Volume 35, Issue 3, 01 March 1964
Index of content:
- HARD MAGNETIC MATERIALS; SUPER CONDUCTORS
35(1964); http://dx.doi.org/10.1063/1.1713557View Description Hide Description
Flux jumping has been investigated in both cold‐worked and heat‐treated (1 h at 700°C) 0.025‐cm‐diam Nb+25% Zr wire wound into solenoids and noninductive coils of approximate dimensions: i.d.=0.3 cm, o.d.=1.2 cm, length=2.6 cm. The measurements taken at 4.2°K were: (1) The induced voltage pulses vs applied magnetic field with both I=0 and a preset transport current. (2) The induced voltage pulses vs coil current for H=0 and with a preset uniform external field.
For the case with no transport current in the coil (I=0) when the external field was increased smoothly from zero, a squence of very sharp voltage pulses (flux jumps) were induced in the coil, and recorded simultaneously on both an X‐Y recorder and an oscilloscope. These flux jumps had the following general characteristics for all the cases discussed in this paper: (1) Starting from well defined initial conditions, the pulse always appeared at about the same place in the field sweeping cycle. (2) The polarity of the pulses is reversed when the direction of field change is reversed. (3) The pattern of these pulses was essentially unchanged when the field sweep rate was increased from 7 to 340 G/sec.
As a result of these measurements, we have obtained new insight into both the multilevel transitions of noninductive coils and the phenomenon known as degradation of solenoids. Two successful methods are described for partially overcoming the excessive degradation of a solenoid made from heat treated wire. The flux jumps are interpreted according to an extension of a model proposed by both Bean and London.
35(1964); http://dx.doi.org/10.1063/1.1713558View Description Hide Description
The properties of permanent magnets together with their theoretical background are reviewed. The study of micromagnetics has shown that because of processes such as fanning, buckling, and curling, the high energies once predicted for elongated single domain particle magnets are unlikely to be obtained by shape anisotropy alone. There may be a better chance of obtaining high energies from materials with crystal anisotropy or if displaced loops due to exchange anisotropy could be produced in a practical material at room temperature. The properties of bariumferrite are due to crystal anisotropy, but its magnetization is low. Other materials with high crystal anisotropy at present require expensive materials or manufacturing processes. The properties of Alnico V and related alloys can be explained in terms of shape anisotropy due to two‐phase structure. Progress has been made in a group of high coercivityalloys containing more than 4% Ti and 29% Co. A single crystal of one of these alloys has given the highest (BH)max so far obtained, but it is only recently that it has been discovered that small additions of S or Se enable these alloys to be made columnar by simple methods.
Magnetic Exchange Anisotropy in Mixtures of Barium, Strontium, or Lead Ferrite with Potassium Ferrite35(1964); http://dx.doi.org/10.1063/1.1713559View Description Hide Description
This paper describes some new systems F 1−x Ax , showing exchange anisotropy at room temperature and above. F is ferrimagnetic lead (P), barium (M), or strontium (S) ferrite, all with the magnetoplumbite structure, PbFe12O19, Tc =455°C. A is antiferromagneticpotassiumferrite (KFe11O17, TN =530°C) which crystallizes in the closely related β‐alumina structure. Mixtures F 1−x Ax were prepared with 0≤x≤1. Specimens were cooled from above 600°C to room temperature in a field of 4 kOe and measurements made of the magnetic saturation σ s , intrinsic coercive force Hci , torqueL, and rotational hysteresisWr , from −197° to 480°C. The observation of nonvanishing rotational hysteresis, sinθ torque curves, displaced hysteresis loops and changes in σ s are interpreted in terms of exchange anisotropy in the M A and P A systems; compositions in the S A system behaved as simple mixtures of magnetic and nonmagnetic phases. X‐ray diffraction indicated the formation of a new phase A′, possibly related to the development of exchange anisotropy. The identity of A′ has not been established; it may be a potassium‐rich superstructure of A with lattice parameters and C 0′≈C 0.
35(1964); http://dx.doi.org/10.1063/1.1713560View Description Hide Description
The hexagonal ferrites, of current interest because of their high‐frequency properties, are being studied by single‐crystal x‐ray methods. A new rhombohedral member of the group, ideally Ba4Zn2Fe52O84, has been identified. The centrosymmetric space group is R3̄m; referred to hexagonal axes, c=153.85 Å, a=5.88 Å, and there are 66 anion layers in the c‐axis repeat period. The stacking sequence was established by (00.l) structure factor and one‐dimensional electron density calculations. The new structure forms a series of mixed‐layer compounds with three of the original phases. All of the hexagonal ferrites are thus characterized by two series, each of which features the c‐axis interlayering of two discrete structural units. Stacking relations within and between the two series are discussed, along with implications for new structures and series.
35(1964); http://dx.doi.org/10.1063/1.1713561View Description Hide Description
The starting torque for hysteresis motors is discussed in terms of the rotational hysteresistorque of the permanent magnet rotors as measured with a dc torque magnetometer. The rotational hysteresistorque was found to agree within experimental error with the starting torque of a rotor suspended in a magnetic field rotating at 60 cps. The rotational hysteresis in tungsten steel was measured for as‐quenched and tempered samples. The rotational hysteresis had a peak value of 2.1×105 ergs/cm3 and decreased with tempering. The applied field at which the peak rotational hysteresis was found was not changed by tempering. The rotational hysteresis integral, a measure of the magnetization reversal mechanism, varied from 2.2 to 3.2, increasing as the coercive force decreased. The maximum anisotropy field in the material was found to be 6700 Oe.
35(1964); http://dx.doi.org/10.1063/1.1713562View Description Hide Description
Electrodepositedfilms of Co–Ni–P containing 14% nickel and 1% phosphorus by weight and having thickness 0.05 to 0.2 μ showed the following magnetic properties—high coercivity (300–550 Oe), high ratio of remanent‐to‐saturation moment (∼0.90), steep‐sided hysteresis loops and isotropy in the film plane.
The process of magnetization reversal was studied by using a sensitive Hall probe to measure and map the horizontal component of the flux above the surface of the film after increasing reverse fields has been applied. This technique was used since neither the Bitter pattern method nor the Kerr effect revealed the natural pattern of magnetization reversal. However, it was possible to introduce, deliberately, a small region of reversed magnetization by means of the field from a magnetic recording head and examine this region by all three methods. This provided a method of checking the interpretations of the Hall effect data.
It was found that these films employ a unique mode of magnetization reversal. Initially, small regions of reversed magnetization are established throughout the film, and these regions grow together to form elongated switched areas whose long dimensions are always perpendicular to the direction of applied field. The reversal pattern is highly reproducible in a given sample for a given field direction and reverses when the field direction reverses.
This switching behavior is dependent on the direction of the field with respect to the sample, the long direction of the reversed regions always being perpendicular to the applied field. The sign of the vertical component of the flux from the sample was compared above and below the sample to determine whether magnetization perpendicular to the film plane existed. No such regions were found with a Hall probe of sensitive area down to 25×25 μ.
35(1964); http://dx.doi.org/10.1063/1.1713563View Description Hide Description
The susceptibility,magnetic moment, and resistivity of an alloy of CeRu2+7.3% GdRu2 prepared by arc melting have been measured in the range 1.3° to 20°K. This alloy has been previously reported to be both ferromagnetic and superconducting. The sample is weakly ferromagnetic, and undergoes a superconducting transition which reduces the positive susceptibility. The resistance becomes smaller at the transition temperature, but does not become zero. The transition temperature in zero field is in good agreement with that previously reported.
35(1964); http://dx.doi.org/10.1063/1.1713564View Description Hide Description
Magnetic susceptibilities of binary Mo‐Tc alloys have been measured in the temperature range from 300°K to the superconducting transitiontemperature. The room‐temperature magnetic susceptibility increases from pure Mo (82.5×10−6 emu/g‐atom) to a small maximum (96.5×10−6 emu/g‐atom) at the equiatomic composition. Further additions of Tc produce a decrease which reaches a minimum at 70 at. % Tc (χ=87.3×10−6 emu/g‐atom). Beyond this composition there is another increase to the value of 120.8×10−6 emu/g‐atom for the pure Tc. No Curie or Curie—Weiss type of temperature dependence of the susceptibility was observed in all the alloys in the temperature range investigated. The results are discussed in relation to the band theory of metals and some generalizations about the d‐band form of the three transition metal series having the same electron concentrations are made.
35(1964); http://dx.doi.org/10.1063/1.1713565View Description Hide Description
A systematic investigation has been made of the sign and magnitude of the g shifts of Gd impurities in the AB 2intermetallic Laves phase compounds. Gd was partially substituted in the A site for the elements Sc, Y, La, Th, U, and Zr. The B sites are occupied by elements of the 4d and 5d group whose positions in the periodic table are such as to correspond to an almost filled d band (e.g., Ru, Os, and Rh). Negative Gd g shifts, Δg, up to 7% were observed; the highest was that found in LaRu2. It was found that the closer a B element was to having a filled d shell the larger was the g value. It appears, in keeping with our previous EPR measurements on Gd in Rh‐Pd, Pd‐Ag alloys and in Cu5Ca intermetallic compounds, that without exception the g shifts are negative when the d band is nearly filled in the host alloy and that the g value has a minimum as a function of the atomic position in the host d band metal. In very fine powders of the hard superconductors (Ce0.99Gd0.01) Ru2 and (Th0.99Gd0.01) Ru2 no change in the Gd g shift was observed in going from the normal to the superconductor state. For the superconductor (La0.15Ce0.85) Ru2 the La Knight shift was also observed not to change in going from well above to well below the superconducting transition temperature. In both experiments Ho≪the upper critical field.
35(1964); http://dx.doi.org/10.1063/1.1713566View Description Hide Description
Critical alternating currents are studied experimentally in 0.96‐cm (⅜‐in.) lengths and coils of superconducting wire.Critical current density vs frequency are measured for three short Nb‐25% Zr wires having the same amount of cold work. For the 5‐, 10‐, and 20‐mil (125‐, 250‐, and 500‐μ) specimens, the critical ac density varies from 290 to 170 kA/cm2, 200 to 110 kA/cm2, and 107 to 54 kA/cm2, respectively, as the frequency is varied from 0.02 to 4.5 kc/sec. A short length of Nb–Ti wire 9 mils in diameter has a critical ac density that varies from 260 to 130 kA/cm2 as the frequency ranges from 0.01 to 4.5 kc/sec. In a 15‐mil Nb3Sn specimen, the critical ac density is constant at 90 kA/cm2 from 0.01 to 4 kc/sec.
In a tightly wound Nb‐25% Zr coil, the critical ac density is found to vary inversely with the one‐fourth power of frequency. In a very loosely wound coil of the same material, the critical ac density varies inversely with the cube root of frequency. In all cases investigated, except for pure niobium, the exponent on the frequency varies from −0.25 to −0.36. In a Nb coil, no variation of critical ac density is observed below 200 cps. Above 200 cps, it behaves in the same way as the other coils tested. The data indicate that the critical ac density occurs such that the coil or short wire always becomes normal at the same level of power dissipation.
35(1964); http://dx.doi.org/10.1063/1.1713567View Description Hide Description
The magnetization of hollow cylinders of niobium‐25 at.% zirconium has been measured at 4.2°K. The experimental results can be described by a model in which the magnetization is derived from an induced supercurrent density given by Jc =α/B 0+H, where H is the transverse magnetic field. This relationship has been verified for Nb‐25%Zr with a microstructure varied by mechanical deformation. The values of α and B 0 for several samples cold worked from 25% to 65% have been determined. Extrapolation of the functional dependence of these parameters to negligible cold work (annealed samples) and maximum cold work (heavily strained wire) are shown to be in agreement with previously reported results on cylinders and wires. The current density in the absence of the Lorentz force, (H=0) is found to be somewhat independent of the cold working, implying that the constant J(o)=α/B 0 may be related to bulk electronic parameters.