It depends on the geometry of your material. If you have rod shaped Ni and the field is alon the long axis of the rod, then a small field should do. The same is true for a thin film of Nickel and an in plane oriented magnetic field.

Otherwise you will have to consider the (magnetostatic) effects of shape anisotropy and domain formation. Pls read about these in magnetism text books (e.g. Coey or O'Handley)

Apart from the important remarks of Kai, It is also necessary consider that the applied magnetic field have to be larger than the coercive field and the anisotropy field, Hk. In particular Hk is around 200 Oe ( equivalent to 20 mT), but the geometry of the sample and grains could play a major role.

I would say that it is very possible, but the precise value strongly depends on the size & geometry of the sample(s). For instance, in nanomagnetic samples, magnetostatic effects (which comes about from dipole-dipole interaction) are quite pronounceable and gives rise to the so-called shape anisotropy, as already mentioned by other. Besides geometry itself, another important point concerns when the sample supports topologically stable patterns, like vortices, solitons and so forth. Just to mention, a thin cylindrical sample (around few hundreds of nanometers in diameter; so-called nanodisc) comprising a vortex configuration tends to be strongly stable against fields applied perpendicular to the disc face. Indeed, a field at least around 0.5 T is necessary to switch vortex polarization in this situation, whereas if the field is (alternate) and applied parallel to the disc face only few hundreds of mT switch vortex polarization. This simple case exemplifies what I said in the very beginning.. in agreement with some other chaps who have written here.

As previous answerers already mentioned it depends on your sample geometry (cube, thin film, long rod, nanostructures etc.) and the field direction (easy axis, hard axis) - see magnetic anisotropy. You can have a look to the literature for instance recommended by Kai Fauth.

But for now some other example for a thin continuous Ni film. As far as the thickness is not too small (below around 1nm) the easy axis will be in-plane and with an in-plane applied field you can easily switch the magnetization (and therewith saturate the sample) with fields of around 1 mT. If you have higher roughness the coercivity might increase a bit due to pinning of domain walls at grain boundaries. Anyway, if you wanna saturate this sample with an out-of-plane applied field you will need way higher fields (0.5T, J.A. Blanco where do you have these 200Oe from? I have measured thin Ni films (below 5 nm) where I need at least 3kOe to saturate in the hard axis direction - that is the so called anisotropy field H_A or H_K...) to saturate the sample. But if you switch of the field, the magnetization of your thin Ni film will switch back to the easy axis direction (in-plane in this case) and your sample might end up in a kind of demagnetized state.

However, for the other ferromagnetic materials (Co, Fe) the behavior will be similar, with the difference that H_A is higher (have a look on saturation magnetization for different materials).

The original question is ambiguous on several score. It does not state size,shape, surrounding conditions etc. Apart from the cases considered in above answers there is additional one. If the particles are single or subdomain and held in a passive matrix e.g. polymer or liquid and if they are blocked then temperature may affect saturation field.

It is possible to use longitudinal magnetic field.