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14 Jul 2008

Volume 93, Issue 2, Articles (02xxxx)

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Appl. Phys. Lett. 93, 023303 (2008); http://dx.doi.org/10.1063/1.2953179 (3 pages)

Takafumi Kawanishi, Takaaki Fujiwara, Megumi Akai-Kasaya, Akira Saito, Masakazu Aono, Junichi Takeya, and Yuji Kuwahara
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Suppression of martensitic phase transition at the Ni2MnGa film surface

P. Pörsch, M. Kallmayer, T. Eichhorn, G. Jakob, H. J. Elmers, C. A. Jenkins, C. Felser, R. Ramesh, and M. Huth

Appl. Phys. Lett. 93, 022501 (2008); http://dx.doi.org/10.1063/1.2957647 (3 pages) | Cited 10 times

Online Publication Date: 14 July 2008

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We investigated magnetic and structural properties at the surface of epitaxial Ni2MnGa(110) Heusler films using x-ray absorption spectroscopy and x-ray magnetic circular dichroism both in transmission and total electron yield mode. The magnetic shape memory films were prepared by dc sputtering from a stoichiometric target onto sapphire substrates at an optimized substrate temperature of 773 K. X-ray diffraction confirms a (110) oriented growth on Al2O3(11math0) and an austenite to martensite transition at 270–280 K. At the surface the martensitic phase transition and the magnetization are strongly suppressed. The deviation in the surface properties is caused by a Mn deficiency near the surface.
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81.30.Kf Martensitic transformations
64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
78.20.Fm Birefringence
78.70.Dm X-ray absorption spectra
61.66.Bi Elemental solids
61.66.Dk Alloys

Exchange bias in nanoscale antidot arrays

D. Tripathy, A. O. Adeyeye, and N. Singh

Appl. Phys. Lett. 93, 022502 (2008); http://dx.doi.org/10.1063/1.2959727 (3 pages) | Cited 14 times

Online Publication Date: 15 July 2008

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Exchange bias effects have been systematically investigated in nanoscale Cu (10 nm)/Ni80Fe20 (30 nm)/Ir75Mn25 (30 nm)/Cu (2 nm) multilayer antidot arrays. The antidot arrays exhibit asymmetric and shifted hysteresis loops along the induced exchange bias direction, with higher coercivity and exchange bias field values as compared to a continuous film deposited under identical conditions. The evolution in exchange bias field with increasing antidot diameter is ascribed to the constraints imposed on the domain size in the Ir75Mn25 layer and reduced ferromagnetic-ferromagnetic interactions in the Ni80Fe20 layer.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Bb Fe and its alloys
75.75.-c Magnetic properties of nanostructures
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetic shape memory effect in thin foils

Oleg Heczko, Aleksandr Soroka, and Simo-Pekka Hannula

Appl. Phys. Lett. 93, 022503 (2008); http://dx.doi.org/10.1063/1.2957675 (3 pages) | Cited 13 times

Online Publication Date: 15 July 2008

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The magnetic shape memory (MSM) effect was observed in Ni–Mn–Ga freestanding thin foils down to 90 μm in thickness using top-down approach. The foils were prepared by thinning the bulk crystals exhibiting MSM effect. The effect was evaluated from the magnetization curves. The significant decrease in magnetic field needed to initiate the MSM effect (magnetic field induced strain or martensite structure reorientation) was observed for the studied foils down to μ0H = 0.088 T or H = 70 kA/m. Observation suggests that the pinning of twin boundaries on the internal obstacles rather than pinning on surface lowers twin boundaries’ mobility.
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75.70.-i Magnetic properties of thin films, surfaces, and interfaces
64.70.kd Metals and alloys
81.30.Kf Martensitic transformations
61.72.Mm Grain and twin boundaries
75.80.+q Magnetomechanical effects, magnetostriction

Synthesis and magnetic properties of self-assembled FeRh nanoparticles

Z. Jia, J. W. Harrell, and R. D. K. Misra

Appl. Phys. Lett. 93, 022504 (2008); http://dx.doi.org/10.1063/1.2952956 (3 pages) | Cited 8 times

Online Publication Date: 15 July 2008

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We report here the synthesis and magnetization behavior of tunable FeRh magnetic nanoparticles with both controllable composition and size. FexRh1−x (x = 0.35,0.44,0.51) nanoparticles of 4–20 nm size range were fabricated using a polyol coreduction process. The stoichiometry of FexRh1−x nanoparticles was altered by tuning the molar ratio of rhodium acetylacetonate and iron acetylacetonate. The particle size was tunable via control of surfactant concentration. Magnetic measurements were made for films of the particles cast onto silicon wafers. The coercivity of Fe51Rh49 nanoparticles was ∼ 250 Oe at room temperature after annealing at 700 °C for 2 h, indicating CsCl-type phase transition. The temperature dependent magnetization measurement of annealed Fe51Rh49 confirmed the antiferromagnetic-ferromagnetic transition and was supported by x-ray diffraction measurements.
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81.16.Dn Self-assembly
81.07.Bc Nanocrystalline materials
75.50.Tt Fine-particle systems; nanocrystalline materials
61.66.Bi Elemental solids
61.66.Dk Alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Spin transfer induced coherent microwave emission with large power from nanoscale MgO tunnel junctions

D. Houssameddine, S. H. Florez, J. A. Katine, J.-P. Michel, U. Ebels, D. Mauri, O. Ozatay, B. Delaet, B. Viala, L. Folks, B. D. Terris, and M.-C. Cyrille

Appl. Phys. Lett. 93, 022505 (2008); http://dx.doi.org/10.1063/1.2956418 (3 pages) | Cited 45 times

Online Publication Date: 15 July 2008

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In low resistance-area product MgO magnetic tunnel junction nanopillars, we observe high integrated power (up to 43 nW), narrow linewidth (down to 10 MHz), spin transfer induced microwave emission at frequencies up to 14 GHz due to precession of the free layer magnetization at room temperature. Although all devices were fabricated on the same wafer, they present bimodal transport and precessional characteristics. The devices in which the narrowest linewidths were observed exhibited low resistance and tunneling magnetoresistance (30%), while maintaining large integrated power.
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78.70.Gq Microwave and radio-frequency interactions
75.50.Tt Fine-particle systems; nanocrystalline materials
75.47.Pq Other materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Measurement of the critical curve of a synthetic antiferromagnet

Cosmin Radu, Dorin Cimpoesu, Alexandru Stancu, and Leonard Spinu

Appl. Phys. Lett. 93, 022506 (2008); http://dx.doi.org/10.1063/1.2953439 (3 pages) | Cited 2 times

Online Publication Date: 16 July 2008

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In this paper, we propose a method for a synthetic antiferromagnet structure’s critical curve determination. The method is based on reversible susceptibility’s singularities detection, as the magnetic field is swept along easy axis, in both positive and negative direction, while a hard axis bias field is also applied. By performing susceptibility measurements with different values of the bias field, the critical curve can be determined. Knowing the critical curve of a synthetic antiferromagnetic structure is essential for devices such as magnetic random access memories.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Cr Saturation moments and magnetic susceptibilities

Switchable magnetic dipole induced guided vortex motion

N. Verellen, A. V. Silhanek, W. Gillijns, V. V. Moshchalkov, V. Metlushko, F. Gozzini, and B. Ilic

Appl. Phys. Lett. 93, 022507 (2008); http://dx.doi.org/10.1063/1.2955824 (3 pages) | Cited 10 times

Online Publication Date: 16 July 2008

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We present evidence of magnetically controlled vortex motion in an Al film on top of a periodic array of Permalloy square rings. The resulting magnetic template generates a strongly anisotropic pinning potential landscape for vortices in the superconducting layer. Transport measurements show that this anisotropy is able to confine the flux motion along the high symmetry axes of the square lattice of dipoles. This guided vortex motion can be rerouted by 90° simply changing the dipole orientation or even suppressed by inducing a flux-closure magnetic state with very low stray fields in the rings.
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74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.78.-w Superconducting films and low-dimensional structures
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)

Scattering of backward spin waves in a one-dimensional magnonic crystal

A. V. Chumak, A. A. Serga, B. Hillebrands, and M. P. Kostylev

Appl. Phys. Lett. 93, 022508 (2008); http://dx.doi.org/10.1063/1.2963027 (3 pages) | Cited 39 times

Online Publication Date: 17 July 2008

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Scattering of backward volume magnetostatic spin waves from a one-dimensional magnonic crystal, realized by a grating of shallow grooves etched into the surface of an yttrium iron garnet film, was experimentally studied. Rejection frequency bands were clearly observed. The rejection efficiency and the frequency width of the rejection bands increase with increasing groove depth. A theoretical model based on the analogy of a spin-wave film waveguide with a microwave transmission line was used to interpret the obtained experimental results.
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75.30.Ds Spin waves
75.50.Gg Ferrimagnetics

Graphene-protected iron layer on Ni(111)

Yu. S. Dedkov, M. Fonin, U. Rüdiger, and C. Laubschat

Appl. Phys. Lett. 93, 022509 (2008); http://dx.doi.org/10.1063/1.2953972 (3 pages) | Cited 30 times

Online Publication Date: 17 July 2008

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Here we report a photoemission study of the Fe intercalation underneath a graphene layer on Ni(111). The process of intercalation was monitored by means of x-ray photoemission of corresponding core levels as well as ultraviolet photoemission of the graphene-derived π states in the valence band. Thin fcc Fe layers (2–5 ML thickness) at the interface between a graphene capping layer and Ni(111) form epitaxial films passivated from the reactive environment.
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79.60.Jv Interfaces; heterostructures; nanostructures
75.70.Ak Magnetic properties of monolayers and thin films
81.65.Rv Passivation
75.50.Bb Fe and its alloys
73.20.At Surface states, band structure, electron density of states

Room-temperature ferromagnetism of Zn0.97Co0.03O pressed nanocrystalline powders

Jifan Hu, Hongwei Qin, Tianfeng Xue, Ensi Cao, and Dengtao Li

Appl. Phys. Lett. 93, 022510 (2008); http://dx.doi.org/10.1063/1.2960350 (3 pages) | Cited 4 times

Online Publication Date: 17 July 2008

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Zn0.97Co0.03O nanocrystalline powders were prepared by sol-gel method with a low annealing temperature of 450 °C. The as-synthesized Zn0.97Co0.03O powders were paramagnetic. However, after compaction under a pressure of 94 MPa, the Zn0.97Co0.03O pressed powders show a weak room-temperature ferromagnetism embedded in the paramagnetic background. The observed ferromagnetism is connected with compaction-induced defects at/near grain boundaries. Meanwhile, a room-temperature magnetoresistance ΔR/R0 as high as −73.7% is observed in Zn0.97Co0.03O pressed powder sample.
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75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.47.De Giant magnetoresistance
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Magnetic characterization of bulk nanostructured iron oxides

J. R. Morales, J. E. Garay, M. Biasini, and W. P. Beyermann

Appl. Phys. Lett. 93, 022511 (2008); http://dx.doi.org/10.1063/1.2959070 (3 pages) | Cited 1 time

Online Publication Date: 18 July 2008

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Iron oxide nanopowders were consolidated using a current activated method at varying temperatures. The resulting bulk dense samples with crystal sizes in the nanometric range were found to be mixtures of cubic and hexagonal phases of iron oxides (nanocomposites). Magnetic characterization reveals one abrupt increase, up to 300%, in the magnetization with increasing temperature. The coercivity Hc steps down 300% at this point as well. The transition temperature, T1, is strongly dependent on the size of the nanopowder employed during the sintering. For the sizes of 8 and 40 nm we detect T1 = (14±2) K and T1 = (122±1) K, respectively.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
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