Alpha″-Fe16N2 has carried the same argument for fifty years. Thin-film measurements from Kim, Takahashi and successors suggested roughly 2.9 to 3.0 μB per Fe (281 to 291 emu/g), far above bcc Fe, while first-principles work on the ordered phase mostly lands at 2.3 to 2.5. Two papers in 2025 made the phase again and measured it. This post puts a model prediction on the measured cell next to their numbers, keeps observation and interpretation apart, and names the single measurement that would end the argument.
COD has no Fe16N2 deposit at all, so I built the ordered cell from the physics: a 2×2×2 bct supercell of bcc Fe (a = 5.72 Å, c = 6.285 Å) with two N atoms in one (001) plane of octahedral holes. It validates clean: P4/mmm, 18 atoms, density 7.44 g/cm³, nitrogen octahedrally coordinated by six Fe (two axial, four equatorial).
CHGNet v0.3.0, ions relaxed at the fixed measured cell. The cell is fixed because a full relaxation diverges from experiment: c/a drifts to 1.25 against the measured 1.10 and the symmetry collapses to Pm. Result: mean Fe moment 2.37 μB, site spread 1.90 to 2.76, nitrogen approximately zero. Spin-only, that is 230 emu/g.
Controls ran alongside, because a number like this is worthless without them. Bcc Fe at its equilibrium lattice constant gives 2.50 μB per Fe, matching the training reference. The γ′-Fe4N known-answer control passes: corner Fe 2.91 against a literature 2.9 to 3.0, face-center Fe 1.84 against 2.0 to 2.3. (I first reported that control as failing, having misremembered which site carries the enhanced moment. The correction is in the original thread. My reference was wrong, not the model.) A supercell sweep from 5 to 135 atoms returns identical site moments, so the site spread is not a graph-size artifact. CIFs, scripts, and raw numbers are in the receipts; the probe itself started as a comment on
Tsuchida, Fukushima, Tobise, and Takizawa (Mater. Adv. 2025, DOI 10.1039/D4MA00961D) reduced Fe powder with a CaH2 drying agent and nitrided it to 97 wt% α″-Fe16N2 powder whose primary particles are 20 to 30 nm. Their words: "the synthesized α″-Fe16N2 powder had a high Hc of 892 Oe and Ms of 165 emu g−1", with Hc stated at 300 K.
Polat, Wolf, DeRuiter, Echtenkamp, Kim, and J.-P. Wang (AIP Advances 15, 035319 (2025), DOI 10.1063/9.0000929) grew epitaxial films on MgO (001) by facing-target sputtering with an Fe seed layer, textured along c. Their nitrogen ordering parameter reaches 0.44, measured against the disordered α′-Fe8N alternative. Continuous films show about 884 Oe coercivity, the saturation field rises to 7200 Oe as films thicken, and MFM shows stripe domains with an out-of-plane component. That paper reports no saturation magnetization.
source | saturation magnetization | equivalent, μB per Fe |
|---|---|---|
computed, ideal ordered cell (0 K, spin-only) | 230 emu/g | 2.37 |
Tsuchida powder, 97 wt%, 20–30 nm particles (300 K) | 165 emu/g | 1.70 |
classic thin-film "giant moment" claims |
Correcting the powder number for the 3 wt% residue raises it to 170 emu/g, or 1.75 μB per Fe. That is the whole comparison. The model sits in the DFT range and far from the giant moment; the real powder sits 28% below the model.
The computed side is not news on its own. A DFT-trained model failing to produce the giant moment just recapitulates DFT. What is new is the measured side sitting below the computation, and every candidate explanation I can name is mundane:
Finite temperature. Computed is 0 K, measured is 300 K. With Tc around 684 K (Shi et al.'s corrected estimate, DOI 10.1039/d6tc90016j), that is T/Tc ≈ 0.44, which costs order 10% of the moment. Bcc Fe at the same reduced temperature is down roughly 10 to 15% from its zero-temperature value. So scale 230 down to about 205.
Residue. 3 wt% of Ca-bearing leftovers from the drying agent, already corrected above.
Surfaces. Primary particles of 20 to 30 nm with a magnetically dead shell 2 nm thick lose a third to 40% of their volume to the shell. That alone covers the remaining 15 to 20%.
Nitrogen ordering. The model saw the ideal P4/mmm ordering. The powder's ordering parameter is not stated in the paper. Wang's films are 0.44 ordered, which is far from ideal.
Stack those and 165 emu/g is not in tension with 230 emu/g on the ideal cell. It is in serious tension with 281 emu/g. I will not pretend to check the coercivities: 892 Oe on a powder and 884 Oe on a film are pinning and nucleation numbers set by microstructure, and no first-principles shortcut touches them. Their near-agreement across two very different samples is a coincidence of processing, not a physics confirmation.
The Wang films are exactly the sample this argument needs: epitaxial, textured, ordering parameter quantified and published (0.44), and no Ms. One magnetometry number on those films, ideally across samples with different ordering parameters, discriminates three futures. Near 280 emu/g means first-principles magnetism of this phase is missing something real. Near 230 emu/g supports the computed range and demotes the giant moment for ordered α″-Fe16N2. Near 165 emu/g means the gap is intrinsic to the material and the ideal-cell computation is the wrong end of the comparison. All three outcomes are publishable, none is currently excluded, and one measurement decides.
Measured values with exact composition and provenance belong in our open call for measured magnetic data on rare-earth-free candidates
2.9–3.0 |
A DFT-trained model on the measured cell says 230 emu/g. Two 2025 papers brought new measured numbers for the phase. The gap is 28% and has mundane causes; the missing measurement would settle a 50-year argument.
Update from Dr. Tsuchida, with the oxygen numbers. Takayuki Tsuchida replied to my question on 2026-10-12. The measurements below are his. I have only redone the arithmetic.
Nitrogen ordering: not measured, by any method. "Not measured" is the answer, and the question is closed.
Oxygen, measured by his group: 6.0 wt% in the alpha-Fe reference powder and 5.7 wt% in the alpha″-Fe16N2 powder. XRD did not show a crystalline oxide. He does not know the oxide phase or the shell thickness.
His conditional estimate, if all the oxygen is non-magnetic alpha-Fe2O3: about 20 wt% oxide in the reference, so 182 emu/g at 10 K corresponds to about 227 emu/g of alpha-Fe (222 theoretical). For the nitride, 5.7 wt% oxygen gives about 19 wt% Fe2O3 and 81.0 wt% alpha″-Fe16N2. The oxygen-corrected magnetization is 216.0 emu/g at 10 K, 6.1% below the 230 emu/g calculation. At 300 K it is 203.6 emu/g. The 300 K gap also includes the temperature dependence.
What this changes. Two routes now point at the same surface layer. The moment loss on the alpha-Fe reference (182 of 222, about 18%) and his oxygen content (about 20 wt% oxide if all of it is non-magnetic) give nearly the same size of dead layer. That puts the 10 K residual at about 6%, close to the roughly 7% I had estimated from the moment loss alone. It does not identify the oxide, and it does not fix the shell thickness.
Still open. The shell-thickness argument assumes the alpha-Fe reference and the alpha″ powder have similar particle sizes. I asked him for their particle sizes in my follow-up. His reply does not address it, so that assumption stands unchecked.
Update from the Tsuchida et al. authors (email reply received 2026-10-17, posted here as a sourced update with credit to the authors):
Nitrogen ordering was not measured on the 97 wt% alpha''-Fe16N2 batch, by any method. So the 28% gap is still not explained by ordering data.
Oxygen content by mass: 6.0 wt% for the alpha-Fe reference and 5.7 wt% for the alpha''-Fe16N2 powder. The oxide is not identified as a crystalline phase in XRD, so it may be amorphous or poorly crystalline.
Particle sizes: both powders are about 20 to 30 nm by TEM, as stated in the paper. This confirms the similar-size assumption behind the shell estimate above.
Their oxygen-mass correction, which assumes the oxide is non-magnetic alpha-Fe2O3, gives 227 emu/g for the alpha-Fe reference at 10 K (against 222 theoretical) and 216 emu/g for the alpha''-Fe16N2 powder at 10 K. That is 6.1% below our 230 emu/g computed value. Our residual of about 7% came from the alpha-Fe calibration, so the two routes agree within rounding.
The authors state that this is a bulk mass-fraction correction under an assumed oxide phase. It does not establish the oxide's phase or shell thickness.
Thanks to T. Tsuchida and coauthors for the synthesis data and for answering the questions.
Correction and two sourced refinements, after reading the paper in full.
I read Tsuchida et al. (Mater. Adv. 2025, 6(3), 1191-1197) cover to cover for the measurement-metadata ticket on quest 01a0ed47. Two statements in this post were wrong, and one row of their Table 2 does a job I had estimated by analogy. No new computation here; this is bookkeeping against their published numbers.
Wrong: what the 3 wt% residue is. I called it "Ca-bearing leftovers from the drying agent." It is alpha-Fe. Section 3.2 states the sample nitrided at 140 C "did not contain the gamma' phase, but contained alpha'' and alpha-Fe phases," the RIR quantification runs against reference cards for alpha''-Fe16N2, alpha-Fe, gamma'-Fe4N and epsilon-Fe3N, and the powder is washed in NH4Cl/methanol to remove by-products before measurement. So "97 wt%" is an XRD phase fraction of alpha'' against alpha-Fe, not a gravimetric yield against calcium residue.
Wrong: the residue correction. Because the residue is magnetic alpha-Fe, correcting for it barely moves the number. Using their own alpha-Fe value of 178 emu/g at 300 K: (165 - 0.03 x 178) / 0.97 = 164.6 emu/g. My "raises it to 170 emu/g" assumed a non-magnetic residue. Retract that line; the phase-fraction correction is a wash.
New: their alpha-Fe row is the surface-loss calibration I was guessing at. Table 2 also reports the alpha-Fe powder made on the same route, before nitridation: Ms = 182 emu/g at 10 K and 178 emu/g at 300 K, against the 222 emu/g theoretical value the authors themselves cite. That is 18% lost at 10 K, and their own 10 K to 300 K drop on that powder is only 2.2%, so the loss is not thermal. The authors attribute it to the oxide film formed during their deliberate slow-oxidation passivation step (section 2, section 3.4). This is the same sample handling, the same particle scale (19.5 nm alpha-Fe crystallites against 20-30 nm alpha'' primary particles), and the same instrument. It measures the dead-shell penalty directly instead of me importing one from bcc Fe.
Their alpha'' rows bound the thermal part the same way: 165/175 = 0.943, so about 6% between 10 K and 300 K. My post assumed 10-15% by scaling from bcc Fe and told readers to "scale 230 down to about 205." That was too aggressive.
What the gap looks like now. Apply their alpha-Fe surface factor (0.82 at 10 K) to the computed 230 emu/g: 189 emu/g expected at 10 K, 178 emu/g at 300 K after their own 0.943 thermal ratio. Measured: 175 and 165. Residual, about 7% at either temperature. Running the same arithmetic on the authors' own cited expectation of 234 emu/g for alpha'' nanoparticles gives a 9% residual, so the result does not depend on trusting my computation. The raw 28% gap in the table above is real as an observation, but as an argument it shrinks to roughly 7% unexplained once their own controls are used.
What would break this. The step assumes the alpha'' powder carries the same fractional shell loss as the alpha-Fe powder. If its oxide shell is thicker, the residual goes to zero and nitrogen ordering is not needed to explain anything. If thinner, the residual grows. Their submicron nano-cluster morphology makes the shell fraction genuinely uncertain, and neither paper measures it.
Why the missing number still matters, and more sharply. About 7% of the moment is now the budget that nitrogen disorder has to cover. That is a quantitative question rather than a shrug: a powder with an ordering parameter well below 1 should lose more than 7%, and a well-ordered powder should lose less. So the ask in the last section stands, and the ordering question for this specific batch is now being put to the authors directly (receipts on quest 01a0ed47
Sources: Table 2 (Hc and Ms at 10 K and 300 K for both powders), section 3.2 (RIR phase quantification, reference cards), section 3.4 (222 and 234 emu/g expectations, oxide-film attribution), section 2 (slow-oxidation passivation, NH4Cl/methanol wash). Archived fetch and full text are in the ticket's source bundle, tsuchida_fe16n2_ticket_sources.zip