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10 Sep 2007

Volume 91, Issue 11, Articles (11xxxx)

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Appl. Phys. Lett. 91, 112501 (2007); http://dx.doi.org/10.1063/1.2780107 (3 pages)

Y. Liu, S. Gliga, R. Hertel, and C. M. Schneider
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Current-induced magnetic vortex core switching in a Permalloy nanodisk

Y. Liu, S. Gliga, R. Hertel, and C. M. Schneider

Appl. Phys. Lett. 91, 112501 (2007); http://dx.doi.org/10.1063/1.2780107 (3 pages) | Cited 32 times

Online Publication Date: 10 September 2007

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The authors report on the switching of a magnetic vortex core in a submicron Permalloy disk, induced by a short current pulse applied in the film plane. Micromagnetic simulations including the adiabatic and nonadiabatic spin-torque terms are used to investigate the current-driven magnetization dynamics. They predict that a core reversal can be triggered by current bursts a tenth of a nanosecond long. The vortex core reversal process is found to be the same as when an external field pulse is applied. The control of a vortex core’s orientation using current pulses introduces the technologically relevant possibility to address individual nanomagnets within dense arrays.
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75.75.-c Magnetic properties of nanostructures
75.30.Ds Spin waves
75.40.Mg Numerical simulation studies
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)

Transverse domain walls in nanoconstrictions

D. Backes, C. Schieback, M. Kläui, F. Junginger, H. Ehrke, P. Nielaba, U. Rüdiger, L. J. Heyderman, C. S. Chen, T. Kasama, R. E. Dunin-Borkowski, C. A. F. Vaz, and J. A. C. Bland

Appl. Phys. Lett. 91, 112502 (2007); http://dx.doi.org/10.1063/1.2779109 (3 pages) | Cited 15 times

Online Publication Date: 11 September 2007

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The spin structure of domain walls in constrictions down to 30 nm is investigated both experimentally with electron holography and with simulations using a Heisenberg model. Symmetric and asymmetric transverse domain walls for different constriction sizes are observed, consistent with simulations. The experimentally observed asymmetric transverse walls can be further divided into tilted and buckled walls, the latter being an intermediate state just before the vortex nucleation. As the constriction width decreases, the domain wall width decreases faster than linearly, which leads to very narrow domain walls for narrow constrictions.
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75.60.Ch Domain walls and domain structure
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.10.Jm Quantized spin models, including quantum spin frustration

Large room-temperature magnetocaloric effects in Fe0.8Mn1.5As

N. K. Sun, S. Ma, Q. Zhang, J. Du, and Z. D. Zhang

Appl. Phys. Lett. 91, 112503 (2007); http://dx.doi.org/10.1063/1.2784170 (3 pages) | Cited 25 times

Online Publication Date: 12 September 2007

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In Fe0.8Mn1.5As compound, an external magnetic field induces a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase above Ts = 285 K, leading to large magnetocaloric effects around room temperature. Instead of showing inverse magnetocaloric effects, the sign of the entropy change ΔSM in the compound is unexpectedly negative, revealing a different mechanism. The maximum value of ΔSM is 6.2 J/kg K at 287.5 K for a magnetic field change of 5 T. The study on systems with antiferromagnetism-related metamagnetic transitions may open an important field in searching good materials for room-temperature magnetic refrigeration.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Gg Ferrimagnetics
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Temperature dependence of the interlayer exchange coupling in epitaxial Fe1/MgO/Fe2/Co tunnel junctions

E. Popova, N. Keller, F. Gendron, C. Tiusan, A. Schuhl, and N. A. Lesnik

Appl. Phys. Lett. 91, 112504 (2007); http://dx.doi.org/10.1063/1.2784942 (3 pages) | Cited 8 times

Online Publication Date: 13 September 2007

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The temperature dependence of the interlayer exchange coupling has been investigated in epitaxial tunnel junctions Fe1/MgO/Fe2/Co/V with thin MgO layers using X-band ferromagnetic resonance (FMR) in the range 2–300 K. Variations of FMR parameters allow concluding that the coupling strength increases with temperature. This is in agreement with predictions of the theories considering pure tunneling mechanisms and contradicts the model of a resonant assisted tunneling related to defects in the insulator. The temperature dependence of the FMR linewidth shows the line narrowing under the sample heating. This may be due to the additional mechanism associated with the coupling.
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75.50.Bb Fe and its alloys
75.50.Cc Other ferromagnetic metals and alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields

Observation of exchange bias in the martensitic state of Ni50Mn36Sn14 Heusler alloy

Zhe Li, Chao Jing, Jiping Chen, Shujuan Yuan, Shixun Cao, and Jincang Zhang

Appl. Phys. Lett. 91, 112505 (2007); http://dx.doi.org/10.1063/1.2784958 (3 pages) | Cited 37 times

Online Publication Date: 13 September 2007

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Exchange bias was observed in the Ni50Mn36Sn14 Heusler alloy after field cooling by means of hysteresis loop measurement. The hysteresis loops shift along the axis of an applied field and its magnitude significantly increased with decreasing temperature below 70 K. This effect could be understood as a result of exchange anisotropy created at the interface between an antiferromagnet and a ferromagnet in the phase separated of martensitic state. Above 70 K, however, the exchange bias field disappeared and the coercivity significantly reduced owing to the fact that the pinning between an antiferromagnet and a ferromagnet becomes weaker with increasing temperature.
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75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
81.30.Kf Martensitic transformations
75.50.Ee Antiferromagnetics
75.50.Cc Other ferromagnetic metals and alloys

Coherent magnetization precession in GaMnAs induced by ultrafast optical excitation

J. Qi, Y. Xu, N. H. Tolk, X. Liu, J. K. Furdyna, and I. E. Perakis

Appl. Phys. Lett. 91, 112506 (2007); http://dx.doi.org/10.1063/1.2785114 (3 pages) | Cited 22 times

Online Publication Date: 14 September 2007

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The authors use femtosecond optical pulses to induce, control, and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ∼ 40 K, they observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ∼ TC/2. The authors find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but independent of ambient temperature. They provide a physical interpretation of the observed light-induced collective Mn-spin precession and relaxation.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Pp Magnetic semiconductors
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
75.50.Dd Nonmetallic ferromagnetic materials

Enhanced stability by field cooling in superconducting levitation with translational symmetry

Nuria Del Valle, Alvaro Sanchez, Enric Pardo, Carles Navau, and Du-Xing Chen

Appl. Phys. Lett. 91, 112507 (2007); http://dx.doi.org/10.1063/1.2785169 (3 pages) | Cited 14 times

Online Publication Date: 14 September 2007

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The force and stability of a levitating system consisting of an infinitely long superconductor and different arrangements of infinitely long parallel permanent magnets are theoretically analyzed for two different cooling processes, field and zero-field cooling, by using a model based on the critical-state model and a magnetic energy minimization procedure. The authors find that by cooling the superconductor near the magnets, one can obtain improved lateral stability as compared to the case of zero-field cooling.
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74.25.Ha Magnetic properties including vortex structures and related phenomena
75.50.Ww Permanent magnets
75.50.Vv High coercivity materials
84.71.Ba Superconducting magnets; magnetic levitation devices
85.70.Rp Magnetic levitation, propulsion and control devices
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