Longitudinal Spin Density Wave Research Articles

Effect of magnetic order on longitudinal Tomonaga-Luttinger liquid spin dynamics in weakly coupled spin- 12 chains

The quantum many-body interactions in one-dimensional spin-$\frac{1}{2}$ systems are subject to Tomonaga-Luttinger liquid (TLL) physics, which predict an array of multiparticle excitations that form continua in momentum-energy space. Here we use inelastic neutron spectroscopy to study the TLL spin dynamics in ${\mathrm{SrCo}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, a compound which contains weakly coupled spin-$\frac{1}{2}$ chains of Co atoms, at 0.05 K and in a longitudinal magnetic field up to 9.0 T. The measurements were performed above 3.9 T, where the ground-state N\'eel antiferromagnetic (AFM) order is completely suppressed and the multiparticle excitations are exclusively of the TLL type. In this region and below 7.0 T, the longitudinal TLL mode---psinon/antipsinon (P/AP)---is unexpectedly well described by a damped harmonic oscillator (DHO) while approaching the zone center defining the static spin-spin correlations. A non-DHO-type, continuumlike signal is seen at higher fields, but deviations from the ideal one-dimensional TLL still remain. This change in the P/AP mode coincides with the phase transition between the longitudinal spin density wave (LSDW) and transverse AFM order. Inside the LSDW state, the DHO-type P/AP spectral weight increases and the linewidth broadens as the magnetic order parameter decreases. These results reveal the impact of three-dimensional magnetic order on the TLL spin dynamics; they call for beyond-mean-field treatment for the interchain exchange interactions.

Metamagnetism and crystal-field splitting in pseudohexagonal CeRh3Si2

CeRh$_3$Si$_2$ has been reported to exhibit metamagnetic transitions below 5~K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) reveal a $J_z$\,=\,1/2 groundstate for Ce when considering the crystallographic $a$ direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78\,meV for the $J$\,=\,5/2 multiplet. The neutron diffraction study in zero field reveals that on cooling from the paramagnetic state, the system first orders at $T_{\text{N}_1}=4.7$\,K in a longitudinal spin density wave with ordered Ce moments along the $b$-axis (i.e. the [0 1 0] crystal direction) and an incommensurate propagation vector $\textbf{k}=(0,0.43,0$). Below the lower-temperature transition $T_{\text{N}_2}=4.48$\,K, the propagation vector locks to the commensurate value $\textbf{k}=(0,0.5,0)$, with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field $H\parallel b$-axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at $H=3$\,kOe, followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at $H = 7$\,kOe. This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behaviour is also observed for the $H\parallel$ [0 1 1] crystal direction.

Crystal electric field and possible coupling with phonons in Kondo lattice CeCuGa3

We investigate the magnetic and crystal electric field (CEF) states of the Kondo lattice system CeCuGa3 by muon spin relaxation (muSR), neutron diffraction, and inelastic neutron scattering (INS) measurements. A noncentrosymmetric BaNiSn3-type tetragonal crystal structure (space group I4mm) is inferred from x-ray as well as from neutron powder diffraction. The low-temperature magnetic susceptibility and heat capacity data show an anomaly near 2.3 - 2.5~K associated with long range magnetic ordering, which is further confirmed by muSR and neutron diffraction data. The neutron powder diffraction collected at 1.7 K shows the presence of magnetic Bragg peaks indexed by an incommensurate magnetic propagation vector k = (0.148, 0.148, 0) and the magnetic structure is best described by a longitudinal spin density wave with ordered moments lying in ab-plane. An analysis of the INS data based on a CEF model reveals the presence of two magnetic excitations near 4.5 meV and 6.9 meV. The magnetic heat capacity data suggest an overall CEF splitting of 20.7 meV, however the excitation between 20 and 30 meV is very broad and weak in our INS data, but could provide an evidence of CEF level in this energy range in agreement with the magnetic entropy. Our analysis of INS data based on the CEF-phonon model indicates that the two excitations at 4.5 meV and 6.9 meV have their origin in CEF-phonon coupling (i.e. splitting of one CEF peak into two peaks, called vibron), with an overall splitting of 28.16 meV, similar to the case of CeCuAl3 and CeAuAl3.

Role of the spin anisotropy of the interchain interaction in weakly coupled antiferromagnetic Heisenberg chains**Project supported by the National Natural Science Foundation of China (Grant No. 11674392), the Ministry of Science and Technology of China, National Program on Key Research Project (Grant No. 2016YFA0300504), and the Research Funds of Remnin University of China (Grant No. 18XNLG24). R.Y. acknowledges the hospitality

In quasi-one-dimensional (q1D) quantum antiferromagnets, the complicated interplay of intrachain and interchain exchange couplings may give rise to rich phenomena. Motivated by recent progress on field-induced phase transitions in the q1D antiferromagnetic (AFM) compound YbAlO3, we study the phase diagram of spin-1/2 Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations, and investigate the role of the spin anisotropy of the interchain coupling on the ground state of the system. We find that the Ising anisotropy of the interchain coupling can significantly enhance the longitudinal spin correlations and drive the system to an incommensurate AFM phase at intermediate magnetic fields, which is understood as a longitudinal spin density wave (LSDW). With increasing field, the ground state changes to a canted AFM order with transverse spin correlations. We further provide a global phase diagram showing how the competition between the LSDW and the canted AFM states is tuned by the Ising anisotropy of the interchain coupling.

Phase diagram and quantum criticality of Heisenberg spin chains with Ising anisotropic interchain couplings

Motivated by recent progress on field-induced phase transitions in quasi-one-dimensional quantum antiferromagnets, we study the phase diagram of $S=1/2$ antiferromagnetic Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations. The interchain interactions is shown to enhance longitudinal spin correlations to stabilize an incommensurate longitudinal spin density wave order at low temperatures. With increasing field the ground state changes to a canted antiferromagnetic order until the magnetization fully saturates above a quantum critical point controlled by the $(3+2)$D XY universality. Increasing temperature in the quantum critical regime the system experiences a fascinating dimension crossover to a universal Tomonaga-Luttinger liquid. The calculated NMR relaxation rate $1/T_1$ indicates this Luttinger liquid behavior survives a broad field and temperature regime. Our results determine the global phase diagram and quantitative features of quantum criticality of a general model for quasi-one-dimensional spin chain compounds, and thus lay down a concrete ground to the study on these materials.

Tomonaga-Luttinger Liquid Spin Dynamics in the Quasi-One-Dimensional Ising-Like Antiferromagnet BaCo_{2}V_{2}O_{8}.

Combining inelastic neutron scattering and numerical simulations, we study the quasi-one-dimensional Ising anisotropic quantum antiferromagnet BaCo_{2}V_{2}O_{8} in a longitudinal magnetic field. This material shows a quantum phase transition from a Néel ordered phase at zero field to a longitudinal incommensurate spin density wave at a critical magnetic field of 3.8T. Concomitantly, the excitation gap almost closes and a fundamental reconfiguration of the spin dynamics occurs. These experimental results are well described by the universal Tomonaga-Luttinger liquid theory developed for interacting spinless fermions in one dimension. We especially observe the rise of mainly longitudinal excitations, a hallmark of the unconventional low-field regime in Ising-like quantum antiferromagnetic chains.

Magnetic phase diagram of the quantum spin chain compound SrCo2V2O8: a single-crystal neutron diffraction study

We explore magnetic order in the quantum spin chain compound SrCo2V2O8 up to 14.9 T and down to 50 mK, using single-crystal neutron diffraction. Upon cooling in zero-field, commensurate antiferromagnetic (C-AFM) order with modulation vector = (0, 0, 1) develops below TN ≃ 5.0 K. Applying an external magnetic field (H∥c axis) destabilizes this C-AFM order, leading to an order-disorder transition between TN and ∼1.5 K. Below 1.5 K, a commensurate to incommensurate (IC-AFM) transition occurs at 3.9 T, above which the magnetic reflections can be indexed by = (0, 0, 1 ± δl). The incommensurability δl scales monotonically with H until the IC-AFM order disappears around 7.0 T. Magnetic reflections modulated by emerge again at higher fields. While the characters of the C-AFM, IC-AFM and the emergent AFM order in SrCo2V2O8 appear to fit the descriptions of the Néel, longitudinal spin density wave and transverse AFM order observed in the related compound BaCo2V2O8, our results also reveal several unique signatures that are not present in the latter, highlighting the inadequacy of mean-field theory in addressing the complex magnetic order in systems of this class.

Incommensurate Magnetism Near Quantum Criticality in CeNiAsO.

We report the discovery of incommensurate magnetism near quantum criticality in CeNiAsO through neutron scattering and zero field muon spin rotation. For T<T_{N1}=8.7(3) K, a second order phase transition yields an incommensurate spin density with a wave vector k=(0.44(4),0,0). For T<T_{N2}=7.6(3) K, we find coplanar commensurate order with a moment of 0.37(5)μ_{B}, reduced to 30% of the saturation moment of the |±1/2⟩ Kramers doublet ground state, which we establish through inelastic neutron scattering. Muon spin rotation in CeNiAs_{1-x}P_{x}O shows the commensurate order only exists for x≤0.1 so we infer the transition at x_{c}=0.4(1) is between an incommensurate longitudinal spin density wave and a paramagnetic Fermi liquid.

Magneto-orbital texture in the perovskite modification of Mn2O3

Crystal and magnetic structures of the high-pressure-stabilized perovskite modification of ${\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$ ($\ensuremath{\zeta}\text{\ensuremath{-}}{\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$) have been studied by neutron powder diffraction combined with symmetry arguments based on the phenomenological Landau theory. This metastable phase exhibits a unique charge disproportionation phenomenon stabilizing the quadruple perovskite structure (${\mathrm{Mn}}^{2+}{\mathrm{Mn}}_{3}^{3+}){\mathrm{Mn}}_{4}^{3.25+}{\mathrm{O}}_{12}$ with an additional charge-ordering and commensurate orbital density wave localized in the B-site perovskite position. The commensurate nature of the orbital density wave is stimulated by a coupling of the orbital ordering to independent structural distortions, which improve poor bonding conditions of ${\mathrm{Mn}}^{2+}$ in the A-site perovskite position. Below ${T}_{1}\ensuremath{\sim}100$ K, an anharmonic longitudinal spin density wave arises and locks to the structural modulation associated with the orbital density. At ${T}_{2}\ensuremath{\sim}50$ K, the magnetic subsystem delocks from the structural modulation giving rise to a multi-$k$ phase-modulated ground state admixing cycloidal and helical components. The complex anharmonic and phase-modulated magnetic structures are discussed based on a phenomenological magneto-orbital coupling scheme, previously developed to explain the multi-$k$ helical ground states with modulated spin chirality observed in ${\mathrm{A}}^{2+}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ (${\mathrm{A}}^{2+}=\text{Ca}$, Sr, Pb, and Cd) quadruple perovskites.

Neutron scattering study of spin density wave and weak ferromagnetic orders in [formula omitted

Microscopic coexistence between weak ferromagnetism (WFM) and superconductivity (SC) in Tb0.47Y0.53Ni211B2C was investigated. Magnetization measurements indicate that antiferromagnetic (AFM), WFM and SC transitions occur in this material. Unpolarized neutron diffraction experiments revealed that the AFM order is best described by a longitudinal spin density wave with a propagation vector of q=0.550a∗. Polarized neutron diffraction experiments further confirmed that the WFM order is originated in Tb0.47Y0.53Ni211B2C.

Crystallite size effect on the monoclinic deformation of the bcc crystal structure of chromium

The modulated spin density wave magnetic orderings observed in chromium suggests that the crystal structure of chromium cannot be described by the cubic space group Im3¯m. Our experimental studies of polycrystalline and nanocrystalline chromium by synchrotron radiation (SR) and neutron powder diffraction show a hkl-dependent Bragg peak broadening which can be interpreted by the low-symmetry monoclinic space group P21/n instead of the high symmetry cubic space group Im3¯m. The monoclinic angle is βm = 90.05(1)° and 90.29(1)° for polycrystalline Cr and nanocrystalline Cr, respectively. The relative monoclinic distortion observed in chromium is 5 times larger than those reported for several oxides: BiFeO3, α-Fe2O3, Cr2O3 and calcite. The symmetry of the magnetic transverse spin density wave (TSDW) and the longitudinal spin density wave (LSDW) observed in Cr are described by using the superspace groups P21/n(0β0)00 and P21′/n′(0β0)00, respectively. These superspace groups describe both the magnetic modulations and the atomic position modulations reported in the literature. The monoclinic symmetry of chromium is a robust effect which is observed in the paramagnetic as well as in the TSDW and LSDW phases.

Magnetism of PrFeAsO parent compound for iron-based superconductors: Mössbauer spectroscopy study

Mössbauer spectroscopy measurements were performed for the temperature range between 4.2 K and 300 K in a transmission geometry applying 14.41-keV resonant line in 57Fe for PrFeAsO the latter being a parent compound of the iron-based superconductors belonging to the ‘1111’ family. It was found that an itinerant 3d magnetic order develops at about 165 K and it is accompanied by an orthorhombic distortion of the chemical unit cell. A complete longitudinal 3d incommensurate spin density wave (SDW) order develops at about 140 K. Transferred hyperfine magnetic field generated by the praseodymium magnetic order on iron nuclei is seen at 12.8 K and below, i.e., below magnetic order of praseodymium magnetic moments. It is oriented perpendicular to the field of SDW on iron nuclei. The shape of SDW is almost rectangular at low temperatures and it transforms into roughly triangular form around “nematic” transition at about 140 K. Praseodymium magnetic order leads to the substantial enhancement of SDW due to the large orbital contribution to the magnetic moment of praseodymium. A transferred field indicates presence of strong magnetic susceptibility anisotropy in the [b-c] plane while following rotation of praseodymium magnetic moments in this plane with lowering temperature. It was found that “nematic” phase region is a region of incoherent spin density wavelets typical for a critical region.

Incommensurate spin density wave at a ferromagnetic quantum critical point in a three-dimensional parabolic semimetal

We explore the ferromagnetic quantum critical point in a three-dimensional semimetallic system with upward- and downward-dispersing bands touching at the Fermi level. Evaluating the static spin susceptibility to leading order in the coupling between the fermions and the fluctuating ferromagnetic order parameter, we find that the ferromagnetic quantum critical point is masked by an incommensurate, longitudinal spin density wave phase. We first analyze an idealized model which, despite having strong spin-orbit coupling, still possesses O(3) rotational symmetry generated by the total angular momentum operator. In this case, the direction of the incommensurate spin density wave propagation can point anywhere, while the magnetic moment is aligned along the direction of propagation. Including symmetry-allowed anisotropies in the fermion dispersion and the coupling to the order parameter field, however, the ordering wavevector instead breaks a discrete symmetry and aligns along either the [111] or [100] direction, depending on the signs and magnitudes of these two types of anisotropy.

Mössbauer studies of the peculiar magnetism in parent compounds of the iron-based superconductors

A review of the magnetism in the parent compounds of the iron-based superconductors is given based on the transmission Mössbauer spectroscopy of 57Fe and 151Eu. It was found that the 3d magnetism is of the itinerant character with varying admixture of the spin-polarized covalent bonds. For the ‘122’ compounds, a longitudinal spin density wave (SDW) develops. In the case of the EuFe2As2, a divalent europium arranges in an anti-ferromagnetical order at a much lower temperature as compared with the onset of SDW. These two magnetic systems remain almost uncoupled one to another. For the non-stoichiometric Fe1+xTe parent of the ‘11’ family, one has a transversal SDW and magnetic order of the interstitial iron with relatively high and localized magnetic moments. These two systems are strongly coupled one to another. For the ‘grand parent’ of the iron-based superconductors FeAs, one observes two mutually orthogonal phase-related transversal SDW on the iron sites. There are two sets of such spin arrangements due to two crystallographic iron sites. The FeAs exhibits the highest covalency among the compounds studied, but it has still a metallic character.

Field-induced magnetic behavior in quasi-one-dimensional Ising-like antiferromagnet BaCo2V2O8: A single-crystal neutron diffraction study

BaCo${}_{2}$V${}_{2}$O${}_{8}$ is a nice example of a quasi-one-dimensional quantum spin system that can be described in terms of Tomonaga-Luttinger liquid physics. This is explored in the present study where the magnetic field-temperature phase diagram is thoroughly established up to 12 T using single-crystal neutron diffraction. The transition from the N\'eel phase to the incommensurate longitudinal spin density wave (LSDW) phase through a first-order transition, as well as the critical exponents associated with the paramagnetic-N\'eel phase transition, and the magnetic order both in the N\'eel and in the LSDW phase are determined, thus providing a stringent test for the theory.

Spin Density Wave Ordering in CeIrSi3

Pressure induced superconductivity discovered in CeRhSi3 and CeIrSi3 has attracted much attention due to their being heavy fermion superconductors without inversion symmetry. It is also pointed out that both of them lie close to the quantum critical point, and interestingly Ce 4f electrons are itinerant even in their antiferromagnetic phase in sharp contrast to CeRhIn5 in which Ce 4f electrons are localized in the antiferromagnetic phase. The itinerant character of the Ce 4f electron in CeRhSi3 was pointed out by de Haas–van Alphen (dHvA) measurements. When Ce 4f electrons are localized, the Fermi surfaces (FSs) in CeRhSi3 will be very similar to those in LaRhSi3. On the other hand, the observed dHvA frequencies and then FSs in CeRhSi3 are totally different to those in LaRhSi3, and these results are interpreted as a strong indication of the itinerant character of Ce 4f electrons in CeRhSi3. The angle-resolved photoemission study in CeIrSi3 indicates that the Ce 4f electrons in CeIrSi3 are also itinerant. The observed FSs in CeIrSi3 are substantially larger than those in LaIrSi3 and they contact each other at Brillouin zone boundaries. The ground states of Ce ions in CeIrSi3 and CeRhSi3 are determined to be 7 doublets through the susceptibility studies, and their magnetic moments order antiferromagnetically at low temperatures. A neutron diffraction study of those magnetic structures may provide important information on the possible itinerant character of Ce 4f electrons in CeIrSi3 and CeRhSi3. The magnetic structure of CeRhSi3 was determined by our previous study. CeRhSi3 shows an antiferromagnetic phase below TN 1⁄4 1:6K with the magnetic moment of ord 1⁄4 0:13 B/Ce. The magnetic moments lie in the c plane and form a longitudinal spin-density wave with a propagation vector 1⁄4 ð0:215; 0; 1=2Þ, being consistent with a picture that the Ce 4f electrons are itinerant. Recently, we also succeeded in observing magnetic peaks in CeIrSi3 by neutron diffraction. The magnetic peaks appear below TN 1⁄4 5:0K at Q 1⁄4 G 1 and at Q 1⁄4 G 2 with 1 1⁄4 ð0:265; 0; 0:43Þ and 2 1⁄4 ð 0:265; 0; 0:43Þ, and G denotes the fundamental reciprocal vector for the BaNiSn3-type structure. In contrast to the magnetic structure of CeRhSi3, the observed magnetic structure is incommensurate both along the a and c directions. The single crystal sample used in the present study was grown by Czochralsky pulling method in a tetra-arc furnace at Tohoku university. The X-ray powder diffraction measurements at room temperature confirmed only the Bragg peak lines expected for the BaNiSn3-type structure. The size of the crystal was 3mm in diameter and 35mm long whose weight was 1.1 g. Neutron diffraction measurements were carried out with the GPTAS triple-axis spectrometer installed at 4G in the JRR-3 research reactor at Japan Atomic Energy Agency. The incident momentum of ki 1⁄4 3:83 A 1 was adopted to avoid large absorption of Ir atoms with a set of collimators of 400-800-400. To reduce the higher order contaminations, two PG filters were utilized. The one was placed at the position before the monochromator and the other just before the analyzer. The crystal was mounted to the He gas closed cycle refrigerator whose base temperature is 0.75K, and it was aligned with the (0 1 0) axis vertical so that the (h0l) zone can be observed. The lattice parameters at 1 K was a 1⁄4 1:487 A , and c 1⁄4 0:642 A . The (h0l) reciprocal plane in CeIrSi3 is shown in Fig. 1 in which the fundamental reflections for the BaNiSn3-type structure are denoted by open circles, while observed magnetic satellites are denoted by yellow filled circles (for the propagation vector 1) and star-shape blue symbols (for the propagation vector 2). The dashed circles indicate the position at which the aluminum powder lines are observed. In order to find the magnetic peaks in CeIrSi3, we carried out a two-dimensional survey scans around the Q 1⁄4 ð0:215; 0; 2:5Þ at which the magnetic peak was observed in CeRhSi3. The scans were carried out for the range with h: 0.10–0.30 and l: 2.34–2.48 which is indicated by a gray hatch in Fig. 1, and the observed intensity distribution is depicted as a contour map in Fig. 2. We discovered a weak peak at Q 1⁄4 ð0:265; 0; 2:43Þ. We noted that the peak is actually a single peak and dips of intensity around Q 1⁄4 Magnetic Bragg Nuclear Bragg 111 200 ( h 0 l ) zone CeIrSi3

PHASE DIAGRAM OF THE ONE-DIMENSIONAL t–U–J MODEL WITH ON-BOND REPULSION AT HALF FILLING

By using the bosonization approach and the renormalization group (RG) technique, we study the half-filled band one-dimensional t–U–J model with additional on-bond repulsion (W&gt;0) in the weak-coupling regime. The presence of on-bond repulsion is responsible for realization of a metallic phase in the system, and the phase diagram is strongly controlled by the symmetry of the model. By analyzing the RG flow diagram and comparing order parameters, the phase boundaries are determined and the structure of the phase diagram is clarified. In the case of SU (2) ⊗ SU (2) symmetry, the phase diagram consists of a metallic phase characterized by a Luttinger liquid (LL) and two insulting phases characterized by the degenerate spin-density-wave (SDW) and the bond-charge-density-wave (BCDW). In the SU (2) ⊗ U(1)-symmetric case, the phase diagram contains two metallic phases: a LL and a Luther–Emery phase, and three insulating phases: the transverse SDW ( SDW ±), the longitudinal SDW ( SDW z) and the dimerized BCDW. The insulating charge-density-wave and bond-spin-density-wave (BSDW) phases are always suppressed in the ground state. In addition, the system show a long-ranged order in the BCDW and SDW z phases.

Incommensurate spin-density wave and magnetic lock-in transition inCaFe4As3

The magnetic structure for the recently synthesized iron-arsenide compound ${\text{CaFe}}_{4}{\text{As}}_{3}$ has been studied by neutron-powder diffraction. Long-range magnetic order is detected below 85 K, with an incommensurate modulation described by the propagation vector $k=(0,\ensuremath{\delta},0)$, $\ensuremath{\delta}\ensuremath{\sim}0.39$. Below $\ensuremath{\sim}25\text{ }\text{K}$, our measurements detect a first-order phase transition where $\ensuremath{\delta}$ locks into the commensurate value $\frac{3}{8}$. A model of the magnetic structure is proposed for both temperature regimes, based on Rietveld refinements of the powder data and symmetry considerations. The structures correspond to longitudinal spin-density waves with magnetic moments directed along the $b$ axis. A Landau analysis captures the change in thermodynamic quantities observed at the two magnetic transitions, in particular, the drop in resistivity at the lock-in transition.

Fluctuation-driven first-order behavior near theT=0two-dimensional stripe to Fermi liquid transition

The possibility is investigated that competition between fluctuations at different symmetry-related ordering wave vectors may affect the quantum phase transition between a fermi liquid and a longitudinal spin density wave state, in particular, giving rise to an intermediate ``nematic'' state with broken rotational symmetry but unbroken translational symmetry. At the marginal dimension the nematic transition is found to be preempted by a first-order transition but a weak symmetry breaking field restores a second-order magnetic transition with an intermediate regime in which correlations substantially enhance the broken rotational symmetry. Comparison to recent experiments is made.

Investigation of the elastic properties of UNI2Si2 as a function of temperature, magnetic field, and pressure

It is now well-established that the strong anisotropy in the magnetic properties of the intermetallic compounds UT2Si2, where T stands for a transition metal, is responsible for their rich magnetic phase diagram. However, within that series of compounds, UNi2Si2 is one that shows an unusual sequence of magnetically ordered states. Thus, to better understand its unusual properties, we have investigated the elastic properties of UNi2Si2 as a function of temperature, magnetic field, and pressure. In all three magnetic phases, our measurements indicate that the sound-velocity temperature dependence is dominated by the magnetoelastic coupling. Moreover, the analysis of the temperature dependence for the incommensurate longitudinal spin-density wave phase is consistent with a critical exponent β = 0.38 ± 0.01. We also present the magnetic phase diagram for UNi2Si2 obtained at 0 and 8 kbar. Our investigation reveals that the triple-point coordinates (Tp, Hp) decrease with pressure at a rate of dTp/dP = –0.1 K/kbar and dHp/dP = –0.1 T/kbar, respectively. PACS Nos.: 75.30.kz, 62.20.Dc, 62.50.+p, 75.40.Cx