Short-range Magnetic Correlations Research Articles

Multiglass properties and magnetoelectric coupling in the uniaxial anisotropic spin-cluster-glassFe2TiO5

The compound, Fe2TiO5 (FTO)is a well-known uniaxial anisotropic spin-glass insulator with two successive glassy freezing temperatures i.e. transverse (TTF= 9K) and longitudinal (TLF= 55 K). In this article, we present the results of measurements of complex dielectric behavior, electric polarization as a function of temperature (T), in addition to characterization by magnetic susceptibility and heat-capacity, primarily to explore magnetoelectric (ME) coupling and multiglass properties in uniaxial anisotropic spin cluster-glass FTO. The existence of two magnetic transitions is reflected in the isothermal magnetodielectric (MD) behavior in the sense that the sign of MD is different in the T regime T<TTF and T>TTF.The data in addition provide evidence for the glassy dynamics of electric-dipoles; interestingly, this occurs at much higher temperature (~100-150 K) than TLF with high remnant polarization at 10 K( 4000 micro C/m2) attributable to short-range magnetic correlations, thereby offering a route to attain ME coupling above 77 K.

Properties near magnetic instability of heavy-electron compounds Ce3M4Sn13 and La3M4Sn13, with M=Co, Rh and Ru

In this review, we report the thermodynamic, magnetic and electronic transport properties of the skutterudite-related Ce3M4Sn13 and La3M4Sn13 intermetallic compounds with M = Co, Rh and Ru, which display a variety of behaviours. Ce3M4Sn13 exhibit a large increase in C/T with a maximum value of about 4 JK-2mol-1Ce due to strong electron and short-range magnetic correlations. These compounds show a crossover from a magnetically correlated heavy-fermion state to a single impurity state in applied magnetic fields. In order to study the proximity of Ce3Co4Sn13 to the possible magnetic quantum critical point (QCP), we investigated the system of Ce3-xLaxCo4Sn13 alloys. We found the critical concentration , which separates the magnetically correlated state () from a single impurity state (), however the low-T C(T)/T and the magnetic susceptibility behaviours are not characteristic of the QCP. With increasing of the magnetic field, resistivity follows power law behaviour for the samples , with n strongly field dependent. The -anomaly is discussed on the base of spin-fluctuation theory of Moriya and Takimoto. Specific heat data show that La3M4Sn13 are typical BCS superconductors, however, La3Rh4Sn13 and La3Ru4Sn13 exhibit a second superconducting phase, characteristic of inhomogeneous superconductors.

Short-range magnetic correlations in the highly correlated electron compoundCeCu4Ga

We present experimental results for the heavy-electron compound CeCu$_{4}$Ga which show that it possesses short-range magnetic correlations down to a temperature of $T = 0.1$ K. Our neutron scattering data show no evidence of long-range magnetic order occurring despite a peak in the specific heat at $T^{*} =1.2$ K. Rather, magnetic diffuse scattering occurs which corresponds to short-range magnetic correlations occurring across two unit cells. The specific heat remains large as $T\sim0$ K resulting in a Sommerfeld coefficient of $\gamma_{0} = 1.44(2)$ J/mol-K$^{2}$, and, below $T^{*}$, the resistivity follows $T^{2}$ behavior and the ac magnetic susceptibility becomes temperature independent. A magnetic peak centered at an energy transfer of $E_{\rm{c}}=0.24(1)$ meV is seen in inelastic neutron scattering data which shifts to higher energies and broadens under a magnetic field. We discuss the coexistence of large specific heat, magnetic fluctuations, and short-range magnetic correlations at low temperatures and compare our results to those for materials possessing spin-liquid behavior.

Possible coupling between magnons and phonons in multiferroicCaMn7O12

Spin and lattice dynamics of CaMn7O12 ceramics were investigated using infrared, THz and inelastic neutron scattering (INS) spectroscopies in the temperature range 2 to 590 K, and, at low temperatures, in applied magnetic fields of up to 12 T. On cooling, we observed phonon splitting accompanying the structural phase transition at Tc = 450K as well as the onset of the incommensurately modulated structure at 250 K. In the two antiferromagnetic phases below T_N1 = 90K and T_N2 = 48 K, several infrared-active excitations emerge in the meV range; their frequencies correspond to the maxima in the magnon density of states obtained by INS. At the magnetic phase transitions, these modes display strong anomalies and for some of them, a transfer of dielectric strength from the higher-frequency phonons is observed. We propose that these modes are electromagnons. Remarkably, at least two of these modes remain active also in the paramagnetic phase; for this reason, we call them paraelectromagnons. In accordance with this observation, quasielastic neutron scattering revealed short-range magnetic correlations persisting within temperatures up to 500K above T_N1.

Electronic structure and low-temperature study of theCe1−xLaxRh4Sn13

Recently, it was reported that the skutterudite-related ${\mathrm{Ce}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ heavy fermion compound has a large electronic specific heat coefficient $C(T)/T$ and short-range magnetic correlations, both suggest a possible quantum critical point (QCP). We present the low-temperature specific heat, resistivity, and susceptibility of ${\mathrm{Ce}}_{1\text{\ensuremath{-}}x}{\mathrm{La}}_{x}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$, which do not confirm the presence of QCP. X-ray photoelectron spectroscopy revealed a stable configuration of Ce atoms consistent with magnetic susceptibility data. Magnetic susceptibility shows that the sixfold-degenerate multiplet of ${\mathrm{Ce}}^{3+}$ ion splits into three doublets due to the lower local symmetry of ${\mathrm{CeSn}}_{12}$ cage in the cubic ${\mathrm{Ce}}_{1\text{\ensuremath{-}}x}{\mathrm{La}}_{x}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ compounds. The resistivity, specific heat, and susceptibility data are consistent with our band-structure calculations. The chemical bondings in the unit cell of ${\mathrm{Ce}}_{1\text{\ensuremath{-}}x}{\mathrm{La}}_{x}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$, analyzed based on the difference charge density maps from band-structure calculations within the virtual crystal approximation explain the deformation of the cages and the crystal electric field properties of the system.

Revealing the nature of magnetic phases in the semi-Heusler alloy Cu0.85Ni0.15MnSb

We report the magnetic, magnetocaloric, and magnetotransport properties of the semi-Heusler alloy Cu0.85Ni0.15MnSb, which exhibits coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) phases. A broad magnetic phase transition is evident from the temperature variations of magnetization, heat capacity, and isothermal magnetic entropy change. This is due to the presence of both AFM and FM phases at low temperatures. The variation of electrical resistivity with temperature shows three distinct regions of magnetic phases. The magnetoresistance (MR) results also show the presence of AFM and FM phases at temperatures below 45K, and a FM phase at temperature above 45K. Though there is no signature of a spin-glass state at low temperatures, various results point towards the presence of short-range magnetic correlations at low temperatures.

Dimensional and coordination number reductions in a large family of lanthanide tellurite sulfates.

Twenty-two new lanthanide tellurite sulfates with five distinct structures, Ln2(Te2O5)(SO4)2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb; LnTeSO-1), Ho3(TeO3)2(SO4)2(OH)(H2O) (LnTeSO-2), Ln2TeO3(SO4)2(H2O)2 (Ln = Dy, Ho, Er; LnTeSO-3), Ln2(Te2O5)(SO4)2 (Ln = Er, Tm, Yb, Lu; LnTeSO-4), and Ln2(Te4O10)(SO4) (Ln = Gd, Dy, Ho, Er, Tm, Yb; LnTeSO-5), have been prepared and characterized. The topologies of LnTeSO-1, LnTeSO-2, LnTeSO-3, LnTeSO-4, and LnTeSO-5 are substantially different with respect to the connectivity between Ln polyhedra and the coordination environments of the lanthanide ions. For the first four topologies, the dimensionality changes from layered (LnTeSO-1) to chains (LnTeSO-2) to tetramers (LnTeSO-3) and finally to a monomer (LnTeSO-4). The coordination numbers of lanthanides decrease from nine (LnTeSO-1) to eight (LnTeSO-2 and LnTeSO-3) to seven and six (LnTeSO-4). We attribute the transitions to a decrease in the ionic radii of the 4f ions. Magnetic susceptibility measurements reveal no evidence for long-range magnetic ordering in these materials. However, diverse short-range magnetic correlations were observed within LnTeSO-1.

Magnetic pair distribution function analysis: introduction and applications

Short-range magnetic correlations play a crucial role in a variety of condensed matter phenomena, yet they remain notoriously difficult to investigate experimentally. Quantitative analysis of the diffuse scattering of neutrons from local magnetic correlations represents a viable but challenging route toward revealing short-range magnetic order in complex materials. Reverse Monte Carlo techniques that iteratively fit randomly generated structural models in momentum space have been used successfully [1], demonstrating that diffuse magnetic scattering can be rich in information. Recently [2], we developed a real-space approach to investigating local magnetic correlations, which we call magnetic pair distribution function (mPDF) analysis in analogy to the more familiar atomic pair distribution function. This experimentally accessible quantity reveals magnetic correlations directly in real space and places diffuse and Bragg scattering on equal footing, thereby gaining sensitivity to both short- and long-range magnetic order. Here we present the basic theory behind mPDF analysis and provide several examples of its utility using both simulated and experimentally measured data on several interesting magnetic systems, including a canonical antiferromagnetic, a spin glass, and a spin ice. We discuss the potential impact that mPDF methods may have on current and future research interests in magnetism.

Crystal electric field and the ground state properties of heavy fermion Ce3Ru4Sn13

We report on the electronic structure, electric transport and basic thermodynamic properties of the skutterudite-related Ce3Ru4Sn13 and La3Ru4Sn13. The calculated density of electronic states within the local spin density approximation (LSDA) method well fit the X-ray photoelectron spectroscopy (XPS) valence band spectra of Ce3Ru4Sn13. Magnetic susceptibility and specific heat measurements reveal that the sixfold degenerated multiplet of Ce3+ ions splits into three doublets, due to the tetragonal Ce point local symmetry in the cubic Ce3Ru4Sn13 system. Ce3Ru4Sn13 exhibits a large increase in the specific heat, C/T, data due to Kondo effect and strong electron and short-range magnetic correlations, we also suggest significant contribution of the crystal field effect. La3Ru4Sn13 is typical BCS superconductor, however, specific heat and electrical resistivity data show that La3Ru4Sn13 also exhibits a second superconducting phase at higher temperatures, which is characteristic of inhomogeneous superconductors.

Displacive-type ferroelectricity from magnetic correlations within spin-chain.

Observation of ferroelectricity among non-d0 systems, which was believed for a long time an unrealistic concept, led to various proposals for the mechanisms to explain the same (i.e. magnetically induced ferroelectricity) during last decade. Here, we provide support for ferroelectricity of a displacive-type possibly involving magnetic ions due to short-range magnetic correlations within a spin-chain, through the demonstration of magnetoelectric coupling in a Haldane spin-chain compound Er2BaNiO5 well above its Néel temperature of (TN = ) 32 K. There is a distinct evidence for electric polarization setting in near 60 K around which there is an evidence for short-range magnetic correlations from other experimental methods. Raman studies also establish a softening of phonon modes in the same temperature (T) range and T-dependent x-ray diffraction (XRD) patterns also reveal lattice parameters anomalies. Density-functional theory based calculations establish a displacive component (similar to d0-ness) as the root-cause of ferroelectricity from (magnetic) NiO6 chain, thereby offering a new route to search for similar materials near room temperature to enable applications.

Spin fluctuations and frustrated magnetism in multiferroicFeVO4

We report $^{51}$V nuclear magnetic resonance (NMR) studies on single crystals of the multiferroic material FeVO$_4$. The high-temperature Knight shift shows Curie-Weiss behavior, $^{51}K = a/(T + \theta)$, with a large Weiss constant $\theta \approx$ 116 K. However, the $^{51}$V spectrum shows no ordering near these temperatures, splitting instead into two peaks below 65 K, which suggests only short-ranged magnetic order on the NMR time scale. Two magnetic transitions are identified from peaks in the spin-lattice relaxation rate, $1/^{51}T_1$, at temperatures $T_{N1} \approx$ 19 K and $T_{N2} \approx$ 13 K, which are lower than the estimates obtained from polycrystalline samples. In the low-temperature incommensurate spiral state, the maximum ordered moment is estimated as 1.95${\mu}_B$/Fe, or 1/3 of the local moment. Strong low-energy spin fluctuations are also indicated by the unconventional power-law temperature dependence $1/^{51}T_1 \propto T^2$. The large Weiss constant, short-range magnetic correlations far above $T_{N1}$, small ordered moment, significant low-energy spin fluctuations, and incommensurate ordered phases all provide explicit evidence for strong magnetic frustration in FeVO$_4$.

Neutron scattering measurements of spatially anisotropic magnetic exchange interactions in semiconducting K0.85 Fe1.54Se2 (TN = 280 K.

We use neutron scattering to study the spin excitations associated with the stripe antiferromagnetic order in semiconducting K(0.85)Fe(1.54)Se(2) (T(N) = 280 K). We show that the spin-wave spectra can be accurately described by an effective Heisenberg Hamiltonian with highly anisotropic inplane couplings at T = 5 K. At high temperature (T = 300 K) above T(N), short-range magnetic correlation with anisotropic correlation lengths are observed. Our results suggest that, despite the dramatic difference in the Fermi surface topology, the inplane anisotropic magnetic couplings are a fundamental property of the iron-based compounds; this implies that their antiferromagnetism may originate from local strong correlation effects rather than weak coupling Fermi surface nesting.

Wiedemann-Franz law and nonvanishing temperature scale across the field-tuned quantum critical point ofYbRh2Si2

The in-plane thermal conductivity kappa(T) and electrical resistivity rho(T) of the heavy-fermion metal YbRh2Si2 were measured down to 50 mK for magnetic fields H parallel and perpendicular to the tetragonal c axis, through the field-tuned quantum critical point, Hc, at which antiferromagnetic order ends. The thermal and electrical resistivities, w(T) and rho(T), show a linear temperature dependence below 1 K, typical of the non-Fermi liquid behavior found near antiferromagnetic quantum critical points, but this dependence does not persist down to T = 0. Below a characteristic temperature T* ~ 0.35 K, which depends weakly on H, w(T) and rho(T) both deviate downward and converge in the T = 0 limit. We propose that T* marks the onset of short-range magnetic correlations, persisting beyond Hc. By comparing samples of different purity, we conclude that the Wiedemann-Franz law holds in YbRh2Si2, even at Hc, implying that no fundamental breakdown of quasiparticle behavior occurs in this material. The overall phenomenology of heat and charge transport in YbRh2Si2 is similar to that observed in the heavy-fermion metal CeCoIn5, near its own field-tuned quantum critical point.

Lattice effects in the quasi-two-dimensional valence-bond-solid Mott insulator EtMe3P[Pd(dmit)2]2

The organic charge-transfer salt EtMe$_3$P[Pd(dmit)$_2$]$_2$ is a quasi-two-dimensional Mott insulator with localized spins $S$ = 1/2 residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients $\alpha_i$ along the in-plane $i$ = $a$- and $c$-axis as well as along the out-of-plane $b$-axis for temperatures 1.4\,K $\leq $T$ \leq$ 200\,K. Particular attention is paid to the lattice effects around the phase transition at $T_{VBS}$ = 25\,K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-range antiferromagnetic correlations can be expected. The salient results of our study include (i) the observation of strongly anisotropic lattice distortions accompanying the formation of the valence-bond-solid, and (ii) a distinct maximum in the thermal expansion coefficients in the paramagnetic regime around 40\,K. Our results demonstrate that upon cooling through $T_{VBS}$ the in-plane $c$-axis, along which the valence bonds form, contracts while the second in-plane $a$-axis elongates by the same relative amount. Surprisingly, the dominant effect is observed for the out-of-plane $b$-axis which shrinks significantly upon cooling through $T_{VBS}$. The pronounced anomaly in $\alpha_i$ around 40\,K is attributed to short-range magnetic correlations. It is argued that the position of this maximum, relative to that in the magnetic susceptibility around 70\,K, speaks in favor of a more anisotropic triangular-lattice scenario for this compound than previously thought.

1212-Molybdo-Cuprates; effect of oxygenation in the structure, properties and electronic states

ABSTRACTThe influence of oxygenation in the magnetism, superconductivity and electronic states for the Mo0.3Cu0.7Sr2RECu2Oy (RE = Y, Er and Tm) compounds are discussed here. The magnetic measurements on the as-prepared (AP) samples suggest the existence of short-range magnetic correlations due to the presence of the paramagnetic MoV cations in the copper chain site. On the other hand, all the oxygenated samples are not magnetic but superconducting. The high pressure oxygenated sample shows the highest superconducting transition temperature of TC = 84 K. The influence of oxygenation in the electronic states for the Mo0.3Cu0.7Sr2YCu2Oy system associated with an oxidation reaction leading from a non-superconducting to a superconducting state has also been investigated by means of X-ray photoelectron spectroscopy (XPS). XPS measurements show the predominance of the MoV oxidation state over the MoVI one in the AP material; annealing under flowing oxygen enhances both the MoVI and CuII amounts. A detailed study of the electronic states for the Mo0.3Cu0.7Sr2YCu2Oy samples has been performed and is also discussed.

Magnetic pair distribution function analysis of local magnetic correlations

The analytical form of the magnetic pair distribution function (mPDF) is derived for the first time by computing the Fourier transform of the neutron scattering cross section from an arbitrary collection of magnetic moments. Similar to the atomic pair distribution function applied to the study of atomic structure, the mPDF reveals both short-range and long-range magnetic correlations directly in real space. This function is experimentally accessible and yields magnetic correlations even when they are only short-range ordered. The mPDF is evaluated for various example cases to build an intuitive understanding of how different patterns of magnetic correlations will appear in the mPDF.

Short-range magnetic correlations and parimagnetism in RCo2

X-ray circular magnetic dichroism, polarized neutron diffraction, ac susceptibility, and Seebeck effect have been measured for several members of the RCo2 series (R = Ho, Tm, Er) as a function of temperature and applied magnetic field. The experimental results show robust parimagnetism (a general behaviour along the RCo2 series with R being a heavy rare earth ion) and two reversal temperatures in some systems, which is an unexpected result. Polarised neutron diffraction show differences between results obtained on single crystals or polycrystalline ingots. We propose an interpretation of parimagnetic RCo2 as a Griffiths phase of the high temperature, magnetically ordered, amorphous RCo2 phase.

Exploring the magneto-volume anomalies in Dy2Fe17 with unconventional rhombohedral crystal structure

We have synthesized the Dy2Fe17 alloy with an unconventional crystal structure (R3¯m space group). Neutron powder thermo-diffraction and X-ray powder diffraction under high pressure (up to 15GPa) show that the rhombohedral crystal structure is stable. Likewise, the alloy with the usual hexagonal crystal structure (P63/mmc), the rhombohedral variant of Dy2Fe17 compound, exhibits a collinear ferrimagnetic order below the Curie temperature (TC≈363K), with antiparallel mutual alignment of the Dy and Fe sublattices. Additionally, we have observed two distinctive issues related to magneto-volume anomalies: (i) the pressure dependence of the cell volume at room temperature shows the existence of a critical pressure at which the compound is no longer ferrimagnetic; and (ii) the cell volume shows invar-like behaviour in a wide temperature range (2–290K), with a minimum at T≈380K. The spontaneous volume magnetostriction reaches ωS=1.6×10−2 at 2K that decreases to zero at T≈500K (≈1.4 TC), which is associated with the existence of short-range magnetic correlations.

Dielectric relaxations and magnetodielectric response in BiMn2O5 single crystal

Magnetic and dielectric properties have been investigated on BiMn2O5 single crystals. Magnetic measurements show that BiMn2O5 becomes antiferromagnetic below TN∼39 K, with short range magnetic correlation existing between TN and T∗∼100 K. Three thermally excited dielectric relaxations at temperatures TN&amp;lt;T&amp;lt;300 K and large intrinsic magnetodielectric effect due to the special arrangement of Mn3+/Mn4+ ions associated with electron hopping between them were observed. The low temperature anisotropic relaxation (TN&amp;lt;T&amp;lt;T∗) along the b-axis, which is insignificant along the c-axis, can be ascribed to electron hopping between the zigzag chains with short range magnetic correlation. The other two dielectric relaxations are also discussed.

Low energy spin dynamics in trimer spin chain compound Ca3Cu2Ni(PO4)4: 31P NMR study

31P nuclear-magnetic-resonance (NMR) study in the trimer spin chain compound Ca3Cu2Ni(PO4)4 is performed in the presence of different external magnetic fields in the temperature range 4–300K. The results suggest that the three magnon mediated scattering process which was the dominant contribution to 1/T1 in the pure compound Ca3Cu3(PO4)4, has negligible contribution in Ca3Cu2Ni(PO4)4, with the two magnon mediated Raman process being dominant. The linear variation of nuclear spin lattice relaxation rate 1/T1T with the magnetic susceptibility χ indicates that the Q component of magnetic susceptibility χ(Q) is nearly proportional to χ, where Q is the antiferromagnetic wave vector. A change in the slope of 1/T1T with respect to χ near 15K suggests a change in the antiferromagnetic spin fluctuation spectrum. Below 15K, the spin diffusion mechanism governs the 1/T1 process. This could be a signature of the development of short range magnetic correlation or due to the contribution arising from the paramagnetic impurity spins.