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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Oxidation process of AlOx-based magnetic tunnel junctions studied by photoconductance

P. H. P. Koller, F. W. M. Vanhelmont, H. Boeve, R. Coehoorn, and W. J. M. de Jonge

J. Appl. Phys. 93, 8549 (2003); http://dx.doi.org/10.1063/1.1555317 (3 pages) | Cited 8 times

Online Publication Date: 9 May 2003

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The oxidation process of Co/AlOx/Co magnetic tunnel junctions has been investigated by photoconductance, in addition to traditional transport measurements. The shape of the photoconductance curves is explained within the framework of a simple qualitative model, assuming an oxidation time dependent imbalance of the incident forward and reverse hot electron fluxes, as well as inelastic scattering processes in the oxide. Due to the large sensitivity of the technique, the presence of unoxidized Al beneath the barrier layer can be monitored very accurately. The disappearance of a negative contribution to the photocurrent indicates the complete oxidation of the barrier layer, which coincides with the maximum magnetoresistance. From a Fowler analysis, the barrier height is determined as a function of oxidation time. The observed disagreement of the effective barrier heights determined by this technique and those found by Simmons fits demonstrates the added value of photoconductance studies. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Np Metals and alloys
73.50.Pz Photoconduction and photovoltaic effects
81.65.Mq Oxidation
85.75.Dd Magnetic memory using magnetic tunnel junctions
73.40.Rw Metal-insulator-metal structures

Ion-beam deposited low resistance magnetic tunnel junctions prepared by a two-step oxidation process

Z. G. Zhang, Z. Z. Zhang, and P. P. Freitas

J. Appl. Phys. 93, 8552 (2003); http://dx.doi.org/10.1063/1.1558660 (3 pages) | Cited 13 times

Online Publication Date: 9 May 2003

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Spin tunnel junctions with ultrathin (<1 nm) AlOx tunnel barriers were fabricated by ion-beam deposition. The tunnel barrier was formed by natural oxidation of an ultrathin Al layer (4–8.5 Å thick) by either a single-step or a two-step oxidation process. The magnetic and transport properties of these tunnel junctions indicate the barrier to be pinhole free for tAl⩾6.5 Å. The minimum Al thickness at which pinholes form depends on ion acceleration voltage used for Al deposition. Single-step oxidation of an 8 Å Al layer leads to a magnetic tunnel junction with a 25% tunnel magnetoresistance (TMR) and R×A of 14 Ω μm2. The two-step oxidation procedure was used for thinner Al layers. TMR ranges from 18%–23% at 8–10 Ω μm2 resistance values, for a total Al thickness of 6.5 Å. Junction resistance has been further decreased down to 4.35 Ω μm2 with a TMR of 10.6% by two-step oxidizing a 6 Å Al layer. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
81.15.Jj Ion and electron beam-assisted deposition; ion plating
81.65.Mq Oxidation
75.47.Pq Other materials

Magnetic tunnel junctions with high magnetoresistance and small bias voltage dependence using epitaxial NiFe(111) ferromagnetic bottom electrodes

Ji Hyung Yu, Hyuck Mo Lee, Masamitsu Hayashi, Mikihiko Oogane, Tadaomi Daibou, Hiroaki Nakamura, Hitoshi Kubota, Yasuo Ando, and Terunobu Miyazaki

J. Appl. Phys. 93, 8555 (2003); http://dx.doi.org/10.1063/1.1544458 (3 pages) | Cited 7 times

Online Publication Date: 9 May 2003

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Magnetic tunnel junctions (MTJs) were fabricated using an Al–O insulating layer prepared on an epitaxially grown Ni80Fe20 bottom electrode and on a polycrystalline Ni80Fe20 bottom electrode. Crystallographic orientations and surface morphology of the films were examined using x-ray diffraction and atomic force microscopy, respectively. The MTJ with an epitaxial bottom electrode showed a tunnel magnetoresistance (TMR) ratio of 51% after annealing at 250 °C. This value was about two times larger than that of the MTJ with a polycrystalline bottom electrode (27%). The applied bias voltage dependences of the TMR ratios were also much different. The Vhalf values of epitaxial and polycrystalline samples were about 750 and 400 mV, respectively. © 2003 American Institute of Physics.
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75.47.Np Metals and alloys
75.50.Cc Other ferromagnetic metals and alloys
75.70.Ak Magnetic properties of monolayers and thin films

Spin dependent tunneling junctions with reduced Neel coupling

Dexin Wang, James M. Daughton, Zhenghong Qian, Cathy Nordman, Mark Tondra, and Art Pohm

J. Appl. Phys. 93, 8558 (2003); http://dx.doi.org/10.1063/1.1556982 (3 pages) | Cited 13 times

Online Publication Date: 9 May 2003

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A new structure of spin dependent tunneling (SDT) junctions has been demonstrated to have a much reduced Neel coupling field between the free and pinned ferromagnetic layers comparing with conventional SDT structures. The new structure consists of a modified synthetic-antiferromagnetic composite layer as the pinned layer with two Ru spacer layers and three ferromagnetic layers. The Neel coupling field is much reduced for both top- and bottom-pinned SDT structures using this new composite pinned layer. Furthermore, the net magnetic moment is kept at zero for the composite pinned layer to minimize the fringe field after patterning. The coupling reduction can be understood by considering the additive contribution from the first two interfaces with Ru in the composite pinned layer, which cancels that from the pinned layer interface with the barrier. By properly spacing these three most important interfaces, reducing the coupling to basically zero is realized. The coupling reduction allows the elimination of an on-chip bias coil used to correct the coupling, therefore simplifying the electronics and reducing the power to operate the SDT sensors. The new SDT structure has potential impacts on many SDT and spin valve devices such as magnetoresistive sensors, galvanic isolators, magnetic logic, and MRAM devices. © 2003 American Institute of Physics.
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72.25.Mk Spin transport through interfaces
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
75.50.Ee Antiferromagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Superparamagnetic NiFeCo layers as free layers in magnetic tunnel junctions

R. Schad, H. Alouach, J. W. Harrell, M. Shamsuzzoha, and D. Wang

J. Appl. Phys. 93, 8561 (2003); http://dx.doi.org/10.1063/1.1544515 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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In order to study the possibility of using superparamagnetic films as a hysteresis-free switching layer in magnetic tunnel junctions or spin valves we prepared thin, discontinuous NiFeCo layers on different buffer layers on Si3N4/Si wafers. The magnetic properties were measured as a function of temperature for various layer thicknesses. We observe a transition from ferromagnetic behavior via superparamagnetic to paramagnetic behavior as either the film thickness is reduced or the temperature is increased. For a 0.8 nm thick (nominal thickness) NiFeCo layer grown on 10 nm Cu the coercivity reaches essentially zero at around 260 K, indicating a transition from ferromagnetic to superparamagnetic behavior. At that point, the slope of the steepest part of the hysteresis loop diverges. A further increase in temperature causes the saturation magnetization Ms (estimated at sufficiently large fields to saturate the sample) to sharply decrease. The drop of Ms with increasing temperature points to a Curie temperature (transition towards paramagnetism) at about T=350–400 K. Thinner films show reduced temperatures for those transitions, whereas thicker films are still ferromagnetic at room temperature. We found that these transition temperatures are not only a function of the film thickness but also vary with the material on which the NiFeCo layers are prepared. This points toward the importance of the microstructure of such films. Transmission electron microscope images do reveal a pronounced granularity of the NiFeCo layer. © 2003 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Bb Fe and its alloys
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Vv High coercivity materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.37.Lp Transmission electron microscopy (TEM)

Surface alloy formation of Co on Al surface: Molecular dynamics simulation

Sang-Pil Kim, Yong-Chae Chung, Seung-Cheol Lee, Kwang-Ryeol Lee, and Kyu-Hwan Lee

J. Appl. Phys. 93, 8564 (2003); http://dx.doi.org/10.1063/1.1544471 (3 pages) | Cited 21 times

Online Publication Date: 9 May 2003

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Control of the interface structure of atomic scale multilayers is a fundamental issue to improve the performance of spin electronic devices. Deposition behavior of Co on Al surface at 300 K was investigated by molecular dynamics simulation. The deposited Co resulted in the formation of CoAl surface alloy regardless of the Al surface orientation. Structure of the surface alloy was dependent on the substrate orientation. Crystalline B2 structure was formed on Al (001) surface. On the contrary, amorphous mixed layers were evolved on Al (011) and (111) surfaces. In the case of Al (001) surface, 4 ML of the surface alloy were observed, which is consistent with the previous experimental observation. The present work shows that the formation of CoAl surface alloy should be considered even at the low substrate temperature and the low incident energy of deposited atoms. © 2003 American Institute of Physics.
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68.35.Fx Diffusion; interface formation
68.65.Ac Multilayers
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Np Metals and alloys
68.35.Ct Interface structure and roughness
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