ScienceWeek

APPLIED PHYSICS: MAGNETORESISTANCE AND MOLECULAR DEVICES

The following points are made by Christoph Strunk (Science 2004 306:63):

1) Advances in nanoelectronics continue to push forward the miniaturization of devices and the improvement of their speed and functionality. Of particular importance are the fields of molecular electronics and spintronics. By studying electronic transport at molecular scales -- for example, through individual carbon nanotubes [1], C60 molecules [2], and single organic molecules [3] -- researchers try to reach the ultimate size limits for devices. In spintronics, the spin rather than the charge is used to store and process classical as well as quantum information [4].

2) Pasupathy et al [5] have succeeded in merging these two fields. They explored molecular quantum dots consisting of single C60 molecules, which are sandwiched between two ferromagnetic nickel electrodes. These new spintronic devices combine two fundamental electron-electron interaction effects of condensed matter physics: the Kondo effect and ferromagnetism. At first sight, these effects seem to exclude each other, but they have now been integrated in one device. Once a controlled assembly of such devices is achieved, they may even outperform more conventional magnetoelectronic devices.

3) Previous experiments have shown that a quantum dot trapping an unpaired electron can display the Kondo effect, which is one of the most prominent many-body effects in condensed matter physics. If the tunneling barriers defining the quantum dot are sufficiently transparent, the wave function of the single electron can leak out of the dot and hybridize with the delocalized electrons in the contacts. The Coulomb repulsion on the dot leads to an antiferromagnetic exchange coupling between the electron spin on the dot and the neighboring electron spins in the electrodes. The corresponding coupling energy can be expressed as a characteristic temperature, the Kondo temperature T(sub-K).

4) Below this temperature, the spin on the dot is screened by the formation of a cloud of electrons on the electrodes, having a spin polarization antiparallel to the spin on the dot. The formation of the screening cloud enhances the density of states in the electrodes and leads to a high-conductance state. The hallmark of the Kondo effect is a pronounced peak in the differential conductance with width k(sub-B)T(sub-K) (where k(sub-B) is the Boltzmann constant), which gradually disappears at temperatures above T(sub-K). In the presence of a magnetic field, this peak shows a Zeeman splitting.

5) Until recently, the prospects for using the Kondo effect in quantum dots in applications were poor because it required temperatures below 1 K. The use of molecules as quantum dots has pushed the Kondo temperature up to 30 K. However, at this high T(sub-K), very large external magnetic fields are required to split the Kondo resonance, again precluding applications in magnetoelectronics. The situation is changed completely by the experiment of Pasupathy et al [5]. The use of ferromagnetic electrodes puts the antiferromagnetic Kondo interaction in competition with the ferromagnetic spin alignment by the ferromagnet's exchange field. The exchange field is responsible for the spontaneous spin polarization of the ferromagnet and acts also on the single spin trapped on the quantum dot. It has an effect similar to that of an external magnetic field. However, the corresponding Zeeman energy is given by the Curie temperature, T(sub-C), which is 20 to 30 times larger than T(sub-K). The exchange field is much larger than laboratory-scale magnetic fields.

References (abridged):

1. S. J. Tans, A. R. M. Verschueren, C. Dekker, Nature 393, 49 (1998).

2. C. Joachim, J. K. Gimzewski, R. Schittler, C. Chavy, Phys. Rev. Lett. 74, 2102 (1995) [APS].

3. J. Reichert et al., Phys. Rev. Lett. 88, 176804 (2002) [APS].

4. S. A. Wolf et al., Science 294, 1488 (2001).

5. A. N. Pasupathy et al., Science 306, 86 (2004).

Science http://www.sciencemag.org

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MATERIALS SCIENCE: ON MAGNETORESISTIVE TUNNEL JUNCTIONS

The following points are made by Barbara Goss Levi (Physics Today 2004 December):

1) The relentless demand for smaller, faster, cheaper, more capable computers continues to drive the development of devices for sensing and storing information. Over the past decade, manufacturers have exploited the phenomenon of giant magnetoresistance (GMR) to build sensors for reading data bits coded as tiny magnetized regions on disk drives. The higher sensitivity of these GMR read heads to magnetic fields has allowed a reduction in the bit size and hence an enormous increase in the storage capacity of magnetic hard disk drives.(1)

2) One of the technologies on the horizon is the magnetic tunnel junction (MTJ). It is possible that every hard-drive manufacturer has some kind of tunnel-junction sensor under development. Among its advantages, MTJs promise even higher sensitivities than GMR devices. Recent experiments now suggest that MTJs will not disappoint.

3) A GMR device comprises two layers of ferromagnetic material, such as cobalt, separated by a thin layer of normal metal -- say, copper. When the magnetic moments of the ferromagnetic layers are parallel, current flows through the sandwiched layers with relatively little resistance. When the two moments are antiparallel, however, the resistance is higher. In today's GMR devices, the magnetoresistance, defined as the percentage difference in resistance between the parallel and antiparallel configurations, is around 10-15%. In applications, the direction of magnetization of one ferromagnetic layer is usually fixed, and the direction of the other layer is determined by the external field, such as that on a data bit. A magnetoresistive device can then detect the direction of that field by its effect on the resistance of the device and hence the current flow through it.

4) MTJs also consist of two layers of ferromagnetic material, but they are separated by an insulating layer. Electrons in one layer must tunnel through the insulator to reach the other layer. The tunneling current typically flows more readily when the ferromagnetic moments are aligned than when they are opposed. Researchers demonstrated in 1975 that the conductance in tunnel junctions would depend on the relative directions of magnetization of the two ferromagnetic electrodes.[2] The work drew on spin-polarized tunneling studies begun in 1971 by Robert Meservey and Paul Tedrow at MIT.[3] Early attempts to produce high tunneling magnetoresistance (TMR) were unsuccessful, however. In 1995, interest was rejuvenated when TMR values of 17% were reported by an MIT team led by Jagadeesh Moodera.[4] More results were reported by a Tohoku University team headed by Terunobu Miyazaki.[5] TMR values have grown by modest jumps since then, and by January 2004 reached 70%.

References (abridged):

1. For a review, see S. S. P. Parkin et al., Proc. IEEE 91, 661 (2003)

2. M. Julliere, Phys. Lett. 54A, 225 (1975)

3. For a review, see R. Meservey, P. M. Tedrow, Phys. Rep. 238, 173 (1994)

4. J. S. Moodera, L. R. Kinder, T. M. Wong, R. Meservey, Phys. Rev. Lett. 74, 3272 (1995)

5. T. Miyazaki, N. Tezuka, J. Magn. Magn. Mater. 139, L231 (1995)

Physics Today http://www.physicstoday.org

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MATERIALS SCIENCE: GIANT MAGNETORESISTANCE AND SPIN-VALVES

Notes by ScienceWeek:

The term "magnetoresistance" refers to a change in the electrical resistance of a conductor or semiconductor upon the application of a magnetic field, a property of certain systems. Giant magnetoresistance is a quantum mechanical effect observed in magnetic thin-film structures composed of alternating ferromagnetic and nonmagnetic layers.

The following points are made by Z.H. Xiong et al (Nature 2004 427:821):

1) A "spin valve" is a layered structure of magnetic and non-magnetic (spacer) materials whose electrical resistance depends on the spin state of electrons passing through the device and so can be controlled by an external magnetic field. The discoveries of giant magnetoresistance(1) and tunnelling magnetoresistance(2) in metallic spin valves have revolutionized applications such as magnetic recording and memory, and launched the new field of spin electronics(3) -- "spintronics". Intense research efforts are now devoted to extending these spin-dependent effects to semiconductor materials. But while there have been noteworthy advances in spin injection and detection using inorganic semiconductors(4,5), spin-valve devices with semiconducting spacers have not yet been demonstrated. Pi-conjugated organic semiconductors may offer a promising alternative approach to semiconductor spintronics by virtue of their relatively strong electron-phonon coupling and large spin coherence.

2) Pi-conjugated organic semiconductors (OSEs) are a relatively new class of electronic materials that are revolutionizing important technological applications including information display and large-area electronics, owing to their ability to be economically processed in large areas, their compatibility with low-temperature processing, the tunability of their electronic properties, and the simplicity of thin-film device fabrication. The virtually limitless flexibility of synthetic organic chemistry allows the fabrication of pi-conjugated OSE structures with a degree of control unattainable with the conventional inorganic semiconductors. In addition, the OSEs have extremely weak spin-orbit interaction and weak hyperfine interaction, so that electron spin diffusion length is especially long. These properties make them ideal for spin-polarized electron injection and transport applications.

3) The authors report the injection, transport, and detection of spin-polarized carriers using an organic semiconductor as the spacer layer in a spin-valve structure, yielding low-temperature giant magnetoresistance effects as large as 40 per cent.

4) The authors have chosen the small pi-conjugated molecule 8-hydroxy-quinoline aluminum (Alq3), most commonly used in organic light-emitting diodes (OLEDs), to serve as an OSE spacer in organic spin-valves, because it can easily be deposited as thin films and integrated with a variety of metallic electrodes. The vertical organic spin-valves that were fabricated consist of three layers: two ferromagnetic electrode films (FM1 and FM2, respectively) and the OSE spacer. By engineering the two FM electrodes to have different coercive fields (Hc1 and Hc2, respectively), their magnetization directions can have either a parallel or anti-parallel alignment configuration when sweeping the external magnetic field, H; this is essential for proving the spin-valve effect.

6) The authors suggest that this study demonstrates that spin-polarized carrier injection, transport, and detection, which are the main ingredients of spintronics, can be successfully achieved using pi-conjugated OSE. The authors suggest this may initiate a variety of new applications in organic spintronics such as spin-OLEDs, enabled by the functionalities of the OSE.

References (abridged):

1. Baibich, M. N. et al. Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices. Phys. Rev. Lett. 61, 2472-2475 (1988)

2. Moodera, J., Kinder, L., Wong, T. & Meservey, R. Magnetic tunnel junction. Phys. Rev. Lett. 74, 3273-3276 (1995)

3. Wolf, S. A. et al. Spintronics: a spin-based electronics vision for the future. Science 294, 1488-1495 (2001)

4. Kikkawa, J. M. & Awschalom, D. D. Lateral drag of spin coherence in gallium arsenide. Nature 397, 139-141 (1999)

5. Ohno, Y. et al. Electrical spin injection in a ferromagnetic semiconductor heterostructure. Nature 420, 790-792 (1999)

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