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Spiraling Antiferromagnetic Spin Structure in Exchange-Coupled Multilayers
Introduction
Exchange coupling in ferromagnet (FM)/antiferromagnet (AF) bilayers, or exchange bias, has been intensively studied in recent years because of the intriguing physics and its center role in spin-valve devices. It is generally recognized that the spin structure of the AF holds the key to the understanding of exchange bias. It has been demonstrated that all models assuming a static and rigid AF spin structure are incompatible with experiments. More realistic models have concluded a spiraling AF spin structure, but without experimental confirmation. We have used FM/AF/ FM trilayers to reveal the spiraling AF spin structure.
Samples
We used Co(100 Å)/wedged FeMn(tAF)/Py(200 Å), a trilayer of hard FM/AF(tAF)/soft FM. The large specimen was cut into many smaller pieces, each with a different tAF.
Special FIeld Cooling Procedure
At 400 K, the Co and Py layers are uncoupled. The large difference in the anisotropy of Co and Py gives the "steps" in the hysteresis loops. At the "steps" (at about + 10 Oe and - 10 Oe), the magnetizations of Co and Py are opposite. Field-cooling at the step field (e.g., at - 9 Oe) causes the AF layer to be coupled to, and bounded by, two FM layers of opposite directions.
tAF = 150 Å
Results with different tAF
For a large tAF (e.g., 300 Å and 150 Å), the axes of the Co and Py magnetizations are opposite to each other and both loops are square. However, at tAF < 90 Å, while the Co loop remains square, the Py loop has become slanted, indicating that the anisotropy axis of the Py layer has changed from being opposite to that of Co. Regardless of the value of tAF, the unidirectional anisotropy of the Co layer is always along the same direction (opposite to the cooling field direction) due to its larger intrinsic magnetic anisotropy.
Determining the anisotropy axis of the Py layer
One can determine the anisotropy axis by measuring angular dependence of either HE or Hc and note the angle at which HE and Hc are maximal. For example, in this sample, the Py loop has the largest width at b = 55º, q = 125º between the Co and Py axes.
Turn angle between Co and Py
The measured turn angles q between Co and Py are shown as a function of tAF. For tAF > 90 Å, the magnetization axes of the Py and the Co layers are opposite, whereas, for tAF < 90 Å, q is less than p, and q = gotAF with go = 1.76 degree/Å.
Thus, a spiraling AF spin structure has been found in Co/FeMn/Py.
Pitch Angle within AF
From the turn angle (q) and the atomic plane spacing of FeMn, one can calculate the pitch angle (Dq) and its dependence on tAF.
tAF < 90 Å: q = (1.76°/Å) tAF
tAF > 90 Å: q = 180°
FeMn layer: (111) texture
Atomic plane spacing: 2.09 Å
Pitch angle between spins of adjacent layers:
tAF < 90 Å: Dq0 = 3.68°
tAF > 100 Å:
Dq = 180°/(tAF/2.09)
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