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The electronic structure and magnetism of individual Dy atom adsorbed on the graphene/Ir(1 1 1) surface is investigated using the combination of the density functional theory with the Hubbard-I approximation to the Anderson impurity model (DFT + HIA). The divalent Dy 2 + adatom is found, with the magnetic moment of 9.4–9.6 μ B , depending on the placement of the graphene on the Ir(1 1 1) surface, in an external magnetic field. The spin and orbital magnetic moments are evaluated, and compared with the X-ray magnetic circular dichroism (XMCD) data. The positive magnetic anisotropy energy determines the out-of-plane orientation of the Dy adatom magnetic moment. Without an external magnetic field, the ground state is a doublet | J = 8 , J z = ± 7.9 〉 , symmetry protected from quantum tunnelling of the magnetization. Calculations show that insertion of the graphene layer promotes reduction of the hybridization between Dy- 4 f -states and the Ir(1 1 1) substrate, and leads to increase of the magnetic moment relaxation time for the Dy adatom.

Correlated band theory implemented as a combination of density functional theory with exact diagonalization [$\mathrm{DFT}+U$(ED)] of the Anderson impurity term with Coulomb repulsion $U$ in the open 14-orbital $5f$ shell is applied to ${\mathrm{UTe}}_{2}$. The small gap for $U=0$, evidence of the half-filled $j=\frac{5}{2}$ subshell of $5{f}^{3}$ uranium, is converted for $U=3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ to a flat-band semimetal with small heavy-carrier Fermi surfaces that will make properties sensitive to pressure, magnetic field, and off-stoichiometry, as observed experimentally. Two means of identification from the Green's function give a mass enhancement of the order of 12 for already heavy (flat) bands, consistent with the common heavy-fermion characterization of ${\mathrm{UTe}}_{2}$. The predicted Kondo temperature around 100 K matches the experimental values from resistivity. The electric field gradients for the two Te sites are calculated by $\mathrm{DFT}+U$(ED) to differ by a factor of 7, indicating a strong site distinction, while the anisotropy factor $\ensuremath{\eta}=0.18$ is similar for all three sites. The calculated uranium moment ${\ensuremath{\langle}{M}^{2}\ensuremath{\rangle}}^{1/2}$ of $3.5\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ is roughly consistent with the published experimental Curie-Weiss values of 2.8 and $3.3\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ (which are field-direction dependent), and the calculated separate spin and orbital moments are remarkably similar to Hund's rule values for an ${f}^{3}$ ion. The $U=3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ spectral density is compared with angle-integrated and angle-resolved photoemission spectra, with agreement that there is strong $5f$ character at, and for several hundred meV below, the Fermi energy. Our results support the picture that the underlying ground state of ${\mathrm{UTe}}_{2}$ is that of a half-filled $j=\frac{5}{2}$ subshell with two half-filled ${m}_{j}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ orbitals forming a narrow gap by hybridization and then driven to a conducting state by configuration mixing (spin-charge fluctuations). ${\mathrm{UTe}}_{2}$ displays similarities to ${\mathrm{UPt}}_{3}$ with its $5f$-dominated Fermi surfaces rather than a strongly localized Kondo lattice system.

The electronic structure and magnetism of individual Dy atoms adsorbed on the graphene/Cu(111) surface is investigated using the combination of the density functional theory with the Hubbard-I approximation to the Anderson impurity model ($\mathrm{DFT}+\mathrm{U}+\mathrm{HIA}$). We find that the results of the $\mathrm{DFT}+\mathrm{U}+\mathrm{HIA}$ depend on the choice of the double-counting term. For fully localized limit, the divalent ${\mathrm{Dy}}^{2+}$ adatom is found, with the total magnetic moment of $9.71\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}$. The spin and orbital magnetic moments are evaluated, and compared with the x-ray magnetic circular dichroism data. The calculated positive magnetic anisotropy energy determines the out-of-plane orientation of the Dy adatom magnetic moment, in agreement with available experimental data.

A new neptunium intermetallic compound, NpPt 2 In 7 , has been synthesized in polycrystalline form and characterized by several macroscopic techniques. A Rietveld analysis of its powder X-ray diffraction pattern shows that NpPt 2 In 7 crystallizes in a tetragonal lattice with I 4/ mmm symmetry and lattice parameters a = 4.58471 ( 3 ) A, c = 21.5065 ( 3 ) A. Magnetic susceptibility, electrical resistivity, Hall effect, and heat capacity measurements indicate a metallic character and the occurrence of antiferromagnetic order below a Neel temperature T N = 23 K. The transition is exceptionally robust and T N decreases by 0.2 K under a magnetic field of 9 T. A modified Curie-Weiss fit of the high-temperature magnetic susceptibility curve χ ( T ) gives an effective magnetic moment μ e f f = 2.54 μ B close to the value expected for trivalent Np. Low temperature heat capacity measurements give a reduced Sommerfeld linear coefficient close to 25 mJ mol −1 K −2 and a Debye Temperature Θ D = 181 K. First principles, correlated-band electronic structure calculations suggest that the neptunium magnetic moment in NpPt 2 In 7 is localized and that a quasi-two-dimensional antiferromagnetic structure could result from the competition of very weak interlayer interactions leading to very anisotropic properties.

We address a recent controversy concerning the magnetic state of holmium adatom on platinum surface. Within a combination of the density functional theory (DFT) with the exact diagonalization (ED) of Anderson impurity model, the 〈J z 〉 = 0 paramagnetic ground state |J = 8, J z = ±8〉 is found. In an external magnetic field, this state is transformed to a spin-polarized state with 〈J z 〉 ≈ 6.7. We emphasize the role of 5d–4f interorbital exchange polarization in modification of the 4f shell energy spectrum.

The orthorhombic uranium dichalcogenide ${\mathrm{UTe}}_{2}$ displays superconductivity below 1.7 K, with the anomalous feature of retaining 50% of normal state (ungapped) carriers, according to heat capacity data from two groups. Incoherent transport that crosses over from above 50 K toward a low-temperature, Kondo lattice Fermi liquid regime indicates strong magnetic fluctuations and the need to include correlation effects in theoretical modeling. We report density functional theory plus Hubbard $U$ ($\mathrm{DFT}+U$) results for ${\mathrm{UTe}}_{2}$ to provide a platform for modeling its unusual behavior, focusing on ferromagnetic (FM, time-reversal breaking) long-range correlations along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{a}$ axis as established by magnetization measurements and confirmed by our calculations. States near the Fermi level are dominated by the $j=\frac{5}{2}$ configuration, with the ${j}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ sectors being effectively degenerate and half-filled. Unlike the small-gap insulating nonmagnetic electronic spectrum, the FM Fermi surfaces are large (strongly metallic) and display low-dimensional features, reminiscent of the FM superconductor ${\mathrm{UGe}}_{2}$.

The electronic and magnetic character, and the magnetic anisotropy in practically important magnetic materials containing the rare earth elements are investigated making use of the correlated band theory implemented as a combination of the relativistic density functional theory with the Anderson impurity model (DFT+U+HIA). First, we demonstrate that DFT+U+HIA provides a good description of the electronic structure, spin and orbital magnetic moments, and the magnetic anisotropy for SmCo5 magnet with 4f5-like Sm3+ shell. Further, the method is applied to theoretical investigation of Dy-doped Bi2Te3 topological insulator. For both ferro- and anti-ferromagnetic Dy-planes in Bi2Te3 we found the in-gap flat f-bands located at the top of the valence band of Bi2Te3. The positive uniaxial MAE is predicted for (DyxBi1−x)2Te3 with x = 0.33. The experimental resonant photoemission spectra are well reproduced by the calculations. These studies can be important to explore the potential use of rare-earth doped topological insulators in the low-power spintronic devises.

The electronic structure of PuB6, an actinide analog of SmB6 , was investigated making use of a combination of the density functional theory (DFT), and the exact diagonalization (ED) of an effective discrete Anderson impurity model. Intermediate valence ground state with the f-shell occupation n4f =5.5 for the Pu atom in PuB6 is calculated. The 5f-shell magnetic moment is completely compensated by the moment carried by the electrons in the conduction band. Already in DFT, PuB6 is an insulator with a small amount of holes near the X-point, and the indirect band gap of ≈60 meV. This band gap becomes direct in DFT+ED calculations supporting the idea of “topological Kondo insulator” in PuB6. Connection between the electronic structure of PuB6 and δ-Pu is established. We propose that these materials belong to a new class of intermediate valence “Racah” materials with the multi-orbital “Kondo-like” singlet ground-state.

A simple scheme for avoiding non-spherical double counting in the combination of density func- tional theory with dynamical mean-field theory (DFT+DMFT)is developed. It is applied to total- energy calculations and structural optimization of the pnictide superconductor LaFeAsO. The results are compared to a recently proposed "exact" double-counting formulation. Both schemes bring the optimized Fe-As interatomic distance close to the experimental value. This resolves the long stand- ing controversy between DFT+DMFT and experiment for the structural optimization of LaFeAsO., 4 pages 2 figures

Antiferromagnetic hexagonal MnTe is a promising material for spintronic devices relying on the control of antiferromagnetic domain orientations. Here we report on neutron diffraction, magnetotransport, and magnetometry experiments on semiconducting epitaxial MnTe thin films together with density functional theory (DFT) calculations of the magnetic anisotropies. The easy axes of the magnetic moments within the hexagonal basal plane are determined to be along $\left$ directions. The spin-flop transition and concomitant repopulation of domains in strong magnetic fields is observed. Using epitaxially induced strain the onset of the spin-flop transition changes from $\sim2$~T to $\sim0.5$~T for films grown on InP and SrF$_2$ substrates, respectively., 9 pages, 7 figures

We measure x-ray magnetic circular dichroism (XMCD) spectra at the Pu M_{4,5} absorption edges from a newly prepared high-quality single crystal of the heavy-fermion superconductor ^{242}PuCoGa_{5}, exhibiting a critical temperature T_{c}=18.7 K. The experiment probes the vortex phase below T_{c} and shows that an external magnetic field induces a Pu 5f magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a superconducting quantum interference device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa_{5} have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5f electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the 5f magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5f moment.

The ferromagnetic superconductor URhGe has been investigated by high field magnetic circular dichroism (XMCD) at the U ${M}_{4,5}$, Rh ${L}_{2,3}$, and Ge $K$ edges at 2.1 K and at applied fields up to 17 T. The XMCD performed at the ${M}_{4,5}$ absorption edges allows us to determine the spectroscopic branching ratio and the $5f$ electron contribution to the valence spin-orbit interaction. Combination with polarized neutron diffraction results allows us to derive the individual $\mathrm{U}$ orbital and spin moments and the magnetic-dipole contribution $\ensuremath{\langle}{T}_{z}\ensuremath{\rangle}$. There is no evidence for any change of the orbital-to-spin moment ratios across the spin reorientation transition at ${H}_{R}=12\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, when the field is applied along the initial hard $b$ axis. We also confirm that the magnetism of URhGe is dominated by $\mathrm{U}$, with the contribution of Rh representing only about $10%$ of the macroscopic moment. The orbital and spin moments at the Rh site are found to be parallel to each other and parallel to the macroscopic magnetization, but an unexpectedly large orbital-to-spin moment ratio is observed. The XMCD at the Ge $K$ edge reveals the presence of a small induced Ge $4p$ orbital moment, parallel to the macroscopic magnetization. The results are discussed against predictions of the electronic band structure calculations by the density functional theory plus Coulomb $U$, including spin-orbit coupling $(\mathrm{DFT}+U+\mathrm{SOC})$.

Electronic structure calculations combining the local density approximation with an exact diagonalization of the Anderson impurity model show intermediate 5f5–5f6-valence ground state and delocalization of the 5f5 multiplet of the Pu atom 5f-shell in PuCoGa5, and δ-Pu. The 5f-local magnetic moment is compensated by a moment formed in the surrounding cloud of conduction electrons. For PuCoGa5 and δ-Pu the compensation is complete and the Anderson impurity ground state is a singlet. This can be important for analyses of the superconducting pairing mechanism in PuCoGa5. It is likely that in PuCoGa5 unconventional d-wave superconductivity is mediated by the 5f-states valence fluctuations, rather than antiferromagnetic fluctuations.

The magnetic phase transitions for Gd(0 0 0 1) grown on W(1 1 0) - a bulk transition at 293 K and a surface transition about 85 K above this - are found to influence the energy separation of the Gd 5s and 4s core-photoelectron doublets. The 5s doublet separation ¿E5s changes over a range of temperatures spanning these transitions, and decreases by a maximum of 60 meV in this region, but then recovers its original value; the 4s doublet shows a smaller change in the reverse direction, which does not recover at high temperature. Some of these effects are semi-quantitatively understood from free-atom multiplet theory and from theoretical calculations based on all-electron LDA+U calculations including 4f electron correlation effects. However, the high-temperature behavior of the data also suggest a dynamical nature to these effects via spin-dependent electron screening that is influenced by magnetic fluctuations. Several avenues for studying such effects in a time-resolved manner in future experiments are discussed., Support was provided by the Director, Office of Energy Research, Mat. Sci. Div., of the U.S. Dept. of Energy, under Contracts No. DE-AC03-76SF00098 and DE-AC02-05CH11231, and the DGICyT (Grant No. PB94-0022-C02-02) MEC, Spain. Work at UC Davis (W.E.P.) was supported in part by DOE grant No. DE-FG03-03NA00071.

The electronic structure of selected rare-earth atoms adsorbed on a free-standing graphene was investigated using methods beyond the conventional density functional theory (DFT+U, DFT+HIA and DFT+ED). The influence of the electron correlations and the spin-orbit coupling on the magnetic properties has been examined. The DFT+U method predicts both atoms to carry local magnetic moments (spin and orbital) contrary to a nonmagnetic $f^6$ ($J=0$) ground-state configuration of Sm in the gas phase. Application of DFT${}+{}$Hubbard-I (HIA) and DFT${}+{}$exact diagonalization (ED) methods cures this problem, and yields a nonmagnetic ground state with six $f$ electrons and $J=0$ for the Sm adatom. Our calculations show that Nd adatom remains magnetic, with four localized $f$ electrons and $J=4.0$. These conclusions could be verified by STM and XAS experiments., Comment: REVTeX 4-1, 8 pages, 4 figures

Rare-earth-based compounds R2T17 (R=Rare earth; T=Transition metal) have been extensively studied and developed for applications as permanent magnets. The actinide-based analogues, however, are much less documented and we report here about the magnetic properties of Np2Co17 and Np2Ni17, as inferred from 237Np Mossbauer spectroscopy, the best resonance in actinides, and specific heat.

Mn(2)Au, a layered bimetal, is successfully grown using molecular beam epitaxy (MBE). The experiments and theoretical calculations presented suggest that Mn(2)Au film is antiferromagnetic with a very low critical temperature. The antiferromagnetic nature is demonstrated by measuring the exchange-bias effect of Mn(2)Au/Fe bilayers. This study establishes a primary basis for further research of this new antiferromagnet in spin-electronic device applications.

In the conventional Ohmic regime, magnetoresistance effects comprise the ordinary responses to the external magnetic field and extraordinary responses to the internal magnetization. Here we study magnetoresistance effects in the Coulomb blockade regime using a ferromagnetic (Ga, Mn)As single electron transistor. We report measurements of the magneto-Coulomb blockade effect due to the direct coupling of high external magnetic fields and the Coulomb blockade anisotropic magnetoresistance associated with magnetization rotations in the ferromagnet. The latter, extraordinary magnetoresistance effect is characterized by low-field hysteretic magnetoresistance which can exceed three orders of magnitude. The sign and size of this magnetoresistance signal is controlled by the gate voltage, and the data are interpreted in terms of anisotropic electrochemical shifts induced by magnetization reorientations. Non-volatile transistor-like applications of the Coulomb blockade anisotropic magnetoresistance are briefly discussed.

We report computational investigations of the electronic structures of the superconducting Pu-compounds PuCoGa5 as well as of the non-superconducting compounds PuFeGa5 and PuNiGa5. To capture the localization behavior of the Pu 5f electrons, we apply two computational approaches, which are both rooted in the density-functional theory: the local spin-density approximation (LSDA) and the around mean field (AMF) LSDA + U approach. The latter is applicable to moderately localized 5f electrons while the former is applicable to delocalized 5f electrons. Our LSDA calculations show that the Fermi surfaces of the three Pu-115 compounds are sensitive to the amount of band filling, i.e., the number of electrons of the 3d element. Precisely at the electron filling corresponding to PuCoGa5 the Fermi surface has a particularly two-dimensional shape. AMF–LSDA + U calculations (with a Coulomb U of about 3 eV and exchange J of 0.6 eV) lead to a non-magnetic ground state for PuCoGa5, in which the 5f states are shifted to a higher binding energy, in better agreement with photoemission data. The Fermi surface of PuCoGa5 computed with the AMF–LSDA + U approach is nonetheless rather two-dimensional and similar to the LSDA Fermi surface. The AMF–LSDA + U approach with a Coulomb U of ≈ 3 eV would thus predict an electronic structure for PuCoGa5 in accord with several experimental data.

The possibility of the 5f count variations and its impact on magnetic properties are discussed on the background of known data on diluted alloys, intermetallic compounds and monopnictides and monochalcogenides. Weakly paramagnetic character of δ-Pu is preserved when Pu is doped by Am, which expands the crystal lattice. Specific-heat data show that the γ -coefficient does not increase. Such behaviour can be understood on the basis of LSDA + U calculations, which yield invariably non-magnetic Pu state due to the proximity to the 5f 6 state. It is also shown that the ground-state density of states should not be directly compared with photoelectron spectra, because the spectral density obtained from Hubbard-I calculations is in the narrow band regime of Pu dramatically different than the underlying density of states. We suggest that a general condition for Pu magnetism is some degree of hybridization with states of other elements in alloys and compounds, which allows for the reduction of the 5f count.

We observe low-field hysteretic magnetoresistance (MR) in a (Ga,Mn)As single electron transistor which can exceed three orders of magnitude. The sign and size of the MR signal is controlled by the gate voltage. Experimental data are interpreted in terms of electrochemical shifts associated with magnetization rotations. This Coulomb blockade anisotropic MR is distinct from previously observed anisotropic MR effects as it occurs when the anisotropy in a band structure derived parameter is comparable to an independent scale, the single electron charging energy. Effective kinetic-exchange model calculations in (Ga,Mn)As show chemical potential anisotropies consistent with experiment and ab initio calculations in transition-metal systems suggest that this generic effect persists to high temperatures in metal ferromagnets with strong spin–orbit coupling. In addition we observe a significant gate voltage dependence of the magnetic switching field which we attribute to carrier density variations in the electric field exposed GaMnAs regions causing a substantial change in the magneto-crystalline anisotropy profile.

The current theoretical understanding of the electronic structures of the CeMIn 5 compounds (with M=Co, Rh, and Ir) and PuCoGa 5 and PuRhGa 5 is reviewed. Both the Ce-115 and Pu-115 compounds have in common that their f-electrons appear to be poised on the edge of a localization–delocalization boundary. We consider here the implications of this characteristic on the emergent electronic structure, addressing, in particular, the computed Fermi surfaces. In order to capture the electronic structures to the best as possible, we employ three different density-functional theory based computational approaches, viz. the local-spin density approximation (LSDA) for delocalized f-states, the open-core approach for localized f's, and the LSDA+ U approach for moderately localized f's. The Fermi surfaces of CeCoIn 5 and CeIrIn 5 are rather well described assuming delocalized 4f electrons. Conversely, the better description is obtained for CeRhIn 5 assuming a localized 4f state. For the Pu-115 compounds, LSDA calculations predict a rather two-dimensional Fermi surface. However, the Fermi surface of PuCoGa 5 computed with the around-mean field LSDA+ U scheme is also quite two-dimensional and, moreover, similar to the LSDA Fermi surface. Taking other available experimental data on PuCoGa 5 into account, it appears that the electronic structure of PuCoGa 5 is best described by the AMF-LSDA+ U approach with a Coulomb U of about 3 eV.

We report on the magnetic properties of Fe and Co adatoms on a Cu2N/Cu(100)-c(2 x 2) surface investigated by x-ray magnetic dichroism measurements and density functional theory (DFT) calculations including the local coulomb interaction. We compare these results with properties formerly deduced from STM spin excitation spectroscopy (SES) performed on the individual adatoms. In particular we focus on the values of the local magnetic moments determined by XMCD compared to the expectation values derived from the description of the SES data. The angular dependence of the projected magnetic moments along the magnetic field, as measured by XMCD, can be understood on the basis of the SES Hamiltonian. In agreement with DFT, the XMCD measurements show large orbital contributions to the total magnetic moment for both magnetic adatoms.

The local-density approximation combined with the dynamical mean-field theory (LDA+DMFT) is applied to the paramagnetic phase of light actinide dioxides: UO2, NpO2, and PuO2. The calculated band gaps and the valence-band electronic structure are in a very good agreement with the optical absorption experiments as well as with the photoemission spectra. The hybridization of the actinide 5f shell with the 2p states of oxygen is found to be relatively large, it increases the filling of the 5f orbitals from the nominal ionic configurations with two, three, and four electrons to nearly half-integer values 2.5, 3.4 and 4.4. The large hybridization leaves an imprint also on the core-level photoemission spectra in the form of satellite peaks. It is demonstrated that these satellites are accurately reproduced by the LDA+DMFT calculations., REVTeX 4-1, 10 pages, 6 figures

The magnetism of FeRh (001) films strongly depends on film thickness and surface terminations. While the magnetic ground state of bulk FeRh is $G$-type antiferromagnetism, the Rh-terminated films exhibit ferromagnetism with strong perpendicular magnetocrystalline anisotropy whose energy $+2.1$ meV/\ensuremath{\square} is two orders of magnitude greater than bulk $3d$ conventional magnetic metals (\ensuremath{\square} is the area of a two-dimensional unit cell). While the Goodenough-Kanamori-Anderson rule on the superexchange interaction is crucial in determining the magnetic ground phases of FeRh bulk and thin films, the magnetic phases are the results of interplay and competition between three mechanisms---the superexchange interaction, the Zener-type direct interaction, and energy gain by Rh magnetization.

We present the results obtained by superconducting quantum interference device (SQUID) magnetometry, specific heat, and M\"ossbauer spectroscopy measurements carried out on ${\mathrm{Np}}_{2}{\mathrm{Ni}}_{17}$ polycrystalline samples. We show that long-range magnetic order, with a moment ${\ensuremath{\mu}}_{(2b)}\ensuremath{\sim}2.25$ ${\ensuremath{\mu}}_{B}$, occurs below ${\mathrm{T}}_{N}=17.5$ K on the Np $(2b)$ sites. A nontrivial situation is observed in that the other Np sites $(2d)$ do not take part to the order transition and carry only an induced moment of about 0.2 ${\ensuremath{\mu}}_{B}$ below ${\mathrm{T}}_{N}$. A combined analysis of the different experimental data sets allowed us to determine key parameters associated with the electronic structure of the system. The experimental results are discussed against first-principles electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction.

We have measured the valence-band densities of states (DOSs) in buried CoFeBandCoFe layers in a magnetic tunnel junction using a novel extension of arecently developed standing wave/wedge soft x-ray photoemission technique.The CoFe DOS at the Fermi level is substantially enhanced when the CoFethickness is reduced from 25 to 15 A. This enhancement, which we suggest˚is due to the amorphous character of the CoFe when 20 Ainthic˚ kness andresults in a spin-polarized peak in the DOS of primarily Co origin, can bedirectly correlated to marked improvementsin magnetic transport and switchingproperties. This technique for studying buried-layer DOSs should also beapplicable to other multilayer nanostructures.(Some figures in this article are in colour only in the electronic version)Magnetic tunnel junctions (MTJs) have received considerable attention due to promisingapplications as magnetic sensors, read heads, and magnetic random access memory(MRAM) [1]. While much effort has been focused on materials research so as to obtainhigher tunnelling magnetoresistance (TMR), the physics of the magnetic tunnelling mechanismandhow it is influenced by various materials choices is by no means fully understood. Forexample, Parkin

The electronic structures of several Pu-115 compounds are investigated computationally, using ab initio density-functional-theory based calculations. We focus here on aspects related to the modification of the transition metal element ( M ) in the Pu M Ga 5 compounds, considering specifically PuFeGa 5 , PuCoGa 5 and PuNiGa 5 . Our calculations show that a peculiarity of the Pu-115 compounds is the energy position of the transition-metal d -states, which are retracted mainly from the Fermi energy, while Pu 5 f states are located at and in the vicinity of the Fermi energy. As a consequence, modifications of the transition metal element, as in the series PuFeGa 5 , PuCoGa 5 and PuNiGa 5 , lead to hole or electron doping, respectively, on the Pu site. The narrow band of 5 f states at the Fermi level is thus sensitively modified through the 5 f band filling, and hence, qualitatively different Fermi surfaces result.

First principles calculations are used to investigate a monatomic Co chain at the Pt(111) surface step edge. We optimize the system geometry minimizing the atomic forces and evaluate the magnetocrystalline anisotropy energy (MAE). The easy magnetization axis is found to align perpendicular to the Co-wire at an angle of ≈ 50° (A-step) and ≈ 30° (B-step) towards the Pt step and the calculated MAE of 2.8meV (A-step) and 4.45 meV (B-step) are in good agreement with experiment. Our calculations show that the structural relaxation improves the quantitative description of the MAE.

We report a first-principles investigation of the anomalous magnetic anisotropy of quasi-one-dimensional ferromagnetic Fe and Co chains decorating the Pt(1 1 1) step edge. Our calculations for both Fe and Co wires show that the easy magnetization is always confined in the plane perpendicular to the wire. The symmetry breaking at the step leads to an easy magnetization axis of the Co wire at an odd angle of ∼ 20 ∘ towards the Pt step, in agreement with the experiment. This canting becomes more pronounced for the Fe wire, for which we obtain an easy axis at 30 ∘ towards the Pt step. Also, the Fe and Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement.

The around-the-mean-field version of the LDA+U method is applied to investigate electron correlation effects in $\delta$-Pu. It yields a non-magnetic ground state of $\delta-$Pu, and provides a good agreement with experimental equilibrium volume, bulk modulus and explains important features of the photoelectron spectra.

The magnetic and electronic properties of NpCoGe are reported. NpCoGe orders antiferromagnetically at T N ≈ 13 K with an average ordered magnetic moment 〈 µN p > = 0.80 µ B . The comparison with NpRhGe and uranium analogues suggests the leading role of f-d hybridization, the rather delocalized character of 5f electrons in NpCoGe and its possible proximity to a magnetic quantum critical point.

Ni/Cu(001) is known as a unique system showing the spin-reorientation transition from an in-plane to out-of-plane magnetization direction when the Ni-overlayer thickness is increased. We investigate different relaxed multilayer structures with a hydrogen adlayer using the full-potential linearized augmented plane-wave method. The relaxed geometries, determined by total energy and atomic force calculations, show that H-monolayer strongly influences the interlayer distance between the Ni-surface and sub-surface layers yielding the outward relaxation of Ni-layer at H/Ni interface. Furthermore, large decrease of local magnetic moments at the top surface area is found for the surface covered by H. The magneto-crystalline anisotropy energies are calculated for fully relaxed H/Ni-films. The spin-reorientation transition critical thickness of 4 ML is found in good quantitative agreement with the experiment.

We address the long-standing mystery of the nonmagnetic insulating state of the intermediate valence compound SmB6. Within a combination of the local density approximation (LDA) and an exact diagonalization (ED) of an effective discrete Anderson impurity model, the intermediate valence ground state with the f-shell occupation 〈n4f〉 = 5.6 is found for the Sm atom in SmB6. This ground state is a singlet and the first excited triplet state ~3 meV higher in the energy. SmB6 is a narrow band insulator already in LDA, with the direct band gap of ~10 meV. The electron correlations increase the band gap which now becomes indirect. Thus, the many-body effects are relevant to form the indirect band gap, crucial for the idea of “topological Kondo insulator" in SmB6. Also, an actinide analog PuB6 is considered and the intermediate valence singlet ground state is found for the Pu atom. We propose that [Sm, Pu]B6 belong to a new class of the intermediate valence materials with the multi-orbital “Kondo-like" singlet ground-state. Crucial role of complex spin-orbital f n–f n+1 multiplet structure differently hybridized with ligand states in such Racah materials is discussed.

Two new transuranium metal boron carbides, NpBC and PuBC, have been synthesized. Rietveld refinements of powder XRD patterns of {Np,Pu}BC confirmed in both cases isotypism with the structure type of UBC. Temperature dependent magnetic susceptibility data reveal antiferromagnetic ordering for PuBC below TN = 44 K, whereas ferromagnetic ordering was found for NpBC below TC = 61 K. Heat capacity measurements prove the bulk character of the observed magnetic transition for both compounds. The total energy electronic band structure calculations support formation of the ferromagnetic ground state for NpBC and the antiferromagnetic ground state for PuBC.

The correlated-band theory implemented as a combination of the local-density approximation with the dynamical mean-field theory is applied to PuO2. An insulating electronic structure, consistent with the experimental valence-band photoemission spectra, is obtained. The calculations yield a nonmagnetic ground state that is characterized by a noninteger filling of the plutonium 5f shell. The noninteger filling as well as the satellites appearing in the 4f core-level photoemission spectra originate in a sizable hybridization of the 5f shell with the 2p states of oxygen., contribution to proceedings of SCES 2014, 6 pages, 3 figures

The recently proposed concept of a Hund's metal - a metal in which electron correlations are driven by Hund's rule coupling - can be used to explain the exotic magnetic and electronic behaviour of strongly correlated electron systems of multi-orbital metallic materials. Tuning the abundance of parameters that determine these materials is, however, experimentally challenging. Here, we show that the basic constituent of a Hund's metal - a Hund's impurity - can be realized using a single iron atom adsorbed on a platinum surface, a system that comprises a magnetic moment in the presence of strong charge fluctuations. The magnetic properties can be controlled by using the tip of a scanning tunnelling microscope to change the binding site and degree of hydrogenation of the 3d transition-metal atom. We are able to experimentally explore a regime of four almost degenerate energy scales (Zeeman energy, temperature, Kondo temperature and magnetic anisotropy) and probe the magnetic excitations with the microscope tip. The regime of our Hund's impurity can be tuned from an emergent magnetic moment to a multi-orbital Kondo state, and the system could be used to test predictions of advanced many-body theories for non-Fermi liquids in quantum magnets or unconventional superconductors. © 2015 Macmillan Publishers Limited. All rights reserved.

The electronic structure, magnetic moments, effective exchange interaction parameter and the magnetic anisotropy energy of [monolayer Co]/Ir(1 1 1) and Co intercalated graphene on Ir(1 1 1) are studied making use of the first-principles density functional theory calculations. A large positive magnetic anisotropy of 1.24 meV/Co is found for [monolayer Co]/Ir(1 1 1), and a high Curie temperature of 1190 K is estimated. These findings show the Co/Ir(1 1 1) system is a promising candidate for perpendicular ultra-high density magnetic recording applications. The magnetic moments, exchange interactions and the magnetic anisotropy are strongly affected by graphene. Reduction of the magnetic anisotropy and the Curie temperature are found for graphene/[monolayer Co]/Ir(1 1 1). It is shown that for graphene placed in the hollow-hexagonal positions over the monolayer Co, the magnetic anisotropy remains positive, while for the placements with one of the C atoms on the top of Co it becomes negative. These findings may be important for assessing the use of graphene for magnetic recording and magnetoelectronic applications.

NpCoGe, the neptunium analogue of the ferromagnetic superconductor UCoGe, has been investigated by dc-magnetization, ac-susceptibility, specific heat, electrical resistivity, Hall effect, 237Np Moessbauer spectroscopy and LSDA calculations. NpCoGe exhibits an antiferromagnetic ground state with a Neel temperature TN = 13 K and an average ordered magnetic moment = 0.80 mB. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (around 3 T). The value of the isomer shift suggests a Np3+ charge state (configuration 5f4). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol-1K-2) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the c-axis. Moessbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogues 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., Comment: 38 pages, 14 figures

The effects of electron correlation on the electronic structure and spin and orbital magnetic properties of the f.c.c. Ce were investigated using the relativistic (with spin–orbit coupling included) full-potential linearized augmented plane wave implementation of correlated band theory (‘LDA+U’). Multiple energy minima of the LDA+U energy functional are obtained for γ-Ce. The lowest energy solution leads to a fully spin polarized 4f state and the lattice constant of γ-Ce. The higher energy local minima (additional self-consistent solutions) are shown to be strongly indicative of low-energy excitations.

Effects of electron correlation on the electronic structure and magnetic properties of the Gd(0001) surface are investigated using of the full-potential linearized augmented plane wave implementation of correlated band theory ("LDA+U"). The use of LDA+U instead of LDA (local density approximation) total energy calculations produces the correct ferromagnetic ground state for both bulk Gd and the Gd surface. Surface strain relaxation leads to an 90 % enhancement of the interlayer surface-to-bulk effective exchange coupling. Application of a Landau-Ginzburg type theory yields a 30 % enhancement of the Curie temperature at the surface, in very good agreement with the experiment., Comment: revised version: minor typos corrected

The ruthenocuprate RuSr2GdCu2O8 orders magnetically at 135 K and then becomes superconducting at 45 K. These transitions have been observed by several groups, but the intrinsically complex nature of the compound, as well as the unexpected coexistence of magnetism with high-temperature superconductivity, makes it uncertain what the magnetic, electronic, and structural character of its ground state is. We find that density functional-based evaluations give a magnetic and structural character that is consistent with the latest data: the RuO6 octahedra rotate (by 7° around the c^ axis) causing a \(\sqrt 2 \times \sqrt 2 \) doubling of the cell, and an antiferromagnetic structure that has this same cell doubling is favored over a ferromagnetic alignment of Ru spins. The minority Ru dxy states are partially occupied, leaving a metallic RuO2 layer that dopes the CuO2 bilayer and may lead to canting of the Ru spins.

Analysis of available photoelectron spectra from Pu compounds indicates the presence of the 5f5 multiplet, which dominates the energy scale close to the Fermi level. As shown by the LDA+U+Hubbard I calculations, its intensity scales with the admixture of the 5f6 manifold in the 5f5 ground state, which can lead to an average occupancy substantially exceeding 5.0. This admixture also leads to the suppression of magnetic order and occurrence of heavy fermion behaviour.

Thin layers of PuCoGax (x = 4 to 18) have been prepared by dc sputtering from a PuCoGa5 single crystal target, and investigated in situ by X-ray and ultraviolet photoelectron spectroscopy. We could achieve broad composition variability (monitored by the Pu-4f, Co-2p and Ga-2p core-level spectra). The results are compared to the valence band spectra reported previously for PuCoGa5. Our experiments reveal that some Ga excess (PuCoGa≈7) was likely for those original data. We demonstrate that there is a tendency to the segregation of Ga at the surface, which has an important effect on the valence band spectra.

Relying on the inhomogeneous (layered) crystal, electronic, and magnetic structure, we show how superconductivity can coexist with the ferromagnetic phase of RuSr2GdCu2O8 as observed by Tallon and coworkers. Since the Cu d_{x^2-y^2} orbitals couple only to apical O p_x, p_y orbitals (and only weakly), which also couple only weakly to the magnetic Ru t_{2g} orbitals, there is sufficiently weak exchange splitting, especially of the symmetric CuO2 bilayer Fermi surface, to allow singlet pairing. The exchange splitting is calculated to be large enough that the superconducting order parameter may be of the Fulde-Ferrell-Larkin-Ovchinnikov type. We also note that \pi-phase formation is preferred by the magnetic characteristics of RuSr2GdCu2O8.

We performed first-principles relativistic full-potential linearized augmented plane-wave calculations for strained tetragonal ferromagnetic $\mathrm{La}(\mathrm{Ba}){\mathrm{MnO}}_{3}$ with an assumed experimental structure of thin strained tetragonal ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ (LCMO) films grown on ${\mathrm{SrTiO}}_{3}[001]$ and ${\mathrm{LaAlO}}_{3}[001]$ substrates. The calculated uniaxial magnetic anisotropy energy (MAE) values are in good quantitative agreement with experiments for LCMO films on ${\mathrm{SrTiO}}_{3}$ substrate. We also analyze the applicability of linear magnetoelastic theory for describing the stain dependence of MAE, and estimate magnetostriction coefficient ${\ensuremath{\lambda}}_{001}.$