Temperature dependence of the spin state of a Co3+ Ion in RCoO3 (R = La,Gd) cobaltites

Changes in the spin state of Co³⁺ ions in LaCoO₃ and GdCoO₃ compounds are studied through the use of the temperature dependence of the magnetic susceptibility and the modified crystal field theory. It is shown that the spin subsystem of Co³⁺ ions in LaCoO₃ and GdCoO₃ undergoes the spin-crossover type transition between the high-spin (S = 2) and low-spin (S = 0) states without any contribution of the intermediate-spin state (S = 1).     

Magnetization studied on spin alignments in organic molecule-based ferrimagnetics

A physics picture of spin alignments in molecule-based ferrimagnets is presented from studying the temperature dependence of the effective sublattice magnetic moments and the total reduced magnetization by means of Green’s function theory combined with the Jordan-Wigner transformation. The ferrimagnetic chain includes an S=1 biradical and an S=1/2 monoradical with antiferromagnetic alternating interactions, and the S=1 site in the chain is composed of two S=1/2 spins coupled by a finite ferromagnetic interaction. From the calculations of the sublattice magnetic moments, the magnetic moment of the S=1 biradical is negative, while that of the S=1/2 monoradical is positive, leading to a ferrimagnetic ground state. With the different kinds of the elementary excitations and the competition between the magnetic interactions and thermal fluctuations, the temperature dependence of the magnetization displays rich thermodynamic properties. Meanwhile, the external magnetic field dependence of the magnetization has a clear plateau at one third of the saturation magnetization, which can be compared with the possible experimental findings.     

Tunable dipolar magnetism in high-spin molecular clusters

We report on the Fe17 high-spin molecular cluster and show that this system is an exemplification of nanostructured dipolar magnetism. Each Fe17 molecule, with spin S=35/2 and axial anisotropy as small as D approximately -0.02 K, is the magnetic unit that can be chemically arranged in different packing crystals while preserving both the spin ground state and anisotropy. For every configuration, molecular spins are correlated only by dipolar interactions. The ensuing interplay between dipolar energy and anisotropy gives rise to macroscopic behaviors ranging from superparamagnetism to long-range magnetic order at temperatures below 1 K.     

Triplet spin excitons in organic charge transfer complexes

Electron paramagnetic resonance (EPR) spectra of single crystals of the 1 : 1 charge transfer (CT) complex of N, N'-dimethyldihydrophenazine (M₂P) and of tetracyanoquinodimethane (TCNQ) provide the first case of thermally activated, with ΔE_p = 0.60 ± 0.06 eV, triplet spin excitons observed in an organic CT solid. The narrow EPR lines and resolved fine structure splittings indicate mobile triplets based on adjacent M₂P⁺TCNQ^- ion radicals above a largely neutral ground state.     

Phenomenological descriptions of nuclear rotational spectra in the spin region I < 30

From semi-empirical considerations, we suggest the application of broken polynomial expressions in terms of the angular momentum for describing rotational spectra at spin values I < 30. A three-parameter expression for the moment of inertia gives satisfactory fits to the ground state bands of several even-even nucleides, including high-spin (I_max = 18) recently identified.     

Benzoic acid and phenol derivatives of nitronyl nitroxide biradical as building blocks of organic molecule-based ferrimagnets

A crystal-engineering approach to organic ferrimagnets is reported. Coulombic energy between an anionic biradical withS = 1 and a cationic monoradical withS = 1/2 can be utilized as a driving force of cocrystallization of open-shell molecules with different spin quantum numbers, leading to organic salt ferrimagnets. In this study, 3,5-substituted phenol and benzoic acid derivatives of nitronyl nitroxide biradicals were synthesized as an ionizableS = 1 component of organic salt ferrimagnets. The molecular ground states of the biradicals in the neutral state were examined by continuous wave electron spin resonance (ESR) spectroscopy and static paramagnetic susceptibility measurements in the solid state. The molecular ground state of the phenol derivative was found to be triplet (S = 1) with the singlet-triplet energy gap of ΔE/k_B ≈ 25 K, indicating that the biradical can be a building block of organic salt ferrimagnetics. The benzoic acid derivative was found to have a singlet (S = 0) ground state (ΔE/k_B ≧ −5 K), exemplifying thatmeta-(3,5)-linkage of unpaired electrons in π-aromatic rings does not necessarily give a triplet ground state for heteroatomic-substituted π conjugation. The molecular ground states of the biradicals determined in the ESR experiments were confirmed by the susceptibility in the solid state.     

Developments of the theory of spin fluctuations and spin fluctuation-induced superconductivity

Theory of spin fluctuations as developed in the past 30 years have played important roles in the theory of magnetism in metals, particularly in elucidating the properties around the magnetic instability or quantum critical points. Recently the theory has been extended to deal with the spin fluctuaion-mediated superconductivity with anisotropic order parameters in strongly correlated electron systems. These theoretical developments are briefly reviewed and the high temperature superconductivity of cuprates and organic and heavy electron superconductors are discussed in the light of these theories.     

Magnetic properties of nanographite with modified zigzag edges

Newly proposed aromatic molecules and graphene fragments are shown to have the high-spin ground state by the first-principles electronic structure calculations. Our strategy to predict magnetic carbon materials is based on our previous conclusion that mono-hydrogenated, di-hydrogenated or mono-fluorinated zigzag edges of honeycomb networks are magnetic. Structural optimization as well as determination of the electronic states was performed for various nanographite ribbons and high-spin molecules, e.g. 1,8,9-di-hydro-anthracene, C₁₉H₁₄ and C₁₄F₁₃. For hydrogenated molecules and ribbons, the total spin S determined by the LSDA calculation coincides with the value expected from a counting rule for the total spin on a bipartite network. However, S depends on structures of fluorinated nanographite.     

On the ground state of Ising models with arbitrary spin quantum number

The ground state of a class of Ising models with site dependent arbitrary spin quantum number is shown to be restricted to ±S_i^MAX state where S_i^MAX is the spin quantum number at the site i.     

Organic magnetoresistance and spin diffusion in organic semiconductor thin film devices

We review recent work in the field of organic spintronics, focusing on our own contributions to this field. There are two principle magnetoresistance effects that occur in organic devices. (i) Organic magnetoresistance (OMAR), which occurs in nonmagnetic organic semiconductor devices. For example, in devices made from the prototypical small molecule Alq₃ OMAR reaches values of 10% or more at room temperature. (ii) Organic spin‐valve effects that occur in devices that employ ferromagnetic electrodes for spin‐polarized current injection and detection. We undertake an analysis of these two types of magnetoresistance with the goal of identifying the dominant spin‐scattering mechanism. Analysis of OMAR reveals that hyperfine coupling is the dominant spin‐coupling mechanism. Spin–orbit coupling, on the other hand, is important only in organic semiconductor materials containing heavy atoms. We explore the reasons why spin–orbit coupling is relatively unimportant in hydrocarbon materials. Next, we present a theory for spin diffusion in disordered organic semiconductors based on hyperfine coupling, taking into account a combination of incoherent carrier hopping and coherent spin precession in the random hyperfine magnetic fields. We compare our findings with experimental values for the spin‐diffusion length. Finally, we demonstrate a criterion that allows the determination whether the organic spin‐valves operate in the tunneling or injection regimes. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Dzyaloshinskii−Moriya interactions on Kagome anti-ferromagnetic clusters

The low energy and low temperature behavior of a few finite size Kagome clusters, including mixed spin systems of S=1/2 and S=1, with the nearest neighbor Heisenberg antiferromagnetic model is studied under the influence of out-of-plane Dzyaloshinskii?Moriya interactions (DMI) within the exact diagonalization formalism. The ground state of all the finite size systems is found to be present in the lowest spin sector with a finite gap to the lowest magnetic excitation irrespective of the strength of out-of-plane DMI. The energy level structures within the non-magnetic ground state and the lowest magnetic state have been studied for all the systems as a function of DMI. The characteristic signature of such low-lying non-magnetic excitations is reflected in the low temperature behavior of the specific heat. It is also found that the ground state chiral structure (characterized by the vector chiral order of the system) in the xy-plane shows sharp changes as a function of out-of-plane DMI at level crossing or avoided crossing regions. The in-plane spin ordering for each system is also studied with the estimation of static structure factor as a response to the varying strength of DMI.     

Truncated forms of the second-rank orthorhombic Hamiltonians used in magnetism and electron magnetic resonance (EMR) studies are invalid—Why it went unnoticed for so long?

This paper deals with the truncated forms of the second-rank orthorhombic Hamiltonians employed in magnetism and electron magnetic resonance (EMR) studies. Consideration of the intrinsic features of orthorhombic Hamiltonians reveals that the truncations, which consist in omission of one of three interdependent orthorhombic terms, are fundamentally invalid. Implications of the invalid truncations are: loss of generality of quantized spin models, misinterpretation of physical properties of systems studied (e.g. maximum rhombicity ratio and relative parameter values), and inconsistent notations for Hamiltonian parameters that hamper direct comparison of data from various sources. Truncated Hamiltonian forms identified in our survey are categorized and systematically reviewed. Examples are taken from studies of various magnetic systems, especially those involving transition ions, as well as model magnetic systems. The pertinent studies include magnetic ordering in three- and lower dimensions, e.g. [(CH₃)₄N]MnCl₃ (TMMC), canted ferromagnets, Haldane gap antiferromagnets, single molecule magnets exhibiting macroscopic quantum tunneling, e.g. Mn₁₂ complexes with spin S=10. Our study provides better insight into magnetic and spectroscopic properties of pertinent magnetic systems, which calls for reconsideration of the experimental and theoretical results based on invalid truncated Hamiltonians. The physical nature of Hamiltonians used in magnetism and EMR studies and other types of inappropriate terminology occurring, especially in model magnetism studies, require separate discussion.     

Excitonic representation: Collective excitation spectra in the quantized Hall regime and spin biexciton

The excitonic representation method for describing collective excitations in the quantized Hall regime makes it possible to simplify analysis of the spectra and to obtain new results in the strong magnetic field limit, when E _C ??ω_c (ω_c is the cyclotron frequency and E_C is the characteristic Coulomb energy). For an integer odd filling factor ν greater than unity (i.e., for ν = 3, 5, 7,...), the spectra of one-cyclotron magneto-plasma excitations are calculated. For unit filling factor, the existence of a spin biexciton (bound state of two spin waves) corresponding to excitation with a spin change (δ_S = δ_Sz = ?2) is proved. The exact equation determining the ground state of the biexciton is derived in the thermodynamic limit N_Φ → ∞ (_N? is the system degeneracy). The exchange energy of this state is lower than for a single spin wave (with δ_S = δ_Sz = ?1) for the same value of the 2D wavevector q. In the limit q → ∞ corresponding to the decay of a biexciton into a pair of quasiparticles one of which is a trion with a spin of ?3/2, the energy is found to be lower than the energy (e²/εl _B )√π/2 required for exciting an electron-hole pair in the strictly 2D case (l_B is the magnetic length and ε is the dielectric constant), although this energy is higher than another “classical” result (e²/εl _B )√π/2, corresponding to the excitation of a skyrmion-antiskyrmion pair (|δS|=|δS _z |?1). The solution of the exact equation gives the trion binding energy and the activation gap for quasiparticles whose excitation corresponds to a change in the total spin by δS = δ S_z =?3. The energy of a spin biexciton is calculated for values of the wavevector such that ql _B ?1.     

2-D electron spin transient nutation spectroscopy of Lanthanoid ion Eu2+(8S7/2) in a CaF2 single crystal on the basis of FT-pulsed electron spin resonance spectroscopy: Transition moment spectroscopy

This work demonstrates the usefulness of pulsed electron spin resonance (ESR)-based two-dimensional electron spin transient nutation (2-D ESN) spectroscopy for complete assignments of complicated fine-structure hyperfine ESR spectra including hyperfine forbidden transitions from electronic and nuclear high-spin systems. The 2-D ESN spectroscopy is termed transition moment spectroscopy as spectra are acquired as a function of transition moment instead of transition energy used in conventional spectroscopy. We have applied the novel spectroscopic technique to Eu²⁺ ion (S=7/2,I=5/2), which has two isotopes (¹⁵¹Eu [47.9%] and¹⁵³Eu [52.1%]), in a CaF₂ single crystal as a model system. We have completely identified the complicated fine-structure hyperfine ESR spectra by invoking the spectral simulation of the 2-D ESN spectra on the basis of transition moment analyses. The analyses are based on exact numerical calculations of the transition moments as well as a perturbation-based analytical approach combined with reduced rotation matrices for the nuclear part of the transition moment. This is the first example of the spectral simulation for 2-D ESN spectra including the hyperfine allowed and forbidden transitions in high-spin systems. In addition, we have made simulation of the fine-structure forbidden transitions, which reproduces the angular variations of the observed spectra at liquid helium temperatures.     

Magnetic properties of 2D nano-islands I: Ising spin model

An Ising spin effective field theory (EFT) is developed as a framework for a detailed analysis of the magnetic properties of two-dimensional (2D) nano-islands on a nonmagnetic substrate with an out of plane magnetization. The Hamiltonian with nearest neighbor exchange interactions and single-atom magnetic anisotropy defines the ground state. The calculation yields the single site spin correlations, the magnetizations, and the isothermal susceptibilities for the core and periphery domains, and the island core phase diagrams. The choice of a spin S=1 for the atoms permits the analysis of the effects of spin fluctuations via the single site spin correlations. In particular we investigate the effects due to the different anisotropies and reduced dimensionalities for the core and periphery domains. The present model calculations are developed for different 2D nano-islands lattices. Detailed theoretical results are presented for the square and hexagonal lattices, with numerical applications for the 2D Co nano-islands on Pt. The derived transition temperature for the hexagonal lattice nano-islands is in good agreement with the experimental data for Co nano-islands on Pt. Though both the core and the periphery domains have the same order-disorder transition temperature, the magnetization of each domain attains this transition differently. The temperature behavior of the spin correlations is also fundamentally different for the periphery and core sites, which entails distinctly different isothermal susceptibilities, and yields statistically averaged nano-islands susceptibilities that do not correspond to a second order phase transition. The experimental susceptibility results for 2D Co nano-islands on Pt can be interpreted within our EFT Ising model without reference to a transition from a blocking state of the particle to a superparamagnetic behavior. The results for the different lattices are formally comparable, and demonstrate the robustness and general character of the model.     

Concepts in high-frequency EPR — Applications to bio-inorganic systems

In this contribution we describe a high-frequency high-field EPR facility which has been developed at the University of Nijmegen. We present the design of a heterodyne quasi-optical bridge based on a millimeter-wave vector network analyzer as source and detection system. The mm-waves are transported in free-space through Gaussian beam optic elements and through a corrugated guide inside the resonator insert. The Fabri-Pérot (TEM_00n) resonator is coupled through a metallic mesh and because of its bimodal property it can be operated using orthogonal detection leading to substantial improvement in sensitivity. In the first stage of the project, a multifrequency CW-facility is realized covering the 100–500 GHz range. In our initial explorative experiments we demonstrate the advantages of HF-EPR of high-spin systems: Due to the large microwave quantum, transitions which would be undetectable at X-band due to the large zerofield splitting can now be observed in good sensitivity. As a model for biological high-spin systems a sample of metmyoglobin was measured at D-band (130 GHz). Theg = 5.9 perpendicular line from theS = 5/2 ferric heme was detected and its line-width was compared to data previously obtained at Q-, X-, S- and L-band. As a model for biological integer spin systems theS = 1 signal of Ni(II) in nickel Tutton salt (Ni(NH₄)₂(SO₄)₂) was studied at 35 and 130 GHz.     

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

Phase diagram of the spin extended model

The quantum phase transition in the ground state of the extended spin S = 1/2 XY model has been studied in detail. Using the exact solution of the model the low temperature thermodynamics, as well as the ground state phase diagram of the model in the presence of applied uniform and/or staggered magnetic field are discussed. Received 29 November 2002 / Received in final form 24 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: japa@iph.hepi.edu.ge     

NDMAP: the best material to study the Haldane's prediction

As predicted by Haldane, spin, S=1 one-dimensional (1D) Heisenberg antiferromagnet (HAF) has an energy gap between the singlet ground state and first excited triplet. On application of magnetic field, the triplet state Zeeman splits and the energy of one of the triplet state becomes zero at a critical field, H_c. Above H_c the system recovers magnetism. Then, we expect that a quasi-1D HAF will show a magnetic long-range ordering (LRO) at low temperatures due to the inter-chain coupling. This field-induced LRO has not been observed before due to complication of the crystal structure in the materials studied so far and/or technical difficulty.From a heat capacity measurement on a single crystal of an S=1 quasi-Q1D HAF, Ni(C₅H₁₄N₂)₂N₃(PF₆), we found an anomaly at a temperature in finite fields indicating a field-induced phase transition. A magnetic LRO is confirmed by a neutron diffraction measurement on the same sample. The temperature versus magnetic field phase diagram of this compound is constructed and discussed.