Ahmad,  We will need the material system from which your magnetic nanoparticles are mode.  We might also need the general size (range) of the particles and the family of processing used to make them. 

What makes you think that the magnetic nanoparticles should be fully oxidized?  (That is, contain zero oxygen vacancies?)

Lattice defects or vacancies are known to induce due to exchange of cations in octahedral or tetrahedral sites in some magnetic oxides i.e spinel type ferrites. According to the occupation of divalent transition metal ions in these sites, results inverse of normal spinel structures. At this point, relative positions of the tetrahedral and octahedral site atoms give rise to superexchange mechanism. And this is closely related to cation, anion, bond angle and interatomic distances. Oxygen atom in the structure determines the hopping mechanism that contributes an exchange of an e- to a half-filled shell. This superexchange with nearest neighbor transition metal results either a contribution to net magnetism or it helps forming of an antiferromagnetic couplic according to the unpaired spin of oxygen (in accordance with Pauli exclusion and Hund's rule).

Attached article should be helpful

You may be helped by reading this article about ZnFe2O4 nanoparticles:

https://www.researchgate.net/file.PostFileLoader.html?id=555ecea160614bfd238b4658&key=045ce2ed-ca8f-4b08-8c63-f250df3cb2cc

When the number of oxygen atoms expected in a compound is less (or missing) than what it should be in its perfect crystal lattice, it is known as oxygen vacancy.

How: When one makes a compound say ZnFe2O4, one needs to choose ZnO and Fe2O3 as starting materials. If one looks at the addition of their atomic ratios, it gives exactly (luckily) what one wants. Be careful, this is nominal, not exact. Even if you consider that the starting materials are perfect, (Which in general may not be possible practically), the final product may not be necessarily exactly what is written here. One can assume that it is just a writing convenience, not the exact, since they (starting materials) may not necessarily make alliances (bonding) to form a perfect lattice, which does not seemingly exist in reality. That is how, one will get impurities in a compound one makes or purity of a compound/ element is generally provided when sold by a standard company, in general.

Coming back to the point, if one wants to remove oxygen from a compound of oxygen, it requires annealing in reducing atmosphere (N2, Ar, etc). Conversely, if one wants to add more, extra oxygenation is normally performed by annealing in oxidizing atmosphere (O2).  

Now, by simple arithmetic, addition of one oxygen atom from its parent compound leads to removal of two electrons from it (hole doping), while removal of one of it leads to addition of two electrons (electron-doping), since an oxygen atom needs two electrons to make an octet. As a result, the final compound's structure or property is decided by its overall charge/ spin state prevailed and hence decides its transport, magnetic or any other properties.

This is expected to be true either in bulk or nanomaterials. However, in nanoparticles, since they have new/ active electronic states/ environment, they may exhibit many more so-called anomalous properties.

oxidation state plays an important role in magnetic coericivity and magnetization increasing oxidation state increase the coericivity this make change in magnetic nature 

the vacancies take place because the exchange of cations in crystal structure sites and affected by bond angle, interatomic distances, cation and anion .This exchange effect on contribution of magnetism

The best answer is given by prof Murat Sertkol