A New "Twist" on Polarized Neutron Reflectivity: Imaging Exchange-Spring
Magnets
Kevin V. O'Donovan, Julie A. Borchers, Charles F. Majkrzak
NIST Center for Neutron Research, National Institute of Standards and Technology,
Gaithersburg, MD 20899
Olav Hellwig, Eric E. Fullerton
IBM Almaden Research Center, San Jose, CA 95120
The need for small yet powerful magnets for a wide variety of devices provides
impetus to design permanent magnets with larger magnetization M and energy
products (BH)max. Kneller and Hawig proposed a model
for an "exchange-spring magnet" in which grains of a magnetically hard
material are imbedded in a magnetically soft matrix. Layered epitaxial
samples show magnetic behavior comparable to that of granular composites.
We used the NG-1 polarized neutron reflectometer at NCNR to examine the
spin structure of an Fe55Pt45|Ni80Fe20
exchange-coupled epitaxial film in fields up to 630 mT. Using the depth-dependent
properties of neutron reflectometry, we observe the spiral structure of
the magnetization and track its evolution with field. We also identified
some inconsistencies with earlier predictions. These measurements directly
probe the complex magnetic microstructure and highlight the competing properties
of the hard and soft components.