Low temperature hysteresis in high anisotropy systems D.J. Branagan a, * , M.J. Kramer b , K.W. Dennis b , R.W. McCallum b a Idaho National Engineering and Environmental Laboratory, Bechtel BWXT Idaho LLC, Idaho Falls, ID 83415, USA b Ames Laboratory, Iowa State University, Ames, IA, USA Received 9 April 2002; accepted 10 April 2002 Abstract This paper is focused on understanding hysteresis in Dy 3 Al 2 , which is a member of a class of high anisotropy compounds containing high fractions of rare earth elements with no 3-d transition metals. Analysis of the data suggests that the principal domain wall pinning mechanism does not involve the grain boundaries but instead arises from interactions between either the ferromagnetic glass or the ferromagnetic Dy 3 Al 2 phase with the antiferromagnetic DyAl second phase through an exchange bias mechanism. Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Magnetic properties; Coercivity; Hard magnets; Melt spinning; Rare earth 1. Introduction The magnetic properties of rare earth rich phases, containing no 3-d transition metals, such as Dy 3 Al 2 , were first studied almost four decades ago [1,2]. Due to the classically weak 4f/4f ex- change coupling, this class of compounds is gen- erally characterized by low Curie temperatures ( <150 K) but due to their high rare earth content, these phases can exhibit incredibly high levels of magnetocrystalline anisotropy coupled with high magnetization. Single crystals of Dy 3 Al 2 still hold the record measured energy product of 73 MGOe at 4 K [1]. However, the hard magnetic properties of these single crystals diminishes rapidly above 4 K and the coercivity is entirely lost at 20 K [1]. Previous studies on single crystals have suggested that the anisotropy dominated monotonically thin domain walls are pinned through an intrinsic pinning mechanism through point defects [3]. There is renewed interest in low temperature rare earth rich phases from both a basic and technological viewpoint. Phases, such as Dy 3 Al 2 or TbNi 1x Cu x , when studied at low temperature may be ideal for basic studies on hysteresis due to their high anisotropy resulting in extremely thin do- main walls, sharp critical nucleation fields, and low magnetic viscosity. Additionally, there are an in- creasing number of emerging applications at low temperature such as satellites, spacecraft, and pos- sibly Maglev train systems where very high energy densities may be needed due to weight or volume restrictions. Many of these applications may re- quire using existing liquid nitrogen cooling which necessitates operating temperatures at least up to 77 K. Currently, maintaining coercivity to this Scripta Materialia 47 (2002) 537–543 www.actamat-journals.com * Corresponding author. E-mail address: brandj@inel.gov (D.J. Branagan). 1359-6462/02/$ - see front matter Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6462(02)00175-6