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     

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     

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     

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     

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     

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     

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     

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     

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     

Temperature dependence of coercivity at recording frequencies:barium ferrite versus oxide

It is demonstrated that materials with comparable thermal dependence of coercivity under normal laboratory measurement conditions, differing only in sign, have quite different properties at recording frequencies. Barium ferrites, with positive dH_c/dT, have augmented thermal dependence, whereas oxides with negative dH_c/dT exhibit weaker thermal dependence at recording frequencies. Experimental data and theoretical analysis confirm this behavior. Two methods of predicting coercivity as a function of temperature, for recording time scales, are presented. The implications for direct overwrite performance while thermally cycling are discussed     

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.%     

High performances boron doped γ-Fe2O3particles prepared from oxalic precursors

γ-Fe₂O₃ particles (L≈0.15 to 0.2 μm, L/D≈2 to 5) with coercivity up to 400 Oe have been prepared from oxalic precursors. To preserve the particle form, the oxalates were doped with boron. The effects due to this element was studied during the different thermal treatments which effect the pseudomorphous transformation of oxalate particles into spinel ferrite. ESCA spectroscopy, and X-ray measurements could point out the presence of a Fe₃BO₅ phase at the surface of γ-Fe₂O₃ particles. In spite of the important mass losses (H₂O, CO, CO₂) and the important crystallographic structure modifications occurring during the transformation of oxalate into magnetic oxide, γ-Fe₂O ₃ particles obtained by this method have very good textural characteristics. Boron-doped γ-Fe₂O₃ particles were used to make 3.5-in diskettes with standard formulation. In these conditions, media properties were comparable to trading diskettes properties     

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     

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     

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     

Magnetic properties of GdBa2Cu3O7crystals in high magnetic field

The magnetic properties of single crystals of GdBa₂Cu ₃O₇ were investigated in a magnetic field of up to 20 T applied parallel to the c axis in the 1.7-300 K temperature range. In the superconducting state, the field and temperature dependences of the critical current densities were deduced from the hysteresis of the half-cycle using Bean's critical state model. The Gd³⁺ paramagnetic moment was then studied. Above about 20 K, the M(H) isotherms were found to be given, at different temperatures, by the Brillouin function of the free Gd³⁺ ion. Below 20 K, the average magnetization does not obey the Brillouin law. The normal-state susceptibility was described by the free-ion Curie-Weiss law     

Automatic technique for measuring the Polder resonance in ferritespheres

An automatic technique for measuring the parameters of Polder resonance in polycrystalline ferrite spheres is proposed. The diagonal elements of the external susceptibility tensor versus DC magnetic field are calculated from the changes in resonance frequency and unloaded Q factor of a microwave cavity by perturbation theory. From these diagonal elements, all the elements of the intrinsic permeability tensor can be obtained. By fitting the theoretical curves to experimental data, the parameters of Polder resonance, ΔH M_s, and the g factor, are simultaneously calculated. The whole measurement procedure is controlled by a desktop computer. An accuracy of 5% is obtained in ΔH and M _s, and of 0.1% in the g factor     

Measurement of frequency response functions in noisy environments

The geometric mean is proposed as an alternative averaging technique for frequency response function (FRF) measurements of a linear system. It is shown that it produces almost unbiased measurements even if the input and output measurements are both disturbed with (normal distributed) noise. The properties of the arithmetic and geometric mean of expression are compared to those of the H₁ and H₂ methods. It is shown that if the S/N ratio on the input and the output measurements is higher than 3 dB, then it is possible to generate measurements without systematic errors     

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