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ABSTRACT: Ar(+) ions have been implanted into Laves phase epitaxial thin films of YFe(2) and DyFe(2). Magneto-optical Kerr effect and vibrating sample magnetometry experiments show that the easy and hard axes of magnetization in both materials rotate through an in-plane angle of 90°, whilst the strength of the magnetic anisotropy remains unaltered. This is supported by OOMMF computational modelling. Atomic force microscopy confirms that the film roughness is not affected by implanted ions. X-ray diffraction data show that the lattice parameter expands upon ion implantation, corresponding to a release of strain throughout the entire film following implantation with a critical fluence of 10(17) Ar(+) ions cm(-2). The anisotropy of the films is linked to the strain and from these data it is concluded that the source of anisotropy alters from one where magnetoelastic and magnetocrystalline effects compete to one which is governed solely by magnetocrystalline effects. The ability to locally tune the source of magnetic anisotropy without affecting the film surface and without inducing or eliminating anisotropy could be important in the fabrication of high density magnetic data storage media, spintronic devices and magneto-optical materials.
Journal of Physics Condensed Matter 01/2013; 25(8):086002. · 2.55 Impact Factor
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ABSTRACT: X-ray magnetic circular dichroism at the Er M4,5 edge is used to study the
switching behavior of the hard ErFe2 layers in an epitaxial
[ErFe2(70{\AA})/YFe2 (150{\AA})]{\times}25 exchange-spring superlattice.
Magnetic hysteresis loops for the Er magnetization, at temperatures T < 200 K,
reveal a switching behavior with a single type of irreversible switch
corresponding to vertical exchange spring states. Experiments at T > 200 K
reveal a crossover to a regime with two types of switching processes.
Computational modelling for this system gives a semi-quantitative agreement
with the experiment and reveals that the observed high temperature switching
behavior is due to a spin-flop like reorientation transition. In contrast to
conventional spin-flop transitions in antiferromagnets, in this exchange spring
system the increase in anisotropy energy of the hard magnetic layers is
overcome by the decrease in Zeeman energy of the soft layers. Computational
studies also reveal the presence of transitions between vertical exchange
spring and spin-flop states with a first-order character as well as continuous
transitions between these states.
02/2011;
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ABSTRACT: Implanted 3.25 keV Ar+ ions have been used to modify the in-plane bulk anisotropy in thin films of epitaxially grown Laves phase YFe2. The magneto optical Kerr effect, vibrating sample magnetometry and computational modeling have been used to show that the dominant source of anisotropy changes from magnetoelastic in as-grown samples to magnetocrystalline in ion implanted samples. This change occurs at a critical fluence of order 1017 Ar+ ions cm-2. The change in source of the anisotropy is attributed to a relaxation of the strain inherent in the epitaxially grown thin-films. Atomic force microscopy shows that the samples' topography remains unchanged after ion implantation. The ability to control the dominant source of magnetic anisotropy without affecting the sample surface could have important consequences in the fabrication of patterned media for high use in density magnetic data storage devices. Comment: 8 pages, 4 images
09/2010;
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ABSTRACT: The current status of magnetic moments used in micro-magnetic modelling of the Laves phase rare earth iron REFe(2) inter-metallic compounds is reviewed. In particular, it is argued that both the neutron scattering results and band structure calculations provide little support for the long-held view that the Fe 3d moments are constant across the REFe(2) series, and for the oft-used rule of thumb that the (57)Fe hyperfine field is proportional to the Fe magnetic moment. Nevertheless, it is argued that it is acceptable to employ a simple ferrimagnetic model, in which the free-ion moment is ascribed to the RE ion and a moment of μ(d) = μ(3d) + μ(5d)≈1.5 µ(B) is used for the combined Fe(3d) and Fe-driven RE(5d) moments.
Journal of Physics Condensed Matter 07/2010; 22(29):291001. · 2.55 Impact Factor
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ABSTRACT: Epitaxially grown intermetallic RFe2 (R = Dy, Er, Y) thin films have been
studied by point contact Andreev reflection. Spin polarization values were
extracted by fitting normalized conductance curves for mechanical Nb/RFe2 point
contacts, using a modified Blonder-Tinkham-Klapwijk (BTK) model. Good agreement
is found between this model and the experimentally obtained data. Extracted
values of spin polarization, which are close to the spin polarization of Fe,
reveal no variation with the rare earth component for the measured
intermetallic compounds. This suggests that using this technique we probe the
Fe sub-lattice, and that this lattice drives spintronic effects in these
compounds.
09/2008;
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ABSTRACT: The demagnetization processes of antiferromagnetic exchange-coupled hard-soft bilayer structures have been studied using a one-dimensional atomic chain model, taking into account the anisotropies of both hard and soft layers. It is found that for very thin soft layers, magnetization/demagnetization involves typical reversible exchange-spring behavior. However as the thickness t of the soft layer is increased, there is a crossover point tc, after which the exchange spring becomes irreversible. The value of the critical thickness tc is determined inter alia by the magnetic anisotropy of the soft layer. However, an important feature of the irreversible exchange spring is the formation of a large angle domain wall, realized immediately at the bending field transition.
Applied Physics Letters 09/2008; 93(10):102505-102505-3. · 3.84 Impact Factor
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ABSTRACT: Magnetic anisotropy basis sets for the cubic Laves phase rare earth intermetallic REFe2 compounds are discussed in some detail. Such compounds can be either free standing, or thin films grown in either (110) or (111) mode using molecular beam epitaxy. For the latter, it is useful to rotate to a new coordinate system where the z-axis coincides with the growth axes of the film. In this paper, three symmetry adapted basis sets are given, for multi-pole moments up to n = 12. These sets can be used for free-standing compounds and for (110) and (111) epitaxial films. In addition, the distortion of REFe2 films, grown on sapphire substrates, is also considered. The distortions are different for the (110) and (111) films. Strain-induced harmonic sets are given for both specific and general distortions. Finally, some predictions are made concerning the preferred direction of easy magnetization in (111) molecular beam epitaxy grown REFe2 films.
Journal of Physics Condensed Matter 06/2008; 20(28):285226. · 2.55 Impact Factor
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ABSTRACT: The effect of inter-layer diffusion between the magnetically hard and soft layers in magnetic exchange spring systems is examined, using 1D and 2D models. It is shown that diffusion across the hard/soft interfaces leads to an increase in the bending field BB. This increase eventually saturates when the bending field BB and the coercivity BC merge. Moreover, if the increase in the bending field BB is large enough, the nature of the magnetic reversal can be affected. This behaviour is illustrated using a YFe2 dominated YFe2/DyFe2 exchange spring system. In this case the 1D model predicts that inter-layer diffusion can drive a magnetic phase change, from negative to positive coercivity. Discrete 2D model calculations of inter-layer diffusion are also presented and discussed. The latter support the predictions of the 1D model. Finally, while the emphasis is on atomic diffusion, some comments are made concerning interface roughness.
Journal of Physics Condensed Matter 03/2008; 20(12):125223. · 2.55 Impact Factor
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ABSTRACT: 1D models of magnetic multilayers, with alternating hard and soft layers, are extended to 2D and 3D, and presented within a common framework of nearest neighbour interactions. Using 2D calculations, it is shown that the properties of magnetic exchange springs can be changed significantly by patterning the hard pinning layers. But, in certain cases the bending field BB is not significantly altered, even when half the pinning layer is removed. 3D calculations are used to probe the effects of defects on the properties of magnetic exchange springs, using epitaxial DyFe2/YFe2 superlattices as an example. It is shown that point defects such as Fe vacancies have little effect on the bending field transition. This is in marked contrast to the 1D model, where an Fe vacancy cuts the magnetic exchange spring into two. Finally, it is demonstrated that significant changes in the properties of magnetic exchange springs can be engineered, by placing rare-earth ions in the centre of the soft YFe2 springs. A new phenomenon, exchange spring collapse, is predicted.
Journal of Physics Condensed Matter 11/2007; 20(1):015209. · 2.55 Impact Factor
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ABSTRACT: Exchange springs are formed in multilayers of alternating hard and soft ferromagnetic materials which are exchange coupled at their interfaces. These systems are rich of interesting physical properties, which can be tuned by selecting suitable geometries and compositions. In this paper, we present a computational study of the dynamics of a tri-layer DyFe<sub>2</sub>/YFe<sub>2</sub>/DyFe<sub>2</sub> exchange spring system near the bending field (the field required to twist the magnetization of the soft YFe<sub>2</sub> layer out of the aligned state). The dynamical reaction of the system to small variations of the applied field is studied and its oscillatory nature is analyzed numerically. The behaviors of the decay times, the frequencies, and amplitudes reveal enhanced responses of the system near the bending field
IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
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ABSTRACT: We study the anisotropic magnetoresistance (AMR) of a 2-D periodic square array of connected permalloy rings with periodicity of 1 mum combining experimental and computational techniques. The computational model consists of two parts: 1) the computation of the magnetization and 2) the computation of the current density. For 1), we use standard micromagnetic methods. For 2), we start from a potential difference applied across the sample, compute the resulting electric potential, and subsequently the corresponding current density based on a uniform conductivity. We take into account the backreaction of the magnetoresistive effects onto the current density by self-consistently computing the current density and conductivity until they converge. We compare the experimentally measured AMR curve (as a function of the applied field) with the numerically computed results and find good agreement. The numerical data provides insight into the characteristics of the AMR data. Finally, we demonstrate the importance of taking into account the spatial variation of the current density when computing the AMR
IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
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ABSTRACT: Single crystal 400nm thick Laves phase [20Å DyFe2/80Å YFe2]40 superlattice have been grown by MBE with a (110) growth direction. VSM measurements performed at room temperature with an
applied field range of ±1.2×105Oe, directed along the [001] direction, reveal a unique single-phase-liked ferrimagnetic behavior. A dominant exchange spring
behavior is revealed by MOKE measurement along the [–110] direction. Furthermore, for striped arrays patterned along the [001]
direction with height-to-width ratio of 0.05, a shape anisotropy of the order of 104erg/cm3 is induced, resulting into a pronounced change of coercivity due to the comparable magnitude with intrinsic anisotropies.
The results demonstrate the feasibility of engineering both single-phase-liked and exchange-spring magnet behavior in Laves
phase epitaxial hard/soft superlattices by patterning.
Applied Physics A 01/2007; 86(3):325-328. · 1.63 Impact Factor
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A.V. Goncharov,
A.A. Zhukov,
V.V. Metlushko,
G. Bordignon,
H. Fangohr,
C.H. de Groot,
J. Unguris,
W.C. Uhlig,
G. Karapetrov,
B. Ilic, P.A.J. de Groot
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ABSTRACT: Magnetization reversal mechanisms and the impact of magnetization direction are studied in square arrays of interconnected circular permalloy nanorings using magnetooptical Kerr effect, local imaging, numerical simulations, and transport techniques
IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
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ABSTRACT: Magnetization loops for (110) ErFe2/YFe2 multilayer films grown by molecular beam epitaxy are presented and discussed. The direction of easy magnetization for the Er layers is out of plane, near a ⟨111⟩-type crystal axis. For fields applied along the (110) crystal growth axis, out-of-plane magnetic exchange springs are set up in the magnetically soft YFe2 layers. For multilayer films that display negative coercivity at low temperatures, there is a crossover temperature above which the coercivity becomes positive, with additional transitions at high fields. These features are interpreted using micromagnetic modeling. At sufficiently high fields, applied perpendicular to the multilayer film plane, the energy is minimized by an exchange spring driven multilayer spin flop. In this state, the average magnetization of the ErFe2 layers switches into a nominally hard in-plane ⟨111⟩ axis, perpendicular to the applied field.
Applied Physics Letters 09/2006; 89(13):132511-132511-3. · 3.84 Impact Factor
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ABSTRACT: Arrays of micron-sized stripes of epitaxial (110) Laves phase YFe2 films have been fabricated using a UV laser direct write system and etching process. The magnetization reversal of the stripe arrays in the sample plane is investigated by magneto-optical Kerr magnetometry. The measured MO Kerr hysteresis loops reveal that a uniaxial magnetic anisotropy can be induced in the materials by the shape of the stripes. The easy axis is found to be rigidly confined parallel to the length axis of the stripe and the hard axis is perpendicular to it. This is correlated with the induced uniaxial anisotropy which dominates the intrinsic four-fold magnetocrystalline anisotropy of the cubic Laves phase structure. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 09/2006; 203(15):3831 - 3835. · 1.46 Impact Factor
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ABSTRACT: Using electrochemical deposition of Pb in the pores of templates prepared by self-assembly from colloidal suspensions of polystyrene latex spheres, well ordered superconducting antidot arrays with spherically shaped holes are fabricated. In contrast to conventional lithographic arrays the samples with spherical cavities demonstrate significantly reduced pinning strength favoring the formation of commensurate states. The ac-flux penetration acquires a hybrid intra- and intervalley regime. For high ac drives an unusual inversion to paramagnetic ac shielding is found at commensurate states.
Applied Physics Letters 08/2006; 89(9):092503-092503-3. · 3.84 Impact Factor
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ABSTRACT: The magnetic anisotropy parameters in [110] MBE-grown films of REFe2 (RE, rare earth) compounds are not the same as those in the bulk. This is due to the presence of a shear strain εxy, frozen-in during crystal growth. In this paper, magnetic anisotropy parameters for [110] MBE-grown REFe2 films, that directly involve the shear strain εxy, are presented and discussed. In addition to the usual first-order Callen and Callen term , there are nine second-order terms, six of which involve cross-terms between εxy and the cubic crystal field terms B4 and B6. Two of the second-order cross-terms are identified as being important: and . Of these, the rank-two term dominates over a large temperature range. It has the same angular dependence as the first-order term , but with a more rapid temperature dependence. The correction at T = 0 K for TbFe2, DyFe2, HoFe2, ErFe2 and TmFe2, amounts to ~+9.2%, −13.9%, −11.6%, +14.3%, and 27.1%, respectively. Similar comments are made concerning the rank-four term.
Journal of Physics Condensed Matter 06/2006; 18(26):5861. · 2.55 Impact Factor
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ABSTRACT: Magnetic ring arrays are promising candidates for application in magnetic random access memory devices. The magnetic reversal processes and anisotropy of the coercivity in arrays of square-shaped nanorings with different spacings were investigated by vector magneto-optical Kerr effect magnetometry, magnetic force microscopy, and micromagnetic simulations. Two-step magnetization reversal demonstrates fourfold symmetry in the film plane resulting from the shape anisotropy in rings. Our numerical simulations show good agreement with the experiment.
Journal of Applied Physics 04/2006; 99(8):08Q508-08Q508-3. · 2.17 Impact Factor
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A. V. Goncharov,
A. A. Zhukov,
V. V. Metlushko,
G. Bordignon,
H. Fangohr,
C.H. de Groot,
J. Unguris,
W. C. Uhlig,
G. Karapetrov,
B. Ilic, P. A. J. de Groot
[show abstract]
[hide abstract]
ABSTRACT: Magnetization reversal mechanisms and impact of magnetization direction are studied in square arrays of interconnected circular permalloy nanorings using MOKE, local imaging, numerical simulations and transport techniques.
04/2006;
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ABSTRACT: Recent developments in magnetic applications, such as data storage, sensors, and transducers, are stimulating intense research into magnetism on submicrometer-length scales. Emerging self-assembly fabrication techniques have been proposed as viable, low-cost methods to prepare such submicron structures. In this letter we present studies on magnetic nanostructures with 3D architectures, fabricated using a self-assembly template method. We find that the patterning transverse to the film plane, which is a unique feature of this method, governs the magnetic behavior. In particular, the coercive field, a key parameter for magnetic materials, was found to demonstrate an oscillatory dependence on film thickness.
Applied Physics Letters 02/2006; 88(6):062511-062511-3. · 3.84 Impact Factor
