Showing papers on "Exchange interaction published in 2012"

Probing the timescale of the exchange interaction in a ferromagnetic alloy.

Abstract: The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.

Generic quantum spin ice

Abstract: We consider possible exotic ground states of quantum spin ice as realized in rare earth pyrochlores. Prior work [Savary and Balents, Phys. Rev. Lett. 108, 037202 (2012).] introduced a gauge mean-field theory (gMFT) to treat spin or pseudospin Hamiltonians for such systems, reformulated as a problem of bosonic spinons coupled to a $U(1)$ gauge field. We extend gMFT to treat the most general nearest-neighbor exchange Hamiltonian, which contains a further exchange interaction. This term leads to interactions between spinons and requires a significant extension of gMFT, which we provide. As an application, we focus especially on the non-Kramers materials Pr${}_{2}T{M}_{2}$O${}_{7}$ ($TM=$ Sn, Zr, Hf, and Ir), for which the additional term is especially important, but for which an Ising-planar exchange coupling discussed previously is forbidden by time-reversal symmetry. In this case, when the planar $\mathit{XY}$ exchange is unfrustrated, we perform a full analysis and find three quantum ground states: a $U(1)$ quantum spin liquid (QSL), an antiferroquadrupolar ordered state and a noncoplanar ferroquadrupolar ordered one. We also consider the case of frustrated $\mathit{XY}$ exchange, and find that it favors a $\ensuremath{\pi}$-flux QSL, with an emergent line degeneracy of low-energy spinon excitations. This feature greatly enhances the stability of the QSL with respect to classical ordering.

Theoretical study of exchange coupling in 3d-Gd complexes: large magnetocaloric effect systems.

Abstract: Polynuclear 3d transition metal-Gd complexes are good candidates to present large magnetocaloric effect. This effect is favored by the presence of weak ferromagnetic exchange interactions that have been investigated using methods based on Density Functional Theory. The first part of the study is devoted to dinuclear complexes, focusing on the nature and mechanism of such exchange interactions. The presence of two bridging ligands is found more favorable for ferromagnetic coupling than a triple-bridged assembly, especially for complexes with small M–O···O–Gd hinge angles. Our results show the crucial role of the Gd 5d orbitals in the exchange interaction while the 6s orbital seems to have a negligible participation. The analysis of the atomic and orbital spin populations reveals that the presence of spin density in the Gd 5d orbital is mainly due to a spin polarization effect, while a delocalization mechanism from the 3d orbitals of the transition metal can be ruled out. We propose a numerical DFT approach...

Nanographene Magnetic Properties: A Monte Carlo Study

Abstract: The magnetic properties of an Ising antiferromagnetic model on a nanographene lattice which have spins that can take the values S=±3/2, ±1/2, are studied by Monte Carlo simulations. We only consider the nearest-neighbor interactions between the sites i and j. The zero field cooled and field cooled magnetizations, magnetic susceptibilities and hysteresis cycle are obtained for a nanographene structure with the effect of exchange interaction between sites i and j. The blocking temperature is also obtained for specific values of the external magnetic field.

Tuning magnetic properties of graphene nanoribbons with topological line defects: From antiferromagnetic to ferromagnetic

Abstract: Zigzag-edged graphene nanoribbons are antiferromagnetic in cross-edge coupling and unsuitable for spintronics applications. Two new strategies of tuning antiferromagnetism (AFM) to ferromagnetism (FM) in graphene nanoribbons are introduced through topological line defects composed of pentagonal and octagonal rings, and their ability to induce magnetic transition is probed by using density functional theory. The resulting exchange energy is found to be large enough for ferromagnetism to be observed at room temperature. Both strategies are experimentally feasible, and the results suggest that defect engineering may provide a novel path to manipulate the magnetic properties of graphene nanoribbons.

Universal quantum computing with spin and valley states

Abstract: We investigate a two-electron double quantum dot with both spin and valley degrees of freedom as they occur in graphene, carbon nanotubes or silicon and regard the 16-dimensional space with one electron per dot as a four-qubit logic space. In the spin-only case, it is well known that the exchange coupling between the dots combined with arbitrary single-qubit operations is sufficient for universal quantum computation. The presence of valley degeneracy in the electronic band structure alters the form of the exchange coupling and, in general, leads to spin-valley entanglement. Here, we show that universal quantum computation can still be performed by exchange interaction and single- qubit gates in the presence of an additional (valley) degree of freedom. We present an explicit pulse sequence for a spin-only controlled-NOT consisting of the generalized exchange coupling and single-electron spin and valley rotations. We also propose state preparations and projective measurements with the use of adiabatic transitions between states with (1,1) and (0,2) charge distributions similar to the spin-only case, but with the additional requirement of controlling the spin and valley Zeeman energies by an external magnetic field. Finally, we demonstrate a universal two-qubit gate between a spin and a valley qubit, allowing universal gate operations on the combined spin and valley quantum register.

Detection of single-electron heat transfer statistics

Abstract: We consider a quantum dot system whose charge fluctuations are monitored by a quantum point contact allowing for the detection of both charge and transferred heat statistics. Our system consists of two nearby conductors that exchange energy via Coulomb interaction. In interfaces consisting of capacitively coupled quantum dots, energy transfer is discrete and can be measured by charge counting statistics. We investigate gate-dependent deviations away from a charge fluctuation theorem in the presence of local temperature gradients (hot spots). Non-universal relations are found for state-dependent charge counting. A fluctuation theorem holds for coupled dot configurations with heat exchange and no net particle flow.

The role of 4f-electron on spin reorientation transition of NdFeO3: A first principle study

Abstract: Magnetic properties and electronic structures of NdFeO3 have been studied by performing accurate first principle calculation based on density functional theory The 4f-electrons of Nd are explicitly treated as valence electrons The simulation results of crystal structure and magnetic structure of this compound agree well with the experimental observations Importantly, our study indicates that the spin reorientation transition of Fe3+ spin sublattice can be ascribed to the exchange interaction between Nd-4f and Fe-3d electrons, which are mediated by O2-2p state in Fe-O plane As the temperature decreases, the Fe-O and Nd-O bonds become more covalent, and the exchange interactions become stronger

Universal Quantum Computing with Spin and Valley

Abstract: We investigate a two-electron double quantum dot with both spin and valley degrees of freedom as they occur in graphene, carbon nanotubes, or silicon, and regard the 16-dimensional space with one electron per dot as a four-qubit logic space. In the spin-only case, it is well known that the exchange coupling between the dots combined with arbitrary single-qubit operations is sufficient for universal quantum computation. The presence of the valley degeneracy in the electronic band structure alters the form of the exchange coupling and in general leads to spin-valley entanglement. Here, we show that universal quantum computation can still be performed by exchange interaction and single-qubit gates in the presence of the additional (valley) degree of freedom. We present an explicit pulse sequence for a spin-only controlled-NOT consisting of the generalized exchange coupling and single-electron spin and valley rotations. We also propose state preparations and projective measurements with the use of adiabatic transitions between states with (1,1) and (0,2) charge distributions similar to the spin-only case, but with the additional requirement of controlling the spin and the valley Zeeman energies by an external magnetic field. Finally, we demonstrate a universal two-qubit gate between a spin and a valley qubit, allowing universal gate operations on the combined spin and valley quantum register.

Trends in the magnetic properties of Fe, Co, and Ni clusters and monolayers on Ir(111), Pt(111), and Au(111)

Abstract: We present a detailed theoretical investigation on the magnetic properties of small single-layered Fe, Co, and Ni clusters deposited on Ir(111), Pt(111), and Au(111). For this, a fully relativistic ab initio scheme based on density functional theory has been used. We analyze the element, size, and geometry specific variations of the atomic magnetic moments and their mutual exchange interactions as well as the magnetic anisotropy energy in these systems. Our results show that the atomic spin magnetic moments in the Fe and Co clusters decrease almost linearly with increasing coordination number on all three substrates, while the corresponding orbital magnetic moments appear to be much more sensitive to the local atomic environment. The isotropic exchange interaction among the cluster atoms is always very strong for Fe and Co exceeding the values for bulk bcc Fe and hcp Co, whereas the anisotropic Dzyaloshinski-Moriya interaction is, in general, one or two orders of magnitude smaller when compared to the isotropic one. For the magnetic properties of Ni clusters, the magnetic properties can show quite a different behavior, and we find in this case a strong tendency towards noncollinear magnetism.

Quantum phase transition from superparamagnetic to quantum superparamagnetic state in ultrasmall Cd(1-x)Cr(II)(x)Se quantum dots?

Abstract: Despite a long history of success in formation of transition-metal-doped quantum dots (QDs), the origin of magnetism in diluted magnetic semiconductors (DMSs) is yet a controversial issue. Cr(II)-doped II-VI DMSs are half-metallic, resulting in high-temperature ferromagnetism. The magnetic properties reflect a strong p-d exchange interaction between the spin-up Cr(II) t(2g) level and the Se 4p. In this study, ultrasmall (~3.1 nm) Cr(II)-doped CdSe DMSQDs are shown to exhibit room-temperature ferromagnetism, as expected from theoretical arguments. Surprisingly, a low-temperature phase transition is observed at 20 K that is believed to reflect the onset of long-range ordering of the single-domain DMSQD.

Electrically controlled magnetization in ferromagnet-topological insulator heterostructures

Abstract: Electrostatic control of magnetic anisotropy is proposed by utilizing a hybrid structure of a topological insulator (TI) and insulating ferromagnet. The concept relies on the exchange interaction at the interface and the subsequent change in the free energy of the combined system. Calculations illustrate that the spin-momentum interlocked nature of TI surface electrons can induce the out-of-plane magnetic anisotropy of the system. Moreover, it is shown that modulation of its strength via surface chemical potential can achieve magnetization rotation between the in-plane and out-of-plane directions with a characteristic time of nanoseconds. A deterministic 180${}^{\ensuremath{\circ}}$ turn is further hypothesized with the aid of a small TI surface current with natural spin polarization.

Exchange-biased magnetic tunnel junctions with antiferromagnetic ε-Mn3Ga

Abstract: Oriented c-axis films of the hexagonal triangular antiferromagnetic e-Mn3Ga have been used in bottom-pinned synthetic antiferromagnet magnetic tunnel junctions with MgO barriers, which show up to 150% tunneling magnetoresistance at room temperature. Exchange bias fields as high as 150 mT can be achieved for samples field-cooled from 100 °C. Thin films of the antiferromagnet have a Neel temperature in excess of 650 K and provide an interface exchange energy with CoFe of 0.09 mJ m−2. They show an isotropic uncompensated magnetization of Ms = 48 kA m−1, with a coercivity μ0Hc > 3 T.

Detection of single-electron heat transfer statistics

Abstract: We consider a quantum dot system whose charge fluctuations are monitored by a quantum point contact allowing for the detection of both charge and transferred heat statistics. Our system consists of two nearby conductors that exchange energy via Coulomb interaction. In interfaces consisting of capacitively coupled quantum dots, energy transfer is discrete and can be measured by charge counting statistics. We investigate gate dependent deviations away from a charge fluctuation theorem in the presence of local temperature gradients (hot spots). Non universal relations are found for state dependent charge counting. A fluctuation theorem holds for coupled dot configurations with heat exchange and no net particle flow.

The effect of distributed exchange parameters on magnetocaloric refrigeration capacity in amorphous and nanocomposite materials

Abstract: The temperature dependent magnetization of nanocomposite alloys has been fit with a modified Handrich-Kobe equation with an asymmetric exchange fluctuation parameter combined with the Arrott-Noakes equation. The two equations of state are combined to calculate the entropy change in the magnetocaloric effect associated with the ferromagnetic to paramagnetic phase transformation. The complete fit for the M(T) of (Fe70Ni30)88Zr7B4Cu nanocomposite powder is accomplished by combining the two theories. We investigate the broadening of the second-order transition arising from asymmetric exchange parameters and resulting from the fluctuations of interatomic spacing found in an amorphous matrix and the asymmetric dependence of exchange energy on interatomic spacing. The magnetic entropy curve revealed extra broadening with a refrigeration capacity (RC) value of 135 J/kg at 5 T, which is comparable to (Fe76Cr8-xMoxCu1B15) ribbons, which have a RC value of 180 J/kg for the same applied field. Broadening of the magne...

Tunable doping of a metal with molecular spins

Abstract: The mutual interaction of localized magnetic moments and their interplay with itinerant conduction electrons in a solid are central to many phenomena in condensed-matter physics, including magnetic ordering and related many-body phenomena such as the Kondo effect(1), the Ruderman-Kittel-Kasuya-Yoshida interaction(2) and carrier-induced ferromagnetism in diluted magnetic semiconductors(3). The strength and relative importance of these spin phenomena are determined by the magnitude and sign of the exchange interaction between the localized magnetic moments and also by the mean distance between them. Detailed studies of such systems require the ability to tune the mean distance between the localized magnetic moments, which is equivalent to being able to control the concentration of magnetic impurities in the host material. Here, we present a method for doping a gold film with localized magnetic moments that involves depositing a monolayer of a metal terpyridine complex onto the film. The metal ions in the complexes can be cobalt or zinc, and the concentration of magnetic impurities in the gold film can be controlled by varying the relative amounts of cobalt complexes (which carry a spin) and zinc complexes (which have zero spin). Kondo and weak localization measurements demonstrate that the magnetic impurity concentration can be systematically varied up to similar to 800 ppm without any sign of inter-impurity interaction. Moreover, we find no evidence for the unwanted clustering that is often produced when using alternative methods.

Spin liquid correlations, anisotropic exchange, and symmetry breaking in Tb 2 Ti 2 O 7

Abstract: We have studied the low-energy spin dynamics between 4.6 and 0.07 K in a Tb${}_{2}$Ti${}_{2}$O${}_{7}$ single-crystal sample by means of inelastic neutron scattering experiments. The spectra consist in a dual response, with a static and an inelastic contribution, showing striking $Q$ dependencies. We propose an interpretation involving an anisotropic exchange interaction in combination with a breaking of the threefold symmetry at the rare earth site. Simulations of the $Q$-dependent scattering in the random phase approximation account well for the inelastic response.

Anion driven modulation of magnetic intermolecular interactions and spin crossover properties in an isomorphous series of mononuclear iron(III) complexes with a hexadentate Schiff base ligand

Abstract: A series of spin crossover iron(III) complexes with the general composition [Fe(4OH-L6)]X (H2-4OH-L6 = 1,8-bis(4-hydroxysalicylaldiminato)-3,6-diazaoctane; X = Cl, 1a; Br, 1b; I, 1c) was prepared. A combination of the results following the single crystal X-ray analysis, infrared and EPR spectroscopy, and temperature dependent magnetic experiments revealed that the Fe(III) atoms occur in the low-spin state below room temperature and the crystal structures of the complexes involve rich networks of non-covalent intermolecular contacts resulting in two-dimensional supramolecular structures. Alterations in the halide anions influence the strength of the non-covalent contacts and affect the magnetic properties of the studied complexes. The antiferromagnetic exchange interaction between the non-covalently bound cations is the most obvious in the case of 1a and it weakens with the growing anionic volume of X. The 1D and 2D spin Hamiltonian models were applied to quantitatively extract the information about the intermolecular magnetic exchange (fit on 1D infinite chain gives J(1a) = −2.86 cm−1, J(1b) = −2.02 cm−1, J(1c) = −1.16 cm−1). Furthermore, gradual spin crossover behaviour for all of the compounds of the series was observed above room temperature in the solid state. Spin crossover accompanied by thermochromism was also demonstrated by EPR experiments in solution.

Quantum confinement-controlled exchange coupling in manganese(II)-doped CdSe two-dimensional quantum well nanoribbons.

Abstract: The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to ∼13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications.

Biradical Paradox Revisited Quantitatively: A Theoretical Model for Self-Associated Biradical Molecules as Antiferromagnetically Exchange Coupled Spin Chains in Solution B

Abstract: An ESR hyperfine splitting pattern of a biradical in solution depends on the magnitude of the intramolecular exchange interaction Jᵢₙₜᵣₐ compared with the hyperfine coupling constant A. Some biradicals exhibit their hyperfine splitting patterns characteristic of a monoradical, even though their exchange interaction is strong enough, |Jᵢₙₜᵣₐ| ≫ |A|. The contradiction in ESR spectroscopy is known as “biradical paradox”, puzzling scientists for a long time. In this study, it is shown from ESR spectral simulations underlain by a theoretical model of a series of spin Hamiltonians that noncovalent aggregation of biradical molecules in solution leads to the appearance of paradoxical ESR spectra. Most of the spins in an aggregate of one dimension lose their contribution to the ESR spectra owing to intermolecular antiferromagnetic interactions Jᵢₙₜₑᵣ, leaving two outermost spins ESR-active in the aggregate of one dimension. Paradoxical ESR spectra appear only when Jᵢₙₜᵣₐ and Jᵢₙₜₑᵣ fall within a particular range of the magnitudes which depends on the number of molecules in the aggregate.

Spin-orbit effects in heavy-atom organic radical ferromagnets

Abstract: We discuss the effects of the spin-orbit interaction on heavy-atom organic magnets with specific reference to a series of isostructural sulfur- and selenium-based radical ferromagnets of tetragonal space group $P\overline{4}{2}_{1}m$. By using a perturbative approach, we show the spin-orbit effects lead to a pairwise anisotropic exchange interaction between neighboring radicals that provides an easy magnetic axis running parallel to the $c$-axis. Estimates of the magnitude of this magnetic anisotropy explain the significant increase in the coercive fields by virtue of selenium incorporation. Complementing this theoretical discussion are the results of ferromagnetic resonance studies, which provide an experimental verification of both the magnitude and symmetry of the spin-orbit terms. Taken as a whole, the results underscore the importance of heavy atoms and crystal symmetry in the design of molecular ferromagnets with large magnetic anisotropy and high ordering temperatures.

d carrier induced intrinsic room temperature ferromagnetism in Nb:TiO2 film

Abstract: High crystalline anatase TiO2 and Nb:TiO2 thin films were fabricated on LaAlO3 (100) substrates by pulsed laser deposition. Room temperature ferromagnetism was obtained in Nb:TiO2 but absent in pure TiO2. The Kondo effect and anomalous Hall effect observed in metallic Nb:TiO2 strongly confirmed the existence of exchange interaction between intrinsic local magnetic moments and carriers. High energy resolution x-ray photoelectron spectroscopy studies of the Nb:TiO2 thin film revealed clear signals of Ti3+ and Nb4+ ions, which had one unpaired d electron responsible for the local magnetic moments. This result consisted quite well with the spin polarized first principle calculation.