction sensors. By conducting micromagnetic simulation, the spatial distribution of thermal mag-noise in the free layer (FL) was obtained under various hard bias (HB) field and applied field. It was demonstrated that the edges are the main contributor of thermal mag-noise in the FL. This result could be explained by the nonuniform distribution of the stiffness field around the edges. It was also found that both HB field and applied field could suppress the thermal mag-noise in edges. A relatively high applied field will decrease the influence of HB field on mag-noise in the edges. [ABSTRACT FROM AUTHOR]

We report the effects of ion-beam bombardment on the room temperature and low temperature magnetic properties of ferromagnetic CoFe/antiferromagnetic (Co,Fe)O thin film bilayers. The films were deposited onto amorphous SiO2 and single crystalline MgO (110)/(100) substrates. Magnetometry showed that ion-beam bombardment was capable of modifying the coercivity and loop shape for the thin film system at room temperature, corresponding to alteration of the effective magneto-crystalline anisotropy field. After field cooling to 50 K, a shifted hysteresis loop was seen for those films containing a proportion of the antiferromagnetic rock-salt (Co,Fe)O phase, with an exchange bias magnitude that depended on the ion-beam bombardment conditions. Our results indicate that matching the substrate with appropriate ion-bombardment conditions provides a promising way to engineer selectively two important types of magnetic anisotropy in ferromagnetic/antiferromagnetic bilayers: magneto-crystalline and exchange bias. [ABSTRACT FROM PUBLISHER]

The fabrication processes for step-edge high-Tc superconducting-quantum-interference-device (SQUID) magnetometers have been developed. A magnetometer consists of three SQUIDs, which can be activated either individually or in series. Thus, the transfer function can be manipulated by activating a single SQUID or multiple SQUIDs in series. Furthermore, such a SQUID magnetometer is benefited by that if one of the three SQUIDs is broken, the other two SQUIDs are still workable. In addition to characterizing the SQUID magnetometer, the application of the SQUID magnetometer in the low-field nuclear magnetic resonance (NMR) has also been investigated. The results has shown a clear NMR signal of 4230 Hz for the water under 99.3 \muT. [ABSTRACT FROM AUTHOR]

Substrate and ion-bombardment effects on exchange bias were explored for CoFe/(Co,Fe)O bilayer films. The (Co,Fe)O component was sputtered onto different substrates and then ion-bombarded before a ferromagnetic CoFe layer was grown. Ferromagnetic resonance reveals that the CoFe is magnetically pinned by the (Co,Fe)O film. We find that substrate type heavily influences the magnitude of exchange bias and also the degree to which the exchange biased system is affected under ion-bombardment. In all cases there is a general decrease in the magnitude of exchange bias and coercivity, and for cases with high energy ion-bombardment unusual changes to the hysteresis loop are observed which may indicate the formation of an additional magnetic phase. [ABSTRACT FROM AUTHOR]

Structural and magnetic properties of FePt thin films were affected strongly by capped MnOx layers prepared by ion-beam bombardment and post-annealing. As-deposited FePt/MnOx bilayer exhibited a magnetically soft fcc phase, and it turned to an ordered fct FePt phase with large coercivity (\sim8 kOe) after annealing at 550^\circC. Increasing the \%O2/Ar in capped MnOx layer during deposition resulted in smaller ordered FePt grains separated by grain boundaries of MnOx. We found that the superlattice (001) peak is broadened considerably with larger amount of MnOx incorporated into FePt, likely due to the hindered formation of hard phase. Our results indicate that FePt/MnOx films deposited with lower \%O2/Ar, the oxygen atoms may occupy the interstitial positions in the FePt lattice to induce a local strain thus enhancing the FePt ordering. Further increased \%O2/Ar in capped MnOx layer, the excess oxygen atoms act a diffusion barrier effectively to inhibit the FePt grain growth and ordering. [ABSTRACT FROM AUTHOR]