Anisotropic Superexchange Interaction Research Articles

Reentrant canonical spin-glass dynamics and tunable field-induced transitions in (GeMn)Co2O4 Kagomé lattice

We report on the reentrant canonical semi spin-glass characteristics and controllable field-induced transitions in distorted Kagomé symmetry of (GeMn)Co2O4. This B-site spinel exhibits complicated, yet interesting magnetic behaviour in which the longitudinal ferrimagnetic (FiM) order sets in below the Néel temperature T FN ∼ 77 K due to uneven moments of divalent Co (↑ 5.33 μ B) and tetravalent Mn (↓ 3.87 μ B) which coexists with transverse spin-glass state below 72.85 K. Such complicated magnetic behaviour is suggested to result from the competing anisotropic superexchange interactions (J AB/k B ∼ 4.3 K, J AA/k B ∼ −6.2 K and J BB/k B ∼ −3.3 K) between the cations, which is extracted following the Néel’s expression for the two-sublattice model of FiM. Dynamical susceptibility (χ ac (f, T)) and relaxation of thermoremanent magnetization, M TRM (t) data have been analysed by means of the empirical scaling-laws such as Vogel–Fulcher law and Power law of critical slowing down. Both of which reveal the reentrant spin-glass like character which evolves through a number of intermediate metastable states. The magnitude of Mydosh parameter (Ω ∼ 0.002), critical exponent zυ = (6.7 ± 0.07), spin relaxation time τ 0 = (2.33 ± 0.1) × 10−18 s, activation energy E a/k B = (69.8 ± 0.95) K and interparticle interaction strength (T 0 = 71.6 K) provide the experimental evidences for canonical spin-glass state below the spin freezing temperature T F = 72.85 K. The field dependence of T F obtained from χ ac (T) follows the irreversibility in terms of de Almeida–Thouless mean-field instability in which the magnitude of crossover scaling exponent Φ turns out to be ∼2.9 for the (Ge0.8Mn0.2)Co2O4. Isothermal magnetization plots reveal two field-induced transitions across 9.52 kOe (H SF1) and 45.6 kOe (H SF2) associated with the FiM domains and spin-flip transition, respectively. Analysis of the inverse paramagnetic susceptibility after subtracting the temperature independent diamagnetic term (=−3 × 10−3 emu mol−1 Oe−1) results in the effective magnetic moment = 7.654 μ B/f.u. This agrees well with the theoretically obtained = 7.58 μ B/f.u. resulting the cation distribution in support of the Hund’s ground state spin configuration and of Mn4+ and Co2+, respectively. The H–T phase diagram has been established by analysing all the parameters (T F(H), T FN(H), H SF1(T) and H SF2(T)) extracted from various magnetization measurements. This diagram enables clear differentiation among the different phases of the (GeMn)Co2O4 and also illustrates the demarcation between short-range and long-range ordered regions.

Structure of the normal state and origin of the Schottky anomaly in the correlated heavy-fermion superconductor UTe2

The recently discovered $\mathrm{U}{\mathrm{Te}}_{2}$ superconductor is regarded as a heavy-fermion mixed-valence system with very peculiar properties within the normal and superconducting (SC) states. It shows no signs of magnetic order, but has a strong anisotropy of magnetic susceptibility and SC critical field. In addition to a heavy-fermion-like behavior in the normal state, it exhibits a distinctive Schottky-type anomaly at $\ensuremath{\sim}12$ K and a characteristic excitations gap $\ensuremath{\sim}35\ensuremath{-}40$ K. Here, we show, by virtue of dynamical mean-field theory calculations with a quasiatomic treatment of electron correlations, that ab initio-derived crystal-field splitting of the $5{f}^{2}$ ionic configuration yields agreement with these experimental observations. A close analogy of the normal paramagnetic state of $\mathrm{U}{\mathrm{Te}}_{2}$ to that of $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ in the Kondo arrest scenario is revealed. We consider the impact of anisotropic superexchange interactions within the U-U structural dimer on magnetization and development of strong orbital and magnetic moment fluctuations in $\mathrm{U}{\mathrm{Te}}_{2}$.

Anisotropic superexchange through nonmagnetic anions with spin-orbit coupling

Anisotropic superexchange interaction is one of the most important interactions in realizing exotic quantum magnetism, which is traditionally regarded to originate from magnetic ions and has no relation with the nonmagnetic ions. In our work, by studying a multi-orbital Hubbard model with spin-orbit coupling on both magnetic cations and nonmagnetic anions, we analytically demonstrate that the spin-orbit coupling on nonmagnetic anions alone can induce antisymmetric Dzyaloshinskii-Moriya interaction, symmetric anisotropic exchange and single ion anisotropy on the magnetic ions and thus it actually contributes to anisotropic superexchange on an equal footing as that of magnetic ions. Our results promise one more route to realize versatile exotic phases in condensed matter systems, long-range orders in low dimensional materials and switchable single molecule magnetic devices for recording and manipulating quantum information through nonmagnetic anions.

Control of the two-electron exchange interaction in a nanowire double quantum dot

The two-electron exchange coupling in a nanowire double quantum dot (DQD) is shown to possess Moriya's anisotropic superexchange interaction under the influence of both the Rashba and Dresselhaus spin-orbit couplings (SOCs) and a Zeeman field. We reveal the controllability of the anisotropic exchange interaction via tuning the SOC and the direction of the external magnetic field. The exchange interaction can be transformed into an isotropic Heisenberg interaction, but the uniform magnetic field becomes an effective inhomogeneous field whose measurable inhomogeneity reflects the SOC strength. Moreover, the presence of the effective inhomogeneous field gives rise to an energy-level anticrossing in the low-energy spectrum of the DQD. By fitting the analytical expression for the energy gap to the experimental spectroscopic detections [S. Nadj-Perge et al., Phys. Rev. Lett. 108, 166801 (2012)], we obtain the complete features of the SOC in an InSb nanowire DQD.

Structural, magnetic and electrical properties of double-doped manganites Y0.5+ySr0.5−yMn1−yCryO3 (0≤y≤0.5)

We report an investigation on the evolution of structural, magnetic and transport properties in double-doped manganites Y0.5+ySr0.5−yMn1−yCryO3 (0≤y≤0.5), remaining eg electron density (x=0.5) unchanged. From a pseudo-cubic perovskite structure for Y0.5Sr0.5MnO3 with y=0, powder X-ray diffraction patterns demonstrate that Cr3+ substitution on the Mn-site leads to a tetragonal distortion, which increases with the doping level y. The temperature dependence of magnetic susceptibility shows that the magnetic state at low temperatures evolves from an antiferromagnetic-spin-glass state for y=0, undergoing a ferromagnetic-spin-glass state, finally to a ferrimagnetic state for y=0.5. The electrical transport properties of all samples show an insulator behavior dominated by the small polaron hopping mechanism and the activation energy increases significantly along with y. Consequently, the enhancement of ferromagnetic component in the Y0.5+ySr0.5−yMn1−yCryO3 series may be attributed to the anisotropic ferromagnetic superexchange interactions of Mn3+ ions collaborating with the ordered arrangement of Mn3+/Cr3+ ions, rather than the double exchange interaction via eg-electron hopping process.

Hydrothermal synthesis, magnetism and resistivity of orthorhombic perovskite manganates Y1−xCaxMnO3 (x = 0, 0.07, 0.55, 0.65)

Orthorhombic Y1−xCaxMnO3 (x=0, 0.07, 0.55, 0.65) crystals were successfully prepared in one step via the mild hydrothermal synthesis, and the magnetic and electric properties were investigated in detail. The formation and composition of Y1−xCaxMnO3 crystals mainly depend on the molar ratios of starting materials, reaction temperature and alkalinity of the reaction system. The magnetic properties of Y1−xCaxMnO3 strongly depend on the doping content of Ca. Y1−xCaxMnO3 (x=0, 0.07 and 0.55) demonstrate the spin-glass-like behaviors. Y1−xCaxMnO3 (x=0.55 and 0.65) show charge ordering and antiferromagnetic transition, and the antiferromagnetic state is stabilized by charge ordering. The signal of charge ordering transition of Y1−xCaxMnO3 (x=0.65) is very strong and its magnetization increases significantly below 46K due to the Dzyaloshinsky–Moriya (DM) anisotropic super-exchange interaction. The charge transport of samples that have charge ordering takes on the behavior of semiconductors. They show a small magnetoresistance effect. Their electric conduction mechanism accords with the variable-range hopping mode of highly correlated electrons.

Effect of orbital symmetry on the anisotropic superexchange interaction

Employing the microscopic superexchange model incorporating the effect of spin–orbit interaction, we have investigated the Dzyaloshinsky–Moriya (DM) interaction in perovskite transition-metal (TM) oxides and explored the interplay between the DM interaction and the TM-3d orbital symmetry. For d3 and d5 systems with isotropic orbital symmetry, the DM vectors are well described by a simple symmetry analysis considering only the bond geometry. In contrast, the DM interaction for d4 systems with anisotropic orbital symmetry shows slightly different behavior, which does not obey simple symmetry analysis. The direction as well as the strength of the DM vector varies depending on the occupied orbital shape. We have understood this behavior based on the orbital symmetry induced by local crystal field variation.

초교환 상호작용과 비등방성 초교환(Dzyaloshinskii-Moriya) 상호작용

초교환 상호작용은 MnO, <TEX>$MnF_2$</TEX>와 같은 전이금속 화합물에서 나타나는 반강자성을 설명하기 위해서 도입된 상호작용이다. 이 초교환 상호작용에 스핀-궤도 결합 효과를 포함시키면 비등방성 초교환(Dzyaloshinskii-Moriya: DM) 상호작용이 된다. 이 비등방성 초교환 DM 상호작용으로 <TEX>${\alpha}-Fe_2O_3$</TEX>, <TEX>$MnCO_3$</TEX>, <TEX>$CrF_3$</TEX> 등에서 나타나는 약강자성을 설명할 수 있다. The superexchange interaction is introduced to explain antiferromagnetic ordering in transition metal compounds such as MnO and <TEX>$MnF_2$</TEX>. The anisotropic superexchange (Dzyaloshinskii-Moriya: DM) interaction can be derived by incorporating the spin-orbit interaction into the superexchange interaction. The anisotropic superexchange DM can account for the weak ferromagnetism observed in transition metal compounds such as <TEX>${\alpha}-Fe_2O_3$</TEX>, <TEX>$MnCO_3$</TEX>, <TEX>$CrF_3$</TEX>.

Spin Interactions in Cd&lt;sub&gt;1-x&lt;/sub&gt;Mn&lt;sub&gt;x&lt;/sub&gt;S Bulk Crystals

Electron paramagnetic resonance (EPR) linewidth in the Cd1-xMnxS bulk crystals with manganese concentrations x = 0.25, 0.33, 0.35 and 0.42 has been studied in the high-temperature region. An analysis of the infinite-temperature EPR linewidth performed within the exchange narrowing model shows that the contributions of the anisotropic superexchange and magnetic dipolar interaction to the linewidth are nearly the same due to weak anion spin-orbit coupling. The single-ion anisotropy originating from the crystal field of axial symmetry with the parameter Dsingle= 0.0295 cm-1, as observed in some CdS:Mn crystals, gives a contribution to the linewidth comparable with the influence of the anisotropic superexchange and magnetic dipolar interaction. The calculated infinite-temperature linewidths, including all anisotropic spin interactions examined in this work, are close to the experimental infinite-temperature linewidths estimated from the existing high-temperature data.

A-type antiferromagnetic order, 2D ferromagnetic fluctuation and orbital order in stoichiometric La 1− xEu xMnO 3

Magnetic, structural and Mn K-edge X-ray absorption near-edge structure (XANES) spectroscopic studies have been made on the stoichiometric La 1− x Eu x MnO 3 (0⩽ x⩽1) system with trivalent Mn 3+. The system undergoes an A-type antiferromagnetic (AF) order with Néel temperature T N ranging from 139 K for LaMnO 3 to 42 K for smaller EuMnO 3, above which a two-dimensional ferromagnetic (2D-FM) short range fluctuation prevails with T 2D> T N at low applied field. This is the result of anisotropic superexchange interaction for the A-type AF order with strong basal plane FM coupling of the orbital-ordered e g electrons and weak c-axis AF coupling of t 2g electrons. The anomalous decrease of T N with increasing smaller Eu substitution is due to more localized t 2g electrons from a stronger crystal field splitting and lattice distortion in the MnO 6 octahedron. Both T 2D and T N are field dependent and nearly merge at higher applied fields.

μ +-SR studies of the weak ferromagnets CoCO 3 and NiCO 3

We present the results of zero-field μ +-SR measurements on CoCO 3 and NiCO 3. Both compounds are rhombohedral antiferromagnets which exhibit a spontaneous weak magnetisation below 22.2 and 17.2 K , respectively. This arises due to an anisotropic superexchange (Dzyaloshinsky–Moriya) interaction. We have studied the temperature dependence of the internal field in the ordered state and the magnetic fluctuations in the paramagnetic state. We describe the possible muon-site obtained from dipolar-field calculations.

Room Temperature Ferromagnetic Behavior in Pressed Pellets of Doped Poly (3-methylthiophene)

Room temperature ferromagnetic behavior has been observed in pressed pellets of doped poly(3-methylthiophene). In this work we show that thermoremance data taken in two different ways favours the interpretation of data in terms of the Dzialoshinski-Moriya anisotropic superexchange interaction of the polarons via dopant anions giving rise to weak ferromagnetism.

Spectral and dynamical effects of octahedral impurities on in garnets

The effects of impurity ions that enter octahedral a sites on the spectra of ions in dodecahedral c sites of garnets are investigated. Two types of such impurities have been studied: nonstoichiometric defects and transition ions . Based on the new data on in YAG it is shown that a nonstoichiometric defect such as determines four different perturbations of the nearby ions (P centres). Due to the low symmetry at the c site, the four directions of perturbation from the nearby centres are nonequivalent with respect to the local symmetry axes and determine particular compositions of perturbed crystal fields. If the ion is a non-Kramers one such as or , the lowering in symmetry determined by octahedral impurities is clearly observed by additional lines (mainly for one of the P centres) forbidden in . A splitting of for one of the P centres in is assigned to the rise of Kramers degeneracy due to a short-range magnetic order determined by an anisotropic superexchange interaction. Other arguments in favour of anisotropic a-c superexchange interaction are the high energy transfer rates measured for one centre and one centre in YAG.

Q-Deformed Anisotropic Superexchange Interaction, Frustration and GLpq(2)

We study a suitable q-deformed version of the Moriya's superexchange interaction theory by means of its underlying quantum group structure. We show that the one-dimensional chain case is associated with the non-standard quantum group GL pq(2), evidencing the integrability structure of the system. This biparametric deformation of GL(2; C) arise as a twisting of GL q(2) and it match exactly the local rotation appearing in the Shekhtman's work.1 This allow us to express the frustration condition in terms of this twisting, also showing that effect of the Moriya's vector amounts to a twisting of the boundary condition.

Magnetic anisotropies and general on-site Coulomb interactions in the cuprates.

This paper derives the anisotropic superexchange interactions from a Hubbard model for excitations within the copper 3d band and the oxygen 2p band of the undoped insulating cuprates. We extend the recent calculation of Yildirim et al. [Phys. Rev. B {\bf VV}, pp, 1995] in order to include the most general on--site Coulomb interactions (including those which involve more than two orbitals) when two holes occupy the same site. Our general results apply when the oxygen ions surrounding the copper ions form an octahedron which has tetragonal symmetry (but may be rotated as in lanthanum cuprate). For the tetragonal cuprates we obtain an easy--plane anisotropy in good agreement with experimental values. We predict the magnitude of the small in--plane aniso

Influence of the Mn2+Ion Cluster Formation on the EPR Spectrum of Zn1−xMnxTe

The second and fourth central moments of the EPR line of the Mn2+ion clusters are calculated for the case when isotropic and anisotropic superexchange interactions exist between paramagnetic ions. The infinite-temperature linewidth for each type of the cluster is found taking Lorentzian lineshape function. Using cluster-method calculations of central moments, infinite-temperature EPR linewidth in Zn1−xMnxTe is evaluated forx≤ 0.20, and the results are compared with the theory of the EPR line broadening developed for randomly diluted magnetic semiconductors. The cluster-method approach to the EPR linewidth offers an explanation of the narrow resonance observed in Zn1−xMnxTe forx> 0.20.

Theory of magnetism in low symmetry crystals: cuprates and nickelates

The effective spin Hamiltonians for the distorted NiO 2 and CuO 2 planes with low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) structures are derived by taking into account the spin-orbit coupling and structural distortion as perturbations. We explain why the weak ferromagnetism occurs in both LTO and LTT phases for cuprates whereas it does only in the LTT phase for nickelates. The spin-wave gaps observed by neutron scattering experiments are analyzed. We find that the gaps of the in-plane and out-of-plane modes in La 1.65Nd 0.35CuO 4 are caused, respectively, by anisotropic superexchange and direct-exchange interactions.

Erratum: Electronic and magnetic structures of cuprates with spin-orbit interaction

The effective Hamiltonian for the ${\mathrm{CuO}}_{2}$ plane of high-${\mathit{T}}_{\mathit{c}}$ cuprates is derived by taking into account the spin-orbit interaction. A finite-size exact-diagonalization technique is applied to the Hamiltonian, and the magnetic and electronic structures of the ${\mathrm{CuO}}_{2}$ plane in the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) phases of the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$-type crystals are examined. It is shown that the contribution from the in-plane oxygen 2${\mathit{p}}_{\mathit{z}}$ orbital perpendicular to the plane is essential for the emergence of the weak ferromagnetism in the LTO-phase ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. In the LTT phase, either the uniaxial antiferromagnetism or the weak ferromagnetism is induced, depending on how the 2${\mathit{p}}_{\mathit{z}}$ orbital contributes to the anisotropic superexchange interaction of the single Cu--O--Cu bond. We study the dynamics of a hole in an extended t-J model, and show that the lattice distortion works to change the symmetry of the electronic ground state via the spin-orbit coupling. The effect is, however, small and seems irrelevant to the anomalous properties of ${\mathrm{La}}_{1.88}$${\mathrm{Ba}}_{0.12}$${\mathrm{CuO}}_{4}$.

Comment on "Moriya's anisotropic superexchange interaction, frustration, and Dzyaloshinsky's weak ferromagnetism"

A Comment on the Letter by L. Shekhtman, O. Entin-Wohlman, and A. Aharony, Phys. Rev. Lett. 69, 836 (1992).

Electronic and magnetic structures of cuprates with spin-orbit interaction.

The effective Hamiltonian for the ${\mathrm{CuO}}_{2}$ plane of high-${\mathit{T}}_{\mathit{c}}$ cuprates is derived by taking into account the spin-orbit interaction. A finite-size exact-diagonalization technique is applied to the Hamiltonian, and the magnetic and electronic structures of the ${\mathrm{CuO}}_{2}$ plane in the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) phases of the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$-type crystals are examined. It is shown that the contribution from the in-plane oxygen 2${\mathit{p}}_{\mathit{z}}$ orbital perpendicular to the plane is essential for the emergence of the weak ferromagnetism in the LTO-phase ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. In the LTT phase, either the uniaxial antiferromagnetism or the weak ferromagnetism is induced, depending on how the 2${\mathit{p}}_{\mathit{z}}$ orbital contributes to the anisotropic superexchange interaction of the single Cu--O--Cu bond. We study the dynamics of a hole in an extended t-J model, and show that the lattice distortion works to change the symmetry of the electronic ground state via the spin-orbit coupling. The effect is, however, small and seems irrelevant to the anomalous properties of ${\mathrm{La}}_{1.88}$${\mathrm{Ba}}_{0.12}$${\mathrm{CuO}}_{4}$.