Near infrared magneto-absorption in the laser material KZn1−xCoxF3

Infrared absorption (1.4–2μm) of KZn_1−xCo_xF₃ (0⩽ x<0.15) at 4.2 K has been studied in a magnetic field (B⩽7 T). Large field-induced splittings were observed for Co²⁺-ion lines. Details of the cobalt concentration dependence of the absorptivity at B = 0 and at field are given along with the dependences on field direction and strength. Assignments are made of the observed Co²⁺ single-ion and pair transitions. The data has enabled the determination of the infrared (IR) transition g values.     

Magnetostriction and other magnetic properties of Co-Ni based amorphous alloys

The magnetostriction coefficients λ_s of (Co_1−xNi_x)₇₅Si₁₅B₁₀ amorphous alloys have been determined in the range 0 ⩽ x ⩽ 0.41. The dependence of the magnetostriction on temperature T has been related to that of the magnetic polarization J_s in a phenomenological way as λ_s(T) = α[J_s(T)]³ + β[J_s(T)]². The compositional dependence of the coefficients α and β has been obtained.From the studies of the magnetostriction in the region close to the Curie temperature it has been possible to evaluate the magnetostriction critical exponent K.     

Induced magnetic anisotropy in Co-doped Finemet-type nanocrystalline materials

Transverse magnetic anisotropy has been induced in the Fe_14.7Co_58.8Cu₁Nb₃Si_13.5B₉ and Fe_13.8Co₆₅Cu_0.6Nb_2.6Si₉B₉ amorphous ribbons by annealing under an external magnetic field. This anisotropy plays a predominant role, compared to magneto-crystalline and magneto-elastic anisotropies, in forming the magnetic properties and shaping the hysteresis loop. The effect of temperature and time of annealing on the induced magnetic anisotropy and magnetic properties (magnetic permeability, coercivity and power losses) in both alloys was investigated. Under this work, measurements of frequency dependence of the real and imaginary parts of magnetic permeability were made within a frequency range up to 110 MHz. It was found that as a result of magnetic field annealing the Snoek limit increases in both alloys compared to the Co-free Finemet.     

Strain- and field-induced magnetic anisotropy in metallic glasses with positive or negative γs

Magnetic anisotropy in Co₇₃Mo₂Si₁₅B₁₀ (γ_s < 0) and (Co_.89Fe_.11)₇₂Mo₃Si₁₅B₁₀ (γ_s > 0) metallic glass ribbons has been induced by strain annealing and by field annealing above and below the Curie-temperature, respectively. The results support very convincingly the assumption that two different mechanisms (frozen-in strain and directional ordering) are responsible for the obtained anisotropies. We propose that a biaxial magnetic anisotropy can be ontained by proper annealing treatments.     

A mean-field study on the thermo-magnetic properties of GdxCo1−x amorphous alloys (0.16⩽x⩽0.25)

We present a mean-field study on the thermo-magnetic properties of Gd_xCo_1−x amorphous alloys in the 0.16⩽x⩽0.25 composition range. A single set of exchange integrals and fixed values of the angular momenta of Gd and Co fairly describe the temperature dependence of magnetization. The magnetic specific heat and magnetic entropy show field and composition dependence. Both the specific heat anomaly and the saturated entropy, at the temperature of the magnetic phase transition, increase with increasing Co concentration. The two magnetic subnetworks and their cross-interactions contribute differently to the specific heat.     

Magnetic annealing effect in amorphous Fe100-xBx and Co100-xBx binary alloys

The uniaxial magnetic anisotropy induced by magnetic annealing was studied for rapidly quenched amorphous Fe_100-xB_x (12?x?25) and Co_100-xB_x (17?x?30) alloys. The value of induced uniaxial anisotropy constant K_ui increased with increasing B content from 1 x 10³ erg/cm³ at x = 12 to about 4 x 10³ erg/cm³ at x = 25 in Fe_100-xB_x alloy s exhibiting an inflection around x=20. While the value of K_ui in Co_100-xB_x alloys exhibits a maximum of about 6 x 10³ erg/cm³ aro round x=20. The compositional dependence of K_ui for both alloy systems can be explained roughly by the model of the directiona; occupation of B atom between ferromagnetic atoms.     

Effect of silicon additions on the magnetic properties of Nd2Fe12Co2B alloy

The results of magnetic measurements performed on Nd₂Fe_12−xSi_xCo₂B alloys with 0⩽x⩽0.6 are presented. All the compound studied crystallize in the tetragonal Nd₂Fe₁₄B structure. The unit cell volume is found to decrease by introducing Si. With substitution of Fe by Si, the saturation moments decrease, but theanisotropy fields and Curie temperatures are found to increase significantly. For example, in Nd₂Fe_11.5Si_0.5Co₂b, at room temperature, μ_s=28.4μ_B, H_A=80 kOe and T_c=736 K.     

Rapid stress annealing dependence of structural and magnetic properties of Fe75 − x Co x Cu1Nb3Si15B6 alloys

Structural and magnetic properties of nanocrystalline Fe_75???x Co _x Cu₁ Nb₃Si₁₅B₆ (x?=?0, 2, 5) alloys are reported using magnetic measurements X-ray diffraction, Mössbauer spectroscopy. Results show that: (1) for the specimens with x?=?0 reveal that the volume fraction of the nanograins and their grain diameter ranges between 56% and 80% and 10 and 18 nm, (2) annealing above 700°C apart from Fe₃Si type nanocrystals, magnetically hard Fe₃B, Fe₂₃B phases also appear, leading to a sharp increase of the coercive field, (3) Co content and applied stress during annealing has considerable effect on relative permeability and stress induced anisotropy, which is perpendicular to the ribbon axis, Mössbauer spectroscopy also suggests changes in spin texture.     

Experiments concerning the origin of stress anneal induced magnetic anisotropy in metallic glass ribbons

The stress anneal induced anisotropy has been studied in metallic glass ribbons of the compositions (Co_1-xFe_x)₇₅Si₁₅B ₁₀, 0≤x≤0.12. The induced anisotropies are found to consist of two contributions, an irreversable one and a recoverable one. From their thermal variations and from their stress dependence the anisotropy constants have tentatively been related to the constituents of the saturation magnetostriction coefficients. The kinetics reveal for the recoverable anisotropy a continuous spectrum of activation energies in the range 1.55–1.8 eV.     

Anomalous low temperature hysteresis properties of ferromagnetic metglasses

We report on measurements of hysteresis loops and initial permeability between room temperature and 1.6 K for the meglass systems Fe_{80 − x}B_x, Fe_{80 − x}Ni_xB₂₀, Fe_{80 − x}Mo_xB₂₀ and (Co₉₃Fe₇)_{75− x} Mo_xB₂₅. Above a temperature of typically 70 K the temperature dependence of the coercive force and the permeability can be well described by the conventional statistical potential theory for Bloch wall movements. At lower temperatures one observes definite deviations from the theoretical curves, which can be partially attributed to the blocking of minute crystalline ferromagnetic or antiferromagnetic inclusions. In addition in all metglasses one observes an exponential temperature dependence of the permeability below typically 20 K which is suggestive of a thermal activation of the Bloch wall movements.     

Relaxation phenomena in amorphous Co75−x Mn x B25 induced by stress annealing

The change of the magnetization direction in amorphous ferromagnets by the application of a magnetic field and a tensile stress is investigated by magnetoresistivity measurements. Attention is focussed on permanently remaining influences on the domain structure induced by stress annealing treatments. To observe any dependence on the sign of the magnetostrictionλ _s, we investigated the amorphous Co_75?x Mn _x B₂₅ system where the sign ofλ _s changes from negative to positive values with increasing Mn content. Surprisingly the stress-annealed samples showed a similar behavior independent of the sign of the magnetostriction.     

Magnetic anisotropy in thin films of metallic glasses Fe1−xBx alloys

Noninduced uniaxial magnetic anisotropy in Fe_1−xB_x thin films of metallic glasses was studied for 26⩽x⩽32. The samples were obtained by rf sputtering technique. Thickness and compositional dependence of uniaxial magnetic anisotropy was determined for films with thickness d⩽120 nm. The existence of the uniaxial magnetic anisotropy can be explained roughly by a directional alignment of free volume and directional order of atomic pairs in the vicinity of the free volume.     

Creep-induced magnetic anisotropy in Co-57Fe-Ni-Si-B amorphous alloys

The effect of creep annealing on the⁵⁷Fe enriched Co-rich low magnetostriction amorphous alloys will be reported. Contrary to the Fe-B-base alloys, after creep annealing of the⁵⁷Fe₅Co₅₇Ni₁₀Si₁₁B₁₇ the ribbon axis becomes the hard direction. Both Mössbauer spectroscopy and direct magnetic domain observations by SEM confirmed an enhanced volume with magnetization oriented in transversal and normal directions to the ribbon axis. Relaxation of the creep-induced state at a temperature slightly below that of the creep annealing leads to the enhancement of the fraction of domains magnetized in the longitudinal direction.     

Structure and magnetism of the R2Fe14−xCoxB ferrimagnetic systems (R = Dy and Er)

R₂Fe_14−xCo_xB ferrimagnetic systems (R = Dy and Er) have been synthesized and studied by X-ray and magnetometry methods to determine the lattice parameters, Curie temperatures, saturation magnetizations, anisotropy fields and spin-reorientation temperatures. It has been established that the single phase materials, exhibiting a tetragonal crystal structure, can be formed in the Dy-based system only for x ⩽ 8, while in the Er-based system only for x ⩽ 5. An average increase of the Curie temperature of 58 K per one substituted Fe atom by Co is observed for both systems (x ⩽ 5). A characteristic maximum in composition dependence of the saturation magnetizations at 295 K is found for x ≃ 2, although this is less pronounced than in light rare earth-based Co-substituted systems. The composition dependencies of the anisotropy fields for the Dy-based system at 77 and 295 K show distinct maxima for low Co concentrations and a sharp decrease for higher Co content. The spin-reorientation temperature for the Er-based system is shifted towards higher temperatures by the Co substitution. A brief comparison between ferro- and ferrimagnetic R₂Fe_14−xCo_xB systems is included.     

Octahedral vs tetrahedral coordination of the co(II) ion in layer compounds: CoxZn1−xIn2S4(O⩽x⩽0.46) solid solution

Large crystals of Co_xZn_1−xIn₂S₄ (0.00 ⩽ x ⩽ 0.46) solid solution were grown from the vapour phase by chemical transport modified by the time variation of the temperature profile procedure. Co²⁺ ions were found, by XPS analysis, to occupy tetrahedral as well as octahedral sites of the ZnIn₂S₄-type structure. The magnetic properties of Co_0.46Zn_0.54In₂S₄, crystallizing in space group R 3m, were explained in terms of the short-range magnetic order arising from the presence of isolated clusters of magnetic ions.     

Magnetostriction of amorphous GdxAg1−x alloys

Magnetostriction measurements above 4.2 K and up to 2.2 T have been performed in the amorphous alloys Gd_xAg_1−x (0.30 ⩽ x ⩽ 0.40). Magnetostriction is purely of volume character. It shows with composition a maximum at x = 0.35, and this behaviour can be explained if forced volume magnetostriction derives form the strain dependence of the two-ion longitudinal spin correlation. The isotropic spin exchange correlation seems irrelevant in these series, as also put forward by thermal expansion measurements.     

Influence of annealing on the magnetoimpedance effect in amorphous FeCoMoSiB ribbons

The effect of annealing by electric current passing through a sample prepared from the amorphous metallic alloy Vitrovac 6025 Z of the composition Fe₄Co₆₇Mo_1.5Si₁₆₅B₁₁ on the form of the dependence of the sample impedance on the external magnetic field is investigated. The results are explained on the basis of the concepts concerning the change in the preferred direction of the sample magnetization.     

Magnetic anisotropy in Co73Mo2Si15B10 and (Co0.89Fe0.11) 72Mo3Si15B10 metallic glasses,induced by stress-annealing

Annealing temperature dependence of the stress-induced magnetic anisotropy in Co₇₃Mo₂Si₁₅B₁₀ (λ_s < 0) and (Co_0.89Fe_0.11) ₇₂Mo₃Si₁₅B₁₀ (λ > 0) metallic glass ribbons has been studied experimentally. The room temperature values of the induced anisotropy constants K depend strongly on the annealing conditions, and reveal a change of sign at annealing temperatures well below T_cryst. It is concluded that transient creep and steady-state creep at the elevated temperature give rise to compressive and tensile stresses, respectively, in the expression for the magnetoelastic coupling energy at room temperature.     

High temperature electrical properties and defect structure of Co1−xMgxO

The electrical conductivity and thermoelectric power of Co_1−xMg_xO solid solutions with x ⩽ 0.85 were investigated in the range 700–1100°C and for oxygen partial pressures in the range 1–10⁻¹⁶ atm. The experimental results show that an ideal point defect model of isolated cobalt vacancies cannot explain nonstoichiometry of Co_1−xMg_xO materials with x, at least, less than 0.3. Furthermore, the hole mobility in Co_1−xMg_xO with x < 0.6 was found to be nonactivated suggesting that holes in these materials are itinerant carriers. For x > 0.6, the mobility is thermally activated indicating a change to small polaron conduction. The experimental evidence for complex defect structure and bandlike conduction in CoO is in agreement with the conclusions of recent theoretical studies of 3-D transition metal monoxides.     

Magnetic properties and specific heat of Tm1−xLuxCu2Si2

The tetragonal TmCu₂Si₂ compound is the only magnetically ordered material of the RECu₂Si₂ group for which crystal field parameters were determined. Quadrupole splitting measured by means of Mössbauer spectroscopy showed that two lowest lying states are nonmagnetic singlets. Therefore, this material is likely to have an induced magnetic moment, mainly due to mixing of the two lowest states. We performed specific heat and magnetic susceptibility measurements of Tm_1−xLu_xCu₂Si₂ (x = 0, 0.025, 0.050, 0.10, 0.25, 0.50 and 1) alloys to determine the crystal field level scheme and compare it with the Mössbauer data. The saturation magnetization of the antiferromagnetic phase was calculated to be 3.2 μ_B and the moment is directed along the tetragonal c axis. No direct experimental evidence is known to support this prediction.