Please check the height adjustment of your sample with respect to the center axis of your XRD set up. You may suffer from partial volume effect.

Please repeate your measurement at different sample heights and compare your baselines; so you will get a 'feeling' on this topic.

What's about your sample surface; is it flat? or even curved?

For the case of declining (with 2 theta) baseline please have a look at the answers of a very recent RG question attached in the link

https://www.researchgate.net/post/why_xrd_data_of_powder_materials_do_not_have_stable_baseline_throughout_2theta_range10-90_Baseline_shifts_from_top_to_down_as_angle_increases

I believe you aswer is in the title of your project; iron containing samples fluoresce under cu Kalpha radiation.

You can have a look at this discussion thread for more details:

https://www.researchgate.net/post/Why_mainly_Cu_Kalpha_radiation_is_used_in_XRD_why_not_other_elements_and_what_is_the_reason_for_using_Kalpha_radiation_why_not_Kbeta