Abstract
The magnetic vortex structure, an important ground state configuration in micron and sub-micron sized ferromagnetic thin film platelets, is characterized by a curling in-plane magnetization and, in the center, a minuscule region with out-of-plane magnetization, the vortex core, which points either up or down. It has already been demonstrated that the vortex core polarity can be reversed with external AC magnetic fields, frequency-tuned to the (sub-GHz) gyrotropic eigenmode or to (multi-GHz) azimuthal spin wave modes, where reversal times in the sub-ns regime can be realized. This fast vortex core switching may also be of technological interest as the vortex core polarity can be regarded as one data bit. Here we experimentally demonstrate that unidirectional vortex core reversal by excitation with sub-100 ps long orthogonal monopolar magnetic pulse sequences is possible in a wide range of pulse lengths and amplitudes. The application of such short digital pulses is the favourable excitation scheme for technological applications. Measured phase diagrams of this unidirectional, spin wave mediated vortex core reversal are in good qualitative agreement with phase diagrams obtained from micromagnetic simulations. The time dependence of the reversal process, observed by time-resolved scanning transmission X-ray microscopy indicates a switching time of 100 ps and fits well with our simulations. The origin of the asymmetric response to clockwise and counter clockwise excitation which is a prerequisite for reliable unidirectional switching is discussed, based on the gyromode - spin wave coupling.
Highlights
In thin soft magnetic layers with thicknesses of a few tens of nanometers and lateral dimensions ranging from 100 nm to 10 μm, the vortex structure is the ground state showing an in-plane curling magnetization and a perpendicularly magnetized core at the center with a size of about 10–20 nm [1,2,3]
By applying rotating in-plane fields this reversal occurs in a unidirectional manner [8,9,10,11,12] since the exciting field couples only to the vortex gyrotropic mode if the sense of rotation of the excitation corresponds to the sense of the vortex gyrotropic mode, which is determined by the core polarity
The vortex structures are investigated by scanning transmission x-ray microscopy at the MAXYMUS end station at BESSY II, Berlin, which provides a lateral resolution of 25 nm and high magnetic contrast based on the x-ray magnetic circular dichroism effect [16] at the Ni L3 edge
Summary
In thin soft magnetic layers with thicknesses of a few tens of nanometers and lateral dimensions ranging from 100 nm to 10 μm, the vortex structure is the ground state showing an in-plane curling magnetization and a perpendicularly magnetized core at the center with a size of about 10–20 nm [1,2,3]. In spite of its high stability with respect to static external magnetic fields the vortex core can be switched dynamically with low-power sine or pulsed magnetic fields or spin-polarized currents [4,5,6,7] This was achieved by resonantly exciting the gyrotropic mode with its eigenfrequency typically in the range from 100 MHz to 1 GHz. By applying rotating in-plane fields this reversal occurs in a unidirectional manner [8,9,10,11,12] since the exciting field couples only to the vortex gyrotropic mode if the sense of rotation of the excitation corresponds to the sense of the vortex gyrotropic mode, which is determined by the core polarity. Hereby unidirectional vortex core reversal was achieved and corresponding micromagnetic simulations indicated a switching time slightly above 200 ps [14,15] when using one-period bursts
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