SPIN WAVE RESONANCE ON PdFe ALLOYS

Spin wave resonance has been performed on PdFe films vacuum evaporated onto glass substrates with the iron concentration ranging from 0.5 to 26 atomic per cent. Measurements were made at 33 GHz at temperatures of 4.2 to 8.1 OK. The high order modes are quadratically spaced for the high iron concentration films, but the spacing becomes nearly linear for the lower iron concentrations. The g-values are found to exhibit a concentration dependence and display a minimum at approximately 18 atomic per cent iron. The incipient ferromagnetism of palladium metal and the consequent existence of a ferromagnetic state in the palladium-iron alloys of low iron concentration has created considerable interest in the magnetic properties of these alloys. Crangle and Scott [I, 21 have measured the composition dependence of the saturation magnetization and the Curie temperature and also found a giant moment of approximately 11 Bohr magnetons per iron atom for a 1 at. % iron alloy, with the giant moment decreasing with increasing iron concentration. Magnetic diffuse neutron scattering experiments by Cable, Wollan, and Koehler [3] have shown that the moment residing on the iron atom itself is approximately 3 Bohr magnetons independent of alloying and that the remainder of the giant moment resides on the palladium atoms. Further neutron scattering work by Low and Holden [4] and Hicks, et. al. [5] has revealed the range of the palladium polarization to be about 10 A for very dilute alloys. A theoretical calculation by Doniach and Wohlfarth [6] of the dynamical magnetic properties of the palladium-iron alloys based on an s-d interaction model has predicted a magnon dispersion having both acoustic and optical branches and an exchange stiffness which increases linearly with increasing iron concentration. Neutron small angle scattering experiments by Stringfellow [7] have verified this compositional dependence for low iron concentrations but reveal an exchange stiffness four times larger than the theoretical figure. Exchange stiffness measurements have also been conducted by Oder [&I using temperature modulation techniques. Fischer, Herr, and Meyer 193 have performed ferromagnetic resonance on the related nickel-palladium alloys ranging in composition from 0 to 90 at. % palladium. They have concluded that their g-value results can be represented by a tightlycoupled two sub-lattice model using constant g-values for nickel and palladium (gNi = 2.18 and g,, = 2.58). We report here the results of ferromagnetic and spin wave resonance studies on palladium-iron alloy (*) Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR number 68-1484. films with compositions ranging from 0.5 to 26 at. % iron. The films, 1 000 to 4 700 A in thickness, were vacuum deposited onto clean glass substrates heated to approximately 100 O C . Alloy sources, prepared from 99.99 % pure iron and 99.99 % pure palladium, were heated in an alumina coated tungsten filament crucible. Pressures during evaporation were on the order of torr. Film compositions were determined from the saturation magnetization data of Crangle [l] using magnetization values obtained from the ferromagnetic resonance data. Resonance measurements in both the parallel and perpendicular field orientations were performed using a reflection cavity microwave spectrometer of standard design and field modulation techniques. The klystron source was frequency stabilized on the cavity at 33.4 Ghz. A liquid helium cryostat and temperature controller were used to maintain isothermal conditions during resonance runs. Resonance measurements were conducted at constant temperatures ranging from 4.2 to 8.1 OK. From the parallel and perpendicular resonance fields the effective g-values of the alloy films may be calculated. The results of such a calculation, depicted as curve A in figure 1, display two striking features. 1.90 0 20 40 60 8 0 100 ATOMIC PERCENT IRON IN PAUAOIUM FIG. 1. Compositional dependence of g-values for palladium (B) and palladium iron alloys (A). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711193 C 1 566 D. L. HARDISON AND E. D. THOMPSON First, the effective g-value displays a minimum at about 18 at. % iron. Second, the effective g-value varies rapidly with composition in the range less than 10 at. % iron and precludes an extrapolation to pure palladium. Curve A in figure 1 has been extrapolated over the entire composition range taking 2.1 as the g-value of pure iron. We have analyzed the g-value data using as a model a two sub-lattice system having strong exchange coupling between the two sub-lattices. The assumption of strong coupling appears valid since the exchange field for a 0.1 at. % Co palladium cobalt alloy is estimated to be greater than lo5 Oe [lo]. Under this assumption a molecular field equation of motion calculation [ l l ] leads to an effective g-value, geff, for the alloy In equation (1) palladium and iron form the two sublattices with their corresponding g-values and magnetizations. Equation (1) corresponds to a weighting of each sub-lattice g-value according to the angular momentum density of that sub-lattice. The fractional sub-lattice magnetization can be calculated using the data of Crangle [I] and Cable, et. al. [3]. Assuming the g-value for iron to be 2.1 and alloy independent, an assumption which appears consistent with the iron moment determinations of Cable, et. al. [3], we may use equation (1) in conjunction with the experimental data to calculate the gvalue for palladium. The results, shown as curve B in figure 1, indicate the palladium g-value to be alloy independent for iron concentrations greater than 25 at. %. Below 25 at. % iron the palladium g-value increases rapidly with decreasing iron content, This behavior correlates with the decreasing palladium atom moment for alloys with less than 25 at. % iron and the palladium moment saturation at 0.34 Bohr magnetons for disordered alloys having greater than 25 at. % iron [3]. If this moment arises from the splitting of the d-conduction bands of palladium, then we are led to the conclusion that the palladium g-value depends strongly on this band splitting, reaching the almost free electron value for widely split bands. This result contrasts sharply with the palladium-nickel alloy results of Fischer, et. al. [9]. Resonance measurements performed in the perpendicular field orientation display evidence of spin Refer waves at all iron concentrations. The spin wave modes appear discrete, however, only for iron concentrations greater than about 12 at. %. Figure 2 depicts typical