Table Of ContentKinetic self-organization of trenched templates for the fabrication of versatile
ferromagnetic nanowires
B. Borca,1 O. Fruchart,1,∗ Ph. David,1 A. Rousseau,1 and C. Meyer1
1
Institut N´eel, Department of Nanosciences (CNRS-UJF-INPG), BP166, F-38042 Grenoble Cedex 9, France
(Dated: February 6, 2008)
7 We have self-organized versatile magnetic nanowires, i.e. with variable period and adjustable
0 magnetic anisotropy energy (MAE). First, using the kinetic roughening of W(110) uniaxial tem-
0 plates of trenches were grown on commercial Sapphire wafers. Unlike most templates used for
2 self-organization, those have a variable period, 4-12nm are demonstrated here. Fe deposition then
n results in the formation of wires in the trenches. The magnitude of MAE could be engineered up
a or down by changing the capping- or underlayer, in turn affecting the mean superparamagnetic
J temperature, raised to 175K so far.
4
1
The bottom-up approach is promising for the fabrica- based on the growth of body-centered-cubic (110) mate-
] tionofnanostructuresatmoderatecost,withbetterreso- rials on nominally-flat Saphir yielding parallel trenches
r
lutionandlessmicroscopicdefectsthanwithlithography. with an adjustable period, which we then use to grow
e
h Inepitaxialself-organization(SO)thebuildingblocksare magnetic nanowires. We could tune the magnitude of
t atomsthataggregatetoeachotherduringgrowth,apro- the MAE of the wires using suitably-chosencapping lay-
o
cess that can be engineered to fabricate wires and dots ers or underlayers. The use of commercial wafers while
.
at with lateral dimensions from the micron size1 down to achievingsomecontrolonboth the period andthe MAE
m the atomic size2,3,4. The interest of SO for fundamen- represents a significant advancement in the versatility of
tal research is often the narrow size dispersion5, so that SO nanowires.
-
d macroscopic measurements reveal the properties of one The samples weregrownin a set ofultra-highvacuum
n single nanostructure. Issues like the increase of orbital chambers using pulsed-laser deposition with a Nd-YAG
o
momentum and magnetic anisotropy energy (MAE) at laser (λ = 532nm). The chambers are equipped with
c
[ atomic edges or kinks have been addressed2,3,5. Con- a quartz microbalance, sample heating and a translat-
cerning applications it is sometimes argued that SO ar- ing mask for the fabrication of wedge-shaped samples.
1 rays could be used by addressing single nanostructures A 10 keV Reflection High Energy Electron Diffraction
v
one by one, to store one bit of information for example. (RHEED) setup with a CCD camera synchronized with
4
Amorerealisticviewistheuseofthearrayasamaterial lasershotspermitsoperationduringdeposition. AnOmi-
0
3 with specific properties arising from the nanoscale. This cron room-temperature Scanning Tunneling Microscope
1 is the case of semiconductor quantum dots with lasing (STM-1) and an Auger Electron Spectrometer (AES)
0 properties6. are available. The metallic films are grown on sapphire
7 DomagneticSO systemsmeetthe requirementsofap- nominally-flat (1120) commercial wafers. A detailed de-
0
plied materials? A prerequisite is that some versatility scription of the chambers and growth procedures can be
/
t of geometrical as well as physical properties is achieved, foundin15. The magneticmeasurementswereperformed
a
m liketuning themagnitudeoftheMAE.We focushereon ex-situ on 5nm-Mo-capped samples, with a Quantum
the fabrication and properties of wires, which lie at the Design Superconducting QUantum Interference Device
-
d backgroundofthefast-developingfieldofspinelectronics (SQUID) magnetometer.
n making use of the propagation of domain walls in wires Ordered arrays of wires were obtained in three steps,
o for storage or logics devices7,8,9. consisting of the fabrication of 1. a non-magnetic flat
c
bufferlayer;2.anon-magnetictemplatedisplayingauni-
: SO magnetic wires are often achieved by step-
v axial array of trenches; 3. Fe wires by layer-by-layer de-
decoration of vicinal surfaces10,11,12. This approach is
i position at the bottom of the trenches.
X not versatile as a new crystal has to be prepared with a
The first step is the preparation of a smooth buffer
r specificmiscutwhenevertheperiodneedstobechanged.
a Templates resulting from kinetic effects are potentially layer. A seed layer of Mo (nominal thickness Θ=1nm)
followedby W (Θ≈10nm) aredeposited onsapphireat
moreversatileastheperiodcanbechangedwithprocess-
◦
ingparameters,liketemperature. Thishasbeenexplored roomtemperature (RT) followed by annealing at 800 C.
This yields a (110) surface of quality similar to that
using ion etching under grazing incidence to create rip-
plesonsurfaces,independentlyofcrystallinedirections14. of metal single crystals, with atomically-flat terrasses of
width up to hundreds of nanometers15,16.
However a significant control of the period has not been
demonstrated so far. Here we explore a new approach, The second step consists of the preparation of a non-
magnetic template. It was inspired by reports of the
kinetic uniaxial roughening of films of the bcc elements
Fe(110)17 and W(110)18 during homoepitaxy at moder-
∗[email protected] atetemperature,explainedbyanisotropicdiffusionalong
2
the steps and the occurrence of an Ehrlich-Schwoebel a) 0] [110] b)
1
-
barrier. FIG. 1a (resp. c) shows the topography of a [1
◦
W(110) layer obtained upon deposition at 150 C (resp.
(110)
◦
following nucleationat 550 C and subsequentgrowthat [ [001]
0
150◦C). A uniaxial arrayof trenches aligned along [001] 01
]
is thus obtained, with a period of 4nm (resp. 12nm).
The STM cross-sections shown as insets reveal a depth
of 0.6 − 0.8nm (resp. 2-3nm). The structure of the
trenches can be probed using RHEED with the electron
20nm
beam along [001] (FIG. 1b,d). The diffraction patterns
consistofarrowswhosehalf-anglerevealstheslopeofthe
microfacets. The arrows are much sharper for trenches
c) [110] d)
of larger depth because the coherence length along the
facets is increased. This allows us to determine accu-
rately the angle of the facets to θ = 18±1◦, in perfect (110) [001]
[001]
agreementwiththetriangular-likeSTMcross-sectionsof
FIG. 2c. This angle is unambiguously ascribed to facets
◦
of type {210}, with θ = 18.43 expected. The monitor-
◦
ing of RHEED patterns during growth shows that 18
is a stationary value, reached after a few nanometers of
nominal thickness. The occurrence of a stationary angle 20nm
garantiesthatthe shapeofthe trenchesisnotinfluenced
by fluctuations of the width of the trenches, and thus
displaysessentiallynodistribution. Thisiscrucialasthe FIG.1: W(110)templates: (a,c)100×100nmSTMimagesof
dispersion of MAE increases tremendously for nanosized templates with small (a) and large (c) period, see text (b,d)
systems in relation with the distribution of structural RHEED patterns along the [001] azimuth for surfaces a and
environments19. The existence of a stationary angle was c, resp. The sample lies vertical, on theleft of thepatterns.
postulated in Ref.18, however the analysis concluded to
◦
facets of type 310 with θ ∼ 26.57 for W. The discrep-
ancy may come from the fact that the LEED patterns
used in Ref.18 were broad because trenches with a small
period were studied. We could produce templates with
the same facets for Mo, Nb, V and Ta by deposition at
room temperature.
Thethirdstepconsistsofthefabricationofwires. Feis
◦
depositedonthe templates at150 C (FIG.2),forwhich
layer-by-layer growth occurs with PLD on a smooth
surface12,15. For all deposits we have used W templates,
which remain perfectly stable up to 300◦C. For tem- a) 2 b)
plates of small period the surface gradually smoothes
1
and becomes essentially flat for Θ = 2AL(Atomic Lay-
)
ers; not shown here). The details of the early stages of nm0
growth are easier to investigate for templates of large (3 c)
t
period. For these STM cross-sections (FIG. 2)b reveal h
g2
a flat level at the bottom of the trenches, suggesting a ei
H
progressive filling and thus yielding Fe wires with a tri- 1
angular cross-section. Disconnected wires can be formed
up to Θ = 2.5AL, beyond which percolation sets in for 20nm 0
0 20 40
the 10nm period. Distance (nm)
We report below the magnetic properties of Fe wires
FIG. 2: (a) 100nm STM image of wires Fe(2.5AL)/W
prepared on W(110) templates with a mean period of
(b) Cross-section along the line shown in a (c) Cross-section
10nm and an Fe nominal thickness of 2.5AL. The mean
along theline shown in FIG. 1c (same scales).
thickness and width of the wires are 1nm and 7nm, re-
spectively, as deduced from STM.
ForMo/Fe/Wwirestheeasyaxisofmagnetizationlies
in-the-plane along the wires, i.e. along [001] (FIG. 3a).
The MAE is deduced from loops along [110] like E =
a
M
µ0R0 S HdM ≈ 3 × 105J/m3 at 10K. Ea originates
3
fromthesumofseveralcontributions,amongwhichonly thebottomofthetrenchesbyamagneticmaterialunder
the dipolar energy E can be estimated reliably. Other layer-by-layer deposition conditions, here Fe. Periods in
d
sources of MAE are surface (N´eel-type) expected to fa- the range4−12nmweredemonstrated. Owingtoaself-
vorthe[110]directionforthe(bottom)Fe/Winterface20 limiting effect the micro-facets are all of type 210 with
and [001] for the (top) Mo/Fe interface21, step-edge for anangleof ≈18±1◦, whichis promisingfor attaining a
Fe/W, expected to favor [110]10; magneto-elastic – un- lowdistributionofphysicalproperties. Concerningmag-
like the case of thin films, here the large density of steps netism the easy axis of magnetization lies in-the-plane
isexpectedtoinduceasignificantout-of-planemismatch along the wires. We have demonstrated the possibility
and shear stress, so that no figure or even sign can be to tailor its magnitude using interface MAE with differ-
reliably predicted; finally the bulk Fe MAE is negligible. ent capping- or underlayers, bringing the mean blocking
Surprisingly E ≈ E for this sample despite this com- temperature to 175K inserting an ultrathin underlayer
a d
plex situation, similarly to (Fe,Ag) self-organized arrays 1 a) FC d)
of wires22. S
M 0 ) ZFC
The loops measured at different temperatures with M/ [001] u. Mo
H//[001] have a rather square shape, and at remanence -1 [1-10] a. W
(
full saturation is observed (FIG. 3b). The coercivity -1 Fiel0d (T) 1 t FC e)
n
continuously decreases with temperature while Ms re- me Mo ZFC
mains essentially unchanged, suggesting a superparam- o
1 b) m W
agneticbehavior. Theultimateblockingtemperaturede- S c
tFeCrm)pinreodcebssyitsh≈e ”1z6e0rKo fi(eFldIGc.o3odli)n.gT/fiheeldmceoaonlivnaglu”e(ZofFTCB- MM/-10 4251K500KK0Kgneti FC f)
is around ≈ 100K, defined as half-way up the zero-field 1 c) MaZFC
S
cooling remagnetization curve. M/ 0 10K Al
InthefollowingwereportontheuseofsurfaceMAEto M-1 2550KK W
100K
tailor the magnitude of the MAE of the wires and move -400 0 400 0 100K 200K 300K
towards features that would be required for applications Field (mT) Temperature
likerecordingmedia,likefunctionalityatroomtempera-
ture and adjustment of MAE. First, the very same wires FIG.3: Magnetization curvesofFe(2.5AL)wires with10nm
period. Mo/Fe/W wires (a) at 10K in-plane along [001]
were fabricated after inserting a 1AL-thick underlayer
and [110] (b) in-plane along [001] at different temperatures;
of Mo on the W trenches. The magnetic easy axis is
(c) same as b, however for Mo/Fe/Mo/W wires, see text;
againalongthewires,howeveritsmagnitudeisincreased,
(d-f) In-plane field-cooled (FC) and zero-field-cooled (ZFC)
which is consistent as Fe/Mo interfaces favoralignement
magnetization of (d) Mo/Fe/W (e) Mo/Fe/Mo/W and
of magnetization along [001]21. This brings the mean (f)Al/Fe/Wwires. Aschematicillustration ofthesamplesis
TB from 100K without underlayer to 175K with under- shown in insets.
layer (FIG. 3e). The coercive field is increased at all
temperatures, compare FIG. 3b-c. On the reverse T is
B
lowered to 40K for Al/Fe/W (FIG. 3f).
of Mo.
B. B. and A. R. acknowledge financial support from
Conclusion
French R´egion Rhoˆne-Alpes (mobility program), and
FP6 EU-NSF program (STRP 016447 MagDot), respec-
We have demonstrated a novel route for the self- tively.
organizationofarraysofplanarnanowiresonnominally-
flat commercial wafers. It is based on the fabrication of
atemplate oftrenchesuponthe depositionunderkinetic
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