Interplay of lattice-spin-orbital coupling and Jahn-Teller effect in noncollinear spinel TixMn1-x(FeyCo1-y)2O4: a neutron diffraction study.

Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel TixMn1‒x(FeyCo1‒y)2O4has been reported by means of neutron diffraction and magnetization studies. Two distinct sets of compositions (i)x(Ti) = 0.20 andy(Fe) = 0.18; (ii)x(Ti) = 0.40 andy(Fe) = 0.435 have been considered for this study. The first compound of equivalent stoichiometry Ti0.20Mn0.80Fe0.36Co1.64O4exhibits enhanced tetragonal distortion across the ferrimagnetic transition temperatureTC= 258 K in comparison to the end compound MnCo2O4(TC∼ 180 K) with a characteristic ratioct/√2atof 0.99795(8) demonstrating robust lattice-spin-orbital coupling. However, in the second case Ti0.40Mn0.60Fe0.87Co1.13O4with higherB-site compositions, the presence of Jahn-Teller ions with distinct behavior appears to counterbalance the strong tetragonal distortion thereby ceasing the lattice-spin-orbital coupling. Both the investigated systems show the coexistence of noncollinear antiferromagnetic and ferrimagnetic components in cubic and tetragonal settings. On the other hand, the dynamical ac-susceptibility,χac(T) reveals a cluster spin-glass state with Gabay-Toulouse (GT) like mixed phases behaviour belowTC. Such dispersive behaviour appears to be sensitive to the level of octahedral substitution. Further, the field dependence ofχac(T) follows the weak anisotropic GT-line behaviour with crossover exponent Φ lies in the range 1.38-1.52 on theH-Tplane which is in contrast to theB-site Ti substituted MnCo2O4spinel that appears to follow irreversible non-mean-field AT-line behaviour (Φ âˆ¼ 3 +δ). Finally, the Arrott plots analysis indicates the presence of a pseudo first-order like transition (T< 20 K) which is in consonance with and zero crossover of the magnetic entropy change within the frozen spin-glass regime.

Spin-liquid state with precursor ferromagnetic clusters interacting antiferromagnetically in frustrated glassy tetragonal spinel Zn0.8Cu0.2FeMnO4.

Spinels (AB2O4) with magnetic ions occupying only the octahedralBsites have inherent magnetic frustration which inhibits magnetic long-range order (LRO) but may lead to exotic states. Here we report on the magnetic properties of the tetragonal spinel Zn0.8Cu0.2FeMnO4, the tetragonality resulting from the Jahn-Teller active Mn3+ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn0.82+Cu0.22+)A[Fe0.42+Fe0.63+Mn3+]BO4‒δ. Analysis of the temperature dependence of magnetization (M), ac magnetic susceptibilities (χ'andχ''), dc susceptibility (χ), heat capacityCp, and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data ofχ vs. Tfits the Curie-Weiss law:χ = C/(T ‒ θ) fromT= 250 K to 400 K withθ≃ 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constantJ/kB= 17 K, andC= 3.29 emu K mol‒1Oe‒1yielding an effective magnetic momentµeff= 5.13µBresulting from the high-spin states of Cu2+(Asite) and Fe2+(Bsite), while theBsite trivalent ions Mn3+and Fe3+are in their low-spin states. The extrapolated saturation magnetization obtained from theM vs. Hdata atT= 2 K is explained using the spin arrangement (Cu2+↓)A[Fe2+↑, Fe3+↓, Mn3+↑]Bleading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d(χT)/dTshows the onset of ferrimagnetism below ∼100 K and peaks near 47 K and 24 K. The relaxation timeτobtained from temperature and frequency dependence ofχ″when fit to the power law and Vogel-Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperatureTSGHfollows the equation:TSGH=TSG01-AH2/ϕwithTSG(0) = 46.6 K,A= 8.6 × 10‒3Oe‒0.593andϕ= 3.37. The temperature dependence of hysteresis loops yields coercivityHC∼ 3.8 kOe at 2 K without exchange-bias, butHCdecreases with increase inTbecoming zero above 24 K, theTSG(H) forH= 800 Oe. Variations ofCpvs. Tfrom 2 K to 200 K inH=0 andH=90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad weak peak typically of SRO becomes evident centered around 40 K. ForT< 9 K,Cpvaries asT2; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetizationMTRM(t) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn0.8Cu0.2FeMnO4, ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state atTSG(0) = 46.6 K, followed by SL behavior below 9 K.

Magnetic-field induced melting of long-range magnetic order akin to Kitaev insulators in the metallic compound Tb5Si3.

There have been constant efforts to find 'exotic' quantum spin-liquid (QSL) materials. Some of the transition metal insulators dominated by the direction-dependent anisotropic exchange interaction ('Kitaev model' for honeycomb network of magnetic ions) are considered to be promising cases for the same. In such Kitaev insulators, QSL is achieved from the zero-field antiferromagnetic state by the application of magnetic-field, suppressing other exchange interactions responsible for magnetic order. Here, we show that the features attributable to long-range magnetic ordering of the intermetallic compound, Tb5Si3, (TN= 69 K), containing honey-comb network of Tb ions, are completely suppressed by a critical applied field,Hcr, in heat-capacity and magnetization data, mimicking the behavior of Kitaev physics candidates. The neutron diffraction patterns as a function ofHreveal that it is an incommensurate magnetic structure that gets suppressed, showing peaks arising from multiple wave vectors beyondHcr. Increasing magnetic entropy as a function ofHwith a peak in the magnetically ordered state is in support of some kind of magnetic disorder in a narrow field range afterHcr. Such a high-field behavior for a metallic heavy rare-earth system to our knowledge has not been reported in the past and therefore is intriguing.

Models of Ni- and Co-ion occupation in LiNi0.5Co0.5PO4orthophosphate and its magnetic structure.

The neutron diffraction, magnetic and heat capacity measurements have been carried out to study the polycrystalline sample LiNi0.5Co0.5PO4prepared by the glycerol-nitrate synthesis method. Models of Ni- and Co-ion occupation the 4coctahedral position in a crystal structure LiNi0.5Co0.5PO4are calculated for a paramagnetic state. The best model is the Ni- and Co-ions occupy the 4csite inPnmapatent space-group in sequence Ni-Co-Ni-Co or Co-Ni-Co-Ni. It is shown that nickel ions formabplanes alternating with the planes of cobalt ions in the direction of theccrystallographic axis. At 7 K, an average magnetic moment of 3d-ions is equal to 1.90 (9)µB. The moments are ordering antiferromagnetically and parallel to thebcplane decreasing to zero at 15 K. In the high-spin state a temperature dependence of the Ni2+/Co2+ion-magnetic moment is well described within the 2D Ising model with order parameterß= 0.198 and Néel temperatureTN= 14.1 (1) K, obtained from heat capacity data. This temperature agrees well withTcr= 14.3 (2) K, determined with magnetic measurement. Maybe the short-range magnetic order exists in LiNi0.5Co0.5PO4over temperature region (14-16) K, that is confirmed by the maximum on a temperature dependence of the magnetization at 16.1 (5) K.

Neutron diffraction evidence for local spin canting, weak Jahn-Teller distortion, and magnetic compensation in Ti1-x Mn x Co2O4 spinel.

A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti1-x Mn x Co2O4 in the temperature interval 1.6 K [Formula: see text] T [Formula: see text] 300 K. Our neutron diffraction study reveals cooperative distortions of the TO6 octahedra in the Ti1-x Mn x Co2O4 system for all the Jahn-Teller active ions T = Mn3+ , Ti3+ and Co3+ , having the electronic configurations 3d 1, 3d 4 and 3d 6, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet-Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a < 1, in which the apical bond length d c (T B -O) is longer than the equatorial bond length d ab (T B -O) due to the splitting of the e g level of Mn3+ ions into [Formula: see text] and [Formula: see text]. For the composition x = 0.4, the distortion in the T B O6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti0.6Mn0.4Co2O4 and Ti0.8Mn0.2Co2O4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti3+ , Mn3+ , and Co3+ occupy the octahedral, whereas, Co2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at T COMP = 25.4 K in Ti0.8Mn0.2Co2O4 and a reentrant spin-glass behaviour in Ti0.6Mn0.4Co2O4 with a freezing temperature of ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below T COMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.

A crystal structure prediction enigma solved: the gallic acid monohydrate system - surprises at 10 K.

The seemingly unpredictable structure of gallic acid monohydrate form IV has been investigated using accurate X-ray diffraction measurements at temperatures of 10 and 123 K. The measurements demonstrate that the structure is commensurately modulated at 10 K and disordered at higher temperatures. Aided by charge-density modeling and periodic DFT calculations we show that the disorder gives a substantial stabilization of the structure.

Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr3Fe2O7.

We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr(3)Fe(2)O(7), which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe(4+) moments adopt incommensurate spiral order below T(N) = 115 K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr(3)Fe(2)O(7) results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.

A cluster-glass magnetic state in R5Pd2 (R = Ho, Tb) compounds evidenced by AC-susceptibility and neutron scattering measurements.

AC- and DC-susceptibility, high-field magnetization and neutron diffraction measurements have been performed in order to study the magnetic state of R5Pd2 (R = Ho, Tb) compounds. The results show that both compounds undergo cluster-glass freezing upon cooling below Tf. According to the neutron diffraction a long-range magnetic order is absent down to 2 K and magnetic clusters with short-range incommensurate antiferromagnetic correlations exist not only below Tf but also in a wide temperature range above the freezing temperature (at least up to 2Tf). A complex cluster-glass magnetic state existing in Ho5Pd2 and Tb5Pd2 down to low temperatures results in rather complicated magnetization behavior in DC and AC magnetic fields. Such an unusual magnetic state in compounds with a high rare-earth concentration may be associated with the layered type of their crystal structure and with substantial atomic disorder, which results in frustrations in the magnetic subsystem.

Propagation of neutron spherical waves through a thick, vibrating Ge single crystal.

The results of experimental and theoretical studies into the influence of ultrasound on the propagation of neutron waves in a thick Ge crystal are presented. The neutron intensity profiles were measured for the case of Laue diffraction inside the Borrmann fan. At low amplitudes of ultrasonic waves interference effects (diffraction intensity beatings) were observed. The observations were possible because of the uniform acoustic-field distribution through the whole bulk of the crystal. As distinct from the classical Shull experiments, wide analysing slits or position-sensitive detectors were used. To explain the results obtained, a modified theory for the spatial distribution of neutron diffraction intensities in the presence of acoustic excitation of the crystal is proposed. A good agreement between experiment and theory is obtained. At high amplitudes of ultrasonic waves the transition to kinematic scattering was not observed, despite the large strains in the crystalline lattice created by ultrasound. This could be connected with the formation of a superlattice having a standing wave period. A strong rise in the diffraction intensity and a sharp constriction of the neutron beam at the centre of the Borrmann fan were observed. This new effect could be used for the creation of ultrasound-controlled monochromators.

Magnetic structures and physical properties of Tm3Cu4Ge4 and Tm3Cu4Sn4.

Tm(3)Cu(4)Ge(4) crystallizes in the orthorhombic Gd(3)Cu(4)Ge(4)-type crystal structure (space group Immm) whereas Tm(3)Cu(4)Sn(4) crystallizes in a distorted variant of this structure (monoclinic space group C2/m). The compounds were studied by means of neutron diffraction, specific heat, electrical resistivity and magnetic measurements. Analysis of experimental data revealed the presence of an antiferromagnetic order below 2.8 K in both compounds. In Tm(3)Cu(4)Ge(4) the magnetic unit cell is doubled in respect to the crystal unit cell and the magnetic structure can be described by a propagation vector k = [0, 1/2, 0]. A larger magnetic unit cell was found in Tm(3)Cu(4)Sn(4), given by a propagation vector k = [1/2, 1/2, 0] (for simplicity the orthorhombic description is used for both the germanide and the stannide). Close to 2 K, in each compound an incommensurate antiferromagnetic order develops. This low-temperature magnetic phase is characterized by a propagation vector k = [1/4, 0, k(z)], where k(z) is close to 0.49 and 0.47 in Tm(3)Cu(4)Ge(4) and Tm(3)Cu(4)Sn(4), respectively. The antiferromagnetic phase transitions are clearly seen in the bulk magnetic and specific heat data of both compounds.

Magnetic behavior of Ba3Cu3Sc4O12.

The chain-like system Ba(3)Cu(3)Sc(4)O(12) has potentially interesting magnetic properties due to the presence of Cu(2+) and a structure-suggested low dimensionality. We present magnetization M versus magnetic field H and temperature T, T- and H-dependent heat-capacity C(p), (45)Sc nuclear magnetic resonance (NMR), muon spin rotation (µSR), neutron diffraction measurements and electronic structure calculations for Ba(3)Cu(3)Sc(4)O(12). The onset of magnetic long-range antiferromagnetic (AF) order at T(N) ∼ 16 K is consistently evidenced from the whole gamut of our data. A significant sensitivity of T(N) to the applied magnetic field H (T(N) ∼ 0 K for H = 70 kOe) is also reported. Coupled with a ferromagnetic Curie-Weiss temperature (θ(CW) ∼ 65 K) in the susceptibility (from a 100 to 300 K fit), it is indicative of competing ferromagnetic and antiferromagnetic interactions. These indications are corroborated by our density functional theory based electronic structure calculations, where we find the presence of significant ferromagnetic couplings between some copper ions whereas AF couplings were present between some others. Our experimental data, backed by our theoretical calculations, rule out the one-dimensional magnetic behavior suggested by the structure and the observed long-range order is due to the presence of non-negligible magnetic interactions between adjacent as well as next-nearest chains.

Core shell structure for solid gas synthesis of LiBD(4).

The formation of LiBD(4) by the reaction of LiD in a diborane/hydrogen atmosphere was analysed by in situ neutron diffraction and subsequent microstructural and chemical analysis of the final product. The neutron diffraction shows that nucleation of LiBD(4) already starts at temperatures of 100 degrees C, i.e. in its low temperature phase (orthorhombic structure). However, even at higher temperatures the reaction is incomplete. We observe a yield of approximately 50% at a temperature of 185 degrees C. A core shell structure of the grains, in which LiBD(4) forms a passivation layer on the surface of the LiD grains, was found in the subsequent microstructural (electron microscopy) and chemical (electron energy loss spectrometry) analysis.

Fine-tuning of the spin-density-wave state in Cr/V heterostructures via hydrogen uptake.

We show that the magnetic state in rather thick Cr films can be finely tuned via hydrogen uptake into adjacent vanadium layers at rather low hydrogen pressures. By changing the hydrogen concentration and, hence, the electronic structure in the V layers, it is possible to affect the global properties of spin-density waves (SDWs) in Cr layers, including the SDW period and the Néel temperature. We provide direct experimental evidence that hydrogen uptake into V layers can be used to switch between incommensurate and commensurate SDW states in a reproducible way.

Neutron scattering study of the field-induced soliton lattice in CuGeO3

CuGeO3 undergoes a transition from a spin-Peierls phase to an incommensurate phase at a critical field of H(c) approximately 12.5 T. In the high-field phase a lattice of solitons forms, with both structural and magnetic components, and these have been studied using neutron scattering techniques. Our results provide direct evidence for a long-ranged magnetic soliton structure which has both staggered and uniform magnetizations with amplitudes that are broadly in accord with theoretical estimates. The magnetic soliton width gamma(m) and the field dependence of the incommensurability deltak(SP) are found to agree well with theoretical predictions.

Hair zinc and copper concentration in survivors of myocardial infarction.

Increased Zn/Cu ratio in the diet, and consequently in the body, was suggested to be of importance in the pathogenesis of atherosclerosis. Head hair of 29 male survivors of myocardial infarction and of 23 control males was studied for the concentration of Zn and Cu. The Zn hair concentration and Zn/Cu ratio in survivors of myocardial infarction was significantly higher in comparison with controls. The inclusion of the Zn/Cu ratio into the discriminant analysis using total cholesterol and HDL cholesterol considerably improved the coefficient R2 and decreased the number of cases not properly classified.