A ferromagnetic oxide featuring copper as the sole transition metal would be very weird. I'm not aware of any examples of the copper block demonstrating ferromagnetism without a more "conventional" ferromagnetic transition metal (from Cr to Ni and cousins).

But ferromagnetism has been demonstrated in black phosphorus:

https://www.sciencedirect.com/science/article/pii/S003810981...

So the most likely explanation for the observation here is that the sample contains black phosphorus. But do all the samples contain black phosphorus?