Magnetic Bragg Peak Intensity Research Articles

4D Visualization of a Nonthermal Coherent Magnon in a Laser Heated Lattice by an X-ray Free Electron Laser.

Ultrafast optical manipulation of magnetic phenomena is an exciting achievement of mankind, expanding one's horizon of knowledge toward the functional nonequilibrium states. The dynamics acting on an extremely short timescale push the detection limits that reveal fascinating light-matter interactions for nonthermal creation of effective magnetic fields. While some cases are benchmarked by emergent transient behaviors, otherwise identifying the nonthermal effects remains challenging. Here, a femtosecond time-resolved resonant magnetic X-ray diffraction experiment is introduced, which uses an X-ray free-electron laser (XFEL) to distinguish between the effective field and the photoinduced thermal effect. It is observed that a multiferroic Y-type hexaferrite exhibits magnetic Bragg peak intensity oscillations manifesting entangled antiferromagnetic (AFM) and ferromagnetic (FM) Fourier components of a coherent AFM magnon. The magnon trajectory constructed in 3D space and time domains is decisive to evince ultrafast field formation preceding the lattice thermalization. A remarkable impact of photoexcitation across the electronic bandgap is directly unraveled, amplifying the photomagnetic coupling that is one of the highest among AFM dielectrics. Leveraging the above-bandgap photoexcitation, this energy-efficient optical process further suggests a novel photomagnetic control of ferroelectricity in multiferroics.

Search for octupolar order in NpO2 by neutron powder diffraction

The magnetic structure of neptunium dioxide (NpO2) remains a mystery despite decades of research. We revisited the search for magnetic ordering in NpO2 by performing a powder neutron diffraction experiment at low temperatures and relatively large values of Q. Diffraction data were collected for 300 ​mK, 15 ​K, and 35 ​K to understand the phase transition near 25 ​K. However, no significant changes in the neutron diffraction pattern were identified. Using the octupolar magnetic form factor j6, the maximum theoretical expected value of the magnetic moment (3.27 μB), the calculated magnetic Bragg peak intensities would not be observable in neutron powder diffraction data. This value is much larger than the previously estimated magnetic moment of ∼0.1 μB. Supported by experimental results, these calculations suggest that unpolarized neutron powder diffraction is unsuitable for measuring octupolar ordering in NpO2.

Magnetic structure of the swedenborgite compound CaBaMn2Fe2O7 derived by powder neutron diffraction and Mössbauer spectroscopy

We present a study combining neutron diffraction and $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy on a powder sample of ${\mathrm{CaBaMn}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{7}$ belonging to the large family of swedenborgite compounds. The undistorted hexagonal crystal structure (space group $P{6}_{3}mc$) is preserved down to low temperatures, and all employed techniques reveal a transition into a magnetically long-range ordered phase at ${T}_{\mathrm{N}}$ = 205 K. The magnetic Bragg peak intensities from the powder diffraction patterns together with a symmetry analysis of the employed models unambiguously reveal the classical $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ magnetic structure on a hexagonal lattice with propagation vector $\mathbf{q}=(\frac{1}{3}\phantom{\rule{0.28em}{0ex}}\frac{1}{3}\phantom{\rule{0.28em}{0ex}}0)$. The nuclear Bragg peak intensities allowed the statistical distribution of Fe and Mn ions on both trigonal and kagome sites of the complex swedenborgite structure to be analyzed which was considered to explain the complex shape of the M\"ossbauer spectra.

Noncollinear antiferromagnetic order in the buckled honeycomb lattice of magnetoelectric Co4Ta2O9 determined by single-crystal neutron diffraction

Co4Ta2O9 exhibits a three-dimensional magnetic lattice based on the buckled honeycomb motif. It shows unusual magnetoelectric effects, including the sign change and non-linearity. These effects cannot be understood without the detailed knowledge of the magnetic structure. Herein, we report neutron diffraction and direction-dependent magnetic susceptibility measurements on Co4Ta2O9 single crystals. Below 20.3 K, we find a long-range antiferromagnetic order in the alternating buckled and flat honeycomb layers of Co2+ ions stacked along the c axis. Within experimental accuracy, the magnetic moments lie in the ab plane. They form a canted antiferromagnetic structure with a tilt angle of ~ 14 degrees at 15 K in the buckled layers, while the magnetic moments in each flat layer are collinear. This is directly evidenced by a finite (0, 0, 3) magnetic Bragg peak intensity, which would be absent in the collinear magnetic order. The magnetic space group is C2'/c. It is different from the previously reported C2/c' group, also found in the isostructural Co4Nb2O9. The revised magnetic structure successfully explains the major features of the magnetoelectric tensor of Co4Ta2O9 within the framework of the spin-flop model.

Magnetic phase diagram of the frustrated spin chain compound linarite PbCuSO4(OH)2 as seen by neutron diffraction and H1 -NMR

We report on a detailed neutron diffraction and $^1$H-NMR study on the frustrated spin-1/2 chain material linarite, PbCuSO$_4$(OH)$_2$, where competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor interactions lead to frustration. From the magnetic Bragg peak intensity studied down to 60 mK, the magnetic moment per Cu atom is obtained within the whole magnetic phase diagram for $H \parallel b$ axis. Further, we establish the detailed configurations of the shift of the SDW propagation vector in phase V with field and temperature. Finally, combining our neutron diffraction results with those from a low-temperature/high-field NMR study we find an even more complex phase diagram close to the quasi-saturation field suggesting that bound two-magnon excitations are the lowest energy excitations close to and in the quasi-saturation regime. Qualitatively and semi-quantitatively, we relate such behavior to $XYZ$ exchange anisotropy and contributions from the Dzyaloshinsky-Moriya interaction to affect the magnetic properties of linarite.

The magnetic properties and structure of the quasi-two-dimensional antiferromagnet CoPS3

The magnetic properties and magnetic structure are presented for CoPS3, a quasi-two-dimensional antiferromagnet on a honeycomb lattice with a Néel temperature of K. The compound is shown to have XY-like anisotropy in its susceptibility, and the anisotropy is analysed to extract crystal field parameters. For temperatures between 2 K and 300 K, no phase transitions were observed in the field-dependent magnetization up to 10 Tesla. Single-crystal neutron diffraction shows that the magnetic propagation vector is k = with the moments mostly along the axis and with a small component along the axis, which largely verifies the previously-published magnetic structure for this compound. The magnetic Bragg peak intensity decreases with increasing temperature as a power law with exponent for .

Coexistence of long-range magnetic ordering and singlet ground state in the spin-ladder superconductor SrCa13Cu24O41

A long-range magnetic order was discovered in the quasi-one-dimensional spin-ladder compound SrCa${}_{13}$Cu${}_{24}$O${}_{41}$ by susceptibility, specific heat, and neutron diffraction experiments. The temperature dependence of the magnetic Bragg peak intensity could be well fitted to the power law with a transition temperature ${T}_{N}$ $=$ 4.23 K and a critical exponent \ensuremath{\beta} $=$ 0.28, indicating a three-dimensional phase transition for a low-dimensional magnet. A computer program was coded and found two possible magnetic structure models fitting best with all the observed magnetic peaks. These models suggest the spin-ladder sublattice is magnetically ordered with Cu moments aligning along the $a$ axis. The spin interactions are primarily antiferromagnetic along rungs and legs, while there are ferromagnetic clusters along legs. Surprisingly, the singlet-triplet spin-gap excitation is observable above and below ${T}_{N}$, indicating a coexistence of the spin-singlet ground state and long-range magnetic ordering state in this compound.

Critical behavior in a Kondo-screening partially-ordered antiferromagnet CePdAl

A rare-earth compound CePdAl is a good example of the Kondo-screening partially-ordered antiferromagnet below TN = 2.7 K. In this compound, the formation of the Kondo-singlet state is realized as a new route of the relief of the frustration. The spin dynamics of CePdAl are, however, still controversial. In order to further investigate the spin dynamics of CePdAl, we have performed 27Al-NMR and specific heat single crystal measurements and reanalysed published powder neutron diffraction data. NMR intensities decrease with decreasing temperature in the ordered state suggesting the slow spin fluctuations below TN = 2.7 K, while the intensity of magnetic Bragg peak measured by neutron diffraction increases monotonously in the same temperature range. The critical exponents derived from the neutron diffraction and the specific heat measurements are not coincident with those of known universality classes. An unconventional magnetic order parameter or a charge degree of freedom originated from the coexistence of frustration and the Kondo effect may influence the critical behavior.

Crystal and magnetic structures of the superconductor CeNi0.8Bi2

We have performed powder neutron diffraction on the new superconductor, CeNi$_{0.8}$Bi$_2$ with a superconducting transtion temperature $T_\textrm{c} \sim$ 4.2 K. The structural parameters of this compound at room temperature are determined by Rietveld analysis. Below about 5 K, the clear magnetic Bragg peaks with propagation vector $q$=(0 0 0) are observed. The observed intensities of magnetic Bragg peaks can be explained by the magnetic structure that the two Ce moments in the unit cell are antiparallel along c axis. The magnetic Bragg peaks are observed in the superconducting state, indicating the coexistence of the antiferromagnetic ordering and the superconductivity in this compound. The intensity of magnetic Bragg peak monotonously increases with decreasing temperature below $T_\textrm{N}$ and does not exhibit apparent anomaly at $T_\textrm{c}$, obviously different from cases of heavy fermion superconductors in which the magnetic ordering and the superconductivity coexist, for example, Cd-doped CeCoIn$_5$. These results suggest that the 4$f$ electron of the Ce atom is not coupled with the superconducting carrier, and the magnetic ordering is almost independent of the superconductivity in CeNi$_{0.8}$Bi$_2$.

Static and Dynamic Magnetism in Underdoped SuperconductorBaFe1.92Co0.08As2

We report neutron scattering measurements on single crystals of BaFe1.92Co0.08As2. The magnetic Bragg peak intensity is reduced by 6% upon cooling through TC. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional interactions. Below TC additional intensity appears at an energy of approximately 4.5(0.5) meV, similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below TC for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance, demonstrating the direct competition between static magnetic order and superconductivity.

Do antiferromagnetism and superconductivity coexist in 2% and 10% Ge doped CeCu2Si2?

Doping the heavy-fermion compound CeCu2Si2 with Ge allows for studying the unconventional superconducting state in the presence of a more stabilized antiferromagnetic phase. We performed heat capacity and elastic neutron scattering measurements on single crystals of 2% and 10% Ge doped CeCu2Si2 at different temperatures and magnetic fields. The zero field superconducting and magnetic transition temperatures in 2% Ge doped CeCu2Si2 are still close to those in pure CeCu2Si2, whereas Tc and Tn differ by an order of magnitude in 10% Ge doped CeCu2Si2. Our neutron scattering results show that the magnetic Bragg peak intensity decreases with decreasing temperature below Tc in CeCu2(Si0.98 Ge0.02)2. However, in CeCu2(Si0.9Ge0.1)2, it stays constant in the superconducting regime down to lowest temperatures. We conclude that, while a phase separation between magnetic and superconducting volumes takes place in 2% Ge doped CeCu2Si2, antiferromagnetism and superconductivity coexist on the microscopic scale in 10% doped CeCu2Si2.

Energetic selection of ordered states in a model of the Er2Ti2O7 frustrated pyrochlore XY antiferromagnet

We consider the possibility that the discrete long-range ordered states of Er2Ti2O7 are selected energetically at the mean field level as an alternative scenario that suggests selection via thermal fluctuations. We show that nearest neighbour exchange interactions alone are not sufficient for this purpose, but that anisotropies arising from excited single ion crystal field states in Er2Ti2O7, together with appropriate anisotropic exchange interactions, can produce the required long range order. However, the effect of the single ion anisotropies is rather weak so we expect thermal or quantum fluctuations, in some guise, to be ultimately important in this material. We reproduce recent experimental results for the variation of magnetic Bragg peak intensities as a function of magnetic field.

Internal Fields Observed by Heavy Itinerant Electrons in the Superconducting Ferromagnet UGe2

There are fascinating classes of materials called heavy fermions which contain a periodic array of localized f electrons interacting with itinerant conduction electrons. Owing to both the strong Coulomb correlation within f shell and the mixing of f -electron and itinerant-electron wave functions, a heavy fermion state is formed at low temperatures, where the electronic specific heat coefficient can be enhanced by several orders of magnitude over that of ordinary transition metals. Among them, some uraniumbased intermetallic compounds have attracted much attention due to the unusual coexistence of superconductivity and magnetism. In particular, the interplay between superconductivity and ferromagnetism in UGe2 is an interesting topic in the solid-state physics. As shown in a schematic phase diagram of UGe2 (see an inset in Fig. 1), a Curie temperature (TFM) monotonically decreases with increasing pressure (P), and collapses to zero temperature at around 1.5GPa. In the ferromagnetic phase, there seems to be another phase transition or crossover at TX, where a magnetic Bragg peak intensity exhibits a steep increase (see Fig. 1). This characteristic temperature TX also decreases with increasing pressure and becomes suppressed to zero at a critical pressure PX ( 1:2GPa). Since the superconducting transition temperature (TSC) shows a maximum at around PX, it has been speculated that the critical point PX plays an important role of the superconductivity (see, for example, refs. 2 and 3). Recently, Nakane et al. provided a supporting evidence for the conjecture by means of ac magnetic susceptibility measurements. In general, superconductivity and magnetism would be antagonistic ground states, because of the competitive nature between the Meissner effect and internal fields generated by magnetic orderings. In contrast with the case of an antiferromagnet, ferromagnetic internal fields can not be cancelled out in the range of the superconducting coherence length, which raises a question of why such nonvanishing internal fields do not destroy the superconducting Cooper pairs. This has motivated us to evaluate the magnitude of the internal fields in UGe2. In a previous paper, Aso et al. reported the temperature dependence of magnetic Bragg peak intensities [IBðTÞ] under pressure, and showed that IBðTÞ below TX can be explained by a conventional Stoner model, a typical example at P 1⁄4 1:1GPa being illustrated in Fig. 1. Furthermore, they argued that the system lies in a perfectly polarized state in a pressure region below PX. In the present Short Note, we show that this simple model also provides useful information about the internal fields observed by itinerant electrons which may carry superconducting currents. The characterization of single crystals grown by the Czochralski pulling method is seen in refs. 4 and 6. Experimental details of the elastic neutron scattering under pressure were reported in ref. 5, and will be given elsewhere. According to the Stoner model, the low temperature magnetization M in the case of the perfectly polarized state is expressed as follows (see, for example, ref. 8), M 1⁄4 M0f1 T expð =TÞg; ð1Þ

Enhancement of Magnetic Order in the Incommensurate Phase of Mg-doped CuGeO3

We present elastic neutron scattering results at high magnetic fields for Mg-doped CuGeO$_{3}$. For low magnesium concentrations, where a spin-Peierls phase is present, we find an enhancement of the long range antiferromagnetic order in the structural incommensurate phase induced by a high magnetic field. In the incommensurate phase, the N\'eel temperature increases with field leading to a gain in the magnetic Bragg peak intensity at low temperatures. We also find that the magnetic Bragg peaks remain commensurate even though the structure is incommensurate at high magnetic fields due to the formation of solitons. We interpret these results in terms of the solitons formed at high-magnetic fields effectively acting as doped impurities. Our results point to a strongly disordered ground state with the the antiferromagnetic order localized around impurity sites.

Magnetic Properties of ZnFe2O4as a 3-D Geometrical Spin Frustration System

Magnetic properties of ZnFe 2 O 4 were studied for various specimens prepared by different thermal and artificial treatments using neutron scattering, Mössbauer spectroscopy and DC and AC susceptibilities. We found a strong correlation between the magnetic Bragg peak intensity and the Mössbauer absorption line-width. Comparing with the susceptibility data, it is discussed what are the intrinsic magnetic properties of ZnFe 2 O 4 as a typical 3-D geometrical spin frustration system.

Field-induced magnetic order inLa2−xSrxCuO4(x=0.10,0.115,0.13)studied by in-plane thermal conductivity measurements

We have measured the thermal conductivity in the $ab$ plane of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ $(x=0.10,0.115,0.13)$ in magnetic fields up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ parallel to the $c$ axis and also parallel to the $ab$ plane. By the application of magnetic fields parallel to the $c$ axis, the thermal conductivity has been found to be suppressed at low temperatures below the temperature ${T}_{\ensuremath{\kappa}}$ which is located above the superconducting transition temperature and is almost independent of the magnitude of the magnetic field. The suppression is marked in $x=0.10$ and 0.13, while it is small in $x=0.115$. Furthermore, no suppression is observed in the $1%$ $\mathrm{Zn}$-substituted ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Cu}}_{0.99}{\mathrm{Zn}}_{0.01}{\mathrm{O}}_{4}$ with $x=0.115$. Taking into account the experimental results that the temperature dependence of the relative reduction of the thermal conductivity is quite similar to the temperature dependence of the intensity of the incommensurate magnetic Bragg peak corresponding to the static stripe order and that the $\mathrm{Zn}$ substitution tends to stabilize the static order, it is concluded that the suppression of the thermal conductivity in magnetic fields is attributed to the development of the static stripe order. The present results suggest that the field-induced magnetic order in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ originates from the pinning of the dynamical stripes of spins and holes by vortex cores.

Interplay of structural and electronic phase separation in single-crystallineLa2CuO4.05studied by neutron and Raman scattering

We report a neutron- and Raman-scattering study of a single crystal of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4.05}$ prepared by high-temperature electrochemical oxidation. Elastic neutron-scattering measurements show the presence of two phases, corresponding to the two edges of the first miscibility gap, all the way up to 300 K. An additional oxygen redistribution, driven by electronic energies, is identified at 250 K in Raman scattering (RS) experiments by the simultaneous onset of two-phonon and two-magnon scattering, which are fingerprints of the insulating phase. Elastic neutron-scattering measurements show directly an antiferromagnetic ordering below a N\'eel temperature of ${T}_{N}=210\mathrm{K}.$ The opening of the superconducting gap manifests itself as a redistribution of electronic Raman scattering below the superconducting transition temperature, ${T}_{c}=24\mathrm{K}.$ A pronounced temperature-dependent suppression of the intensity of the (100) magnetic Bragg peak has been detected below ${T}_{c}.$ We ascribe this phenomenon to a change of relative volume fraction of superconducting and antiferromagnetic phases with decreasing temperature caused by a form of a superconducting proximity effect.

Relations of crystal structure to magnetic properties in the quasi-one-dimensional compound PbNi 1.88Mg 0.12V 2O 8

The crystal structure of the quasi-one-dimensional oxide PbNi 1.88Mg 0.12V 2O 8 has been studied by Rietveld analysis of combined high-resolution neutron and X-ray powder diffraction data at 300 K and at low temperatures. The (Ni/Mg)O 6 octahedral units share a common edge and form spiral chains along the c-axis of the tetragonal unit cell, without deviating the I4 1 cd ( Z=8) symmetry upon cooling. DC magnetic susceptibility measurements show that the system undergoes a magnetic phase transition below T N≅3.4 K. Rietveld analysis of the medium resolution neutron powder diffraction data confirms that impurity-induced antiferromagnetic order (with propagation vector, κ =[000] ) takes over from the Haldane ground state of the parent compound. The power-law [ β=0.31(3)] temperature evolution of the strongest magnetic Bragg peak intensity indicates three-dimensional Ising-type magnetic interactions, while the reduced magnitude of the Ni 2+ moment [〈 μ〉=0.98(3) μ B] suggests important zero-point spin fluctuations. Structural considerations are consistent with small changes in the interatomic distances around the bridging tetrahedral VO 4 entities separating the chains. However, no bulk structural phase transition concurrent to the Néel ordering is found. We show that the modification of intra- and inter-chain Ni–Ni distances upon cooling promotes the magnetic coupling of the end-of-chain liberated S=1/2 spins and leads to antiferromagnetic ordering.

High- Tc superconductors with antiferromagnetic order: limitations on spin-fluctuation pairing mechanism

The antagonistic interplay of antiferromagnetism (AF) and superconductivity (SC), recently discovered in high-temperature superconductors, is studied in the framework of a microscopic theory. We explain the surprisingly large increase of the magnetic Bragg peak intensity I Q at Q∼( π, π) in the magnetic field H≪ H c2 at low temperatures 0< T≪ T c, T AF in La 2− x Sr x CuO 4. Good agreement with experimental results is found. The theory predicts large anisotropy of the relative intensity R Q ( H)=( I Q ( H)− I Q (0))/ I Q (0), i.e. R Q( H ∥c -axis)≫R Q( H ⊥c -axis) . The quantum ( T=0) phase diagram at H=0 is constructed. The theory also predicts: (i) that the magnetic field can induce the AF order in the SC state; (ii) that the spin-fluctuation (SF) effective coupling constant g<0.1 eV is small, which gives small SC critical temperature T c (≪40 K)––thus questioning the SF mechanism of pairing in HTS oxides.

Comment on “Magnetic field effects on neutron diffraction in the antiferromagnetic phase ofUPt3”

Moreno and Sauls [Phys. Rev. B 63, 024419 (2000)] have recently tried to reanalyze earlier neutron scattering studies of the antiferromagnetic order in UPt3 with a magnetic field applied in the basal plane. In their calculation of the magnetic Bragg peak intensities, they perform an average over different magnetic structures belonging to distinct symmetry representations. This is incorrect. In addition, they have mistaken the magnetic field direction in one of the experiments, hence invalidating their conclusions concerning the experimental results.