\begin{tikzpicture}
\begin{semilogxaxis}[
xmin = 10, xmax = 100,
ymin = -7, ymax = -4,
width= 11cm,
height= 9.5cm,
y dir=reverse,
xlabel = {Period (days)},
ylabel = {Absolute Magnitude},
xtick = {10,50,80,90,100},
minor y tick num = 4,
ymajorgrids,
xmajorgrids,
%minor grid style={solid,black!10, line width=0.05pt},
%major grid style={solid,black!40, line width=0.05pt}
xticklabel style={/pgf/number format/fixed},
xticklabel={%
\pgfmathfloatparsenumber{\tick}%
\pgfmathfloatexp{\pgfmathresult}%
\pgfmathprintnumber{\pgfmathresult}%
},
xticklabel style={font=\footnotesize}
]
\addplot [color=blue, mark size=2pt, only marks] coordinates {
(10,-4.09)
(15,-4.59)
(20,-4.94)
(25,-5.22)
(30,-5.45)
(40,-5.80)
(50,-6.08)
(60,-6.30)
(80,-6.65)
(100,-6.93)
};
\end{semilogxaxis}
\end{tikzpicture}