Very low frequency resistance fluctuations in thin films of ${\text{La}}_{0.67}{\text{Ca}}_{0.33}{\text{MnO}}_3$ with quenched disorder

In this paper we report the appearance of very low frequency $\left(<1\text{mHz}\right)$ resistance fluctuations (noise) in strain relaxed rare-earth perovskite manganite films of ${\text{La}}_{0.67}{\text{Ca}}_{0.33}{\text{MnO}}_3$ with quenched disorder grown on ${\text{SrTiO}}_3$ (STO). The films show high degree of orientation and extremely low $1/f$ noise. The observed spectral power clearly consists of two components: a Lorentzian part (arising from discrete fluctuators) that peaks near the ferromagnetic transition and a broadband $1/f$ part that is essentially temperature independent. The noise power in the films with quenched disorder, interestingly, is orders of magnitude less than that seen in nearly strain free and uniformly strained thin films (thickness $\le 50\text{nm}$) grown on ${\text{NdGaO}}_3$ and STO, respectively. The discrete fluctuators seen in the films with quenched disorder are thermally activated with a high activation energy, $\sim 0.7\text{eV}$. Strain accommodation has been suggested as one of the likely mechanisms that leads to such a high thermal activation energy. The resistance fluctuation can be changed even by a small magnetic field $\left(\le 0.1\text{T}\right)$. The relative variance of the fluctuation has been observed to be proportional to the measured magnetoresistance $\left(dR/dH\right)$. We propose that the fluctuations arise from magnetization fluctuations, which couple to the resistance fluctuations by the magnetoresistance. The magnetization fluctuations arise from magnetic domains as well as from the fluctuating magnetization of the coexisting phases.

Figure 1

(Color online) Topography (t) on LCMO/STO (200 nm) film at 265 K, $\text{t}:\left[\left(\Delta x\mu \text{m}\times \Delta y\mu \text{m}\times \Delta z\text{nm}\right)\right]\left(0.16\mu \text{m}\times 0.16\mu \text{m}\times 0.6\text{nm}\right)$.

Figure 2

(Color online) Temperature variation of $\beta$ for LCMO/STO (50 nm), LCMO/NGO, and LCMO/STO (500 nm) film.

Figure 3

(Color online) The scaled spectral power $S_V\left(f\right)/V^2$ measured at few representative frequencies as a function of $T$ along with the sample resistivity $\rho \left(T\right)$.

Figure 4

(Color online) The spectral power $f{S}_V\left(f\right)/V^2$ measured at few representative temperatures as a function of $f$.

Figure 5

(Color online) The temperature dependence of $B\left(T\right)$ and Lorentzian corner frequency $f_C\left(T\right)$.

Figure 6

The time series data at two representative temperatures. $V_{220\text{K}}$ and $V_{250\text{K}}$ are the sample bias voltages at 220 and 250 K, respectively.

Figure 7

(Color online) $\beta$ as function of magnetic field $H$ at few representative temperatures. The upper panel shows the saturation of the magnetization at 250 K.

Figure 8

(Color online) MR as a function of magnetic field for few representative temperatures. MR as a function of temperature (measured with field 0.1 T) is shown as an inset.

Figure 9

$\sqrt{\frac{⟨{\left(\Delta R\right)}^2⟩}{R^2}}$ as a function of $dR/dH$ at temperatures $T<T_C$.

Figure 10

${⟨\Delta g^2⟩}^{0.5}/g$ calculated from line scans across LCMAPs of LCMO/STO (50 nm), LCMO/STO (200 nm), and LCMO/NGO films as a function of temperature.