Effect of Mn incorporation for Ni on the properties of melt spun off-stoichiometric compositions of NiMnGa alloys

The investigation addresses the effect of Mn incorporation for Ni on the properties of a series of Ni_77−xMn_xGa₂₃ (x=22-29; at%) ferromagnetic shape memory alloys prepared in the form of ribbons by a melt spinning technique. Phase transformation studies in these ribbons by differential scanning calorimetry revealed that austenitic start and martensitic start temperatures decreased with the increase in Mn content. The Curie temperature (T_C) of these alloys determined from thermal variation of magnetisations was found to rise with increasing Mn content. The martensitic transformation temperatures were above T_C in low Mn containing (x=22 and 23) alloys. Morphology observed through transmission electron microscopy manifested complex martensitic features in the alloy with x=22 while x=29 had an austenitic phase. The alloys with intermediate Mn content (x=24, 25) had overlapping magnetic and martensitic transformations close to room temperature. The thermal lag between austenitic and martensitic characteristic temperatures in these alloys has been corroborated to their structural state. X-ray diffraction indicated a predominant martensite phase and austenite phase in low and high Mn containing alloys respectively. In-situ diffraction studies during thermal cycle indicate martensite-austenite transformations.     

Phase transition and extended X-ray absorption fine structure of melt-spun amorphous Fe100−xYx alloys

The phase diagram and local structure of melt-spun amorphous (a-) Fe_100−xY_x (22?x?62) alloys were investigated using AC and DC magnetic and extended X-ray absorption fine structure (EXAFS) measurements. The a-Fe–Y system shows reentrant spin glass (RSG) behavior for 42?x?58 and spin glass (SG) behavior for 60?x. Two SG transition temperatures, T_g and T_f, were obtained in the RSG state. The T_g, T_f and Curie temperature T_C decrease with increasing x, and the T_C and T_g vanish at x=60. A new magnetic phase diagram for the melt-spun a-Fe_100−xY_x alloys was obtained from magnetic measurements for higher Y concentration. The magnetic states of the a-Fe_100−xY_x alloys change remarkably around x=60 and an EXAFS study revealed that the average atomic distance between nearest-neighboring Fe atoms changes at approximately x=60.     

Effect of Mn doping on structural and magnetic properties of GaFeO3

GaFe_1−xMn_xO₃ polycrystalline materials have been prepared by a solid state reaction (SR) and by the sol-gel (SG) method. The maximum Mn content amounts up to 10% and 40% for SR and SG preparation, respectively. All compounds in these composition ranges crystallize in space group P c 2₁ n derived from Rietveld refinement of X-ray powder patterns. The gradual incorporation of manganese is accompanied by a decrease in the cell volume. The ferrimagnetic transition temperature of T_c=282 K for GaFeO₃ decreases with Mn content and reaches T_c=149 K for x=0.4.     

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.     

Giant magnetoresistance in Co–Al2O3 granular films prepared by self-organized growth

Co–Al₂O₃ granular films with a narrow distribution in cluster size of Co clusters embedded in Al₂O₃ matrix were prepared by sequential deposition based on self-organized growth. Resistivity dependence of giant magnetoresistance (GMR) was studied. The GMR takes a maximum of 5.2% at room temperature and 9.4% at 13 K and 5700 Gs when the resistivity of the sample is 4×10⁵–7×10⁵ μΩ cm. The temperature dependence of resistivities and GMR were discussed especially. A temperature dependence of conductance ρ∼exp[T₁/(T+T₀)] was found, which indicates the dominant conduction mechanism is fluctuation-induced tunneling. A linear relationship of GMR versus T was observed, GMR=a–kT, in applied magnetic field 5700 Gs. The remarkable character of temperature dependence of GMR should be due to the special microstructure that the clusters are monodispersed in the films.     

Magnetic properties of Mn-rich Rh2Mn1+xSn1−x Heusler alloys

X-ray powder diffraction and magnetization measurements have been carried out on Rh₂Mn_1+xSn_1−x (0≤x≤0.3) alloys. The alloys, which crystallize in the L2₁ structure, were found to exhibit ferromagnetic behavior. The lattice constant a at room temperature decreases with increasing x, whereas the Curie temperature T_C decreases linearly. At 5 K the magnetic moment per formula unit first increases with increasing x and then saturates for x≥0.2. The experimental results are discussed in terms of the influence of the Mn-Mn exchange interactions between the Mn atoms on the Sn and Mn sites.     

Magnetocaloric effect in high Ni content Ni52Mn48−xInx alloys under low field change

Ni-rich Heusler alloys Ni₅₂Mn_48−xIn_x (x=15.5, 16 and 16.5) were prepared by the arc melting method. X-ray diffraction analysis revealed that the martensite has orthorhombic structure (S.G. Pmm2) at room temperature. The only alloy with x=15.5 has structural transmission from martensite to austenite without any magnetic transmission. The temperature dependence and the field dependence of the magnetization measurement indicated that the magnetization increased with the decreasing of the concerntration of Mn. The lesser the Mn atoms located in the In atom sites, the weaker the total AFM interaction in the system. Giant entropy changes ΔS_M(T, H) were found in Ni₅₂Mn_48−xIn_x alloys with the maximum ΔS_M value of 22.3 J kg K for the sample with x=16.5 at 270 K under the magnetic field change of 1.5 T.     

Magnetocaloric effects in Ni-Mn-X based Heusler alloys with X=Ga, Sb, In

The Mn-based Heusler alloys encompass a rich collection of useful materials from highly spin-polarized systems to shape memory alloys to magnetocaloric materials. In this work we have summarized our studies of magnetostructural transitions from paramagnetic austenite to ferromagnetic martesite phases at TMC in Ni₂MnGa-based alloys (Ni₂Mn_0.75Cu_0.25-xCo_xGa, Ni₂Mn_0.70Cu_0.30Ga_0.95Ge_0.05, Ni₂Mn_1-xCu_xGa, Ni_2+xMn_1-xGa, and Ni₂Mn_0.75-xCu_xGa), and martensitic transitions from the ferromagnetic austenite to the martesite state in off-stoichiometric Ni-Mn-(In/Sb) Heusler alloys. The phase transition temperatures and respective magnetic entropy changes (ΔS) depend on composition in these systems and have been determined from magnetization measurements in the temperature interval 5-400 K, and in magnetic fields up to 5 T. It is shown that, depending on the composition and doping scheme the “giant” ΔS=40-60 J/(kgK) (for a field change of 5 T) can be observed in the temperature range (300-360 K) for the Ga-based alloys. The interplay between or coupling of the various transitions in Ni₂Mn(Mn,X) systems with X=Sb and In leads to exchange bias effects, giant magnetoresistance, and both inverse and “normal” magnetocaloric effects.     

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.     

Microstructure and magnetocaloric effect in cast LaFe11.5Si1.5Bx (x=0.5, 1.0)

Phase formation, structure, and the magnetocaloric effect (MCE) in as-cast LaFe_11.5Si_1.5B_x (x=0.5, 1.0) compounds have been studied. The Curie temperatures, T_C, are ∼211 and 230 K for x=0.5 and 1.0, respectively, which are higher than that of annealed LaFe_11.5Si_1.5 (T_C=183 K), while the maximum magnetic entropy changes at the respective T_C under a magnetic field change of 0-5 T are 7.8 and 5.8 J/(kg K). Wavelength dispersive spectrometry (WDS) analysis shows that only a small fraction of boron atoms is dissolved in the NaZn₁₃-type structure phase, and that the compositions of the as-cast LaFe_11.5Si_1.5B_x (x=0.5, 1.0) alloys are much different from the intended nominal compositions. These as-cast alloys exhibit second-order magnetic phase transitions and low MCEs. However, based on the relative cooling power, the as-cast LaFe_11.5Si_1.5B_x alloys are promising candidates for magnetic refrigerants over a wide temperature range.     

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.     

Susceptibility behaviour of Mn dissolved in liquid Cu—Al-alloys

Measurements of the magnetic susceptibility of liquid Mn_0.05—Cu—Al alloys up to 1700 K indicate Curie—Weiss like behaviour at the Cu-rich end and minima of the reciprocal Mn susceptibility values as a function of the temperature 1/_ξMn(T) at the A1-rich end of the series. The occurrence of 1/_ξMn(T) minima at temperatures, which are sensitively depending on the alloy composition, hints at a transition to localized magnetic moment behaviour even in the A1-based alloys at high temperatures. Both, the aspects of extremely increasing Kondo temperature and that of decreasing spin fluctuation times, allow to discuss the susceptibility behaviour uniformly for the whole alloy series. Thus, the results do not support a fundamental distinction (magnetic—nonmagnetic) between the alloy systems $\text{Cu}$—Mn and $\text{A1}$—Mn.     

Magnetostriction, Curie temperature and microstructure of Tb0.29(Dy1−xPrx)0.71Fe1.97 oriented alloys

The Tb_0.29(Dy_1−xPr_x)_0.71Fe_1.97 (x=0, 0.1, 0.2 and 0.3) alloys were prepared by directional solidification method. The orientation, magnetostriction λ, Curie temperature T_c and microstructure of alloys were characterized by XRD, standard resistant strain gauge technique, VSM and SEM-EDS. The results reveal that the alloys have a preferred orientation of 〈1 1 0〉 and 〈1 1 3〉 direction when x>0. With the increase in Pr content, the T_c of alloys decreases gradually and the non-cubic phase appears, resulting in the decline of λ dramatically, from 1935.2×10⁻⁶ for x=0 to 695.9×10⁻⁶ for x=0.3 at a compressive stress of 6 MPa and a magnetic field of H=240 kA m⁻¹.     

Near room temperature magnetocaloric properties of melt-spun Gd100−xBx (x=0, 5, 10, 15, and 20 at%) alloys

The magnetocaloric properties of melt-spun Gd-B alloys were examined with the aim to explore their potential application as magnetic refrigerants near room temperature. A series of Gd_100−xB_x (x=0, 5, 10, 15, and 20 at%) alloys were prepared by melt spinning. With the decrease in Gd/B ratio, Curie temperature (T_C) remains constant at ∼293 K, and saturation magnetization, at 275 K, decreases from ∼100 to ∼78 emu/g. Negligible magnetic hysteresis was observed in these alloys. The peak value of magnetic entropy change, (−ΔS_M)_max, decreased from ∼9.9 J/kg K (0-5 T) and ∼5.5 J/kg K (0-2 T) for melt-spun Gd to ∼7.7 J/kg K (0-5 T) and ∼4.0 J/kg K (0-2 T), respectively for melt-spun Gd₈₅B₁₅ and Gd₈₀B₂₀ alloys. Similarly, the refrigeration capacity (q) decreased monotonously from ∼430 J/kg (0-5 T) for melt-spun Gd to ∼330 J/kg (0-5 T) for melt-spun Gd₈₀B₂₀ alloy. The near room temperature magnetocaloric properties of melt-spun Gd_100−xB_x (0≤x≤20) alloys were found to be comparable to few first-order transition based magnetic refrigerants.     

Phase transitions and the magnetocaloric effect in Mn rich Ni–Mn–Ga Heusler alloys

In Mn rich polycrystalline Heusler alloys, Ni₅₀Mn_25+−xGa_25−x, prepared by Arc melting, it is found that the structural/first-order magnetic transition temperature T_m increases as the Mn content increases. The Curie temperature T_c is higher than that of Ni rich alloys (Ni_50+xMn_25−xGa₂₅ ) of the same series, and is less affected by composition x. Magnetic entropy change of |ΔS_M| also increases as Mn content increases, while behaviour of the field dependence of ΔS_M is similar to that of single crystal Ni_52.6Mn_23.1Ga_24.3.     

High-temperature ferromagnetism in laser-deposited layers of silicon and germanium doped with manganese or iron impurities

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high-T_c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T_g=(20-550) °C on single-crystal GaAs or Al₂O₃ substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5-500 K. The optimum growth temperature, T_g, for Si:Mn/GaAs layers with T_c≈400 K is shown to be about 400 °C. The Si:Mn/Al₂O₃ layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.     

非晶态Fe90-xMnxZr10合金的磁性

本文报道用单辊急冷方法制备的非晶态合金Fe_90-xMn_xZr₁₀(x=0,4,6,10,15)的磁性,讨论了样品中每个原子的平均磁矩和居里温度T_c随Mn含量x的变化以及类自旋玻璃特性,给出了非晶态Fe_90-xMn_xZr₁₀合金的磁相图。观察到非晶态Fe₈₄Mn₆Zr₁₀合金晶化后的热磁曲线 关键词：     

Magnetic properties of the Ce2Fe17−xMnx helical magnets up to high magnetic fields

Magnetic properties of the Ce₂Fe_17−xMn_x, x=0–2, alloys in magnetic fields up to 40 T are reported. The compounds with x=0.5–1 are helical antiferromagnets and those with 1<x?2 are helical ferromagnets or helical antiferromagnets at low and high T, respectively. Mn ions in the system carry average magnetic moment of 3.0±0.2 μ_B that couple antiparallelly to the Fe moments. Easy-plane magnetic anisotropy in the Ce₂Fe_17−xMn_x compounds weakens upon substitution of Mn for Fe. The absolute value of the first anisotropy constant in the Ce₂Fe_17−xMn_x helical ferromagnets decreases slower with increasing temperature than that calculated from the third power of the spontaneous magnetization. Noticeable magnetic hysteresis in the Ce₂Fe_17−xMn_x, x=0.5–2, helical magnets over the whole range of magnetic fields reflects mainly irreversible deformation of the helical magnetic structure during the magnetization of the compounds. A contribution from short-range order (SRO) magnetic clusters to the magnetic hysteresis of the helical magnets has been also estimated.     

Magnetic properties of metastable Gd-Cr alloys

We report on the magnetization, magnetocaloric effect, magnetic ordering temperatures, saturation magnetic moments and anisotropy of sputter-deposited Gd_xCr_1−x alloys with Gd atomic concentrations, x, ranging from 0.13 to 0.52. The complex magnetic nature of the Gd-Cr films was revealed from the M×H isotherms, which do not show saturation even at an applied field of 70 kOe and a temperature of 2 K and do not exhibit a linear behavior at higher temperatures. For some of the samples, the isotherms were used to determine the isothermal entropy variation as a function of temperature, for a change of 50 kOe in the applied magnetic field. The saturation magnetic moment varies with x and follows the dilution law, implying that the Cr atoms do not contribute to the total moment of the Gd-Cr alloys. Both static magnetization and dynamic susceptibility measurements reveal the existence of a magnetic glassy behavior in the alloys, which occurs below a freezing temperature. The existence of anisotropy at low temperatures for all samples was revealed by their M×H hysteresis loops from which the in-plane coercive fields, H_c, were determined. A monotonical increase in H_c with increasing Gd concentration was observed.     

Magnetic/structural phase diagram and enhanced giant magnetoresistance in Zn-doped antiperovskite compound Ga1−xZnxCMn3

The structural, magnetic and electrical transport properties of Zn-doped antiperovskite compounds Ga_1−xZn_xCMn₃ (0≤x≤0.30) have been investigated. After partial substitution of Zn for Ga, the Curie temperature increases monotonously and the ground antiferromagnetic (AFM)-ferromagnetic intermediate (FI) phase transition is gradually suppressed. With increasing the doping level x, the saturated magnetizations decreases gradually firstly for x≤0.20, then increases with increasing x. The electrical transport properties of Ga_1−xZn_xCMn₃ are studied at different magnetic fields. Enhanced giant magnetoresistance (GMR) was observed around the AFM-FI transition. With increasing x, the maximal values and peak widths of GMR increase. Particularly, for x=0.20, GMR reaches a maximum value of 75%, spanning a temperature range of 80 K at 50 kOe and displays the behavior of strongly depending on the magnetization history. The possible origins are discussed.