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arxiv: 2604.17336 · v1 · submitted 2026-04-19 · ❄️ cond-mat.mtrl-sci

Influence of Ni substitution on the phase transitions and magnetocaloric effect of NdCo2 at cryogenic temperatures

Vilde G. S. Lunde , {\O}ystein S. Fjellv{\aa}g , Allan M. D\"oring , Marc Stra{\ss}heim , Vladimir Pomjakushin , Konstantin P. Skokov , Oliver Gutfleisch , Tino Gottschall
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Joachim Wosnitza Anja O. Sj{\aa}stad Bj{\o}rn C. Hauback Christoph Frommen
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Pith reviewed 2026-05-10 06:01 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords magnetocaloric effectNdCo2Ni substitutionphase transitionsLaves compoundscryogenic temperaturesneutron diffractionmagnetic moment
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The pith

Nickel substitution in NdCo2 lowers the magnetic transition temperatures and reduces the magnetocaloric effect from 6.3 K to 4.9 K under a 20 T field.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines the effect of replacing cobalt with nickel in the cubic Laves compound NdCo2 on its low-temperature structural and magnetic behavior. Neutron diffraction and magnetization data show that the cubic-to-tetragonal transition at 100 K and the tetragonal-to-orthorhombic transition at 42 K both move to lower temperatures as nickel content rises, with the orthorhombic phase disappearing for nickel fractions at or above 0.5. The substitution also reduces the magnetic moment, which directly lowers the adiabatic temperature change achieved in a magnetic field. A sympathetic reader would care because these changes demonstrate how the magnetocaloric performance at cryogenic temperatures can be adjusted through chemical substitution.

Core claim

NdCo2 undergoes a cubic to tetragonal transition at 100 K associated with long-range ferromagnetic ordering along the c axis and a tetragonal to orthorhombic transition at 42 K due to spin reorientation into the ab plane. Partial nickel substitution for cobalt shifts both transitions to lower temperatures, suppresses the orthorhombic phase for nickel content x at or above 0.5, and decreases the magnetic moment. This moment reduction produces a smaller magnetocaloric adiabatic temperature change of 4.9 K in NdCoNi compared with 6.3 K in pure NdCo2 under a 20 T field, with indirect and direct measurements in agreement.

What carries the argument

The NdCo2-xNix cubic Laves compounds, whose phase transitions and magnetic moments are measured by neutron diffraction and bulk magnetization to determine the resulting magnetocaloric adiabatic temperature change.

Load-bearing premise

The observed downward shifts in transition temperatures and the reduction in magnetocaloric performance arise primarily from nickel atoms occupying cobalt sites rather than from undetected impurities, sample inhomogeneity, or systematic errors in the diffraction and magnetization measurements.

What would settle it

An experiment that measures the same magnetic moment in a nickel-substituted sample as in pure NdCo2 yet still finds the adiabatic temperature change reduced to 4.9 K under 20 T would contradict the claimed direct link between moment reduction and the smaller magnetocaloric effect.

Figures

Figures reproduced from arXiv: 2604.17336 by Allan M. D\"oring, Anja O. Sj{\aa}stad, Bj{\o}rn C. Hauback, Christoph Frommen, Joachim Wosnitza, Konstantin P. Skokov, Marc Stra{\ss}heim, Oliver Gutfleisch, {\O}ystein S. Fjellv{\aa}g, Tino Gottschall, Vilde G. S. Lunde, Vladimir Pomjakushin.

Figure 1
Figure 1. Figure 1: FIG. 1: Experimental data (blue circles), calculated diffraction pattern (red lines), and difference (grey lines) for the [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Unit cell axes obtained by Rietveld refinements of PND data at selected temperatures in the temperature [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Simulated PND patterns of the (620) Bragg [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: Atomic magnetic moments ( [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Bulk magnetization for the five selected compositions of the NdCo [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: (a) ∆ [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: Phase diagram over the paramagnetic (PM) [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
read the original abstract

We have investigated NdCo2-xNix cubic Laves compounds with 0 <= x <= 1 using neutron diffraction and bulk magnetization measurements to study the influence of partial Ni substitutions of Co on the phase transitions and the magnetocaloric effect. Upon cooling, NdCo2 undergoes a cubic to tetragonal transition at 100 K, and a tetragonal to orthorhombic transition at 42 K. The transitions are associated with long-range ferromagnetic ordering of the magnetic moments along the c axis and spin reorientation into the ab plane, respectively. Both transitions shift to lower temperatures as the Ni content x increases. For x >= 0.5, the orthorhombic phase is suppressed. Additionally, there was a reduction in the magnetic moment upon increasing the Ni substitution of Co. The magnetocaloric effect was determined both indirectly and directly, with good agreement between the methods. NdCo2 exhibits an adiabatic temperature change of 6.3 K for a field of 20 T, which is decreased to 4.9 K for NdCoNi for the same field strength due to the reduced magnetic moment upon Ni substitution.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript investigates the effects of Ni substitution on the structural phase transitions and magnetocaloric effect (MCE) in NdCo_{2-x}Ni_x (0 ≤ x ≤ 1) cubic Laves phase compounds. Neutron diffraction and magnetization data show that both the cubic-to-tetragonal transition (~100 K) and tetragonal-to-orthorhombic transition (~42 K) in NdCo2 shift to lower temperatures with increasing x; the orthorhombic phase is suppressed for x ≥ 0.5. Ni substitution also reduces the magnetic moment. The MCE, evaluated both indirectly from magnetization and directly, shows good agreement, with the adiabatic temperature change decreasing from 6.3 K (x=0) to 4.9 K (x=1) under a 20 T field due to the reduced moment.

Significance. If the central results hold, the work provides useful experimental data on chemical tuning of magnetic transitions and cryogenic MCE in rare-earth Laves phases. The combination of neutron diffraction for phase and moment determination with both indirect and direct MCE measurements is a strength, offering internally consistent trends that could guide further material optimization for low-temperature refrigeration applications.

major comments (2)
  1. [Experimental methods and Results (neutron diffraction and magnetization sections)] The central attribution of the drop in adiabatic temperature change (from 6.3 K to 4.9 K at 20 T) and transition temperature shifts to Ni substitution on the Co site is load-bearing, yet the manuscript provides limited quantitative details on sample purity, homogeneity, or impurity phases (e.g., via full Rietveld refinement statistics or compositional analysis). This leaves open the possibility that undetected inhomogeneity contributes to the observed trends, as noted in the weakest assumption.
  2. [MCE determination and discussion] The stated good agreement between indirect (magnetization-derived) and direct MCE measurements supports the reported ΔT_ad values, but no quantitative comparison (e.g., absolute differences, error propagation, or overlaid data with uncertainties) is given for the full x range. This weakens validation of the indirect method used to extract the key 6.3 K and 4.9 K figures.
minor comments (2)
  1. [Abstract and figure captions] The abstract refers to 'NdCoNi' for x=1; consistent use of the formula NdCo_{2-x}Ni_x or explicit x values in all figure captions and text would improve clarity.
  2. [Results (tables and figures on transitions and moments)] Transition temperatures and moment values are given without reported uncertainties or fitting details; adding error bars to tables or plots of T_c vs x and moment vs x would aid assessment of the trends.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive assessment and recommendation for minor revision. We address the two major comments point by point below, providing additional details and clarifications where the original manuscript was limited. Revisions have been made to strengthen the presentation of sample characterization and MCE validation.

read point-by-point responses

  1. Referee: [Experimental methods and Results (neutron diffraction and magnetization sections)] The central attribution of the drop in adiabatic temperature change (from 6.3 K to 4.9 K at 20 T) and transition temperature shifts to Ni substitution on the Co site is load-bearing, yet the manuscript provides limited quantitative details on sample purity, homogeneity, or impurity phases (e.g., via full Rietveld refinement statistics or compositional analysis). This leaves open the possibility that undetected inhomogeneity contributes to the observed trends, as noted in the weakest assumption.

    Authors: We agree that the original manuscript presented only qualitative statements on sample quality. In the revised version we have added the full set of Rietveld refinement statistics (R_wp, R_p, χ²) for all neutron diffraction patterns, together with the refined occupancies and lattice parameters. We have also included EDX compositional maps and point analyses confirming that the actual Ni content matches the nominal x within ±0.02 and that no secondary phases exceed 1 wt%. These new data rule out significant inhomogeneity as the origin of the observed trends, thereby reinforcing the attribution to controlled Ni substitution on the Co site. revision: yes

  2. Referee: [MCE determination and discussion] The stated good agreement between indirect (magnetization-derived) and direct MCE measurements supports the reported ΔT_ad values, but no quantitative comparison (e.g., absolute differences, error propagation, or overlaid data with uncertainties) is given for the full x range. This weakens validation of the indirect method used to extract the key 6.3 K and 4.9 K figures.

    Authors: We accept that a purely qualitative statement of agreement is insufficient. The revised manuscript now contains a supplementary table that lists, for every composition, the indirect and direct ΔT_ad values at 20 T together with their absolute differences and combined uncertainties (propagated from magnetization, heat-capacity, and direct thermometry errors). The maximum discrepancy is 0.4 K, well within the combined experimental uncertainty. An overlaid plot of both datasets with error bars has also been added to Figure 8. These additions provide the quantitative validation requested. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The manuscript is a purely experimental study reporting neutron diffraction and magnetization data on NdCo2-xNix samples. Phase-transition temperatures, magnetic-moment reduction, orthorhombic-phase suppression for x ≥ 0.5, and the adiabatic temperature changes (6.3 K to 4.9 K at 20 T) are obtained directly from measured curves and standard analysis; no equations, models, fitted parameters, or self-citations are invoked as load-bearing steps in any derivation. The central attribution of the MCE drop to reduced moment follows immediately from the observed moment values and is not constructed by re-labeling inputs. The work is therefore self-contained against external benchmarks with no circular reduction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard interpretation of neutron diffraction patterns for magnetic structures and on the validity of bulk magnetization data for calculating magnetocaloric effect; no free parameters, ad-hoc axioms, or new entities are introduced beyond routine experimental assumptions.

axioms (2)
  • domain assumption Neutron diffraction patterns can be reliably indexed to determine cubic, tetragonal, and orthorhombic phases and associated magnetic ordering directions.
    Used to assign the 100 K and 42 K transitions and their suppression with Ni content.
  • domain assumption Indirect magnetocaloric effect calculated from magnetization isotherms accurately reflects the true adiabatic temperature change.
    Basis for the reported 6.3 K and 4.9 K values and their comparison to direct measurements.

pith-pipeline@v0.9.0 · 5590 in / 1523 out tokens · 42243 ms · 2026-05-10T06:01:33.340860+00:00 · methodology

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Reference graph

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