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14 May 2012

Volume 100, Issue 20, Articles (20xxxx)

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Appl. Phys. Lett. 100, 203104 (2012); http://dx.doi.org/10.1063/1.3701731 (4 pages)

Z. Y. Jiang, X. X. Jiang, S. Su, X. P. Wei, S. T. Lee, and Y. He
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Effect of oxygen deficiency on room temperature ferromagnetism in Co doped ZnO

Hao Gu, Wen Zhang, Yongbing Xu, and Mi Yan

Appl. Phys. Lett. 100, 202401 (2012); http://dx.doi.org/10.1063/1.4717741 (4 pages) | Cited 6 times

Online Publication Date: 15 May 2012

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We investigated the correlation between magnetization and oxygen vacancies in Zn0.95Co0.05O nanoparticles. Enhanced magnetizations were found in SiO2 nanopowders and carbon nanotubes (CNTS) treated Zn0.95Co0.05O, which are attributed to minimizing nanoparticle size and increasing oxygen vacancy concentration. After oxygen annealing, the magnetization of both non-treated Zn0.95Co0.05O and CNTS treated Zn0.95Co0.05O decreased sharply with the filling of the oxygen vacancies, while the SiO2 treated Zn0.95Co0.05O was influenced little as the amorphous SiO2 shell prevents the diffusion of oxygen into magnetic particles. It demonstrated that the ferromagnetism comes from the interfacial oxygen deficiency and is tunable by changing the oxygen vacancies.
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81.05.Dz II-VI semiconductors
61.72.jd Vacancies
61.72.Cc Kinetics of defect formation and annealing
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Effect of cold working in a magnetic field on the shape of a ferromagnetic nanocontact

Marc Müller, Richard Montbrun, Christoph Sürgers, and Hilbert v. Löhneysen

Appl. Phys. Lett. 100, 202402 (2012); http://dx.doi.org/10.1063/1.4718307 (5 pages)

Online Publication Date: 15 May 2012

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The role of a magnetic field present during formation of a ferromagnetic nanocontact—established by mechanically pulling a thin dysprosium wire—is investigated. We demonstrate that the shape of the nanocontact depends on the strength and direction of the magnetic field applied during plastic deformation of the contact. The different contact shapes obtained after cycles of tensile or compressive stress are attributed to the rearrangement of magnetic domains during formation of the nanocontact by magnetoelastic coupling.
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81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity
75.50.Cc Other ferromagnetic metals and alloys
75.80.+q Magnetomechanical effects, magnetostriction
81.07.Lk Nanocontacts

Anomalies of magnetic properties and magnetovolume effect in Cd1−xMnxGeAs2 at hydrostatic pressure

A. Yu. Mollaev, I. K. Kamilov, R. K. Arslanov, T. R. Arslanov, U. Z. Zalibekov, V. M. Novotortsev, S. F. Marenkin, and V. M. Trukhan

Appl. Phys. Lett. 100, 202403 (2012); http://dx.doi.org/10.1063/1.4718419 (4 pages) | Cited 1 time

Online Publication Date: 15 May 2012

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We present the experimental results of the effect hydrostatic pressure up to P ≤ 7 GPa applied at the room temperatures in diluted magnetic semiconductor Cd1−xMnxGeAs2 (x = 0.06 − 0.3). We have found the pressure areas at which anomalies in magnetic properties were observed. Induced by hydrostatic pressure at P > 1.5 GPa magnetic phase transitions, interpreted as metamagnetic transition, were observed. The transitions from magnetic-ordered into magnetic disordered phases in region P > 4.1 GPa on the pressure dependences of relative volume compressibility were detected. We estimated the values of bulk modulus and volume magnetostriction. It is shown that high pressures significantly decrease the Curie temperature with values dTC/dP ≈ (−14.0 ÷ −6.8) K/GPa.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Pp Magnetic semiconductors
81.40.Lm Deformation, plasticity, and creep
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.80.+q Magnetomechanical effects, magnetostriction
81.40.Jj Elasticity and anelasticity, stress-strain relations

Reversibility in the inverse magnetocaloric effect in Mn3GaC studied by direct adiabatic temperature-change measurements

Ö. Çakιr and M. Acet

Appl. Phys. Lett. 100, 202404 (2012); http://dx.doi.org/10.1063/1.4717181 (3 pages) | Cited 3 times

Online Publication Date: 15 May 2012

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Thermal-hysteresis affects adversely the reversibility of the magnetocaloric effect around a first order magnetostructural transition. In Mn3GaC, a first order antiferromagnetic-ferromagnetic transition is accompanied by a volume-change and a 5 K thermal-hysteresis. We study the reversibility of the magnetocaloric effect in the transition region by direct adiabatic temperature-change measurements. The magnetic field is cycled between 0 and 3 T, and the temperature-change is observed. We find that the system exhibits a temperature-change of 3.1 K in the virgin state, and all subsequent cycling leads to a 2.8 K warming and cooling when the field is decreased and increased, respectively.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Ee Antiferromagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.78.-n Magnetization dynamics
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Temperature and concentration-dependent relaxation of ferrofluids characterized with a high-Tc SQUID-based nuclear magnetic resonance spectrometer

Hong-Chang Yang, Chieh-Wen Liu, S. H. Liao, Hsin-Hsien Chen, M. J. Chen, K. L. Chen, Herng-Er Horng, S. Y. Yang, and L. M. Wang

Appl. Phys. Lett. 100, 202405 (2012); http://dx.doi.org/10.1063/1.4718032 (4 pages) | Cited 3 times

Online Publication Date: 16 May 2012

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We investigated the relaxation of protons in magnetic fluids using a high-Tc SQUID magnetometer. It was found that the longitudinal relaxation rate, 1/T1, is slower than the transverse relaxation rate, 1/T2, for ferrofluids in the same field. This is due to the fact that the 1/T1 process involves returning the magnetization to the z-direction, which automatically involves the loss of magnetization in the x-y plane governed by the 1/T2 process. Additionally, 1/T1 and 1/T2 at high temperatures are slower than the corresponding relaxation rates at low temperatures, which is due to the enhanced Brownian motion of nanoparticles at high temperatures.
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75.50.Mm Magnetic liquids
76.60.Es Relaxation effects
05.40.Jc Brownian motion

The study of perpendicular magnetic anisotropy in CoFeB sandwiched by MgO and tantalum layers using polarized neutron reflectometry

T. Zhu, Y. Yang, R. C. Yu, H. Ambaye, V. Lauter, and J. Q. Xiao

Appl. Phys. Lett. 100, 202406 (2012); http://dx.doi.org/10.1063/1.4718423 (4 pages) | Cited 3 times

Online Publication Date: 16 May 2012

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The perpendicular magnetic anisotropy (PMA) in CoFeB sandwiched by MgO and tantalum layers was investigated using anomalous Hall effect and polarized neutron reflectometry. It was found that a large PMA in the CoFeB above MgO layer was related to its low magnetization compared to the case of CoFeB under MgO layer. Using the sensitivity of neutrons to the absorption cross-section of boron, we unambiguously determined the depth profile of the boron distribution and showed that after annealing, most of the boron diffused to form a 2-nm-thick interface layer between the CoFeB and tantalum layers.
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75.30.Gw Magnetic anisotropy
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
68.65.Ac Multilayers
68.35.Ct Interface structure and roughness
68.35.Fx Diffusion; interface formation
72.15.Gd Galvanomagnetic and other magnetotransport effects

Temperature dependence of carrier spin polarization determined from current-induced domain wall motion in a Co/Ni nanowire

K. Ueda, T. Koyama, R. Hiramatsu, D. Chiba, S. Fukami, H. Tanigawa, T. Suzuki, N. Ohshima, N. Ishiwata, Y. Nakatani, K. Kobayashi, and T. Ono

Appl. Phys. Lett. 100, 202407 (2012); http://dx.doi.org/10.1063/1.4718599 (3 pages) | Cited 2 times

Online Publication Date: 17 May 2012

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We have investigated the temperature dependence of the current-induced magnetic domain wall (DW) motion in a perpendicularly magnetized Co/Ni nanowire at various temperatures and with various applied currents. The carrier spin polarization was estimated from the measured domain wall velocity. We found that it decreased more with increasing temperature from 100 K to 530 K than the saturation magnetization did.
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81.05.Bx Metals, semimetals, and alloys
81.07.Gf Nanowires
75.75.-c Magnetic properties of nanostructures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Ch Domain walls and domain structure
75.50.Tt Fine-particle systems; nanocrystalline materials
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GaMnAs: Position of Mn-d levels and majority spin band gap predicted from GGA-1/2 calculations

R. R. Pelá, M. Marques, L. G. Ferreira, J. Furthmüller, and L. K. Teles

Appl. Phys. Lett. 100, 202408 (2012); http://dx.doi.org/10.1063/1.4718602 (4 pages) | Cited 2 times

Online Publication Date: 17 May 2012

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Among all magnetic semiconductors, GaMnAs seems to be the most important one. In this work, we present accurate first-principles calculations of GaMnAs within the GGA-1/2 approach: We concentrate our efforts in obtaining the position of the peak of Mn-d levels in the valence band and also the majority spin band gap. For the position of the Mn-d peak, we find a value of 3.3 eV below the Fermi level, in good agreement with the most recent experimental results of 3.5 and 3.7 eV. An analytical expression that fits the calculated Eg(x) for majority spin is derived in order to provide ready access to the band gap for the composition range from 0 to 0.25. We found a value of 3.9 eV for the gap bowing parameter. The results agree well with the most recent experimental data.
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71.20.Nr Semiconductor compounds
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
75.50.Pp Magnetic semiconductors

d carrier induced intrinsic room temperature ferromagnetism in Nb:TiO2 film

J. Y. Yang, Y. L. Han, L. He, R. F. Dou, C. M. Xiong, and J. C. Nie

Appl. Phys. Lett. 100, 202409 (2012); http://dx.doi.org/10.1063/1.4707378 (4 pages) | Cited 3 times

Online Publication Date: 17 May 2012

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High crystalline anatase TiO2 and Nb:TiO2 thin films were fabricated on LaAlO3 (100) substrates by pulsed laser deposition. Room temperature ferromagnetism was obtained in Nb:TiO2 but absent in pure TiO2. The Kondo effect and anomalous Hall effect observed in metallic Nb:TiO2 strongly confirmed the existence of exchange interaction between intrinsic local magnetic moments and carriers. High energy resolution x-ray photoelectron spectroscopy studies of the Nb:TiO2 thin film revealed clear signals of Ti3+ and Nb4+ ions, which had one unpaired d electron responsible for the local magnetic moments. This result consisted quite well with the spin polarized first principle calculation.
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72.25.-b Spin polarized transport
75.50.Dd Nonmetallic ferromagnetic materials
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Et Exchange and superexchange interactions
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

Heating asymmetry induced by tunneling current flow in magnetic tunnel junctions

E. Gapihan, J. Hérault, R. C. Sousa, Y. Dahmane, B. Dieny, L. Vila, I. L. Prejbeanu, C. Ducruet, C. Portemont, K. Mackay, and J. P. Nozières

Appl. Phys. Lett. 100, 202410 (2012); http://dx.doi.org/10.1063/1.4719663 (4 pages) | Cited 2 times

Online Publication Date: 18 May 2012

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In this work, exchange bias was used as a probe to characterise the temperature profile induced by the inelastic relaxation of electrons tunnelling across a MgO barrier. Thermally assisted magnetic random access memory (TA-MRAM) cells comprising a magnetic tunnel junction (MTJ) with a reference pinned layer and a FeMn exchange biased storage layer were used. The pinning direction of the ferromagnetic storage layer is reversed when heated above the blocking temperature of the antiferromagnetic layer (FeMn). The power density required to reach this blocking temperature in the FeMn layer depends on the current polarity, indicating that the heat source term associated with the current flowing through the barrier depends itself on the current direction in contrast to simple Joule heating. This effect is due to the mechanism of energy dissipation in tunnelling. The tunnelling itself is ballistic i.e., without dissipation. However, after tunnelling, the hot electrons very quickly relax to the Fermi energy thereby loosing their excess energy in the receiving electrode. Therefore, the heat is essentially generated on one side of the barrier so that the whole profile of temperature throughout the pillar depends on the current direction. Full 3D thermal simulations also confirmed the temperature profile asymmetry. The proper choice of heating current direction (i.e., voltage polarity applied to the MTJ) can yield a reduction of about 10% in the heating power density required to enable writing in thermally assisted MRAM cells.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
73.23.Ad Ballistic transport
75.50.Bb Fe and its alloys
75.50.Ee Antiferromagnetics
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