VOLUME 65, NUMBER 23 PHYSICAL REVIEW LETTERS 3 DECEMBER 1990 New Antiferromagnetic Insulator Superlattices: Structural and Magnetic Characterization of (FeFQ (CoFQ „ C. A. Ramos, D. Lederman, A. R. King, and V. Jaccarino Department of Physics, University of California, Santa Barbara, California 93106 (Received 15 August 1990) High-quality antiferromagnetic (AF) superlattices (FeF2) (CoF2)„as judged from high-resolution x-ray diffraction (HRXD), were grown on (001) ZnFz substrates. Using HRXD, the thermal-expansion coefficient a[ooi](T) vs T was measured. While two shifted AF anomalies are seen for (m, n) =(25, 30), only one is observed for (m, n) =(19,6). Comparison of alooii(T) in the two superlattices with that of FeF& and CoF2 indicates that a significant redistribution of the magnetic entropy occurs. PACS numbers: 75.70. Fr, 68.65.+g, 75.50.Rr There has been a growing interest in thin films and su- perlattices of ionic materials from the fundamental point of view' as well as for possible device applications. A parallel theoretical effort has been directed towards un- derstanding the magnetic properties of multilayer mag- netic materials. The simplicity and importance of ionic magnetic superlattices cannot be overemphasized. The short-range exchange (J~~ and Jtttt) and anisotropy (Kz and Ktt) interactions are usually well known for the A and 8 hosts, as is often Jztt from impurity studies. Hence, all interactions necessary to characterize multi- layer A B„systems are known beforehand; as such they become ideal candidates for quantitative studies of critical-phenomena crossover behavior, Brillouin-zone folding, and new optical modes in superlattices. Among the most studied and best understood ionic systems are the rutile-type antiferromagnets (AF): FeF2, CoF2, MnF2, and their nonmagnetic isomorph ZnF2. There has already been extensive eAorts to prepare, characterize, and measure the properties of ep- itaxial films of these materials in our laboratory. This lead to the first observation of AF resonance and AF standing-spin-wave resonances (Kittel modes)' in MnFq thin films. Interdigitated capacitance measurements have yielded the specific heat of FeF2 thin films near the critical point. We have now succeeded in growing high- quality FeF2/CoF2 superlattices. Here we report on the structural characterization and thermal-expansion mea- surements of (FeF2) (CoFq)„xl superlattices on ZnF2 substr ates. FeF2 and CoF2 were chosen for their prototypical 3D Ising critical behavior. ZnF2 was chosen as the sub- strate because its a axis is almost identical with that of FeF2 and CoF2 (a =4. 7034, 4.6966, and 4. 6951 A, re- spectively), and its obvious advantage for studies of su- perlattice magnetic properties. The substrates were c- axis oriented to within 0.5 before cutting and polish- ing. The (FeF&) (CoF2)„samples were thermally evaporated under high vacuum (approximately I x 10 Torr) at rates between 1 and 2 A/sec, on substrates heat- ed to 325 C. A PC computer controlled the two shutters of the individual sources, by measuring the frequency change of a calibrated quartz-crystal monitor. A structural characterization of each substrate and su- perlattice was performed with high-resolution x-ray diffraction (HRXD), utilizing a rotating-anode source, with a beam intensity of 40 kV and 80 mA set in point focus (0.3x3-mm beam). The Cu Kai radiation was collimated using slits at the entrance and output of a GaAs(400) wafer monochromator. Counting rates at the peak of the (002) substrate peak were typically =5x10 counts/sec. The substrate peaks exhibited full widths at half maximum (measured in 28 for 8-28 10 4 10 3 ~ & 0 0 10 1 10 6 N 10 10 io I I I I I 55 56 57 58 I I I I I (b) t l 1 't \ \ T ~ ZnF~ 58 54 56 58 60 28(degrees) FIG. I. (a) 8-28 scan of the (FeF2)~s(CoF2)30 superlattice. The central peak h, . =0.04' indicates a coherence length of g =2200 A. The adjusted superlattice period is A =89.2 A. A total of 100 layers were deposited. (b) 8-28 scan of the (FeFt)i9(CoF2)6 superlattiee. The central peak 6 =0. 12' in- dicates a coherence length of (=750 A. The superlattice period is A =41.3 A. A total of 670 layers were deposited. The ZnFt (002) substrate peak is shown. 1990 The American Physical Society 2913