-
[show abstract]
[hide abstract]
ABSTRACT: Determination of spin-dependent Seebeck coefficients of CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars J. Appl. Phys. 111, 07C520 (2012) Effect of packing fraction on ferromagnetic resonance in NiFe2O4 nanocomposites J. Appl. Phys. 111, 07E348 (2012) General approach to the magnetostatic force and interaction between cylindrically shaped nanoparticles J. Appl. Phys. 111, 07D131 (2012) Effect of rounded corners on the magnetic properties of pyramidal-shaped shell structures The performance of hard-magnetic nanostructures is investigated by analyzing the size and geometry dependence of thin-film hysteresis loops. Compared to bulk magnets, weight and volume are much less important, but we find that the energy product remains the main figure of merit down to very small features sizes. However, hysteresis loops are much easier to control on small length scales, as epitomized by Fe-Co-Pt thin films with magnetizations of up to 1.78 T and coercivities of up to 2.52 T. Our numerical and analytical calculations show that the feature size and geometry have a big effect on the hysteresis loop. Layered soft regions, especially if they have a free surface, are more harmful to coercivity and energy product than spherical inclusions. In hard-soft nanocomposites, an additional complication is provided by the physical properties of the hard phases. For a given soft phase, the performance of a hard-soft composite is determined by the parameter (M s -M h)/K h . V C 2012 American Institute of Physics. [doi:
Journal of Applied Physics 02/2013; 111(111):7-345. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We use magnetic thin film hard/non/soft-magnetic trilayer systems to probe the nature of the hard–soft phase interaction and
the role played by dipolar fields in one-dimensional (d) magnetic systems. We have systematically investigated six wedge samples where the thickness of a Cu spacer layer (t
Cu) was gradually changed to create a varying interfacial effect on the interaction between a CoPt hard layer and a Fe soft
layer. Magneto-optical Kerr effect was used to obtain the magnetization loops at 28 points on each sample, and the nucleation
field (H
N
) as a function of t
Cu was employed to characterize the layer interaction as a function of t
Cu. H
N
(t
Cu) show a RKKY oscillatory behavior in addition to a non-negligible dipolar contribution, which had an exponential dependence.
The dipolar term, which cannot be always neglected, is affected by the interface roughness and also by the CoPt crystallinity.
Therefore, we cannot always consider exchange coupling to be the dominant interaction in one-d hard–soft magnetic bilayer systems, particularly, during magnetic reversal.
Applied Physics A 04/2012; 103(4):1183-1187. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Two-phase nanostructures of hard L10-ordered FePt and soft iron-rich fcc Fe-Pt are investigated experimentally and by model calculations. The Fe-Pt thin films were produced by epitaxial co-sputtering onto MgO and have a thickness of about 10 nm. They form two-phase dots that cover a large fraction of the surface but are separated from each other. X-ray diffraction and TEM show that the c-axis of the phase FePt is aligned in the direction normal to the film plane. The experimental and theoretical hysteresis loops indicate archetypical exchange coupling, and excellent magnetic properties are obtained. The largest values of coercivity, saturation magnetization, and nominal energy product obtained in the samples studied are 51 kOe, 1287 emu/cc, and 54 MGOe, respectively.
Applied Physics Letters 10/2011; 99(17):172504-172504-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The effect of nanostructuring on magnetostatic interactions in permanent magnets is investigated by model calculations. Emphasis is on the energy product as a function of packing fraction of the magnetic phase, of the magnet’s macroscopic shape, and of the nanoscale feature size. The main difference between nanostructured and macroscopic magnetic bodies, namely, the transition between coherent and incoherent reversal, has a far-reaching impact on demagnetizing field and energy product. For small magnet sizes, the energy product is substantially enlarged, up to μ<sub>0</sub>M<sub> s </sub><sup arrange="stagger">2</sup>/4 for soft magnetic materials, but this effect is difficult to exploit in real devices. In bulk magnets, the energy product depends on the packing fraction f of the soft phase and exhibits a maximum μ<sub>0</sub>M<sub> s </sub><sup arrange="stagger">2</sup>/12 for f=2/3 . Nanoscale magnetization processes involve demagnetizing factors different from the macroscopic ones used to determine the optimum shape of permanent magnets. Confusion of these two types of demagnetizing fields yields unphysical mechanisms, such as hysteresis-loop overskewing and the addition of self-interaction fields to the external field.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The interfacial effect on the magnetization in the magnetron sputtered Co/FeMn/Co multilayers with Pt spacer layer upon and beneath the FeMn layer, respectively, was investigated. The variation in the magnetization with the thickness of the Co layer strongly depends on the orientation and the rotation of the interfacial spins at the ultrathin limit of the Co layer as well as the FeMn layer, which is determined by the magnetic anisotropy at the ferromagnetic/antiferromagnetic interface. The perpendicular magnetization is considerably enhanced by the FeMn layer either with or without the Pt spacer, and the enhancement is cut down by inserting the Pt spacer at the Co/FeMn interface, whereas nearly not changed by the Pt spacer at the FeMn/Co interface. Although a perpendicular inducing field is applied during the deposition, the perpendicular exchange bias is not induced at the Co/FeMn interface unless a 0.4 nm Pt spacer is inserted, indicating that the interfacial uncompensated spins could hardly be pinned into the hard axis.
Journal of Applied Physics 02/2010; · 2.17 Impact Factor
-
H. Oguchi,
A. Zambano,
M. Yu,
J. Hattrick-Simpers,
D. Banerjee, Y. Liu,
Z. L. Wang,
J. P. Liu,
S. E. Lofland,
D. Josell,
I. Takeuchi
[show abstract]
[hide abstract]
ABSTRACT: The effect of the crystallinity and the grain texturing of CoPt hard layers on exchange coupled Fe/CoPt soft/hard magnetic systems was studied using gradient thickness multilayer thin films. We have studied the hard layer structures by transmission electron microscopy and x-ray diffraction, and characterized the exchange coupling interaction through magnetization loops obtained by the magneto-optical Kerr effect measurement. We found that exchange coupling strongly depends on the crystalline characteristics of the CoPt hard layer. There is correlation between the mixture of different grain orientations of the CoPt hard layer and coupling efficiency. In particular, interlayer coupling is enhanced when there is only one orientation, namely, the L 1<sub>0</sub> CoPt structure with its c -axis inclined at 45° with respect to the substrate plane. An increased degree of mixture of the latter with the in-plane- c -axis L 1<sub>0</sub> CoPt structure is detrimental to obtaining one-phase-like magnetization loops. The present work points to the importance of controlling the crystalline properties of the hard layer in order to enhance the maximum energy product (BH)<sub> max </sub> in hard/soft nanocomposite magnets.
Journal of Applied Physics 02/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In exchange-coupled two-phase permanent magnets, the length scale of soft phase is limited to about twice of the domain-wall width of the hard phase. To optimize the energy product, it is important to realize this length experimentally. In this work, we investigate the Sm – Co / Co hard/soft multilayers with varying thickness of soft phase layers. On rapidly annealing, the multilayered hard-soft structure forms. Transmission electron microscopy micrography confirms that the multilayer structure is retained after the annealing. Single-phase-like hysteresis loops are obtained for samples with Co layers up to 13 nm thick. This behavior indicates that the soft phase is well exchange coupled to the neighboring Sm Co <sub> z </sub> hard phase. An optimal energy product of 16.6 MGOe has been obtained. Longer annealing time results in more diffusion at the interface and yields two-phase-like hysteresis behavior. Direct current demagnetization measurement shows exchange-spring behavior of the samples annealed for longer time. Micromagnetic simulations with varying interface exchange coupling have been performed to compare with the experimental results.
Journal of Applied Physics 06/2005; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A combined experimental and theoretical investigation of advanced nanostructured magnetic materials has been carried out. Novel physical systems have been synthesized and studied including: hard/soft nanocomposites, magnetic nanowires, thermally processed multilayer films, and nanoparticle-assembled composites. Emphasis has been focused on exchange coupling at grain boundaries, incoherent magnetization reversal, and magnetization dynamics. The design of nanomagnets with controlled properties has been emphasized, and unprecedented advances have been made in high-temperature permanent magnets, high-coercivity nanocluster magnets, and hard and soft magnetic nanotubes prepared with chemical methods.
10/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: Permanent magnets with the general composition Pr<sub>9</sub>(Fe<sub>0.95</sub>Co<sub>0.05</sub>)<sub>85.5</sub>M<sub>0.5</sub>B<sub>5</sub>, where M denotes a transition metal, have been produced by melt spinning. The effect on the microstructure and magnetic properties of eight different alloying elements has been examined. The grain size and distribution have been correlated to the maximum energy product obtained. The grain size of the Pr<sub>9</sub>(Fe<sub>0.95</sub>Co<sub>0.05</sub>)<sub>85.5</sub>B<sub>5</sub> control alloy was determined to be 17.5 nm, and it was found that the grain size was not significantly reduced compared to the control alloy by any of the additions. However, some additions resulted in significant increases in grain size, notably Cu, Ti, and Cr, which had grain sizes between 26 and 36 nm, and an increase in the grain size distribution. The increase in grain size resulted in a loss in hysteretic squareness, leading to reduced energy products in these alloys. In other alloys, fine grain size did not always correlate to improved maximum energy products. For example, the Mo-added alloy had the finest grain size but a decreased maximum energy product, likely due to solute segregation to grain boundaries that inhibited intergranular exchange interactions. In general, Group VB additions had a more positive effect on the microstructure and magnetic properties than did other alloying additions. This paper will summarize the overall effects of the additions on the microstructures and magnetic properties, enabling further alloy design efforts in nanocomposite alloy systems.
IEEE Transactions on Magnetics 08/2004; · 1.36 Impact Factor
-
Advanced Materials 12/2002; 14(24):1832 - 1834. · 13.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A cluster-beam technique is used to produce nearly monodispersed nanoclusters of Co while independently varying their size and concentration. The Co clusters are embedded in Cu and SiO2 to form films with cluster size varying from 300 to 9000 atoms and Co concentration varying from 10 to 50 vol %. The Co magnetization (Ms) increases with increasing cluster size and decreases with increasing Co concentration for a given cluster size but is always less than the bulk value. First-principles calculations are used to analyze the experimental data in terms of the local environment effects at the cluster-matrix interface and spin-glass-like behavior due to the strong ferro- and antiferromagnetic intercluster exchange interactions for small separations.
Phys. Rev. B. 08/2002; 66(6).
-
[show abstract]
[hide abstract]
ABSTRACT: Periodic magnetic nanodot arrays have been produced on an area as large as 1 cm×1 cm by direct nanolithography using interferometric laser radiation. The dots are formed by the local annealing of sputtered amorphous Co–C films in regions where the laser intensity is highest. At room temperature the dots exhibit ferromagnetic order and are embedded in a paramagnetic matrix. The onset of room-temperature ferromagnetism is caused by nanoscale chemical and morphological changes during dot formation and reflects the phase separation of magnetic Co-rich clusters. The present single-step nanolithography is potentially an efficient method for fabrication of patterned magnetic arrays. © 2001 American Institute of Physics.
Applied Physics Letters 10/2001; 79(16):2606-2608. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper presents results on the synthesis and properties of
FePt- and CoPt-based high-anisotropy nanocomposite films. These films
consist of high anisotropy L1<sub>0</sub> CoPt or FePt particles
embedded in a nonmagnetic matrix such as C, SiO<sub>2</sub>, or B<sub>2
</sub>O<sub>3</sub>. The grain size and magnetic properties of these
films ran he controlled by the processing temperatures and the film
compositions. Particularly in the FePt:B<sub>2</sub>O<sub>3</sub> films
the c-axes of the FePt grains can be made to align along the film normal
direction, resulting in films with perpendicular anisotropy.
Temperature-dependence of magnetic properties and activation volumes are
investigated to understand the magnetization reversal and
thermal-activation effects. The potential of these films as high-density
recording media is discussed
IEEE Transactions on Magnetics 08/2001; · 1.36 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Nanocomposite Co<sub>x</sub>Pt<sub>100-x</sub>:C thin films with
x=57 and x=50 were prepared by magnetron sputtering. Structures and
activation volumes were studied systematically in order to better
understand the magnetic behavior. We found that activation volumes of
these two samples were almost the same. However, physical grain sizes of
these two samples were different. Magnetic grains in the sample with
equiatomic Co and Pt composition were almost independently switched and
magnetic grains in the sample with higher Co content were exchange
coupled and switched together. The reduction of the physical grain size
in the sample does not always cause the reduction of the exchange
coupling in the sample. Our results indicate that the Co concentration
is an additional factor to be used for controlling the properties of
nanocomposite CoPt:C films
IEEE Transactions on Magnetics 08/2001; · 1.36 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Magnetic properties of Co-C nanodot arrays produced by direct
interferometric laser annealing are investigated by magnetic force
microscopy (MFM) and magnetization measurements. The dots are formed by
locally annealing sputtered amorphous Co-C films in regions where the
laser intensity is highest. As-sputtered Co-C films do not exhibit
ferromagnetic order at room temperature, but MFM shows that the dots
become magnetic upon annealing, possibly due to the agglomeration or
phase separation of Co-rich clusters. The dots are embedded in either a
paramagnetic or weakly magnetic matrix. The magnetic properties of the
generated pattern can be changed by varying the laser power. The present
results show that direct interferometric lithography may become a useful
tool for fabricating future patterned magnetic nanostructures
IEEE Transactions on Magnetics 08/2001; · 1.36 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Fe1-xCox (0 < or = x < or = 1) nanowires have been self-assembled by electrodeposition in porous alumina films. The crystal structure is bee at the Fe end. With increased addition of Co, the crystal structure remains bcc until about 67% addition of Co. At the Co end, the structure is a mixture of hcp and fcc. Magnetic studies show very high coercivities for the Fe-Co alloys in the bcc phase. For Fe0.67Co0.33 nanowires of diameter 9 nm, the coercivity is about 2900 Oe, whereas for Fe0.33Co0.67 nanowires, it is about 2850 Oe. Temperature and size dependence of magnetic properties show no indication of superparamagnetic effects down to wire diameters of 9 nm.
Journal of Nanoscience and Nanotechnology 06/2001; 1(2):149-52. · 1.56 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Magnetic properties of Ni nanowires electrodeposited into self-assembled porous alumina arrays have been investigated. By anodizing aluminum in sulfuric acid and immersing the as-anodized template into phosphoric acid for different lengths of time, we are able to vary the diameters of the subsequently deposited nanowires between 8 and 25 nm. The coercivity measured along wire axis first increases with the wire diameter, reaches a maximum of 950 Oe near a diameter of 18 nm, and then decreases with further increase of wire diameter. The dependence of the magnetization of Ni nanowires is found to follow Bloch's law at low temperature but with the Bloch exponent decreasing from the bulk value and the Bloch constant increasing from the bulk value by an order of magnitude.
Phys. Rev. B. 10/2000; 62(18).
-
[show abstract]
[hide abstract]
ABSTRACT: Magnetization processes in Ni nanowire arrays are investigated. The wires are produced by electrodepositing Ni into porous anodic alumina and exhibit coercivities of the order of 0.05 T (500 Oe) along the wire axis. Transmission-electron microscopy of freed wires shows that the wires are polycrystalline and resemble a chain of nanocrystallites. To model the hysteresis loops, the wires are treated as one-dimensional random-anisotropy magnets where the magnetocrystalline bulk anisotropy is a weak perturbation to the leading anisotropy contribution. The calculation yields an analytical equation for the magnetization as a function of the applied magnetic field. For small and moderate reversed fields, the agreement between theory and experiment is very good, but the applicability of the model breaks down close to the coercive field. This failure is explained by the neglect of higher-order perturbation terms describing, for example, magnetic localization effects.
Journal of Physics Condensed Matter 07/2000; 12(30):L497. · 2.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The nanostructure and magnetic properties of composite CoPt:C films at room temperature were investigated as a function of annealing temperature, carbon concentration, and film thickness. CoPt:C films with a variety of carbon concentrations were fabricated by cosputtering Co, Pt, and C onto water-cooled Si(100) substrates followed by annealing. X-ray diffraction and transmission electron microscopy analyses indicate that nanocrystallites of face-centered-tetragonal (fct) CoPt phase, which has a uniaxial magnetocrystalline anisotropy constant of about 5×10<sup>7</sup> erg/cm <sup> 3 </sup>, can be formed in carbon matrix when the annealing temperature is higher than 600 °C. The grain sizes of the fct CoPt crystallites are about 10 nm and the coercivities can be as high as 12 kOe. Higher annealing temperature and lower carbon concentration generally lead to larger grain sizes and perhaps more complete formation of the fct CoPt phase, and therefore higher coercivities. The coercivity is insensitive to the film thickness until the thickness is smaller than the CoPt grain size, when the coercivity starts to decrease with film thickness. The magnetic activation volumes are typically around 1×10<sup>-18</sup> cm <sup> 3 </sup>. The nanostructure and the associated magnetic properties of these composite CoPt:C films are promising as potential media for extremely high-density recording. © 2000 American Institute of Physics.
Journal of Applied Physics 06/2000; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Ferromagnetic Co nanowires have been electrodeposited into self-assembled porous anodic alumina arrays. Due to their cylindrical shape, the nanowires exhibit perpendicular anisotropy. The coercivity, remanence ratio, and activation volumes of Co nanowires depend strongly on the length, diameter, and spacing of the nanowires. Both coercivity and thermal activation volume increase with increasing wire length, while for constant center-to-center spacing, coercivity decreases and thermal activation volume increases with increasing wire diameter. The behavior of the nanowires is explained qualitatively in terms of localized magnetization reversal. © 2000 American Institute of Physics.
Journal of Applied Physics 06/2000; · 2.17 Impact Factor
