Magnetic Transition Metal Research Articles

Magnetoelectric effect arising from a field-induced pseudo Jahn-Teller distortion in a rare-earth magnet

Magnetoelectric materials are attractive for several applications, including actuators, switches, and magnetic field sensors. Typical mechanisms for achieving a strong magnetoelectric coupling are rooted in transition metal magnetism. In sharp contrast, here we identify $\mathrm{CsEr}{({\mathrm{MoO}}_{4})}_{2}$ as a magnetoelectric material without magnetic transition metal ions, thus ensuring that the Er ions play a key role in achieving this interesting property. Our detailed study includes measurements of the structural, magnetic, and magnetoelectric properties of this material. Bulk characterization and neutron powder diffraction show no evidence for structural phase transitions down to 0.3 K and therefore $\mathrm{CsEr}{({\mathrm{MoO}}_{4})}_{2}$ maintains the room temperature $P2/c$ space group over a wide temperature range without external magnetic field. These same measurements also identify collinear antiferromagnetic ordering of the ${\mathrm{Er}}^{3+}$ moments below ${T}_{N}=0.87\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Complementary dielectric constant and pyroelectric current measurements reveal that a ferroelectric phase with a maximum polarization $P\ensuremath{\sim}0.6\phantom{\rule{4pt}{0ex}}\mathrm{nC}/{\mathrm{cm}}^{2}$ emerges when applying a modest external magnetic field, which indicates that this material has a strong magnetoelectric coupling. We argue that the magnetoelectric coupling in this system arises from a pseudo Jahn-Teller distortion induced by the magnetic field.

Magnetism of Layered TiO2

Room temperature ferromagnetism with large saturation magnetism was first observed in layered TiO2 (LT). The LT is in rich of oxygen vacancy and free of magnetic transition metal elements. The temperature dependence of magnetism indicates that carrier mobility and carrier number of LT are larger than that of anatase TiO2 (AT) and amorphous TiO2 (AMT). The room temperature ferromagnetism is an intrinsic property of LT since there are no magnetic transition metal elements in LT. LT is a good platform to study two-dimensional (2D) magnetism.

Vapor Deposition of Magnetic Van der Waals NiI2 Crystals.

The recent discovery of van der Waals magnetic materials has attracted great attention in materials science and spintronics. The preparation of ultrathin magnetic layers down to atomic thickness is challenging and is mostly by mechanical exfoliation. Here, we report vapor deposition of magnetic van der Waals NiI2 crystals. Two-dimensional (2D) NiI2 flakes are grown on SiO2/Si substrates with a thickness of 5-40 nm and on hexagonal boron nitride (h-BN) down to monolayer thickness. Temperature-dependent Raman spectroscopy reveals robust magnetic phase transitions in the as-grown 2D NiI2 crystals down to trilayer. Electrical measurements show a semiconducting transport behavior with a high on/off ratio of 106 for the NiI2 flakes. Lastly, density functional theory calculation shows an intralayer ferromagnetic and interlayer antiferromagnetic ordering in 2D NiI2. This work provides a feasible approach to epitaxy 2D magnetic transition metal halides and also offers atomically thin materials for spintronic devices.

Strengthening the Magnetic Interactions in Pseudobinary First-Row Transition Metal Thiocyanates, M(NCS)2.

Understanding the effect of chemical composition on the strength of magnetic interactions is key to the design of magnets with high operating temperatures. The magnetic divalent first-row transition metal (TM) thiocyanates are a class of chemically simple layered molecular frameworks. Here, we report two new members of the family, manganese(II) thiocyanate, Mn(NCS)2, and iron(II) thiocyanate, Fe(NCS)2. Using magnetic susceptibility measurements on these materials and on cobalt(II) thiocyanate and nickel(II) thiocyanate, Co(NCS)2 and Ni(NCS)2, respectively, we identify significantly stronger net antiferromagnetic interactions between the earlier TM ions-a decrease in the Weiss constant, θ, from 29 K for Ni(NCS)2 to -115 K for Mn(NCS)2-a consequence of more diffuse 3d orbitals, increased orbital overlap, and increasing numbers of unpaired t2g electrons. We elucidate the magnetic structures of these materials: Mn(NCS)2, Fe(NCS)2, and Co(NCS)2 order into the same antiferromagnetic commensurate ground state, while Ni(NCS)2 adopts a ground state structure consisting of ferromagnetically ordered layers stacked antiferromagnetically. We show that significantly stronger exchange interactions can be realized in these thiocyanate frameworks by using earlier TMs.

First-principles study of magnetism of 3d transition metals and nitrogen co-doped monolayer MoS2**Project supported by the Key Project of the National Natural Science Foundation of China (Grant No. 51702089), the National Natural Science Foundation of China (Grant Nos. 21603109 and 11804081), the Henan Joint Fund of the National Natural Science Foundation of China (Grant No. U1404216), China Postdoctoral Science Foundation

The electronic structures and magnetic properties of diverse transition metal (TM = Fe, Co, and Ni) and nitrogen (N) co-doped monolayer MoS2 are investigated by using density functional theory. The results show that the intrinsic MoS2 does not have magnetism initially, but doped with TM (TM = Fe, Co, and Ni) the MoS2 possesses an obvious magnetism distinctly. The magnetic moment mainly comes from unpaired Mo:4d orbitals and the d orbitals of the dopants, as well as the S:3p states. However, the doping system exhibits certain half-metallic properties, so we select N atoms in the V family as a dopant to adjust its half-metal characteristics. The results show that the (Fe, N) co-doped MoS2 can be a satisfactory material for applications in spintronic devices. On this basis, the most stable geometry of the (2Fe–N) co-doped MoS2 system is determined by considering the different configurations of the positions of the two Fe atoms. It is found that the ferromagnetic mechanism of the (2Fe–N) co-doped MoS2 system is caused by the bond spin polarization mechanism of the Fe–Mo–Fe coupling chain. Our results verify that the (Fe, N) co-doped single-layer MoS2 has the conditions required to become a dilute magnetic semiconductor.

Molecular molds for regularizing Kondo states at atom/metal interfaces

Adsorption of magnetic transition metal atoms on a metal surface leads to the formation of Kondo states at the atom/metal interfaces. However, the significant influence of surrounding environment presents challenges for potential applications. In this work, we realize a novel strategy to regularize the Kondo states by moving a CoPc molecular mold on an Au(111) surface to capture the dispersed Co adatoms. The symmetric and ordered structures of the atom-mold complexes, as well as the strong dπ–π bonding between the Co adatoms and conjugated isoindole units, result in highly robust and uniform Kondo states at the Co/Au(111) interfaces. Even more remarkably, the CoPc further enables a fine tuning of Kondo states through the molecular-mold-mediated superexchange interactions between Co adatoms separated by more than 12 Å. Being highly precise, efficient and reproducible, the proposed molecular mold strategy may open a new horizon for the construction and control of nano-sized quantum devices.

Anomalous Hall effect in the kagome ferrimagnet GdMn6Sn6

We present magnetotransport data on the ferrimagnet GdMn$_6$Sn$_6$. From the temperature dependent data we are able to extract a large instrinsic contribution to the anomalous Hall effect $\sigma_{xz}^{int} \sim$ 32 $\Omega^{-1}cm^{-1}$ and $\sigma_{xy}^{int} \sim$ 223 $\Omega^{-1}cm^{-1}$, which is comparable to values found in other systems also containing kagome nets of transition metals. From our transport anisotropy, as well as our density functional theory calculations, we argue that the system is electronically best described as a three dimensional system. Thus, we show that reduced dimensionality is not a strong requirement for obtaining large Berry phase contributions to transport properties. In addition, the coexistence of rare-earth and transition metal magnetism makes the hexagonal MgFe$_6$Ge$_6$ structure type a promising system to tune the electronic and magnetic properties in future studies.

Chemical migration and dipole formation at van der Waals interfaces between magnetic transition metal chalcogenides and topological insulators

Metal and magnetic overlayers alter the surface of the topological insulator (TI) bismuth selenide (Bi$_2$Se$_3$) through proximity effects but also by changing the composition and chemical structure of the Bi$_2$Se$_3$ sub-surface. The interface between Bi$_2$Se$_3$ and Mn metal or manganese selenide was explored using x-ray photoelectron spectroscopy (XPS) revealing chemical and electronic changes at the interface. Depositing Mn metal on Bi$_2$Se$_3$ without an external source of Se shows unexpected bonding within the Mn layer due to Mn-Se bonding as Se diffuses out of the Bi$_2$Se$_3$ layer into the growing Mn film. The Se out-diffusion is further evidenced by changes in Bi core levels within the Bi$_2$Se$_3$ layers indicating primarily Bi-Bi bonding over Bi-Se bonding. No out-diffusion of Se occurred when excess Se is supplied with Mn, indicating the importance of supplying enough chalcogen atoms with deposited metals. However, Bi$_2$Se$_3$ core level photoelectrons exhibited a rigid chemical shift toward higher binding energy after depositing a monolayer of MnSe$_{2-x}$, indicating a dipole within the overlayer. Stoichiometry calculations indicated that the monolayer forms MnSe preferentially over the transition metal dichalcogenide (TMD) phase MnSe$_2$, providing a consistent picture of the dipole formation in which a plane of Se anions sits above Mn cations. This study shows that chemical diffusion and dipole formation are important for Mn-Bi$_2$Se$_3$ and MnSe$_{2-x}$-Bi$_2$Se$_3$ and should be considered carefully for TMD/TI interfaces more generally.

Goodenough–Kanamori rules applied to wurtzite crystals

Goodenough–Kanamori (GK) criteria have provided a significant contribution to our understanding of the importance of the symmetry and the electron orbital characteristics in the development of the magnetic superexchange coupling [antiferromagnetic (AFM) or ferromagnetic (FM)] applied primarily to systems with bond angles of 180° and 90°. In the present work, we quantify and apply the GK criteria to wurtzite systems. Our approach is based on calculations of (i) the spin electron densities of the anions which are first nearest neighbors (1nn) to the magnetic dopants and, (ii) the generalized exchange integrals which are derived by investigating the electronic properties of the systems under a magnetization density constraint. We demonstrate that the magnetization constraint can be used as a probe in investigating the magnetic properties of the materials under magnetization constraints. Our results indicate that the GK criteria applied to diluted magnetic semiconductors and transition metal oxides of wurtzite structures always lead to a FM coupling between two 1nn dopants of the same type. This is justified by ab initio calculations obtained for ZnO and GaN doped with 3d-transition metal dopants.

Reprint of “Photoinduced dynamics in an exchange-coupled trinuclear iron cluster”

We present a joint experimental and computational study of the trinuclear basic carboxylate iron complex FeIII2FeIIO(CH3CO2)6(H2O)3, which is a model system for understanding photoinduced ultrafast spin dynamics in magnetic iron-based transition metal oxides. We have carried out femtosecond optical transient absorption spectroscopy of molecules in solution at room-temperature exciting either at 400 or 520 nm and observed a long-lived excited-state absorption (ESA) signal from ca. 400–670 nm. The ESA signal is composed of several broad un-resolved bands at 405, 440 and 530 nm. The decay dynamics are complicated and three exponentials with corresponding decay time constants of τ1=360±30 fs, τ2=5.3±0.6 ps, τ3=65±5 ps and a constant offset (τ4>500 ps) were needed to fit the data over the full wavelength range. The data indicate that the lowest excited state is populated within the duration of the excitation pulse (<120 fs). Highly correlated coupled-cluster calculations can satisfactorily reproduce the experimental vibrational spectrum and highlight the role of the μ3-oxo bridge connecting the Fe ions to create a highly correlated ground-state and identify the excited state as having a mixture of both charge-transfer and ligand-field/d-orbital characters.

Magnetization of Epitaxial Graphene Induced by Magnetic Metallic Substrate

A model of the energy diagram is proposed for graphene (Gr) monolayer formed on a magnetic transition metal (MTM). The regimes of the strong and weak Gr–MTM coupling are considered; the analytical expressions have been derived for the magnetization induced by a substrate in epitaxial graphene in these regimes. It is shown that, in the strong coupling regime, a quasi-local state gives the main contribution to the magnetization; in this case, the coupling has an antiferromagnetic character. In the weak coupling regime, the main contribution to the magnetization is given by states of the d band of MTM, which leads to the ferromagnetic character of the coupling. The numerical estimations are performed for the Gr/Ni(111) and Gr/Co(poly) and they are compared with the experimental data for Gr/Ni(111) and the calculated results for Cr/Co/Ni(111). In the first case, the results agree quantitatively and, in the second case, qualitatively.

Photoinduced dynamics in an exchange-coupled trinuclear iron cluster

We present a joint experimental and computational study of the trinuclear basic carboxylate iron complex FeIII2FeIIO(CH3CO2)6(H2O)3, which is a model system for understanding photoinduced ultrafast spin dynamics in magnetic iron-based transition metal oxides. We have carried out femtosecond optical transient absorption spectroscopy of molecules in solution at room-temperature exciting either at 400 or 520 nm and observed a long-lived excited-state absorption (ESA) signal from ca. 400–670 nm. The ESA signal is composed of several broad un-resolved bands at 405, 440 and 530 nm. The decay dynamics are complicated and three exponentials with corresponding decay time constants of τ1=360±30 fs, τ2=5.3±0.6 ps, τ3=65±5 ps and a constant offset (τ4>500 ps) were needed to fit the data over the full wavelength range. The data indicate that the lowest excited state is populated within the duration of the excitation pulse (<120 fs). Highly correlated coupled-cluster calculations can satisfactorily reproduce the experimental vibrational spectrum and highlight the role of the μ3-oxo bridge connecting the Fe ions to create a highly correlated ground-state and identify the excited state as having a mixture of both charge-transfer and ligand-field/d-orbital characters.

Successive spin polarization contribution to the magnetic coupling in diluted magnetic semiconductors: A quantitative verification

Using ab initio numerical results, we present a quantitative verification of the proposed contribution to the magnetic coupling in diluted magnetic semiconductors and transition metal oxides originated via successive spin polarizations over dopants and their first nearest neighboring (1nn) anion ligands. This is achieved through an examination of the variation with the magnetization density of both the magnetic moments of 3-d transition metal dopants and those of their 1nn anion ligands in various hosts. These results are then used to derive approximate exchange coupling constants assigned to the magnetic coupling between a dopant and its first nearest neighbor anion ligands for each host-dopant pair. This allows us to justify and/or predict the spin polarization state of an anion ligand which is common to two or three magnetic dopants and, in turn, to justify and/or predict the spin configuration and the intercoupling (ferromagnetic and/or antiferromagnetic) among the atoms of a cluster of dopants sharing common anion ligands.

Намагниченность эпитаксиального графена, наведенная магнитной металлической подложкой

Model of energy diagram for the graphene monolayer (Gr) fabricated on magnetic transition metal (MTM) is proposed. Regimes of the strong and weak Gr-MTM coupling are discussed and corresponding analytical expressions for the substrate induced magnetization of epigraphene are obtained. It is shown that in the case of strong coupling the main contribution in magnetization are given by the quasi-local state which leads to the antiferromagnetic Gr-MTM bond, while in the case of weak coupling the main contribution in magnetization are given by the MTM d-band which leads to the ferromagnetic Gr-MTM bond. Numerical estimates for the Gr/Ni(111) and Gr/Co(poly) heterostructures are fulfilled and compared with the experimental data for Gr/Ni(111) and numerical calculations for Gr/Co/Ni(111). In the first case we have obtained the quantitative agreement, in the second case only the qualitative correspondence is achieved.

Electronic, magnetism, and optical properties of transition metals adsorbed g-GaN

Abstract The electronic, magnetism, and optical absorption behaviors of transition metals adsorbed g-GaN systems were investigated by employing density functional theory based on first-principles calculations. The results demonstrate that the most stable adsorption sites of different transition metals on g-GaN are different. The observed the Ni, Pt, and Zn adsorbed g-GaN systems exbibit nonmagnetic semiconductors, whereas the Au, Co, Cr, Cu, Fe, and Mn adsorbed g-GaN systems appear magnetic semicondutors, and the Sc, Ti, and V adsorbed g-GaN systems show magnetic metal behaviours. Furthermore, the charge transfer occurs between the g-GaN and TM, demonstrating a covalent bonding features in the appeared between TM and under-coordinated atoms. The work function of the Co, Cr, Cu, Fe, Mn, Ni, Sc, Ti, and V adsorbed g-GaN systems are lower than that of pristine g-GaN. Importantly, the absorption spectrum of transition metals adsorbed g-GaN systems have many strong visible light absorption peak, which located about in 471.3 nm, 495.8 nm, 517.6 nm, 574.4 nm, 627.5 nm, 672.3 nm and 717.1 nm. The adsorption of TM on g-GaN has a wide range of adjustment in solar energy spectrum. Thus, above results indicate that transition metals adsorbed g-GaN systems will facilitate design of nano-spintronics and high efficiency photocatalysts device.

Complex magnetic structure and related thermodynamic properties of Mn2SnS4

Magnetic chalcogenide compounds have a great potential in the field of spintronics and biomedical applications. Unlike magnetic transition metal oxides, the study on magnetic chalcogenides is sparse. Here we investigate the magnetic, thermodynamic and electronic properties of a ternary chalcogenide, Mn2SnS4. Contrary to most of the 214-chalcogenide compounds which crystallize in well-known olivine structure with space group Pnma, the title compound, Mn2SnS4, crystallizes in the orthorhombic space group, Cmmm. Magnetic measurements reveal two distinct magnetic transitions: (i) an antiferromagnetic ordering below 152 K, (ii) a weak ferromagnetic ordering, possibly due to spin canting and frustration below 53 K. Neutron diffraction study at 100 K suggest that the magnetic moments (spins) within {1 1 1} planes are ordered ferromagnetically and the adjacent {1 1 1} planes are coupled antiferromagnetically, however, the magnetic spins are not collinear rather canted. LMTO-ASA calculation on an antiferromagnetic model indicates that the compound is poorly metallic in nature and the manganese spins are completely spin polarized.

Enhancement of intrinsic pinning in the high-temperature superconductor TmBa2Cu3Oy: Manifestation of the interaction between vortices and a magnetic rare-earth ion

The magnetic-field dependences of the low-frequency complex permeability have been measured for thin-plate single crystals of the high-temperature superconductor TmBa2Cu3Oy in a static external magnetic field H and an alternating magnetic field with the amplitude h (h ≪ H) both directed along the plate plane (crystallographic ab plane). Two different contributions to the permeability have been revealed. The first contribution (μv) is independent of the amplitude and is associated with the vibrational motion of vortices along the c axis. The second contribution (μc) linearly depends on the amplitude and is due to the occurrence of a critical state along the structure layers. The field dependences of μv and the critical current exhibit features indicating the appearance of an additional intrinsic pinning mechanism, which is initiated by the magnetic field and is caused by the interaction of the vortex system with a magnetic rare-earth ion layer. This effect is enhanced as the temperature increases and approaches the critical value Tc. A comparison of the results for two single crystals shows that the interaction of the vortex and magnetic systems weakly depends on the quality of the crystals. The interaction decreases only slightly for the sample with the smaller Tc value. A method has been proposed to increase pinning by the partial substitution of magnetic transition metal ions, primarily iron ions for ions at the yttrium position. Such a substitution is possible up to 20%, keeping a single phase and without a decrease in Tc.

Preparation and magnetic properties of Mn-doped porous carbon nitride sheets

We report a simple method to fabricate Mn-doped poly(triazine imide) (PTI) by using melamine and cyanuric chloride as the building blocks, and Mn as the coupling agent. This material forms a two-dimensional imide-bridged triazine (C6N9H3) network, with an ordered arrangement of 24-atom voids. The synthesized PTI exhibits a large magnetization due to the localized d-electrons of Mn2+ ions, which suggests that PTI contains a large number of Mn2+ ions in the 24-atom voids. In addition, on being treated with HNO3, PTI absorbs NO3− ions in the 24-atom voids, which replace the Mn2+ ions present there, and forms a structurally well-ordered host-guest conjugated sheet, PTI·NO3. X-ray diffraction measurements of PTI·NO3 indicate that it crystallizes in the hexagonal space group P63cm, with lattice parameters a = 8.43 Å and c = 6.52 Å. The remaining Mn2+ ions are intercalated within the interlayer 24-atom void center sites to maintain charge balance, and constitute an Mn–NO3 column along the c-axis. The Mn2+-intercalated PTI·NO3 structure exhibits Curie paramagnetism, with a negative Weiss temperature. The present study reports the bulk synthesis of a well-characterized, crystalline, graphitic carbon nitride containing magnetic transition metal (TM) ions, that has potential applications in carbon-based magnetic materials and catalysis.

Structure and magnetic properties of Mn doped α-Fe2O3

Magnetic transition metal oxides are interesting candidates for both academic as well as industrial researches. In the present work, we study the influence of partial Mn doping on structural and magnetic and dielectric properties of α-Fe2O3 using x-ray and neutron diffraction, magnetization and dielectric measurements. In particular, this paper deals structural, magnetic and dielectric properties of Fe1.95Mn0.05O3. The compound is found to crystallize in corundum type (α-Al2O3) having rhombohedral lattice structure with hexagonal setting in the space group (R-3c). It is seen that partial substitution of Fe by Mn in α-Fe2O3 (known as hematite) drastically alters the Morin transition in the hematite compound by reducing the ordering temperature from a value of 260 K to about 220 K. Information about ferrimagnetic structure of Fe1.95Mn0.05O3 has been obtained from the magnetic part of the neutron diffraction analysis. The results of room temperature neutron diffraction, temperature dependent magnetization and dielectric studies are reported and discussed in the present work in order to understand the correlation between structure and magnetism in this compound and ways in which magnetism and other physical properties can be controlled.

The chiral biquadratic pair interaction

Magnetic interactions underpin a plethora of magnetic states of matter, hence playing a central role both in fundamental physics and for future spintronic and quantum computation devices. The Dzyaloshinskii–Moriya interaction, , being chiral and driven by relativistic effects, leads to the stabilization of highly-noncollinear spin textures such as skyrmions, which thanks to their topological nature are promising building blocks for magnetic data storage and processing elements. Here, we reveal and study a new chiral pair interaction, , which is the biquadratic equivalent of the DMI. First, we derive this interaction and its guiding principles from a microscopic model, and we connect the atomistic form to the micromagnetic one. Second, we study its properties in the simplest prototypical systems, magnetic 3d transition metal dimers deposited on the Pt(111), Pt(100), Ir(111), and Re(0001) surfaces, resorting to systematic first-principles calculations. Lastly, we discuss its importance and implications not only for magnetic dimers but also for extended systems, namely one-dimensional spin spirals and complex two-dimensional magnetic structures, such as a nanoskyrmion lattice found in an Fe monolayer on Ir(111).