Artificially Modulated Chemical Order in Thin Films: A Different Approach to Creating Ferro/Antiferromagnetic Interfaces

Authors

Thomas Saerbeck (University of Western Australia), Frank Klose, Anton Stampfl, Sergey Danilkin and Mohana Yethiraj (ANSTO) Dieter Lott and Andreas Schreyer (GKSS), Gary Mankey, Zhihong Lu and Patrick LeClair (University of Alabama) and Wolfgang Schmidt (Forschungszentrum Juelich)

Introduction

Artificially modulated magnetic structures, such as thin-film multilayers, heterostructures and superlattices have become an important field in scientific research with numerous applications in the field of spin-electronics. Typically, the desired physical properties are obtained by chemical modulation of different materials, which inevitably affects the behaviour of the system due to incommensurate growth, strain and interface roughness. We have come up with a new approach to create a modulated antiferromagnetic (AFM)/ferromagnetic (FM) structure by modulation of chemical order in the same material [1].

In thin films of FePt3, we control the degree of chemical order through temperature modulation during growth, creating a single crystalline FePt3 film of homogeneous composition, but consisting of alternating chemically ordered antiferromagnetic and chemically disordered ferromagnetic layers. As a direct consequence of interfacing AFM and FM material, due to the order modulation, the system shows a unique magnetic self-exchange bias, unaffected by structural roughness or material incommensurability.

In order to investigate the magnetic structure within a mono-stoichiometric thin film, a unique combination of polarized neutron reflectometry (PNR), X-ray and neutron diffraction and conventional magnetometry has to be used. The intrinsic magnetic moment of the neutron makes PNR and high-angle neutron diffraction an excellent tool for the study of depth-dependent chemical and magnetic structures. High-angle neutron diffraction is employed to fully resolve the antiferromagnetic structure on the atomic scale, using the triple-axis spectrometers TAIPAN (ANSTO) and IN12 (Institute Laue Langevin, Grenoble).

The onset of a half-order magnetic peak along the (½ ½ 0) direction confirms the direction of AFM order below the Néel temperature (Fig. 1). Depth-resolved magnetizations are obtained by analysis of spin-dependent neutron reflectivity of the sample (Fig. 2), which allows for separation of AFM and FM regions along the sample normal and confirms the purely magnetic multilayered structure (Fig. 3).  

Figure 1.  Integrated antiferromagnetic Bragg peak intensities as a function of temperature. The squares represent the data recorded at IN12, which are shown in the inset (symbols = data and lines = Gaussian fits). The circles have been recorded with TAIPAN in similar geometry to IN12.

Figure 2. Recorded polarized neutron reflectivity (open symbols) in R+ (red) and R- (blue) polarization and simulations (lines). The data have been background corrected and normalized to 1. Individual datasets have been offset from one another by two orders of magnitude for clarity.

Figure 3. Schematic representation of a FePt3 chemically ordered/disordered bilayer. The chemically ordered phase shows antiferromagnetic alignment of individual Fe moments along the (½ ½ 0) direction (top) while the disordered phase shows ferromagnetic alignment (bottom). Incomplete ordering in the AFM layer leads to extra ferromagnetic grains, contributing to the exchange bias. The central structure is repeated 5 times in the multilayer.

Reference:

1. T. Saerbeck, F. Klose, D. Lott, G.J. Mankey, Z. Lu, P.R. LeClair, W. Schmidt, A.P.J. Stampfl, S. Danilkin, M. Yethiraj, and A. Schreyer, Phys. Rev. B 82, 134409 (2010)