Oscillatory Exchange Coupling in All-Compound Superlattices

It is observed that the magnetic behavior of two magnetic layers separated by a thin non-magnetic spacer material (either a metal or an insulator) is interdependent due to the communicating electronic states in the heterostructure, and the fact that such states are dependent on spin, which also controls the magnetic properties. This interdependence is termed as "exchange coupling". This is to be clearly distinguished in its strength as well as origin from the negligible dipolar interaction between the layers.

An interesting feature about this exchange coupling which was noticed rather early was the oscillation in the strength of the coupling with the thickness of the spacer layer. Of course, as the thickness increases there is damping of the coupling strength as well. The origin of this effect was traced to the elements of physics attributed to the nature of interaction between magnetic impurities in a non-magnetic host metal. This physics was developed by Ruderman, Kittel, Kasuya, and Yoshida, and is therefore known as the RKKY model. More recently, detailed quantum mechanical theories have been worked out, wherein the nature and the strength of coupling are ascribed to the dependence of the distribution of electronic states on the interplay of the geometric and crystallographic aspects pertaining to the system (as they appear in k space). Much of the work done on exchange coupling till to date has focused on metals and metal alloys. These systems have fairly low coercivity values, and hence the changes in the relative magnetizations of the exchange coupled layers due to the application of small fields can be used to control the magnetotransport in these layers. However, the Fermi surfaces of metals are almost spherical and hence somewhat uninteresting from the physics point of view, as also from the technological point of view of maneuverability and control. Compounds on the other hand have interestingly diverse Fermi surfaces as well as a broad range of physical properties. It is therefore of considerable interest to explore the possibilities of building high quality epitaxial All-Oxide heterostructures and superlattices, and to examine them for exchange coupling and transport effects. This remained an untried area for a long time because it is not easy to grow such structures using conventional deposition techniques. The technique of pulsed laser deposition (PLD) however offers considerable control and flexibility in this regard.

We have grown superlattices of Fe₃O₄/TiN using pulsed laser ablation, and have demonstrated for the first time, the occurrence of exchange coupling effects in these. Fe3O4 is a half-metal implying 100% spin polarization, and it has a very high Curie temperature (840K), making it suitable for room temperature magnetotransport applications. TiN is a very good metal and has a fairly good epitaxial match with Fe₃O₄. In spite of being a good metal, its Fermi surface is not spherical.

In the three figures below, the essential results are summed up. In fig. 1 is shown the TEM cross section of the superlattice grown by PLD, with the high resolution image in the inset showing the atomistic interface quality. Fig. 2 shows the changes in the nature of hysterisis loop with the changes in the thickness of the TiN spacer layers, reflecting the thickness dependence of the saturation field. The corresponding data for films of several thicknesses are summarized in Fig. 3, which clearly shows the oscillation of exchange coupling similar to what is seen in the case of well-studied metallic superlattices.

Reference : Oscillatory exchange coupling and giant positive magnetoresistance in TiN/Fe₃O₄superlattices, Orozco A, Ogale SB, Li YH, Fournier P, Li Eric, Asano H, Solyaninova V, Greene RL, Sharma RP, Ramesh R and Venkatesan T. Phys. Rev. Letts. 83, 1680 (1999).

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