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November 20, 2003
Magnetic Switching in Multilayer ‘Nanomagnets’
F.J. Castańo1, Y. Hao1, S. Haratani1, C.A. Ross1, B. Vögeli2, H.I. Smith2, C. Sánchez-Hanke3, C-C. Kao3, X. Zhu4, and P. Grütter4
1Department of Materials Science and Engineering, MIT, Cambridge, MA;
2Department of Electrical Engineering and Computer Science, MIT, Cambridge, MA;
3National Synchrotron Light Source, BNL, Upton, NY;
4Center for the Physics of Materials, Department of Physics, McGill University, Montreal, Canada
This study investigated the magnetization reversal processes
that take place in arrays of lithographically-patterned magnetic
bars. Each bar is 70 nm x 550 nm in dimensions, and is made from a NiFe 6nm/ Cu 3 nm/ Co 4 nm multilayer stack. Arrays of these
nanomagnets were characterized using a combination of magnetic force
microscopy (MFM), alternating gradient magnetometry (AGM), and
scattering experiments using synchrotron radiation. Both magnetic
layers (i.e. the Co and the NiFe) form single-domain states at
remanence and switch abruptly, but the collective magnetization
reversal of the array shows a wide distribution of switching fields
due to variability between the elements. Elementally-specific
hysteresis loops obtained from synchrotron scattering experiments
enable the separate reversal of the Ni, Fe, and Co to be followed.
The
magnetic properties of lithographically-defined multilayered
magnetic solids are of considerable interest for the development of
high-density magnetoresistive random access memory (MRAM) devices.
The nanoscale bar-shaped magnets used as memory cells in MRAMs are
composed of at least two magnetic layers sandwiching a non-magnetic
insulating or metallic spacer. Future high-density MRAM devices will
require layered magnetic structures with thicknesses below a few
tens of nanometers and in-plane dimensions in the sub-100 nm regime.
Within these structures, the individual magnetic layers are
magnetized parallel to their length, and their switching field
depends on their dimensions and compositions. Additionally, the
magnetic layers interact by both exchange and magnetostatic
coupling, which modifies the switching field and stabilizes specific
remanent states.

In this work, we first used MFM to show that individual Co/Cu/NiFe
sandwich nanomagnets could be magnetized into four distinct states,
labelled A-D in Figure 1. For comparison, AGM measurements, measured
on a piece of the array containing ~109 nanomagnets, show the
structures switching from state A to B to D as the field is swept
from +1000 Oe to –1000 Oe. As the field sweeps from –1000 Oe to
+1000 Oe the structures switch from state D to C to A (Figure 1).
Minor loops (shown as solid points) allow the switching field of the
NiFe layers, and the interaction field (i.e. the field that the Co
exerts on the NiFe) to be measured. In this sample the Co hard
layers reverse over a range of fields centered at 410 Oe. The
interaction field was 60 Oe and the switching field of the NiFe
layers was 125 Oe.

In addition, utilizing modulated circularly polarized soft x-rays
from the X-13A beamline, both reflectivity and magnetic circular
dichroism (MCD) patterns were measured for these arrays. The
magnetic parameters deduced from elementally-specific hysteresis
loops, obtained from the difference signal close to the L3
absorption edge of the Co and Ni present in the sample, agreed well
with those measured by AGM. In the data of Figure 2, the reversal of
the Ni and Fe occurs at identical fields, as expected, while the Co
reverses at a higher field. The Ni and Fe loops are both offset from
zero by 40 Oe, as a result of the interaction field. All three
elements show a distribution of switching fields consistent with AGM
data. The average coercive field for the Ni was 120 Oe and for the
Co was 410 Oe, which is similar to the results obtained from AGM.
These experiments show that synchrotron experiments provide a
valuable measurement of magnetization reversal, even in buried
layers, and can be used to track the behavior of complex
multilayered magnetic structures.
BEAMLINE
X13A
FUNDING
MIT, TDK Corp., Japan, DARPA, and at McGill, NSERC and FCAR.
PUBLICATION
F. J. Castańo, Y. Hao, S. Haratani, C. A. Ross, B. Vögeli, H. I. Smith, C. Sánchez-Hanke, C-C. Kao, X. Zhu,
P. Grütter. "Magnetic force microscopy and X-ray scattering study of 70 x 550
nm2 pseudo-spin-valve nanomagnets", J. Appl. Phys. 93 7927 (2003).
FOR MORE INFORMATION
C.A. Ross
Department of Materials Science and Engineering
Massachusetts Institute of Technology
Cambridge, MA
Email: caross@mit.edu
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