PHYSICAL REVIEW B VOLUME 43, NUMBER 16 1 JUNE 1991 Exchange interactions at the surface of a ferromagnet D. Scholl, M. Donath, * D. Mauri, and E. Kay IBM Almaden Research Center, San Jose, California 95120-6099 J. Mathon and R. B. Muniz~ Department of Mathematics, City Uniuersity, London, United Kingdom H. C. Siegmann Swiss Federal Institute of Technology, Zii rich, Switzerland (Received 1 November 1990) The local magnetization in magnetic surfaces obeys a Bloch T' 1aw with a prefactor kz; that varies from one plane i to another. kz; reAects the relative exchange interactions Jz;~~/J and J&;&/J on a path parallel and perpendicular to the surface, with J the exchange constant in the bulk. The spin polarization P of the low-energy cascade electrons yields k, ii. , which is related to k&; by the probing depth. By measurement of P(T), k, ~ was obtained with various surfaces of polycrystalline fcc Ni»Fe». Deposition of submonolayers of Fe, Cr, and Ta on the clean Ni-Fe sur- face produced dramatic changes in Jz /J. A submonolayer of Fe increased J&~~ /J to as much as = 3, in which case at T ) 0 the relative magnetization is larger at the surface than in the bulk. However, deposition of additional Fe reduced Jz/J again to =1. These effects are analogous to what has been observed in the fcc Fe-Ni alloys as a function of composition, but much stronger. A submonolayer of Cr did not greatly affect Jz/J in contrast to a submonolayer of Ta which reduced Jz/J by a fac- tor of — 10. INTRODUCTION The quantum mechanical exchange energy JS, . S be- tween the magnetic moments S; and S~ of atoms i and j in a solid may lead to magnetic ordering below a critical temperature T&. The exact meaning of the exchange constant J depends on the theoretical model. J cannot be reliably calculated from first principles nor can it be mea- sured directly. However, in the bulk of a ferromagnetic material, J is related to the Curie temperature Tc~ and directly linked to measurable quantities such as the spin wave energy, the thickness of a domain wall, or the spec- troscopic splitting between spin-up and spin-down states. General theoretical considerations show the J must be ex- tremely sensitive to the distance and direction of the nearest-neighbor atoms as well as to the fine details of the electronic structure. Therefore, it is expected that the ex- change interaction Jz at the surface of a ferromagnet is different from the spherically averaged J in the bulk, and that this difference depends on the crystallography and the chemical composition of the surface. The altered J at the surface may lead to dramatic phenomena. For exam- ple, the surface may exhibit ordering at Tzz ) Tcz, or an- tiferromagnetic order might exist at the surface of a fer- romagnet. Magnets traditionally provide the prototype examples for surface critical phenomena such as order- disorder transitions and unmixing in binary alloys, gas- liquid condensations in a Auid, and structural transitions, to name a few. ' Moreover, the surface is the source of the exchange field transferred across an interface to a second magnetic material or crystallite, so surface magnetism is the key to the exciting phenomena observed in magnetic multilayers that are a central interest of present day magnetic research. For these reasons, there is considerable interest in the direct experimental study of the surface exchange interaction Jz. Following up on previous studies in this area, ' it is the purpose of this paper to show that J& can be determined in units of the bulk exchange J by measuring the temperature depen- dence of the local magnetization Ms(T) at the surface. In the case of Permalloy, Ni7~Fe2z, it will be demonstrat- ed that dramatic increases or decreases of Js /J can be obtained by depositing fractional layers of additional metal atoms on the surface. This observation opens a largely unexplored area for further basic investigations, and even suggests the possibility of magnetic interfaces designed to meet specific requirements. PHYSICAL BACKGROUND At low temperatures T/Tc (0. 4, the thermal decrease of the relative magnetization M(T)/M(0) is caused by excitation of noninteracting spin waves. At the surface of the ferromagnet, the spin waves are reAected. Since the surface is a free end, the spin waves of any wavelength will exhibit an antinode in the last layer S. This leads to a characteristic profile of the local magnetization Ms, ( T) /Ms; (0), where i = 0, 1, 2, . .. . The relative probability kz, of finding a spin wave in the layer S — i compared to the probability in the bulk depends on the local exchange interaction Jz, . If Jz, is equal to the average exchange interaction J in the bulk, k& = 2. If, however, Js /J ) 1( ( 1) the local density of spin waves in the surface decreases (increases) and the relative probabil- 43 13 309 1991 The American Physical Society