skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Pinned magnetization in the antiferromagnet and ferromagnet of an exchange bias system

Abstract

Using polarized neutron reflectometry, we obtained separate depth profiles for pinned and unpinned magnetization across the interface of a ferromagnet/antiferromagnet bilayer as a function of the sign of exchange bias. The pinned and unpinned magnetization depth profiles are nonuniform and extend well beyond the chemical interface, suggesting an interfacial region magnetically distinct from its surroundings. A model that includes pinned and unpinned moments in the ferromagnet and antiferromagnet is developed for a complete description of the data.

Authors:
;  [1]; ; ; ;  [2];  [1];  [3]
  1. Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
  2. Department of Physics, University of California at San Diego, La Jolla, California 92093 (United States)
  3. (United States)
Publication Date:
OSTI Identifier:
20951522
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 75; Journal Issue: 21; Other Information: DOI: 10.1103/PhysRevB.75.214412; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ANTIFERROMAGNETISM; DEPTH; EXCHANGE INTERACTIONS; FERROMAGNETISM; INTERFACES; MAGNETIC MOMENTS; MAGNETIZATION; NEUTRONS

Citation Formats

Fitzsimmons, M. R., Kirby, B. J., Roy, S., Li Zhipan, Roshchin, Igor V., Schuller, Ivan K., Sinha, S. K., and Department of Physics, University of California at San Diego, La Jolla, California 92093. Pinned magnetization in the antiferromagnet and ferromagnet of an exchange bias system. United States: N. p., 2007. Web. doi:10.1103/PHYSREVB.75.214412.
Fitzsimmons, M. R., Kirby, B. J., Roy, S., Li Zhipan, Roshchin, Igor V., Schuller, Ivan K., Sinha, S. K., & Department of Physics, University of California at San Diego, La Jolla, California 92093. Pinned magnetization in the antiferromagnet and ferromagnet of an exchange bias system. United States. doi:10.1103/PHYSREVB.75.214412.
Fitzsimmons, M. R., Kirby, B. J., Roy, S., Li Zhipan, Roshchin, Igor V., Schuller, Ivan K., Sinha, S. K., and Department of Physics, University of California at San Diego, La Jolla, California 92093. Fri . "Pinned magnetization in the antiferromagnet and ferromagnet of an exchange bias system". United States. doi:10.1103/PHYSREVB.75.214412.
@article{osti_20951522,
title = {Pinned magnetization in the antiferromagnet and ferromagnet of an exchange bias system},
author = {Fitzsimmons, M. R. and Kirby, B. J. and Roy, S. and Li Zhipan and Roshchin, Igor V. and Schuller, Ivan K. and Sinha, S. K. and Department of Physics, University of California at San Diego, La Jolla, California 92093},
abstractNote = {Using polarized neutron reflectometry, we obtained separate depth profiles for pinned and unpinned magnetization across the interface of a ferromagnet/antiferromagnet bilayer as a function of the sign of exchange bias. The pinned and unpinned magnetization depth profiles are nonuniform and extend well beyond the chemical interface, suggesting an interfacial region magnetically distinct from its surroundings. A model that includes pinned and unpinned moments in the ferromagnet and antiferromagnet is developed for a complete description of the data.},
doi = {10.1103/PHYSREVB.75.214412},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 21,
volume = 75,
place = {United States},
year = {Fri Jun 01 00:00:00 EDT 2007},
month = {Fri Jun 01 00:00:00 EDT 2007}
}
  • This paper reports experimental results obtained on an unconventional exchange bias (EB) system where the ferromagnetic layer, Ni{sub 0.75}Cu{sub 0.25}, has lower ordering temperature (T{sub C}) than that (T{sub N}) of the antiferromagnetic one, NiO, with emphasis on modifying EB through either magnetic annealing or light-ion irradiation. Samples were cooled from temperatures higher than T{sub N} or in between T{sub C} and T{sub N} to room temperature with magnetic field applied in different in-plane directions. Upon ion irradiation, magnetic fields, parallel or antiparallel to the orientation of the field present during the films deposition, were applied to explore different effectsmore » on EB. We found that the EB direction can be completely reversed by means of either annealing or ion bombardment; however, both postdeposition treatments provide very little variation of the EB field value over that produced during the film's growth. The importance of the annealing field strength was also discussed. The results were interpreted based on a mechanism which assumes that the interfacial moments adjacent to the antiferromagnetic layer are responsible for establishing the exchange biasing in the paramagnetic state.« less
  • Monte Carlo spin simulations were coupled to a Voronoi microstructure-generator to predict the magnitude and behavior of exchange bias in a ferromagnet/antiferromagnet (AF) thin film bilayer with a nanocrystalline microstructure. Our model accounts for the effects of irregular grain-shapes, finite-sized particles, and the possible presence of local random-fields originating from the antiferromagnet's grain-boundary regions. As the grain-boundary represents a crystal-structure distortion, we model the local effect on the exchange constants in the Gaussian approximation which can cause regions resembling a spin glass confined to an unusual 2D topology. Although an ensemble of completely disconnected AF grains isolated by non-magnetic barriersmore » provides a small exchange bias, the introduction of a spin-glass network at the boundaries causes a four-fold enhancement in the magnitude of the loop-shift. This implies the importance of local grain-boundary behavior in defect-engineered antiferromagnets.« less
  • We report magnetic and exchange bias studies on Ni-NiO nanoparticulate systems synthesized by a two-step process, namely, chemical reduction of a Ni salt followed by air annealing of the dried precipitate in the temperature range 400–550 °C. Size of Ni and NiO crystallites as estimated from X–ray diffraction line broadening ranges between 10.5–13.5 nm and 2.3–4 nm, respectively. The magneto-thermal plots (M-T) of these bi-magnetic samples show a well developed peak in the vicinity of 130 K. This has been identified as the superparamagnetic blocking temperature “T{sub B}” of NiO. Interestingly, all samples exhibit exchange bias even above their respective NiO blocking temperatures, rightmore » up to 300 K, the maximum temperature of measurement. This is in contrast to previous reports since exchange bias requires the antiferromagnetic NiO to have a stable direction of its moment in order to pin the ferromagnet (Ni) magnetization, whereas such stability is unlikely above T{sub B} since the NiO is superparamagnetic, its moment flipping under thermal activation. Our observation is elucidated by taking into account the core-shell morphology of the Ni-NiO nanoparticles whereby clustering of some of these nanoparticles connects their NiO shells to form extended continuous regions of NiO, which because of their large size remain blocked at T > T{sub B}, with thermally stable spins capable of pinning the Ni cores and giving rise to exchange bias. The investigated samples may thus be envisaged as being constituted of both isolated core-shell Ni-NiO nanoparticles as well as clustered ones, with T{sub B} denoting the blocking temperature of the NiO shell of the isolated particles.« less
  • The effect of spin-polarized current on the steady-state magnetization and oscillations of antiferromagnet magnetization in a ferromagnetic-antiferromagnetic magnetic junction is analyzed. The macrospin approximation is generalized to describe antiferromagnets. The canted configuration of the antiferromagnet and the resultant magnetic moment are produced by the application of an external magnetic field. The resonance frequency, damping, and threshold current density corresponding to the emergence of instability are calculated. The possibility of generating weakly damped magnetization oscillations in the terahertz range is demonstrated. The effect of fluctuations on the canted configuration of the antiferromagnet is discussed.
  • Artificial ferrimagnets have many applications as, e.g., pinned reference electrodes in magnetic tunnel junctions. It is known that the application of ion bombardment (IB) induced patterning of the exchange bias coupling of a single layer reference electrode in magnetic tunnel junctions with He ions is possible. For applications as, e.g., special types of magnetic logic, a combination of the IB induced patterning of the exchange bias coupling and the implementation of an artificial ferrimagnet as reference electrode is desirable. Here, investigations for a pinned artificial ferrimagnet with a Ru interlayer, which is frequently used in magnetic tunnel junctions, are presented.more » It is shown that in this kind of samples the exchange bias can be increased or rotated by IB induced magnetic patterning with 10 keV He ions without a destruction of the antiferromagnetic interlayer exchange coupling. An IrMn/Py/Co/Cu/Co stack turned out to be more sensitive to the influence of IB than the Ru based artificial ferrimagnet.« less