Inelastic Spin Flip Research Articles

Nickelocene SPM tip as a molecular spin sensor.

Functionalization of a scanning microscopy probe with a single nickelocene allows reproducible
spin-sensitive measurements of magnetic systems on surfaces. The triplet ground state of the nicke-
locene tip gives rise to a characteristic inelastic electron spin-flip excitation, which can change upon
interaction with spin systems on the surface. These changes, together with theoretical simulations,
enable us to determine the local spin moment on the surface. In this paper, we discuss the exper-
imental and theoretical aspects of nickelocene-tip measurements. We rationalize the interactions
between the nickelocene spin and the magnetic centers using a spin Heisenberg and dipole model,
complemented by cotunneling theory, and compare the simulated dI/dV with selected experimental
results. We developed a Python script to simulate this magnetic interaction, which is available on
GitHub.

Engineering of Magnetic Coupling in Nanographene.

Nanographenes with sublattice imbalance host a net spin according to Lieb's theorem for bipartite lattices. Here, we report the on-surface synthesis of atomically precise nanographenes and their atomic-scale characterization on a gold substrate by using low-temperature noncontact atomic force microscopy and scanning tunneling spectroscopy. Our results clearly confirm individual nanographenes host a single spin of S=1/2 via the Kondo effect. In covalently linked nanographene dimers, two spins are antiferromagnetically coupled with each other as revealed by inelastic spin-flip excitation spectroscopy. The magnetic exchange interaction in dimers can be well engineered by tuning the local spin density distribution near the connection region, consistent with mean-field Hubbard model calculations. Our work clearly reveals the emergence of magnetism in nanographenes and provides an efficient way to further explore the carbon-based magnetism.

Simulation of inelastic spin flip excitations and Kondo effect in STM spectroscopy of magnetic molecules on metal substrates

Single-ion magnetic anisotropy in molecular magnets leads to spin flip excitations that can be measured by inelastic scanning tunneling microscope (STM) spectroscopy. Here I present a semi ab initio scheme to compute the spectral features associated with inelastic spin flip excitations and Kondo effect of single molecular magnets. To this end density functional theory calculations of the molecule on the substrate are combined with more sophisticated many-body techniques for solving the Anderson impurity problem of the spin-carrying orbitals of the magnetic molecule coupled to the rest of the system, containing a phenomenological magnetic anisotropy term. For calculating the STM spectra an exact expression for the in the ideal STM limit, when the coupling to the STM tip becomes negligibly small, is derived. In this limit the is simply related to the spectral function of the molecule–substrate system. For the case of an Fe porphyrin molecule on the Au(1 1 1) substrate, the calculated STM spectra are in good agreement with recently measured STM spectra, showing the typical step features at finite bias associated with spin flip excitation of a spin-1 quantum magnet. For the case of Kondo effect in Mn porphyrin on Au(1 1 1), the agreement with the experimental spectra is not as good due to the neglect of quantum interference in the tunneling.

Renormalization of single-ion magnetic anisotropy in the absence of the Kondo effect

Inelastic spin flip excitations associated with single-ion magnetic anisotropy of quantum spins, can be strongly renormalized by Kondo exchange coupling to the conduction electrons in the substrate, as shown recently for the case of Co adatoms on CuN$_2$ islands. In this case differential conductance spectra show zero-bias anomalies due to a Kondo effect of the doubly degenerate ground state, and finite-bias step features due to spin flip excitations. Here I consider spin-1 quantum magnets with positive uniaxial anisotropy, where the ground state is non-degenerate and hence the Kondo effect does not take place. Nevertheless the renormalization of inelastic spin excitations due to exchange coupling by hybridization of the quantum spin with the conduction electrons still takes place despite the complete absence of the Kondo effect in the ground state. Additionally, I show that away from particle-hole symmetry, charge fluctuations have a similar effect to Kondo exchange coupling, leading to the renormalization of spin flip excitations. However, in contrast to the renormalization by Kondo exchange, charge fluctuations lead to asymmetric spectra, which for strong charge fluctuations can mimic Fano behavior.

Controlled spin switching in a metallocene molecular junction

The active control of a molecular spin represents one of the main challenges in molecular spintronics. Up to now spin manipulation has been achieved through the modification of the molecular structure either by chemical doping or by external stimuli. However, the spin of a molecule adsorbed on a surface depends primarily on the interaction between its localized orbitals and the electronic states of the substrate. Here we change the effective spin of a single molecule by modifying the molecule/metal interface in a controlled way using a low-temperature scanning tunneling microscope. A nickelocene molecule reversibly switches from a spin 1 to 1/2 when varying the electrode–electrode distance from tunnel to contact regime. This switching is experimentally evidenced by inelastic and elastic spin-flip mechanisms observed in reproducible conductance measurements and understood using first principle calculations. Our work demonstrates the active control over the spin state of single molecule devices through interface manipulation.

Imaging single electron spin in a molecule trapped within a nanocavity of tunable dimension

Control of magnetism at the nanoscale is shown by a reversible transfer of an electron to and from a single molecule within the tunable gap of a scanning tunneling microscope. The addition of an electron to magnesium porphine changes the molecule from the diamagnetic state to the paramagnetic state. The existence of the single unpaired electron in the molecule is confirmed by spectroscopy and spatial imaging of the many body Kondo state and inelastic spin excitation between the Zeeman levels at 600 mK and up to 9 Tesla magnetic field. Here, we show that the spin is delocalized in an extended molecular orbital, in contrast to the spatially confined d and f states in atoms and magnetic centers in molecules. Furthermore, by tuning the dimension of the tunneling gap and visualizing the spectroscopic images, the inelastic spin-flip scatterings are shown to underlie the formation of the Kondo state.

Spin dynamics of current-driven single magnetic adatoms and molecules

A scanning tunneling microscope can probe the inelastic spin excitations of a single magnetic atom in a surface via spin-flip assisted tunneling in which transport electrons exchange spin and energy with the atomic spin. If the inelastic transport time, defined as the average time elapsed between two inelastic spin flip events, is shorter than the atom spin relaxation time, the STM current can drive the spin out of equilibrium. Here we model this process using rate equations and a model Hamiltonian that describes successfully spin flip assisted tunneling experiments, including a single Mn atom, a Mn dimer and Fe Phthalocyanine molecules. When the STM current is not spin polarized, the non-equilibrium spin dynamics of the magnetic atom results in non-monotonic $dI/dV$ curves. In the case of spin polarized STM current, the spin orientation of the magnetic atom can be controlled parallel or anti-parallel to the magnetic moment of the tip. Thus, spin polarized STM tips can be used both to probe and to control the magnetic moment of a single atom.

Tunnel magnetoresistance in epitaxially grown magnetic tunnel junctions using Heusler alloy electrode and MgO barrier

Epitaxially grown magnetic tunnel junctions (MTJs) with a stacking structure of Co2MnSi/MgO/CoFe were fabricated. Their tunnel magnetoresistance (TMR) effects were investigated. The TMR ratio and tunnelling conductance characteristics of MTJs were considerably different between those with an MgO barrier prepared using sputtering (SP-MTJ) and those prepared using EB evaporation (EB-MTJ). The EB-MTJ exhibited a very large TMR ratio of 217% at room temperature and 753% at 2 K. The bias voltage dependence of the tunnelling conductance in the parallel magnetic configuration for the EB-MTJ suggests that the observed large TMR ratio at RT results from the coherent tunnelling process through the crystalline MgO barrier. The tunnelling conductance in the anti-parallel magnetic configuration suggests that the large temperature dependence of the TMR ratio results from the inelastic spin–flip tunnelling process.

Spin-dependent transport through quantum dot with inelastic interactions

Using the Keldysh nonequilibrium Green function method, we theoretically investigate the electron transport properties of a quantum dot coupled to two ferromagnetic electrodes, with inelastic electron-phonon interaction and spin flip scattering present in the quantum dot. It is found that the electron-phonon interaction reduces the current, induces new satellite polaronic peaks in the differential conductance spectrum, and at the same time leads to oscillatory tunneling magnetoresistance effect. Spin flip scattering suppresses the zero-bias conductance peak and splits it into two, with different behaviors for parallel and anti-parallel magnetic configuration of the two electrodes. Consequently, a negative tunneling magnetoresistance effect may occur in the resonant tunneling region, with increasing spin flip scattering rate.

Measurements of Kondo and Spin Splitting in Single-Electron Transistors

We measure the spin splitting in a magnetic field B of localized states in single-electron transistors using a new method, inelastic spin-flip cotunneling. Because it involves only internal excitations, this technique gives the most precise value of the Zeeman energy Delta=/g/mu(B)B. In the same devices we also measure the splitting with B of the Kondo peak in differential conductance. The Kondo splitting appears only above a threshold field as predicted by theory. However, the magnitude of the Kondo splitting at high fields exceeds 2/g/mu(B)B in disagreement with theory.

Spin-resolved neutron spectroscopy from the heavy Fermion compound CeCu 6

Neutron time-of-flight spectroscopy with neutron polarisation analysis permits the in situ separation of magnetic and lattice vibrational energy spectra. Preliminary experiments on the heavy Fermion compound, CeCu 6, in which the Ce magnetic moment is suppressed by the Kondo effect, allow an indicative separation of a broadened crystal field transition and features due to lattice vibrations. An inelastic spin-flip feature at −12 meV is due to the crystal field while an inelastic non-spin-flip feature at −6 meV is predominantly due to phonon scattering.

The neutron peak in the extended saddle point model of high temperature superconductors

An explanation is proposed for the maximum at 41 meV in the inelastic spin-flip neutron scattering from YBa 2Cu 3O 7− δ , based on the extended saddle point model developed by the author in his previous works. It is shown that, for appearance of the maximum in the imaginary part of the spin susceptibility, close proximity of the Fermi energy to the extended saddle point is necessary. The energy of the maximum is then close to 2 Δ max, in agreement with experiment. Theoretical and experimental evidence concerning the energy of the extended saddle point (flat region) is discussed. Different limiting cases are calculated. A proof is given within the present model that interaction in the final state is small, and hence no collective modes are formed. A general discussion of the experimental situation is presented.

Neutron peak in the extended-saddle-point model of high-temperature superconductors

An explanation is proposed of the maximum at 41 meV in the inelastic spin-flip neutron scattering from YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, based on the extended-saddle-point model developed by the author in his previous works. It is shown that for appearance of the maximum in the imaginary part of the spin susceptibility a close proximity of the Fermi energy to the extended saddle point is necessary. The energy of the maximum is then close to 2{Delta}{sub max} in agreement with experiment. Theoretical and experimental evidence concerning the energy of the extended saddle point (flat region) is discussed. Different limiting cases are calculated. A proof is given within the present model that interaction in the final state is small, and hence, no collective modes are formed. A general discussion of the experimental situation is presented. {copyright} {ital 1998} {ital The American Physical Society}

Inelastic excitation and spin flip in heavy ion reactions

Coupled channel calculations were carried out for the inelastic scattering of $^{13}\mathrm{C}$ on $^{24}\mathrm{Mg}$ using form factors generated in a single folding model. We studied the population probability ${P}_{1}$ for $^{24}\mathrm{Mg}$ in the 1.37 MeV ${2}^{+}$ level with $|m|=1$ and $^{13}\mathrm{C}$ in its ground state. Using spin-independent potentials the excitation of various states in $^{13}\mathrm{C}$ gave rise to a maximum ${P}_{1}(6\ifmmode^\circ\else\textdegree\fi{})$ of 0.2%. This increased to 0.3% when spin-dependence was allowed for in an approximate manner. At 10\ifmmode^\circ\else\textdegree\fi{} the calculations fell significantly short of the experimental data. Large polarizations were found when excited states of $^{13}\mathrm{C}$ were populated; these effects are discussed.NUCLEAR REACTIONS $^{13}\mathrm{C}$+$^{24}\mathrm{Mg}$ inelastic scattering, $E=35$ MeV; coupled channel study of $|m|=1$ population probability for the $^{24}\mathrm{Mg}$ 1.37 MeV ${2}^{+}$ level and polarization phenomena.

Point contact spectroscopy of g-values in metals

It has previously been demonstrated that the I-V characteristic associated with current flow through a constriction whose dimension is less than the electron mean free path is nonlinear. The nonlinearity is largely associated with electron-phonon scattering. We have analyzed the case in which the contacts contain localized impurity spins. We show that, in the presence of an applied magnetic field, the inelastic spin-flip, scattering should be readily observable in the I-V characteristic, thereby providing an alternative technique for obtaining g values of localized spins in metals.

Microscopic magnetic properties of non-stoichiometric V 2O 3+x-NMR and inelastic spin-flip neutron scattering measurements

Microscopic magnetic properties of non-stoichiometric V 2O 3+x-NMR and inelastic spin-flip neutron scattering measurements

Microscopic magnetic properties of nonstoichiometric V 2O 3+x—NMR and inelastic spin-flip neutron scattering measurements

The local magnetic properties of the V sites in the nonstoichiometric V 2O 3+ x (0 ⩽ x <0.08) have been examined by nuclear magnetic resonance and inelastic spin-flip neutron scattering techniques. The samples with x = 0.01 and 0.02 show a paramagnetic metal (PM)-antiferromagnetic insulator (AFI) transition. In the AFI phase, two distinct 51V NMR signals with hyperfine fields H n = 184.9±0.5 kOe and 71±1 kOe were observed at 1.8 K, which were assigned as due to V 3+ and V 3+ sites, respectively. On the other hand, the samples with x = 0.04 and 0.06 were metallic down to 1.4K, and showed a paramagnetic (PM)-antiferromagnetic (AFM) transition at about 10 K. In these samples, a 51V NMR signal with H n = 58±2 k0 e and one with 〈 H n 〉 = 9 kOe were observed at 1.8 K, which were assigned as due to V 3+-like sites and the matrix V sites, respectively. These results are entirely consistent with those obtained from the neutron experiment. We propose that in the metallic phase (0.04 ⩽ x < 0.08) the minority V 4+-like sites are magnetically localized in the delocalized V matrix and may be responsible for the antiferromagnetic long range order below 10 K.

Investigation of the hyperfine fields in the compounds LaCo13, LaCo5, YCo5 and ThCo5 by means of inelastic neutron scattering

The Co hyperfine fields have been determined in the intermetallic compounds LaCo13, LaCo5, YCo5 and ThCo5 by means of inelastic spin flip scattering of neutrons. The relatively low hyperfine fields observed for the two Co sites in LaCo5, YCo5 and ThCo5 have been interpreted as resulting from a partial cancellation of the contributions of core polarization, orbital moments and conduction electron polarization.

Investigation of the hyperfine splitting of protons in TbH 1.9 by means of incoherent spin flip scattering of neutrons

For the first time, the hyperfine field at a proton was measured by means of inelastic spin flip scattering of neutrons. The temperature dependence of the hyperfine field at the proton in TbH 1.9 was determined. The measured values could be explained by the polarization of the conduction electrons.

Effect of complex coupling in inelastic cross sections, asymmetries and spin flip in the DWA

The effects of adding an effective imaginary part to real inelastic microscopic form factors in DWA are investigated. Calculations are compared to data for the proton differential cross sections and asymmetries of the lowest-lying states of 120Sn, 58Ni, and 208Pb. Proton spin flip is calculated for 120Sn. Both collective imaginary and effective microscopic imaginary contributions were considered. The most substantial improvements were in fits to asymmetry data. Collective-model calculations indicate the effects of complex coupling are comparable to those of the deformed spin-orbit well.