The magnetic properties of polycrystaline La0.5Ba0.5CoO2.5+d (d = 0.44 and 0.485) where investigated using muon spin relaxation and bulk magnetization experiments. Below a Curie temperature of T_C = 160 K and 180 K, respectively, a magnetic phase with a macroscopic ferromagnetic moment forms. muSR proves that the full volume inhomogeneously orders at T_C but that there is microscopically a phase separation into phase volumes possessing a fully static and a slow magnetic dynamic behavior. A peak in the dynamic relaxation rate 35 K respectively 55 K below the Curie temperature indicates the freezing of spin components. The spin fluctuations are thermally activated with a typical Jahn-Teller like phonon energy of E_a = 340 K. Together with co-operative Jahn-Teller distortions appearing below the spin freezing temperature, this points to a strong magneto elastic coupling as the cause for the magnetic fluctuations and the spin freezing.
The effect of local structure on the magnetic and transport properties of the layered perovskites has been investigated. The samples PrBaCo$_{2}$O$_{5+δ}$, ($δ$ = 0.80 and 0.67) though crystallize in the same 112 type tetragonal structure have different magnetic ground states. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy has been employed to explain the changes in magnetic interactions based on the rotation and tilting of CoO polyhedra in these oxygen rich double perovskites.
We present first principles LDA+U calculations of electronic structure and magnetic state for LaCoO3 and HoCoO3. Low Spin to Intermediate Spin state transition was found in our calculations using experimental crystallographic data for both materials with a much higher transition temperature for HoCoO3, which agrees well with the experimental estimations. Low Spin state t6e0 (non-magnetic) to Intermediate Spin state t5e1 (magnetic) transition of Co(3+) ions happens due to the competition between crystal field t_2g-e_g splitting and effective exchange interaction between 3$d$ spin-orbitals. We s…
We report the observation of unusual exchange bias phenomena in the doped perovskite cobaltites La_{1-x}Sr_{x}CoO_{3} (x = 0.15, 0.18, and 0.30) in which a spontaneous phase separation into ferromagnetic clusters embedded in a non-ferromagnetic matrix occurs. When the La_{1-x}Sr_{x}CoO_{3} samples are cooled in a static magnetic field through a freezing temperature, the magnetization hysteresis loops exhibit exchange bias, i.e., the loops shift to the negative field and the magnetization becomes asymmetric. Moreover, exchange bias disappears when the measuring magnetic field is high enough. Th…
The effect of oxygen content on the magnetic properties of the layered perovskites has been investigated. The samples, PrBaCo$_{2}$O$_{5+δ}$ (0.35 $\leq δ\leq$ 0.80) were prepared by sol-gel method and characterized by X-ray diffraction and DC magnetization. A detailed magnetic phase diagram for PrBaCo$_{2}$O$_{5+δ}$ is presented. It is found that unlike in the case of heavier rare-earths, ferromagnetic interactions are present at all temperatures below Tc and even in the antiferromagnetically ordered phase. Moreover, in compounds with lower oxygen content, short range ferromagnetic interactio…
We systematically studied the magnetic and transport properties for the polycrystalline samples of Fe-doped perovskite cobaltites: Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$ ($y$=0.3, $x$=0-0.15; $y$=0.45, $x$=0-0.3) and Gd$_{0.55}$Sr$_{0.45}$Co$_{1-x}$Fe$_x$O$_{3}$ ($x$=0-0.3). Fe doping leads to an enhancement of the ferromagnetism in the systems of Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$, while the ferromagnetism is suppressed with further increasing Fe content and spin-glass behavior is observed at high doping level of Fe. In contrast, the ferromagnetism is suppressed in the system Gd$_{0.55}…
The oxygen-deficient perovskite cobaltite SrCo1-xNbxO3-d was synthesized by direct solid-state reaction and its magnetotransport properties were investigated. This cobaltite exhibits an unusual ferromagnetic behavior with a transition temperature Tm = 130-150 K and a spin glass like behavior below Tm. Importantly, this phase reaches a large magnetoresistance (MR) value, MR = -(rH - r0) / r0 = 30% at 5 K in 7 T. The large MR effect is believed to be related to the disordered magnetic state induced by the Nb-for-Co substitution.
The electronic and magnetic states in doped perovskite cobaltites, (La, Sr)CoO_3, are studied in the numerically exact diagonalization method on Co_2O_{11} clusters. For realistic parameter values, it is shown that a high spin state and an intermediate spin state coexist in one-hole doped clusters due to strong p-d mixing. The magnetic states in the doped cobaltites obtained in the calculation explain various experimental results.
Two different kinds of metal transition oxides have been studied for their large thermopower values. The first one corresponds to the Tl-based misfit cobaltite which is a hole-doped metal. We demonstrate that the partial Bi-substitution for Tl in this phase induces an increase of the room temperature (RT) thermopower (TEP) value. Same result is obtained with the new Pb_{1/3}SrCoO_{3+delta} misfit corresponding to the Tl complete replacement by lead. Simultaneously, the T dependence of their resistivity exhibits a re-entrance below 70-90K where a large negative magnetoresistance is observed. Ma…
We have investigated cobaltite relatives of the layered perovskite cuprates and nickelates, Pr$_{2-x}$Ca$_x$CoO$_4$ ($0.39 \leq x \leq 0.73$) and La$_{2-x}$Sr$_x$CoO$_4$ ($x = 0.61$), using elastic neutron scattering. We have discovered doping-dependent incommensurate short-range ordering of charges and magnetic moments, which in cobaltites occur in the range of heavy doping, $ 0.5 \lesssim x \lesssim 0.75$. The charge order exists already at room temperature and shows no change on cooling. The incommensurability of its propagation vector, ${\bf Q}_c = (ε_c,0,l)$, roughly scales with the conce…
The magnetic and magnetotransport properties of the oxygen deficient perovskites, SrCo1-xMxO3-d with M = Nb and Ru, were investigated. Both Nb- and Ru-substituted cobaltites are weak ferromagnets, with transition temperatures Tm of 130-150 K and 130-180 K, respectively, and both exhibit a spin glass behavior at temperatures below Tf = 80-90 K. It is demonstrated that there exists a strong competition between ferromagnetism and spin glass state, where Co4+ induces ferromagnetism, whereas Nb or Ru substitution at the cobalt sites induces magnetic disorder, and this particular magnetic behavior i…
Doped perovskite cobaltites (e.g., La$_{1-x}$Sr$_x$CoO$_3$) have been extensively studied for their spin-state physics, electronic inhomogeneity, and insulator-metal transitions. Ferromagnetically-interacting spin-state polarons emerge at low $x$ in the phase diagram of these compounds, eventually yielding long-range ferromagnetism. The onset of long-range ferromagnetism ($x \approx 0.18$) is substantially delayed relative to polaron percolation ($x \approx 0.05$), however, generating a troubling inconsistency. Here, Monte-Carlo simulations of a disordered classical spin model are used to esta…
The layered perovskite PrBaCo2O5.5+x demonstrates a strong negative thermal expansion (NTE) which holds potential for being fabricated into composites with zero thermal expansion. The NTE was found to be intimately associated with the spontaneous magnetic ordering, known as magnetovolume effect (MVE). Here we report with compelling evidences that the continuous-like MVE in PrBaCo2O5.5+x is intrinsically of discontinuous character, originating from an magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic metallic small-volume (FMSV) phase.…
Coexistence of magnetic and dielectric glassy states in alternating kagome and triangular lattice LuBaCo$_4$O$_7$ cobaltite
Indian Institute of Technology Bombay（印度理工学院孟买分校）；National Sun Yat-sen University（国立中山大学）；Center of Crystal Research, National Sun Yat-sen University（国立中山大学晶体研究中心）；Indian Institute of Science（印度科学研究所）；Indian Institute of Technology, Kharagpur（印度理工学院卡拉格普尔分校）；Tilk Manjhi Bhagalpur University（蒂尔克曼吉巴加尔布尔大学）
In this article we study the effects of a partial substitution of Ba with the smaller cation Ca in the layered cobaltites YBaCo_2O_{5+δ} for δ\approx 0.5. Neutron thermodiffractograms are reported for the compounds YBa_{0.95}Ca_{0.05}Co_2O_{5.5} (x_{Ca}=0.05) and YBa_{0.90}Ca_{0.10}Co_2O_{5.5} (x_{Ca}=0.10) in the temperature range 20 K \leq T \leq 300 K, as well as high resolution neutron diffraction experiments at selected temperatures for the samples x_{Ca}=0.05, x_{Ca}=0.10 and the parent compound x_{Ca}=0. We have found the magnetic properties to be strongly affected by the cationic subst…
Dimethylammonium zinc formate (DMAZnF) is the precursor for a large family of multiferroics, materials which display co-existing magnetic and dielectric ordering. However, the mechanism underlying these orderings remains unclear. While it is generally believed that the dielectric transition is related to the freezing of the order-disorder dynamics of the dimethylammonium (DMA+) cation, no quantitative data on this motion are available. We surmise that this is due to the fact that the timescale of this cationic motion is on the borderline of the timescales of experimental techniques used in ear…
We present elastic and quasielastic neutron scattering measurements characterizing peculiar short-range charge-orbital and spin order in the layered perovskite material La1.5Sr0.5CoO4. We find that below Tc~750 K holes introduced by Sr doping lose mobility and enter a statically ordered {\it charge glass} phase with loosely correlated checkerboard arrangement of empty and occupied d{3z2-r2} orbitals (Co3+ and Co2+). The dynamics of the resultant mixed spin system is governed by the anisotropic nature of the crystal-field Hamiltonian and the peculiar exchange pattern produced by the orbital ord…
We report on the unconventional magnetism in the cubic B-site ordered double perovskite Ba2YMoO6, using AC and DC magnetic susceptibility, heat capacity and muSR. No magnetic order is observed down to 2 K while the Weiss temperature is ~ -160 K. This is ascribed to the geometric frustration in the lattice of edge-sharing tetrahedra with orbitally degenerate Mo5+ s=1/2 spins. Our experimental results point to a gradual freezing of the spins into a disordered pattern of spin-singlets, quenching the orbital degeneracy while leaving the global cubic symmetry unaffected, and providing a rare exampl…
We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-δ}$. By using the relation of synthesis temperature and oxygen partial pressure to oxygen stoichiometry obtained from thermogravimetric analysis, we have synthesized a series of samples with precisely controlled $δ=0.00-0.49$. These samples show three structural phases at $δ=0.00-0.15$, $\approx0.25$, $\approx0.5$, and two-phase behavior for other oxygen contents. The stoichiometric material with $δ=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_{\rm C}=274$ K. The increas…
The quantum spin continuum and classical spin freezing, associated with a glassy state, represent two opposite extremes of a correlated electronic material. Here, we report the coexistence of a quantum spin continuum with a weak spin glass order in Co-doped CaRuO$_{3}$ perovskite near the chemical doping dependent metal-insulator transition boundary. Inelastic neutron measurements on Ca(Co$_{0.15}$Ru$_{0.85}$)O$_{3}$ at low temperature, $T$ = 1.5 K, reveal a continuum spectrum in the $Q-E$ space due to uncorrelated spin fluctuations. This persists across the glass transition at $T_G \simeq$23 …
Simultaneous study of magnetic and electrocatalytic properties of cobaltites under extreme conditions expands understanding of physical and chemical processes proceeding in them with the possibility of their further practical application. Therefore, La0.6Sr0.4CoO3 (LSCO) nanopowders have been synthesized at different annealing temperatures tann = 850, 875, 900 C, and their multifunctional properties have been studied comprehensively. As tann increases, the rhombohedral perovskite structure of the LSCO becomes more single-phase, whereas average particle size and dispersion grow. Co ions are in …
Emergent Random Spin Singlets in Disordered Spin-1/2 perovskite BaCu$_{1/3}$Ta$_{2/3}$O$_3$
Indian Institute of Science Education and Research（印度科学教育研究院）；Universitá Roma Tre（罗马第三大学）；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）；Deutsches Elektronen-Synchrotron (DESY)（德国电子同步辐射中心）；IFW Dresden（德累斯顿亥姆霍兹材料研究所）
This paper reports the observations of long-time relaxation, aging, and temperature memory behaviors of resistance and magnetization in the ferromagnetic state of a polycrystalline La0.7Ca0.3Mn0.925Ti0.075O3 compound. The observed glassy dynamics of the electrical transport appears to be magnetically originated and has a very close association with the magnetic glassiness of the sample. Phase separation and strong correlation between magnetic interactions and electronic conduction play the essential roles in producing such a resistive glassiness. We explain the observed effects in terms of a c…
Motivated by experimental observation of the non-magnetic phase in the compounds with frustration and disorder, we study the ground state of the spin-$1/2$ square-lattice Heisenberg model with randomly distributed nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ couplings. By using the density matrix renormalization group (DMRG) calculation on cylinder system with circumference up to $10$ lattice sites, we identify a disordered phase between the Néel and stripe magnetic phase with growing $J_2 / J_1$ in the presence of strong bond randomness. The vanished spin-freezing parameter indicate…
Iridates with the 5$d^4$ electronic configuration have attracted recent interest due to reports of magnetically-ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $J = 0$ electronic ground state for each Ir$^{5+}$ ion. The major focus of prior research has been on the double perovskite iridates Ba$_2$YIrO$_6$ and Sr$_2$YIrO$_6$, where the nature of the ground states (i.e. ordered vs non-magnetic) is still controversial. Here we present neutron powder diffraction, high energy resolution fluorescence detected x-ray absorption spectrosco…
Tunability of the partially ordered double perovskite (PODP) and coexisting spin-glass phase in SrMn1-xWxO3 (x=0.20 to 0.40) have been studied using neutron powder diffraction (NPD), muon spin relaxation (MuSR), and magnetic susceptibility (X) measurements. Structural studies reveal that SrMn1-xWxO3 undergoes a quasi-continuous transformation from simple perovskite (Pm-3m) to PODP (P21/n) phase as x increases. Xdc(T) and Xac(T) measurements show a sharp cusp-like peak at a spin glass transition, Tg. The muon relaxation rate (λ) peaks at Tg following a critical growth, given by λ=λ(0).t-w [t=(T…
We present Co $L_3$-edge resonant inelastic x-ray scattering (RIXS) of bulk LaCoO$_3$ across the thermally-induced spin-state crossover around 100~K. Owing to a high energy resolution of 25~meV, we observe unambiguously the dispersion of the intermediate-spin (IS) excitations in the low temperature regime. Approaching the intermediate temperature regime, the IS excitations are damped and the bandwidth is reduced. The observed behavior can be well described by a model of mobile IS excitons with strong attractive interaction, which we solve using dynamical mean-field theory for hard-core bosons.…
The layered perovskite cobaltite RBaCo$_2$O$_{5.5}$ (R: rare-earth elements or Yttrium) exhibits an abrupt temperature-induced metal$-$insulator transition (MIT) and has been attributed to spin-state ordering (SSO) of Co$^{3+}$ ions. Here we investigated the hole doping member of PrBaCo$_2$O$_{5.5+x}$ ($0 \le x \le 0.24$) with multiple techniques. The analysis on crystal and magnetic structures by electron and neutron diffraction confirm the SSO in the insulating phase of undoped PrBaCo$_2$O$_{5.5}$, which is melted by increasing the temperature across the MIT. In addition, we discovered that …
We have measured the electrical resistivity and the thermopower of the electron-doped perovskite cobaltites LaCo$_{1-y}$Te$_y$O$_3$. In contrast to the hole-doped systems such as metallic ferromagnets La$_{1-x}M_x$CoO$_3$ ($M$ = Ca, Sr, Ba), the electron-doped samples show an insulating behavior even in a heavily doped range due to a spin-state blockade mechanism that an electron hopping from high-spin Co$^{2+}$ to low-spin Co$^{3+}$ site is energetically suppressed. We find that, despite the electron doping, the thermopower shows relatively large positive values above $y=0.05$, strikingly dis…
We report on the observation of exchange bias and memory effect in double perovskite Sr$_2$FeCoO$_6$. Antiphase boundaries between the ferromagnetic and antiferromagnetic regions in the disordered glassy phase is assumed as responsible for the observed effect which reflects in the cooling field dependence and temperature evolution of exchange bias field and in training effect. The spin glass phase itself is characterized through memory, ageing and magnetic relaxation experiments. The spin glass transition temperature, $T_g$, versus $H_{dc}^{2/3}$ follows the Almeida-Thouless line yielding a fr…
A detailed study of the low-temperature magnetic state and the relaxation in the phase-separated colossal magnetoresistance Nd2/3Ca1/3MnO3 perovskite has been carried out. Clear experimental evidence of the cluster-glass magnetic behavior of this compound has been revealed. Well defined maxima in the in-phase linear ac susceptibility $χ^{/}(T)$ were observed, indicative of the magnetic glass transition at $T_{g}$ = 60 K. Strongly divergent zero-field-cooled and field-cooled static magnetizations and frequency dependent ac susceptibility are evident of the glassy-like magnetic state of the comp…
The magnetic and magnetotransport properties of the Sr3Fe2-xCoxO7-d system (0.2 <= x <= 1.0) were systematically investigated. This oxide system exhibits a giant magnetoresistance (GMR) effect at low temperatures, reaching up to 80% in 7 T at 5 K. Ac-susceptibility measurements show that there exists a strong competition between ferromagnetic (F) and spin glass states, and the balance between these two magnetic states can be controlled by varying cobalt (x) and/or oxygen contents (d). Importantly, the MR effect is closely related to the magnetic property: the development of magnetic disorderin…
$B$-site ordered 4$d^1$ and 5$d^1$ double perovskites have a number of potential novel ground states including multipolar order, quantum spin liquids and valence bond glass states. These arise from the complex interactions of spin-orbital entangled $J_{eff}$ = 3/2 pseudospins on the geometrically frustrated fcc lattice. The 4$d^1$ Mo$^{5+}$ perovskite Ba$_2$YMoO$_6$ has been suggested to have a valence bond glass ground state. Here we report on the low temperature properties of powder samples of isostructural Ba$_2$LuMoO$_6$: the only other known cubic 4$d^1$ perovskite with one magnetic catio…
The family of hole-doped Pr-based perovskite cobaltites, Pr$_{0.5}$Ca$_{0.5}$CoO$_{3}$ and (Pr$_{1-y}$RE$_{y}$)$_{0.3}$Ca$_{0.7}$CoO$_{3}$ (where RE is rare earth) has recently been found to exhibit simultaneous metal-insulator, spin-state, and valence transitions. We have investigated magnetic-field-induced phase transitions of (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ by means of magnetization measurements at 4.2$-$100 K up to an ultrahigh magnetic field of 140 T with the chemical pressure varied by $y$ = 0.0625, 0.075, 0.1. The observed magnetic-field-induced transitions were found to …
Structure with orbital degeneracy is unstable toward spontaneous distortion. Such orbital correlation usually has a much higher energy scale than spins, and therefore, magnetic transition takes place at a much lower temperature, almost independently from orbital ordering. However, when the energy scales of orbitals and spins meet, there is a possibility of spin-orbital entanglement that would stabilize novel ground state such as spin-orbital liquid and random singlet state. Here we review on such a novel spin-orbital magnetism found in the hexagonal perovskite oxide Ba$_3$CuSb$_2$O$_9$, which …
Microscopic Origin of Random Singlet Behavior in B-site Disordered Spin-1/2 Perovskite BaCu_1/3Nb_2/3O_3 Revealed by EXAFS and Thermodynamics
Indian Institute of Science Education and Research（印度科学教育研究所）；Universitá Roma Tre（罗马第三大学）；Deutsches Elektronen-Synchrotron (DESY)（德国电子同步加速器）；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）
Emergent Dynamic Magnetic Ground State in a Mixed 3d/5d Heavy Fermion System CaCu3Ir4O12
ISIS Neutron and Muon Source, STFC, Rutherford Appleton Laboratory（ISIS中子与缪子源，STFC，拉瑟福德阿普尔顿实验室）；Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences（北京凝聚态物理国家实验室，中国科学院物理研究所）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Highly Correlated Matter Research Group, Physics Department, University of Johannesburg（约翰内斯堡大学物理系强关联物质研究组）
We carried out AC magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK. Below ~1 K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the AC susceptibility revealed a dilute-spin-glass like transition below ~ 1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously bee…
