Interactions Within Arrays of NiCuCu Superlattice Nanowires

1
Interactions Within Arrays of Ni-Cu/Cu
Superlattice Nanowires A. Robinson, W.
Schwarzacher H H Wills Physics Laboratory,
Bristol University, BS8 1TL, UK
Abstract The remanent magnetisation of Ni-Cu/Cu
superlattice nanowire arrays with a range of
layer thicknesses has been studied using a SQUID
magnetometer. It is observed that magnetostatic
anti-ferromagnetic interactions occur between the
nickel layers within each wire, demagnetising the
array. In addition there appears to be domain
formation within each nickel layer creating
vortices that further demagnetise the array.
Simulations indicate that the net effect of these
interactions is that the sense of vortices in
adjacent layers is opposite
Introduction Ferromagnetic/non-ferromagnetic
superlattice nanowires form an ideal system for
the study of magnetic interactions between
particles in one-dimension. The thickness of each
type of layer can be independently controlled,
allowing for fine tuning of both ferromagnetic
particle size and the distance between
neighbouring magnetic particles, using the
non-ferromagnetic layers as spacers.
Superlattice nanowires were prepared by template
electrodeposition through a polycarbonate
nano-porous membrane 6 ?m thick with track etched
pores 80nm in diameter, using a single
electrolyte containing both metal ions to be
deposited. A schematic diagram of the set up used
is shown below in figure 1. The current flowing
in the cell during deposition was monitored by a
computer, used to calculate the volume of
material deposited onto the working electrode.
The potential was switched between those required
to deposit each metal after the desired thickness
of each layer had been deposited. Several arrays
of wires were deposited with a nominal tNi and
tCu ranging from 25Å to 600Å.
Discussion Plot (a) shows the magnitude of the
deviations from the non-interacting case, and
hence strength of the interactions taking place,
decreasing with increasing copper layer
thickness. This suggests that the magneto-static
interactions are taking place between
ferromagnetic nickel layers within the individual
wires. Plot (b) shows a negative value for the
quantity ?I, which indicates that the
interactions taking place between the nickel
layers are demagnetising, such as dipolar
interactions which lead to flux closure
configurations between layers, i.e.
anti-ferromagnetic coupling. An idealised
schematic diagram of this type of interaction is
shown in figure 2 below. This does not explain
the existence of a minimum interaction level,
however. It is possible that the limit is caused
by the production of magnetic domains within each
nickel layer, which arrange into flux closure
configurations (vortices) after the applied field
is removed. These demagnetising interactions
between domains within each layer would then
cause the deviations. Preliminary micromagnetic
simulations using NIST's SimulMag indicate the
existence of vortices within nickel layers for
80nm diameter wires. These simulations indicate
that the net effect of both anti-ferromagnetic
interactions between nickel layers and vortex
formation within each layer is that the sense of
vortices in adjacent layers is opposite. This
arrangement is shown schematically in figure 3
below.
Results Plot (a) below shows Henkel plots for
wires with tNi of 200Å, and values of tCu ranging
from 25Å to 600Å, taken at 20K. The straight line
demonstrates the ideal non-interacting case
considered by Wohlfarth. It is clear that the
magnitude of the deviations from the
non-interacting case decrease with increasing
tCu, suggesting that the interactions are taking
place between the ferromagnetic nickel layers
within the wires. Note the existence of a minimum
interaction level at tCu of 200Å, beyond which an
increase in copper layer thickness does not
correspond to a decrease in interaction strength.
(a)
Plot (b) below shows a plot of ?I against field
applied before remanence measurement for the same
array, showing a negative value for the quantity
?I. This indicates that the interactions taking
place between the nickel layers are
demagnetising, such as dipolar interactions which
lead to flux closure configurations between
layers, i.e. anti-ferromagnetic coupling.
Figure 1. Set up for electrodeposition, bath
contains Cu and Ni ions
Figure 2. Anti-ferromagnetic coupling
Figure 3. Vortex formation with coupling between
layers
(b)
Experiment All remanence measurements were taken
using a Super-conducting QUantum Interference
Device (SQUID) magnetometer supplied by Quantum
Design. Each array was subject to AC
demagnetisation at 350K before cooling to 20K.
Two modes of the remanent magnetisation were then
recorded, the remanence after applying a small
field (less than saturation) to a demagnetised
state (Ir) and the remanence after applying the
same small field to a saturated state (Id). A
study of the Wohlfarth relation (1) then provides
evidence of interactions occurring within the
array, where Is is the remanence after saturation
used to normalise the results. Deviations from
this relation in real systems are due to
interactions between the particles, with a larger
deviation corresponding to stronger
interactions. Id(H)/Is 1-2(Ir(H)/Is). (1
) The effective magnitude of magnetisation
arising from interaction effects relative to the
non-interacting case is then given by ?I(H)
Id(H)/Is 1-2(Ir(H)/Is). (2) A positive
value of ?I corresponds to interactions promoting
the magnetised state, whereas a negative value is
due to demagnetising interactions.
More advanced simulations using the commercially
available Advanced Recording Model software are
currently underway to confirm the preliminary
findings. Remanence measurements using 40nm
diameter wire arrays are also planned, in order
to study the effect of layer diameter on domain
formation
Acknowledgements Euxine Technologies for use of
the ARM micromagnetic simulator I. Kazeminezhad
for his help and assistance in electrodeposition
EPSRC for funding the work NanoMagnetics for
financial support
The high level of noise after removal of high
fields in plot (b) appears to be a defect of the
SQUID, further measurements are planned to
confirm this. Note that peak demagnetisation
occur after removal of low applied fields, when
few dipoles had switched in the applied field.
Similar results, including the existence of a
minimum interaction level, are observed for
arrays with tNi of 50Å