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Layered Magnetic Structures: Evidence for Antiferromagnetic Coupling of Fe Layers across Cr Interlayers
Grünberg, P., Schreiber, R., Pang, Y., Brodsky, M.B., & Sowers, H. (1986) Layered Magnetic Structures: Evidence for Antiferromagnetic Coupling of Fe Layers across Cr Interlayers. Phys. Rev. Lett. 57, 2442.
Essay about this article
In Albert Fert’s thinking that lead to a realization of GMR, it was necessary to have an antiferromagnetic coupling between layers so that an external magnetic field could switch them to parallel alignment. It took another 20 years to achieve antiferromagnetically aligned magnetic layers sandwiched between non-magnetic metallic spacers. This achievement was due largely to a parallel development in multilayers that used semiconducting elements, in this case heterostructures. Semiconducting elements have a lower affinity to interdiffuse at the interfaces between layers than do metallic elements; therefore one was able to grow heterostructures by 1980. By following the recipe used for semiconductors several groups were able to achieve a controlled growth for metallic multilayers that demonstrated antiferromagnetic coupling between magnetic layers. Notably Peter Grünberg grew Fe/Cr and Fe/Au double layers consisting of two Fe layers separated by a thin spacer layer of Cr or Au, and demonstrated that the alignment between the Fe layers was antiparallel before applying an external magnetic field.
To determine the alignment and evaluate the interlayer coupling between the iron layers Grünberg used the Brillouin light scattering technique, in which light either excites or absorbs spin wave modes [small deviations of the magnetization of the iron layers from their equilibrium positions], and thereby either lowers or raises the frequency of the light. To ascertain the alignment of the iron layers Grünberg noted whether an applied field shifted the light’s frequency. No shift indicated the magnetizations of the iron layers were parallel because either there is no coupling or a ferromagnetic coupling between layers.
By varying the thickness of the spacer layer between a few and 20 Å he identified the thicknesses where the alignments were antiparallel in zero field; in particular he found that the coupling was definitively antiferromagnetic for 8 Å of chromium. With this feat Peter Grünberg provided the material, a Fe/Cr multilayer, to test the idea of Albert Fert on GMR.
Parenthetically, the origin of the exchange coupling between magnetic layers across a nonmagnetic metallic spacer layer is similar to the Ruderman-Kittel-Kasuya-Yosida [RKKY] interaction. That is, a magnetic moment immersed in a Fermi sea of electrons creates a local polarization in the gas; as only the electrons at the Fermi level can produce the polarization, due to the availability of empty electron states to rearrange the electron distribution in the vicinity of the local moment, the induced magnetic polarization oscillates between being parallel and opposite to the local magnetic moment. This is reminiscent of the charge oscillations induced in a Fermi sea of electrons by an added charge, which is known as Friedel oscillations. When a second magnetic moment is in the proximity of the first it feels the magnetic polarization induced by it, i.e., it senses a local magnetic field, and aligns itself in its direction; hence one establishes an indirect exchange interaction between the two moments through the metal’s electron gas.
When a magnetic layer [iron] is in contact with a nonmagnetic metallic spacer it polarizes the electron gas in the spacer and provides the exchange field to couple to the magnetization of the magnetic layer at the opposite end of the spacer. This interlayer exchange coupling between magnetic layers is sizeable only for thin spacers of the order of a few nanometers, and oscillates between ferromagnetic and antiferromagnetic coupling due to the RKKY or “Friedel” oscillations.
It should be mentioned that an interlayer exchange coupling was found about the same time as Grünberg’s demonstration on Fe/Cr. Groups at Bell Labs [1] and at the University of Illinois [2] found an oscillatory coupling in multilayers containing rare-earth metals, e.g., dysprosium-yttrium [Dy-Y], where Dy is magnetic and Y nonmagnetic. After the discovery of GMR in magnetic multilayers there was tremendous interest in interlayer exchange coupling. Stuart Parkin [3] developed a “periodic table” of the interlayer coupling for a broad range of transition-metal magnetic multilayers, and Patrick Bruno [4] developed a theory of this coupling based on the spin-dependent reflection coefficients at the interfaces between the ferromagnetic and nonmagnetic layers.
References
[1] C. F. Majkrzak, J. W. Cable, J. Kwo, M. Hong, D. B. McWhan, Y. Yafet, and J. V. Waszczak, and C. Vettier, Phys. Rev. Lett. 56, 2700 (1986).
[2] M. B. Salamon, Shantanu Sinha, J. J. Rhyne, J. E. Cunningham, Ross W. Erwin, Julie Borchers, and C. P. Flynn, Phys. Rev. Lett. 56, 259 (1986).
[3] S.S.P. Parkin, Phys. Rev. Lett. 67, 3598 (1991).
[4] P. Bruno Phys. Rev.B 52, 411 (1995).
Discussion Questions
a. Why is an antiferromagnetic alignment between magnetic layers so important to achieve the GMR effect?
b. What role, if any, does the spin density wave that exists in Cr layers play in the interlayer coupling across these layers?
c. Do spin density waves affect the GMR ratio?
The above article is reprinted with permission from Grünberg, P., Schreiber, R., Pang, Y., Brodsky, M.B., & Sowers, H. Physical Review Letters 57, 2442 (1986). Copyright (1986) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.
Select articles citing this paper
Himpsel, F. J., J. E. Ortega, et al. (1998). "Magnetic nanostructures." Advances in Physics 47(4): 511-597.
Bruno, P. (1995). "THEORY OF INTERLAYER MAGNETIC COUPLING." Physical Review B 52(1): 411-439.
Unguris, J., R. J. Celotta, et al. (1991). "OBSERVATION OF 2 DIFFERENT OSCILLATION PERIODS IN THE EXCHANGE COUPLING OF FE/CR/FE(100)." Physical Review Letters 67(1): 140-143.
Parkin, S. S. P., Z. G. Li, et al. (1991). "GIANT MAGNETORESISTANCE IN ANTIFERROMAGNETIC CO/CU MULTILAYERS." Applied Physics Letters 58(23): 2710-2712.
Parkin, S. S. P., N. More, et al. (1990). "OSCILLATIONS IN EXCHANGE COUPLING AND MAGNETORESISTANCE IN METALLIC SUPERLATTICE STRUCTURES - CO/RU, CO/CR, AND FE/CR." Physical Review Letters 64(19): 2304-2307.
Binasch, G., P. Grunberg, et al. (1989). "ENHANCED MAGNETORESISTANCE IN LAYERED MAGNETIC-STRUCTURES WITH ANTIFERROMAGNETIC INTERLAYER EXCHANGE." Physical Review B 39(7): 4828-4830.
Baibich, M. N., J. M. Broto, et al. (1988). "GIANT MAGNETORESISTANCE OF (001)FE/(001) CR MAGNETIC SUPERLATTICES." Physical Review Letters 61(21): 2472-2475.
