Exchange Bias and Asymmetric Reversal in Nanostructured Dot Arrays

Eisenmenger, Johannes; Li, Zhi-Pan; Macedo, W. A. A.; Schuller, Iván K.

doi:10.1103/physrevlett.94.057203

Cited by 55 publications

(47 citation statements)

References 31 publications

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“…[1][2][3][4] In addition, nanoscale studies may shed light on the heavily investigated mechanism of exchange bias [5][6][7][8][9] existing in ferromagnet ͑FM͒-antiferromagnet ͑AF͒ bilayers. Fabrication of sub-100-nm FM single layer and FM/AF bilayer nanostructures offers a great opportunity for research on fundamental magnetism at the nanoscale.…”

Section: Introductionmentioning

confidence: 99%

Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area

Roshchin

Batlle

et al. 2006

Journal of Applied Physics

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Porous alumina masks are fabricated by anodization of aluminum films grown on both semiconducting and insulating substrates. For these self-assembled alumina masks, pore diameters and periodicities within the ranges of 10-130 and 20-200 nm, respectively, can be controlled by varying anodization conditions. 20 nm periodicities correspond to pore densities in excess of 10 12 per square inch, close to the holy grail of media with 1 Tbit/ in. 2 density. With these alumina masks, ordered sub-100-nm planar ferromagnetic nanodot arrays covering over 1 cm 2 were fabricated by electron beam evaporation and subsequent mask lift-off. Moreover, exchange-biased bilayer nanodots were fabricated using argon-ion milling. The average dot diameter and periodicity are tuned between 25 and 130 nm and between 45 and 200 nm, respectively. Quantitative analyses of scanning electron microscopy ͑SEM͒ images of pore and dot arrays show a high degree of hexagonal ordering and narrow size distributions. The dot periodicity obtained from grazing incidence small angle neutron scattering on nanodot arrays covering ϳ2.5 cm 2 is in good agreement with SEM image characterization. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2356606͔ INTRODUCTIONNanostructured magnets have attracted great attention recently due to their unique magnetic properties which are completely different from those of continuous films and bulk materials. [1][2][3][4] In addition, nanoscale studies may shed light on the heavily investigated mechanism of exchange bias 5-9 existing in ferromagnet ͑FM͒-antiferromagnet ͑AF͒ bilayers. Fabrication of sub-100-nm FM single layer and FM/AF bilayer nanostructures offers a great opportunity for research on fundamental magnetism at the nanoscale.A variety of methods are used for nanofabrication, including electron beam and optical lithography. The low throughput and high cost of electron beam lithography lead to limited applicability for the fabrication of nanopatterns with large coverage area. Although optical lithography has high throughput, the smallest size of features that can be produced is limited by the long optical wavelength. Among many nanofabrication methods, 10-15 a promising technique is based on masks with sub-100-nm self-assembled pores. Such masks, for example, can be fabricated by the anodization of aluminum under appropriate experimental conditions. [16][17][18][19][20][21][22] Arrays of nanopores with tunable pore diameter covering more than 1 cm 2 demonstrate the potential application of porous alumina ͑AlO x ͒ masks for nanostructure fabrication. Previously, alumina membranes obtained from the anodization of thick ͑Ͼ100 m͒ aluminum foils were mostly used for the fabrication of nanowires. [23][24][25][26][27][28][29][30] These nanowires were grown by electrodeposition, which cannot be easily adapted for the fabrication of nanodots with controlled and uniform thickness. Moreover, this method requires transfer of the alumina membranes onto a substrate. Insufficient adhesion between the membrane and substrate,...

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Section: Introductionmentioning

confidence: 99%

Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area

Roshchin

Batlle

et al. 2006

Journal of Applied Physics

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“…The absence of transverse moments in the increasing branch reversal was interpreted as domain wall propagation [2,3]. Different, even opposite, scenarios were also found [6][7][8]. Despite the well established experimental evidence and proposed theoretical models [9][10][11], the origin of this asymmetry remains a controversial and highly debated issue [12].…”

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confidence: 99%

Asymmetric Reversal in Inhomogeneous Magnetic Heterostructures

Petracic

Morales

et al. 2006

Phys. Rev. Lett.

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Asymmetric magnetization reversal is an unusual phenomenon in antiferromagnet/ferromagnet (AF/FM) exchange biased bilayers. We investigated this phenomenon in a simple model system experimentally and by simulation assuming inhomogeneously distributed interfacial AF moments. The results suggest that the observed asymmetry originates from the intrinsic broken symmetry of the system, which results in local incomplete domain walls parallel to the interface in reversal to negative saturation of the FM. The magneto-optical Kerr effect unambiguously confirms such an asymmetric reversal and a depth-dependent FM domain wall in accord with the magnetometry and simulations. DOI: 10.1103/PhysRevLett.96.217205 PACS numbers: 75.25.+z, 75.60.Jk, 75.70.Cn, 78.20.Jq Exchange coupling between a ferromagnet (FM) and an antiferromagnet (AF) has been intensely studied due to the fundamental interest in inhomogeneous magnetic systems and its central role as a magnetic reference in various devices. In most magnetic systems, time reversal symmetry is present and manifested by a symmetric magnetization curve relative to the origin. This symmetry also requires that the magnetization reversal from positive to negative saturation be identical to the reverse process. However, in a FM/AF system, exchange bias (EB) develops below the AF Néel temperature T N producing a shift (H EB ) of the hysteresis loop along the magnetic field axis [1]. Therefore, with the shift breaking the time reversal symmetry, magnetization reversal symmetry is no longer required. In fact, asymmetric reversal was observed by polarized neutron reflectometry [2], photoemission electron microscopy [3], magneto-transport [4], magneto-optical indicator film [5], and magneto-optical Kerr effect [6]. In some systems the reversal along the decreasing branch is dominated by transverse magnetic moments, a phenomenon interpreted as due to coherent magnetic rotation. The absence of transverse moments in the increasing branch reversal was interpreted as domain wall propagation [2,3]. Different, even opposite, scenarios were also found [6][7][8]. Despite the well established experimental evidence and proposed theoretical models [9][10][11], the origin of this asymmetry remains a controversial and highly debated issue [12]. This situation is further complicated by the lack of knowledge of the interface, crystal imperfections, complex FM and AF anisotropy energies, and training effect. While these factors are important for each individual system, the fundamental connection of the reversal asymmetry to the broken symmetry intrinsic in the inhomogeneous system is overlooked.In this Letter, we investigate a simple model system using a variety of experimental techniques combined with numerical simulations. We establish a critical link between this unusual reversal asymmetry with the time reversal asymmetry in these systems. Namely, in reversal toward the two FM saturated states, the intrinsic asymmetry gives rise to different competing mechanisms, thus different reversal processes.Fe...

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“…Coercivity in the front side T-MOKE is not the same as that in the back side, and the amplitudes at the left and right coercivities are asymmetric, indicating that the amount of F spins differ from each other during magnetization rotation between the decreasing and increasing field branches of the T-MOKE loops from the front and back sides of the NiFe/FeMn bilayer. This asymmetric magnetization reversal has been experimentally and theoretically attributed to typical phenomena in a number of exchange-biased F/AF systems [3,[7][8][9][10][11][12][13][14][15][16]. The asymmetric T-MOKE loop for a NiFe/FeMn bilayer seems to originate from the competition between the uniaxial and unidirectional anisotropies [3,13,15].…”

Section: Methodsmentioning

confidence: 99%

“…Various measurement techniques such as VSM, anisotropic magnetoresistance, polarized neutron reflectivity, soft x-ray scattering, and MOKE have revealed the asymmetric form of the magnetization reversal of the exchangebiased F layer [3,[7][8][9][10][11][12][13][14][15][16]. The magnetization reversal takes place via magnetization rotation or via nucleation and domain wall propagation along the same or different sides of an applied field during the decreasing (from positive to negative saturation) and increasing (from negative to positive saturation) field branches of the M-H loop.…”

Section: Introductionmentioning

confidence: 99%

Magnetization Reversal of Exchange-biased Bilayers and Trilayers Probed using Front and Back LT-MOKE

Kim¹,

Kim²,

Choi³

et al. 2009

Journal of Magnetics

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Magneto-optical Kerr effect (MOKE) magnetometry was used to investigate magnetization reversal dynamics in 30-nm NiFe/15-nm FeMn, 15-nm FeMn/30-nm CoFe bilayers, and 30-nm NiFe/(2,10)-nm FeMn/30-nm CoFe trilayers. The in-plane magnetization components of each ferromagnetic layer, both parallel and perpendicular to the applied field, were separately determined by measuring the longitudinal and transverse MOKE hysteresis loops from both the front and back sides of the film for an oblique incident s-polarized beam. The magnetization of the FeMn/CoFe bilayer was reversed abruptly and symmetrically through nucleation and domain wall propagation, while that of the NiFe/FeMn bilayer was reversed asymmetrically with a dominant rotation. In the NiFe/FeMn/CoFe trilayers, the magnetic reversal of the two ferromagnetic layers proceeded via nucleation and domain wall propagation for 2-nm FeMn, but via asymmetric rotation for 10-nm FeMn. The exchange-biased ferromagnetic layers showed the magnetization reversal along the same path in the film plane for the decreasing and increasing field branches from transverse MOKE hysteresis loops, which can be qualitatively explained by the theoretical model of the exchange-biased ferromagnetic/antiferromagnetic systems.

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Exchange Bias and Asymmetric Reversal in Nanostructured Dot Arrays

Cited by 55 publications

References 31 publications

Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area

Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area

Asymmetric Reversal in Inhomogeneous Magnetic Heterostructures

Magnetization Reversal of Exchange-biased Bilayers and Trilayers Probed using Front and Back LT-MOKE

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