Frustrated kagome-based intermetallic compound Fe32+delta Ge35-xSix was chemically doped using Sn both in single crystal and polycrystalline powder forms, thus forming a solid solution Fe32+delta Ge35-x-ySixSny. Our structural analysis of a single crystal of Fe32.8Ge30.2Si4Sn0.8 showed that the crystal structure of the compound is a result of two-dimensional intergrowth of two structure types: MgFe6Ge6 and Co2Al5, the former one carrying the hexagram-shaped fragments of the kagome lattice. Due to the different environments of the p-element sites, the Sn atoms are isolated in a single crystallographic site in between two MgFe6Ge6-type blocks, linking the kagome fragments, while the Si atoms replace the Ge atoms at another site inside the MgFe6Ge6-type blocks. Magnetic measurements of a larger Fe32.8Ge30.2Si4Sn0.8 single crystal revealed that despite relatively low total Sn concentration, the special position of the Sn atoms has a profound effect on the magnetic properties. While the Sn-doping conserves the magnetic frustration and low ordering temperature TN of 150 K, it disrupts spin reorientation significantly reducing its extent and lowering its temperature by around 50 K. The Sn doing also creates small uncompensated magnetic moments leading to a weak anisotropic ferromagnetic response in low magnetic fields not observed in the parent phase.

Single crystals of Mn-based kagome metals Ti3Mn3Sn4Ga and Hf3Mn3Sn4Ga were grown from Sn flux. The X-ray structural analysis showed that the two compounds are isostructural to recently reported Zr3Mn3Sn4Ga and feature a kagome lattice of Mn linear chains linked by Sn atoms. The structural analysis also revealed noticeable changes in the interatomic distances, primarily T-Sn ones, originating from differences in atomic radii of T = Ti, Zr, and Hf. Ti3Mn3Sn4Ga and Hf3Mn3Sn4Ga showed magnetic ordering at 110 K and 160 K, respectively, which is higher than that reported for Zr3Mn3Sn4Ga. While Hf3Mn3Sn4Ga demonstrates antiferromagnetic ordering below the transition temperature, Ti3Mn3Sn4Ga displays a small uncompensated moment directed along the c axis (0.02μB per Mn at 2 K), and exhibits several characteristic features of a spin-glass state, including sensitivity to the magnetic history of a sample and a pronounced shift of the magnetic hysteresis if cooled with the field.

Isostructural kagome metals Ti 3 Mn 3 Sn 4 Ga and Hf 3 Mn 3 Sn 4 Ga are reported. Hf 3 Mn 3 Sn 4 Ga undergoes AFM ordering at 160 K, while Ti 3 Mn 3 Sn 4 Ga shows possible coexistence of AFM and spin glass states below 110 K.

Fe32+δGe35-xSix was synthesized using solid-state and chemical vapor transport reactions in both powder and single crystalline forms. Single crystal and high-resolution powder X-ray diffraction experiments revealed Fe32+δGe35-xSix to be the third member of the Fe32+δGe35-xEx (E = p-element) family of ternary compounds alongside Fe32+δGe33As2 and Fe32+δGe35-xPx. Fe32+δGe35-xSix features a two-dimensional intergrowth structure of two parent structure types: MgFe6Ge6 and Co2Al5. Similar to the other members, the stabilisation of the intergrowth structure in Fe32+δGe35-xSix occurs as a result of p-element substitution in the MgFe6Ge6-type block. The intergrowth breaks the kagome net of MgFe6Ge6 into individual hexagrams while providing additional layers of geometrically frustrated atomic arrangements. Magnetic measurements showed antiferromagnetic ordering at TN ∼ 150-160 K and spin reorientation below 80-90 K owing to the competition between magnetic interactions in the frustrated magnetic lattice of Fe32+δGe35-xSix.

Stoichiometric single crystals of Mn4Al11 were synthesized from the elements using Sn as a flux. The crystal structure of Mn4Al11 was investigated using single crystal X-ray diffraction and showed a complex triclinic structure with a relatively small unit cell and interpenetrating networks of Mn and Al atoms. While our results generally agree with the previously reported data in the basic structure features such as triclinic symmetry and structure type, the atomic parameters differ significantly, likely due to different synthetic techniques producing off-stoichiometry or doped crystals used in the previous works. Our structural analysis showed that the view of the Mn substructure as isolated zigzag chains is incomplete. Instead, the Mn chains are coupled in corrugated layers by long Mn-Mn bonds. The high quality of the crystals with the stoichiometric composition also enabled us to study magnetic behavior in great detail and reveal previously unobserved magnetic ordering. Our magnetization measurements showed that Mn4Al11 is an antiferromagnet with TN of 65 K. The presence of the maximum above TN also suggests strong local interactions indicative of low-dimensional magnetic behavior, which likely stems from lowered dimensionality of the Mn substructure.

Layered chalcogenides are interesting from the point of view of the formation of two-dimensional magnetic systems for relevant applications in spintronics. High-spin Mn2+ or Fe3+ cations with five unpaired electrons are promising in the search for compounds with interesting magnetic properties. In this study, a new layered modification of the Mn2In2Se5 compound from the A2B2X5 family (“225”) was synthesized and investigated. A phase transition to the polymorph with primitive trigonal lattice was recorded at a temperature of 711 °C, which was confirmed by simultaneous thermal analysis, X-ray powder diffraction at elevated temperatures, and sample annealing and quenching. The stability of Mn2In2Se5 in air at high temperatures was investigated by thermal gravimetric analysis and powder X-ray diffraction. The new polymorph of Mn2In2Se5 crystallizes in the Mg2Al2Se5 structure type, as revealed by the Rietveld refinement against powder X-ray diffraction data. The crystal structure can be viewed as a close-packing of Se anions, in which indium and manganese cations are enclosed inside tetrahedral and octahedral voids, respectively, according to the AMnBInCBInCMnA… sequence. Magnetization measurements reveal an antiferromagnetic-like transition at a temperature of 6.3 K. The same magnetic properties are reported in the literature for the low-temperature R-centered trigonal polymorph. An approximation by the modified Curie–Weiss law yields a significant ratio of |θ|/TN = 28, which indicates strong magnetic frustration.

Cryogenic temperature controllers are essential devices for all experiments that require precise temperature measurements and temperature stabilization of samples. This job often can't be simply offloaded to generic hardware such as a multimeter paired with a computer-controlled heater. Several world-known vendors (such as Lake Shore, Oxford Instruments, Omega, Stanford Research) produce such devices, often as a part of more complex experimental setup. But sometimes these devices require careful attention to fine details of the experiment, including temperature sensor itself. In our case automatic change of the resistive sensor excitation current during the basic experiment lead us to non-negligible measurement errors. Dealing with the sensor mode and conditions without interfering the main experiment is the job of the equipment, not the researcher. Several years ago we developed a temperature controller specifically for our measurements and in the present work we describe the key points of development a more universal and commercially ready device.

A manganese monosilicide (MnSi) single crystal has been studied by the high-frequency (60 GHz) electron paramagnetic resonance method. The most currently detailed temperature dependences of the linewidth and g-factor have been obtained in the temperature range of 2–40 K. They indicate that a spin fluctuation transition occurs in the spin-polarized phase of МnSi, formed by magnetic polarons in a magnetic field of B ~ 2 T at $${{T}_{{\text{L}}}} \sim 15{\kern 1pt} $$ K. This transition corresponds to change in the regime of magnetic fluctuations at a temperature much lower than the transition temperature Tc ~ 30 K from the paramagnetic phase existing at T > Tc to the magnetically ordered phase occurring at T < Tc. This result extends the region of applicability of the concept of spin fluctuation transitions, which were previously considered only for magnetic states without long-range magnetic order. A model for the description of the electron paramagnetic resonance parameters in the system of magnetic polarons has been discussed.

Seebeck effect in the crystalline samples of Eu

Seebeck effect in the crystalline samples of EuxYb1-xB6(x= 0, 0.082, 0.127, 0.9, 1) was investigated at temperatures 2-300 K. For all the compounds thermopower is shown to be well described by the sum of diffusion (S-d=AT) and phonon drag components. The latter contribution is induced by quasilocal (Einstein) modes of ytterbium and europium ions with characteristic temperatures Theta(E)(YbB6) approximate to 91 K and Theta(E)(EuB6) approximate to 122 K. The estimation of effective massm*of the charge carriers proves that increasing of Eu content induces crossover from 'heavy' holes withm(h)*(x <= 0.127) approximate to 0.3-0.36m(0)to 'light' electrons withm(e)*(x > 0.9) approximate to 0.12-0.13m(0)(m(0)-free electron mass). For the Eu-rich compounds we propose the existence of additional point on the phase diagram, which corresponds to short-range magnetic order with enhanced spin fluctuations preceding the stabilization of magnetic polarons.

We have investigated the doping-induced changes of magnetic properties of the archetypal heavy fermion compound CeB6 on a series of substitutional solid solutions CexLa1-xB6 (0.01 <= x <= 1). For these strongly correlated electron systems high precision measurements of magnetic susceptibility chi(T) were undertaken in small external magnetic field aligned along [100], [110], and [111] directions in the simple cubic lattice of these hexaborides. It was shown that for all crystals and all magnetic field orientations the chi(T) dependences obey a power law chi similar to T-alpha at temperatures between 10 and 200 K. The observed critical exponents alpha = 0.77-0.93 demonstrate a monotonous decrease with the increase of Ce content. It is concluded that such a kind of Griffiths phase-type behavior that differs considerably from the convenient Curie-like dependence should be attributed to the emergence of disordered nanometer-size magnetic clusters of Ce-ions in the RB6 matrix.

The magnetic properties of substitutional solid solutions Eu 0.9 Yb 0.1 B 6 have been investigated in the temperature range of 2–300 K in fields up to 5 T. The data obtained confirm that the state with the electronic and magnetic phase separation (typical of europium hexaboride) is implemented in Eu 0.9 Yb 0.1 B 6 when the system metallization ( T M ≈ 15 K) precedes the ferromagnetic ordering ( T C ≈ 11.4 K). An analysis of the curves M ( H ) in the Belov–Arrott coordinates makes it possible to evaluate spontaneous magnetization M sp and zero field susceptibility χ 0 and determine the character of their critical behavior near the Curie point. The calculated critical indices (γ ≈ 1.28 and β ≈ 0.34) are in agreement with the predictions of the three-dimensional Heisenberg model.

Comparative analysis of Hall effect in substitutional solid solutions Ho0.5Lu0.5B12, Mn1 – xFexSi (0 ≤ x ≤ 1) and Eu1 – xGdxB6 (x < 0.04) is carried out at temperatures 2–300 K in magnetic fields up to 8 T. Anomalous contribution to the Hall effect $$\rho _{{xy}}^{{\text{A}}}$$ ~ $${{\rho }_{{xx}}}M$$ has been identified for systems with various types of magnetic frustration. The linear scaling $$\rho _{{xy}}^{{\text{A}}}$$ ~ ρxx is detected in the resistivity range ρxx ~ 0.01–1 mΩ cm lying outside the range of applicability of the classical model of asymmetric scattering. It is associated with the increase in the amplitude of spin fluctuations in the paramagnetic phase of the investigated compounds with noncollinear magnetic structure. The topological contribution to the Hall effect is extracted for Ho0.5Lu0.5B12 and Eu1 – xGdxB6. Its amplitude is found to vary from 80 nΩ  cm (Ho0.5Lu0.5B12) to 7.5 μΩ  cm (Eu0.97Gd0.03B6).

AbstractThe magnetic properties of substitutional solid solutions Eu_0.9Yb_0.1B_6 have been investigated in the temperature range of 2–300 K in fields up to 5 T. The data obtained confirm that the state with the electronic and magnetic phase separation (typical of europium hexaboride) is implemented in Eu_0.9Yb_0.1B_6 when the system metallization ( T _M ≈ 15 K) precedes the ferromagnetic ordering ( T _C ≈ 11.4 K). An analysis of the curves M ( H ) in the Belov–Arrott coordinates makes it possible to evaluate spontaneous magnetization M _sp and zero field susceptibility χ_0 and determine the character of their critical behavior near the Curie point. The calculated critical indices (γ ≈ 1.28 and β ≈ 0.34) are in agreement with the predictions of the three-dimensional Heisenberg model.

The model strongly correlated electron system Ho0.8Lu0.2B12 which demonstrates a cooperative Jahn-Teller instability of the boron sub-lattice in combination with rattling modes of Ho(Lu) ions, dynamic charge stripes and unusual antiferromagnetic (AF) ground state has been studied in detail at low temperatures by magnetoresistance, magnetization and heat capacity measurements. Based on received results it turns out that the angular H-fi-T magnetic phase diagrams of this non-equilibrium AF metal can be reconstructed in the form of a Maltese cross. The dramatic AF ground state symmetry lowering of this dodecaboride with fcc crystal structure can be attributed to the redistribution of conduction electrons which leave the RKKY oscillations of the electron spin density to participate in the dynamic charge stripes providing with extraordinary changes in the indirect exchange interaction between magnetic moments of Ho3+ ions and resulting in the emergence of a number of various magnetic phases. It is also shown that the two main contributions to magnetoresistance in the complex AF phase, the (i) positive linear on magnetic field and the (ii) negative quadratic component can be separated and analyzed quantitatively, correspondingly, in terms of charge carrier scattering on spin density wave (5d) component of the magnetic structure and on local 4f-5d spin fluctuations of holmium sites.

The high-frequency (60 GHz) electron paramagnetic resonance is studied in Ho x Lu 1− x B 12 single crystals at low temperatures (2–80 K) in an applied magnetic field up to 7 T within a wide doping range, 0.01 ≤ x ≤ 1. These compounds are characterized by the presence of a low-temperature cage glass phase with random displacements of magnetic holmium ions from the centrosymmetric position in B 12 boron cuboctahedra. For the samples with x ≥ 0.1, it is demonstrated that the electron paramagnetic resonance in the form of a single broad line with a g -factor of about 5 appears in the cage glass phase at T < T *= 60 K because of the decrease in the relaxation rate for magnetic moments of Ho 3+ ions. For the compounds with x > 0.3, the pronounced broadening of the resonance line and a steep decrease in the g -factor related to antiferromagnetic correlations is observed on cooling at T < 30 K. The performed simulation of the electron paramagnetic resonance spectra suggests the dominant role of the exchange interaction effects and positional disorder in determining the characteristic features of spin dynamics in Ho x Lu 1− x B 12 .

The low temperature (T < 5 K) electron spin resonance in topological Kondo insulator SmB6 is investigated. It is shown that spin relaxation time of the paramagnetic centers responsible for low temperature electron spin resonance in SmB6 is about ~ 10−8 s, which exceeds by five orders of magnitude the widely accepted spin and charge fluctuation time ~ 10−13 s in the homogeneous mixed valence state of this material. This discrepancy may be attributed to the formation of the specific paramagnetic centers in the sample surface–spin polarons.

A comparative analysis of Hall effect in substitutional solid solutions Ho0.5Lu0.5B12, Mn1-xFexSi (0≤ x≤1) and Eu1-xGdxB6 (x<0.04) is carried out at temperatures 2-300 K in magnetic fields up to 8 T. Anomalous Hall effect rhoxyA~rhoxxM is identified for the systems with different kinds of magnetic frustration. Linear scaling rhoxyA~rhoxx, which is detected at resistivity rhoxx~0.01-1 mOmega·cm out from the region of applicability of classical skew-scattering model, is associated with the increase of spin fluctuations amplitude in paramagnetic phases of the investigated compounds with noncollinear magnetic structure. Topological contribution to Hall effect is extracted for Ho0.5Lu0.5B12 and Eu1-xGdxB6, its amplitude varying from 80 nOmega·cm (for Ho0.5Lu0.5B12) up to 7.5 μOmega·cm (for Eu0.97Gd0.03B6).