Fe/Rh „100… multilayer magnetism probed by x-ray magnetic circular dichroism M. A. Tomaz, D. C. Ingram, and G. R. Harp Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701 D. Lederman and E. Mayo Department of Physics, West Virginia University, Morgantown, West Virginia 26506-6315 W. L. O’Brien Synchrotron Radiation Center, University of Wisconsin-Madison, 3731 Schneider Drive, Stoughton, Wisconsin 53589 Received 10 April 1997 We report the layer-averaged magnetic moments of both Fe and Rh in sputtered Fe/Rh 100 multilayer thin films as measured by x-ray magnetic circular dichroism. We observe two distinct regimes in these films. The first is characterized by Rh moments of at least 1 B , Fe moments enhanced as much as 30% above bulk, and a bct crystal structure. The second regime is distinguished by sharp declines of both Fe and Rh moments accompanied by a transition to an fct crystal lattice. The demarcation between the two regions is identified as the layer thickness for which both bct and fct phases first coexist, which we term the critical thickness t crit . We attribute the change in magnetic behavior to the structural transformation.S0163-18299707433-X Thin-film multilayer systems are an area of intense re- search today partly due to the applications potential of mag- netic properties such as the giant magnetoresistance. 1,2 Such systems also provide a venue for the exploration of funda- mental issues in magnetism. One such issue is that of in- duced moments in normally paramagnetic materials in prox- imity to a ferromagnetic material. The late 4d transition metals TM are interesting candidates in light of their chemical similarity to the 3d ferromagnetic TM. In spite of this similarity, the 4d TM lack intrinsic magnetic moments in the bulk. There is a plethora of work addressing induced moments on 4d TM atoms in magnetic alloys. 3 Here we employ the thin-film multilayer geometry to place the para- magnetic material in proximity to the ferromagnet. This ad- ditionally provides for the observation of effects due to re- duced dimensions and/or the existence of interfaces. Epitaxial growth of metal films may lead to lattice strain and atomic volume variations due to lattice mismatches. These in turn may impact the system’s magnetics. Moruzzi and Marcus 4 calculated that Rh and Pd exhibit an anomolous magnetic behavior with respect to the increase in atomic vol- ume. They had previously shown that all transition metals exhibit a well defined onset to magnetic behavior at some critical volume. They found that the magnetic moments for Rh and Pd exceeded the Hund’s rule atomic limit at the onset of magnetism and decreased toward this limit as the atomic volume was further increased. All other TM elements in- crease toward the Hund’s rule magnetic moment limit as the atomic volume increases. There is also a simple rule that relates magnetism to the local atomic environment: the lower the coordination number the larger the atomic moment. 5 These two factors have been the motivation behind a body of theoretical and experimental work to determine under what conditions Rh may be magnetic. Calculations predict a fer- romagnetic Rh ground state in a free-standing monolayer, 6–8 as an overlayer on MgO 001, 9 and more recently in an expanded bcc lattice. 10 Conflicting reports are found for Rh when in proximity to a noble-metal substrate. Some calcula- tions suggest a ferromagnetic ground state for Rh on Ag and Au, 6,11 while others do not. 12,13 Experimentally, magnetism of Rh on Ag has been observed by some researchers 14 but not by others. 13,15 Also, no magnetism has been measured in Rh grown on Au. 16 Another possible method for inducing magnetic moments is to place a nonmagnetic material in proximity to a ferromagnet. In addition to the mechanisms mentioned above, the Rh moment may be further enhanced by a spin polarization due to band hybridization at the inter- face. We pursue this approach here. A previous study of Rh/Fe 100 by Kachel et al. 17 showed that overlayers of Rh on Fe 100 are ferromagnetic at one monolayer ML coverage. They also found that the ferromagnetic order in the Rh overlayer persists at a cover- age of 2 ML, although only the interface Rh atoms appear to be magnetic. They attributed the ferromagnetic FM order- ing of the Rh overlayer to direct overlayer-substrate hybrid- ization, and used a full linear augmented plane-wave calcu- lation to predict a moment of 0.82 B for a monolayer of Rh on an Fe100 substrate. The above result was further inves- tigated theoretically by Chouairi et al. 5 who found the polar- ization of the Rh overlayers, observed by Kachel et al., to be more consistent with bct and fct rather than bcc or fcc con- figurations. Additionally, Chouairi and co-workers found an oscillatory antiferromagnetic behavior in the polarization of the Rh atoms in all cases they studied. In the present work, we describe element specific mag- netic moment measurements in Fe/Rh 100 multilayers us- ing x-ray magnetic circular dichroism XMCD. 18 Applying XMCD to 4d elements is a nontrivial, yet viable technique for such measurements. 19 This paper presents quantitative, element specific moment measurements of Fe/Rh 100 mul- tilayers. We find that the Rh in these films carries a nearly constant magnetic moment as a function of t Rh up to t crit which varies from 3 to 5 Å depending on the Fe layer thick- ness. The measurements do not support the notion of an os- PHYSICAL REVIEW B 1 SEPTEMBER 1997-I VOLUME 56, NUMBER 9 56 0163-1829/97/569/547410/$10.00 5474 © 1997 The American Physical Society