Electron Spin Resonance Linewidth Research Articles

Partial Acquisition of Spectral Projections Accelerates Four-dimensional Spectral-spatial EPR Imaging for Mouse Tumor Models: A Feasibility Study.

Our study aimed to accelerate the acquisition of four-dimensional (4D) spectral-spatial electron paramagnetic resonance (EPR) imaging for mouse tumor models. This advancement in EPR imaging should reduce the acquisition time of spectroscopic mapping while reducing quality degradation for mouse tumor models. EPR spectra under magnetic field gradients, called spectral projections, were partially measured. Additional spectral projections were later computationally synthesized from the measured spectral projections. Four-dimensional spectral-spatial images were reconstructed from the post-processed spectral projections using the algebraic reconstruction technique (ART) and assessed in terms of their image qualities. We applied this approach to a sample solution and a mouse Hs766T xenograft model of human-derived pancreatic ductal adenocarcinoma cells to demonstrate the feasibility of our concept. The nitroxyl radical imaging agent 2H,15N-DCP was exogenously infused into the mouse xenograft model. The computation code of 4D spectral-spatial imaging was tested with numerically generated spectral projections. In the linewidth mapping of the sample solution, we achieved a relative standard uncertainty (standard deviation/| mean |) of 0.76 μT/45.38 μT = 0.017 on the peak-to-peak first-derivative EPR linewidth. The qualities of the linewidth maps and the effect of computational synthesis of spectral projections were examined. Finally, we obtained the three-dimensional linewidth map of 2H,15N-DCP in a Hs766T tumor-bearing leg in vivo. We achieved a 46.7% reduction in the acquisition time of 4D spectral-spatial EPR imaging without significantly degrading the image quality. A combination of ART and partial acquisition in three-dimensional raster magnetic field gradient settings in orthogonal coordinates is a novel approach. Our approach to 4D spectral-spatial EPR imaging can be applied to any subject, especially for samples with less variation in one direction.

Hot ion implantation to create dense NV center ensembles in diamond

Creating dense and shallow nitrogen-vacancy (NV) ensembles with good spin properties is a prerequisite for developing diamond-based quantum sensors exhibiting better performance. Ion implantation is a key enabling tool for precisely controlling spatial localization and density of NV color centers in diamond. However, it suffers from a low creation yield, while higher ion fluences significantly damage the crystal lattice. In this work, we realize N2+ ion implantation in the 30–40 keV range at high temperatures. At 800 °C, NV's ensemble photoluminescence emission is three to four times higher than room temperature implanted films, while narrow electron spin resonance linewidths of 1.5 MHz, comparable to well-established implantation techniques, are obtained. In addition, we found that ion fluences above 2 × 1014 ions/cm2 can be used without graphitization of the diamond film, in contrast to room temperature implantation. This study opens promising perspectives in optimizing diamond films with implanted NV ensembles that could be integrated into quantum sensing devices.

Anisotropic microwave response in Cr1∕3TaS2 hosting chiral magnetic soliton

Chromium-intercalated hexagonal Cr1∕3TaS2 possesses exotic properties, such as high-temperature magnetic soliton, crossover of critical behavior, and nontrivial magnetism. In this study, we employ the electron spin resonance (ESR) technique to modulate and study the chiral magnetic soliton lattice (CSL) in Cr1∕3TaS2 using microwave at X-band. The experimental results show that Cr1∕3TaS2 has a particularly strong microwave response when H⊥c but very weak feedback when H//c, revealing an anisotropic microwave response. When H⊥c, the resonance peak at lower fields results from the surface barrier induced by the competition between Dzyaloshinsky–Moriya interaction, Zeeman energy, and exchange energy. The dominant resonance is caused by the ferromagnetic mode with the twisted region as the boundary condition, which transforms into a paramagnetic resonance above the phase transition temperature (TC). Furthermore, an unusual temperature dependence of ESR spectra is revealed. The crystalline electric field (CEF) excitation owing to large strong spin–orbit coupling results in the widening of the ESR linewidth above TC. Furthermore, an abrupt shift in the resonance field Hr2 and ESR linewidth ΔHPP2 is observed around TC, which is attributed to a sudden change in spin-exchange interactions and magnetic anisotropy around TC, as Cr1∕3TaS2 was previously observed for spin interaction transition from strong anisotropic 3D-Ising type to isotropic 3D-Heisenberg magnetic interaction below TC. This work demonstrates that the ESR can detect the CSL state and investigate materials with nontrivial magnetism.

Spin exchange dynamics in 4H SiC monocrystals with different nitrogen donor concentrations

4H silicon carbide (SiC) polytype is preferred over other SiC polytypes for high-power, high-voltage, and high-frequency applications due to its superior electrical, thermal, and structural characteristics. In this manuscript, we provide a comprehensive study of the spin coupling dynamics between conduction electrons and nitrogen (N) donors in monocrystalline 4H SiC with various concentrations of uncompensated N donors from 1017 to 5 × 1019 cm−3 by continuous wave, pulsed electron paramagnetic resonance (EPR), and microwave perturbation techniques at T = 4.2–300 K. At low temperatures, two triplets due to N donors in cubic (Nk) hexagonal (Nh) positions and triplet arisen from spin-interaction between Nh and Nk were observed in 4H SiC having Nd − Na ≈ 1017 cm−3. A single S-line (S = 1/2) dominates the EPR spectra in all investigated 4H SiC monocrystals at high temperatures. It was established that this line occurs due to the exchange coupling of localized electrons (dominate at low temperatures) and non-localized electrons (dominate at high temperatures). The localized electrons were attributed to Nh for Nd − Na ≈ 1017 cm−3 and Nk donors for Nd − Na ≥ 5 × 1018 cm−3. We have concluded that the conduction electrons in 4H SiC monocrystals are characterized by g|| = 2.0053(3) and g⊥ = 2.0011(3) for Nd − Na ≤ 5 × 1018 cm−3 and g|| = 2.0057(3) and g⊥ = 2.0019(3) for Nd – Na ≈ 5 × 1019 cm−3. Using the theoretical fitting of the temperature variation of S-line EPR linewidth in 4H SiC having Nd – Na ≤ 5 × 1018 cm−3, the energy levels of 57–65 meV that correlate with the valley-orbit splitting values for Nk donors in 4H SiC monocrystals were obtained.

Symmetric carbon tetramers forming spin qubits in hexagonal boron nitride

Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy-related defects are at the forefront of high spatial resolution and low-dimensional sensing. On the other hand, it is expected that impurity-related defect structures may give rise to new features that could further advance quantum sensing in low dimensions. Here, we study the symmetric carbon tetramer clusters in hexagonal boron nitride and propose them as spin qubits for sensing. We utilize periodic-DFT and quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect. We show that the nitrogen-centered symmetric carbon tetramer gives rise to spin state-dependent optical signals with strain-sensitive intersystem crossing rates. Furthermore, the weak hyperfine coupling of the defect to their spin environments results in a reduced electron spin resonance linewidth that can enhance sensitivity.

Isotopic Control of the Boron-Vacancy Spin Defect in Hexagonal Boron Nitride.

We report on electron spin resonance (ESR) spectroscopy of boron-vacancy (V_{B}^{-}) centers hosted in isotopically engineered hexagonal boron nitride (hBN) crystals. We first show that isotopic purification of hBN with ^{15}N yields a simplified and well-resolved hyperfine structure of V_{B}^{-} centers, while purification with ^{10}B leads to narrower ESR linewidths. These results establish isotopically purified h^{10}B^{15}N crystals as the optimal host material for future use of V_{B}^{-} spin defects in quantum technologies. Capitalizing on these findings, we then demonstrate optically induced polarization of ^{15}N nuclei in h^{10}B^{15}N, whose mechanism relies on electron-nuclear spin mixing in the V_{B}^{-} ground state. This work opens up new prospects for future developments of spin-based quantum sensors and simulators on a two-dimensional material platform.

Electron spin resonance study of mixed Bi[formula omitted]Se[formula omitted] topological insulator–magnetic insulator nanoparticles

We investigated Bi2Se3 topological insulator nanoparticles mixed with room-temperature ferromagnetic insulator nanoparticles by using electron spin resonance (ESR) of the surface conduction electrons in the Bi2Se3 nanoparticles. The ESR intensity and linewidth as a function of the molar ratio of the magnetic insulator could be well understood in terms of the magnetic proximity effect due to the short-range exchange field of the magnetic insulator nanoparticles along with the long-range magnetic dipole field effect and the surface-to-volume ratio effect.

Influence of iron oxide substitution on the magnetic interactions in sol–gel‐derived 45S5 bioactive glass–ceramics

AbstractMagnetic interactions in sol–gel‐derived bioactive magnetic glass–ceramics (MGCs) with compositions of (45 − x)SiO2·24.5CaO·24.5Na2O·6P2O5 xFe2O3 (2 ≤ x ≥ 15 wt.%) have been investigated using electron paramagnetic resonance (EPR), and temperature‐dependent magnetic susceptibility and magnetization (M–T) techniques. EPR spectra of the MGC samples revealed strong composition dependence in the intensity and linewidth of resonance absorptions at g ≈ 2.0 and g ≈ 4.3. EPR linewidth analysis showed the dominance of dipole–dipole interaction in MGC samples with iron oxide content ≤4 wt.% and a crossover to super‐exchange type interaction in samples with higher iron oxide content. Composition‐dependent magnetic interaction in these MGC could be related to Fe2+ and Fe3+ ion concentrations using high‐temperature magnetic susceptibility studies. Zero‐field cooled and field cooled M–T curves indicate different magnetic behavior for MGC samples with x ≤ 6 and x ≥ 8 wt.% iron oxide. Although the former show weak magnetic behavior, the latter exhibit superparamagnetic behavior which could be correlated with the percentage of magnetic phases present in each sample. These studies reveal composition‐dependent variation in dipolar and super‐exchange type interaction in the samples which could help in assessing these MGC for biomedical applications.

Anomalous spin dynamics of the S=32 kagome ferromagnet Li9Cr3(P2O7)3(PO4)2

We report the dynamical magnetic properties of the $s=\frac{3}{2}$ kagome ferromagnet ${\mathrm{Li}}_{9}{\mathrm{Cr}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{7})}_{3}{({\mathrm{PO}}_{4})}_{2}$ using magnetic susceptibility, specific heat, electron spin resonance (ESR), and nuclear magnetic resonance techniques. The magnetic susceptibility and specific heat show ferromagnetic ordering at ${T}_{\mathrm{C}}=2.7$ K. The in-plane ferromagnetic correlation length ${\ensuremath{\xi}}_{ab}$ deduced from the magnetization curves reveals that ${\ensuremath{\xi}}_{ab}$ increases steeply from 18.2 to 41.3 \AA{} as the temperature is lowered from 50 K to ${T}_{\mathrm{C}}$, indicative of the persistence of two-dimensional magnetic correlations up to temperatures as high as $\ensuremath{\sim}18\phantom{\rule{0.16em}{0ex}}{T}_{\mathrm{C}}$. The ESR linewidth $\mathrm{\ensuremath{\Delta}}{H}_{\mathrm{pp}}$ and magnetic specific heat data indicate a change in spin dynamics at about ${T}^{*}\ensuremath{\approx}4.4{T}_{\mathrm{C}}$. Intriguingly, for temperatures below 50 K, we observe an unexpected suppression of the $^{7}\mathrm{Li}$ spin-lattice relaxation rate $1/{T}_{1}$. The decrease in $1/{T}_{1}$, which occurs well above ${T}_{\mathrm{C}}$, is at odds with the critical slowing down of magnetic correlations typically observed in conventional ferromagnets. This anomalous thermal evolution of spin fluctuations is interpreted in terms of the concerted interplay of flat and Dirac magnon bands inherent in kagome ferromagnets.

Electron spin resonance on a 2D van der Waals CrBr3 uniaxial ferromagnet

We report on the electron spin resonance (ESR) properties of a van der Waals bulk CrBr3 single crystal. Angular- and temperature-dependent studies are performed in the range of the critical temperature for ferromagnetic order. The angular-dependent data enable the determination of a strong uniaxial magnetic anisotropy constant, K, with the easy axis of magnetization being parallel to the crystallographic c axis. The peak-to-peak ESR linewidth below the Curie temperature shows contributions from the term (3 cos2 θ−1)2, thus suggesting the occurrence of long-wavelength (q → 0) modes of spin fluctuations typical for 2D systems. Finally, we analyze the temperature dependence of the double integrated ESR intensity, resonance field, and linewidth for the direction of the magnetic field parallel and perpendicular to the c easy axis. We determine Weiss's constant and the thermal dependence of K and observe the crossover region from linewidth narrowing to broadening approaching the critical temperature. Finally, we discuss our findings in the view of the potential application of CrBr3 in perpendicular magnetic tunneling junctions.

Enhanced quantum sensing with room-temperature solid-state masers.

Quantum sensing with solid-state electron spin systems finds broad applications in diverse areas ranging from material and biomedical sciences to fundamental physics. Exploiting collective behavior of noninteracting spins holds the promise of pushing the detection limit to even lower levels, while to date, those levels are scarcely reached because of the broadened linewidth and inefficient readout of solid-state spin ensembles. Here, we experimentally demonstrate that such drawbacks can be overcome by a reborn maser technology at room temperature in the solid state. Owing to maser action, we observe a fourfold reduction in the electron paramagnetic resonance linewidth of an inhomogeneously broadened molecular spin ensemble, which is narrower than the same measured from single spins at cryogenic temperatures. The maser-based readout applied to near zero-field magnetometry showcases the measurement signal-to-noise ratio of 133 for single shots. This technique would be an important addition to the toolbox for boosting the sensitivity of solid-state ensemble spin sensors.

Chemical Gardens Mimic Electron Paramagnetic Resonance Spectra and Morphology of Biogenic Mn Oxides.

Manganese (Mn) oxides are ubiquitous in nature and occur as both biological and abiotic minerals, but empirically distinguishing between the two remains a problem. Recently, electron paramagnetic resonance (EPR) spectroscopy has been proposed for this purpose. It has been reported that biogenic Mn oxides display a characteristic narrow linewidth in contrast to their pure abiotic counterparts, which is explained in part by the large number of cation vacancies that form within the layers of biogenic Mn oxides. It was, therefore, proposed that natural samples that display a narrow EPR linewidth, ΔHpp < 580G, could be assigned to a biogenic origin. However, in poorly crystalline or amorphous solids, both dipolar broadening and exchange narrowing simultaneously determine the linewidth. Considering that the spectral linewidth is governed by several mechanisms, this approach might be questioned. In this study, we report synthetic chemical garden Mn oxide biomorphs that exhibit both morphologically life-like structures and narrow EPR linewidths, suggesting that a narrow EPR line may be unsuitable as reliable evidence in assessment of biogenicity.

Microstructure and Conduction Electron Quantum Properties of Small Diamond Cubic α-Sn Nanocrystals Embedded in Cubic Boron Nitride Crystals.

The morphology, structure, composition, and conduction electron properties of quasi-spherical tin nanocrystals (NCs) of 2.5 nm average diameter, with unstrained, bulk-like α-Sn diamond cubic structure, observed in dark cubic boron nitride (cBN) crystallites, were determined by correlated analytical high-resolution scanning transmission electron microscopy and multifrequency electron spin resonance (ESR) investigations. The narrow Lorentzian ESR line with g = 2.0028 is attributed to the conduction ESR of the α-Sn NCs, consistent with the temperature- and frequency-independent small g-shift and intensity reduction under high temperature (950 °C) vacuum annealing when the α-Sn NCs are thermally dissolved in the host cBN crystallites. The ESR linewidth and line intensity vs temperature dependences recorded in the 20 to 295 K range are quantitatively described considering the presence of discrete, quantum confinement-induced conduction electron energy levels with ΔQC/k B = 125 K separation, close to the theoretical value for conductive α-Sn NCs of 2.5 nm in diameter. The observed properties are tentatively explained with the predicted nanosize induced band-gap opening and change of band ordering from bulk α-Sn to small unstrained α-Sn NCs, resulting in a topological phase transition that also explains the predominantly s-like character of the conduction band electron orbitals.

Revealing 2D Magnetism in a Bulk CrSBr Single Crystal by Electron Spin Resonance

Abstract2D magnets represent material systems in which magnetic order and topological phase transitions can be observed. Based on these phenomena, novel types of computing architectures and magnetoelectronic devices can be envisaged. Unlike conventional magnetic films, their magnetism is independent of the substrate and interface qualities, and 2D magnetic properties manifest even in formally bulk single crystals. However, 2D magnetism in layered materials is rarely reported often due to weak exchange interactions and magnetic anisotropy, and low magnetic transition temperatures. Here, the electron spin resonance (ESR) properties of a layered antiferromagnetic CrSBr single crystal are reported. The W‐like shape angular dependence of the ESR linewidth provides a signature for room temperature spin–spin correlations and for the XY spin model. By approaching the Néel temperature the arising of competing intralayer ferromagnetic and interlayer antiferromagnetic interactions might lead to the formation of vortex and antivortex pairs. This argument is inferred by modeling the temperature dependence of the ESR linewidth with the topological Berezinskii‐Kosterlitz‐Thouless phase transition. These findings together with the chemical stability and semiconducting properties, make CrSBr a promising layered magnet for future magneto‐ and topological‐electronics.

Electron Spin Resonance Study of Hydrogen‐Free Germanium‐Doped Diamond‐Like Carbon Films

Hydrogen‐free germanium (Ge)‐doped diamond‐like carbon (DLC) films deposited on SiO2 substrates are studied by electron spin resonance (ESR) technique at T = 5–295 K. The Ge‐free and DLC films with 1 and 2.5 at% Ge content having low cytotoxicity are investigated. A single Lorentzian ESR line resulting from exchange interaction between localized (LE) and nonlocalized electrons (NE) is observed. The spin susceptibility temperature dependence deviates from the Curie–Weiss law at T &gt; 75 K due to Pauli contribution caused by NE. The antiferromagnetic low‐temperature spin ordering in DLC films, decreasing with Ge content, is observed. The LE contribution is significant for Ge‐free and DLC:1%Ge, whereas NE makes the main contribution to DLC:2.5%Ge. The LEs are related to sp 2/sp carbon‐related centers, while the NEs are assigned with electrons in the sp 3 orbital. The correlation between the sp 3/sp 2 ratio and the shift of g‐factor for NE to lower values with Ge doping is established. The Korringa relaxation model is provided for the ESR linewidth temperature dependence. The higher density of LE states on Fermi energy level caused by Ge doping of DLC films is proposed.

Exchange interaction between localized magnetic moments and conduction-electrons in Er doped gold nanoparticles synthesized by laser ablation in water

In this work, we report a fundamental study on the exchange interaction between localized rare earth magnetic moments and conduction electrons of Er3+ diluted in Au metallic nanoparticles (NPs) produced by laser ablation in liquid. The study was carried out in Au1−xErx (x ≤ 0.026) bulk metallic alloys and NPs with a mean size of 20 nm. The samples were characterized by means of x-ray diffraction, transmission electron microscopy, magnetic susceptibility, and electron spin resonance (ESR) experiments. The obtained results showed that, despite the high temperature and being far away from chemical equilibrium throughout the laser ablation process, in the AuNPs, the Er3+ (J = 15/2) ground state of the crystal electric field split multiplet remains a Γ7 (g = 6.79) Kramers doublet with the expected g-shift and T-dependence of the ESR linewidth, preserving the general bulk properties and the cubic symmetry. In addition, the Au1−xErx NPs present narrow ESR residual linewidth suggesting homogeneous Er3+ doping and negligible strain distribution in the Au1−xErx NPs. This new methodology may certainly provide relevant insight into the study of the intrinsic physical properties of dilute rare earth metallic alloys at the nanometer scale seeking quantum size effects and motivates novel technological applications.

Slow crystalline electric field fluctuations in the Kondo lattice SmB6

This work reports on the temperature dependence of the electron spin resonance (ESR) of Gd$^{3+}$-doped SmB$_{6}$ single crystals at X- and Q-band microwave frequencies in different crystallographic directions. We found an anomalous inhomogeneous broadening of the Gd$^{3+}$ ESR linewidth ($\Delta H$) within 5.3 K $\leq T \leq$ 12.0 K which is attributed to slow crystalline electric field (CEF) fluctuations, slower than the timescale of the ESR microwave frequencies used ($\sim$10 GHz). This linewidth inhomogeneity may be associated to the coupling of the Gd$^{3+}$ $S$-states to the breathing mode of the SmB$_{6}$ cage, and can be simulated by a random distribution of the 4$^{th}$ CEF parameter, $b_4$, that strikingly takes negative and positive values. The temperature at which this inhomogeneity sets in, is related to the onset of a continuous insulator-to-metal phase transition. In addition, based on the interconfigurational fluctuation relaxation model, the observed exponential $T$-dependence of $\Delta H$ above $T\simeq$ 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB$_{6}$ ($\Delta\simeq$ 60 K). This charge fluctuation scenario provides important ingredients to the physical properties of SmB$_{6}$. We finally discuss the interplay between charge and valence fluctuations under the view of slow CEF fluctuations in SmB$_{6}$ by coupling the Gd$^{3+}$ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism.

Diluting a triangular-lattice spin liquid: Synthesis and characterization of NaYb1−xLuxS2 single crystals

Yb-based magnets, with a perfect triangular lattice of pseudospin-$\frac{1}{2}\phantom{\rule{4pt}{0ex}}{\mathrm{Yb}}^{3+}$ ions, have emerged as candidates for realizing a quantum spin-liquid state, with ${\mathrm{NaYbS}}_{2}$ being a prominent example. Here, we present the solid-solution series ${\mathrm{NaYb}}_{1\ensuremath{-}x}{\mathrm{Lu}}_{x}{\mathrm{S}}_{2}$ with well-defined single crystals over the entire substitution range $0\ensuremath{\le}x\ensuremath{\le}1$. Chemical and structural analysis indicate a statistically homogeneous replacement of ${\mathrm{Yb}}^{3+}$ by ${\mathrm{Lu}}^{3+}$ ions. We magnetically characterize the relatively small single crystals using electron spin resonance (ESR). Below $30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the ESR intensity can be well described by a Curie-Weiss function for all $x$, with a decreasing Weiss temperature with increasing Lu content. This reduction of the average magnetic interaction upon Lu substitution is also supported by magnetization measurements. Importantly, no signs of magnetic or spin-glass order are detected down to $2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for any $x$. For $x&gt;0.5$, the ESR linewidth strongly increases, indicating the breakup of the magnetic system into disconnected clusters, as expected from percolation physics. The experimental magnetization data are found to be in good agreement for all $x$ with results of classical Monte Carlo simulations for a triangular-lattice Heisenberg model, amended with a small second-neighbor interaction. Taken together, our results establish ${\mathrm{NaYb}}_{1\ensuremath{-}x}{\mathrm{Lu}}_{x}{\mathrm{S}}_{2}$ as a family of diluted triangular-lattice spin liquids.

Timescale distributions of spin fluctuations in the S=2 kagome antiferromagnet CsMn3F6 ( SeO3)2

We report the static and dynamical properties of a newly discovered $S=2$ kagome antiferromagnet ${\mathrm{CsMn}}_{3}{\mathrm{F}}_{6}{({\mathrm{SeO}}_{3})}_{2}$. By combining dc and ac magnetic susceptibilities, specific heat, electron spin resonance (ESR), and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$), we identify two characteristic temperatures ${T}^{*}=8$ K and $T=|{\mathrm{\ensuremath{\Theta}}}_{\text{CW}}|=49(1)$ K, thereby three distinct regimes: a paramagnetic, a cooperative paramagnetic, and a quasistatic ordered state. At high temperature ($T&gt;|{\mathrm{\ensuremath{\Theta}}}_{\text{CW}}|$), the ac susceptibility and ESR linewidth show a power-law dependence, reflecting short-range spin-spin correlations of the paramagnetic ${\mathrm{Mn}}^{3+}$ ions. In the cooperative paramagnetic regime (${T}^{*}&lt;T&lt;|{\mathrm{\ensuremath{\Theta}}}_{\text{CW}}|$), the ESR signals evince the development of dichotomic spin correlations, which is interpreted in terms of the distinct timescales of in-plane and out-of-plane spin fluctuations. Remarkably, we observe a broad maximum at the characteristic temperature scale of ${T}_{\text{cl}}=20$ K ($\ensuremath{\approx}|{J}_{\text{cl}}|$) in the specific heat and ESR results, suggesting short-range ordering. At low temperatures below ${T}^{*}$, the $\ensuremath{\mu}\mathrm{SR}$ data suggest the presence of dynamically fluctuating fields with partially frozen moments, consistent with the absence of long-range order evidenced by specific heat and magnetic susceptibility data. A magnetic behavior that depends on the chosen time window points to the presence of multiple timescales and temporally anisotropic spin correlations, as predicted for a classical kagome antiferromagnet.

Microscopic probe of magnetic polarons in antiferromagnetic Eu5In2Sb6

Colossal magnetoresistance (CMR) emerges from intertwined spin and charge degrees of freedom in the form of ferromagnetic clusters also known as trapped magnetic polarons. As a result, CMR is rarely observed in antiferromagnetic materials. Here we use electron spin resonance (ESR) to reveal microscopic evidence for the formation of magnetic polarons in antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$. First, we observe a reduction of the Eu$^{2+}$ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the Eu$^{2+}$ lineshape changes markedly below T' ~ 200 K, a temperature scale that coincides with the onset of CMR. The combination of these two effects provide strong evidence that magnetic polarons grow in size below T' and start influencing the macroscopic properties of the system.