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We study the magnetic coupling in a thin film bilayer exchange-spring magnet composed of an amorphous ${\mathrm{Tb}}_{10}{\mathrm{Co}}_{90}$ layer with a large perpendicular anisotropy and an amorphous ${\mathrm{Co}}_{85}{({\mathrm{Al}}_{70}{\mathrm{Zr}}_{30})}_{15}$ layer with uniaxial in-plane anisotropy. The CoAlZr is directly exchange coupled to the TbCo with an interface region of at least 7.5 nm within which the CoAlZr magnetization is perpendicular to the plane and switches with the underlying TbCo. The influence of the coupling extends up to 30 nm into the CoAlZr resulting in an effective tilted anisotropy. The coercivity of the CoAlZr is greatly enhanced due to the coupling and is tuneable over a large range by varying its thickness.

We report on our evaluation of field-induced magnetic moments in the paramagnetic state of the $\text{spin-}\frac{1}{2}$ antiferromagnetic trimerized (${J}_{1}\ensuremath{-}{J}_{2}\ensuremath{-}{J}_{2}$) chain compound ${\mathrm{Cu}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{6}\mathrm{OD})}_{2}$ with ${J}_{1}=111\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${J}_{2}=30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ [M. Hase et al., Phys. Rev. B 76, 064431 (2007)]. Magnetic reflections with integer indices, generated by field-induced magnetic moments, were observed at neutron-diffraction experiments performed in an applied magnetic field, and we evaluated the magnitudes of the moments to be ${M}_{1}=0.43(2)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Cu}$ and ${M}_{2}=0.013(10)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Cu}$ on the two crystallographic ${\mathrm{Cu}}^{2+}$ (Cu1 and Cu2) sites, respectively, at 6 T and 1.6 K. The resulting ratio ${M}_{2}/{M}_{1}=0.03(2)$ is in good agreement with the theoretical value for the ratio of the magnetization at the two sites. We thus conclude that for this well-understood spin system (which has the advantage of being amenable to detailed theoretical calculations) the extracted field-induced magnetic moments reproduce the correct information on the magnetization. Our result leads us to believe that we can precisely evaluate exchange interactions in paramagnets with multiple exchange interactions and multiple crystallographic magnetic-ion sites by using field-induced magnetic moments in combination with macroscopic physical quantities. This idea is expected to be applicable to a wide variety of quantum and frustrated magnets without long-range order.

We propose a data-driven technique to estimate the spin Hamiltonian, including uncertainty, from multiple physical quantities. Using our technique, an effective model of ${\mathrm{KCu}}_{4}{\mathrm{P}}_{3}{\mathrm{O}}_{12}$ is determined from the experimentally observed magnetic susceptibility and magnetization curves with various temperatures under high magnetic fields. An effective model, which is the quantum Heisenberg model on a zigzag chain with eight spins having ${J}_{1}=\ensuremath{-}8.54\ifmmode\pm\else\textpm\fi{}0.51\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${J}_{2}=\ensuremath{-}2.67\ifmmode\pm\else\textpm\fi{}1.13\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${J}_{3}=\ensuremath{-}3.90\ifmmode\pm\else\textpm\fi{}0.15\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, and ${J}_{4}=6.24\ifmmode\pm\else\textpm\fi{}0.95\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, describes these measured results well. These uncertainties are successfully determined by the noise estimation. The relations among the estimated magnetic interactions or physical quantities are also discussed. The obtained effective model is useful to predict hard-to-measure properties such as spin gap, spin configuration at the ground state, magnetic specific heat, and magnetic entropy.

We report experimental and theoretical evidence that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a novel class of ferroelectricity induced by a vector spin chirality order., Comment: main text (6 pages, 5 figures) + supplemental material (10 pages, 8 figures), to appear in Phys. Rev. B (R)

We investigate the spectral profiles of time signals attributed to coherent phonon generation in an undoped Si crystal. Here, the retarded longitudinal-optical (LO) phonon Green function relevant to the temporal variance of induced charge density of ionic cores is calculated by employing the polaronic quasiparticle model developed by the authors [Y. Watanabe et al., Phys. Rev. B 95, 014301 (2017); ibid., 96, 125204 (2017)]. The spectral asymmetry is revealed in the frequency domain of the signals under the condition that an LO phonon mode stays almost energetically resonant with a plasmon mode in the early time region; this lasts for approximately 100 fs immediately after the irradiation of an ultrashort pump-laser pulse. It is understood that based on the adiabatic picture in time, this asymmetry is caused by the Rosen-Zener coupling between both modes. The associated experimental results are obtained by measuring time-dependent electro-optic reflectivity signals, and it is proved that these are in harmony with the calculated ones. The spectra become more symmetric, as the photoexcited carrier density further changes from that meeting the above condition to higher and lower sides of carrier densities. Moreover, the effect of optical nutation of carrier density on the CP signals is addressed, and the present results are compared with the asymmetry caused by transient Fano resonance, and the spectral profiles observed in a GaAs crystal in the text., 9 pages, 7 figures

In order to explain and predict the properties of many physical systems, it is essential to understand the interplay of different energy scales. Here we present investigations of the magnetic order ...

We measured the temperature dependence of the magnetic susceptibility $\ensuremath{\chi}(T)$ and the specific heat $C(T)$ and the magnetic-field dependence of the magnetization $M(H)$ of ${A}_{2}{\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}\phantom{\rule{4pt}{0ex}}(A$ = Rb or K) powder. We consider that the probable spin model is an interacting spin-1 antiferromagnetic tetramer model. We evaluated values of the intratetramer interactions as ${J}_{1}=9$ K and ${J}_{2}=18$ K, and the effective intertetramer interaction as ${J}_{\mathrm{eff}}=4$ K for ${\mathrm{Rb}}_{2}{\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}$. The susceptibility and magnetization at 1.3 K of ${\mathrm{K}}_{2}{\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}$ are very close to those of ${\mathrm{Rb}}_{2}{\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}$. We observed a phase transition to a magnetically ordered state in $C(T)/T$ in magnetic fields above 3 T. The transition temperature increases with magnetic field. Probably, the ordered state appears around 1.8 K even in 0 T. The ordered state in 0 T, however, is not stable enough like an order in the vicinity of a quantum critical point. Longitudinal-mode magnetic excitations may be observable in single crystalline ${A}_{2}{\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}\phantom{\rule{4pt}{0ex}}(A$ = Rb or K).

A Rb-NMR study has been performed on the quasi-one-dimensional competing spin chain $\mathrm{R}{\mathrm{b}}_{2}\mathrm{C}{\mathrm{u}}_{2}\mathrm{M}{\mathrm{o}}_{3}{\mathrm{O}}_{12}$ with ferromagnetic and antiferromagnetic exchange interactions on nearest-neighboring and next-nearest neighboring spins, respectively. The system changes from a gapped ground state at zero field to a gapless state at ${H}_{\mathrm{C}}\ensuremath{\simeq}2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, where the existence of magnetic order below 1 K was demonstrated by a broadening of the NMR spectrum, associated with a critical divergence of $1/{T}_{1}$. In the higher-temperature region, ${T}_{1}^{\ensuremath{-}1}$ showed a power-law-type temperature dependence, from which the field dependence of the Luttinger parameter $K$ was obtained and compared with theoretical calculations based on the spin nematic Tomonaga-Luttinger liquid (TLL) state.

We measured magnetization, specific heat, electron spin resonance, neutron diffraction, and inelastic neutron scattering of ${\mathrm{CrVMoO}}_{7}$ powder. An antiferromagnetically ordered state appears below ${T}_{\mathrm{N}}=26.5\ifmmode\pm\else\textpm\fi{}0.8$ K. We consider that the probable spin model for ${\mathrm{CrVMoO}}_{7}$ is an interacting antiferromagnetic spin-$\frac{3}{2}$ dimer model. We evaluated the intradimer interaction $J$ to be $25\ifmmode\pm\else\textpm\fi{}1$ K and the effective interdimer interaction ${J}_{\mathrm{eff}}$ to be $8.8\ifmmode\pm\else\textpm\fi{}1$ K. ${\mathrm{CrVMoO}}_{7}$ is a rare spin dimer compound that shows an antiferromagnetically ordered state at atmospheric pressure and zero magnetic field. The magnitude of ordered moments is $0.73(2){\ensuremath{\mu}}_{\mathrm{B}}$. It is much smaller than a classical value $\ensuremath{\sim}3{\ensuremath{\mu}}_{\mathrm{B}}$. Longitudinal-mode magnetic excitations may be observable in single crystalline ${\mathrm{CrVMoO}}_{7}$.

We measured the temperature dependence of the magnetic susceptibility and specific heat and the magnetic-field dependence of the magnetization of CuInVO$_5$. An antiferromagnetically ordered state appears below $T_{\rm N} = 2.7$ K. We observed a $\frac{1}{2}$ quantum magnetization plateau above 30 T at 1.3 K. We show that the spin system consists of antiferromagnetic spin-$\frac{1}{2}$ tetramers with $J_1 = 240 \pm 20$ and $J_2 = -142 \pm 10$ K for the intratetramer interactions.

We performed inelastic neutron scattering experiments on ${\mathrm{Cu}}_{2}^{114}\mathrm{Cd}{}^{11}{\text{B}}_{2}{\mathrm{O}}_{6}$ powder. The magnetic excitations at low temperatures are similar to those of the interacting spin-$\frac{1}{2}$ tetramers in the ordered state. The weak excitations existing above 3 meV suggest that the Higgs mode appears in ${\mathrm{Cu}}_{2}{\mathrm{CdB}}_{2}{\mathrm{O}}_{6}$ at ambient pressure and zero magnetic field. We evaluated ${J}_{1}=27.3\ifmmode\pm\else\textpm\fi{}1.0$ and ${J}_{2}=\ensuremath{-}14.0\ifmmode\pm\else\textpm\fi{}1.4$ meV for the intratetramer interactions and ${J}_{3}=\ensuremath{-}0.4\ifmmode\pm\else\textpm\fi{}0.2$ and ${J}_{4}=1.4\ifmmode\pm\else\textpm\fi{}0.2$ meV for the intertetramer interactions. The spin gap in the isolated spin tetramer was calculated to be 1.6 meV, which is less than the effective intertetramer interaction value $(3.6\ifmmode\pm\else\textpm\fi{}0.8$ meV). Therefore, antiferromagnetic long-range order is possible, although the ground state of the isolated spin tetramer is the spin-singlet state. We discuss the temperature dependence of the magnetic excitations.

We investigated the crystal and magnetic structures of the $\text{spin}\ensuremath{-}\frac{1}{2}$ frustrated antiferromagnet ${\mathrm{Cu}}_{3}{\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$ in which the spin system consists of antiferromagnetic chains and dimers. The space group at room temperature has been reported to be orthorhombic $Pnma$ (No. 62). We infer that the space group above ${T}_{\mathrm{N}}=7.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ is monoclinic $P{2}_{1}/m$ (No. 11) from the observation of reflections forbidden in $Pnma$ in x-ray powder diffraction experiments at room temperature. We determined the magnetic structure of ${\mathrm{Cu}}_{3}{\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$ in neutron powder diffraction experiments. Magnetic moments on dimer sites lie in the $ac$ planes. The magnitudes are $(0.50\ensuremath{\sim}0.74){\ensuremath{\mu}}_{\mathrm{B}}$. Moments on chain sites may exist but the magnitudes are very small. The magnetic structure indicates that a partially disordered state is realized. We consider the origin of the magnetic structure, weak ferromagnetism, and electric polarization.

Coherent phonons (CPs) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, nonthermal switching in phase-change materials such as ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ (GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100-ps x-ray pulses from the European Synchrotron Radiation Facility (ESRF) storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal; however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone, implying the presence of excited-state effects in addition to temperature-driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase transitions in GST.

The element-specific magnetization and ordering in trilayers consisting of 0.3–1.4 monolayer (ML) thick Fe layers embedded in Pd(001) has been determined using x-ray resonant magnetic scattering. The proximity to Fe induces a large moment in the Pd which extends ∼2nm from the interfaces. The magnetization as a function of temperature is found to differ significantly for the Fe and Pd sublattices: The Pd signal resembles the results obtained by magneto-optical techniques with an apparent three-dimensional (3D) to two-dimensional (2D) transition in spatial dimensionality for Fe thickness below ∼1ML. In stark contrast, the Fe data exhibits a 2D behavior. No ferromagnetic signal is obtained from Fe below the 2D percolation limit in Fe coverage (∼0.7ML), while Pd shows a ferromagnetic response for all samples. The results are attributed to the temperature dependence of the susceptibility of Pd and a profound local anisotropy of submonolayered Fe.

A spin-singlet ground state with a spin gap has been discovered in antiferromagnetic spin chain substances when the spin value is 1/2, 1, or 2. To find spin-gap (singlet-triplet) excitations in spin-$\frac{3}{2}$ chain substances, we performed inelastic neutron scattering and magnetization measurements on $R\mathrm{CrGeO}{}_{5}$ ($R=\text{Y}$ or Sm) powders. As expected, we observed spin-gap excitations and the dispersion relation of the lowest magnetic excitations. We proved that the spin system of $\mathrm{Cr}{}^{3+}$ was an antiferromagnetic alternating spin-$\frac{3}{2}$ chain. We describe the influence of frustration between nearest-neighbor and next-nearest-neighbor exchange interactions in the chain.

The electronic energy band structure for cubic palladium oxides ${\mathrm{CaPd}}_{3}{\mathrm{O}}_{4}$ in the ${\mathrm{NaPt}}_{3}{\mathrm{O}}_{4}$ structure has been calculated by the full-potential linearized augmented-plane-wave method within the local-density approximation. ${\mathrm{CaPd}}_{3}{\mathrm{O}}_{4}$ is almost semiconducting, but has a very small number of carriers. We propose that this compound is a good candidate for being an excitonic insulator, similar to ${\mathrm{CaB}}_{6}.$ Moreover, the bands near the Fermi level have one-dimensional character.

Polarized far-infrared (FIR) spectroscopic measurements and FIR magneto-optical studies were performed on the inorganic spin-Peierls compound ${\mathrm{CuGeO}}_{3}.$ An absorption line, which was found at $98 {\mathrm{cm}}^{\ensuremath{-}1}$ in the dimerized phase $(D$ phase), was assigned to a folded phonon mode of ${B}_{3u}$ symmetry. The splitting of the folded mode into two components in the incommensurate phase (IC phase) has been observed. A broad absorption centered at $63 {\mathrm{cm}}^{\ensuremath{-}1}$ was observed only in the $\mathbf{E}\ensuremath{\Vert}b$ axis polarization, which was assigned to a magnetic excitation from singlet ground state to a continuum state.

The electronic energy-band structures for layered oxichloride ${\mathrm{BiPbO}}_{2}\mathrm{Cl}$ have been calculated by using the scalar-relativistic full-potential linearized augmented plane-wave method within the local-density approximation. It is found that (i) ${\mathrm{BiPbO}}_{2}\mathrm{Cl}$ is a band insulator with 1.3 eV indirect gap and its electronic structure has strong two dimensionality. (ii) The lowest portion of the conduction band is dominated only by $\mathrm{Bi}\ensuremath{-}p$ bands, and the p band of lowest energy is looked like an ``s band'' due to the cancellation of \ensuremath{\sigma} and \ensuremath{\pi} bonding. An effective model of ${\mathrm{BiPbO}}_{2}\mathrm{Cl}$ can be regarded as a two-dimensional analogue of ${\mathrm{BaPb}}_{x}{\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{O}}_{3}.$

By separately identifying magnetic and charge scatter, we find conclusive evidence for conformality in magnetic roughness in {Co (8 \AA{}) Cu (9 \AA{})} multilayers. For layers magnetized in the easy direction, the magnetic roughness equals the structural roughness but increases when magnetized in the hard direction. The in-plane magnetic correlation length, which changes on magnetization, is several orders of magnitude larger than the structural roughness length scales. The magnetic length scale is of the same order as magnetic ripple observed in Lorentz microscopy and is not associated with domains.

The oscillatory interlayer indirect exchange coupling in Co/Cu multilayers is known to be highly sensitive to structural defects. In this paper the dependence of the antiferromagnetic exchange coupling on the background pressure in the vacuum chamber is investigated. Co/Cu multilayers were grown by dc magnetron sputtering in a system equipped with a leak valve to allow the introduction of low levels of air or ${\mathrm{O}}_{2}.$ Below a relatively narrow band of pressure the samples exhibit excellent antiferromagnetic coupling and consequently a high giant magnetoresistance, $\ensuremath{\sim}40%$ for {Co(8 \AA{})/Cu(8 \AA{})}$\ifmmode\times\else\texttimes\fi{}25$ samples. Above this transitional band of pressure no antiferromagnetic coupling, and hence no giant magnetoresistance, is observed. X-ray diffraction measurements reveal no change of any significance in any of the layer thicknesses or roughnesses. Whilst the high-field magnetic behavior is found to be isotropic in the sample plane, the reversal of the remanent moment around zero field is found to show a varying degree of uniaxial anisotropy. Lower remanent moments are found to be associated with a more isotropic reversal mechanism. A thorough characterization of the physical and magnetic microstructure by means of various modes of transmission electron microscopy is presented. Cross-sectional images reveal subtle changes in the crystallinity and layer quality of the samples as the background pressure is increased. Plan-view Lorentz microscopy reveals that the isotropic reversal mechanism of the low remanence samples involves a complete domain structure. The reversal mechanism of multilayers with a significant remanent moment varies markedly with field direction and can be dominated by rotation or comparatively simple domain processes. Samples with a significant giant magnetoresistance $\ensuremath{\sim}20%$ and remanent fractions $\ensuremath{\sim}0.7$ are found to still show highly anisotropic reversal mechanisms around zero field. Indeed when demagnetized along the easy axis the samples are in a single domain state at remanence. This is compelling evidence for substantial noncollinear ordering of the moments through biquadratic coupling.

The coherent folded longitudinal acoustic (FLA) phonons in a GaAs/AlAs superlattice have been studied using a two-color pump-probe technique and a designed sample in which the superlattice layer is sandwiched by ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layers. The two-color pump-probe technique makes it possible to separate regions of generation and detection of coherent phonons, which provides a powerful tool to examine the spatial dynamics of coherent phonons. We have found two types of coherent FLA phonons: the first-order FLA phonons with finite wave vectors $(q\ensuremath{\ne}0)$ and the zone-center mode of the first-order FLA phonons $(q=0).$ When ballistic acoustic phonons are generated near the surface region of the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layer and propagate through the superlattice layer, the first-order FLA phonons with $q\ensuremath{\ne}0$ are observed in the superlattice layer. On the other hand, when the FLA phonons are generated within the superlattice layer, the zone-center FLA phonon extending from the superlattice layer is detected near the surface region of the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layer.

The electronic energy-band structures for \ensuremath{\beta}-ZrNCl and \ensuremath{\beta}-HfNCl, which can be superconducting by intercalation, have been calculated by using the scalar-relativistic full-potential linearized augmented-plane-wave method within the local-density approximation. For \ensuremath{\beta}-ZrNCl, we have calculated the electronic structure for the four proposed crystal structures, and obtained two kinds of qualitatively different energy bands. For the first two structures, the calculations show that this compound is a band insulator with a gap of \ensuremath{\sim}0.2 eV, which is significantly smaller than the experimental value. For the last two structures, this band gap is increased to \ensuremath{\sim}1.6 and \ensuremath{\sim}0.8 eV, which is closer to the experiment. A comparison between other experimental data and the calculated density of states also supports the crystal structure of the recent neutron-diffraction experiment. A tight-binding analysis has revealed that direct $d\ensuremath{-}d$ hopping is important for the gap formation of the mother material. We have compared the experimental ${T}_{c}$ gap calculated band gap, and density of states for several intercalation-derived superconductors, and found some empirical trends.

We have measured photoemission and inverse-photoemission spectra of the ferromagnetic metal ${\mathrm{SrRuO}}_{3}.$ The observed Ru $4d$-derived conduction band above and below the Fermi level ${(E}_{F})$ is found to be widely spread and the emission intensity at ${E}_{F}$ to be weakened compared to band-structure calculations. Here, the calculations have been made for the ferromagnetic state with the actual distorted perovskite structure. We compare the spectra with the spectral density of states (DOS) obtained by modifying the band DOS with a phenomenological self-energy correction, which is chosen to be compatible with the measured electronic specific heat coefficient $\ensuremath{\gamma}.$ Although the mass enhancement factor is only moderately large, $\ensuremath{\sim}4.4,$ the renormalization factor Z is found to be as small as $\ensuremath{\sim}0.08$ ${(Z}^{\ensuremath{-}1}\ensuremath{\sim}13).$ These indicate a significant momentum dependence of the self-energy and a highly incoherent metallic state in ${\mathrm{SrRuO}}_{3}.$ We have also compared the photoemission and inverse-photoemission spectra taken below and above the Curie temperature, but significant changes have not been identified.