The electric field component of a monochromatic radiation is given by $\overrightarrow{E}=2E_0\cos kz\cos \omega t \widehat{\mathrm{i}}$, Its magnetic field $\overrightarrow{B}$ is then given by:

$\frac{dE}{dz}=-\frac{dB}{dt}$

$\frac{dE}{dz}=-2E_{0}K\sin kz\cos \omega t=-\frac{dB}{dt}$

$dB=+2E_{0}K\sin kz\cos \omega t dt$

$B= +2E_{0}K\sin kz\int \cos \omega t dt$

$=+2E_0\frac{k}{\omega }\sin kz\sin \omega t$

$\frac{E_0}{B_0}=\frac{\omega }{k}=C$

$B=\frac{2E_0}{C}\sin kz\sin \omega t$

Propagation of EM radiation, Electric field oscillation and Magnetic field oscillation are perpendicular to each other.

Since propagation of EM radiation is $x-$axis and electric field oscillation is along $z-$axis, the Magnetic field oscillation is along $y-$axis.