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Interfacing ultracold atoms with
domain
walls
nanomagnetic
A. D. West1, K. J. Weatherill1, T. Hayward2, D. Allwood2 and I. G. Hughes1
1Joint Quantum Centre (JQC) Durham – Newcastle, Department of Physics,
Durham University, Durham, DH1 3LE, UK
2Department of Engineering Materials, University of Sheffield, Sheffield, UK
Abstract:
We present a first demonstration of interaction of ultracold atoms with domain walls within magnetic nanowires. Planar permalloy
nanowires host oppositely orientated domains, divided by domain walls. Fringing fields are produced at the walls [1]. An array of
serpentine nanowires gives a 2D array of domain walls, mimicing the sinusoidal magnetisation pattern required for an ideal magnetic
mirror [2]. The domain wall structure is reconfigurable by external magnetic fields [3]. A cloud of ~107 87Rb atoms at 10 mK, optically
pumped into the weak field seeking F=2, mF = 2 state is reflected by these fields. Monte Carlo simulations accurately predict the
signal observed from an interrogating light sheet. Using the bounce signal we infer characteristics of the nanowire array. Future work
aims to probe van der Waals potentials at short distances. We also present planned work to use the field from a single domain wall to
create a tight (~1 MHz) atom trap. Once in such a trap, atoms can be transported above nanowires by application of a small magnetic
field, inducing domain wall motion. Such a setup bears all the hallmarks
of a scheme
quantum information processing.
Magnetic
AtomforMirror:
Nanowires:
Made from Permalloy (Ni81Fe19)
• Very soft ferromagnetic material
• Negligible
magnetocrystalline
anisotropy
• Lithographically fabricated using
e-beam and lift off processing.
Host head-to-head or tail-to-tail
domain walls.
Quasi-discontinuous magnetisation
reversal (left) yields out of
plane magnetic fields (bottom
left) [1].
Macroscopic shape
determines the domain
structure.
An atom’s intrinsic magnetic moment,
, interacts with field produced by
nanowires:
mFgF > 0 gives weak-field-seeking
state; atoms entering fringing fields
are repelled.
Ideal mirror has sinusoidal magnetisation, giving an evanescent field
proportional to the period of the pattern.
A domain wall based mirror represents a quantum
combining ultracold physics and spintronic technology.
Domain Wall
Array
interface
Periodic B
Field
Can image the
resulting fringing
fields using MOKE or
scanning Hall probe
microscopy (right).
Mobile Atom Trap:
Field gradients up to
106 T/m.
The field from a single domain
can be approximated by [7]:
wall
Choose a serpentine pattern of nanowires which produces a 2D
array of domain walls.
External fields toggle ‘on’ (lower) and ‘off’ (upper) states.
Adding
a
bias
field
produces
trapping potential (left):
a
Resulting periodic field mimics an ideal magnetic mirror (above
right).
Overvie
Experimental
We approximate
the interaction as a point one,
w giving an
Setup
effective surface (below right).
The massive field gradients then give
a very high trap frequency.
The presence of a magnetic zero leads to rapid Majorana losses.
Many conventional techniques are not applicable for nanoscale
traps – we propose a new method of creating a time-averaged
potential, using piezoelectric actuation [8]:
Mechanical
rotation
of
the field source produces
a harmonic potential.
We have observed the diffuse reflection of a cloud of ultracold
atoms, shown in fluorescence images below [7]:
The trap is also deeper
and more adiabatic than
conventional
timeaveraged potentials.
We anticipate wTrap ~MHz
and depths of around 200
R = 0.25
R =
mK.
mm
mm
0.50
R = 0.75
mm
R = 1.00
mm
We also use a weak light sheet to
quantitatively
analyse
the
dynamics.
Results (left) are seen to agree
well with Monte Carlo simulations.
By increasing the amplitude of movement (R) the trap topology
changes to being toroidal (above and below left).
Colder atoms give a larger, better
resolved signal.
By reconfiguring the mirror
we
can
tune
the
atomnanowire interaction.
Traps based on domain walls are inherently mobile – they can be
moved by currents or external fields.
Networks of such traps present an ideal architecture for quantum
information processing.
References:
The atoms then act as a
probe of the micromagnetic
reconfiguration (right).
Results
agree
well
with
conventional magnetometry.
The closest approach is very
Acknowledgments:
small
(~50
nm)
–
enter
van
calculating magnetic nanowire domain wall fringing
der Waals region.
[1] A. D. West, T. J. Hayward, K. J. Weatherill, T. Schrefl, D. A. Allwood and I. G. Hughes, A simple model for
fields, J. Phys. D. 45, 095002 (2012).
[2] E. A. Hinds and I. G. Hughes, Magnetic atom optics: mirrors, guides, traps and chips for atoms, J. Phys. D, 32 R119 (1999).
[3] T. J. Hayward, A. D. West, K. J. Weatherill, P. J. Curran, P. W. Fry, P. M. Fundi, M. R. J. Gibbs, T. Schrefl, C. S. Adams, I. G. Hughes, S. J. Bending and D.
A. Allwood, Design and characterization of a field-switchable nanomagnetic atom mirror, J. Appl. Phys. 108, 043906 (2010)
[4] D. A. Allwood, T. Schrefl, G. Hrkac, I. G. Hughes and C. S. Adams, Mobile atom traps using magnetic nanowires, Appl. Phys. Lett. 89, 014102 (2006).
[5] J. Fortágh and C. Zimmerman, Magnetic microtraps for ultracold atoms, Rev. Mod. Phys. 79 235 (2005) and references therein.
[6] R. Folman, P. Krüger, J. Schmiedmayer, J. Denschlag and C. Henkel, Microscopic atom optics: from wires to an atom chip, Adv. Atom. Mol. Opt. Phys. 48 263
(2002) and references therein.
[7] A. D. West, K. J. Weatherill, T. J. Hayward, P. W. Fry, T. Schrefl, M. R. J. Gibbs, C. S. Adams, D. A. Allwood and I. G. Hughes, Realization of the
manipulation of ultracold atoms with a reconfigurable nanomagnetic system of domain walls, Nano Letters DOI: 10.1021/nl301491m.
[8] A. D. West, C. G. Wade, K. J. Weatherill and I. G. Hughes, Piezoelectrically-actuated time-averaged atomic microtraps, Appl. Phys. Lett., 3 023115 (2012).
This work was carried out in
collaboration with colleagues
at the University of Sheffield.
This project is funded by
EPSRC grants EP/F025459/1
and EP/F024886/1.

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