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23 Apr 2012

Volume 100, Issue 17, Articles (17xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 171903 (2012); http://dx.doi.org/10.1063/1.4704193 (3 pages)

Y. Peng and K. Kempa
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Surface photoluminescence and magnetism in hydrothermally grown undoped ZnO nanorod arrays

Xiaoyong Xu, Chunxiang Xu, Yi Lin, Tao Ding, Shengjiang Fang, Zengliang Shi, Weiwei Xia, and Jingguo Hu

Appl. Phys. Lett. 100, 172401 (2012); http://dx.doi.org/10.1063/1.4705412 (5 pages) | Cited 6 times

Online Publication Date: 23 April 2012

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ZnO nanorod arrays were synthesized by a hydrothermal method on the Si substrate with ZnO thin film as seed layer prepared by magnetron sputtering. The presence of -OH ligands on the surface of the as-grown sample was confirmed, and its dominant role in both suppressing the visible emission and boosting the room-temperature ferromagnetism (FM) was revealed. Through alternative H2 and O2 annealing to remove the -OH ligands, reconstruct surface-states and tune the oxygen occupancy in ZnO nanorods, the clear correlation between the characteristic green emission and ferromagnetism was established.
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81.16.-c Methods of micro- and nanofabrication and processing
68.55.ag Semiconductors
81.15.Cd Deposition by sputtering
78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors

Role of donor-acceptor complexes and impurity band in stabilizing ferromagnetic order in Cu-doped SnO2 thin films

Yongfeng Li, Rui Deng, Yufeng Tian, Bin Yao, and Tom Wu

Appl. Phys. Lett. 100, 172402 (2012); http://dx.doi.org/10.1063/1.4705419 (4 pages) | Cited 7 times

Online Publication Date: 23 April 2012

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Our complementary magnetic and photoluminescence measurements reveal the correlation between the donor-acceptor complex and the ferromagnetic order in Cu-doped SnO2 thin films. Oxygen vacancies (VO) and Cu dopants form defect complexes of donor-acceptor pairs, and the associated spin-polarized impurity band leads to the narrowing of bandgap. Electronic structure calculations based on the first-principles method demonstrate that the Cu-VO complex has low formation energy and can stabilize the ferromagnetic coupling. Our results suggest that intrinsic defects and their complexes with dopants play a key role for establishing the ferromagnetic order in doped wide-bandgap oxides.
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61.72.up Other materials
75.50.Dd Nonmetallic ferromagnetic materials
61.72.jd Vacancies
75.50.Pp Magnetic semiconductors
71.20.Nr Semiconductor compounds
78.55.Hx Other solid inorganic materials

The role of Ni-Mn hybridization on the martensitic phase transitions in Mn-rich Heusler alloys

Mahmud Khan, J. Jung, S. S. Stoyko, Arthur Mar, Abdiel Quetz, Tapas Samanta, Igor Dubenko, Naushad Ali, Shane Stadler, and K. H. Chow

Appl. Phys. Lett. 100, 172403 (2012); http://dx.doi.org/10.1063/1.4705422 (4 pages) | Cited 3 times

Online Publication Date: 23 April 2012

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Room temperature x-ray diffraction, dc magnetization, and ac susceptibility measurements have been performed on a series of Mn rich Ni50Mn37-xCrxSb13 and Ni50+xMn37-xSb13 Heusler alloys. Depending on the value of x, the room temperature crystal structures of the samples are either L21 cubic or orthorhombic. It is a commonly accepted idea that the martensitic transition temperatures in Ni-Mn-Z (Z = Ga, In, Sb, Sn) based Heusler alloys decrease (increase) with decreasing (increasing) valence electron concentration, e/a. However, the present work shows that regardless of the change in e/a, the martensitic transition temperature (TM) decreases with increasing Cr or Ni concentration. These results support the model where, in the case of Mn rich Heusler alloys, it is the hybridization between the Ni atoms and the Mn atoms in the Z sites that plays the dominant role in driving the martensitic transformation.
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81.30.Kf Martensitic transformations
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
64.70.kd Metals and alloys
61.66.Dk Alloys

Phase diagram of magnetic domain walls in spin valve nano-stripes

N. Rougemaille, V. Uhlíř, O. Fruchart, S. Pizzini, J. Vogel, and J. C. Toussaint

Appl. Phys. Lett. 100, 172404 (2012); http://dx.doi.org/10.1063/1.4704665 (4 pages)

Online Publication Date: 23 April 2012

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We investigate numerically the transverse versus vortex phase diagram of head-to-head domain walls in Co/Cu/Py spin valve nano-stripes (Py: permalloy), in which the Co layer is mostly single domain while the Py layer hosts the domain wall. The range of stability of the transverse wall is shifted towards larger thickness compared to single Py layers, due to a magnetostatic screening effect between the two layers. An approached analytical scaling law is derived, which reproduces faithfully the phase diagram.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ch Domain walls and domain structure
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.75.-c Magnetic properties of nanostructures

Reversible switching of room temperature ferromagnetism in CeO2-Co nanoparticles

J. Sacanell, M. A. Paulin, V. Ferrari, G. Garbarino, and A. G. Leyva

Appl. Phys. Lett. 100, 172405 (2012); http://dx.doi.org/10.1063/1.4705045 (3 pages) | Cited 2 times

Online Publication Date: 23 April 2012

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We investigated the reversible ferromagnetic (FM) behavior of pure and Co doped CeO2 nanopowders. The as-sintered samples displayed an increasing paramagnetic contribution upon Co doping. Room temperature FM is obtained simply by performing thermal treatments in vacuum at temperatures as low as 500 °C and it can be switched off by performing thermal treatments in oxidizing conditions. The FM contribution is enhanced as we increase the time of the thermal treatment in vacuum. Those systematic experiments establish a direct relation between ferromagnetism and oxygen vacancies and open a path for developing materials with tailored properties.
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75.78.Jp Ultrafast magnetization dynamics and switching
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.20.Ck Nonmetals
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials
81.07.Wx Nanopowders

Effect of oxidizing the ferromagnetic electrode in magnetic tunnel junctions on tunneling magnetoresistance

Sungjung Joo, K. Y. Jung, B. C. Lee, Tae-Suk Kim, K. H. Shin, Myung-Hwa Jung, K.-J. Rho, J.-H. Park, Jinki Hong, and K. Rhie

Appl. Phys. Lett. 100, 172406 (2012); http://dx.doi.org/10.1063/1.4704557 (3 pages) | Cited 1 time

Online Publication Date: 24 April 2012

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The ferromagnetic layer in magnetic tunnel junctions (MTJs) was oxidized with varying O2 concentrations, and the corresponding effect on spin-dependent transport was studied. As expected from our previous results for MTJs with an over-oxidized AlOx tunnel barrier, a partially oxidized ferromagnetic layer plays an important role in spin-dependent transport. As the temperature is lowered, the junction resistance increases dramatically, and the tunneling magnetoresistance (TMR) is strongly suppressed. Increasing the O2 concentration enhances the increase of resistance and suppression of TMR. This work supports our previous conclusion that oxidizing the ferromagnetic layer generates localized magnetic moments, which act as a scattering center for spin-polarized tunneling electrons.
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75.47.-m Magnetotransport phenomena; materials for magnetotransport
81.65.Mq Oxidation
82.45.-h Electrochemistry and electrophoresis
72.25.-b Spin polarized transport

Reducing spin torque switching current density by boron insertion into a CoFeB free layer of a magnetic tunnel junction

Xin Jiang, Rai Moriya, and Stuart Parkin

Appl. Phys. Lett. 100, 172407 (2012); http://dx.doi.org/10.1063/1.4704916 (3 pages) | Cited 1 time

Online Publication Date: 24 April 2012

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The spin torque switching current density is measured for magnetic tunnel junctions containing a CoFeB free layer. We find that the insertion of an ultra-thin boron layer near the free layer/tunnel barrier interface gives rise to an increased resistance-area product and to a reduction in the switching current density. This is attributed to a lower tunneling matrix element near the inserted boron. As a result, the injected current is concentrated within smaller areas of the free layer, which leads to an overall decrease in the switching current density.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
72.15.Gd Galvanomagnetic and other magnetotransport effects
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Nanoscale spin wave valve and phase shifter

Y. Au, M. Dvornik, O. Dmytriiev, and V. V. Kruglyak

Appl. Phys. Lett. 100, 172408 (2012); http://dx.doi.org/10.1063/1.4705289 (4 pages) | Cited 4 times

Online Publication Date: 24 April 2012

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We have used micromagnetic simulations to demonstrate a method for controlling the amplitude and phase of spin waves propagating inside a magnonic waveguide. The method employs a nanomagnet formed on top of a magnonic waveguide. The function of the proposed device is controlled by defining the static magnetization direction of the nanomagnet. The result is a valve or phase shifter for spin waves, acting as the carrier of information for computation or data processing within the emerging spin wave logic architectures of magnonics. The proposed concept offers such technically important benefits as energy efficiency, non-volatility, and miniaturization.
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85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.

Strong ferromagnetism in Pt-coated ZnCoO: The role of interstitial hydrogen

Jong Moon Shin, Ho Sik Lee, Su Young Cha, Seunghun Lee, Ji Young Kim, Noejung Park, Yong Chan Cho, Su Jae Kim, Sung-Kyu Kim, Jong-Seong Bae, Sungkyun Park, Chae Ryong Cho, Hideomi Koinuma, and Se-Young Jeong

Appl. Phys. Lett. 100, 172409 (2012); http://dx.doi.org/10.1063/1.4705304 (4 pages)

Online Publication Date: 24 April 2012

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We observed strong ferromagnetism in ZnCoO as a result of high concentration hydrogen absorption. Coating ZnCoO with Pt layer, and ensuing hydrogen treatment with a high isostatic pressure resulted in a highly increased carrier concentration of 1021/cm3. This hydrogen treatment induced a strong ferromagnetism at low temperature that turned to superparamagnetism at about 140 K. We performed density functional method computations and found that the interstitial H dopants promote the ferromagnetic ordering between scattered Co dopants. On the other hand, interstitial hydrogen can decrease the magnetic exchange energy of Co-H-Co complexes, leading to a reduction in the blocking temperature.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Pp Magnetic semiconductors
75.30.Et Exchange and superexchange interactions
75.50.Dd Nonmetallic ferromagnetic materials
73.61.Le Other inorganic semiconductors
61.72.jj Interstitials

Magnetic-field-induced reverse martensitic transformation and large magnetoresistance in Ni50−xCoxMn32Al18 Heusler alloys

H. C. Xuan, L. J. Shen, T. Tang, Q. Q. Cao, D. H. Wang, and Y. W. Du

Appl. Phys. Lett. 100, 172410 (2012); http://dx.doi.org/10.1063/1.4707161 (4 pages) | Cited 3 times

Online Publication Date: 25 April 2012

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We have studied the magnetic and magnetoresistance properties in polycrystalline Ni50−xCoxMn32Al18 Heusler alloys. A unique martensitic transformation (MT) from the ferromagnetic austenite to the weak-magnetic martensite phase was obtained and magnetic-field-induced reverse MT was confirmed in these alloys. A magnetic difference up to 45 emu/g was measured across the MT for Ni42Co8Mn32Al18 alloy. In the same quaternary alloy, the large magnetoresistance effect of 67% at 225 K was observed through this field-induced magnetic phase transition.
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81.30.Kf Martensitic transformations
64.70.kd Metals and alloys
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Cc Other ferromagnetic metals and alloys
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

Co/Ni multilayers with perpendicular anisotropy for spintronic device applications

L. You, R. C. Sousa, S. Bandiera, B. Rodmacq, and B. Dieny

Appl. Phys. Lett. 100, 172411 (2012); http://dx.doi.org/10.1063/1.4704184 (4 pages) | Cited 2 times

Online Publication Date: 26 April 2012

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This letter presents a study of perpendicular anisotropy in Co/Ni multilayers, which could constitute a thick polarizer in spin torque oscillators or a magnetic electrode in magnetic tunnel junctions (MTJ) with perpendicular anisotropy. Perfectly square perpendicular loops are observed for as-deposited Co/Ni multilayers with various sublayer thicknesses and bilayer repetition numbers using a Pt buffer layer. An anisotropy energy of 1.0 × 106 erg·cm−3 is obtained for 9 nm thick Co/Ni multilayers. For Co/Ni multilayers deposited on MgO, no perpendicular magnetization component is observed in the as-deposited state, but it develops (even in 2.1 nm Co/Ni multilayers) after annealing at 250 °C.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Gw Magnetic anisotropy
75.47.Np Metals and alloys
81.40.Gh Other heat and thermomechanical treatments

High sensitivity low field magnetically gated resistive switching in CoFe2O4/La0.66Sr0.34MnO3 heterostructure

Vishal Thakare, Guozhong Xing, Haiyang Peng, Abhimanyu Rana, Onkar Game, P. Anil Kumar, Arun Banpurkar, Yesappa Kolekar, Kartik Ghosh, Tom Wu, D. D. Sarma, and Satishchandra B. Ogale

Appl. Phys. Lett. 100, 172412 (2012); http://dx.doi.org/10.1063/1.4707373 (4 pages) | Cited 5 times

Online Publication Date: 26 April 2012

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The phenomenon of resistive switching (RS) has been demonstrated in several non-magnetic and some magnetic oxide systems, however the “magnetic” aspect of magnetic oxides has not been emphasized especially in terms of low field tunability. In our work, we examined the CoFe2O4/La0.66Sr0.34MnO3 all-magnetic oxide interface system for RS and discovered a very sharp (bipolar) transition at room temperature that can be gated with high sensitivity by low magnetic fields (∼0–100 mT). By using a number of characterizations, we show that this is an interface effect, which may open up interesting directions for manipulation of the RS phenomenon.
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85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
75.50.Gg Ferrimagnetics
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Radial-spin-wave-mode-assisted vortex-core magnetization reversals

Myoung-Woo Yoo, Jehyun Lee, and Sang-Koog Kim

Appl. Phys. Lett. 100, 172413 (2012); http://dx.doi.org/10.1063/1.4705690 (4 pages) | Cited 1 time

Online Publication Date: 27 April 2012

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The dynamic behaviors of vortex-core magnetization reversals in soft magnetic nanodisks driven by oscillating magnetic fields applied perpendicularly to the disk plane were studied by means of micromagnetic numerical simulations. It was found that when the field frequencies are tuned to the eigenfrequencies of radial spin-wave modes, the threshold field amplitudes required for vortex-core switching are an order of magnitude smaller than those of static perpendicular fields. The reversal mechanism and associated underlying physics are completely different from those of vortex-antivortex-pair-mediated core reversals. The results reflect the achievement of an alternative efficient means of ultrafast vortex-core switching.
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75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.30.Ds Spin waves
75.50.Tt Fine-particle systems; nanocrystalline materials
75.78.Jp Ultrafast magnetization dynamics and switching
75.78.Cd Micromagnetic simulations
75.60.Jk Magnetization reversal mechanisms

Investigation of perpendicular magnetic anisotropy of CoFeB by x-ray magnetic circular dichroism

W. C. Tsai, S. C. Liao, H. C. Hou, C. T. Yen, Y. H. Wang, H. M. Tsai, F. H. Chang, H. J. Lin, and Chih-Huang Lai

Appl. Phys. Lett. 100, 172414 (2012); http://dx.doi.org/10.1063/1.4707380 (4 pages) | Cited 1 time

Online Publication Date: 27 April 2012

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We have studied the variation of perpendicular magnetic anisotropy (PMA) of CoFeB layers with the oxidation degree of the MgO buffer layers. After annealing at 330 °C, the out-of-plane anisotropy field (Hk) of 1.5-nm CoFeB could be optimized to 3.1 kOe. To investigate the orbital hybridization of the Fe-3d and O-2p, the orbital moments per hole (morb/Nh) of Fe in annealed CoFeB layers were extracted by using x-ray magnetic circular dichroism. The dependence of morb/Nh on the oxidation time of MgO coincided with that of Hk, which revealed that PMA of annealed CoFeB originated from the interfacial orbital hybridization.
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75.30.Gw Magnetic anisotropy
75.60.Nt Magnetic annealing and temperature-hysteresis effects
75.70.Ak Magnetic properties of monolayers and thin films
78.20.Ls Magneto-optical effects
75.30.Cr Saturation moments and magnetic susceptibilities
78.66.Bz Metals and metallic alloys
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