J. E. Noakes

Initial Susceptibility of Iron and Iron Alloys Just Above Their Curie Temperatures

The volume susceptibility of Fe and dilute alloys of Fe with various solutes have been measured from within 0.05°K of the Curie temperature Tc to 15°K above Tc. The results have been analyzed using the equation χv−1 = [(T−Tc)/T1]γ. The results yield a value of γ = 1.333±0.015 for Fe and its dilute alloys. For Fe T1 = 1.27°±0.01°K, T1 appears to vary rapidly with solute concentration for some solutes. The measurements do not show any temperature dependence of γ within 15°K of Tc.

Initial Susceptibility of Ferromagnetic Iron and Iron‐Vanadium Alloys just above Their Curie Temperatures

Induction measurements of magnetization of spherical samples of Fe and Fe(V) alloys in low applied fields are used to deduce the temperature dependence of initial susceptibility near the critical temperature. The initial susceptibility follows an expression of the form χ=A(T−Tc)−γ, where Tc is the Curie temperature. The value of γ for Fe is 1.37±0.04. In dilute Fe(V) alloys γ decreases with increasing concentration of solute.

Surface of Magnetization, Field, and Temperature for Nickel near Its Curie Temperature

A nickel single‐crystal sphere is placed in a solenoid with a 100 axis along the solenoid axis. The solenoid is placed in an electromagnet with the field at right angles to the solenoid axis and along another 100 axis of the nickel crystal. A detector coil is placed with its axis parallel to the solenoid in order to measure the change in the component of the magnetization of the sphere along the solenoid when the 20‐G field in the solenoid is reversed. This component is measured as a function of the transverse field of the electromagnet to obtain the effective susceptibility in applied fields from 20 to 5000 G. A sufficient time is allowed between each change of the electromagnet field for the large magnetocaloric effects to decay and the sample to return to the furnace temperature. The temperature dependence of the spontaneous magnetization in zero internal field and of the initial rate of increase with field of magnetization above the spontaneous magnetization below the Curie temperature, the temperatur...

Saturation Magnetization and Curie Points in Dilute Alloys of Iron

A report of the initial stages of an investigation of the intrinsic properties of dilute alloys of iron is given. Methods of determining spontaneous magnetization and Curie temperatures with precision of one part in 30 000 are discussed. The results for Fe‐Ti alloys are singled out for attention. The decrease of spontaneous magnetization at a rate of more than 1.6 times that of simple dilution implies either a negative moment on the Ti atoms or more probably that the occupation of states contributing to the moment on the iron is decreased by the addition of Ti. The increase in the Curie temperature at a rate of 3.7°C per atomic percent Ti is discussed on the basis of the Bethe‐Slater curve of exchange interaction as a function of atomic separation.

Vibrating Sample Magnetometers

In order to eliminate the sensitivity to field noise and microphonics of a vibrating sample magnetometer it is sufficient to add trimming coils which are adjusted to produce no signal when the applied field is deliberately driven at the frequency ω0 which is to be used for vibrating the sample. A mechanical chopper which discriminates against all even harmonics and against third harmonics is discussed. The use of eddy currents to precisely align a sample in a null measurement of magnetization is described.

Magnetization of Nickel near its Critical Temperature

Magnetic properties of a single‐crystal sphere of nickel as a function of temperature and applied field have been measured in the critical region. The dependent variable, directly measured, is proportional to MV/B where B is the applied field and MV is the magnetic moment of the sphere in the direction of B. Below TC and for B <4πMs(T) /3, where Ms(T) is the spontaneous magnetization, M/B=3/4π. This serves as an absolute calibration of the measurement. Three types of data have been taken: (1) M/B at constant T for 20 G<B<1000 G in 2‐G steps for steps of ΔT/TC=3×10−4 from TC−0.01 TC to TC+0.01 TC; (2) M/B at constant B in steps of ΔT/TC=3×10−6 from TC−0.01 TC to TC+0.01 TC for B=20, 240, 500, and 5000 G; (3) B at constant M/B in various steps of ΔT/TC for various values of M/B near 3/4π and near 0.2. From a partial analysis of the data we find an approximate equation of state for Ni. It is: (H/M)0.764=(T−TC)/0.309+ |M/39.5| 2.59. This equation of state fits well to the data of Weiss and Forrer also. But it...

Antiferromagnetic‐Ferromagnetic Transition in the Alloy System Au4(Mn1−xCrx)

The nature of the transition from ferromagnetic Au4Mn to antiferromagnetic Au4Cr has been investigated in detail in the ternary system Au4(Mn1−xCrx) by means of electrical resistivity, magnetization and magnetic susceptibility measurements. Previously published data over the composition range 0<x<0.8 have been corrected and new data for 0.8<x<1 are reported. Both the ferromagnetic moments and the Curie temperatures decrease with increasing x to a value near zero at x≈0.8 where the transition to antiferromagnetism takes place. The results indicate that the magnetic moments of both Mn and Cr are well localized and that the second‐nearest‐neighbor magnetic interaction plays the important role in determining whether the structure is ferromagnetic or antiferromagnetic.

Parametric Equation of State Applied to the Magnetization of Nickel Near its Curie Temperature

Data for the surface of magnetization, temperature, and field for single crystal nickel are compared with the parametric equation of state of Schofield, Litster, and Ho [Phys. Rev. Lett. 23, 1098 (1969)]. An excellent fit to the magnetic isotherms is obtained by choosing a reduced temperature for each isotherm, and the variation of the reduced temperature with measured temperature is linear above and below the critical temperature. But whereas the five parameter equation would demand that the slope be the same above and below Tc, the experiments show a 15% change in slope. It is argued that a six parameter equation is needed for the experimental fit and a suitable equation is given.

Precision Measurement of Magnetic Flux Changes

Systems which employ two amplifiers—one to amplify, the other to integrate—for precise and/or sensitive determination of magnetic flux changes are described and their operating characteristics reported. One system measures flux changes in time intervals of 10−2 to 10−1 sec using moderately broad band amplifiers and gated circuits. This system discriminates against dc thermals with a gated self‐balancing potentiometer, and combines sensitivity and precision with the convenience of rapid readout on a digital voltmeter. The sensitivity of the system is 10−7 V sec, the accuracy better than one part in 104, and combined with pickup coils, permits detection of changes of 10−3 G. This system has been applied to measurements of saturation magnetization, Curie temperature, and susceptibility of iron and its dilute alloys. An alternative system uses a highly stable, dc chopper amplifier in which case the flux changes are produced in seconds and can be followed for minutes. This method has been essential for measure...

Measurements of Pressure Dependence of Magnetic Moment in Iron‐Aluminum Alloys

A method of measuring the change in magnetic moment with pressure for spherical samples at a relatively low pressure level is described. Special attention is paid to the problem of avoiding the effects of sample motion with respect to the detector coil to insure the accuracy in such a highly sensitive measurement. The results of measurements on iron‐aluminum alloys up to 30 at.% Al are presented here. There is a sharp increase in the pressure dependence of the magnetic moment with the aluminum concentration. This is interpreted as favoring a competing antiferromagnetic interaction with increasing pressure.