Magnetic Anisotropy and Texture of Polycrystalline Permanent Magnet Materials

• A fitting routine has been develpoed which allows the simultaneous determination of anisotropy constants, the saturation polarization and a texture parameter of polycrystalline, hard magnetic material. Therein, the reversible rotational magnetization processes determining the demagnetization curves in the first quadrant of the J-H-plane after initial saturation of the sample are analyzed. Advantages: polycrystalline material can be investigated and the maximum field neccessary can be smaller than the anisotropy field of the material.
  
• In its basic version, the two anisotropy constants K₁ and K₂, the texture parameter of an assumed rotationally symmetric texture function, the saturation magnetization J_S as well as the contributions of a high-field susceptibility and a soft phase to the sample magnetization can be determined. The texture of permanent magnets has been studied extensively [3]. Results on the formation of texture in NdFeB material have been compared to X-ray measurements [11].
  
• Extensive studies of Sm₂Fe₁₇Z_X compounds (with Z = N, C, H) have been performed by means of this procedure [7], [12], [14]. Main result: accurate values for the crystalline electric field. Also NdFeB materials have been studied, mainly to establish the procedure [4].
  
• The fit procedure has been adapted to allow for the investigation of Sm₃Fe₂₉N_X compound which show a monoclinic crystalline structure [13].
  
• Another adaption accounts for biaxial textures (as e.g. in hot rolled NdFeB materials) [10].
  
• A special case of field and orientation dependence of the spontaneous magnetization has been taken into account.
  
• Statistical mathematics has been applied to check on the applicability of the fit procedure as well as to find the accuracy of the results.
  
• An overview has been given in [20].
  

• The formation and structural properties of Sm₂Fe₁₇Z_x (Z = N, C, H) compounds have been studied [2]. It has been shown that Sm₂Fe₁₇N_x with intermediate N content can be prepared as single phase material - an important issue for application of this material in permanent magnets. The interstitial modification of rare-earth intermetallics has been investigated in detail [B1], especially the diffusion leading to the formation of the modified compounds [18].
  
• The magnetic properties, especially the crystalline electric field of these compound have been studied [14].
  

• An experimental procedure for determination of the angular and field distribution of the magnetization reversal in polycrystalline samples has been developed [1]. The procedure has been established by application to strongly diluted Ba-ferrite samples in which the angular dependence of the switching field corresponds to the predictions of the Stoner-Wohlfarth model.
  
• The experimental procedure has been generalized to account for magnetostatic interactions in the global approximation of a demagnetizing field [9]. Moreover, the switching probability in dependence on the magnitude and orientation of the applied field (with respect to the grain's easy magnetization direction) has been experimentally determined. Basically, this probability generalizes the switching field distribution (for arbitrary angle) and the angular dependence of the switching field (by taking into account the distribution of the switching field). From this, the magnetization reversal process can be analyzed in more detail.
  

• The magnetic viscosity of sintered (Nd,Dy)₁₅(Fe,Mo,Co,Al)₇₇B₈ has been investigated (mainly during my stay in Grenoble ) [5]. This material shows the curious effect that its coercivity can reversibly be changed by a heat treatment (minimum coercivity at room temperature 1.3 T after a heat treatment at 850 K, maximum coercivity 2.4 T after treatment at 1025 K). The intrinsic properties of this material show no significant change due to the heat treatment. A possible explaination for this effect supported by viscosity measurements is that the grains are chemically or/and topologically change very close to their surface infuencing the nucleation of reversed domains.
  
• Viscosity and magnetization processes of the exchange coupled, two phase material Nd₄Fe₇₇B₁₉ has been investigated [6].
  

• Investigation of the structural and magnetic properties of the shape-memory ferromagnetic Heusler alloy Ni₂MnGa [15]. It has been found that the cubic phase can be suppressed by a certain Ni excess substituting Mn. The tetragonal phase is thus established at room temperature showing easy-axis anisotropy. The Curie temperature (376 K) is not influenced by this substitution.
  
• The stray field in Sm₂Fe₁₇N₃ has been evaluated by comparison of ac- and differential dc- susceptiblity [8].
  
• A VSM has been designed using permanent magnets for field generation. The eight pick-up coils measure the magnetization components in two perpendicular directions (vector measurement) and were optimized for maximum signal sensitivity and minimum sensitivity against small sample displacement by numerical simulations.