Porous Silicon Nanocomposites with Combined Hard and Soft Magnetic Properties
Magnetic nanostructures of two ferromagnetic metals have been combined within porous silicon, and the magnetic switching behavior of the resulting porous silicon/metal nanocomposite has been modified by varying the arrangement. The two magnetic materials are Ni and Co, whereas Co is the magnetic harder one. These “hard/soft” magnetic nanocomposites have been achieved by two different routes. On the one hand, double-sided porous silicon has been used whereas one side has been filled with Ni nanostructures and the other one with Co nanostructures. On the other hand, Ni and Co have been deposited within one porous layer alternatingly. The filling of the pores has been carried out by electrodeposition with varying the deposition parameters. In systems which offer two distinct slopes of the hysteresis curves due to the different saturation behavior of the two types of deposited metal, magnetic exchange coupling is not present. For samples which show smooth hysteresis curves exchange, coupling between the Ni and Co nanostructures seems to be present. The aim is to control especially the structure size of the soft and the hard magnetic materials and the distance between them at the nanoscale to optimize exchange coupling resulting in a maximum energy product.
KeywordsPorous silicon Electrodeposition Magnetic nanostructures Magnetic interactions
Back scattered electrons
Scanning electron microscopy
Superconducting quantum interference device
The utilization of low-dimensional structures becomes more and more important due to the miniaturization of devices and also due to the novel arising nanoscopic properties. The fabrication of such nanostructures is often carried out by nanopatterning using lithographic methods. On the other hand, self-assembling techniques are of great interest due to the uncomplex and low-cost fabrication process. Quite common are nanoparticles grown on a substrate by self-organization. Also, three-dimensional arrays of nanostructures (nanowires, nanotubes) have been formed without prestructuring in using hexagonal arranged porous alumina as matrices . Also, quasi-regular arranged porous silicon has been used for the incorporation of metal deposits [2, 3]. In this context, ferromagnetic nanostructures are an important part in basic research but also in nanotechnological applications such as magneto-optical devices, magnetic sensors, or high-density data storage . A further ambition is the production of exchange-coupled nanostructures usable as permanent magnets. One approach is the self-organization of hard and soft magnetic nanoparticles which are tunable in their size and inter-particle distance .
Considering the achieved nanocomposites, magnetization reversal processes with the concomitant domain wall motion within the deposited metal nanostructures, the interactions among them, and also transport phenomena like magnetoresistance in spin valves are of great interest. Magnetic materials in the nanometer scale exhibit changed properties compared to bulk material and therefore offer great potential for novel nanotechnological applications. The nanoscopic systems consist either of particles or wires with magnetic properties dependent on their geometry and arrangement. For technical application of the system, the magnetic nanostructures should be ferromagnetic at room temperature. In some cases, a high anisotropy between the two magnetization directions, perpendicular and parallel to the surface, is of interest and thus needle-like structures are favorable due to their high demagnetizing field. One method to achieve low-dimensional structures is the deposition of metal nanostructures on patterned surfaces or into porous membranes with channels perpendicular to the surface, and therefore, the metal structures exhibit a high density with respect to the sample surface. Templates like porous alumina or polycarbonate foils are usually electrochemically fabricated and afterwards filled with a magnetic material by electrochemical deposition. In commercial microelectronics, most devices are based on silicon technology and thus for process compatibility, a silicon substrate is a good precondition for applicability.
In the present work, a combined porous silicon/“hard-soft” magnetic nanocomposite is investigated and the results concerning the magnetic properties with respect to the switching behavior are figured out.
Two kinds of porous silicon templates have been fabricated by anodization of an n+ silicon wafer. On the one hand, a silicon wafer has been porosified on one side with an average thickness of the porous layer of 40 μm. On the other hand, ultrathin wafers have been etched on both sides (front and back) in using an electrolytic double tank cell. The thickness of the porous layers was around 20 μm on each side. As anodization electrolyte, an 10 wt% aqueous hydrofluoric acid solution has been used. To achieve pore diameters of about 60 nm, a current density of 100 mA/cm2 has been applied. The etching rate was around 4 μm/min.
The prepared porous silicon templates offer a morphology in the mesoporous regime with a quasi-regular pore arrangement. Within the pores of these templates, nanostructures consisting of two different magnetic materials have been incorporated by electrodeposition. The double-sided porous silicon has been filled with Ni on one side and with Co on the other side, also using a double tank cell. Depending on the polarity of the applied current, either Ni on the one side or Co on the other side is deposited. For the Ni deposition, a current density of 25 mA/cm2 with 0.05 Hz and for the Co deposition, a current density of 20 mA/cm2 with 0.1 Hz has been applied. In the case of the one-sided sample, both metals have been electrochemically deposited alternately inside the pores resulting in a stacked arrangement of Co and Ni nanostructures. As electrolytes, a Ni and Co metal salt solution (NiCl2, NiSO4, CoSO4) has been used. The deposition time for each metal was up to 10 min. Magnetic characterization of the samples has been performed by SQUID (superconducting quantum interference device) magnetometry (field range ± 6 T, temperature range 4–300 K).
Results and Discussion
Coercivity and squareness in dependence on the elongation of Ni deposits
Shape and elongation of Ni deposits
M R/M S
Spherical, ~60 nm
Ellipsoidal, ~500 nm
Wire-like, >1 μm
The presented “hard/soft” magnetic nanocomposite is fabricated during a low-cost two-step electrochemical process whereas double-sided as well as one-sided porous silicon acts as template. Considering the magnetic behavior of double-sided systems and single-sided systems gained from a combined electrolyte solution, two characteristic terms are observed. The first one is caused by the magnetic properties of the “softer” magnetic metal (Ni) and the second one is caused by the higher saturation magnetization of the deposited “harder” magnetic nanostructures (Co). In the case of depositing Co and Ni alternatingly from a Watts-solution and CoSO4-solution, a smooth hysteresis curve is observed due to exchange coupling between the different metal deposits. Furthermore, the magnetic properties are determined by the geometry of the deposits and their arrangement and thus the coercivity, remanence, and magnetic anisotropy of the samples (both types) can be modified by varying the size and shape of the Ni and Co deposits. Therefore, the magnetic characteristics of the specimens can be broadly tailored, as desired. The magnetic properties of the resulting nanocomposites are not only correlated with the size, shape, and spatial distribution of the deposited metal structures but also strongly depend on the filling ratio between the “softer” (Ni) and “harder” (Co) magnetic materials as well as on the way how the electrodeposition is performed (from a combined or separate solutions).
The authors thank Dr. Armando Loni, pSivida, Malvern, UK, for providing the thinned wafers and Dr. Peter Pölt, Institute for Electron Microscopy, University of Technology Graz, for performing the electron microscopy.
KR and PG fabricated the nanocomposite samples by anodization and subsequent electrodeposition and carried out the magnetization measurements. All authors discussed the data and prepared the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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