2. Rare earth-transition metal systems
2.3.2. Ternary alloys
2.3.2.2. Gadolinium-cobalt-gold films
In the same way Chaudari and Herd (1976) and Herd (1976) have studied G d - C o - A u films. For Q > 1 they showed strip contrast due to Bloch line walls and the formation of a dense rafter of unichiral bubbles (packing with hexagonal order) with increasing perpendicular to in-plane ratio. For Q ~ 1 the films exhibit circular domains with multiple (eight) Bloch lines (materials with lower Q exhibit only two Bloch lines). Herd (1979) has shown that the magnetic domain struc- tures in GdlTCo76Au7 films prepared by bias sputtering exhibit a dependence on the bias voltage.
Hischii et al. (1977) have prepared by rf sputtering a specimen with a perpendicular uniaxial anisotropy. They observed various kinds of bubble domains under a bias field: unichiral bubbles, bubbles with two or four Bloch lines, and also maze domains for thicker films, and white and black bubbles for higher bias field. They discussed the variation of the intensity of the wall image as a function of the angle between the easy axis and the film normal.
66 M. GASGNIER 3. Rare earth-noble metal systems
3.1. The copper systems 3.1.1. Binary systems 3.1.1.1. Amorphous state
3.1.1.1.1. Gadolinium-copper alloys
The first magnetic properties for such materials have been reported by Heiman and Lee (1976), who prepared samples by sputtering. The measurements on three different compositions show a decrease of Tc (53 to 35 K) with increasing Cu content (0.76 to 0.86 at%). They conclude that the R - R exchange coupling is ferromagnetic with an exchange constant about half as large as that in Gd metal or Gd alloys. Mizoguchi et al. (1977b) and McGuire et al. (1978) have established that alloys deposited by rf sputtering exhibit at low temperature a magnetic phase diagram quite close to that of GdxAll-x. The GdxCul-x system has a spin-glass behavior (with a wide variation of exchange interaction) for x = 0.21 and 0.31, and a ferromagnetic behavior for higher concentrations, with intermediate states. The structures are discussed in terms of partially ferro- magnetic regions, small ferromagnetic clusters and random anisotropy effects.
Popplewell and Charles (1978) have shown that alloys which are prepared by vapor quenching deposition are metastable and always amorphous after deposi- tion at 77 K and annealing at 293 K. They concluded that these materials are ferromagnetic, and that the added Cu decreases Tc b y simply increasing the Gd-Gd distance. Heiman and Kazama (1978) have determined the magnetization and the Curie temperature of alloys prepared by co-evaporation with 30 to 60 at% Gd. As Mizoguchi et al. (1977) and Heiman and Lee (1976) they found that their samples were not saturated at low Gd contents. Except for 40 at% Gd, the magnetization is near the expected value. The calculation of Tc using a percolation model is shown to be consistent with the experimental results, i.e. T~
increases nearly linearly with the Gd content, with the two end values being 0 K at x = 0.25 and 200 K for pure amorphous Gd (for crystalline Gd, Tc = 290 K).
3.1.1.1.2. Terbium-copper alloys
Heiman and Kazama (1978) have noticed that the magnetization is reduced by local anisotropy effects (such as lack of saturation) for TbCu alloys.
Boucher (1980) has studied amorphous (Tb0.25Cu0.75) thick films (20-40/xm), which were prepared by sputtering, using neutron or X-ray small angle scatter- ing, He concluded that magnetic short range ordering occurs at temperatures much higher than the asymptotic Curie temperature, and that there is a micro- scopic atomic structure as a dense stacking of atoms (the film contained 2 at%
Ar).
3.1.1.1.3. Dysprosium-copper alloys
Coey and von Molnar (1978), by rf sputtering deposition, have studied a DyCu alloy with the following composition: OyCul.n4Ar0.0500.23. They observed a shar- ply defined spin-freezing temperature at 18 K marked by a cusp in the low-field dc susceptibility (as for the Gd-A1 system). They developed and discussed a
RE ALLOYS AND COMPOUNDS AS THIN FILMS 67 model to explain the spin-glass behavior. Tissier et al. (1980) also observed the spin freezing transition at 18 K for DyCu amorphous films (41 at% Dy). At 100 mK they recorded the hysteresis loops which show large Barkhausen jumps giving direct evidence of asperomagnetic (random ferromagnetic) domains con- taining ~ 10 TM Dy atoms.
Heiman and Kazama (1978), as for terbium, noticed a reduction of the magnetization of DyCu and HoCu samples.
3.1.1.1.4. Magnetic and transport properties
The magnetic moment, spontaneous Hall effect and resistivity of amorphous films such as Pr0.4Cu0.6 and Rl-xCu~ (R = Pr, Nd, Gd, Tb, Dy, Ho, Er and Tm) have been studied by McGuire et al. (1977a, 1979). In the later paper the authors conclude that alloys with Nd, Gd, Tb, Dy and Ho are ferromagnetic at 4.2 K, with a Curie temperature below 25 K, except for Gd, for which it is near 77 K.
The magnetization is one third to one half of the expected saturation, except again for Gd. The highest resistivity is obtained for GdCu films. The Hall resistivity is positive except for Gd and Tb, and the calculated values have the same sign as those for the pure elements. The ratio PH/P at 4.2 K is shifted toward positive values, but the authors can only speculate about this shift, and they think that its origin is due to a polarization of Cu by the rare earth moments.
3.1.1.2. Crystalline state
Domyshev et al. (1974) have given some properties (crystalline magnetic and magnetoelastic) for GdCu2 thin films (1000-1500 ]k) prepared by thermal evaporation. The films have the MgCu2-type structure. The'authors show, by comparison with GdMn2, GdFe2, GdCo2 and GdNi2 thin films, that GdCu2 has the highest Young's modulus, resistivity and density of dislocations.
3.1.2. Ternary systems
3.1.2.1, Amorphous state (gadolinium-( cobalt )-copper alloys)
Such samples have been studied for magnetic bubble devices and magneto- optic applications.
Chaudari et al. (1974) have prepared GdCoCu amorphous materials by rf sputtering. Their films are richer in Co, because there is a Gd resputtering effect.
They have observed that a "drive field" for bubble domain propagation in T-bar devices is proportional to the magnetization. They have studied particularly the magnetic quality factor and the exchange stiffness versus the temperature.
Potter et al. (1976) have measured the bubble velocity as a function of drive field for bubbles in rf sputtered films. They have shown that magnetic annealing lowers the coercivity. They have measured the wall mobility and dynamic coercive force and presented the first conclusive data on the existence of gyrotropic deflection in an amorphous material. Minkiewicz et al. (1976) have shown that for samples prepared by rf sputtering, Cu dilutes the transition metal sublattices much differently than Mo. But overall the static coercivity in as-deposited films is larger than one would desire for device applications, and it is lowered by annealing treatments (Potter et al., 1976).
68 M. GASGNIER
The influence of the deposition parameters on the properties of the samples have also been studied. Burilla et al. (1976) have used a multiple-target method for sputtered amorphous films. They speculate on the exact mechanism that determines Ku. Albert and Guarnieri (1977) have studied the influence of biased magnetron deposition parameters (target current, sputtering pressure and sub- strate bias voltage). This technique yields a film composition close to the one of the target. The authors show the dependence of 47rM, Ko and Q on dc target current for different argon sputtering pressures and for 0 and - 1 0 0 V bias voltage. Their specimens deposited with substrate bias show an increase of Ku without addition of Ar and a composition shift. Such results appear to eliminate a possible Ar incorporation mechanism for the anisotropy. Heiman et al. (1978) have shown that the fundamental mechanisms responsible for Ku and the effects of annealing are not understood. For GdCoCu alloys they have observed that the addition of Cu seems to increase Ku after annealing.
One should notice that Potter et al. (1976) did not observe a significant change in Ku after annealing, but on the other hand, Heiman et al. (1978) have shown a large change in magnetization after annealing. They have tentatively explained their results by the nature of the chemical bonding of Cu with GdCo alloys for the Ku increase, and by the insolubility of Cu in Co for the 4~rM change (see sections 2.2 and 3.3.2 for a comparison with Mo and Au). In the same way Nishihara et al. (1979) have studied the effects of copper on the magnetic uniaxial anisotropy and compared this effect to that induced by molybdenum.
These authors have reported that Cu as well as Gd atoms are easily resputtered in bias sputtering of GdCoCu films. So, the anisotropy is larger than that of GdCoMo and changes greatly with annealing. They conclude that the magnetic anisotropy strongly depends on the resputtering probability of copper.
As for R - T M amorphous films, the various authors have given numerous magnetic results relative to GdCoCu materials. But it seems evident that the various experimental procedures used to examine their fundamental properties induce different mechanisms which are responsible for the varying results (Heiman et al., 1978).
3.1.2.2. Crystalline state (SmCos_xCux alloys)
Theuerer et al. (1969) have studied SmCo3.65Cul.35 films deposited by vapor deposition. Unfortunately the authors have only noticed the existence of a fine grained structure, with no mention of the crystalline one. However, their results are interesting because they observe a very high coercive force of 30 000 Oe (12 000 for the bulk) for a thin film (4000 A) deposited onto glass substrates heated to 770 K. A thick film (5/xm) exhibited a coercive force of 13 350 Oe. They attribute this large value to the magnetic anisotropy and to the fine grained structure.
Shelkovnikov et al. (1976), for (Sm, Gd)Cos_xCux films prepared by flash evaporation onto substrates heated to 470 K, have observed large magnetic after effects. They discuss th~ origin of such properties and show that these effects are characteristics of high coercivity in the materials.
RE ALLOYS AND COMPOUNDS AS THIN FILMS 69 3.2. The silver systems
3.2.1. Amorphous and crystalline gadolinium-silver alloys
Bates et at. (1970a,b), Charles et al. (1971, 1973) and Popplewell et al. (1975, 1977, 1978) have studied the properties of ferromagnetic GdAg alloys prepared by vapor-quenched evaporation. They have investigated the alloys series from 0 to 100 at% Gd. In this range they notice that alloys with 76-100 at% Gd are amorphous, and for Gd content less than 66 at% there is formation of a fcc structure with a lattice parameter increasing from 3.94 to 4.04 A (pure Ag), These authors have studied the amorphous phase by RDF and defined a binary hard-sphere model. From ESR experiments they have studied the variation of Tc with composition. They report various changes (increase or decrease) of this parameter versus the Gd content and the substrate temperature during the deposition. A special feature is observed for 87 at% Gd (also defined by RDF):
for such a composition T~ is equal to 40 K. Hauser (1975) has reported that amorphous GdxAgl_~ (20 ~ x ~<70) prepared by sputtering onto cold substrates (77 K) are ferromagnets. When the Gd content increases from 20 to 100, Tc linearly increases from 20 to 253 K. The pure Gd films are quasi-amorphous.
(Notice that for pure amorphous Gd films, Heiman and Kazama (1978) report a Tc of 200 K, while for polycrystalline Gd Tc = 290 K.)
3.2.2. Amorphous thick films 3.2.2.1. Neodymium-silver alloys
Friederich et al. (1979) have reported the first data obtained by E P R for non-s rare earth ions (Nd3+), e.g. Nd17Ag83, which is paramagnetic down to 4.2 K. The experiments were done at 293, 100 and 4.2 K in a magnetic field of 1000 to 5000 G, and showed the presence of some complicated structure. The results are discussed in terms of the g factor. The resonance observed above 100K indicates the existence of sites having a non-axial crystal field. The authors think that E P R is promising for investigation of the electrical field gradients (or crystal fields) in amorphous rare earth alloys (see next section for transport properties).
3.2.2.2. Heavy rare earth-silver alloys (R = Gd to Er)
Magnetic properties, such as suceptibility, magnetization, coercive field and Curie temperature, have been compared with the crystalline state properties for such alloys as Gd54Ag46, Tb48Ag52 and (Dy, Ho or Er)xAgl-x, in the form of sheets (30-100/xm) prepared by diode sputtering (Boucher, 1976, 1977a,b). The amor- phous samples show a spontaneous magnetization at low temperatures cor- responding to a ferromagnetic state (To = 122 and 64 K for GdAg and TbAg, respectively), the crystalline state is antiferromagnetic. The absence of aniso- tropy in GdAg alloys does not permit one to observe the hysteresis effect. For TbAg, DyAg and HoAg alloys such a phenomenon is only observed at tem- peratures less than 40, 15 and 8 K, respectively. The author interprets the low temperature magnetic properties (magnetic ordering and magnetic after effect) in terms of exchange interaction, anisotropy and applied fields. He believes that a complex amorphous state involving both ferromagnetic and antiferromagnetic
70 M. GASGNIER
interactions exists. Boucher and Barbara (1979) have made a detailed in- vestigation of the magnetic after effect in TbsEAg48 sheets: this phenomenon exists only in a peculiar range of the magnetization (medium applied fields), is due to the reversal of the moments and can be represented by an activation energy law. Chappert et al. (1980) have done M6ssbauer experiments to study the relaxation effects in crystalline and amorphous DyAg alloys. In the amor- phous state the magnetic interactions are predominantly positive with T~=
18 K, but there is no collinear magnetic arrangement. The interpretation of the relaxation effects is discussed in terms of the spin-lattice relaxation, spin-flips between two different spin systems and mutual spin-flip models. Asomoza et al.
(1980) have studied the extraordinary Hall effect (EHE) for RsoAgs0 alloys (R = Pr, Nd, Gd, Tb, Dy and Er), prepared by sputtering. They report numerous data about the resistivity, the Hall resistivity, the magnetization per ion, the normalized Hall resistivity, the magnetic moment at saturation and the nor- malized Hall angle. They conclude that EHE varies through the rare earth series approximately as for rare earth impurities in silver, and that it is due to orbital exchange and 5d spin-orbit interactions. Ousset et al. (1980) have observed in (Gd, Dy or Er)50Ags0 an up-turn of the resistivity at low temperature and a negative magnetoresistance. Their results indicate a coherent exchange scatter- ing with a predominant contribution from anti-ferromagnetic pairs. They have tested their results with a ternary alloy such as AgsoLus0-xDyx (x = 50 and 35) and shown that the resistivity increase at low temperature is larger for x = 50.
The magnetoresistance is also larger for the same composition (see also Asomoza, 1980).
3.2.3. Dilute thin films: dysprosium or erbium-silver system
Barberis et al. (1977) and Pela et al. (1980, 1981) have studied the stress effects on the electron spin resonance (ESR) spectra (at 1.5 K) of diluted Dy or Er (1 to 2 at%) in epitaxially grown single crystal or microcrystalline oriented Ag thin films. They report that the numerous data about the anisotropic g-value, the line width and lineshape can be explained by the mixing of excited crystalline field levels with the ground state--these admixtures are caused by the planar strain induced by the difference in the thermal expansion coefficients between the film and the substrate. The authors estimate the lower limits for the tetragonal and trigonal second order orbit-lattice coupling parameters. Numerous mathematical results support their conclusions. They discuss the results of Dodds and Sunny (1978) and think that these authors, who studied microcrystalline films, have confused the tetragonal second order OLCP with the trigonal one.
3.3. The gold systems 3.3.1. Binary systems
3.3.1.1. Amorphous thin films
McGuire and Gambino (1979, 1980) have reported numerous data on transport properties for several RxAUy (R = Pr to Tm) alloys. They made the following observations: Tc is below 20 K for all the elements, except for Gd (To -~ 100 K);
RE ALLOYS AND COMPOUNDS AS THIN FILMS 71 only Gd alloys have almost full magnetization, while it is one-third or one-half of the expected saturation for the other alloys; the resistivity ranges between 90 and 230 txf~ cm for HoAu and NdAu, respectively; the spontaneous Hall effect, which is proportional to the paramagnetic 0 value, is positive for the Pr to Eu alloys and negative for the heavy rare earth alloys. They propose a model to explain the last parameter by including both the total spin of the rare earth ion and a factor which contains the exchange evergy via the conduction electrons.
They have shown that such a model is valid for numerous GdxMl-x (M = Mg, Si, Ti, V, Cr, Ge, Nb) alloys, which are ferromagnets (some data are reported in sections 2 and 7). The ratio pH/O increases linearly with 0, the only exception being the GdAu alloy. However, there are some discrepancies with the first results given by McGuire et al. (1977a,b) for several pieces of data: for example, they reported for the Tb47Au53 alloy a resistivity of 385/x~ cm, while in their 1979 and 1980 papers it is equal to 190/x12 cm; the authors do not explain this rather large change.
3.3.1.2. Terbium-gold alloy
Boucher (1977c) has reported the magnetic susceptibility, the Curie tem- perature (32 K) and the hysteresis loops below 15 K for a Tb52Au48 sheet alloy prepared by diode sputtering with high deposition rate. The author notices that the first-neighbour R-R interactions are ferromagnetic and predominant, that the second-neighbour ones are antiferromagnetic, and that this explains the spon- taneous magnetization of this alloy.
3.3.1.3. Amorphous and crystalline gadolinium-gold system
Bostanjoglo and R6hkel (1971) have obtained amorphous samples by evaporation "in situ" in an electron microscope at low temperature (30 K) onto carbon foils. By annealing they observed at 160 K, as a critical temperature, the formation of small crystallites. They discuss their results in terms of ferro- magnetic clusters. At low temperature clusters are aligned forming a super- ferromagnetic state and at the critical temperature they are decoupled forming a superparamagnetic state. Such conclusions are consistent with the magnetic domains observed by Lorentz microscopy (see section 2.3).
3.3.1.4. Dilute thin films: erbium or ytterbium-gold system
Suassana et al. (1977), Gandra et al. (1980a) and Pelh et al. (1981) have studied, as for ErAg, the stress effect on the ESR spectra of Er in Au. They report the lower limits for the tetragonal and trigonal second order OLCP. In comparison to silver the first one is ~-2.5 times larger and the second one is positive (negative for Ag). There is a maximum anisotropy of the g-factor for films with a thickness of 550/~. They discuss their results relative to the sticking effect, the nature of the substrates (quartz or NaC1) inducing deformation, and cracking or sliding effects. Gandra et al. (1980b) report for YbAu positive values for tetragonal and trigonal parameters. For such an alloy they show that the Kondo temperature is low.
3.3.2. Ternary systems
3.3.2.1. Amorphous state: gadolinium-cobalt-gold alloys
As for copper, Chaudari et al. (1974) have reported the magnetic properties of
72 M. GASGNIER
such samples for bubble domain applications. Cuomo and Gambino (1975) and Heiman et al. (1978) have shown the effects of the sputtering parameters (such as substrate bias, target voltage, pressure, system geometry and target com- position) and annealing on changes in magnetization, anisotropy, composition, X-ray diffraction patterns and Ar content in the films. Heiman et al. (1978) interpret their results, in the same way as for copper, by noticing that gold is insoluble in cobalt. Hasegawa et al. (1975b) have studied the magnetic properties of (Gdx_xCox)l_rAu r films prepared by rf sputtering. From magnetization data versus temperature and composition, they discuss the sublattice moments and the spin values of Gd and Co and the exchange constants, and they con- clude that the origin of the perpendicular uniaxial anisotropy is due to cobalt atomic pair ordering promoted by a dipolar energy. Ku has larger values in GdCoAu alloys than in GdCoMo alloys. McGuire et al. (1977b) have given some transport data for an alloy such as CoToGd19Au10. They have observed that pr~
shifts and this can be interpreted by inhomogeneous sample composition, and Tcump is not well defined.
Miyama et al. (1978) and Matsushita et al. (1978) have reported numerous magnetic data for GdCoAu films prepared by rf sputtering and coated with a permalloy (NiFe) film. Near Tcomp (285 K) they determined the dependence of He. Miyama et al. (1978) studied the dependence of the domain wall velocity on the presence of an overlayered permalloy film, the temperature and the drive field. They measured wall velocities up to 35 000 cm s -1 and showed that the saturation of the wall velocity is suppressed by the permalloy overlay. Mat- sushita et al. (1978) showed that Toomp is lowered by about 5 K by coating of a permalloy film. They also found numerous domain patterns for different mag- netized states.
We remind the reader briefly of the work done by Lorentz microscopy: Herd (1976, 1979), Chaudari and Herd (1976), and Hishii et al. (1977) have studied GdCoAu samples in order to observe magnetic bubbles (see section 2.3).
3.3.2.2. Amorphous and crystalline state: gadolinium-iron-gold alloys
Gasgnier (1981) has studied the evolution of GdFe amorphous films over- layered by a gold film. In X-ray diffraction two line systems are observed: one for amorphous G d - F e alloy (broad haloes) and one for Au. Upon ageing at ambient atmosphere there is no transformation to the crystalline state. Upon heating of different gold films of various thicknesses with an electron beam in an electron microscope, the samples crystallized. In the first stage of the annealing three ring systems which correspond to C-Gd203, Fe and Au were found. Upon further heating there is recrystallization and the following structures have been defined:
- fcc (a = 3.60 A);
o
- fcc (a = 3.75 A) (with the thicker Au films);
- primitive cubic (a = 7.60 ]k) (with the thinner Au films);
- hcp (a = 3.70 A, c = 6.05 ]k);
- an unknown system with the following interplanar distances: 7.11, 5.69, 5.53, 4.35, 2.97, 2.60 and 2.36 ,~.