Inverted magnetoresistance in dual spin valve structures with a synthetic antiferromagnetic free layer

Abstract

We report an oscillation of the giant magnetoresistance ͑GMR͒ ratio as a function of Ru layer thickness in the CoFe/Cu/͓CoFe/Ru/CoFe͔SAF/Cu/CoFe/IrMn dual spin valve ͑SV͒ structure. A normal GMR with a positive sign is observed for the thickness of Ru providing a ferromagnetic interlayer exchange coupling ͑IEC͒. The inverted GMR is observed for the thickness of Ru providing an antiferromagnetic IEC, which is consistent with IEC period across the Ru spacer as well as the electrical separation of the overall structure into two SVs connected in parallel. © 2009 American Institute of Physics. ͓doi:10.1063/1.3266522͔Following the discovery of giant magnetoresistance ͑GMR͒, 1,2 extensive experimental and theoretical investigations were performed on spin valve ͑SV͒ systems. In a typical SV two ferromagnetic ͑FM͒ layers are separated by a nonmagnetic spacer, in which one FM layer is pinned by exchange anisotropy by using an antiferromagnetic ͑AF͒ layer, while the other FM layer ͑free layer͒ can be switched with a small external magnetic field. However in a SV with submicrometer scale, the interlayer magnetostatic field arising from the pinned layer on the free layer becomes larger. In order to eliminate this effect, a synthetic antiferromagnetic ͑SAF͒ structure has been used to replace the pinned or the free layer. Since in the SAF structure, the magnetizations of the two FM layers separated by nonmagnetic spacer have strong AF interlayer exchange coupling ͑IEC͒ resulting in a closed flux loop between the two layers of the SAF and a effective reduction in the dipolar field on the free layer.The MR ratio is defined as ͑R ap −R p ͒ / R p , where R p and R ap is the resistance when the magnetizations of the two FM layers are aligned in parallel and antiparallel, respectively. Generally, when the magnetizations of the two FM layers are parallel the resistance is lower and higher when they are antiparallel configuration. This negative MR is termed the normal GMR ͑the resistance is higher for an antiparallel alignment͒. In some cases, however, a positive MR ͑inverted GMR͒ response is seen ͑the resistance is lower for an antiparallel alignment͒. The inverted GMR effect was first reported by George et al. 3 The MR, which was positive at low field and negative at high field, resulted from a magnetic spin-flop transition. Recently, several experimental and theoretical studies have shown an inverted GMR effect in different systems. [4][5][6][7][8][9] In the rare-earth transition metal multilayers, inverted GMR is due to the direct AF coupling across a Co/Dy interface. 4 Due to the increase of the density of states at the Fermi level for majority spin electrons in the Fe 1−x V x / Au/ Co system 5 and doping effect in the Co/Ru/ CoRu system, 6 the spin polarization of the conduction band having opposite sign in alternate FM layers results in inverted GMR. Inverted GMR in the SAF pinned layer structure was observed by Marrows et al. 9 and they explain the observed inverted GMR by the pinning field direction being opposite to the grow...

Crystal and magnetic structure of antiferromagnetic Mn2PtPd

“…Antiferromagnets (AFMs) are special among magnetic mat erials as they display magnetic ordering but with zero magn etic moment. Their practical use has been established in many fields especially in spintronics, for instance as pinning layers in giant magnetoresistance (GMR) and tunnel magnetoresist ance (TMR) devices [1][2][3][4][5][6][7][8][9][10][11][12][13]. Here, the AFM acts as a passive component.…”