Without knowing anything about the graphics standards you have to work to, and steeped in the study of Tufte, my personal preference would be, FWIW:
Plot[{B1@z, B2@z, B3@z, B4@z, B5@z, B6@z}, {z, -0.3, 0.3},
PlotStyle -> Table[GrayLevel[0.1 + 0.13 j], {j, 1, 6}],
PlotLegends -> {"d = 0.1mm", "d = 0.09mm", "d = 0.08mm",
"d = 0.07mm", "d = 0.06mm", "d = 0.05mm"}
, LabelStyle -> "Background" -> White,
AxesLabel -> {"z(mm)", "Magnetic Field at(Bo=0.01T)"}]
Cheers,
Fred