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.