PHYSICAL REVIEW 8 VOLUME 52, NUMBER 14 1 OCTOBER 1995-II Magnetic ordering in Fe-containing spinodally decomposing materials synthesized from laser plasma Yuri Blyakhman Department of Physics, ¹w York University, g Washington Place, No. 8, New York, New York 10008 N. I. Polushkin, A. D. Akhsakhalyan, S. A. Gusev, and N.N. Salashchenko Institute for Physics of Microstructures, Russian Academy of Sciences, 608600 Nizhny Novgorod, $6 Ujlanov St , Ru. ssia V. G. Semenov Chemical Institute, St Pe. tersburg State University, 198g0$ University, Universitetsky pr. 8, St. Petersburg, Russia (Received 31 January 1995; revised manuscript received 9 May 1995) Using Mossbauer spectroscopy and the magneto-optical Kerr efFect, we investigate how magnetic order arises and develops at room temperature in Fe/C and Fe/Cr layered mixtures prepared by pulsed laser deposition. Two difFerent magnetic transformations are found to occur in these systems under their thermal annealing. The 6rst is paramagnetic-ferromagnetic crossover that takes place at annealing temperatures of 420 to 570 K in dependence on the average iron content. Varying the latter quantity, we also observed the same crossover to the ferromagnetic state. The second transformation is magnetic hardening at 700 — 800 K. The speci6c feature of both transformations is that they occur within narrow ranges of annealing temperature (( 10 K). The first transformation is supposed to result from an additional iron enrichment of Fe-rich clusters and is associated with magnetic phase transition near the Curie temperature. The cluster enrichment was directly observed with transmission electron microscopy and was also shown in small angle I-ray scattering patterns of the compositionally modulated structures. The observed sharpening of the magnetic behavior near the Curie temperature is presumably a manifestation of the clustered structure. The second transformation is found to be induced by the onset of crystallization and the formation of particles of the Fe-rich phase. I. INTH. ODUCTION The eÃects arising in supersaturated solid solutions have attracted considerable attention in the past few decades. Recently a research activity on mechanisms of phase separation and its kinetics in solid materials was stimulated by an enhanced interest in the formation of nanosized particles with physical properties that dif- fer &om those of the bulk counterparts. It is of interest to follow the process of phase separation &om the view- point of modification in physical properties of a phase- separating system. In magnetic materials, phase separa- tion leads to formation of spatially separated regions with nanometer scales and varying Curie temperature, thus al- lowing the tailoring of the magnetic nanostructures as well as their fabrication, for example, with local heat actions on a 61m surface. Earlier we reported our stud- ies of the modi6cation in magnetic behavior of composi- tionally modulated Fe/C films under thermal annealing and short-pulse laser treatments. It was found out that the observed transformation into ferromagnetic phase is caused by agglomeration of the iron atoms. However, the interpretation of the threshold feature of this transfor- mation seems to lack clarity. On the other hand, under- standing of the mechanism underlying this transforma- tion is a necessary condition that would provide a basis for the nanostructure fabrication. In this work we address supersaturated solid solutions upon the basis of iron as an example of how a magnetic order develops due to phase separation. The proper- ties of the resulting materials will depend on a phase- separation mechanism, i.e. , whether spinodal decompo- sition or nucleation and growth of clusters takes place. Spinodal decomposition leads to the changes rather in a continuous paramagnetic medium, whereas a nucle- ation process is expected to induce a superparamagnetic- ferromagnetic crossover in. a clustered medium. ' Su- perparamagnetic or ferromagnetic behavior manifests it- self depending on the relation between the relaxation time and the characteristic time scale of observation. Us- ing a variety of methods with specific time scales, such as magnetic susceptibility measurements (10 s — 10 sec), Mossbauer spectroscopy (10 — 10 r sec), neutron scat- tering (10 i2 — 10 io sec), one can study the characteristic features and properties of clustered systems. ' These techniques, however, can be used along with the more conventional and accessible methods like transmission electron microscopy (TEM), small angle x-ray scattering (SAXS), and the magneto-optical Kerr effect (MOKE). In particular, by SAXS it is possible to detect even the slightest (with a characteristic scale of several nanome- ters) fluctuations of the electron density. We used this approach to study various phase-separating systems pre- pared by means of pulsed laser deposition (PLD) as mul- 0163-1829/95/52(14)/10303(12)/$06. 00 52 10 303 1995 The American Physical Society