The synthesis and magnetic properties of a nanostructured Ni-MgO system

Abstract

Research Summary Spintronic Materials and DevicesWe have investigated the magnetic properties of the Ni-MgO system with an Ni concentration of 0.5 at.%. In asgrown crystals, Ni ions occupy substitutional Mg sites. Under these conditions the Ni-MgO system behaves as a perfect paramagnet. By using a controlled annealing treatment in a reducing atmosphere, we were able to induce clustering and form pure Ni precipitates in the nanometer size range. The size distribution of precipitates or nanodots is varied by changing annealing time and temperature. Magnetic properties of specimens ranging from perfect paramagnetic to ferromagnetic characteristics have been studied systematically to establish structure-property correlations. The spontaneous magnetization data for the samples, where Ni was precipitated randomly in MgO host, fi ts well to Bloch's T 3/2 -law and has been explained within the framework of spin wave theory predictions.

Multi-frequency ferromagnetic resonance investigation of nickel nanocubes encapsulated in diamagnetic magnesium oxide matrix

“…Nonetheless, assuming nickel nanocubes in MgO:Ni retain the magnetization axes of the bulk nickel, we believe that set 3 nickel nanocubes are small sized with their internal field oriented along the hard magnetization axis of h100i Ni . Further, an observation of multiple FMR lines with large linewidths agrees very well with the bulk magnetic susceptibility studies, 5,7 which indicated a very broad distribution of blocking temperatures consistent with a broad nanocube size distribution. More importantly, remarkable sensitivity of the FMR line position and linewidth to temperature, sample orientation, and microwave frequency allowed us to distinguish between the three major types of nickel nanocubes in MgO:Ni.…”

“…23 At T ¼ 298 K, K 1 ¼ 4.5 Â 10 4 erg/cm 3 (bulk nickel) 21 and M s ¼ 11.80 emu/cm 3 in MgO:Ni. 7 These values yield B int (3) % 95.3 mT, which is about an order of magnitude smaller than the observed value from FMR. This estimate shows that the magnetocrystalline anisotropy alone is insufficient for explaining the observed values of B int (3).…”

“…Large linewidths are consistent with the broad size distribution of Ni nanocubes observed in TEM and magnetic susceptibility data. 5,7 However, an observation of the three FMR lines (referred to as 1, 2, and 3 from here on) with some of the lines only partially resolved at certain orientations is somewhat surprising, especially since nickel is considered to be a soft ferromagnet. If the magnitude of the Zeeman field jB ext j !…”

“…6 Previous variable temperature DC magnetic susceptibility studies 5,7 revealed that under an applied magnetic field of 0.05 T, MgO:Ni exhibits superparamagnetism with the blocking temperature above room temperature. 5,7 As expected, a difference between magnetization vs. temperature curves measured for the zero-field and field-cooled samples vanished when the applied magnetic field was increased to 1 T. 5,7 Further, the variable temperature magnetization studies of MgO:Ni from 5 to 350 K revealed changes in the coercive field from 31.5 to 0.6 mT and in the saturation field from 570 to 375 mT, respectively. These observations suggest that MgO:Ni exhibits a strong bulk ferromagnetism below ca.…”

“…These observations suggest that MgO:Ni exhibits a strong bulk ferromagnetism below ca. 300 K. 7 Once the bulk ferromagnetic behavior of Ni nanocubes in a diamagnetic MgO host has been established by the DC magnetometry, it is imperative to further characterize the spin exchange and local anisotropy field phenomena in such macroscopically organized anisotropic nanoscale system. Thus, we turned to ferromagnetic resonance (FMR) as a spectroscopic tool to differentiate and characterize the magnetic interactions originating from various components of MgO:Ni as well as to evaluate the strength of the local magnetic anisotropy fields in Ni nanocubes.…”