First-principle study of magnetism in Co-doped SnO2

The effects of Co dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO₂ are studied by the first-principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. The Co atoms favorably substitute on neighboring sites of the metal sublattice. Without oxygen vacancies, the Co atoms are at low spin state independent of concentration and distribution of Co atoms, and only the magnetic coupling between nearest-neighbor Co atoms is ferromagnetic through direct exchange and super-exchange interaction. Oxygen vacancies tend to locate near the Co atoms. Their presence strongly increases the local magnetic moments of Co atoms, which depend sensitively on the concentration and distribution of Co atoms. Moreover, oxygen vacancies can induce the long-range ferromagnetic coupling between well-separated Co atoms through the spin-split impurity band exchange mechanism. Thus the room temperature ferromagnetism observed experimentally in the Co-doped SnO₂ may originate from the combination of short-range direct exchange and super-exchange interaction and the long-range spin-split impurity band exchange model.     

Magnetic properties of (Ga,Mn)As

A summary of experimental findings and theoretical modelling of micromagnetic properties of zinc-blende ferromagnetic semiconductor (Ga,Mn)As is presented. It is shown that the Zener p–d model explains quantitatively observed Curie temperatures in compensation free samples and that major strain-related effects are correctly accounted for, including the presence of the magnetization reorientation transition, observed as a function of hole concentration and temperature. It is evidenced that a presence of a small trigonal distortion could account for both the presence and properties of uniaxial in-plane magnetic anisotropy.     

Magnetic and transport properties of Mn-doped Bi2Se3 and Sb2Se3

We report on the single crystal growth and thermoelectric and magnetic properties of Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals prepared by the temperature gradient solidification method. The composition and crystal structure were determined using electron probe microanalysis and θ–2θ powder X-ray diffraction studies, respectively. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the undoped sample due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin-glass and paramagnetic properties, respectively.     

Interplay of local structure and magnetism in Co-doped TiO2 anatase

We report ab initio density-functional theory investigations on the local structure and magnetization of Co ions doped in TiO₂ anatase. The calculated formation energy of the pair of substitutional Co ions indicates that they have a tendency to cluster; but clustering has no noticeable effect on the low-spin state of Co. The interstitial Co, which is energetically unstable in reference to bulk cobalt, is found to be strongly attracted to a substitutional Co, and even more strongly to a substitutional Co pair. Interestingly, in a one-to-one binding, the interstitial Co enhances the magnetization of the two; whereas in a one-to-two binding, it destroys the magnetic moment of the substitutional Co pair and therefore reduces the average magnetic moment of Co ions. Our results could explain the strong sample-to-sample variability of the magnetic moment of Co measured in experiments. The magnetic interaction between substitutional and interstitial Co is discussed with bonding analysis.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

The influence of hydrogen annealing on magnetism of Co-doped TiO2 films prepared by sol–gel method

Co-doped TiO₂ (Co_xTi_1−xO₂, 0.05?x?0.2) films have been prepared on Si (0 0 1) substrates by sol–gel method. When heat treated in air, Co_xTi_1−xO₂ films are non-ferromagnetic at room temperature. However, after further annealed in a flowing hydrogen atmosphere, Co_xTi_1−xO₂ films show room-temperature ferromagnetism (RTFM). Measurements of magnetization (M) vs. temperature (T), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) fail to detect Co clusters in the hydrogenated Co_0.1Ti_0.9O₂ films, suggesting that RTFM in the hydrogenated Co_0.1Ti_0.9O₂ films may be intrinsic. But, metal Co appears in the hydrogenated Co_0.2Ti_0.8O₂ films, showing that RTFM in the hydrogenated Co_0.2Ti_0.8O₂ films is as least partly due to metal Co. These results indicate that hydrogen annealing can produce room-temperature ferromagnetism in Co_xTi_1−xO₂ films, but it should be carefully designed to avoid the formation of metal Co in the hydrogenated Co_xTi_1−xO₂ films.     

On the room-temperature ferromagnetism in (ZnO)0.98(MnO2)0.02

Room-temperature ferromagnetism (RTFM) is investigated in the polycrystalline bulk (ZnO)_0.98(MnO₂)_0.02 samples prepared by a modified solid-state sintering route. Successive sintering of a sample was carried out in air at different temperatures in the range of 400-1000 °C. The study of magnetization and phase-investigation in the sample was carried out after each sintering step. The progressive suppression of impurities and the consequent reduction in RTFM is clearly observed in the samples with increase in the sintering temperature up to 800 °C. The subsequent successive sintering of the (ZnO)_0.98(MnO₂)_0.02 sample up to 1000 °C yields fully paramagnetic sample exhibiting wurtzite structure. The studies support the conjecture (Kundaliya et al., Nat. Mater. 3 (2004) 709 [18]) that RTFM in this system has an origin related to a randomly distributed impurity phase produced by local dissolution of ZnO and MnO₂.     

Anisotropy of magnetoresistance in Be Co-doped GaMnAs

Magnetotransport properties of GaMnAs co-doped with Be have been studied as a function of applied magnetic field orientations and temperature. It was shown that magnetoresistivity phenomena in GaMnAs:Be depend not only on the relative orientation of the current and applied magnetic field, but also on the respective orientation of these two vectors to the crystalline axis.     

Influence of temperature on critical fields in ZnCr2Se4

The complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr₂Se₄ spinel. An antiferromagnetic order with a Néel temperature T_N=20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie-Weiss temperature θ_CW=55.1 K were established. An increasing static magnetic field shifts T_N to lower temperatures while a susceptibility peak at T_m in the paramagnetic region—to higher temperatures. The non-zero and negative values both of the second and third harmonics of susceptibility suggest only a parallel spin coupling in ferromagnetic clusters in the range between the Néel and Curie-Weiss temperatures. Below T_N the magnetic field dependence of susceptibility, χ_ac(H), shows two peaks at critical fields H_c1 and H_c2. The values of H_c1 decrease slightly with temperature while the values of H_c2 drop rapidly with temperature. The strong changes of H_c2 temperature induced are mainly responsible for a spin frustration of the re-entrant type in the spinel under study.     

Origin of ferromagnetism via hole doping in SnO2: First-principles calculation

First-principles calculations are carried out in order to find the ferromagnetism dependence on the number of holes substituted for Sn sites. The results show that strong localization of defect states of the p bands of the oxygen atoms near the dopants favors high-spin states and local moment formation. These states appear to be ferromagnetically coupled with a rather long-range magnetic interaction, resulting in a half-metallic ferromagnetic ground state for the whole systems. Analysis of the total energies indicates that the induced well-confined ferromagnetism in the oxygen p orbitals due to hole doping is quite possible and easily controlled in these systems, which indicate a new way to develop a half-metallic ferromagnet in nonmagnetic d⁰ oxides.     

Nonmagnetic impurities induced magnetism in SnO2

A density functional study is performed to investigate the magnetism induced by the nonmagnetic impurity substitution for the cation in SnO₂. The calculated results show that the K impurity substitution leads to a robust magnetism in SnO₂, and the induced magnetic moments are mainly attributed to the first shell of oxygen atoms surrounding the impurity atom. Meanwhile, no magnetism is observed in SnO₂ doped with Ca which implies a decreasing tendency of induced magnetic moments for Sn substituted by vacancy, K, and Ca. It is also demonstrated that the magnetic coupling constant oscillates as a function of K-K separation distance, and the Curie temperature above room temperature can be obtained in K-doped SnO₂.     

Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films

Co-doped TiO₂ films were fabricated under different conditions using reactive facing-target magnetron sputtering. Co doping improves the transformation of TiO₂ from anatase phase to rutile phase. The chemical valence of doped Co in the films is +2. All the films are ferromagnetic with a Curie temperature above 340 K. The average room-temperature moment per Co of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases from 0.74 μ_B at x=0.03 to 0.02 μ_B at x=0.312, and decreases from 0.54 to 0.04 μ_B as x increases from 0.026 to 0.169 for the Co-doped TiO₂ films fabricated at 0.27 Pa. The ferromagnetism originates from the oxygen vacancies created by Co²⁺ dopants at Ti⁴⁺ cations. The optical band gaps value (E_g) of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases linearly from 3.35 to 2.62 eV with the increasing x from 0 to 0.312. For the Co-doped TiO₂ films fabricated at 1.86 Pa, the E_g decreases linearly from 3.26 to 2.53 eV with increasing x from 0 to 0.350.     

Effect of SnO2 coating on the magnetic properties of nanocrystalline CuFe2O4

Nanocrystalline CuFe₂O₄ and CuFe₂O₄/xSnO₂ nanocomposites (x=0, 1, 5 wt%) have been successfully synthesized by one-pot reaction of urea-nitrate combustion method. The transmission electron microscope study reveals that the particle size of the as synthesized CuFe₂O₄ and CuFe₂O₄/5 wt%SnO₂ are 10 and 20 nm, respectively. The SnO₂ coating on the nanocrystalline CuFe₂O₄ was confirmed from HRTEM studies. The resultant products were sintered at 1100 °C and characterized by XRD and SQUID for compound formation and magnetic studies, respectively. The X-ray diffraction pattern shows the well-defined sharp peak that confirms the phase pure compound formation of tetragonal CuFe₂O₄. The zero field cooled (ZFC) and field cooled (FC) magnetization was performed using SQUID magnetometer from 2 to 350 K and the magnetic hysteresis measurement was carried out to study the magnetic properties of nanocomposites.     

Effects of Ag addition on electrical transport and magnetic properties of La0.67Ba0.33Mn0.88Cr0.12O3

We have prepared a series of polycrystalline manganites with the nominal compositions, La_0.67Ba_0.33Mn_0.88Cr_0.12O₃/Ag_x (LBMCO/Ag_x) (x   is the mole fraction) with x=0$x=0$, 0.05, 0.1, 0.15, 0.2, 0.23, 0.27, 0.3, 0.35. The X-ray diffraction patterns show that the samples with x>0.05$x>0.05$ are two-phase composites. The Ag addition in LBMCO improves the properties of grain surfaces/boundaries and reduces the resistivity of the composites. For x=0.30$x=0.30$ sample, a minimum resistivity is obtained and a maximum room temperature magnetoresistance up to −54.5% was observed at 288 K, 1 T field. The room temperature T_C and the reduced resistivity are responsible for the enhancement of room temperature MR.     

Large enhancement of room temperature magnetoresistance in Ag-added La0.67(Ca0.65Ba0.35)0.33MnO3

The electrical and magnetoresistant properties of La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃/Ag_x (abbreviated by LCBMO/Ag_x) have been studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for x=0.27 sample in 10 kOe magnetic field is 41%, almost 20 times larger than that for x=0 sample. This enhancement is related to that the Curie temperature (T_c) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of experimental results for x=0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.     

Magnetic properties of the quasi-one-dimensional system BaMn2V 2O8

Magnetic properties of BaMn₂V ₂O₈ are investigated by means of susceptibility, magnetization, and heat capacity measurements. Our experimental results show that BaMn₂V ₂O₈ is a one-dimensional canted antiferromagnet. The antiferromagnetic transition at a relatively high T_N of 37 K may be due to an enhancement of interchain interaction in the system, since Mn²⁺ ions have large spin moment of S=5/2. Weak ferromagnetism in BaMn₂V ₂O₈ may be due to Dzyaloshinskii-Moriya interactions arising from its noncentrosymmetric crystal structure.     

Magnetic behavior of nanocrystalline LaMn2Ge2

The compound, LaMn₂Ge₂, crystallizing in ThCr₂Si₂-type tetragonal crystal structure, has been known to undergo ferromagnetic order below (T_C=) 326 K. In this article, we report the magnetic behavior of nanocrystalline form of this compound, obtained by high-energy ball milling. T_C of this compound is reduced marginally for the nanoform, whereas there is a significant reduction of the magnitude of the saturation magnetic moment with increasing milling time. The coercive field however increases with decreasing particle size. Thus, this work provides a route to tune these parameters by reducing the particle size in this ternary family.     

Magnetic properties of doped ZnO prepared by different synthetic routes

We have studied the magnetic properties of Zn_0.96M_0.04O (M=Mn, Fe, Co) compounds prepared using several routes. The low temperature ceramic synthesis gave multiphasic samples and show ferromagnetic behavior. Single phases can be obtained by heating at higher temperatures (∼900–1100 °C). The use of very low oxygen pressure also favours the preparation of single-phases. We were also successful in preparing single-phase samples at very low temperature (∼400 °C) by using a sol-gel method. All of the samples without noticeable secondary phases in the X-ray patterns behave as conventional paramagnets. This is true even for the samples with very low grain size. Samples exhibiting secondary phases reveal spontaneous magnetization even at room temperature in some cases. Our results strongly support that ferromagnetism at room temperature is always due to the presence of secondary phases and not to the doping of ZnO.     

Structural, electronic and magnetic properties of Cr-doped (ZnTe)12 clusters

We have studied the energetics and magnetism in Cr-doped (ZnTe)₁₂ clusters by first principles density functional calculations. Total energy calculations suggest that it is energetically most favourable for Cr atoms to substitute at Zn sites. Both ferromagnetic and anti-ferromagnetic coupling between the Cr atoms exist depending on the Cr-Cr distance in the clusters. The magnetic exchange coupling between Cr atoms is short-ranged.     

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.