Showing papers on "Exchange interaction published in 2013"

Short-Range Quantum Magnetism of Ultracold Fermions in an Optical Lattice

Abstract: Quantum magnetism originates from the exchange coupling between quantum mechanical spins. Here, we report on the observation of nearest-neighbor magnetic correlations emerging in the many-body state of a thermalized Fermi gas in an optical lattice. The key to obtaining short-range magnetic order is a local redistribution of entropy, which allows temperatures below the exchange energy for a subset of lattice bonds. When loading a repulsively interacting gas into either dimerized or anisotropic simple cubic configurations of a tunable-geometry lattice, we observe an excess of singlets as compared with triplets consisting of two opposite spins. For the anisotropic lattice, the transverse spin correlator reveals antiferromagnetic correlations along one spatial axis. Our work facilitates addressing open problems in quantum magnetism through the use of quantum simulation.

Exchange couplings in magnetic films

Abstract: Recent advances in the study of exchange couplings in magnetic films are introduced. To provide a comprehensive understanding of exchange coupling, we have designed different bilayers, trilayers and multilayers, such as anisotropic hard-/soft-magnetic multilayer films, ferromagnetic/antiferromagnetic/ferromagnetic trilayers, [Pt/Co]/NiFe/NiO heterostructures, Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide (AAO) template. The exchange-coupling interaction between soft- and hard-magnetic phases, interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer, ferromagnetic layer, and antiferromagnetic layer. Some particular physical phenomena have been observed and explained.

Exchange bias up to room temperature in the antiferromagnetic bulk hexagonal Mn3Ge

Abstract: This work reports an exchange bias (EB) effect up to room temperature in the binary intermetallic bulk compound Mn3.04Ge0.96. The sample annealed at 700 K crystallizes in a tetragonal structure with ferromagnetic ordering, whereas, the sample annealed at 1073 K crystallizes in a hexagonal structure with antiferromagnetic ordering. The hexagonal Mn3.04Ge0.96 sample exhibits an EB of around 70 mT at 2 K that continues with a non-zero value up to room temperature. The exchange anisotropy is proposed to be originating from the exchange interaction between the triangular antiferromagnetic host and the embedded ferrimagnetic like clusters. The ferrimagnetic clusters develop when excess Mn atoms occupy empty Ge sites in the original triangular antiferromagnet structure of Mn3Ge.

Effect of the magnetic order on the room-temperature band-gap of Mn-doped ZnO thin films

Abstract: Exchange interaction between localized magnetic moments mediated by free charge carriers is responsible for a non-monotonic dependence of the low-temperature energy band-gap in dilute magnetic semiconductors. We found that in weakly doped Mn-ZnO films, increasing the exchange interaction by increasing the concentration of free charge carriers results in a red-shift of the near-band-edge emission peak at room temperature. An increase of Mn concentration widens the band gap, and a blue-shift prevails. Exchange interaction can be used to tune the room-temperature optical properties of the wide-band gap semiconductor ZnO.

Quantum spin-liquid behavior in the spin-1/2 random Heisenberg antiferromagnet on the triangular lattice

Abstract: Experimental quest for the hypothetical "quantum spin liquid" state has recently met a few promising candidate materials including organic salts \kappa-(ET)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, S=1/2 triangular-lattice Heisenberg antiferromagnets consisting of molecular dimers. These compounds exhibit no magnetic ordering nor the spin freezing down to very low temperature, while various physical quantities exhibit gapless behaviors. Recent dielectric measurements revealed the glassy dielectric response suggesting the random freezing of the electric polarization degrees of freedom. Inspired by this observation, we propose as a minimal model of the observed quantum spin-liquid behavior the S=1/2 antiferromagnetic Heisenberg on the triangular lattice with a quenched randomness in the exchange interaction. We study both zero- and finite-temperature properties of the model by an exact diagonalization method, to find that when the randomness exceeds a critical value the model exhibits a quantum spin-liquid ground state. This randomness-induced quantum spin-liquid state exhibits gapless behaviors including the temperature-linear specific heat. The results provide a consistent explanation of the recent experimental observations on organic salts.

Interfacial ferromagnetism in LaNiO3/CaMnO3 superlattices.

Abstract: We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.

Magnetic blocking from exchange interactions: slow relaxation of the magnetization and hysteresis loop observed in a dysprosium-nitronyl nitroxide chain compound with an antiferromagnetic ground state.

Abstract: The combination of the ani- sotropic Dy III ion and organic radicals as spin carriers results in discrete and one-dimensional lanthanide-radical magnetic materials, namely, (Dy- A3(NITThienPh)2 )( 1) and (Dy2- A6(NITThienPh)2)n (2; hfac = hexa- fluoroacetylacetonate, NITThienPh = 2- (5-phenyl-2-thienyl)-4,4,5,5-tetrameth- yl-imidazoline-1-oxyl-3-oxide). Linking monomeric 1 with the Dy III ion leads to the formation of polymeric 2, and the transformation between them is chemically controllable and reversible. The characterization of both static and dynamic magnetic properties shows that the dominant intrachain exchange interaction is important to observe magnetic bistability in 2 rather than that in 1. Monomeric 1 exhibits para- magnetic behavior, whereas polymeric 2 shows the unusual coexistence of su- perparamagnetic and two-step field-in- duced metamagnetic behaviors. The antiferromagnetic ground state of 2 does not prevent the dynamic relaxa- tion of the magnetization with the finite-sized effect in the lanthanide- radical system. Energy barriers to ther- mally activated relaxation for 2 are 53 and 98 K in the low- and high-tempera- ture regimes, respectively. A hysteresis loop is observed with the coercive field of 99 Oe at 2 K.

Probing the spin pumping mechanism: exchange coupling with exponential decay in Y3Fe5O12/barrier/Pt heterostructures.

Abstract: It is widely believed that the mechanism for spin pumping in ferromagnet-nonmagnet bilayers is the exchange interaction between the ferromagnet and nonmagnetic material. We observe 1000-fold exponential decay of spin pumping from thin Y3Fe5O12 films to Pt across insulating barriers, from which exponential decay lengths of 0.16, 0.19, and 0.23 nm are extracted for oxide barriers having band gaps of 4.91, 3.40, and 2.36 eV, respectively. This archetypal signature of quantum tunneling through a barrier underscores the importance of exchange coupling for spin pumping and reveals its dependence on the characteristics of the barrier material.

Exchange bias driven by Dzyaloshinskii-Moriya interactions

Abstract: The exchange bias effect in a compensated IrMn3/Co(111) system is studied using multiscale modeling from ab initio to atomistic spin model calculations. We evaluate numerically the out-of-plane hysteresis loops of the bilayer for different thicknesses of the ferromagnetic layer. The results show the existence of a perpendicular exchange bias and an enhancement of the coercivity of the system. To identify the origin of the exchange bias, we analyze the hysteresis loops of a selected bilayer by tuning the different contributions to the exchange interaction across the interface. Our results indicate that the exchange bias is primarily induced by Dzyaloshinskii-Moriya interactions, while the coercivity is increased mainly due to a spin-flop mechanism.

Meson Spectrum in Strong Magnetic Fields

Abstract: We study the relativistic quark-antiquark system embedded in magnetic field. The Hamiltonian containing confinement, one gluon exchange and spin-spin interaction is derived. We analytically follow the evolution of the lowest meson states as a functions of MF strength. Calculating the one gluon exchange interaction energy and spin-spin contribution we have observed, that these corrections remain finite at large magnetic fields, preventing the vanishing of the total rho-meson mass at some B_crit, as previously thought. We display the rho masses as functions of magnetic field in comparison with recent lattice data.

Dimerized Solids and Resonating Plaquette Order in SU(N)-Dirac Fermions

Abstract: We study the quantum phases of fermions with an explicit SU(N)-symmetric, Heisenberg-like nearest-neighbor flavor exchange interaction on the honeycomb lattice at half filling. Employing projective (zero temperature) quantum Monte Carlo simulations for even values of N, we explore the evolution from a weak-coupling semimetal into the strong-coupling, insulating regime. Furthermore, we compare our numerical results to a saddle-point approximation in the large-N limit. From the large-N regime down to the SU(6) case, the insulating state is found to be a columnar valence bond crystal, with a direct transition to the semimetal at weak, finite coupling, in agreement with the mean-field result in the large-N limit. At SU(4) however, the insulator exhibits a subtly different valence bond crystal structure, stabilized by resonating valence bond plaquettes. In the SU(2) limit, our results support a direct transition between the semimetal and an antiferromagnetic insulator.

Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates

Abstract: One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N $K$ x-ray absorption (XA) and Fe ${L}_{2,3}$ x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state ($S=2$) from an intermediate spin state ($S=1$) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

Origin of Selective Guest-Induced Magnetism Transition in Fe/MOF-74

Abstract: The experimentally observed selective guest-induced (olefin-induced) magnetism transition from ferromagnetism (FM) to antiferromagnetism (AFM) on Fe/MOF-74 is investigated using the first principles calculations. We reveal that FM in bare MOF is due to direct exchange interaction between neighboring Fe ions while the olefin-adsorbed MOF prefers AFM through the superexchange interaction. We find that geometrical changes (such as Fe–Fe distance and Fe–O–Fe angle) associated with strong interaction of olefin π-bonds with open metal sites in MOF framework define magnetic coupling while electronic effects such as charge transfer and orbital interaction play a minor role. Additionally, the weak interchain magnetic coupling whose magnitude is smaller than the intrachain coupling by an order of magnitude can be switched by introducing a paramagnetic ligand between metal atoms. This work offers the origin of magnetism in MOFs with open metal sites and its subtle alteration due to selective guest binding that can b...

Memory effects on the magnetic behavior of assemblies of nanoparticles with ferromagnetic core/antiferromagnetic shell morphology

Abstract: Monte Carlo simulations of the dynamic magnetic behavior of an assembly of ferromagnetic core/antiferromagnetic shell nanoparticles are reported and compared with the experimental results on a system of Co nanoparticles in Mn matrix. Memory effects on low-field zero-field-cooled magnetization curves have been investigated. Our simulations show that the memory effects increase with the concentration and that both the interface exchange coupling and the dipolar interparticle interactions contribute to the observed dynamic behavior. In particular the interface exchange interaction provides an additive source for the frustration of the system resulting in an enhancement of the memory effect. The numerical data reproduce well the experimental results confirming the glassy behavior of the investigated nanoparticle systems.

Spin resolution and evidence for superexchange on NiO(001) observed by force microscopy.

Abstract: The spin order of the nickel oxide (001) surface is resolved, employing noncontact atomic force microscopy at 4.4 K using bulk Fe and SmCo tips mounted on a qPlus sensor that oscillates at sub-50 pm amplitudes. The spin-dependent signal is hardly detectable with Fe tips. In contrast, SmCo tips yield a height contrast of 1.35 pm for Ni ions with opposite spins. SmCo tips even show a small height contrast on the O atoms of 0.5 pm within the 2×1 spin unit cell, pointing to the observation of superexchange. We attribute the increased signal-to-noise ratio to the increased magnetocrystalline anisotropy energy of SmCo, which stabilizes the magnetic moment at the apex. Atomic force spectroscopy on the Ni↑, Ni↓, and O lattice site reveals a magnitude of the exchange energy of merely 1 meV at the closest accessible distance with an exponential decay length of λexc=18 pm.

Effective-field and Monte Carlo studies of mixed spin-2 and spin-1/2 Ising diamond chain

Abstract: The magnetic properties of a mixed spin-2 and spin-1/2 ferromagnetic diamond chain are studied by effective-field theory and Monte Carlo (MC) simulation based on the Ising model. The temperature dependences of magnetization, magnetic susceptibility, internal energy, and specific heat are studied, respectively. The exchange interaction dependences of magnetization and the critical temperature are calculated by MC simulation. The changes of magnetization depending on the field increasing and then the field decreasing under steady-static conditions are also given.

The electronic structure of organic-inorganic hybrid compounds : (NH4)(2)CuCl4, (CH3NH3)(2)CuCl4 and (C2H5NH3)(2)CuCl4

Abstract: Hybrid organic-inorganic compounds are an intriguing class of materials that have been experimentally studied over the past few years because of a potential broad range of applications. The electronic and magnetic properties of three organic-inorganic hybrid compounds with compositions (NH4)2CuCl4, (CH3NH3)2CuCl4 and (C2H5NH3)2CuCl4 are investigated for the first time with density functional theory plus on-site Coulomb interaction. A strong Coulomb interaction on the copper causes a relatively weak exchange coupling within the layers of the octahedral network, in good agreement with experiment. The character of the exchange interaction (responsible for magnetic behavior) is analyzed. The calculations reveal that (C2H5NH3)2CuCl4 has the strongest Jahn-Teller (JT) distortion in comparison with the two other compounds. The easy axis of magnetization is investigated, showing a weak anisotropic interaction between inter-layer Cu(2+) ions in the (C2H5NH3)2CuCl4 structure. Orbital ordering is concluded from our partial density of states calculations: a cooperation of the JT distortion with an antiferro-distortive pattern.

Two-qubit gates for resonant exchange qubits.

Abstract: A new approach to single-qubit operations using exchange interactions of single electrons in gate-defined quantum dots has recently been demonstrated: the resonant exchange qubit. We show that two-qubit operations, specifically the controlled phase gate, can be performed between resonant exchange qubits very straightforwardly, using a single exchange pulse. This is in marked contrast to the best known protocols for exchange qubits where such a gate requires many pulses so that leakage processes arising from the exchange interaction can be overcome. For resonant exchange qubits a simple two-qubit gate is possible because in this mode of operation energy conservation suppresses leakage.

Spin interactions in mesons in strong magnetic field

Abstract: Spin interactions in relativistic quark-antiquark system in magnetic field is considered in the framework of the relativistic Hamiltonian, derived from the QCD path integral. The formalism allows to separate spin-dependent terms from the basic spin-independent interaction, contained in the Wilson loop, and producing confining and gluon exchange interaction. As a result one obtains relativistic spin-spin interaction $V_{ss}$, generalizing its nonrelativistic analog. It is shown, that in large magnetic field $eB, V_{ss}$ modifies and produces hyperfine shifts which grow linearly with $eB$ and preclude the use of perturbation theory. We also show, that tensor forces for $eB eq 0$ are active in all meson states, but do not grow with $eB$

Interfacial exchange coupling in Fe-Tb/[Co/Pt] heterostructures

Abstract: The exchange interaction and magnetization reversal processes in exchange-biased heterostructures consisting of amorphous ferrimagnetic Fe-Tb alloy films and ferromagnetic Co/Pt multilayers were investigated. Both film systems exhibit an easy axis magnetization perpendicular to the film plane. The dependence of the interfacial exchange coupling on the stoichiometry of the Fe-Tb layer as well as the number of repetitions in the Co/Pt multilayers were analyzed. The net magnetization and effective magnetic anisotropy of the Fe-Tb alloy films have an influence on the exchange energy per unit area. A large exchange-bias field up to 8 kOe is found to be accompanied by an interfacial domain wall as probed by element specific x-ray magnetic circular dichroism absorption measurements. This domain wall exhibits a total thickness between 3 and 4 nm and affects strongly the magnetization reversal in the heterostructure. Furthermore, a linear relation between the thickness of the ferromagnetic Co/Pt multilayers and the exchange-bias field is observed and can be attributed to a change of the location where the interfacial domain wall nucleates in the heterostructure.

Phase diagrams in a ultra-thin transverse Ising film with bond or site dilution atsurfaces

Abstract: The phase diagrams and magnetizations of two nanoscaled thin films with bond and site dilutions at the surfaces, described by the transverse Ising model, are investigated by the use of the effective field theory with correlations. Some characteristic phenomena have been found in them; The transition temperature (TC) of bond dilution is always larger than the corresponding one of site dilution. There exists some region in the phase diagram where the longitudinal magnetization mT for the site dilution is always given by mT=0.0 in the whole temperature region, while the mT for the bond dilution takes a finite value below its Tc. Even in a uniformly applied transverse field, the appearance of a broad maximum in a system with site dilution can be also found, when the variation of TC is plotted as a function of r (r=J1/J, where J is the exchange interaction in inner layer and J1 is the exchange interaction between the surface and the next inner layer). But, such a phenomenon cannot be obtained in the system with bond dilution.

First-principles prediction of magnetic superatoms in 4d-transition-metal-doped magnesium clusters.

Abstract: We theoretically predict magnetic superatoms in the 4d-transition-metal-doped Mg8 clusters using a spin-polarized density functional theory method. We demonstrate that TcMg8 is highly energetically stable in both structure and magnetic states, and identify it as a magnetic superatom with a magnetic moment as large as 5 μB. The magnetic TcMg8 with 23 valence electrons has a configuration of 1S(2)1P(6)1D(10) closed shell and 2S(1)2D(4) open shell, complying with Hund's rule similar to the single atom. We elucidate the formation mechanism of the magnetic TcMg8 superatom based on the detailed analysis of molecular orbitals, and attribute it to the large exchange interaction and moderate crystal field effect. Finally, we predict that the magnetic TcMg8 may exhibit semiconductor-like property with spin polarization characteristics.

Dynamical Splayed Ferromagnetic Ground State in the Quantum Spin Ice Yb2Sn2O7

Abstract: From magnetic, specific heat, 170Yb Mossbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb2Sn2O7, we show that below the first order transition at 0.15 K all of the Yb3+ ions are long-range magnetically ordered and each has a moment of 1.1?B which lies at ?10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [ Phys. Rev. Lett. 108 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.