Magnetic and transport properties of Fe-Ag granular multilayers

Results of magnetization, magnetotransport, and Mössbauer spectroscopy measurements of sequentially evaporated Fe-Ag granular composites are presented. The strong magnetic scattering of the conduction electrons is reflected in the sublinear temperature dependence of the resistance and in the large negative magnetoresistance. The simultaneous analysis of the magnetic properties and the transport behavior suggests a bimodal grain size distribution. A detailed quantitative description of the unusual features observed in the transport properties is given.

Figure 1

Cross-sectional TEM analysis of the combined Fe-Ag sample. (a) The bright field image shows the separable bottom and upper part of the sample. The slight 200 texture of the sample is indicated on SAED pattern by the inserted ellipse. (b) The HRTEM image from the bottom part with $\mathrm{Ag}\left(2.6\mathrm{nm}\right)$ and $\mathrm{Fe}\left(1.5\mathrm{nm}\right)$ layers. The FFT image (inset) indicates both Ag and Fe reflections. (c) The HRTEM image from the top part of the sample. The FFT image contains only Ag reflections.

Figure 2

(Color online) Transmission Mössbauer spectra of sample A at various temperatures and at $T=4.2\mathrm{K}$ in various magnetic fields. For the $T=4.2\mathrm{K}$, $B_{\mathit{ext}}=0\mathrm{T}$ spectrum the individual six-line patterns of the fitted distribution are also plotted.

Figure 3

Zero field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature.

Figure 4

(Color online) Magnetization measured by a SQUID magnetometer up to $B=5\mathrm{T}$ magnetic field at various temperatures. The solid lines correspond to Langevin-fits, the characteristic magnetic moments determined from the fits at various temperatures are shown in the inset.

Figure 5

(Color online) Field dependence of the magnetization obtained from anomalous Hall effect data (red dots) normalized to the SQUID measurements (open black circles). The continuous lines are the same Langevin fits as in Fig. 3. The inset shows the measured Hall-resistance (black dots), and the separation of the linear Lorentz-term (blue curve) and the anomalous Hall signal (red curve).

Figure 6

Magnetic field dependence of the resistivity at various temperatures.

Figure 7

Resistivity vs temperature in zero (a) and $12\mathrm{T}$ magnetic fields (b). The solid lines are the experimental data, dashed lines indicate the magnetic field independent phonon scattering, as calculated from the high temperature slope of $\rho \left(\mathrm{T}\right)$. The dotted curves are the differences of the former two (shifted upwards by the residual resistivity), and are attributed to the temperature-dependent magnetic scattering on small Fe clusters.

Figure 8

(Color online) Temperature dependence of the resistivity contribution attributed to magnetic scattering on tiny Fe clusters in $B=12\mathrm{T}$ magnetic field. Symbols are experimental data after subtracting the phonon contribution as shown in Fig. 7, solid curves are calculated from Eq. (5) with fitting parameter $S=16.6{\mu }_{\mathrm{B}}$.

Figure 9

(Color online) Magnetic field dependence of the resistivity in the superparamagnetic range, at various temperatures. The open symbols are experimental data, the solid line is the fit according to Eq. (7). The dashed and dash-dotted curves correspond to the first and second terms of Eq. (7), respectively. The magnetic moments of the clusters and the grains were taken from independent experiments, see text for details.

Figure 10

(Color online) Magnetoresistance vs the square of the magnetization in the superparamagnetic range (data taken from Figs. 5, 9). Deviation from the simple $\Delta R\left(B\right)\propto M^2$ quadratic behavior signifies the effect of the small clusters, which barely contribute to the magnetization but dominate the scattering at high fields.

Figure 11

Zero field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature for smaller spacer thickness (samples B and C).

Figure 12

Magnetoresistance of samples A–C at room temperature (a) and at $T=4.2\mathrm{K}$ (b).

Figure 13

(Color online) Temperature dependence of the resistivity contribution attributed to magnetic scattering on tiny Fe clusters for samples B and C. Symbols are the high field experimental data after subtracting the phonon contribution, solid curves are calculated from Eq. (5).