Reproducible formation of single magnetic bubbles

By Dean Williams on May 13, 2016

Lead authors Tao Liu and Vito Puliafito, working at the University of Lorraine and University of Messina, explain how their research aims to breathe new life into a once-abandoned area of research and explore potential applications in the latest magnetic devices.

A soliton is a mathematical concept used to study general non-linear effects in physics, from optics to magnetism. In magnetism, particularly, one of the oldest observed solitons is the magnetic bubble, namely a static magnetic configuration achievable in materials with perpendicular magnetic anisotropy (PMA).

Solitons are characterized by a central region that holds downward moments and an outer ring region with upward moments, separated by an in-plane domain wall. Bubble-based memory started out as a promising technology in the 1970s, however it was abandoned at the beginning of 1980s for a number of reasons, including the introduction of Flash memory technology.

(a) MFM image of a 45×45 μm2 array of 1.25 μm diameter [Co/Ni]10 dots after demagnetization under AC in-plane field. The red, blue and green circles show single bubble state, single domain state, and multi-domain state, respectively. (b-d) Corresponding magnetic states, as obtained by means of micromagnetic simulations.

This work reports a procedure that enables a reproducible nucleation of well-defined single magnetic bubbles in confined circular Co/Ni dots patterned in an array. Appearance of bubble state had been already achieved in micrometer-sized cylindrical Co, Permalloy, Ni dots with weak PMA, and in circular dots of L1₀FePt with large PMA energy. Nevertheless, at remanence, for these systems, only a small number of dots in an assembly were in a bubble state. Our work overcomes this lack of reproducibility and gives hope of achieving both spin-transfer-torque based devices, analogous to the vortex based nano-oscillators, and reprogrammable magnonic crystals for the control of spin-wave propagation.

The method we propose consists in applying a magnetic field in the plane of the patterned dots up to certain amplitude H_max at which a wavy magnetic state is obtained. We point out there is a range of H_max for which, at remanence, a very high percentage of dots contain a magnetic bubble.

We present magnetic force microscopy images as well as calculations to explain the physics behind this procedure. The experiments have been performed on an assembly of Co/Ni circular dots, patterned on a Pt layer, with diameters ranging from 4 microns down to 200 nm and with thicknesses from 20 to 7 repeats (i.e. 16 nm down to 5 nm). The Dzyaloshinskii-Moriya interaction, typical in ferromagnet/heavy metal bilayers, is negligible here for two main reasons: the large thickness of the ferromagnet and the coupling to the same heavy metal (Pt) both in the lower and upper interface of the ferromagnet.

Up to 90% of bubbles can be formed by tuning the dot size, material thickness and intrinsic material parameters, but also the bubble nucleation path. We use an in-plane field AC demagnetization procedure whose success rate strongly depends on the starting field, H_max. Micromagnetic calculations show that it originates from the existence of a field range for which dipolar field favors a wavy magnetic configuration that transforms into single bubble state when the field is swept back towards remanence. Experimentally, we demonstrate that success rate higher than 90% would be achieved by suppressing dot edge defects.

The main experimental activities of this paper have been carried out at the University of Lorraine, France, while the theoretical ones have been performed at the University of Messina, Italy.

Read the full article in Journal of Physics D: Applied Physics.

About the authors

Tao Liu received his PhD degree in Physics from Institute of Physics in Beijing in 2013. After his graduation, he worked as a postdoctoral research fellow for two years in Université de Lorraine, France. Now, he is doing another postdoctoral fellowship in Colorado State University, USA. His main research interests are effective magnetization and spin wave manipulating methods and their applications in spintronics devices.

Vito Puliafito received his PhD degree in Advanced Technologies for Optoelectronics, Photonics and Electromagnetic Modeling from University of Messina, Italy, in 2011. Later, he has worked as postdoctoral research fellow in Messina, in Politecnico di Bari, Italy, and again in Messina. His main research interests are numerical methods for micromagnetic computations, soliton dynamics, and spintronics applications.