Tunable electronic structure and magnetic coupling in strained two-dimensional semiconductor MnPSe<Subscript>3</Subscript>

The electronic structures and magnetic properties of strained monolayer MnPSe₃ are investigated systematically via first-principles calculations. It is found that the magnetic ground state of monolayer MnPSe₃ can be significantly affected by biaxial strain engineering, while the semiconducting characteristics are well-preserved. Owing to the sensitivity of the magnetic coupling towards structural deformation, a biaxial tensile strain of approximately 13% can lead to an antiferromagnetic (AFM)- ferromagnetic (FM) transition. The strain-dependent magnetic stability is mainly attributed to the competition of the direct AFM interaction and indirect FM superexchange interaction between the two nearest-neighbor Mn atoms. In addition, we find that FM MnPSe₃ is an intrinsic half semiconductor with large spin exchange splitting in the conduction bands, which is crucial for the spin-polarized carrier injection and detection. The sensitive interdependence among the external stimuli, electronic structure, and magnetic coupling makes monolayer MnPSe₃ a promising candidate for spintronics.     

Correction: Ab initio study of anisotropic mechanical and electronic properties of strained carbon-nitride nanosheet with interlayer bonding

The magnetic and electronic properties of strontium titanate with different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation (GGA) and the GGA+U approach. Our results show that the structural stability, electronic properties and magnetic properties of C-doped SrTiCO₃ strongly depend on the distance between carbon dopants. In both GGA and GGA+U calculations, the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors, which can be ascribed to the formation of a C-C dimer pair accompanied by stronger C-C and weaker C-Ti hybridizations as the C-C distance becomes smaller. As the C-C distance increases, C-doped SrTiCO₃ changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations, while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method. Our work demonstrates the possibility of tailoring the magnetic and electronic properties of C-doped SrTiO₃, which might provide some guidance to extend the applications of strontium titanate as a magnetic or optoelectronic material.

     

Strain-dependent electronic and magnetic of Co-doped monolayer of WSe2

We perform first-principles calculation to investigate electronic and magnetic properties of Co-doped WSe₂ monolayer with strains from −10% to 10%. We find that Co can induce magnetic moment about 0.894 μ_B, the Co-doped WSe₂ monolayer is a magnetic semiconductor material without strain. The doped system shows half-metallic properties under tensile strain, and the largest half-metal gap is 0.147 eV at 8% strain. The magnetic moment (0.894 μ_B) increases slightly from 0% to 6%, and jumps into about 3 μ_B at 8% and 10%, which presents high-spin state configurations. When we applied compressive strain, the doped system shows a half-metallic feature at −2% strain, and the magnetic moment jumps into 1.623 μ_B at −4% strain, almost two times as the original moment 0.894 μ_B at 0% strain. The magnetic moment vanishes at −7% strain. The Co-doped WSe₂ can endure strain from −6% to 10%. Strain changes the redistribution of charges and magnetic moment. Our calculation results show that the Co-doped WSe₂ monolayer can transform from magnetic semiconductor to half-metallic material under strain.     

Ferromagnetism in epitaxial orthorhombic YMnO3 thin films

Epitaxial orthorhombic YMnO₃ thin films, (0 0 1) oriented, have been grown by pulsed laser deposition on (0 0 1)SrTiO₃ substrates. Their crystal structure and magnetic response have been studied in detail. Although bulk o-YMnO₃ is antiferromagnetic, our magnetic measurements reveal intriguing thermal hysteresis between the zero-field-cooled and field-cooled curves below the onset of the antiferromagnetic ordering temperature, thus signaling a more complex magnetic structure with net ferromagnetic moments. We discuss on the possible origin of this net magnetization and we have found a correlation of the magnetic response with the strain state of the films. We propose that substrate-induced strain modifies the subtle competition of magnetic interactions and leads to a non-collinear magnetic state that can thus be tuned by strain engineering.     

Quantum fluctuations and ground state of weakly interacting helix spin chains in a magnetic field

Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J₁ develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J₂ is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit. Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997     

Electronic and magnetic properties of Mn-doped WSe2 monolayer under strain

Electronic and magnetic properties of Mn-doped WSe₂ monolyer subject to isotropic strain are investigated using the first-principles methods based on the density functional theory. Our results indicate that Mn-doped WSe₂ monolayer is a magnetic semiconductor nanomaterial with strong spontaneous magnetism without strain and the total magnetic moment of Mn-doped system is 1.038μ_B. We applied strain to Mn-doped WSe₂ monolayer from -10% to 10%. The doped system transforms from magnetic semiconductor to half-metallic material from −10% to −2% compressive strain and from 2% to 6% tensile strain. The largest half-metallic gap is 0.450 eV at −2% compressive strain. The doped system shows metal property from 7% to 10%. Its maximum magnetic moment comes to 1.181μ_B at 6% tensile strain. However, the magnetic moment of system decreases to zero sharply when tensile strain arrived at 7%. Strain changes the redistribution of charges and arises to the magnetic effect. The coupling between the 3d orbital of Mn atom, 5d orbital of W atom and 4p orbital of Se atom is analyzed to explain the strong strain effect on the magnetic properties. Our studies predict Mn-doped WSe₂ monolayers under strain to be candidates for thin dilute magnetic semiconductors, which is important for application in semiconductor spintronics.     

First-principles electronic-structure calculation on the spin distribution and the half-metallic properties of the mixed valence iron compound Ba2F2Fe1.5S3

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) approach were applied to study the new mixed valence compound Ba₂F₂Fe_1.5S₃. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the Fe^III and Fe^II ions arising from the bridging S atoms, which validate the experimental assumptions that there is a low-dimensional antiferromagnetic interaction in Ba₂F₂Fe_1.5S₃. The spin magnetic moment mainly comes from the Fe^III and Fe^II ions with smaller contribution from S anion. By analysis of the band structure, we find that the compound has half-metallic property.     

Strain effect on the electronic properties of 1T-HfS2 monolayer

We perform first-principles based on the density function theory to investigate electronic and magnetic properties of 1T-HfS₂ monolayer with biaxial tensile strain and compressive strain. The results show that HfS₂ monolayer under strains doesn’t display magnetic properties. When the strain is 0%, the HfS₂ monolayer presents an indirect band gap semiconductor with the band gap is about 1.252 eV. The band gap of HfS₂ monolayer decreases quickly with increasing compressive strain and comes to zero when the compressive strain is above −7%, the HfS₂ monolayer system turns from semiconductor to metal. While the band gap increases slowly with increasing tensile strain and comes to 1.814 eV when the tensile strain is 10%. By comparison, we find that the compressive strain is more effective in band engineering of pristine 1T-HfS₂ monolayer than the tensile strain. And we notice that the extent of band gap variation is different under tensile strain. The change of band gap with strain from 1% to 5% is faster than that of the strain 6–10%. To speak of, the conduction band minimum (CBM) is all located at M point with different strains. While the valence band maximum (VBM) turns from Γ point to K point when the strain is equal to and more than 6%.     

First-order magnetic phase transition in layered Sr<Subscript>3</Subscript>YCo<Subscript>4</Subscript>O<Subscript>10.5 + δ</Subscript>-type cobaltites

In layered Sr₃YCo₄O_{10.5 + δ}-type cobaltites with different oxygen contents, we have observed a first order magnetic phase transition from the high-temperature “ferromagnetic” state to the low-temperature antiferromagnetic state. The transition can be induced by an applied magnetic field. It is accompanied by a significant hysteresis in the magnetic field (∼10 T) and temperature (∼10 K). A decrease and an increase in the yttrium content lead to a purely “ferromagnetic” and antiferromagnetic behavior, respectively.     

Effect of strain on electronic and magnetic properties of n-type Cr-doped WSe2 monolayer

Using first-principles calculations based on the density functional theory, we study the effect of strain on the electronic and magnetic properties of Cr-doped WSe₂ monolayer. The results show that no magnetic moment is induced in the Cr-doped WSe₂ monolayer without strain. For the Cr substitutions, the impurity states are close to the conduction bands, which indicate n-type doping occurs in this case. Then we applied strain (from −10% to 10%) to the doped system, and find that a little magnetic moment is induced with tensile strain from 6% to 9% and negligible. We find that the influence of strain on the magnetic properties is inappreciable in Cr-doped WSe₂. Moreover, the tensile strain appears to be more effective in reducing the band gap of Cr-doped WSe₂ monolayer than the compressive strain.     

Electron transition in intercalated disulfide CuCrS<Subscript>2</Subscript>

Electrical, resonant, and magnetic properties of intercalated copper chromium disulfide CuCrS₂ are studied. It is established that CuCrS₂ is an antiferromagnetic semiconductor with Néel temperature T_N=40.7 K and an effective magnetic moment of 4.3µ_B. Anomalies in the electrical, magnetic, and resonant properties of CuCrS₂ are found at T_c=110 K, which suggest an electron transition accompanied by alteration of the valences of the 3d-metal ions.     

Electronic, magnetic, and vibrational properties of the molecular magnet Mn4 monomer and dimer

A new type of the single-molecule magnet [Mn₄O₃Cl₄(O₂CEt)₃(py)₃] forms dimers. Recent magnetic hysteresis measurements on this single-molecular magnet revealed interesting phenomena: an absence of quantum tunneling at zero magnetic field and tunneling before magnetic field reversal. This is attributed to a significant antiferromagnetic exchange interaction between different monomers. To investigate this system, we calculate the electronic structure, magnetic properties, intramolecular and intermolecular exchange interactions using density-functional theory within the generalized-gradient approximation. Our calculations agree with experiment. We find that the calculated threefold symmetric structure is vibrationally stable. We also calculate vibrational infrared absorption and Raman scattering intensities for the monomer which can be tested experimentally.     

Evidence of a core–shell structure in the antiferromagnetic La0.2Ce0.8CrO3 nanoparticles by neutron scattering

We report the evidence of a core?Cshell structure in the antiferromagnetic La_0.2Ce_0.8CrO₃ nanoparticles by using a combination of neutron diffraction, polarized neutron small angle scattering (SANSPOL), and dc magnetization techniques. The neutron diffraction study establishes that the present nanoparticles are antiferromagnetic in nature. The magnetic scattering in the SANSPOL study arises from the shell part of the nanoparticles due to the disordered surface spins. The analysis of the SANSPOL data shows that these nanoparticles have a mean core diameter of 12.3±1.1?nm, and a shell thickness of 2.8±0.4?nm, giving a core?Cshell structure with an antiferromagnetic core, and a shell with a net magnetic moment under an applied magnetic field.     

Magnetoelectric effect in perovskite quantum paraelectric EuTiO3

On the basis of successful theoretical explanation of the observed large magnetic-field effect (by ∼7% with 1.5 T) on the dielectric constant below the Néel temperature T_N of 5.5 K, we have demonstrated convincingly the magnetoelectric effect in an antiferromagnetic quantum paraelectric EuTiO₃ system. The mutual control of electric and magnetic properties is revealed by the variation of the electric-field-induced polarization with applied magnetic fields as well as the change of the magnetic-field-induced spin moments under the control of electric fields. It is found that the applied electric field (magnetic field) acts like a fictitious magnetic field (electric field) on the EuTiO₃ system. The magnetoelectric susceptibility is deduced to be proportional to the product of the magnetization, electrical polarization, magnetic susceptibility and dielectric susceptibility.     

Possibility of two-stage flux penetration in antiferromagnetic superconductors

An induced ferromagnetism in antiferromagnetic superconductors is possible, caused by a magnetic structure of vortex lines appearing in an external magnetic field strong enough to “flip over” the spins in the vortex core from their antiferromagnetic configuration. If the magnetic field is less than the “flip over” field the vortex line is entirely in the antiferromagnetic phase. Therefore the vortex interaction with the surface of a sample is altered when an applied magnetic field exceeds the “flip over” field. This mechanism makes the appearance of a new energy barrier that strongly influences the flux penetration possible. An estimation of the second critical entry field is made for DyMo₆S₈.     

Doping level and environment dependence of structural stability and magnetic properties in Mn-doped WS2 bilayer in first principles

We carried out first-principles electronic structure calculation to study the structural stability and magnetic properties of Mn-doped WS₂ ultra-thin films within the density functional theory. Adopting various configurations of Mn doping into WS₂ bilayer, we find that the magnetic phase can be manipulated among the ferromagnetic, antiferromagnetic, or ferrimagnetic phases by altering doping level and growth environment. Magnetic phase and strength are determined by magnetic coupling of Mn dopants 3d electrons which can be attributed crucially to the exchange interaction mediated by neighboring S atoms 3p electrons. Accompanying to the magnetic phase transition, the electronic structure reveals that transport properties switch from semiconducting with various bandgap to half-metallic states. This result implicates possible way to develop magnetic semiconductors based on Mn doped 2D WS₂ ultra-thin films for spintronics applications.     

Magnetic interactions in the monoclinic ludwigite Cu FeO BO

The system Cu₂FeO₂BO₃ is an oxyborate belonging to the family of the ludwigites. In this paper we present AC susceptibility, magnetization measurements and M?ssbauer spectroscopy on this material which allows for a complete characterization of its complex magnetic behavior. We find an hierarchy of interactions which clearly defines three regimes with decreasing temperature. These are associated with, the freezing of the Fe moments, the antiferromagnetic ordering of the Cu sub-lattice and finally the coupling between both systems. Received 25 September 1998     

Mechanical and magnetic properties of ZnO/Fe2O3 ceramic varistors

Mechanical and magnetic properties of the ZnO/Fe₂O₃ ceramic varistors have been examined by using mechanical analyzer, digital microhardness tester and vibrating sample magnetometer. The initial stress–strain behavior is found to be linear (elastic) then becomes nonlinear (plastic deformation) without reaching the failure limit up to the maximum available stress (0.07 MPa). The compressive elastic modulus varies between 0.2 and 0.8 MPa with Fe addition up to 0.50. Furthermore, an approximately monotonically linear decrease in VHN with increasing Fe content up to 50% has been observed for all applied loads, which closely resembles the behavior of the true hardness and the surface energy. The magnetic measurements revealed an antiferromagnetic to paramagnetic to transition for all Fe doped samples. The Fe free sample showed paramagnetic behavior down to 2 K. The Neel temperature moderately increased from 18 K at 0.05% Fe to 25 K at 0.5% Fe. The magnetization (M) versus applied magnetic field (H) did not reach saturation for all samples up to 9 Tesla. The saturated magnetization (per Fe contents) is low and found to decreases linearly at a rate of (−35 emu/g-Fe) in a clear manifestation of the strengthening of the antiferromagnetic exchange interaction with increasing Fe contents.     

Phase transitions in FeBO<Subscript>3</Subscript> under pressure: DFT + DMFT study

We present a theoretical study of spectral, magnetic, and structural properties of the iron borate FeBO₃. Within the DFT + DMFT method combining density functional theory with dynamical mean-field theory FeBO₃ was investigated under pressures up to 70 GPa at 300 K. We found that FeBO₃ is an insulator with a gap of 2.0 eV with antiferromagnetic ordering at ambient pressure in agreement with experiments. In our calculations, we showed that Fe ions in FeBO₃ undergo a high-spin to low-spin transition under pressure with change from antiferromagnetic to paramagnetic state, and demonstrate that the spin and magnetic transitions occur simultaneously with an isostructural transition at 50.4 GPa with the volume collapse of 13%.     

Effective-medium approach for surface-localized magnetic polaritons in magnetic (ferromagnetic and antiferromagnetic) superlattices

The general effective-medium dispersion relations are derived for surface-localized magnetic polaritons which propagate parallel to the surface between a superlattice and semi-infinite bulk material, as applied to ferromagnetic and antiferromagnetic superlattices, in the situation when a static magnetic field is applied in the plane of the layers and parallel to the magnetization. The dependence of the energy of the surface waves on the volume fraction of the ferromagnetic superlattice component and the influence of the external magnetic field on the spectrum of the surface magnetic polaritons for the antiferromagnetic superlattice are investigated. The spectrum of the surface-localized magnetic polaritons which appear at the junction of the magnetic (ferromagnetic and antiferromagnetic) superlattice with the magnetic material are more complex, in contrast to the cases of semi-infinite magnetic material or semi-infinite magnetic SL. It is essential that in all cases in the presence of the external magnetic field the spectrum of the magnetic polaritons are non-reciprocal. The properties of surface polaritons are discussed in detail for the system ferromagnetic superlattice (YIG/non magnet)/YAG and the antiferromagnetic superlattice (MnF₂/ZnF₂)/FeF₂.