The paper claims that replacing Co²⁺ with Zn²⁺ increases M_s due to Zn²⁺’s diamagnetic nature and preference for tetrahedral (T_d) sites, reducing the T_d sublattice magnetization (∑μ_T_d) and unbalancing the total moment. However, the reported M_s values for the Zn-series (Zn19, Zn32, Zn48) show a monotonic increase with Zn substitution (Table 2: 104 → 114 → 143 A m² kg⁻¹ at 5 K).
In contrast, the Neel model (eq. 4: μ_tot^Zn = 7 + 3x – 4y) predicts that M_s should peak and then decrease beyond a certain Zn²⁺ threshold (x ~ 0.5) due to the breakdown of superexchange interactions, as Zn²⁺ cannot participate in magnetic coupling. This is supported by literature.
Problem: The experimental data does not show this expected decrease, even at Zn48 (x = 0.48), which is close to the theoretical limit. This inconsistency suggests either:
Overestimation of M_s for Zn48 (143 A m² kg⁻¹), or Incorrect assumptions in the Neel model (e.g., ignoring surface spin canting or Zn²⁺ occupancy in octahedral sites).
The paper cites Mameli et al. (2016), who observed M_s peaking at Zn ~ 0.5 and then decreasing, aligning with theory. The authors’ data, however, shows no such peak, which is physically implausible for high Zn²⁺ content.
The authors use Mössbauer data (for Ni31 and Zn38) to validate the Neel model, but the calculated M_s values for these samples (red dots in Fig. 5b) fall below the experimental trendline. This discrepancy is handwaved as due to “spin canting” or “surface effects,” but no quantitative correction is applied.