Magnetization reversal in circularly exchange-biased ferromagnetic disks
M. Tanase,
1
A. K. Petford-Long,
1
O. Heinonen,
2
K. S. Buchanan,
3,
* J. Sort,
4
and J. Nogués
5
1
Materials Science Division, Argonne National Laboratory, 9700 S Cass Avenue, Lemont, Illinois 60439, USA
2
Recording Heads Operation, Seagate Technology, 7801 Computer Avenue, Bloomington, Minnesota 55435, USA
3
Center for Nanoscale Materials, Argonne National Laboratory, 9700 S Cass Avenue, Lemont, Illinois 60439, USA
4
Institució Catalana de Recerca i Estudis Avançats (ICREA) and Departament de Física, Universitat Autònoma de Barcelona,
08193 Bellaterra, Spain
5
Institució Catalana de Recerca i Estudis Avançats (ICREA) and Centre d’Investigació en Nanociència i Nanotecnologia (ICN-CSIC),
Campus Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
Received 24 September 2008; published 27 January 2009
We investigate the reversal behavior of circularly exchange-biased micron-sized bilayer disks of Permalloy
Py/IrMn and CoFe/IrMn. A circular exchange bias is induced by imprinting the vortex configuration of the
ferromagnetic layer into the IrMn when the disks are cooled in zero external field through the blocking
temperature of IrMn. The resulting circular exchange bias has a profound effect on the reversal behavior of the
ferromagnetic magnetization. In Py/IrMn disks the reversal takes place via vortex motion only, and the behav-
ior is controlled by the exchange bias; it is reversible over a range of small fields and the vortex maintains a
single chirality throughout reversal, determined by the chirality of the exchange bias. In CoFe/IrMn disks the
non-negligible magnetocrystalline anisotropy causes a reversal via both vortices and domain walls resulting in
a finite coercivity, and the behavior is controlled by microstructure. We verify that circular exchange bias does
not give rise to a hysteresis loop shift. It lowers coercivity with respect to the field-cooled case, and in Py/IrMn
disks it even causes completely reversible magnetic behavior. In both Py/IrMn and CoFe/IrMn disks, circular
exchange bias removes the randomness i.e., stochastic processes due to thermal activation inherent in single-
layer ferromagnetic disks and causes the magnetic behavior to be reproducible over time.
DOI: 10.1103/PhysRevB.79.014436 PACS numbers: 75.60.Jk, 85.70.Kh, 75.75.+a, 85.75.Dd
I. INTRODUCTION
Ferromagnetic FM nanostructures supporting vortex
magnetization
1
are of considerable interest because of their
fundamental properties and because of their potential appli-
cations in ultrahigh-density recording media,
2
magnetic ran-
dom access memories,
3,4
and spintronic logic devices.
5
Mag-
netic vortices carry information in their chirality and
polarity
6,7
each having two possible orientations. Thus, they
have four different magnetic states rather than the conven-
tional two magnetic states of other ferromagnetic nanostruc-
tures. Methods to control the chirality in single FM layer
elements exploit an asymmetry either in the element
shape
8–12
or in the applied field, such as using a magnetic
force microscopy MFM tip,
13
magnetic pulses,
14
or a mag-
netic field gradient,
15
as well as the magnetization history.
16
More recently exchange bias EB has been explored in com-
bination with a homogeneous external field to control rever-
sal in Co/IrMn elliptical dots
17
and rings.
18
A combination of
spin torque and Oersted field has been used to switch the
chirality in pseudospin valve rings
19
and the spin torque ef-
fect alone has been predicted to switch the chirality in
vortex-supporting spin-valve disks.
20
EB Ref. 21 obtained by cooling the antiferromagnet/
ferromagnet AF/FM bilayer in a saturating field through
the blocking temperature of the AF has been extensively
studied in both continuous thin films
22–24
and
nanostructures.
25
EB can add an extra mechanism by which
the reversal behavior of the FM vortex can be controlled. EB
patterned magnetic elements have been shown to support FM
vortex structures with shifted constricted loops,
26–28
and the
EB effect was reported to pin the circulating direction of
magnetization in Permalloy Py/IrMn/Py asymmetric rings
after the application of saturating fields.
29
Recently it has
been proposed that exchange bias itself can take on a vortex-
like or circular configuration by zero-field cooling ZFC
patterned Py/IrMn micron-sized disks exhibiting vortex mag-
netization through the blocking temperature of the antiferro-
magnet. The FM vortex field becomes “imprinted” into the
AF resulting in exchange bias with circular symmetry.
30,31
This results in an enhanced stability of the vortex state over
a wider applied field range and a reversible central part of the
hysteresis loop. Unlike conventional exchange bias obtained
by cooling in external fields, the ZFC treatment does not
result in a macroscopic hysteresis loop shift. A displaced
vortex state can be maintained at remanence by reducing the
strength of the magnetic field applied during the cooling pro-
cedure. Magnetization reversal via vortex formation is ob-
served even upon cooling in saturating fields, but a critical
angle appears between the applied and the exchange bias
field, beyond which vortex reversal is replaced by coherent
rotation.
26
The circular exchange bias geometry provides a
low-energy vortex reference state, which has been shown via
micromagnetic modeling to control the magnetization dy-
namics vortex core precession and spin-wave dynamics of
NiFe/IrMn disks.
32
In this paper the magnetization reversal
in micron-sized disks of CoFe and Py, both as single FM
layers and exchange biased to 5 nm of IrMn, has been inves-
tigated using Lorentz microscopy, micromagnetic simula-
tions and magneto-optical Kerr magnetometry MOKE.
This combination of techniques
33,34
allows us to address both
the collective behavior of arrays of disks and the behavior of
individual disks and to make a quantitative determination of
PHYSICAL REVIEW B 79, 014436 2009
1098-0121/2009/791/0144369 ©2009 The American Physical Society 014436-1