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arXiv:1112.0485 · Nano Letters

Realization of the Manipulation of Ultracold Atoms with a Reconfigurable Nanomagnetic System of Domain Walls

Adam D. West, Kevin J. Weatherill, Thomas J. Hayward, Paul W. Fry, T. Schrefl, Mike R. J. Gibbs, Charles S. Adams, Dan A. Allwood, Ifan G. Hughes

Published Jul 11, 2012• 29 citations
arXivHTMLPDF

Abstract

Planar magnetic nanowires have been vital to the development of spintronic technology. They provide an unparalleled combination of magnetic reconfigurability, controllability, and scalability, which has helped to realize such applications as racetrack memory and novel logic gates. Microfabricated atom optics benefit from all of these properties, and we present the first demonstration of the amalgamation of spintronic technology with ultracold atoms. A magnetic interaction is exhibited through the reflection of a cloud of (87)Rb atoms at a temperature of 10 μK, from a 2 mm × 2 mm array of nanomagnetic domain walls. In turn, the incident atoms approach the array at heights of the order of 100 nm and are thus used to probe magnetic fields at this distance.

DOI
10.1021/nl301491m

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Planar magnetic nanowires have been vital to the development of spintronic technology. They provide an unparalleled combination of magnetic reconfigurability, controllability, and scalability, which has helped to realize such applications as racetrack memory and novel logic gates. Microfabricated atom optics benefit from all of these properties, and we present the first demonstration of the amalgamation of spintronic technology with ultracold atoms. A magnetic interaction is exhibited through the reflection of a cloud of (87)Rb atoms at a temperature of 10 μK, from a 2 mm × 2 mm array of nanomagnetic domain walls. In turn, the incident atoms approach the array at heights of the order of 100 nm and are thus used to probe magnetic fields at this distance.
This thesis presents the first realisation of a new type of hybrid quantum device based on spintronic technology. We demonstrate an interaction between the magnetic fringing fields produced by domain walls within planar permalloy nanowires and a cloud of ultracold Rubidium 87 atoms. This interaction is manifested through the realisation of a magnetic atom mirror produced by a two-dimensional domain wall array. The interaction is tuned through the reconfiguration of the micromagnetic structure. Analytic modelling of the fringing fields is developed and shows good agreement with calculations based on micromagnetically simulated structures. The accurate and rapid calculation of the fringing fields permits simulation of the resulting atom dynamics, which agrees well with data. In turn, we use the atom dynamics as a probe of the micromagnetic reconfiguration processes that take place and observe a collective behaviour which is both reliably reproducible and in agreement with alternative, conventional magnetometry. We also observe evidence of stochastic behaviour, characteristic of superparamagnetic systems. We consider the development of a more advanced spintronics-based atom chip which will allow for the creation of extremely tight mobile atom traps. We consider the problems associated with ensuring that the trapping potential is adiabatic, sufficiently deep, and technically feasible. In particular we examine techniques to circumvent losses due to Majorana spin-flip transitions. As a result of this study we propose a novel scheme for creating time-averaged potentials via the piezoelectric actuation of magnetic field sources. We show that this technique presents significant fundamental and technical advantages over conventional time-averaging schemes.
The interaction of ultra-cold atoms with magnetic microstructures has become the focus of many recent experiments [1]. We present progress towards the interaction of cold Rb atoms with the magnetic fields from domain walls in nanowires. Fig. 1 a) shows the out of plane field from such a domain wall. Micromagnetic simulations show that nanowires could be used to create atom traps with frequencies in the MHz regime [2]. Transparent substrates can be used so that the microscopic traps can be loaded from an optical lattice and the small amount of metal present means that the usual Johnson noise heating effects can be avoided and lifetimes of many seconds achieved. Since domain walls are mobile, transportable traps can be envisaged which can allow atoms to be brought into close proximity to allow interactions for quantum information processes.
We present a design for a switchable nanomagnetic atom mirror formed by an array of 180° domain walls confined within Ni80Fe20 planar nanowires. A simple analytical model is developed which allows the magnetic field produced by the domain wall array to be calculated. This model is then used to optimize the geometry of the nanowires so as to maximize the reflectivity of the atom mirror. We then describe the fabrication of a nanowire array and characterize its magnetic behavior using magneto-optic Kerr effect magnetometry, scanning Hall probe microscopy, and micromagnetic simulations, demonstrating how the mobility of the domain walls allow the atom mirror to be switched “on” and “off” in a manner which would be impossible for conventional designs. Finally, we model the reflection of R87b atoms from the atom mirror’s surface, showing that our design is well suited for investigating interactions between domain walls and cold atoms.
We have used the results of micromagnetic simulations to investigate the effects of nanowire geometry and domain wall magnetization structure on the characteristic parameters of magnetic atom traps formed by domain walls in planar ferromagnetic nanowires. It is found that when traps are formed in the near-field of a domain wall both nanowire geometry and wall structure have a substantial effect on trap frequency (how tightly atoms are spatially confined) and adiabaticity (how closely the atoms’ magnetic moments track the applied field direction within the trap). We also show that in certain regimes a trap’s depth depends only on the amplitude of an externally applied rotating magnetic field, thus allowing it to be tuned independently of the trap’s other critical parameters.
We show that the magnetic field produced by a chiral ferromagnetic film can be applied to control ultracold atoms. The film will act as a magnetic mirror or a reflection grating for ultracold atoms when it is in the helical phase or the skyrmion crystal phase, respectively. By applying a bias magnetic field and a time-dependent magnetic field, one-dimensional or two-dimensional magnetic lattices including honeycomb, Kagome, and triangular types can be created to trap the ultracold atoms. We have also discussed the trapping height, the potential barrier, the trapping frequency, and the Majorana loss rate for each lattice. Our results suggest that the chiral ferromagnetic film can be a platform to develop artificial quantum systems with ultracold atoms based on modern spintronics technologies.
We report on an experimental study of the dynamics of the reflection of ultracold atoms from a periodic one-dimensional magnetic lattice potential. The magnetic lattice potential of period $10\text{ }\ensuremath{\mu}\text{m}$ is generated by applying a uniform bias magnetic field to a microfabricated periodic structure on a silicon wafer coated with a multilayered TbGdFeCo/Cr magneto-optical film. The effective thickness of the magnetic film is about 960 nm. A detailed study of the profile of the reflected atoms as a function of externally induced periodic corrugation in the potential is described. The effect of angle of incidence is investigated in detail. The experimental observations are supported by numerical simulations.