High Field Torque Magnetometry Measurements on Y 1.8 Er 0.2 Fe 14 B
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HIGH FIELD TORQUE MAGNETOMETRY MEASUREMENTS ON YI8ErO2 Fel 4B
C.M. WILLIAMS, N.C. KOON, AND B.N. DAS U.S. Naval Research Laboratory, Washington, D.C. 20375-5000
ABSTRACT The magnetocrystalline anisotropy energy has been determined for single crystal Y 1.8 Er 0 .2 Fel 4 B in the (001) and (100) planes between 5 K and 300 K using torque magnetometry techniques. The results are compared with a model based on crystal field theory. Excellent agreement was obtained between the model and experiment. Both experiment and model showed a first order spin reorientation between the [001] and an angle 70 degrees from the [001] in the (100) plane.
INTRODUCTION There has been considerable interest in tetragonal R 2Fel 4 B compounds as the basis of a new class of permanent magnet materials because of the unusually large energy products they exhibit at room temperature for certain rare earth substitutions. The origin of the high energy products is directly related to the large saturation magnetization and magnetic anisotropy energy. The single crystal magnetic anisotropy energy has been investigated by several groups[1- 5]. The origin of the magnetocrystalline anisotropy energy is believed to be directly related to an interaction between the 4f-electrons and the crystal field; however, to date there have been few if any direct comparisons made between the experimental magnetic anisotropy and crystal field theory. The reason being that the large anisotropy energy makes it very difficult to use conventional torque magnetometry techniques to determine the angular dependence of the magnetic free energy, particularly at low temperatures where the higher order terms become more important. Most anisotropy energies measurements have been obtained from magnetization data which do not have the angular fidelity required for a direct comparison with theory. In this investigation we determine the angular dependence of the magnetic free energy as a function of temperature using conventional torque magnetometry techniques and make a direct comparison of the free energy with the energy calculated using a model based on crystal field theory. We circumvent the high anisotropy energy problem to some extent by considering YI.8Er 0 .2Fel 4 B. The rationale for using this composition is the large iron sublattice anisotropy favors the c-axis and the erbium anisotropy favors the basal plane; at low temperatures the Fe and Er sublattice anisotropies nearly cancel. The ultimate aim of this investigation is to determine the crystal field constants for this dilute system and use these constants to predict the anisotropy energies of other R 2 Fe 1 4 B structures which may be potentially useful in high energy product permanent magnet applications.
EXPERIMENTAL The most direct method of determining the magnetic free energy is by torque magnetometry; however, this method requires well characterized, good quality single crystals. The torque method consist of measuring the torque required to rotate the magnetization away from a principal crystallographic directi
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