SPMR measurements depend on the characteristics of the nanoparticle (NP) that is used. The NP must have the property that the bulk material is normally ferromagnetic in the bulk. Magnetite (FeO) is one such example as it is ferromagnetic when below its Curie temperature. However, if the NPs are single domain, and of a size less than ~50 nm, they exhibit paramagnetic properties even below the Curie temperature due to the energy of the NP being dominated by thermal activity rather than magnetic energy. If an external magnetic field is applied, the NPs align with that field and have a magnetic moment now characteristic of ferromagnetic behavior. When this external field is removed, the NPs relax back to their paramagnetic state.
The size of the NP determines the rate of decay of the relaxation process after the extinction of the external magnetization field. The NP decay rate also depends on whether the particle is bound (tethered) to a surface, or is free to rotate. The latter case is dominated by thermal activity, Brownian motion.
For the bound case, the decay rate is given by the Néel equation
Here the value of  is normally taken as &thinsp;≈&thinsp;10&thinsp;s,  is the anisotropy energy density of the magnetic material (1.35&thinsp;×&thinsp;10&thinsp;J/m),  the magnetic core volume,  is Boltzmann’s constant, and  is the absolute temperature.  This exponential relationship between the particle volume and the decay time implies a very strong dependence on the diameter of the NP used in SPMR studies, requiring precise size restrictions on producing these particles.
For magnetite, this requires a particle diameter of ~25 nm. The NP also require high monodispersity around this diameter as NP a few nm below this value will decay too fast and a few nm above will decay too slowly to fit into the time window of the measurement.
The value of the time constant, , depends on the method of fabrication of the NP. Different chemical procedures will produce slightly different values as well as different NP magnetic moments. Equally important characteristics of the NP are monodispersity, single domain character, and crystalline structure.