Antisite Disorder Research Articles

Physical properties of the full Heusler-type Ru2-Fe NbAl (x = 0.00–0.50) alloys

We report the physical properties of a series of full Heusler-type Ru2-xFexNbAl (x = 0.00–0.50) alloys using magnetization, electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements. Structure and phase analysis via x-ray diffraction and scanning electron microscopy indicate that Fe replaces the Ru in Ru2NbAl, and the antisite disorders reduce. The dc magnetization shows predominantly paramagnetic behavior for all Fe-doped samples, as in the pristine Ru2NbAl sample. Interestingly, the electrical resistivity measurements reveal that the conduction behavior of Ru2NbAl alloy changes from semiconducting-like to metal-like with Fe substitution. In addition, a minimum in resistivity at low temperatures is observed in the Fe-doped samples. It is noted that the minimum shifts to low temperatures with Fe content, presumably due to the weak localization effect. The Seebeck coefficient measurements show that charge carriers of these alloys remain hole type with a maximum S value of ∼27 µV/K at 300 K for x = 0.13, a result of the modification of the electronic band structure. The Fe substitution is found to reduce thermal conductivity significantly. An analysis of the lattice thermal conductivity indicates that the reduction in κ is primarily due to more grain boundaries, Umklapp phonons, and electron scatterings. These observations lead to an increase in the thermoelectric performance, i.e., the figure of merit and power factor, to about 0.007 and 1.7 μW/cm K2 at 300 K for the optimized composition of Fe with x = 0.13.

Anti-site disorder driven short-range order and canted antiferromagnetism in inverse spinel LiNiVO4

Frustration driven short-range magnetic correlations/fluctuations in quantum materials usually host exotic ground states like spin glass, low dimensional magnetism and quantum spin liquid. In this context, different members of spinel (both normal and inverse) family have gained enormous interest and an inverse spinel vanadate, LiNiVO4, seems to be an interesting candidate. Structural investigations reveal that unlike usual cation ordering, Ni and Li ions are randomly distributed at octahedral sites and results in reduced magnetic frustration. Further, an additional anti-site disorder is present, as a small fraction of Ni ions occupies tetrahedral sites. Magnetic susceptibility and heat capacity studies show that short-range magnetic correlations develop below 30 K, followed by a canted antiferromagnetism below 2.2 K. Different competing magnetic interactions due to the presence of disorder and reduced magnetic frustration result in the short-range ordering as well as canted antiferromagnetism in LiNiVO4.

Antisite disorder mediated exchange bias effect and spin-glass state in La[formula omitted]Sm[formula omitted]NiMnO[formula omitted

We report the synthesis of La2−xSmxNiMnO6 (x = 0, 0.1, 0.2) polycrystalline compounds through the sol–gel process using all-aqueous precursors. The influence of Sm-substitution on magnetic properties of La2−xSmxNiMnO6 (LSNMO) compounds are investigated through X-ray diffraction (XRD), dc-magnetization (Mdc), ac-magnetic susceptibility measurements, and Raman spectroscopy. XRD analysis confirms that the phase stability of LSNMO is improved significantly upon Sm-substitution as evident from a structural phase transformation of the compound from a biphasic (monoclinic + rhombohedral) into an aphasic (monoclinic) structure. The magnetic measurements indicate a formation of a metastable magnetic state with random ferromagnetic (FM) and antiferromagnetic (AFM) interactions below 100 K. The metastable spin configuration exhibits critical slow-down dynamics and memory effect that signifies a spin-glass (SG) state. The influence of Sm-doping reduces the Curie temperature (Tc) from 284 K → 245 K and enhances saturation magnetization (Ms). These findings are attributed to the combined effect of a reduced degree of antisite disorder upon the addition of Sm. A profound variation (45.5 → 72.9 K) in SG transition temperature (TSG) is also reported. Magnetization loops M(H) confirm the presence of an exchange bias (EB) effect that value suppressed prominently from 120 → 28 Oe upon Sm-substitution. The temperature-dependent magnetization M(T) and frequency-dependent ac-susceptibility χ[ω, T] are used to study the antisite-driven SG phase, spin relaxation dynamics, magnetic memory, and rejuvenation effect. Raman spectroscopy is employed to probe the degree of antisites disorder that reduces after Sm-addition. The phase stability, lower Tc, tunable Ms, and EB in the single-phase compound make LSNMO a potential candidate for its applications in realizing energy-efficient devices.

Disorder-driven cluster glass state in a geometrically frustrated hexagonal perovskite

We report the observation of cluster glass-like properties in a double perovskite ruthenate Ba$_2$CoRuO$_6$ through structural (neutron and synchrotron X-ray diffraction), magnetic and transport measurements. The system exhibits classic glassy characteristics like a frequency dependence in ac Susceptibility, aging and memory effects along with persistance of short-range correlations upto room temperature. The significant ($\sim30\%$) anti-site disorder on the dimer sites, coupled with the inherent geometrical frustration, allows a variety of exchange (both antiferro- and ferromagnetic) interactions to be distributed randomly across the lattice. On cooling, locally dominant interactions cause spins to nucleate and form local, short-range ordered clusters which grow in size until a global freezing occurs at about $T_f \sim 43K$.

Bulk electronic structure of Mn2NiGa using hard x-ray photoelectron spectroscopy and density functional theory

Mn2NiGa is a potential magnetic shape memory alloy with austenite to the martensite phase transition. Here, we have investigated the bulk electronic structure of Mn2NiGa in the austenite and martensite phases studying its valence band using hard x-ray photoelectron spectroscopy (HAXPES). In the austenite phase, we observe a wide (≈10 eV) valence band (VB) spectrum with several prominent features. In order to explain the HAXPES VB spectra, we have compared our experimental VB spectra with the theoretical VB calculated using the partial density of states from our existing density functional theory (DFT) calculations. The shape of the experimental VB and energy positions of all features are in excellent agreement with the calculated VB and we find that the former is dominated by Ni 3d as well as 4s states of Mn, Ni, and Ga. An important observation is that experimental VB combined with the DFT-based VB calculation establishes the prevalence of the anti-site disorder in Mn2NiGa. Compared to the austenite phase, in the martensite phase, the VB shows a marginal decrease in the density of states around −0.5 eV below the Fermi level (E F ), and the main peak is slightly shifted towards E F . These experimental observations have been explained by considering the tetragonally distorted structure with the anti-site disorder in the martensite phase.

Temperature‐induced magnetic compensation and negative magnetization in a cluster spin glass Gd2Co0.5Mn1.5O6 system

AbstractExperiments for orthorhombic double perovskite Gd2Co0.5Mn1.5O6 revealed intrinsic effects of magnetic compensation characterized by M(Tcomp) = 0 at Tcomp = 20 K and negative magnetization depicted by M(T) < 0 under positive magnetic fields, which were experimentally investigated by different protocols of direct current magnetization measurements. Compared to Gd2CoMnO6, the excessive ratio of Mn in Gd2Co0.5Mn1.5O6 promotes the antisite disorder and preferably generates magnetic clusters owing to the intrinsic inhomogeneity. The clusters exhibit spin glass (SG) properties as demonstrated by alternating current susceptibility and aging measurements. A possible physical mechanism for evolution of the spin configuration with temperature is proposed. The effects of magnetic compensation and negative magnetization are attributed to the negative exchange coupling among the abundant ferromagnetic clusters. The additional pinning force provided by the cluster SG is an essential factor to prevent the flipping of the spins from aligning with the applied magnetic field.

A combined study on structural, magnetic and specific heat on double perovskite iridates Ln2CoIrO6 [Ln = Pr, Nd

We report rich magnetic behavior for Co–Ir based double perovskites consisting of different rare earth cations Pr and Nd: Pr2CoIrO6 (PCIO) and Nd2CoIrO6 (NCIO). Both oxides show an antisite disorder of 10% and a ferrimagnetic transition, T around 96 K and 98 K respectively. The long range magnetic ordering is arising from the canted antiferromagnetic ordering between the Co2+ and Ir4+ ions. A prominent peak around 27 K in magnetization data of NCIO indicates that the total moment of Nd ion is antiferromagnetically coupled to the Co–Ir sublattice. The long range order of the Nd sublattice is corroborated by the evidence of an anomaly in specific heat at very low temperature. The compounds exhibit a maximum change of magnetic entropy of 0.57 (0.48) J kg.K−1 at T in a magnetic field of 5 T. The strong spin–orbit coupling in 5d states of Ir and cation disorder lead to the Mott insulating phase as found from the analysis of temperature dependent resistivity. These unique behaviors suggest an interesting interplay between localized Pr/Nd-4f, itinerant Co-3d and Ir-5d electrons.

Unveiling the structure-property relationship in metastable Heusler compounds by systematic disorder tuning

Heusler compounds represent a unique class of materials that exhibit a wide range of fascinating and tuneable properties such as exotic magnetic phases, superconductivity, band topology, or thermoelectricity. An exceptional, but for Heusler compounds common, feature is that they are prone to antisite defects and disorder. In this regard, the $\mathrm{Fe}{}_{2}\mathrm{VAl}$ Heusler compound has been a particularly interesting and disputed candidate. Even though various theoretical scenarios for the interplay of physical properties and disorder have been proposed, the metastable disordered A2 phase hitherto precluded experimental investigation in bulk samples. Here, we report experimental results on disorder-tuned $\mathrm{Fe}{}_{2}{\mathrm{VAl}}_{0.9}{\mathrm{Si}}_{0.1}$ alloys all the way toward the A2 phase, which we realized via rapidly quenching our samples from high temperatures. We measured the thermoelectric properties of these materials in a wide temperature range (4 to 700 K); they suggest a gradual semimetal/narrow-gap semiconductor $\ensuremath{\rightarrow}$ metal transition upon increasing the disorder. We also find a large anomalous Hall effect in the disordered A2 phase, arising from the side-jump scattering of charge carriers at the antisite magnetic moments. This is corroborated by measurements of the temperature- and field-dependent magnetization, which increases dramatically up to $\ensuremath{\approx}2.5\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}/\mathrm{f}.\mathrm{u}.$ as compared to the ordered compound ($<0.1\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}/\mathrm{f}.\mathrm{u}.$). This study provides an experimental realization of the metastable A2 structure in bulk $\mathrm{Fe}{}_{2}\mathrm{VAl}$-based alloys and grants insight into the structure-property relationship of these materials. Our work confirms that temperature-induced antisite disorder, occurring during thermal heat treatment, can be a precisely tuneable parameter in the family of Heusler compounds.

Resistivity minima in disordered Co2FeAl0.5Si0.5 Heusler alloy thin films

An exhaustive study of the effect of anti-site disorder on the ‘zero-field’, ρ(T, H = 0), and ‘in-field’, ρ(T, H = 80 kOe), electrical resistivity in 50 nm thick Co2FeAl0.5Si0.5 (CFAS) Heusler alloy thin films (deposited on the Si(100) or SiO2/Si(100) substrates at fixed substrate temperatures in the range 27°C≤TS≤550°C), has been carried out. Irrespective of the strength of disorder, resistivity goes through a minimum as a function of temperature at T = Tmin. With increasing substrate temperature, the crystalline order of the CFAS thin films improves so much so that the films deposited at 500 °C have the lowest anti-site disorder within the B2 structure and the least residual resistivity. A quantitative comparison of our results with the predictions of the existing theoretical models permits us to unambiguously identify the diffusive and ballistic transport mechanisms, responsible for ρ(T, H = 0) and ρ(T, H = 80 kOe) in different temperature ranges and accurately determine their relative magnitudes. The electron-diffuson (e – d) scattering and weak localization (WL) mechanisms, responsible for negative temperature coefficient of resistivity (TCR) for T < Tmin, compete with the positive TCR mechanisms, electron-magnon (e – m) and electron–phonon (e – p) scattering, to produce the resistivity minimum at Tmin. The e – d and WL contributions to ρ, ρe-d and ρwl, dominate over the e – m and e – p contributions, ρe-m and ρe-p, for T<Tmin whereas the reverse is true for T>Tmin. At any given temperature, ρe-d, ρwl and ρe-m decrease while ρe-p increases as the atomic order improves with increasing substrate temperature, TS. ρe-p and ρe-m originate from the phonon-induced non-spin-flip two-band (s↑↓- d↑↓) scattering and magnon - induced spin-flip s - d interband (s↑↓- d↓↑) transitions, respectively. In all the CFAS films, except for the one deposited at TS = 27°C, the thermal renormalization of the spin-wave stiffness, due to the electron - magnon interaction, contributes significantly to ρe-m(T). Furthermore, we demonstrate that the negative magnetoresistance (MR) is a consequence of a progressive suppression of the WL effect and e – m scattering by external magnetic field. However, the WL contribution to MR turns out to be negligibly small as the e – m contribution almost entirely accounts for MR over the temperature range 5K≤T≤300K.

Rational design, crystal structure, and frustrated magnetism of the Ge-containing YbFe2O4-type layered oxides In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3).

YbFe2O4-type layered oxides have attracted tremendous interest because the unique crystal comprises two distinct geometrically frustrated triangular cation-sublattices. Herein, a series of YbFe2O4-type materials In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3) were rationally designed and experimentally synthesized for the first time. The crystal structures of In2Zn3-xCoxGeO8 were investigated comprehensively by Rietveld refinements against high-resolution monochromatic Cu Kα1 XRD data. Zn2+, Co2+, and Ge4+ cations are distributed randomly on the [MO]2 bilayer and possess a trigonal bipyramid (TBP) coordination geometry. Because Co2+ has an unpaired electron in the dz2 orbital and a larger electronegativity than Zn2+, Co2+-to-Zn2+ equivalent substitution in In2Zn3-xCoxGeO8 results in more compact MO5-TBPs, which is the origin of anisotropic lattice expansion and contraction along the a and c axes, respectively. The Co2+ moments in the [MO]2 bilayer are strongly AFM coupled and geometrically frustrated, therefore resulting in a spin-glass magnetic transition at around Tg = 20 K for In2ZnCo2GeO8, while a long-range AFM ordering is established for In2Co3GeO8 with a Néel temperature of 53 K, attributed to the significantly enhanced AFM interactions and increased In3+/Co2+ anti-site disordering, as compared to those in In2ZnCo2GeO8.

Structural phase transformation of quantum spin liquid herbertsmithite via pressure induced enhancement of the cooperative Jahn-Teller effect and antisite disorder.

Herbertsmithite, Cu3Zn(OH)6Cl2, serves as one of the most promising candidates for quantum spin liquids with a perfect quantum kagome Heisenberg antiferromagnetic system. It can comprise an ideal model system for studying the compression response of the unique structure as well as exotic properties of kagome quantum spin liquid materials, which is of fundamental importance from both scientific and technological viewpoints. In this work, the structural evolution of herbertsmithite was investigated via in situ X-ray diffraction and Raman scattering techniques up to 30 GPa. The trigonal herbertsmithite structure transformed into a monoclinic clinoatacamite-like structure at 12.6 GPa. High pressure seems to act in a reverse way as Zn-doping for herbertsmithite, with the distortion degree of the system changing continuously. The occurrence of the displacive and reversible phase transition between the polymorphs is a consequence of the interplay between the external pressure and cooperative Jahn-Teller (JT) effect, aided by the presence of antisite mutual substitution of magnetic Cu2+ ions and nonmagnetic Zn2+ ions between the kagome layer and interlayer sites.

Li ion diffusion behavior of Li3OCl solid-state electrolytes with different defect structures: insights from the deep potential model.

Li3OX (X = Cl, Br), a lithium-rich anti-perovskite material developed in recent years, has received tremendous attention due to its high ionic conductivity of >10-3 S cm-1 at room temperature. However, the origin of the high ionic conductivity of the material at the atomic level is still not clear. In this work, we investigated the dynamic behavior of the Li3OCl system with three different defect structures (Li-Frenkel, LiCl-Schottky, and Cl-O anti-site disorder) at seven temperature intervals and calculated its ionic conductivity using the deep potential (DP) model. The results show that the presence of LiCl-Schottky defects is the main reason for the high performance of the Li3OCl system, and the Li vacancy is the main carrier. The ionic conductivity obtained from the DP model is 0.49 × 10-3 S cm-1 at room temperature and it can reach 10-2 S cm-1 above the melting point, which is in the same order of magnitude as the experimentally reported results. We also explored the effect of different defect concentrations on the ionic conductivity and migration activation energy. This work also demonstrates the feasibility of the DP method for solving the accuracy-efficiency dilemma of ab initio molecular dynamics (AIMD) and classical molecular dynamics simulations.

CaFeFeNbO6 - an iron-based double double perovskite.

Ordering of cations is important for controlling properties of ABO3 perovskites, and CaFeFeNbO6 is the first example of an Fe-based AA'BB'O6 double double perovskite, with Ca2+/Fe2+ ordered on A-site columns, and Fe3+/Nb5+ at the octahedral B-sites. Substantial (37%) antisite disorder of the latter cations leads to spin glassy magnetism below a freezing transition at 12 K. The CaMnFeNbO6 analogue also shows substantial cation disorder and spin glassy behaviour. Comparison of synthesis pressures for ordered materials based on different A-site transition metals, suggests that pressures of at least 14-18 GPa will be required to discover the expected plethora of double double perovskites based on A' cations smaller than Mn2+.

Antisite disorder and Berry curvature driven anomalous Hall effect in the spin gapless semiconducting Mn2CoAl Heusler compound

Spin gapless semiconductors exhibit a finite band gap for one spin channel and a closed gap for another spin channel, and they have emerged as a new state of magnetic materials with a great potential for spintronic applications. The first experimental evidence for spin gapless semiconducting behavior was observed in an inverse Heusler compound ${\mathrm{Mn}}_{2}\mathrm{CoAl}$. Here, we report a detailed investigation of the crystal structure and anomalous Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ using experimental and theoretical studies. The analysis of the high-resolution synchrotron x-ray diffraction data shows antisite disorder between Mn and Al atoms within the inverse Heusler structure. The temperature-dependent resistivity shows semiconducting behavior and follows Mooij's criteria for disordered metal. The scaling behavior of the anomalous Hall resistivity suggests that the anomalous Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ is primarily governed by an intrinsic mechanism due to the Berry curvature in momentum space. The experimental intrinsic anomalous Hall conductivity (AHC) is found to be $\ensuremath{\sim}35$ S/cm, which is considerably larger than the theoretically predicted value for ordered ${\mathrm{Mn}}_{2}\mathrm{CoAl}$. Our first-principles calculations conclude that the antisite disorder between Mn and Al atoms enhances the Berry curvature and hence the value of intrinsic AHC, which is in very good agreement with the experiment.

Interplay of spin, phonon, and lattice degrees in a hole-doped double perovskite: Observation of spin–phonon coupling and magnetostriction effect

Unlike a typical spin–phonon coupling, an exhibition of unconventional spin–phonon coupling, which is mediated via magnetostriction effect, is reported in a hole-doped double perovskite Pr1.5Sr0.5CoMnO6. Various investigations including electronic and crystal structures, spin structure, transport property, lattice dynamics, and theoretical density of states analysis by density-functional theory (DFT) have been performed. A substantial increase in the mean oxidation states of Co ions and a concurrent abrupt decrease in the resistivity upon Sr doping is observed, thus altering its underlying transport mechanism. An insulating and ferromagnetic (FM) ground state is predicted by DFT calculations. The neutron diffraction data analysis reveals a complex crystal structure of Pr1.5Sr0.5CoMnO6, which consists of B-site disordered monoclinic (P21/n) and orthorhombic (Pnma) structures, highlighting the presence of an anti-site disorder in the system. The analysis also suggests an overall FM ordering of Co/Mn spins below 150 K for the monoclinic phase, whereas no such magnetic ordering is found for the orthorhombic phase. More interestingly, the neutron powder diffraction study perceives the presence of a strong magnetostriction effect in the system. Raman spectroscopy unravels the presence of a spin–phonon coupling, which is essentially mediated by the magnetostriction effect.

Impact of magnetic and antisite disorder on the vibrational densities of states in Ni2MnSn Heusler alloys

We have performed a combined experimental and theoretical investigation of the vibrational properties of ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Sn}$ Heusler alloys. Sn-partial vibrational density of states (VDOS) of $^{119}\mathrm{Sn}$ was measured by nuclear resonant inelastic x-ray scattering at temperatures of 15 and 300 K, while magnetism and local environment of Sn was resolved by $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy. Using density functional theory, we associate the peaks in the VDOS with particular features in the element-resolved phonon dispersion of $\mathrm{L}{2}_{1}$ ordered ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Sn}$. The good agreement between theory and experiment in the low-energy region provides the evidence that the inversion of optical modes at $\mathrm{\ensuremath{\Gamma}}$ involving the displacement of Ni and the heavier main group element atoms, which was predicted previously for other Ni-Mn-based Heusler compounds, is also a characteristic property of ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Sn}$. Introducing different types of magnetic and antisite disorder in our calculations results in a distinctive redistribution and broadening of the Sn-VDOS, suggesting that considering partial disorder further improves the agreement with the experiment in particular at the highest phonon energies.

Dielectric and Spin-Glass Magnetic Properties of the A-Site Columnar-Ordered Quadruple Perovskite Sm2CuMn(MnTi3)O12

Perovskite-type ABO3 oxides show a number of cation-ordered structures, which have significant effects on their properties. The rock-salt-type order is dominant for B cations, and the layered order for A cations. In this work, we prepared a new perovskite-type oxide, Sm2CuMn(MnTi3)O12, with a rare columnar A-site order using a high-pressure, high-temperature method at about 6 GPa and about 1700 K. Its crystal structure was studied with synchrotron powder X-ray diffraction. The compound crystallizes in space group P42/nmc (No. 137) at room temperature with a = 7.53477 Å and c = 7.69788 Å. The magnetic properties of the compound were studied with dc and ac magnetic susceptibility measurements and specific heat. Spin-glass (SG) magnetic properties were found with TSG = 7 K, while specific heat, in the form of Cp/T, showed a strong, very broad anomaly developing below 20 K and peaking at 4 K. The dielectric constant of Sm2CuMn(MnTi3)O12 was nearly frequency and temperature independent between 8 K and 200 K, with a value of about 50. Cu2+ doping drastically modified the magnetic and dielectric properties of Sm2CuMn(MnTi3)O12 in comparison with the parent compound Sm2MnMn(MnTi3)O12, which showed a long-range ferrimagnetic order at 34-40 K. The antisite disorder of Cu2+ and Mn2+ cations between square-planar and octahedral sites was responsible for the SG magnetic properties of Sm2CuMn(MnTi3)O12.

Universal scaling of specific heat in the S=12 quantum kagome antiferromagnet herbertsmithite

Despite tremendous investigations, a quantum spin liquid (QSL) state realized in a spin-1/2 kagome Heisenberg antiferromagnet remains largely elusive. In herbertsmithite $\mathrm{Zn}{\mathrm{Cu}}_{3}{(\mathrm{O}\mathrm{H})}_{6}{\mathrm{Cl}}_{2}$, a quantum spin liquid candidate on the perfect kagome lattice, precisely characterizing the intrinsic physics of the kagome layers is extremely challenging due to the presence of interlayer Cu/Zn antisite disorder within its crystal structure. Here we measured the specific heat and thermal conductivity of single-crystal herbertsmithite in magnetic fields with high resolution. Strikingly, intrinsic magnetic specific heat contribution arising from the kagome layers exhibits excellent scaling collapse as a function of $T/H$ (temperature/magnetic field). In addition, no residual linear term in the thermal conductivity $\ensuremath{\kappa}/T(T\ensuremath{\rightarrow}0)$ is observed in zero and applied magnetic fields, indicating the absence of itinerant gapless excitations. These results capture a new essential feature of the QSL state of the kagome layers; localized orphan spins are induced by exchange bond randomness, surrounded by a nonitinerant QSL.

Role of A site doped rare earth (Gd3+ and Ho3+) ions in the lattice structure and magnetic properties of biphasic La2−xRExNiMnO6 (RE = Gd, Ho, x = 0.1) double perovskites

The doping effects of a small amount of RE (Gd and Ho) atoms at A (La) sites of La2NiMnO6 double perovskite have been studied by preparing the materials using mechanical alloying of oxide powders and thermal treatment (800–1200 °C), and a direct sintering of the oxide powders at 1300 °C. The synchrotron x-ray diffraction patterns have been used to confirm the formation of single (monoclinic) phase or bi-phased lattice structure depending on thermal treatment temperature. The bi-phased samples are consisting of primary monoclinic phase (P 21/n space group) and secondary orthorhombic phase (Pnma space group). The x-ray photoelectron spectroscopy has confirmed the presence of trivalent La, RE ions, and mixed-valence state of Ni and Mn ions in the samples. The existence of bi-phasic lattice structure and mixed-valence state of Ni and Mn ions have determined magnetic properties. The monoclinic phase has shown ferromagnetic Curie point around 280 K, which has been reduced to low temperature by Gd and Ho doping. The orthorhombic phase has shown weak ferro/ antiferromagnetic spin ordering temperature around 150 K. The experimental results in RE doped La2NiMnO6 double perovskite support a strong correlation between lattice structure and magnetic properties, which depend on doping of RE ions at A sites, electronic charge state of Ni and Mn ions, structural phase components and anti-site disorder of Mn/Ni ions at octahedral sites.

Gapless triangular-lattice spin-liquid candidate PrZnAl11O19

A quantum spin liquid (QSL) is an exotic state in which electron spins are highly entangled, yet keep fluctuating even at zero temperature. Experimental realization of model QSLs has been challenging due to imperfections, such as antisite disorder, strain, and extra or a lack of interactions in real materials compared to the model Hamiltonian. Here we report the magnetic susceptibility, thermodynamic, inelastic neutron scattering (INS), and muon-spin relaxation studies on a polycrystalline sample of PrZnAl$_{11}$O$_{19}$, where the Pr$^{3+}$ ions form an ideal two-dimensional triangular lattice. Our results demonstrate that this system does not order nor freeze, but keep fluctuating down to 50 mK despite large antiferromagnetic couplings ($\sim$ -10 K). Furthermore, the INS and specific-heat data suggest that PrZnAl$_{11}$O$_{19}$ is best described as a gapless QSL.