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     

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     

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     

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     

Hard magnetic phase in rapidly quenched Zr-Co-B alloys

In order to identify a hard magnetic phase in rapidly quenched Zr-Co-B alloys and clarify its magnetic properties, Zr-Co-B ribbons, Zr-Co ribbons, and Zr-Co ingots were studied. The hard magnetic phase is interpreted as a Zr₂Co₁₁ intermetallic compound. This compound has a Curie temperature of 500°C and uniaxial magnetic anisotropy with an anisotropy field of 34 kOe. The magnetization of this compound was estimated to be 67 emu/g at 15 kOe. In addition to the hard magnetic phase, the low and high T_c phases appear in both binary and ternary alloys. The low T_c phase is FCC Zr₆Co₂₃ with T_c=180°C. The magnetization of Zr₆Co₂₃ was estimated to be 44 emu/g at 15 kOe. The high T_c phase is cobalt including a small amount of zirconium. In Zr-Co-B alloys, suitable boron addition is shown to enhance the coercive force. On the other hand, the addition increases the magnetization. While the boron addition produces cobalt, it reduces Zr₆Co₂₃ or quenches its ferromagnetism     

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     

The Effect of Hydrogen on Fatigue Properties of Metals used for Fuel Cell System

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and two annealed medium-carbon steels (0.47 and 0.45%C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behaviour of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the case of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little degradation due to hydrogen. A marked increase in the amount of martensitic transformation occurred in the hydrogen-charged SUS304 specimens compared to specimens without hydrogen charge. In case of SUS316L, little martensitic transformation occurred in either specimens with and without hydrogen charge. The results of S-N testing showed that in the case of the 0.7C–13Cr stainless steel and the Cr–Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steels hydrogen did not cause a reduction in fatigue behaviour. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. Thus, it is presumed that a synergetic effect of hydrogen and martensitic structure enhances degradation of fatigue crack resistance.     

Stripe and bubble domains in CoCr films

Using the colloid-scanning-electron-microscope (SEM) method, an investigation was conducted of the domain structure of CoCr films (400-1740 nm thick) at different points of the hysteresis loop. In some cases, an AC field exceeding the value of the coercive force was applied in addition to the DC field. For low-coercivity samples (H_c/H_k~0.02) a bubble structure, which is energetically more favorable above a certain critical field, was observed. From the dependence of the bubble density on the DC field the value of the anhysteretic collapse field was estimated and compared with the theory. Attempts were made to observe the field dependence of the domain structure of high-coercivity samples (H_c/ H_k~0.08) with different thicknesses. For one sample the results were compared with those determined using the Kerr effect. For thicker films the colloid-SEM method permits the simultaneous observation of the magnetic domains and the columnar structure at the surface of the sample     

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-energy Co-Cr thin films sputtered on glass disks forperpendicular recording

The authors have developed high-energy Co-Cr thin-film perpendicular recording media for rigid disks. They obtained high perpendicular coercivity (Hc⊥) exceeding 2000 Oe with Co-Cr films sputtered on glass disks. They examined recording characteristics obtained with double-layered media and single-pole heads. Readout voltages were proportional to Hc⊥ up to 2000 Oe and not dependent on saturation magnetization. The authors explain the experimental results using the hysteresis curve of the Co-Cr film and the permeance factor determined by the magnetic reluctance of the head and medium. Using a magnetic circuit model, they clarify the effect of the difference in the operating point on the hysteresis curves of rigid- and flexible-disk systems     

Effect of stress on the law of approach to saturation magnetizationin carbon steels

In high fields where the magnetization is approaching saturation, the resolved magnetic polarization I can be expressed by a power series of the inverse of external field H as: I=I_s-b/H ²-c/H³ . . ., or alternatively in terms of the reversible permeability μ_rev=1+(2b/H³+3c/H⁴+ . . .)/μ₀ where I_s is the spontaneous magnetic polarization and b, c, are constants. These equations express the law of approach to saturation magnetization. The coefficient b for a cubic crystal has been deduced as b=0.0762/I_s[K+1.5(λ₁₀₀-λ ₁₁₁)σ]², where σ is the applied stress and others are magnetic constants. The values of reversible permeability μ_rev under biasing field H were measured for carbon steels with applied stresses. The results showed that the square root of 2b changed linearly with the applied stress, The values of magnetic constant K andλ₁₀₀ -λ₁₁₁ were calculated, They agreed with the standard values, but were affected by chemical composition and heat treatment of materials, especially in λ₁₀₀-λ₁₁₁     

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     

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     

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     

Annealing behavior of magnetic anisotropy in CoNbZr films

The effect of magnetic and nonmagnetic annealing on the magnetic anisotropy in CoNbZr films, formed by a DC opposing-targets sputtering method, was investigated. It was revealed that the origin of the magnetic anisotropy is the directional ordering of the magnetic atoms. The anisotropy fields and the direction of the easy axis obtained when the films are annealed in zero magnetic field are almost the same as those for the magnetic field parallel to the easy axis of the as-deposited films. When the films are annealed in a magnetic field perpendicular to the easy axis, the anisotropy field induced in parallel with the magnetic field, H_k(t), is well represented by the following formula: ln {1-H_k(t )/H_k(∞)∝-√Tt, where H_k(∞) is the thermal equilibrium value of the anisotropy field and D is the diffusion constant. The activation energy of the as-deposited film is 0.86 eV. Annealing the film increases the activation energy which is 2.1 eV when the film is annealed at a temperature of 450°C for 2 h     

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     

Demagnetization processes and magnetic viscosity of amorphous FeTbfilms

The magnetic viscosity coefficient, S_V, of RF sputtered FeTb films was measured at fields between -5000 and +200 Oe at room temperature. The films, which are amorphous, have a perpendicular anisotropy and a typical stripe domain structure. S_V depends strongly on the demagnetization processes of the films, S _V of the domain-lengthening magnetization process being larger than that of the domain-widening process     

Ta additive effect on RF magnetron sputtered CoCr films

Magnetic properties for sputtered CoCrTa films (18 at.% Cr and 2.0-3.0 at.% Ta), which were deposited under various background pressures P_i, and argon sputtering pressures, P _Ar, have been examined. The perpendicular anisotropy field H_k for CoCrTa films maintains high values of 5-6 kOe in a wide range of P_i and P_Ar , as compared with that for CoCr films. In order to optimize Ta composition, magnetic properties and crystalline microstructures for Ta additive content (0-4.0 at.%) have been investigated. H_k and perpendicular coercivity H_c⊥ increase with increasing Ta concentration above 2.0 at.% Ta. C-axis orientation is improved by adding Ta to CoCr films. However, above 3.0 at.% Ta, H_c⊥ steeply decreases and domain wall motion is observed, owing to the increase in crystalline grain size. The appropriate Ta composition is 2.0-3.0 at.%     

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