Study of the antiferromagnetism of Mn5Si3: an inverse magnetocaloric effect material

The intermetallic compound Mn5Si3 has been studied by high-resolution Time-of-Flight (TOF) neutron powder diffraction. At room temperature, Mn5Si3 is paramagnetic and it crystallizes in the P6(3)/mcm hexagonal space group. Magnetic susceptibility and specific heat measurements show clearly two major anomalies. At 100(1) K, a transition (Tm-1) corresponds to a collinear antiferromagnetic ordering (AF1). The second transition at 62(1) K (Tm-2), which was still unclear, highlights a magneto-structural distortion from an orthorhombic symmetry (AF1) to a monoclinic symmetry (AF2), which could be influenced by a low magnetic field. Such a magneto-structural change is directly associated with the inverse magnetocaloric effect behaviour of this material. A new description by means of the commensurate magnetic superspace groups, Ccmm1'(0 beta 0)00ss and C2(1)/m1'(alpha beta 0)0ss, has been used to refine properly the low temperature antiferromagnetic structures. Band structure calculations using the self-consistent, spin-polarized TB-LMTO method were accomplished to support the magnetic properties observed at low temperature

Possible magnetic-polaron-switched positive and negative magnetoresistance in the GdSi single crystals

Magnetoresistance (MR) has attracted tremendous attention for possible technological applications. Understanding the role of magnetism in manipulating MR may in turn steer the searching for new applicable MR materials. Here we show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive MR value (PMRV), up to ~415%, accompanied by a large negative thermal volume expansion (NTVE). Around T(N) the PMRV translates to negative, down to ~-10.5%. Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins. Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices

Magnetic correlations in HoxTb2-xTi2O7

Polycrystalline samples of HoxTb2-xTi2O7 (0 Ising spins, nearest-neighbor exchange, and dipolar interactions. Inelastic neutron scattering on HoTbTi2O7 reveals two dispersionless excitations, one of similar to 2.5 meV out of the ground state, and a 4-meV transition out of an excited state. We argue that these data suggest that the very strong single-ion effects of Ho2Ti2O7 and Tb2Ti2O7 persist in the HoxTb2-xTi2O7(0 < x < 2) solid solution, whereas the Tb-Ho correlations are weak, resulting in small shifts in the energy scales but with no dramatic effect on the bulk properties

Unconventional magnetoresistance and electronic transition in Mn 3 Ge Weyl semimetal

Weyl semimetals are well known for their anomalous transport effects caused by a large fictitious magnetic field generated by the non-zero Berry curvature. We performed the analysis of the electrical transport measurements of the magnetic Weyl semimetal Mn$_{3}$Ge in the a-b and a-c plane. We have observed negative longitudinal magneto-resistance (LMR) at a low magnetic field ($B<1.5$ T) along all the axes. The high field LMR shows different behavior along x and z axes. A similar trend has been observed in the case of planar Hall effect (PHE) measurements as well. The nature of high field LMR along the x axis changes near 200 K. Dominant carrier concentration type, and metallic to semimetallic transition also occur near 200 K. These observations suggest two main conclusions: (i) The high field LMR in Mn$_3$Ge is driven by the metallic - semimetallic nature of the compound. (ii) Mn$_3$Ge compound goes through an electronic band topological transition near 200 K. Single crystal neutron diffraction does not show any change in the magnetic structure below 300 K. However, the in-plane lattice parameter (a) shows a maximum near 230 K. Therefore, it is possible that the change in electronic band structure near 200 K is driven by the a lattice parameter of the compound.Comment: Accepted in Physical Review

Elasticity and magnetocaloric effect in MnFe4Si3\mathrm{MnFe_4Si_3}

The room temperature magnetocaloric material MnFe4Si3 was investigated with nuclear inelastic scattering (NIS) and resonant ultrasound spectroscopy (RUS) at different temperatures and applied magnetic fields in order to assess the influence of the magnetic transition and the magnetocaloric effect on lattice dynamics. The NIS data give access to phonons with energies above 3 meV, whereas RUS probes the elasticity of the material in the MHz frequency range and thus low-energy, ∼ neV, phonon modes. A significant influence of the magnetic transition on the lattice dynamics is observed only in the low-energy, long-wavelength limit. MnFe4Si3 and other compounds in the Mn5−xFexSi3 series were also investigated with vibrating sample magnetometry, resistivity measurements, and Mössbauer spectroscopy in order to study the magnetic transitions and to complement the results obtained on the lattice dynamics

121 Sb and 125 Te nuclear inelastic scattering in Sb 2 Te 3 under high pressure424

We investigated the lattice dynamics of Sb2Te3 under high pressure using 121Sb and 125Te nuclear inelastic scattering of synchrotron radiation. We measured the room temperature 121Sb and 125Te inelastic spectra at 15(1) GPa and 77(3) GPa and extracted the Te and Sb element specific density of phonon states of δ-Sb2Te3 at 77(3) GPa. X-ray diffraction confirms the sample to be in the cubic δ-Sb2Te3 phase with space group $Im\bar{3}m$ and lattice constant $a=3.268(4)\;\overset{\circ}{A}$. The total density of phonon states of δ-Sb2Te3 strongly resembles the one of amorphous GeSb2Te4, suggesting the presence of covalent bonding in contrast to the resonance bonding in α-Sb2Te3. From the density of phonon states of δ-Sb2Te3 a mean speed of sound of 2.61(6) ${\rm km}\ {{{\rm s}}^{-1}}$ and Debye temperatures of 278(10) K for Te and 296(10) K for Sb were determined

Anomalous Hall effect and magnetic structure of the topological semimetal ( Mn 0.78 Fe 0.22 ) Ge 3

Me3+δGe, being a Weyl semimetal, shows a large anomalous Hall effect (AHE), which decreases slowly with an increase in δ from 0.1 to 0.4. However, AHE in this compound remains significantly large in the whole range of δ because of the robust nature of the topology of bands. To explore the possibility of tuning the anomalous transport effects in Weyl semimetals, we have studied the single-crystal hexagonal-(Mn0.78Fe0.22)3Ge compound. Magnetization of this compound shows two magnetic transitions at 242 K (TN1) and 120 K (TN2). We observed that the AHE persists between TN2−TN1 and vanishes below TN2. Further, we performed single-crystal neutron diffraction experiments (using spherical neutron polarimetry and unpolarized neutron diffraction) to determine the magnetic structures of (Mn0.78Fe0.22)3Ge at different temperatures. Our neutron diffraction results show that the sample possesses a collinear antiferromagnetic structure below TN2. However, the magnetic structure of the sample remains noncollinear antiferromagnetic, the same as Mn3Ge, between TN1 to TN2. The presence of AHE, and noncollinear magnetic structure in (Mn0.78Fe0.22)3Ge, between TN1 and TN2, suggest the existence of Weyl points in this temperature regime. Below TN2, AHE is absent, and the magnetic structure also changes to a collinear antiferromagnetic structure. These observations signify a strong link between the magnetic structure of the sample and AHE

Anisotropy of the magnetocaloric effect: Example of Mn 5 Ge 3

We have investigated the field direction dependence of thermo-magnetic behavior in single crystalline Mn5Ge3. The adiabatic temperature change ΔTad in pulsed fields, the isothermal entropy change ΔSiso calculated from static magnetization measurements, and heat capacity have been determined for fields parallel and perpendicular to the easy magnetic direction [001]. The isothermal magnetization measurements yield, furthermore, the uniaxial anisotropy constants in second and fourth order, K1 and K2. We discuss how the anisotropy affects the magneto-caloric effect (MCE) and compare the results to the related compound MnFe4Si3, which features an enhanced MCE, too, but instead exhibits strong easy plane anisotropy. Our study reveals the importance of magnetic anisotropy and opens new approaches for optimizing the performance of magnetocaloric materials in applications