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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Competitive effects of dipolar interactions and a bias magnetic field on the magnetic relaxation times of Co clusters

F. Luis, J. Bartolomé, F. Petroff, L. M. García, A. Vaurès, and J. Carrey

J. Appl. Phys. 93, 7032 (2003); http://dx.doi.org/10.1063/1.1557411 (3 pages) | Cited 7 times

Online Publication Date: 9 May 2003

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We have investigated, by means of ac susceptibility experiments, the magnetic relaxation of layers of Co nanoclusters (D≃2.6 nm) embedded in Al2O3. Superparamagnetic blocking takes place at higher temperatures as the number of layers increases. These results are interpreted using a simple model in which dipole–dipole interactions between nearest neighbor particles increase the relaxation time. The influence of interactions is affected by the application of bias magnetic field H. As the magnetic moments of the particles become polarized by H, the blocking temperature approaches the behavior expected for noninteracting particles. © 2003 American Institute of Physics.
Show PACS
75.50.Tt Fine-particle systems; nanocrystalline materials
75.20.En Metals and alloys
75.30.Cr Saturation moments and magnetic susceptibilities
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Zero magnetization states in electrodeposited Co0.45Fe0.55 nanowire arrays

P. S. Fodor, G. M. Tsoi, and L. E. Wenger

J. Appl. Phys. 93, 7035 (2003); http://dx.doi.org/10.1063/1.1557400 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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Co0.45Fe0.55 alloy nanowires with 12 to 35 nm diameter and 12 μm length were fabricated by electrodeposition in porous anodic alumina templates. The initial magnetization curves reveal that the zero magnetization state is not unique and is determined by the field history (ac demagnetization process) leading to the zero average moment state. For ac demagnetization processes with the field applied parallel to the nanowire axis, the subsequent magnetization curves suggest that an individual nanowire behaves as a single domain with neighboring nanowires being antiparallel to each other in the zero magnetization state. However, for a demagnetization process with the field applied perpendicular to the nanowires, a different zero magnetization state is created in which the individual nanowires consist of multidomains having opposite axial orientations. These results are consistent with the asymmetric (symmetric) behavior found in the minor hysteresis loops measured after perpendicular (parallel) ac demagnetization on these nanowire arrays. © 2003 American Institute of Physics.
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75.75.-c Magnetic properties of nanostructures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Bb Fe and its alloys
75.60.Ch Domain walls and domain structure
81.15.Pq Electrodeposition, electroplating
81.07.-b Nanoscale materials and structures: fabrication and characterization

Defect related switching field reduction in small magnetic particle arrays

M. J. Donahue, G. Vértesy, and M. Pardavi-Horvath

J. Appl. Phys. 93, 7038 (2003); http://dx.doi.org/10.1063/1.1557399 (3 pages) | Cited 4 times

Online Publication Date: 9 May 2003

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An array of 42 μm square, 3 μm thick garnet particles has been studied. The strong crystalline uniaxial anisotropy of these particles results in the stable remanent state being single domain with magnetization parallel to the film normal. Magneto-optic measurements of individual particles provide distribution statistics for the easy-axis switching field Hsw, and the in-plane hard-axis effective anisotropy field, Heff, which induces the formation of a metastable stripe domain structure. Both Hsw and Heff are much smaller than the crystalline anisotropy field. Micromagnetic simulations show that the small Hsw cannot be attributed to shape anisotropy, but is consistent with smooth, localized reductions in the crystalline anisotropy caused by defects in either the particles or the substrate. © 2003 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Gg Ferrimagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure
75.60.Jk Magnetization reversal mechanisms
78.20.Ls Magneto-optical effects

Coercivity and remanence in self-assembled FePt nanoparticle arrays

T. Schrefl, G. Hrkac, D. Suess, W. Scholz, and J. Fidler

J. Appl. Phys. 93, 7041 (2003); http://dx.doi.org/10.1063/1.1557398 (3 pages) | Cited 13 times

Online Publication Date: 9 May 2003

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FePt-based nanostructured materials are excellent candidates for high density recording beyond 1 Tbit/in2. We calculate remanence, coercivity, and loop shape of annealed monodisperse FePt nanocrystals, using a modified Stoner–Wohlfarth model. To justify the simplifications of a Stoner–Wohlfarth model detailed finite element micromagnetic simulations were performed. Magnetic measurements on arrays of chemically synthesized FePt nanoparticles show remanence ratios of about 0.6 which is greater than that predicted for a series of noninteracting Stoner–Wohlfarth particles. A small fraction of the particles (5%) is assumed to remain in the disordered fcc phase with low magnetocrystalline anisotropy. Both remanence and coercivity are highly sensitive to the strength of the exchange interactions within a multiple twined nanocrystal. The calculated values are in the range from Jr/Js=0.52, Hc=0.77 MA/m to Jr/Js=0.61, Hc=1.2 MA/m. The results of the modified Stoner–Wohlfarth model are confirmed by finite element micromagnetic simulations taking into account magnetostatic interactions and allowing nonuniform magnetic structures within a particle. © 2003 American Institute of Physics.
Show PACS
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Bb Fe and its alloys
75.50.Ss Magnetic recording materials
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy

Magnetization reversal of individual Fe nanowires in alumites studied by magnetic force microscopy

T. G. Sorop, C. Untiedt, F. Luis, L. J. de Jongh, M. Kröll, and M. Raşa

J. Appl. Phys. 93, 7044 (2003); http://dx.doi.org/10.1063/1.1557397 (3 pages) | Cited 7 times

Online Publication Date: 9 May 2003

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We have studied the magnetization reversal of two-dimensional (2D) arrays of parallel ferromagnetic Fe nanowires in nanoporous alumina templates. Combining bulk magnetization measurements using a superconducting quantum interference device with field-dependent magnetic force microscopy (MFM), we decomposed the macroscopic hysteresis loop in terms of irreversible magnetic responses of individual nanowires. The field-dependent MFM provides a microscopic method by which to obtain the hysteresis curve by registering the fraction of upward and downward magnetized wires for each field. The nanowire system proves to be an excellent example of the 2D classical Preisach model, well-known from the field of hysteresis modeling and micromagnetism. © 2003 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Bb Fe and its alloys
75.60.Jk Magnetization reversal mechanisms
68.37.Rt Magnetic force microscopy (MFM)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetic properties of Fe nanocubes with magnetostatic interactions

H. K. Lee, T. C. Schulthess, G. Brown, D. P. Landau, K. D. Sorge, and J. R. Thompson

J. Appl. Phys. 93, 7047 (2003); http://dx.doi.org/10.1063/1.1557396 (3 pages) | Cited 5 times

Online Publication Date: 9 May 2003

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Recent experiments indicate that the magnetic properties of single domain Fe nanoparticles (dispersed in an insulating matrix) may be dominated by magnetostatic interactions at packing fractions as low as 10%, where the separation between particles is of the same order as the particle size [Sorge et al., IEEE Trans. Magn. 37, 2197 (2001)]. We use extensive Monte Carlo simulations to calculate the temperature dependence of the remnant magnetization as a direct test of this hypothesis [Sorge et al., IEEE Trans. Magn. 37, 2197 (2001)]. The particle distribution is constructed with a computer model that imitates the experimental system for which data were obtained from transmission electron microscopy images, and the Fe particles are modeled as point dipoles with cubic anisotropy. Using bulk values for the anisotropy and the Fe magnetization, our simulations reproduce very well the experimental remnant magnetization. Furthermore, we find that the magnetic properties are dominated by the effects of dipole–dipole interactions and that the experimental results cannot be reproduced with noninteracting particles. © 2003 American Institute of Physics.
Show PACS
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Bb Fe and its alloys
75.40.Mg Numerical simulation studies
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetic nanowire arrays: A study of magneto-optical properties

Yong Peng, Tiehan-H. Shen, and Brian Ashworth

J. Appl. Phys. 93, 7050 (2003); http://dx.doi.org/10.1063/1.1557395 (3 pages) | Cited 11 times

Online Publication Date: 9 May 2003

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Nanocomposite films of highly ordered magnetic nanowires embedded in anodic aluminum oxide templates were fabricated and magneto-optical properties were studied. Following our previous work, studies on Co nanowire arrays were carried out, where the MO properties in relationship with nanowire lengths and diameters were investigated. The results were found to be considerably different from corresponding bulk metal. We demonstrated that the Faraday effect was a convenient and useful probe for the study of the magnetic properties of these semitransparent nanocomposite films. A preliminary study on the photon energy dependence of the magneto-optical properties of Fe nanowire arrays was also conducted in the visible spectrum regime. © 2003 American Institute of Physics.
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78.20.Ls Magneto-optical effects
75.75.-c Magnetic properties of nanostructures
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.40.Kc Metals, semimetals, and alloys
75.50.Bb Fe and its alloys
75.50.Cc Other ferromagnetic metals and alloys

Bit isolation in periodic antidot arrays using transverse applied fields

M. B. A. Jalil

J. Appl. Phys. 93, 7053 (2003); http://dx.doi.org/10.1063/1.1557394 (3 pages) | Cited 15 times

Online Publication Date: 9 May 2003

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Ferromagnetic films etched with a periodic array of holes (antidots) may be used as high-density storage media, where memory bits are stabilized by shape fields Hs near the edges of the antidots. A micromagnetic simulation is performed to study the conditions for well-defined bits at remanent state. The parameter under consideration is the bit signal-to-noise ratio (SNR), which is defined w.r.t. the ideal magnetization alignment, and is calculated for different values of intrinsic anisotropy Ku and transverse applied field Hy. Unlike previously thought, a transverse Ku hardly improves the SNR (<10%) due to its sign independence, which leads to vortex formation around the antidots and, hence, increased noise in the interbit regions. By contrast, a relatively weak Hy field of 50 to 100 Oe can effectively separate neighboring bits by aligning the interbit regions, leading to a 250%–400% improvement in SNR. Further improvement in SNR is achieved by increasing the bit-aspect ratio. The SNR however, degrades sharply when the anisotropy Hk and transverse Hy fields approach the shape field value obtained via an analytical model. The model used is corroborated by the coercivity trend of different antidot size, obtained by micromagnetics. © 2003 American Institute of Physics.
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75.75.-c Magnetic properties of nanostructures
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Ss Magnetic recording materials
75.40.Mg Numerical simulation studies
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.30.Gw Magnetic anisotropy

Magnetization reversal in arrays of individual and coupled Co-rings

U. Welp, V. K. Vlasko-Vlasov, G. W. Crabtree, J. Hiller, N. Zaluzec, V. Metlushko, and B. Ilic

J. Appl. Phys. 93, 7056 (2003); http://dx.doi.org/10.1063/1.1557393 (3 pages) | Cited 14 times

Online Publication Date: 9 May 2003

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The magnetization behavior of arrays on 10 μm Co rings has been studied using magnetometry, magneto-optical imaging, and Lorentz microscopy. Square arrays of individual rings and arrays of chains of interacting, touching rings have been investigated. In fields transverse to the chains the switching of the rings occurs always in pairs. This coupling introduces a broad distribution of switching fields and correspondingly a broad magnetization loop. Lorentz microscopy reveals that the switching for both, the isolated and the coupled rings, occurs through the formation of a buckled state, and the nucleation and propagation of a vortex domain wall. © 2003 American Institute of Physics.
Show PACS
75.60.Jk Magnetization reversal mechanisms
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ch Domain walls and domain structure
75.70.Kw Domain structure (including magnetic bubbles and vortices)
78.20.Ls Magneto-optical effects

Control of domain patterns in square shaped nickel rings

Xiaobin Zhu, P. Grütter, V. Metlushko, and B. Ilic

J. Appl. Phys. 93, 7059 (2003); http://dx.doi.org/10.1063/1.1557392 (3 pages) | Cited 16 times

Online Publication Date: 9 May 2003

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Magnetic rings in a square shape are studied by magnetic force microscopy with in situ in plane magnetic fields. Well defined domain structures are accessible by changing the orientation of the magnetic field. Magnetic domain wall can easily be trapped at corners. The domain patterns can be controlled by the magnetic field strength and field direction. © 2003 American Institute of Physics.
Show PACS
75.75.-c Magnetic properties of nanostructures
75.60.Ch Domain walls and domain structure
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.50.Cc Other ferromagnetic metals and alloys
68.37.Rt Magnetic force microscopy (MFM)
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