Suppression Of Magnetic Order Research Articles

Structure, Spin Correlations, and Magnetism of the S = 1/2 Square-Lattice Antiferromagnet Sr2CuTe1-xWxO6 (0 ≤ x ≤ 1).

Quantum spin liquids are highly entangled magnetic states with exotic properties. The S = 1/2 square-lattice Heisenberg model is one of the foundational models in frustrated magnetism with a predicted, but never observed, quantum spin liquid state. Isostructural double perovskites Sr2CuTeO6 and Sr2CuWO6 are physical realizations of this model but have distinctly different types of magnetic order and interactions due to a d10/d0 effect. Long-range magnetic order is suppressed in the solid solution Sr2CuTe1-xWxO6 in a wide region of x = 0.05-0.6, where the ground state has been proposed to be a disorder-induced spin liquid. Here, we present a comprehensive neutron scattering study of this system. We show using polarized neutron scattering that the spin liquid-like x = 0.2 and x = 0.5 samples have distinctly different local spin correlations, which suggests that they have different ground states. Low-temperature neutron diffraction measurements of the magnetically ordered W-rich samples reveal magnetic phase separation, which suggests that the previously ignored interlayer coupling between the square planes plays a role in the suppression of magnetic order at x ≈ 0.6. These results highlight the complex magnetism of Sr2CuTe1-xWxO6 and hint at a new quantum critical point between 0.2 < x < 0.4.

Structural and Magnetic Properties of the B-Site-Ordered Double Perovskites Ln2NiTiO6 (Ln = La, Pr, and Nd)

Abstract Structural and magnetic properties of B-site-ordered double perovskites Ln2NiTiO6 (Ln = La, Pr, and Nd) were investigated. The charge formulas were revealed to be Ln23+Ni2+Ti4+O6, which gave a geometrically frustrated face-centered cubic lattice consisting of magnetic Ni2+ (S = 1) cations. In spite of the geometrical spin frustration of Ni2+ spins, the samples show an antiferromagnetic-like magnetic transition at temperatures TN close to their absolute values of the Weiss temperature θ, giving small frustration indices f = |θ|/TN ∼ 1. The small f values are considered to be due to some amount of anti-site disorder between Ni2+ and Ti4+ originating in the small valence difference. The suppression of magnetic ordering in the geometrically frustrated system is strongly affected by the B-site cation ordering in the double perovskites.

Magnetic behavior of Ru substituted skyrmion metal MnSi

We report on the structural and magnetic properties of Ru substituted skyrmion metal MnSi i.e. MnRu x Si for the nominal compositions of . The composition–temperature (x–T) phase diagram illustrates the substitution-driven changes in the magnetic behavior. It is confirmed that the magnetic ordering temperature (para-to helimagnetic) and the effective magnetic moment decrease with increasing x. This indicates the suppression of magnetic order by the substitution of Ru in MnSi. However, the magnetic nature is sustained up to a concentration of about x = 0.1 above which the system exhibits spin-glass like nature as inferred from the negative Curie–Weiss temperature , reduced magnetic moment (of the order 10) and linear M–H (at 2 K) in x = 0.5. MnRu x Si is found to avoid the quantum phase transition and exhibits a composition-driven magnetic to spin-glass like transition.

Pressure-induced structural and magnetic phase transitions in La0.75Ba0.25CoO2.9 studied with scattering methods and first-principle calculations

We studied structural and magnetic phase transitions under applied pressure for the doped cobaltite ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$. Neutron and x-ray diffraction experiments established the coexistence of rhombohedral and cubic phases in the sample. The magnetic state at 2 K is best described as a long-range ordered antiferromagnet (AFM) with small ferromagnetic (FM) clusters. With application of pressure, the rhombohedral phase gradually transforms into a cubic one. At room temperature and the highest applied pressure of 16 GPa, the cubic phase accounts for 70% of the sample volume. Quantum mechanical modeling confirmed the experimental findings and provided more insights into the structural and magnetic phase transitions at pressures exceeding 16 GPa. While the cubic crystal structure was preserved above 10 GPa, the AFM to FM phase transition was found at around 16 GPa. Further increase of the pressure resulted in suppression of magnetic order above 45 GPa. Using density functional theory (DFT)$+\mathrm{U}$ calculations, we were able to relate macroscopic magnetic properties induced by pressure with corresponding spin-state transitions in Co ions.

Synthesis and physical properties of the new iridium oxyfluoride Sr2Ir(O,F)6−δ using a topochemical reaction method

We report the synthesis of a new layered iridium oxyfluoride, $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{(\mathrm{O},\mathrm{F})}_{6\ensuremath{-}\ensuremath{\delta}}$, using a topochemical fluorination method. The $c$ axis is elongated compared to that in the mother compound $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$, because the fluorine layer is inserted into the rock-salt layer while tetragonal symmetry is preserved. Resistivity measurements for the compound show nonmetallic behavior similar to that of $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$. In magnetization measurements, suppression of antiferromagnetic order and paramagnetic behavior was observed after topochemical fluorination. Meanwhile, muon spin relaxation (\ensuremath{\mu}SR) experiments suggest the development of antiferromagnetic spin correlation between Ir spins. Furthermore, fluorine content was estimated at 1.5 the chemical composition of the fluorinated product being $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{x}{\mathrm{F}}_{1.5}$ with $x=3.25$, assuming $\mathrm{I}{\mathrm{r}}^{4+}$. Suppression of magnetic order in $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{(\mathrm{O},\mathrm{F})}_{6\ensuremath{-}\ensuremath{\delta}}$ is attributed to antiferromagnetic instability between $\mathrm{Ir}{\mathrm{O}}_{2}$ layers by elongation of the $c$ axis due to fluorine intercalation.

Cation-ordered Pb2-xBixMnO4 solid solutions with magnetic frustration

A new solid solution of composition Pb2-xBixMnO4 (0 ≤ x ≤ 0.3) has been synthesised by solid-state reaction at 730 °C. The crystal structures have been investigated using X-ray and neutron diffraction. The replacement of Pb2+ by Bi3+ ions induces reduction of Mn4+ to Mn3+ with an increase in lattice volume. The Bi3+ ions preferentially occupy the five-coordinated rather than four-coordinated Pb sites. Magnetic measurements and neutron diffraction reveal antiferromagnetic transitions in these materials. A clear increase in magnetic frustration and suppression of magnetic order is evidenced for Pb1.7Bi0.3MnO4 through the long-range order of a small ~1.0 µB AFM moment below TN.

Disentangling superconducting and magnetic orders in NaFe1−xNixAs using muon spin rotation

Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering.

Electrodynamics in Organic Dimer Insulators Close to Mott Critical Point

Organic layered charge-transfer salts κ -(BEDT-TTF) 2 X form highly frustrated lattices of molecular dimers in which strong correlations give rise to Mott insulating states situated close to the metal-to-insulator phase boundary. The salts κ -(BEDT-TTF) 2 Cu 2 (CN) 3 and κ -(BEDT-TTF) 2 Ag 2 (CN) 3 have been considered as prime candidates for a quantum spin liquid, while κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl has been suggested as a prototypical charge-order-driven antiferromagnet. In this paper, we summarize and discuss several key results, including some not reported previously, obtained in search to clarify the competition of these two ground states. The origin of anomalous dielectric response found at low temperatures in all three salts is also discussed. We conclude by pointing out the relevant new insights into the role of frustration and random disorder in the suppression of magnetic ordering and formation of the spin liquid state.

Suppression of Magnetic Order before the Superconducting Dome in MnP

We have performed neutron diffraction experiments on the manganese superconductor, MnP, under applied pressure. Higher harmonics of the previously reported double helix (2δ and 3δ) at ambient pressure were observed and a new magnetic phases was discovered as hydrostatic pressure was applied to a polycrystalline sample below the pressure required to induce superconductivity. The double helix magnetic structure is suppressed by 0.7 GPa. A new incommensurate magnetic structure with propagation vector \( {\sim} (0.25,0.25,0.125)\) was found at 1.5 GPa. The application of higher pressures results in the quenching of the incommensurate phase and broad, diffuse magnetic scattering develops before the superconducting phase. Single crystal studies complement the polycrystalline data confirming the magnetic propagation vector in the low pressure phase.

Probing α-RuCl_{3} Beyond Magnetic Order: Effects of Temperature and Magnetic Field.

Recent studies have brought α-RuCl_{3} to the forefront of experimental searches for materials realizing Kitaev spin-liquid physics. This material exhibits strongly anisotropic exchange interactions afforded by the spin-orbit coupling of the 4d Ru centers. We investigate the dynamical response at finite temperature and magnetic field for a realistic model of the magnetic interactions in α-RuCl_{3}. These regimes are thought to host unconventional paramagnetic states that emerge from the suppression of magnetic order. Using exact diagonalization calculations of the quantum model complemented by semiclassical analysis, we find a very rich evolution of the spin dynamics as the applied field suppresses the zigzag order and stabilizes a quantum paramagnetic state that is adiabatically connected to the fully polarized state at high fields. At finite temperature, we observe large redistributions of spectral weight that can be attributed to the anisotropic frustration of the model. These results are compared to recent experiments and provide a road map for further studies of these regimes.

Pressure-tuned magnetic quantum critical point and unconventional superconductivity

Magnetic quantum critical point (QCP) arises when a long-range magnetic order occurring at finite temperature can be suppressed to absolute zero temperature by using chemical substitutions or exerting high pressure. Exotic phenomena such as the non-Fermi-liquid behaviors or the unconventional superconductivity are frequently observed near the magnetic QCP. In comparison with chemical substitutions, the application of high pressure has some advantages in the sense that it introduces no chemical disorder and can approach the QCP in a very precise manner. In this article, our recent progress in exploring the unconventional superconductors in the vicinity of pressure-induced magnetic QCP is reviewed. By utilizing the piston-cylinder and cubic-anvil-cell apparatus that can maintain a relatively good hydrostatic pressure condition, we first investigated systematically the effect of pressure on the electrical transport properties of the helimagnetic CrAs and MnP. We discovered for the first time the emergence of superconductivity below Tc=2 K and 1 K near their pressure-induced magnetic QCPs at Pc0.8 GPa and 8 GPa for CrAs and MnP, respectively. They represent the first superconductor among the Cr- and Mn-based compounds, and thus open a new avenue to searching novel superconductors in the Cr- and Mn-based systems. Then, we constructed the most comprehensive temperature-pressure phase diagram of FeSe single crystal based on detailed measurements of high-pressure resistivity and alternating current magnetic susceptibility. We uncovered a dome-shaped magnetic phase superseding the nematic order, and observed the sudden enhancement of superconductivity with Tcmax=38.5 K accompanied with the suppression of magnetic order. Our results revealed explicitly the competing nature of nematic order, antiferromagnetic order, and superconductivity, and how the high-Tc superconductivity is achieved by suppressing the long-range antiferromagnetic order, suggesting the important role of antiferromagnetic spin fluctuations for the Cooper paring. These aforementioned results demonstrated that high pressure is an effective approach to exploring or investigating the anomalous phenomena of strongly correlated electronic systems by finely tuning the competing electronic orders.

Tailoring magnetic frustration in strained epitaxial FeRh films

We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances.

Pressure dependence of the magnetic order in CrAs: A neutron diffraction investigation

The suppression of magnetic order with pressure concomitant with the appearance of pressure- induced superconductivity was recently discovered in CrAs. Here we present a neutron diffraction study of the pressure evolution of the helimagnetic ground-state towards and in the vicinity of the superconducting phase. Neutron diffraction on polycrystalline CrAs was employed from zero pressure to 0.65 GPa and at various temperatures. The helimagnetic long-range order is sustained under pressure and the magnetic propagation vector does not show any considerable change. The average ordered magnetic moment is reduced from 1.73(2) {\mu}B at ambient pressure to 0.4(1) {\mu}B close to the critical pressure Pc=0.7 GPa, at which magnetic order is completely suppressed. The width of the magnetic Bragg peaks strongly depends on temperature and pressure, showing a maximum in the region of the onset of superconductivity. We interpret this as associated with competing ground-states in the vicinity of the superconducting phase.

On the Antiferromagnetic Superconductor YbPd $$_2$$ 2 Sn: The Case for Local-Moment Quantum Criticality

The extraordinary Heusler compound YbPd\(_2\)Sn is a near-full magnetic moment Yb\(^{+3}\) paramagnet at high temperature, becomes superconducting below \(T_\mathrm{sc}\simeq {2~}\)K, and orders long-range antiferromagnetic at \(T_\mathrm{N}~=~0.2~\)K. \(T_\mathrm{sc}\) is of re-entrant nature for fields between \(0.04\) and \(0.05~\)T. The \(T\rightarrow 0\) superconductivity is quenched in a field of \(0.04~\)T, whereas the field suppression of \(T_\mathrm{N}\) requires upward of \(0.3~\)T. This presents the possibility to study the stability of magnetic order and its \(T_\mathrm{N}\rightarrow 0\) behavior in the normal region of phase space. In this work, we are concerned with critical effects accompanying the suppression of magnetic order in YbPd\(_2\)Sn as a local-moment antiferromagnet, and we formulate a description of the specific heat of YbPd\(_2\)Sn in terms of field-driven quantum criticality.

Anisotropy of incommensurate magnetic excitations in slightly overdopedBa0.5K0.5Fe2As2probed by polarized inelastic neutron scattering experiments

Polarized neutron scattering experiments on the slightly overdoped superconductor ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ reveal broad magnetic resonance scattering peaking at approximately 15 meV. In spite of doping far beyond the suppression of magnetic order, this compound exhibits dispersive and anisotropic magnetic excitations. At energies below the resonance maximum, magnetic correlations polarized parallel to the layers but perpendicular to the propagation vector are reduced by a factor of 2 compared to those in the two orthogonal directions; in contrast, correlations at the peak maximum are isotropic.

Evolution of magnetic and crystal structure of FeTe doped with Cr and Ni

Due to inherent phase separation, it has been so far impossible to grow ideally stoichiometric (1:1) tetragonal (P 4/n m m) iron telluride. The excess iron ions are located in the inter-planar positions and usually represented as a fraction x in a general formula Fe1+xTe [1], where x ranges from about 4% to 17%. The additional iron has been found to negatively correlate with the level of anion site doping and subsequently with hindering the superconductivity (SC), for example in the Fe(Te,Se,S) [2] series, where SC can be induced by doping with selenium or small amounts of sulfur. A binary Fe1+xTe orders magnetically into incommensurate magnetic structure with the transition temperature and crystal structure in the magnetic state depending on the excess iron. For x&lt;0.12 a monoclinic (P 21/m) distortion was observed and for x&gt;0.12 an orthorhombic one (P m m n). In our work, we attempted to create and investigate compounds electronically equivalent to variable iron stoichometry by substituting Fe with chromium (3d electron deficient) or nickel (3d electron rich). Single crystal samples several millimeter in size were grown by solidification from melt method in the substitution range 0.025 to 0.1 and only in the case of nickel the incorporation of dopant into host lattice was confirmed. Despite low effective Cr content in the single crystal form [3], neutron powder diffraction (NPD) of polycrystaline specimens revealed systematic decrease of long range magnetic moment and gradual suppression of monoclinic (M) distortion in both series. In the Cr doped series, the structure evolved through a mixed phase region into orthorhombic (O) one (Fig. 1), whereas in the nickel system a complete restoration of tetragonal symmetry was found. The suppression of magnetic ordering and lack of the structural distortion did not result in the SC. This work is supported by the Polish National Science Centre grant No 2011/01/B/ST3/00425

Single crystal synthesis and magnetism of the BaLn2O4 family (Ln = lanthanide)

The series of compounds in the BaLn2O4 family (Ln = La–Lu, Y) has been synthesized for the first time in single crystalline form, using a molten metal flux. The series crystallizes in the CaV2O4 structure type with primitive orthorhombic symmetry (space group Pnma, #62), and a complete structural study of atomic positions, bonds, angles, and distortions across the lanthanide series is presented. With the exception of the Y, La, Eu, and Lu members, magnetic susceptibility measurements were performed between 2 K and 300 K. BaCe2O4 and BaYb2O4 display large crystal fields effects and suppression of magnetic ordering. All compounds show signs of magnetic frustration due to the trigonal arrangements of the trivalent lanthanide cations in the structure.

Segregation of antiferromagnetism and high-temperature superconductivity inCa1−xLaxFe2As2

We report the effect of applied pressures on magnetic and superconducting order in single crystals of the aliovalent La-doped iron pnictide material Ca$_{1-x}$La$_{x}$Fe$_{2}$As$_{2}$. Using electrical transport, elastic neutron scattering and resonant tunnel diode oscillator measurements on samples under both quasi-hydrostatic and hydrostatic pressure conditions, we report a series of phase diagrams spanning the range of substitution concentrations for both antiferromagnetic and superconducting ground states that include pressure-tuning through the antiferromagnetic (AFM) quantum critical point. Our results indicate that the observed superconducting phase with maximum transition temperature of $T_{c}$=47 K is intrinsic to these materials, appearing only upon suppression of magnetic order by pressure tuning through the AFM critical point. In contrast to all other intermetallic iron-pnictide superconductors with the ThCr$_2$Si$_2$ structure, this superconducting phase appears to exist only exclusively from the antiferromagnetic phase in a manner similar to the oxygen- and fluorine-based iron-pnictide superconductors with the highest transition temperatures reported to date. The unusual dichotomy between lower-$T_{c}$ systems with coexistent superconductivity and magnetism and the tendency for the highest-$T_{c}$ systems to show non-coexistence provides an important insight into the distinct transition temperature limits in different members of the iron-based superconductor family.

Dilute magnetism and spin-orbital percolation effects inSr2Ir1−xRhxO4

We have used a combination of resonant magnetic x-ray scattering (RMXS) and x-ray absorption spectroscopy (XAS) to investigate the properties of the doped spin-orbital Mott insulator Sr2Ir(1-x)Rh(x)O4 (0.07 < x < 0.70). We show that Sr2Ir(1-x)Rh(x)O4 represents a unique model system for the study of dilute magnetism in the presence of strong spin-orbit coupling, and provide evidence of a doping-induced change in magnetic structure and a suppression of magnetic order at x_c ~ 0.17. We demonstrate that Rh-doping introduces Rh3+/Ir5+ ions which effectively hole-dope this material. We propose that the magnetic phase diagram for this material can be understood in terms of a novel spin-orbital percolation picture.

Field-induced long-range magnetic order in the spin-singlet ground-state system YbAl3C3: Neutron diffraction study

The $4f$-electron system YbAl$_3$C$_3$ with a non-magnetic spin-dimer ground state has been studied by neutron diffraction in an applied magnetic field. A long-range magnetic order involving both ferromagnetic and antiferromagnetic components has been revealed above the critical field H$_C\sim $ 6T at temperature T=0.05K. The magnetic structure indicates that the geometrical frustration of the prototype hexagonal lattice is not fully relieved in the low-temperature orthorhombic phase. The suppression of magnetic ordering by the remanent frustration is the key factor stabilizing the non-magnetic singlet ground state in zero field. Temperature dependent measurements in the applied field H=12T revealed that the long-range ordering persists up to temperatures significantly higher than the spin gap indicating that this phase is not directly related to the singlet-triplet excitation. Combining our neutron diffraction results with the previously published phase diagram, we support the existence of an intermediate disordered phase as the first excitation from the non-magnetic singlet ground state. Based on our results, we propose YbAl$_3$C$_3$ as a new material for studying the quantum phase transitions of heavy-fermion metals under the influence of geometrical frustration.