Specific features of the properties of half-metallic ferromagnetic Heusler alloys Fe<Subscript>2</Subscript>MnAl,Fe<Subscript>2</Subscript>MnSi,and Co<Subscript>2</Subscript>MnAl

The structural, magnetic, and electrical properties of half-metallic Heusler alloys Fe₂MnAl, Fe₂MnSi, and Co₂MnAl have been investigated in the temperature range of 4–900 K. According to the X-ray diffraction analysis, these alloys have the B2 and L2₁ structures with different degrees of atomic order. The magnetic state of the alloys is considered as a two-sublattice ferrimagnet. The electrical resistivity and thermoelectric power have been discussed in the framework of the two-current conduction model taking into account the existence of an energy gap in the electronic spectrum of the alloys near the Fermi level for the subband with spin-down (minority) electrons.     

Low-temperature electron properties of Heusler alloys Fe<Subscript>2</Subscript>VAl and Fe<Subscript>2</Subscript>CrAl: Effect of annealing

We present the results of measurements of low-temperature heat capacity, as well as electrical and magnetic properties of Heusler alloys Fe₂VAl and Fe₂CrAl prepared in different ways using various heat treatment regimes. The density of states at the Fermi level is estimated. A contribution of ferromagnetic clusters in the low-temperature heat capacity of the Fe₂VAl alloy is detected. The change in the number and volume of clusters as a result of annealing of an alloy affects the behavior of their low-temperature heat capacity, resistivity, and magnetic properties.     

Electronic and magnetic properties of Fe<Subscript>2</Subscript>SiC

The electronic structure and magnetic properties of Fe₂SiC compound have been studiedusing the framework of an all-electron full-potential linearized augmented-plane wave(FP-LAPW) method within the local density (LSDA) and + U corrected(LSDA + U)approximations. An antiferromagnetic spin ordering of Fe atoms is shown to be the groundstate for this compound. From the electronic band structures and density of states (DOS),Fe₂SiC has ametallic character and from the analysis of the site and momentum projected densities, itis deduced that the bonding is achieved through hybridization of Fe-3d with C-2p states andFe-3d withSi-3pstates. It is also pointed out that the Fe-C bonding is more covalent than Fe-Si. In theFM phase, the spin polarized calculations indicate that the total magnetic moment ofFe₂SiC increasesfrom 0.41 to 4.33μ _B when the Hubbard U parameter for iron isconsidered.     

Influence of magnetism on the structural stability of cubic L2<Subscript>1</Subscript>Ni<Subscript>2</Subscript>MnGa

We present calculations of magnetic field-dependent properties of magnetic shape memory (MSM) Heusler alloys by means of density functional theory calculations. The effects of an external magnetic field on structural properties are simulated by fixing the magnetic moments within the framework of the fixed spin moment (FSM) method. We calculate the binding surface as a function of the magnetic moment and the tetragonal distortion. For magnetizations of 10% below the equilibrium value, the energy of the martensitic L1₀ phase steeply increases leading to a relative stabilization of the L2₁ phase in a confined magnetization range. Calculations of the phonon dispersion in the direction [ξξ0]2π/a suggest that the instability at ξ≈1/3 disappears with decreasing magnetization, allowing a nearly stable spectrum in a small magnetization interval.     

Synthesis and structural,magnetic, and resonance properties of the LiCuFe<Subscript>2</Subscript>(VO<Subscript>4</Subscript>)<Subscript>3</Subscript> compound

Complex studies have been performed for the structural, static magnetic, and resonance properties of a new magnet LiCuFe₂(VO₄)₃ prepared by solid-phase synthesis. The temperature dependence of the susceptibility has an anomaly at temperature T_max = 9.6 K. At high temperatures, the LiCuFe₂(VO₄)₃ sample is in the paramagnetic state described by the Curie–Weiss law at T > 50 K and mainly determined by iron ions with effective magnetic moment μ_eff(exp) = 8.6μ_B per formula unit. At low temperatures, a long-range magnetic order is observed in the magnetic subsystem of the sample; the order is predominantly characterized by the antiferromagnetic exchange interaction and high frustration level. The exchange interaction parameters are estimated in a six-sublattice representation of the LiCuFe₂(VO₄)₃ magnet. It is shown that the LiCuFe₂(VO₄)₃ compound is an antiferromagnet with strong intrachain and frustrating interchain exchange interactions.     

The effect of defects on the electronic structure and magnetic map of the Fe<Subscript>2</Subscript>CrSi Heusler alloy: ab-initio calculations

Density functional theory (DFT) calculations are performed using the full-potential linearized augmented plane wave (FP-LAPW) and generalized gradient approximation (GGA) to study the electronic and magnetic properties of perfect and defected Fe₂CrSi Heusler alloy. The perfect structure was found to be a half-metallic ferromagnet with a total magnetic moment of 2 μ _B and a band gap 0.6 eV. The Fermi level is found to be in the middle of this gap, which is promising for fabricating tunneling magnetoresistance (TMR) devices. Among the studied defected structures Fe_Si and Cr_Si antisite defects as well as Fe-Si and Cr-Si defects destroyed the half metallicity. However the remaining antisite, swap and vacancy defects retained the half metallicity with band gaps lower than the perfect case.     

The evolution of electronic configuration and magnetic characterization of Fe<Subscript>9</Subscript>Ni<Subscript>1</Subscript>, Fe<Subscript>8</Subscript>Ni<Subscript>2</Subscript> alloy in theoretical calculation

To analyze the origin of the magnetic enhancement of Fe-Ni alloy, the electronicconfigurations and magnetic properties were investigated using density functional theorybased on the first-principle. The supercell (5 × 1 × 1) of Fe,Fe₉Ni₁ and Fe₈Ni₂ were constructed. Thedefect formation energy, band structure, density of states and electron density differencewere calculated. The results showed that Ni doping changed the electronic configuration ofFe atoms, resulting in the enhancement of spin polarization of Fe and the larger Bohrmagnetic moment in Fe-Ni alloys (Fe₉Ni₁). The results showed thatthe charge transfer and the atomic spacing between Fe atoms and the dopant Ni atoms playedan important role in determination of magnetic moment. The value of Fe supercell(5 × 1 × 1), Fe₉Ni₁ and Fe₈Ni₂ were 23.14,23.34 and 22.61μ _B, respectively.     

Field dependence of orbital magnetic moments in the Heusler compounds Co<Subscript>2</Subscript>FeAl and Co<Subscript>2</Subscript>Cr<Subscript>0.6</Subscript>Fe<Subscript>0.4</Subscript>Al

Orbital and spin magnetic moments of the Heusler compounds Co₂FeAl and Co₂Cr_0.6Fe_0.4Al were measured by magnetic circular dichroism in X-ray absorption (XMCD). The orbital magnetic moments per spin are quite large (0.1–0.2) compared to bulk values of Fe and Co metals, indicating a considerable spin–orbit coupling in these Heusler compounds. A strong localization of the 3d electron states might be responsible for this observation. The Co and Fe orbital to spin moment ratio shows a distinct decrease of r(Fe)=0.04±0.02 and r(Co)=0.06±0.02 with increasing external field for the ternary compound Co₂FeAl, while the ratio is within error limits independent of the field for Co₂Cr_0.6Fe_0.4Al. This is discussed in terms of a relation to magnetocrystalline anisotropies. PACS 75.50.Cc; 71.20.Lp; 78.40.Kc     

Controlled-synthesis,characterization, and magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanostructures

Fe₃O₄ nanostructures with different morphologies, including uniform nanoparticles, nanorods and nanowire bundles, have been successfully synthesized via a facile hydrothermal route. Based on the observation of TEM images, the growth mechanism of one-dimensional Fe₃O₄ nanostructures is in accordance with Ostwald ripening process. From the hysteresis loops of as-prepared Fe₃O₄ products, we found that the morphology has great influence on the magnetic properties. The uniform Fe₃O₄ nanoparticles have higher saturation magnetization and lower coercivity than that of Fe₃O₄ nanorods and nanowires bundles. These phenomena attribute to the high shape anisotropy of nanorods and nanowire bundles, which prevent them from magnetizing in directions other than along their easy magnetic axes. PACS 81.07.-b; 75.50.Bb; 75.30.Gw; 81.10.Dn; 81.16.Be     

Effect of change in structural and magnetic states of Pt<Subscript>74.1</Subscript>Fe<Subscript>25.9</Subscript> alloy on its optical properties

The effect of atomic disordering on the optical properties of Pt_74.1Fe_25.9 alloy, whose stoichiometry is close to that of Pt₃Fe, has been investigated. The optical constants of ordered and plastically deformed alloys, which are, respectively, in the paramagnetic and ferromagnetic states, have been measured by the polarimetric method. The frequency dispersions of the permittivity, optical conductivity, and reflectivity, as well as the microscopic characteristics of conduction electrons (plasma and relaxation frequencies), have been calculated. The energy dependences of the optical conductivity are compared to the calculated energy-band structure of atomically ordered and disordered Pt₃Fe compounds.     

Structural and magnetic properties of size-controlled Mn<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles and magnetic fluids

Mn_0.5Zn_0.5Fe₂O₄ ferrite nanoparticles with tunable Curie temperature and saturation magnetization are synthesized using hydrothermal co-precipitation method. Particle size is controlled in the range of 54 to 135 Å by pH and incubation time of the reaction. All the particles exhibit super-paramagnetic behaviour at room temperature. Langevin’s theory incorporating the interparticle interaction was used to fit the virgin curve of particle magnetization. The low-temperature magnetization follows Bloch spin wave theory. Curie temperature derived from magnetic thermogravimetric analysis shows that Curie temperature increases with increasing particle size. Using these particles magnetic fluid is synthesized and magnetic characterization is reported. The monolayer coating of surfactant on particle surface is confirmed using thermogravimetric measurement. The same technique can be extended to study the magnetic phase transition. The Curie temperature derived using this measurement complies with the low-temperature magnetic measurement. The room-temperature and high-temperature magnetization measurements are also studied for magnetic fluid systems. The magnetic parameters derived for fluid are in good agreement with those obtained for the particle system.     

Synthesis and magnetic properties of Laves phase Fe<Subscript>2</Subscript>Nb amorphous alloy

The formation process and magnetic properties of the Laves phase compound of nominal composition Fe₂Nb in its amorphous phase prepared by mechanical alloying have been investigated. The effect of milling time on the formation of amorphous phase has been studied using an X-ray diffraction technique. Further characterizations were carried out by particle size measurement, dc magnetization, ac susceptibility and ferromagnetic resonance studies. Magnetization and susceptibility studies show soft ferromagnetic behaviour, whereas ferromagnetic resonance studies show some sort of disorder/strain introduced during the mechanical alloying process. PACS 74.70.Ad; 61.82.Bg; 74.25.Ha; 75.60.Ej; 76.50.+g     

First principles study of structural,magnetic and electronic properties of half-metallic CrO<Subscript>2</Subscript> under pressure

A first-principles tight-binding linear muffin tin orbital (TB-LMTO) method within the local-density approximation is used to calculate the total energy, lattice parameter, bulk modulus, magnetic moment, density of states and energy band structures of half-metallic CrO₂ at ambient as well as at high pressure. The magnetic and structural stabilities are determined from the total energy calculations. From the present study we predict a magnetic transition from ferromagnetic (FM) state to a non-magnetic (NM) state at 65 GPa, which is of second order in nature. We also observe from our calculations that CrO₂ is more stable in tetragonal phase (rutile-type) at ambient conditions and undergoes a transition to an orthorhombic structure (CaCl₂-type) at 9.6 GPa, which is in good agreement with the experimental results. We predict a second structural phase transition from CaCl₂- to fluorite-type structure at 89.6 GPa with a volume collapse of 7.3%, which is yet to be confirmed experimentally. Interestingly, CrO₂ shows half metallicity under ambient conditions. After the first structural phase transition from tetragonal to orthorhombic, half metallicity has been retained in CrO₂ and it vanishes at a pressure of 41.6 GPa. Ferromagnetism is quenched at a pressure of 65 GPa.     

Mössbauer studies of magnetic Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> nanocomposites

A large variety of glass and glass ceramics may be obtained by sol-gel process from hydrolysis of tetraethoxysilane. The transformation involves hydrolysis and polycondensation reactions leading to the growth of clusters that eventually collide together to form a gel. The structure and properties of the final product have been found to be strongly dependent on the initial conditions of preparation. Silica nanocomposites based on Fe₂O₃/SiO₂ were prepared with the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 to 30wt.%.     

Structural,magnetic, and magnetothermal properties of the Tb<Subscript>0.3</Subscript>Dy<Subscript>0.7</Subscript>Co<Subscript>2</Subscript> compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb_0.3Dy_0.7Co₂ compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb_0.3Dy_0.7Co₂ compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb_0.3Dy_0.7Co₂ have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.     

Vibrational spectra of Fe<Subscript>3</Subscript>P and Fe<Subscript>2</Subscript>P metal-metalloid compound crystallites: Phonon and breather excitations

The spectra of small-amplitude single-phonon vibrations and large-amplitude nonlinear multiphonon vibrations of Fe₃P and Fe₂P metal-metalloid compound crystallites are investigated. The vibrational spectra of 3D crystallites of gradually increasing volumes are calculated within a microscopic approach using previously tested interatomic interaction potentials. The calculated energy of the main low-frequency peak is closer to its experimental value in comparison with previous calculations, which reduces the significant discordance between the experimental and theoretical spectra. The fine structure of the high-frequency part of the vibrational spectrum is discussed. A study of large-amplitude nonlinear vibrations (which determine the diffusion of atoms and dispersive processes in materials) showed that a specific nonlinear breather mode of vibration can be generated in the crystallites in question, which is a genetic precursor of nonlinear self-localized vibrations.     

Synthesis and properties of barium ferrigermanate Ba<Subscript>2</Subscript>Fe<Subscript>2</Subscript>GeO<Subscript>7</Subscript>

The magnetic susceptibility and specific heat of single crystals of the Ba₂Fe₂GeO₇ barium ferrigermanate are investigated. It is revealed that the temperature dependence of the magnetic susceptibility exhibits a kink at a temperature T = 8.5 K. The number of nonequivalent positions of Fe³⁺ ions and their occupancies are determined using Mössbauer spectroscopy. It is shown that the Fe³⁺ ions located in tetrahedral positions T2 are ordered incompletely, which is inconsistent with the results obtained previously. An assumption is made regarding the possible ground magnetic state of the Ba₂Fe₂GeO₇ compound.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Dielectric dispersion and magnetic properties of Ba-modified Pb(Fe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>)O<Subscript>3</Subscript>

The polycrystalline samples of Ba-modified Pb(Fe_1/2Nb_1/2)O₃ (i.e., (Pb_1-xBa_x)(Fe_1/2Nb_1/2)O₃ PBFN, with x=0,0.05,0.07) were synthesized by a mechanosynthesis (i.e., high-energy ball milling) route followed by a mixed oxide method. Structural analysis provides the information on formation of single-phase orthorhombic structure on substitution of a small amount (x=0.07) of Ba at the Pb-site of Pb(Fe_0.50Nb_0.50)O₃ (PFN). The ferroelectric–paraelectric phase transition in PFN was observed at 383 K, which decreases on increasing Ba-concentration in PBFN. Detailed studies of dielectric properties of PBFW show the following: (i) diffuse phase transition, (ii) low loss tangent, (iii) low activation energy, and (iv) low frequency dielectric dispersion. An anomaly in the ac conductivity was found very close to phase transition temperature. The activation energy is found to decrease from 0.19 to 0.01 eV on increasing Ba-concentration to 7% (x=0.07). Temperature field-dependent magnetization measurements of all the samples showed antiferromagnetic transition at ∼15 K (for x=0.07). PBFN sample showed a slight increase in the coercivity (i.e., from 400 Oe (PFN) to 500 Oe (PBFN, for x=0.07) at 2 K. PACS 61.10.Nz; 68.37.Hk; 75.50.Ss; 75.60.Ej; 77.22.Ch; 77.22.Gm     

Investigation of the magnetic properties and magnetic structure parameters of nanocrystalline Fe<Subscript>79</Subscript>Zr<Subscript>10</Subscript>N<Subscript>11</Subscript> films

The magnetic properties (magnetization curve, ferromagnetic resonance spectrum) of nanocrystalline Fe₇₉Zr₁₀N₁₁ films obtained by RF magnetron sputtering with subsequent annealing were studied experimentally, along with the fundamental magnetic constants of these films (saturation magnetization M _S, local magnetic anisotropy energy K, and the exchange coupling constant A). The magnetic properties are discussed within the random magnetic model, which determines the correlation of the magnetic properties with the fundamental magnetic constants and nanostructure parameters (grain size, magnetic anisotropy, and correlation radius R _C). The exchange correlation length 2R _L for the film magnetic microstructure was determined by correlation magnetometry.