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Magnetization of thin Fe films on V(1 1 0) and Cr(1 1 0)
Journal of Magnetism and Magnetic Materials 212 (2000) 337 } 346
Magnetization of thin Fe "lms on V(1 1 0) and Cr(1 1 0) T. Nawrath, H. Fritzsche, H. Maletta* Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany Received 12 April 1999; received in revised form 23 November 1999
Abstract The magnetization of thin Fe(1 1 0) "lms was measured by magneto-optical Kerr e!ect (MOKE) magnetometry and polarized neutron re#ectometry (PNR). In order to further understand the in#uence of strain and roughness on the Fe magnetization di!erent surfaces were prepared by evaporating Cr and V on V(1 1 0) single crystals by molecular beam epitaxy. By varying the growth temperature both smooth and faceted Cr and V surfaces were created. For the facets we obtained an up and down staircase structure with the ridges along the [0 0 1] direction that consisted approximately of subsequent (1 0 0) and (0 1 0) surfaces. For the thin Fe "lms that were evaporated on the smooth V(1 1 0) surface a relatively high reduction of the magnetization was obtained, independently of the "lm thickness. In terms of thickness the observed reduction is equivalent to an o!set of *t"!(4.0$1.0) As . A V layer evaporated on this structure does not further reduce the magnetization of the Fe. For comparison Fe evaporated on a smooth Cr "lm was investigated, and the reduction amounted to *t"!(1.4$0.4) As . For thin Fe "lms on faceted V and Cr substrates *t"!(3.3$0.4) and !(2.6$0.3) As were measured, respectively. For samples with a 2 As Cr bu!er layer between the V(1 1 0) single crystal and the Fe "lm we obtained *t"!(4.08$0.10) As , a value which is close to the result obtained for the smooth V(1 1 0) substrate. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ha; 61.14.Hg; 68.35.Ct; 75.30; 75.70; 78.20.Ls Keywords: Magnetization; Thin "lms; Magneto-optical Kerr e!ect; Neutron re#ectometry
1. Introduction The changes occurring in the magnetism of thin "lms in comparison to the bulk material have always been of great interest. This is e. g. re#ected by early works of NeH el [1] in a study on a possible perpendicular anisotropy in thin "lms, or Ising [2], who investigated the critical behavior of magnetic
* Corresponding author. Tel.:#49-30-8062-2058; fax: #4930-8062-2523. E-mail address: [email protected] (H. Maletta)
systems in lower dimensions. More recent studies predicted an enlargement of the magnetic moment of free magnetic layers [3,4], and have motivated further experimental studies. And last but not least magnetism at interfaces is also technologically interesting, since antiferromagnetically aligned bilayers or multilayers are known to create a giant magnetoresistance e!ect [5,6]. The magnetism of iron studied in this paper is characterized by the delocalized 3d-electrons, which carry the main magnetic moment. Thus, it can be expected that the hybridization with the 3d orbitals of V or Cr at the interface strongly
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 8 2 5 - 2
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in#uences the magnetism of the system. Theoretical studies, however, mainly focusing on the (1 0 0) interface, predicted that the magnetic moments of the iron situated next to the interface in a Fe/Cr system are reduced, while the Cr atoms carry an enlarged magnetic moment that is antiparallelly aligned to the magnetic moments of the iron [7}12]. At the Fe/V interface the magnetic moment is also reduced in the "rst Fe layer with an antiparallelly induced magnetic moment in the "rst V layer [10}14]. For both cases this results in a total reduction of the magnetization that can be expressed in terms of a thickness and amounts to +!0.2 As for the Fe/Cr(1 0 0) and +!0.35 As for the Fe/V(1 0 0) interface. The experimental "ndings show more variations than the theoretical predictions: Even in the rather simple case, where thin single V and Cr layers are evaporated on a Fe(1 0 0) surface, the magnetic moments in the "rst V and Cr layer range from !0.3 l to !1.0 l /atom [15,16] for V and from B B !0.5 l to !2.0 l /atom [17}22] for Cr, respecB B tively. Although neither of these methods directly measure the magnetization of the V and Cr and all publications mentioned above apply a model or an estimate derived from known materials, it is more likely that the discrepancy between these results originates from small variations in the structure of the Fe surface. This is e.g. reported in a discussion on the Fe/Cr(1 0 0) system in Refs. [23,24], where the preparation of the substrate does not only lead to topologically di!erent interfaces, but also drastically e!ects the magnetic moments. In previous studies on the magnetization of ultrathin Fe "lms on V(1 1 0) [25}27], we found a relatively high reduction of the magnetization compared to bulk Fe. An o!set independent of the thickness was observed, equivalent approximately to two &magnetically dead' monolayers. We measured a reduction in the magnetization of a wedged sample which in terms of a thickness is equivalent to an o!set of *t"!(4.0$1.0) As . Because of this high reduction the growth of this system was studied in more detail by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), and a special growth mode characterized by a collapse in the island size in the thickness range of
0}6 As Fe was found [28]. Starting from a typical terrace width of the V substrate with 89 As in [0 0 1] and 50 As in [1 11 0] there is a collapse in the island size of the Fe "lm down to values of 21 As in [0 0 1] and 9 As in [1 11 0] for t "4 As . Above t "10 As F% F% the island size of the Fe "lms increases again to 30 As in [0 0 1] and 20 As in [1 11 0]. Contrary to what one might have expected, this behaviour is not connected to a Vollmer}Weber (island) growth, since the intensities of Fe and V measured dependently of the Fe thickness by AES show a coverage typical of a Frank}van der Merwe (layer) growth, and thus it is very probable that the magnetic behaviour of the thin Fe "lm is a result of this special growth mode. In our studies the substrate on which the Fe "lm was evaporated was varied, while the growth temperature and rate were not changed throughout the preparation. The results of the magnetization of Fe on a faceted V(1 1 0) substrate will be presented below. This faceting (described in more detail in Ref. [28]) is characterized by a quasi-periodic sequence of up and down staircases in [1 11 0] inclined to the "lm plane at an angle of 503 and ridges orientated along [0 0 1]. Thus the substrate can be regarded as a subsequent sequence of up- and down-going (1 0 0) and (0 1 0) surfaces. Additionally, Fe layers on Cr(1 1 0) were studied, and experiments on smooth and faceted Cr surfaces were performed with the facets orientated in the same direction as in the before mentioned V substrate. The results of these experiments shed more light on the magnetic behaviour of the Fe/V(1 1 0) system.
2. Sample preparation The V(1 1 0) single crystals were cleaned during repeated sputtering and annealing cycles, with 700 eV argon ions used in the sputtering process and at a temperature of 1400 K during the annealing cycles in order to recrystallize the surface. This process was repeated for 120 h and resulted in a clean surface with contamination below the detection limit of the Auger spectrometer, i.e. below 2% of a monolayer for sulphur or carbon and below 5% for oxygen (because the oxygen transition coincides with the vanadium
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LMM-transition). No other contamination was detected during the sputtering and annealing cycles. All "lms were prepared under ultra-high vacuum (UHV) conditions at a base pressure of (10~10 mbar. The surface of the "lms were not contaminated either. In all experiments the Fe was evaporated at a rate of 0.07 As /s at a sample temperature of 320 K. For the preparation of smooth and faceted V and Cr substrates we exploited a special behavior which has been observed for many BCC(1 1 0) surfaces: a facetting with staircases in [1 11 0] direction and ridges along [0 0 1] [28}32]. The conditions of preparation were varied as follows: (a) The smooth V surface was prepared by covering a clean single crystal with a 10 As V layer at an evaporation temperature of ¹ "320 K at % a rate of 0.07 As /s. This resulted in a typical terrace width of 80 As in the [0 0 1] and 50 As in the [1 11 0] direction [28]. (b) The faceted V surface with ridges orientated in the [0 0 1] direction and inclinations of 503 to the "lm plane was also prepared on the clean V(1 1 0) crystal by evaporating 50 As V at ¹ "570 K and a rate of 0.07 As /s. Here the % terrace width in [0 0 1] direction is about 52 As [28]. (c) The smooth Cr surface was prepared by evaporating 200 As Cr on a V(1 1 0) single crystal at ¹ "470 K. With respect to the terrace width % and the roughness of the Cr "lm, best results were obtained at a growth rate of 2 As /s. A LEED picture of these "lms is shown in Fig. 1 and allows for determining the terrace width to be 28.8$1.5 As in the [0 0 1] and 23.8$2.0 As in the [1 11 0] direction. (d) The faceted Cr surface was prepared at ¹ "320 K and 0.07 As /s on a clean V(1 1 0) % crystal. The resulting faceting was identical to that of V prepared at ¹ "570 K with the % ridges orientated in [0 0 1], a terrace width in [0 0 1] direction of about 50 As [28] and a quasi-periodic sequence of up and down staircases along [1 11 0] now inclined at an angle of 403. For our experiment a 16 As Cr "lm was evaporated on the V(1 1 0) single crystal. (e) For the last experiment a 2 As thick buwer layer of Cr (one monolayer) was evaporated on the
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Fig. 1. LEED picture (E"70 eV) of a 200 As thick Cr layer evaporated on V(1 1 0) at ¹"470 K. The surface directions of the crystal are also sketched.
V(1 1 0)/10 As V substrate (see paragraph a) before the Fe "lm was evaporated at a growth temperature of 320 K and a rate of 0.07 As /s. Because Cr grows facetedly on V(1 1 0) at 320 K, it is likely that the Cr "lm is not perfectly closed and that parts of the evaporated Fe are in contact with the Cr "lm and the V substrate.
3. Experiment The magnetic properties of the "lm were studied by polarized neutron re#ectometry (PNR) and magneto-optical Kerr e!ect (MOKE) magnetometry. PNR probes the "lm with polarized neutrons and is sensitive to the in-plane magnetization [33]. For details of how to apply this method as an in situ technique under UHV conditions see Ref. [27]. MOKE measurements are based on the rotation of the polarization axis of light, which results from the magnetization of the thin "lm. Our set-up is displayed in Fig. 2 in the longitudinal geometry
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Fig. 2. Set-up of the MOKE magnetometer in the longitudinal geometry.
with the applied magnetic "eld in-plane with the incident and re#ected LASER beam. The measurements were performed with an He}Ne-LASER (j"632.8 nm) in the s-polarization. A lock-in ampli"er was used in order to increase the sensitivity, and the LASER amplitude was modulated (l" 100.24 kHz) by a subsequent arrangement of a polarizer, a photoelastic modulator, and a second polarizer. To generate the modulated and polarized light, the "rst polarizer was adjusted to an s-con"guration, with the two axes of the photoelastic modulator adjusted to this polarization axis at an angle of 453. The second polarizer was arranged parallelly or perpendicularly to the "rst polarizer to create s-polarized or p-polarized light, respectively. For the experiments presented here the longitudinal geometry sensitive to the in-plane component of the magnetization was used and a magnetic "eld was applied in the [0 0 1] direction, which is the easy axis of the Fe "lms. The measurements were carried out in situ and on Fe wedges that were prepared under the same conditions as the "lms for the in situ PNR measurements. The wedges were inclined by 0.6 As /mm with a diameter of the LASER spot of +0.3 mm. Fig. 3 displays the Kerr hysteresis loops for an Fe thickness of 11.2 or 11.8 As for di!erent substrates. Fig. 3a shows the hysteresis loop of a 11.2 As Fe layer on V(1 1 0)/200 As Cr (smooth Cr surface; the preparation of this substrate is described in paragraph c), while Figs. 3b and c show the hysteresis loops of V(1 1 0)/16 As Cr/11.8 As Fe (faceted Cr
Fig. 3. MOKE hysteresis loops of Fe "lms on di!erent substrates: (a) 11.2 As Fe on V(1 1 0)/200 As Cr (For the surface structure of the Cr "lm see Fig. 1); (b) 11.8 As Fe on V(1 1 0)/16 As Cr; (c) 11.2 As Fe on V(1 1 0)/10 As V/2 As Cr.
surface, see paragraph d) and V(1 1 0)/2 As Cr/ 11.2 As Fe (see paragraph e). Despite the di!erent surface structures of the Cr substrate it can be seen that the coercive "elds of Fe increase with increasing Cr thickness. It is an interesting property of Fe on Cr(1 1 0) "lms, that the coercive "elds increase in dependence on the Cr thickness. As shown in Fig. 3, a higher Cr thickness results in a higher coercive "eld. Further experiments have shown that this also holds for Cr layers evaporated on Fe "lms [34]. Most likely this property, which may also be of technological interest, can be explained by the antiferromagnetism of the Cr layers.
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In the following paragraphs a}e the results of the magnetic measurement on the various substrates shall be presented. The preparation of the substrates is described in the preceding section in the paragraphs a}e. Except for the wedge covered with 50 As V, all "lms were measured in situ under UHV conditions. After the MOKE and PNR measurements were "nished the "lms were tested for surface contamination with AES and no contamination was detected. In MOKE magnetometry in the thin "lm limit the maximum Kerr rotation is proportional to the magnetization. This value is obtained from the different Kerr rotations measured at the two saturation values in the MOKE loops, and will be presented in the following "gures. The proportionality factor depends on the experimental set-up (angle of incidence, s- or p-polarization of the light, etc.) and on the optical constants of the "lm and the substrate. (a) In Figs. 4a}c the measured neutron re#ectivities of Fe "lms deposited on V(1 1 0) with a thickness of 6, 10 and 19.5 As are presented. The spin-up re#ectivities are plotted as up and the spin down re#ectivities as down triangles, respectively. The Fe "lms of a thickness of 6 and 10 As were measured at 80 K, whereas the 19.5 As "lm was measured at 110 and 300 K. Fig. 4 shows that for all measurements best "ts are obtained at reduced magnetic moments. Averaged values of k "1.8 and F% k "1.3 l per Fe atom are obtained for t " F% B F% 19.5 As and t "10 As , respectively, while the magF% netization vanishes at a "lm thickness of 6 As . In Fig. 5 these results of the PNR measurements are plotted (as squares) as a product of the magnetic moment per atom, l , times the Fe thickness F% t versus t . This product is directly proportional F% F% to the in-plane component of the magnetic moment of the "lm and therefore increases with increasing "lm thickness t . F% Additionally, the MOKE data were plotted as circles in Fig. 5 and adjusted to the PNR data by multiplying the Kerr intensities by a factor speci"c to the set-up. In order to compare these results to the MOKE data of the other "lms (e.g. the covered "lm) a normalized Kerr rotation was plotted in the following "gures. After "tting the Kerr data by a straight line they were divided by the inclination
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Fig. 4. PNR measurements of Fe "lms of various thicknesses of 19.5, 10, and 6 As on V(1 1 0) are shown in parts (a), (b), and (c), respectively. The re#ectivities of the spin-up neutrons are given as up triangles, the spin-down re#ectivities as down triangles. The simulations with the magnetic moment of bulk iron are shown for comparison as dotted lines, whereas the best "ts are performed with a magnetic moment per atom of 1.8 l , 1.3 l , B B and 0 l . B
of the line to get the normalized signals. The resulting inclination always equals 1 rel. unit/As . For both measurements shown in Fig. 5 the (in-plane) magnetization vanishes at an Fe thickness of t "6 As . By "tting a straight line to the F% data points of the "nite magnetic signals a negative o!set equivalent to *t"!(4.0$1.0) As is obtained. In order to test whether a "nite magnetization component perpendicular to the "lm layer (that might correspond to the detected magnetic o!set)
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Fig. 5. Magnetization of Fe "lms on V(1 1 0)/10 As V versus Fe "lm thickness t . The PNR results are plotted (as squares) as F% the product of the magnetic moment per atom k times t (left F% F% side) which is directly proportional to the magnetic moment. In addition the maximum MOKE signals obtained on a similarly prepared Fe wedge are plotted as full circles and adjusted to the PNR data by multiplication with a set-up speci"c factor (right side). *t"!(4.0$1.0) As .
remains MOKE measurements were performed also in the polar geometry that is sensitive to a perpendicular magnetization. In this experiment no magnetic signal was detected although the polar Kerr signal was expected to be by one order of magnitude more intense than the longitudinal signal in the current set-up. This leads to the conclusion that the Fe "lms on V(1 1 0) with t up to F% 6 As have indeed zero magnetization at a measuring temperature of 300 K. In the next experiment the "lm shown in Fig. 5 was additionally covered with 50 As V to "nd out if this results in an increased reduction of the magnetization. For this purpose the V layer was evaporated at ¹ "320 K and a rate of 0.07 As /s. The % MOKE measurements of this wedge are shown in Fig. 6, where the maximum Kerr signal is plotted versus t . The data reveal no further reduction, the F% o!set is *t"!(3.8$0.3) As . (b) In Fig. 7 the maximum Kerr signal of an Fe wedge evaporated on a faceted V(1 1 0) surface is plotted versus t . The V surface was prepared by F% evaporating 50 As V on the V(1 1 0) single crystal (as described in paragraph b). An o!set of *t"!(3.3$0.4) As was obtained. (c) Fig. 8 shows the maximum Kerr signal of an Fe wedge evaporated on a smooth 200 As Cr "lm. The Cr "lm was prepared at ¹ "470 K (as %
Fig. 6. The maximum Kerr signals of Fe on V(1 1 0)/10 As V coated with a 50 As V "lm is plotted versus the Fe thickness t . *t"!(3.8$0.3) As . F%
Fig. 7. Maximum Kerr signals of Fe on a faceted V surface plotted versus the Fe thickness t . *t"!(3.3$0.4) As . F%
Fig. 8. Maximum Kerr signals of Fe on a smooth Cr surface (V(1 1 0)/200 As Cr) plotted versus the Fe thickness t . *t" F% !(1.4$0.4) As .
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Table 1 Terrace widths for the various substrates as described in paragraphs a}e in the text. The magnetically dead layers *t obtained for the di!erent substrates are also included
(a) (b) (c) (d) (e)
Substrate
Terrace width along [0 0 1] (As )
Terrace width along [1 11 0] (As )
*t (As )
V V/50 As V V/200 As Cr V/16 As Cr V/2 As Cr
80 52 29 50 80
50 2 24 2 50
!4.0 !3.3 !1.4 !2.6 !4.1
Fig. 9. Maximum Kerr signals of Fe on a faceted Cr surface plotted versus the Fe thickness t . *t"!(2.6$0.4) As . F%
4. Discussion In general the magnetic saturation moment m of S thin "lms follows the relation [35]
Fig. 10. Maximum Kerr signals of Fe on V(1 1 0)/10 As V/2 As Cr plotted versus the Fe thickness t . *t"!(4.08$0.10) As . F%
described in paragraph c). An o!set of *t"!(1.4$0.4) As was obtained. (d) Fig. 9 shows the maximum Kerr signal of an Fe wedge evaporated on a faceted Cr "lm. The faceting has the same orientation as the V surface (described in paragraph b). An o!set of *t"!(2.6$0.3) As was obtained. (e) In the last experiment a 2 As thick Cr layer was evaporated on the V(1 1 0) crystal that had been covered with 10 As V before (see paragraph e). The results are presented in Fig. 10. An o!set of *t"!(4.08$0.10) As was obtained. The magnetic and structural properties of the di!erent systems described in paragraphs (a)}(e) are compiled in Table 1.
m "J A(t#*t) (1) S S with the bulk saturation magnetization J , the S sample area A, and the o!set *t which is called the e!ective magnetically dead layer thickness for negative values. The parameter *t is obtained as o!set from a linear extrapolation of the data according to Eq. (1). Magnetically dead layers have been observed e.g. for Ni(1 1 1) "lms on Re( 0 0 0 1) [36] or Co "lms on W(1 1 0) [37]. The reasons for a non-vanishing *t may be (i) a deviation of the ground state magnetic moment of the magnetic "lm from its bulk value in the surface of the magnetic "lm (&surface e!ect') [38]; (ii) a deviation of the ground state magnetic moment of the magnetic "lm from its bulk value at the interface between substrate and magnetic "lm (&interface e!ect'); (iii) a polarization of the substrate with ferromagnetically or antiferromagnetically coupling with respect to the magnetization of the magnetic "lm (&polarization e!ect'); (iv) enhanced thermal decrease of magnetization in thin "lms (&size e!ect') [35,38]. A deviation from Eq. (1) is only observed near the region of the critical thickness where the "lm becomes ferromagnetically. The critical thickness itself is in#uenced by the growth mode of the epitaxial system under investigation.
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We shall "rst discuss the MOKE and PNR measurements of Fe on V(1 1 0), which reveal a high reduction in the magnetization independent of the Fe thickness (an o!set of *t" !(4.0$1.0) As ) as shown in Fig. 5. Additional PNR studies revealed that the increase of the magnetization at lower temperatures can be neglected [26,27]. Hence, *t cannot be explained by a temperature-dependent size e!ect, which would actually lead to the observation of a reduced magnetic moment due to a non-saturated magnetization (in case of a very low Curie temperature of the thin "lm). The reduced magnetization might be interpreted as a result either of the V/Fe interface or of the Fe/vacuum interface, or as a combination of both e!ects. Theory predicts an increase in the magnetization at the Fe surface [3,8,9,39,40] and a reduction at the Fe/V interface [10}14]. Experimental investigations on the magnetization of the free Fe surface show in agreement with theory an increase of the magnetic moments at the Fe surface [41,42] corresponding to *t"0.5 As . Experiments in dependence of the roughness show an additional slightly increasing magnetization of rough Fe(1 1 0) surfaces [43]. All these e!ects mentioned above are small compared to our "ndings and cannot explain the large o!set of *t"!4 As . To shed more light on that large o!set we evaporated 50 As V onto a V(1 1 0)/Fe wedge, what did not result in a further reduction of the magnetization of the Fe "lm (see Fig. 6). This experiment clearly demonstrates that the observed large reduction of the magnetization is not due to an interface e!ect of the V/Fe interface. Otherwise, we should have observed an increase of *t approximately by a factor of two. This leads to the conclusion that there is a qualitative di!erence between the upper Fe/V and the lower V/Fe interface, as it was also revealed in Ref. [24]. The reduced magnetization values of V(1 1 0)/Fe and V(1 1 0)/2 As Cr/Fe were measured in the same thickness range as the observed structural anomalies for the V(1 1 0)/Fe system which were described in detail in Ref. [28]. Similar to Ref. [24], where also reduced magnetization values were reported, we also observed a relatively high disorder at the interface. For our system this is due to the
thermodynamic conditions during the layer growth. In V/Fe the observed collapse in the island size can be explained by a higher lattice parameter of the V substrate (with d "3.02 As , d "2.87 As V F% and d "2.88 As ), and a smaller free surface enC3 thalpy of V compared to Fe [44]. Due to this condition the strain, which would occur in large Fe islands on V(1 1 0), cannot be compensated for by a gain in the free surface enthalpy. The resulting optimum in the free enthalpy is established for small islands. Between these islands gaps occur which are not closed by additional iron atoms (see also Ref. [28]). With respect to this model it is not yet clear, whether the magnetization vanishes totally near the interface, i.e. if Fe is nonmagnetic indeed or if the magnetization vanishes only on a macroscopic scale. Further experimental and theoretical studies on disordered interfaces might provide an answer to this question. It is to be emphasized, however, that this magnetic behavior does not contradict theory, because most of the theoretical publications are based on perfect (in the sense of highly symmetric) interfaces, where the variation of the magnetization results from a magnetic polarization at the interface, while in our system the structure of the interface is obviously the most important parameter. Vega et al. [9] describe how the magnetic arrangement at the interface is in#uenced by the structural properties. This interpretation is strongly supported by the experiments performed with a smooth Cr bu!er. For the 2 As Cr bu!er (see Fig. 10) we observe the same o!set *t"(!4.08$0.1) As as for the V/Fe system because the Cr layer adopts the lattice constant of the V substrate. For the case of the smooth 200 As Cr bu!er (see Fig. 8) the epitaxial strain is nearly completely relaxed and the growth of Fe on Cr is similar to a homoepitaxial growth because of the small mis"t. We next discuss the e!ects observed by using the faceting during the growth of BCC-structured (1 1 0) surfaces. As already stated, the facets of the V and the Cr "lms have the same orientation, since the ridges are in the [0 0 1] direction with an up and down going sequence in [1 11 0]. The inclination angle of the up- and down-going sequences amounts to 50$23 for the V and to 40$23 for the Cr surface, respectively.
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In the following, we discuss the results of the faceted substrates. MOKE measurements reveal a *t"!(3.3$0.4) As for the Fe "lms on the faceted V layer (Fig. 7). This smaller reduction compared to the smooth V substrate is not in agreement with theoretical predictions of rough V/Fe interfaces [11]. But the main problem in comparing theoretical and experimental results is that the experimentally prepared "lms are not as perfect as assumed in theoretical calculations. Besides that more informations on the detailed structure of the "lms (e.g. STM investigations) are needed. For the 16 As thick faceted Cr layer, where *t amounts to !(2.6$0.3) As , the situation is even more complicated because in addition to the roughness we have to consider that the Cr "lm is in a transition region with respect to the epitaxial strain. The Cr lattice constant is no longer that of the V substrate but it is not yet the bulk value. So, it is reasonable that the amount of the reduction is between the case of the 2 As Cr bu!er and the 200 As Cr bu!er. In contrast to our experimental "ndings with respect to rough and smooth Cr surfaces the roughness at the Fe/Cr interface can drastically reduce the total magnetization due to a reduced magnetization in the Fe and an enhanced antiparallel magnetization in the Cr layer [9]. Experimental studies have shown that the magnetization near disordered interfaces is considerably reduced for Fe/Cr(1 0 0) with *t+!7 As [24]. In conclusion, we want to state that the epitaxial growth conditions like roughness, free surface enthalpy and strain dramatically in#uence the magnetic moment of thin Fe "lms. Hence, for a system X/Fe/X, as demonstrated in the present work for the case of X"V, the magnetic properties of the X/Fe interface can strongly deviate from the ones of the Fe/X interface.
Acknowledgements This work was funded by the Verbundforschung of BMBF under grant No. 03-MA4 HMI-1. The authors are indebted to K. Diederichsen for editing the manuscript.
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References [1] L. NeH el, J. Phys. Radium 15 (1954) 225. [2] E. Ising, Z. Phys. 31 (1925) 253. [3] C.L. Fu, A.J. Freeman, T. Oguchi, Phys. Rev. Lett. 54 (1985) 2700. [4] S. BluK gel, Phys. Rev. Lett. 68 (1992) 851. [5] P. GruK nberg, R. Schreiber, Y. Pang, M.B. Brodsky, H. Sowers, Phys. Rev. Lett. 57 (1986) 2442. [6] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen van Dau, F. Petro!, P. Eitenne, G. Creuzet, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [7] R.H. Victora, L.M. Falicov, Phys. Rev. B 31 (1985) 7335. [8] D. Stoe%er, F. Gautier, J. Magn. Magn. Mater. 147 (1995) 260. [9] A. Vega, C. Demangeat, H. DreysseH , A. Chouairi, Phys. Rev. B 51 (1995) 11546. [10] P. Alvarado, J. Dorantes-DaH vila, G.M. Pastor, Phys. Rev. B 58 (1998) 12216. [11] P. Martin, A. Vega, C. Demangeat, H. DreysseH , J. Magn. Magn. Mater. 148 (1995) 177. [12] R. Coehoorn, J. Magn. Magn. Mater. 151 (1995) 341. [13] A. Vega, A. Rubio, L.C. Balbas, J. Dorantes-Davila, S. Bouarab, C. Demangeat, A. Mokrani, H. DreysseH , J. Appl. Phys. 69 (1991) 4544. [14] A. Vega, L.C. BalbaH s, H. Nait-Laziz, C. Demangeat, H. DreysseH , Phys. Rev. B 48 (1993) 985. [15] T.G. Walker, H. Hopster, Phys. Rev. B 49 (1994) 7687. [16] P. Fuchs, K. Totland, M. Landolt, Phys. Rev. B 53 (1996) 9123. [17] R. Jungblut, C. Roth, F.U. Hillebrecht, E. Kisker, J. Appl. Phys. 70 (1991) 5923. [18] F.U. Hillebrecht, C. Roth, R. Jungblut, E. Kisker, A. Bringer, Europhys. Lett. 19 (1992) 711. [19] T.G. Walker, A.W. Pang, H. Hopster, S.F. Alvarado, Phys. Rev. Lett. 69 (1992) 1121. [20] Y.U. Idzerda, L.H. Tjeng, H.J. Lin, C.J. Gutierrez, G. Meigs, C.T. Chen, Phys. Rev. B 48 (1993) 4144. [21] Y.U. Idzerda, L.H. Tjeng, H.J. Lin, G. Meigs, C.T. Chen, J. Gutierrez, J. Appl. Phys. 73 (1993) 6204. [22] Z. Xu, Y. Liu, P.D. Johnson, B.S. Itchkawitz, Phys. Rev. B 52 (1995) 15393. [23] C. Turtur, G. Bayreuther, Phys. Rev. Lett. 72 (1994) 1557. [24] S. Miethaner, G. Bayreuther, J. Magn. Magn. Mater. 148 (1995) 42. [25] T. Nawrath, H. Fritzsche, F. Klose, J. Nowikow, C. Polaczyk, H. Maletta, Physica B 234}236 (1997) 505. [26] H. Fritzsche, T. Nawrath, H. Maletta, H. Lauter, Physica B 241}243 (1998) 707. [27] T. Nawrath, H. Fritzsche, F. Klose, J. Nowikow, H. Maletta, Phys. Rev. B 60 (1999) 9525. [28] T. Nawrath, H. Fritzsche, H. Maletta, Surf. Sci. 414 (1998) 209. [29] M. Albrecht, H. Fritzsche, U. Gradmann, Surf. Sci. 294 (1993) 1.
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[30] P. Hahn, J. Clabes, M. Henzler, J. Appl. Phys. 51 (1980) 2079. [31] W. Folkerts, F. Hakkens, J. Appl. Phys. 73 (1993) 3922. [32] H. Fritzsche, U. Gradmann, Mater. Res. Soc. Symp. Proc. 312 (1993) 321. [33] G.P. Felcher, R.O. Hilleke, R.K. Crawford, J. Haumann, R. Kleb, G. Ostrowski, Rev. Sci. Instrum. 58 (1987) 609. [34] T. Nawrath, In-situ-Neutronenre#ektometrie und KerrMagnetometrie an ultraduK nnen Eisenschichten auf Vanadium(1 1 0), Ph.D. thesis, TU Berlin, 1998. [35] U. Gradmann, in: K.H.J. Buschow (Ed.), Magnetism in Ultrathin Films in Handbook of Magnetic Materials, Vol. 7, Elsevier Science Publishers, Amsterdam, 1993. [36] R. Bergholz, U. Gradmann, J. Magn. Magn. Mater. 45 (1984) 389.
[37] H. Fritzsche, J. Kohlhepp, U. Gradmann, Phys. Rev. B 51 (1995) 15933. [38] B.R. Cooper, A.J. Bennett, Phys. Rev. B 1 (1970) 4654. [39] C.L. Fu, A.J. Freeman, J. Magn. Magn. Mater. 69 (1987) L1. [40] R.H. Victora, L.M. Falicov, S. Ishida, Phys. Rev. B 30 (1984) 3896. [41] E. Tamura, R. Feder, G. Waller, U. Gradmann, Phys. Stat. Sol. (b) 157 (1990) 627. [42] H. Fritzsche, Struktur und Magnetismus ultraduK nner Eisen- und Kobalt-Epitaxieschichten, Ph.D. Thesis, TU Clausthal, 1995. [43] M. Albrecht, U. Gradmann, T. Furubayashi, W.A. Harrison, Europhys. Lett. 20 (1992) 65. [44] L.Z. Mezey, J. Giber, Jpn. J. Appl. Phys. 21 (1982) 1569.
Magnetization of thin Fe "lms on V(1 1 0) and Cr(1 1 0) T. Nawrath, H. Fritzsche, H. Maletta* Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany Received 12 April 1999; received in revised form 23 November 1999
Abstract The magnetization of thin Fe(1 1 0) "lms was measured by magneto-optical Kerr e!ect (MOKE) magnetometry and polarized neutron re#ectometry (PNR). In order to further understand the in#uence of strain and roughness on the Fe magnetization di!erent surfaces were prepared by evaporating Cr and V on V(1 1 0) single crystals by molecular beam epitaxy. By varying the growth temperature both smooth and faceted Cr and V surfaces were created. For the facets we obtained an up and down staircase structure with the ridges along the [0 0 1] direction that consisted approximately of subsequent (1 0 0) and (0 1 0) surfaces. For the thin Fe "lms that were evaporated on the smooth V(1 1 0) surface a relatively high reduction of the magnetization was obtained, independently of the "lm thickness. In terms of thickness the observed reduction is equivalent to an o!set of *t"!(4.0$1.0) As . A V layer evaporated on this structure does not further reduce the magnetization of the Fe. For comparison Fe evaporated on a smooth Cr "lm was investigated, and the reduction amounted to *t"!(1.4$0.4) As . For thin Fe "lms on faceted V and Cr substrates *t"!(3.3$0.4) and !(2.6$0.3) As were measured, respectively. For samples with a 2 As Cr bu!er layer between the V(1 1 0) single crystal and the Fe "lm we obtained *t"!(4.08$0.10) As , a value which is close to the result obtained for the smooth V(1 1 0) substrate. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ha; 61.14.Hg; 68.35.Ct; 75.30; 75.70; 78.20.Ls Keywords: Magnetization; Thin "lms; Magneto-optical Kerr e!ect; Neutron re#ectometry
1. Introduction The changes occurring in the magnetism of thin "lms in comparison to the bulk material have always been of great interest. This is e. g. re#ected by early works of NeH el [1] in a study on a possible perpendicular anisotropy in thin "lms, or Ising [2], who investigated the critical behavior of magnetic
* Corresponding author. Tel.:#49-30-8062-2058; fax: #4930-8062-2523. E-mail address: [email protected] (H. Maletta)
systems in lower dimensions. More recent studies predicted an enlargement of the magnetic moment of free magnetic layers [3,4], and have motivated further experimental studies. And last but not least magnetism at interfaces is also technologically interesting, since antiferromagnetically aligned bilayers or multilayers are known to create a giant magnetoresistance e!ect [5,6]. The magnetism of iron studied in this paper is characterized by the delocalized 3d-electrons, which carry the main magnetic moment. Thus, it can be expected that the hybridization with the 3d orbitals of V or Cr at the interface strongly
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 8 2 5 - 2
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in#uences the magnetism of the system. Theoretical studies, however, mainly focusing on the (1 0 0) interface, predicted that the magnetic moments of the iron situated next to the interface in a Fe/Cr system are reduced, while the Cr atoms carry an enlarged magnetic moment that is antiparallelly aligned to the magnetic moments of the iron [7}12]. At the Fe/V interface the magnetic moment is also reduced in the "rst Fe layer with an antiparallelly induced magnetic moment in the "rst V layer [10}14]. For both cases this results in a total reduction of the magnetization that can be expressed in terms of a thickness and amounts to +!0.2 As for the Fe/Cr(1 0 0) and +!0.35 As for the Fe/V(1 0 0) interface. The experimental "ndings show more variations than the theoretical predictions: Even in the rather simple case, where thin single V and Cr layers are evaporated on a Fe(1 0 0) surface, the magnetic moments in the "rst V and Cr layer range from !0.3 l to !1.0 l /atom [15,16] for V and from B B !0.5 l to !2.0 l /atom [17}22] for Cr, respecB B tively. Although neither of these methods directly measure the magnetization of the V and Cr and all publications mentioned above apply a model or an estimate derived from known materials, it is more likely that the discrepancy between these results originates from small variations in the structure of the Fe surface. This is e.g. reported in a discussion on the Fe/Cr(1 0 0) system in Refs. [23,24], where the preparation of the substrate does not only lead to topologically di!erent interfaces, but also drastically e!ects the magnetic moments. In previous studies on the magnetization of ultrathin Fe "lms on V(1 1 0) [25}27], we found a relatively high reduction of the magnetization compared to bulk Fe. An o!set independent of the thickness was observed, equivalent approximately to two &magnetically dead' monolayers. We measured a reduction in the magnetization of a wedged sample which in terms of a thickness is equivalent to an o!set of *t"!(4.0$1.0) As . Because of this high reduction the growth of this system was studied in more detail by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), and a special growth mode characterized by a collapse in the island size in the thickness range of
0}6 As Fe was found [28]. Starting from a typical terrace width of the V substrate with 89 As in [0 0 1] and 50 As in [1 11 0] there is a collapse in the island size of the Fe "lm down to values of 21 As in [0 0 1] and 9 As in [1 11 0] for t "4 As . Above t "10 As F% F% the island size of the Fe "lms increases again to 30 As in [0 0 1] and 20 As in [1 11 0]. Contrary to what one might have expected, this behaviour is not connected to a Vollmer}Weber (island) growth, since the intensities of Fe and V measured dependently of the Fe thickness by AES show a coverage typical of a Frank}van der Merwe (layer) growth, and thus it is very probable that the magnetic behaviour of the thin Fe "lm is a result of this special growth mode. In our studies the substrate on which the Fe "lm was evaporated was varied, while the growth temperature and rate were not changed throughout the preparation. The results of the magnetization of Fe on a faceted V(1 1 0) substrate will be presented below. This faceting (described in more detail in Ref. [28]) is characterized by a quasi-periodic sequence of up and down staircases in [1 11 0] inclined to the "lm plane at an angle of 503 and ridges orientated along [0 0 1]. Thus the substrate can be regarded as a subsequent sequence of up- and down-going (1 0 0) and (0 1 0) surfaces. Additionally, Fe layers on Cr(1 1 0) were studied, and experiments on smooth and faceted Cr surfaces were performed with the facets orientated in the same direction as in the before mentioned V substrate. The results of these experiments shed more light on the magnetic behaviour of the Fe/V(1 1 0) system.
2. Sample preparation The V(1 1 0) single crystals were cleaned during repeated sputtering and annealing cycles, with 700 eV argon ions used in the sputtering process and at a temperature of 1400 K during the annealing cycles in order to recrystallize the surface. This process was repeated for 120 h and resulted in a clean surface with contamination below the detection limit of the Auger spectrometer, i.e. below 2% of a monolayer for sulphur or carbon and below 5% for oxygen (because the oxygen transition coincides with the vanadium
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LMM-transition). No other contamination was detected during the sputtering and annealing cycles. All "lms were prepared under ultra-high vacuum (UHV) conditions at a base pressure of (10~10 mbar. The surface of the "lms were not contaminated either. In all experiments the Fe was evaporated at a rate of 0.07 As /s at a sample temperature of 320 K. For the preparation of smooth and faceted V and Cr substrates we exploited a special behavior which has been observed for many BCC(1 1 0) surfaces: a facetting with staircases in [1 11 0] direction and ridges along [0 0 1] [28}32]. The conditions of preparation were varied as follows: (a) The smooth V surface was prepared by covering a clean single crystal with a 10 As V layer at an evaporation temperature of ¹ "320 K at % a rate of 0.07 As /s. This resulted in a typical terrace width of 80 As in the [0 0 1] and 50 As in the [1 11 0] direction [28]. (b) The faceted V surface with ridges orientated in the [0 0 1] direction and inclinations of 503 to the "lm plane was also prepared on the clean V(1 1 0) crystal by evaporating 50 As V at ¹ "570 K and a rate of 0.07 As /s. Here the % terrace width in [0 0 1] direction is about 52 As [28]. (c) The smooth Cr surface was prepared by evaporating 200 As Cr on a V(1 1 0) single crystal at ¹ "470 K. With respect to the terrace width % and the roughness of the Cr "lm, best results were obtained at a growth rate of 2 As /s. A LEED picture of these "lms is shown in Fig. 1 and allows for determining the terrace width to be 28.8$1.5 As in the [0 0 1] and 23.8$2.0 As in the [1 11 0] direction. (d) The faceted Cr surface was prepared at ¹ "320 K and 0.07 As /s on a clean V(1 1 0) % crystal. The resulting faceting was identical to that of V prepared at ¹ "570 K with the % ridges orientated in [0 0 1], a terrace width in [0 0 1] direction of about 50 As [28] and a quasi-periodic sequence of up and down staircases along [1 11 0] now inclined at an angle of 403. For our experiment a 16 As Cr "lm was evaporated on the V(1 1 0) single crystal. (e) For the last experiment a 2 As thick buwer layer of Cr (one monolayer) was evaporated on the
339
Fig. 1. LEED picture (E"70 eV) of a 200 As thick Cr layer evaporated on V(1 1 0) at ¹"470 K. The surface directions of the crystal are also sketched.
V(1 1 0)/10 As V substrate (see paragraph a) before the Fe "lm was evaporated at a growth temperature of 320 K and a rate of 0.07 As /s. Because Cr grows facetedly on V(1 1 0) at 320 K, it is likely that the Cr "lm is not perfectly closed and that parts of the evaporated Fe are in contact with the Cr "lm and the V substrate.
3. Experiment The magnetic properties of the "lm were studied by polarized neutron re#ectometry (PNR) and magneto-optical Kerr e!ect (MOKE) magnetometry. PNR probes the "lm with polarized neutrons and is sensitive to the in-plane magnetization [33]. For details of how to apply this method as an in situ technique under UHV conditions see Ref. [27]. MOKE measurements are based on the rotation of the polarization axis of light, which results from the magnetization of the thin "lm. Our set-up is displayed in Fig. 2 in the longitudinal geometry
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Fig. 2. Set-up of the MOKE magnetometer in the longitudinal geometry.
with the applied magnetic "eld in-plane with the incident and re#ected LASER beam. The measurements were performed with an He}Ne-LASER (j"632.8 nm) in the s-polarization. A lock-in ampli"er was used in order to increase the sensitivity, and the LASER amplitude was modulated (l" 100.24 kHz) by a subsequent arrangement of a polarizer, a photoelastic modulator, and a second polarizer. To generate the modulated and polarized light, the "rst polarizer was adjusted to an s-con"guration, with the two axes of the photoelastic modulator adjusted to this polarization axis at an angle of 453. The second polarizer was arranged parallelly or perpendicularly to the "rst polarizer to create s-polarized or p-polarized light, respectively. For the experiments presented here the longitudinal geometry sensitive to the in-plane component of the magnetization was used and a magnetic "eld was applied in the [0 0 1] direction, which is the easy axis of the Fe "lms. The measurements were carried out in situ and on Fe wedges that were prepared under the same conditions as the "lms for the in situ PNR measurements. The wedges were inclined by 0.6 As /mm with a diameter of the LASER spot of +0.3 mm. Fig. 3 displays the Kerr hysteresis loops for an Fe thickness of 11.2 or 11.8 As for di!erent substrates. Fig. 3a shows the hysteresis loop of a 11.2 As Fe layer on V(1 1 0)/200 As Cr (smooth Cr surface; the preparation of this substrate is described in paragraph c), while Figs. 3b and c show the hysteresis loops of V(1 1 0)/16 As Cr/11.8 As Fe (faceted Cr
Fig. 3. MOKE hysteresis loops of Fe "lms on di!erent substrates: (a) 11.2 As Fe on V(1 1 0)/200 As Cr (For the surface structure of the Cr "lm see Fig. 1); (b) 11.8 As Fe on V(1 1 0)/16 As Cr; (c) 11.2 As Fe on V(1 1 0)/10 As V/2 As Cr.
surface, see paragraph d) and V(1 1 0)/2 As Cr/ 11.2 As Fe (see paragraph e). Despite the di!erent surface structures of the Cr substrate it can be seen that the coercive "elds of Fe increase with increasing Cr thickness. It is an interesting property of Fe on Cr(1 1 0) "lms, that the coercive "elds increase in dependence on the Cr thickness. As shown in Fig. 3, a higher Cr thickness results in a higher coercive "eld. Further experiments have shown that this also holds for Cr layers evaporated on Fe "lms [34]. Most likely this property, which may also be of technological interest, can be explained by the antiferromagnetism of the Cr layers.
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In the following paragraphs a}e the results of the magnetic measurement on the various substrates shall be presented. The preparation of the substrates is described in the preceding section in the paragraphs a}e. Except for the wedge covered with 50 As V, all "lms were measured in situ under UHV conditions. After the MOKE and PNR measurements were "nished the "lms were tested for surface contamination with AES and no contamination was detected. In MOKE magnetometry in the thin "lm limit the maximum Kerr rotation is proportional to the magnetization. This value is obtained from the different Kerr rotations measured at the two saturation values in the MOKE loops, and will be presented in the following "gures. The proportionality factor depends on the experimental set-up (angle of incidence, s- or p-polarization of the light, etc.) and on the optical constants of the "lm and the substrate. (a) In Figs. 4a}c the measured neutron re#ectivities of Fe "lms deposited on V(1 1 0) with a thickness of 6, 10 and 19.5 As are presented. The spin-up re#ectivities are plotted as up and the spin down re#ectivities as down triangles, respectively. The Fe "lms of a thickness of 6 and 10 As were measured at 80 K, whereas the 19.5 As "lm was measured at 110 and 300 K. Fig. 4 shows that for all measurements best "ts are obtained at reduced magnetic moments. Averaged values of k "1.8 and F% k "1.3 l per Fe atom are obtained for t " F% B F% 19.5 As and t "10 As , respectively, while the magF% netization vanishes at a "lm thickness of 6 As . In Fig. 5 these results of the PNR measurements are plotted (as squares) as a product of the magnetic moment per atom, l , times the Fe thickness F% t versus t . This product is directly proportional F% F% to the in-plane component of the magnetic moment of the "lm and therefore increases with increasing "lm thickness t . F% Additionally, the MOKE data were plotted as circles in Fig. 5 and adjusted to the PNR data by multiplying the Kerr intensities by a factor speci"c to the set-up. In order to compare these results to the MOKE data of the other "lms (e.g. the covered "lm) a normalized Kerr rotation was plotted in the following "gures. After "tting the Kerr data by a straight line they were divided by the inclination
341
Fig. 4. PNR measurements of Fe "lms of various thicknesses of 19.5, 10, and 6 As on V(1 1 0) are shown in parts (a), (b), and (c), respectively. The re#ectivities of the spin-up neutrons are given as up triangles, the spin-down re#ectivities as down triangles. The simulations with the magnetic moment of bulk iron are shown for comparison as dotted lines, whereas the best "ts are performed with a magnetic moment per atom of 1.8 l , 1.3 l , B B and 0 l . B
of the line to get the normalized signals. The resulting inclination always equals 1 rel. unit/As . For both measurements shown in Fig. 5 the (in-plane) magnetization vanishes at an Fe thickness of t "6 As . By "tting a straight line to the F% data points of the "nite magnetic signals a negative o!set equivalent to *t"!(4.0$1.0) As is obtained. In order to test whether a "nite magnetization component perpendicular to the "lm layer (that might correspond to the detected magnetic o!set)
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Fig. 5. Magnetization of Fe "lms on V(1 1 0)/10 As V versus Fe "lm thickness t . The PNR results are plotted (as squares) as F% the product of the magnetic moment per atom k times t (left F% F% side) which is directly proportional to the magnetic moment. In addition the maximum MOKE signals obtained on a similarly prepared Fe wedge are plotted as full circles and adjusted to the PNR data by multiplication with a set-up speci"c factor (right side). *t"!(4.0$1.0) As .
remains MOKE measurements were performed also in the polar geometry that is sensitive to a perpendicular magnetization. In this experiment no magnetic signal was detected although the polar Kerr signal was expected to be by one order of magnitude more intense than the longitudinal signal in the current set-up. This leads to the conclusion that the Fe "lms on V(1 1 0) with t up to F% 6 As have indeed zero magnetization at a measuring temperature of 300 K. In the next experiment the "lm shown in Fig. 5 was additionally covered with 50 As V to "nd out if this results in an increased reduction of the magnetization. For this purpose the V layer was evaporated at ¹ "320 K and a rate of 0.07 As /s. The % MOKE measurements of this wedge are shown in Fig. 6, where the maximum Kerr signal is plotted versus t . The data reveal no further reduction, the F% o!set is *t"!(3.8$0.3) As . (b) In Fig. 7 the maximum Kerr signal of an Fe wedge evaporated on a faceted V(1 1 0) surface is plotted versus t . The V surface was prepared by F% evaporating 50 As V on the V(1 1 0) single crystal (as described in paragraph b). An o!set of *t"!(3.3$0.4) As was obtained. (c) Fig. 8 shows the maximum Kerr signal of an Fe wedge evaporated on a smooth 200 As Cr "lm. The Cr "lm was prepared at ¹ "470 K (as %
Fig. 6. The maximum Kerr signals of Fe on V(1 1 0)/10 As V coated with a 50 As V "lm is plotted versus the Fe thickness t . *t"!(3.8$0.3) As . F%
Fig. 7. Maximum Kerr signals of Fe on a faceted V surface plotted versus the Fe thickness t . *t"!(3.3$0.4) As . F%
Fig. 8. Maximum Kerr signals of Fe on a smooth Cr surface (V(1 1 0)/200 As Cr) plotted versus the Fe thickness t . *t" F% !(1.4$0.4) As .
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343
Table 1 Terrace widths for the various substrates as described in paragraphs a}e in the text. The magnetically dead layers *t obtained for the di!erent substrates are also included
(a) (b) (c) (d) (e)
Substrate
Terrace width along [0 0 1] (As )
Terrace width along [1 11 0] (As )
*t (As )
V V/50 As V V/200 As Cr V/16 As Cr V/2 As Cr
80 52 29 50 80
50 2 24 2 50
!4.0 !3.3 !1.4 !2.6 !4.1
Fig. 9. Maximum Kerr signals of Fe on a faceted Cr surface plotted versus the Fe thickness t . *t"!(2.6$0.4) As . F%
4. Discussion In general the magnetic saturation moment m of S thin "lms follows the relation [35]
Fig. 10. Maximum Kerr signals of Fe on V(1 1 0)/10 As V/2 As Cr plotted versus the Fe thickness t . *t"!(4.08$0.10) As . F%
described in paragraph c). An o!set of *t"!(1.4$0.4) As was obtained. (d) Fig. 9 shows the maximum Kerr signal of an Fe wedge evaporated on a faceted Cr "lm. The faceting has the same orientation as the V surface (described in paragraph b). An o!set of *t"!(2.6$0.3) As was obtained. (e) In the last experiment a 2 As thick Cr layer was evaporated on the V(1 1 0) crystal that had been covered with 10 As V before (see paragraph e). The results are presented in Fig. 10. An o!set of *t"!(4.08$0.10) As was obtained. The magnetic and structural properties of the di!erent systems described in paragraphs (a)}(e) are compiled in Table 1.
m "J A(t#*t) (1) S S with the bulk saturation magnetization J , the S sample area A, and the o!set *t which is called the e!ective magnetically dead layer thickness for negative values. The parameter *t is obtained as o!set from a linear extrapolation of the data according to Eq. (1). Magnetically dead layers have been observed e.g. for Ni(1 1 1) "lms on Re( 0 0 0 1) [36] or Co "lms on W(1 1 0) [37]. The reasons for a non-vanishing *t may be (i) a deviation of the ground state magnetic moment of the magnetic "lm from its bulk value in the surface of the magnetic "lm (&surface e!ect') [38]; (ii) a deviation of the ground state magnetic moment of the magnetic "lm from its bulk value at the interface between substrate and magnetic "lm (&interface e!ect'); (iii) a polarization of the substrate with ferromagnetically or antiferromagnetically coupling with respect to the magnetization of the magnetic "lm (&polarization e!ect'); (iv) enhanced thermal decrease of magnetization in thin "lms (&size e!ect') [35,38]. A deviation from Eq. (1) is only observed near the region of the critical thickness where the "lm becomes ferromagnetically. The critical thickness itself is in#uenced by the growth mode of the epitaxial system under investigation.
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We shall "rst discuss the MOKE and PNR measurements of Fe on V(1 1 0), which reveal a high reduction in the magnetization independent of the Fe thickness (an o!set of *t" !(4.0$1.0) As ) as shown in Fig. 5. Additional PNR studies revealed that the increase of the magnetization at lower temperatures can be neglected [26,27]. Hence, *t cannot be explained by a temperature-dependent size e!ect, which would actually lead to the observation of a reduced magnetic moment due to a non-saturated magnetization (in case of a very low Curie temperature of the thin "lm). The reduced magnetization might be interpreted as a result either of the V/Fe interface or of the Fe/vacuum interface, or as a combination of both e!ects. Theory predicts an increase in the magnetization at the Fe surface [3,8,9,39,40] and a reduction at the Fe/V interface [10}14]. Experimental investigations on the magnetization of the free Fe surface show in agreement with theory an increase of the magnetic moments at the Fe surface [41,42] corresponding to *t"0.5 As . Experiments in dependence of the roughness show an additional slightly increasing magnetization of rough Fe(1 1 0) surfaces [43]. All these e!ects mentioned above are small compared to our "ndings and cannot explain the large o!set of *t"!4 As . To shed more light on that large o!set we evaporated 50 As V onto a V(1 1 0)/Fe wedge, what did not result in a further reduction of the magnetization of the Fe "lm (see Fig. 6). This experiment clearly demonstrates that the observed large reduction of the magnetization is not due to an interface e!ect of the V/Fe interface. Otherwise, we should have observed an increase of *t approximately by a factor of two. This leads to the conclusion that there is a qualitative di!erence between the upper Fe/V and the lower V/Fe interface, as it was also revealed in Ref. [24]. The reduced magnetization values of V(1 1 0)/Fe and V(1 1 0)/2 As Cr/Fe were measured in the same thickness range as the observed structural anomalies for the V(1 1 0)/Fe system which were described in detail in Ref. [28]. Similar to Ref. [24], where also reduced magnetization values were reported, we also observed a relatively high disorder at the interface. For our system this is due to the
thermodynamic conditions during the layer growth. In V/Fe the observed collapse in the island size can be explained by a higher lattice parameter of the V substrate (with d "3.02 As , d "2.87 As V F% and d "2.88 As ), and a smaller free surface enC3 thalpy of V compared to Fe [44]. Due to this condition the strain, which would occur in large Fe islands on V(1 1 0), cannot be compensated for by a gain in the free surface enthalpy. The resulting optimum in the free enthalpy is established for small islands. Between these islands gaps occur which are not closed by additional iron atoms (see also Ref. [28]). With respect to this model it is not yet clear, whether the magnetization vanishes totally near the interface, i.e. if Fe is nonmagnetic indeed or if the magnetization vanishes only on a macroscopic scale. Further experimental and theoretical studies on disordered interfaces might provide an answer to this question. It is to be emphasized, however, that this magnetic behavior does not contradict theory, because most of the theoretical publications are based on perfect (in the sense of highly symmetric) interfaces, where the variation of the magnetization results from a magnetic polarization at the interface, while in our system the structure of the interface is obviously the most important parameter. Vega et al. [9] describe how the magnetic arrangement at the interface is in#uenced by the structural properties. This interpretation is strongly supported by the experiments performed with a smooth Cr bu!er. For the 2 As Cr bu!er (see Fig. 10) we observe the same o!set *t"(!4.08$0.1) As as for the V/Fe system because the Cr layer adopts the lattice constant of the V substrate. For the case of the smooth 200 As Cr bu!er (see Fig. 8) the epitaxial strain is nearly completely relaxed and the growth of Fe on Cr is similar to a homoepitaxial growth because of the small mis"t. We next discuss the e!ects observed by using the faceting during the growth of BCC-structured (1 1 0) surfaces. As already stated, the facets of the V and the Cr "lms have the same orientation, since the ridges are in the [0 0 1] direction with an up and down going sequence in [1 11 0]. The inclination angle of the up- and down-going sequences amounts to 50$23 for the V and to 40$23 for the Cr surface, respectively.
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In the following, we discuss the results of the faceted substrates. MOKE measurements reveal a *t"!(3.3$0.4) As for the Fe "lms on the faceted V layer (Fig. 7). This smaller reduction compared to the smooth V substrate is not in agreement with theoretical predictions of rough V/Fe interfaces [11]. But the main problem in comparing theoretical and experimental results is that the experimentally prepared "lms are not as perfect as assumed in theoretical calculations. Besides that more informations on the detailed structure of the "lms (e.g. STM investigations) are needed. For the 16 As thick faceted Cr layer, where *t amounts to !(2.6$0.3) As , the situation is even more complicated because in addition to the roughness we have to consider that the Cr "lm is in a transition region with respect to the epitaxial strain. The Cr lattice constant is no longer that of the V substrate but it is not yet the bulk value. So, it is reasonable that the amount of the reduction is between the case of the 2 As Cr bu!er and the 200 As Cr bu!er. In contrast to our experimental "ndings with respect to rough and smooth Cr surfaces the roughness at the Fe/Cr interface can drastically reduce the total magnetization due to a reduced magnetization in the Fe and an enhanced antiparallel magnetization in the Cr layer [9]. Experimental studies have shown that the magnetization near disordered interfaces is considerably reduced for Fe/Cr(1 0 0) with *t+!7 As [24]. In conclusion, we want to state that the epitaxial growth conditions like roughness, free surface enthalpy and strain dramatically in#uence the magnetic moment of thin Fe "lms. Hence, for a system X/Fe/X, as demonstrated in the present work for the case of X"V, the magnetic properties of the X/Fe interface can strongly deviate from the ones of the Fe/X interface.
Acknowledgements This work was funded by the Verbundforschung of BMBF under grant No. 03-MA4 HMI-1. The authors are indebted to K. Diederichsen for editing the manuscript.
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