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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Repulsive Tomonaga-Luttinger liquid in the quasi-one-dimensional alternating spin-$\frac{1}{2}$ antiferromagnet NaVOPO 4

Here, we probe the magnetic field-induced Tomonaga-Luttinger liquid (TLL) state in the bond-alternating spin1/2 antiferromagnetic (AFM) chain compound NaVOPO 4 using thermodynamic as well as local μSR and 31 P NMR probes down to mK temperatures in magnetic fields up to 14 T. The μSR and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power-law behavior, enabling us to directly determine the interaction parameter K as a function of the magnetic field. These estimates are crosschecked using magnetization and specific heat data. The field-dependent K lies in the range of 0.4 < K < 1 and indicates the repulsive nature of interactions between the spinless fermions, in line with the theoretical predictions. This renders NaVOPO 4 the first experimental realization of TLL with repulsive fermionic interactions in hitherto studied S = 1/2 bond-alternating AFM-AFM chain compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

High field magnetization of FePS 3

High field magnetization measurements in pulsed fields up to 65 T have been performed on FePS 3 , which is nominally a good example of a two-dimensional Ising-like antiferromagnet on a honeycomb lattice. Measurements with the field parallel to the moment direction confirm the presence of two first-order transitions above 35 T, to M/M sat = 1/2 and M/M sat = 1, respectively, at 4 K. The measurements are in contradiction with published estimates for the magnetic exchange parameters, but the contradiction can be resolved by allowing for anisotropic exchange parameters in the Hamiltonian. Furthermore, the magnetization with the field perpendicular to the moment direction is anisotropic, with no transitions observed for fields along the a axis while a cascade of first-order transitions is observed for fields above 50 T along the b axis, the latter case also showing a strong degradation of the sample after repeated pulses. The results indicate a strong magnetolattice coupling in FePS 3 . Temperature-dependent measurements hint at a possible tricritical point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Investigation of magnetic and electrical transport properties of YMB 4 (M = Cr, Fe, and Co) compounds

We have studied the structural, magnetic, and electrical transport properties of the YMB 4 series of compounds (M= Cr, Fe, and Co). These materials exhibit high stability and possess notable refractory and thermoelectric properties. Furthermore, recent theoretical predictions suggest that some of these compounds may display quantum magnetism and dimer formation among the magnetic ions. We have synthesized almost single phase YMB 4 compounds in the orthorhombic crystal structure (space group Pbam) using arc-melting and annealing. The Rietveld analysis of the room-temperature X-ray diffraction patterns reveals a clear change in the lattice parameter c, which correlates with the atomic radii of Cr, Fe, and Co. The temperature variation of resistivity measured between 2 K and 300 K reveals a metallic electron transport in all three YMB 4 compounds. However, no abrupt change in resistivity due to structural or magnetic phase transitions is observed. Additionally, a weak positive magnetoresistance of 1%–2% has been measured at 2.5 K, the contribution from the metallic impurity cannot be ruled out. The carrier concentration, on the order of 10 21 cm −3 , has been determined using conventional Hall measurements. The thermomagnetic curves recorded between 2 K and 350 K reveal the non-magnetic behavior of all these compounds. However, a small magnetic moment from paramagnetic/ferromagnetic impurity phases is detected at low temperatures. Above room temperature, no magnetic transition associated with the breaking of a dimer is observed.

borides↗
DOE OSTI2021

Disorder-induced time effect in the antiferromagnetic domain state of Fe 1+ y Te

Herein we report on temperature-dependent soft X-ray absorption spectroscopy (XAS) measurements utilizing linearly polarized synchrotron radiation to probe magnetic phase transitions in iron-rich Fe 1+y Te (y ≈ 0.12). X-ray magnetic linear dichroism (XMLD) signals, which sense magnetic ordering processes at surfaces, start to increase monotonically below the Néel temperature T N = 57 K. This increase is due to a progressive bicollinear antiferromagnetic (AFM) alignment of Fe spins of the monoclinic Fe 1+y Te parent phase. This AFM alignment was achieved by a [1 0 0]-oriented biasing field favoring a single-domain state during cooling across T N . Our specific heat and magnetization measurements confirm the bulk character of this AFM phase transition. On longer time scales, however, we observe that the field-biased AFM state is highly unstable even at the lowest temperature of T = 3 K. After switching off the biasing field, the XMLD signal decays exponentially with a time constant τ = 1506 s. The initial XMLD signal is restored only upon repeating a cycle consisting of heating and field-cooling through T N . We explain the time effect by a gradual formation of a multi-domain state with 90° rotated AFM domains, promoted by structural disorder, facilitating the motion of twin-domains. Significant disorder in our Fe 1+y Te sample is evident from our X-ray diffraction and specific heat data. The stability of magnetic phases in Fe-chalcogenides is an important material property, since the Fe(Te 1-x Se x ) phase diagram shows magnetism intimately connected with superconductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Dynamical vertex approximation for many-electron systems with spontaneously broken SU(2) symmetry

We generalize the formalism of the dynamical vertex approximation (DΓA)—a diagrammatic extension of the dynamical mean-field theory (DMFT)—to treat magnetically ordered phases. To this aim, we start by concisely illustrating the many-electron formalism for performing ladder resummations of Feynman diagrams in systems with broken SU(2) symmetry associated to ferromagnetic (FM) or antiferromagnetic (AF) order. We then analyze the algorithmic simplifications introduced by taking the local approximation of the two-particle irreducible vertex functions in the Bethe-Salpeter equations, which defines the ladder implementation of DΓA for magnetic systems. The relation of this assumption with the DMFT limit of large coordination-number/high dimensions is explicitly discussed. As a last step, we derive the expression for the ladder DΓA self-energy in the FM- and AF-ordered phases of the Hubbard model. The physics emerging in the AF-ordered case is explicitly illustrated by means of approximated calculations based on a static mean-field input for DΓA equations. The results obtained capture fundamental aspects of both metallic and insulating ground states of two-dimensional antiferromagnets, providing a reliable compass for future, more extensive applications of our approach. Furthermore, possible routes to further develop diagrammatic-based treatments of magnetic phases in correlated electron systems are briefly outlined in the Conclusions.

2-dimensional systems↗
DOE OSTI2023

Dielectric Relaxation by Quantum Critical Magnons

We report the experimental observation of dielectric relaxation by quantum critical magnons. Complex capacitance measurements reveal a dissipative feature with a temperature-dependent amplitude due to low-energy lattice excitations and an activation behavior of the relaxation time. The activation energy softens close to a field-tuned magnetic quantum critical point at H = H c and follows single-magnon energy for H > H c , showing its magnetic origin. Furthermore, our study demonstrates the electrical activity of coupled low-energy spin and lattice excitations, an example of quantum multiferroic behavior.

1-dimensional spin chains↗
DOE OSTI2021

Magnetic field induced phase transitions of the triangular spin model

In this work, motivated by a recent inelastic neutron scattering experiment on YbMgGaO 4 , I study the homogeneous spin model on the triangular lattice using the cluster mean-field theory and exact diagonalization. Phase diagrams with and without magnetic field are computed. The results show that the magnetic field can induce the phase transition from the spin liquid state (or the stripe state) to the 120 ° antiferromagnetic state. These phase transitions are suppressed by the next-nearest-neighbor exchange interaction J 2 / J 1 and vanish as J 2 / J 1 > 0.11 . I analyze a parameter space at J 2 / J 1 = 0.1 , where a field induced phase transition can occur, and find that the deviation of the theoretical spin excitation energy from the experimental data is only about 0.054. These results imply that an effective homogeneous spin model still applies to YbMgGaO 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Strong magnetoelastic coupling in Mn 3 X ( X = Ge , Sn)

Here we measure the full elastic tensors of Mn 3 Ge and Mn 3 Sn as a function of temperature through their respective antiferromagnetic phase transitions. Large discontinuities in the bulk moduli at the Néel transitions indicate strong magnetoelastic coupling in both compounds. Strikingly, the discontinuities are nearly a factor of 10 larger in Mn 3 Ge than in Mn 3 Sn. We use the magnitudes of the discontinuities to calculate the pressure derivatives of the Néel temperature, which are 39 K/GPa 14.3 K/GPa for Mn 3 Ge and Mn 3 Sn, respectively. We measured the in-plane shear modulus c 66 , which couples strongly to the magnetic order, in magnetic fields up to 18 T and found quantitatively similar behavior in both compounds. Recent measurements have demonstrated strong piezomagnetism in Mn 3 Sn : Our results suggest that Mn 3 Ge may be an even better candidate for this effect.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Crystal structure, magnetism, and specific heat of lightly depleted layered honeycomb oxides, Na$_2$(Ni$_{2-x}$D$_x$)TeO$_6$ ($D$ = Mg, Ga, Co; 0.10 ≤ $x$ ≤ 0.25)

Layered oxides, in which transition metal atoms form a honeycomb lattice, are known for complex magnetic phase diagrams. In this work we study the doped compounds, Na$_2$(Ni$_{2-x}$D$_x$)TeO$_6$, $D$ = Mg, Ga, and Co (0.10 ≤ $x$ ≤ 0.25), to understand how chemical dilution at the nickel site influences the stability of magnetic exchanges within the honeycomb layer of the parent, Na$_2$ Ni$_2$ TeO$_6$. Here, the studied compounds were found to crystallize in $P6_3/mcm$ space group common to P2 type layered oxides. In general, the lattice parameters and the unit cell volume expanded with increased doping. The oxidation states determined from X-ray photoelectron spectroscopy were consistent with bond valence sum estimates, that confirmed the presence of Ni$^{2+}$, Mg$^{2+}$, Ga$^{3+}$, Co$^{2+}$ in our samples. Irrespective of magnetic or non-magnetic doping, the magnetic phase transition temperature ($T_N$) of the doped compounds were reduced from that of Na$_2$ Ni$_2$ TeO$_6$ or Na$_2$ Co$_2$ TeO$_6$, and lay between 21.7 K and 26.8 K. The effective paramagnetic moments extracted from Curie–Weiss analysis were in the range 3.17 μ B to 3.84 μ B and the Weiss temperatures were between $-15$ K and $-25$ K, suggesting antiferromagnetism. Mg- and Ga-doped compounds showed free spin formation, indicated by Curie–Weiss tails in their magnetic susceptibility. Our results suggest emergence of short-range magnetic correlations in Na$_2$(Ni$_{2-x}$D$_x$)TeO$_6$, enhanced due to the depletion of the Ni honeycomb lattice.

75 - CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY A↗
DOE OSTI2022

Magnetic transition behavior in epitaxial Fe 47 Rh 47 Pd 6 films

The properties of Fe–Rh–Pd epitaxial thin films grown on MgO(001) were studied as a function of growth temperature. Films grown above 400°C exhibit a first-order antiferromagnetic to ferromagnetic magnetic phase transition with a transition temperature that decreases as the growth temperature is increased. The chemical order parameter computed from the ratio of intensities of the (001) and (002) diffraction peaks is nearly independent of the growth temperature, while the lattice constants change slightly. A comparison of our structural, magnetic, and electrical transport results with first-principle-based calculations as well as literature results indicates that the transition temperature of Fe–Rh-based alloy films depends sensitively on the lattice parameters and is of electronic origin. The transition temperature and its width can be tuned over a wide range by controlling the crystal structure via growth conditions or postdeposition annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Structure and properties of NdCuGa 3 single crystals

Here this manuscript reports on the structural and magnetic properties of NdCuGa 3 using powder and single crystal X-ray diffraction (XRD), zero-field single crystal neutron diffraction, magnetization, and specific heat measurements. Our XRD on a single crystal specimen of NdCuGa3 confirmed that it crystallizes in the tetragonal BaNiSi 3 -type structure. A magnetic phase transition at T N = 3.3 K is assessed using specific heat and ac magnetic susceptibility measurements. No additional anomaly below T N down to 50 mK was detected by performing specific heat measurements. Neutron single crystal diffraction data collected at T = 300 mK confirm the antiferromagnetic phase below T N = 3.3 K with the propagation vector $\vec{\tau}$ = (0.2, 0, 0). Possible magnetic structure solutions of NdCuGa 3 are discussed

36 MATERIALS SCIENCE↗
DOE OSTI2026

Altermagnetic polar metallic phase in ultrathin epitaxially strained RuO 2 films

Altermagnetism refers to a wide class of magnetic orders featuring magnetic sublattices with opposite spins related by rotational symmetries, resulting in nontrivial spin splitting and magnetic multipoles. However, the direct observation of the altermagnetic transition remains elusive. Here, by combining theoretical analysis, electrical transport, X-ray, and optical spectroscopies, we establish a phase diagram in hybrid molecular beam epitaxy-grown RuO 2 /TiO 2 (110) films, mapping symmetries along with altermagnetic/electronic/structural phase transitions as functions of film thickness and temperature. This features an altermagnetic metallic polar phase in epitaxially strained 2 nm films, suggesting a potential link between polar metals and altermagnetic materials. Such a clear signature of a magnetic phase transition at ~500 K is observed exclusively in ultrathin strained films, unlike in bulk RuO 2 single crystals. These results highlight the power of epitaxial heterostructure engineering to induce altermagnetism in systems initially nonmagnetic, opening avenues for realizing emergent quantum phases with multifunctional properties.

RuO2 thin film↗
DOE OSTI2025

Collinear antiferromagnetic order in a quasi-two-dimensional triangular lattice compound DyNiAl 4 Ge 2

The two-dimensional magnetic triangular lattice, with rare-earth local moments coupled through long-range interactions and hosting strong spin-orbit coupling, serves as an ideal platform for exploring novel quantum phenomena. Here, we report the physical properties of DyNiAl 4 Ge 2 single crystals through comprehensive structural, magnetic, heat capacity, electrical transport, and neutron powder diffraction measurements. DyNiAl 4 Ge 2 crystallizes in a trigonal lattice with the space group $R\bar{3}m$, where Dy atoms form two-dimensional triangular lattice layers within the 𝑎𝑏 plane and stack along the 𝑐 axis. Magnetic susceptibility measurement reveals a magnetic phase transition at 𝑇 𝑁 = 9.0 K, further confirmed by the 𝜆-shaped peak in the heat capacity curve. The obtained temperature-magnetic field phase diagram is relatively simple compared to other isostructural compounds, suggesting the absence of magnetic frustration. This is further evidenced by neutron powder diffraction, which shows a collinear antiferromagnetic ground state with a propagation vector 𝒌 = (0, 0, 1.5), implying the absence of magnetic frustration in DyNiAl 4 Ge 2 . In this magnetic structure, Dy moments are aligned ferromagnetically within each triangular layer and arranged antiferromagnetically between adjacent layers along the 𝑐 axis. In conclusion, our results establish DyNiAl 4 Ge 2 as a model system for understanding the interplay between RKKY interaction and geometrical frustration in this family of compounds.

Wu, HengHeng [Chinese Academy of Sciences (CAS), S↗
DOE OSTI2025

Magnetic order and physical properties of the kagome metal UNb 6 ⁢Sn 6

The 𝑅⁢𝑀 6 ⁢𝑋 6 family of materials (𝑅 = rare-earth, 𝑀 = transition metal, 𝑋 = Ga, Si, Ge, Sn) produces an array of emergent phenomena, such as charge density waves, intrinsic Hall effects, and complex magnetic order, due to its kagome net of transition metal atoms, its local-moment magnetic anisotropies, and its extensive chemical tunability. Here, in this study, we report UNb 6⁢ Sn 6 , a new “166” material containing both an actinide and a 4⁢𝑑 transition metal, along with its nonmagnetic analog ThNb6⁢Sn6, to investigate the properties of a 5⁢𝑓−4⁢𝑑 electron 166 system. UNb 6 ⁢Sn 6 crystallizes in the hexagonal 𝑃⁢6/𝑚⁢𝑚⁢𝑚 space group with a small degree of disorder due to shifts in the size of the CoSn-like cages along the 𝑐 axis. Upon cooling at zero magnetic field, the material undergoes two magnetic phase transitions at 𝑇 2 = 46K and 𝑇 N = 43K. The low-temperature, zero-field phase is an antiferromagnet with ordered uranium moments and a k = (0,0,1/2) propagation vector determined by neutron diffraction. Remarkably, with a magnetic field applied along the 𝑐 axis, five additional magnetic transitions occur, evidenced by magnetization and resistivity data, before the moment saturates at 2.62 𝜇 B /U at 2 K and ≥ 13.6T. In two magnetic phase regions, the Hall resistivity of UNb 6⁢ Sn 6 significantly deviates from the magnetization, suggesting that the phases have a large Berry curvature or a change in the Fermi surface. The unknown magnetic ordering of the field-dependent phases of UNb 6⁢ Sn 6 demonstrates the complexity of the 5⁢𝑓−4⁢𝑑 166 system and encourages further study of its properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Nonperturbative study of bulk photovoltaic effect enhanced by an optically induced phase transition

Solid systems with strong correlations and interactions under light illumination have the potential for exhibiting interesting bulk photovoltaic behavior in the non-perturbative regime, which has remained largely unexplored in the past theoretical studies. We investigate the bulk photovoltaic response of a perovskite manganite with strongly coupled electron-spin-lattice dynamics, using real-time simulations performed with a tight-binding model. The transient changes in the band structure and the photoinduced phase transitions, emerging from spin and phonon dynamics, result in a nonlinear current versus intensity behavior beyond the perturbative limit. The current rises sharply across a photoinduced magnetic phase transition, which later saturates at higher light intensities due to excited phonon and spin modes. The predicted peak photoresponsivity is orders of magnitude higher than other known ferroelectric oxides such as BiFeO$_3$. We disentangle phonon-and spin-assisted components to the ballistic photocurrent, showing that they are comparable in magnitude. Our results illustrate a promising alternative way for controlling and optimizing the bulk photovoltaic response through the photoinduced phase transitions in strongly-correlated systems.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

LaFeSi–LaFe 13-x Si x composites: Modulating magnetic and magnetocaloric properties through inherent stress manipulation

We examine structural and magnetic properties of a series of La–Fe–Si alloys in the region of concentrations where they naturally form two-phase LaFeSi–LaFe 13-x Si x composites with variable content and connectivity of LaFe 13-x Si x grains distributed within the LaFeSi matrix. Theoretical calculations confirm that the LaFeSi constituent is magnetically and structurally inert below room temperature and at pressures between -10 and 10 GPa. The LaFe 13-x Si x constituent, on the other hand, is magnetically and structurally active: it exhibits first-order magnetostructural transformations that, in addition to x Si , can be controlled with temperature, magnetic field, and pressure. In composites where the concentration of the inactive constituent is ~70 wt. % or greater, the standard, single-step, LaFe 13-x Si x first-order phase transformation proceeds in two steps separated by over 30 K in a zero magnetic field. Increasing the magnetic field recouples the two steps and restores the single-step phase transformation pathway. We analyze the roles of stresses caused by both thermal expansion mismatch and the first-order magnetic phase transition in LaFe 13-x Si x to rationalize the observed physical behaviors that emerge as the temperature or/and magnetic field vary.

36 MATERIALS SCIENCE↗