The Role of Disorder in the Magnetic Properties of Mechanically Milled Nanostructured Alloys
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The Role of Disorder in the Magnetic Properties of Mechanically Milled Nanostructured Alloys Diandra L. Leslie-Pelecky1, Elaine M. Kirkpatrick1, Tom Pekarek2, Richard L. Schalek3, Paul Shand4, Deborah S. Williams1, and Lanping Yue1 1
Center for Materials Research and Analysis and Department of Physics & Astronomy, University of Nebraska, Lincoln NE 68588-0111 2 Department of Chemistry and Physics, University of North Florida, Jacksonville, FL 32224 3 Composite Materials and Structures Center, Michigan State University, East Lansing MI, 48824 4 Physics Department, University of Northern Iowa, Cedar Falls, Iowa 50614 ABSTRACT Mechanical milling provides a unique means of studying the influence of grain size and disorder on the magnetic properties of nanostructured alloys. This paper compares the role of milling in the nanostructure evolution of two ferromagnets – SmCo5 and GdAl2 – and the subsequent impact of nanostructure on magnetic properties and phase transitions. The ferromagnetic properties of SmCo5 are enhanced by short (< 2 hours) milling times, producing up to an eight-fold increase in coercivity and high remanence ratios. The coercivity increase is attributed to defect formation and strain. Additional milling increases the disorder and produces a mix of ferromagnetic and antiferromagnetic interactions that form a magnetically glassy phase. GdAl2, which changes from ferromagnetic in its crystalline form to spin-glass-like in its amorphous form, is a model system for studying the dependence of magnetically glassy behavior on grain size and disorder. Nanostructured GdAl2 with a mean grain size of 8 nm shows a combination of ferromagnetic and magnetically glassy behavior, in contrast to previous studies of nanostructured GdAl2 with a grain size of 20 nm that show only spin-glass-like behavior. INTRODUCTION Mechanical milling is a high-energy deformation process that progressively introduces defect structures (dislocations and vacancies), atomic-scale chemical disorder and elastic strain energy into the initially crystalline starting powders through the shearing actions of ball-powder collisions [1,2]. Mechanical milling can be used to produce a variety of effects in intermetallic alloys due to the complex dependence of the nanostructure on milling intensity, temperature, and other factors [3]. The magnetic properties of a nanostructure depend on the intrinsic material properties, but also on grain size and grain size distribution, the magnetic character of the interphase (the region between grains), and the intergrain magnetic coupling. The challenge is to separate the different contributions to the overall magnetic properties by carefully controlling and characterizing nanostructure. The versatility of mechanical milling makes it a valuable tool for altering a material’s structure and thus improving our understanding of correlations between nanostructure and magnetism. U5.1.1
This paper compares the magnetic behavior of two mechanically milled alloys. SmCo5 and GdAl2 are both ferromagnetic in their crystalli
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