778 zyxwvutsrqponm IEEETRANSACTIONS ON MAGNETICS, VOL. 30, NO. zyxw 2, MARCH zyx 1994 A Mossbauer Study of Fe zyxwv (5A) + Cu (50& Multilayers Q. A. Pankhurst, M. F. Thomas, C. E. Johnson Department of Physics, University of Liverpool, M9 3BX, England R. Zquiak, X. X. Zhang, J. Tejada Departament de Fisica Fonamental, Universitat de Barcelona, Barcelona 08028, Spain Abstract--Recent magnetic viscosity studies of FdCu multilayer systems containing 5 A layers of Fe sandwiched between 50 A layers of Cu(ll1) have shown behaviour that is consistent with macroscopic quantum tunnelling of the magnetization. As part of a wider program of measurements aimed at characterising this system, 57Fe Miissbauer spectra of two samples grown in different laboratories were recorded at temperatures ranging from room temperature zyxwvutsr to 4.2 K, and at 4.2 K in applied fields of up to 5 T. The Mossbauer data show that one sample contains iron oxide impurities, but that both samples contain roughly equal quantities of ferromagnetic bcc Fe and antiferromagnetic fcc Fe. The Fe moments in both the bcc and fcc phases lie preferentially in the plane of the multilayer film. I. INTRODUCTION The observation of quantum behaviour in macroscopic variables is an elusive goal which is usually realised only in superconductingsystems. However, as long ago as 1959, Bean and Livingston proposed that quantum tunnelling of the magnetization of small ferromagnetic particles could be an observable phenomenon [l]. In single domain particles the magnetization vector could reverse direction as the result of tunnelling through the anisotropy energy barrier. In multi- domain particles a similar magnetization reversal could result from the tunnelling of a domain wall through its pinning energy barrier. In the past few years there have been major developments in both theoretical and experimentalresearch into the quantum tunnelling of magnetization. In 1988 Chudnovskyand Gunther 121 presented a model which allowed them to predict the rate of quantum switching in a single domain ferromagnetic particle. In 1990 the model was extended to single domain antiferromagnetic particles [3,41, and much larger effects, observable at temperatures of the order of a few Kelvin, were predicted. Initial SQUID susceptibility experiments on nanometre scale FeCO particles, were suggestive of quantum effects, but were inconclusive [5]. Magnetic relaxation measurements on ferromagnetic and antiferromagnetic Manuscript received May 31, 1993. clusters, performed at the Universitat de Barcelona, revealed behaviour consistent with quantum effects, with crossover temperatures between the thermal-activation and non-thermal regimes of several Kelvin. The Barcelona samples comprised magnetic clusters in thin film alloys of Fe/Sm [6,71, CdSm and Fe,Tb [8]. Similar experiments at the CNRS in Grenoble on 15 nm particles of ferromagnetic Tb,,%,Fe, showed similar effects, with a crossover temperature - 1.5 K [91. The most recent experiments in Barcelona, on Fe/Cu multilayer systems comprising 5 A layers of Fe sandwiched between 50 8, layers of Cu(lll), show magnetic viscosity properties consistent with macroscopic quantum tunnelling at temperatures as high as 5 K [lo]. In order that this possibil- ity be thoroughly examined, it is necessary to collate as much infcxmation as possible on the material. We have therefore conducted an 57Fe Mossbauer study of two Fe/Cu multilayer system of the type used in [lo]. Experiments using alternate techniques, such as transmission electron microscopy, are planned. EXPERIMENTAL The Fe/Cu multilayers were prepared by thermal evapor- ation onto a mica substrate in a vacuum of torr. One set of specimens was prepared by Dr G Fratucello and co- workers at the Universiti di Ferrara [ll], and a second set was prepared in Barcelona. The Fmara samples were grown to two specifications: (i) a 500A undfrlayer of crystalline Cu(ll1) coated with 7 bi-layers of (5A Fe + 50A Cu), and (ii) a 50A Cu underlayer with a coating of 100 bi-layers of (5A Fe + 50A Cu). The Barcelona samples comprised a 20081 Cu underlayer and 18 bi-layers of zyxw (58, Fe + 50A Cu). followed by a covering layer of 200A Cu. Two Miissbauer absorbers were prepared. Sample 1 con- tained the Ferrara multilayers: two 18mmxlOmm sections of type (i) and an 18mmxllmm section of type 6). Sample 2 contained the Barcelona multilayers: twelve 12mmx 12" sections. The total Fe thicknesses were therefore 570A for sample 1 and 10808, for sample 2. All the samples were grown using natural Fe. Transmission 57Fe Mossbauer spectra were recorded using conventionalconstant acceleration spectrometers.A triangular drive waveform was used, and the spectra were folded to 0018-9464/94$04.00 0 1994 IEEE 1 - . ..