Intersite Disorder Research Articles

First principles calculations of intersite disorder influence on the electronic structure and X-ray magnetic circular dichroism in CoFeMnSi Heusler alloy

Electronic structure, X-ray absorption, and magnetic circular dichroism (XMCD) spectra in the CoFeMnSi Heusler alloy were studied from first principles. Fully relativistic Dirac linear muffin-tin orbital band structure method was implemented with various exchange–correlation functionals tested. The supercell approach was used to study the influence of intersite disorder, at the levels of 6.25%, 12.5%, and 25% within transition metal sites, on the XMCD spectra at [Formula: see text] edges and spin polarization (SP) at the Fermi level. It is found that most sensitive to Fe–Mn and Co–Fe disorder are XMCD spectra at [Formula: see text] edges of Fe, while the sensitivity decreases from Mn to Co. It is shown that magnetic moments estimated with the use of magneto-optical (MO) sum rules agree with the ab initio calculated ones to within [Formula: see text], [Formula: see text], and [Formula: see text], for Co, Fe, and Mn, respectively. The calculated SP decreases from 99% for ordered CoFeMnSi alloy, to 96% upon 25% Co–Fe disorder, to 83% for Fe–Mn disorder, and to 42% in the case of Co–Mn disorder. The calculated spectra agree well with the available experimental data. The rich XMCD spectral structures are predicted from first principles at Fe, Co, Mn and Si [Formula: see text] edges.

Q=0 long-range magnetic order in centennialite CaCu3(OD)6Cl2·0.6D2O : A spin- 12 perfect kagome antiferromagnet with J1−J2−Jd

Crystal and magnetic structures of the mineral centennialite CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O crystallizes in the $P\bar{3}m1$ space group and Cu$^{2+}$ ions form a geometrically perfect kagome network with antiferromagnetic $J_1$. No intersite disorder between Cu$^{2+}$ and Ca$^{2+}$ ions is detected. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O enters a magnetic long-range ordered state below $T_\text{N}=7.2$~K, and the $\mathbf{q}=\mathbf{0}$ magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3~K is suppressed to $0.58(2)\mu_\text{B}$. Our DFT calculations indicate the presence of antiferromagnetic $J_2$ and ferromagnetic $J_d$ superexchange couplings of a strength which places the system at the crossroads of three magnetic orders (at the classical level) and a spin-$\frac{1}{2}$ PFFRG analysis shows a dominance of $\mathbf{q}=\mathbf{0}$ type magnetic correlations, consistent with and indicating proximity to the observed $\mathbf{q}=\mathbf{0}$ spin structure. The results suggest that this material is located close to a quantum critical point and is a good realization of a $J_1$-$J_2$-$J_d$ kagome antiferromagnet.

Quantum magnetisms in uniform triangular lattices Li2AMo3O8 (A\u2009=\u2009In, Sc)

Molecular based spin-1/2 triangular lattice systems such as LiZn2Mo3O8 have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li2AMo3O8 (A = In or Sc) is such a compound where spin-1/2 Mo3O13 clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the A site. Li2InMo3O8 undergoes conventional 120° long-range magnetic order below TN = 12 K whereas isomorphic Li2ScMo3O8 exhibits no long-range magnetic order down to 0.5 K. Here, we report exotic magnetisms in Li2InMo3O8 and Li2ScMo3O8 investigated by muon spin rotation (μSR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li2InMo3O8 and Li2ScMo3O8 show completely different behaviors observed in both μSR and INS measurements, representing their different ground states. Li2InMo3O8 exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the 120° spin structure. In contrast, Li2ScMo3O8 undergoes short-range magnetic order below 4 K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions.

μSRinsight into the impurity problem in quantum kagome antiferromagnets

Impurities, which are inherently present in any real material, may play an important role in the magnetism of frustrated spin systems with spin-liquid ground states. We address the impurity issue in quantum kagome antiferromagnets by investigating ZnCu$_3$(OH)$_6$SO$_4$ (Zn-brochantite) by means of muon spin spectroscopy. We show that muons couple to the impurity magnetism, originating from Cu-Zn intersite disorder, and that the impurities are highly correlated with the kagome spins, allowing us to probe the intrinsic kagome physics via a Kondo-like effect. The low-temperature plateau in local susceptibility identifies the spin-liquid ground state as being gapless. The corresponding spin fluctuations exhibit an unconventional spectral density and an intriguing field dependence.

Effects of inter-site chemical disorder on the magnetic properties of MnBi

Stoichiometric and ordered hexagonal phase of MnBi are difficult to obtain due to the peritectic reaction of the materials constituents. Thus, inter-site mixing of Mn and Bi is expected. Given that the material is a promising rare-earth free permanent magnet, we study the effects of inter-site chemical disorder on its magnetic properties, using ab initio density functional based electronic structure methods. Our calculations, focusing on inter-site substitutional disorder, show that excess of Mn and Bi ions in MnBi have detrimental effects on its magnetic properties. We find that inter-site chemical disorder not only decreases the total magnetization, but also reduces the materials׳ ferromagnetic transition temperature. The latter is deduced from the calculated magnetic pair-exchange parameters by means of linear response technique. Our calculations suggest that the inter-site disorder of Mn and Bi ions in MnBi must be minimized for its optimum performance in permanent magnet applications.

Energetics and electronic structure of La/Sr disorder at the interface of SrTiO3/LaTiO3 heterostructure

In the artificially tailored heterostructures of certain materials, a polar discontinuity across the interface introduces a large energy cost. The total energy of such systems may be reduced either by electronic reconstruction leading to the interface phases or by simple atomic reconstruction by inter-site cation mixing. While most of the experiments and theoretical calculations assume an abrupt interface, in this work, we consider the La/Sr inter-site disorder across the interface of SrTiO3/LaTiO3 heterostructures and study its energetics and electronic structure properties. The calculations find that inter-site mixing of La/Sr atoms across the interface also reduces the total energy. However, the extent of such disorder is found to be dramatically minimized by allowing the interfacial atoms to relax fully and that for such systems, the changes in the electronic structure are negligible.

The A-site ordered manganese perovskite and its colossal magnetoresistance

Abstract The A-site ordered manganites show the charge/orbital order (CO) transition far above room temperature. By using this advantage, we have engaged in the realization of colossal magnetoresistance (CMR) at room temperature by introducing some sophisticated disorders to SmBaMn 2 O 6 . The inter-site disorder of Sm/Ba results in no significant magnetoresistance (MR) effect. The La-substitution for both Sm- and Ba-sites is effective to realize MR effect. At room temperature and 9 T, we have achieved the record of MR effect about 340% in the sintered Sm 0.90 La 0.24 Ba 0.86 Mn 2 O 6 . In a single crystal of the compound, the perfect CO insulator to ferromagnetic metal (FM) transition is observed at 9 T, showing the room temperature CMR over 1000%. TEM study reveals that the formation of microdomains with CO and FM or magnetic glassy states is crucial for CMR.