The structural and magnetic properties of Ni2Mn1−xBxGa Heusler alloys

The influence of the substitution of manganese by boron on the crystal structure and magnetic properties of Ni₂Mn_1−xB_xGa Heusler alloys with 0?x?0.5 has been investigated using X-ray diffraction, thermal expansion, resistivity, and magnetization measurements. The samples with concentrations x<0.25 were found to be of single phase and belonged to the cubic L2₁ crystal structure at room temperature. Crystal cell parameters of the alloys decreased from 5.830 to 5.825 Å with increasing boron concentration (x) from 0 to 0.25. The alloys were ferromagnetically ordered at 5 K and the saturation magnetization decreased with increasing boron concentration. The ferromagnetic ordering and structural transition temperatures for 0?x?0.3 have been observed and the phase (x−T) diagram of the Ni₂Mn_1−xB_xGa system was constructed. The phase (x−T) diagram indicates that the ground state of Ni₂Mn_1−xB_xGa alloys belongs to ferromagnetic martensitic, premartensitic, and austenitic phases in x?0.12, 0.12<x?0.18, and 0.18<x?0.3, respectively. The relative influence of cell parameters and electron concentrations on the phase diagram is discussed.     

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.     

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.     

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 behavior of MnAs precipitates in Ga1−xMnxAs diluted magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x≈4.7–5.5%) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy. These p-type (Ga,Mn)As films were revealed to have a ferromagnetic structure and ferromagnetism is observed up to a Curie temperature of 318 K, which is ascribed to the presence of MnAs secondary magnetic phases within the film. It is highly likely that the phase segregation occurs due to the high Mn cell temperature around 890–920 °C, as it is well established that GaMnAs is unstable at such a high temperature. The MnAs precipitate in the samples with x≈4.7–5.5% has a Curie temperature T_c≈318 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.     

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.     

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.     

Growth and magnetism in amorphous Si1−xMnx thin films grown by thermal deposition

Si_1−xMn_x   (x?0.22$x?0.22$) thin films were grown by using a thermal evaporator, and their magnetic and electrical properties were investigated. The Si_1−xMn_x semiconductors are amorphous when Mn concentration is 9.0 at% and less. The electrical resistivities of amorphous Si_1−xMn_x   (x?0.09$x?0.09$) semiconductor thin films are in the range of 9.86–6.59×10⁻⁴ Ω cm at room temperature and decrease with increasing Mn concentration. The amorphous Si_1−xMn_x   (x?0.09$x?0.09$) semiconductor thin films are p-type and hole densities are 3.73×10¹⁸–1.33×10²² cm⁻³ at room temperature. Low temperature magnetization characteristics reveal that amorphous Si_1−xMn_x   (x?0.09$x?0.09$) semiconductor thin films are paramagnetic.     

Characterization of magnetic phases in the FexMn0.65−xAl0.35 disordered alloys

Melted alloys of the Fe_xMn_0.65−xAl_0.35 disordered system, 0.25?x?0.65, were experimentally studied by Mössbauer spectrometry, vibrating sample magnetometry and AC magnetic susceptibility. All the alloys exhibit the BCC structure with a nearly constant lattice parameter (2.92 Å). Mössbauer studies at room temperature (RT) show that in the 0.25 ?x?0.45 range the alloys are paramagnetic (P) while in the 0.50?x?0.65 range, they are ferromagnetic. At 77 K, Mössbauer studies show that the alloy with x=0.25$x=0.25$ presents weak magnetic character that is consistent with an antiferromagnetic (AF) behavior due to the high Mn content, while those with 0.30?x?0.40 are paramagnetic, and those in the 0.45?x  ?0.65 range are ferromagnetic (F) with a mean field increasing with the Fe content. Hysteresis cycles at RT prove the paramagnetic character of the alloys between x=0.25$x=0.25$ and 0.40 and the ferromagnetic character for x?0.45$x?0.45$. Complementary measurements using AC magnetic susceptibility permit a magnetic phase diagram to be proposed, with the P phase for high temperature and all the compositions, the AF phase for low Fe content and at low temperature, the F phase for high Fe content above RT and the spin glass phase for all the compositions and at temperatures lower than 46 K. In addition, the mean field renormalization group (MFRG) method, applied to a random competitive and site dilute Ising model with nearest-neighbor, gives rise to magnetic phase diagram, which fairly agrees with previous experimental one.     

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.     

Magnetocaloric effect of Gd5SixSn4−x Alloys

The crystal structure and magnetocaloric effect of Gd₅Si_xSn_4−x   (with x=2.4$x=2.4$, 2.6 and 2.8) alloys were studied by means of X-ray power diffraction (XRD) and magnetic measurements. From the XRD results, these alloys adopt a Gd₅Si₄-type structure for x=2.8$x=2.8$, Gd₅Si₄-type and Gd₅Si₂Ge₂-type mixed structures for x=2.4$x=2.4$ and 2.6, while some minor phases can also be found. The Curie temperatures of the Gd₅Si_xSn_4−x increases gradually when x increases from 276 K for x=2.4$x=2.4$, to 301.5 K for x=2.8$x=2.8$. Magnetic entropy changes of these alloys at a magnetic field change of 0–1.8 T are 1.88, 2.26 and 1.69 J/kg K for x=2.4$x=2.4$, 2.6 and 2.8, respectively. The temperature-dependent XRD analysis shows that there is no crystallographic transition for these alloys, which can explain their low magnetic entropy changes.     

Magnetic domain structure in Fe78.8−xCoxCu0.6Nb2.6Si9B9 nanocrystalline alloys studied by Lorentz microscopy

The magnetic domain structures of Fe_78.8−xCo_xCu_0.6Nb_2.6Si₉B₉ (x=0, 20, 40, 60) alloys are investigated by Lorentz microscopy coupled with the focused ion beam method. The specimen prepared using the FIB method is found to have a considerably more uniform thickness compared to that prepared using the ion-milling method. In Fe_38.8Co₄₀Cu_0.6Nb_2.6Si₉B₉ and Fe_18.8Co₆₀Cu_0.6Nb_2.6Si₉B₉ alloys, 180° domain walls extending in the direction of the induced magnetic anisotropy are observed. Analysis with Lorentz microscopy reveals that the width of the magnetic domains decreases with an increase in the cobalt content or the induced magnetic anisotropy K_u, that is, the domain width d is proportional to the induced magnetic anisotropy (K_u)^−1/4. On the other hand, in the in situ Lorentz microscopy observation as a function of temperature, magnetic ripple structures are found to appear in a localized area due to the fluctuation of magnetization vectors from 423 K. It is observed that the induced magnetic anisotropy caused by the applied magnetic field at 803 K is not suppressed by the magnetic ripple structures observed at 423–443 K.     

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.     

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.     

Magnetic properties of electron-doped Y1−xCexMnO3 compounds

Cerium-doped Y_1−xCe_xMnO₃ compounds have been prepared in single-phase form for x=0 to 0.10. X-ray diffraction (XRD) patterns could be analyzed by using P6₃cm space group. Temperature variations of ac susceptibility and magnetization measurements show that these Ce-doped materials exhibit weak ferromagnetic transition. The observed ferromagnetic transition is attributed to the double exchange ferromagnetic interaction between Mn²⁺ and Mn³⁺ ions due to electron doping. The M–H loops exhibit hysteresis along with linear contribution and were analyzed based on bound magnetic polaron (BMP) model. Increase in saturation magnetization and decrease in BMP concentrations have been observed with increase in Ce doping.     

Ferromagnetic spin fluctuations in antiferromagnetic Pr1−xCaxMnO3: An ESR study

Electron spin resonance (ESR) measurements have been performed on polycrystalline samples of Pr_1−xCa_xMnO₃ (x=0.4, 0.5) in the temperature range of 100-300 K. The temperature dependence of ESR intensity, g value and linewidth shows the existence of ferromagnetic spin correlations in the paramagnetic state. With decreasing temperature, the ferromagnetic spin correlations switch to antiferromagnetic spin correlations in the charge ordering state and vanish at the antiferromagnetic ordering temperature T_N.     

Effect of Cr doping on the magnetic and electrical transport properties of charge-ordered Bi0.5Ca0.5MnO3

An experimental study on the magnetic and electrical transport properties of the manganites Bi_0.5Ca_0.5Mn_1−xCr_xO₃ (BCMCO) (0≤x≤0.12) is carried out. The results show that Cr doping can suppress the charge-ordering transition, favoring the ferromagnetic clusters. For x=0.12, the charge-ordering transition disappears but a very broad paramagnetic-ferromagnetic-like transition is detected at the Curie temperature T_C=72.6 K. It is caused by phase separation or coexistence of the charge-ordering and ferromagnetic phase. Moreover, the critical Cr content to destroy charge ordering phase in BCMCO does not match the general monotonous tendencies shown by Cr-doped Re_0.5Ca_0.5MnO₃ (Re-rare-earth). These differences are ascribed to the fact that the ground state in BCMCO differs markedly from the ferromagnetic metallic phase in Cr-doped Re_0.5Ca_0.5MnO₃ compounds.     

Effect of illumination on magnetization and microwave absorption of La1−xCaxMnO3 (x<0.2) films

Light-induced changes of the hysteresis loops of magnetization and microwave absorption are investigated in low-doped La_1−xCa_xMnO₃ (x<0.2) thin films. The width of the hysteresis loops decreases clearly under illumination with visible or near-infrared light at temperatures below 50 K. The microwave conductivity has a minimum value at magnetic fields corresponding to the magnetization reversal and is shifted towards weaker fields under illumination. These effects show complex nonexponential time evolution and dependence on strength of the magnetic field. The results can be explained by assuming that small ferromagnetic metallic regions exist within the insulating ferromagnetic phase of the sample, and that these regions are expanded by optically induced charge transfer between Jahn–Teller split e_g states of neighboring Mn³⁺ ions. Decrease of the Mn³⁺ XPS core level spectrum is observed in the samples under illumination with a HeNe laser.     

Low temperature thermodynamical properties of ErCu2Si2

ErCu₂Si₂ crystallises in the tetragonal ThCr₂Si₂-type crystal structure. In this paper results of magnetometric, electrical transport, specific heat as well as neutron diffraction are reported. Results of electrical resistivity and specific heat measurements performed at low temperature yield existence of magnetic ordering roughly at 1.3 K. These results are in concert with neutron diffraction measurements, which reveal simple antiferromagnetic ordering between 0.47 and 1.00 K. At temperatures ranging from 1.00 up to 1.50 K an additional incommensurate magnetic structure was observed. The propagation vector k=(0;0;0.074) was proposed to describe magnetic reflections within the amplitude modulated magnetic structure. Basing on specific heat studies the crystal field levels splitting scheme and magnetic entropy were calculated.     

Magnetic characterization of Mn5SiB2 and Mn5Si3 phases

In this work the Mn₅Si₃ and Mn₅SiB₂ phases were produced via arc melting and heat treatment at 1000 °C for 50 h under argon. A detailed microstructure characterization indicated the formation of single-phase Mn₅Si₃ and near single-phase Mn₅SiB₂ microstructures. The magnetic behavior of the Mn₅Si₃ phase was investigated and the results are in agreement with previous data from the literature, which indicates the existence of two anti-ferromagnetic structures for temperatures below 98 K. The Mn₅SiB₂ phase shows a ferromagnetic behavior presenting a saturation magnetization M_s of about 5.35×10⁵ A/m (0.67 T) at room temperature and an estimated Curie temperature between 470 and 490 K. In addition, AC susceptibility data indicates no evidence of any other magnetic ordering in 4-300 K temperature range. The magnetization values are smaller than that calculated using the magnetic moment from previous literature NMR results. This result suggests a probable ferrimagnetic arrangement of the Mn moments.