Browsing by Author "DiTusa, J. F."
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Item Helical magnetic order and Fermi surface nesting in non-centrosymmetric ScFeGe(American Physical Society, 2021) Karna, Sunil K.; Tristant, D.; Hebert, J. K.; Cao, G.; Chapai, R.; Phelan, W. A.; Zhang, Q.; Wu, Y.; Dhital, C.; Li, Y.; Cao, H. B.; Tian, W.; Dela Cruz, C. R.; Aczel, A. A.; Zaharko, O.; Khasanov, A.; McGuire, M. A.; Roy, A.; Xie, W.; Browne, D. A.; Vekhter, I.; Meunier, V.; Shelton, W. A.; Adams, P. W.; Sprunger, P. T.; Young, D. P.; Jin, R.; DiTusa, J. F.; Physics, School of ScienceAn investigation of the structural, magnetic, thermodynamic, and charge transport properties of noncentrosymmetric hexagonal ScFeGe reveals it to be an anisotropic metal with a transition to a weak itinerant incommensurate helimagnetic state below 𝑇𝑁=36 K. Neutron diffraction measurements discovered a temperature and field independent helical wave vector 𝒌 = (0 0 0.193) with magnetic moments of 0.53 𝜇𝐵 per Fe confined to the 𝑎𝑏 plane. Density functional theory calculations are consistent with these measurements and find several bands that cross the Fermi level along the 𝑐 axis with a nearly degenerate set of flat bands just above the Fermi energy. The anisotropy found in the electrical transport is reflected in the calculated Fermi surface, which consists of several warped flat sheets along the 𝑐 axis with two regions of significant nesting, one of which has a wave vector that closely matches that found in the neutron diffraction. The electronic structure calculations, along with a strong anomaly in the 𝑐 -axis conductivity at 𝑇𝑁, signal a Fermi surface driven magnetic transition, similar to that found in spin density wave materials. Magnetic fields applied in the 𝑎𝑏 plane result in a metamagnetic transition with a threshold field of ≈6.7 T along with a sharp, strongly temperature dependent discontinuity and a change in sign of the magnetoresistance for in-plane currents. Thus, ScFeGe is an ideal system to investigate the effect of in-plane magnetic fields on a helimagnet with a 𝑐 -axis propagation vector, where the relative strength of the magnetic interactions and anisotropies determine the topology and magnetic structure.Item Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn2O4(American Chemical Society, 2023-03-14) Zhang, Qiang; Tian, Wei; Nepal, Roshan; Huq, Ashfia; Nagler, Stephen; DiTusa, J. F.; Jin, Rongying; Physics, School of ScienceSpinel compounds AB2X4 consist of both tetrahedral (AX4) and octahedral (BX6) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn2O4 has been reported to exhibit one structural transition and two successive magnetic transitions. Here, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn2O4 by employing the high-resolution neutron powder diffraction. The cation distribution is found to be (Mn0.92+Fe0.13+)A(Mn3+Fe0.93+Mn0.12+)BO4. While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at TS ≈ 750 K, driven by the Jahn-Teller effect of the orbital active B-site Mn3+ cation. Strong magnetoelastic coupling is unveiled at TN1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to TN2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn3+/Fe3+ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ∼25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O-B-O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn2O4. Our study indicates that FeMn2O4 is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.Item Quantum Oscillations with Angular Dependence in PdTe2 Single Crystals(IOP, 2020-10-16) Chapai, Ramakanta; Browne, D. A.; Graf, David E.; DiTusa, J. F.; Jin, Rongying; Physics, School of ScienceThe layered transition-metal dichalcogenide PdTe2 has been discovered to possess bulk Dirac points as well as topological surface states. By measuring the magnetization (up to 7 T) and magnetic torque (up to 35 T) in single crystalline PdTe2, we observe distinct de Haas-van Alphen (dHvA) oscillations. Eight frequencies are identified with H||c, with two low frequencies (Fα= 8 T and Fβ= 117 T) dominating the spectrum. The effective masses obtained by fitting the Lifshitz-Kosevich (LK) equation to the data are mα*=0.059m0 and mβ*=0.067m0 where m0 is the free electron mass. The corresponding Landau fan diagrams allow the determination of the Berry phase for these oscillations resulting in values of ∼0.67π for the 3D α band (hole-type) (down to the 1st Landau level) and ∼0.23π-0.73π for the 3D β band (electron-type) (down to the 3rd Landau level). By investigating the angular dependence of the dHvA oscillations, we find that the frequencies and the corresponding Berry phase (ΦB) vary with the field direction, with a ΦB∼ 0 when H is 10°-30° away from the a b plane for both α and β bands. The multiple band nature of PdTe2 is further confirmed from Hall effect measurements.Item Topological Hall effect and magnetic states in the Nowotny chimney ladder compound Cr11Ge19(American Physical Society, 2021) Li, Yu; Gui, Xin; Khan, Mojammel A.; Xie, Weiwei; Young, David P.; DiTusa, J. F.; Physics, School of ScienceWe have investigated the magnetic and charge transport properties of single crystals of the Nowotny chimney ladder compound Cr11Ge19 and mapped out a comprehensive phase diagram reflecting the complicated interplay between the Dzyaloshinskii-Moriya (DM) interaction, the dipolar interaction, and the magnetic anisotropy. We have identified a set of interesting magnetic phases and attributed a finite topological Hall effect to the recently discovered biskyrmion phase. These data also suggest the existence of an antiskyrmion state at finite fields for temperatures just below the magnetic ordering temperature 𝑇𝑐 as indicated by a distinct change in sign of the topological Hall effect. Above 𝑇𝑐, we discovered a region of enhanced magnetic response corresponding to a disordered phase likely existing near the ferromagnetic critical point under small magnetic fields. Strong spin chirality fluctuations are demonstrated by the large value of the topological Hall resistivity persisting up to 1 T, which is most likely due to the existence of the DM interaction. We argue that changes to the topological Hall effect correspond to different topological spin textures that are controlled by magnetic dipolar and DM interactions that vary in importance with temperature.Item Toward tunable quantum transport and novel magnetic states in Eu1−xSrxMn1−zSb2 (z < 0.05)(Springer Nature, 2022) Zhang, Qiang; Liu, Jinyu; Cao, Huibo; Phelan, Adam; Graf, David; DiTusa, J. F.; Tennant, D. Alan; Mao, Zhiqiang; Physics, School of ScienceMagnetic semimetals are very promising for potential applications in novel spintronic devices. Nevertheless, realizing tunable topological states with magnetism in a controllable way is challenging. Here, we report novel magnetic states and the tunability of topological semimetallic states through the control of Eu spin reorientation in Eu1−xSrxMn1−zSb2. Increasing the Sr concentration in this system induces a surprising reorientation of noncollinear Eu spins to the Mn moment direction and topological semimetallic behavior. The Eu spin reorientations to distinct collinear antiferromagnetic orders are also driven by the temperature/magnetic field and are coupled to the transport properties of the relativistic fermions generated by the 2D Sb layers. These results suggest that nonmagnetic element doping at the rare earth element site may be an effective strategy for generating topological electronic states and new magnetic states in layered compounds involving spatially separated rare earth and transition metal layers.