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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Exchange through nonmagnetic insulating matrix

R. Skomski, A. Kashyap, Y. Qiang, and D. J. Sellmyer

J. Appl. Phys. 93, 6477 (2003); http://dx.doi.org/10.1063/1.1541633 (3 pages) | Cited 5 times

Online Publication Date: 9 May 2003

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Exchange interactions between hard-magnetic particles in a nonmagnetic matrix are investigated by model calculations. A Landau–Ginzburg approach is developed to describe the net exchange interactions between spheres of arbitrary diameters. Introducing cylindrical coordinates and integrating over the surfaces of the adjacent spheres yields an exchange coupling which decreases with a decay length depending on interatomic exchange, intra-atomic exchange, and temperature. Typically, the decay length does not exceed a few interatomic distances. The decay is exponential but also contains a prefactor depending on the surface curvature of the grains. It increases with decreasing curvature, but this dependence is only a small correction to the leading exponential term. © 2003 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Et Exchange and superexchange interactions
75.75.-c Magnetic properties of nanostructures
75.50.Ww Permanent magnets
75.50.Vv High coercivity materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Modeling the magnetic properties of DyFe2/YFe2 superlattices

G. J. Bowden, J.-M. L. Beaujour, A. A. Zhukov, B. D. Rainford, P. A. J. de Groot, R. C. C. Ward, and M. R. Wells

J. Appl. Phys. 93, 6480 (2003); http://dx.doi.org/10.1063/1.1539072 (3 pages) | Cited 10 times

Online Publication Date: 9 May 2003

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The Stoner–Wohlfarth model has proved reasonably successful in describing the coercivities of antiferromagnetically coupled DyFe2/YFe2 hard/soft superlattices in the absence of magnetic exchange springs. In particular, the coercivity rises sharply as the net magnetic moment of the superlattice approaches zero. However the situation becomes more complicated as the thickness of the YFe2 layers is increased. Two distinct “instability fields” can be identified: the bending field BB, signifying the onset of a magnetic exchange spring, and the irreversible switching field BIS associated with magnetic reversal. We have developed a computational model to address this problem. In particular, it is shown that the two instability fields in question are characterized by vanishing eigenvalues in the matrix formed by the double energy derivatives ∂2E/∂θi∂θj, where E is the total energy and θi the angle of each individual monolayer. It is shown that the model provides a very good description of the M–Bapp loops of DyFe2/YFe2 multilayer films. In particular, the coercivity of a nearly magnetically compensated multilayer (75 ÅDyFe2/150 ÅYFe2) is much reduced below the prediction of the Stoner–Wolhfarth model, in accord with experiment. © 2003 American Institute of Physics.
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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
75.60.Jk Magnetization reversal mechanisms

Ferromagnetic resonance in exchange spring thin films

D. C. Crew and R. L. Stamps

J. Appl. Phys. 93, 6483 (2003); http://dx.doi.org/10.1063/1.1558244 (3 pages) | Cited 4 times

Online Publication Date: 9 May 2003

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The ferromagnetic resonance frequencies have been calculated for a soft/hard Co/CoPt exchange spring thin film system. In this geometry the magnetostatics are crucial to understanding the variation of resonant frequency with field due to the out-of-plane component of magnetization of the CoPt hard layer. When exchange coupling between films exists, significant changes in the resonant frequency occur. For in-plane reverse applied fields, a spiral is formed in the soft Co layer. This appears as a double minimum in the resonant frequency as a function of the in-plane applied field. The positions of the frequency minima correspond to the beginning and end of the spiral formation in the Co layer. The frequency of the maximum provides a sensitive measure of the perpendicular anisotropy in the cobalt film. © 2003 American Institute of Physics.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.50.Cc Other ferromagnetic metals and alloys
75.30.Gw Magnetic anisotropy
75.50.Vv High coercivity materials

Remagnetization processes in SmCo/NdCo exchange springs

V. K. Vlasko-Vlasov, U. Welp, Z. J. Guo, J. S. Jiang, J. E. Pearson, J. P. Liu, D. J. Miller, Y. Tang, and S. D. Bader

J. Appl. Phys. 93, 6486 (2003); http://dx.doi.org/10.1063/1.1541634 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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Remagnetization processes in a SmCo/NdCo exchange spring system are studied using superconducting quantum interference device magnetometry and magneto-optical imaging. The magneto-optical images reveal that the reversal of the soft NdCo layer and its return to the polarized state at cycling around the exchange bias field occur due to the asymmetric nucleation and growth of twisted-phase domains. The observed peculiarities of the domain-wall kinetics are discussed and a way of tailoring exchange biased systems by regulating the motion of the twisted-phase domains is suggested. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.60.Jk Magnetization reversal mechanisms
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

Reversible magnetization processes and energy density product in Sm–CoFe and Sm–Co/Co bilayers

T. Schrefl, H. Forster, R. Dittrich, D. Suess, W. Scholz, and J. Fidler

J. Appl. Phys. 93, 6489 (2003); http://dx.doi.org/10.1063/1.1558245 (3 pages) | Cited 10 times

Online Publication Date: 9 May 2003

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The hysteresis properties of epitaxial SmCo/Co and SmCo/Fe bilayers are calculated by the solution of the Landau–Lifshitz Gilbert equation. The thin film grain structure is taken into account using appropriate finite element techniques. The J(H) curve shows the typical exchange spring behavior for the bilayer if the soft magnetic layer thickness exceeds 10 nm. However, the reversible rotations of the magnetization for low external field deteriorate the maximum energy density product. Straight B(H) curves are obtained only for a Fe layer thickness of 5 nm. Magnetization reversal starts with the reversible rotation of the soft layer magnetization. Initially, the magnetization rotates in opposite directions in different regions of the film. The reversible rotations penetrate substantially into the hard layer. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Jk Magnetization reversal mechanisms
75.30.Et Exchange and superexchange interactions
75.50.Cc Other ferromagnetic metals and alloys
75.50.Bb Fe and its alloys
75.50.Vv High coercivity materials
75.50.Ww Permanent magnets

The improved magnetic properties in phosphorus substituted Pr–Fe–P–B nanocomposites

Z. Q. Jin, Y. Zhang, H. L. Wang, A. Klaessig, M. Bonder, and G. C. Hadjipanayis

J. Appl. Phys. 93, 6492 (2003); http://dx.doi.org/10.1063/1.1558335 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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Phosphorus substituted (Pr,Tb)8(Fe,Nb,Zr,P)88B4 nanocomposites have been produced by melt-spinning. The effects of phosphorus substitution as well as wheel speed on the crystallization behavior and magnetic properties of the melt-spun samples have been investigated. With the substitution of phosphorus, the crystallization temperature of amorphous phase increases. The optimum wheel speed was found to be around 25 m/s for as-spun ribbons and 40 m/s for the annealed samples, both of which present excellent second quadrant hysteresis loop shapes due to the fine grain size of α-Fe which is around 20 nm. The addition of phosphorus also greatly improves the coercivity of Pr–Fe–B nanocomposites without a significant loss of saturation magnetization. A higher coercivity of 9.2 kOe in P-substituted samples was obtained as compared to 8.1 kOe in P-free samples. This is attributed to a narrower temperature span between the crystallization into TbCu7 structure and the transformation into the 2:14:1 phase caused by the phosphorus substitution. © 2003 American Institute of Physics.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
81.07.Bc Nanocrystalline materials
81.05.Bx Metals, semimetals, and alloys
61.43.Dq Amorphous semiconductors, metals, and alloys
61.46.-w Structure of nanoscale materials
81.40.Rs Electrical and magnetic properties related to treatment conditions

Magnetic hysteresis of mechanically alloyed Sm–Co nanocrystalline powders

J. Zhou, R. Skomski, and D. J. Sellmyer

J. Appl. Phys. 93, 6495 (2003); http://dx.doi.org/10.1063/1.1558587 (3 pages) | Cited 7 times

Online Publication Date: 9 May 2003

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Mechanically alloyed Sm–Co powders and Sm12(Co,Cu,Ti)88 powders are investigated. X-ray diffraction patterns show that after annealing, structures of 2:7, 1:5, and 1:7 phases form. A room-temperature coercivity of 41 kOe was obtained in Sm2Co7 powders. Magnetic hysteresis was investigated by the method of δm curves. Positive value δm curves were obtained in Sm2Co7 and SmCo5, while negative values were found in SmCo7.3 and SmCo8 indicating different magnetization reversal mechanisms. Nanocrystalline powders of Sm12(Co,Cu,Ti)88 with a mixture of 1:5 and 2:17 phases form after long-time heat treatment. The intrinsic coercivity of the powders increases with an increasing amount of Cu. Short annealing time produces 1:7 phase with higher crystalline anisotropy, which results in larger coercivity. © 2003 American Institute of Physics.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Cc Other ferromagnetic metals and alloys
81.07.Wx Nanopowders
75.50.Ww Permanent magnets
75.30.Gw Magnetic anisotropy
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation

Fabrication of Sm–Co/Co (Fe) composites by electroless Co and Co–Fe plating

Q. Zeng, Y. Zhang, M. J. Bonder, and G. C. Hadjipanayis

J. Appl. Phys. 93, 6498 (2003); http://dx.doi.org/10.1063/1.1558246 (3 pages) | Cited 3 times

Online Publication Date: 9 May 2003

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Sm–Co/Co and Sm–Co/Fe–Co nanocomposites have been fabricated by electroless plating of Sm–Co powders with Co and Co–Fe, respectively. The influence of electroless plating conditions on the magnetic properties and structure of the coatings were studied. The saturation magnetization (Ms) and morphology of the deposited material were found to be dependent on the concentration of hypophosphite, the temperature of the solution, and plating time. By plating at an elevated temperature and decreasing the NaH2PO2⋅H2O concentration of the bath, the phosphorus content decreases, and it is possible to form Co particles with a Ms of up to 142 emu/g as compared to 160 emu/g for bulk Co. For 50 μm Sm(Co,Cu,Fe,Zr)7.5 particles, coating with Co has had little effect on the coercivity, maintaining a value of 7 kOe, while there is a substantial increase in the value of Ms that varies with Co plating time. However, SmCo5 particles with a size of a few microns, plating with the bcc Co–Fe alloy increases Ms significantly, at the expense of the coercivity which decreases drastically. © 2003 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
81.07.Wx Nanopowders
81.15.Pq Electrodeposition, electroplating
75.50.Cc Other ferromagnetic metals and alloys
75.50.Vv High coercivity materials
75.50.Ww Permanent magnets
75.50.Bb Fe and its alloys
81.05.Ni Dispersion-, fiber-, and platelet-reinforced metal-based composites
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.46.-w Structure of nanoscale materials
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