Ultrafast modulation of the anomalous Hall conductivity by coherent magnetization precession in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>Co</mml:mi> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/M

Ultrafast dynamics of magnetic properties in the high gigahertz (GHz) or even terahertz (THz) frequency range may be exploited in next-generation data storage and processing devices, and therefore suitable methods for detecting ultrafast dynamics of magnetization need to be developed. In this work, we investigate the impact of optically induced ultrafast precession of magnetization on the anomalous Hall conductivity in a ferromagnetic <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>Co</a:mi> <a:mstyle displaystyle="false" scriptlevel="0"> <a:mtext>−</a:mtext> </a:mstyle> <a:mi>Pt</a:mi> </a:math> multilayer structure using THz-based pump-probe measurements. From the measured Faraday rotation, the time-dependent anomalous Hall conductivity <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>σ</e:mi> <e:mrow> <e:mi>x</e:mi> <e:mi>y</e:mi> </e:mrow> </e:msub> </e:math> is extracted, and our data reveals an initial ultrafast quench due to laser-induced demagnetization and a subsequent oscillation with a frequency that increases with the strength of the applied external magnetic field <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msub> <g:mtext mathvariant="bold">H</g:mtext> <g:mi>ext</g:mi> </g:msub> </g:math> . for <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msub> <j:mi>H</j:mi> <j:mi>ext</j:mi> </j:msub> </j:math> = 30 kOe, the oscillation frequency reaches approximately 100 GHz. The magnetic-field dependence of the oscillation frequency is in good agreement with the prediction from the ferromagnetic resonance model, confirming that the observed oscillation is caused by coherent magnetization precession. These findings are considered to be useful for studies that aim to integrate spintronics and THz electronics.