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     

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     

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     

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     

Magnetic properties of Sm2Fe17Nxpowder and bonded magnet

Sm₂Fe₁₇N_x powders were prepared through metallurgical processes such as melting, casting, pulverizing, and gas nitrogenation. Epoxy resin and zinc-bonded magnets were prepared from the powder by a compression molding process. The magnetic properties of magnetically aligned Sm₂Fe₁₇N_x powder were (BH)_max=170 kJ/m³ (21.4 MG-Oe), Br=1.20 T, and H_cJ=547 kA/m (6.87 kOe). Those of an epoxy-bonded magnet were (BH)_max =103 kJ/m³ (13.0 MG-Oe) and Br=0.835 T. H _cJ values in excess of 2 MA/m (25 kOe) were observed in the case of a zinc-bonded magnet     

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     

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     

Hot-pressed Mn-Zn-Ni and Mn-Zn-Co ferrites for magnetic recordingheads

Ni²⁺ and Co²⁺ substituted Mn-Zn ferrites have been developed as high-density magnetic recording materials by hot pressing using optimum sintering parameters. Ferrite series of the composition Mn_0.6Zn_0.4-xNi_xFe₂ O₄ and Mn_0.6Zn_0.4-yCo_yFe₂O₄ were prepared by the hot-pressing technique wherein x and y varied from 0.0 to 0.4 in steps of 0.05. It yielded ferrites with improved magnetic properties having higher hardness (⩾650 Vickers units), low porosity (<0.1%), and small grain size (≈10 μm). The initial permeability increased, whereas the coercive field decreased for an Ni²⁺ concentration x or a Co ²⁺ concentration y equal to 0.05. Maximum values of saturation magnetization equal to 4850 and 5250 G were obtained for x and y equal to 0.225 and 0.275, respectively. The Curie temperature increased appreciably, whereas the DC resistivity decreased for a larger substitution of Ni²⁺ and Co²⁺ ions     

Determination of the time-dependence of ΩNBS usingthe quantized Hall resistance

The quantum Hall effect is being used to monitor the US legal representation of the ohm, or as-maintained ohm, Ω_NBS. Measurements have been made on a regular basis since August 1983. Individual transfers between the quantized Hall resistance R _H and the five 1-Ω resistors which comprise Ω_NBS can be made with a total of one standard deviation (1σ) uncertainty of ±0.014 p.p.m. This uncertainty is the root-sum-square of 32 individual components. The time-dependent expression for R_H in terms of Ω_NBS is: R_H=25812.8[1+(1.842±0.012)×10^-6 =(0.0529±0.0040)(t-0.7785)×10^-6 /year] Ω_NBS, where t is measured in years from January 1, 1987. The value of Ω_NBS is, therefore, decreasing at the rate of (0.0529±0.0040) p.p.m./year     

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     

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     

Digital correction of A/D conversion error due to multiplexingdelay

A measurement channel which consists of a multiplexer, sample-and-hold circuit, and analog-to-digital (A/D) converter is studied. It is designed for the synchronous sampling and measurement of two or more voltage signals V₁(t), V ₂(t), . . ., but the finite time of A/D conversion (ΔT) makes it impossible to acquire consecutive samples closer in time than ΔT. This can become a source of measurement error if further processing of the measurement data is based on the assumption of ideal synchronism. It has been found that interpolation filters, developed from the Lagrange polynomial interpolation, are useful tools for solving the problem of correction. An illustrative example of their use is presented     

Magnetic properties of BaFe12O19 smallparticles

The magnetic properties of small BaFe₁₂O₁₉ particles about 50 nm in size have been studied; the emphasis was on small-size effects. The compound formation of the hexagonal structure was made at a relatively low temperature; the small particle size made this result possible. As compared to the bulk value, a significantly reduced saturation magnetization is observed for these particles; this agrees with similar reports made earlier by several authors. To examine these phenomena further, Mossbauer spectra were obtained with high magnetic field applied longitudinally (H_ex=16.4 and 50 kOe). The spectra show the presence of recalcitrant spins where the spin configuration has a noncollinear arrangement that even a 50-kOe magnetic field does not remove. This hard-to-saturate component may lie in the surface layer of the Ba-ferrite small particles. The data also provide evidence that the Fe ions in the bipyramidal (2b) sites undergo magnetic-field-induced oscillations at 4.2 K     

A low field determination of the proton gyromagnetic ratio in water

The proton gyromagnetic ratio in H₂O is measured by the low-field method. γ'_p(low)=2.67513376×10⁸ s^-1 T^-1 (0.11 p.p.m.), leads to a value of the fine structure constant of α^-1=137.0359840 (0.037 ppm) and a value for the quantized Hall resistance in SI units of R_H=25812.80460 Ω (0.037 p.p.m.). To achieve this result, the dimensions of a 2.1-m solenoid were measured with an accuracy of 0.04 μm, and the NMR (nuclear magnetic resonance) frequency of a water sample was measured in the field of the solenoid     

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     

Grain size dependence of low field microwave absorption in Y1Ba2Cu3O7 powders

The authors report investigations on the influence of low (≲65 Oe) magnetic fields on microwave absorption in several Y₁ Ba₂Cu₃O₇ powders at 2, 10, 22, and 35 GHz at temperatures varying between 1.3 K and 77 K. Even powders which show negligible zero-field absorption have a significant induced absorption at low fields. At low T there is a large frequency-dependent magnetic hysteresis in the field-induced absorption     

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     

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     

Threshold dosages for damage to mammalian liver by high intensity focused ultrasound

The threshold dosages for high-intensity focal lesion production were determined at 3 MHz in the cat liver for exposure durations covering the range 0.003-35 s. The liver threshold was found to parallel that for the brain over the range of exposure durations 0.1 to 10 s, but to be more than twice the intensity level, approximately following the relation IT^0.5=460 W-cm²/s², where I is the peak intensity and T is the exposure duration. At shorter exposure durations the threshold curve deviates from this relation, probably due to a change to a transient cavitation mechanism of damage