Magnetic instabilities in fcc Fe x Ni 1 Àx thin films E. Foy, 1, * S. Andrieu, 2 M. Finazzi, 3 R. Poinsot, 4 C. M. Teodorescu, 5 F. Chevrier, 6 and G. Krill 1 1 Laboratoire pour l’Utilisation du Rayonnement Electromagne ´tique, Ba ˆtiment 209D, Centre Universitaire, Boı ˆte Postal 34, 91898 Orsay Cedex, France 2 Laboratoire de Me ´tallurgie Physique et Sciences des Mate ´riaux, CNRS-Universite ´ Henri Poincare ´, Boı ˆte Postal 239, 54506 Vandoeuvre, France 3 INFM– Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy 4 Institut de Physique et Chimie des Mate ´riaux de Strasbourg, UMR 7504, Boı ˆte Postal 20 CR, 67037 Strasbourg, France 5 Daresbury Laboratory, Daresbury, Warrington, Cheshire WA44AD, United Kingdom 6 Centre d’Electronique et de Micro-optoe ´lectronique de Montpellier, UMR 5507, CNRS-MEN, Place Euge `ne Bataillon, 34095 Montpellier Cedex 5, France Received 25 November 2002; revised manuscript received 7 April 2003; published 17 September 2003 We present the results obtained on Fe x Ni 1-x alloy films epitaxially grown on Cu100. They are character- ized by a fcc structure pseudomorphic to the substrate over a wide range of concentration and thickness. In particular, the martensitic transition which in bulk alloys occurs around the ‘‘Invar’’concentration ( x 0.65) is suppressed. We report the concentration dependence at low temperature of the total magnetic moment and of its Fe-3 d and Ni-3 d projected components in such thin fcc Fe x Ni 1-x alloy films. Magnetic instabilities that might be associated with noncollinear spin alignments of Fe atoms are clearly observed for x 0.73, where the magnetic moment decreases with increasing Fe concentration. In this Fe-rich concentration range the layers are still ferromagnetic and a magnetic moment is still observed, even on Ni atoms and at room temperature, up to x =0.86. We also show how the variation of the magnetization in this region is correlated with a very small variation of the atomic volume 1%. DOI: 10.1103/PhysRevB.68.094414 PACS numbers: 75.70.-i, 61.10.Ht, 61.14.Hg, 75.50.Bb I. INTRODUCTION For more than two decades, the study of artificial systems such as multilayers and/or superlattices, as well as metallic ultrathin films deposited on appropriate substrates has been attracting increasing interest. As the magnetic properties of thin films are very sensitive to the constraints imposed by the substrate, the first challenge was to change the crystallo- graphic structure of a system in order to favor the appearance of a magnetic order which is different from the one in the bulk. Indeed, the magnetic and the structural properties are strongly correlated, especially in the case of 3 d metals. Many 3 d compounds present magnetic-volume instabilities characterized by high-magnetic-moment–high-atomic- volume HM states and/or low-magnetic-moment–low- atomic-volume LM states. In many cases, the energy dif- ference between those states is so small that fluctuations between them become thermally accessible via low-energy excitations. 1 This situation results in several anomalies of the physical properties observed as a function of composition and external parameters such as temperature, magnetic field, and pressure. 2 The Fe x Ni 1 -x alloys are considered as arche- types of this class of materials: for concentrations near x =0.65, the thermal expansion coefficient is very small in a wide range of temperature 0–450 K. This is the so-called ‘‘Invar’’ effect. 3 The prevailing explanation attributes this ef- fect to anharmonic lattice vibrations compensating the ther- mal excitations between a HM ferromagnetic state and a LM state. 2,4 Similarly, the departure of the temperature depen- dence of the total magnetic moment from a Bloch behavior, in the Fe 0.65 Ni 0.35 Invar alloy, has been related to the exis- tence of almost degenerate HM and LM states. Many theoretical models have predicted the existence, in Fe x Ni 1 -x systems, of stable or metastable states separated by a very small total energy difference only 10 meV but char- acterized by different equilibrium volumes. However, the transition from a HM to a LM phase as a function of tem- perature has not so far received any experimental evidence. Moreover, while there is a general agreement about the na- ture of the HM state, the magnetic order in the LM state is still controversial. Ab initio calculations done for the HM phase at T =0 K in the case of an ordered Fe 3 Ni alloy, indi- cate that the magnetic moments range from 1.4 B to 1.9 B for Fe and from 0.5 B to 0.6 B for Ni. 5–8 The LM phase is predicted nonmagnetic 5,6 or magnetic with 0.5 B on Fe and no moment on Ni. 7,8 More recently, the dependence of the total energy with magnetization and volume of fcc Fe x Ni 1 -x alloys has been calculated as a function of concentration. 9,10 At T =0 K, a HM ground state is found in the case of Ni-rich alloys, with a concentration dependence of its moment fol- lowing a Slater-Pauling curve. The energy difference be- tween the HM and the LM states decreases as the iron con- centration is increased. Above the instability concentration where the HM and LM states are degenerate the ground state is expected nonmagnetic. The understanding of the magnetic behavior of Fe x Ni 1 -x alloys is complicated by the fact that, as the Fe concentration x is increased, a fcc-bcc martensitic transition takes place near the instability limit of x =0.70. The change in the crys- tal structure is accompanied by a strong downward deviation of the magnetic moment from the Slater-Pauling curve and a fast decrease of the Curie Temperature T c . Simultaneously, a deviation of the lattice parameter from the Vegard law is observed. It has been suggested that such behavior may be PHYSICAL REVIEW B 68, 094414 2003 0163-1829/2003/689/0944147/$20.00 ©2003 The American Physical Society 68 094414-1