Anisotropic Magnetic Moments Research Articles

Manipulation of Cooper-Pair Tunneling via Domain Structure in a van der Waals Ferromagnetic Josephson Junction.

Ferromagnetic Josephson junctions play a key role in understanding the interplay between superconductivity and ferromagnetism in condensed matter physics. The magnetic domain structures of the ferromagnet in such junctions can significantly affect the tunneling of the superconducting Cooper pairs due to the strong interactions between Cooper pairs and local magnetic moments in the ferromagnetic tunnel barrier. However, the underlying microscopic mechanism of relevant quasiparticle tunneling processes with magnetic domain structures remains largely unexplored. Here, the manipulation of Cooper-pair tunneling in the NbSe2/Cr2Ge2Te6/NbSe2 ferromagnetic Josephson junction is demonstrated by using a multidomain ferromagnetic barrier with anisotropic magnetic moments. The evolution of up-, down-magnetized domain and Bloch domain structures in Cr2Ge2Te6 barrier under external magnetic fields leads to the enhancement of the critical tunneling supercurrent and an unconventional dual-peak feature with two local maxima in the field-dependent critical current curve. The phenomenon of magnetic-field-modulated critical tunneling supercurrent can be well explained by the competition between the coherence length of tunneling Cooper pairs and the size of magnetic domain walls in Cr2Ge2Te6 barrier. This kind of ferromagnetic Josephson junction provides an intriguing material system for manipulating Cooper-pair tunneling by tuning the local magnetic moments within magnetic Josephson junction devices.

Magnetic anisotropy driven by ligand in 4d transition-metal oxide SrRuO3

The origin of magnetic anisotropy in magnetic compounds is a longstanding issue in materials science, and nonmagnetic ligand ions are considered to contribute little to magnetic anisotropy. Here, we introduce the concept of ligand-driven magnetic anisotropy in a complex transition-metal oxide. We conducted x-ray magnetic circular dichroism spectroscopies at the Ru and O edges in the 4d ferromagnetic metal SrRuO3. Systematic variation of the sample thickness in the range of ≤10 nm allowed us to control the localization of Ru 4d t2g states, which affects the magnetic coupling between the Ru and O ions. We observe that the orbital magnetic moment of the ligand O 2p orbitals induced through the charge transfer to the Ru 4d t2g states becomes anisotropic first, and the anisotropic magnetic moment of Ru and, therefore, the entire system is induced via magnetic coupling between Ru 4d and O 2p orbitals.

The magnetocaloric effect of the lanthanide fluorides: Using polarized neutron scattering to probe a magnetocaloric suitable for hydrogen liquefaction

This work reports the competitive magnetocaloric effect of some simple lanthanide fluoride materials with cations with high magnetic anisotropy. Of these, HoF3 is particularly promising due to exhibiting a high magnetocaloric entropy change under modest applied fields at higher temperatures, which only decreases modestly with temperature such that it has the potential for cooling for hydrogen liquefaction. Spin-polarized neutron spectroscopy indicates that its promising conventional magnetocaloric effect is likely due to the presence of ferromagnetic fluctuations of highly anisotropic magnetic moments, while its singlet electronic ground state and low-temperature magnetic ordering lead to a decrease in its magnetocaloric performance below 4 K.

NMR-like effect on the anisotropic magnetic moment of surface bound states in the topological superfluid He3−B

We present experimental observation of a phenomenon that we interpret as a NMR-like effect on an anisotropic magnetic moment of the surface Andreev bound states in topological superfluid $^{3}\mathrm{He}\text{\ensuremath{-}}\mathrm{B}$ at zero temperature limit. We show that an anisotropic magnetic moment formed near the horizontal surface of a mechanical resonator due to symmetry violation of the superfluid $^{3}\mathrm{He}\text{\ensuremath{-}}\mathrm{B}$ order parameter by the resonator's surface may lead to anomalous damping of the resonator motion in magnetic field. In difference to the classical NMR technique, here NMR was excited using own harmonic motion of the mechanical resonator and nuclear magnetic resonance was detected as a maximum in damping when the resonator's angular frequency satisfied the Larmor resonance condition.

Cloaking the magnons

We propose two approaches to cloak the spin waves (magnons) by investigating magnetization dynamics. One approach is based on a spatially inhomogeneous anisotropic magnetic moment tensor. The other mechanism is using a spatially inhomogeneous anisotropic gyromagnetic factor tensor and an inhomogeneous external magnetic field. For both approaches, the damping tensor is also inhomogeneous and anisotropic. The magnetic characteristic functions of the magnetic materials have been theoretically derived for both mechanisms. A non-magnetic core, which prevents magnons from entering and consequently distorts the spin wave propagation, can be cloaked by a structured magnetic shell to redirect the spin wave around the core using the above design mechanisms. We discuss the feasibility of the proposed mechanisms in an ensemble of quantum dot molecules and magnetic semiconductors. The proposed approaches shed light on transformation magnonics, and can be utilized for future spin-wave lenses, concentrators, low back-scattering waveguides, and ultimately quantum computing.

Energy levels of metal porphyrins upon molecular alignment during layer-by-layer electrostatic assembly: scanning tunneling spectroscopy vis-à-vis optical spectroscopy

Energy levels of metal-porphyrins having an anisotropic magnetic moment respond when they are aligned on electrodes during the layer-by-layer adsorption process.

ChemInform Abstract: Lanthanide Ion and Tetrathiafulvalene‐Based Ligand as a “Magic” Couple Toward Luminescence, Single Molecule Magnets, and Magnetostructural Correlations

AbstractReview: 66 refs.

Lanthanide ion and tetrathiafulvalene-based ligand as a "magic" couple toward luminescence, single molecule magnets, and magnetostructural correlations.

The synthesis of molecules featuring different properties is a perpetual challenge for the chemists' community. The coexistence and even more the synergy of those properties open new perspectives in the field of molecular devices and molecular electronics. In that sense, coordination chemistry contributed to the development of new functional molecules through, for instance, single-molecule magnets (SMMs) and light emitting molecules with potential applications in high capacity data storage and OLEDs, respectively. The appealing combination of both electronic properties into one single object may offer the possibility to have magnetized luminescent entities at nanometric scale. To that end, lanthanides seem to be one of the key ingredients since their peculiar electronic structures endow them with specific magnetic and luminescence properties. Indeed, lanthanides cover a wide range of emission wavelengths, from infrared to UV, which add up to a large variety of magnetic behaviors, from the fully isotropic spin (e.g., Gd(III)) to highly anisotropic magnetic moments (e.g., Dy(III)). In lanthanide complexes, ligands play a fundamental role because on one hand they govern the orientation of the magnetic moment of anisotropic lanthanides and on the other hand they can sensitize efficiently the luminescence. The design of appropriate organic ligands to elaborate such chemical objects with the desired property appears to be essential but remains a perpetual challenge. In this Account, we describe the design of lanthanide-based complexes that emit light, behave as SMMs, or combine both properties. We have paid peculiar attention to the design of ligands based on the tetrathiafulvalene (TTF) moiety. TTF and its derivatives are well-known chemical entities, stable at different oxidation states, and employed mainly in the synthesis of molecular conductors and superconductors. In addition to their redox properties, TTF-based derivatives act as organic chromophores for the sensitization of visible and near-infrared (NIR) luminescence of lanthanides. The mechanism of sensitization involves either antenna effect (energy transfer from the excited state) or photoinduced electron transfer. TTF-based ligands act also as structural agents in the conception of SMM in crystals. Such objects are obtained with the highly anisotropic Dy(III) ion in crystalline phase as well as in frozen solution with magnetic memory at helium-4 temperature (4 K). We highlight the influence of the magnetic dilution (both in amorphous solution and in diamagnetic crystalline matrix) and, particular case of dysprosium based SMMs, the effect of metal-centered isotope enrichment on the SMM properties. Our aim is not only to realize functional molecules but to rationalize both luminescence and magnetic properties on the basis of the structure of the molecules. These two properties are intimately intricate and governed by the electronic structure, which can be calculated and interpreted using modern quantum chemistry tools.

Single molecule magnetism in a family of mononuclear β-diketonate lanthanide(iii) complexes: rationalization of magnetic anisotropy in complexes of low symmetry

The use of an amino-pyridyl substituted β-diketone, N-(2-pyridyl)-ketoacetamide (paaH), has allowed for the isolation of two new families of isostructural mononuclear lanthanide complexes with general formulae: [Ln(paaH*)2(H2O)4][Cl]3·2H2O (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5) and Y (6)) and [Ln(paaH*)2(NO3)2(MeOH)][NO3] (Ln = Tb (7), Dy (8), Ho (9) and Er (10)). The dysprosium members of each family (3 and 8) show interesting slow magnetic relaxation features. Compound 3 displays Single Molecule Magnet (SMM) behaviour in zero DC field with an energy barrier to thermal relaxation of Ea = 177(4) K (123(2) cm−1) with τ0 = 2.5(8) × 10−7 s, while compound 8 shows slow relaxation of the magnetization under an optimum DC field of 0.2 T with an energy barrier to thermal relaxation of Ea = 64 K (44 cm−1) with τ0 = 6.2 × 10−7 s. Ab initio multiconfigurational calculations of the Complete Active Space type have been employed to elucidate the electronic and magnetic structure of the low-lying energy levels of compounds 2–5 and 8. The orientation of the anisotropic magnetic moments for compounds 2–5 are rationalized using a clear and succinct, chemically intuitive method based on the electrostatic repulsion of the aspherical electron density distributions of the lanthanides.

Characterisation of the paramagnetic [2Fe–2S]+ centre in palustrisredoxin-B (PuxB) from Rhodopseudomonas palustris CGA009: g-matrix determination and spin coupling analysis

Palustrisredoxin-B (PuxB) from Rhodopseudomonas palustris (CGA009) is a [2Fe-2S] ferredoxin which is able to accept electrons from NADH via the flavin-dependent palustrisredoxin reductase (PuR); these electrons can then be transferred to the P450 enzyme (CYP199A2). This work reports on the paramagnetic state of the [2Fe-2S](+) cluster in PuxB, both alone and in the PuR-PuxB complex. Aided by the X-ray crystal structure of PuxB, the protons nearest to the reduced [2Fe-2S](+) cluster were used as magnetic probes to quantify the g-matrix orientation and anisotropic magnetic moment of the paramagnetic centre. (1)H hyperfine couplings were measured with W-band Davies ENDOR and X-band HYSCORE spectroscopy and fitted to a model in which (1)H dipolar couplings were calculated assuming point magnetic moments located at the Fe ions, and bridging and coordinating cysteine sulfur atoms. The absolute sign of a (1)H hyperfine coupling was measured using a variable mixing time ENDOR experiment to confirm the assignment of the Fe(3+) and Fe(2+) ions. For the anti-ferromagnetically coupled cluster the magnetic moment is described in terms of spin projection factors, and our analysis yields values of K(exp)(A) = +2.33 to +1.85 (ferric site), and K(exp)(B) = -1.33 to -0.85 (ferrous site). These values are discussed in terms of the delocalisation of the spin density and hence the limitations of applying a local site spin coupling model to calculate the spin projection factors in a complex with considerable overlap of the α- and β-spin magnetic oribitals. The accurate description of the g-matrix orientation and magnetic moment of this [2Fe-2S](+) cluster enable it to be utilised as a paramagnetic spin probe, for example, to measure electron-electron distances. In the pdb reference frame of PuxB (code ) the g(∥) axis vector is g(∥) = [-0.6524 ± 0.0248, -0.6269 ± 0.0115, 0.4259 ± 0.0405], with the principal g-values of g(⊥) = 1.9328 ± 0.0003, g(∥) = 2.0233 ± 0.0003.

3d−4f Combined Chemistry: Synthetic Strategies and Magnetic Properties

Heterometallic 3d-4f complexes are of high interest in molecular magnetism: the lanthanide ions bring large and, in most cases, anisotropic magnetic moments. The combination of 3d and 4f metal ions, which differ through their chemistries and stereochemical preferences, leads to a rich variety of heterometal complexes, ranging from discrete entities to 3-D coordination polymers. This paper reviews recent achievements in this field: (i) oligonuclear complexes for studying the nature of the 3d-4f exchange interaction; (ii) construction of single-molecule magnets; (iii) magnetic properties of 3d-4f coordination polymers.

Superconducting anisotropy in Nd 1.85Ce 0.15CuO 4 single crystals

The magnetic torque of the Nd 1.85Ce 0.15CuO 4 crystal consists of three contributions in the superconducting state, i.e., (1) a torque of the anisotropic magnetic moment of the Nd 3+ ions, (2) a torque coming from the mass difference between m ab and m c and (3) a torque of the vortex lock-in states appeared when the field is applied parallel to the CuO 2 planes. We analyze a torque component arising from the mass anisotropy by using the anisotropic London model and obtain an anisotropy parameter γ c =28±3. The irreversibility line of Nd 1.85Ce 0.15CuO 4 is in excellent accordance with that of the La-based cuprate.

Exchange Mechanisms in Ferrocinium Complexes

Abstract We examine kinetic and potential exchange in 1:1 ion-radical complexes of ferrocene (Fc) and π-acceptors like tetracyanoethylene (TCNE). Spin-orbit and asymmetry split the (e2g)3 configuration of Fc+ and lead to anisotropic magnetic moments. The partly-filled Fc+ and TCNE− orbitals are almost orthogonal and suggest alternative charge-transfer processes involving the acceptor and ligands.

Correlation of proton chemical shifts in proteins using two-dimensional exchange correlated spectroscopy

The full utilization of the information contained in protein NMR spectra requires the assignment of resonances to specific residues. Many one-dimensional and twodimensional NMR experiments can be used to give assignments to amino acid type (1-5) but, with the exception of relatively small proteins, assignments to specific residues can only be obtained by the use of chemical modification or from prior knowledge of the three-dimensional structure of the protein. In some cases, such considerations lead to a considerable simplification of the assignment procedure. A simplification is also possible where the study requires the protein to be compared in two or more different states, such as the native or nonnative states (6, 7) or the ligand-free and ligand-bound states (8). In such cases, provided the spectrum of one of the states has been previously characterized and there is exchange between the two states, the spectrum of the second state can be assigned. Tuna cytochrome c is a heme-containing electron transfer protein of 103 amino acids that may exist in either a diamagnetic oxidation state, ferrocytochrome c or a paramagnetic oxidation state, ferricytochrome c. The iron is low-spin in both oxidation states, and in ferricytochrome c it acts as a shift probe with a highly anisotropic magnetic moment. Consequently, the NMR spectra of ferricytochrome c and ferrocytochrome care very different (9). Because of the paramagnetism of ferricytochrome c, its ‘H NMR spectrum is relatively well resolved, and this has allowed resonances of approximately 45% of its carbon-bonded protons to be specifically assigned using a combination of one-dimensional NMR techniques and specific chemical modification (IO, 1Z). In contrast, the ‘H NMR spectrum of ferrocytochrome c has a considerably smaller chemical-shift dispersion, which allows fewer selective double-resonance experiments, and consequently there are fewer specific assignments available. In the present communication, making use of the assignments of ferricytochrome c, we demonstrate the utility of chemical exchange NMR to assign the corresponding resonances of ferrocytochrome c. An equimolar mixture of ferricytochrome c (CIII) and ferrocytochrome c (CII) undergoes electron exchange according to km *c111 + CII z=t *c11 + CIII. ill

Magnetic anisotropy of single crystals of rocksalttype uranium compounds

The magnetic properties of uranium pnictides and chalcogenides depend on the orientation of the applied field. As an example the anisotropic magnetic moments of USe (a ferromagnet) and the anisotropic susceptibility of UAs (an antiferromagnet) are discussed. Furthermore the existence of an intermediate ferrimagnetic spin structure of UAs is put into evidence.

Anisotropic magnetic susceptibilities for octahedral copper (II) salts with tetragonal distortion

The method of calculating molecular susceptibilities for tetragonally distorted octahedral copper (II) complexes using a model based on molecular orbitals is described. The method involves a number of parameters among which are orbital energy levels in copper, orbital reduction factor caused by formation of molecular orbitals, temperature and the spin orbit coupling parameter. The effect on the anisotropic magnetic moments when these are varied is demonstrated and discussed. The model is fitted to available anisotropic susceptibility data for K 2[Cu(H 2O) 6](SO 4) 2 and the best fit values for the orbital reduction parameters are derived through an error analysis.