Nonmagnetic Doping Research Articles

Magnetic properties in BiFeO3 doped with Cu and Zn first-principles investigation

Based on first-principles spin-polarized density functional theory calculations, the electronic structures, and magnetic properties of Cu and Zn-doped BiFeO3 are investigated. The calculated formation energies show that Cu prefers to occupy Fe site, while the Zn prefer to occupy Bi site. All the doped BiFeO3 systems turn out to be favorable for G-type antiferromagnetic arrangement. The substitution of Cu and Zn for Fe produces a magnetic moment of 3.0 and 4.0 μB per dopant, respectively. The net magnetic moments are from the broken symmetry of the AFM spin ordering network. For the substitution of Cu and Zn for Bi, the net magnetic moment originates from Cu/Zn itself and hole introduced by Cu/Zn. Two-Cu/Zn-doped cases show various magnetic behaves. If O vacancy is far away from dopants, the O vacancies don't affect the net magnetic moment of the substitution of Cu and Zn for Fe, but have notable effect for Bi site doping. The O vacancies result in metallicity in all doped cases. Our study demonstrates that the nonmagnetic Cu and Zn doping will lead to the diversity and complexity of magnetic properties depending on doping sites, distance between dopants, intrinsic defect, and so on, which could be responsible for the observed various magnetic behaviors in Cu/Zn-doped BiFeO3 samples.

Effect of strain on the magnetic states of transition-metal atoms doped monolayer WS2

The effects of transition-metal atoms doping and elastic strain on the magnetic properties of monolayer WS2 are investigated by the first-principles calculation. Firstly, we investigate the magnetic properties of a series of 3d transition-metal atoms (from Sc to Zn) doped monolayer WS2. Our calculations show that doping of transition-metal atoms from Sc to Cr results in nonmagnetic states, while Mn, Fe, Co, Ni, Cu and Zn doping can induce magnetism in WS2 monolayer. The localized nonbonding 3d electrons of TM atoms lead to the formation of magnetic moments. Meanwhile, our calculations indicate that the elastic strain can be used to manipulate the spin polarization of TM-3d orbital. Specifically, the magnetic moment of Fe and Co-doped monolayer WS2 increase with the increase of tensile strain, while the magnetic moment of Mn, Ni, Cu and Zn-doped WS2 monolayer initially increase and then decrease with the increase of tensile strain. On the other hand, the magnetic moment of Fe, Co, Mn, Ni, Cu and Zn-doped WS2 monolayers decrease with the increase of compressive strain. While the magnetic moment of Sc, Ti, V and Cr-doped WS2 monolayers remain 0μB independent of tensile strain and compressive strain.

Robust topological surface transport with weak localization bulk channels in polycrystalline Bi2Te3 films

Bi2Te3 polycrystalline topological insulator films have provided an attractive material platform to investigate topological insulator properties and created new opportunities for novel magneto-electronic device applications. In order to confirm that Bi2Te3 polycrystalline film has a robust topological surface state, and whether such surface Dirac fermions can be protected from localization in transport, we performed a systematic transport measurement and analysis based on a Sn-doped Bi2Te3 polycrystalline film with Hall configuration electrodes. We demonstrated that the electron–electron interaction effect is very strong, which can help realize an insulating ground state. The surface state of the film always exhibits stable weak anti-localization features despite the presence of many structural defects and non-magnetic doping, and the Hall resistance can present a significant nonlinear dependence on magnetic fields. These two characteristics provide significant experimental evidence that the polycrystalline film has a robust topological surface state, and that such surface electrons cannot be localized. Owing to the lack of topological protection, the weak anti-localization transport of bulk electrons cannot be guaranteed, and weak localization behavior may appear in the bulk channels. These results verify that robust topological surface transport in topological insulator polycrystalline films can be accompanied by weak localization bulk channels.

Tuning magnetism of monolayer GaN by vacancy and nonmagnetic chemical doping

In view of important role of inducing and manipulating the magnetism in 2D materials for the development of low-dimensional spintronic devices, the magnetism of GaN monolayer with Ga vacancy and nonmagnetic chemical doping are investigated using first-principles calculations. It is found that pure GaN monolayer has graphene-like structure and is nonmagnetic. While, a neutral Ga vacancy can induce 3μB intrinsic magnetic moment, localized mainly on the neighboring N atoms. Interestingly, after one Mg or Si atom doping in g-GaN with Ga vacancy, the magnetic moment can be modified to 4μB or 2μB respectively due to the change in hole number. Meantime, Mg-doped g-GaN with Ga vacancy shows half-metal character. With the increasing of doping concentrations, the magnetic moment can be further tuned. The results are interesting from a theoretical point of view and may open opportunities for these 2D GaN based materials in magnetic devices.

Effect of magnetic and nonmagnetic nano metal oxides doping on the critical temperature of a YBCO superconductor

Bulk superconductor samples of YBa2Cu3O7−δ (YBCO) doped with nano metal oxides of Mn3O4, Co3O4, Cr2O3, CuO and SnO2 respectively with 0.2 wt% are synthesized by a solid-state reaction route. The structural characterization of all samples has been carried out by x-ray diffraction (XRD) and scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The XRD patterns indicate that the magnetic doping of nano metal oxides gives a high value of orthorhombicity of the YBCO samples which is the result of high oxygen content, and consequently could give better superconducting properties contrary to the non magnetic nano oxides (CuO, SnO2). The critical temperature (Tc) of the studied samples was found to improve by nano magnetic doping and lower with nano nonmagnetic doping. The superconducting transition temperature Tc determined from electrical resistivity measurements was found to increase for Mn3O4 (5.27 μB) doping and decrease for other metal oxides doping.

Effect of isovalent non-magnetic Fe-site doping on the electronic structure and spontaneous polarization of BiFeO3

We report the results of our first-principles calculations on the effect of isovalent, non-magnetic, Al3+ ion doping on the electronic structure and spontaneous polarization of multiferroic BiFeO3. Our calculations reveal that Al3+ doping in BiFeO3 results in the reduction of Fe–O–Fe bond angle, leading to the weakening of antiferromagnetic superexchange interaction, further substantiated by the reduction of exchange interaction constant with increasing doping level. Lowering of well-depth is suggestive of reduced switching potential and improved P-E loop with lowered coercivity. Chemical bonding analysis by electron localization function shows that cation–oxygen bonding is of mixed ionic–covalent character, with marginal increase in the covalent character with increasing doping concentration. Large spontaneous polarization of undoped BiFeO3 is retained with lower doping level (6.25%), while for higher doping content (31.25%), the spontaneous polarization is reduced, primarily due to larger c/a ratio at higher doping level.

Manipulation of magnetic state in phosphorene layer by non-magnetic impurity doping

Using the full-potential linearized augmented plane wave method, we have explored the structural, electronic band structure, binding energy, and magnetic properties of a non-magnetic impurity (Al, Si, S, and Cl) doped phosphorene layer. The binding energy showed a tendency to decrease with increasing atomic number. Nonetheless, we still found large binding energies of 5.1 eV ∼ 8.3 eV. We observed that the electronic band structure and magnetic property were strongly dependent on the specific impurity atom. For instance, we propose that the Al doped system will show a semiconducting transport property. In all other systems, we found metallic band structures. We predict that the Al and Si impurity act as donor elements whereas the S and Cl impurities behave as acceptor elements. Interestingly, the magnetic state appeared in Si, S, and Cl doped systems while the Al impurity induced no spin polarized state. In the Si doped system, the major contribution to a magnetic moment originated from Si impurity itself, while the presence of dangling bonds at a neighboring P atom near the impurity site plays a crucial role for magnetism in S and Cl doped systems. On the other hand, interestingly, we observed a half metallic feature in the S doped layer. Overall, we have found that the magnetic state can be manipulated by even non-magnetic impurity doping in a phosphorene layer.

Effect of transition metal doping on multiferroic ordering inFeVO4

$\mathrm{FeV}{\mathrm{O}}_{4}$ is a multiferroic that undergoes two antiferromagnetic transitions at ${T}_{\mathrm{N}1}\ensuremath{\sim}21\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}\ensuremath{\sim}15\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, with a small ferroelectric polarization developing at the 15-K transition. We have studied the effect of magnetic and nonmagnetic transition metal dopants on these magnetic phase transitions in order to investigate the microscopic mechanisms for magnetoelectric coupling in this system as well as probing the evolution of the phase transitions. We have synthesized polycrystalline $\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}\mathrm{x}}T{M}_{\mathrm{x}}\mathrm{V}{\mathrm{O}}_{4}\phantom{\rule{0.28em}{0ex}}(TM=\mathrm{Zn},\phantom{\rule{0.28em}{0ex}}\mathrm{Cr},\phantom{\rule{0.28em}{0ex}}\mathrm{Mn})$ samples up to $x=0.2$ and studied dynamics of these two ferroic phase transitions using thermodynamic, magnetic, and pyrocurrent measurements. We find that the magnetic ordering temperatures in $\mathrm{FeV}{\mathrm{O}}_{4}$ are remarkably stable, showing only a minimal suppression in transition temperatures even for larger doping fractions. We also observe clear reversible polarization at $x=0.05$ for all samples, which also persists to the largest doping for Zn, suggesting that the multiferroic order persists over a large range of compositions.

Antiferromagnetism ofZn2VO(PO4)2and the dilution withTi4+

We report static and dynamic properties of the antiferromagnetic compound Zn$_{2}$(VO)(PO$_{4}$)$_{2}$, and the consequences of non-magnetic Ti$^{4+}$ doping at the V$^{4+}$ site. $^{31}$P nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rate ($1/T_1$) consistently show the formation of the long-range antiferromagnetic order below $T_N= 3.8-3.9$\,K. The critical exponent $\beta=0.33 \pm 0.02$ estimated from the temperature dependence of the sublattice magnetization measured by $^{31}$P NMR at 9.4\,MHz is consistent with universality classes of three-dimensional spin models. The isotropic and axial hyperfine couplings between the $^{31}$P nuclei and V$^{4+}$ spins are $A_{\rm hf}^{\rm iso} = (9221 \pm 100)$ Oe/$\mu_{\rm B}$ and $A_{\rm hf}^{\rm ax} = (1010 \pm 50)$ Oe/$\mu_{\rm B}$, respectively. Magnetic susceptibility data above 6.5\,K and heat capacity data above 4.5\,K are well described by quantum Monte-Carlo simulations for the Heisenberg model on the square lattice with $J\simeq 7.7$\,K. This value of $J$ is consistent with the values obtained from the NMR shift, $1/T_1$ and electron spin resonance (ESR) intensity analysis. Doping Zn$_2$VO(PO$_4)_2$ with non-magnetic Ti$^{4+}$ leads to a marginal increase in the $J$ value and the overall dilution of the spin lattice. In contrast to the recent \textit{ab initio} results, we find neither evidence for the monoclinic structural distortion nor signatures of the magnetic one-dimensionality for doped samples with up to 15\% of Ti$^{4+}$. The N\'eel temperature $T_{\rm N}$ decreases linearly with increasing the amount of the non-magnetic dopant.

Effects of Doping on the Magnetic Properties and Frustration of Hexagonal YMn0.9A0.1O3 (A=Al, Fe, and Cu)

The doping effects on the frustration and the magnetic properties in hexagonal compounds of YMn0.9A0.1O3 (A=Al, Fe and Cu) are investigated. Experimental results indicate that both the non-magnetic and magnetic ion dopants lead to the increase of magnetic moments and the decrease of the absolute value of Curie—Weiss temperature (|θcw|). Compared with pure YMnO3, the geometrical frustration of YMn0.9A0.1O3 is greatly suppressed and the magnetic coupling in that exhibits dopant-dependent. In addition, for the doped YMn0.9A0.1O3, the antiferromagnetic transition temperature (TN) is also suppressed slightly, which shows an abnormal dilution effect and it may be ascribed to the reduction of frustration due to the chemical substitution.

Magnetism in spin models for depleted honeycomb-lattice iridates: Spin-glass order towards percolation

Iridates are characterized by a fascinating interplay of spin-orbit and electron-electron interactions. The honeycomb-lattice materials ${A}_{2}{\mathrm{IrO}}_{3}$ $(A=\text{Na},\text{Li})$ have been proposed to realize pseudospin-1/2 Mott insulating states with strongly anisotropic exchange interactions, described by the Heisenberg-Kitaev model, but other scenarios involving longer-range exchange interactions or more delocalized electrons have been put forward as well. Here we study the influence of nonmagnetic doping, i.e., depleted moments, on the magnetic properties of experimentally relevant variants of the Heisenberg-Kitaev and Heisenberg ${J}_{1}\text{\ensuremath{-}}{J}_{2}\text{\ensuremath{-}}{J}_{3}$ models. We generically find that the zigzag order of the clean system is replaced, upon doping, by a spin-glass state with short-ranged zigzag correlations. We determine the spin-glass temperature as a function of the doping level and show that this quantity allows one to assess the importance of longer-range exchange interactions when the doping is driven across the site-percolation threshold of the honeycomb lattice.

Magnetic and magnetocaloric properties of TbMnO3 and Tb0.67R0.33MnO3 (R=Dy, Y, and Ho) bulk powders

Abstract Orthorhombically distorted perovskite rare-earth manganites with smaller rare-earth ions, such as TbMnO3, have mostly been explored for their multiferroic properties. In this work, magnetic and magnetocaloric properties in both TbMnO3 and A-site substituted Tb0.67R0.33MnO3 (R=Dy, Y, Ho) powder samples, prepared via solution route, were investigated. A weak ferromagnetic component was observed in the dc magnetic data of Dy- and Ho-substituted TbMnO3 that may be due to the change in average ionic radius at the A-site (Tb-site) as well as the larger magnetic moment of the substituting ion (~10.6 μB for Dy3+ and Ho3+ as compared to 9.7 μB for Tb3+). The magnetic entropy was found to decrease with the dilution of the Tb3+ ordering by the introduction of both magnetic and non-magnetic dopants at the A-site. Magnetocaloric properties of these samples were evaluated and for pure TbMnO3, a maximum relative cooling power of ~103 J/kg with 5 T applied field was observed that shows its potential for magnetic refrigeration.

Interlayer coupling enhancement in graphene/hexagonal boron nitride heterostructures by intercalated defects or vacancies

Hexagonal boron nitride (hBN), a remarkable material with a two-dimensional atomic crystal structure, has the potential to fabricate heterostructures with unusual properties. We perform first-principles calculations to determine whether intercalated metal atoms and vacancies can mediate interfacial coupling and influence the structural and electronic properties of the graphene/hBN heterostructure. Metal impurity atoms (Li, K, Cr, Mn, Co, and Cu), acting as extrinsic defects between the graphene and hBN sheets, produce n-doped graphene. We also consider intrinsic vacancy defects and find that a boron monovacancy in hBN acts as a magnetic dopant for graphene, whereas a nitrogen monovacancy in hBN serves as a nonmagnetic dopant for graphene. In contrast, the smallest triangular vacancy defects in hBN are unlikely to result in significant changes in the electronic transport of graphene. Our findings reveal that a hBN layer with some vacancies or metal impurities enhances the interlayer coupling in the graphene/hBN heterostructure with respect to charge doping and electron scattering.

Correlation Between Temperature Dependent Photoluminescence and Origin of Room Temperature Ferromagnetism in Li Doped ZnO Nano-Flowers

Nanostructured ZnO is one of the most important dilute magnetic semiconductor materials (DMS) where room temperature ferromagnetism has been achieved by the doping of transition metals. However, transition metal doped ZnO are predominantly n-type materials. ZnO nanomaterial based DMS with p-type majority charge carrier are of great research interest as one can make use of both the magnetic as well as the optical proprieties for opto-electronic and spintronics applications. Recently, it has been reported that nonmagnetic Li doping in ZnO thin films show stable p-type characteristics with room temperature ferromagnetism. Thus there is a great need to understand both the optical and the magnetic properties in Li doped ZnO nanomaterials and a correlation between them. Here we report on the synthesis and studies of room temperature ferromagnetism and optical properties of Li doped ZnO nano-flowers (NFs). The room and low temperature SQUID measurement confirms the ferromagnetism in Li doped ZnO NFs. The temperature dependent photoluminescence measurements shed light on the origin of ferromagnetism. The comparison of the low temperature PL spectra with those of Li doped ZnO NFs shows appearance of distinct new peaks related to acceptor bound exciton and Zn vacancy in Li doped ZnO. Based on these findings from the optical studies we have correlated the origin of room temperature ferromagnetism with stable Zn vacancy in Li doped ZnO NFs. We also discuss the role of Li in stabilizing the Zn vacancy in particular lowering of the formation energy of the latter.

Ferromagnetism and topological surface states of manganese doped Bi2Te3: Insights from density-functional calculations

Based on first-principles calculations, the electronic, magnetic, and topological characters of manganese (Mn) doped topological insulator Bi2Te3 were investigated. The Mn substitutionally doped Bi2Te3, where Mn atoms tend to be uniformly distributed, was shown to be p-type ferromagnetic, arising from hole-mediated Ruderman-Kittel-Kasuya-Yosida interaction. Mn doping leads to an intrinsic band splitting at Γ point, which is substantially different from that of nonmagnetic dopant. The topological surface state of Bi2Te3 is indeed gapped by Mn doping; however, the bulk conductance limits the appearance of an insulating state. Moreover, the n-type doping behavior of Bi2Te3 is derived from Mn entering into the van der Waals gap of Bi2Te3.

Topological surface state in the Kondo insulator samarium hexaboride

Topological invariants of electron wavefunctions in condensed matter reveal many intriguing phenomena. A notable example is provided by topological insulators, which are characterized by an insulating bulk coexisting with a metallic boundary state. Although there has been intense interest in Bi-based topological insulators, their behaviour is complicated by the presence of a considerable residual bulk conductivity. Theories predict that the Kondo insulator system SmB6, which is known to undergo a transition from a Kondo lattice metal to a small-gap insulator state with decreasing temperature, could be a topological insulator. Although the insulating bulk and metallic surface separation has been demonstrated in recent transport measurements, these have not demonstrated the topologically protected nature of the metallic surface state. Here we report thickness-dependent transport measurements on doped SmB6, and show that magnetic and non-magnetic doping results in contrasting behaviour that supports the conclusion that SmB6 shows virtually no residual bulk conductivity.

High-frequency magnetic properties and their thermal stability in diluted IrMn–Al2O3/FeCo exchange-biased multilayers

In this work, we prepare and characterize FeCo/IrMn–Al2O3 multilayers using magnetron sputtering method with various doping concentrations in order to study the doping effect of the antiferromagnetic layer in exchange-biased films on their high frequency magnetic properties and their thermal stability for fundamental research as well as for application. The influences of the nonmagnetic Al2O3 doping in the antiferromagnetic layers on the high frequency magnetic properties from 1GHz up to 8GHz and their thermal stability of the FeCo/IrMn exchange-biased multilayer have been investigated in the temperature range from room temperature up to 420K. The temperature behavior of exchange bias, static magnetic anisotropy field, dynamic magnetic anisotropy field, rotatable magnetic anisotropy field, resonant frequency and damping of the samples are presented and interpreted by classifying the antiferromagnet grains into random spin grains, frozen spin grains and rotatable spin grains. In particular, the roles and changes of each grain type with the doping amount and with temperature are discussed in conjunction with the variations of the magnetic and microwave parameters.

On the enhancement of magnetic anisotropy in cobalt clusters via non-magnetic doping

We show that the magnetic anisotropy energy (MAE) in cobalt clusters can be significantly enhanced by doping them with group IV elements. Our first-principles electronic structure calculations show that Co4C2 and Co12C4 clusters have MAEs of 25 K and 61 K, respectively. The large MAE is due to controlled mixing between Co d- and C p-states and can be further tuned by replacing C by Si. Larger assemblies of such primitive units are shown to be stable with MAEs exceeding 100 K in units as small as 1.2 nm, in agreement with the recent observation of large coercivity. These results may pave the way for the use of nano-clusters in high density magnetic memory devices for spintronics applications.

Mechanism of ferromagnetism in non-magnetic ion-doped zinc oxides

Zinc oxide (ZnO) that contains non-magnetic ionic dopants, such as nitrogen (N)-doped zinc oxide (ZnO:N), has been observed to exhibit ferromagnetism. Ferromagnetism is proposed to arise from the Coulomb excitation in the localized states that is induced by the oxygen vacancy, V O. A model based on the Coulomb excitation that is associated with the electron–phonon interaction theoretically explains the ferromagnetic mechanism of ZnO:N. This study reveals that the ferromagnetism will be induced by either deep localized states with a small V O concentration or shallow localized states with a high V O concentration. Additionally, electron–phonon coupling either suppresses the ferromagnetism that is induced by the deep donor states of V O or enhances the ferromagnetism that is induced by the shallow donor states of V O.

Effect on Magnetic Properties of Zinc Doped Nano Ferrites Synthesized by Precursor or Method

Nanocrystalline Cu-Zn ferrites have been synthesized using precursor method. Cu-Zn ferrites were formed at low temperature without any impurities. The particle sizes were observed to decrease from 60 nm to 50 nm with increasing non-magnetic Zn doping. Cu is used to decrease the sintering temperature. The X-ray diffraction (XRD) and IR analysis of Cu-Zn revealed the formation of Single-Phase Spinel structure at very low annealing temperature. The particle sizes observed from XRD is very well in agreement with SEM analysis. Cu-Zn ferrite nanoparticles were observed to be dependent on the particle size. Saturation (Ms) and Remanence (Mr) magnetization of ferrites increases due to the modifications occurred among the A-B, A-A and B-B interactions of Spinel structure. The Coercive force (Hc) decreases with increase of Zn ions concentration.