Corrosion study in TbFeCoCr amorphous magneto-optical films

(Tb₂₀Fe₆₅Co₁₅)₉₄Cr ₆ film shows high corrosion resistance with good magnetooptical properties, H_c=4.9 kOe, &thetas;_k=0.3°, K_u=1.4×10⁶ erg/cm³ and T_c=230°C. Pit corrosion in 1N-NaCl aqueous solution and wet corrosion in 85°C and 85%RH atmosphere were studied for TbFeCo films with and without Cr addition. It is suggested that pit corrosion occurs by a chemical reaction between the anodic pit wall and the cathodic film surface, resulting in a piling up of corroded products inside the pits. On the other hand, wet corrosion in TbFeCo and TbFeCoCr films shows a bubble-chain-like morphology similar to conventional filiform corrosion observed in a coated film. This seems to take place by discontinuous movement of the anodic reaction area ahead of the bubbles     

Cr migration in CoNiTa/Cr films by annealing

CoNiTa/Cr double-layered films for longitudinal magnetic recording media have been prepared by using the facing targets sputtering apparatus. Substrate heating and postannealing were performed in order to increase the coercive force H_c. While the as-deposited film at the substrate temperature T_s of 400°C had H_c of 780 Oe, the films deposited at T_s of room temperature, exposed to atmosphere, and annealed at 400°C, had H_c as high as 1700 Oe. It has been found that the same amount of Cr atoms migrated from the underlayer to the magnetic layer and the vicinity of the film surface. Consequently, such a large increase of H_c seemed to be caused by isolation of magnetic grains by Cr atoms in their boundaries     

Development of high-quality ceramic powders for Sr0.9Ca0.1Zn2-W type hexagonal ferrite for permanentmagnet devices

Fine ceramic powders of Sr_0.9Ca_0.1Zn₂ Fe₁₆O₂₇ (Sr_0.9Ca_0.1Zn ₂-W) ferrite, of almost single domain grain size ~1 μm, have been synthesized successfully by sintering for a few hours a mixture of SrCO₃, CaCO₃, ZnO, and α-Fe₂ O₃ in stoichiometric amounts at an effectively low temperature (T_s) of ~1100°C. Carbon dioxide evolves in the reaction Sr(Ca)CO₃+2ZnO+8 α-Fe₂D₃ &lrarr2;Sr(Ca)Zn₂Fe₁₆O₂+C₂ , making fine pores in the sample, which prevent large grain growth of the material and result in a very loose powder. The small ⩽10% substitution of Sr by Ca activates the reaction, but preserves the crystallization of the small-sized grains. This yields magnetic properties useful for most permanent magnet applications, with a promisingly high coercivity H_c~3650 Oe and saturation magnetization M_s~65 emu/g. A considerably better saturation magnetization M_s~85 emu/g can be obtained by sintering the samples at higher T_s~1300°C, but that results in a rather very low coercivity H_c~105 Oe     

Hexagonal ferrite particles for perpendicular recording prepared byion exchange

Particles of nonstoichiometric M-type barium hexaferrite with the chemical formula BaFe_10.5M_0.25O_17.05 (M=Mg, Cd, Co, Ni, Nd Zn) have been prepared by ion exchange in Ba containing molten salts from precursor ferrites with the β- or β"-alumina structure. The particles possess a plateletlike shape with diameters as low as 0.2 μm and as diameter-to-thickness ratio between 5 and 10. The Curie temperature is close to 470°C. Saturation magnetization values σ_s up to 64 emu g and coercive fields jH_c ranging from 0.5 to 1.5 kOe were found     

Properties of the W-type hexaferritePbZn2Fe16O27

W-type Pb-hexaferrites were prepared by standard ceramic methods. The lattice constants found by refinement were a=0.59140±0.00006 nm and c=3.29209±0.00041 nm. The X-ray density of a typical composition PbZn_1.9Fe_15.3O_25.8 was ρ=5.32 g/cm³ and the Vickers microhardness value h _v=6 kN/mm². A plot of the saturation magnetization versus temperature is given. The extrapolated value of the saturation magnetization (H→∞, T→0) was σ_s=108 emu×g^-1, and the Curie temperature was T_c=600±20 K     

Lateral magnetic inhomogeneities in Co-Cr layers detected by FMR

Co-Cr layers deposited on Ge underlayers were investigated with ferromagnetic resonance (FMR). The thickness of the Co-Cr layers and the substrate temperature (T_s) during deposition were varied. Measurements with a vibrating sample magnetometer showed that an increase of T_s results in an increase of both the bulk saturation magnetization M_s and the perpendicular coercivity H_c^⊥. The FMR spectra, obtained with the applied field perpendicular to the sample, showed two resonances for all layers. The effective anisotropy field is positive for both resonances. The difference in effective anisotropy field does not change with thickness of the layer, but increases with increasing T_s. From this it is clear that the two resonances are not due to magnetic inhomogeneities in the growth direction, but must be due to lateral magnetic inhomogeneities. Inhomogeneities in composition, resulting in different values of M _s, or in geometric structure, resulting in different values of the demagnetizing factor N_d, are the most probable causes of the observed effects     

NiTi substituted hexaferrites for magnetic recording

In order to obtain hexagonal ferrites with improved characteristics for application in the field of the magnetic recording, the ferrimagnetic Sr(NiTi)_x Fe_12-2xO₁₉ system is studied. Preliminary investigations suggested that NI is more effective than Co in reducing the magnetic anisotropy of hexaferrites. The system was analyzed as a function of the degree of substitution x and of the reaction temperature T_R and time τ. The reactivity of the mixtures and their magnetic characteristics (σ, H_c, H_A, T_c) were measured, and from the _χT(H) data it was possible to distinguish between single-domain or polydomain particles. The results are presented. The influence of the extraneous magnetic phases on the coercive field value was also studied and a preliminary investigation of the relation between coercive and anisotropy fields was carried out     

Analysis of temperature-dependent magnetization in Sm-Fe-Ccompounds using a two-sublattice model

The temperature dependence of magnetization in Sm₃Fe ₂₀C_x (x =0.3, 0.6, 0.8) and Sm₂Fe_16.5C_1.0 intermetallic compounds with rhombohedral structure has been analyzed using molecular field theory. On the basis of a two-sublattice model, the molecular field coefficients are calculated using a numerical fitting method. The Curie temperature and the Fe-Fe, Sm-Fe, Sm-Sm magnetic interaction energies for the compounds are determined from these coefficients. The results show that the increase of the Curie temperature (T_c) with the increase of carbon content x is attributed mainly to the enhancement of the Fe-Fe exchange interaction energy caused by carbon atoms added     

Field modulated microwave absorption in high Tcsuperconductors

Modified electron spin resonance (ESR) techniques were used to measure low-field (<200 Oe) microwave absorption of high-T _c cuprates YBa₂Cu₃O_7-x. A series of experiments on polycrystalline and single-crystal samples in which the angle between the DC and the modulating magnetic fields was varied showed that the microwave absorption obeys results predicted from thermodynamic considerations of fluxoids. The absorption takes place in the normal regions created by flux penetration at low fields in these samples. The change in magnetically modulated microwave absorption on passing through T_c is explained by the decrease in absorption that occurs when part of the sample becomes superconducting and by the rapid variation of the penetration depth near T_c. The technique is an extremely sensitive method of detecting superconductivity in very small samples and for studying the superconducting state. Possible applications such as microwave attenuators, magnetic memories, and gaussmeters are discussed     

Grain size dependence of coercivity and permeability innanocrystalline ferromagnets

Amorphous ribbons of composition Fe_74.5-xCu_xNb₃Si_13.5B₉ (x=0, 1 at.%) have been annealed between about 500°C and 900°C. This produced a series of crystallized samples with grain sizes between about 10 nm and 300 nm and with coercivities H _c and initial permeabilities μ_i varying over several orders of magnitude. The best soft magnetic properties (H _c≈0.01 A/cm and μ_i≈80×10³ ) were observed for the smallest grain sized of about 10 nm. With increasing grain size D, coercivity steeply increases following a D⁶-power law (up to D≈50 nm). H_c then runs through a maximum of H_c≈30 A/cm and decreases again for grain sizes above 150 nm according to the well-known 1/D law for polycrystalline magnets. The initial permeability was found to vary in a similar manner, essentially being inversely proportional to coercivity. The variation of the soft magnetic properties with the average grain size is discussed and compared with the predictions of the random anisotropy model and other theories for the magnetization reversal     

Magnetic particles derived from iron nitride

Due to its high saturation magnetization (σ_s(0K)≈208 emu/g) and to the ease of obtaining it as a particulate material, Fe₄N could be a suitable magnetic material for replacing highly reactive metallic iron pigments (σ _s(0K)≈218 emu/g) in high-density recording. Throughout a nitriding process of properly selected precursors Fe₄N can be obtained in either of two morphologies: acicular or isotropic. Further protection of the magnetic particles can be ensured through coating of the precursor particles prior to thermal decomposition in NH₃/H₂ atmosphere or through substitutions with selected elements. The obtained passivated nitrides are characterized by high magnetic properties (H_c≈600 Oe; σ_s≈150 emu/g) and chemical stability well adapted for use as magnetic pigments for high-density recording     

High coercivity in melt-spun SmFe10(Ti,M)2 ribbons (M=V/Cr/Mn/Mo)

Magnetic properties of SmFe₁₀(Ti,M)₂ melt-spun ribbons were studied, where M is V, Cr, Mn, and Mo. The ribbons (M=V/Cr/Mo) quenched at 20 m/s exhibit the high coercivities of 4.2-5.5 kOe. Annealing the ribbons quenched at 40 m/s enhances their coercivities in the range of 5.9-10.0 kOe. In particular, SmFe₁₀ (TiV) and SmFe₁₀(TiCr) ribbons yield coercivities of 10.0 kOe and 7.9 kOe, respectively. This is the highest value among the reported melt-spun ThMn₁₂-type structure ribbons. The importance of Sm atmosphere during annealing in minimizing the Sm evaporation from ribbons is also demonstrated     

Sintered Sm2(Co, Fe,Cu,Zr)17 magnets withlight rare earth substitutions

The effects of partial substitution of light rare earths for Sm in (Sm_1-xR_x)₂ (Co,Fe,Cu,Zr)₁₇ have been investigated where R=Ce, Pr, Nd, Pr_0.5Nd _0.5, Ce_0.2Pr_0.4Nd_0.4 and x=0-0.5. Experiments show that both remanence and maximum energy product are improved for x=0.1-0.2, depending upon compositions and heat treatments. For a composition Sm_0.8R_0.2(Co_0.633Fe_0.286 Cu_0.061Zr_0.020)_7.59 with R =Ce_0.2Pr_0.4Nd_0.4, the authors obtained B_r=11.57 kG, _MH_c =15.5 kOe, _BH_c=10.33 kOe, H_k=10.03 kOe, and (BH)_max=30 MGOe     

Magnetic characteristics of ferromagnetic stainless steels

The magnetic characteristics of two representatives of martensitic and one of ferritic stainless steels, i.e. SUS 403, TAF, and SUS 405, were investigated in connection with development of high-temperature and nuclear resistive sensors. The test pieces were machined out from rod materials and heat-treated in an argon atmosphere. They were placed in an electric furnace, and their magnetic characteristics were measured at room temperature and elevated temperatures. The examined characteristics were of initial magnetization curve, hystersis loop, AC permeability, Curie point, and inverse magnetostriction. The martensitic stainless steels show somewhat semihard characteristics (H_c: 5 to 15 Oe) and the ferritic one is rather soft (H_c; 2 to 5 Oe). The Curie point ranges from 720 to 750°C. The magnetization is about 15000 G at a field strength of 100 Oe. A large inverse magnetostriction is observed in SUS 403 and SUS 405. Their magnetic characteristics were observed to change with heat treatment     

Magnetic properties of melt spun NdFe10Cr2-Nd0.67B0.33 pseudobinary alloys

The coercivity of the melt-spun pseudobinary (NdFe₁₀Cr ₂)_1-x (Nd₂B)_x alloys, in which Nd₂B represents a composition of Nd_0.67B_0.33, was systematically studied. It was found that significant coercivities are possible with optimal additions of the Nd-B composition at x=0.5-0.7. The as-spun (substrate velocity=10 m/s) coercivity increased from 0.1 kOe at x=0.0 to maxima of 7.0 and 7.5 kOe at x=0.5 and 0.7, due to the Nd₂ Fe₁₄B and a new Fe-Nd phase, respectively, as evidenced from thermomagnetic analysis and X-ray diffraction. Annealing the overquenched amorphous x=0.7 alloy led to the crystallization of the Nd₂Fe₁₄B phase, and a coercivity of 8.5 kOe was obtained     

Computation of 3-D current driven skin effect problems using acurrent vector potential

A finite element formulation of current-driven eddy current problems in terms of a current vector potential and a magnetic scalar potential is developed. Since the traditional T-Ω method enforces zero net current in conductors, an impressed current vector potential T₀ is introduced in both conducting and nonconducting regions, describing an arbitrary current distribution with the prescribed net current in each conductor. The function T ₀ is represented by edge elements, while nodal elements are used to approximate the current vector potential and the magnetic scalar potential. The tangential component of T is set to zero on the conductor-nonconductor interfaces. The method is validated by computing the solution to an axisymmetric problem. Problems involving a coil with several turns wound around an iron core are solved     

Development of a microprocessor-based four probe DC resistivitysetup for Tc measurement of superconductingmaterials

The design of a microprocessor-based resistivity measurement system for high transition temperature (T_c) superconducting materials is described. This design includes interfacing an Intel 8085 microprocessor system with an IBM-compatible personal computer (PC) through RS-232C serial communication. This design is used to measure resistivity for superconducting samples of YBa₂Cu ₃O_7-x and strontium/potassium substituted YBa₂Cu₃O_7-x. A correction method is described for the parameters whose values are changing throughout the measurement period to minimize the measurement errors     

The role of the compensation temperature in thermomagneticswitching

Amorphous rare-earth-transition-metal alloys with different compensation temperatures, Tcom_p, exhibit pronounced differences in the temperature characteristics of the coercivity in the high-temperature range. This is of primary importance for the thermomagnetic switching process in which the domain radius, domain regularity, or domain jitter must be controlled. The influence of the coercivity profile on these parameters has been investigated for alloys with Tcom_p>100 K. The radial dependences of the driving fields acting on the domain wall are calculated using experimental data for the saturation magnetization, uniaxial anisotropy, and coercivity     

Formation and magnetostructural characterization of the novelternary compound Ce3Fe29-xMox

Formation and magnetostructural properties of the novel ternary ferromagnetic compound Ce₃Fe_29-xMo_x is reported. It was obtained in alloys with nominal compositions Ce₃ Fe_29-xMo_x, x=1.4, 1.5, and 1.6, prepared by arc melting and annealing during three weeks at 950°C. The crystal structure was well resolved as monoclinic with space group A2/m (No 12). Quantitative phase analysis, based on the processing of the X-ray diffraction patterns by the Rietveld method, was performed. Ce₃ Fe_29-xMo_x is the main phase (78-72 wt.%) with lattice parameters a=10.553(1) Å, b=8.511(1) Å, c=9.688(1) Å, and β=96.78(1)°. It coexists with Ce(Fe,Mo) ₁₂ as main spurious phase. Atomic positions and mean Fe-Fe interatomic distances are reported. Temperature-dependent magnetic susceptibility shows well-defined transitions at 7 and 115°C, which correspond to the 3:29 and 1:12 phases, respectively. Saturation magnetization at 4.2 K is 123 emu/g     

Magnetic anisotropy of perpendicular media on grooved structuresubstrate

Magnetic anisotropies were investigated for CoCr perpendicular magnetic films on grooved structure substrates (pitch length 2.1 μm) prepared by CF₄-reactive-ion etching. Perpendicular magnetic anisotropy decreases in the region where groove depth GD >650 A. The reduction is due to the inclination of the crystal growth axis on the groove surface. In-plane magnetic anisotropy, K_u∥, strongly depends on GD and saturation magnetization M_s. In a region of large GD and large M_s, Ku_∥ is mainly due to shape anisotropy. In a region of small GD and small M_s , K_u∥ is caused by perpendicular magnetic anisotropy, in addition to shape and magnetorestrictive anisotropy