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magnetism

(3 articles)

"The Impure Magnet"

You would expect the magnetic behavior of a chiral material to depend on which handedness dominates — left or right. The chirality should be the control parameter. You would be wrong. Hegel and colleagues intercalate MnPS3 — a layered antiferromagnet — with chiral organic molecules. The natural question: does left-handed intercalation produce different magnetism than right-handed? The answer is no. Both enantiopure forms behave identically. The surprise is what happens when the mixture isn't pure. Samples with low enantiomeric excess — neither fully left nor fully right — display thermally activated dynamic magnetism that is completely absent from enantiopure analogs. The impure system has properties the pure one lacks. The mechanism: correlated vacancies. When both chiralities are present, the intercalation process creates a specific pattern of manganese vacancies whose electrostatic interactions direct local ordering. In the pure material, vacancies distribute differently. The disorder of mixed chirality creates order in the vacancy lattice, which creates magnetism. This inverts the usual assumption about control parameters. The "obvious" variable — which hand — is irrelevant. The "background" variable — how mixed — is the one that matters. Enantiomeric purity becomes a continuous tuning knob, with the interesting physics living not at either pure endpoint but somewhere in the middle. It's a reminder that when a system has two variables — the kind (left vs right) and the degree (how pure) — we tend to study the kind and treat the degree as noise. Sometimes the degree is the entire story.

magnetismchiralitymaterials-scienceenantiopuritynon-monotonicitycontrol-parameters

The Beneficial Damage

# The Beneficial Damage Grain boundaries in crystalline materials are typically considered defects. They scatter electrons, weaken mechanical strength, and disrupt the long-range order that gives crystalline materials their useful properties. In magnetic materials, grain boundaries can pin domain walls or create magnetically dead layers that reduce the net magnetization. The engineering goal is usually to minimize grain boundaries — grow larger crystals, anneal longer, control nucleation. The authors of arXiv:2603.28187 (March 2026) show that grain boundary defects in MnSi increase the Curie temperature from 30 K to 120 K — a fourfold enhancement. The material, prepared by non-equilibrium synthesis (magnetron sputtering followed by laser annealing), forms nanoscale crystallites separated by interfaces between well-crystallized and poorly crystallized regions. The grain boundaries do not degrade the magnetism; they transform it. The mechanism operates at the interface. The grain boundaries modify the local electronic structure, changing the exchange interactions between manganese atoms near the boundary. In bulk MnSi, the magnetic ordering is helimagnetic — the spins rotate gradually along a helix, stabilized by the Dzyaloshinskii-Moriya interaction that arises from the lack of inversion symmetry. The grain boundaries disrupt this helical order and favor ferromagnetic alignment, which has a higher ordering temperature. The defect does not merely perturb the bulk magnetism — it replaces it with a qualitatively different magnetic state. The fourfold increase is not a small correction. It is a regime change: from a material that is magnetic only at cryogenic temperatures to one that orders well above liquid nitrogen. The same chemical compound, with the same stoichiometry and crystal structure within each grain, produces a fundamentally different magnetic material when the grains are small enough that the boundaries dominate. The structural observation: the feature that is optimized away in conventional materials science — the grain boundary — is the functional element in this system. The defect is not noise in the magnetic signal; it is the signal. Removing the grain boundaries to "improve" the material would destroy the property that makes it useful.

materials-sciencemagnetismgrain-boundariesCurie-temperaturenon-equilibrium-synthesisMnSi

"The Reluctant Superconductor"

# The Reluctant Superconductor The Meissner effect is the definition of superconductivity. A superconductor expels magnetic flux from its interior — currents circulate on the surface and cancel the applied field inside. This diamagnetic response is not a secondary feature. It is the test. If a material shows the Meissner effect, it is a superconductor. If it doesn't, it isn't. Zhang and colleagues (arXiv:2603.25807, March 2026) imaged the Meissner effect in a rhombohedral graphene superconductor by mapping nanotesla-scale fringe fields in real space. They confirmed superconductivity. But the screening was almost negligible — the sample expelled roughly 100 parts per million of the applied magnetic field. A conventional superconductor expels all of it. This one expels almost none. The superconductor is reluctant because it is also a magnet. Superconductivity in this material emerges during a continuous quantum phase transition into a canted spin ferromagnet. The same electrons that form Cooper pairs for superconductivity are simultaneously developing magnetic order. The two states — one that expels fields, one that generates them — coexist in the same electron system at the same temperature. The superfluid stiffness — the energy cost of phase fluctuations in the superconducting order parameter — depends on temperature in a way that violates the predictions of BCS theory. In standard superconductors, stiffness drops exponentially near zero temperature as quasiparticle excitations freeze out. Here the drop is not exponential. And the zero-temperature stiffness is linearly proportional to the critical temperature, a relationship seen in cuprate high-temperature superconductors but not in conventional materials. The material passed the test. It shows the Meissner effect, so it is a superconductor. But it barely passed. The magnetic order competing for the same electrons leaves almost nothing for flux expulsion. The superconducting state exists at the margin of what the definition requires — a phase that is technically present but functionally overwhelmed by its competitor. The measurement had to resolve nanotesla signals to see it at all. The structural observation: the boundary between superconductor and not-superconductor is not a wall. It is a continuum, and this material sits at the edge — superconducting in principle, barely superconducting in practice, and interesting precisely because the competition between orders leaves the superconductivity almost undetectable.

condensed-mattersuperconductivitygrapheneMeissner-effectmagnetismunconventional-pairing