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iron multilayers
M. A
Journal of Magnetism and Magnetic Materials 119 (1993) 141-149 North-Holland
Magnetic and structural properties of cerium/iron multilayers J. T h i e l e a, F. K l o s e a, A. S c h u r i a n a, O. S c h u l t e a, W. F e l s c h
a
a n d O. B r e m e r t b
a I. Physikalisches Institut and b Institut flir Metal~Thysik, Universitiit GSttingen, 3400 Gi~ttingen, Germany
Received "27 April 1992; in revised form 7 July 1992
Ce/Fe multilayers with modulation lengths between 18 and 200 A were prepared by ion-beam sputtering in an UHV system and structurally characterized by X-ray diffraction at small angles and 57Fe conversion-electron Mfssbaner spectroscopy. Good periodicity and sharp concentration profiles at the interfaces are found. The magnetic properties are unusual. For Fe-layer thicknesses below 25 .~, relatively independent of the Ce-layer thickness, the Curie temperature is reduced to values below 180 IC This is accompanied by a distinct decrease of the spontaneous magnetization and the appearence of hysteresis in the low-field susceptibility at low temperatures. Isothermal magnetization curves point to noncollinear magnetic order. Evidently, these phenomena are closely related to the transition in the Fe layers from the bcc cystaUine to an amorphous structure occurring near 25 ~, according to X-ray diffraction at large angles and RHEED diagrams.
1. Introduction Artificial multilayers composed of alternately stacked transition and rare-earth metals exhibit a large variety of novel magnetic properties, such as perpendicular anisotropy (e.g. T b / F e [1]) or coupling of magnetic order of the rare earth across a nonmagnetic transition metal (e.g. G d / Y [2]). The special features of these structures arise for example from the strong magnetocrystalline anisotropy of the rare earth, the rich diversity of their magnetic phases, or from the interplay of their 4f-electronic states with the transition-metal 3d states (the exotic properties of r a r e - e a r t h transition-metal compounds are well known). It is very attractive that the properties of the layered systems can be designed, on a large scale, by varying the relative thickness of the constituents. In this p a p e r we present results for multilayers composed of the very different magnetic elements cerium and iron. The electronic states of Ce in Correspondence to: Prof. W. Felsch, I. Physikalisches Institut,
Universit;it G6ttingen, Bunsenstr. 9, 3400 G6ttingen, Germany. Tel.: +49-551-397620.
intermetallic compounds depend sensitively on its local environment, i.e. on the number, distance a n d / o r chemical nature of the neighbouring atoms. In CeFe2 the Ce-4f states are delocalized and strongly hybridized with the Fe-3d states. As a result, the compound is an itinerant ferromagnet with an unusually low Curie t e m p e r a t u r e (~- 230 K) and saturation magnetization. T h e Ce4f moments are coupled antiferromagneticaUy to the Fe-3d moments, which is anomalous for such r a r e - e a r t h / i r o n compounds with light rare earths. It is a consequence of 4f-orbital-moment quenching by band formation! [3]. At low temperature, the compound has the tendency towards an antiferromagnetic instability, which is enhanced by metal additives substituting Fe sites [4]. Thus it appears interesting to combine Ce and Fe in an artificial periodic stacked-layer structure and study the influence of its length scale on the magnetic properties. It~ is to be expected that, as in other cases, these properties will sensitively depend on the microstructure of the individual layers and their interfaces. So structural characterization is of particular importance for an understanding of this system.
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
142
J. Thiele et al. / Properties of Ce/Fe multilayers
2. Experimental details
The multilayers were prepared by ion-beam sputtering in an ultrahigh vacuum system (base pressure < 5 x 10-10 Torr), using alternately targets of 99.9% pure cerium and 99.998% pure iron. Modulation lengths A =tce -t- tFe, where tce and tFe are the nominal thicknesses of the Ce and Fe layers, were varied in the range between 18 and 200 A. During operation of the ion-beam source the pressure rose to 4 x 10 -5 Torr due to the incoming argon gas (6 N purity), and the partial pressures of oxygen and hydrogen (reactive gases for Ce) were = 4 x 10 -11 and 5 × 10 -8 Torr, respectively. The deposition process was controlled by a computer: film thicknesses were measured by a quartz rate monitor, and at designed values a target change was effected by a step motor while the substrate was protected by a shutter. Typical deposition rates were close to 0.5 .A/s. The total thickness of the multilayers was always 2000 ,A. Layering pattern and total number of bilayers are denoted as [CeX/FeY] X n, where X, Y represent the nominal thicknesses /Ce, tFe in A and n denotes the number of repetition. The substrates were single-crystalline Si wafers with (100) orientation, Prior to sample deposition, they were cleaned by irradiation with lowenergy Ar ions and covered with a buffer layer of Cr which acts as a diffusion barrier. The samples were condensed at liquid-nitrogen temperature to suppress diffusion (a solid-state amorphization reaction has been observed in Ce-Fe sandwich structures during deposition at room temperature, where diffusion of Fe in Ce is involved [5]). Before removal from the sputtering system, the samples were overcoated with 60 .A of Cr to prevent oxidation on exposure to atmosphere. Characterization of the samples regarding periodicity, interface quality and lattice structure of the individual layers was performed by X-ray diffraction, RHEED, resitivity measurements and M6ssbauer spectroscopy. The X-ray diffractometer was operated in the 0-20 reflection mode, both in the low- and high-angle regions, using Cr-K~ radiation. RHEED observations were made in situ, they permit to study the surface
[CeX/FeY]xn
4-J
0
[11/07]x111
•
O
i
l
5
i
i
r
i
I
IO
i
i
i
,
1
15
2 @ (deg.) Fig. 1. Small-angle X-ray diffraction diagrams (Cr K a) for different C e / F e multilayers of = 2000 A total thickness.
morphology of the layers during the condensation process. 57Fe conversion,electron M6ssbauer spectra were obtained at room temperature using a conventional constant-acceleration spectrometer, with a gas-flow proportional counter and a source of 57Co in Rh. Long-time stability of the samples was investigated by Rutherford backscattering spectroscopy (RBS) with 900 keV He 2+. A vibrating-sample magnetometer operated between 4.2 and 300 K in fields up to 50 kOe provided magnetization curves of the samples.
3. Results
The layered structure of the samples is revealed by X-ray diffraction diagrams for small scattering angles (fig. 1) which show a number of peaks associated with the multilayer periodicity. The modulation lengths A calculated from the position of these peaks agree, within a few per-
J. Thiele et al. / Properties of Ce/Fe multilayers
cent, with the nominal ones designed using the thickness monitor. The appearance of reflections to relatively high orders, in particular for large values of A, prove rather sharp concentration profiles at the C e / F e interfaces. Even for [ C e l l / F e 7 ] x 111, the sample with the shortest periodicity investigated, the second-order peak can be resolved, indicating that the modulation in composition is steeper than sinusoidal. A detailed comparison of the measured diffraction diagrams with those resulting from a numerical simulation of the multilayer structure involving effects from diffusion and roughness at the interfaces will be published elsewhere [6]. The modulated structure of the samples was also confirmed by RBS experiments. The X-ray diffraction diagrams recorded for large scattering angles (which are not shown here) reveal that the structure of the individual layers depends on their thickness. Up to a critical value to, which is near 25 ,~ for Fe and 70 ,& for Ce, only broad intensity appears, with no evidence of crystalline phases, i.e. the films grow in an amorphous structure. Above this thickness, the (110) diffraction peak of bcc ot-Fe and the (111) peak of fcc ~/-Ce are observed. From the related scattering angles it can be concluded that both the Fe and Ce lattices are expanded by 0.5 to 1% in the direction of growth (in the 0-20 mode, only this direction is probed). There is no signature of the compounds CeFe 2 and CezFe17, allowed by the equilibrium phase diagram, for any of the multilayers. The amorphous-to-(poly)crystalline transition of the individual layers is clearly visible in the R H E E D experiments of the Fe- and Ce-layer surfaces performed in situ. The diffracted intensity in the patterns changes from diffuse to an arrangement of concentric half circles. The structural transformation is also reflected (for the Fe layers) in the variation of the electrical resistivity p and the resistivity ratio p (300 K)/p(4.2 K) with the modulation !ength A (fig. 2, tce= tFe = A/2). Up to A = 60 A, where the multilayers are entirely amorphous, the resistivity is near 150 ~ c m , and the resistivity ratio is below 1, i.e. the temperature coefficient is negative, as is frequently observed for amorphous alloys. Upon
143
160
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•I ' " ' i .............
~' 100 0 0
I]"
--
)
0D
. . . .
1
I
0
70 40
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I
I
40
80
I
I
120 :1.60 200
Fig. 2. Electrical resistivity at 300 K and resistivity ratio of = 2000 ,~ thick C e / F e multi!ayers as a function of the modulation length A = t c e + tFe. T h e lines are guides to the eye.
crossing the critical thickness t c for crystallization of the Fe layers, p drops rather steeply and the temperature coefficient changes sign. The magnetic properties of the C e / F e multilayers are unusual. Fig. 3 shows the magnetization data for different, modulation lengths A of equal thicknesses of the Ce and Fe layers, tce= tFe = A / 2 , with the same total thickness of the samples. For these measurements, the magnetic field was applied in the plane of the films. Note that the amount of Ce and Fe is the same in each case. As is immediately apparent (fig. 3a), the magnetization curves at 4.2 K are not a simple summation of the individual Fe-layer magnetizations. While the magnetizaton of the samples with single-layer thicknesses A / 2 > 30 A saturates readily in a moderate field, with no measurable high-field slope, all of t h e samples with smaller modulation lengths exhibit a considerable slope at high fields in their magnetization curves, which fail to saturate even in 50 kOe at 4.2 K. Fig. 3b shows the magnetization Ms(0) obtained by extrapolating the high-field data to zero field ("spontaneous" magnetization hereafter) as a function of temperature. For large thicknesses A / 2 > 50 ,~ the spontaneous magnetization of the multilayers, if attributed to the Fe part only, approaches that of bflk Fe. But Ms(0) at 10w temperatures drops rather abruptly when A / 2 decreases to values be53w 30 A. This is accompa-
J. Thiele et al. / Properties of Ce / Fe multilayers
144
nied by a precipitous fall of the Curie temperature T¢. Values estimated from Arrott plots are below 180 K. As is evident in fig. 3, Ms(0) increases again when the smallest modulation lengths are approached. Variation of the individual layer thicknesses has shown that the thicknesses of the Ce layers is of minor importance for the observed drop of Ms(0) and Tc, i.e. the effect is mainly governed by the thickness of the Fe layers and evidently occurs at their transition to an amorphous structure. The marked change in the magnetic properties of the multilayers with relatively small changes in A is further documented by the appearance of considerable hysteresis and coercivity at low temperature. While for tFe > 30/~ the coercive fields are near 100 Oe between 4.2 K and room temper-
1000
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X=Y=50 800
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600
& 200
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100
T(K)
200
300
Fig. 3. Magnetization M vs.magnetic field H applied in the film planes at 4.2 K (a) and "spontaneous" magnetization M,(0) vs. temperature T (b) for mu!tilayers [CeX/FeY].x n of equal Ce- and Fe-layer thicknesses X = Y and -- 2000 A total thickness. Curie temperatures resulting from Arrott plots are Tc = 177 K ( X = 10), 165 K ( X = 1 5 ) , 158 K ( X = 22). The bulk-Fe value of M,(0) at 4.2 K would be 1743/2 emu/cm 3. The lines are guides to the eye.
850
I
I
~°°°°°o0OOo0o oO
~*****..... ~, ~,~0,,~,,. E501503x20 *~e~**,, f,c.
,., 750
~ 200 ~ [ 1 0 / 2 7 3 x 5 4 H=gO Oe
:~ loo 0
,
0
I
~
100
I
200
300
T0~) Fig. 4. Magnetization M in an applied field H = 90 Oe of C e / F e multilayers as a function of temperature T. The samples were cooled in zero field (z.f.c.) and in the measuring field H (f.c.). The Curie temperature of the samples 50/50 and 10/27 are above room temperature, and Te = 164 K for the sample 30/27.
ature, they raise to 1 kOe at 4.2 K for smaller modulation lengths, declining rapidly with temperature to 100 Oe near 20 K. Fig. 4 shows that the low-field magnetization depends on the magnetic history of the samples. The irreversibility observed is particularly pronounced for the multilayers with tFe = 27 A, where the magnetization at 4.2 K differs by about 50% for the field-cooled and the zero-field-cooled cases. Measurements with the magnetic field applied perpendicular to the planes of the films revealed that the easy axis of the magnetization is in the film planes, for all of the samples investigated, as a direct consequence of shape anisotropy~ While for large modulation lengths A (tFe > 30 A) the anisotropy fields are close to the demagnetization field 4"n-Ms, they are considerably larger for small A, i.e. for the low-Tc samples. M6ssbauer spectra were obtained at room temperature in zero magnetic field for samples with Fe-layer thicknesses oabove and below the critical value, tc,Fe-----25 A, for the structural change in the Fe layers. Data for [Ce30/Fe35] × 31, [Ce30/Fe27] × 35 and [Ce10/Fe27] × 54 are shown in fig. 5, together with a reference spectrum of a 1000 A thick Fe film prepared under the same conditions. This latter spectrum
145
J. Thiele et al. / Properties o f C e / F e multilayers
(fig. 5a) compares well with that of bulk bcc Fe, but shows some line broadening due to the disorder induced by the low deposition temperature of the sample. The two spectra in figs. 5b and d are magnetically split revealing the ferromagnetic nature of the samples. In contradistinction, the spectrum in fig. 5c consists of an asymmetric quadrupole-split doublet of a paramagnetic sample. This behaviour is in agreement with the room-temperature magnetization curves. Note that the loss of ferromagnetism with decreasing Fe-layer thickness at constant Ce-layer thickness (figs. 5b,c) is rather abrupt, and that on diminishing the Ce-layer thickness for the samples with 27 A thick Fe layers (figs. 5c,d) the Curie temperature is raised to above room temperature. The magnetization of this latter sample saturates only above 30 kOe, which is an indication of the close proximity of the individual Fe layers to their critical thickness tc,Fe.
b
,
C30/35]x31
.Id
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° 2 -8
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0
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4
8
Velocity (ram IS) Fig. 5. Conversion-electronM6ssbauer spectra taken at room
temperature for three different Cc/Fe multilayers (total thickness ~ 2000 ,~). Shown for comparison is the spectrum of a 1000A thick Fe f~m (a).
Table 1 Hyperfine fields Hat at room temperature, relative fraction of 57Fe sites and partial Fe-layer thicknesses t i for multilayers [CeX/FeY]×n and a 1000 A thick Fe film ("bulk Fe") resulting from the conversi0n-electron M6ssbauer spectra in
fig. 5 Hi
bulk Fe
(kOe) 330
Ri
t i = RitFe
1
(A) 1000
[Ce30/Fe35] X31 site 1 325 site 2 285 site 3 -
0.39 0.41 0.20
14 14 7
[Ce10/Fe27]x 54 site 1 325 site 2 299 site 3 -
0.32 0.37 0.31
9 10 8
The procedure employed in fitting the spectra of the ferromagnetic samples was based on using the minimum number o f Fe sites needed to obtain a good representation of the data. The inner lines exhibit a pronounced left-right asymmetry suggestive of a nonzero quadrupole interaction. Furthermore, a certain distribution of magnetic hyperfine fields can be expected. In order to approximate the distribution and the asymmetry we used a simple three-component model to fit the spectra. Each contponent involves a site of the 57Fe nuclei with a differing local surrounding. The (mean) hyperf'm~ fields Hhf obtained and the relative fraction o t 57Fe sites, R i (i ffi 1, 2, 3), resulting from the relative intensity contribution of the subspectra are given in table 1. The subspectra are: (i) A six-line pattern with the same hyperfine field as in bulk bcc Fe (site 1). (ii) A broadened six-line pattern with a mean hyperfine field depleted by -- 10% (site 2) as compared to the :former one. This compo, nent gives rise to the shoulders of the outer lines. The fit results in a considerable linewidth and thus indicates a distribution of hyperfme fields. (iii) A doublet representing the effect from the interaction of the 57Fe nuclear quadrupole
146
3".Thiele et al. / Properties of Ce/ Fe multilayers moment with the electric-field gradient, in a noncubic environment (site 3).
The quadrupole-split doublet of the paramagnetic sample (fig. 5c) could be represented by a simple two-single-line fit, yielding a mean quadrupole splitting of 0.4 m m / s and a mean isomer shift of -0.1 mm/s. Similar values were found for the site-3 subspectra of both ferromagnetic samples. The negative sign of the isomer shift implies an enhanced s-electron density at the sites of the 57Fe nuclei involved.
4. Discussion
An important problem to address in view of the magnetic properties of the C e / F e multilayers is the nature of the interfaces. Their role is, a priori, of particular importance, since they represent the region where Ce and Fe are in immediate contact. According to RBS performed on multilayers with tFe > tc,Fe, the crystalline Fe films exhibit island growth [6], but details of their structure are not known. Simple numerical modeling of the small-angle X-ray diffraction diagrams of samples with Fe-layer thicknesses immediately above and below the critical value to,re reveals that (i) local layer-thickness fluctuations and (ii) interfacial diffusion are restricted to nominally 5:1 monolayer [6]. Further information about the interfaces is provided, for the samples being ferromagnetic at room temperature (tF~ > t~,~), by their M6ssbauer spectra which are sensitive to the local environment of the 57Fe nuclei. The sequential deposition of the Fe and Ce layers suggests that site 1 may be attributed to the core part of the Fe layers and sites 2 and 3 to the interface part. If the relative fraction of sites, R i (i--1, 2, 3), is translated into partial thicknesses t i = RitFe (table 1), then on site 3 nominally only ---4 atom layers of Fe are involved, taking both interfaces together. The absence of ferromagnetism on this site and the negative isomer shift point to a mixing with Ce. The formation of compounds at the interface (the equilibrium phase diagram al-
lows CeFe 2 and Ce2Fe17 with magnetic ordering temperatures below room temperature) may be ruled out since cubic CeFe 2 would not show a quadrupole interaction and Ce2Fel7 shows a much larger isomer shift [7] than observed for site 3. A distinct possibility is that Fe has diffused into the amorphous Ce layers (here tce < tclce = 70 A; from simple thermodynamic arguments the smaller Fe atoms are expected to be the diffusing species [8]), or that short-range collisional mixing has occurred at the interface during sputter deposition. As pointed out before, small-angle X-ray scattering reveals that the extension of the intermixed zone is limited to nominally 2 monolayers at each interface [6], in agreement with other rare-earth iron multilayers [9,10]. The equilibrium phase diagram of Ce and Fe rules out the formation of solid solutions, but Fukamichi et al. [11] have shown that amorphous alloys exist for Ce concentrations between 10 and 40% which are paramagnetic at room temperature. The observed mean isomer shift and quadrupole splitting attributed to site 3 are very similar as for a number of amorphous transition-metal-iron alloys [12]. Hence this site is likely to be located in an amorphous Ce-Fe environment. The reduced coordination with Fe atoms at the C e / F e interface, together with the Ce-Fe interaction, must be responsible for the --10% reduction (referred to the core of the Fe layers) of the average hyperfine field attributed to site 2 of the 57Fe nuclei. The relatively large contribution of this site to the total intensity of the spectra, i.e. the large partial thickness 12 of 14 and 10 A of Fe (table 1) is amazing and may indicate that imperfections of the stacked structure are at the origin of the effect. Small-angle X-ray diffraction reveals that the local thickness fluctuations in the individual layers are very small [6] and hence can be discounted as an explanation of the large values of t 2. An enlarged interface area could result from the island structure of the Fe layers suggested by RBS [6], but their morphology remains to be elucidated. A further possibility is that nonlocal Ce-Fe interactions are effective which extend the influence of the interface into the Fe films. The reduction of the hyperfine field attributed to site 2 and the fraco
J. Thiele et al. / Properties of Ce/ Fe multilayers
tion of Fe atoms involved are similar as in D y / F e multilayers, where nominally 12 ,~ of Fe are affected by the influence of the interface [13]. The hyperfine subspectra contributed by sites 1 and 2 of the Fe layers display a line-intensity ratio 3 : 4 : 1 : 1 : 4: 3 and hence reveal that the magnetic easy direction for these multilayers with tFe > t~,Fe is in the film plane. The collapse of the hyperfine field at room temperature of sample [Ce30/Fe27] × 35 (fig. 5c) coincides with the loss of crystallinity of the Fe layers, as is demonstrated by X-ray diffraction and RHEED. The asymmetric broadened doublet, revealing a quadrupole interaction due to a distribution of local electric field gradients at the sites of the Fe nuclei, must contain information on the local symmetry and short range order, but due to the complex mierostructure of the artificial C e / F e heterostructures no attempt was made to analyze these spectra. It would be highly desirable to extend M6ssbauer spectroscopy to lower temperatures into the ferromagnetic state of these multilayers with small modulation lengths A, to shed light on the distribution of the hyperfine fields and the related sites of the 57Fe nuclei. The small-angle X-ray diagrams vary smoothly towards low A on crossing the critical thickness t¢,Ve of the Fe layers for their crystalline-to-amorphous transition (fig. 1), and their numerical simulation reveals that the local fluctuations in the thickness of the individual layers and the interdiffused zone are equal just above and below t¢,F,. Hence the quality of the interfaces must essentially be preserved in this transition. Comparison of the samples [Ce30/Fe27] × 35 and [Ce10/Fe27] × 54 is interesting. It shows that to a certain extent the growth conditions for the Fe layers, i.e. their critical thickness tc,Fe, depend on the thickness of the Ce layers, since according to the structural investigations the individual Fe layers are amorphous for 30 ,~ thick Ce layers but crystalline for the thinner one. The mechanisms which favour growth of an amorphous structure of the individual Fe and Ce layers are largely unknown. Growth of amorphous Fe below a critical thickness seems to be a common observation in many layered structures based on iron and rare-earth metals [13-15]. This may be due to the
147
similar chemical and structural properties of the rare earth. For a sandwich structure of Fe and Gd it has been suggested [15] that the large structural misfit at the interface of the constituents is at the origin of the effect. The same growth properties were found for layered structures of Fe and Y [16,17]. The M6ssbauer spectra in fig. 5 clearly demonstrate that the loss of ferromagnetism at room temperature of the C e / F e multilayers is, as pointed out before, closely related to the crystalline-to-amorphous transition in the Fe layers. The behaviour is consistent with the steep drop of the Curie temperature Tc and spontaneous magnetization M s ( 0 ) o n crossing the critical thickness tc,Fe (fig. 3). As is evident from the appearance of considerable hysteresis in the magnetization curves at low temperatures, of irreversibility in the low4ield magnetization (fig. 4) and from the large slope of the high-field magnetization at low temperature, the magnetic character of the multilayers is distinctly modified below tc,Fe. The magnetic irreversibility is not directly related to the hysteresis, as would be expected for an ordinary ferromagnet. At the temperature where the magnetization begins to depend on the sample history, the coercive field is much smaller than the cooling or measuring field. Hence the onset of irreversibility may indicate a freezing phenomenon, i.e. a transition into a spin-glas-like magnetic state. In fact, the large high-field susceptibility reveals that the material cannot be a collinear ferromagneti As we have pointed out before, the concentration gradient in the multilayers must be very similar immediately above and below tc,Fe, and there must remain a core of pure Fe in the center of the amorphous Fe layers somewhat below t~,F~. Thus the magnetic anomalies are essentially not the result of alloy formation, but of the structural transition in the Fe layers itself. Similar effects found in D y / F e multilayers were interpreted in the same way [13,14]. There is no evidence ifor inter-layer coupling of the Fe moments in the C e / F e multilayers. There has been a 19ng debate on the magnetic properties of pure amorphous Fe. Arguments rely on extrapolations of experimental data obtained for amorphous Fe alloys containing a non-negligi-
148
J. Thiele et al. / Properties of Ce/ Fe multilayers
ble amount of a second element. The saturation moment seems to be related to the polymorphism of bulk crystalline Fe [12,18], but there is evidence that ferromagnetic ordering is noncollinear, presumably due to the presence of antiferromagnetic exchange. It has been argued that the high sensitivity of the distribution of exchange integrals to the nearest-neighbour distance is at the origin of this behaviour [19]. Estimates of the Curie temperature range somewhat above 100 K [19]. The magnetic properties of the C e / F e multilayers just below te.Fe where the structure is entirely amorphous are very similar to those of hypothetical pure amorphous Fe. However, there must be a non-negligible contribution of the interfaces. This can be conjectured from the behaviour above tc.ve, i.e. from the presence of irreversibility in the low-field magnetization (fig. 4, [Ce50/Fe50] x 20), from the reduction of Ms(0) below the value of bulk Fe (fig. 3, [Ce30/Fe30] × 33), or from the presence of the subspectrum attributed to site 2 in the M6ssbauer spectra (fig. 5b,d). Exchange and anisotropy fluctuations experienced by the Fe atoms in the interracial zone due to bonding with neighbouring Ce atoms can be expected to contribute to noncollinearity of magnetic order and to the magnetic irreversibilities. The contribution of the Ce atoms to the interracial anisotropy is not clear. In amorphous Ce-Fe alloys [11], there is evidence that the 4f states of Ce are delocalized, as they are in the compound CeFe z. Hence in the amorphous intermixed part of the interfaces the Ce ions presumably do not carry an orbital 4f moment and cannot give rise to strong single-ion anisotropies frequently encountered in amorphous rare-earthtransition-metal alloys. This agrees with the relatively low coercive field at low temperatures. A t the lowest periodic lengths A, e.g. for [Cel0/Fel0] × 100, where a modulation in composition is clearly present, values of the magnetization Ms(0) and the Curie temperature Tc of homogeneous amorphous Ce-Fe alloys [11] with compositions corresponding to the mean multilayer compositions are approached, which explains the observed increase of Ms(0) and Tc towards low A (fig. 3).
5. Conclusion We have shown that systems with alternately stacked Ce and Fe layers with good periodicity and sharp concentration profiles at the interfaces may be grown by ion-beam sputtering on Crcoated Si wafers cooled to liquid-nitrogen temperature,.for a wide range of modulation lengths (18-200 A). The Fe and Ce layers first grow in an amorphous structure up to a critical thickness, which is near 25 A for Fe and 70 ~k for Ce, and then change to the crystalline bcc and fcc structures, respectively. Samples with Fe layers below the critical thickness, i.e. in the amorphous state, are ferromagnets, presumably with a noncollinear spin structure, with an unusually low Curie temperature and spontaneous magnetization. Many of the properties are similar to those of hypothetical pure amorphous Fe, extrapolated from data for homogeneous amorphous Fe alloys. In addition, the influence of the C e / F e interfaces is clearly visible. There is no evidence of inter-layer coupling of the Fe moments.
Acknowledgements The authors would like to thank T. Kacsich, II. Physikalisches Institut, for RBS measurements, M. Steins, Mineralogisch-Kristallographisches Institut, for help with the interpretation of the small-angle X-ray diffraction experiments, and U. Krebs, Institut fiir Metallphysik, for discussions concerning CEMS. We acknowledge the support of the Deutsche Forschungsgemeinschaft within SFB 345.
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J. Thiele et al. / Properties of Ce /Fe multilayers [6] F. Klose, M. Steins, T. Kacsich, A. Schurian and W. Felsch, to be published. [7] K.H.J. Buschow and J.S. van Wieringen, Phys. Stat. Sol. 42 (1970) 231. [8] A.R. Miedema, A.K. Niessen and K.H.J. Buschow, J. Less-Common. Met. 100 (1984) 71. [9] T. Baczewski, M. Piecuch, J. Durand, G. Marchal and P. Delcroix, Phys. Rev. B 40 (1989) 11237. [10] K. Cherifi, C. Dufour, M. Piecuch, A. Bruson, Ph. Bauer, G. Marchal and Ph. Mangin, J. Magn. Magn. Mater. 93 (1991) 609. [11] K. Fukamichi, H. Komatsu, T. Goto and H. Wakabayashi, Physica B 149 (1988) 276. [12] G. Xiao and C.L Chien, Phys. Rev. B 35 (1987) 8763. [13] K. Yoden, N. Hosoito, K. Kawaguchi, K. Mibu and T. Shinjo, Jpn. J. Appl. Phys. 27 (1988) 1680.
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[14] S. Honda, S. Nishimura and T. Kusuda, IEEE Trans. Magn. MAG-25 (1989) 4027. [15] J. Landes, Ch. Sauer, B. Kabius and W. Zinn, Phys. Rev. B 44 (1991) 8342. [16] T. Morishita, Y. Togami and K. Tsuhima, Proc. Intern. Syrup. on Physics of Magnetic Materials, Sendai 1987, eds. M. Takahashi, S. Maekawa, Y. Gondo and H. Nose (World Scientific, Singapore, 1987) p. 295. [17] S. Handschuh, J. Landes, U. K6bler, Ch. Sauer, G. Kisters, A. Fuss and W. Zinn, preprint. [18] W. Felsch, Z. Angew. Phys. 29 (1970) 217. [19] D.H. Ryan, J.M.D. Coey, E. Batalla, Z. Altounian and J.O. Str6m-Olsen, Phys. Rev. B 35 (1987) 8630.
Journal of Magnetism and Magnetic Materials 119 (1993) 141-149 North-Holland
Magnetic and structural properties of cerium/iron multilayers J. T h i e l e a, F. K l o s e a, A. S c h u r i a n a, O. S c h u l t e a, W. F e l s c h
a
a n d O. B r e m e r t b
a I. Physikalisches Institut and b Institut flir Metal~Thysik, Universitiit GSttingen, 3400 Gi~ttingen, Germany
Received "27 April 1992; in revised form 7 July 1992
Ce/Fe multilayers with modulation lengths between 18 and 200 A were prepared by ion-beam sputtering in an UHV system and structurally characterized by X-ray diffraction at small angles and 57Fe conversion-electron Mfssbaner spectroscopy. Good periodicity and sharp concentration profiles at the interfaces are found. The magnetic properties are unusual. For Fe-layer thicknesses below 25 .~, relatively independent of the Ce-layer thickness, the Curie temperature is reduced to values below 180 IC This is accompanied by a distinct decrease of the spontaneous magnetization and the appearence of hysteresis in the low-field susceptibility at low temperatures. Isothermal magnetization curves point to noncollinear magnetic order. Evidently, these phenomena are closely related to the transition in the Fe layers from the bcc cystaUine to an amorphous structure occurring near 25 ~, according to X-ray diffraction at large angles and RHEED diagrams.
1. Introduction Artificial multilayers composed of alternately stacked transition and rare-earth metals exhibit a large variety of novel magnetic properties, such as perpendicular anisotropy (e.g. T b / F e [1]) or coupling of magnetic order of the rare earth across a nonmagnetic transition metal (e.g. G d / Y [2]). The special features of these structures arise for example from the strong magnetocrystalline anisotropy of the rare earth, the rich diversity of their magnetic phases, or from the interplay of their 4f-electronic states with the transition-metal 3d states (the exotic properties of r a r e - e a r t h transition-metal compounds are well known). It is very attractive that the properties of the layered systems can be designed, on a large scale, by varying the relative thickness of the constituents. In this p a p e r we present results for multilayers composed of the very different magnetic elements cerium and iron. The electronic states of Ce in Correspondence to: Prof. W. Felsch, I. Physikalisches Institut,
Universit;it G6ttingen, Bunsenstr. 9, 3400 G6ttingen, Germany. Tel.: +49-551-397620.
intermetallic compounds depend sensitively on its local environment, i.e. on the number, distance a n d / o r chemical nature of the neighbouring atoms. In CeFe2 the Ce-4f states are delocalized and strongly hybridized with the Fe-3d states. As a result, the compound is an itinerant ferromagnet with an unusually low Curie t e m p e r a t u r e (~- 230 K) and saturation magnetization. T h e Ce4f moments are coupled antiferromagneticaUy to the Fe-3d moments, which is anomalous for such r a r e - e a r t h / i r o n compounds with light rare earths. It is a consequence of 4f-orbital-moment quenching by band formation! [3]. At low temperature, the compound has the tendency towards an antiferromagnetic instability, which is enhanced by metal additives substituting Fe sites [4]. Thus it appears interesting to combine Ce and Fe in an artificial periodic stacked-layer structure and study the influence of its length scale on the magnetic properties. It~ is to be expected that, as in other cases, these properties will sensitively depend on the microstructure of the individual layers and their interfaces. So structural characterization is of particular importance for an understanding of this system.
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
142
J. Thiele et al. / Properties of Ce/Fe multilayers
2. Experimental details
The multilayers were prepared by ion-beam sputtering in an ultrahigh vacuum system (base pressure < 5 x 10-10 Torr), using alternately targets of 99.9% pure cerium and 99.998% pure iron. Modulation lengths A =tce -t- tFe, where tce and tFe are the nominal thicknesses of the Ce and Fe layers, were varied in the range between 18 and 200 A. During operation of the ion-beam source the pressure rose to 4 x 10 -5 Torr due to the incoming argon gas (6 N purity), and the partial pressures of oxygen and hydrogen (reactive gases for Ce) were = 4 x 10 -11 and 5 × 10 -8 Torr, respectively. The deposition process was controlled by a computer: film thicknesses were measured by a quartz rate monitor, and at designed values a target change was effected by a step motor while the substrate was protected by a shutter. Typical deposition rates were close to 0.5 .A/s. The total thickness of the multilayers was always 2000 ,A. Layering pattern and total number of bilayers are denoted as [CeX/FeY] X n, where X, Y represent the nominal thicknesses /Ce, tFe in A and n denotes the number of repetition. The substrates were single-crystalline Si wafers with (100) orientation, Prior to sample deposition, they were cleaned by irradiation with lowenergy Ar ions and covered with a buffer layer of Cr which acts as a diffusion barrier. The samples were condensed at liquid-nitrogen temperature to suppress diffusion (a solid-state amorphization reaction has been observed in Ce-Fe sandwich structures during deposition at room temperature, where diffusion of Fe in Ce is involved [5]). Before removal from the sputtering system, the samples were overcoated with 60 .A of Cr to prevent oxidation on exposure to atmosphere. Characterization of the samples regarding periodicity, interface quality and lattice structure of the individual layers was performed by X-ray diffraction, RHEED, resitivity measurements and M6ssbauer spectroscopy. The X-ray diffractometer was operated in the 0-20 reflection mode, both in the low- and high-angle regions, using Cr-K~ radiation. RHEED observations were made in situ, they permit to study the surface
[CeX/FeY]xn
4-J
0
[11/07]x111
•
O
i
l
5
i
i
r
i
I
IO
i
i
i
,
1
15
2 @ (deg.) Fig. 1. Small-angle X-ray diffraction diagrams (Cr K a) for different C e / F e multilayers of = 2000 A total thickness.
morphology of the layers during the condensation process. 57Fe conversion,electron M6ssbauer spectra were obtained at room temperature using a conventional constant-acceleration spectrometer, with a gas-flow proportional counter and a source of 57Co in Rh. Long-time stability of the samples was investigated by Rutherford backscattering spectroscopy (RBS) with 900 keV He 2+. A vibrating-sample magnetometer operated between 4.2 and 300 K in fields up to 50 kOe provided magnetization curves of the samples.
3. Results
The layered structure of the samples is revealed by X-ray diffraction diagrams for small scattering angles (fig. 1) which show a number of peaks associated with the multilayer periodicity. The modulation lengths A calculated from the position of these peaks agree, within a few per-
J. Thiele et al. / Properties of Ce/Fe multilayers
cent, with the nominal ones designed using the thickness monitor. The appearance of reflections to relatively high orders, in particular for large values of A, prove rather sharp concentration profiles at the C e / F e interfaces. Even for [ C e l l / F e 7 ] x 111, the sample with the shortest periodicity investigated, the second-order peak can be resolved, indicating that the modulation in composition is steeper than sinusoidal. A detailed comparison of the measured diffraction diagrams with those resulting from a numerical simulation of the multilayer structure involving effects from diffusion and roughness at the interfaces will be published elsewhere [6]. The modulated structure of the samples was also confirmed by RBS experiments. The X-ray diffraction diagrams recorded for large scattering angles (which are not shown here) reveal that the structure of the individual layers depends on their thickness. Up to a critical value to, which is near 25 ,~ for Fe and 70 ,& for Ce, only broad intensity appears, with no evidence of crystalline phases, i.e. the films grow in an amorphous structure. Above this thickness, the (110) diffraction peak of bcc ot-Fe and the (111) peak of fcc ~/-Ce are observed. From the related scattering angles it can be concluded that both the Fe and Ce lattices are expanded by 0.5 to 1% in the direction of growth (in the 0-20 mode, only this direction is probed). There is no signature of the compounds CeFe 2 and CezFe17, allowed by the equilibrium phase diagram, for any of the multilayers. The amorphous-to-(poly)crystalline transition of the individual layers is clearly visible in the R H E E D experiments of the Fe- and Ce-layer surfaces performed in situ. The diffracted intensity in the patterns changes from diffuse to an arrangement of concentric half circles. The structural transformation is also reflected (for the Fe layers) in the variation of the electrical resistivity p and the resistivity ratio p (300 K)/p(4.2 K) with the modulation !ength A (fig. 2, tce= tFe = A/2). Up to A = 60 A, where the multilayers are entirely amorphous, the resistivity is near 150 ~ c m , and the resistivity ratio is below 1, i.e. the temperature coefficient is negative, as is frequently observed for amorphous alloys. Upon
143
160
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~' 100 0 0
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--
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1
I
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70 40
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I
40
80
I
I
120 :1.60 200
Fig. 2. Electrical resistivity at 300 K and resistivity ratio of = 2000 ,~ thick C e / F e multi!ayers as a function of the modulation length A = t c e + tFe. T h e lines are guides to the eye.
crossing the critical thickness t c for crystallization of the Fe layers, p drops rather steeply and the temperature coefficient changes sign. The magnetic properties of the C e / F e multilayers are unusual. Fig. 3 shows the magnetization data for different, modulation lengths A of equal thicknesses of the Ce and Fe layers, tce= tFe = A / 2 , with the same total thickness of the samples. For these measurements, the magnetic field was applied in the plane of the films. Note that the amount of Ce and Fe is the same in each case. As is immediately apparent (fig. 3a), the magnetization curves at 4.2 K are not a simple summation of the individual Fe-layer magnetizations. While the magnetizaton of the samples with single-layer thicknesses A / 2 > 30 A saturates readily in a moderate field, with no measurable high-field slope, all of t h e samples with smaller modulation lengths exhibit a considerable slope at high fields in their magnetization curves, which fail to saturate even in 50 kOe at 4.2 K. Fig. 3b shows the magnetization Ms(0) obtained by extrapolating the high-field data to zero field ("spontaneous" magnetization hereafter) as a function of temperature. For large thicknesses A / 2 > 50 ,~ the spontaneous magnetization of the multilayers, if attributed to the Fe part only, approaches that of bflk Fe. But Ms(0) at 10w temperatures drops rather abruptly when A / 2 decreases to values be53w 30 A. This is accompa-
J. Thiele et al. / Properties of Ce / Fe multilayers
144
nied by a precipitous fall of the Curie temperature T¢. Values estimated from Arrott plots are below 180 K. As is evident in fig. 3, Ms(0) increases again when the smallest modulation lengths are approached. Variation of the individual layer thicknesses has shown that the thicknesses of the Ce layers is of minor importance for the observed drop of Ms(0) and Tc, i.e. the effect is mainly governed by the thickness of the Fe layers and evidently occurs at their transition to an amorphous structure. The marked change in the magnetic properties of the multilayers with relatively small changes in A is further documented by the appearance of considerable hysteresis and coercivity at low temperature. While for tFe > 30/~ the coercive fields are near 100 Oe between 4.2 K and room temper-
1000
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X=Y=50 800
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300
Fig. 3. Magnetization M vs.magnetic field H applied in the film planes at 4.2 K (a) and "spontaneous" magnetization M,(0) vs. temperature T (b) for mu!tilayers [CeX/FeY].x n of equal Ce- and Fe-layer thicknesses X = Y and -- 2000 A total thickness. Curie temperatures resulting from Arrott plots are Tc = 177 K ( X = 10), 165 K ( X = 1 5 ) , 158 K ( X = 22). The bulk-Fe value of M,(0) at 4.2 K would be 1743/2 emu/cm 3. The lines are guides to the eye.
850
I
I
~°°°°°o0OOo0o oO
~*****..... ~, ~,~0,,~,,. E501503x20 *~e~**,, f,c.
,., 750
~ 200 ~ [ 1 0 / 2 7 3 x 5 4 H=gO Oe
:~ loo 0
,
0
I
~
100
I
200
300
T0~) Fig. 4. Magnetization M in an applied field H = 90 Oe of C e / F e multilayers as a function of temperature T. The samples were cooled in zero field (z.f.c.) and in the measuring field H (f.c.). The Curie temperature of the samples 50/50 and 10/27 are above room temperature, and Te = 164 K for the sample 30/27.
ature, they raise to 1 kOe at 4.2 K for smaller modulation lengths, declining rapidly with temperature to 100 Oe near 20 K. Fig. 4 shows that the low-field magnetization depends on the magnetic history of the samples. The irreversibility observed is particularly pronounced for the multilayers with tFe = 27 A, where the magnetization at 4.2 K differs by about 50% for the field-cooled and the zero-field-cooled cases. Measurements with the magnetic field applied perpendicular to the planes of the films revealed that the easy axis of the magnetization is in the film planes, for all of the samples investigated, as a direct consequence of shape anisotropy~ While for large modulation lengths A (tFe > 30 A) the anisotropy fields are close to the demagnetization field 4"n-Ms, they are considerably larger for small A, i.e. for the low-Tc samples. M6ssbauer spectra were obtained at room temperature in zero magnetic field for samples with Fe-layer thicknesses oabove and below the critical value, tc,Fe-----25 A, for the structural change in the Fe layers. Data for [Ce30/Fe35] × 31, [Ce30/Fe27] × 35 and [Ce10/Fe27] × 54 are shown in fig. 5, together with a reference spectrum of a 1000 A thick Fe film prepared under the same conditions. This latter spectrum
145
J. Thiele et al. / Properties o f C e / F e multilayers
(fig. 5a) compares well with that of bulk bcc Fe, but shows some line broadening due to the disorder induced by the low deposition temperature of the sample. The two spectra in figs. 5b and d are magnetically split revealing the ferromagnetic nature of the samples. In contradistinction, the spectrum in fig. 5c consists of an asymmetric quadrupole-split doublet of a paramagnetic sample. This behaviour is in agreement with the room-temperature magnetization curves. Note that the loss of ferromagnetism with decreasing Fe-layer thickness at constant Ce-layer thickness (figs. 5b,c) is rather abrupt, and that on diminishing the Ce-layer thickness for the samples with 27 A thick Fe layers (figs. 5c,d) the Curie temperature is raised to above room temperature. The magnetization of this latter sample saturates only above 30 kOe, which is an indication of the close proximity of the individual Fe layers to their critical thickness tc,Fe.
b
,
C30/35]x31
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0
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8
Velocity (ram IS) Fig. 5. Conversion-electronM6ssbauer spectra taken at room
temperature for three different Cc/Fe multilayers (total thickness ~ 2000 ,~). Shown for comparison is the spectrum of a 1000A thick Fe f~m (a).
Table 1 Hyperfine fields Hat at room temperature, relative fraction of 57Fe sites and partial Fe-layer thicknesses t i for multilayers [CeX/FeY]×n and a 1000 A thick Fe film ("bulk Fe") resulting from the conversi0n-electron M6ssbauer spectra in
fig. 5 Hi
bulk Fe
(kOe) 330
Ri
t i = RitFe
1
(A) 1000
[Ce30/Fe35] X31 site 1 325 site 2 285 site 3 -
0.39 0.41 0.20
14 14 7
[Ce10/Fe27]x 54 site 1 325 site 2 299 site 3 -
0.32 0.37 0.31
9 10 8
The procedure employed in fitting the spectra of the ferromagnetic samples was based on using the minimum number o f Fe sites needed to obtain a good representation of the data. The inner lines exhibit a pronounced left-right asymmetry suggestive of a nonzero quadrupole interaction. Furthermore, a certain distribution of magnetic hyperfine fields can be expected. In order to approximate the distribution and the asymmetry we used a simple three-component model to fit the spectra. Each contponent involves a site of the 57Fe nuclei with a differing local surrounding. The (mean) hyperf'm~ fields Hhf obtained and the relative fraction o t 57Fe sites, R i (i ffi 1, 2, 3), resulting from the relative intensity contribution of the subspectra are given in table 1. The subspectra are: (i) A six-line pattern with the same hyperfine field as in bulk bcc Fe (site 1). (ii) A broadened six-line pattern with a mean hyperfine field depleted by -- 10% (site 2) as compared to the :former one. This compo, nent gives rise to the shoulders of the outer lines. The fit results in a considerable linewidth and thus indicates a distribution of hyperfme fields. (iii) A doublet representing the effect from the interaction of the 57Fe nuclear quadrupole
146
3".Thiele et al. / Properties of Ce/ Fe multilayers moment with the electric-field gradient, in a noncubic environment (site 3).
The quadrupole-split doublet of the paramagnetic sample (fig. 5c) could be represented by a simple two-single-line fit, yielding a mean quadrupole splitting of 0.4 m m / s and a mean isomer shift of -0.1 mm/s. Similar values were found for the site-3 subspectra of both ferromagnetic samples. The negative sign of the isomer shift implies an enhanced s-electron density at the sites of the 57Fe nuclei involved.
4. Discussion
An important problem to address in view of the magnetic properties of the C e / F e multilayers is the nature of the interfaces. Their role is, a priori, of particular importance, since they represent the region where Ce and Fe are in immediate contact. According to RBS performed on multilayers with tFe > tc,Fe, the crystalline Fe films exhibit island growth [6], but details of their structure are not known. Simple numerical modeling of the small-angle X-ray diffraction diagrams of samples with Fe-layer thicknesses immediately above and below the critical value to,re reveals that (i) local layer-thickness fluctuations and (ii) interfacial diffusion are restricted to nominally 5:1 monolayer [6]. Further information about the interfaces is provided, for the samples being ferromagnetic at room temperature (tF~ > t~,~), by their M6ssbauer spectra which are sensitive to the local environment of the 57Fe nuclei. The sequential deposition of the Fe and Ce layers suggests that site 1 may be attributed to the core part of the Fe layers and sites 2 and 3 to the interface part. If the relative fraction of sites, R i (i--1, 2, 3), is translated into partial thicknesses t i = RitFe (table 1), then on site 3 nominally only ---4 atom layers of Fe are involved, taking both interfaces together. The absence of ferromagnetism on this site and the negative isomer shift point to a mixing with Ce. The formation of compounds at the interface (the equilibrium phase diagram al-
lows CeFe 2 and Ce2Fe17 with magnetic ordering temperatures below room temperature) may be ruled out since cubic CeFe 2 would not show a quadrupole interaction and Ce2Fel7 shows a much larger isomer shift [7] than observed for site 3. A distinct possibility is that Fe has diffused into the amorphous Ce layers (here tce < tclce = 70 A; from simple thermodynamic arguments the smaller Fe atoms are expected to be the diffusing species [8]), or that short-range collisional mixing has occurred at the interface during sputter deposition. As pointed out before, small-angle X-ray scattering reveals that the extension of the intermixed zone is limited to nominally 2 monolayers at each interface [6], in agreement with other rare-earth iron multilayers [9,10]. The equilibrium phase diagram of Ce and Fe rules out the formation of solid solutions, but Fukamichi et al. [11] have shown that amorphous alloys exist for Ce concentrations between 10 and 40% which are paramagnetic at room temperature. The observed mean isomer shift and quadrupole splitting attributed to site 3 are very similar as for a number of amorphous transition-metal-iron alloys [12]. Hence this site is likely to be located in an amorphous Ce-Fe environment. The reduced coordination with Fe atoms at the C e / F e interface, together with the Ce-Fe interaction, must be responsible for the --10% reduction (referred to the core of the Fe layers) of the average hyperfine field attributed to site 2 of the 57Fe nuclei. The relatively large contribution of this site to the total intensity of the spectra, i.e. the large partial thickness 12 of 14 and 10 A of Fe (table 1) is amazing and may indicate that imperfections of the stacked structure are at the origin of the effect. Small-angle X-ray diffraction reveals that the local thickness fluctuations in the individual layers are very small [6] and hence can be discounted as an explanation of the large values of t 2. An enlarged interface area could result from the island structure of the Fe layers suggested by RBS [6], but their morphology remains to be elucidated. A further possibility is that nonlocal Ce-Fe interactions are effective which extend the influence of the interface into the Fe films. The reduction of the hyperfine field attributed to site 2 and the fraco
J. Thiele et al. / Properties of Ce/ Fe multilayers
tion of Fe atoms involved are similar as in D y / F e multilayers, where nominally 12 ,~ of Fe are affected by the influence of the interface [13]. The hyperfine subspectra contributed by sites 1 and 2 of the Fe layers display a line-intensity ratio 3 : 4 : 1 : 1 : 4: 3 and hence reveal that the magnetic easy direction for these multilayers with tFe > t~,Fe is in the film plane. The collapse of the hyperfine field at room temperature of sample [Ce30/Fe27] × 35 (fig. 5c) coincides with the loss of crystallinity of the Fe layers, as is demonstrated by X-ray diffraction and RHEED. The asymmetric broadened doublet, revealing a quadrupole interaction due to a distribution of local electric field gradients at the sites of the Fe nuclei, must contain information on the local symmetry and short range order, but due to the complex mierostructure of the artificial C e / F e heterostructures no attempt was made to analyze these spectra. It would be highly desirable to extend M6ssbauer spectroscopy to lower temperatures into the ferromagnetic state of these multilayers with small modulation lengths A, to shed light on the distribution of the hyperfine fields and the related sites of the 57Fe nuclei. The small-angle X-ray diagrams vary smoothly towards low A on crossing the critical thickness t¢,Ve of the Fe layers for their crystalline-to-amorphous transition (fig. 1), and their numerical simulation reveals that the local fluctuations in the thickness of the individual layers and the interdiffused zone are equal just above and below t¢,F,. Hence the quality of the interfaces must essentially be preserved in this transition. Comparison of the samples [Ce30/Fe27] × 35 and [Ce10/Fe27] × 54 is interesting. It shows that to a certain extent the growth conditions for the Fe layers, i.e. their critical thickness tc,Fe, depend on the thickness of the Ce layers, since according to the structural investigations the individual Fe layers are amorphous for 30 ,~ thick Ce layers but crystalline for the thinner one. The mechanisms which favour growth of an amorphous structure of the individual Fe and Ce layers are largely unknown. Growth of amorphous Fe below a critical thickness seems to be a common observation in many layered structures based on iron and rare-earth metals [13-15]. This may be due to the
147
similar chemical and structural properties of the rare earth. For a sandwich structure of Fe and Gd it has been suggested [15] that the large structural misfit at the interface of the constituents is at the origin of the effect. The same growth properties were found for layered structures of Fe and Y [16,17]. The M6ssbauer spectra in fig. 5 clearly demonstrate that the loss of ferromagnetism at room temperature of the C e / F e multilayers is, as pointed out before, closely related to the crystalline-to-amorphous transition in the Fe layers. The behaviour is consistent with the steep drop of the Curie temperature Tc and spontaneous magnetization M s ( 0 ) o n crossing the critical thickness tc,Fe (fig. 3). As is evident from the appearance of considerable hysteresis in the magnetization curves at low temperatures, of irreversibility in the low4ield magnetization (fig. 4) and from the large slope of the high-field magnetization at low temperature, the magnetic character of the multilayers is distinctly modified below tc,Fe. The magnetic irreversibility is not directly related to the hysteresis, as would be expected for an ordinary ferromagnet. At the temperature where the magnetization begins to depend on the sample history, the coercive field is much smaller than the cooling or measuring field. Hence the onset of irreversibility may indicate a freezing phenomenon, i.e. a transition into a spin-glas-like magnetic state. In fact, the large high-field susceptibility reveals that the material cannot be a collinear ferromagneti As we have pointed out before, the concentration gradient in the multilayers must be very similar immediately above and below tc,Fe, and there must remain a core of pure Fe in the center of the amorphous Fe layers somewhat below t~,F~. Thus the magnetic anomalies are essentially not the result of alloy formation, but of the structural transition in the Fe layers itself. Similar effects found in D y / F e multilayers were interpreted in the same way [13,14]. There is no evidence ifor inter-layer coupling of the Fe moments in the C e / F e multilayers. There has been a 19ng debate on the magnetic properties of pure amorphous Fe. Arguments rely on extrapolations of experimental data obtained for amorphous Fe alloys containing a non-negligi-
148
J. Thiele et al. / Properties of Ce/ Fe multilayers
ble amount of a second element. The saturation moment seems to be related to the polymorphism of bulk crystalline Fe [12,18], but there is evidence that ferromagnetic ordering is noncollinear, presumably due to the presence of antiferromagnetic exchange. It has been argued that the high sensitivity of the distribution of exchange integrals to the nearest-neighbour distance is at the origin of this behaviour [19]. Estimates of the Curie temperature range somewhat above 100 K [19]. The magnetic properties of the C e / F e multilayers just below te.Fe where the structure is entirely amorphous are very similar to those of hypothetical pure amorphous Fe. However, there must be a non-negligible contribution of the interfaces. This can be conjectured from the behaviour above tc.ve, i.e. from the presence of irreversibility in the low-field magnetization (fig. 4, [Ce50/Fe50] x 20), from the reduction of Ms(0) below the value of bulk Fe (fig. 3, [Ce30/Fe30] × 33), or from the presence of the subspectrum attributed to site 2 in the M6ssbauer spectra (fig. 5b,d). Exchange and anisotropy fluctuations experienced by the Fe atoms in the interracial zone due to bonding with neighbouring Ce atoms can be expected to contribute to noncollinearity of magnetic order and to the magnetic irreversibilities. The contribution of the Ce atoms to the interracial anisotropy is not clear. In amorphous Ce-Fe alloys [11], there is evidence that the 4f states of Ce are delocalized, as they are in the compound CeFe z. Hence in the amorphous intermixed part of the interfaces the Ce ions presumably do not carry an orbital 4f moment and cannot give rise to strong single-ion anisotropies frequently encountered in amorphous rare-earthtransition-metal alloys. This agrees with the relatively low coercive field at low temperatures. A t the lowest periodic lengths A, e.g. for [Cel0/Fel0] × 100, where a modulation in composition is clearly present, values of the magnetization Ms(0) and the Curie temperature Tc of homogeneous amorphous Ce-Fe alloys [11] with compositions corresponding to the mean multilayer compositions are approached, which explains the observed increase of Ms(0) and Tc towards low A (fig. 3).
5. Conclusion We have shown that systems with alternately stacked Ce and Fe layers with good periodicity and sharp concentration profiles at the interfaces may be grown by ion-beam sputtering on Crcoated Si wafers cooled to liquid-nitrogen temperature,.for a wide range of modulation lengths (18-200 A). The Fe and Ce layers first grow in an amorphous structure up to a critical thickness, which is near 25 A for Fe and 70 ~k for Ce, and then change to the crystalline bcc and fcc structures, respectively. Samples with Fe layers below the critical thickness, i.e. in the amorphous state, are ferromagnets, presumably with a noncollinear spin structure, with an unusually low Curie temperature and spontaneous magnetization. Many of the properties are similar to those of hypothetical pure amorphous Fe, extrapolated from data for homogeneous amorphous Fe alloys. In addition, the influence of the C e / F e interfaces is clearly visible. There is no evidence of inter-layer coupling of the Fe moments.
Acknowledgements The authors would like to thank T. Kacsich, II. Physikalisches Institut, for RBS measurements, M. Steins, Mineralogisch-Kristallographisches Institut, for help with the interpretation of the small-angle X-ray diffraction experiments, and U. Krebs, Institut fiir Metallphysik, for discussions concerning CEMS. We acknowledge the support of the Deutsche Forschungsgemeinschaft within SFB 345.
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[14] S. Honda, S. Nishimura and T. Kusuda, IEEE Trans. Magn. MAG-25 (1989) 4027. [15] J. Landes, Ch. Sauer, B. Kabius and W. Zinn, Phys. Rev. B 44 (1991) 8342. [16] T. Morishita, Y. Togami and K. Tsuhima, Proc. Intern. Syrup. on Physics of Magnetic Materials, Sendai 1987, eds. M. Takahashi, S. Maekawa, Y. Gondo and H. Nose (World Scientific, Singapore, 1987) p. 295. [17] S. Handschuh, J. Landes, U. K6bler, Ch. Sauer, G. Kisters, A. Fuss and W. Zinn, preprint. [18] W. Felsch, Z. Angew. Phys. 29 (1970) 217. [19] D.H. Ryan, J.M.D. Coey, E. Batalla, Z. Altounian and J.O. Str6m-Olsen, Phys. Rev. B 35 (1987) 8630.