I. S. Jacobs

Magnetic Anisotropy and Rotational Hysteresis in Elongated Fine‐Particle Magnets

Various aspects of the magnetic anisotropy of electrodeposited elongated single‐domain Fe and FeCo alloy particles are examined with a view to better understanding their process of magnetization. The initial increase of coercivity with increase of the angle between alignment direction and measuring field is in qualitative accord with the prediction of the chain‐of‐spheres model with fanning, as previously proposed, and in contrast to coherent rotation models. Analyses of high‐field torque curves and of the fields at which rotational hysteresis vanishes suggest that an anisotropy is present which is a little greater than predicted by the chain‐of‐spheres model but less than that predicted by the Stoner‐Wohlfarth ellipsoid model, for the observed dimensional ratios. Calculations of the rotational hysteresis in single domain‐particles are extended to the chain‐of‐spheres model. A study of the rotational hysteresis enables a relatively sensitive choice between several models of the magnetization process. Comp...

Magnetic Granulometry and Super‐Paramagnetism

The magnetic technique of particle size measurement based on the work of Elmore consists of a comparison of a modified Langevin curve with the observed magnetization curve for an appropriate system of ferromagnetic particles. The average magnetic moment per particle is obtained from this analysis, and the average particle size is thus determined. The prediction by Neel of thermal relaxation in single‐domain ferromagnetic particles broadens the range of applicability of the granulometry method, with special emphasis on the size range from 20 A to about 100 A. A possible objection to the method, arising from evidence for an abnormal temperature dependence of the saturation magnetization of very small magnetic particles, is re‐examined experimentally and found to be invalid. Several examples are presented that demonstrate the predicted thermal relaxation and that point to a wide utilization of the granulometry technique.

Metamagnetism of Siderite (FeCO3)

A uniaxial antiferromagnet with high anisotropy should show a sharp transition to a ferromagnetic configuration in fields sufficient to overcome the antiferromagnetic exchange. Examples of this restricted type of metamagnetism are rare, but are here extended to ferrous carbonate in its natural mineral form, siderite. Magnetic susceptibility measurements on powder and oriented crystals at 10 kOe between 4.2° and 300°K confirm in detail the high anisotropy and indicate a Neel point at 38°K for this mineral. Pulsed field magnetization measurements to 200 kOe at 4.2°K exhibit a metamagnetic transition toward ferromagnetism starting near 100 kOe for both the powder and c‐axis oriented crystal. The transition in this crystal is nearly complete at 200 kOe where the magnetization approaches a saturation of about 6μB per iron ion. From an elementary statistical model of the broad transition, the critical field for pure FeCO3 is estimated at about 200 kOe. These data and other factors lead to a conclusion that the predominant exchange interaction in FeCO3 is antiferromagnetic.

Susceptibility calculations for alternating antiferromagnetic chains

Earlier work of Duffy and Barr consisting of exact calculations on alternating antiferromagnetic Heisenberg spin‐1/2 chains is extended to longer chains of up to 12 spins, and subsequent extrapolations of thermodynamic properties, particularly the susceptibility, are extended to the weak alternation region close to the uniform limit. This is the region of interest in connection with the recent experimental discovery of spin‐Peierls systems. The extrapolated susceptibility curves are compared with corresponding curves calculated from the model of Bulaevskii, which has been used extensively in approximate theoretical treatments of a variety of phenomena. Qualitative agreement is observed in the uniform limit and persists for all degrees of alternation, but quantitative differences of about 10% are present over the whole range, including the isolated dimer limit. Potential application of the new susceptibility calculations to experiment is discussed.

Spin pinning at ferrite‐organic interfaces

We have previously reported a drastic moment decrease in NiFe2O4 fine particles (∠100A dia.) coated with an organic surfactant such as oleic acid. Removal of the organic coating restored the moment of the particles. Continued investigation has demonstrated that the apparent moment decrease is due to strong pinning of the spins of those ferrite cations that are bonded to the organic molecules. The evidence for this model includes: (1) UNcoated NiFe2O4 particles, prepared in an otherwise identical fashion and with the same size distribution showed no decrease in moment. This observation eliminates the possibility that defects or abnormal surface morphology are responsible for the decreased moment. (2) Low temperature Mossbauer measurements in zero field of coated and uncoated particles showed identical spectra associated with ordered fine particle NiFe2O4. (3) Mossbauer data taken on coated particles in a field of 68.6 kOe applied along the direction of γ‐ray emission showed only a small decrease of the Pm=...

Field‐Induced Spin Reorientation in YFeO3 and YCrO3

Application of a sufficiently strong magnetic field Hx along the antiferromagnetic easy axis (x) of the isomorphic, distorted perovskites YFeO3 and YCrO3, results in the smooth rotation of the antiferromagnetic vector in the x‐z plane, reaching the z axis at the critical fields Hcr=74±1 kOe and Hcr=40±1 kOe at 4.2°K, for YFeO3 and YCrO3, respectively. A detailed analysis of the magnetization curve mx(Hx) provides an unambiguous determination of the quartic and uniaxial‐type anisotropy in the x‐z plane of YFeO3 (spin= 52). In agreement with theory, quartic anisotropy terms are not found in YCrO3 (spin= 32). Only in the latter case are the uniaxial anisotropy and the reorientation field calculable from the initial susceptibilities. Comparison of mx(Hx) with mz(Hz) for H>Hcr provides a quantitative separation between the antisymmetric exchange (HD) and single‐ion anisotropy (Hxz) contributions to the weak ferromagnetic canting. We find |Hxz/HD| <0.02 for YFeO3 and |Hxz/HD| <0.05 for YCrO3, at 4.2°K.

Magnetic and Mössbauer spectroscopic characterization of coal

We sketch a role that solid state science, especially magnetic and Mossbauer spectroscopic characterization, can play in understanding coal, with its impurities. The Mossbauer characterization has potential impact on essentially all precombustion coal clean‐up procedures. We review concurrent Mossbauer studies on coal in order to demonstrate that this technique has much to offer as an improved analytical method for determining forms of sulfur in coal. Our magnetization analysis throws light on the magnetic properties of coal impurities, ash and pyrite, which facilitate magnetic separation clean‐up. It is demonstrated that simple crushing of coal can be responsible for transformations which aid in the separation. The two methods of analysis are shown to be useful for understanding coal liquefaction residues from the point of view of their removal magnetically.

The Spin-Peierls Transition

In a spin-Peierls transition, a spin-lattice system consisting of one-dimensional antiferromagnetic linear chains in a 3-D lattice progressively dimerizes and thereby becomes nonmagnetic at T = 0. Like the usual Peierls transition, this is a soft-mode transition associated with a “fermi-surface-driven” instability (in a pseudo-fermion representation). We discuss the character of the transition and make predictions concerning the dynamic structure factor.

Magnetization of a Dilute Suspension of a Multidomain Ferromagnetic

The observed magnetization curve of a dilute suspension of carbonyl iron powder, type E, is shown to be closely that one would derive from consideration of a dilute assembly of randomly oriented, single‐crystal, multidomain spheres. The deviations are in the directions expected from the effects of stress and non‐spherical clumping. The effects of packing and the applicability of various laws of approach to saturation are discussed.

Spin-Flopping in MnF2 by High Magnetic Fields

Observations of spin-flopping, or decoupling between the direction of antiferromagnetism and the easy axis of the crystal, are presented for MnF2 in single crystal and powder form at low temperatures. Magnetization measurements in pulsed fields to 140 koe are employed to explore the critical interaction energy in this antiferromagnet, as an alternative tool to microwave resonance. The orientation dependence predicted by Neel in 1936 is confirmed. The critical field for spin-flopping given by the combination of anisotropy and exchange, (2HEHA)12 is found to be 93±2 koe in excellent agreement with resonance results of S. Foner [(a) Phys. Rev. 107, 683 (1957); (b) J. phys. radium 20, 336 (1959)], and F. M. Johnson and A. H. Nethercot [Phys. Rev. 104, 847 (1956); 114, 705 (1959)].