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26 Jun 2000

Volume 76, Issue 26, pp. 3849-4013

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Magnetic tunnel junction performance versus barrier thickness: NiFe/AlOx/NiFe junctions fabricated from a wedged Al layer

M. Covington, J. Nowak, and D. Song

Appl. Phys. Lett. 76, 3965 (2000); http://dx.doi.org/10.1063/1.126836 (3 pages) | Cited 24 times

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The resistance–area product (R∗A) and the magnetoresistance (MR) of NiFe/AlOx/NiFe spin-dependent tunnel junctions exhibit a strong dependence on the thickness of Al before oxidation. We obtain these data from wafers where we uniformly oxidize an Al layer with a wedged thickness profile, enabling us to reliably characterize the effect of Al thickness variations with subangstrom precision. The R∗A drops from 104 to 102 Ω μm2 as the Al thickness decreases from 9 to 4 Å, respectively. The MR is highest (21%) for an Al thickness of 7 Å, where the Al layer is fully oxidized and the oxidation of the bottom NiFe electrode is minimal. © 2000 American Institute of Physics.
Show PACS
75.47.De Giant magnetoresistance
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Direct measurement of the dependence of granular giant magnetoresistance on the relative orientation of magnetic granules

Jianbiao Dai and Jinke Tang

Appl. Phys. Lett. 76, 3968 (2000); http://dx.doi.org/10.1063/1.126837 (3 pages) | Cited 2 times

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Experiments have been designed to vary the relative angle between the magnetic moments of different Co granules in Cu80Co20 granular system. The moments of granules are mostly aligned in the same direction by field cooling to low temperature in a high magnetic field. A small field applied at an angle relative to the cooling field rotates the moments of a portion of the granules that have small particle size and coercivity. It is found that the giant magnetoresistance (GMR) varies linearly with cos ϕ, where ϕ is the relative angle between the magnetic axes of granules. This behavior disappears if the sample is cooled in zero fields, or if the rotating field is too large or small, or if the measuring temperature is higher than the blocking temperature. Our results show that the GMR in granular structures has the same angular dependence as the layered films and confirm the existing theories and recent microscopic models of granular GMR suggesting a crucial role of the relative orientations of the magnetic granules in determining the spin dependent scattering. © 2000 American Institute of Physics.
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72.15.Gd Galvanomagnetic and other magnetotransport effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Microstructure and magnetic properties of FePt–Al–O granular thin films

M. Watanabe, T. Masumoto, D. H. Ping, and K. Hono

Appl. Phys. Lett. 76, 3971 (2000); http://dx.doi.org/10.1063/1.126838 (3 pages) | Cited 81 times

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We have investigated the microstructures and magnetic properties of L10 ordered FePt granular thin films prepared by ion beam sputtering and subsequent annealing. As-deposited FePt–Al–O films are composed of fine spherical FePt particles of ∼2 nm diameter with disordered face-centered-cubic structure. These particles are embedded in the amorphous aluminum oxide matrix with average spacing of ∼1.3 nm, and the film exhibits superparamagnetism. By annealing above 650 °C, the nanoparticles transform to the L10 ordered phase and the grains are coarsened to ∼20 nm, which results in a high coercivity up to 10 kOe. In the films with a high Al content, most of the particles are isolated by the amorphous aluminum oxide matrix, while the grains are interconnected in the films with a low Al content. The changes in magnetic properties are discussed based on the microstructural observation results. © 2000 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
68.55.-a Thin film structure and morphology
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.15.Cd Deposition by sputtering
61.72.Cc Kinetics of defect formation and annealing

Structural origin of coercivity enhancement and exchange-bias field in double antiferromagnet/ferromagnet bilayers

C. Hou, H. Fujiwara, and K. Zhang

Appl. Phys. Lett. 76, 3974 (2000); http://dx.doi.org/10.1063/1.126839 (3 pages) | Cited 5 times

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An experimental method to identify the structural origins of the coercivity enhancement and the exchange-bias field of polycrystalline ferro-antiferromagnetic (F/AF) coupled systems is proposed. The exchange-bias field and the coercivity of the FI layer of the samples of the structure FI/AFI are compared with those of the FII layer of the samples of the structure FII/AFII/FIII/AFIII. It is concluded that, for a given temperature, it is those AF grains which stop their growth at a critical thickness that contribute to the coercivity enhancement, and those which grow over the critical thickness that contribute to the exchange-bias field. Meanwhile the effective magnetic surface anisotropy introduced to the top surface of the AFII layer by the FIII layer can increase the exchange-bias field of the FII layer when the AFII layer is thin. © 2000 American Institute of Physics.
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75.50.Ee Antiferromagnetics
75.50.Bb Fe and its alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy
75.70.Rf Surface magnetism
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