A. Arrott

Remarks on magnetically dilute systems

Abstract Because of the inadequacies of previous treatments of the magnetic properties of magnetically dilute systems when used to interpret experimental results, a re-examination of the problem starting from a simple model is made. The treatments of the problem discussed here, using an Ising model, show that a Curie or a Neel temperature does not appear until a finite concentration of magnetic atoms is obtained if the atomic distribution is random. This concentration depends on the co-ordination number of the lattice and on the range of interaction, but not on the strength of the interaction. The results given here for nearest-neighbor interactions describe the general behavior observed in magnetically dilute solutions. Such things as anomalously high values of “effective magnetic moments” per magnetic atom and its concentration-dependence, curvature in inverse susceptibility against temperature plots, and parasitic paramagnetism in the weakly ferromagnetic alloys, &c., are reasonably well explained. When the system has antiferromagnetic interactions, it is found that the inverse susceptibility shows a complicated temperature-dependence varying with concentration and that the existence of a maximum in the susceptibility does not necessarily mean the onset of antiferromagnetism. Special references are made to iron in gold and chromium and to manganese in copper.

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.

Antiferromagnetism in fcc and hcp Iron—Manganese Alloys: Mössbauer Effect

That metallic antiferromagnetism can be verified by employing the Mossbauer effect has been demonstrated for fcc FeMn alloys near the compositions of equal atomic fractions and for a hcp Fe‐Mn alloy with 17% Mn. The hyperfine fields deduced from measurements of line broadening of incompletely resolved spectra give 33, 39, 43, 42, and 22 kG for 60, 50, 45, and 43% Mn in the fcc phase and 17% Mn in the hcp phase, respectively. The uncertainty is estimated at ±2.5 kG. Neel temperatures of 500°K are found for the four fcc alloys. The hcp alloy has a Neel temperature of 240°K. These values agree with resistivity and susceptibility data. In the light of this treatment of incompletely resolved spectra, a suggested reinterpretation of previous data on antiferromagnetic Cr is offered.

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.

Antiferromagnetic Transition in γ‐Phase Mn Alloys

Specific heat measurements on Mn‐rich Mn‐Cu alloys reveal first‐order antiferromagnetic phase transitions at around room temperature and above. Magnetic entropy increases with Mn content but in all alloys it is smaller than would be expected from localized spins. The same is true of an equiatomic FeMn alloy. However the specific heat exhibits a well‐defined cusp at 475°K, indicating a transition of second or higher order. By extrapolating the Mn‐Cu results to pure Mn, the thermodynamic properties of γ‐Mn in the region of the transition are derived.

Antiferromagnetism of α-Mn and Its Alloys

Low-temperature magnetic isotherms of α-Mn and its alloys with Fe, N, and H have been examined. Hysteresis loops and effects of cooling in an applied field are reported. Similar properties are found for single crystals of Cu-Mn. It is pointed out that the spin density wave mechanism of antiferromagnetism, proposed by Overhauser, provides explanations for these phenomena. Neutron diffraction data are reinterpreted and provide further evidence for spin density waves in α-Mn and its alloys.

Studies of Au4X‐Ordered Alloys: Electron and Neutron Diffraction, Resistivity and Specific Heat

Micro‐, chemical and magnetic structures of ordered alloys Au4X, where X stands for various transition elements and their mixtures, have been studied using electron, x‐ray, and neutron diffraction. Specific heat and electrical resistivity measurements have been carried out for Au4Ti, Au4(Ti0.5Cr0.5)Au4V, Au4(V0.5Mn0.5)Au4Cr, Au4(Cr0.5Fe0.5), and Au4Mn. The magnetic form factors are given for Au4Cr and Au4Mn.

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...

The determination of errors in polarized neutron diffractometry

Abstract The polarized neutron method of determining the magnetic form factor of magnetic materials is examined in detail with special attention given to the way in which statistical errors are propagated. Because of the nonlinear relation between the polarizing efficiency of a crystal and the magnetic scattering length, the usual methods of linear error theory will not work. However, this difficulty can be circumvented by using the nonlinear formulas directly. The statistical error analysis is applied to the practical problem of how long one should count on a particular Bragg peak. This time will depend on the ratio of the magnetic to the nuclear scattering lengths, p b , and on the accuracy with which the beam polarizations and flipping efficiencies have been determined.

Rapid inverting of the polarization of a neutron beam using large amplitude oscillating magnetic fields

Abstract During the course of designing and constructing a pulsed-neutron-polarization-inverter (PNPI) to be used in the study of magnetic excitations in solids, it was necessary to re-examine the oscillating field method of magnetic resonance in the region where the amplitude of the rf field B1 is comparable to the dc field B0. It is found that the phase of the rf field at the instant the neutron enters the spatial region of the field becomes an increasingly important considerations as B 1 B 0 becomes comparable to 1. Analog and digital computer solutions are given for the equations of motion of the neutrons magnetic moment in the presence of various field configurations important in the construction of a PNPI.