In-plane Magnetic Field Pulse Research Articles

Correlated gyrotropic motion of skyrmion clusters in ultrathin ferromagnetic nanodisks

We report the correlated gyrotropic motion of two-dimensional skyrmion clusters in ferromagnetic nanodisks. Based on the massless Thiele equation, we calculate the gyrotropic eigenmodes for the multi-skyrmion system. Micromagnetic simulations are performed to verify theoretical predictions with a good agreement. By applying an in-plane magnetic field pulse on four-skyrmion system, four gyrotropic modes have been excited. One of them has all skyrmions moving in-phase; two modes have the face-to-face skyrmions pair moving out-of-phase; the rest one has a pair of face-to-face skyrmions moving in-phase with side skyrmions rotating out-of-phase. Meanwhile, we identify a reversal of phase and rotation direction originating from the complex interaction between skyrmions for five-skyrmion system. By applying an out-of-plane magnetic field pulse, we observe the correlated breathing modes for four-skyrmion system with the face-to-face skyrmions breathing in-phase or out-of-phase, and all skyrmions breathing consistently in-phase. In particular, for the five-skyrmion system, a hybridization mode has been clarified with side skyrmions breathing and central skyrmion rotation. Our results provide useful insights for controlling the dynamic behavior of multiple skyrmions that are indispensable in the future skyrmion-based spintronic devices.

Room temperature manipulation of exchange bias in magnetic heterojunctions

Emerging hybrid in-plane and out-of-plane devices have exposed the urgent demand for simultaneously in-plane and out-of-plane exchange biased layers in a single functional device. In this work, we have performed detailed studies on the manipulation of dual exchange bias in the IrMn/Co/Pt/CoPt/IrMn heterojunction. The in-plane exchange bias between IrMn and Co layers can be reversibly controlled by simultaneous application of colinear in-plane magnetic field and current pulse along x-direction. And the out-of-plane exchange bias between CoPt and IrMn layers can be manipulated by the application of out-of-plane external magnetic field along z-direction or colinear in-plane magnetic field and current pulse along x-direction. Moreover, thanks to the interlayer exchange coupling between Co/Pt/CoPt, a purely electrical induced spin-orbit torque switching can be used to manipulate the out-of-plane exchange bias. Based on this purely electrical manipulation of out-of-plane exchange bias, multistate nonvolatile memory has been demonstrated. Our work has revealed the promising application prospects of flexible manipulation of exchange bias in future multifunctional and multidimensional spintronic devices.

Magnonic key based on skyrmion clusters

In this work, we report the excitation of spin-waves modes in skyrmion clusters hosted in Co/Pt nanodots by applying an in-plane magnetic field pulse. The direction at which the magnetic field is applied enables the excitation of five main spin-waves modes that are understood in terms of only azimuthal-like modes, which are shown to be strongly dependent on the number of skyrmions stabilized in the system. This feature converts the present system in a mechanism to activate and suppress a set of given modes, which in turn we propose to be utilized as a magnonic key based skyrmion cluster. Our results could be useful in manufacturing potential magnonic logic devices based in skyrmionic textures.

Field driven magnetic racetrack memory accompanied with the interfacial Dzyaloshinskii-Moriya interaction

The in-plane magnetic field pulse driven domain wall motion on a perpendicularly magnetized nanowire is numerically investigated by performing micromagnetic simulations and magnetic domain wall dynamics are evaluated analytically with one-dimensional collective coordinate models including the interfacial Dzyaloshinskii-Moriya interaction. With the action of the precession torque, the chirality and the magnetic field direction dependent displacements of the magnetic domain walls are clearly observed. In order to move Bloch type and Neel type domain walls, a longitudinal and a transverse in-plane magnetic field pulse are required, respectively. The domain wall type (Bloch or Neel) can easily be determined by the dynamic motion of the domain walls under the applied pulse fields. By applying a temporally asymmetric in-plane field pulse and successive notches in the perpendicularly magnetized nanowire strip line with a proper interval, the concept of racetrack memory based on the synchronous displacements of the chirality dependent multiple domain walls is verified to be feasible. Requirement of multiple domain walls with homogeneous chirality is achieved with the help of Dzyaloshinskii-Moriya interaction.

Magnetic Vortex Core Dynamics in Correlated Double Plates of Permalloy

In this work, we theoretically investigate the dynamics of magnetic vortex in a double-plate nanostructure, in which two square permalloy plates are joined together by imposing closely on a thin permalloy film. It is found that magnetic vortices have been formed in the two square permalloy plates. Due to the involving of the joint, the out-of-plane magnetization can also be observed at the sidewalls between the square plates and the joint area. The ground states with the vortex polarities (1,1) and (1,-1) are obtained when the thickness of the joint area is varied. By applying a short in-plane magnetic field pulse, the vortices deviate from their equilibrium positions and gyrate with damping radius. Interestingly, the vortex polarity can be switched if the strength of magnetic field approaches the threshold of switching field. Particularly by changing the strength of magnetic field, the two vortices can be switched alternatively in the double-plate nanostructure. The investigations may provide potential applications in magnetic vortex memories.

Signal transfer in a chain of stray-field coupled ferromagnetic squares

We study the vortex-core dynamics in a chain of three stray-field coupled permalloy squares. Time-resolved scanning transmission x-ray microscopy is employed to image the out-of-plane magnetization of the cores. After exciting the first element via a short in-plane magnetic field pulse, the excitation can be transferred through the chain via dipolar interaction. The transfer efficiency of the gyrotropic vortex motion strongly depends on the configuration of the core polarizations. For alternating polarizations, a transfer efficiency of about 56% to the third square is achieved. The chain can be switched back and forth between the transmitting and a locking state.

Switching Behavior of Vortex Cores in Bilayer Nanodots by Uniform Magnetic Field Pulses

We investigate the response of magnetic vortex cores in bilayer dots, a vertical stack of disk and square sublayers, to ultrashort in-plane magnetic field pulses by micromagnetic simulations. The bilayer dots adopt the advantage of switching behaviors of constituent dots; the low switching field threshold of the square is still maintained and the operating switching field range is intermediate, compared to the single-layer disk and square. Besides the usual creation and subsequent annihilation of vortex-antivortex pair reported previously, subsequent antivortex or vortex and antivortex pair during the vortex core switching process can be observed in the bilayer dot with a lateral size of about 100 nm.

Vortex Core Switching by Coherent Excitation with Single In-Plane Magnetic Field Pulses

The response of magnetic vortex cores to subnanosecond in-plane magnetic field pulses was studied by time-resolved x-ray microscopy. Vortex core reversal was observed and the switching events were located in space and time. This revealed a mechanism of coherent excitation by the leading and trailing edges of the pulse, lowering the field amplitude required for switching. The mechanism was confirmed by micromagnetic simulations and can be understood in terms of gyration around the vortex equilibrium positions, displaced by the applied field.

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.

Vortex Core Switching Dynamics in Submicron Permalloy Disk

The switching dynamics of the magnetic vortex core in a submicrometer-scale Permalloy disk is investigated by micromagnetic modeling. At zero magnetic field, the vortex state is formed with a single vortex core at the disk center. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, an additional two vortices, i.e., a vortex and an anti-vortex, with an opposite polarity are created near the original vortex core. Subsequently, the original vortex core annihilates with the antivortex core, and the final magnetic state contains a single core with the reversed polarity. The dynamics of the vortex core switching are discussed in terms of the nonlocal torque exerted to spins in response to an externally applied magnetic field pulse

Magnetic Bubble Domain Phase at the Spin Reorientation Transition of UltrathinFe/Ni/Cu(001)Film

Magnetic domain phases of ultrathin Fe/Ni/Cu(001) are studied using photoemission electron microscopy at the spin reorientation transition (SRT). We observe a new magnetic phase of bubble domains within a narrow SRT region after applying a nearly in-plane magnetic field pulse to the sample. By applying the magnetic field pulse along different directions, we find that the bubble domain phase exists only if the magnetic field direction is less than approximately 10 degrees relative to the sample surface. A temperature dependent measurement shows that the bubble domain phase becomes unstable above 370 K.

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.

Vortex pumps in the crossing lattices regime of highly anisotropic layered superconductors

It is now well established that vortex dynamics in samples with a spatially asymmetric pinning potential can lead to rectifying vortex ‘diode’ behaviour. Spatial asymmetry is not a fundamental requirement for the control of vortex motion, however, and we demonstrate that vortex ‘lensing’ is possible in highly anisotropic layered superconductors simply under the action of non time-reversible trains of in-plane magnetic field pulses. Our devices depend crucially on the existence of ‘crossing’ pancake vortex (PV) and Josephson vortex (JV) lattices in Bi2Sr2CaCu2O8+δ (BSCCO) single crystals under tilted magnetic fields. An attractive interaction between these two sub-lattices makes it possible to indirectly manipulate the PV distribution by modifying the JV lattice, and a number of functional devices based on this principle have been proposed. In our experiments a BSCCO single crystal is placed on a Hall probe array, and cooled below Tc in a small out-of- plane magnetic field. Trains of sawtooth in-plane field pulses are then applied to the system and different elements of the Hall array used to demonstrate PV lensing or antilensing behaviour, depending on the pulse shape. The mechanism leading to lensing will be discussed and results compared with molecular dynamics simulations.

Dynamic interactions between pancake vortex stacks and Josephson vortices inBi2Sr2CaCu2O8 single crystals: relaxation and ratchets

We present a detailed study of the dynamic interactions between Josephson vortices and stacks of pancakevortices in a Bi2Sr2CaCu2O8 single crystal, obtained by measuring the effect of applied in-plane magnetic field pulses on thec-axis magnetization. The predominant interaction relaxes the system towards equilibrium.However, using a highly sensitive AC technique we are able to measure also the forcesacting to drive the system away from equilibrium, consistent with the existence of dragginginteractions between the Josephson and pancake systems. Such forces were discussedrecently as the basis of possible flux ratchet devices.

Time Resolved Magnetization Dynamics of Ultrathin Fe(001) Films: Spin-Pumping and Two-Magnon Scattering

The time-resolved magnetic response of ultrathin epitaxial Fe(001) films grown on GaAs(001) and covered by Au, Pd, and Cr capping layers was investigated by time and spatially resolved Kerr effect measurements. The magnetization was excited by an in-plane magnetic field pulse using the transient internal field generated at a Schottky barrier while the wavelength of the excitation (resonant mode) was roughly 4 microm. Each of the three cap layers affected the spin relaxation in a unique way. Au cap layers resulted in the bulk Gilbert damping of the Fe film. Pd cap layers caused an additional Gilbert damping due to spin-pump or spin-sink effects. Cr cap layers lead to a strong extrinsic damping which can be described by two-magnon scattering. In this case the strength of the extrinsic damping can be controlled by a field induced shift of the spin wave manifold with respect to the excited k vector.

Picosecond precessional magnetization reversal by magnetic field pulse shaping

We have developed a method to vary the shape of magnetic field pulses on a picosecond time scale and to monitor the magnetization response in all three dimensions by using time-resolved linear and nonlinear magneto-optical techniques. With this, we have measured the temporal response of rectangular 10/spl times/20 /spl mu/m and elliptical 8/spl times/16 /spl mu/m, 8 nm thin Permalloy (Ni/sub 80/Fe/sub 20/) elements due to in-plane magnetic field pulses with varying length and amplitudes. The data shows coherent precession of the magnetic spins in the elliptical elements, for large and small angle excitations. The shaping of the in-plane field pulses allows the control of the magnetization-motion on a picosecond timescale, so that stopping of the precessional motion of the underdamped magnetic system after one full precessional oscillation could be achieved. The large angle excitation showed a 160/spl deg/ switching of the magnetization within 300 ps. Further, it is demonstrated that the present technique can successfully be applied to improve the data integrity in magnetic random access memories (MRAMs).

Picosecond control of coherent magnetisation dynamics in permalloy thin films by picosecond magnetic field pulse shaping

We have measured the response of a 20×10 μm, 8 nm thin NiFe (80:20) permalloy film due to excitation by short in-plane magnetic field pulses. We will show that using a two-pulse-technique a complete control of the precessional motion of the magnetisation can be achieved on picosecond timescales. Furthermore, we will present numerical calculations which show that a complete suppression of magnetisation ringing after switching can only be realised by a cascade of short field pulses.

Magnetization dynamics in NiFe thin films induced by short in-plane magnetic field pulses

The magnetization dynamics in a thin NiFe film was investigated by applying short in-plane magnetic field pulses while probing the response using a time-resolved magneto-optical Kerr effect setup. In-plane magnetic field pulses, with duration shorter than the relaxation of the system, were generated using a photoconductive switch and by subsequent propagation of current pulses along a waveguide. The field pulses with typical rise and decay times of 10–60 and 500–700 ps, respectively, have a maximum field strength of 9 Oe, by which Permalloy elements of 16 nm thickness and lateral dimensions of 10×20 μm were excited. The observed coherent precession of a ferromagnetic NiFe system had precession frequencies of several GHz and relaxation times on a nanosecond time scale. The dynamic properties observed agree well the Gilberts’s precession equation and the static magnetic properties of the elements

Magnetization Reversal in Ultrashort Magnetic Field Pulses

Strong in-plane magnetic field pulses of 2--4.4 ps duration are used to study magnetization reversal in perpendicularly magnetized Co/Pt films. Ring domains, reminiscent of the field contour during exposure, are observed later with Kerr microscopy. Their radii represent switching fields which are in quantitative agreement with the coherent rotation model. The observation of intrinsic transition broadening is attributed to the existence of static and dynamic fluctuations of the magnetic anisotropy.