Localized Magnetic States Research Articles

Formation of localized magnetic states in graphene in hollow-site adsorbed adatoms

By applying tight binding model of adatoms in graphene, we study theoretically the localized aspects of the interaction between transition metal atoms and graphene. Considering the electron-electron interaction by adding a Hubbard term in the mean-field approximation, we find the spin-polarized localized and total density of states. We obtain the coupled system of equations for the occupation number for each spin in the impurity and we study the fixed points of the solutions. By comparing the top site and hollow site adsorption, we show that the anomalous broadening of the latter allows to obtain magnetization for small values of the Hubbard parameter. Finally, we model the magnetic boundaries in order to obtain the range of Fermi energies at which magnetization starts.

Magnetism in Boron Nitride Monolayer Induced by Cobalt or Nickel Doping

We have investigated the electronic structure and magnetic properties of cobalt (Co)- or nickel (Ni)-doped hexagonal boron nitride (h-BN) monolayer using density functional theory calculations. The h-BN monolayer without any doping is a nonmagnetic insulator. Our studies show that isolated Co or Ni atom can both induce the local magnetic state in h-BN monolayer. And the impurity energy levels will be formed in the band gap. Results of our first-principle calculations reveal that isolated Co atom can result in a magnetic moment of 3.57 μ B, while the Ni atom is 0.87 μ B. And the magnetic moments mainly come from d orbitals of the doped atoms. The studies of magnetic coupling show that the two Co atoms at different distances in h-BN monolayer do not always couple ferromagnetically. When the distances are 2.504 and 6.625 A, the spins induced by the two Co atoms will form antiferromagnetic ground states. While the two Ni atoms at different distances in the monolayer always couple ferromagnetically. So our results imply that the Ni-doped h-BN is more suitable for spintronic material than Co doping. These results are useful for spintronic application.

Morphological and local magnetic properties of cobalt clusters electrodeposited onto indium tin oxide substrates

Magnetic force microscopy was employed in order to investigate the local magnetic distribution of individual cobalt clusters electrodeposited onto indium tin oxide surfaces. In addition, the size and shape of the clusters were analyzed by using scanning electron microscopy and atomic force microscopy at various electrodeposition potential values or growth stages. From these results, the cluster magnetic state at different cluster dimensions was identified and the critical cluster size for the magnetic transition from the single to the multi domain state was obtained. By analyzing the local magnetic states of the aggregates in terms of a theoretical single-domain ferromagnetic model, various magnetic parameters were inferred. Finally, in order to validate the experimental results, a micromagnetic simulation study was employed which correctly confirmed the experimental magnetic transition diagram, and also, provided some insights of other intrinsic magnetic properties of the aggregates.

Transition Jitter in Heat-Assisted Magnetic Recording by Micromagnetic Simulation

In this paper we apply an extended Landau-Lifschitz equation, as introduced by Ba\v{n}as et al. for the simulation of heat-assisted magnetic recording. This equation has similarities with the Landau-Lifshitz-Bloch equation. The Ba\v{n}as equation is supposed to be used in a continuum setting with sub-grain discretization by the finite-element method. Thus, local geometric features and nonuniform magnetic states during switching are taken into account. We implement the Ba\v{n}as model and test its capability for predicting the recording performance in a realistic recording scenario. By performing recording simulations on 100 media slabs with randomized granular structure and consecutive read back calculation, the write position shift and transition jitter for bit lengths of 10nm, 12nm, and 20nm are calculated.

Origin of the large positive magnetoresistance of Ge1−xMnx granular thin films

GeMn granular thin films are a unique and promising material for spintronics applications due to large positive magnetoresistance (MR). Previous studies on GeMn have suggested that the large MR is related to nanospinodal decomposition of GeMn into Mn-rich ferromagnetic nanoparticles and Mn-poor paramagnetic matrix. However, its microscopic origin of the MR has not been clarified yet. Here, using X-ray magnetic circular dichroism (XMCD), which is extremely sensitive to the local magnetic state of each atom, we investigate the magnetic properties of the nanoparticles and the matrix in GeMn separately. We find that the MR ratio is proportional to the product of the magnetizations originating from the nanoparticles and the matrix. This result indicates that spin-polarized holes in the nanoparticles penetrate into the matrix and that these holes undergo spin-disorder magnetic scattering by the paramagnetic Mn atoms in the matrix, which induces the large MR.

Probing the spiral magnetic phase in 6 nm textured erbium using polarised neutron reflectometry

We characterise the magnetic state of highly-textured, sputter deposited erbium for a film of thickness 6 nm. Using polarised neutron reflectometry it is found that the film has a high degree of magnetic disorder, and we present some evidence that the film’s local magnetic state is consistent with bulk-like spiral magnetism. This, combined with complementary characterisation techniques, show that thin film erbium is a strong candidate material for incorporation into device structures.

Confined spin wave spectra of Kagome artificial spin ice arrays

The spin wave modes of elongated magnetic islands arranged in Kagome artificial spin-ice arrays are micromagnetically simulated in the frequency regime between 3 and 16GHz. The edge modes are more suitable in order to detect the signatures of various types of local order of the spin-ice lattice as they are much more sensitive to the magnetic configurations of neighboring elements. The spectra of arrays consisting up to 30 elements can be decomposed to those originating from local magnetic states of their vertices.

Ultrafast dynamics of localized magnetic moments in the unconventional Mott insulator Sr2IrO4

We report a time-resolved study of the ultrafast dynamics of the magnetic moments formed by the states in Sr2IrO4 by directly probing the localized iridium 5d magnetic state through resonant x-ray diffraction. Using optical pump–hard x-ray probe measurements, two relaxation time scales were determined: a fast fluence-independent relaxation is found to take place on a time scale of 1.5 ps, followed by a slower relaxation on a time scale of 500 ps–1.5 ns.

Rewritable artificial magnetic charge ice.

Artificial ices enable the study of geometrical frustration by design and through direct observation. However, it has proven difficult to achieve tailored long-range ordering of their diverse configurations, limiting both fundamental and applied research directions. We designed an artificial spin structure that produces a magnetic charge ice with tunable long-range ordering of eight different configurations. We also developed a technique to precisely manipulate the local magnetic charge states and demonstrate write-read-erase multifunctionality at room temperature. This globally reconfigurable and locally writable magnetic charge ice could provide a setting for designing magnetic monopole defects, tailoring magnonics, and controlling the properties of other two-dimensional materials.

Localized Magnetic States in Three Dimensional Dirac Solids

Formation of localized magnetic states in a metallic host is a classic problem ofcondensed matter physics formalized by P. W. Anderson within the so called single impurity Anderson model (SIAM). The general picture in a host of a simple one-band metal is that a large Hubbard $U$ in the impurity orbital is pre-requisite for the formation of localized magnetic states. In recent years three dimensional (3D) Dirac solids have emerged the hallmark of which is strong spin-orbit interaction. In this work we show that such a strong spin-orbit interaction allows to form localized magnetic states even with small values of Hubbard $U$. This opens up the fascinating possibility of forming magnetic states with $s$ or $p$ orbital impurities -- different from traditional paradigms of $d$ or $f$ orbital based magnetic moments.

Specific features of local structural, valence, and magnetic states of iron ions in the perovskite Bi0.5Ca0.5FeO3

The perovskite Bi0.5Ca0.5FeO3 has been investigated using the Mossbauer effect at temperatures of 295 and 675 K. The measured temperature of the magnetic phase transition (Neel temperature) is TN = 640 ± 10 K. Above the Neel temperature, there are two nonequivalent structural states of iron ions. In the perovskite Bi0.5Ca0.5FeO3 at room temperature, there are seven most probable nonequivalent magnetic states of iron ions with significantly different values of the hyperfine interaction parameters. Four iron states correspond to Fe3+ ions in the octahedral oxygen environment, and three iron states correspond to Fe3+ ions in the tetrahedral oxygen environment.

Origin of enhanced magnetoelectric coupling inNiFe2O4/BaTiO3multilayers studied by x-ray magnetic circular dichroism

${\text{NiFe}}_{2}$${\text{O}}_{4}$/${\text{BaTiO}}_{3}$ (NFO/BTO) multilayer heterostructures grown on (001)-${\text{SrTiO}}_{3}$ substrates with alternating ferroelectric BTO and ferrimagnetic NFO layers exhibit magnetoelectric (ME) coupling, which increases with the number of layers and, hence, with the number of NFO/BTO interfaces. We have studied the local electronic and magnetic states of Ni and Fe ions in the NFO/BTO multilayers with various NFO and BTO thicknesses by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. With an increasing number of layers, both Ni and Fe magnetic moments decrease. With decreasing NFO layer thickness, the average magnetic moment of Ni decreases, while the average magnetic moment of Fe ions remains unaffected, meaning that Ni ions at the interface lose ferromagnetic moments. This suggests that the enhancement of ME coupling is associated with Ni ions at the interface whose ferromagnetic behavior is weakened through hybridization with the BTO electronic states.

Ab initio calculations on magnetism induced by composite defects in magnesium oxide

The local magnetic state induced by the composite defects, composed of an oxygen vacancy and a nitrogen substituting oxygen, in magnesium oxide has been studied by using ab initio calculation based on density functional theory. The calculated results show that local magnetic moment can be induced by the composite defects around the oxygen vacancy, when the exchange split of the oxygen vacancy is enhanced either by the hybridization between the N-p and nearest neighbor O-p orbitals or by applying on-site Coulomb repulsion (U) and exchange interaction (J). We show that the magnetic state induced by the composite defect is energetically more stable than the non-magnetic state. In addition, we show that the U and J applied on the p-orbitals of N and O atoms may significantly impact the calculated magnetic state of the composite defect, resulting in magnetic state for a configuration that is non-magnetic by generalized gradient approximation.

Studying local structural, valence, and magnetic states of iron ions in Bi0.9Ca0.1FeO3 perovskite

Mossbauer method was used to study a perovskite compound Bi0.9Ca0.1FeO3 at T = 295 K and at temperature above TN. It has been established that Bi0.9Ca0.1FeO3 has a rhombohedral crystal structure similar to that of BiFeO3. The substitution of Ca2+ for Bi3+ ions leads to the formation of three states of Fe3+ ions with an octahedral surroundings and one state with a tetrahedral oxygen surroundings with substantially different hyperfine magnetic fields. All Fe ions are in a trivalent state; the compensation of the charge deficit occurs via the formation of oxygen vacancies. Above TN, two structurally nonequivalent states of Fe3+ ions exist in the Bi0.9Ca0.1FeO3 sample, which correspond to the Fe3+ ions with an octahedral and tetrahedral oxygen coordination.

NMR study of magnetic properties of Eu1−x Sr x MnO3 (0 ≤ x ≤ 0.5)

The perovskite manganite EuMnO3 is known to be a compound with a narrow eg-band width due to small ionic radius of Eu3+ compared to La3+. The Eu1 −xSrxMnO3 compounds show complex magnetic properties, depending on the Sr concentration x. We have carried out 55Mn spin-echo nuclear magnetic resonance (NMR) measurements on polycrystalline Eu1−xSrxMnO3 (0 ≤ x≤ 0.5) compounds at 4.2 or 1.4 K in order to clarify the microscopic local magnetic state of the Mn ions. NMR spectra have been observed in the frequency range from ∼250 MHz to ∼400 MHz. For the A-type antiferromagnetic EuMnO3, the resonance frequency is about 260 MHz, which is much lower than that observed for Mn3+ in La1−xSrxMnO3. For 0.1 ≤ x ≤ 0.4, the resonance lines corresponding to Mn4+ and Mn3+ have been observed at about 315 MHz and 390 MHz, respectively. Furthermore, the spectra originating from Mn2+ have been observed around 590 MHz for 0.1 ≤ x ≤ 0.4. For Eu0.5Sr0.5MnO3, the NMR spectrum spreads widely with several distinct peaks in the frequency range from 250 MHz to 410 MHz, which shows that the compound is in an inhomogeneous state due to competition between the antiferromagnetic and the ferromagnetic states.

Effect of uniaxial strain on ferromagnetic instability and formation of localized magnetic states on adatoms in graphene

We investigate the effect of an applied uniaxial strain on the ferromagnetic instability due to long- range Coulomb interaction between Dirac fermions in graphene. In case of undeformed graphene the ferromagnetic exchange instability occurs at sufficiently strong interaction within the Hartree- Fock approximation. In this work we show that using the same theoretical framework but with an additional applied uniaxial strain, the transition can occur for much weaker interaction, within the range in suspended graphene. We also study the consequence of strain on the formation of localized magnetic states on adatoms in graphene. We systematically analyze the interplay between the anisotropic (strain- induced) nature of the Dirac fermions in graphene, on- site Hubbard interaction at the impurity and the hybridization between the graphene and impurity electrons. The polarization of the electrons in the localized orbital is numerically calculated within the mean- field self- consistent scheme. We obtain complete phase diagram containing non- magnetic as well as magnetic regions and our results can find prospective application in the field of carbon- based spintronics.

Effect of interaction between impurities on localized magnetic states in graphene

We study effect of interaction between impurities on localized magnetic states in graphene. It is shown that the modifications of the impurity self-energy induced by the inter-impurity interaction can push the impurity energy across the zero energy point, which leads to splitting of the impurity magnetization boundary into the three peaks. With increasing the inter-impurity interaction strength, the two side peaks in the magnetic region are sharply skewed away from the central one, and their amplitudes exhibit the different dependence on interaction strength. The magnetic moment and corresponding magnetic transition of the impurities are also investigated.

Mixed-valence-driven heavy-fermion behavior and superconductivity in KNi2Se2

Based on specific heat and magnetoresistance measurements, we report that a ``heavy'' electronic state exists below $T$ \ensuremath{\approx} 20 K in KNi${}_{2}$Se${}_{2}$, with an increased carrier mobility and enhanced effective electronic band mass, ${m}^{*}$ = 6${m}_{\mathrm{b}}$ to 18${m}_{\mathrm{b}}$. This heavy state evolves into superconductivity at ${T}_{\mathrm{c}}$ = 0.80(1) K. These properties resemble that of a many-body heavy-fermion state, which derives from the hybridization between localized magnetic states and conduction electrons. Yet, no evidence for localized magnetism or magnetic order is found in KNi${}_{2}$Se${}_{2}$ from magnetization measurements or neutron diffraction. Instead, neutron pair-distribution-function analysis reveals the presence of local charge-density-wave distortions that disappear on cooling, an effect opposite to what is typically observed, suggesting that the low-temperature electronic state of KNi${}_{2}$Se${}_{2}$ arises from cooperative Coulomb interactions and proximity to, but avoidance of, charge order.

NMR of localized magnetic states in graphene

The NMR relaxation rate is studied on the magnetic states of an impurity in bilayer graphene within a tight-binding scenario. The dependencies of the relaxation rate on temperature, interlayer interaction and also the chemical potential have been considered. Although for low temperatures we observe the usual Korringa relation, a characteristic of the conventional fermions, the rate increases with the increase in temperature and tends to saturate for high temperatures. For small interlayer interactions (t⊥) the system can be either magnetic or non-magnetic. However for higher t⊥ we observe the existence of only a pure magnetic state. In graphene this transition is also observed with two cusps related to the magnetic to non-magnetic transition, which modifies to a single hump for higher t⊥, where the system is purely magnetic for any value of chemical potential.

Influence of dipolar energy on the magnetization reversal in magnetization-modulated thin film systems: Model and experiment

Laterally patterned magnetic hybrid structures display novel magnetic reversal properties, which are related to the fundamental exchange coupling between material interfaces. We present an analytical model that depicts the influence of dipolar fields in mesoscopic structures with modulated saturation magnetization on the magnetization reversal and the local magnetic states, as well as the occurrence of a lateral exchange-spring effect. This is done by confining a lateral array of stripes with alternating saturation magnetization ${M}_{S}$ in a micrometer-sized square, introducing external boundary conditions to the system. The calculations were performed for distinct stripe and array sizes, as well as different ${M}_{S}$ values. From the calculations a stability region of array and stripe sizes is derived, in which the lateral exchange-spring effect occurs. The obtained modeling results were found to be in agreement with the experimental data. The model adds a building block to the fundamental understanding of magnetic hybrid structures.