Earl Callen

Mössbauer evidence of spin reorientation in single crystal Nd2Fe14B

Abstract Using 57 Fe Mossbauer spectroscopy, we have measured the canting angle of the iron spins in Nd 2 Fe 14 B. Our measurements are made on a single crystal sample cut with its c -axis normal to the sample face. The iron spins rotate away from the c -axis by about 27° at 4.2 K.

First order phase transition in orthohydrogen and paradeuterium

Abstract We display a third order term in the orientational free energy expansion of solid orthohydrogen and paradeuterium. According to Landaus theory of phase transitions, this makes the order-disorder transition first order, even in the absence of distortion or of a change in crystal structure or volume.

The temperature dependence of magnetic anisotropy energy and magnetostriction of spins with n degrees of freedom

For spins of fixed length rotating in three space (two degrees of freedom, n=2) the anisotropy and magnetostriction coefficients of degree l vary at low temperatures as the l(l+1)/2 power of the magnetization. We demonstrate that for arbitrary number of degrees of freedom n the power of the magnetization is l(l+n−1)/n. We further demonstrate that at high temperatures, where the magnetization is small, the power of the magnetization is l, for all numbers of degrees of freedom.

The magnetic anisotropy phase diagram of R2(Co1−xFex)17 compounds

Abstract The easy axis-easy plane magnetic anistropy phase diagram (temperature, composition) of the lanthanide rare earth pseudobinary compounds R 2 (Co 1− x Fe x ) 17 can be rationalized when one takes into account: (i) the shape — oblate or prolate — of the rare earth 4f charge density; (ii) the composition dependence of the Curie temperature, and; (iii) the magnetic anisotropy of the background cobalt-iron matrix. At each composition x , there is a temperature below which the rare earth dominates and above which the background determines the easy direction. The larger x , the lower this temperature. Thus, at room temperature, toward the cobalt end, the rare earth determines the easy direction, because T c is high. But toward the iron end the magnetization lies in the plane, because T c is low, the rare earth is significantly unmagnetized, and the transition metal background dominates.

Mössbauer study of spin reorientation in Y1.8Er0.2Fe14B

Fe57 Mossbauer spectra were taken from 300 K down to 4.2 K for two single‐crystal samples of Y1.8Er0.2Fe14B, one cut with its [001] axis normal to the surface, the other with its diagonal of [100], [010], and [001] axes normal to the surface. The former sample has shown that the angle between the Fe spin and the [001] increases continuously below ∼90 K. This qualitatively agrees with the spin reorientation (i.e., the easy axis of magnetization rotates away from the [001] toward [100] on cooling below ∼90 K) which have already been observed by torque magnetometry. The latter sample has shown that the Fe spin actually rotates continuously; abrupt change of Fe spin direction, which have been already suggested to occur in Er2Fe14B, does not occur in Y1.8Er0.2Fe14B. The samples were so thick that the intensity of strong absorption lines were saturated. We have derived a formula with which the saturation effect can be corrected in order to determine the angle of the Fe spin direction correctly.

Competing anisotropy in rare earth intermetallics

Abstract The manner in which crystalline electric fields compete in cubic ferromagnets is discussed. In the first section it is shown how competition between fourth and sixth-degree terms in the crystalline field energy lead to re-entrant behavior of the magnetization process in an applied magnetic field. For one special value of the ratio of sixth-to fourth-degree coupling strengths a new type of critical point is found to occur in a field applied in a [111] direction. The influence of temperature fluctuations is briefly discussed. The second section is devoted to a discussion of the effects of competition in alloys between ions with opposite signs of crystalline coupling. Using rare-earth Laves phase pseudobinary alloys as an example, the fluctuations in the sign of the crystalline field are shown to lead to unusual magnetic phases in concentrated alloys. The effects of mixing different angular momentum states is shown to lead, in special cases, to high-order anisotropy (eight and greater) and possibly, unusual magnetic phases in pure binary compounds. The influence of applied fields on canted-spin phases is also discussed and field-concentration phase diaplayed in a model case. In the last section, the important influence of randomness of the crystalline field is noted.

Amorphous ferrimagnets in large magnetic fields

Abstract In the amorphous rare earth-transition metals the rare earth sublattice is asperomagnetic with the transition metal spins aligned antiparallel to the rare earth moment. Application of a magnetic field produces an angled phase between two critical fields as in crystalline ferrimagnets, but with the rare earth moment dependent on field.

Temperature Dependence of Magnetic Excitons in Singlet‐Triplet Systems

Recent observations1 indicate that the magnetic exciton energies in the singlet ground state systems fcc Pr and Pr3Tl are nearly temperature independent up to several times the ferromagnetic ordering temperature. Since the lowest excited crystal field state is a triplet, assuming that higher levels can be ignored one can describe these systems in terms of two pseudospins 1/2 on each site2. We have investigated the temperature dependence of the exciton energies using a decoupling scheme for the pseudospin equations of motion which consistently retains pair correlations, including those between pseudospin operators on adjacent sites. Numerical calculations have been performed using this scheme and a boson formalism for the correlation functions. The results are in agreement with experiment in that the exciton energies do not show any appreciable temperature dependence up to about twice the ordering temperature. The physical reason for this behavior is that kBTc is small compared to a typical exciton energy,...

Japan, magnetism, and technological competition (invited) (abstract)

Magnetism has been the fountainhead of ideas for understanding cooperative phenomena and magnetics is at the foundation of

First‐order line and critical end point of cubic rare‐earth pseudobinaries in RxR’1−xT2 in the H, X plane (abstract)

60 billion of annual trade (Mallinson). And yet, led by the National Science Foundation, the federal science contracting agencies years ago dropped support of magnetism and magnetics. A few months ago I returned from 3 years in Tokyo as an ONR scientific cooperation and exchange officer, monitoring magnetism and superconductivity. Unlike the Americans, the Japanese, traditionally very strong in magnetics, continue to concentrate resources there. World‐wide sales of magnetically soft electrical steels come to