Abstract: Magnetic properties of rare earth−transition metal amorphous thin films
Authors: Lee Kenneth , Heiman Neil 
Published: July 2002

Considerable interest has been created recently in the preparation and magnetic properties of rare earth(RE) −transition metal(TM) binary amorphous thin films. The specific binary systems of primary interest have been Gd−Co1 for its induced uniaxial anisotropy, Ku, and its associated properties which make it attractive for magnetic bubble and magneto−optic devices, and TbFe22 for its large coercivity, magnetostriction, energy product, and permanent magnet applications. In addition to their technological significance, their properties have resulted in the two prevailing, but contradicting, theories of magnetic exchange in amorphous systems, namely the local anisotropy3 and charge transfer4 models. This report presents the results of an investigation in which (1) we systematically studied the magnetic properties of Ho−Co and Ho−Fe amorphous thin films over a wide compositional range in order to judge the validity of the two theories, and (2) we measured the temperature dependence of Ku in Gd−Co, Ho−Co, and Ho−Fe alloys in order to help identify its origin.5

Thin films of amorphous Ho−Co, Ho−Fe, and Gd−Co were prepared by thermal evaporation of the two metals in a vacuum system with a base pressure of ∠7×10−9 Torr. Deposition rates were 2−5 A/sec and the fused quartz and carbon−coated electron microscope grid substrates were precooled to and maintained at either −15 or −150°C. System pressure during deposition ranged from 3×10−7 to 8×10−8 Torr. Compositions varied from 17 at.% Ho to 60 at.% Ho and from 17 at.% Gd to 33 at.% Gd. In addition, Gd−Co films were prepared by rf bias sputter deposition1 utilizing different targets which yielded films with compositions ranging from 16 at.% Gd to 27 at.% Gd. Electron and x−ray diffraction spectra showed the films to be amorphous, and microprobe analysis confirmed the compositions to within 2%. Ho−Ni films were also prepared by evaporation. Only films of more than 85 at.% Ni were magnetic near room temperature, and films with this and more Ni concentration showed signs of microcrystallinity (grain size ∠70 Å). Therefore, the Ho−Ni films will not be discussed here. Polar Faraday rotation measurements at λ = 0.6328 μ as a function of temperature show that for the Ho−Co alloys, the Curie temperatures, Tc, are much higher, and the compensation temperatures, Tcomp, are much lower in the amorphous state as compared to the same compositions in the crystalline alloys. For example, Tc for amorphous Ho0.33 Co0.67 is greater than 500 K, where Tc = 80 K for crystalline HoCo2. On the other hand, Tc and Tcomp of the amorphous Ho−Fe films are drastically lower than the corresponding crystalline alloys. For example, Tc = 270 K for Ho0.33 Fe0.67 in the amorphous state as compared with Tc = 600 K for crystalline HoFe2. These results, when combined with low−temperature magnetization results, clearly show the magnetic moments of Co and Fe to increase in the amorphous state.

The T dependence of the polar Faraday and Kerr rotation hysteresis loops also show that for small temperature ranges, bubble type hysteresis loops are observed, indicating bubbles can be sustained at those temperatures. In the case of a 0.2 μ thick Ho0.33 Co0.67 film, these loops are observed from 200 to 275 K, disappearing near Tcomp = 293 K, and the magnetization relaxes into the film plane above Tcomp. A 0.3 μ thick Ho0.25 Co0.75 film with Tcomp = 233 K showed bubble loops from 280 to 325 K, and at room temperature, Hc ? 5 Oe, H (nucleation) = 360 Oe, H (collapse) = 625 Oe, and 2ϑSAT (Kerr) = 0.57°. Similar hysteresis loops were observed in Ho−Fe, but over a narrower temperature range.

The Gd−Co films prepared by thermal evaporation do not have their magnetization oriented normal to the film plane. That is, either Ku is much smaller or is absent in these films which are prepared under the same conditions as those used to obtain Ho−TM films with Ku. The magnetization being parallel to the plane of the evaporated films is compatible with the result that the sputter−deposited films also possess in−plane magnetization except when a bias voltage is applied during deposition. The Ku in Gd−Co appears to be directly related to a preferential arrangement of the atoms resulting from the resputtering caused by the bias voltage applied to the substrates.

The Ku in Ho−TM evaporated films does not originate from the preferential arrangement apparently produced in bias sputter deposited GdCo. This does not preclude the possibility that the same mechanism may be responsible for Ku in both alloys and that the resputtering process is required in GdCo simply to enhance this mechanism sufficiently to produce Ku ≳ 2πMs2. However, it is also possible that different mechms are involved. In any event, when comparing evaporated Gd−Co and Ho−Co films, the differences in magnetic anisotropy are clearly due to the Ho. In addition, the temperature dependence of Ku in bias−sputtered GdCo indicate that Co−Co pair ordering may not be the mechanism, as previously suggested. 1,4 Furthermore, temperature variation of Ku in evaporated films indicate the rare earth atom to be the major source of Ku. In summary, this investigation has yielded (i) two new amorphous materials which can sustain bubble domains, (ii) evidence that the change in the magnetic properties in these RE−TM amorphous alloys are due primarily to the altered electronic configuration of the TM, and (iii) the indication that Ku may not result from TM−TM pair ordering but rather from the RE atom.