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Dynamic Interface Formation in Magnetic Thin Film Heterostructures

Komori Group

In magnetic thin film heterostructures, interaction at the interface plays a dominant role in the development of novel electronic and magnetic properties. The interlayer coupling strength primarily relies on the interfacial structure at the atomic scale, including atomic roughness, steps, and intermixing. However, the impact of the atomic scale interfacial structure on the magnetic coupling in magnetic thin film heterostructures has not yet been elucidated well in relation to the macroscopic magnetic properties. We use scanning tunneling microscopy (STM) and x-ray absorption spectroscopy/x-ray magnetic circular dichroism (XAS/XMCD) as complementary tools for clarifying the correlation between the atomic structure at the interface and the magnetism. Successive atomically-resolved STM characterizations of not only structural but also electronic and magnetic properties during the growth of magnetic thin film heterostructures provide crucial information on the atomic-scale interfacial factors in the dynamical process of the interface formation. The element-specific, quantitative and macroscopic observations of electronic and magnetic properties by XAS/XMCD can be linked with the microscopic interface characteristics.

Fig. 1. (a) Fe L2,3 remanent XAS (upper) and XMCD (middle) of a 7-ML Fe film on Cu(001), and XMCD (lower) of a 5-ML Mn/7-ML Fe film in the normal incidence (NI) and grazing incidence (GI) geometries. Here, external magnetic field is parallel to the incident x-ray. (b) Iron magnetization curves in the heterostructures of Mn (x ML, x = 0, 1, 2, 3, and 5 ) overlayers on the 7-ML Fe film measured in the NI (left) and GI (right) geometries. The L3 XAS peak intensity normalized by the L2 one is plotted as a function of the magnetic field. In the GI geometry, the magnetic field is applied along the [100] direction of the Cu(001) substrate.

Fig. 2. (a-c) STM images of 7-ML Fe films with 1.0-ML (a), 1.5-ML (b), 1.8-ML (c) Mn overlayers. (d-g) High-resolution surface STM images of 1.0-ML (d), 1.5-ML (e), 1.8-ML films (first (level-1) (f) and second (level-2) (g) levels,). The disorder alloy can be seen in each image as bright protrusions. (h) Statistical plots of the fraction of the ordered alloy region with the (4 × 2) and (4 × 4) reconstructions in level-1 and level-2 as a function of the average thickness of the Mn overlayer. The dashed lines are linear extrapolations of the fractions of the ordered alloy.

We have studied fcc Fe thin films grown on Cu(001) with Mn overlayers (Mn/Fe films) as a unique system for investigating the interface interaction. [1] The electronic and magnetic properties of ferromagnetically-coupled top two layers in the fcc Fe thin film on Cu (001) [2] is susceptible to the structural changes on the atomic scale. At the interface, a collinear and homogeneous antiferromagnetic/ferromagnetic coupling is expected at the Mn/Fe interface as observed in the reference system of Mn thin films on the bulk bcc Fe(001) substrate. Thus, the fcc Fe thin film highlights the role of atomic-scale interfacial factors with the Mn overlayers.

The films were fabricated by successive deposition of Fe and Mn on Cu(001) at room temperature (RT) in an ultrahigh vacuum. The thickness of the fcc Fe layer was fixed to ~ 7 monolayer (ML). The Fe layer in the Mn/Fe film exhibits a two-step spin reorientation transition (SRT) from out-of-plane to in-plane direction with increasing the thickness of the Mn overlayer as observed by XAS/XMCD measurements shown in Fig. 1. After 1 ML deposition of Mn, the coercive field in the magnetic field perpendicular to the film decreases to a value less than that in the in-plain magnetic field. Furthermore, the in-plane anisotropy gradually increases up to 3-ML Mn deposition. Corresponding atomically-resolved STM observations reveal the dynamic structure change from a disordered surface to an ordered one by the Mn deposition at RT as in Fig. 2. First, a disordered surface alloy with the Fe film is formed by the Mn deposition, which induces the first SRT. The ordered-alloy area of the surface gradually increases with increasing the amount of the deposited Mn atoms more than 1 ML on average. The ordering of the alloy completes around 3-ML Mn deposition, which corresponds to the completion of the second SRT.

The present complementary approach by XAS/XMCD and STM successfully disentangles the hidden functionality of the interface alloying, which changes and enhances the magnetic anisotropy in the Fe layer. The results will pave new ways to understand novel phenomena emerging at the interface on the atomic scale, and to improve the electronic and magnetic properties of the magnetic thin film heterostructure.

  • [1] S. Nakashima et al., Adv. Func. Matter. 29, 1804594 (2019).
  • [2] H. L. Meyerheim et al., Phys. Rev. Lett. 103, 267202 (2009).
  • S. Nakashima, T. Miyamachi, Y. Tatetsua, Y. Takahashi, Y. Takagib, Y. Gohdaa, T. Yokoyamab, F. Komori
  • aTokyo Institute of Technology
  • bInstitute for Molecular Science