Onset Of Long-range Antiferromagnetic Order Research Articles

One-dimensional quantum magnetism in the S=12 Mo(V) system KMoOP2O7

We present a comprehensive experimental and ab-initio study of the $S=1/2$ Mo$^{5+}$ system, KMoOP$_2$O$_7$, and show that it realizes the $S = 1/2$ Heisenberg chain antiferromagnet model. Powder neutron diffraction reveals that KMoOP$_2$O$_7$ forms a magnetic network comprised of pairs of Mo$^{5+}$ chains within its monoclinic $P2_1/n$ structure. Antiferromagnetic interactions within the Mo$^{5+}$ chains are identified through magnetometry measurements and confirmed by analysis of the magnetic specific heat. The latter reveals a broad feature centred on $T_\textrm{N} = 0.54$ K, which we ascribe to the onset of long-range antiferromagnetic order. No magnetic Bragg scattering is observed in powder neutron diffraction data collected at 0.05 K, however, which is consistent with a strongly suppressed ordered moment with an upper limit $\mu_\textrm{ord} < 0.15 \mu_\textrm{B}$. The one-dimensional character of the magnetic correlations in KMoOP$_2$O$_7$ is verified through analysis of inelastic neutron scattering data, resulting in a model with $J_\textrm{1} \approx 34$ K and $J_\textrm{2} \approx -2$ K for the intrachain and interchain exchange interactions, respectively. The origin of these experimental findings are addressed through density-functional theory calculations.

Synthesis, Optical, Magnetic and Thermodynamic Properties of Rocksalt Li1.3Nb0.3Mn0.4O2 Cathode Material for Li-Ion Batteries

Since the discovery of the reversible intercalation of lithium-ion materials associated with promising electrochemical properties, lithium-containing materials have attracted attention in the research and development of effective cathode materials for lithium-ion batteries. Despite various studies on synthesis, and electrochemical properties of lithium-based materials, fairly little fundamental optical and thermodynamic studies are available in the literature. Here, we report on the structure, optical, magnetic, and thermodynamic properties of Li-excess disordered rocksalt, Li1.3Nb0.3Mn0.4O2 (LNMO) which was comprehensively studied using powder X-ray diffraction, transient absorption spectroscopy, magnetic susceptibility, and low-temperature heat capacity measurements. Charge carrier dynamics and electron–phonon coupling in LNMO were studied using ultra-fast laser spectroscopy. Magnetic susceptibility and specific heat data are consistent with the onset of long-range antiferromagnetic order at the Neel temperatures of 6.5 (1.5) K. The effective magnetic moment of LNMO is found to be 3.60 μB. The temperature dependence of the inverse magnetic susceptibility follows the Curie–Weiss law in the high-temperature region and shows negative values of the Weiss temperature 52 K (3), confirming the strong AFM interactions.

Synthesis, Optical, Magnetic and Thermodynamic Properties of Rocksalt Li1.3Nb0.3Mn0.4O2 Cathode Material for Li-Ion Batteries

Since the discovery of the reversible intercalation of lithium-ion materials associated with promising electrochemical properties, lithium-containing materials have attracted attention in the research and development of effective cathode materials for lithium-ion batteries. Despite various studies on synthesis, and electrochemical properties of lithium-based materials, fairly little fundamental optical and thermodynamic studies are available in the literature. Here, we report on the structure, optical, magnetic, and thermodynamic properties of Li-excess disordered rocksalt, Li1.3Nb0.3Mn0.4O2 (LNMO) which was comprehensively studied using powder X-ray diffraction, transient absorption spectroscopy, magnetic susceptibility, and low-temperature heat capacity measurements. Charge carrier dynamics and electron–phonon coupling in LNMO were studied using ultra-fast laser spectroscopy. Magnetic susceptibility and specific heat data are consistent with the onset of long-range antiferromagnetic order at the Néel temperatures of 6.5 (1.5) K. The effective magnetic moment of LNMO is found to be 3.60 μB. The temperature dependence of the inverse magnetic susceptibility follows the Curie–Weiss law in the high-temperature region and shows negative values of the Weiss temperature 52 K (3), confirming the strong AFM interactions.

High-pressure optical floating-zone growth of Li2FeSiO4 single crystals

We report the growth of mm-sized Pmnb-Li2FeSiO4 single crystals by means of the optical floating-zone method at high argon pressure and describe the conditions required for a stable growth process. The crystal structure is determined and refined by single-crystal X-ray diffraction. The lattice constants amount to a = 6.27837(3) Å, b = 10.62901(6) Å and c = 5.03099(3) Å at 100 K. In addition, we present high-resolution neutron powder diffraction data that suggest that the slight Li – Fe site exchange seems to be intrinsic to this material. High quality of the crystal is confirmed by very sharp anomalies in the static magnetic susceptibility and in the specific heat associated with the onset of long-range antiferromagnetic order at TN = 17.0(5) K and pronounced magnetic anisotropy for the three crystallographic axes. Furthermore, magnetic susceptibility excludes the presence of sizable amounts of magnetic impurity phases.

Magnetism and the phase diagram of MnSb2O6

Static and dynamic magnetic properties of P\={3}1$m$-phase MnSb$_2$O$_6$ have been studied by means of muon spin relaxation ($\mu$SR), high-frequency electron spin resonance (HF-ESR), specific heat, and magnetisation studies in magnetic fields up to 25\,T. The data imply onset of long-range antiferromagnetic order at $T_N$ =~8~K and a spin-flop-like transition at $B_{SF}\approx 0.7 - 1$~T. Below $T_N$, muon asymmetry exhibits well-defined oscillations indicating a narrow distribution of the local fields. A competing antiferromagnetic phase appearing below $T_2$ =~5.3~K is evidenced by a step in the magnetisation and a slight kink of the relaxation rate. Above $T_N$ , both $\mu$SR and HF-ESR data suggest short-range spin order. HF-ESR data show that local magnetic fields persist up to at least $12\cdot T_{\rm N}\approx 100$\,K. Analysis of the antiferromagnetic resonance modes and the thermodynamic spin-flop field suggest zero-field splitting of $\Delta \approx 18$\,GHz which implies small but finite magnetic anisotropy.

Nernst effect in the electron-doped cuprate superconductor La2−xCexCuO4

We report a systematic study of the Nernst effect in films of the electron-doped cuprate superconductor $\mathrm{L}{\mathrm{a}}_{2\ensuremath{-}x}\mathrm{C}{\mathrm{e}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ as a function of temperature and magnetic field (up to 14 T) over a range of doping from underdoped ($x=0.08$) to overdoped ($x=0.16$). We have determined the characteristic field scale ${H}_{C2}^{*}$ of superconducting fluctuation which is found to track the domelike dependence of superconductivity (${T}_{C}$). The fall of ${H}_{C2}^{*}$ and ${T}_{C}$ with underdoping is most likely due to the onset of long-range antiferromagnetic order. We also report the temperature onset, ${T}_{\mathrm{onset}}$, of superconducting fluctuations above ${T}_{C}$. For optimally doped $x=0.11{T}_{\mathrm{onset}}$ ($\ensuremath{\cong}39\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is high compared to ${T}_{C}$ (26 K). For higher doping ${T}_{\mathrm{onset}}$ decreases and tends to zero along with the critical temperature at the end of the superconducting dome. The superconducting gap closely tracks ${H}_{C2}^{*}$ measured from the temperature- and field-dependent Nernst signal.

Weak itinerant antiferromagnetism in PuIn3 explored using 115In nuclear quadrupole resonance

The results of 115In nuclear quadrupole resonance (NQR) measurements on PuIn3 are reported. Three of the four NQR lines of 115In expected for nuclear spin I = 9/2 are observed. The equal spacing of these lines at 20 K yields the NQR frequency of νQ = 10.45 MHz, and the asymmetry parameter of the electric field gradient η = 0. The NQR line profile and the nuclear spin–lattice relaxation rate 1/T1 display an abrupt change at 14 K, which is associated with the onset of long-range antiferromagnetic order. The temperature dependences of the staggered magnetization MQ(T), extracted from the NQR spectra, and 1/T1 below TN = 14 K are well explained by the self-consistent renormalization (SCR) theory for spin fluctuations. In addition, the scaling between T1T and MQ(T)/MQ(0) is also consistent with the predictions of SCR theory, providing further evidence that PuIn3 is a weak itinerant antiferromagnet in which spin fluctuations around the antiferromagnetic wavevector play a major role in the system’s behavior at finite temperatures.

Universal probes for antiferromagnetic correlations and entropy in cold fermions on optical lattices

We determine antiferromagnetic (AF) signatures in the half-filled Hubbard model at strong coupling on a cubic lattice and in lower dimensions. Upon cooling, the transition from the charge-excitation regime to the AF Heisenberg regime is signaled by a universal minimum of the double occupancy at entropy $s\ensuremath{\equiv}S/(N{k}_{\text{B}})={s}^{*}\ensuremath{\approx}\mathrm{ln}(2)$ per particle and a linear increase of the next-nearest-neighbor (NNN) spin correlation function for $s&lt;{s}^{*}$. This crossover, driven by a gain in kinetic exchange energy, appears as the essential AF physics relevant for current cold-atom experiments. The onset of long-range AF order (at low $s$ on cubic lattices) is hardly visible in nearest-neighbor spin correlations versus $s$, but could be detected in spin correlations at or beyond NNN distances.

Pressure-inducedFe↔Cucationic valence exchange and its structural consequences: High-pressure studies of delafossiteCuFeO2

The present high-pressure studies of ${\text{CuFeO}}_{2}$ to 30 GPa using x-ray diffraction, along with $^{57}\text{F}\text{e}$ M\"ossbauer and Fe and $\text{Cu}\text{ }K$-edge x-ray absorption spectroscopy methods, reveal a sequence of intricate structural/electronic-magnetic pressure-induced transitions. The low-pressure $R\overline{3}m$ structure (0--18 GPa) is composed of sheets of ${\text{Fe}}_{S=5/2}^{3+}$ ions alternating with layers of ${\text{O-Cu}}_{S=0}^{1+}\text{-O}$ dumbbells, the latter oriented along the $c$ axis. This structure is characterized by an unusual positive $d(c/a)/dP$. At 18 GPa a structural transition takes place to a more isotropic $C2/c$ structure with the ${\text{O-Cu}}_{S=0}^{1+}\text{-O}$ axis tilted $28\ifmmode^\circ\else\textdegree\fi{}$ from the $c$ axis and with negative $d(c/a)/dP$. This transition corroborates with the onset of long-range antiferromagnetic order. Starting at $\ensuremath{\sim}23\text{ }\text{GPa}$, with an initial volume reduction in $\ensuremath{\sim}|\ensuremath{\Delta}V/{V}_{0}|=0.16$, the Cu-Fe bands overlap and this leads to a $({\text{Cu}}_{S=0}^{1+}{\text{Fe}}_{S=5/2}^{3+})\ensuremath{\rightarrow}({\text{Cu}}_{S=1/2}^{2+}{\text{Fe}}_{S=2}^{2+})$ interionic valence exchange in about 1/3 of the $C2/c{\text{-CuFeO}}_{2}$ at 27 GPa. As a result: (i) the ${\text{Cu}}^{2+}\text{-O}$ becomes fourfold coordinated and is in a new crystallographic structure with space group $P\overline{3}m$, and (ii) the N\'eel temperature increases above twofold $[{T}_{\text{N}}({\text{Cu}}_{S=1/2}^{2+}{\text{Fe}}_{S=2}^{2+})\ensuremath{\approx}2.2{T}_{\text{N}}({\text{Cu}}_{S=0}^{1+}{\text{Fe}}_{S=5/2}^{3+})]$. This sequence of transitions is reversible with minimal hysteresis.

Neutron diffraction study of the magnetic and structural phase transitions inBaFe2As2

We present the results of an investigation of both the magnetic and structural phase transitions in a high quality single crystalline sample of the undoped, iron pnictide compound BaFe$_2$As$_2$. Both phase transitions are characterized via neutron diffraction measurements which reveal simultaneous, continuous magnetic and structural orderings with no evidence of hysteresis, consistent with a single second order phase transition. The onset of long-range antiferromagnetic order can be described by a simple power law dependence $\phi(T)^2\propto(1-\frac{T}{T_N})^{2\beta}$ with $\beta=0.103\pm0.018$; a value near the $\beta=0.125$ expected for a two-dimensional Ising system. Biquadratic coupling between the structural and magnetic order parameters is also inferred along with evidence of three-dimensional critical scattering in this system.

Electron spin resonance and resistivity studies of charge-ordered Bi1−xCaxMnO3

In this paper we report the temperature dependent electron spin resonance (ESR) and electrical resistivity studies on Bi(1−x)CaxMnO3 (x = 0.4, 0.45, 0.5, 0.6). The values of charge-ordering temperature TCO and Neel temperature TN estimated from ESR data are comparable to values obtained by other techniques and reported in the literature. The double integrated (DI) intensity of the ESR signal versus temperature and ln DI versus 1000/T plots have been used to get information about magnetic interactions. It is found that the magnetic phase for T > TCO is dominated by ferromagnetic (FM) correlations which can be attributed to activated Mn3+–Mn4+ hopping of a small polaron. In the temperature range TCO > T > TN, the domains of FM and antiferromagnetic (AFM) spin correlations coexist. The contribution of AFM correlations progressively increase with the increase in Ca content. The sharp change in the slope of ln DI versus 1000/T plot at ∼170–190 K indicates the onset of long-range AFM order. For T < TN, temperature independent ln DI indicates the freezing of FM microdomains/inhomogeneities in the AFM long-range ordered state. The temperature dependent resistivity data are analysed in view of the polaron model. The resistivity data analysed in view of Mott's VRH show that lnρ varies linearly with T−1/4, with a change in slope at TN. It is also found that ln ρ varies linearly with T−1/2 in the entire temperature range below TCO for all the compositions. The Griffiths phase physics may be an appropriate theoretical model for describing the ESR and resistivity data in the present system.

Thermodynamic properties of NdFe 3(BO 3) 4

We present results on thermodynamic properties of single crystalline NdFe 3(BO 3) 4 studied by means of the specific heat c p ( T) and the magnetic susceptibility χ( T). At high temperatures, we observe no anomalies in c p ( T) and χ( T) obeys a Curie–Weiss law. The onset of long-range antiferromagnetic order at 29 K is accompanied by anomalies in both magnetization and specific heat. A Schottky anomaly is visible in c p at ∼4 K. At ∼6 K a low-temperature anomaly is also found in χ( T) if a magnetic field is applied perpendicular to the c-axis. Both phenomena are related to the depopulation of the upper component of the ground Kramers doublet of Nd 3+ split in the magnetically ordered state.

Evidence for strong spin-lattice coupling in multiferroic RMn2O5 (R=Tb,Dy,Ho) via thermal expansion anomalies

Thermal expansion measurements were done on single-crystal RMn2O5 (R=Tb,Dy,Ho) along the principal crystallographic axes. Distinctive anomalies were observed in the linear thermal expansivities at critical temperatures marking the onset of long-range antiferromagnetic order (TN1), ferroelectricity (TC1), as well as at temperatures when anomalous changes in the polarization, dielectric constant, and spin wave incommensurability have been previously reported. These observations suggest that the coupling between the magnetic orders and the dielectric properties is mediated by the lattice distortion. In particular, a possible scenario as to how ferroelectricity arises and subsequently is altered as a result of the subtle changes in the highly frustrated magnetic order, is discussed in the light of the strong spin-lattice coupling observed in these materials. Neutron diffraction measurements on HoMn2O5 show a spin reorientation at 23K corresponding to a step-like anomaly in the dielectric constant.

A reinterpretation of the magnetic properties of the mixed-valence (Nb(V)/Nb(IV)) Zintl phase, Cs(9)Nb(2)As(6).

A new mixed-valent (Nb(V)/Nb(IV)) Zintl phase, Cs(9)Nb(2)As(6), has been prepared and characterized, recently (Gascoin, F.; Sevov, S. C. Inorg. Chem. 2002, 41, 5920). Niobium is present in the form of isolated, edge-sharing tetrahedral, Nb(2)As(6)(9)(-) dimers. The reported magnetic susceptibility features a broad maximum at approximately 36 K which has been interpreted as the onset of long-range antiferromagnetic order. Such a high transition temperature is difficult to understand as the compound is insulating and the interdimer Nb-Nb distance is 7.2 A. It is shown here that the observed magnetic properties follow straightforwardly from a statistical occupation of the equivalent intradimer Nb sites by equal concentrations of Nb(IV)(4d(1), S = (1)/(2)) and Nb(V)(4d(0)). From this analysis the broad maximum arises from intradimer antiferromagnetic exchange with an exchange constant, J/k = -40 K, and there is no long-range magnetic order except, possibly, below 5 K.

Thermal expansion and magnetovolume effects in the heavy-fermion systemCe2RhIn8

We present the results of thermal expansion $\ensuremath{\alpha}(T),$ magnetostriction $\ensuremath{\lambda}(H,T),$ and specific-heat ${C}_{p}(T)$ measurements made on the heavy-fermion antiferromagnet ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}.$ The effects of magnetic order are clearly evident as anomalies in $\ensuremath{\alpha}(T)$ at ${T}_{N}=2.8\mathrm{K}$ and at ${T}_{m}=1.65\mathrm{K}.$ ${C}_{p}(T)$ data indicate that the upper transition corresponds to the onset of long-range antiferromagnetic order while the lower transition involves only a subtle rearrangement of the ordered state. Both ${C}_{p}/T$ and $\ensuremath{\alpha}/T$ grow with decreasing temperature below 20 K in a manner consistent with Kondo renormalization. Kondo interactions appear to be responsible for the large electronic Gr\uneisen parameter which extrapolates to ${\ensuremath{\Omega}}_{e}\ensuremath{\approx}48$ as $\stackrel{\ensuremath{\rightarrow}}{T}0,$ while the characteristic Kondo energy is ${T}_{K}\ensuremath{\approx}10\mathrm{K}$ as determined from the ${C}_{p}/T$ ratio at ${T}_{N}.$ Above 20 K $\ensuremath{\alpha}(T)$ is dominated by crystalline-electric-field (CEF) effects. The data are consistent with a CEF level scheme consisting of excited ${\ensuremath{\Gamma}}_{7}^{(1)}$ and ${\ensuremath{\Gamma}}_{6}$ doublets lying $71\ifmmode\pm\else\textpm\fi{}6\mathrm{K}$ and $195\ifmmode\pm\else\textpm\fi{}10\mathrm{K}$ above a ${\ensuremath{\Gamma}}_{7}^{(2)}$ ground state. In the paramagnetic state $(Tg{T}_{N})$ the volume magnetostriction follows a simple scaling law, $\ensuremath{\lambda}\ensuremath{\propto}[{H/(T+T}_{\ensuremath{\lambda}}){]}^{2}.$ The scaling parameter ${T}_{\ensuremath{\lambda}}=5\mathrm{K}$ is consistent with the Kondo temperature determined from $\ensuremath{\alpha}(T)$ and ${C}_{p}(T)$ data. Analysis of $\ensuremath{\lambda}(H,T)$ data also indicates that the magnetic and electronic energy scales associated with the Kondo state in ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$ are equivalent. These thermodynamic data indicate that the physical properties of ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$ result from competition between magnetic exchange, Kondo, and CEF interactions.

Anion Vacancy Distribution and Magnetism in the New Reduced Layered Co(II)/Co(I) Phase LaSrCoO3.5-x

The new reduced K2NiF4 Co(II) phase, LaSrCoO3.5-x, is prepared by reduction of the Co(III) containing LaSrCoO4 using H2 gas and the new hydride solid−solid reduction methods. The hydride route is superior in both crystallinity and control of phase purity and allows access to reduced phases containing substantial concentrations of Co(I). The anion vacancies are located within the MO2 planes of the K2NiF4 structure but do not prevent the onset of long-range antiferromagnetic order: the magnetic structure is refined using a noncollinear model consistent with the tetragonal crystal symmetry.

Optical Study of the Evolution of Charge and Spin Ordering in the Manganese PerovskiteBi1−xCaxMnO3(x&gt;0.5)

We report an optical reflectivity study of charge and spin ordering in ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ( $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.74$ and 0.82) single crystals. In the high temperature phase, $T&gt;{T}_{\mathrm{co}}$, which is characterized by ferromagnetic correlations, the low-energy contribution to the optical conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ is dominated by small polaron dynamics. For ${T}_{\mathrm{co}}&gt;T&gt;{T}_{\mathrm{N}}$, the presence of both small polaron and charge-gap-like features signifies ``phase separation'' behavior in which domains of ferromagnetic and antiferromagnetic order coexist. Below ${T}_{\mathrm{N}}$, the polaron response is completely suppressed, and a complete charge gap develops, coincident with the onset of long-range antiferromagnetic order.

Magnetoelastic tetragonal-to-orthorhombic distortion inErNi2B2C

We have performed synchrotron x-ray scattering experiments on single crystals of ErNi{sub 2}B{sub 2}C. Below 6.3 K, the (2,0,0) Bragg peak splits, indicating a tetragonal-to-orthorhombic distortion. The mismatch between the a and b lattice parameters appears coincident with the onset of long-range antiferromagnetic order and increases continuously with decreasing temperature, reaching a value of a/b{minus}1{approx}0.2{percent} at 3.7 K. {copyright} {ital 1997} {ital The American Physical Society}

A Study of the Structure and Electronic and Thermal Properties of Quasi-One-Dimensional La3MoO7

The structure of La3MoO7was solved inP212121withZ=4 anda=7.597(1) Å,b=7.7192(4) Å andc=11.0953(8) Å and refined to the reliability factors ofR(F)=0.0366 andwR(F2)=0.0782 for 102 variables and 5352 reflections. A structural feature of interest is the presence of zigzag chains oftrans-corner-sharing octahedra of composition MoO5−5parallel to theb-axis. Resistivity data taken along theb-axis direction show semiconducting behavior in the range 140 to 298 K with an activation energy of 0.16 eV. The magnetic susceptibility is quite complex. The main feature is a broad maximum at 655 K which is interpreted as due to intrachain spin correlations of the Mo(5 +) ions. Assuming theS=1/2 Heisenberg model this implies aJ/k=−511 K. Several other anomalies are observed at 483, 140, and 100 K. Differential scanning calorimetry data also show the 483 K feature and disclose another at 373 K not seen in the susceptibility. There is no evidence from powder neutron diffraction data of any structural changes from 298 to 10 K but no data are available above room temperature. There is an indication from neutron diffraction data for the onset of long-range antiferromagnetic order below 100 K.

A magnetic environment model of moment defects in antiferromagnetic γ-MnNi alloys

A model which includes the effects of magnetic as well as atomic environment on the spatial magnetic moment fluctuations in binary alloys has been applied to measurements of the diffuse magnetic scattering from antiferromagnetic gamma -MnNi alloys. The application of this model has shown that the effects of magnetic environment are particularly important at concentrations approaching the critical concentration for the onset of long-range antiferromagnetic order and signifies some further success of the formalism of a model which has previously accounted for the moment defects in nickel-based ferromagnetic alloys.