Latest ATLAS results on $\phi_s$

New Physics effects beyond the predictions of the Standard Model may manifest in the $CP$-violation of $b$-hadron decays. This paper presents the latest analysis of $B^0_s \to J/\psi\phi$ decay at the ATLAS experiment, measuring the $CP$-violating phase $\phi_s$, the decay width $\Gamma_s$ and the difference of widths between the mass eigenstates $\Delta\Gamma_s$. The latest results are using integrated luminosity of 14.3 fb$^{-1}$ collected by the ATLAS detector from $\sqrt{s}$ = 8 TeV $pp$ collisions at the Large Hadron Collider, and are statistically combined with the results from 4.9 fb$^{-1}$ of $\sqrt{s}$ = 7 TeV data, leading to: \begin{eqnarray*} \phi_s & = & -0.090 \pm 0.078 \;\mathrm{(stat.)} \pm 0.041 \;\mathrm{(syst.)~rad} ,\;\;\\ \Delta\Gamma_s & = & 0.085 \pm 0.011 \;\mathrm{(stat.)} \pm 0.007 \;\mathrm{(syst.)~ps}^{-1} ,\;\;\\ \Gamma_s & = & 0.675 \pm 0.003 \;\mathrm{(stat.)} \pm 0.003 \;\mathrm{(syst.)~ps}^{-1}. \end{eqnarray*} The results are also presented in the form of likelihood contours in the $\phi_s-\Delta\Gamma_s$ plane. The measurement includes the transversity amplitudes and corresponding strong phases. All the results are compatible with the Standard Model predictions. The paper is concluded with an overview of the detector performance relevant for this particular analysis in current and future phases of the ATLAS experiment and the Large Hadron Collider.