Calculating Critical Buckling Load for 3D Rectangular Beam - Ixx or Iyy?

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I should probably know the answer to this but anyway...

I want to compute the critical buckling load for a simple 3D rectangular cross-section beam, though the thickness in the z-direction is small.

If a load is applied in the negative y direction, which direction do I use for my second moment of area value? Ixx or Iyy?

Thanks for any info.

 

[Mapes]

How about trying both, then using the results to decide what buckling plane is most likely?