Magnetic vortices in Sub-100 nm magnets

Abstract

Presented is a quantitative study of the magnetic vortex state and the vortex core in sub-100 nm magnetic dots. Arrays of magnetic nanodots covering over 1 cm² are fabricated using self-assembled nanopores in anodized alumina. Transition from a vortex to a single domain state for the Fe dots is studied by magnetization measurements (SQUID, VSM, and MOKE) as a function of the dots size and magnetic field. Micromagnetic and Monte Carlo simulations confirm the experimental observations. Thermal activation and exchange bias have a large effect on the vortex nucleation field. Quantitative analysis of grazing incidence small angle neutron scattering measurements with polarization analysis, performed on 65 nm Fe dots yields the magnetization of the vortex core of 140plusmn50 emu/cm³ and its diameter of 19plusmn4 nm, in agreement with the simulations results.

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Magnetic Vortices in Sub-100 nm Magnets
Igor V. Roshchin1, Chang-Peng Li2, Harry Suhl3, Xavier Batlle4, S. Roy5, Sunil K. Sinha3,
S. Park6, Roger Pynn7, M. R. Fitzsimmons8, Jose Mejía-López9, Dora Altbir10,
A. H. Romero11, R. Dumas12, Kai Liu12, Ivan K. Schuller3


Abstract − − Presented is a quantitative study of the magnetic
vortex state and the vortex core in sub-100 nm magnetic dots.
Arrays of magnetic nanodots covering over 1 cm2 are
fabricated using self-assembled nanopores in anodized
alumina. Transition from a vortex to a single domain state for
the Fe dots is studied by magnetization measurements
(SQUID, VSM, and MOKE) as a function of the dots size and
magnetic field. Micromagnetic and Monte Carlo simulations
confirm the experimental observations. Thermal activation
and exchange bias have a large effect on the vortex nucleation
field. Quantitative analysis of grazing incidence small angle
neutron scattering measurements with polarization analysis,
performed on 65 nm Fe dots yields the magnetization of the
vortex core of 140±50 emu/cm3 and its diameter of 19±4 nm, in
agreement with the simulations results.

1 Physics Department, Texas A&M University, College Station, TX, 77843-4242, USA,
e-mail: riv@tamu.edu, tel.: +1 979-845-8520.
2 Physics Department, University of Michigan, MI, USA.
3 Physics Department, University of California - San Diego, La Jolla, CA 92093-0319, USA.
4 Departament de Fisica Fonamental, Universitat de Barcelona, 08028 Barcelona, Catalonia, Spain.
5 Advanced Light Source, LBNL, Berkeley, CA, USA.
6 Physics Department, Pusan National University, Korea.
7 Physics Department, University of Indiana, USA.
8 Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
9 Facultad de Física, Pontificia Universidad Católica de Chile, Santiago, Chile.
10 Departamento de Física, Universidad de Santiago de Chile (USACH), Santiago, Chile.
11 Materials Department, CINVESTAV, Querétaro, Mexico.
12 Physics Department, University of California, Davis, California 95616, USA.
1 EXTENDED ABSTRACT
Magnetism of the nanostructured magnets, which size
is comparable to or smaller than ferromagnetic
domain size offers a great potential for new physics.
Detailed knowledge of magnetization reversal and
possible magnetic configurations in
nanostructures is essential for high-density magnetic
memory. Many theoretical and experimental studies
are focused on a magnetic vortex which in addition to
a circular in-plane configuration of spins has a core, –
the region with the out-of-plane magnetization.
Presented is a quantitative study of the magnetic
vortex state and the vortex core in sub-100 nm
magnetic dots. Arrays of magnetic nanodots covering
over 1 cm2 are fabricated using self-assembled
nanopores in anodized alumina, with a good control
over the dot size and periodicity [1,2]. Magnetization
measurements, performed using SQUID, VSM, and
MOKE indicate transition from a vortex to a single
domain state for the Fe dots (Figure 1) [1,3]. This
transition is studied as a function of the dots size and
magnetic field. Micromagnetic and Monte Carlo
simulations confirm the experimental observations
magnetic
[4]. Thermal activation and exchange bias are found
to have a large effect on the vortex nucleation field
[5]. Direct imaging of magnetic moments in sub-
100 nm dots is extremely difficult and has not been
reported yet. Using grazing incidence small angle
neutron scattering measurements with polarization
analysis, dots are imaged in the reciprocal space.
Quantitative analysis of
performed on 65 nm Fe dots yields the magnetization
of the vortex core of 140±50 emu/cm3 and its
diameter of 19±4 nm (Figure 2), in a reasonable
agreement with the simulations results [1].

such measurements

Figure 1: In-plane hysteresis loops for 20-nm-thick Fe
dots that are 43 (□) and 65 (●) nm in diameter at 10 K
(○- virgin curve for 65 nm dots).

978-1-4244-3386-5/09/$25.00 ©2009 IEEE
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Page 2

Figure 2: (a) Sketch of magnetization direction in the
20 nm-thick Fe dot with the diameter of 65 nm (per
neutron measurements).
magnetization (MZ) profile of the vortex in the same
dot, obtained using micromagnetic simulation.
(b) Out-of-plane


Acknowledgments
This work has benefited from the use of the Lujan
Neutron Scattering Center (Asterix spectrometer) at
LANSCE funded by US DOE-BES. We acknowledge
support from US AFOSR and DOE (DMR-BES);
Texas A&M University, Texas A&M University -
CONACYT Collaborative Research Grant Program,
KOSEF (R01-2008-000-21092-0),
(1050066, and 7070149), Millennium Science
Nucleus (P06-022F); CONACYT
Spanish CYCIT (MAT2006-03999), Catalan Dursi
(2005BE00028, 2005SGR00969), University of
Barcelona (International Cooperation), NSF, and
CITRIS.


FONDECYT
(J-59853-F),
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