Magnetic properties of half-metallic semi Heusler Co1−xCuxMnSb compounds

A study of the half-metallic character of the semi Heusler alloys Co_1−xCu_xMnSb (0?x?0.9) is presented. We investigated the saturation magnetization M_S at temperatures from 5 K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature T_C. The magnetic moments at 5 K, for most compositions are very close to the quantized value of 4 μ_B for Mn³⁺ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with M_S (5 K)=3.78 μ_B/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90%. Above T_C, the χ⁻¹ vs T curves follow very well the Curie–Weiss law. The effective moment μ_eff and paramagnetic Curie temperature θ are derived. A comparison between the values of M_S at 5 K and μ_eff shows a transition from localized to itinerant spin system in these compounds.     

Structural and magnetic properties of ferromagnetic In1−xMnxAs1−yPy layers

The structural and magnetic properties of epitaxial In_1−xMn_xAs_1−yP_y quaternary layers with Mn content ranging from 0.01 to 0.04 and phosphorous content ranging from 0.11 to 0.21 were studied. X-ray diffraction indicated that the films were two phase consisting of an InMnAsP solid solution and hexagonal MnAs nanoprecipitates. Addition of phosphorus promoted precipitate formation. Films were ferromagnetic showing hysteretic behavior in the field dependence of magnetization at 5 and 298 K. From field-cooled magnetization measurements ferromagnetic transitions were observed at 280 and 325 K. The zero field-cooled magnetization versus temperature measurements showed irreversibility for T<300 K that was attributed to the presence of MnAs nanoprecipitates. The calculated coercivity using the Neel model was 1380 G compared to the experimental value of 380 G at 5 K. The difference was attributed to a strong inter-cluster exchange that stabilizes the ferromagnetic state.     

Structural,magnetic and electrical properties of Co1−xZnxFe2O4 synthesized by co-precipitation method

Nanoparticles of Co_1−xZn_xFe₂O₄ with stoichiometric proportion (x) varying from 0.0 to 0.6 were prepared by the chemical co-precipitation method. The samples were sintered at 600 °C for 2 h and were characterized by X-ray diffraction (XRD), low field AC magnetic susceptibility, DC electrical resistivity and dielectric constant measurements. From the analysis of XRD patterns, the nanocrystalline ferrite had been obtained at pH=12.5–13 and reaction time of 45 min. The particle size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The size of precipitated particles lies within the range 12–16 nm, obtained at reaction temperature of 70 °C. The Curie temperature was obtained from AC magnetic susceptibility measurements in the range 77–850 K. It is observed that Curie temperature decreases with the increase of Zn concentration. DC electrical resistivity measurements were carried out by two-probe method from 370 to 580 K. Temperature-dependent DC electrical resistivity decreases with increase in temperature ensuring the semiconductor nature of the samples. DC electrical resistivity results are discussed in terms of polaron hopping model. Activation energy calculated from the DC electrical resistivity versus temperature for all the samples ranges from 0.658 to 0.849 eV. The drift mobility increases by increasing temperature due to decrease in DC electrical resisitivity. The dielectric constants are studied as a function of frequency in the range 100 Hz–1 MHz at room temperature. The dielectric constant decreases with increasing frequency for all the samples and follow the Maxwell–Wagner's interfacial polarization.     

Ferromagnetic semiconductor Ge1−x−ySnxMnyTe with phase transformation of ferroelectric type

It is expected that joint existence of ferromagnetic properties and ferroelectric structural phase transition in diluted magnetic semiconductors IV-VI leads to new possibilities of these materials. Temperature of ferroelectric transition for such crystals can be tuned by the change of Sn/Ge ratio. Magnetic susceptibility, Hall effect, resistivity and thermoelectric power of Ge_1−x−ySn_xMn_yTe single crystals grown by Bridgeman method (x=0.083-0.115; y=0.025-0.124) were investigated within 4.2-300 K. An existence of FM ordering at T_C∼50 K probably due to indirect exchange interaction between Mn ions via degenerated hole gas was revealed. A divergence of magnetic moment temperature dependences at T?T_C in field-cooled and zero-field-cooled regimes is obliged to magnetic clusters which are responsible for superparamagnetism at T>T_C≈T_f (freezing temperature) and become ferromagnetic at T_C arranging spin glass state at T<T_f≈T_C. Phase transition of ferroelectric type at T≈46 K was revealed. Anomalous Hall effect which allows to determine magnetic moment was observed.     

Effect of Co or Mn addition on the soft magnetic properties of amorphous Fe89−xZr11Bx (x=5, 10) alloy ribbons

The temperature and field dependent magnetic properties of melt-spun amorphous Fe_89−x−yZr₁₁B_x(Co,Mn)_y (x=5, 10 and 0≤y≤10) alloys in the temperature range 5-1200 K are reported. The Curie temperature and saturation magnetization at room temperature increase (decrease) almost linearly with Co (Mn) addition. With increasing Co concentration, the room temperature coercivity increases at the rate of 2.26 (0.28) A/m per at% for the x=5 (10) samples. The high-field magnetic susceptibility and local magnetic anisotropy decrease (increases) rapidly with increasing Co (Mn) concentration. The thermomagnetic curves show a marked increase in magnetization above 850 K corresponding to the crystallization of α-FeCo (α-Fe) phase in samples containing Co (Mn). The Curie temperature of the crystalline phase increases (remains same) with increasing Co (Mn) concentration with the formation of α-FeCo (α-Fe). Addition of Co up to 10 at% in Fe-Zr-B improves the room temperature saturation magnetization from 0.56 to 1.2 T, and Curie temperature from 315 to 476 K. Also, the coercivity increases with Co addition from 1.27 to 23.88 A/m for x=5 and from 7.64 to 10.35 A/m for x=10 alloy. The non-collinear spin structures that characterize Fe rich Fe-Zr-B amorphous alloys have been used to describe the observed results.     

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.     

A Hall coefficient investigation of ferromagnetic Ga1−xMnxAs layers on (100) GaAs substrates

Ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fraction in the range of x≈2.2-4.4% were grown on semi-insulating (100) GaAs substrates using the molecular beam epitaxy technique. The transport properties of these epilayers were investigated through Hall effect measurements. The measured hole concentration of Ga_1−xMn_xAs layers varied from 4.4×10¹⁹ to 3.4×10¹⁹ cm⁻³ in the range of x≈2.2-4.4% at room temperature. From temperature dependent resisitivity data, the sample with x≈4.4% shows typical behavior for insulator Ga_1−xMn_xAs and the samples with x≈2.2 and 3.7% show typical behavior for metallic Ga_1−xMn_xAs. The Hall coefficient for the samples with x≈2.2 and 4.4% was fitted assuming a magnetic susceptibility given by Curie-Weiss law in a paramagnetic region. This model provides good fits to the measured data up to and the Curie temperature T_c was estimated to be 65, 83 K and hole concentration p was estimated to be 5.1×10¹⁹, 4.6×10¹⁹ cm⁻³ for the samples with x≈2.2 and 4.4%, respectively, confirming the existence of an anomalous Hall effect for metallic and insulating samples.     

Magnetic properties of the compounds ThCO2Ge2 and ThCo2Si2

Rather old preparation of the compounds ThCo₂Ge₂ and ThCo₂Si₂ and their magnetic study in the temperature range 100–570 K, published by Omejec and Ban [Z. Anorg. Allg. Chem. 380 (1971) 111], indicated that both compounds ordered ferrromagnetically below 100 K. In order to verify the old data, polycrystalline samples of ThCo₂Ge₂ and ThCo₂Si₂ have been prepared by arc melting and subsequent annealing, and studied by X-ray diffraction at room temperature (RT), by superconducting quantum interference device (SQUID)-magnetization and AC-susceptibility measurements at 2–320 K, and by dc-magnetization measurements in variable magnetic fields up to 120 kOe at 5, 80, and 283 K. The magnetic measurements confirm the ferromagnetic ordering in both compounds, but with totally different Curie temperatures: ≈120(20) K for ThCo₂Ge₂ and above 320 K for ThCo₂Si₂. The paramagnetic values of ThCo₂Ge₂ and the ordering of both compounds are discussed and compared with the old results of Omejec and Ban.     

Magnetic and electrical properties of amorphous Ge1−xCrx thin films grown by low temperature vapor deposition

Amorphous Ge_1−xCr_x thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10⁻³–0.96×10² Ω cm at 300 K, and decrease with Cr concentration. The Ge_1−xCr_x thin films are p-type. The hole concentrations are 5×10¹⁶–7×10²¹ cm⁻³ at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm³ at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge_1−xCr_x thin films are paramagnetic.     

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.     

Spin-glass-like behavior in ZnxCryAlzSe4

The magnetic and electrical properties of the Al-doped polycrystalline spinels Zn_xCr_yAl_zSe₄ (0.13≤z≤0.55) with the antiferromagnetic (AFM) order and semiconducting behavior were investigated. A complex antiferromagnetic structure below a Néel temperature T_N≈23 K for the samples with z up to 0.4 contrasting with the strong ferromagnetic (FM) interactions evidenced by a large positive Curie-Weiss temperature θ_CW decreasing from 62.2 K for z=0.13 to 37.5 K for z=0.55 was observed. Detailed investigations revealed a divergence between the zero-field-cooling (ZFC) and field-cooling (FC) susceptibilities at temperature less than T_N suggesting bond frustration due to competing ferromagnetic and antiferromagnetic exchange interactions in the compositional range 0.13≤z≤0.4. Meanwhile, for z=0.55 a spin-glass-like behavior of cluster type with randomly oriented magnetic moments is observed as the ZFC-FC splitting goes up to the freezing temperature T_f=11.5 K and the critical fields connected both with a transformation of the antiferromagnetic spin spiral via conical magnetic structure into ferromagnetic phase disappear.     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Analysis of Raman scattering of Ga1−xMnxAs dilute magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x=1.4-3.0%) grown on (1 0 0) GaAs substrates by molecular beam epitaxy were characterized using Raman spectroscopy. As Mn is introduced into GaAs, a marked increase in disorder in the material occurs, as indicated by the growth of the disorder-allowed transverse-optical Raman line. Another important result is that as the Mn concentration in Ga_1−xMn_xAs increases further beyond ca. 2%, Raman-active coupled-plasmon-longitudinal-optical phonon modes arise, which signals the increasing presence of holes, and thus provides a useful tool for determining their concentration. Using the depletion-layer approach from the Raman spectroscopy data, we determined the carrier concentration for samples with x=2.2% and 3.0% was to be 7.2×10¹⁹ and 8.3×10²⁰ cm⁻³, respectively.     

Monodispersed nanocrystalline Co1–xZnxFe2O4 particles by forced hydrolysis: Synthesis and characterization

Zinc-substituted cobalt ferrites, Co_1–xZn_xFe₂O₄, were for the first time successfully prepared by forced hydrolysis method. The obtained materials are single phase, monodispersed nanocrystalline with an average grain size of about 3 nm. These materials are superparamagnetic at room temperature and ferrimagnetic at temperature lower than the blocking temperature. When the zinc substitution increases from x=0 to 0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g. The high saturation magnetization of these samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications.     

The role of iron in the formation of the magnetic structure and related properties of La0.8Sr0.2Co1−xFexO3 (x=0.15, 0.2, 0.3)

La_0.8Sr_0.2Co_1−xFe_xO₃ (x=0.15, 0.2, 0.3) samples were studied by means of AC magnetic susceptibility, magnetization, magnetoresistance and ⁵⁷Fe Mössbauer spectrometry. Iron was found to take on a high spin 3d^5−α electronic state in each of the samples, where α refers to a partly delocalized 3d electron. The compounds were found to exhibit a spin-cluster glass transition with a common transition temperature of ∼53 K. The spin-cluster glass transition is visualized in the ⁵⁷Fe Mössbauer spectra as the slowing down of magnetic relaxation below ∼70 K, thereby showing that iron takes part in the formation of the glassy magnetic phase. The paramagnetic-like phase found at higher temperatures is identified below T_c≈195 K as being composed of weakly interacting, magnetically ordered nanosized clusters of magnetic ions in part with a magnetic moment oriented opposite to the net magnetic moment of the cluster. For each of the samples a considerable low-temperature negative magnetoresistance was found, whose magnitude in the studied range decreases with increasing iron concentration. The observed results obtained on the present compounds are qualitatively explained assuming that the absolute strengths of magnetic exchange interactions are subject to the relation ∣J_Co–Co∣<∣J_Fe–Co∣<∣J_Fe–Fe∣.     

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.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

Ferromagnetism in MnxGe1−x films prepared by magnetron sputtering

Mn_xGe_1−x thin films were prepared by magnetron sputtering with a substrate temperature of 673 K and subsequently annealed at 873 K. The X-ray diffraction (XRD) measurements showed that all samples had a single Ge cubic structure. No films showed clear magnetic domain structure under a magnetic force microscope (MFM). Atom force microscope (AFM) measurements showed that the films had an uniform particle size distribution, and a columnar growth pattern. X-ray photoelectron spectroscopy (XPS) measurements indicated that the valences of both Mn and Ge atoms increase with the Mn concentration. The resistance decreased with increasing temperature, suggesting that the films were typical semiconductors. Magnetic measurements carried out using a Physical Property Measurement System (PPMS) showed that all samples exhibited ferromagnetism at room temperature. There was a small concentration of Mn₁₁Ge₈ in the films, but the ferromagnetism was mainly induced by Mn substitution for Ge site.