Refining the exchange anisotropy paradigm: Magnetic and microstructural heterogeneity at the Permalloy-CoO interface

Berkowitz, A. E.; Hong, Jung‐Il; McCall, S.; Shipton, Erik; Chan, Keith; Leo, Titus; Smith, David J.

doi:10.1103/physrevb.81.134404

Cited by 34 publications

(29 citation statements)

References 31 publications

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“…Berkowitz et al 24 have found strong indications of a hard Co-ferrite-like phase at the permalloy/ CoO interface resulting from chemical reactions; for IrMn/ CoFe systems, O'Grady et al 25 have inferred that the UCSs are AFM spins. Given that our experiments do not provide atomic level information about the microstructure and chemistry of the interfacial region between Co and CoO, we are unable to determine the origin of the interface UCSs in our films.…”

Section: Resultsmentioning

confidence: 99%

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Rotatable anisotropy driven training effects in exchange biased Co/CoO films

Dias

Menéndez

Liu

et al. 2014

Journal of Applied Physics

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Determination of exchange anisotropy by means of ac susceptometry in Co/CoO bilayers J. Appl. Phys. 81, 5003 (1997) The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Previous experiments on the same system have shown that, starting from the ascending branch of the first hysteresis loop, coherent magnetization rotation is the dominant reversal mechanism. This is confirmed by the performed numerical simulations, which also indicate that the training is predominantly caused by changes of the rotatable anisotropy parameters of uncompensated spins at the Co/CoO interface. Moreover, in contrast with what is commonly assumed, the exchange coupling between the rotatable spins and the ferromagnetic layer is stronger than the coupling between the ferromagnet and the spins responsible for the bias. Thus, uncompensated spins strongly coupled to the ferromagnet contribute to the coercivity rather than to the bias, whatever the strength of their magnetic anisotropy. V C 2014 AIP Publishing LLC.

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Section: Resultsmentioning

confidence: 99%

“…It is currently accepted [23][24][25] that at the FM/AFM interfaces of polycrystalline bilayers there are UCSs which behave as single FM domains and interact with the adjacent FM and/ or interfacial AFM spins. Stable UCSs do no rotate irreversibly and add to the bias (from now on, the corresponding quantities will be indexed by set).…”

Section: Modelmentioning

confidence: 99%

Rotatable anisotropy driven training effects in exchange biased Co/CoO films

Dias

Menéndez

Liu

et al. 2014

Journal of Applied Physics

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“…This suggests that it is unlikely that chemical interdiffusion or formation of foreign magnetic interface phases could explain the experimental observations. 26 Moreover, decreasing or increasing the permalloy moment within 1-2 nm of this region increased the error. Despite the probable existence of an induced interface moment, significant exchange bias is not seen in the magnetic hysteresis.…”

Section: B Room-temperature Magnetic Propertiesmentioning

confidence: 96%

“…Work on similar oxide multilayers has also sometimes reported anomalous magnetic moments in the antiferromagnet layers 25 or double-step hysteresis loops. 26 This elicits a second possibility that uncompensated spins or foreign phases embedded within the antiferromagnet, such as Fe 25 or Fe 3 O 4 clusters, 27 may also play a part in the observed exchange bias. To better understand the mechanism for exchange bias, we investigated a nanocrystalline permalloy/hematite bilayer using a depth-sensitive magnetic neutron scattering technique.…”

Section: Introductionmentioning

confidence: 99%

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

et al. 2012

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We investigated a hematite α-Fe2O3/permalloy Ni80Fe20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe2O3 is known to be very high (860 K), we attribute the lowtemperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μB per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains. We investigated a hematite α-Fe 2 O 3 /permalloy Ni 80 Fe 20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe 2 O 3 is known to be very high (860 K), we attribute the low-temperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μ B per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains.

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“…These clusters are thought to transmit the anisotropy from the AFM layer to the FM layer, allowing for interactions via the exchange coupling to the AFM and FM layers that gives rise to the coercivity of the FM layer. In FeNi/CoO FM/AFM films, Berkowitz et al estimate that ~75% of interfacial spins belong to magnetically hard particles (e.g., CoFe 2 O 4 ) which are exchange-coupled to CoO and are responsible for the EB and coercivity enhancement by virtue of their exchange coupling to the FeNi layer [11]. The interfacial spins within the magnetically soft FeNi phase do not contribute to the EB.…”

Section: Introductionmentioning

confidence: 99%

Exchange Bias Effects in Iron Oxide-Based Nanoparticle Systems

et al. 2016

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The exploration of exchange bias (EB) on the nanoscale provides a novel approach to improving the anisotropic properties of magnetic nanoparticles for prospective applications in nanospintronics and nanomedicine. However, the physical origin of EB is not fully understood. Recent advances in chemical synthesis provide a unique opportunity to explore EB in a variety of iron oxide-based nanostructures ranging from core/shell to hollow and hybrid composite nanoparticles. Experimental and atomistic Monte Carlo studies have shed light on the roles of interface and surface spins in these nanosystems. This review paper aims to provide a thorough understanding of the EB and related phenomena in iron oxide-based nanoparticle systems, knowledge of which is essential to tune the anisotropic magnetic properties of exchange-coupled nanoparticle systems for potential applications.

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Refining the exchange anisotropy paradigm: Magnetic and microstructural heterogeneity at the Permalloy-CoO interface

Cited by 34 publications

References 31 publications

Rotatable anisotropy driven training effects in exchange biased Co/CoO films

Rotatable anisotropy driven training effects in exchange biased Co/CoO films

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

Exchange Bias Effects in Iron Oxide-Based Nanoparticle Systems

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