Investigations of the Magnetic Perpendicular Exchange Bias in L1 0 FePt/NiO Bilayer Thin Films
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.421
Investigations of the Magnetic Perpendicular Exchange Bias in L10 FePt/NiO Bilayer Thin Films Zachary B. Leuty1 and Robert A. Mayanovic1 1
Physics and Materials Science, Missouri State University, Springfield, MO 65897
ABSTRACT
We report on the exploration of perpendicular exchange bias in iron platinum/nickel oxide (FePt/NiO) bilayer thin films grown using pulsed laser deposition (PLD) on MgO (100) substrates. Exchange bias is an important property for giant magnetoresistance, and, as such has promise for applications in spin valves, magnetic sensors and magnetic random access memory. The magnetic L10 phase of FePt is known for having high perpendicular magnetic anisotropy, tunable coercivity/grain size and large magnetic storage density. The FePt layer was first deposited directly on MgO, followed by the deposition of the NiO layer on top of the FePt layer. The coercivity of the L10 FePt layer was tuned during growth to form a hard or soft magnetic layer. The FePt/NiO thin films grown for this study exhibit perpendicular exchange bias at 5K, as quantified using our SQUID measurements. XRD confirms parallel plane ordering between the MgO (200), FePt (002) and NiO (111) atomic planes while cross sectional TEM confirms the epitaxial growth of L1 0-FePt(001)//MgO(100) and the preferential growth of NiO on top of the FePt. Films of only FePt were grown to examine the surface architecture of the ferromagnetic layer and thus the interface of the FePt/NiO bilayer. The results from our XRD, TEM and magnetometry characterization of the FePt films and FePt/NiO bilayer thin films will be discussed.
INTRODUCTION Exchange bias is generally accepted to be due to spin-spin coupling occurring at the interface between ferromagnetic (FM) and anti-ferromagnetic (AFM) layers. Although exchange bias can occur in many types of systems, it most commonly occurs in core-shell nanoparticles or layered thin films. The effect has been studied computationally and experimentally since its discovery in 1956 by Meiklejohn and Bean[1]. Exchange bias has applications in magnetic storage devices, such as magnetic RAM, spin-valves, GMR and other spintronic devices. The origin of exchange bias is still debated today mainly due to various competing theories that have been offered and because there may be a number of potential mechanisms that are responsible for the effect. A leading theory for a structure with FM magnetic ordering orthogonal to the bulk AFM easy axis suggests that an incomplete domain wall is created in the FM layer leading to exchange bias (EB)[2]. This type of system is referred to as the “Frozen Interface Model” by Kiwi et al. and also explains the AFM spin canting at the interface[3]. Calculations often overestimate the experimental exchange bias because it
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