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15 Mar 2010

Volume 96, Issue 11, Articles (11xxxx)

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Appl. Phys. Lett. 96, 111901 (2010); http://dx.doi.org/10.1063/1.3358107 (3 pages)

B. Rožič, S. Krause, H. Finkelmann, G. Cordoyiannis, and Z. Kutnjak
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Temperature dependent nucleation and annihilation of individual magnetic vortices

G. Mihajlović, M. S. Patrick, J. E. Pearson, V. Novosad, S. D. Bader, M. Field, G. J. Sullivan, and A. Hoffmann

Appl. Phys. Lett. 96, 112501 (2010); http://dx.doi.org/10.1063/1.3360841 (3 pages) | Cited 6 times

Online Publication Date: 15 March 2010

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We studied the temperature dependence of the magnetization reversal in individual submicron permalloy disks with micro-Hall and bend-resistance magnetometry. The nucleation field exhibits a nonmonotonic dependence with positive and negative slopes at low and high temperatures, respectively, while the annihilation field monotonically decreases with increasing temperature, but with distinctly different slopes at low and high temperatures. Our analysis suggests that at low temperatures vortex nucleation and annihilation proceeds via thermal activation over an energy barrier, while at high temperatures they are governed by a temperature dependence of the saturation magnetization.
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75.60.Jk Magnetization reversal mechanisms
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.50.Bb Fe and its alloys

Effects of annealing on the local structure of Fe and Co in CoFeB/MgO/CoFeB tunnel junctions: An extended x-ray-absorption fine structure study

Abdul K. Rumaiz, J. C. Woicik, W. G. Wang, Jean Jordan-Sweet, G. H. Jaffari, C. Ni, John Q. Xiao, and C. L. Chien

Appl. Phys. Lett. 96, 112502 (2010); http://dx.doi.org/10.1063/1.3364137 (3 pages) | Cited 6 times

Online Publication Date: 16 March 2010

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Extended x-ray-absorption fine structure (EXAFS) was used to study the evolution of the local structure of Fe and Co as a function of annealing time in CoFeB/MgO/CoFeB magnetic tunnel junctions. EXAFS indicates B depletion and crystallization of the CoFeB layers within a few seconds of the post growth high temperature anneal. The decrease in first-shell Debye–Waller factor and hence the increase in structural order during annealing explains the increase in tunnel magnetoresistance observed as a result of post deposition annealing.
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75.47.Pq Other materials
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.25.Dk Orbital, charge, and other orders, including coupling of these orders
61.72.Cc Kinetics of defect formation and annealing
78.70.Dm X-ray absorption spectra

Irreversibility of field-induced magnetostructural transition in NiCoMnSb shape memory alloy revealed by magnetization, transport and heat capacity studies

Ajaya K. Nayak, K. G. Suresh, and A. K. Nigam

Appl. Phys. Lett. 96, 112503 (2010); http://dx.doi.org/10.1063/1.3365181 (3 pages) | Cited 8 times

Online Publication Date: 17 March 2010

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The effects of magnetic field on the martensitic transition have been studied in Ni45Co5Mn38Sb12. We find a large field-induced irreversibility in this system, as revealed by the field dependence of resistivity, magnetization, and heat capacity data. At the critical temperature, the field-induced conversion of the martensitic to austenite phase is not reversible under any field variation. At this temperature any energy fluctuation induces nucleation and growth of the equilibrium austenite phase at the expense of the metastable martensitic phase and gets arrested. All these three measurements completely rule out the coexistence of austenite and martensitic phases in the irreversibility regime.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
81.30.Kf Martensitic transformations
81.40.Lm Deformation, plasticity, and creep
64.70.K- Solid-solid transitions
62.20.fg Shape-memory effect; yield stress; superelasticity

Superparamagnetic and superspin glass behaviors in the martensitic state of Ni43.5Co6.5Mn39Sn11 magnetic shape memory alloy

D. Y. Cong, S. Roth, J. Liu, Q. Luo, M. Pötschke, C. Hürrich, and L. Schultz

Appl. Phys. Lett. 96, 112504 (2010); http://dx.doi.org/10.1063/1.3367721 (3 pages) | Cited 10 times

Online Publication Date: 17 March 2010

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The magnetic state of the low-temperature martensite in a Ni43.5Co6.5Mn39Sn11 magnetic shape memory alloy (MSMA) is disclosed. At temperatures (T) above a critical temperature Tf, the magnetization versus field [M(H)] curves display a sigmoid shape, show no magnetic hysteresis, and can be well fitted according to the Langevin model, confirming that the martensite shows superparamagnetic behavior at T>Tf. On the other hand, the observation of a memory effect during the stop-and-wait protocol and the analysis of dynamic magnetic properties probed by ac susceptibility measurements unequivocally corroborate the superspin glass behavior of interacting magnetic clusters at T<Tf. The present study provides significant insights into the magnetism of martensite in MSMAs.
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81.30.Kf Martensitic transformations
81.40.Rs Electrical and magnetic properties related to treatment conditions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
64.70.kd Metals and alloys
62.20.fg Shape-memory effect; yield stress; superelasticity

Electrode artifacts in low resistance organic spin valves

Alberto Riminucci, Mirko Prezioso, Patrizio Graziosi, and Carol Newby

Appl. Phys. Lett. 96, 112505 (2010); http://dx.doi.org/10.1063/1.3309599 (3 pages) | Cited 4 times

Online Publication Date: 17 March 2010

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Artifacts can originate from the inherent shortcomings of the cross bar configuration, when the resistance of the device is small compared to that of one of the electrodes. This is particularly relevant to the field of organic spintronics, in which at least one recent work overlooked this effect. We use a simplified one-dimensional resistor model and a full three-dimensional finite element method simulation to show that an increase in the resistance of one electrode appears as a decrease of the measured resistance. We found that the model agrees qualitatively but not quantitatively with observation.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
02.70.Dh Finite-element and Galerkin methods

In-plane reorientation of magnetization in epitaxial exchange biased Fe/MnPd bilayers

Qing-feng Zhan and Kannan M. Krishnan

Appl. Phys. Lett. 96, 112506 (2010); http://dx.doi.org/10.1063/1.3367705 (3 pages) | Cited 5 times

Online Publication Date: 18 March 2010

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We investigated the in-plane reorientation of magnetization in epitaxial Fe/MnPd bilayers. The samples with various thicknesses of MnPd and Fe layers present a conventional square and an unusual two-step exchange biased hysteresis loops at different temperature. The shape of the loops is reproduced using the coherent rotation model and including the relative orientation of the uniaxial anisotropy with respect to the exchange bias. The parallel and perpendicular uniaxial anisotropies in the ferromagnetic layer are linked to the aligned and the reoriented states, respectively. The magnetic reorientation between the aligned and the reoriented states, which is determined by the competition between the interface exchange coupling and the intrinsic uniaxial energies, is shown to be driven by the temperature, as well as the thickness of MnPd and Fe layers.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.30.Gw Magnetic anisotropy

Comparing magnetotransport and surface magnetic properties of half-metallic CrO2 films grown by low pressure and atmospheric pressure chemical vapor deposition

M. Pathak, X. Zhong, J. Wang, X. Zhang, T. M. Klein, P. LeClair, and A. Gupta

Appl. Phys. Lett. 96, 112507 (2010); http://dx.doi.org/10.1063/1.3367783 (3 pages)

Online Publication Date: 18 March 2010

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CrO2 films prepared by low pressure chemical vapor deposition (LPCVD) using Cr(CO)6 precursor have been investigated and compared with epitaxial half metallic CrO2 films prepared at atmospheric pressure (APCVD) using CrO3 precursor for their magnetotransport and surface magnetic properties. LPCVD films showed higher resistivity than APCVD epitaxial (100) CrO2 films prepared on (100) TiO2 substrates. Magnetoresistance of LPCVD films is comparable to that of APCVD films. X-ray magnetic circular dichroism suggests a reduced surface magnetic moment for LPCVD films. This reduced magnetic moment is attributed to antiferromagnetic alignment of the uncompensated Cr spins in the Cr2O3 surface layer.
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73.61.Ng Insulators
75.70.Ak Magnetic properties of monolayers and thin films
75.70.Rf Surface magnetism
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

Epitaxial stabilization of ε-Fe2O3 (00l) thin films on SrTiO3 (111)

M. Gich, J. Gazquez, A. Roig, A. Crespi, J. Fontcuberta, J. C. Idrobo, S. J. Pennycook, M. Varela, V. Skumryev, and M. Varela

Appl. Phys. Lett. 96, 112508 (2010); http://dx.doi.org/10.1063/1.3360217 (3 pages) | Cited 2 times

Online Publication Date: 18 March 2010

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Thin films of the metastable and elusive ε-Fe2O3 have been epitaxially stabilized on SrTiO3 (111) substrates. The ε-Fe2O3 films present a (001) orientation perpendicular to the substrate and three in-plane domains measuring a few nanometers and showing atomically sharp interfaces. We argue that this domain structure, rather than the epitaxial-strain, plays an essential role in stabilizing the ε-Fe2O3 by minimizing the energy of (100) surfaces. The ε-Fe2O3 films show a large in-plane coercivity ∼ 8 kOe which combined with the magnetoelectric character claimed for this oxide may lead to novel applications in spintronics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.78.-n Magnetization dynamics
75.50.Vv High coercivity materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.85.+t Magnetoelectric effects, multiferroics

Hot electron transport in magnetic tunnel transistors with an epitaxial MgO tunnel barrier

Taro Nagahama, Hidekazu Saito, and Shinji Yuasa

Appl. Phys. Lett. 96, 112509 (2010); http://dx.doi.org/10.1063/1.3360222 (3 pages) | Cited 3 times

Online Publication Date: 18 March 2010

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We fabricated fully epitaxial magnetic tunnel transistors (MTTs) consisting of a Fe(001)/MgO(001) emitter and a Fe(001) based on a n-GaAs(001) collector. We observed much higher transfer ratio (α) and magnetocurrent (MC) ratio than those of the previously reported MTTs that had an amorphous AlO barrier. These improvements are due to the coherent tunneling and single-crystalline base layer. In addition, a pronounced peak structure appeared in the emitter bias dependence of α, which might be attributed to the interface resonance states at the Fe(001)/GaAs(001) interface.
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85.70.-w Magnetic devices
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
85.30.Pq Bipolar transistors
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)

Spin wave mode transition induced by surface anisotropy and characteristic length in magnetic nanoparticles

Peiheng Zhou (周佩珩), Li Zhang (张丽), and Longjiang Deng (邓龙江)

Appl. Phys. Lett. 96, 112510 (2010); http://dx.doi.org/10.1063/1.3360882 (3 pages) | Cited 2 times

Online Publication Date: 18 March 2010

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In magnetic nanoparticles with cylindrical symmetry of the magnetization configurations, spin wave mode transition (SWMT) is predicted for the case of a surface anisotropy parallel to the uniform magnetization. Theoretical investigation has shown that a characteristic length can be determined to evaluate the spatial influence of the spin wave mode excited by surface anisotropy. This knowledge implies SWMT occurs as the particle size exceeds the corresponding characteristic length. An indirect experimental observation of the SWMT is also provided. These results can be utilized for ferromagnetic resonances control in magnetic devices.
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75.75.Jn Dynamics of magnetic nanoparticles
75.30.Ds Spin waves
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Rf Surface magnetism

Correlated d0 ferromagnetism and photoluminescence in undoped ZnO nanowires

Guozhong Xing, Dandan Wang, Jiabao Yi, Lili Yang, Ming Gao, Mi He, Jinghai Yang, Jun Ding, Tze Chien Sum, and Tom Wu

Appl. Phys. Lett. 96, 112511 (2010); http://dx.doi.org/10.1063/1.3340930 (3 pages) | Cited 53 times

Online Publication Date: 19 March 2010

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We report the correlated d0 ferromagnetism and photoluminescence in undoped single-crystalline ZnO nanowires synthesized by using a vapor transport method. We systematically tune the oxygen deficiency in the ZnO nanowires from 4% to 20% by adjusting the growth conditions, i.e., selecting different catalyst (Au or Ag) and varying the growth temperature. Our study suggests that oxygen vacancies induce characteristic photoluminescence and significantly boost the room-temperature ferromagnetism. Such undoped ZnO nanowires with tunable magnetic and optical properties are promising to find applications in multifunctional spintronic and photonic nanodevices.
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78.55.Et II-VI semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
78.67.Uh Nanowires
81.07.Gf Nanowires
75.75.-c Magnetic properties of nanostructures
75.75.Cd Fabrication of magnetic nanostructures
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