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.     

Suppression of low-temperature magnetic states in Mn3O4 nanoparticles

We have investigated the low-temperature magnetic properties of Mn₃O₄ nanoparticles using thermodynamic and magnetic measurements. While bulk Mn₃O₄ exhibits three magnetic transitions close to 42, 40 and 34 K, the two lower temperature transitions appear to be absent above 15 K in Mn₃O₄ nanoparticles. The magnetization and spin entropy associated with the ferrimagnetic transition at 42 K is smaller in the Mn₃O₄ nanoparticles than bulk Mn₃O₄, which is consistent with roughly 30-50% of the spins not contributing to the magnetic order. We tentatively attribute this suppression of the lower temperature transitions to a combination of finite size effects and effects arising from amorphous surface spins on the nanoparticles.     

Magnetic properties and charge ordering in Pr0.75Na0.25MnO3 manganite

The structural and magnetic properties of Pr_0.75Na_0.25MnO₃ have been investigated experimentally. At room temperature, the compound shows paramagnetic characteristic. Along with decreasing temperature, a peak appears in the magnetization versus temperature curve around 220 K. To clarify whether this peak is associated with the ordering arrangement of Mn³⁺ and Mn⁴⁺ ions, electron diffraction experiments were carried out below and above 220 K respectively. Only basic Brag diffraction spots can be observed at high temperatures, however, superlattice diffraction appears below 220 K. This provides direct evidence for the existence of charge ordering in Pr_0.75Na_0.25MnO₃. We find the Mn³⁺ and Mn⁴⁺ cations form zigzag chains in a-c plane by analyzing the diffraction patterns. Combining with the magnetization measurements and the results of electron spin resonance, we confirm the antiferromagnetic phase and ferromagnetic component coexist in Pr_0.75Na_0.25MnO₃ below 120 K.     

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 response of NiFe2O4 nanoparticles in polymer matrix

We report the magnetic properties of magnetic nano-composite, consisting of different quantity of NiFe₂O₄ nanoparticles in polymer matrix. The nanoparticles exhibited a typical magnetization blocking, which is sensitive on the variation of magnetic field, mode of zero-field-cooled/field-cooled experiments and particle quantity in the matrix. The samples with lower particle quantity showed an upturn of magnetization down to 5 K, whereas the blocking of magnetization dominates at lower temperatures as the particle quantity increases in the polymer. We examine such magnetic behaviour in terms of the competitive magnetic ordering between core and surface spins of nanoparticles, taking into account the effect of inter-particle (dipole-dipole) interactions on nanoparticle magnetic dynamics.     

Magnetic properties of MnO nanocrystals dispersed in a silica matrix

Magnetic nanocrystalline MnO particles have been synthesized in a silica glass matrix by the sol-gel method at calcination temperatures up to 1000 °C. EPR spectra of 0.1 mol% MnO doped silica gel and glasses studied in the temperature range 10-290 K show with the exception of those samples calcined at 900 and 1000 °C 6-line characteristic Mn(II) hyperfine (HF) lines. Additionally five spin-forbidden doublets have been observed at 100 K and below. Small spreads in spin Hamiltonian parameters (D and E) imply that the ligand field environments of Mn(II) ions embedded in the silica glass are nearly uniform. Monotonous decrease in HF linewidth in going from 120 °C gel to 800 °C calcined glass has been interpreted as the continuous decrease in population of isolated Mn²⁺ ions in silica glass matrix resulting in the decrease of magnetic dipolar interactions leading to the observed decrease in HF linewidth. XRD and TEM of sample calcined at 1000 °C shows the presence of nanocrystals of MnO having orthorhombic crystalline phase and sizes about 10 nm. The thermal behavior of magnetization (zero-field-cooled and field-cooled) and magnetic hysteresis of MnO nanocrystals in the 5-300 K temperature interval have demonstrated that the MnO nanocrystals display superparamagnetic-ferromagnetic transition at low temperatures. X-band EPR linewidth data plotted versus inverse of temperature (1/T) for samples calcined at 900 and 1000 °C (EPR recorded in the vicinity of 0.35 T applied field) depict similar transitions.     

Understanding the magnetic ground state of rare-earth intermetallic compound Ce4Sb3: Magnetization and neutron diffraction studies

Magnetization and neutron diffraction studies have been performed on Ce₄Sb₃ compound (cubic Th₃P₄-type, space group I4¯3d, no. 220). Magnetization of Ce₄Sb₃ reveals a ferromagnetic transition at ∼5 K, the temperature below which the zero-field-cooled and field-cooled magnetization bifurcate in low applied fields. However, a saturation magnetization (M_S) value of only ∼0.93μ_B/Ce³⁺ is observed at 1.8 K, suggesting possible presence of crystal field effects and a paramagnetic/antiferromagnetic Ce³⁺ moment. Magnetocaloric effect in this compound has been computed using the magnetization vs. field data obtained in the vicinity of the magnetic transition, and a maximum magnetic entropy change, −ΔS_M, of ∼8.9 J/kg/K is obtained at 5 K for a field change of 5 T. Inverse magnetocaloric effect occurs at ∼2 K in 5 T indicating the presence of antiferromagnetic component. This has been further confirmed by the neutron diffraction study that evidences commensurate antiferromagnetic ordering at 2 K in zero magnetic field. A magnetic moment of ∼1.24μ_B/Ce³⁺ is obtained at 2 K and the magnetic moments are directed along Z-axis.     

High concentration of hematite nanoparticles in a silica matrix: Structural and magnetic properties

The α-Fe₂O₃/SiO₂ nanocomposite containing 45 wt% of hematite was prepared by the sol-gel method followed by heating in air at 200 °C. The so-obtained composite of iron(III) nanoparticles dissolved in glassy silica matrix was investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRPD confirms the formation of a single-phase hematite sample, whereas TEM reveals spherical particles in a silica matrix with an average diameter of 10 nm. DC magnetization shows bifurcation of the zero-field-cooled (ZFC) and field-cooled (FC) branches up to the room temperature with a blocking temperature T_B=65 K. Isothermal M(H) dependence displays significant hysteretic behaviour below T_B, whereas the room temperature data were successfully fitted to a weighted Langevin function. The average particle size obtained from this fit is in agreement with the TEM findings. The small shift of the T_B value with the magnetic field strength, narrowing of the hysteresis loop at low applied field, and the frequency dependence of the AC susceptibility data point to the presence of inter-particle interactions. The analysis of the results suggests that the system consists of single-domain nanoparticles with intermediate strength interactions.     

Magnetic properties of CdSb doped with Ni

Magnetic properties of the group II–V semiconductor CdSb single crystals doped with Ni (2 at%) are investigated. Deviation of the zero-field-cooled susceptibility, χ_ZFC, from the field-cooled susceptibility is observed below 300 K, along with a broad maximum of χ_ZFC (T) at T_b in fields below the anisotropy field B_K∼4 kG. T_b(B) obeys the law [T_b(B)/T_b(0)]^1/2=1–B/B_K with T_b(0)∼100 K. The magnetization exhibits saturation above ∼20–30 kG, a weak temperature dependence and anisotropy of the saturation value M_s. The coercive field is much smaller then B_K and displays anisotropy inverted with respect to that of M_s. Such magnetic behavior is expected for spheroidal Ni-rich Ni_1−xSb_x nanoparticles with high aspect ratio, broad distribution of the sizes and with orientations of the major axis distributed around a preferred direction.     

Exchange bias and negative magnetoresistance in [Co/Bi/Co]/IrMn thin films]

The artificial control of grain-boundary resistance and its contribution to magnetic and magneto-transport properties in [Co(3 nm)/Bi(2.5 nm)/Co(3 nm)]Ir₂₀Mn₈₀(12 nm) thin films that exhibit exchange bias is studied. Transverse magnetoresistance (MR) loops exhibit a negative MR in thin films grown by magnetron sputtering on Si/SiN_x(100 nm) substrates. This negative MR effect is of the giant-MR (GMR) type, although its magnitude is less than 1%. A considerable exchange bias (EB) effect is observed only at lower temperatures, where both, GMR and isothermal magnetization loops exhibit a shift of −600 Oe at 5 K.     

Spin-glass-like behavior of uncompensated surface spins in NiO nanoparticulated powder

Nickel oxide nanoparticles successfully synthesized by a polymer precursor method are studied in this work. The analysis of X-ray powder diffraction data provides a mean crystallite size of 22±2 nm which is in a good agreement with the mean size estimated from transmission electron microscopy images. Whereas the magnetization (M) vs. magnetic field (H) curve obtained at 5 K is consistent with a ferromagnetic component which coexists with an antiferromagnetic component, the presence of two peaks in the zero-field-cooled trace suggests the occurrence of two blocking process. The broad maximum at high temperature was associated with the thermal relaxation of uncompensated spins at the particle core and the low temperature peak was assigned to the freeze of surface spins clusters. Static and dynamic magnetic results suggest that the correlations of surface spins clusters show a spin-glass-like behavior below T_g=7.3±0.1 K with critical exponents zν=9.7±0.5 and β=0.7±0.1, which are consistent with typical values reported for spin-glass systems.     

Investigation of magnetic properties of interacting Fe2O3 nanoparticles

The magnetic properties of Fe₂O₃ nanoparticles (average diameter ∅≅3 nm) in alumina (68% Fe₂O₃ in weight) have been investigated by magnetization measurements. The results indicate a superparamagnetic behavior of interacting particles, which block with decreasing temperature (the zero-field-cooled susceptibility shows a maximum at T≅145 K) with a distribution of relaxation times. A change of magnetic regime is observed below ∼60 K, due to the increasing interparticle interactions and local surface anisotropy.     

Synthesis and magnetic properties of hexagonal Y(Mn,Cu)O3 multiferroic materials

Single-phase hexagonal-type solid solutions based on the multiferroic YMnO₃ material were synthesized by a modified Pechini process. Copper doping at the B-site (YMn_1−xCu_xO₃; x<0.15) and self-doping at the A-site (Y_1+yMnO₃; y<0.10) successfully maintained the hexagonal structure. Self-doping was limited to y(Y)=2 at% and confirmed that excess yttrium avoids formation of ferromagnetic manganese oxide impurities but creates vacancies at the Mn site. Chemical substitution at the B-site inhibits the geometrical frustration of the Mn³⁺ two-dimensional lattice. The magnetic transition at T_N decreases from 70 K down to 49 K, when x(Cu) goes from 0 to 15 at%. Weak ferromagnetic Mn³⁺-Mn⁴⁺ interactions created by the substitution of Mn³⁺ by Cu²⁺, are visible through the coercive field and spontaneous magnetization but do not modify the overall magnetic frustration. Presence of Mn³⁺-Mn⁴⁺ pairs leads to an increase of the electrical conductivity due to thermally-activated small-polaron hopping mechanisms. Results show that local ferromagnetic interactions can coexist within the frustrated state in the hexagonal polar structure.     

Hydrogen absorption and field-dependent diamagnetism of SmNiAl

Hydrogen absorption properties of SmNiAl were systematically investigated between room temperature and 673 K over the pressure range from 0 to 10 MPa. It absorbs hydrogen without clear plateaus, forms hydride SmNiAlH_x (x=0.75–2.04) without structural change below 623 K and decomposes into SmH₂ and NiAl at higher temperatures. Then, the magnetic curves, the field-cooled (FC) and the zero-field-cooled (ZFC) susceptibilities of SmNiAl and its hydride were investigated. SmNiAl behaves as paramagnetic above 65 K, but has three antiferromagnetic transitions at lower temperatures. Hydrogenation enhances its magnetism, but weakens the interaction between magnetic moments. In the FC process, hydrogenation especially induces an anomalous diamagnetism. Mechanisms for the hydrogen absorption, structural change and magnetism were discussed.     

Ferromagnetic resonance observation of a phase transition in magnetic field-aligned Fe2O3 nanoparticles

Fe₂O₃ hematite (alpha) nanoparticles suspended in the liquid phase of the liquid crystal 4,4-azoxyanlsole (PAA) are cooled below the freezing temperature (397 K) in a 4000 G dc magnetic field. The in field solidification locks the direction of maximum magnetization of the particles parallel to the direction of the applied dc magnetic field removing the effects of dynamical fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the nanoparticles as well as the anisotropic behavior of the ferromagnetic resonance (FMR) signal. Freezing in PAA allows temperature-dependent measurements to be made at much higher temperature than previous measurements. The field position, line width and intensity of the FMR signal as a function of temperature as well as the magnetization show anomalies in the vicinity of 200 K indicative of a magnetic transition, likely the previously observed Morin transition shifted to lower temperature due to the small particle size. Weak ferromagnetism is observed below T_c in contrast to the bulk material where it is antiferromagnetic below T_c. The Raman spectrum above and below 200 K shows no evidence of a change in lattice symmetry associated with the magnetic transition.     

Magnetic properties of Cr-Fe-Mn alloys

The magnetic behaviour of a Cr_80−xFe₂₀Mn_x alloy system with x=2, 7, 10, 13 and 22 has been investigated in the temperature range 2-400 K through measurements of magnetization, electrical resistivity, magnetoresistivity, specific heat and thermal expansion. The temperature vs. Mn concentration magnetic phase diagram of the system is rich in magnetic behaviour with ferromagnetic (FM), antiferromagnetic (AFM) and paramagnetic phase regions and a spin-glass (SG) region at the lowest temperatures. Phase transition temperatures amongst these different magnetic phases could be identified from well-defined anomalies of magnetic origin that are displayed by graphs of the above-mentioned physical properties as a function of temperature. The time relaxation of the thermoremanent, isothermal remanent and field-cooled magnetizations below and above the SG freezing temperature show unusual aspects. These relaxations do not follow the usual superposition principle that is expected for typical SG materials. Negative giant magetoresistance (GMR) is observed in the alloys at 4 K. The GMR initially increases sharply on increasing the Mn content in the alloy system, followed by a tendency towards a saturation negative value for concentrations of more than about 10 at% Mn. Low-temperature plots of C_p/T vs. T², where C_p is the specific heat, present anomalous behaviour for alloys with x=2, 10 and 22. For x=2 the plot shows an upturn at the lowest temperatures that changes over to a prominent downturn for x=10 and 22. This behaviour is attributed to Fe concentration fluctuations in the alloys, confirming the theoretical model of Matthews.     

Spin-glass-like behaviour and magnetic order in Mn[Cr0.5Ga1.5]S4 spinels

Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr_0.5Ga_1.5]S₄ spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θ_CW=−55 K and an effective magnetic moment of 5.96 μ_B close to the spin-only value of 6.52 μ_B for Cr³⁺ and Mn²⁺ ions in the 3d³ and 3d⁵ configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature T_N=3.9 K, a minimum at the Yafet-Kittel temperature T_YK=5 K, a broad maximum at the freezing temperature T_f=7.9 K, and an inflection point at the Curie temperature T_C=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than T_C suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.     

Temperature dependent coercivity and magnetization of nickel ferrite nanoparticles

Magnetic nanoparticles of nickel ferrite (size: 24±4 nm) have been synthesized by chemical coprecipitation method using stable ferric and nickel salts. Coercivity of nanoparticles has been found to increase with decrease in temperature of the samples. It has been observed that the coercivity follows a simple model of thermal activation of particle’s moment over the anisotropy barrier in the temperature range (10-300 K), in accordance with Kneller’s law for ferromagnetic materials. Saturation magnetization follows the modified Bloch’s law in the temperature range from 300 to 50 K. However, below 50 K, an abrupt increase in magnetization of nanoparticles was observed. This increase in magnetization at lower temperatures was explained with reference to the presence of freezed surface-spins and some paramagnetic impurities at the shell of nanoparticles that are activated at lower temperatures in core-shell nickel ferrite nanoparticles.     

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.     

Recent developments in the formation and structure of tin-iron oxides by laser pyrolysis

Complex oxides demonstrate specific electric and magnetic properties which make them suitable for a wide variety of applications, including dilute magnetic semiconductors for spin electronics. A tin-iron oxide Sn_1−xFe_xO₂ nanoparticulate material has been successfully synthesized by using the laser pyrolysis of tetramethyl tin-iron pentacarbonyl-air mixtures. Fe doping of SnO₂ nanoparticles has been varied systematically in the 3-10 at% range. As determined by EDAX, the Fe/Sn ratio (in at%) in powders varied between 0.14 and 0.64. XRD studies of Sn_1−xFe_xO₂ nanoscale powders, revealed only structurally modified SnO₂ due to the incorporation of Fe into the lattice mainly by substitutional changes. The substitution of Fe³⁺ in the Sn⁴⁺ positions (Fe³⁺ has smaller ionic radius as compared to the ionic radius of 0.69 Å for Sn⁴⁺) with the formation of a mixed oxide Sn_1−xFe_xO₂ is suggested. A lattice contraction consistent with the determined Fe/Sn atomic ratios was observed. The nanoparticle size decreases with the Fe doping (about 7 nm for the highest Fe content). Temperature dependent ⁵⁷Fe Mössbauer spectroscopy data point to the additional presence of defected Fe³⁺-based oxide nanoclusters with blocking temperatures below 60 K. A new Fe phase presenting magnetic order at substantially higher temperatures was evidenced and assigned to a new type of magnetism relating to the dispersed Fe ions into the SnO₂ matrix.