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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Interactions of ultracold atoms with magnetic fields from domain walls in nanowires

Kevin J. Weatherill, Thomas J. Hayward, Adam West, T. Schrefl, P. W. Fry, Charles S. Adams, D. A. Allwood, Ifan G. Hughes

Published Jun 1, 2009• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-eqec.2009.5192206

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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.
By solving the Landau-Lifshitz-Gilbert equation using a finite element method we show that an atom trap can be produced above a ferromagnetic nanowire domain wall. Atoms experience trap frequencies of up to a few megahertz, and can be transported by applying a weak magnetic field along the wire. Lithographically defined nanowire patterns could allow quantum information processing by bringing domain walls in close proximity at certain places to allow trapped atom interactions and far apart at others to allow individual addressing.
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.
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.
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.
We report the trapping of ultracold 87Rb^{87}\mathrm{Rb}87Rb atoms in a 0.7-$\ensuremath{\mu}\text{m}$-period two-dimensional triangular magnetic lattice on an atom chip. The magnetic lattice is created by a lithographically patterned magnetic Co/Pd multilayer film plus bias fields. Rubidium atoms in the $|F=1,{m}_{F}=\ensuremath{-}1\ensuremath{\rangle}$ low-field seeking state are trapped at estimated distances down to about 100nm100\phantom{\rule{0.28em}{0ex}}\mathrm{nm}100nm from the chip surface and with calculated mean trapping frequencies up to about 800kHz800\phantom{\rule{0.28em}{0ex}}\mathrm{kHz}800kHz. The measured lifetimes of the atoms trapped in the magnetic lattice are in the range 0.4--1.7ms1.7\phantom{\rule{0.28em}{0ex}}\mathrm{ms}1.7ms, depending on distance from the chip surface. Model calculations suggest the trap lifetimes are currently limited mainly by losses due to one-dimensional thermal evaporation following loading of the atoms from the ZZZ-wire trap into the very tight magnetic lattice traps, rather than by fundamental loss processes such as surface interactions, three-body recombination, or spin flips due to Johnson magnetic noise. The trapping of atoms in a $0.7\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-period magnetic lattice represents a significant step toward using magnetic lattices for quantum tunneling experiments and to simulate condensed matter and many-body phenomena in nontrivial lattice geometries.
Basic periodic trapping configurations is presented, which can be realized with multilayer microstructures. The trapping fields for the atoms (Ioffe Pritchard type potentials for magnetic trapping of atoms as well as quadrupole fields for magneto-optical trapping) are formed by two layers of crossed wires, which can be individually addressed. The possibility of producing multiple magneto optical traps (MOT) next to a surface in a controlled manner will be shown. This could be a first step to load ultracold atoms into an array of microstructured magnetic traps in a parallel way.