Improvement of Gmr in NiFeCo/Cu Multilayers by a Layer - by - Layer Magnetic Field Sputtering
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ABSTRACT NiFeCo/Cu multilayers fabricated by an improved magnetic field sputtering were investigated in order to achieve the soft GMR (giant magnetoresistance) with a high sensitivity at low magnetic fields. A magnetic field was applied to the film during sputter-deposition, and its field direction was changed alternately from layer to layer. Such an alternate field sputtering is called hereafter layer-by-layer magnetic field sputtering. The best GMR characteristics (large MR at low magnetic fields) were achieved when the angle between the directions of magnetic field applied to neighboring two magnetic layers was 90' . As one of the speculation, it has been considered that the result is attributed to the induced composite magnetic anisotropy which causes the magnetization to occur more dominantly by spin rotation than by domain wall movement. INTRODUCTION Giant magnetoresistance (GMR) multilayered thin films are now considered to be useful for various kinds of electric, magnetic and mechanical sensors, if one can improve the GMR so as to occur easily at low magnetic fields. The NiFeCo/Cu system has been anticipated to be the most suitable GMR multilayers [1,2]. Kanda et al. [3] and Jimbo et al. [4] have reported that the combination of elements of Ni, Fe and Co has opportunities to make both magnetocrystalline anisotropy and magnetostriction negligibly small and to induce a fairly large uniaxial magnetic anisotropy by magnetic field sputter-deposition, leading to a magnetically soft GMR in NiFeCo/Cu multilayers. However, the magnetic fields needed for exciting GMR are still large and a large hysteresis is observed in the MR vs. field curves. The bar-like 180" magnetic domains arising from the induced uniaxial magnetic anisotropy might be responsible for these disadvantages, particularly for the large hysteresis. In order to solve this problem, we have attempted to control the magnetic domain structure by using an improved sputtering technique. That is, during sputter-deposition for fabricating GMR multilayers (NiFeCo/Cu, for example), a magnetic field has been applied to magnetic layers by changing the field direction alternately from layer to layer. The angle between the field directions changed from a given layer to the nearest neighbor layer has been selected to be 0' , 45' and 90° . In addition, we have also investigated the case that field is not applied. We have found that the MR vs. magnetic field curves are different in these four cases, and that the most prominent MR response with a considerably reduced hysteresis can be obtained in the case of the 90* alternate field sputtering. As one of the speculation, it may be considered that such an alternate magnetic field sputtering (layer-by-layer magnetic field sputtering, in another word) causes the multilayer to have a composite magnetic anisotropy consisting of the uniaxial magnetic anisotropies piled up spirally from layer to layer. This induced composite magnetic anisotropy may compete to the exchange coupling between magnetic layers and then affects to th
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