The team, led by Reena Sharma and Satish Khasa of Deenbandhu Chhotu Ram University of Science and Technology together with colleagues at Parul University and Ramdeobaba University, synthesized Nd2CrMnO6 using a modified sol-gel Pechini method, a wet-chemistry route that allows fine control over particle formation and produces nanoscale crystallites. Rather than yielding a perfectly ordered double perovskite, the synthesis produced a material with a strikingly mixed structure. Rietveld refinement of the powder X-ray diffraction data revealed that the sample is not a single phase at all: roughly 63 percent of it adopts the monoclinic P21/n symmetry typically expected for an ordered double perovskite, while the remaining 37 percent crystallizes in the orthorhombic Pbnm structure, a signature of significant B-site disorder.

That structural duality matters because double perovskites of the formula A2BB’O6 are designed to have two different transition-metal cations—here chromium and manganese—occupying alternating sites in a rock-salt-like ordered pattern. When that ordering breaks down, antisite defects appear: chromium ions sitting where manganese should be and vice versa. These defects change the network of superexchange interactions, the quantum-mechanical coupling mechanisms through which magnetic moments communicate via intervening oxygen ions. In an ordered lattice, the dominant Cr3+-O-Mn3+ superexchange is ferromagnetic, following the Goodenough-Kanamori-Blume rules articulated by Blasse in the 1960s. But antisite disorder creates neighboring Cr3+-O-Cr3+ and Mn3+-O-Mn3+ pathways, which favor antiferromagnetic alignment, seeding competing interactions throughout the material.

The nanoscale character of the powder added another layer of complexity. A size-strain plot analysis of the diffraction profiles gave an average crystallite size of about 22 nanometers, accompanied by a lattice strain of roughly 10.14 × 10−3. Strain at this level, arising from the coexistence of two structural phases and the finite size of the crystallites, can locally distort the Cr-O-Mn bond angles and further modulate the exchange interactions. Raman spectroscopy supported the structural picture, showing two characteristic modes consistent with the P21/n monoclinic symmetry, confirming that at least a substantial fraction of the sample retains the rock-salt cation ordering that gives double perovskites their distinctive vibrational fingerprint.

Magnetic measurements, performed with a physical property measurement system, revealed the consequences of this delicate balance. As the sample was cooled in a small field of 100 oersted, it underwent a transition from a paramagnetic state to a long-range ferromagnetic phase at a Curie temperature of about 98 kelvin. That transition reflects the establishment of the ferromagnetic Cr3+-Mn3+ superexchange network across the bulk of the material. But the story did not end there. Below a compensation temperature of roughly 29 kelvin, the net magnetization of the sample flipped sign, becoming negative with respect to the applied field—a phenomenon known as negative magnetization that is as counterintuitive as it is visually dramatic in the measurement data.

Negative magnetization arises when two magnetic sublattices with different temperature dependences oppose each other and the weaker one temporarily wins. In Nd2CrMnO6, the researchers attribute the effect to the interplay of antisite disorder-induced antiferromagnetic interactions and strong 3d-4f coupling between the transition-metal sublattice of chromium and manganese ions and the rare-earth sublattice of neodymium ions. The magnetic moments of the Nd3+ ions, carried by well-shielded 4f electrons, align antiparallel to the net 3d moment through this coupling. Because the 4f moments follow their own temperature dependence, governed by crystal-field splitting and the weakening of thermal fluctuations, there comes a point during cooling at which the antiparallel rare-earth contribution outweighs the ferromagnetic 3d background, and the measured magnetization crosses zero and turns negative.

The field dependence of this behavior underscores how fragile the balance is. When the measurement field was raised to 500 oersted, the Curie temperature shifted slightly upward to 102 kelvin, and the compensation temperature dropped dramatically to about 10 kelvin. A modest external field partially aligns the competing moments and suppresses the sign reversal, pushing the compensation point to lower temperatures. This sensitivity to field is a hallmark of systems in which ferromagnetic and antiferromagnetic contributions are nearly matched in strength, and it provides experimentalists with a knob for tuning the crossover between conventional and inverted magnetic responses.

Equally intriguing is the appearance of a Griffiths phase in the material. Above the ferromagnetic transition, in the paramagnetic regime, the sample does not behave as a simple collection of independent moments. Instead, short-range ferromagnetic clusters persist within the antiferromagnetic-influenced background, producing the non-analytic magnetic behavior first predicted by Robert Griffiths in 1969 for dilute magnets. The team observed a stronger Griffiths-like phase at 100 oersted, which weakened at 500 oersted as the field aligned the clusters. Similar Griffiths phenomenology has been reported in related disordered perovskites such as La2CrMnO6, Gd2CrMnO6, and Y2CrMnO6, suggesting that antisite disorder is a general recipe for creating these exotic cluster states in the chromium-manganese double perovskite family.

Quantitatively, the magnetization reached a maximum of 39.25 emu per gram, corresponding to about 3.45 Bohr magnetons per formula unit, at 1.8 kelvin, and declined gradually to 8.59 emu per gram, or roughly 0.76 Bohr magnetons per formula unit, at 125 kelvin. Arrott plot analysis at every measurement temperature indicated that the ferromagnetic transition is second order, meaning the magnetization grows continuously rather than jumping discontinuously—a classification with practical consequences, because second-order transitions are associated with minimal hysteresis losses, an essential property for refrigerant materials that must be cycled repeatedly.

That practical angle brings the magnetocaloric effect into focus. When a magnetic material is magnetized and demagnetized, its entropy changes, producing heating and cooling that can be harnessed for refrigeration. In Nd2CrMnO6, the low-field magnetocaloric response showed a conventional peak near the ferromagnetic transition, with a maximum change in magnetic entropy of about 0.88 × 10−2 joules per kilogram-kelvin at 400 oersted. More remarkably, an inverse magnetocaloric effect appeared in the low-temperature region below the compensation temperature, where the sign reversal of the magnetization inverts the entropy change. While the magnitudes are modest at these small fields, the coexistence of conventional and inverse effects in a single material, switchable by temperature and field, illustrates the rich functional landscape that antisite disorder can unlock. For engineers pursuing cryogenic magnetic cooling and for physicists probing frustrated magnetism, Nd2CrMnO6 now stands as a compelling example of how imperfection, carefully understood, becomes a design tool rather than a flaw.

Subject of Research: Antisite disorder, negative magnetization, and competing exchange interactions in the Nd2CrMnO6 double perovskite

Article Title: Negative magnetization and competing magnetic exchange interactions driven by B/Bʹ-site antisite disorder in Nd2CrMnO6 double perovskite: probing structural and magnetic properties

Article References: Sharma, R., Hooda, A., Hooda, N., Hooda, A., & Khasa, S. (2026). Negative magnetization and competing magnetic exchange interactions driven by B/Bʹ-site antisite disorder in Nd2CrMnO6 double perovskite: probing structural and magnetic properties. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13852-z

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13852-z

Keywords: double perovskite, Nd2CrMnO6, antisite disorder, negative magnetization, competing exchange interactions, ferromagnetism, antiferromagnetism, Griffiths phase, magnetocaloric effect, 3d-4f coupling, Rietveld refinement, cryogenic refrigeration