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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 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.
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.
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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.
