as far as I know, zerovalent iron nanoparticles does not exists in the nature. Since the nature is surrounded by a huge amount of water/oxygen, all zerovalent nanoparticles would get oxidized rapidly. However, to answer your question correctly, sure they are magnetic, why not? (When they exist)
The most common magnetic particles in the nature are the magnetite one, which are called Magnetosome. Organisms (Cells) which create this arrangement of magnetite nanoparticles, use them for the orientation in the magnetic field of the earth (like Birds or Fish).
Not many people know, but humans also have Magnetosome Cells. But due to the evolution the ability to use them, get lost
The following few lines and one latest reference give an insight into its magnetic natue.
Finely divided iron has long been known to be pyrophoric; which being a major reason that Fe NPs have not been more fully studied to date. There extreme reactivity has traditionally made them difficult to study because of their being most suscsptible to oxidation .
Iron however has a great deal to offer at the nanoscale, including very potent magnetic and catalytic properties.
Given the highly reactive nature of zero valent Fe, all materials made
consisted of mixtures of nc-Fe and Fe oxides (FeO, Fe2O3, Fe3O4) in the size range 50-100 nm. But zerovalent has been observed to fall in the range of ferromagnetism[Pls see page 4 of 6 of following reference].
[Rosická and Šembera Nanoscale Res Lett 2011, 6:10]
Further, zero valent iron nanoparticles can achieve unique property of exhibiting superparamagnetism at sizes less than about 10-20 nm in size.
Years ago we have lots of studies on the iron oxide and iron ZV nanoparticles. ZVI NPs showed no magnetic properties and we reflected them in our publications. Moreover, the size of particles were mainly different. We could also synthesize ZVI NPs via in-situ and ex-situ reduction method. You can find more in our articles:
Article Anti-bacterial assay of doped membrane by zero valent Fe nan...
Article Sublayer assisted by hydrophilic and hydrophobic ZnO nanopar...
Article PVDF Membrane assisted by Modified ZnO/ZIF Nanoparticle for ...
Article A support assisted by photocatalytic Fe 3 O 4 /ZnO nanocompo...
Article PVDF membrane assisted by modified hydrophobic ZnO nanoparti...