Magnetic properties of nitrogen doped ZnO

Using the first principle method based on density functional theory,this paper studies the electronic structure and the ferromagnetic stability in N-doped ZnO.The calculated results based on local density approximation(LDA) and LDA+U method show that ferromagnetism coupling between N atoms is more energetically favourable for eight geometrically distinct configurations.The dominant ferromagnetic interaction is due to the hybridization between O 2p and N 2p.The origin of the ferromagnetic state in N doped ZnO is discussed by analysing coupling between N 2p levels.We also analyse N dopant concentration and lattice strain effect on ferromagnetism.     

Ferromagnetism of V-doped ZnO nanowires

The geometry structures,electronic structures,and magnetic properties of Zn46V2O48 nanowires are studied by density functional theory(DFT) calculations.We find that the ferromagnetic(FM) coupling is more stable for six configurations of Zn46V2O48 nanowires,and is mediated by neighboring O as evidenced from the strong hybridization of V 3d and O 2p states,exhibiting strong spin polarization.The spin polarization is found to be 100% in the Zn46V2O48 nanowires,which confirms that it is a half-metallic ferromagnet and very suitable for the injection of the spin carriers,which shows that Zn46V2O48 nanowire is one of the ideal materials to realize spin electronic devices.At the same time,the magnetic coupling mechanisms of Zn46V2O48 nanowires are analyzed with V 3d and O 2p orbitals and their magnetic moments mainly come from the contributions of the unpaired electrons of V 3d orbitals.The above results provide a theoretical basis for the preparation of 3d transition metal-doped ZnO nanowire materials.     

Influence of the Jahn-Teller distortion on magnetic ordering in TbMn_1-xFe_xO₃

The Jahn-Teller distortion plays an important role in determining the exchange interaction in rare-earth manganites.In this work we study the influence of the Jahn-Teller distortion on the magnetic structures of TbMn1-xFexO3(x = 0,0.02,0.05,0.10,and 0.20) single crystals in the basal MnO2 plane.The decrease in the quadruple splitting with the increasing Fe doping indicates the reduction of the Jahn-Teller distortion,which makes the nearest neighboring(NN) FM interaction dominant over the next nearest neighbor(NNN) AFM interaction.This alteration is favorable for the development of A-type AFM ordering instead of the spiral magnetic ordering,which collapses when x ≥ 0.05.The analysis of dielectric data indicates that the ferroelectricity is arising from the peculiar spiral magnetic ordering.     

Strain-tuned magnetic properties in (Ga,Fe)Sb:First-principles study

In view of the importance of enhancing ferromagnetic(FM) coupling in dilute magnetic semiconductors(DMSs),the effects of strain on the electronic structures and magnetic properties of(Ga,Fe)Sb were examined by a first-principles study.The results of the investigation indicate that Fe_(Ga) substitution takes place in the low-spin state(LSS) with a total magnetic moment of 1μB in the strain range of-3% to 0.5%,which transitions to the high-spin state(HSS) with a total magnetic moment of 5μB as the strain changes from 0.6% to 3%.We attribute the changes in the amount and distribution of the total moment to the influence of the crystal field under different strains.The FM coupling is strongest under a strain of about0.5%,but gradually becomes weaker with increasing compressive and tensile strains.The magnetic coupling mechanism is discussed in detail.Our results highlight the important contribution of strain to magnetic moment and FM interaction intensity,and present an interesting avenue for the future design of high Curie temperature(T_C) materials in the(Ga,Fe)Sb system.     

Berry Phase of Composite System Induced by Time-Dependent Interaction

The Berry phase in a composite system induced by the time-dependent interaction is discussed. We choose two coupled spin-1/2 systems as the composite system： one of the subsystems is subjected to a static magnetic field, and the coupling parameters between two spins are controllable in time. We show that the time-dependent interaction can induce the Berry phase in a similar way as that a spin-1/2 system （qubit） is driven by an effective time-dependent magnetic field. Furthermore, using two consecutive cycles with opposite directions of both the static magnetic field as well as opposite signs of the coupling parameters, a nontrivial two-qubit unitary transformation purely based on Berry phases can be constructed.     

Electronic structures and magnetic properties of Zn-and Cd-doped AlN nanosheets:A first-principles study

In this paper, the magnetic properties, electronic structures and the stabilities of Zn/Cd incorporated two-dimensional Al N nanosheets are investigated by the first-principles method. Numerical results indicate that Zn and Cd substituting Al atom in Al N nanosheets introduce some holes into the 2p orbitals of the N atoms, and the holes mainly come from spindown 2p orbitals of the N atoms. The magnetic moment of 1.0 μBis produced by Zn/Cd doping Al N nanosheets, and the main component of the magnetic moment of the system is contributed by the partially filled 2p states of the N atoms around the dopant. In particular, when Zn/Cd substituting Al atoms, the magnetic coupling is found to be ferromagnetic. We attribute the hole-mediated p–d interaction to the created ferromagnetic coupling. More importantly, the result of formation energy indicates that Al atom is more inclined to be replaced by Zn atom rather than Cd. This finding is beneficial to developing the spin electronic devices.     

Influence of the Jahn—Teller distortion on magnetic ordering in TbMn1-xFexO3

The Jahn-Teller distortion plays an important role in determining the exchange interaction in rare-earth manganites.In this work we study the influence of the Jahn-Teller distortion on the magnetic structures of TbMn1-xFexO3（x = 0,0.02,0.05,0.10,and 0.20） single crystals in the basal MnO2 plane.The decrease in the quadruple splitting with the increasing Fe doping indicates the reduction of the Jahn-Teller distortion,which makes the nearest neighboring（NN） FM interaction dominant over the next nearest neighbor（NNN） AFM interaction.This alteration is favorable for the development of A-type AFM ordering instead of the spiral magnetic ordering,which collapses when x ≥ 0.05.The analysis of dielectric data indicates that the ferroelectricity is arising from the peculiar spiral magnetic ordering.     

Ferrimagnetic Properties of Bond Dilution Mixed Blume-Capel Model with Random Single-Ion Anisotropy

We study the ferrimagnetic properties of spin 1/2 and spin-1 systems by means of the effective field theory. The system is considered in the framework of bond dilution mixed Blume-Capel mode/ （BCM） with random single-ion anisotropy. The investigation of phase diagrams and magnetization curves indicates the existence of induced magnetic ordering and single or multi-compensation points. Special emphasis is placed on the influence of bond dilution and random single-ion anisotropy on normal or induced magnetic ordering states and single or multi-compensation points. Normal magnetic ordering states take on new phase diagrams with increasing randomness （bond and anisotropy）, while anisotropy induced magnetic ordering states are always occurrence no matter whether concentration of anisotropy is large or small. Existence and disappearance of compensation points rely strongly on bond dilution and random singleion anisotropy. Some results have not been revealed in previous papers and predicted by Néel theory of ferrimagnetism.     

Antiferromagnetic ferromagnetic transition in zigzag graphene nanoribbons induced by substitutional doping

Using first-principles calculations based on density functional theory, we show that the ground state of zigzag-edged graphene nanoribbons(ZGNRs) can be transformed from antiferromagnetic(AFM) order to ferromagnetic(FM) order by changing the substitutional sites of N or B dopants. This AFM–FM transition induced by substitutional sites is found to be a consequence of the competition between the edge and bulk states. The energy sequence of the edge and bulk states near the Fermi level is reversed in the AFM and FM configurations. When the dopant is substituted near the edge of the ribbon, the extra charge from the dopant is energetically favorable to occupy the edge states in AFM configuration. When the dopant is substituted near the center, the extra charge is energetically favorable to occupy the bulk states in FM configuration. Proper substrate with weak interaction is necessary to maintain the magnetic properties of the doped ZGNRs. Our study can serve as a guide to synthesize graphene nanostructures with stable FM order for future applications to spintronic devices.     

Dynamic magnetic behavior of the mixed spin （2, 5/2） Ising system with antiferromagnetic/antiferromagnetic interactions on a bilayer square lattice

Using the mean-field theory and Glauber-type stochastic dynamics, we study the dynamic magnetic properties of the mixed spin （2, 5/2） Ising system for the antiferromagnetic/antiferromagnetic （AFM/AFM） interactions on the bilayer square lattice under a time varying （sinusoidal） magnetic field. The time dependence of average magnetizations and the thermal variation of the dynamic magnetizations are examined to calculate the dynamic phase diagrams. The dynamic phase diagrams are presented in the reduced temperature and magnetic field amplitude plane and the effects of interlayer coupling interaction on the critical behavior of the system are investigated. We also investigate the influence of the frequency and find that the system displays richer dynamic critical behavior for higher values of frequency than that of the lower values of it. We perform a comparison with the ferromagnetic/ferromagnetic （FM/FM） and AFM/FM interactions in order to see the effects of AFM/AFM interaction and observe that the system displays richer and more interesting dynamic critical behaviors for the AFM/AFM interaction than those for the FM/FM and AFM/FM interactions.     

Computational materials design of filled tetrahedral compound magnetic semiconductors

Based on first-principles calculations within the density functional theory, materials design of filled tetrahedral compound magnetic semiconductors is proposed. By using the Korringa–Kohn–Rostoker coherent potential approximation, electronic structures of Mn-doped LiZnAs, LiZnP and LiZnN are calculated. First, by estimating free energy, phase diagrams of these systems are predicted. It is shown that these systems are phase separating systems and favor spinodal decomposition. However, by introducing Li vacancies, spinodal decomposition is strongly suppressed and Mn can be doped up to high concentration. Moreover, the introduced Li vacancies induce ferromagnetic interaction between Mn and thus we can expect high Curie temperature (T_C) in these systems. To see the chemical trend, electronic structure and T_C of Li(Zn, Cr)As are also calculated.     

Magnetic ordering in CeMnCuSi2

Temperature and field-dependent magnetization measurements on polycrystalline CeMnCuSi₂ reveal that the Mn moments in this compound exhibit ordering with a ferromagnetic (FM) component ordered instead of the previously reported purely antiferromagnetic (AFM) ordering. The FM ordering temperature, T_c, is about 120 K and almost unchanged with external fields up to 50 kOe. Furthermore, an AFM component (such as in a canted spin structure) is observed to be present in this phase, and its orientation is modified rapidly by the external magnetic field. The Ce L₃-edge X-ray absorption result shows that the Ce ions in this compound are nearly trivalent, very similar to that in the heavy fermion system CeCu₂Si₂. Large thermomagnetic irreversibility is observed between the zero-field-cooled (ZFC) and field-cooled (FC) M(T) curves below T_c indicating strong magnetocrystalline anisotropy in the ordered phase. At 5 K, a metamagnetic-type transition is observed to occur at a critical field of about 8 kOe, and this critical field decreases with increasing temperature. The FM ordering of the Mn moments in CeMnCuSi₂ is consistent with the value of the intralayer Mn–Mn distance R^a_Mn–Mn=2.890 Å, which is greater than the critical value 2.865 Å for FM ordering. Finally, a magnetic phase diagram is constructed for CeMnCuSi₂.     

The electronic and magnetic properties of （Mn,C）-codoped ZnO diluted magnetic semiconductor

The electronic and magnetic properties of (Mn,C)-codoped ZnO are studied in the Perdew-Burke-Ernzerhof form of generalized gradient approximation of the density functional theory. By investigating five geometrical configurations, we find that Mn doped ZnO exhibits anti-ferromagnetic or spin-glass behaviour, and there are no carriers to mediate the long range ferromagnetic (FM) interaction without acceptor co-doping. We observe that the FM interaction for (Mn,C)-codoped ZnO is due to the hybridization between C 2p and Mn 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. Meanwhile, we demonstrate that ZnO co-doped with Mn and C has a stable FM ground state and show that the (Mn,C)-codoped ZnO is FM semiconductor with super-high Curie temperature (T C = 5475 K). These results are conducive to the design of dilute magnetic semiconductors with codopants for spintronics applications.     

Interaction between the magnetic moments of the 3d and the 4f electrons in manganite, probed by Ga substitution

The substitution of Ga for Mn in manganite Nd_0.6Dy_0.1Sr_0.3MnO₃ with a ferromagnetic (FM) ground state has been performed to study the influence of the Mn-sublattice magnetic ordering on the magnetic rare-earth sublattice. It is found that the substitution of Mn³⁺ with Ga³⁺ ions results in a sharp decrease of T_C, reflecting the reduction of the double-exchange interactions strength J_Mn-Mn. At the same time, a depinning effect of the rare-earth magnetic moment has been observed. This behavior unambiguously proves that the exchange interaction between Mn and rare-earth ions J_Mn-R strongly influences the rare-earth magnetic ordering at temperatures below T_C and stabilizes the rare-earth magnetic ground state.     

Microstructural and magnetic properties of Ax:Zn1-xO (A=Mn,Gd and Mn/Gd) nanocrystals

We report the microstructural and magnetic properties of transition (3d) and rare earth (4f) metal substituted into the A_x:Zn_1?xO (A=Mn, Gd and Mn/Gd) nanocrystal samples synthesized by solgel method. The structural properties and morphology of all samples have been analysed using X-ray diffraction (XRD) method and scanning electron microscopy. The impurity phase in the XRD patterns for all samples is not seen, except (Mn/Gd):ZnO sample where a very weak secondary phase of Gd₂O₃ is observed. Due to the large mismatch of the ionic radii between Mn²⁺ and Gd³⁺ ions, the strain inside the matrix increases, unlike the crystallite size decreases with the substitution of Mn and Gd into ZnO system. A couple of additional vibration modes due to the dopant have been observed in Raman spectrum. The magnetic properties have been studied by vibrating sample magnetometer. The magnetic hysteresis shows that Mn:ZnO and Gd:ZnO have soft ferromagnetic (FM) behaviour, whereas (Mn/Gd):ZnO has strong FM behaviour at room temperature (RT). The enhancement of ferromagnetism (FM) in (Mn/Gd):ZnO sample might be related to short-range FM coupling between Mn²⁺ and Gd³⁺ ions via defects potential and/or strain-induced FM coupling due to the expansion lattice by doping. The experimental results indicate that RTFM can be achieved by co-substitution of 3d and 4f metals in ZnO which can be used in spintronics applications.     

A first-principles study of ferromagnetism in Pd-doped ZnO

The electronic structures and magnetic properties of Pd-doped ZnO have been studied by the full-potential linear augmented plane wave (FLAPW) method using the generalized gradient approximation (GGA). We find that Pd-doped ZnO becomes 100% spin polarized when Pd substitutes for Zn in the 2×2×2 ZnO supercell. The supercell magnetic moment reaches 2.0μ_B. Long-range ferromagnetic (FM) coupling is obtained with all Pd dopant configurations, and a Curie temperature as high as 860 K is predicted for the ground-state configuration. The hybridized Pd(4d)-O(2p)-Zn(4d)-O(2p)-Pd(4d) chain formed through p-d-like coupling is responsible for the long-range room-temperature FM coupling. We discuss the effect of O vacancies or Zn vacancies on the magnetism as well.     

Observation of mixed-phase behavior in the Mn-doped cobaltite La0.7Sr0.3Co1−xMnxO3 (x=0-0.5)

Studies on La_0.7Sr_0.3Co_1−xMn_xO₃ (x=0-0.5) compounds evidence that the interaction between Mn and Co ions in this system is antiferromagnetic super-exchange and not ferromagnetic (FM) double-exchange (DE). As a result, antiferromagnetism and magnetic glassiness develop steadily with increasing Mn content and the system becomes a spin glass at x∼0.1. Analyses of high-field magnetization data indicate that the system consists of two major phases: a metallic FM phase which magnetically saturates in rather low field, and an insulating non-FM phase which has a linear dependence of magnetization on magnetic field. In the low doping regime, the fraction of the non-FM component expands with temperature at the expense of the FM phase and becomes maximal at T_C. Ferromagnetism reappears in highly doped (x≥0.2) compounds due to the presence of DE interaction between the Mn ions. The small volume fraction of the FM phase derived from the M(H) data in high-field region supports the coexistence of insulating and FM behaviors in the highly doped samples.     

Intrinsic magnetic inhomogeneity of Eu substituted La0.7Pb0.3Mno3 single crystals

The study of magnetic and magnetotransport properties of the crystals of (La_1−yEu_y)_0.7Pb_0.3MnO₃ system has been carried out. Eu ions enter the crystals being in trivalent nonmagnetic state. Europium ions possessing of smaller ionic radius in comparison with La ions, induce local distortions of Mn–O–Mn bonds in the system that cause random distribution of magnetic exchange interactions in magnitude and, probably, in sign. The competition of magnetic interactions leads to the appearance of the inhomogeneous magnetic state in the crystals. The enhancement of concentration of Eu ions results in decrease of the Curie temperature and broadening of the inhomogeneous magnetic state area. At y=0–0.4 the coexistence of the paramagnetic phase with conductivity of the polaronic type and the ferromagnetic metallic phase is observed in a bounded temperature interval both above and below T_C. Below T_C the increasing of y up to 0.6 induces the magnetic state representing the coexistence of two different FM phases. These phases are spatially separated due to frustration of FM and AFM exchange interactions on phase boundaries. Above T_C, up to 1.6T_C ferromagnetic clusters exist in a paramagnetic matrix similar to the case of samples with y=0–0.4. Concerning electric properties, the samples with y=0–0.4 reveal the metal–insulator transition at temperature that practically coincides with T_C. The sample with y=0.6 has conductivity of insulator character up to the lowest temperatures. For all investigated compositions y=0–0.6 the CMR effect is observed in the area where the inhomogeneous magnetic state exists. The effect is determined by different conductivity of the coexisting phases, as well as by sensitivity of the inhomogeneous state to external magnetic field.     

First-principles study of ferromagnetism in Zn- and Cd-doped SnO2

The electronic structures and magnetic properties of Zn- and Cd-doped SnO₂ are investigated using first-principles calculations within the generalized gradient approximation (GGA) and GGA+U scheme. The substitutional Zn and Cd atoms introduce holes in the 2p orbitals of the O atoms and the introduced holes are mostly confined to the minority-spin states. The magnetic moment induced by doping mainly comes from the 2p orbitals of the O atoms, among which the moment of the first neighboring O atoms around the dopant are the biggest. The U correction for the anion-2p states obviously increases the moment of the first neighboring O atoms and transforms the ground states of the doped SnO₂ from half-metallic to insulating. The magnetic coupling between the moments induced by two dopants is ferromagnetic and the origin of ferromagnetic coupling can be attributed to the p–d hybridization interaction involving holes.     

Theoretical study of Mn doping effects and O or Zn vacancies on the magnetic properties in wurtzite ZnO

The effects of including the exchange interaction (J) and Hubbard on-site Coulombic interaction (U) on the structural parameters and magnetic moment of Mn-doped ZnO were explored. The calculations were performed with the plane-wave pseudopotential method along with generalized-gradient approximations (GGA). Using the GGA+U + +J method by applying Hubbard corrections U_d to the Zn 3d states and U_p to the O 2p states, the lattice constants were calculated for various reported Hubbard parameters. The difference in the lattice constants between the calculated results and experimental measurements is within 1% for pure ZnO and pure MnO. This study considers three cases: (i) substitution of Mn for Zn, (ii) substitution of Mn for Zn combined with Zn vacancy, and (iii) substitution of Mn for Zn with O vacancy. Results are shown that the system is ferromagnetic (FM) when zinc vacancies are present. For three cases with oxygen vacancies, only one of them is FM. It was also found that the Hubbard U and exchange interaction J improved the calculated results, allowing it to exhibit good agreement properties for Mn-doped ZnO with the experimental data.