Se p 19 99 Magnetic properties of disordered CoCu alloys : A first principles approach

CoCu alloys have been extensively studied experimentally. Earlier investigations, particularly those of the magnetic properties, include high-temperature magnetic moment for 4085% Cu rich Co-Cu solid solutions [1] and variation of magnetic moments for Co rich alloys at room temperature [2,3]. Recently, the focus of attention has been on giant magneto-resistance (GMR) studies. Several experimental groups have been carrying studies of GMR in this system. Most remarkable works include investigation of GMR in bulk CoxCu1−x alloys for x = 0.05-0.2 [4,5], in heterogeneous thin film CoCu alloys [6,7] and in as-grown, epitaxial Co-Cu alloy layers [8]. Other magnetic studies on this system include magnetic anisotropy of Co on Cu(110) [9] and of Co-Cu ultra thin films [10]. An altogether different type of work by Childress and Chien [11] is, to our knowledge, the only one of its kind which involved low-temperature magnetic studies of CoxCu1−x alloys. This work is worth mentioning because it revealed very interesting features of the magnetic phases over the entire concentration range from x = 0 to x= 0.8. It indicated a low-temperature spin glass phase upto x = 0.23 ; a low-temperature re-entrant spinglass or mixed phase, stable in the range 0.24 < x < 0.40 and the normal low-temperature long-ranged random ferromagnetic phase stable beyond x = 0.4. Mandal and Ghatak [12] have reported calculations on the periodic Anderson model for a binary alloy, working within the Hartree-Fock, coupled with the Virtual Crystal Approximation (VCA). They have been able to produce some of the features observed experimentally. The model used by these authors have many empirical fitted parameters. The existence of a spin-glass and, at higher concentrations, a re-entrant mixed phase has been known from several mean-field approaches [13,14]. To our knowledge, the system has never been investigated from a first-principles electronic structure point of view. The aim of this communication is to present such a calculation of magnetic properties of the fcc, ferromagnetic phase of CoxCu1−x alloys in the concentration range from 40% to 80% of Co. We have used the fully self-consistent, tight-binding linearized muffin-tin orbitals augmented space recursion (TB-LMTO-ASR) technique as the basis of our calculations. We have restricted ourselves to the ferromagnetic phase only leaving the spin-glass and mixed phases for a later study. Our studies involved calculation of magnetic moments, Curie temperatures and the spin susceptibilities.