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15 May 2002

Volume 91, Issue 10, pp. 6227-8917

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Microstructural observation on effect of oxidation method of AlOx in magnetic tunnel junction by high resolution transmission electron microscopy

Jun Soo Bae, Kyung Ho Shin, and Hyuck Mo Lee

J. Appl. Phys. 91, 7947 (2002); http://dx.doi.org/10.1063/1.1447200 (3 pages) | Cited 5 times

Online Publication Date: 13 May 2002

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The formation of the insulating AlOx tunnel barrier is a critical and sensitive process in magnetic tunnel junctions. Both the natural oxidation and the plasma oxidation methods were employed to fabricate the AlOx insulating layer in this study. In the natural oxidation, the Al layer was exposed to pure oxygen gas at 20 Torr for up to 50 min to produce the AlOx tunnel barrier. It was revealed by high resolution transmission electron microscopy that the oxidation occurred preferentially through the grain boundary of Al grains. Also, the AlOx grains expanded isotropically when fully oxidized, thereby making the surface of the AlOx layer modulated. In plasma oxidation, the oxygen plasma was used at 20 mTorr of pure oxygen gas for up to 30 s and the flat AlOx layer formed uniformly on the Al layer. It had sharp interfaces with the underlying metallic Al and the rate of oxide layer growth decreased as the oxidation proceeded. © 2002 American Institute of Physics.
Show PACS
81.05.Bx Metals, semimetals, and alloys
81.65.Mq Oxidation
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
52.77.-j Plasma applications
61.72.Mm Grain and twin boundaries
68.35.Ct Interface structure and roughness

Intermixing effects in ultrathin barrier magnetic tunneling junctions

Yun Li and Shan X. Wang

J. Appl. Phys. 91, 7950 (2002); http://dx.doi.org/10.1063/1.1452231 (3 pages) | Cited 6 times

Online Publication Date: 13 May 2002

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Ultrathin tunneling barrier magnetic tunneling junctions were fabricated to study the intermixing effects of ferromagnetic layer, NiFe, and barrier precursor aluminum. Both bottom ferromagnetic layer (NiFe) and aluminum were grown epitaxially on a Pt buffered sapphire (0001) substrate. Grazing incidence x-ray reflectometry indicated an intermixing layer of 0.6 nm (∼3 atomic layers) between the NiFe and Al. Junction barrier height and width were extracted from both R(T) curve and zero-bias conductance. Nonideal oxidation of the intermixing region degrades the device performance, and poses a fundamental limit to the optimization of ultrathin barrier tunnel junctions. © 2002 American Institute of Physics.
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68.65.Ac Multilayers
68.35.Fx Diffusion; interface formation
66.30.Ny Chemical interdiffusion; diffusion barriers
81.05.Bx Metals, semimetals, and alloys
72.25.Ba Spin polarized transport in metals
72.25.Mk Spin transport through interfaces
73.40.Gk Tunneling
85.75.Mm Spin polarized resonant tunnel junctions
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.50.Bb Fe and its alloys
61.05.cm X-ray reflectometry (surfaces, interfaces, films)
81.65.Mq Oxidation
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Enhanced performance of CoFe/AlOx/CoFe magnetic tunnel junctions prepared by an off-axis rf remote plasma oxidation method

K. S. Yoon, J. H. Park, J. Y. Yang, C. O. Kim, and J. P. Hong

J. Appl. Phys. 91, 7953 (2002); http://dx.doi.org/10.1063/1.1452647 (3 pages) | Cited 6 times

Online Publication Date: 13 May 2002

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A new off-axis rf remote oxidation technique for the insulating barrier was carried out to enhance performance of CoFe/AlOx/CoFe magnetic tunneling junctions. The rf remote plasma method was designed to reduce self-bias voltage effect on the barrier during the rf oxidation process and to increase atomic oxygen concentration at high power operation, compared with a conventional rf plasma oxidation method. In addition, the off-axis geometry was chosen to give rise to high uniformity of the insulating layer. Experimentally observed root mean square of the barrier was decreased from 5 Å to 1.5 Å in our method. Electrical breakdown voltage and magnetoresistance of our MTJ devices increased from 0.8 V up to 1.4 V and from 20.2% up to 33.6%, respectively, correlated with the improved structural information of the barrier. © 2002 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
52.77.-j Plasma applications
73.40.Rw Metal-insulator-metal structures
75.47.De Giant magnetoresistance
81.65.Mq Oxidation
77.22.Jp Dielectric breakdown and space-charge effects

Temperature dependence of tunneling magnetoresistance: Double-barrier versus single-barrier junctions

J. H. Lee, K. I. Lee, W. L. Lee, K.-H. Shin, J. S. Lee, K. Rhie, and B. C. Lee

J. Appl. Phys. 91, 7956 (2002); http://dx.doi.org/10.1063/1.1452232 (3 pages) | Cited 6 times

Online Publication Date: 13 May 2002

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The temperature dependence of tunneling magnetoresistance (TMR) is studied for spin valve type double-barrier tunnel junctions. Normalized TMR values for double-barrier tunnel junctions (DBTJs) and single-barrier junctions (SBTJs) are plotted as functions of temperature and it is found that the DBTJ shows stronger temperature dependence of TMR than the SBTJ. The strong temperature dependence of TMR for the DBTJ is explained in terms of temperature dependence of the spin polarization of the middle magnetic layer and decrease of the spin coherence length with increasing temperature. © 2002 American Institute of Physics.
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85.75.Mm Spin polarized resonant tunnel junctions
75.47.De Giant magnetoresistance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Temperature dependence of magnetoresistance for tunnel junctions with high-power plasma-oxidized barriers: Effects of annealing

K. I. Lee, J. H. Lee, W. L. Lee, K. H. Shin, Y. B. Sung, J. G. Ha, K. Rhie, and B. C. Lee

J. Appl. Phys. 91, 7959 (2002); http://dx.doi.org/10.1063/1.1452233 (3 pages) | Cited 10 times

Online Publication Date: 13 May 2002

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Magnetic tunnel junctions (MTJ) were fabricated with high oxygen-plasma power and the effects of annealing on the temperature dependence of tunneling magnetoresistance (TMR) were investigated experimentally. As grown, TMR increases, peaks around 160 K, and decreases with increasing temperature from 80 K to 300 K. When MTJs are annealed, Tmax, the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain the temperature dependence of TMR, the difference of conductance between parallel and antiparallel alignments of magnetizations as a function of temperature is also analyzed. The shifts of Tmax due to annealing are described phenomenologically with spin-dependent transfer rates of electrons through the barrier. © 2002 American Institute of Physics.
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72.25.Mk Spin transport through interfaces
73.40.Gk Tunneling
75.47.De Giant magnetoresistance
85.75.Bb Magnetic memory using giant magnetoresistance
85.75.Dd Magnetic memory using magnetic tunnel junctions
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

Creation and annihilation of 360° domain walls in magnetic tunnel junctions with exchange-biased artificial ferrimagnet

H. Boeve, L. Esparbe, G. Gieres, L. Bär, J. Wecker, and H. Brückl

J. Appl. Phys. 91, 7962 (2002); http://dx.doi.org/10.1063/1.1447876 (3 pages) | Cited 4 times

Online Publication Date: 13 May 2002

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In state-of-the-art read heads, exchange-biased artificial ferrimagnet hard layers are extensively used. In this paper, we analyze different magnetic tunnel junction stacks with respect to the formation and annihilation of 360° walls in their hard subsystem, visualized using magnetic force microscopy. The existence of 360° walls in a magnetic tunnel junction has a microscopic origin, i.e., the local competition between different coupling mechanisms in the layered system, and can qualitatively be interpreted from a strong but reversible reduction in minor loop tunnel magnetoresistance. Therefore, the magnetic history of a tunnel junction is of great importance in the interpretation of minor loop measurements. © 2002 American Institute of Physics.
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75.70.Kw Domain structure (including magnetic bubbles and vortices)
81.05.Bx Metals, semimetals, and alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.50.Ss Magnetic recording materials
75.30.Et Exchange and superexchange interactions

PtMn-based spin-dependent tunneling materials with thin alumina barrier fabricated by two-step natural oxidation

Ki-Seok Moon, Yingjian Chen, and Yiming Huai

J. Appl. Phys. 91, 7965 (2002); http://dx.doi.org/10.1063/1.1456390 (3 pages) | Cited 10 times

Online Publication Date: 13 May 2002

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Spin-dependent tunneling (SDT) materials with bottom-pinned structure (substrate/Ta/NiFeCr/PtMn/CoFe/t Al2O3/CoFe/NiFe/Ta) are fabricated by magnetron sputtering in ultrahigh vacuum. In this study, a two-step natural oxidation was used, in which the second Al layer was deposited and naturally oxidized after the natural oxidation of the first Al layer. The top and bottom leads were also patterned into bow-tie shaped structures. The two-step oxidation process results in a perfectly decoupled pinned and free layer in a film with a total as-deposited aluminum thickness of 7 Å, whereas, the one-step oxidation process gives rise to strongly coupled magnetic layers in a film with this thickness of aluminum. By using this two-step natural oxidation technique, an optimum tunneling magnetic resistance (TMR) ratio of 29.3% and resistance×area (RA) product of 34 Ω μm2 were achieved in junctions with 8 Å barrier (5+3 Å). In conclusion, a two-step oxidation process was used to fabricate spin-dependent tunneling devices with low RA product and high TMR ratio for head applications. © 2002 American Institute of Physics.
Show PACS
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.De Giant magnetoresistance
81.65.Mq Oxidation
85.75.Dd Magnetic memory using magnetic tunnel junctions

Magnetic tunnel junctions utilizing diamond-like carbon tunnel barriers

F. J. Cadieu, Li Chen, and Biao Li

J. Appl. Phys. 91, 7968 (2002); http://dx.doi.org/10.1063/1.1456395 (3 pages)

Online Publication Date: 13 May 2002

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We have devised a method whereby thin particulate-free diamond-like carbon films can be made with good adhesion onto even room-temperature substrates. The method employs a filtered ionized carbon beam created by the vacuum impact of a high-energy, approximately 1 J per pulse, 248 nm excimer laser onto a carbon target. The resultant deposition beam can be steered and deflected by magnetic and electric fields to paint a specific substrate area. An important aspect of this deposition method is that the resultant films are particulate free and formed only as the result of atomic species impact. The vast majority of magnetic tunnel junctions utilizing thin metallic magnetic films have employed a thin oxidized layer of aluminum to form the tunnel barrier. This has presented reproducibility problems because the indicated optimal barrier thickness is only approximately 13 Å thick. Magnetic tunnel junctions utilizing Co and permalloy films made by evaporation and sputtering have been fabricated with an intervening diamond-like carbon tunnel barrier. The diamond-like carbon thickness profile has been tapered so that seven junctions with different barrier thickness can be formed at once. Magnetoresistive (MR) measurements made between successive permalloy strip ends include contributions from two junctions and from the permalloy and Co strips that act as current leads to the junctions. Magnetic tunnel junctions with thicker carbon barriers exhibit MR effects that are dominated by that of the permalloy strips. Since these tunnel barriers are formed without the need for oxygen, complete tunnel junctions can be formed with all high-vacuum processing. © 2002 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
85.75.Dd Magnetic memory using magnetic tunnel junctions
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
75.50.Bb Fe and its alloys
75.47.De Giant magnetoresistance
75.50.Cc Other ferromagnetic metals and alloys

Tunnel magnetoresistance in NiFe/TaOx/Al2O3/Co junctions with a thin TaOx layer

L. S. Dorneles, R. L. Sommer, and L. F. Schelp

J. Appl. Phys. 91, 7971 (2002); http://dx.doi.org/10.1063/1.1452234 (3 pages) | Cited 1 time

Online Publication Date: 13 May 2002

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We have measured the magnetoresistance versus bias voltage (Vbias) in NiFe/TaOx/Al2O3/Co magnetic tunnel junctions. The TaOx layer was produced exposing a thin metallic Ta film (0.25 or 0.5 nm) to atmosphere before the deposition of Al2O3. The samples with 0.5 nm of Ta present a faster decrease of the magnetoresistance for increasing Vbias. They also present negative tunnel magnetoresistance values for Vbias larger than 250 mV. The results are discussed in terms of the oxidation of the Ta layers and may suggest quantum well state effects. © 2002 American Institute of Physics.
Show PACS
75.47.De Giant magnetoresistance
75.45.+j Macroscopic quantum phenomena in magnetic systems
81.65.Mq Oxidation

Ultrafast switching of magnetic nanoelements using a rotating field

J. Fidler, T. Schrefl, V. D. Tsiantos, W. Scholz, D. Suess, and H. Forster

J. Appl. Phys. 91, 7974 (2002); http://dx.doi.org/10.1063/1.1452648 (3 pages) | Cited 2 times

Online Publication Date: 13 May 2002

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Nanostructured granular Ni80Fe20 and Co films are studied using a 3D hybrid finite element/boundary element model. Switching dynamics are calculated for external fields applied unidirectional after a rise time of 0.1 ns and for a 10 GHz rotational field with a field strength of Hext=0.2 Js/μ0 (160 kA/m for NiFe and 280 kA/m for Co). The transient magnetization patterns show that reversal in the unidirectional field proceeds by the nucleation and propagation of end domains towards the center of the element. The switching time strongly depends on the Gilbert damping parameter α. Materials with uniaxial anisotropy (Co), require larger field, but exhibit shorter switching times. Reversal in rotational fields involves inhomogeneous rotation of the end domains towards the rotational field direction leading to partial flux-closure structures. Shorter switching times are obtained by the application of the 10 GHz rotational field (tsw=0.05 ns). Precessional oscillation effects after abruptly switching off the external field which occurred in the NiFe square element, were suppressed in the granular Co film. Reducing the field to zero slowly inhibits the high frequency excitations in NiFe. © 2002 American Institute of Physics.
Show PACS
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.60.Ch Domain walls and domain structure
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Jk Magnetization reversal mechanisms
75.30.Gw Magnetic anisotropy
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