Macroscopic Domain Structure in a Wedged Permalloy/FeMn Bilayer


S. M. Zhou*, Kai Liu, and C. L. Chien, The Johns Hopkins University, Baltimore, MD 21218
*Fudan University, Shanghai, China

     I. Motivation

Switching of an isolated ferromagnetic layer magnetization (M) from +M to -M generally involves the evolution of a complex domain pattern consisting of a large number of magnetic domains, whose size and shape change rapidly with the external magnetic field H.  Likewise, in exchange-coupled ferromagnet (FM) / antiferromagnet (AF) bilayers, the FM layer also breaks up into many domains with complex wall motions during switching.
Recently, it has been experimentally shown that the exchange field HE and the coercivity Hc of exchange-coupled FM/AF bilayers exhibit a 1/tFM dependence (Fig. 1), where tFM is the thickness of the FM layer.

Fig. 1: Exchange field HE (squares) and coercivity Hc (circles) of Py/FeMn bilayers as a function of 1/ tFM, where tFM is the Py thickness.

  By taking advantage of the 1/tFM dependence, we have achieved a very simple domain structure during switching in bilayers consisting of a wedge-shaped FM layer (40 Å — 300 Å) exchange-coupled to a uniform antiferromagnetic layer (300 Å) (Fig. 2A). During switching, the domain wall moves from the thick end to the thin end of the FM layer. For H * - 40 Oe, the entire specimen has all the moments pointing up (Fig. 2B). At a slightly more negative field, the magnetization at the thick end of the Py layer begins to switch to the opposite direction. For example, at H = - 90 Oe, the moments to the right of point b are pointing down, while the moments to the left of point b remain pointing up, i.e., there is a 180° domain wall at point b when H = - 90 Oe (Fig. 2C). The 180° wall moves further to the left of point b at a more negative field until H ? - 250 Oe, at which the entire specimen has the magnetization pointing down (Fig. 2D).
Fig. 2: (A) Schematic descriptions of the exchange-coupled Py(wedge)/FeMn(300 Å) specimen, 5 cm x 2 cm in size, with a wedged Py layer (40 - 300 Å) from left to right. The deposition field, cooling field (Hcool), and measuring field are perpendicular to the wedge direction. MOKE measurements are taken at various spots along the line marked a, b, c. The VSM measurement are taken from individual samples cut along the line marked a’, b’, c’. (B) In the field range of H * - 40 Oe, the entire sample has one domain with up magnetization. (C) In the intermediate field range, there are two domains with opposite magnetization, separated by a 180° wall. (D) In the field range of H ? - 250 Oe, the entire sample has one domain with down magnetization.

   II. Results and Discussions

This unique situation has been achieved because of the 1/tFM dependence, with which the motion of the domain wall is impeded by the thinning FM layer. As a result, there are only two macroscopic domains (several cm’s in size), extending across the entire sample, separated byone180° domain wall (Fig. 2C). Under a magnetic field of an increasingly more negative value, the wall moves along the wedge direction (Fig. 2C) until the entire FM layer has switched. Equally important, at a constant H, the 180° wall remains stationary.
This unique macroscopic domain pattern has been determined using magneto-optical Kerr effect (MOKE) and vibrating sample magnetometry (VSM). In MOKE, the laser beam was directly at different locations on a large uncut sample along the line a, b, c in Fig. 2A. For the VSM measurements, many small samples cut from the large wedged sample along the line a’, b’, c’ in Fig. 2A were used. The macroscopic domain pattern was revealed by the fact that the scanning MOKE measurements on an uncut wedged specimen give the same results as those obtained from VSM using many separate samples at corresponding locations (Fig. 3).

Fig. 3: Representative hysteresis loops measured by MOKE (left) and VSM (right), where a, b, c, d and a’, b’, c’ refer to the locations on the specimen described in Fig. 2A.

Because of the unique domain pattern, a hysteresis loop in the present case does not involve many domains. Instead, it is just a signature of the movement of one180° wall sweeping across the sample. The switching field as a function of H (Fig. 4) allows us to determine the rate of wall moment due to the external field (dx/dH), which increases with tFM in a non-linear manner.
These domain patterns of two macroscopic domains separated by one 180° have recently been confirmed by the NIST group (R. Shull, V. Nikitenko, A. Shapiro, and V. Gornakov) using advanced Magneto-Optic Indicator Film (MOIF) technique. The dynamics of the 180° domain wall and other aspects of the domain dynamics are currently being studied by this imaging technique.

Fig. 4: The switching field measured from VSM (open symbols) and MOKE (solid symbols) for increasing-field branch and decreasing-field branch as a function of Py thickness and location on the wedged Py layer.
 

References:
    1. S. M. Zhou, Kai Liu, and C. L. Chien, Phys. Rev. B 58 (Rapid Communications), R14717 (1998). (Full article, PDF file)
    2. V. Nikitenko, A. Shapiro, V. Gornakov, R. Shull, Kai Liu, S. M. Zhou, and C. L. Chien, Phys. Rev. Lett. 84, 765 (2000).

    Contact Us

      S. M. Zhou:    smzhou@fudan.ac.an
      Kai Liu:    kliu@pha.jhu.edu
      C. L. Chien:   clc@pha.jhu.edu