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22 Oct 2007

Volume 91, Issue 17, Articles (17xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 173113 (2007); http://dx.doi.org/10.1063/1.2801551 (3 pages)

Nanfang Yu, Ertugrul Cubukcu, Laurent Diehl, Mikhail A. Belkin, Kenneth B. Crozier, Federico Capasso, David Bour, Scott Corzine, and Gloria Höfler
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Magnetic nanowires as permanent magnet materials

T. Maurer, F. Ott, G. Chaboussant, Y. Soumare, J.-Y. Piquemal, and G. Viau

Appl. Phys. Lett. 91, 172501 (2007); http://dx.doi.org/10.1063/1.2800786 (3 pages) | Cited 14 times

Online Publication Date: 22 October 2007

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We present the fabrication of metallic magnetic nanowires using a low temperature chemical process. We show that pressed powders and magnetically oriented samples exhibit a very high coercivity (6.5 kOe at 140 K and 4.8 kOe at 300 K). We discuss the magnetic properties of these metamaterials and show that they have the suitable properties to realize “high temperature magnets” competitive with AlNiCo or SmCo permanent magnets. They could also be used as recording media for high density magnetic recording.
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81.07.Wx Nanopowders
75.50.Ww Permanent magnets
75.50.Vv High coercivity materials
75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Ss Magnetic recording materials

Ferroelectricity of multiferroic hexagonal TmMnO3 ceramics synthesized under high pressure

L. J. Wang, S. M. Feng, J. L. Zhu, R. C. Yu, C. Q. Jin, W. Yu, X. H. Wang, and L. T. Li

Appl. Phys. Lett. 91, 172502 (2007); http://dx.doi.org/10.1063/1.2800816 (3 pages) | Cited 2 times

Online Publication Date: 22 October 2007

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Dense hexagonal TmMnO3 ceramics were synthesized by solid-state reaction technique combined with high-pressure treatment which significantly increased the density of ceramic samples. The crystal structure of the hexagonal TmMnO3 oxide was refined by using Rietveld analysis based on powder x-ray diffraction experiment. We observed obvious dielectric peaks through dielectric measurement on the specimen subjected to postannealing in oxygen atmosphere. A ferroelectric-paraelectric transition around 348 °C is identified. Polarization-electric field hysteresis (P-E) loop measurement proved the ferroelectricity of the sample at room temperature.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
61.66.Fn Inorganic compounds
77.80.B- Phase transitions and Curie point
77.80.Dj Domain structure; hysteresis

A mean-field scaling method for first- and second-order phase transition ferromagnets and its application in magnetocaloric studies

J. S. Amaral, N. J. O. Silva, and V. S. Amaral

Appl. Phys. Lett. 91, 172503 (2007); http://dx.doi.org/10.1063/1.2801692 (3 pages) | Cited 13 times

Online Publication Date: 22 October 2007

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We present a method based on mean-field theory, where from scaling of experimental magnetization data, the mean-field exchange parameter λ and the f function of the equation of state M = f[(H+Hexch)/T] are directly determined. The scaling approach allows finding the dependence of Hexch on T or higher powers of M, which determine the order of the phase transition. Within the thermodynamics of the mean-field theory, the magnetocaloric effect is obtained, in a straightforward way, from the magnetic equation of state. Results are presented for first- and second-order magnetic phase transitions in manganites and metastable nanocrystalline metal alloy.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Sg Magnetocaloric effect, magnetic cooling
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions

The spin polarization of CrO2 revisited

K. A. Yates, W. R. Branford, F. Magnus, Y. Miyoshi, B. Morris, L. F. Cohen, P. M. Sousa, O. Conde, and A. J. Silvestre

Appl. Phys. Lett. 91, 172504 (2007); http://dx.doi.org/10.1063/1.2801693 (3 pages) | Cited 10 times

Online Publication Date: 22 October 2007

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Here, we use Andreev reflection spectroscopy to study the spin polarization of high quality CrO2 films. We study the spin polarization as a function of growth temperature, resulting in grain size and electrical resistivity. In these films low temperature growth appears to be a necessary but not sufficient condition to guarantee the observation of high spin polarization, and this is only observed in conjunction with suppressed superconducting gap values and anomalously low interface properties. We suggest that this combination of observations is a manifestation of the long range spin triplet proximity effect.
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74.78.-w Superconducting films and low-dimensional structures
74.25.Jb Electronic structure (photoemission, etc.)
74.25.Ha Magnetic properties including vortex structures and related phenomena
74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions
72.25.-b Spin polarized transport
68.55.A- Nucleation and growth

Evidence of short-range magnetic ordering above TC in the double perovskite La2NiMnO6

Shiming Zhou, Lei Shi, Haipeng Yang, and Jiyin Zhao

Appl. Phys. Lett. 91, 172505 (2007); http://dx.doi.org/10.1063/1.2801694 (3 pages) | Cited 23 times

Online Publication Date: 22 October 2007

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The magnetic properties of the double perovskite La2NiMnO6 polycrystalline sample have been investigated by magnetometry and electron spin resonance. The magnetization curve shows a paramagnetic-ferromagnetic transition at TC ∼ 280 K. However, the electron spin resonance spectra reveal that a pure paramagnetic regime only exists above T* ∼ 390 K in which the behaviors of the spectra can be well described by the small polaron hopping model. Between TC and T*, a short-range magnetic ordering appears which maybe derives from the correlation of those magnetic polarons. The possible relationship between those polarons and the observed multifunctions in La2NiMnO6 is briefly discussed.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Dd Nonmetallic ferromagnetic materials
75.20.Ck Nonmetals
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
71.38.Ht Self-trapped or small polarons
72.20.Ee Mobility edges; hopping transport

Isothermal low-field tuning of exchange bias in epitaxial Fe/Cr2O3/Fe

S. Sahoo, T. Mukherjee, K. D. Belashchenko, and Ch. Binek

Appl. Phys. Lett. 91, 172506 (2007); http://dx.doi.org/10.1063/1.2801519 (3 pages) | Cited 14 times

Online Publication Date: 23 October 2007

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Moderate dc magnetic fields of less than 1 T allow tuning the exchange bias in an epitaxially grown Fe 10 nm/Cr2O3 2.7 nm/Fe 10 nm trilayer between negative and positive bias fields. Remarkably, this tunable exchange bias is observed at least up to 395 K which exceeds the Néel temperature of bulk Cr2O3 (307 K). The presence of spontaneous exchange bias and the absence of training effects at room temperature suggest the existence of stable interface moments independent of antiferromagnetic long range order in Cr2O3. Furthermore, the coercivity remains constant, independent of the exchange bias field. In contrast, large training associated with nonequilibrium spin configurations of antiferromagnetically ordered Cr2O3 appears below 50 K.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Bb Fe and its alloys
75.50.Ee Antiferromagnetics
75.30.Et Exchange and superexchange interactions
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Significant reduction of coercivity without reduction of tunneling magnetoresistance ratio of CoFeB/MgO/CoFeB-based magnetic tunnel junction using sandwich-structured free layer

Young-suk Choi and Koji Tsunekawa

Appl. Phys. Lett. 91, 172507 (2007); http://dx.doi.org/10.1063/1.2801703 (3 pages) | Cited 4 times

Online Publication Date: 23 October 2007

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We report the significant reduction of coercivity (Hc) without huge drop of tunneling magnetoresistance ratio (TMR) in the CoFeB/MgO/CoFeB-based magnetic tunnel junction by employing sandwich-structured free layer of CoFeB/metallic spacer/NiFe. Strong magnetostatic coupling without physical contact induces magnetic softness from NiFe to CoFeB and avoids considerable reduction of TMR. This achievement satisfies the requirements given to the magnetic read head in hard disk drive and the spin torque transfer magnetic random access memory for the application to the higher areal density and the reduction of critical current density for current-induced magnetization switching, respectively.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Vv High coercivity materials
75.50.Ss Magnetic recording materials
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
85.75.Dd Magnetic memory using magnetic tunnel junctions

FePt nanodot arrays with perpendicular easy axis, large coercivity, and extremely high density

Chaehyun Kim, Thomas Loedding, Seongjin Jang, Hao Zeng, Zhen Li, Yucheng Sui, and David J. Sellmyer

Appl. Phys. Lett. 91, 172508 (2007); http://dx.doi.org/10.1063/1.2802038 (3 pages) | Cited 23 times

Online Publication Date: 23 October 2007

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Ordered FePt nanodot arrays with extremely high density have been developed by physical vapor deposition using porous alumina templates as evaporation masks. Nanodot diameter of 18 nm and periodicity of 25 nm have been achieved, resulting in an areal density exceeding 1×1012 dots/in.2. Rapid thermal annealing converts the disordered fcc to L10 phase, resulting in (001)-oriented FePt nanodot arrays with perpendicular anisotropy and large coercivity, without the need of epitaxy. High anisotropy and coercivity, perpendicular easy axis orientation and extremely high density are desirable features for future magnetic data storage media applications.
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81.16.-c Methods of micro- and nanofabrication and processing
75.50.Vv High coercivity materials
75.50.Tt Fine-particle systems; nanocrystalline materials
61.46.-w Structure of nanoscale materials
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)

Separation of the strain and finite size effect on the ferromagnetic properties of La0.5Sr0.5CoO3 thin films

Changkun Xie, J. I. Budnick, B. O. Wells, and J. C. Woicik

Appl. Phys. Lett. 91, 172509 (2007); http://dx.doi.org/10.1063/1.2803220 (3 pages) | Cited 7 times

Online Publication Date: 23 October 2007

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The ferromagnetic properties of epitaxial La0.5Sr0.5CoO3 thin films have been studied. The magnetic transition is affected by both strain and finite thickness. We have used a series of films of different thicknesses and on different substrates in order to quantitatively determine the change in Curie temperature contributed by each effect. The phase diagram of TC versus in-plane strain suggests that the ferromagnetic transition temperature is suppressed by tensile strain and enhanced by compressive strain. The general method of separating strain and finite thickness effects should be applicable to any ordering phase transition in thin films.
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75.70.Ak Magnetic properties of monolayers and thin films
68.55.-a Thin film structure and morphology
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
75.50.Dd Nonmetallic ferromagnetic materials

Manifestation of anisotropy in melting systematics of RBa2Cu3O7−δ (R = lanthanides)

H. B. Su, D. O. Welch, W. Wong-Ng, L. P. Cook, and Z. Yang

Appl. Phys. Lett. 91, 172510 (2007); http://dx.doi.org/10.1063/1.2799242 (3 pages) | Cited 3 times

Online Publication Date: 24 October 2007

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The conventional isotropic Debye temperature fails to account for the trend of melting temperatures for the high Tc superconductors, RBa2Cu3O7−δ (R-123), as a function of the ionic radius of R3+. We overcame this problem by calculating Debye temperatures using mean sound velocity along the c axis that features an anisotropic layered structure. Using the “improved” Debye temperature, the trend of derived melting temperatures based on the “Lindemann law” matches well with experimental data. This trend is also confirmed by comparing theoretical and experimental Raman active modes corresponding to the Cu–O (plane copper and apical oxygen) and Ba–O (in-plane) bonds in R-123 series.
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74.25.Bt Thermodynamic properties
74.72.-h Cuprate superconductors
74.25.Gz Optical properties
64.70.D- Solid-liquid transitions

Heat assisted magnetic recording by combined field emission and moderate ionization in air

K. W. Ng, S. H. Leong, Z. Yuan, B. Liu, and Y. Ma

Appl. Phys. Lett. 91, 172511 (2007); http://dx.doi.org/10.1063/1.2802050 (3 pages) | Cited 1 time

Online Publication Date: 24 October 2007

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The authors report the enhancement of magnetic recording process by the combined field emission and moderate ionization between the write pole and magnetic media in a heat assisted magnetic recording experiment. When appropriate bias voltages are applied across the head-media gap, field emission from the write pole results in a sustainable moderate ionization without catastrophic air breakdown. The elevated media temperature from electron bombardment leads to reduced media coercivity. Our results show a 25% improvement to writing ability on perpendicular magnetic recording media using a modified read/write head. The effect of different combined emission/ionization currents with different read/write linear velocities was also studied.
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75.50.Ss Magnetic recording materials
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
79.70.+q Field emission, ionization, evaporation, and desorption
75.50.Vv High coercivity materials

Including effects of microstructure and anisotropy in theoretical models describing hysteresis of ferromagnetic materials

H. Hauser, Y. Melikhov, and D. C. Jiles

Appl. Phys. Lett. 91, 172512 (2007); http://dx.doi.org/10.1063/1.2802556 (3 pages)

Online Publication Date: 24 October 2007

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Two recent theoretical hysteresis models (Jiles-Atherton model and energetic model) are examined with respect to their capability to describe the dependence of the magnetization on magnetic field, microstructure, and anisotropy. It is shown that the classical Rayleigh law for the behavior of magnetization at low fields and the Stoner-Wohlfarth theory of domain magnetization rotation in noninteracting magnetic single domain particles can be considered as limiting cases of a more general theoretical treatment of hysteresis in ferromagnetism.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy

Room temperature exchange bias and spin valves based on BiFeO3/SrRuO3/SrTiO3/Si (001) heterostructures

Lane W. Martin, Ying-Hao Chu, Qian Zhan, R. Ramesh, Shu-Jen Han, Shan X. Wang, Maitri Warusawithana, and Darrell G. Schlom

Appl. Phys. Lett. 91, 172513 (2007); http://dx.doi.org/10.1063/1.2801695 (3 pages) | Cited 32 times

Online Publication Date: 25 October 2007

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We report the growth and characterization of exchange bias and spin valve heterostructures based on the multiferroic antiferromagnet BiFeO3 on Si (001) substrates. Using Co0.9Fe0.1 as the ferromagnet, we demonstrate heterostructures with large negative exchange bias and negligible training (or a decrease in exchange bias field as a function of repeated magnetic cycling) at room temperature. We additionally report the manufacture of spin valve structures that have been found to have current in-plane magnetoresistance of over 2.25% at room temperature.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.50.Ee Antiferromagnetics
75.50.Bb Fe and its alloys
75.80.+q Magnetomechanical effects, magnetostriction
75.47.-m Magnetotransport phenomena; materials for magnetotransport

Size dependence in the magnetization reversal of Fe/Gd multilayers on self-assembled arrays of nanospheres

E. Amaladass, B. Ludescher, G. Schütz, T. Tyliszczak, and T. Eimüller

Appl. Phys. Lett. 91, 172514 (2007); http://dx.doi.org/10.1063/1.2802075 (3 pages) | Cited 5 times

Online Publication Date: 25 October 2007

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The size dependent magnetization reversal of Fe/Gd multilayers on self-assembled silica nanospheres has been studied by scanning transmission x-ray microscopy. Caps on 800 nm spherules show S-shaped hysteresis loops while the caps on 330 and 160 nm spherules exhibit rectangular loops. This drastic change is attributed to a transition from a radial to a uniaxial magnetization distribution with decreasing diameter and results from the increasing exchange interaction between adjacent moments. Furthermore, element selective studies reveal a transition from an aligned to a twisted magnetic state with reducing size of the nanocaps.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
81.16.Dn Self-assembly
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Polarization-independent directional anisotropic optical effect in magnetophotonic crystal

H. X. Da, J. C. Wu, and Z. Y. Li

Appl. Phys. Lett. 91, 172515 (2007); http://dx.doi.org/10.1063/1.2802571 (3 pages) | Cited 2 times

Online Publication Date: 25 October 2007

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Polarization-independent directional anisotropic optical effect, a feature that has not been appreciated in the literature so far, in one dimensional magnetophotonic crystals consisting of ferromagnetic materials and anisotropic dielectric layers with misaligned in-plain anisotropy is investigated. We have known that such a configuration exhibits asymmetric spectrum at the certain frequency region [ A. Figotin and I. Vitebskiy, Phys. Rev. B 67, 165210 (2003) ]. In this paper, the existence of the directional anisotropic optical effect is examined according to 4×4 Muller matrix method based on the asymmetric properties of the system. Our results show that this effect can be realized in such magnetophotonic crystals without the existence of magnetization and polarization simultaneously. Furthermore, the order of magnitude of this unique electromagnetic effect can be up to 10−3.
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78.20.Ls Magneto-optical effects
42.70.Qs Photonic bandgap materials
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Gw Magnetic anisotropy

Multifunctional dual-tunable low loss ferrite-ferroelctric heterostructures for microwave devices

Jaydip Das, Boris A. Kalinikos, Arkajit Roy Barman, and Carl E. Patton

Appl. Phys. Lett. 91, 172516 (2007); http://dx.doi.org/10.1063/1.2802577 (3 pages) | Cited 22 times

Online Publication Date: 25 October 2007

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Oriented barium ferrite (BaM) and polycrystalline ferroelectric barium strontium titanate (BSTO) layered structures have been fabricated by pulsed laser deposition. The 0.5 μm thick BaM layer has a saturation induction of 4 kG, a uniaxial effective anisotropy field of 16 kOe, and a relatively low ferromagnetic resonance linewidth of about 25 Oe, values that are indicative of a high quality film. The dielectric constant of the 0.9 μm thick BSTO layer drops by a factor of 2 for an applied voltage of 3 V.
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77.80.-e Ferroelectricity and antiferroelectricity
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ch Permittivity (dielectric function)
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

Epitaxial growth of one-dimensional Ca3Co2O6 thin films prepared by pulsed laser deposition

R. Moubah, A. Bouaine, C. Ulhaq-Bouillet, G. Schmerber, G. Versini, S. Barre, J. L. Loison, M. Drillon, S. Colis, and A. Dinia

Appl. Phys. Lett. 91, 172517 (2007); http://dx.doi.org/10.1063/1.2802731 (3 pages) | Cited 8 times

Online Publication Date: 25 October 2007

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We report on the growth and structural properties of Ca3Co2O6 thin films deposited by pulsed laser ablation on SrTiO3 substrates heated at 700 °C. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy observations reveal that Ca3Co2O6 grows in a three-dimensional (3D) mode with a surface roughness of about 1.5 nm rms. X-ray diffraction and cross-section transmission electron microscopy characterizations show that the deposited films are epitaxial without secondary phases and with a preferential growth orientation perpendicular to the (220) plane. Temperature dependent magnetization measurements reveal that the ferrimagnetic-ferromagnetic transition in the Ca3Co2O6 film is shifted toward higher temperatures with respect to the bulk cobaltite.
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81.05.-t Specific materials: fabrication, treatment, testing, and analysis
75.50.Gg Ferrimagnetics
75.50.Dd Nonmetallic ferromagnetic materials
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
75.70.Ak Magnetic properties of monolayers and thin films

Magnetic characterization of a single superparamagnetic bead by phase-sensitive micro-Hall magnetometry

Goran Mihajlović, Khaled Aledealat, Peng Xiong, Stephan von Molnár, Mark Field, and Gerard J. Sullivan

Appl. Phys. Lett. 91, 172518 (2007); http://dx.doi.org/10.1063/1.2802732 (3 pages) | Cited 18 times

Online Publication Date: 25 October 2007

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Employing phase sensitive micro-Hall magnetometry at room temperature, we map the susceptibility of a single superparamagnetic bead, 1.2 μm in diameter, as a function of magnetic field. We find that the dependence can be explained by modeling the bead as an ensemble of noninteracting superparamagnetic nanoparticles with log-normal distribution of magnetic moments. We also discuss the effect of possible dipolar interactions between the nanoparticles on the obtained results.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.20.Ck Nonmetals
75.50.Tt Fine-particle systems; nanocrystalline materials
72.20.My Galvanomagnetic and other magnetotransport effects
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
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