MAGNETOELECTRONIC DEVICES

Transcription

1 45 CHAPTER 6 MAGNETOELECTRONIC DEVICES James M. Daughton OVERVIEW Since the discovery of giant magnetoresistance (GMR), magnetoresistive devices have progressed rapidly from the anisotropic magnetoresistance (AMR) structures that were the dominant thin film magnetoresistive material into the 1990s. GMR development, followed by work in magnetic tunnel junctions (MTJ), is now being amplified by the latest work in spin electronic devices (SPINS). The resulting improvements in magnetoresistance have been accompanied by rapid exploitation of these new structures in sensors, read heads, galvanic isolators, and nonvolatile memory (MRAM). The ultimate drivers for research in magnetoelectronics that devices be faster, smaller, and cheaper ultimately also define the technological challenges. One of the ways to make magnetoelectronics faster is to achieve higher signals. Magnetoresistance has risen from about 2% for AMR thin films to 20 50% for GMR layers to 50% for MTJ structures. Even higher magnetoresistance is being sought, and considerable effort is being expended toward that end, both in academia and in industry. Another critical means to increase the speed of memory and galvanic isolators is the integration of magnetic devices with integrated circuits. This integration reduces parasitic capacitance and inductance significantly compared to the alternative of separating the magnetic device from the integrated circuit onto separate substrates or into separate electronic packages. The integration of magnetic devices and integrated circuits has largely been the purview of larger companies. Integration of magnetic devices with integrated circuits can also make magnetoelectronics smaller by reducing parts counts and interconnects. The new SPINS research may represent the ultimate in shrinking device size through integration. In addition, the magnetic devices themselves can and are being reduced in size. Line widths of the scale of 0.15 µm are now under development for hard disk drive read heads and MRAM. Three technical challenges that result from microscale device sizes are contending with (1) demagnetizing fields, (2) the ultimate stability and noise of magnetic structures, and (3) the problem of switching fields (currents). The use of antiferromagnetic/ferromagnetic film coupling has significantly improved device stability and is still an active area of research. The use of spin momentum transport to reduce currents for switching is a new area of strong research interest around the world. Provided smaller devices can be made to work, shrinking the size of devices should eventually also make them less expensive. The development of practical devices of very high density for read heads and MRAM is very active in many companies, but news about progress and results is very tightly held for competitive reasons.

2 46 6. Magnetoelectronic Devices SALIENT FEATURES OF MAGNETOELECTRONICS RESEARCH Much of the practical R&D in magnetoelectronics in Europe and Japan, as well as in the United States, is being carried out in companies reluctant to reveal very much about their work, and thus the view of magnetoelectronics based on WTEC-sponsored visits and published information is bound to underestimate the practical efforts in this field. The pre-product development work in general in Europe and Japan seems to be about on a par with the efforts in the United States. However, the very large lead that the United States has in magnetoelectronics applications probably conceals a superior R&D base associated with products and product development, much of which remains proprietary to companies. Despite the difficulty of seeing a complete picture of magnetoelectronic device development in the United States and abroad, it is clear that advances in both spin-dependent transport devices and their applications have been extremely rapid over the past 13 years. Magnetoelectronic Device Structures Giant magnetoresistive (GMR) multilayers (Fig. 6.1a), first discovered in 1988 (Baibich et al. 1988; Barnas et al. 1990), consist of nanometer-thickness, interleaved, alternating layers of ferromagnetic and nonferromagnetic metals, such as cobalt and copper layers or iron and chromium layers. With the proper thickness of the nonmagnetic metal, indirect electron exchange provides a mechanism to induce an antiparallel alignment of the magnetic layers (Parkin, More, and Roche 1991). The resistivity of the structure drops significantly if a sufficient magnetic field (typically 100 Oe to 1000 Oe) is applied to overcome the antiparallel coupling between magnetic layers, with the change in resistivity ( ρ) divided by the resistivity at large fields (ρ) being typically in the range of 10% 60%. This very large ( giant ) magnetoresistance is attributed to changes in the scattering of polarized conduction electrons as the relative magnetization orientations in the ferromagnetic layers change from antiparallel to parallel. A spin valve (Fig. 6.1b) is a simpler structure consisting of only two ferromagnetic layers sandwiching a thin nonmagnetic metal, with one of the two magnetic layers being pinned, so that the magnetization in that layer is relatively insensitive to moderate magnetic fields (Dieny et al. 1991). The other magnetic layer is called the free layer, and its magnetization can be changed by application of a relatively small magnetic field. As the magnetizations in the two layers change from parallel to antiparallel alignment, the resistance of the spin valve rises, typically 5% to 10%. In the spin valve, the magnetic layers are typically alloys of nickel, iron, and cobalt, and the nonmagnetic interlayer is usually copper. Pinning is usually accomplished by using an antiferromagnetic layer that is in intimate contact with the pinned magnetic layer. The two films form an interface that acts to resist changes to the pinned magnetic layer s magnetization. Recent improvements to the basic spin valve are illustrated in Figure 6.1c. The simple pinned layer is replaced with a synthetic antiferromagnet, two magnetic layers separated by a very thin (~10 Å) nonmagnetic conductor, usually ruthenium (Parkin and Mauri 1991). The magnetizations in the two magnetic layers are strongly antiparallel coupled and are thus effectively immune to external magnetic fields. This structure improves both standoff magnetic fields and the operation temperature of the spin valve. Another improvement is the nanooxide layer or NOL, which is formed at the outside surface of the soft magnetic film. This layer reduces resistance due to surface scattering (Egelhoff et al. 1999), thus reducing background resistance and thereby increasing the percentage change in magnetoresistance of the structure. NOL layers have allowed spin valves to increase GMR from about 10% to about 20% over the two years prior to this WTEC study, and their insertion into GMR structures is still a major development area. The structures described above are all termed current-in-the-plane (CIP) structures. In relatively recent developments, NOLs have been inserted into current-perpendicular-to-the-plane (CPP) structures (creating vertical rather than horizontal current flow) with the intriguing results of moderately high GMR and intermediate specific resistance values (Takagishi 2001). The work on NOLs has been largely motivated by applications in read heads, and most of the leading work in this area has been done in Japan and the United States; there is little NOL activity in Europe. Figure 6.1d illustrates a magnetic tunnel junction (Moodera et al. 1995; Miyazaki et al. 1995), or MTJ, in which a pinned layer and a free layer are separated by a very thin insulating layer, commonly aluminum

3 James M. Daughton 47 oxide. The tunneling resistance is modulated by a magnetic field in much the same way as the resistance of a spin valve. The MTJ has typical magnetoresistance ratios of 20% to 40% and requires a saturating magnetic field equal to or somewhat less than that of spin valves. Because the tunneling current density is usually small, MTJ devices tend to have high resistances. Recent results are showing that greater than 40% magnetoresistance is attainable down to about 100 Ohm-µm 2. Research is concentrating on increasing magnetoresistance and decreasing the tunneling resistance, both factors being essential for high performance memory and read heads (Tehrani et al. 2001). Applications of Magnetoelectronic Structures Figure 6.1. Spin dependent transport structures. Important GMR and MTJ applications include magnetic field sensors, read heads for hard drives, galvanic isolators, and magnetoresistive random access memory (MRAM). General purpose GMR sensors have been introduced in the past five years (Daughton et al. 1994; Infineon ND), and several companies are producing GMR sensors for internal use. Figure 6.2a illustrates a sensor chip with GMR multilayer material as the basic sensing material. The GMR multilayer is etched to form resistors, which are connected as a Wheatstone bridge. The substrate is an integrated circuit that is then connected to the bridge using integrated circuit wiring techniques. Resistors on the chip can be trimmed with lasers to allow the circuit to trigger at the desired magnetic field. No commercial sensors using MTJ structures are yet available but are under development (Tondra et al. 1998). GMR spin valve read heads are dominating applications in hard drives. Although some alternative configurations have been proposed, nearly all commercial GMR heads use the spin valve format as originally proposed by IBM (Tsang et al. 1994). There has been development interest in MTJ and GMR multilayers for read head applications, but no significant products have appeared. The magnetoresistance of spin valves has increased dramatically from about 5% in early heads to about 15 20% today using synthetic antiferromagnets and NOL (Koui et al. 2001; Huai et al. 2001). As hard drive storage densities approach 100 Gbits/square inch, sensor stripe widths are approaching 0.1 µm, and current densities are becoming very

4 48 6. Magnetoelectronic Devices high. It is unclear if the conventional spin valve read head can be extended to those levels, or if a new form of read head will have to be introduced. Tunnel junctions with low tunneling resistance and CPP structures with NOL are at the forefront of read head research. Of growing concern is the noise introduced by thermal instabilities in very small read heads (Xue and Vitora 2001). GMR-based galvanic isolators (Fig. 6.2c) are a combination of an integrated coil and a GMR sensor on an integrated circuit chip. They were introduced as products in 2000 (NVE ND). This product can eliminate ground noise in communications between electronics blocks, thus performing a function similar to that of opto-isolators. The GMR isolator is ideally suited for integration with other communications circuits or in the packaging of a large number of isolation channels on a single chip. The speed of the GMR isolator can eventually be 10 to 100 times as fast as today s opto-isolators, depending primarily on the switching speed of the magnetic materials and the speed of the associated electronics. a) Digital Integrated GMR b) Read Head c) GMR Isolator d) 1T1MTJ MRAM Cell Figure 6.2. Device applications (reprinted by permission from Daughton et al. 1994; 1994 IEEE). MRAM uses magnetic hysteresis to store data and magnetoresistance to read data. Memory cells are integrated on an integrated circuit chip, and the function of the resulting device is like a static semiconductor RAM chip, with the added feature that the data are retained with power off. MRAM is not yet available commercially, but development activity is high. Many developers are using MTJ cells (Scheuerlein et al. 2000; Naji 2001), while some are using the pseudospin valve GMR cell (Katti et al. 2001). Motorola has announced a 256K development level MRAM (Naji 2001), using the MRAM cell illustrated in Figure 6.2d. Spin tunneling current is used to test the magnetic state of the cell, and current through one of the electrodes of the tunneling device, in combination with a current through an external strip line, is used to write the cell

5 James M. Daughton 49 to the desired state. Improvements in density have been proposed with vertical cells of cylindrical shape (Zhu, Zheng, and Prinz 2000) and thermally assisted writing (Beech et al. 2000). Producibility improvements using a transistor per cell for writing result in a lower density. Smaller capacity memory has also been suggested (Daughton 2000). Potential advantages of the MRAM compared with silicon EEPROM and flash memory are 1000 times faster write times; no wear out with write cycling (EEPROM and flash wear out at about one million write cycles); lower energy for writing; and data access times that are about 1/10,000 of the access times of hard disk drives. Key challenges in the development of MRAM are improving yield (related to cell uniformity), improving density, and attaining thermal stability at small dimensions. Methods have been proposed for eliminating the read-select transistor in the MTJ cell (Zheng 2001) and for improving cell yield (Arrott 2001). Future Devices and Applications Beyond the progression over the past decade from the use of bulk (AMR) materials with a thin film magnetoresistance limited to about 2%, to GMR and spin tunneling and now to SPINS devices offering 15% to 40% magnetoresistance, several possible new structures suggest startling additional improvements in magnetoresistance (Verslujs and Coey. 2001; Akinaga et al. 2000; Jo et al. 2000). These new structures or materials hint at hundreds of percent changes (at room temperature), with the ultimate promise of on-off devices controlled by magnetism. Most of these new structures are being developed in Europe and Japan; little work on new materials is being done in the United States. Future device applications for SPINS are very promising. In order to use the spin information of electrons in semiconductors, it will be necessary to control spin injection, spin transport, and spin detection. Three spin injection techniques have already been demonstrated: (1) injection using a polarized light source (Kikawa et al. 1997); (2) injection from a ferromagnetic semiconductor (Ohno et al. 1999); and (3) some limited success with injection from a ferromagnetic metal (Jonker et al. 2001). Injection from a contact with a high resistivity ferromagnetic or a half-metallic ferromagnetic material may also be possible. Detection of spin states in semiconductors has been achieved through optical detection (Jonker 2001), and some limited success has been achieved with a diffuse iron contact into GaAs (Crowell 2001). Other potential detection schemes include the extraordinary Hall effect and the use of tunnel junctions. The use of electric fields for manipulating spin-polarized electrons should be possible. One of the recent remarkable results in SPINS has been the modification of the Curie point of ferromagnetic semiconductors with a gate voltage (Ohno et al. 2000), which offers the possibility of magnetizing the ferromagnetic semiconductor with a combination of a magnetic field and a gate voltage. It has recently been shown that spin-polarized currents from one ferromagnet can switch another (Katine et al. 2000), and for small devices, spin current-induced switching is projected at lower currents than by passing currents through etched windings. It is within the realm of possibility that a combination of controlled spin current from a ferromagnet with a gate voltage to raise and lower the Curie point could be used to switch a ferromagnetic semiconductor with very small currents. In these new SPINS research areas, Japan and Europe have some lead over efforts in the United States. COMPARISON OF RESEARCH IN JAPAN AND EUROPE WITH THAT IN THE UNITED STATES In read heads for hard drives, IBM and Seagate in the United States are the two largest producers; Read Rite is a third supplier. In Japan, Fujitsu and TDK (which has purchased Headway in the United States) are major producers. There are no major GMR head producers in Europe. Motorola, IBM, Micron, Honeywell, and Cypress have MRAM efforts in the United States. Infinion has teamed with IBM, and Philips has a research effort in MRAM. NEC and Toshiba have some MRAM development in Japan. NVE sells GMR sensors in the United States, and Infinion sells GMR sensors in Europe. A number of other companies make GMR sensors for internal use.

6 50 6. Magnetoelectronic Devices Japan and Europe both lead the United States in published work on alternative and very high magnetoresistance structures. Akinaga s work in Japan on manganese antimonide, Coey s work in Ireland on iron oxide particles (see Trinity College site report in Appendix B), Garcia s work in Spain on constricted ferromagnets, and Blamire s manganate tunnel junctions all have yielded magnetoresistance over 300%. There is very little published work in the United States on these alternative structures. Europeans seem to be very strong in magnetoresistance theory as well as in spin injection. There is a general realization that GMR and MTJ theory still have some major unexplained results; the Europeans are working through details while researchers in the United States have shifted toward the newer frontiers such as tunneling magnetoresistance, spin current switching, and spin injection into semiconductors. An example of new thoughts on GMR is the work coming from Brian Hickey and his group at Leeds University (see special section on EPSRC Spintronics Network included in University of Nottingham site report, Appendix B). Overall, the device application work in the United States is stronger than that in Japan and Europe. REFERENCES Akinaga, H., M. Mizuguchi, K. Ono, and M. Oshima Appl. Phys. Lett. 76, 357. Arrott, A.S Paper EC-11 at MMM in Seattle, November 14. Baibich, M., J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas Phys. Rev. Lett. 61, Barnas, J., A. Fuss, R. Camley. P. Grunberg, and W. Zinn Phys. Rev. B. 42, Beech, R., J. Anderson, A. Pohm, and J. Daughton J. Appl. Phys. 87, Crowell, P Paper CA-04 at MMM in Seattle, November 14. Daughton, J Advanced MRAM Concepts, Paper presented at Nonvolatile Memory Symposium, Washington DC, November 16. (Also available at < Daughton, J., J. Brown, R. Beech, A. Pohm, and W. Kude IEEE Trans. Mag. 30, Dieny, B., V.S. Speriosu, S. Metin, S.S.P. Parkin, B.A. Gurney, P. Baumgart, and D.R. Wilhoit J. Appl. Phys. 69, Egelhoff, W.F., Jr., P.J. Chen, C.J. Powell, D. Parks, G. Serpa, R.D. McMichael, D. Martien, and A.E. Berkowitz J. Vac. Sci. Technol. B 17, Huai, Y., et al paper presented at The 8th Joint MMM-Intermag Conference, San Antonio, Texas, January Infineon Technologies (Infineon). ND. GMR Magnetic Sensors. Jo, M.-H., N.D. Mathur, N.K. Todd, and M.G. Blamire Phy. Rev. B. 61, R Jonker, B.T Naval Research Laboratory, paper presented at the 1st International Conference on Spintronics and Quantum Information Technology, May 15. Jonker, B.T. et al Paper GD-01 at MMM in Seattle, November 16. Katine, J.A., F.J. Albert, R.A. Buhrman, E.B. Myers, and D.C. Ralph Phys. Rev. Lett. 84, Katti, R., et al Paper FD-04 Presented at the 8th Joint MMM-Intermag Conference, San Antonio, Texas, January Kikawa, J.M., I.P. Smorchkova, N. Samarth, and D. Awschalom Science. 277, Koui, K., et al paper presented at The 8th Joint MMM-Intermag Conference, San Antonio, Texas, January Miyazaki, T. and N. Tezuka J.M.M.M. 151, 403. Moodera, J.S., L.R. Kinder, T.M. Wong, and R. Meservey Phys. Rev. Lett. 74, Naji, P., M. Durlam, S. Tehrani, J. Cader, and M. DeHerrera ISSCC Digest of Technical Papers, p122. NVE Corporation (NVE). ND. (Isolators). Ohno, H., D. Chiba, F. Matsukura, T. Omiya, E. Abe, T. Dietl, Y. Ohno, K. Ohtani Nature 408, 944.

7 James M. Daughton 51 Ohno, Y., D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom Nature 402, 790. Parkin, S., and D. Mauri Phys. Rev. B. 44, Parkin, S., N. More, and K. Roche Phys. Rev. Lett. 64, Scheuerlein, R.E., et al IEEE International Solid State Circuits Conference. Digest of Technical Papers (Cat. No.00CH37056), 128. Takagishi, M Paper CB-01 at MMM in Seattle, November 14. Tehrani, S., et al Paper BZ-01 at MMM in Seattle, November 13. Tondra, M., J. Daughton, D. Wang, and A. Fink J. Appl. Phys. 83, Tsang, C., et al IEEE Trans. Mag. 30, Verslujs, J.J., and J.M.D. Coey J. Magn and Magn Mat 226, Xue, J., and R.H. Vitora Paper CB-08 at MMM in Seattle, November 14. Zheng, Y., X. Wang, D. You, and Y. Wu Paper EC-02 at MMM in Seattle, November 14. Zhu, J., Y. Zheng, and G. Prinz J. Appl. Phys. 87, 6668.

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