Calculation Of Exchange Interaction Research Articles

Pyridyl-Substituted Nitronyl Nitroxides: Comprehensive Magnetochemical and Quantum Chemical Study.

A series of pyridyl-substituted nitronyl nitroxides was synthesized and structurally characterized. A comprehensive magnetochemical and quantum chemical study of extended raw of the nitroxides with different substituents R in the pyridine fragment was performed. It was shown, that temperature-dependent magnetic properties are determined by the short contacts between nitroxide groups of adjacent molecules as well as between nitroxide group and methyl substituents in the pseudo axial positions of imidazoline fragments. Quantum chemistry allows to select the appropriate model of exchange cluster for analysis of experimental magnetic data and evaluation of the exchange interaction parameters. For NN-PyCl the "order-disorder" transition was detected by means of low temperature XRD. The difference in the experimental and calculated exchange interaction energies may serve as an indicator of temperature-induced structural rearrangements. For instance, for methyl substituted nitronyl nitroxide NN-PyMe structural transformations and significant changes in exchange interaction energies were observed.

First principles investigation of spin-orbit coupling driven magnetism of the 3d−5d double-perovskite (Sr/Ca)2FeIrO6

The interplay of spin-orbit coupling (SOC) with the other energy scales gives rise to various novel and interesting quantum phenomena in the Ir based double perovskites. In recent findings the double-perovskites ${\mathrm{Sr}}_{2}{\mathrm{FeIrO}}_{6}$ (SFIO) and ${\mathrm{Ca}}_{2}{\mathrm{FeIrO}}_{6}$ (CFIO) have been reported to fall under the category of a SOC driven antiferromagnetic (AFM) Mott insulator. Using density functional theory based first principles calculations, we have performed a comparative electronic structure investigation of these compounds. We reveal the microscopic origin of the difference in the AFM transition temperature observed in the experiments through the calculated exchange interactions and the Wannier functions. We addressed the role of SOC in dictating the ground state properties of these two compounds. Although both of the compounds are isoelectronic and isovalent, however, the effective strength and the impact of SOC is very different and nonmonotonic in these compounds, due to the delicate balance with the other energy scales. Our study advances the understanding of the nontrivial role of SOC in driving ground state magnetic properties of the Ir based oxides in general and explores the opportunities for further designing of the SOC driven quantum phases.

Structural stability, electronic, magnetic and thermoelectric properties for half-metallic quaternary Heusler alloys CrLaCoZ

Using first-principles calculations, we predict the CrLaCoZ (Z = Al, Ga, In, Ge, Sn, Pb) Heusler alloys to be half-metals, and their total magnetic moments follow the Slater-Pauling rule Mt = Zt − 18. Structural stability is estimated in terms of thermodynamics, dynamic, mechanical and thermal. A comparison in total energy shows that the CrLaCoZ alloys have ferrimagnetic ground states, and the competition between d-d exchange, super-exchange and RKKY exchange interactions leads to the appearance of magnetic order phase. Phase separation shows that CrLaCoAl, CrLaCoGa, CrLaCoIn and CrLaCoSn are easier to be synthesized in experiment. The formation of half-metallic gap ascribes to the d-d, p-d orbital hybridizations. Furthermore, we find that the magnitude of gap is decided by the strength of exchange splitting between eg and t2g states according the distribution and occupation of electronic states. Presence of X and DO3 disorders destroys their half-metallicity. In addition, we examine the effects of spin-orbit coupling on electronic structure, and calculate their thermoelectric properties by using Boltzmann transport theory. From the calculated exchange interactions, it is found that the Cr(A)-Cr(A), Cr(A)-La(B) and Cr(A)-Co(C) exchanges play a leading role in all interactions, and finally determine the Curie temperatures. Apart from the CrLaCoSn alloy, other alloys have evidently higher Curie temperatures than room temperature, which is favorable as an electrode materials in magnetic tunnel junction.

Erratum: Coqblin-Schrieffer model for an ultracold gas of ytterbium atoms with metastable state [Phys. Rev. B 93 , 115143 (2016)

In our paper, calculation of exchange interaction is carried out for two itinerant atoms $^{173}\mathrm{Yb}(^{1}S_{0})$ and $^{173}\mathrm{Yb}(^{3}P_{0})$. The fact that the exchange interaction is modified when the $^{173}\mathrm{Yb}({}^{3}{P}_{0})$ atom is localized in an optical potential was overlooked. This flaw is corrected in this Erratum, and the claim that the exchange constant is antiferromagnetic (a necessary condition for achieving the Kondo effect) is established.

Tetragonal CuMnAs alloy: Role of defects

The antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure is a promising material for the AFM spintronics. The resistivity measurements indicate the presence of defects about whose types and concentrations is more speculated as known. We confirmed vacancies on Mn or Cu sublattices and Mn$_{\rm Cu}$ and Cu$_{\rm Mn}$ antisites as most probable defects in CuMnAs by our new ab initio total energy calculations. We have estimated resistivities of possible defect types as well as resistivities of samples for which the X-ray structural analysis is available. In the latter case we have found that samples with Cu- and Mn-vacancies with low formation energies have also resistivities which agree well with the experiment. Finally, we have also calculated exchange interactions and estimated the N\'eel temperatures by using the Monte Carlo approach. A good agreement with experiment was obtained.

High-temperature expansion method for calculating paramagnetic exchange interactions

The method for calculating the isotropic exchange interactions in the paramagnetic phase is proposed. It is based on the mapping of the high-temperature expansion of the spin-spin correlation function calculated for the Heisenberg model onto Hubbard Hamiltonian one. The resulting expression for the exchange interaction has a compact and transparent formulation. The quality of the calculated exchange interactions is estimated by comparing the eigenvalue spectra of the Heisenberg model and low-energy magnetic part of the Hubbard model. By the example of quantum rings with different hopping setups we analyze the contributions from the different part of the Hubbard model spectrum to the resulting exchange interaction.

Modeling the sublattice magnetizations for the layered bcc nanojunction [formula omitted] systems

Ferromagnetic nanojunctions …Fe[Fe1−cCoc]ℓFe…, with ℓ is the number of layers which constitute the nanojunction, based on Fe/Co alloy are considered for the first time in this work. We model the salient magnetic properties of the layered ferromagnetic nanostructures between magnetically ordered iron leads. The effective field theory (EFT) Ising spin method is used to compute reliable Jav exchange values for the VCA Fe/Co alloy materials in comparison with experimental data and compared to existing DFT calculated exchange interactions. The new set of exchange interaction values between pairs of nearest neighbors atom in the alloy are deduced and agree with previous known measurement of lattice constant for this alloy. Using the combined EFT and mean field theory (MFT) spin methods, the sublattice magnetizations of the Fe and Co sites on the individual bcc basal planes of the layered nanostructures, are calculated and analyzed. The sublattice magnetizations, effective magnetic moments per site, and the possible ferromagnetic order of the layers [Fe1−cCoc]ℓ on the individual bcc atomic planes of the embedded nanostructures for all temperatures and in particular for TcFe≤T≤Tα→γ are presented as a function of temperature and thicknesses of the layered ferromagnetic nanostructures, for different stable concentrations c=0.25, 0.5 and 0.75. In the absence of first principles calculations for these basic physical variables for the layered nanostructures between iron leads, the combined EFT and MFT approach yields the only available information for them at present in the absence of a possible Curie temperature for these alloys. These variables are necessary for certain spin dynamic computations, as for the ballistic magnon transport across embedded nanojunctions in magnonics. The model is general, and may applied directly to other composite magnetic elements and embedded nanostructures.

2D Compound Constructed by Cu3 Units with Mixed Azido and Tetrazole Double Bridges: Synthesis, Structure, and Theoretical Study on Magnetic Properties

AbstractThe two‐dimensional (2D) layer CuII compound [Cu3(L)2(N3)4] (1) [L = 2‐amino‐3‐(5‐tetrazole)‐methyate‐N‐pyridine] was synthesized by in‐situ hydrothermal reaction of CuCl2·2H2O, NaN3, and 3‐(5‐tetrazole)‐methyate‐N‐pyridine. The central Cu1 and Cu2 atoms are located in five‐coordinate and six‐coordinate arrangements, respectively. Three CuII ions are linked by mixed double EO (end‐on)‐azido‐tetrazole bridges to give trinuclear CuII clusters, which are further extended by EE (end‐to‐end) azido bridges to form 2D metal‐organic layers. The magnetic exchange interactions in complex 1 were investigated by DFT calculations, and the calculated exchange interaction (J = –849 cm–1) revealed that the double EO‐azido‐tetrazole bridges transmit antiferromagnetic coupling between CuII ions.

Double End‐On Azido Copper(II) Dinuclear Complex with Oxazoline Ligand: Synthesis, Structure, Magnetic Properties and Theoretical Study

AbstractA double azido‐bridged CuII dinuclear complex with the chelating chiral ligand, [Cu2(L)2(N3)4] (1) [L = (+)‐2, 2′‐isopropylidene‐bis((4R)‐4‐benzyl‐2‐oxazoline)] was synthesized and characterized by single‐crystal X‐ray diffraction, IR spectroscopy, magnetic measurements, and theoretical studies. The asymmetric double end‐on azido bridges in complex 1 lead to a weak antiferromagnetic behavior with J = –7.4 cm–1. The exchange interactions in complex 1 were investigated by DFT calculations, and the calculated exchange interaction (J = –8.0 cm–1) is in good agreement with the experimental value.

Mn-dimer 분자자성체의 전자구조 및 자기구조 계산

We have studied electronic and magnetic structure of Mn-dimer molecule using OpenMX method based on density functional method. The calculated density of states shows that the four O atoms split and energy levels. The energy splitting by the crystal field is smaller than bulk MnO with cubic structure, because of small coordination number of atoms. Total energy with antiferromagnetic spin configuration is lower than that of ferromagnetic configurations. Calculated exchange interaction J between Mn atoms is one order larger than that of the other Mn-O magnetic molecules. That comes from the direct exchange interaction between Mn 3d orbitals and the super-exchange interactions caused by strong -bonding of Mn-O orbitals.

Antiferromagnetic ground state in NpCoGe

NpCoGe, the neptunium analog of the ferromagnetic superconductor UCoGe, has been investigated by dc magnetization, ac susceptibility, specific heat, electrical resistivity, Hall effect, Np M\"ossbauer spectroscopy, and local spin-density approximation (LSDA) calculations. NpCoGe exhibits an antiferromagnetic ground state with a N\'eel temperature ${T}_{N}\ensuremath{\approx}13$ K and an average ordered magnetic moment $\ensuremath{\langle}{\ensuremath{\mu}}_{\mathrm{Np}}\ensuremath{\rangle}=0.80\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (\ensuremath{\approx}3 T). The value of the isomer shift suggests a Np${}^{3+}$ charge state (configuration $5{f}^{4}$). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}2}$) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the $c$ axis. M\"ossbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin-spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogs suggests the leading role of 5f-d hybridization, the rather delocalized character of 5f electrons in NpCoGe, and the possible proximity of NpRuGe or NpFeGe to a magnetic quantum critical point.

Computational materials design of defect-induced ferrimagnetic MnO

We present a computational materials design for defect-induced ferrimagnetic MnO. The magnetic properties of MnO containing Mn vacancies were investigated using first-principle calculations. For these electronic structure calculations, we employed a pseudo-self-interaction-corrected local density approximation (PSIC-LDA). We used the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) to create a random distribution of atoms at the assigned sites. Having described the magnetic properties with a classic Heisenberg model, we calculated the effective exchange coupling constants by applying the magnetic force theorem to two magnetic sites embedded in the CPA medium. We estimated the Curie temperatures from the calculated exchange interactions. This study found that the Mn vacancies induced ferrimagnetic ground states in MnO, and that the Curie temperature could reach room temperature at Mn vacancy concentrations above 20%. These findings suggest a new route for designing ferrimagnetic materials from anti-ferromagnetic host materials.

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Using a methodology based on noncollinear coupled-perturbed density functional theory [J. Chem. Phys. 2013, 138, 174115] we calculate the magnetic exchange coupling parameters in a recently synthesized set of Fe7III disklike clusters [Inorg. Chem. 2011, 50, 3849–3851] to explain the unusually high ground-state spin found in the experiments. We show that the calculated exchange interactions for the new series of Fe7III disks present strikingly different trends compared to prior Fe7III disks. These differences are attributed to variations in the bridging ligands and the consequent structural changes in the complexes. The impact of these differences on the experimental ground-state spin of these complexes is rationalized using a simple classical spin model system and the calculated magnetic exchange couplings.

Ferrimagnetism, antiferromagnetism, and magnetic frustration in La2−xSrxCuRuO6(0⩽x⩽1)

We studied structural and magnetic properties of a series of insulating double perovskite compounds, La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuRuO${}_{6}$ (0 $\ensuremath{\leqslant}$ $x$ $\ensuremath{\leqslant}$ 1), representing doping via $A$-site substitution. The end members La${}_{2}$CuRuO${}_{6}$ and LaSrCuRuO${}_{6}$ form in monoclinic structure while the intermediate Sr doped compounds stabilize in triclinic structure. The Cu and Ru ions sit on alternate $B$ sites of the perovskite lattice with $\ensuremath{\sim}15%$ antisite defects in the undoped sample while the Sr-doped samples show a tendency to higher ordering at $B$ sites. The undoped ($x$ $=$ 0) compound shows a ferrimagnetic-like behavior at low temperatures. In surprising contrast to the usual expectation of an enhancement of ferromagnetic interaction on doping, an antiferromagnetic-like ground state is realized for all doped samples ($x$ $>$ 0). Heat capacity measurements indicate the absence of any long-range magnetic order in any of these compounds. The magnetic relaxation and memory effects observed in all compounds suggest glassy dynamical properties associated with magnetic disorder and frustration. We show that the observed magnetic properties are dominated by the competition between the nearest-neighbor Ru-O-Cu 180${}^{\ensuremath{\circ}}$ superexchange interaction and the next-nearest-neighbor Ru-O-O-Ru 90${}^{\ensuremath{\circ}}$ superexchange interaction as well as by the formation of antisite defects with interchanged Cu and Ru positions. Our calculated exchange interaction parameters from first principles calculations for $x$ $=$ 0 and $x$ $=$ 1 support this interpretation.

An ab-initio study of the structural, electronic and magnetic properties of half-metallic ferromagnetism in Cr-doped BeSe and BeTe

We systematically explore Cr-doped zinc blende BeSe and BeTe compounds in terms of their structural, electronic and magnetic properties by using the first-principles full-potential linearized augmented plane waves plus local orbitals (FP-LAPW + lo) method. The state stability of the doped materials has been studied by calculating the heat of formation and analysing minimum total energies in ferromagnetic (FM) and anti-ferromagnetic (AFM) coupling. A non linear dependency of Cr concentration on the lattice parameters and bulk modulus is found which shows small deviation from Vegard's law. Our calculations for each Cr concentration were performed using two different density functional theory functionals which are the Wu and Cohen (WC) and the modified Becke and Johnson local (spin) density approximation (mBJLDA). The functionals generate spin dependent band structures, electronic density of state (DOS) and magnetic moments that exhibit occurrence of half-metallic ferromagnetism which is caused by spin polarization of electron in Cr-d orbitals. Importantly, comparison between the two functionals shows that the mBJLDA functional provides a better description of the electronic structure of the Cr-doped binary systems. The calculated exchange interaction and exchange constant, N0α and N0β, respectively, are consistent with the values from typical magneto-optical experiment and conversely, the calculated charge spin densities allowed us to understand the bonding nature. In addition, the calculations of spin exchange splitting, Δx(d), exhibit important indication regarding the attractive effective potential for minority spin rather than majority spin.

Cubane-type Mn4클러스터의 전자구조 및 자기구조 계산

We have studied electronic and magnetic structure of cubane-type Mn4 cluster using OpenMX method based on density functional method. The calculated density of states shows that the octahedron of O atoms split and energy levels like bulk MnO with cubic structure. Total energy with antiferromagnetic spin configuration is lower than those of other spin configurations because of super exchange interaction. Calculated exchange interaction J between Mn atoms with anti-parallel spin is larger than between Mn atoms with parallel spin.

Magnetism and half-metallicity of some Cr-based alloys and their potential for application in spintronic devices

We present results of the exchange interactions and Curie temperatures in some Cr-based chalcogenides and pnictides. The calculations are performed mostly for Zinc Blende (ZB) structure with the lattice parameter varying between 5.45 and 6.6 Å, appropriate for some typical II-VI and III-V semiconducting substrates. Electronic structure is calculated via the linear muffintin-orbitals method and disorder effects, when appropriate, are taken into account via the coherent potential approximation. Calculated exchange interactions are used to estimate the Curie temperature by employing the mean-field and the random phase approximations. For CrTe we compare the results of the ZB and Rock-Salt (RS) structures.

Microscopic origins of the ferromagnetic exchange coupling in oxoverdazyl-based Cu(II) complex

The exchange channels governing the experimentally reported coupling constant (J(expt)=6 cm(-1)) value in the verdazyl-ligand based Cu(II) complex [Cu(hfac)(2)(imvdz)] are inspected using wave function-based difference dedicated configuration interaction calculations. The interaction between the two spin 1/2 holders is summed up in a unique coupling constant J. Nevertheless, by gradually increasing the level of calculation, different mechanisms of interaction are turned on step by step. In the present system, the calculated exchange interaction then appears alternatively ferromagnetic/antiferromagnetic/ferromagnetic. Our analysis demonstrates the tremendously importance of some specific exchange mechanisms. It is actually shown that both parts of the imvdz ligand simultaneously influence the ferromagnetic behavior which ultimately reaches J(calc)=6.3 cm(-1), in very good agreement with the experimental value. In accordance with the alternation of J, it is shown that the nature of the magnetic behavior results from competing channels. First, an antiferromagnetic contribution can be essentially attributed to single excitations involving the pi network localized on the verdazyl part. In contrast, the sigma ligand-to-metal charge transfer (LMCT) involving the imidazole moiety affords a ferromagnetic contribution. The distinct nature sigma/pi of the mechanisms is responsible for the net ferromagnetic behavior. The intuitively innocent part of the verdazyl-based ligands is deeply reconsidered and opens new routes into the rational design of magnetic objects.

First-principles material design and perspective on semiconductor spintronics materials

We investigate the electronic structure and magnetic properties of III–V compound semiconductor based dilute magnetic semiconductors (DMS) from first-principles. The electronic structure of DMS is calculated by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA). Describing the magnetic properties by a classical Heisenberg model, effective exchange interactions are calculated by applying magnetic force theorem for two impurities embedded in the CPA medium. With the calculated exchange interactions, TC is estimated by using the mean field approximation, the random phase approximation and the Monte Carlo simulation. In the above compounds, the magnetic interactions are well described from double exchange picture. Due to the short-range interactions, high-TC is difficult to achieve in the presently investigated materials. Based on the present results, two strategies towards high-TC are proposed to realize useful DMS materials. One uses spinodal decomposition to realize high blocking temperature in super-paramagnetic blocking phenomena and the other uses co-doping method to realize high concentration doping of magnetic impurities for high-TC.

Competing Large and Small Angles in a Double End‐On Azido Copper(II) Binuclear Complex: A Combined Experimental and Theoretical Study of Magnetic Interactions

AbstractThe structure and magnetic properties of double azido‐bridged CuII binuclear complex 1 with the chelating chiral ligand (S,S)‐2,2′‐isopropylidenebis(4‐phenyl‐2‐oxazoline) were analyzed by combining experimental and theoretical techniques. The CuII ions adopt a square pyramidal geometry with different degrees of distortion, whereas the two end‐on azido bridges disposed at the equatorial positions exhibit significantly different Cu–N–Cu angles, 110.6 and 97.3°, which are respectively smaller and larger than the critical 108° value distinguishing the ferromagnetic and antiferromagnetic regimes. The asymmetry in 1 arises from the use of the bulky asymmetric ligand, giving rise to two different magnetic pathways between the CuII ions. The magnetic pathway along the large Cu–N–Cu angle value dominates over the small one, resulting in a net antiferromagnetic behavior with J = –78.6 cm–1. On the basis of wavefunction calculations, we investigate the exchange interactions in synthetic compound 1 and fictitious analogs 2 and 3 holding either two large (i.e., 110.6°) or two small (i.e., 97.3°) Cu–N–Cu angles. The calculated exchange interaction in 1(–104 cm–1) is in relatively good agreement with the experimental value and corresponds precisely to the average between the antiferromagnetic value in 2 (–218 cm–1) and the ferromagnetic one in 3 (21 cm–1). The significant enhancement in the antiferromagnetic contribution accompanying the expansion of one of the Cu–N–Cu bridging angles is undoubtedly the driving force for the observed antiferromagnetic behavior in 1. The control of the local metal environments allowed us to monitor the exchange coupling interactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)