Influence of the thickness of the carbon layer on the magnetization and structure of Fe/C multilayers

Abstract

Multilayered films of a heavy metal and carbon have long been investigated. Major interest arises from the use of multilayers as soft X-ray monochromators and reflectors [1, 2]. Studies have shown that the X-ray reflectance of the multilayers and their stability over time are determined by the extent of any interracial reaction, including diffusion, amorphization or compound formation. In addition to the X-ray optical characteristics, magnetic properties of multilayer Fe/C, Ni/C and Co/C have been studied [3-7]. In particular, Fe/C multilayer films show good soft-magnetic properties and nearly zero magnetostriction. These properties are suitable for the application to thin-film heads. Studies have revealed that amorphous alloy or carbide layers are formed at metal-amorphous carbon interfaces, and magnetic properties are strongly influenced by these interfaces. Changes of structure of the interfaces of Fe/C multilayers with the Fe layer thickness have been noticed in some of the reported experiments, although the influence of the C layer thickness on the structure and magnetic properties, such as magnetization, of the multilayers has not been reported so far to our knowledge. In this letter we report the variation of magnetization and structure of Fe/C multilayers with individual carbon layer thickness. The Fe/C multilayers were prepared on silicon wfifers and rock salt by alternately depositing Fe and C in an ion beam sputtering system [8]. The base pressure was 9 x 10 .5 Pa, and sputtering was performed at 7 x 10 .3 Pa of argon. Prior to the deposition the targets of iron and graphite were cleaned with ion beam sputtering. The multilayers were deposited on an interlayer of carbon (about 2 nm). The deposition rates were kept at 0.08 nms 1 for Fe and 0.02nms -1 for C, which were calibrated by Rutherford back-scattering measurement of the thickness of single-layer Fe and C films. The deposition rate of Fe was also verified by the magnetization of single-layer Fe films. Strictly accurate knowledge of the deposition rate of C was not required for the present studies. The ion beam sputtering technique is known as a quantitative method, in which the deposition rate can be well controlled. Alternate deposition was accomplished by rotating a target holder, which could contain four different target materials. The individual layer thickness of Fe anad C was determined by the deposition time. Ten bilayers of Fe/C were deposited for each film. The substrates were cooled with water, and the substrate temperatures were below 50 °C during deposition. The magnetic measurements were conducted using a vibrating sample magnetometer (model 9500 VSM) at room temperature, with an applied field of 5 kOe. Low-angle X-ray diffraction (LXRD) was carried out on a Rigaku D/max-RB diffractometer with CuK~ (0.15405 nm) radiation. Electron diffraction patterns of samples were observed by using a JEM-200CX electron microscope. Electrical sheet resistances were measured using the four-point technique. Fig. 1 shows the saturation magnetization (47rMs) of the films as a function of the individual carbon layer thickness tc, where individual Fe layer thickness tFe was fixed at 4.7 nm. The Ms data was calculated by the volume of the Fe layers. It can be seen that the magnetization decreases remarkably with the increase of t c and reaches a minimum at t c = 1.6 nm, then the magnetization increases steadily with the increase of tc. Further increase of tc results in the magnetization approaching the bulk value. It should be noted here that a similar situation is also observed in other series of Fe/C multilayers s u c h as tFe = 2 nm. The crystallographic and layered structures of the samples in Fig. 1 were investigated by electron diffraction and LXRD. Electron diffraction results revealed that in all samples only some rings belonging to body-centred cubic (b.c.c.) Fe were presented in the patterns. Therefore, the Fe layers in these samples existed basically in the b.c.c. Fe structure.