Short-range Magnetic Correlations Research Articles

From intermediate valence to magnetic behavior without long-range order by hydrogenation of the ternary gallide CeNiGa

We have studied both the crystal chemistry and magnetic, transport, and thermal properties of the hydride $\mathrm{Ce}\mathrm{Ni}\mathrm{Ga}{\mathrm{H}}_{1.1(1)}$. This compound crystallizes in the hexagonal $\mathrm{Al}{\mathrm{B}}_{2}$-type structure with a random distribution of nickel and gallium atoms on the B site, which has an important influence upon the macroscopic properties. Its thermoelectric power versus temperature indicates that cerium is in a trivalent state. The electrical resistivity displays two minima, which could be expected for the Kondo-type interactions in the presence of crystal field effects. Specific heat measurements up to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ allow us to determine the splitting energies ${\ensuremath{\Delta}}_{1}=100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${\ensuremath{\Delta}}_{2}=159\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A broad maximum is observed around $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the specific heat in the low temperature region. This maximum and its evolution with the applied magnetic field, are discussed in the framework of the existing theories, which point toward the existence of short-range magnetic correlations and spin glasslike freezing below $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This study reveals: (i) that the hydrogenation of the intermediate valence gallide CeNiGa induces a valence transition for cerium which is purely trivalent in the hydride and (ii) the absence above $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of long-range magnetic ordering resulting from structural disorder around Ce atoms.

Possible Quantum Critical Point inLa2/3Ca1/3Mn1−xGaxO3

We study the magnetic ground state in La(2/3)Ca(1/3)Mn(1-x)Ga x O3 manganites, where a quantum critical point (QCP) has been theoretically predicted. The metallic ferromagnetic ground state for low Ga doping breaks down for x > or = 0.11, an insulating state being established at low temperatures. Long-range ferromagnetism coexists with short-range magnetic correlations in the concentration range 0.11 < or = x < or = 0.145 while only the short-range correlations survive for x > or = 0.16. We discuss the implications of such a QCP to the physics of manganites and compare to other QCP systems.

U(1)spin liquids and valence-bond solids in a large-Nthree-dimensional Heisenberg model

We study possible quantum ground states of the Sp(N) generalized Heisenberg model on a cubic lattice with nearest-neighbor and next-nearest-neighbor exchange interactions. The phase diagram is obtained in the large-N limit and fluctuation effects are considered via appropriate gauge theories. In particular, we find three U(1) spin liquid phases with different short-range magnetic correlations. These phases are characterized by deconfined gapped spinons, gapped monopoles, and gapless ``photons''. As N becomes smaller, a confinement transition from these phases to valence bond solids (VBS) may occur. This transition is studied by using duality and analyzing the resulting theory of monopoles coupled to a non-compact dual gauge field; the condensation of the monopoles leads to VBS phases. We determine the resulting VBS phases emerging from two of the three spin liquid states. On the other hand, the spin liquid state near J_1 \approx J_2 appears to be more stable against monopole condensation and could be a promising candidate for a spin liquid state in real systems.

Quenching and Recovery of Spin in Quasi-One-dimensional Itinerant Electron Magnet LaMn4Al8

The electron correlation in the 3d heavy-electron compound with linear spin chains, LaMn 4 Al 8 , has been investigated. Like a similar compound, YMn 4 Al 8 , we found a broad maximum in the temperature dependence of the susceptibility. This is interpreted by assuming the presence of a narrow pseudogap with the width Δ≃280 K in the spin excitation spectrum, which leads to the quenching of spin at low temperatures. Anomalies in susceptibility, resistivity and thermal expansion data, observed below approximately 50 K in spite of the absence of any phase transition, suggest the recovery of spin and the development of short-range magnetic correlation in this low-temperature range. This is in contrast to the case of YMn 4 Al 8 . No apparent long-range ordering in spite of the markedly developed magnetic correlation is probably due to the one-dimensional nature of the Mn spin arrangement.

Short-range magnetic correlation with Kondo-lattice behavior in Ce3Ir2Ge2 and superconductivity in La3Ir2Ge2

We present the results of the magnetic susceptibility, magnetization, specific-heat, and electrical resistivity measurements on rare-earth iridium germanides Ce3Ir2Ge2 and La3Ir2Ge2, crystallizing in the La3Ni2Ga2-type orthorhombic crystal structure. The results reveal that Ce3Ir2Ge2 is a Kondo-lattice system showing characteristic −lnT dependence of magnetic resistivity at high temperatures, while there does not exist evident sign of long-range magnetic ordering at least down to 1.8 K. Instead, a short-range magnetic correlation occurs at about 3.5 K. The La-based compound was found to be a type-II superconducting material with a relatively high critical temperature TC=4.7K. A number of the relevant parameters, such as lower critical field, upper critical field, coherence length, penetration depth, and Ginzburg–Landau parameter, were estimated for the La3Ir2Ge2 sample.

Disturbing the spin liquid state in Tb 2Ti 2O 7: Heat capacity measurements on rare earth titanates

One of the best-known spin liquids is the geometrically frustrated pyrochlore Tb 2Ti 2O 7. In a spin liquid, there are short-range dynamic magnetic correlations and an absence of static long-range magnetic order down to 0 K. We have measured heat capacity at low temperatures in magnetic field to 80 kOe on Nd 2Ti 2O 7, Tb 2Ti 2O 7, and Tb 2TiO 5 to better understand the spin liquid state. Nd 2Ti 2O 7 displays antiferromagnetic order at T = 0.6 K much lower than the paramagnetic Curie temperature. Application of magnetic field or distortion of the Tb sublattice destroys the spin liquid as evidenced by static spins.

Magnetic properties of tapiolite (FeTa2O6); a quasi two-dimensional (2D) antiferromagnet

The possibilities of two-dimensional (2D) short-range magnetic correlations and frustration effects in the mineral tapiolite are investigated using bulk-property measurements and neutron Laue diffraction. In this study of the magnetic properties of synthetic single-crystals of tapiolite, we find that single crystals of FeTa2O6 order antiferromagnetically at TN = 7.95 ± 0.05 K, with extensive two-dimensional correlations existing up to at least 40 K. Although we find no evidence that FeTa2O6 is magnetically frustrated, hallmarks of two-dimensional magnetism observed in our single-crystal data include: (i) broadening of the susceptibility maximum due to short-range correlations, (ii) a spin-flop transition and (iii) lambda anomalies in the heat capacity and d(χT)/dT. Complementary neutron Laue diffraction measurements reveal 1D magnetic diffuse scattering extending along the c* direction perpendicular to the magnetic planes. This magnetic diffuse scattering, observed for the first time using the neutron Laue technique by VIVALDI, arises directly as a result of 2D short-range spin correlations.

Slave-rotor mean-field theories of strongly correlated systems and the Mott transition in finite dimensions

The multiorbital Hubbard model is expressed in terms of quantum phase variables (``slave rotors'') conjugate to the local charge, and of auxiliary fermions, providing an economical representation of the Hilbert space of strongly correlated systems. When the phase variables are treated in a local mean-field manner, similar results to the dynamical mean-field theory are obtained, namely a Brinkman-Rice transition at commensurate fillings together with a ``preformed'' Mott gap in the single-particle density of states. The slave- rotor formalism allows to go beyond the local description and take into account spatial correlations, following an analogy to the superfluid-insulator transition of bosonic systems. We find that the divergence of the effective mass at the metal- insulator transition is suppressed by short range magnetic correlations in finite-dimensional systems. Furthermore, the strict separation of energy scales between the Fermi- liquid coherence scale and the Mott gap found in the local picture, holds only approximately in finite dimensions, due to the existence of low-energy collective modes related to zero-sound.

Correlation between spin-glass-like behavior and lattice distortion in itinerant Ca1−xSrxRuO3 metal

The long-range ferromagnetic order was observed only for x⩾0.25 in the Ca1−xSrxRuO3 (0⩽x⩽1) metallic system, while short-range magnetic correlation of ferromagnetic double-exchange interaction persists for x&amp;lt;0.25. Temperature, field, and time dependent magnetization m(T,Ba,t) for exchange-enhanced paramagnetic Ca0.8Sr0.2RuO3 near the boundary show similar electronic-inhomogeneous, spin-glass-like behavior as in Sr-free CaRuO3. X-ray Rietveld analysis and x-ray absorption near-edge structure data indicate that weaker double-exchange coupling strength Jde in the Ca-rich regime is closely related with stronger RuO6 tilting and distortion, which results with weaker Ru 4d–O 2p hybridization, narrower bandwidth W, and stronger on-site Coulomb repulsion Udd.

Magnetic properties of quasi-one-dimensional magnets AM3+Si2O6 (A=Li, Na; M=Ti, V, Cr)

The magnetic properties of the quasi-one-dimensional magnet AM3+Si2O6 (A=Li, Na; M=Ti, V, Cr) have been investigated in the light of the number of d-electron (spin number). The Ti(d1)-compound shows a spin-Peierls like transition at the temperature higher than that (TMAX) at the maximum point of the Bonner–Fisher curve, and the ground state is spin-singlet. The V(d2)- and Cr(d3)-compounds do not show such transition and the ground states are long range antiferromagnetic ordered states. The TMAX, where a short-range magnetic correlation within the chain is fully developed, decreases with increasing the M–M distance within the chain in the compounds with a fixed dn. On the other hand, it decreases with increasing the number of d-electron in the Na-based compounds, although the M–M distance within the chain becomes shorter with increasing the number of d-electron. Any other factor such as the second nearest interaction or orbital degree of freedom should be taken into the consideration for the explanation of the TMAX variation with the number of d-electron.

Yb–Yb correlations and crystal-field effects in the Kondo insulatorYbB12 and its solid solutions

We have studied the effect of Lu substitution on the spin dynamics of the Kondo insulatorYbB12 to clarify the origin of the spin-gap response previously observed at lowtemperature in this material. Inelastic neutron spectra have been measured inYb1−xLuxB12 compounds for fourLu concentrations, x = 0, 0.25, 0.90 and 1.0. The data indicate that the disruption of coherence on the Ybsublattice primarily affects the narrow peak structure occurring near 15–20 meV in pureYbB12, whereas the spin gap and the broad magnetic signal around 38 meV remain almostunaffected. It is inferred that the latter features reflect mainly local, single-site processes,and may be reminiscent of the inelastic magnetic response reported for mixed-valenceintermetallic compounds. On the other hand, the lower component at 15 meV is most likelydue to dynamic short-range magnetic correlations. The crystal-field splitting inYbB12 estimated fromthe Er3+ transitionsmeasured in a Yb0.9Er0.1B12 sample has the same order of magnitude as other relevant energy scales of the system,and is thus likely to play a role in the form of the magnetic spectral response.

Long- and short-range magnetic order, charge transfer, and superconductivity inYBa2Cu3−xCoxO7

We have performed powder-neutron-diffraction measurements in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{7}$ for a large range of Co concentrations $(x=0.04--0.72)$ and determined the magnetic phase diagram. As previously observed, superconductivity disappears for $x&gt;~0.42$ and antiferromagnetic long-range order emerges above this value. The antiferromagnetic order shows noticeable differences compared to that in unsubstituted ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}$ and surprisingly survives to a much higher hole doping. In contrast to recent results in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{7}$ single crystals, there is no evidence of any long-range antiferromagnetic order in the superconducting phase. The Co valence evolves from 4+ to 3+ with increasing Co concentration. We studied the short-range chemical and magnetic correlations between the Co ions in superconducting $(x&lt;~0.36)$ and antiferromagnetic $(x&gt;~0.42)$ samples. The Co ions predominantly occupy the Cu1 sites and their moments remain mostly disordered, forming random antiferromagnetic next nearest neighbor pairs, which compete with the long-range order. This can explain the low value for the ordered Cu1 site moment.

Magnetic properties of quasi-one-dimensional magnets AM3+Si2O6 (A=Li, Na; M=Ti, V, Cr)

The magnetic properties of the quasi-one-dimensional magnet AM3+Si2O6 (A=Li, Na; M=Ti, V, Cr) have been investigated in the light of the number of d-electron (spin number). The Ti(d1)-compound shows a spin-Peierls like transition at the temperature higher than that (TMAX) at the maximum point of the Bonner–Fisher curve, and the ground state is spin-singlet. The V(d2)- and Cr(d3)-compounds do not show such transition and the ground states are long range antiferromagnetic ordered states. The TMAX, where a short-range magnetic correlation within the chain is fully developed, decreases with increasing the M–M distance within the chain in the compounds with a fixed dn. On the other hand, it decreases with increasing the number of d-electron in the Na-based compounds, although the M–M distance within the chain becomes shorter with increasing the number of d-electron. Any other factor such as the second nearest interaction or orbital degree of freedom should be taken into the consideration for the explanation of the TMAX variation with the number of d-electron.

Synthesis, structure, and properties of randomly mixed and layer-ordered SrMn 1− xGa xO 3− δ perovskites

We report the synthesis of SrMn 1− x Ga x O 3− δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques, we have extended the solubility limit of Ga 3+ in the cubic perovskite phase to x≈0.33. Higher Ga concentrations lead to mixed phases until a single-phase ordered double-perovskite structure is obtained at x=0.5, i.e., Sr 2MnGaO 6− δ . In the cubic perovskite phase the maximum oxygen content is 3− x/2, which corresponds to 100% Mn 4+. All maximally oxygenated solid solution compounds are found to order antiferromagnetically, with the transition temperature linearly decreasing as Ga content increases. Reducing the oxygen content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn 0.7Ga 0.3O 2.5 below 30 K. The brownmillerite phase at low oxygen content, Sr 2MnGaO 5, is found to have Icmm crystallographic symmetry. At 12 K its magnetic structure is found to order in the Icm′m′ magnetic symmetry corresponding to a G-type antiferromagnetic structure of Mn 3+ ions. At higher oxygen content, Sr 2MnGaO 5.5 is found to have Cmmm crystallographic symmetry with disordered oxygen vacancies. At 12 K two competing long-range magnetic structures are found for the Mn 4+ sublattice having C I m′m′m symmetry (G-type), and C P m′m′m symmetry (C-type), together with a G-type short-range magnetic correlations.

Nuclear and magnetic short-range order in some dilute β-Mn alloys

Using full XYZ neutron polarization analysis, we have studied the nuclear and magnetic diffuse scattering of various β-Mn alloys. This has enabled the study of not only the magnetic short-range spin-correlations in isolation, but also the crucial role of the interplay between the magnetic and nuclear disorder in the lattice as the impurity atoms are introduced. Using reverse Monte-Carlo methods, we deduce the nuclear short-range order parameters and magnetic correlations of these alloys over a large number of near neighbour shells.

Magnetic behaviour of the tetravalent praseodymium compoundSr2PrO4

We report on investigations of the magnetic properties of the tetravalent praseodymium(Pr4+)compound Sr2PrO4 by dc and ac magnetic susceptibility and heat capacity measurements.The compound shows signatures of long range magnetic order atTN = 3 K in the heat capacity and magnetization data. In addition, the temperature dependence ofthe field cooled and the zero-field cooled susceptibility is hysteretic below 10 K. Heat capacityversus temperature data show that a significant part of the magnetic entropy is released aboveTN.This reveals that short range magnetic correlations among the Pr moments exist well aboveTN.The magnetic entropy obtained from the heat capacity measurements indicates that onlythe crystal field ground state of Pr with effective spin- is significantly populated at low temperatures.

Double magnetic transition and anomalous magnetoresistance in Er 2Ni 3Si 5

Synthesis and magnetic properties of a new material, Er 2Ni 3Si 5, are reported here. This compound exhibits a double magnetic transition ( T m ∼4 K and 3 K ) as revealed by heat capacity measurement. The magnetic ground state in Er 2Ni 3Si 5 is found to be a doublet. Magnetoresistance of Er 2Ni 3Si 5 above T m exhibits anomalous behaviour, suggesting occurrence of short range magnetic correlations above T N.

Anomalous magnetic scattering in tetragonal RB2C2 (R = rare earth)observed by means of neutron diffraction

Detailed neutron diffraction experiments have been performed on tetragonal R11B2C2 (R = rare-earth)compounds which show characteristic antiferroquadrupolar orderings. TbB2C2, ErB2C2 andHoB2C2,which have magnetic long-periodic states, show anomalous magnetic diffusescattering with flat intensity. The anomalous diffuse scattering exists in asmall region in the reciprocal space, surrounded by satellite peaks, dueto the long-periodic magnetic structures. The profile and temperaturedependence of the anomalous diffuse scattering indicate that the origincannot be understood on the basis of magnetic short-range correlations.

Synthesis, Crystal Structure, Crystal Chemistry, and Magnetic Properties of PbMBO4 (M: Cr, Mn, Fe): A New Structure Type Exhibiting One‐Dimensional Magnetism.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electron spin resonance and relaxation studies of double-layered manganites

Electronic properties of ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(x=0.4$ and 0.5) single crystals are studied by electron spin resonance (ESR) and spin-lattice relaxation time measurements. Spin susceptibility $\ensuremath{\chi}(T)$ determined from the ESR signal intensity and macroscopically measured static-susceptibility data are in good agreement, thus ESR detects all spin species in the system. In both compounds, the ESR spectra contain a single, nearly isotropic Lorentzian line associated with the exchange coupled ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ ions. For the $x=0.5$ compound, the fingerprints of charge ordering (CO) transition at ${T}_{\mathrm{CO}}=226\mathrm{K}$ are detected. In addition, strongly anisotropic ferromagnetic resonance spectra are found in both materials, suggesting the presence of extrinsic ferromagnetic phases. For $x=0.4,$ the longitudinal relaxation time ${T}_{1}$ and the transversal relaxation time ${T}_{2}$ are equal around room temperature that is a sign of exchange narrowing. The ${T}_{1}{/T}_{2}$ ratio increases to about 5 approaching the Curie temperature ${T}_{C}=126\mathrm{K}.$ No sign of critical speeding up of ${T}_{1}$ is detected. Instead, the slowing down of the relaxation rate takes place and ${T}_{1}$ is proportional to $T\ensuremath{\chi}(T).$ This is attributed to the freezing of short-range magnetic correlations in the external field.