Metamagnetism, a sudden increase in the magnetization of a material with a small change of external magnetic field, are calling more attentions because of their rich magnetic phenomena and scientific significance . There are quite different physical causes for different types of metamagnets . In this work, metamagnetism has been found in arc-melting Sm(Ni 0.5 Fe 0.4 Cu 0.1 ) 7 . The physical property measurement system (PPMS) is used to measure the magnetic properties . The hysteresis loops at different temperatures (T>5K) are shown in Fig .1 . Both sides of the hysteresis loops exhibit obvious metamagnetic behaviors . The hysteresis loops show wasp-waisted character . At the beginning of the curves the magnetization increases rapidly and then gets saturation at about 1T . When field increases continuously to the critical magnetic fields (H cm ) metamagnetic behavior appears . The lower the temperature is, the higher the H cm is . The magnetization appears to saturate when field increases to a higher value . The magnetization stays at high magnetization state until the field decreases to about 1T . The magnetizatic behaviors at another side of the hysteresis loops are the same . The critical magnetic fields (H cm ) corresponding to the metamagnetic points increases with the temperature decreasing by an exponential dependence. The spin reverse model with thermal activation (TA) is used to explain the relation of the critical field to temperature . The expression can be written as H cm (T)=H cm (0)exp(-kT/U), where Hcm(T) is the critical field at T, Hcm(0) is the calculated value at 0K, k is the Boltzmann constant, U is the energy needed to turnover one spin. The fitting results show U≈6 .6×10 -15 erg . At temperatures, below 5K, the smooth jumps turn into step-like jumps. The number of the steps and the values of critical fields vary with different samples . The inset in Fig .2 is the enlargement of one step with a field interval of 0.05T . When repeating the magnetization process, the hysteresis loops can coincide compactly, which is different from the usual Barkhausen jumps. The XRD results show that the main phase is hexagonal P6/mmm structure and the easy magnetization direction at room temperature is along c axis, which may show high anisotropy constants, where macroscopic quantum tunneling (MQT) may happen. As temperature gets extremely low, the thermal activation can be ignored and the quantum behavior becomes obvious . One possible explanation is as follows: the MQT happens first, which leads to a release of thermal energy and increase of sample temperature, followed by a huge magnetization reversal due to the external magnetic field . A step-like magnetic jump appears . The differences of step numbers and H cm values between different polycrystalline samples are thought to be related to the relative orientation of the crystalline grain and the field direction . It seems that the MQT and TA models have solved the problem well . Another possible explanation is the narrow domain-wall pinning, but such a mechanism would have difficulty accounting for the presence of multistep jumps and the transition from one smooth jump to several sharp jumps just by changing few kelvins .
Crystals of FeSb2 correlated narrow-gap semiconductor host colossal thermopower values. By tuning the impurity level here, we show that electron–phonon scattering that transfers phonon momentum to electrons is efficient only for certain optimal carrier concentration in the low-mobility band. Phonon drag acting on such states in crystals with high phonon mean free path enhances thermopower to colossal values, whereas for different carrier concentration, dominant thermal transport mechanism is electronic diffusion. This highlights the dual nature of correlated in-gap states that take part in the phonon drag but also reduce phonon mean free path.
Abstract Two-dimensional van der Waals (vdW) magnetic materials have emerged as possible candidates for future ultrathin spintronic devices, and finding a way to tune their physical properties is desirable for wider applications. Owing to the sensitivity and tunability of the physical properties to the variation of interatomic separations, this class of materials is attractive to explore under pressure. Here, we present the observation of direct to indirect band gap crossover and an insulator–metal transition in the vdW antiferromagnetic insulator CrPS 4 under pressure through in-situ photoluminescence, optical absorption, and resistivity measurements. Raman spectroscopy experiments revealed no changes in the spectral feature during the band gap crossover whereas the insulator–metal transition is possibly driven by the formation of the high-pressure crystal structure. Theoretical calculations suggest that the band gap crossover is driven by the shrinkage and rearrangement of the CrS 6 octahedra under pressure. Such high tunability under pressure demonstrates an interesting interplay between structural, optical and magnetic degrees of freedom in CrPS 4 , and provides further opportunity for the development of devices based on tunable properties of 2D vdW magnetic materials.
The antiferromagnetic kagome semimetals Mn3X (X = Ge, Sn, Ga) are of great interest due to properties arising from their Berry curvature, such as large anomalous Nernst and anomalous Hall coefficients, and spin to charge conversion efficiencies at ambient temperatures. However, the synthesis of epitaxial thin films of Mn3Ge in the desired hexagonal phase has been challenging because they do not wet insulating substrates, necessitating the use of a metallic buffer layer. Furthermore, a ferrimagnetic tetragonal phase also forms readily under typical growth conditions, interfering with hexagonal phase properties. We have synthesized atomically smooth and continuous epitaxial thin films of hexagonal Mn3Ge directly on insulating LaAlO3 (111) substrates using electron beam assisted molecular beam epitaxy, using a three-step process that mitigates the formation of the tetragonal phase. The anomalous Nernst coefficient is found to be more than six times larger in our films than in sputtered thin films of Mn3Ge and significantly larger than that of Fe. Our approach can be used to grow thin layers of kagome materials, without interference from a buffer layer in transport properties, and may be applicable to a broader range of materials with large surface energies that do not grow readily on insulating substrates.
The discovery of two-dimensional superconductivity in LAO/KTO (111) and (110) interfaces has raised significant interest in this system. Here we report the fabrication of nanoscale direct current superconducting quantum interference devices (DC-SQUID), created by using a conductive atomic force microscope (c-AFM) to lithographically control the conductivity at the LAO/KTO(110) interface. The field modulation of its critical current, I_c (B), corresponds well with our theoretical modeling, which reveals a large kinetic inductance of the superconducting 2DEG in KTO. The kinetic inductance of the SQUID is tunable by electrical gating from the back, due to the large dielectric constant of KTO. The demonstration of SQUID effect in KTO opens up the possibilities for probing the underlying physics of KTO superconductivity, such as the role of spin-orbit-coupling, pairing symmetry, and inhomogeneity. It also further implies that KTO could serve as a foundation for future quantum devices.
Carbon was successfully introduced into NdMn x Cr2−x Si2 serial compounds as an interstitial atom. The compounds with x ≤ 0.5 keep a CeMg2Si2-type structure while the structure changes slowly with increasing manganese content. Finally, a new superstructure was found and determined for x around 1.7. All the compounds for x = 1.1, 1.3, 1.5 and 1.7 have two magnetic phase transition temperatures around 43 and 314 K. What is more, a giant magnetocaloric effect has been observed around the lower phase transition temperature, T R ~ 51 K, in NdMn1.7Cr0.3Si2C. The maximum values of the magnetic entropy change −ΔS M for the field change of 2 T and 5 T are 14.9 J kg−1 K−1 and 26.0 J kg−1 K−1, which means the interstitial atom effect enhanced the MCE values for 85% and 63% compared to NdMn1.7Cr0.3Si2, respectively. NdMn1.7Cr0.3Si2C with first order magnetic transition but low hysteresis losses (thermal < ~0.58 K; magnetic field < ~0.12 T) can be a candidate for magnetic refrigerator applications in the temperature region near 50 K. Besides, two field-induced magnetic phase transitions were found in NdMn1.7Cr0.3Si2C among the temperature range from 53 to 60 K, and a linear relation between the phase transition field and temperature was found and analyzed.
The crystal structure and magnetic properties of MnCoxFe1−xSi (x = 0–0.5) compounds were investigated. With increasing Fe content, the unit cell changes anisotropically and the magnetic property evolves gradually: Curie temperature decreases continuously, the first-order metamagnetic transition from a low-temperature helical antiferromagnetic (AFM) state to a high-temperature ferromagnetic state disappears gradually and then a spin-glass-like state and another AFM state emerge in the low-temperature region. The Curie transition leads to a moderate conventional entropy change. The metamagnetic transition not only yields a larger negative magnetocaloric effect at lower applied fields than in MnCoSi but also produces a very large temperature span (103 K for Δμ0H = 5 T) of ΔS(T), which results in a large refrigerant capacity. These phenomena were explained in terms of crystal structure change and magnetoelastic coupling mechanism. Because of the large isothermal entropy change, the wide working temperature span and the low cost, MnCo1−xFexSi compounds are promising candidates for near room-temperature magnetic refrigeration applications.
We have examined size effect on thermal, transport, and thermodynamic properties of a ${\mathrm{CrSb}}_{2}$ single crystal. We demonstrate highly anisotropic quasi-one-dimensional electrical conductivity, quasiballistic phonons, and giant thermopower of $\ensuremath{-}6$ mV/K at 15 K. A thermopower peak is suppressed to $\ensuremath{-}1.6$ mV/K by changing crystal dimensions and shows linear dependence on the phonon mean-free path. Whereas electronic diffusion thermopower is significant, the bulk of the giant thermopower in ${\mathrm{CrSb}}_{2}$ stems from the coupling of the very long mean-free path phonons with the in-gap states.
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