Magnetic topological insulators provide a unique platform to explore the interplay between magnetism and topology. MnBi2Te4, known for its A-type antiferromagnetic (AFM) ground state, undergoes a striking transformation when single crystals are grown in an applied magnetic field. Despite retaining the same crystal structure, field-grown MnBi2Te4 exhibits a ferromagnetic (FM) ground state with a Curie temperature of ~12.5 K, confirmed by magnetization, magnetic torque, electrical resistivity, and specific heat measurements. First-principles calculations support these findings, revealing that magnetic-field-assisted synthesis can effectively reconfigure the ground-state spin order and thereby modify the material’s electronic properties, as reflected in the de Haas-van Alphen oscillation seen in the magnetic torque. Magnetic topological insulators offer a promising avenue to study the interaction between magnetism and topology. Here, the authors demonstrate that MnBi₂Te₄, typically an antiferromagnet, develops a ferromagnetic ground state despite retaining the same crystal structure when synthesized in a magnetic field, highlighting the potential of magnetic-field-assisted synthesis in designing materials with tunable electronic properties.

Hyperfine interactions couple nuclear and electronic degrees of freedom. The present work explores how hyperfine coupling within the Tm ions in TmVO4 single crystals affects an electronic ferroquadrupole ordered ground state and its associated field-tuned quantum phase transition. As temperature is progressively reduced through the hyperfine energy scale, the nuclear moments reduce the critical field for the electronic order, resulting in a dramatic back-bending of the phase boundary delineating the ferroquadrupole order. This behavior is well described by a single-ion semiclassical mean-field model down to approximately 50 mK. Analysis of the effective Hamiltonian leads to a prediction of spontaneous nuclear magnetic order mediated by 4[Formula: see text] electrons, which in principle persists with the application of orthogonal antisymmetric strain, yielding a proposed electro-nuclear tetracritical point.

A design and implementation of "in-cell" magnetic refrigeration to achieve sub-10 mK temperatures T in cryogen-free dilution refrigerators is presented. The ultra-low temperatures below 5 mK are attained in finite magnetic fields B up to 1 T. The holding time below 5 mK varies between about 3 and 30 h, depending on the final magnetic field after the demagnetization process. The developed technique can be used to study low dimensional devices at ultra-low electron temperatures in the high B/T regime.

The intriguing structural and magnetic properties of the low-dimensional, chain-like anion-radical salt (Et-3,5-diMe-Pz)[TCNQ]2, where Et-3,5-diMe-Pz is N-ethyl-3,5-dimethylpyrazinium and TCNQ is 7,7,8,8-tetracyanoquinodimethane, are reported. The nature of the low-temperature transition near 164 K, which may be misidentified as a traditional spin-Peierls event, is clarified with X-ray diffraction, magnetometry, specific heat studies, and numerical calculations. Specifically, this complex is identified as a spin-Peierls-like system due to a subtle intrinsic magnetic dimerisation present at room temperature. The dimerisation of the antiferromagnetic spin chains in (Et-3,5-diMe-Pz)[TCNQ]2 predicted by DFT calculations is much stronger in the low-temperature phase (α = 0.087) than in the high-temperature phase (α = 0.628), as observed in the magnetic properties.

The static magnetic properties of the silica-based aerogels of Cryogel (R) and Pyrogel (R), manufactured by Aspen Aerogels (R), were measured over a range of temperatures (2K <= T <= 400K\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2\,\textrm{K} \le T \le 400\,\textrm{K}$$\end{document}) and in magnetic fields up to 70 kG. These data and a model of the responses are reported, so these properties are familiar to others who may benefit from knowing them before the materials are employed in potential applications.

Samples of Co were grown directly in the ferromagnetic state under equilibrium conditions using a cobalt sulfide flux. Magnetic fields up to 9 T were applied during growth, and isolated Co products exhibit progressively elongated morphologies, from cubes to rectangular rods to needle-like tendrils with poorly-defined facets. The degree of elongation of the major axis was found to correlate with magnetic field direction, strength, and gradient. Two-dimensional X-ray diffraction data indicate some level of polycrystalline-like samples, and quantitative analyses (Le Bail and Rietveld) of the one-dimensional data confirm the presence of hcp and fcc phases. The magnetic responses indicate a partial alignment of the magnetic easy-axis of the hcp phase along the magnetic field present during growth.

The thermomagnetic processing methods require an understanding of the in situ temperatures experienced by the workpieces. When commonly used thermocouples (TCs) are employed in induction furnaces, RF fields can contribute additional temperature uncertainties to the ones arising from the use of strong magnetic fields. Focusing on temperatures between 300 degrees C and 1000 degrees C produced by induction and resistive furnaces, the readings generated by Type-K TCs were contrasted to the ones produced by Type-S and Type-N sensors for magnetic fields up to 9 T. Overall, when comparing Type-K response to temperatures above 700 degrees C in both zero field and high field (<= 9 T), the differences amounted to less than 1% and when continuously measured has a linear relationship to the strength of the applied field. The relative invariance of Type-N and Type-S TCs was confirmed. These findings suggest that the use of TCs in high magnetic fields remains a viable option for applications, although the precision depends on the type used.

Homoleptic complexes [Fe(4bt)3](ClO4)2 (1), [Fe(2bt)3](ClO4)2 (2), and [Fe(3tpH)3](ClO4)2 (3) were obtained by a reaction between the Fe(II) precursor salt and the corresponding thiazole-based bidentate ligand (L = 4bt = 4,4'-bithiazole, 2bt = 2,2'-bithiazole, 3tpH = 3-(thiazol-2-yl)pyrazole). X-ray crystal structure determination revealed crystallization of solvent-free complex 1, a solvate 2·MeOH, and a co-crystal 3·2(3tpH). The crystal packing of all these complexes is dominated by one-dimensional interactions between the [Fe(L)3]2+ cations. These interactions are stronger in 2·MeOH and 3·2(3tpH), leading to cooperative and slightly hysteretic transitions between the high-spin and low-spin electronic configurations at ∼235 K and 159 K, respectively. In contrast, weaker intermolecular interactions in 1 result in a gradual spin crossover above 300 K, with the maximum fraction of the HS state ∼25% achieved at 400 K. Complexes 2 and 3·2(3tpH) exhibit light-induced excited spin state trapping (LIESST) under irradiation with white light or a 532 nm laser at 5 K. After the photoexcitation, the trapped metastable HS state relaxes to the ground LS state with the average relaxation temperature of 81 K and 68 K, respectively. Examination of the relaxation dynamics by optical absorption spectroscopy on a single crystal of 3·2(3tpH) revealed the sigmoidal shape of the relaxation curves at lower temperatures, attributed to cooperative effects, as well as a plateau at ∼10% of the HS fraction at intermediate temperatures, hinting at a more complex mechanism for the relaxation of the LIESST phase in this material.

CrMnFeCoNi, also called the Cantor alloy, is a well-known high-entropy alloy whose magnetic properties have recently become a focus of attention. We present a detailed muon spin relaxation study of the influence of chemical composition and sample processing protocols on the magnetic phase transitions and spin dynamics of several different Cantor alloy samples. Specific samples studied include a pristine equiatomic sample, samples with deficient and excess Mn content, and equiatomic samples magnetized in a field of 9 T or plastically deformed in pressures up to 0.5 GPa. The results confirm the sensitive dependence of the transition temperature on composition and demonstrate that post-synthesis pressure treatments cause the transition to become significantly less homogeneous throughout the sample volume. In addition, we observe critical spin dynamics in the vicinity of the transition in all samples, reminiscent of canonical spin glasses and magnetic materials with ideal continuous phase transitions. Application of an external magnetic field suppresses the critical dynamics in the Mn-deficient sample, while the equiatomic and Mn-rich samples show more robust critical dynamics. The spin-flip thermal activation energy in the paramagnetic phase increases with Mn content, ranging from 3.1(3) x 10-21 J for 0% Mn to 1.2(2) x 10-20 J for 30% Mn content. These results shed light on critical magnetic behavior in environments of extreme chemical disorder and demonstrate the tunability of spin dynamics in the Cantor alloy via chemical composition and sample processing.

A surrounding matrix is known to alter nanoparticle spin transitions, the solid state structural phase changes associated with transitions between high spin (HS) and low spin(LS) states. To better quantify how the spin transition solid and surrounding matrix interact, several series of core-shell particles were prepared based on RbxCo[Fe(CN)6]z & sdot; nH2O, RbCoFe-PBA, as spin transition core with isostructural shells of different compositions and thicknesses. Synchrotron PXRD through the thermal high HS to LS, the LS to photoexcited high spin (PXHS), and thermal PXHS to LS transitions, show the activation energy is lowered as shells become thicker and stiffer. Calorimetry data coupled with transition state theory analysis indicate the core stiffens in the core-shell particles relative to the uncoated particles. The conclusion is supported by microstrain analysis that shows stiffer shells limit the extent to which the core distorts as individual sites transition, leading to the lower activation energy. Finally, differences in lattice mismatch with different shell materials are shown to alter the mechanism by which the transition progresses.

Magnetic properties of more than twenty Cantor alloy samples of varying composition were investigated over a temperature range of 5 K to 300 K and in fields of up to 70 kOe using magnetometry and muon spin relaxation. Two transitions are identified: a spin-glass-like transition that appears between 55 K and 190 K depending on composition, and a ferrimagnetic transition that occurs at approximately 43 K in multiple samples with widely varying compositions. The magnetic signatures at 43 K are remarkably insensitive to chemical composition. A modified Curie-Weiss model was used to fit the susceptibility data and to extract the net effective magnetic moment for each sample. The resulting values for the net effective moment were either diminished with increasing Cr or Mn concentrations or enhanced with decreasing Fe, Co, or Ni concentrations. Beyond a sufficiently large effective moment, the magnetic ground state transitions from ferrimagnetism to ferromagnetism. The effective magnetic moments, together with the corresponding compositions, are used in a global linear regression analysis to extract element-specific effective magnetic moments, which are compared to the values obtained by ab-initio based density functional theory (DFT) calculations. These moments provide the information necessary to controllably tune the magnetic properties of Cantor alloy variants.

Matúš Mihalik, ∗ Marián Mihalik, Andreas Hoser, Daniel M. Pajerowski, † Dominik Kriegner, Dominik Legut, Kristof M. Lebecki, Martin Vavra, Magdalena Fitta, and Mark W. Meisel 9, 10, ‡ Institute of Experimental Physics SAS, Watsonova 47, 040 01 Košice, Slovak Republic Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany Quantum Condensed Matter Division, Neutron Sciences Directorate,

Magnetic, specific heat, and structural properties of the equiatomic Cantor alloy system are reported for temperatures between 5 kelvin and 300 kelvin, and up to fields of 70 kilo-oersted. Magnetization measurements performed on as-cast, annealed, and cold-worked samples reveal a strong processing history dependence and that high-temperature annealing after cold-working does not restore the alloy to a pristine state. Measurements on known precipitates show that the two transitions, detected at 43 kelvin and 85 kelvin, are intrinsic to the Cantor alloy and not the result of an impurity phase. Experimental and ab initio density functional theory (DFT) computational results suggest that these transitions are a weak ferrimagnetic transition and a spin-glass-like transition, respectively, and magnetic and specific heat measurements provide evidence of significant Stoner enhancement and electron-electron interactions within the material.

The Heusler compound ScInAu2 was previously reported to have a superconducting ground state with a critical temperature of 3.0K. Recent high throughput calculations have also predicted that the material harbors a topologically non-trivial band structure similar to that reported for beta-PdBi2. In an effort to explore the interplay between the superconducting and topological properties properties, electrical resistance, magnetization, and xray diffraction measurements were performed on polycrystalline ScInAu2. The data reveal that high-quality polycrystalline samples lack the superconducting transition present samples that have not been annealed. These results indicate the earlier reported superconductivity is non-intrinsic. Several compounds in the Au-In-Sc ternary phase space (ScAu2, ScIn3, and Sc2InAu2) were explored in an attempt to identify the secondary phase responsible for the non-intrinsic superconductivity. The results suggest that elemental In is responsible for the reported superconductivity in ScInAu2.

A mononuclear complex [Fe(tBu2qsal)2] has been obtained by a reaction between an Fe(II) precursor salt and a tridentate ligand 2,4-di(tert-butyl)-6-((quinoline-8-ylimino)methyl)phenol (tBu2qsalH) in the presence of triethylamine. The complex exhibits a hysteretic spin transition at 117 K upon cooling and 129 K upon warming, as well as light-induced excited spin-state trapping at lower temperatures. Although the strongly cooperative spin transition suggests substantial intermolecular interactions, the complex is readily sublimable, as evidenced by the growth of its single crystals by sublimation at 573 → 373 K and ∼10-3 mbar. This seemingly antagonistic behavior is explained by the asymmetric coordination environment, in which the tBu substituents and quinoline moieties appear on opposite sides of the complex. As a result, the structure is partitioned in well-defined layers separated by van der Waals interactions between the tBu groups, while the efficient cooperative interactions within the layer are provided by the quinoline-based moieties. The abrupt spin transition is preserved in a 20 nm thin film prepared by sublimation, as evidenced by abrupt and hysteretic changes in the dielectric properties in the temperature range comparable to the one around which the spin transition is observed for the bulk material. The changes in the dielectric response are in excellent agreement with differences in the dielectric tensor of the low-spin and high-spin crystal structures evaluated by density functional theory calculations. The substantially higher volatility of [Fe(tBu2qsal)2], as compared to a similar complex without tBu substituents, suggests that asymmetric molecular shapes offer an efficient design strategy to achieve sublimable complexes with strongly cooperative spin transitions.

Strain is often used to alter material properties in applications ranging from bandgap tuning for semiconductor electronics to performing work through mechanical actuation. Spin transitions are accompanied by volume changes in the solid state and are being explored in actuation as a source of mechanical strain inducible by the many controllable stimuli known to trigger spin state changes. There is still much to understand, especially at small length scales, about how strain is transmitted from one material to another across a mechanically coupled interface. Theoretical efforts modeling spin-transition particles in a matrix provide significant insights, but this remains an area where systematic experimental studies are limited. This Perspective highlights the progress using cobalt hexacyanoferrate network solids, or Prussian blue analogues (CoFe-PBA), as a framework for investigating spin transition induced strain in nanometer scale and mesoscale heterostructures. Using a family of isostructural cyanometallate networks to form heterostructures with well-defined interfaces, measurements of the altered properties in response to strain generated by the thermally or optically induced spin state change of the CoFe-PBA provide the chance to experimentally interrogate factors that control interface transmitted strain.

Spin-transition heterostructures have shown promise for inducing large switchable stresses at the nanoscale with a volumetric work density similar to piezoelectrics, but before practical applications are feasible, how heterostructure interfaces and geometry influence the transmission of stress and, in return, how they affect the spin-transition actuator itself, must be better understood. Here, four series of cubic spin-transition Prussian blue analogue (PBA) core-shell heterostructures were developed in order to probe the scaling behavior of the strain induced in the shell by the spin transition of the core. Cubic RbxCo[Fe(CN)(6)](y)center dot nH(2)O (RbCoFe-PBA) particles ranging 100-600 nm were used to prepare separate series of RbxCo[Fe(CN)(6)](y)center dot nH(2)O@KjNi[Cr(CN)(6)](k)center dot mH(2)O (RbCoFe@KNiCr-PBA) core-shell particles with magnetic KNiCr-PBA shells ranging from 15 to 130 nm. A model fit to the strain-modified magnetization extracts the "strained volume" of the shell, and the results are compared with structural changes observed with powder X-ray diffraction. A linear relationship is found between the strained volume of the shell and the volume of the core for thicker shells, where the magnetic KNiCr-PBA shell is influenced to depths greater than 100 nm in response to the spin transition of the RbCoFe-PBA core. For thin shells, the relationship is more complicated, as the volume change in the actuating core and the strain it induces in the shell become interdependent and a function of shell thickness.

Core–shell particles with a Prussian blue analogue RbaCob[Fe(CN)6]c·mH2O core and different shell thicknesses are studied as the cores undergo both thermal and light-induced spin transitions, revealing different mechanisms for accommodating strain.