Observation of spin glass state in weakly ferromagnetic Sr$_2$FeCoO$_6$ double perovskite
R. Pradheesh, Harikrishnan S. Nair, C. M. N. Kumar, Jagat Lamsal, R. Nirmala, P. N. Santhosh, W. B. Yelon, S. K. Malik, V. Sankaranarayanan, K. Sethupathi
We report the observation of spin glass state in the double perovskite oxide Sr$_{2}$FeCoO$_{6}$ prepared through sol-gel technique. Initial structural studies using x rays reveal that the compound crystallizes in tetragonal $I 4/m$ structure with lattice parameters, $a$ = 5.4609(2) Åand $c$ = 7.7113(7) Å. The temperature dependent powder x ray studies reveal no structural phase transition in the temperature range 10 -- 300 K. However, the unit cell volume shows an anomaly coinciding with the magnetic transition temperature thereby suggesting a close connection between lattice and magnetism. Neutron diffraction studies and subsequent bond valence sums analysis show that in Sr$_{2}$FeCoO$_{6}$, the $B$ site is randomly occupied by Fe and Co in the mixed valence states of Fe$^{3+}$/Fe$^{4+}$ and Co$^{3+}$/Co$^{4+}$. The random occupancy and mixed valence sets the stage for inhomogeneous magnetic exchange interactions and in turn, for the spin glass like state in this double perovskite which is observed as an irreversibility in temperature dependent dc magnetization at $T_f\sim$ 75 K. Thermal hysteresis observed in the magnetization profile of Sr$_{2}$FeCoO$_{6}$ is indicative of the mixed magnetic phases present. The dynamic magnetic susceptibility displays characteristic frequency dependence and confirms the spin glass nature of this material. Dynamical scaling analysis of $χ'(T)$ yields a critical temperature $T_{ct}$ = 75.14(8) K and an exponent $zν$ = 6.2(2) typical for spin glasses. The signature of presence of mixed magnetic interactions is obtained from the thermal hysteresis in magnetization of Sr$_{2}$FeCoO$_{6}$. Combining the neutron and magnetization results of Sr$_2$FeCoO$_6$, we deduce the spin states of Fe to be in low spin while that of Co to be in low spin and intermediate spin.
The study of hyperfine interaction by high-resolution inelastic neutron scattering is not very well known compared to the other competing techniques viz. NMR, Mössbauer, PACS etc. Also the study is limited mostly to magnetically ordered systems. Here we report such study on Sr$_{2-x}$La$_x$FeCoO$_6$ (x = 0, 1, 2) of which first (Sr$_2$FeCoO$_6$ with x = 0) has a canonical spin spin glass, the second (SrLaFeCoO$_6$ with x = 1) has a so-called magnetic glass and the third (La$_2$FeCoO$_6$ with x = 2) has a magnetically ordered ground state. Our present study revealed clear inelastic signal for S…
Neutron diffraction measurement on the spin glass double perovskite Sr$_2$FeCoO$_6$ reveals site disorder as well as Co$^{3+}$ intermediate spin state. In addition, multiple valence states of Fe and Co are confirmed through Mössbauer and X-ray photoelectron spectroscopy. The structural disorder and multiple valence lead to competing ferromagnetic and antiferromagnetic interactions and subsequently to a spin glass state, which is reflected in the form of an additional $T$-linear contribution at low temperatures in specific heat. A clear evidence of Jahn-Teller distortion at the Co$^{3+}$-O$_6$ …
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…
The magnetism of the double perovskite compounds \SLFCOx\ ($x$ = 0, 1, 2) are contrasted using magnetization, neutron diffraction and electron paramagnetic resonance with the support from density functional theory calculations. \LFCO\ is identified as a long-range ordered antiferromagnet displaying a near-room temperature transition at $T_N$ = 270~K, accompanied by a low temperature structural phase transition at $T_S$ = 200~K. The structural phase transformation at $T_S$ occurs from $R\overline{3}c$ at 300~K to $Pnma$ at 200~K. The density functional theory calculations support an insulating …
In this paper we address many of the fundamental open questions regarding the glassy behavior of the magnetic/electronic phase segregated state in rare earth perovskites. In particular, magnetic relaxation experiments support that the collective effects (memory, ageing, etc.) are due to interparticle interactions, rather than the double-exchange vs. superexchange competition. A careful study of the non-linear susceptibility in the critical region is performed, and the critical exponents contrasted with those of conventional spin-glasses and concentrated quenched ferrofluids. The phase segregat…
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…
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 …
We report structural, magnetic and dielectric properties of layered perovskite materials LnBaCuFeO5 (Ln = La and Lu). LaBaCuFeO5 shows magnetic cluster glass behavior below 60 K owing to the competing ferromagnetic and antiferromagnetic exchange interactions. Glassy dynamics of electric dipoles has also been observed in the vicinity of the magnetic glass transition temperature. The presence of significant coupling between spin and polar degrees of freedom results in the multiglass feature in LaBaCuFeO5. The LuBaCuFeO5 compound undergoes YBaCuFeO5 like commensurate to incommensurate antiferroma…
We have investigated the electronic and magnetic properties of perovskite SrRu1-xIrxO3 thin films grown by pulsed laser deposition on atomically-flat (001) SrTiO3 substrates. SrRuO3 has the properties of a ferromagnetic metal with Curie temperature 150 K. Substituting Ir for Ru in SrRuO3, films showed fully-metallic behavior and ferromagnetic ordering, although resistivity increased and the ferromagnetic TC decreased. Films with x = 0.25 underwent the metal-to-insulator transition at 75 K, and spin-glass-like ordering at 45 K with the elimination of ferromagnetic long-range ordering caused by …
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…
A new 3d-5d hybridized quadruple perovskite oxide, CaCu3Mn2Ir2O12, was synthesized by high-pressure and high-temperature methods. The Rietveld structure analysis reveals that the compound crystallizes in an AA'3B4O12-type perovskite structure with space group Im-3, where the Ca and Cu are 1:3 ordered at fixed atomic positions. At the B site the 3d Mn and the 5d Ir ions are disorderly distributed due to a rare equal +4 charge states for both of them as determined by X-ray absorption spectroscopy. The competing antiferromagnetic and ferromagnetic interactions among Cu2+, Mn4+, and Ir4+ ions give…
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…
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…
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…
SrTi0.5Mn0.5O3 (STMO) is a chemically disordered perovskite having random distribution of Ti and Mn over 1b site. Striking discrepancies about the structural and magnetic properties of STMO demands detailed analysis which is addressed. To explore the magnetic ground state of STMO, static and dynamic magnetic properties were studied over a broad temperature range (2-300 K). The dc, ac magnetization show a cusp like peak at Tf ~ 14 K, which exhibits field and frequency dependence. The thermoremanent magnetization is characterized by using stretched exponential function and characteristic time su…
We study the magnetic properties of CaFeTi$_2$O$_6$ (CFTO) by high-field magnetization and specific heat measurements. While the magnetic susceptibility data yield a vanishingly small Curie-Weiss temperature, the magnetic moments are not fully polarized in magnetic field up to 60 T, which reveals a large spin exchange energy scale. Yet, the system shows no long range magnetic order but a spin-glass-like state below 5.5 K in zero field, indicating strong magnetic frustration in this system. Applying magnetic field gradually suppresses the spin-glass-like state and gives rise to a potential quan…
We have performed magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation experiments to investigate the magnetic ground states of the 5$d^2$ double perovskites Sr$_2$YReO$_6$ and Sr$_2$InReO$_6$. We find that Sr$_2$YReO$_6$ is a spin glass, while Sr$_2$InReO$_6$ hosts a non-magnetic singlet state. By making detailed comparisons with other 5$d^2$ double perovskites, we argue that a delicate interplay between spin-orbit coupling, non-cubic crystal fields, and exchange interactions plays a key role in the great variation of magnetic ground states observed for…
$B$-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…
We combine the results of magnetic and transport measurements with Mossbauer spectroscopy and room-temperature diffraction data to construct the magnetic phase diagram of the new family of cubic perovskite manganites SrMn_1-xFe_xO_3. We have found antiferromagnetic ordering for lightly and heavily Fe-substituted material, while intermediate substitution leads to spin-glass behavior. Near the SrMn_0.5Fe_0.5O_3 composition these two types of ordering are found to coexist and affect one another. The spin glass behavior may be caused by competing ferro- and antiferromagnetic interactions among Mn^…
High purity polycrystalline Na0.5La0.5RuO3 was synthesized by a solid state method, and its properties were studied by magnetic susceptibility, heat capacity and resistivity measurements. We find it to be a tetragonal perovskite, in contrast to an earlier report, with random La/Na mixing. With a Curie-Weiss temperature of -231 K and effective moment of 2.74 uB/mol-Ru, there is no magnetic ordering down to 1.8 K. A broad hump at 1.4 K in the heat capacity, however, indicates the presence of a glassy magnetic transition, which we attribute to the influence of the random distribution of Na and La…
Ruthenates provide comprehensive platform to study a plethora of novel properties, such as quantum magnetism, superconductivity and magnetic fluctuation mediated metal-insulator transition. In this article, we provide an overview of quantum mechanical phenomenology in calcium ruthenium oxide with varying compositions. While the stochiometric composition of CaRuO$_{3}$ exhibits non-Fermi liquid behavior with quasi-criticality, chemically doped compounds depict prominent signatures of quantum magnetic fluctuations at low temperature that in some cases are argued to mediate in metal-insulator tra…
We report a muon spin rotation ($μ^{+}$SR) study of the magnetic properties of the double perovskite compound LaSrNiReO$_{6}$. Using the unique length and time scales of the $μ^{+}$SR technique, we successfully clarify the magnetic ground state of LaSrNiReO$_{6}$, which was previously deemed as a spin glass state. Instead, our $μ^{+}$SR results point towards a long-range dynamically ordered ground state below $T_{\rm C}= 23$ K, for which a static limit is foreseen at $T=0$. Furthermore, between 23 K$<T\leq$300 K, three different magnetic phases are identified: a dense ($23$ K$<T\leq75$ K), a d…
We report the comprehensive experimental results identifying the magnetic spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6 by employing the (dc and ac) magnetization, powder neutron diffraction (NPD) and X-ray magnetic circular dichroism (XMCD) techniques. X-ray diffraction and neutron diffraction studies revealed that Pr2CoFeO6 adopts a B-site disordered orthorhombic structure with space group Pnma. Additionally, ab initio band structure calculations performed on this system suggested an insulating anti-ferromagnetic (Fe-Fe) ground state. Magnetometry study showed…
Antisite-disorder driven tuning of magnetic properties and exchange-bias in Nd$_{2-x}$Sr$_{x}$CoMnO$_{6-δ}$ $(0 \leq x \leq 1)$ ($δ\sim 0.5$) double perovskites
Indian Institute of Technology Kharagpur（印度理工学院卡拉格普尔分校）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器）；TU Delft Reactor Institute（代尔夫特理工大学反应堆研究所）
We have investigated the peculiar structure of charge and spin ordering in the half-doped layered perovskite oxide La$_{1.5}$Sr$_{0.5}$CoO$_4$ by elastic neutron scattering. Two samples with different preparation histories were studied. We find that the generic features of the ordered states, such as their short-range, glassy nature and the spin incommensurability, are sample-independent. At the same time, some subtle features of the ordered phases, such as the correlation lengths, differ.
We report results of magnetization and ac susceptibility measurements down to very low fields on a single crystal of the perovskite manganite, La$_{0.82}$Ca$_{0.18}$MnO$_3$. This composition falls in the intriguing ferromagnetic insulator region of the manganite phase diagram. In contrast to earlier beliefs, our investigations reveal that the system is magnetically (and in every other sense) single-phase with a ferromagnetic ordering temperature of $\sim$ 170 K. However, this ferromagnetic state is magnetically frustrated, and the system exhibits pronounced glassy dynamics below 90 K. Based on…
The magnetic ground state in the double perovskite system Sr2-xCaxCoOsO6 changes from an antiferromagnet (x = 0), to a spin glass (x = 1), to a ferrimagnet (x = 2) as the Ca content increases. This crossover is driven by chemical pressure effects that control the relative strength of magnetic exchange interactions. The synthesis, crystal structure, and magnetism of SrCaCoOsO6 and Ca2CoOsO6 are investigated and compared with Sr2CoOsO6. Both compounds adopt a monoclinic crystal structure with rock salt ordering of Co2+ and Os6+ and a-a-b+ octahedral tilting, but the average Co-O-Os bond angle ev…
Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase segregation with temperature and magnetic field was studied in the model manganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic neutron scattering. Intra-granular strain-driven mesoscopic segregation between two insulating phases, one of which is charge ordered (CO), sets in below the CO temperature in zero field. The CO phase orders antiferromagnetically, while the other insulating phase shows spin-glass behavior. After field-induced metallization, the CO phase coexists with a ferromagnetic metallic ph…
The synthesis and characterization of the previously unknown material LaCaScIrO$_6$ is reported. LaCaScIrO$_6$ presents a new example of the rare case of a double perovskite with the strongly spin-orbit coupled 5\textit{d}-ion Ir$^{4+}$ as its only magnetic species, forming a monoclinically distorted version of the frustrated fcc lattice. Magnetization measurements show a weak anomaly at 8~K. The Curie-Weiss temperature Θ$_{CW}$ and effective magnetic moment μ$_{eff}$ of LaCaScIrO$_6$ are in close proximity to the related compound La$_2$MgIrO$_6$ but differ from La$_2$ZnIrO$_6$. This suggests …
Magnetic frustration and disorder are key ingredients to prevent the onset of magnetic order. In the disordered hexagonal double perovskite BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$, Mn$^{4+}$ cations, with $S=3/2$ spins, can either form highly correlated states of magnetic trimers or dimers or remain as weakly interacting orphan spins. At low temperature ($T$), the dimer response is negligible, and magnetism is dominated by the trimers and orphans. To explore the role of magnetic frustration, disorder and possibly of quantum fluctuations, the low-$T$ magnetic properties of the remaining magnetic degrees …
The single-layer perovskite cobaltates have attracted enormous attention due to the recent observation of hour-glass shaped magnetic excitation spectra which resemble the ones of the famous high-temperature superconducting cuprates. Here, we present an overview of our most recent studies of the spin and charge correlations in floating-zone grown cobaltate single crystals. We find that frustration and a novel kind of electronic and magnetic nano phase separation are intimately connected to the appearance of the hour-glass shaped spin excitation spectra. We also point out the difference between …
We present a systematic optical study for a bandwidth-controlled series of nearly half doped colossal magnetoresistive manganites RE$_{0.55}$AE$_{0.45}$MnO$_3$ (RE and AE being rare earth and alkaline earth ions, respectively) under the presence of quenched disorder over a broad temperature region $T=10-800$ K. The ground state of the compounds ranges from the charge and orbital ordered insulator through the spin glass to the ferromagnetic metal. The enhanced phase fluctuations, namely the short-range charge and orbital correlations dominate the paramagnetic region of the phase diagram above a…
The double-perovskite A$_2$BB'O$_6$ with heavy transition metal ions on the ordered B' sites is an important family of compounds to study the interplay between electron correlation and spin-orbit coupling (SOC). Here we prepared high-quality Sr$_2$MgReO$_6$ powder and single-crystal samples and performed non-resonant and resonant synchrotron x-ray diffraction experiments to investigate its magnetic ground state. By combining the magnetic susceptibility and heat capacity measurements, we conclude that Sr$_2$MgReO$_6$ exhibits a layered antiferromagnetic (AF) order at temperatures below $\sim$ 5…
Electronic structure, electrical transport, dc and ac magnetization properties of the hole substituted (Sr2+) partially B-site disordered double perovskite Pr2-xSrxCoMnO6 system have been investigated. Electronic structure was probed by employing X-ray photoemission spectroscopy (XPS) measurements. The study suggested the presence of mixed valence states of the B-site ions (Co2+/Co3+ and Mn3+/Mn4+) with significant enhancement of the average oxidation states due to hole doping. The mere absence of electronic states near the Fermi level in the valence band (VB) spectra for both of the pure (x=0…
The dynamic magnetic properties of the distorted perovskite system La{0.7-x}Y{x}Ca{0.3}MnO{3} (0 <= x <= 0.15) have been investigated by ac-susceptibility and dc magnetization measurements, with emphasis on relaxation and aging studies. They evidence for x >= 0.10 the appearance of a metallic cluster glass phase, that develops just below the ferromagnetic transition temperature. The clusters grow with decreasing temperature down to a temperature T(f0) at which they freeze due to severe intercluster frustration. The formation of these clusters is explained by the presence of yttrium induced loc…
The structural and magnetic properties of double perovskiteTb2CoMnO6 have been investigated. Electronic structure analysis by XPS study reveals the presence of mixed oxidation state (Mn4+/Mn3+ and Co2+/Co3+) of B-site ions. The dc and ac magnetization measurements reveal different interesting phases such as Griffith phase, re-entrant spin glass, metamagnetic steps, Hopkinson like peak and also unusual slow relaxation. The M-H curve indicates the presence of competing AFM/FM interactions. The disorder in Tb2CoMnO6 leads to spin frustration at low temperature giving rise to the re-entrant spin g…
Double perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. Herein, we report the electronic structure and magnetic properties of an ordered double perovskite material Ho2CoMnO6. In the double perovskite with general class A2BB'O6, the octahedral B and B'-site has a distinct crystallographic site. The Rietveld refinement of XRD data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. The X-ray photoelectron spectroscopy confirms the charge state of cations present i…
We combine the results of magnetic and transport measurements with neutron diffraction data to construct the structural and magnetic phase diagram of the entire family of SrMn$_{1-x}$Ru$_{x}$O$_3$ ($0 \leqslant x \leqslant 1$) perovskites. We have found antiferromagnetic ordering of the C type for lightly Ru-substituted materials ($0.06 \leqslant x \leqslant 0.5$) in a similar manner to $R_{y}$Sr$_{1-y}$MnO$_3$ ($R$=La, Pr), due to the generation of Mn$^{3+}$ in both families of manganite perovskites by either $B$-site substitution of Ru$^{5+}$ for Mn$^{4+}$ or $A$-site substitution of $R^{3+}…
The crystallographic and magnetic properties of La{0.5+delta}A{0.5-delta}Mn{0.5+epsilon}Ru{0.5-epsilon}O{3} (A= Ca, Sr, Ba) were investigated by means of neutron powder diffraction. All studied samples show the orthorhombic perovskite crystal structure, space group Pnma, with regular (Mn,Ru)O{6} octahedra and no chemical ordering of the Mn3+ and Ru4+ ions. Ferromagnetic spin structures were observed below Tc ~ 200-250 K, with an average ordered moment of ~ 1.8-2.0 Bohr magnetons per (Mn,Ru). The observation of long-range ferromagnetism and the absence of orbital ordering are rationalized in te…
In the family of double perovskites, colossal magnetoresistance (CMR) has been so far observed only in half-metallic ferrimagnets such as the known case Sr$_2$FeMoO$_6$ where it has been assigned to the tunneling MR at grain boundaries due to the half-metallic nature. Here we report a new material-Tl$_2$NiMnO$_6$, a relatively ordered double perovskite stablized by the high pressure and high temperature synthesis-showing CMR in the vicinity of its Curie temperature. We explain the origin of such effect with neutron diffraction experiment and electronic structure calculations that reveal the ma…
We present a detailed magnetic study of the perovskite manganite Pr0.7Ca0.3MnO3 at low temperatures including magnetization and a.c. susceptibility measurements. The data appear to exclude a conventional spin glass phase at low fields, suggesting instead the presence of correlated ferromagnetic clusters embedded in a charge-ordered matrix. We examine the growth of the ferromagnetic clusters with increasing magnetic field as they expand to occupy almost the entire sample at H ~ 0.5 T. Since this is well below the field required to induce a metallic state, our results point to the existence of a…
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…
The double-perovskite compounds Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are magnetically described as quasi-two-dimensional spin-1/2 square-lattice $J_1{-}J_2$ Heisenberg antiferromagnets with predominant $J_1$ and $J_2$ exchange interactions, respectively. We report the low-temperature magnetic properties of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ with randomness in the magnitudes of $J_1$ and $J_2$. It was found that the low-temperature specific heat for $0.1\leq x \leq 0.5$ has a large component proportional to the temperature $T$ above 1.2 K, although the low-temperature specific heat for the two parent s…
We studied structural and magnetic properties of a series of insulating double perovskite compounds, La$_{2-x}$Sr$_{x}$CuRuO$_{6}$ (0 $\le$ x $\le$ 1), representing doping via \textit{A}-site substitution. The end members La$_{2}$CuRuO$_{6}$ and LaSrCuRuO$_{6}$ form in monoclinic structure while the intermediate Sr doped compounds stabilise in triclinic structure. The Cu and Ru ions sit on alternate B-sites of the perovskite lattice with $\sim$15% anti-site defects in the undoped sample while the Sr doped samples show a tendency to higher ordering at B-sites. The undoped (x = 0) compound shows…
We report the detailed analysis of specific heat [C$_{\rm P}$(T)] and ac-susceptibility for magnetically frustrated Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--1) double perovskites to understand low temperature complex magnetic interactions and their evolution with $x$. Interestingly, the observed Schottky anomaly in the $x\leqslant$ 0.4 samples shifts gradually towards higher temperature with magnetic field as well as $x$, and the analysis reveal the persistence of the discrete energy states in these samples resulting from the spin-orbit coupling and octahedral distortion. Moreover, the extracted val…
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…
The layered perovskite Sr$_2$RuO$_4$ is a most intensively studied superconductor, but its pairing mechanism, which is often coupled intimately with magnetic fluctuations in correlated materials, is still an open question. Here we present a systematic evolution of the Seebeck coefficient in Co- and Mn-substituted Sr$_2$RuO$_4$ single crystals, in which ferromagnetic and antiferromagnetic glassy states respectively emerge in proximity to the superconducting phase of the parent compound. We find that the Seebeck coefficient $S$ divided by temperature $T$, $S/T$, shows a maximum near characterist…
We report detailed magnetization measurements on the perovskite oxide NdNiO$_3$. This system has a first order metal-insulator (M-I) transition at about 200 K which is associated with charge ordering. There is also a concurrent paramagnetic to antiferromagnetic spin ordering transition in the system. We show that the antiferromagnetic state of the nickel sublattice is spin canted. We also show that the concurrency of the charge ordering and spin ordering transitions is seen only while warming up the system from low temperature. The transitions are not concurrent while cooling the system throug…
CaRuO_3 is perovskite with an orthorhombic distortion and is believed to be close to magnetic ordering. Magnetic studies of single crystal and polycrystalline CaRu_{1-x}Cu_xO_3 (0\le x \le 15 at.%Cu) reveal that spin-glass-like transition develops for x\le 7 at.%Cu and obtained value for effective magnetic moment p_{eff}=3.55 mu_B for x=5 at.% Cu, single crystal, indicates presence of Ru^{5+}. At higher Cu concentrations more complex magnetic behaviors are observed. Electrical resistivity measured on polycrystalline samples shows metal-to-insulator transition (MIT) at 51 K for only 2 at.% Cu. …
We report the effect of Ga substitution on magnetization, magnetoresistance, and magnetostriction in polycrystalline Pr0.6Sr0.4Co1-yGayO3 (y=0.0-0.3) samples. Upon substitution of the non-magnetic Ga3+ cation for magnetic Co3+, the low-temperature ground state transforms from ferromagnetic metallic for y = 0 to cluster glass semiconductor for y = 0.2. Magnetoresistance at 7T is negative and its magnitude increases from 2 percentage for y = 0 to 30 percentage for y = 0.3 at 10 K. On the other hand, magnetostriction at 10 K is positive and its value decreases with increasing y. Interestingly, th…
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）
Using detailed synchrotron diffraction, magnetization, thermodynamic and transport measurements, we investigate the relationship between the mixed valence of Ir, lattice strain and the resultant structural and magnetic ground states in the geometrically frustrated triple perovskite iridate Ba$_{3}$NaIr$_{2}$O$_{9}$. We observe a complex interplay between lattice strain and structural phase co-existence, which is in sharp contrast to what is typically observed in this family of compounds. The low temperature magnetic ground state is characterized by the absence of long range order, and points t…
Doping at the rare-earth site by divalent alkaline-earth ions in perovskite lattice has witnessed a variety of magnetic and electronic orders with spatially correlated charge, spin and orbital degrees of freedom. Here, we report an antisite disorder driven spontaneous exchange bias effect as a result of hole carrier (Sr2+) doping in La2-xSrxCoMnO6 (0 < x < 1) double perovskites. X-ray diffraction and Raman spectroscopy have evidenced an increase in disorder with the increase of Sr content up to x = 0.5 and thereby decreases from x = 0.5 to 1. X-ray absorption spectroscopy has revealed that onl…
In this work, a temperature-induced valence-state transition is studied in a narrow composition range of Ba$_{2-x}$Sr$_x$TbIrO$_6$. The valence-state transition involves an electron transfer between Tb and Ir leading to the valence-state change between Tb$^{3+}$/Ir$^{5+}$ and Tb$^{4+}$/Ir$^{4+}$ phases. This first-order transition has a dramatic effect on the lattice, transport properties, and the long-range magnetic order at low temperatures for both Tb and Ir ions. Ir$^{5+}$ ion has an electronic configuration of 5$d^4$ ($J\rm_{eff}$ = 0) which is expected to be nonmagnetic. In contrast, Ir$…
