Dynamic magnetic anisotropy at the onset of exchange bias: The NiFe/ IrMn ferromagnet/antiferromagnet system Jeffrey McCord* Leibniz Institute for Solid State and Materials Research IFW Dresden, Institute for Metallic Materials, Helmholtzstrasse 20, 01069 Dresden, Germany Roland Mattheis Institute for Physical High Technology IPHT Jena, Albert-Einstein-Strasse 9, 07745 Jena, Germany Dieter Elefant Leibniz Institute for Solid State and Materials Research IFW Dresden, Institute for Solid State Research, Helmholtzstrasse 20, 01069 Dresden, Germany (Received 26 April 2004; published 27 September 2004) The strength of exchange bias and rotatable anisotropy in polycrystalline NiFe- IrMn ferromagnet/ antiferromagnet systems is quantified from dc down to the picosecond time scale by regular quasistatic and microwave magnetometry, as well as magnetic domain observation. A transition from superparamagnetic to antiferromagnetic behavior with increasing IrMn thickness is derived from the magnetic resonance frequency and the effective magnetic damping parameter. A discrepancy between magnetic loop shift and dynamically obtained exchange bias strength is explained by asymmetric rotatable anisotropy contributions with different relaxation times in the antiferromagnetic layer. The time-dependent relaxation is directly confirmed by mag- netic domain observations. Partially switching in the IrMn layer even with strong exchange bias is concluded. The increase of coercivity rises solely from the rotatable anisotropy contribution. DOI: 10.1103/PhysRevB.70.094420 PACS number(s): 75.70.Cn, 75.70.Kw, 75.60.2d, 75.50.Ee I. INTRODUCTION The interfacial exchange coupling 1 between the spins of a ferromagnetic (F) layer and an antiferromagnetic (AF) layer has been extensively investigated in the past years. Reviews on exchange bias can be found in Refs. 2–5. Experimentally, the exchange bias phenomenon manifests itself in a field shift of the magnetic hysteresis loop, referred to as the ex- change bias field H eb , and an increase of coercivity H c mea- sured in the F film. Numerous theories have been developed that predict values for H eb in reasonable agreement with ex- perimental results. The models 6–14 consider compensated or uncompensated interfaces, polycrystalline thin-film systems, spin-flop coupling, and interface roughness. The internal AF grain and domain structure add many additional aspects to the exchange bias phenomena. Several mechanisms have been proposed to explain the enhanced H c in F/AF bilayers. Models include FM domain wall pinning at AF domains 15 and other F/AF interactions. 16,17 Particularly, time-dependent effects are re- ported in F/AF systems related to thermally activated switch- ing of AF grains. McMichael et al. 18 connected the exhibited rotatable anisotropy H rot (Ref. 19) to changes in the antifer- romagnetic domain structure. This anisotropy can be under- stood as an anisotropy that has an energetic minimum which aligns parallel to a (possibly changing) F layer magnetization direction. It is related to irreversible changes in the AF layer and should not be confused with stripe domain effects ob- served in out-of-plane anisotropy films. 20 The phenomena can be modeled assuming two components in the AF layer. 2,21,22 When changing the magnetic state of the ferro- magnetic layer, one part of the AF grains follows the F layer magnetization, leading to a rotatable anisotropy contribution. The other is fixed, not following the F magnetization, there- fore resulting in exchange bias. Obviously related is the training effect, 23,24 manifesting itself in changes in the hys- teretic characteristics depending on magnetic history. These effects become most pronounced for thin AF layers, 25–27 where enhanced uniaxial anisotropy effects are reported. A detailed discussion on the various aspects of rotatable aniso- tropy, the related enhancement of uniaxial anisotropy, and the training effect can be found in the introduction of Ref. 22. In the present paper we separate the rotatable and the unidirectional anisotropy contributions with varying AF layer thickness by complementary dc and rf measurements. We focus on time-dependent relaxation effects in the AF, important for understanding of the F/AF magnetic structure. The stability of exchange bias and rotatable anisotropy is probed by different techniques. The discussed results are rel- evant for the understanding of exchange bias in polycrystal- line thin films. II. SAMPLE PREPARATION Magnetic bilayer Si/SiO 2 /Tas5 nmd /Ni 81 Fe 19 s40 nmd / Ir 22 Mn 78 s0.2 nm...9 nmd /Rus3 nmd structures are prepared by dc-magnetron sputtering in a multitarget UHV sputter system with a base pressure below 2 3 10 -8 Torr at an Ar pressure of 5 3 10 -3 Torr. The Ta seed layer insures good k111l texture. The distribution of texture is within s =6% and the grain size is typically distributed tightly around d grain PHYSICAL REVIEW B 70, 094420 (2004) 1098-0121/2004/70(9)/094420(8)/$22.50 ©2004 The American Physical Society 70 094420-1