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Parallel and Antiparallel Interfacial Coupling in AF-FM Bilayers Print

Cooling an antiferromagnetic–ferromagnetic bilayer in a magnetic field typically results in a remanent (zero-field) magnetization in the ferromagnet (FM) that is always in the direction of the field during cooling (positive Mrem). Strikingly, when FeF2 is the antiferromagnet (AF), cooling in a field can lead to a remanent magnetization opposite to the field (negative Mrem). A collaboration led by researchers from the Stanford Synchrotron Radiation Laboratory working at ALS elliptically polarizing undulator Beamline 4.0.2 has verified a proposed explanation involving a small magnetic moment at the AF interface, but in the process also found both positive and negative moments and a way to distinguish them.

How to Tell Up from Down

Imagine getting up in the morning and finding that everything around you either is upside down or runs backwards—your bed hangs from the ceiling or the sun is going down. You would be justifiably confused, and you would certainly demand to know what was going on. The perplexity you experience is very similar to what scientists must have thought when they first studied magnetic interactions between thin layers of certain magnetic materials and found behavior upside down or backwards relative to everything they had known up to then about the magnetic properties of so called “antiferromagnetic–ferromagnetic exchange-coupled” systems. Because these systems are at the heart of many of today’s most modern magnetic storage disk technologies, the bewilderment involved more than just pride. Ohldag et al. have now provided direct experimental evidence that supports a subsequently proposed explanation. Moreover, they have begun to unravel a situation that is even more complex than imagined, showing that the magnetic properties at the boundary between an antiferromagnet and ferromagnet are the result of complicated and competing processes.

To understand the occurrence of either preferred direction, one has to assume that the AF layer exhibits a small magnetic moment at the interface that can be aligned by the cooling field. After cooling, this moment will not change its direction, even upon reversal of the external field, since it is now strongly anchored (pinned) in the magnetic system of the AF. This very small moment at the interface then aligns the FM via an internal magnetic field called magnetic exchange coupling. This exchange-bias phenomenon is useful in modern magnetic devices.

Two years ago, the Stanford group was able to provide the first direct experimental evidence for the existence of this small interfacial moment in the AF by using Beamline 4.0.2. To explain the anomalous behavior with FeF2, it has been postulated that the coupling between the pinned moment in FeF2 and a FM layer is not parallel but antiparallel and thus reverses the favored direction of the FM. In the new experiment, the researchers verified this proposition in a high-quality 2.5-nm FeF2–Co bilayer grown at West Virginia University. They used x-ray magnetic circular dichroism (XMCD) spectroscopy, a technique that can selectively detect the magnetic properties of sites close to the interface and distinguish between the magnetic properties of cobalt and iron atoms.

Ideal hysteresis loops (magnetization vs. applied magnetic field) for the FM component of “typical” field-cooled AF-FM bilayers with “positive” remanent magnetization and of FeF2–FM bilayers with “negative” remanent magnetization. The magnetizations in each of the component layers and the interface are represented by the arrows in the diagrams.

Hysteresis loops (magnetization vs. applied magnetic field) of the interfacial iron spins and the FM cobalt layer taken at 300 K (well above the ordering temperature, TN =78 K) for FeF2 show that both loops are symmetric and the entire moment at the FeF2 interface follows the Co moment. Since one would expect the orientations of all iron moments within the FeF2 lattice to be in constant motion above TN, it is clear that the magnetic moment of the cobalt layer can be used to induce magnetic order in the topmost layer of the FeF2.

At 15 K, the investigators observed a partial shift in the cobalt hysteresis loop indicating a positive remanent magnetization. Most of the spins at the interface simply seem to follow the FM Co layer as they do above TN, and the overall shape of the interfacial hysteresis loop closely resembles that of the cobalt layer. However, it appears that the interfacial iron loop shifts down towards negative magnetization. This shows that a small fraction of the spins at the interface generate a magnetic moment that always point opposite to the favored magnetization of the FM.

Left: Element-specific cobalt (black) and iron (red) hysteresis loops (as measured by XMCD) hysteresis loops acquired at 300 K and 15 K after cooling in a weak field that only partially aligns the surface moment. Right: Schematic of moments in the cobalt and at the interface.

In sum, the group found that a small fraction of interfacial magnetic moments tend to align the FM layer opposite to their moment, which can lead to a reversal of the preferred magnetization direction of the FM with respect to the cooling field. These moments appear only below the antiferromagnetic ordering temperature. On the other hand, the FM is able to align some of the moments at the AF surface so that they point in the same direction as the FM even above TN. All in all, the magnetic properties at AF–FM interfaces are the result of complicated and competing processes. The present results provide clear evidence to distinguish between these two types of coupling mechanisms.

Research conducted by H. Ohldag and J. Stöhr (Stanford Synchrotron Radiation Laboratory); H. Shi and D. Lederman (West Virginia University); and E. Arenholz (ALS).

Research funding: National Science Foundation and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: H. Ohldag, H. Shi, E. Arenholz, J. Stöhr, and D. Lederman, “Parallel versus antiparallel interfacial coupling in exchange-biased Co/FeF2 bilayers,” Phys. Rev. Lett. 96, 027203 (2006).