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Title: Combined magnetic X-ray and polarized neutron reflectivity studyof the origins of exchange bias in the Co/FeF2 system

Abstract

We discuss studies of the magnetic specular reflectivity ofneutrons and X-rays from the exchange bias system consisting of a singlecrystal film of antiferromagnetic FeF2 capped with a ferromagnetic Cofilm cooled in an applied magnetic field below the TN of the FeF2. Thissystem exhibits a shift of the magnetic hysteresis loop along thedirection of the cooling field Hc (positive exchange bias) or in theopposite direction (negative exchange bias) depending on the magnitude ofthe cooling field. The use of neutrons with polarization analysis enablesthe spatial distribution of different vector components of themagnetization to be determined, and the use of resonant magnetic X-rayscattering enables magnetization in a compound system to be determinedelement-selectively. Our results show that the coupling across theinterface between the relatively few uncompensated Fe spins and the morenumerous Co spins is antiferromagnetic. In a large cooling field whichoverrides this coupling, the Fe spins are oriented along Hc and some getpinned in this direction, in turn pinning the Co spins above theinterface oppositely and thus creating positive exchange bias. For sallvalues of Hc the Fe spins get locked in the opposite direction producingnegative exchange bias. In addition a significant fraction of the Fespins at the interface are unpinned and always align oppositemore » to the Comagnetization.« less

Authors:
; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
COLLABORATION - UCSD
OSTI Identifier:
932975
Report Number(s):
LBNL-63340
Journal ID: SUMIEK; R&D Project: 504103; TRN: US200814%%185
DOE Contract Number:
DE-AC02-05CH11231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Superlattices and Microstructures; Journal Volume: 41; Journal Issue: 2-3; Related Information: Journal Publication Date: online05/07/2007
Country of Publication:
United States
Language:
English
Subject:
75; 36; HYSTERESIS; MAGNETIC FIELDS; MAGNETIZATION; MONOCRYSTALS; NEUTRONS; POLARIZATION; REFLECTIVITY; SCATTERING; SPATIAL DISTRIBUTION; VECTORS; exchange bias, uncompensated spins, antiferromagnetism, x-rayreflectivity, neutron reflectivity

Citation Formats

Fitzsimmons, M.R., Roy, S., Kirby, B.J., Park, S., Roshchin, IgorV., Li, Zhi-Pan, Kortright, J.B., Sinha, S.K., and Schuller, Ivan K.. Combined magnetic X-ray and polarized neutron reflectivity studyof the origins of exchange bias in the Co/FeF2 system. United States: N. p., 2007. Web. doi:10.1016/j.spmi.2007.02.004.
Fitzsimmons, M.R., Roy, S., Kirby, B.J., Park, S., Roshchin, IgorV., Li, Zhi-Pan, Kortright, J.B., Sinha, S.K., & Schuller, Ivan K.. Combined magnetic X-ray and polarized neutron reflectivity studyof the origins of exchange bias in the Co/FeF2 system. United States. doi:10.1016/j.spmi.2007.02.004.
Fitzsimmons, M.R., Roy, S., Kirby, B.J., Park, S., Roshchin, IgorV., Li, Zhi-Pan, Kortright, J.B., Sinha, S.K., and Schuller, Ivan K.. Wed . "Combined magnetic X-ray and polarized neutron reflectivity studyof the origins of exchange bias in the Co/FeF2 system". United States. doi:10.1016/j.spmi.2007.02.004.
@article{osti_932975,
title = {Combined magnetic X-ray and polarized neutron reflectivity studyof the origins of exchange bias in the Co/FeF2 system},
author = {Fitzsimmons, M.R. and Roy, S. and Kirby, B.J. and Park, S. and Roshchin, IgorV. and Li, Zhi-Pan and Kortright, J.B. and Sinha, S.K. and Schuller, Ivan K.},
abstractNote = {We discuss studies of the magnetic specular reflectivity ofneutrons and X-rays from the exchange bias system consisting of a singlecrystal film of antiferromagnetic FeF2 capped with a ferromagnetic Cofilm cooled in an applied magnetic field below the TN of the FeF2. Thissystem exhibits a shift of the magnetic hysteresis loop along thedirection of the cooling field Hc (positive exchange bias) or in theopposite direction (negative exchange bias) depending on the magnitude ofthe cooling field. The use of neutrons with polarization analysis enablesthe spatial distribution of different vector components of themagnetization to be determined, and the use of resonant magnetic X-rayscattering enables magnetization in a compound system to be determinedelement-selectively. Our results show that the coupling across theinterface between the relatively few uncompensated Fe spins and the morenumerous Co spins is antiferromagnetic. In a large cooling field whichoverrides this coupling, the Fe spins are oriented along Hc and some getpinned in this direction, in turn pinning the Co spins above theinterface oppositely and thus creating positive exchange bias. For sallvalues of Hc the Fe spins get locked in the opposite direction producingnegative exchange bias. In addition a significant fraction of the Fespins at the interface are unpinned and always align opposite to the Comagnetization.},
doi = {10.1016/j.spmi.2007.02.004},
journal = {Superlattices and Microstructures},
number = 2-3,
volume = 41,
place = {United States},
year = {Wed Feb 07 00:00:00 EST 2007},
month = {Wed Feb 07 00:00:00 EST 2007}
}
  • Polarized neutron and X-ray reflectivity are complementary tools used to measure the depth profile of magnetic thin films. In this work, permalloy thin films were epitaxially deposited on Mo buffered Al{sub 2}O{sub 3} (1--100) substrates with a Co interlayer and without a Co interlayer. The interface structure between the permalloy and the Co layers, determined by X-ray reflectivity alone, was insensitive, due to the low contrast of the electron density between these two adjacent layers. It can however, be determined by polarized neutron reflectivity, due to the large difference of the nuclear scattering lengths and magnetic moments between these twomore » layers.« less
  • We have measured the specular reflectivity of spin-polarized neutrons from two (111) superlattices, [Pd 80 A[vert bar]Co 60 A][times]20/Pd 300 A/Co 10 A/GaAs and [Pd 53 A[vert bar]Co 32 A][times]10/Pd 30 A/Cu 354 A/Si, grown by molecular-beam epitaxy. In a saturating field of 2300 Oe, we extract for the first superlattice a Co moment value of corresponding to 78% of the bulk Co moment. At temperatures ranging from 38 to 295 K, no excess magnetic moment from polarization of the Pd atoms is evident in the nominal Pd layer. Comparable reflectivity data from the second sample support these conclusions.
  • We report about the influence of pulsed laser irradiation on the structural and magnetic properties of NiMn/Co thin films. Rocking curve measurements showed a significant improvement of the (111) texture of NiMn after laser irradiation which was accompanied by grain growth. We have studied the ordering transition in as-prepared and irradiated (laser fluence of 0.15 J/cm{sup 2}) samples during subsequent annealing. The onset of the fcc to fct phase transformation occurs at 325 deg. C irrespective of laser irradiation. Exchange bias fields for the laser irradiated samples are higher than those of the as-prepared samples. The observed increase in themore » exchange bias field for laser irradiated samples has been attributed to the increased grain size and the improved (111) texture of the NiMn layer after laser irradiation.« less
  • The magnetic structure of Fe{sub 1-x}Co{sub x}Si single crystals with x=0.10,0.15,0.20,0.50 has been studied by small angle polarized neutron diffraction and superconducting quantum interference device measurements. Experiments have shown that in zero field the compounds with x=0.1,0.15 have a well-defined tendency to order in the one-handed spiral along <100> axes due to the anisotropic exchange, that, however, decreases with increasing Co concentration x. The magnetic structure of Fe{sub 1-x}Co{sub x}Si with x=0.2,0.5 consists of spiral domains with randomly oriented spiral wave vector k. The applied magnetic field produces a single domain helix oriented along the field. The process of themore » reorientation starts at the field H{sub C1}. Further increase of the field leads to a magnetic phase transition from a conical to a ferromagnetic state near H{sub C2}. In the critical range near T{sub C} the integral intensity of the Bragg reflection shows a well-pronounced minimum at H{sub fl} attributed to a k flop of the helix wave vector. On the basis of our experiments we built the H-T phase diagram for each compound. It is shown that the same set of the parameters governs the magnetic properties of these compounds k, H{sub C1}, H{sub fl}, and H{sub C2}. Our experimental findings are well interpreted in the framework of a recently developed theory [Phys. Rev. B 73, 174402 (2006)] for cubic magnets with Dzyaloshinskii-Moriya (DM) interaction. In particular, the theory suggests an additional quantum term in the magnetic susceptibility caused by the DM interaction which is in good agreement with the experiment.« less