Abstract
Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current–field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.
Highlights
Static and dynamic magnetic solitons play a critical role in applied nanomagnetism
Magnon drops can exist in magnetic thin films with perpendicular magnetic anisotropy (PMA), where the PMA leads to an attractive force between the magnons
Real magnetic materials always exhibit non-zero damping, spin transfer torque (STT)[28,29] can locally cancel the damping in devices known as nanocontact spin-torque oscillators (NC-STO)[22,30,31,32]
Summary
A type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Two-dimensional solitons, such as vortices[2,3], skyrmions[4,5,6,7] and dynamical skyrmions[8] have received significant attention for both memory and oscillator applications[8,9,10] Common to these solitons is their nonzero topological charge, which affords them particular stability in extended films. We study these opposing trends by first mapping out the complete and detailed current–field phase diagram for droplet nucleation in a wide range of NC size, and deriving a theoretical model that includes both the applied field, and the field-dependent perpendicular component of the fixed-layer spin polarization. We find an excellent agreement between model and experiment, including the existence of a minimum droplet nucleation current at intermediate fields
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