Nature of the magnetism of iridium in the double perovskite Sr2CoIrO6
S. Agrestini, K. Chen, C. -Y. Kuo, L. Zhao, H. -J. Lin, C. -T. Chen, A. Rogalev, P. Ohresser, T. -S. Chan, S. -C. Weng, A. C. Komarek, K. Yamaura, M. W. Haverko… 展开作者
S. Agrestini, K. Chen, C. -Y. Kuo, L. Zhao, H. -J. Lin, C. -T. Chen, A. Rogalev, P. Ohresser, T. -S. Chan, S. -C. Weng, A. C. Komarek, K. Yamaura, M. W. Haverkort, Z. Hu, L. H. Tjeng
We report on our investigation on the magnetism of the iridate double perovskite Sr$_2$CoIrO$_6$, a nominally Ir$^{5+}$ Van Vleck $J_{eff}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-$L_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the $J_{eff}=0$ ground state. By carrying out also XAS and XMCD experiments at the Co-$L_{2,3}$ edges and by performing detailed full atomic multiplet calculations to simulate all spectra, we discovered that the compound consists of about 90% Ir$^{5+}$ ($J_{eff}=0$) and Co$^{3+}$ ($S=2$) and 10% Ir$^{6+}$ ($S=3/2$) and Co$^{2+}$ ($S=3/2$). The magnetic signal of this minority Ir$^{6+}$ component is almost equally strong as that of the main Ir$^{5+}$ component. We infer that there is a competition between the Ir$^{5+}$-Co$^{3+}$ and the Ir$^{6+}$-Co$^{2+}$ configurations in this stoichiometric compound.
Cubic double perovskites that host heavy ions with total angular momentum J = 2 can exhibit a singular magnetic state epitomized by a lone octupole and bulk ferro-type magnetism. It exists in the Chen - Balents Hamiltonian with a quadrupole interaction and competing exchange forces between the ions. Our symmetry inspired analysis mirrors the Dzyaloshinskii - Man'ko theory of latent antiferromagnetic ordering, and a 3-k collinear structure. Experimental tests of the singular state include neutron and x-ray Bragg diffraction.
Magnetism-induced negative thermal expansion (NTE) observed in inverse perovskite antiferromagnets Mn3AN (A=Zn, Ga, etc.) is theoretically studied by a classical spin model with competing bond-length-dependent exchange interactions. We numerically reproduce the crystal-volume expansion upon cooling triggered by a non-coplanar antiferromagnetic order and show that the expansion occurs so as to maximize an energy gain of the nearest-neighbor antiferromagnetic interactions. This mechanism is not specific to inverse perovskite magnets and might also be expected in magnets with other crystal struct…
We use the LDA+U method to study the possibility of exciton condensation in perovskites of transition metals with $d^6$ electronic configuration such as LaCoO$_3$. For realistic interaction parameters we find several distinct solutions exhibiting spin-triplet exciton condensate, which gives rise to a local spin density distribution while the ordered moments are vanishingly small. Rhombohedral distortion from the ideal cubic structure suppresses the ordered state, contrary to the spin-orbit coupling which enhances the excitonic condensation energy. We explain the trends observed in the numerica…
The magnetic structure of Ca$_2$MnReO$_6$ double perovskite is investigated by neutron powder diffraction and bulk magnetization, showing dominant non-collinear Mn magnetic moments [$4.35(7)$ $μ_B$] that are orthogonally aligned with the small Re moments [$0.22(4)$ $μ_B$]. $Ab$-initio electronic structure calculations show that the strong spin-orbit coupling for Re $5d$ electrons combined with a relatively modest on-site Coulomb repulsion ($U_{eff}^{Re} \gtrsim 0.6$ eV) is sufficient to render this material insulating. This is a rare example of spin-orbit assisted Mott insulator outside the re…
The quest for Kitaev spin liquids in particular three dimensional solids is long sought goal in condensed matter physics, as these states may give rise to exotic new types of quasi-particle excitations carrying fractional quantum numbers namely Majorana Fermionic excitations. Here we report the experimental signature of this characteristic feature of the Kitaev spin liquid via Raman measurements. Sm2ZnIrO6 is a strongly spin orbit coupled Mott insulator, where Jeff = 1/2 controls the physics, which provide striking evidence for this characteristic feature of the Kitaev spin liquid. As the temp…
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…
We report electron spin resonance (ESR) spectroscopy results on the double perovskite Ba$_2$YIrO$_6$. On general grounds, this material is expected to be nonmagnetic due to the strong coupling of the spin and orbital momenta of Ir$^{5+}$ (5$d^4$) ions. However, controversial experimental reports on either strong antiferromagnetism with static order at low temperatures or just a weakly paramagnetic behavior have triggered a discussion on the breakdown of the generally accepted scenario of the strongly spin-orbit coupled ground states in the 5$d^4$ iridates and the emergence of a novel exotic ma…
The coexistence of the strongly correlated 3$d$ transition metal (TM) atom and the strong spin-orbit coupling (SOC) of 5$d$ TM atom is potential to host exotic physical phenomena. In the present work, we have studied the magnetism resulting from such a coexistence in La$_2$CoIrO$_6$ (LCIO), a representative of bulk 3$d$-5$d$ double perovskites. In order to gain further insight into the effect of Co-$d$ states on the Ir-$d$ states, comparisons are carried out with the isostructural La$_2$ZnIrO$_6$, where nonmagnetic Zn atom replaces the Co atom. An in-depth analysis of the magnetic states in th…
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 Fe electronic structure and magnetism in (i) monoclinic Ca$_2$FeReO$_6$ with a metal-insulator transition at $T_{MI} \sim 140$ K and (ii) quasi-cubic half-metallic Ba$_2$FeReO$_6$ ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe $L_{2,3}$ XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to Fe$^{3+}$ for Ca$_2$FeReO$_6$ and intermediate between Fe$^{2+}$ and Fe$^{3+}$ for Ba$_2$FeReO$_6$) despite being related by an isoelectron…
The 3$d$-5$d$ based double perovskites are of current interest as they provide model system to study the interplay between electronic correlation ($U$) and spin-orbit coupling (SOC). Here we report detailed structural, magnetic and transport properties of doped double perovskite material (Sr$_{1-x}$Y$_x$)$_2$FeIrO$_6$ with $x$ $\leq$ 0.2. With substitution of Y, system retains its original crystal structure but structural parameters modify with $x$ in nonmonotonic fashion. The magnetization data for Sr$_2$FeIrO$_6$ show antiferromagnetic type magnetic transition around 45 K, however, a close i…
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 …
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 …
Robust Half-Metallic Character and Large Oxygen Magnetism in a Perovskite Cuprate
新型钙钛矿铜酸盐材料Sr $_{8}$ CaRe $_{3}$ Cu $_{4}$ O $_{24}$具有铁磁性质，居里温度高（$T_c \sim$ 440 K），通过密度泛函理论计算发现，在空穴掺杂或化学取代后，表现出几个令人惊讶的性质：（1）通过用W或Mo取代Re实现了半金属（HM），而$T_c$保持较高;（2）空穴掺杂的Sr $_{8}$ CaRe $_{3}$ Cu $_{4}$ O $_{24}$也是具有高$T_c$的HM。此外，我们发现，O原子将携带一个大的磁矩空穴掺杂后，这是在强烈的对比，在固体磁性的普遍接受的概念，需要部分填充的$d$或$f$电子在阳离子的壳。因此，材料Sr $_8$ CaRe $_3$ Cu $_4$ O $_{24}$有望为材料设计和开发提供非常有用的平台。
Investigation of elementary excitations has advanced our understanding of many-body physics governing most physical properties of matter. Recently spin-orbit excitons have drawn much attention, whose condensates near phase transitions exhibit Higgs mode oscillations, a long-sought physical phenomenon [Nat. Phys. {\bf 13}, 633 (2017)]. These critical transition points resulting from competing spin-orbit coupling (SOC), local crystalline symmetry and exchange interactions, are not obvious in Iridium based materials, where SOC prevails in general. Here, we present results of resonant inelastic x-…
The structural and magnetic properties of the face-centered cubic double perovskite Ba2MnWO6 were investigated using neutron powder diffraction, DC-magnetometry, muon spin relaxation and inelastic neutron scattering. Ba2MnWO6 undergoes Type II long-range antiferromagnetic ordering at a Neel temperature of 8(1) K with a frustration index, f = 8. Inelastic neutron scattering was used to identify the magnetic coupling constants J1 and J2, which were found to equal -0.080 meV and -0.076 meV respectively. This indicated that both of the magnetic coupling constants were antiferromagnetic with simila…
AMX$_3$ compounds are structurally diverse, a notable example being the post-perovskite structure which adopts a two-dimensional framework with corner- and edge-sharing octahedra. Few molecular post-perovskites are known and of these, none have reported magnetic structures. Here we report the synthesis, structure and magnetic properties of molecular post-perovskites: CsNi(NCS)$_3$, a thiocyanate framework, and two new isostructural analogues CsCo(NCS)$_3$ and CsMn(NCS)$_3$. Magnetisation measurements show that all three compounds undergo magnetic order. CsNi(NCS)$_3$ (Curie temperature, $T_\ma…
We present a comprehensive experimental study of magnetization and magnetocaloric effect (MCE) in double perovskite (DP) materials $R_2$NiMnO$_6$ with $R =$ Pr, Nd, Sm, Gd, Tb, and Dy. While a paramagnetic to ferromagnetic transition, with T$_{\rm C}$ in the range $\sim 100 - 200~$K, is a common feature that can be attributed to the ordering of Mn$^{4+}$ and Ni$^{2+}$ magnetic moments, qualitatively distinct behavior depending on the choice of $R$ is observed at low temperatures. These low-temperature anomalies in magnetization are also manifest in the change in magnetic entropy, $-ΔS_{M}$, wh…
Understanding the controlling principles of band gaps trends in d electron perovskites is needed both for gauging metal-insulator transitions, as well as their application in catalysis and doping. The magnitude of this band gap is rather different for different magnetic spin configurations. We find via electronic structure theory that the factors that connect gapping magnitudes to magnetism depend on the nature of the band edge orbital character (BEOC) and surprisingly scale with the number of antiferromagnetic contacts z$_i$ between neighboring transition metal ions. The dependence is weak wh…
Strongly frustrated 2D magnetism in a 3D hexagonal perovskite
East China Normal University（华东师范大学）；University of Science and Technology of China（中国科学技术大学）；Chinese Academy of Sciences（中国科学院）；Huazhong University of Science and Technology（华中科技大学）；Shanghai Jiao Tong University（上海交通大学）
Spin-orbit-entangled frustrated magnetism in fcc Ba$_2$(Yb,Nd)NbO$_6$ double perovskites
Shiv Nadar Institution of Eminence（Shiv Nadar卓越学院）；Bennett University（贝内特大学）；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）；Indian Institute of Science Education and Research, Thiruvananthapuram（印度科学教育与研究研究所，特里凡得琅）；Japan Atomic Energy Agency（日本原子能机构）
Multiferroics with coupling between ferroelectricity and magnetism have been pursued for decades. However, their magnetoelectric performances remain limited due to the common trade-off between ferroelectricity and magnetism. Here, a family of nitride perovskites is proposed as multiferroics with prominent physical properties and nontrivial mechanisms. Taking GdWN$_3$ as a prototype, our first-principles calculations found that its perovskite phases own large polarizations (e.g. $111.3$ $μ$C/cm$^2$ for the $R3c$ phase) and a magnetic moment $7$ $μ_{\rm B}$/Gd$^{3+}$. More interestingly, its fer…
Lattice dynamics and structural correlations in antiferromagnet SrLaCoNbO$_6$ double perovskite
Ames National Laboratory, U.S. Department of Energy, Iowa State University（艾姆斯国家实验室）；Department of Physics, Indian Institute of Technology Delhi（德里印度理工学院物理系）；School of Physics, The University of New South Wales (UNSW)（新南威尔士大学物理学院）
Nontrivial magnetic orders in the inverse-perovskite manganese nitrides are theoretically studied by constructing a classical spin model describing the magnetic anisotropy and frustrated exchange interactions inherent in specific crystal and electronic structures of these materials. With a replica-exchange Monte-Carlo technique, a theoretical analysis of this model reproduces the experimentally observed triangular Γ^{5g} and Γ^{4g} spin ordered patterns and the systematic evolution of magnetic orders. Our work solves a 40-year-old problem of nontrivial magnetism for the inverse-perovskite mang…
Resonant inelastic X-ray scattering (RIXS) experiments performed at the oxygen-$K$ edge on the iridate perovskites {\SIOS} and {\SION} reveal a sequence of well-defined dispersive modes over the energy range up to $\sim 0.8$ eV. The momentum dependence of these modes and their variation with the experimental geometry allows us to assign each of them to specific collective magnetic and/or electronic excitation processes, including single and bi-magnons, and spin-orbit and electron-hole excitons. We thus demonstrated that dispersive magnetic and electronic excitations are observable at the O-$K$…
We report a neutron powder diffraction study of $R$Mn$_7$O$_{12}$ quadruple perovskite manganites with $R$ = La, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the $A$ and $B$ manganese sublattices occurs on cooling below the N$\mathrm{\acute{e}}$el temperature. The respective magnetic structures are collinear, with one uncompensated Mn$^{3+}$ moment per formula unit as observed in bulk magnetisation measurements. We show that both LaMn$_7$O$_{12}$ and NdMn$_7$O$_{12}$ undergo a second magnetic phase transition at low temperature, which introduces a…
