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Hardmagnetic Amorphous Alloys

Amorphous materials have been scrutiny of several researchers due to their auspicious mechanical properties and high corrosion resistance as compared to their crystalline counterparts. These properties appear due to the absence of obvious crystalline defcets e.g., grain boundaries and dislocations. Besides good mechanical properties, amorphous materials are also known for their favorable soft magnetic properties which can not be achieved in crystalline alloys. Amorphous materials have following positive points w.r.t. soft magnetic properties:

In beginning amorphous alloys were mainly considered for soft magnetic applications. In 1980 Croat and  later Inoue in 1996 discovered hard magnetic properties in Nd-Fe melt-spun ribbons and Nd-Fe-Al bulk amorphous alloys, respectively. The novel combination of good mechanical properties and relatively good hard magnetic properties have led to several other investigations in this field. These materials show coercivity of ~ 300 kAm-1 at room temperature. These alloys do not display an endothermic signal due to glass transition in the DSC curves. On the basis of these propertise Cluster Structure has been proposed for these materials. These clusters have crystalline-like short range order. The cluster are expected to consist of Nd and Fe atoms with large random anisotropy. The intra-cluster coupling has been presumed to be ferromagnetic in nature. All these speculations successfully explain the observed properties in the present alloys. Further quantitative explanation of the cluster structure in amorphous Nd-Fe-Al alloys needs more experimental evidences. The major difficulties are the higher cooling rates of the order of 106 K/m to prevent crystallization.  The convincing cause for hard magnetic properties in Nd-Fe-Al is still remaining as a question which has not yet been satisfactorically answered.
In my M.Tech thesis work, we synthesized following two series of alloys by three different processing routes i.e., Mechanical milling, melt spinning, and copper mold casting.

  1. Nd-Fe-(Co, Cu)-Al-B
  2. Nd60FexCo30-xAl10 (x = 0, 2.5, 5, 7.5, 10)

We found that Nd-Fe-(Co, Cu)-Al-B alloys prepared by copper mold casting exhibit similar hard magnetic properties at room temperature as reported by Inoue. Whereas, melt-spun ribbons and milled alloys do not exhibit hard magnetic properties, irrespective of similar XRD patterns. It implies the peculiar atomic structure responsible for hard magnetic properties can only be accessed by copper mold casting realizing moderate cooling rates. The second series of alloys also shows similar discrepencies in magnetic properties. In addition, some of the alloys in second series exhibit distinct glass transition in DSC curves. The increase of Fe content beyond 5 at% leads to disappearance of glass transitions. But at the same time increase of Fe content improves hard magnetic properties. It implies that the hard magnetic properties are very sensitive to Fe/Co ratio in these alloys. The increase of Fe content beyond  5 at% must have stimulated phase separation in these alloys. My Ph.D work is to characterize different phases in present alloys and to further invesigate the magnetic prpertise in present alloys.

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