Dynamical disorder in negative thermal expansion compound Zn(CN)2 is investigated by quasielastic neutron scattering technique in the temperature range 170-320 K. Significant quasielastic broadening is observed above the phase transition temperature of about 250 K, however no broadening is observed at 220 K and below. Data at high temperatures are analyzed assuming the CN reorientation. Characteristic time associated with the CN orientation is estimated as 16 ps and 11 ps at 270 and 320 K respectively.
We show through a variational calculation that in a large range of parameters the paramagnetic to ferromagnetic transition in colossal magnetoresistance compounds is accompanied by a collapse of polaronic lattice disorder in addition to that of spin disorder. The spin-lattice disordered state is shown to be localized and the ordered state itinerant as observed. The observed dramatic change in the diffuse scattering at the transition as well as the isotope effect are also explained.
Polycrystalline samples of the half-metallic ferromagnet Heusler compound Co$_2$TiSn have been prepared and studied using bulk techniques (X-ray diffraction and magnetization) as well as local probes ($^{119}$Sn Mössbauer spectroscopy and $^{59}$Co nuclear magnetic resonance spectroscopy) in order to determine how disorder affects half-metallic behavior and also, to establish the joint use of Mössbauer and NMR spectroscopies as a quantitative probe of local ion ordering in these compounds. Additionally, density functional electronic structure calculations on ordered and partially disordered Co…
We develop a variational mean-field theory of the ferromagnetic transition in compounds like Lanthanum-Manganite within the framework of the Double-Exchange Model supplemented by modest disorder. We obtain analytical expressions for the transition temperature, its variation with the valence electron-density and its decrease with disorder. We derive an expression for the conductivity for both the paramagnetic and the ferromangetic metallic phases, and study its dependence on the temperature and magnetic field. A simple relation between the resistivity in the ferromagnetic phase and the spontane…
Co2-based Heusler compounds are the promising materials for the spintronics application due to their high Curie temperature, large spin-polarization, large magnetization density, and exotic transport properties. In the present manuscript, we report the anomalous Hall effect (AHE) in a polycrystalline Co2FeAl Heusler compound using combined experimental and theoretical studies. The Rietveld analysis of high-resolution synchrotron x-ray diffraction data reveals a large degree (~50 %) of antisite disorder between Fe and Al atoms. The analysis of anomalous transport data provides the experimental …
The local atomic environments and magnetic properties were investigated for a series of Co(1+x)Fe(2-x)Si (0<x<1) Heusler compounds. While the total magnetic moment in these compounds increases with the number of valance electrons, the highest Curie temperature (Tc) in this series was found for Co1.5Fe1.5Si, with a Tc of 1069 K (24 K higher than the well known Co2FeSi). 57Fe Mössbauer spectroscopy was used to characterize the local atomic order and to estimate the Co and Fe magnetic moments. Consideration of the local magnetic moments and the exchange integrals is necessary to understand the tr…
Half-metallic ferromagnets and especially the full-Heusler alloys containing Co are at the center of scientific research due to their potential applications in spintronics. For realistic devices it is important to control accurately the creation of defects in these alloys. We review some of our late results on the role of defects and impurities in these compounds. More precisely we present results for the following cases (i) doping and disorder in Co$_2$Cr(Mn)Al(Si) alloys, (ii) half-metallic ferrimagnetism appeared due to the creation of Cr(Mn) antisites in these alloys, (iii) Co-doping in Mn…
The role of controlled disorder in the strong correlated narrow gap semiconductor candidate FeGa$_3$ has been investigated. Polycrystalline samples were synthesized by the combination of arc-melting furnace and successive annealing processes. Deviations of the occupation number of Fe and Ga sites from those expected in the pristine compound were quantified with X-ray analysis. Besides that, electrical transport and magnetization measurements reveal that hierarchy in Fe and Ga site disorder tunes the ground state of FeGa$_3$ from paramagnetic semiconducting to a magnetic metal. These findings a…
The influence of antisite disorder occupancies on the magnetic properties of the half-metallic Co$_2$MnSi compound was studied by experimental techniques and first-principles calculations. The neutron diffraction studies show almost identical amount of Mn and Co disorders of 6.5\% and 7.6\%, which was found to be in good agreement with density functional theory (DFT) calculations of the stable Co$_2$MnSi system with the corresponding disorders. DFT studies reveal that antiferromagnetic interactions introduced by Mn disorder lead to a reduction of the net magnetic moment. The results are discus…
Spin gapless semiconductors exhibit a finite band gap for one spin channel and closed gap for other spin channel, emerged as a new state of magnetic materials with a great potential for spintronic applications. The first experimental evidence for the spin gapless semiconducting behavior was observed in an inverse Heusler compound Mn2CoAl. Here, we report a detailed investigation of the crystal structure and anomalous Hall effect in the Mn2CoAl using experimental and theoretical studies. The analysis of the high-resolution synchrotron x-ray diffraction data shows anti-site disorder between Mn a…
Many mixed ionic/electronic conductors (MIECs) applied in fuel cell electrodes can be considered as alloys between perovskite oxides and ordered oxygen vacancy compounds. For example, in the model MIEC (STF), low oxygen diffusion barrier exist in SrTiO3 lattice, when it has been mixed with SrFeO2.5 with intrinsic oxygen deficiency, the ionic conductivity can be greatly improved. Meanwhile, the electronic conductivity can be optimized by controlling the defect chemistry of the alloy. However, the configurational space is too large in such alloys so that it is difficult for direct atomic modelin…
Zn(CN)2 is known to have an isotropic negative thermal expansion (NTE) coefficient (about -51 x 10-6 K-1) over 10-370 K that is twice as large as that of ZrW2O8. We have measured the pressure dependence of the phonon spectra up to 30 meV from a polycrystalline sample of Zn(CN)2 at pressures of 0, 0.3, 1.9 and 2.8 kbar at temperatures of 165 and 225 K. The measurements enabled us to estimate the energy dependence of the ratios Gamma/B (Gamma are Gruneisen parameters as a function of phonon energy Ei at ambient pressure and B is the bulk modulus), which reflect the anharmonicity of phonons. We c…
Compositional disorder is common in crystal compounds. In these compounds, some atoms are randomly distributed at some crystallographic sites. For such compounds, randomness forms many non-identical independent structures. Thus, calculating the energy of all structures using ordinary quantum ab initio methods can be significantly time-consuming. Machine learning can be a reliable alternative to ab initio methods. We calculate the energy of these compounds with an accuracy close to that of density functional theory calculations in a considerably shorter time using machine learning. In this stud…
The temperature dependent stability of the magnetic phases of FeRh were investigated by means of total energy calculations with magnetic disorder treated within the uncompensated disordered local moment (uDLM) approach. In addition, Monte Carlo simulations based on the extended Heisenberg model have been performed, using exchange coupling parameters obtained rom first principles. The crucial role and interplay of two factors in the metamagnetic transition in FeRh has been revealed, namely the dependence of the Fe-Fe exchange coupling parameters on the temperature-governed degree of magnetic di…
The quaternary Heusler compound NiFeTiSn can be considered to be derived from the exotic pseudogap-compound Fe2TiSn by the replacement of one Fe atom by Ni. In contrast to Fe2TiSn, which shows a disorder-induced ferromagnetic phase, the ground state of NiFeTiSn is antiferromagnetic with the signature of spin canting. Interestingly, NiFeTiSn shows a Griffiths-like phase characterized by isolated ferromagnetic clusters before attaining the antiferromagnetic state. The Griffiths-like phase is possibly associated with the antisite disorder between Fe and Ti sites as evident from our powder X-ray d…
Disorder induced by chemical inhomogeneity and Jahn-Teller (JT) distortions is often observed in mixed valence perovskite manganites. The main reasons for the evolution of this disorder are connected with the cationic size differences and the ratio between JT active and non-JT active ions. The quenched disorder leads to a spin-cluster state above the magnetic transition temperature. The effect of Cu, a B-site substitution in the La0.8Sr0.2MnO3 compound, on the disordered phase has been addressed here. X-ray powder diffraction reveals rhombohedral (R-3c) structures for the two compounds with ne…
Near-bandgap photoemission spectroscopy experiments were performed on p-GaN and p-InGaN/GaN photocathodes activated to negative electron affinity. The photoemission quantum yield of the InGaN samples drops by more than one order of magnitude when the temperature is decreased while it remains constant on the GaN sample. This indicates a freezing of photoelectron transport in p-InGaN that we attribute to electron localization in the fluctuating potential induced by the alloy disorder. This interpretation is confirmed by the disappearence at low temperature of the peak in the photoemission spectr…
The dielectric constant of the Kagome staircase-like Co3V2O8 polycrystalline compound has been measured as function of temperature and magnetic field up to 14T. It is found that the application of an external magnetic field suppresses the anomaly for the dielectric constant beyond 6.1K. Furthermore, its magnetic field dependence reveals a negative magnetocapacitance which is proportional to the magnetic susceptibility, suggesting a common magnetostrictive origin for the magnetic field dependence of the two quantities. This result is very different from that obtained from the isostructural comp…
We report the synthesis and crystal structure of an organic inorganic compound, ethylenediammonium lead iodide, NH3CH2CH2NH3PbI4. Synchrotron based single crystal X-ray diffraction experiments revealed that the pristine and thermally treated crystals differ in the organic cation behaviour, which is characterized by a partial disorder in the thermally treated crystal. Based on current voltage measurements, increased disorder of the organic cation is associated with enhanced photoconductivity. This compound could be a potential candidate for interface engineering in lead halide perovskite-based …
The quaternary full Heusler compound Mn(2-x)Co(x)VAl with x = 1 is predicted to be a half-metallic antiferromagnet. Thin films of the quaternary compounds with x = 0...2 were prepared by DC and RF magnetron co-sputtering on heated MgO (001) substrates. The magnetic structure was examined by x-ray magnetic circular dichroism and the chemical disorder was characterized by x-ray diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn2VAl (x = 0). For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to Mn. The observed reduced magnetic moments are interpreted with t…
Mixed spin chain compounds, ACuFe2(VO4)3 (A= Li,Na), reach magnetically ordered state at TN ~ 11 K (Li) or ~ 9 K (Na) and experience further transformation of magnetic order at T* ~ 7 K (Li) or ~ 5 K (Na), evidenced in magnetic susceptibility chi and specific heat Cp measurements. While no anomaly has been detected in dielectric property of NaCuFe2(VO4)3, the step-like feature precedes a sharp peak in permittivity epsilon at TN in LiCuFe2(VO4)3. These data suggest the spin-order-induced ferroelectricity in Li compound and no such thing in Na compound. On the contrary, the Moessbauer spectrosco…
The magnetic moments per Fe atom in high-$T{_\textrm{c}}$ iron-based superconducting compounds, BaFe$_{2}$As$_{2}$ and LaFeAsO obtained from the first-principles calculation with local-spin-density approximation are much larger than those obtained from experiments. To resolve the contradictory results between theory and experiment we employed the so-called LDA$+ U$ (or more exactly GGA$+ U$) technique with negative \emph{U} in the first-principles calculation. The calculated values with negative \emph{U}, $- 0.09$ Ry and $- 0.10$ Ry for BaFe$_{2}$As$_{2}$ and LaFeAsO, respectively, are in exce…
An infrared study of the phonon spectra of ZrW2O8 as a function of temperature which includes the low energy (2-10 meV) region relevant to negative thermal expansion is reported and discussed in the context of specific heat and neutron density of states results. The prevalence of infrared active phonons at low energy and their observed temperature dependence are highly unusual and indicative of exotic low-energy lattice dynamics. Eigenvector calculations indicate a mixing of librational and translational motion within each low-frequency IR mode. The role of the underconstrained structure in es…
The present work reports on the new soft ferromagnetic Heusler phases Fe2NiGe, Fe2CuGa, and Fe2CuAl, which in previous theoretical studies have been predicted to exist in a tetragonal regular Heusler structure. Together with the known phases Fe2CoGe and Fe2NiGa these materials have been synthesized and characterized by powder XRD, 57 Fe Mössbauer spectroscopy, SQUID and EDX measurements. In particular Mössbauer spectroscopy was used to monitor the degree of local atomic order/disorder and to estimate magnetic moments at the Fe sites from the hyperfine fields. It is shown that in contrast to th…
Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)${}_2$Sr${}_{3}$Co${}_2$O${}_9$, which has been believed to be a Co analogue of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+δ}$, revealed that it has a more complex structure including a CoO${}_2$ hexagonal layer [T. Yamamoto {\it et al.}, Jpn. J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO${}_2$ layers but also creates a certain amount of localized spins. Ferrom…
The competition between magnetic ordering and the Kondo effect in Ce2RhSi3, ordering antiferromagnetically at 7 K, is investigated by the measurements of magnetization, heat capacity and electrical resistivity on the solid solutions, Ce(2-x)La(x)RhSi3, Ce(2-y)Y(y)RhSi3, and Ce2RhSi(3-z)Ge(z), as well as by high pressure studies on this compound. The trends in the Kondo and Neel temperature variations among these alloys are compared to infer the roles of unit-cell volume and electronic structure changes. On the basis of the results, we infer that this compound lies at the peak of Doniach-magnet…
We systematically investigate the magnetic properties and local structure of Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this $d^4$ compound. We compare the magnetic properties and atomic imaging of (1) a slow cooled crystal, (2) a crystal quenched from 900\degree C after growth, and (3) a crystal grown using a faster cooling rate than the slow cooled one. Atomic imaging by scanning transmission electron microscopy (STEM) shows that quenching from 900oC introduces antiphase boundary to…
Several doped 6H hexagonal ruthenates, having the general formula Ba3MRu2O9, have been studied over a significant period of time in order to understand the unusual magnetism of ruthenium metal. However, among them, the M=Fe compound appears different since it is observed that unlike others, the 3d Fe ions and 4d Ru ions can easily exchange their crystallographic positions and as a result many possible magnetic interactions become realizable. The present study involving several experimental methods on this compound establish that the magnetic structure of Ba3FeRu2O9 is indeed very different fro…
Time-of-flight neutron powder diffraction and specific heat measurements were used to study the nature of thermal expansion in rhenium trioxide, an electrically conducting oxide with cubic symmetry. The temperature evolution of the lattice parameters show that ReO3 can exhibit negative thermal expansion at low temperatures and that the transition from negative to positive thermal expansion depends on sample preparation; the single crystal sample demonstrated the highest transition temperature, 300 K, and largest negative value for the coefficient of thermal expansion, alpha = -1.1(1)x 10^-6 K^…
We expand our study on the full-Heusler compounds [I. Galanakis \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 042502 (2006)] to cover also the case of doping and disorder in the case of Co$_2$MnAl and Co$_2$MnGa half-metallic Heusler alloys. These alloys present a region of very small minority density of states instead of a real gap. Electronic structure calculations reveal that doping with Fe and Cr in the case of Co$_2$MnAl retains the half-metallicity contrary to the Co$_2$MnGa compound. Cr impurities present an unusual behavior and the spin moment of the Cr impurity scales almost linearl…
Half-Heusler compounds have attracted significant attention because of their topologically non-trivial electronic structure, which leads to unusual electron transport properties. We thoroughly investigated the magnetotransport properties of high-quality single crystals of two half-Heusler phases, TbPtBi and HoPtBi, in pursuit of the characteristic features of topologically non-trivial electronic states. Both studied compounds are characterized by the giant values of transverse magnetoresistance with no sign of saturation in magnetic field up to 14 T. HoPtBi demonstrates the Shubnikov-de Haas e…
Exchange bias effect in the ferromagnetic double perovskite compound Y$_2$CoMnO$_6$, which is also a multiferroic, is reported. The exchange bias, observed below 8~K, is explained as arising due to the interface effect between the ferromagnetic and antiferromagnetic clusters created by {\it antisite} disorder in this material. Below 8~K, prominent ferromagnetic hysteresis with metamagnetic "steps" and significant coercive field, $H_c \approx$ 10~kOe are observed in this compound which has a $T_c \approx$ 75~K. A model based on growth of ferromagnetic domains overcoming the elastic energy of st…
We perform a theoretical study of a non-collinear Ising ferrimagnetic spin chain inspired by the compound Co(hfac)2NITPhOMe. The basic building block of its structure contains one Cobalt ion and one organic radical each with a spin 1/2. The exchange interaction is strongly anisotropic and the corresponding axes of anisotropy have a period three helical structure. We introduce and solve a model Hamiltonian for this spin chain. We show that the present compound is very close to a so-called disorder point at which there is a massive ground state degeneracy. We predict the equilibrium magnetizatio…
The results of dc magnetization, electrical and magnetoresistance and heat capacity measurements (2-300 K) on Tb7Rh3, crystallizing in Th7Fe3-type hexagonal structure, are reported. In this compound, magnetic ordering sets in around 90 K with additional transitions at low temperatures and the temperature coefficient of resistivity (R), dR/dT, is negative over a wide temperature range in the paramagnetic state. The present magnetization results reveal that this compound is apparently characterized by rich features in the magnetic-field-temperature phase diagram. A point of major emphasis is tha…
Detailed magnetic properties of the compound Sr2YbRuO6 are presented here. The compound belongs to the family of double perovskites forming a monoclinic structure. Magnetization meas-urements reveal clear evidence for two components of magnetic ordering aligned opposite to each other, leading to a magnetization reversal, compensation temperature (T* = 34 K) and neg-ative magnetization at low temperatures and low magnetic fields. Heat capacity measurements corroborate the presence of two components in the magnetic ordering and a noticeable third anomaly at low temperatures (~15 K) which cannot …
We measure the transient strain profile in a nanoscale multilayer system composed of Yttrium, Holmium and Niobium after laser excitation using ultrafast X-ray diffraction. The strain propagation through each layer is determined by transient changes of the material-specific Bragg angles. We experimentally derive the exponentially decreasing stress profile driving the strain wave and show that it closely matches the optical penetration depth. Below the Neel temperature of Ho, the optical excitation triggers negative thermal expansion, which is induced by a quasi-instantaneous contractive stress,…
A quasi one-dimensional compound, Ca3CuRuO6, has been synthesized by solid-state reaction method and studied using magnetization (M) and heat capacity (C) measurements. This compound undergoes magnetic ordering (T_o) around 40 K, as evidenced by the dc magnetic susceptibility (chi) behavior. However, the magnitude of the paramagnetic Curie temperature (theta_p) obtained from the high temperature linear region is large (-277 K, with the negative sign indicating antiferromagnetic interaction). The reduction of T_o, compared to theta_p, is attributed to geometrical frustration effect arising from…
The magnetic behavior of a solid solution, Ca3-xYxCo2O6, based on an exotic spin-chain compound, Ca3Co2O6, crystallizing in K4CdCl6-derived rhombohedral structure is investigated. Among the compositions investigated (x= 0.0, 0.3, 0.5, 0.75 and 1.0), single phase formation persists up to x= 0.75, with the elongation of the c-axis. The present investigations reveal that the temperature at which the partially disordered antiferromagnetic structure sets in (which occurs at 24 K for the parent compound, x= 0.0) undergoes gradual reduction with the substitution of Y for Ca, attaining the value of ab…
Using first principles calculations, the energetic stability of two-dimensional (2D) binary compounds $XY$ is investigated, where $X$ and $Y$ indicate the metallic element from Li to Pb in the periodic table. Here, 1081 compounds in the buckled honeycomb (BHC), buckled square, B2, L1$_0$, and B$_h$ structures are studied. For the compounds that have negative formation energy in the BHC structure or the compounds that can have the B$_h$ structure, the phonon dispersions of the 2D structures are also calculated. We demonstrate that (i) a negative formation energy is neither a sufficient nor nece…
The compound, Sr3NiPtO6, belonging to a K4CdCl6-type rhombohedral structure, has been reported not to exhibit magnetic ordering at least down to 1.8 K, despite a relatively large value of paramagnetic Curie temperature. This is attributable to geometrical frustration. Here we report the results of our efforts to gradually replace Sr by Ba and to probe the influence of positive (external) and negative (chemical) pressure on the magnetic behavior of this compound. In the Ba substituted series, single phase is formed up to x= 1.0 with Ba substituting for Sr. The magnetic properties of the parent …
Cyanide based framework compounds are known to show large negative thermal expansion behaviour. Here we report the phonon and anomalous lattice behavior of two metal cyanide framework compounds i.e. KMnAg3(CN)6 and KNiAu3(CN)6. We have studied the role of van der Waals dispersion and magnetic interactions on structural stability of these compounds. The behavior of these compounds under isotropic compression shows the presence of negative linear compressibility. The calculated phonon spectra, validated by inelastic neutron scattering measurements and elastic constants are used to study the nega…
Cr$^{3+}$ doped spinel compounds AB$_2$O$_4$ with A=Zn, Mg and B=Ga, Al exhibit a long near infrared persistent luminescence when excited with UV or X-rays. In addition, persistent luminescence of ZnGa$_2$O$_4$ and to a lesser extent MgGa$_2$O$_4$, can also be induced by visible light excitation via $^4$A$_2$ $ \rightarrow $ $^4$T$_2$ transition of Cr$^{3+}$, which makes these compounds suitable as biomarkers for in vivo optical imaging of small animals. We correlate this peculiar optical property with the presence of antisite defects, which are present in ZnGa$_2$O$_4$ and MgGa$_2$O$_4$. By u…
Single-phase films of the full Heusler compound Fe2TiSi have been prepared by magnetron sputtering. The compound is found to be a semiconductor with a gap of 0.4eV. The electrical resistivity has a logarithmic temperature dependence up to room temperature due to Kondo scattering of a dilute free electron gas off superparamagnetic impurities. The origin of the electron gas is extrinsic due to disorder or off-stoichiometry. Density functional theory calculations of the electronic structure are in excellent agreement with electron energy loss, optical, and x-ray absorption experiments. Fe2TiSi ma…
A total negative field dependence of hole mobility down to low temperature was observed in N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'diamine (TPD) doped in Polystyrene. The observed field dependence of mobility is explained on the basis of low values of energetic and positional disorder present in the sample. The low value of disorder is attributed to different morphology of the sample due to aggregation/crystallization of TPD. Monte Carlo simulations were also performed to understand the influence of aggregates on charge transport in disordered medium with correlated site ene…
Exchange bias-like effect observed in the intermetallic compound TbFeAl, which displays a magnetic phase transition at $T^h_c \approx$ 198~K and a second one at $T^l_c \approx$ 154~K, is reported. {\em Jump}-like features are observed in the isothermal magnetization, $M (H)$, at 2~K which disappear above 8~K. The field-cooled magnetization isotherms below 10~K show loop-shifts that are reminiscent of exchange bias, also supported by {\em training effect}. Significant coercive field, $H_c \approx$ 1.5~T at 2~K is observed in TbFeAl which, after an initial increase, shows subsequent decrease wit…
In this work the theoretical and experimental investigations of Co2TiZ (Z = Si, Ge, or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfill the Slater-Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting like behavior. A large negative magnetoresistance of 55% is observed for Co2TiSn at room…
This study explores the relationship between phonon dynamics and negative thermal expansion (NTE) in Cu$_x$Ni$_{2-x}$(CN)$_4$. The partial replacement of nickel (II) by copper (II) in Ni(CN)$_2$ leads to a line phase, CuNi(CN)$_4$ (x = 1), and a solid solution, Cu$_x$Ni$_{2-x}$(CN)$_4$ (0 $\leq$ x $\leq$ 0.5). CuNi(CN)$_4$ adopts a layered structure related to that of Ni(CN)$_2$ (x = 0), and interestingly exhibits 2D NTE which is about 1.5 times larger. Inelastic neutron scattering (INS) measurements combined with first principles lattice dynamical calculations provide insights into the effect…
We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular we study RMnO3 (R= La, Pr, Nd, Tb, Ho, Er, Y and Ca) and the different phases of Ca or Sr doped RMnO3 compounds as well as cation deficient RMnO3. The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of MnO6 octahedra and independently of the average structure obtained by diffraction techniques. The main peaks of compounds with regular MnO6 octahedra, as CaMnO3, highly Ca …
The low temperature magnetic properties of pyrochlore compound Dy2Ti2O7 in magnetic fields applied along the [111] direction are reported. Below 1 K, a clear plateau has been observed in the magnetization process in the field range 2~9 kOe, followed by a sharp moment jump at around 10 kOe that corresponds to a breaking of the spin ice state. We found that the plateau state is disordered with the residual entropy of nearly half the value of the zero-field state, whose macroscopic degeneracy comes from a frustration of the spins on the kagome layers perpendicular to the magnetic field.
The temperature dependent ac and dc magnetization and heat capacity data of Ca$_3$CoIrO$_6$, a spin-chain compound crystallizing in a K$_4$CdCl$_6$-derived rhombohedral structure, show the features due to magnetic ordering of a frustrated-type below about 30 K, however without exhibiting the signatures of the so-called "partially disordered antiferromagnetic structure" encountered in the isostructural compounds, Ca$_3$Co$_2$O$_6$ and Ca$_3$CoRhO$_6$. This class of compounds thus provides a variety for probing the consequences of magnetic frustration due to topological reasons in stoichiometric…
Both amorphous and crystalline materials frequently exhibit low temperature specific heats in excess of what is predicted using the Debye model. The signature of this excess specific heat is a peak observed in $C/T^3$ \textit{versus} $T$. To understand the curious absence of long-range ordering of local distortions in the crystal structure of pyrochlore Bi$_2$Ti$_2$O$_7$, we have measured the specific heat of crystalline Bi$_2$Ti$_2$O$_7$ and related compounds. We find that the peak in $C/T^3$ versus $T$ in Bi$_2$Ti$_2$O$_7$ falls at a substantially lower temperature than other similar compoun…
