= Solution
Only the magnetic-response physics is addressed here; \b[biological and clinical application is user-deferred].
A single-domain particle has an approximately coherent <magnetic dipole moment>, but single-domain does not by itself imply rapid thermal reversal. If moments equilibrate within the measurement time, a dilute, noninteracting ensemble is <superparamagnetic>: its quasistatic <magnetization> is reversible, odd in field and approaches saturation with negligible <remanence> or <magnetic coercivity>.
For freely orientable classical moments,
$$
M(H)=n_p\mu\,\mathcal L\!\left(\frac{\mu_0\mu H}{k_BT}\right),
\qquad
\mathcal L(z)=\coth z-\frac1z.
$$
At small field, $\mathcal L(z)\simeq z/3$; at large field moments align. This Langevin form is an ideal limit, not a universal exact curve for anisotropic fixed-axis particles.
\b[The missing timescale/<magnetic anisotropy> information must be stated:] blocked single-domain particles can already show <hysteresis>. Also, a reversible quasistatic curve does not preclude a finite-frequency loss loop when relaxation lags an alternating field. The original curves illustrate an equilibrated dilute limit and possible blocked/interacting alternatives, not measured treatment performance.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2018/paper-2-magnetism-and-screening.png]
{title=Conditional magnetic loops and electrostatic screening profiles}
{description=Original illustrative magnetic curves and calculated planar mean-field ion profiles. Magnetic-loop broadening is one possible interaction outcome, not a concentration-dependent experimental fit or a clinical prediction.}
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