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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Probing ultrafast spin dynamics through a magnon resonance in the antiferromagnetic multiferroic HoMnO 3

Here, we demonstrate an approach for directly tracking antiferromagnetic (AFM) spin dynamics by measuring ultrafast changes in a magnon resonance. We also test this idea on the multiferroic HoMnO 3 by optically photoexciting electrons, after which changes in the spin order are probed with a THz pulse tuned to a magnon resonance. This reveals a photoinduced change in the magnon line shape that builds up over 5–12 picoseconds, which we show to be the spin-lattice thermalization time, indicating that electrons heat the spins via phonons. We compare our results to previous studies of spin-lattice thermalization in ferromagnetic manganites, giving insight into fundamental differences between the two systems. Finally, our work sheds light on the microscopic mechanism governing spin-phonon interactions in AFMs and demonstrates a powerful approach for directly monitoring ultrafast spin dynamics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Spin-lattice coupling mediated multiferroicity in (ND 4 ) 2 FeCl 5 • D 2 O

In this paper, we report a neutron diffraction study of the multiferroic mechanism in (ND 4 ) 2 FeCl 5 • D 2 O, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at T N = 7.3 K, followed by a second transition to an IC cycloidal spin state at T FE = 6.8 K, the latter of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling, as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The second-order harmonic appears at T FE , thereby providing unambiguous evidence that the onset of the electric polarization is accompanied by a lattice modulation due to spin-lattice interaction. The neutron results, in conjunction with the negative thermal expansion and large magnetostriction observed, indicate that spin-lattice coupling plays a critical role in the ferroelectric mechanism of (ND 4 ) 2 FeCl 5 • D 2 O.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Terahertz dielectric analysis and spin-phonon coupling in multiferroic GeV 4 S 8

We present an investigation of the multiferroic lacunar spinel compound GeV 4 S 8 using time-domain terahertz spectroscopy. We find three absorptions which either appear or shift at the antiferromagnetic transition temperature, T N =17K, as S=1 magnetic moments develop on vanadium tetrahedra. Two of these absorptions are coupled to the magnetic state and one only appears below the Néel temperature, and is interpreted as a magnon. We also observe isosbestic points in the dielectric constant in both the temperature and frequency domains. Further, we perform an analysis on the isosbestic features to reveal an interesting collapse into a single curve as a function of both frequency and temperature, behavior which exists throughout the phase transitions. This analysis suggests the importance of spectral changes in the terahertz range which are linear in frequency and temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Magnetic field-temperature phase diagram of multiferroic [ ( CH 3 ) 2 NH 2 ] Mn ( HCOO ) 3

We combined here pulsed field magnetization and first-principles spin-density calculations to reveal the magnetic field-temperature phase diagram and spin state character in multiferroic $[{({\mathrm{CH}}_{3})}_{2}{\mathrm{NH}}_{2}]\mathrm{Mn}{(\mathrm{HCOO})}_{3}$. Despite similarities with the rare earth manganites, the phase diagram is analogous to other Mn-based quantum magnets with a 0.31 T spin flop, a 15.3 T transition to the fully polarized state, and short-range correlations that persist above the ordering temperature. The experimentally accessible saturation field opens the door to exploration of the high-field phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Phonon mode links ferroicities in multiferroic [ ( CH 3 ) 2 NH 2 ] Mn ( HCOO ) 3

In this work, we combined magnetoinfrared spectroscopy, magnetization, and lattice dynamics calculations to explore the phase transitions in multiferroic [ ( CH 3 ) 2 NH 2 ] Mn ( HCOO ) 3 Both the 185 K ferroelectric transition and the magnetically driven transition to the fully saturated state at 15.3 T involve the formate bending mode, direct evidence of a common connection between the two types of ferroicities and for how lattice distortions advance high-temperature magnetoelectric coupling in this system.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Soft antiphase tilt of oxygen octahedra in the hybrid improper multiferroic Ca 3 Mn 1.9 Ti 0.1 O 7

Here, we report a single crystal neutron and x-ray diffraction study of the hybrid improper multiferroic Ca 3 Mn 1.9 Ti 0.1 O 7 (CMTO), a prototypical system where the electric polarization arises from the condensation of two lattice distortion modes. With increasing temperature ( T ), the out-of-plane, antiphase tilt of MnO 6 decreases in amplitude while the in-plane, in-phase rotation remains robust and experiences abrupt changes across the first-order structural transition. Application of hydrostatic pressure ( P ) to CMTO at room temperature shows a similar effect. The consistent behavior under both T and P reveals the softness of antiphase tilt and highlights the role of the partially occupied d orbital of the transition-metal ions in determining the stability of the octahedral distortion. Polarized neutron analysis indicates the symmetry-allowed canted ferromagnetic moment is less than the 0.04μ B /Mn site, despite a substantial out-of-plane tilt of the MnO 6 octahedra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Polarization analysis of magnetic excitation in multiferroic Ba 2 CoGe 2 O 7

Magnetic excitations in the multiferroic Ba 2 CoGe 2 O 7 are measured by combination of unpolarized and polarized neutron-scattering techniques. The unpolarized neutron spectrum reveals an anisotropic Q dependence of the intensity of the electromagnon mode at 4 meV. The analysis of the polarized neutron spectra identifies the directions of the magnetic fluctuations of the observed modes. The determined modes are consistent with the calculation by the extended spin-wave theory. A couple of transverse modes and a single longitudinal mode are involved in the magnetic excitation of Ba 2 CoGe 2 O 7 . In conclusion, the longitudinal fluctuation takes on a main role in the electromagnon mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Comprehensive inelastic neutron scattering study of the multiferroic Mn 1 - x Co x WO 4

Here, using high-resolution inelastic neutron scattering, we examine the spin dynamics of Mn 1-x Co x WO 4 in the collinear AF1, the ac-b spiral AF2, and the ac cycloidal AF5 phases. The spin wave excitations are well described by a Heisenberg model with competing long-range exchange interactions (J i up to 12th nearest neighbors) and the single-ion anisotropy K induced by the spin-orbit interaction. Additionally, while the exchange constants are relatively unchanged, the dominant effect of doping is to change the single-ion anisotropy from easy axis (K>0) in the collinear AF1 phase to easy plane (K<0) in the two multiferroic phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Weak exchange striction between the 4 f and 3 d ions in the multiferroic GdMn 2 O 5

We used single-crystal neutron diffraction to investigate the magnetic structures of the multiferroic GdMn 2 O 5 . The system undergoes a first-order incommensurate to commensurate magnetic transition below 33 K, accompanied by the appearance of electric polarization P . Upon cooling, P increases smoothly while the magnetic order shows an abrupt enhancement in intensity below 20 K owing to the large increase of the rare-earth Gd moment. The contrasting temperature evolution of the magnetic order and polarization indicates the polarization is mainly driven by the exchange striction between the magnetic Mn ions. The incommensurate phase in the intermediate temperature range has a cycloidal modulation along the c axis, with spin configuration projected in the ab plane similar to the commensurate phase. The lack of observable bulk polarization suggests the exchange striction is not the dominant mechanism for the polarization in the incommensurate phase.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Topological Semimetal features in the Multiferroic Hexagonal Manganites

Using first-principles calculations we investigate the band structures of ferromagnetic hexagonal manganites $\mathrm{YXO_3}$ (X=V, Cr, Mn, Fe and Co) in the nonpolar nonsymmorphic $P6_3/mmc$ space group. For $\mathrm{YVO_3}$ and $\mathrm{YCrO_3}$ we find a band inversion near the Fermi energy that generates a nodal ring in the $k_z=0$ mirror plane. We perform a more detailed analysis for these compounds and predict the existence of the topological "drumhead" surface states. Finally, we briefly discuss the low-symmetry polar phases (space group $P6_3cm$) of these systems, and show they can undergo a $P6_3/mmc \rightarrow P6_3cm$ transition by condensation of soft $K_3$ and $\Gamma_2^-$ phonons. Based on our results, stabilizing these compnds in the hexagonal phase could offer a promising platform for studying the interplay of topology and multiferroicity, and the coexistence of real-space and reciprocal-space topological protection in the same phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Competing magnetic states in multiferroic BaYFeO 4 : A high magnetic field study

Spin-driven ferroelectricity phenomena have drawn great interest in the scientific community due to potential application in spintronics and their complex physical mechanisms. A noticeable example of this is multiferroic BaYFeO 4 that exhibits an unconventional magnetoelectric (ME) coupling due to the uncorrelated behavior of the ferroelectric and cycloidal states under an applied magnetic field. To shed more light on this spin-driven ME effect, a high-quality sample of BaYFeO 4 was synthesized by a standard solid-state reaction method, and its high-field (up to 9 T) magnetic properties have been systematically investigated by means of magnetometry, magnetocaloric effect, and Mössbauer measurements over a wide temperature range (5–400 K). In addition, its crystal and magnetic structures have been studied using x-ray and neutron powder diffraction. Results obtained in this work indicate that Fe spins form a long-range spin density wave (SDW) antiferromagnetic (AFM) order at $T_{\text{N1}}$ ~ 50K, which transforms into the cycloidal AFM order at $T_{\text{N2}}$ ~ 35K. A spin-glass-like state emerges below $T^\ast$ ~ 17K, and coexists with the long-range cycloidal AFM one in this temperature range. Magnetocaloric and Mössbauer measurements consistently confirm the robustness of both the long-range SDWand cycloidal AFM orders under applied magnetic fields up to 6 T, whereas the spin-glass state is converted into the ferromagnetic (FM) state when the applied magnetic field exceeds 1 T. These findings pinpoint the fact that the magnetic field evolution of spin correlations from the AFM to FM character in the spin-glass state is responsible for the magnetic field dependence of ferroelectricity in BaYFeO 4 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Voltage control of unidirectional anisotropy in ferromagnet-multiferroic system

Demonstration of ultralow energy switching mechanisms is imperative for continued improvements in computing devices. Ferroelectric (FE) and multiferroic (MF) order and their manipulation promise an ideal combination of state variables to reach attojoule range for logic and memory (i.e., ~30× lower switching energy than nanoelectronics). In BiFeO 3 (BFO), the coupling between the antiferromagnetic (AFM) and FE order is robust at room temperature, scalable in voltage, stabilized by the FE order, and can be integrated into a fabrication process for a beyond-CMOS (complementary metal-oxide semiconductor) era. The presence of the AFM order and a canted magnetic moment in this system causes exchange interaction with a ferromagnet such as Co 0.9 Fe 0.1 or La 0.7 Sr 0.3 MnO 3 . Previous research has shown that exchange coupling (uniaxial anisotropy) can be controlled with an electric field. However, voltage modulation of unidirectional anisotropy, which is preferred for logic and memory technologies, has not yet been demonstrated. Here, we present evidence for electric field control of exchange bias of laterally scaled spin valves that is exchange coupled to BFO at room temperature. We show that the exchange bias in this bilayer is robust, electrically controlled, and reversible. We anticipate that magnetoelectricity at these scaled dimensions provides a powerful pathway for computing beyond modern nanoelectronics by enabling a new class of nonvolatile, ultralow energy computing elements.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Phase transition enhanced superior elasticity in freestanding single-crystalline multiferroic BiFeO 3 membranes

The integration of ferroic oxide thin films into advanced flexible electronics will bring multifunctionality beyond organic and metallic materials. However, it is challenging to achieve high flexibility in single-crystalline ferroic oxides that is considerable to organic or metallic materials. Here, we demonstrate the superior flexibility of freestanding single-crystalline BiFeO 3 membranes, which are typical multiferroic materials with multifunctionality. They can endure cyclic 180° folding and have good recoverability, with the maximum bending strain up to 5.42% during in situ bending under scanning electron microscopy, far beyond their bulk counterparts. Such superior elasticity mainly originates from reversible rhombohedral-tetragonal phase transition, as revealed by phase-field simulations. This study suggests a general fundamental mechanism for a variety of ferroic oxides to achieve high flexibility and to work as smart materials in flexible electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2016

X-ray diffraction study of Ba{sub 3}TaFe{sub 3}Si{sub 2}O{sub 14} single crystal—a promising langasite-type multiferroic

Ba{sub 3}TaFe{sub 3}Si{sub 2}O{sub 14} single crystals (sp. gr. P321, Z = 1), promising langasite-type multiferroics, have been grown by floating zone melting. An accurate X-ray diffraction study of Ba{sub 3}TaFe{sub 3}Si{sub 2}O{sub 14} single crystal has been performed using two datasets, obtained independently for two different orientations of the same sample on a diffractometer equipped with a CCD area detector at 295 K. Structure refinement is performed based on an averaged dataset: a = 8.5355(1) Å, c = 5.2332(1) Å, sp. gr. P321, Z = 1; the R factors of model structure refinement were found to be R/wR = 1.02/1.23% for 4552 independent reflections. Disordering asymmetry is revealed for the magnetic Fe ion in the 3f site and the Ba cation in the 3e site.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Elastic, magnetic, and magnetoelectric properties of the CaBaCo{sub 4}O{sub 7} multiferroic

The structural, elastic, magnetic, and magnetoelectric properties of the CaBaCo{sub 4}O{sub 7} multiferroic are experimentally studied and compared with the properties of the related YBaCo{sub 4}O{sub 7} cobaltite, where Y{sup 3+} ions substitute for Ca{sup 2+} ions. Unlike the frustrated YBaCo{sub 4}O{sub 7} magnet, the softening of Young’s modulus and the hysteresis in the ΔE(T)/E{sub 0} curve of ferrimagnetic CaBaCo{sub 4}O{sub 7} in the paramagnetic region are weak, and the anomaly during the magnetic transition increases by almost an order of magnitude. This difference can point to different characters of the development of a long-range magnetic order in these two cobaltites. The distortion of the crystal structure that removes the frustrations of exchange interactions is found to correlate with the magnetic behavior of the cobaltites under study. The magnetization curves of the Ca cobaltite have two steps below 15 K, which can point to the presence of a metastable state in a high magnetic field. The study of the longitudinal and transverse magnetoelectric effects in a pulsed magnetic field demonstrates that their magnitudes are maximal near T{sub C} and change their character from linear to quadratic during passage through this temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

μSR study of Eu{sub 0.8}Ce{sub 0.2}Mn{sub 2}O{sub 5} and EuMn{sub 2}O{sub 5} multiferroics

A comparative μSR study of ceramic samples of the EuMn{sub 2}O{sub 5} and Eu{sub 0.8}Ce{sub 0.2}Mn{sub 2}O{sub 5} multiferroics is performed in the temperature range from 15 to 300 K. It is found that the Ce doping of the EuMn{sub 2}O{sub 5} sample slightly reduces the temperature of the magnetic phase transition from T{sub N} = 45 K for the EuMn{sub 2}O{sub 5} sample to T{sub N} = 42.5 K for the Eu{sub 0.8}Ce{sub 0.2}Mn{sub 2}O{sub 5} sample. Below the temperature T{sub N} for both samples, there are two types of localization of a thermalized muon with different temperature dependences of the precession frequency of the magnetic moment of the muon in an internal magnetic field. The higher frequency in both samples refers to the initial antiferromagnetic matrix. The behavior of this frequency in Eu{sub 0.8}Ce{sub 0.2}Mn{sub 2}O{sub 5} follows the Curie–Weiss law with the exponent β = 0.29 ± 0.02, which differs from the value β = 0.39 standard for 3D Heisenberg magnetics and is observed in EuMn{sub 2}O{sub 5}, because of the strong frustration of the doped sample. The temperature-independent low frequency is due to the presence of Mn{sup 3+}–Mn{sup 4+} ferromagnetic pairs located along the b axis of the antiferromagnetic matrix and in the regions of phase separation, which contain such ion pairs and e{sub g} electrons recharging them. In both samples, polarization losses are the same (about 20%) and are associated with the formation of Mn{sup 4+}–Mn{sup 4+} + Mu complexes near Mn{sup 3+}–Mn{sup 4+} ferromagnetic pairs. In the temperature interval from 25 to 45 K, the separation of the Eu{sub 0.8}Ce{sub 0.2}Mn{sub 2}O{sub 5} structure into two fractions where the relaxation rates of polarization of muons differ by an order of magnitude is revealed. This effect is due to a change in the state of regions of phase separation (1D superlattices) at the indicated temperatures. Such effect in EuMn{sub 2}O{sub 5} is significantly weaker.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Features of the low-frequency polarization response in the region of the ferroelectric phase transition in multiferroic TbMnO 3

The unusual behavior of the low-frequency (10 Hz–1 MHz) permittivity in single crystals of ferroelectric multiferroic TbMnO3 has been experimentally and theoretically studied in detail in the region of the narrow temperature peak of the permittivity, associated with the ferroelectric phase transition (T C ~ 27.4 K). It has been found that the ε c (ω, T) maximum sharply decreases with increasing measured field frequency, while the temperature position of the maximum is independent of frequency. It has been shown that the observed features of the polarization response can be satisfactorily described within the Landau–Khalatnikov polarization relaxation theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effect of Frequency of the Alternating Electric Field on Temperature Impedance Spectra of LuFe{sub 2}O{sub 4} Ceramic Multiferroic

Temperature impedance spectra are measured on LuFe{sub 2}O{sub 4} ceramic multiferroics over a temperature range of 100–400 K, upon applying a sinusoidal voltage with a frequency varying between 20 Hz and 120MHz. At signal frequencies of 30–70 MHz, spectra exhibit features such as an abnormal positive temperature resistance coefficient within the temperature range of 200–260 K, which are interpreted in the context of the generalized barrier model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗