B. C. Choi

Dynamics of vortex core switching in ferromagnetic nanodisks

Dynamics of magnetic vortex core switching in nanometer-scale Permalloy disk, having a single vortex ground state, was investigated by micromagnetic modeling. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, additional two vortices, i.e., a circular and an antivortex, are created near the original vortex core. Sequentially, the vortex-antivortex pair annihilates. A spin wave is created at the annihilation point and propagated through the entire element; the relaxed state for the system is the single vortex state with a switched vortex core.

Definition of Magnetic Exchange Length

The magnetostatic exchange length is an important parameter in magnetics as it measures the relative strength of exchange and self-magnetostatic energies. Its use can be found in areas of magnetics including micromagnetics, soft and hard magnetic materials, and information storage. The exchange length is of primary importance because it governs the width of the transition between magnetic domains. Unfortunately, there is some confusion in the literature between the magnetostatic exchange length and a similar distance concerning magnetization reversal mechanisms in particles known as the characteristic length. This confusion is aggravated by the common usage of two different systems of units, SI and cgs. This paper attempts to clarify the situation and recommends equations in both systems of units.

Magnetization configuration and switching behavior of submicron NiFe elements: Pac-man shape

Two types of submicron permalloy element, namely Pac-man, were investigated by a magnetic force microscope for magnetization configuration and switching behaviors. Two distinct domain configurations, bidomain for Pac-man type I and single domain for Pac-man type II, were observed in arrays of Pac-man elements. The domain configuration depends on the slot angle for the Pac-man type I, but is independent of the slot angle for the Pac-man type II. Array of Pac-man elements with a slot angle of 180° shows the highest switching field and the narrowest switching field distribution, as compared to rectangular and hexagonal elements of the same overall dimensions.

Spin-current pulse induced switching of vortex chirality in permalloy∕Cu∕Co nanopillars

Dynamic response of the vortex magnetization in multilayered magnetic nanopillars to the spin-polarized current pulse has been investigated numerically. The equilibrium magnetization configurations in both magnetic layers are the vortex states with single magnetization cores at the disk center. It was found that the chirality of the vortex state in magnetic free layer can be controllably switched by applying current pulse with appropriate amplitude, polarity, and duration. The critical current density required for the chirality switching is found to be on the order of 108A∕cm2.

Effect of shape anisotropy on switching behaviors of Pac-man NiFe submicron elements

We investigated the shape anisotropy effect on switching the behaviors of submicron permalloy Pac-man (PM) elements with a thickness of 40 nm by a magnetic force microscope. The probability to form a vortex configuration at an as-patterned state was lower in an elongated PM type-I (EPM-I) (10%) than a PM-I element (47%). The switching process in PM-I, PM-II, and EPM-I elements was governed by a vortex-driven reversal, while the magnetization of a EPM-II element switches through a single-domain reversal. It was found that a PM element involving a vortex-driven switching process shows a nonreproducible reversal.

High-Quality Factor Ni-Zn Ferrite Planar Inductor

Ferrite and air-core planar inductor arrays were fabricated on 4 inch Si wafer to characterize inductor performance. Three micron thick Ni0.5Zn0.5Fe2O4 film was deposited by a low temperature electrophoresis ferrite deposition process. All ferrite inductors in the array showed 35% higher inductance (L) and 130% higher quality factor (Q) than air-core inductor. The maximum Q of ferrite inductor was found to be 53 at 2 MHz. The superimposed DC current and the rated power were measured to be about 3 A and 15 W, respectively, for a 5% drop in L. The fabricated ferrite inductor has a higher current capability than the air-core inductor. In addition, the power efficiency of the buck DC-DC converter was predicted to be 94.3% at 1.93 W.

Integrated Ferrite Film Inductor for Power System-on-Chip (PowerSoC) Smart Phone Applications

An array of ferrite and air-core inductors was fabricated on silicon wafer to characterize inductor performance. The 1 μm and 2.5 μm thick ferrite films for the fabrication of inductors were prepared by dc magnetron sputtering. The inductance of the ferrite inductor increased with the thickness of ferrite film from 45.5 nH for 1 μm thick ferrite to 50 nH for 2.5 μm thick ferrite. The maximum Q-factor was obtained to be 59 at 2.87 MHz from 2.5 μm thick ferrite inductor, which is higher than 49.3 at 2.26 MHz for 1 μm thick ferrite inductor and 23.2 at 1.56 MHz for air-core inductor. Superimposed dc current of 1 μm and 2.5 μm thick ferrite inductors was estimated to be 2.5 A and 2.15 A, respectively, corresponding to a 5% drop in L at 10 MHz. In addition, the power efficiency of the buck dc-dc converter based on the studied ferrite inductors was calculated to be 91.7% for 2.5 μm thick ferrite inductor and 90.1% for 1 μm thick ferrite inductor at load current of 0.647 A.

Micromagnetic configurations and switching mechanism in Pac-man-shaped submicron Ni80Fe20 magnets

Micromagnetic modeling analysis and magnetic force microscopy studies were performed in order to characterize the magnetization configuration and magnetic switching behavior in two types, PM I and PM II, of submicron Pac-man-shaped Ni80Fe20 magnetic elements. It was found that a slight variation in the shape of the elements has a striking influence on the internal magnetic structures and switching field distribution. In particular, the vortex-formation driven switching is replaced by quasicoherent reversal by removing the central core part at the center of element. The sensitive dependence of remanent magnetic configuration and switching behavior on sample geometry is discussed in terms of the competition between the exchange and demagnetizing energy terms.

Dynamics of magnetic vortex core switching in Fe nanodisks by applying in-plane magnetic field pulse

We investigated the influence of the magnetic field pulse parameters and the size of the Fe element to the vortex core switching by micromagnetic modeling. When the magnetic field pulse with an appropriate strength and duration is applied to 30nm thick Fe circular disks with diameters between 100nm and 1μm, the vortex configuration is perturbed away from the equilibrium state, and the circular symmetric distribution of the in-plane magnetization around the vortex core deforms. This leads to the creation of a new vortex core with the opposite polarity and an antivortex. With increasing time, the vortex-antivortex pair annihilates. As a result of the annihilation, a single vortex core with opposite polarity remains and a vortex core switch is realized. The process of core switching, however, strongly depends on the amplitude and duration of the magnetic pulse.

Effect of a magnetic field pulse on ultrafast magnetization reversal in a submicron elliptical Permalloy thin film

The characteristics of a magnetic field pulse, its magnitude, direction, and rise and fall time on the mechanism of ultrafast magnetization reversal have been studied by micromagnetic simulations. An elliptically shaped, Permalloy thin film, having dimensions of 400nm long axis, 112nm short axis, and 3.2nm, thickness, was considered. A plot of the magnetic field pulse components describes three types of reversal behaviors, quasicoherent, incoherent, and nonreversal. The optimum magnetic field pulse for the suppression of magnetization ringing is found in the incoherent precession area by finding the lowest remanent total energy at the moment the pulse is cut off. The rise and fall time of the pulse has a significant effect on the reversal behavior only in the incoherent precession region. In this region, whether a reversal can occur depends on the rise time of the pulse. Extending the rise and fall time of the pulse, in the noncoherent rotation region, does not lead to a clear change of the remanent total...