Ordered Ni nanohole arrays with engineered geometrical aspects and magnetic anisotropy

Article abstract of DOI:10.1063/1.2737373

Ni nanohole arrays are prepared by a replication process involving sputtering, polymer molding pressing, and electroplating techniques, using anodic alumina membranes as templates. Nanohole diameter to interhole distance ratio is engineered by suitable template processing. From the analysis of the magnetization curves for increasing nanohole diameter, it is concluded that coercivity increases due to the pinning of domain walls to nanoholes, while in-plane anisotropy decreases owing to local shape anisotropy effects.

Full text of DOI:10.1063/1.2737373

Ordered Ni nanohole arrays with engineered geometrical aspects and
magnetic anisotropy
D. Navas, M. Hernández-Vélez, M. Vázquez, W. Lee, and K. Nielsch
Citation: Appl. Phys. Lett. 90, 192501 (2007); doi: 10.1063/1.2737373
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APPLIED PHYSICS LETTERS 90, 192501 ͑2007͒
Ordered Ni nanohole arrays with engineered geometrical aspects
and magnetic anisotropy
D. Navas,a͒ M. Hernández-Vélez,b͒ and M. Vázquez
Instituto de Ciencia de Materiales, CSIC, 28049 Cantoblanco (Madrid), Spain
W. Lee and K. Nielsch
Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany
͑Received 8 February 2007; accepted 13 April 2007; published online 7 May 2007͒
Ni nanohole arrays are prepared by a replication process involving sputtering, polymer molding
pressing, and electroplating techniques, using anodic alumina membranes as templates. Nanohole
diameter to interhole distance ratio is engineered by suitable template processing. From the analysis
of the magnetization curves for increasing nanohole diameter, it is concluded that coercivity
increases due to the pinning of domain walls to nanoholes, while in-plane anisotropy decreases
owing to local shape anisotropy effects. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2737373͔
Highly ordered nanostructures consisting of nanopore
and nanohole arrays are recently attracting great attention
due to their application in high density magnetic storage
media.1 Different magnetic hole arrays are prepared by litho-
graphic patterning1,2 and by alternative less expensive
techniques.3 For example, nanoporous membranes have been
used as templates.4 The geometry of nanoporous membrane
can be identically transferred to different films by successive
replica/antireplica processes. Various routes have been pro-
posed where the final replicated nanostructures consist of
highly ordered nanohole or nanochannel arrays ͑e.g., in glass
or polymeric membranes͒.5 Recently, metal nanohole arrays
based on the replication of anodic alumina membrane
͑AAM͒ have been reported.6 In nanohole magnetic films, the
holes act themselves as pinning centers for the wall displace-
ments and their periodic distribution determines the whole
magnetization process.1,2
The objectives of the present report have been ͑i͒ the
design and fabrication of ordered Ni nanohole arrays, ͑ii͒ the
tailoring of nanohole diameter while keeping constant the
interhole distance, and ͑iii͒ the magnetic characterization of
the magnetization process analyzing the coercivity and mag-
netic anisotropy.
Two-step anodized AAM has been used as initial tem-
plates. A scheme of preparation processes is presented in Fig.
1. Nanoporous anodic alumina membranes were grown from
high-purity ͑99.999%͒ Al foils by two-step anodization pro-
cess, as described elsewhere.6,7 Main features of AAM tem-
plates ͓Fig. 1͑a͔͒ are pore diameter ͑d͒ of around 35 nm,
interpore distance ͑D͒ of 105 nm, and pores self-assembling
into close packed structure in 2ϫ2 ␮m2 size domains. To
gradually increase the hole diameter, a controlled widening
process was performed by immersing the AAMs in 5 wt. %
phosphoric acid solution at 35 °C for a range of time inter-
vals, with a dissolution speed of 0.75 nm/min. Thus, the
final pore diameters of nanoholes ranged from 35 to 70 nm.
Afterwards, a 15 nm thick Au layer was sputtered onto the
surface of the AAM, serving as an active electrode for sub-
sequent electroplating ͓Fig. 1͑b͔͒. Then, melted poly͑methyl
methacrylate͒ ͑PMMA͒ was pressed into the AAM to fill in
the holes ͓Fig. 1͑c͔͒. Two selective chemical etchings were
used to remove both the Al substrate ͑a solution of 50 ml
HCl, 3.4 g CuSO4, and 100 ml H2O͒ and the alumina mem-
brane ͑dipping the sample into a 30 wt. % phosphoric acid
solution͒. Hexagonally ordered arrays of PMMA nanopillars
were thus obtained with a Au thin film at the bottom ͓Fig.
1͑d͔͒. Subsequently, Ni was electroplated using the Au layer
as cathode ͓Fig. 1͑e͔͒ and Watts-Bath ͑300 g/l
NiSO4·6H2O, 45 g/l NiCl2·6H2O, and 45 g/l H3BO3, pH
=4.5͒ as electrolyte. Electroplating was performed at 35 °C
for 10 min with 1 mA/cm2 current density and magnetic
stirring. Finally, the PMMA nanostructure was removed by
chemical action of chloroform, and a freestanding Ni mem-
brane was obtained ͓Fig. 1͑f͔͒. Typical thickness of Ni mem-
branes was around 200 nm. To complete the study, a continu-
ous Ni thin film was prepared under the same electroplating
conditions.
Morphological characterization was carried out with a
Jeol High Resolution Scanning Electron Microscopy
͑HRSEM͒ model JM6400. Figure 2 shows the HRSEM im-
ages of the Au surfaces of Ni membranes with different hole
diameters. The morphological features of the AAMs tem-
plates are exactly replicated at the Au surface. However,
some differences are observed in the Ni surfaces HRSEM
FIG. 1. Preparation of Ni nanohole arrays from porous alumina membrane
͑a͒: a Au nanolayer is sputtered onto the top of the membrane ͑b͒; PMMA
polymer is pressed to fill in the pores ͑c͒; after flipping, a polymeric pillar
array on a solid polymeric layer remains after Al and Al2O3 are removed ͑d͒;
Ni is electroplated using the Au layer electrode ͑e͒; Ni nanohole array is
obtained after removing the polymer ͑f͒.
a͒
Author to whom correspondence should be addressed; electronic mail:
danavas@icmm.csic.es
b͒
Permanent address: Applied Physics Department, C-XII, UAM, Canto-
blanco 28049, Madrid, Spain.
0003-6951/2007/90͑19͒/192501/3/$23.00
90, 192501-1
© 2007 American Institute of Physics
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192501-2
Navas et al.
Appl. Phys. Lett. 90, 192501 ͑2007͒
FIG. 3. In-plane ͑᭿͒ and out-of-plane ͑ᮀ͒ hysteresis loops of Ni nanohole
array with 50 nm pore diameter.
netic saturation and relatively small coercivity͒. Previous
studies on lithographed nanohole arrays2–4 were done with
in-plane magnetic field applied along different azimuthal ori-
entations in order to determine the magnetization easy axis.
It was concluded that the induced easy axes are the directions
along which the holes are furthest separated. Here, such mea-
surements are not needed since the holes are well ordered
along given orientations but only within domains about 2
ϫ2 ␮m2 average size. Therefore, the effective anisotropy
induced by different domains is averaged out for the whole
array.
Figure 4 shows the in-plane loops for a continuous Ni
film and two Ni nanohole arrays with 35 and 62 nm hole
diameters. The presence of the holes changes the shape of
the loops. While coercivity of the continuous Ni film is
39 Oe, those of Ni hole arrays with 35 and 62 nm hole di-
ameters are 107 and 126 Oe, respectively. That increase of
coercivity is ascribed to the pinning of the domain walls to
the nanoholes. This variation is consistent with the relation-
ship between coercivity and packing fraction predicted by
Hilzinger and Kronmüller.8. Coercivity follows a law Hc
ϰ1/͑D−d͒, ͑see inset of Fig. 4͒, similarly as reported
elsewhere.2 This is explained considering that a stronger pin-
ning effect is expected for decreasing interhole distance ͑D
−d͒. Hence, stronger magnetic field is needed to overcome
the pinning and created by the nanoholes. The possibility is
FIG. 2. HRSEM images of Ni nanohole arrays replicating the template hole
diameters of 35 nm ͑a͒, 50 nm ͑b͒, 62 nm ͑c͒, and 70 nm ͑d͒.
images. The ordering of the AAM is only perfectly replicated
into the metallic membranes with higher diameters ͑d=62
and 70 nm͒. To understand it, we should consider that for
AAM templates with the smallest pore diameters ͑d
=35 nm͒, the process of filling the pores with PMMA ͓Fig.
1͑c͔͒ becomes harder and, subsequently, the height of the
polymeric pillars ͓Fig. 1͑d͔͒ is shorter. In this way, during the
Ni electroplating, the metal can overflow the polymeric col-
umns, giving rise to the growth of a metallic film covering
the PMMA template, and a Ni antidot film is created instead
of a nanohole film. For samples with intermediate pore di-
ameter ͑d=50 nm͒, only some of the pores in the polymeric
template are overflowed with Ni after the electroplating.
Therefore, we conclude that a minimum hole diameter of
about 60 nm is required to prepare metallic membranes by
this technique. To overcome such limitations, a diluted sol-
vent of PMMA and an organic solvent could be alternatively
used, which would probably enable an easier filling of the
pores even with smaller diameters.
From the technological viewpoint, the proposed fabrica-
tion method is proven to be a relative simple and low cost
procedure. In Ref. 6 methyl methacrylate monomer was in-
jected into the holes under vacuum and its polymerization
was achieved by ultraviolet radiation or by heating ͑40 °C
for 24 h͒. In the present case, PMMA polymer is melted,
pressed ͓Fig. 1͑c͔͒ and after a few minutes the polymeric
template is ready for use. Xiao et al.4 reported an alternative
way to prepare Ni nanohole arrays with tailored pore diam-
eter and constant interhole distance by sputtering Ni onto
AAM templates. It was there observed that for Ni mem-
branes with thickness larger than tens of nanometers, the
hole diameter was irregular ͑gradually decreased from the
bottom to the top surface͒.
Magnetic properties of the continuous Ni film and the
different Ni nanohole arrays have been studied by Vibrating
Sample Magnetometer ͑VSM͒, EG&G Princeton Applied
Research, model 155. As an example, Fig. 3 shows the hys-
teresis loops of Ni nanohole array with 50 nm hole diameter
for in-plane and out-of-plane applied field. For in-plane ap-
plied field the rather square shaped loop denotes an in-plane
magnetization easy axis, while out-of-plane hysteresis loop
is typical for hard anisotropy axis ͑high field to reach mag-
FIG. 4. In-plane loops for continuous film ͑᭿͒ and for two Ni nanoholes
arrays ͓d=35 ͑᭺͒ and 62 nm ͑᭡͒ hole diameters͔. Linear evolution of co-
ercivity Hc with 1/͑D−d͒ ͑D=interhole distance͒ ͑Inset͒.
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192501-3
Navas et al.
Appl. Phys. Lett. 90, 192501 ͑2007͒
an out-of-plane anisotropy component has been recently con-
firmed by Magnetic Force Microscopy imaging.9
In summary, Ni nanohole arrays have been fabricated by
two-step/positive replication process, using AAM templates
with tailored hole diameter. The increase of coercivity in
patterned films denotes the pinning effect of the nanoholes.
The reduction of the in-plane anisotropy with hole diameter
is ascribed to local shape anisotropy effects. Finally, pattern-
ing of Ni hole arrays offers possibilities to control magnetic
anisotropy and coercivity of these films.
The authors thank U. Gösele who facilitated the work at
MPI and J. L. Baldonedo for HRSEM images. The work was
supported by the Spanish Ministry of Education and Culture
under Project No. MAT04-00150 and Integrated Action
Spain-Hungary 2005-7 HH04-0003, and by the German Fed-
eral Ministry of Education and Research under project
BMBF, FKZ 03N8701.
FIG. 5. Coercivity Hc, ͑b͒, reduced remanence ͑MR /Msat͒ ͑᭺͒ and in-plane
anisotropy ͑᭿͒ as a function of hole diameter.
thus opened to control the coercivity by tailoring the hole
diameters and the interhole distances.
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nanohole diameter. Figure 5 shows the evolution of coerciv-
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