The electric field of a plane electromagnetic wave is given by y = E₀icos(kz)(ωt). The corresponding magnetic field B is then given by:

∇ × E = -∂B/∂t
Given that wave is propagating along positive z-axis and E along positive x-axis. Hence B along y-axis.
From Maxwell equation
∇ × E = -∂B/∂t
i.e., ∂E/∂z = -∂B/∂t
and B₀ = E₀/c
so, B = -E₀c⁻¹jsin(kz)cos(ωt)