The magnetic behavior of La1−xTbxMn2Si2 (0≤x≤0.3) in weak magnetic fields

The structure and magnetic properties of La_1−xTb_xMn₂Si₂ (0≤x≤0.3) were studied by X-ray powder diffraction and DC magnetization measurements. All the compounds crystallize in ThCr₂Si₂-type structure. Substitution of Tb for La led to a linear decrease in the lattice constants and the unit-cell volume. A ferromagnetic phase for x≤0.15, and an antiferromagnetic phase for x=0.3 have been observed at about room temperature, whereas the compounds with x=0.2 and 0.25 exhibit a magnetic phase transition from ferromagnetism to antiferromagnetism.     

Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Synthesis and magnetic properties of Zn1−xMnxO films prepared by the sol-gel method

We report on the ferromagnetic characteristics of Zn_1−xMn_xO films (x=0.1-0.3) prepared by the sol-gel method on silicon substrates using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and superconducting quantum interference device (SQUID) magnetometry at various temperatures. Magnetic measurement show that the Curie temperature (T_C) and the coercive field (H_C) were ∼39 K and ∼2100 Oe for the film of x=0.2, respectively. EDS and TEM measurements indicate that Mn content at the interface is significantly higher than that at the center of the Zn_0.8Mn_0.2O film showing the ratio, Zn:Mn:O≅1:12:15. This experimental evidence suggests that ferromagnetic precipitates containing manganese oxide may be responsible for the observed ferromagnetic behavior of the film.     

Magnetic and electrical properties of new magnetic phase in Si1−xMnx crystals

We have investigated the magnetic and electrical transport properties of Si_1−xMn_x single crystals grown by the vertical Bridgman method. The alloys with Mn concentrations up to x=0.64 have weak ferromagnetic ordering around T_C∼30 K. However, Si_0.25Mn_0.75 alloys show weak ferromagnetic ordering at 70 K and antiferromagnetic ordering at 104 K, which is confirmed by magnetization and electrical transport studies.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

Magnetic behavior of the II-V-diluted magnetic semiconductor Cd1−xMnxSb

Polycrystalline samples of the II-V-diluted magnetic semiconductor Cd_1−xMn_xSb (x=0.05-0.20) were synthesized. Standard high temperature ceramic methods under an inert atmosphere were utilized for sample fabrication. Structural characterization was done using X-ray diffractometry (XRD), which indicated that a simple substitution of Mn for Cd is probably not occurring. Hysteresis, ac susceptibility, dc magnetization, and spontaneous magnetization measurements were performed for Cd_0.90Mn_0.10Sb. The hysteresis data indicated the presence of a ferromagnetic component. Ferromagnetism in the Cd_0.90Mn_0.10Sb system is likely due to two sources: Mn spins in small Mn-rich regions and a small amount of MnSb in a minority phase. Analysis of the spontaneous magnetization as a function of temperature for Cd_0.90Mn_0.10Sb yielded the value 0.172 for the critical exponent β. In MnSb, β was found to have the value 0.379, which is close to the theoretical value for 3D-Heisenberg systems. Thus, in Cd_0.90Mn_0.10Sb, the ferromagnetism is not of the 3D-Heisenberg type; rather, it is closer to 2D Ising behavior, indicating reduced effective dimensionality.     

Influence of Ho substitution for Nd on magnetic properties of the Nd1−xHoxMn2Ge2 germanides

The structure and magnetic properties of Nd_1−xHo_xMn₂Ge₂ (0.0≤x≤1.0) germanides were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Ho for Nd leads to a linear decrease in the lattice constants and the unit cell volume, and the magnetic interactions in the Mn sublattice cross over from a ferromagnetic character to an antiferromagnetic one. A typical SmMn₂Ge₂-like behavior is observed for x=0.6 and 0.8. The results are collected in a phase diagram.     

Magnetic phase transitions of antiperovskite Mn3−xFexSnC (0.5≤x≤1.3)

The magnetization and electrical resistivity of Mn_3−xFe_xSnC (0.5≤x≤1.3) were measured to investigate the behavior of the complicated magnetic phase transitions and electronic transport properties from 5 to 300 K. The results obtained demonstrate that Fe doping at the Mn sites of Mn₃SnC induces a more complicated magnetic phase transition than that in its parent phase Mn₃SnC from a paramagnetic (PM) state to a ferrimagnetic (FI) state consisting of antiferromagnetic (AFM) and ferromagnetic (FM) components, while, with the change of Fe-doped content and magnetic field, there is a competition between the AFM component and FM component in the FI state. Both the Curie temperature (T_C) and the saturated magnetization M_s increase with increasing x. The FM component region becomes broader with further increasing Fe-doped content x. The external magnetic field easily creates a saturated FM state (and increased T_C) when . Fe doping quenches the negative thermal expansion (NTE) behavior from 200 to 250 K reported in Mn₃SnC.     

The effect of Fe doping on structural, magnetic and electrical transport properties of CaMn1−xFexO3 (x=0-0.35)

A comprehensive study of the effect of Fe doping on CaMnO₃ is carried out by means of experiments on the structural, transport conduction, and magnetic properties of CaMn_1−xFe_xO₃ (0≤x≤0.35). With a sol-gel process for sample preparation, Fe is substituted for Mn up to x=0.35. This substitution substantially brings out the lattice expansion and gradually suppresses the antiferromagnetism. For x=0.08 and 0.10 in particular, the magnetization curves with a field-cooled mode under the field of 1 kOe behave as those of a ferrimagnetic-like system and present low-temperature negative magnetization. For x≥0.15, the negative magnetization phenomenon disappears, and a ferromagnetic component coexists with an antiferromagnetic one, but the antiferromagnetic interaction still dominates in these compounds. Electrical transport measurements show insulating behavior for all compositions. Fe doping, even at a level as low as x=0.02, can cause a marked resistivity increase in the temperature range studied. Further increasing the Fe content causes the resistivity to gradually decrease due to the increasing carrier presence.     

Formation mechanism of ferromagnetism in Si1−xMnx diluted magnetic semiconductors

Si_1−xMn_x diluted magnetic semiconductor (DMS) bulks were formed by using an implantation and annealing method. Energy dispersive X-ray fluorescence, transmission electron microscopy (TEM), and double-crystal rocking X-ray diffraction (DCRXD) measurements showed that the grown materials were Si_1−xMn_x crystalline bulks. Hall effect measurements showed that annealed Si_1−xMn_x bulks were p-type semiconductors. The magnetization curve as a function of the magnetic field clearly showed that the ferromagnetism in the annealed Si_1−xMn_x bulks originated from the interaction between interstitial and substitutional Mn⁺ ions, which was confirmed by the DCRXD measurements. The magnetization curve as a function of the temperature showed that the ferromagnetic transition temperature was approximately 75 K. The present results can help to improve understanding of the formation mechanism of ferromagnetism in Si_1−xMn_x DMS bulks.     

Metastable ferromagnetic phases with predictable Tcs in La2MnCo1−xNixO6

Metastable ferromagnetic phases, for different compositions in La₂MnCo_1−xNi_xO₆, are obtained for samples synthesized by a low-temperature method and annealed in air at different temperatures in the range 200-1350 °C. The T_cs of the ferromagnetic phases vary linearly between those of the phases of the end members. T_cs of the different phases of La₂MnCo_1−xNi_xO₆ can be predicted based on the T_cs and spin states of Mn, Co and Ni in the different phases of the end members, La₂MnCoO₆ and La₂MnNiO₆.     

Optical and magnetic properties of (Ga1−xMnx)N/GaN digital ferromagnetic heterostructures

(Ga_1−xMn_x)N/GaN digital ferromagnetic heterostructures (DFHs) and (Ga_1−xMn_x)N/GaN grown on GaN buffer layers by using molecular beam epitaxy have been investigated. The photoluminescence (PL) spectra showed band-edge exciton transitions. They also showed peaks corresponding to the neutral donor-bound exciton and the exciton transitions between the conduction band and the Mn acceptor, indicative of the Mn atoms acting as substitution. The magnetization curves as functions of the magnetic field at 5 K indicated that the saturation magnetic moment in the (Ga_1−xMn_x)N/GaN DFHs decreased with increasing Mn mole fraction and that the saturation magnetic moment and the coercive field in the (Ga_1−xMn_x)N/GaN DFHs were much larger than those in (Ga_1−xMn_x)N thin films. These results indicate that the (Ga_1−xMn_x)N/GaN DFHs hold promise for potential applications in spintronic devices.     

Band-edge exciton transitions in (Ga1−xMnx)N diluted magnetic semiconductor films with above room temperature ferromagnetic transition

(Ga_1−xMn_x)N thin films grown on GaN buffer layers by using molecular beam epitaxy were investigated with the goal of producing diluted magnetic semiconductors (DMSs) with band-edge exciton transitions for applications in optomagnetic devices. The magnetization curve as a function of the magnetic field at 5 K indicated that ferromagnetism existed in the (Ga_1−xMn_x)N thin films, and the magnetization curve as a function of the temperature showed that the ferromagnetic transition temperature of the (Ga_1−xMn_x)N thin film was above room temperature. Photoluminescence and photoluminescence excitation spectra showed that band-edge exciton transitions in (Ga_1−xMn_x)N thin films appeared. These results indicate that the (Ga_1−xMn_x)N DMSs with a magnetic single phase hold promise for potential applications in spin optoelectronic devices in the blue region of the spectrum.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

Microstructure, magnetic and optical properties of sputtered polycrystalline ZnO films with Fe addition

Microstructure, magnetic and optical properties of polycrystalline Fe-doped ZnO films fabricated by cosputtering with different Fe atomic fractions (x_Fe) have been examined systematically. Fe addition could affect the growth of ZnO grains and surface morphology of the films. As x_Fe is larger than 7.0%, ZnFe₂O₄ grains appear in the films. All the films are ferromagnetic. The ferromagnetism comes from the ferromagnetic interaction activated by defects between the Fe ions that replace Zn ions. The average moment per Fe ion reaches a maximum value of 1.61 μ_B at x_Fe = 4.8%. With further increase in x_Fe, the average moment per Fe ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Fe ions. The optical band gap value decreases from 3.245 to 3.010 eV as x_Fe increases from 0% to 10%. Photoluminescence spectra analyses indicate that many defects that affect the optical and magnetic properties exist in the films.     

Analysis of magnetic field dependent mobility in ferromagnetic Ga1−xMnxAs layers

The magneto-transport properties of ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fractions in the range of x≈2.2-4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0-300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.     

Large magnetoresistance in low temperature metallic region of manganite compounds (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05≤x≤0.15)

Magnetoresistance (MR) and magnetization (dc and ac) measurements have been carried out on the manganites, (La_0.7−2xEu_x)(Ca_0.3Sr_x)MnO₃ (0.05≤x≤0.15), in the temperature range of 5-320 K. At 5 K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4-5 T. This large high-field MR occurs in the metallic region, far below the insulator-metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained in the light of various possibilities such as phase segregation, cluster spin-glass behavior, etc.     

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.     

Room temperature ferromagnetism behavior of Sn1−xMnxO2 powders

In this paper, we have investigated Mn-doped SnO₂ powder samples prepared by solid-state reaction method. X-ray diffraction showed a single phase polycrystalline rutile structure. The atomic content of Mn ranged from ∼0.8 to 5 at%. Room temperature M-H loops showed a ferromagnetic behavior for all samples. The ferromagnetic Sn_0.987Mn_0.013O₂ showed a coercivity H_c=545 Oe, which is among the highest reported for dilute magnetic semiconductors. The magnetic moment per Mn atom was estimated to be about 2.54 μ_B of the Sn_0.9921Mn_0.0079O₂ sample. The average magnetic moment per Mn atom sharply decreases with increasing Mn content, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn_1−xMn_xO₂ are discussed based on the competition between the antiferromagnetic superexchange coupling and the F-center exchange coupling mechanism, in which both oxygen vacancies and magnetic ions are involved.     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.