- Email: [email protected]
LaHx multilayers
Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
Hydrogen-controlled interlayer exchange coupling in Fe/LaHx multilayers W. Lohstroha,*, F. Leuenbergera, W. Felscha, H. Fritzscheb, H. Malettab a
I. Physikalisches Institut, Universitat Bunsenstrae 9, 37073 Gottingen, Germany . Gottingen, . . b Hahn-Meitner-Institut Berlin, Glienicker Str. 100, 14109 Berlin, Germany Received 11 May 2001; received in revised form 26 July 2001
Abstract Magneto-optic Kerr magnetometry and neutron reflectometry reveal that Fe layers exhibit magnetic exchange coupling through LaHx spacer layers. Ferromagnetic and antiferromagnetic coupling is observed on multilayers of these materials depending on the thickness of the hydride layers, but without oscillatory behavior. Starting from metallic La dihydride spacer layers the effect of dissolving increasingly more hydrogen was examined. Sign and value of the coupling depend crucially on the hydrogen content x: The coupling can be inverted from antiferromagnetic to ferromagnetic and vice versa. These alterations are due to modifications of the electronic structure of the hydride. When the hydrogen absorption saturates the hydride layers become insulating and the exchange coupling is likely to disappear. In this final state the multilayers are always characterized by a very soft ferromagnetic rectangular hysteresis curve. Upon removal of the hydrogen to the initial concentration the original magnetic structure is restored. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Metallic multilayers; Interlayer exchange coupling; Spin structures; Rare-earth hydrides; Hydrogen in thin films
1. Introduction Interlayer exchange coupling is a property common to many multilayer systems [1–3]. Mediated by the spin polarization of the conduction electrons in a non-magnetic metallic spacer material the magnetization vectors of neighboring ferromagnetic layers are aligned either parallel or antiparallel. The relative orientation generally
*Corresponding author. Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK. Tel.: +44-1865-272-367; fax: +44-1865-282-221. E-mail address: [email protected] (W. Lohstroh).
oscillates as a function of the thickness of the spacer layers. Various theoretical models have been proposed to explain the effect, for an overview see Ref. [3]. They all agree in that the periods of the oscillatory coupling are determined by critical spanning vectors of the Fermi surface of the material that makes up the spacer [4]. Furthermore, it is now widely accepted that the different models can be unified into an approach based upon quantum interference due to spindependent electron confinement in the spacer layers caused by the magnetic layers [5]. This means that matching of the energy bands at the interfaces is important. In this more general model interlayer exchange coupling is described in terms
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 4 9 1 - 7
78
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
of spin-dependent reflection of electrons at the interfaces. Strength and phase of the coupling are sensitive to details of the interfacial structure. Recent experiments on Fe/Nb [6] and Fe/V [7,8] multilayers have shown that the introduction of hydrogen into the Nb or V spacers opens a new way for tuning the exchange coupling between the Fe layers. By simply changing the hydrogen pressure surrounding the sample the magnetic structure can be reversibly inverted from antiferromagnetic (AFM) to ferromagnetic (FM) and vice versa. The change of interlayer coupling energy is not simply due to the variation of the interatomic distances in the Nb or V spacer layers with their hydrogen content but to a modification of the Fermi surface. In view of these observations multilayers comprising rare-earth (RE) metals are of particular interest. The electronic structure of these elements and their metallic character changes drastically upon hydrogen absorption. The light RE metals like La form a cubic dihydride REH2 (CaF2-type structure) with hydrogen atoms on tetrahedrally co-ordinated sites (b-phase), and dissolve in a single phase further hydrogen on octahedral sites (g-phase) up to the cubic trihydride phase REH3 (BiF3-type structure) [9]. Both phases have almost the same lattice parameter, but a very different electronic structure: the dihydride is metallic whereas the trihydride is insulating. The transition is accompanied by a change of the optical properties from metallic reflecting to transparent for photon energies below B2 eV. The binding energies for hydrogen are very different at the tetrahedral and octahedral positions: it is an exciting recent discovery that for RE hydrides in thin film geometry the transition between the di- and trihydride phases is completely reversible in a relatively short time, whereas in the dihydride the hydrogen is irreversibly bound. This lends considerable technological importance to these materials, permitting to construct switchable mirrors for visible light [10]. Quite generally, it has caused renewed interest in RE hydride systems among both experimentalists and theoreticians. We have studied and discuss here the magnetic structures in Fe/LaHx multilayers, xX2 d; and how they are altered by a variation of the
hydrogen content x: The experimental observations were carried out by magneto-optic Kerr (MOKE) magnetometry and by neutron reflectometry at room temperature. In contrast to the pure Fe/La system1 these heterostructures exhibit an interlayer exchange coupling for the metallic phase of the LaHx spacer. The addition of small amounts of atomic hydrogen is sufficient to alter the coupling between the Fe layers, from AFM to FM and vice versa. Upon further hydrogen uptake, LaHx looses its metallic character and the magnetic layers are thought to decouple. When the hydrogen content is reduced to the initial concentration, the magnetic structures are restored. Hence interlayer coupling may be reversibly switched off and on in this multilayer system, which is quite unique.
2. Sample characteristics ( A series of multilayer samples [Fe(15 A)/ ( LaHx(tLaH )] n (tLaH ¼ 10260 A) was grown at room temperature by reactive ion beam sputtering with argon in an ultra-high vacuum chamber (base pressure o5 1010 mbar), after hydrogen had been introduced (partial pressure 8 106 mbar). The number of bilayers, n; was chosen so that the ( High-purity total thickness was close to 2000 A. gases Ar(6N), H2(6N) and target metals La(3N), Fe(4N8) were used. During the layer deposition the partial pressure of reactive impurity gases (O2, N2 and H2O) was below 1010 mbar. Si(1 0 0) ( Cr buffer layer served wafers covered with a 40 A as substrates. For the measurements of the optical transmittance the samples were deposited on thin Cr-coated glass plates. Unlike the La layers the Fe layers do not dissolve hydrogen since the heat of solution is endothermic for this metal, whereas it is strongly exothermic for La. The samples were ( thick Pd cover layer to terminated with a 100 A prevent oxidation and to ensure additional hydrogen uptake and release after preparation. 1 In Fe/La multilayers exchange coupling between the Fe layers is absent, but artificial magnetic configurations may be induced by a small magnetric field acting on the samples during growth (see Ref. [11]).
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
Analysis of selective samples by the resonant nuclear reaction 1H(15N, ag)12C revealed a stable lanthanum hydride in the multilayers after preparation, with composition LaHx where dE0:5 [12]. Such deviation from the perfect stoichiometry of a b-phase rare-earth hydride is not uncommon and has been attributed to effects from the interfaces and/or lattice disorder in the thin-film structure [9]. Structural characterization of the multilayers was performed by X-ray diffraction in y=2ygeometry using Cu-Ka radiation. The small-angle spectra (Fig. 1) reveal layered stacks with good periodicity and sharp interfaces. Fits to the data (solid line) employing an algorithm based on optical theory [13,14] yield a RMS roughness ( (FWHM) at the interfaces. The values of 2.5–3 A high-angle spectra show that both components are crystalline: Fe grows in the a-phase BCC structure, LaHx in the b-phase CaF2-like FCC structure with (1 1 1) texture. Rocking scans around the (1 1 1) reflection show a width of B51 at half maximum. No satellites around the main reflections are observed which indicates the absence of coherent growth in the multilayers, in spite of the small mismatch (B3%) of the component-layer lattices. The exposure of the as-prepared multilayer samples (with x ¼ 2 d) to an atmosphere of
( Fig. 1. Low-angle X-ray reflections of the multilayer [Fe(15 A)/ ( 35, xE1:5: The sharp peaks are the Bragg LaHx(20 A)] reflectivities of the mutilayer; the broader interference fringes in between are due to the Pd capping layer. Solid line: fit to the data based on optical theory.
79
hydrogen gas at room temperature in a previously evacuated chamber leads to additional uptake of atomic hydrogen in the LaHx sublayers. In this way the insulating state of the hydride (x > 2:8) can be reached even for the thinnest layers at moderate hydrogen pressures (p1000 mbar). This is ascertained by measurements of the transmittance of visible light that were carried out in parallel with the MOKE magnetometry: a steep increase of the transmitted light was observed when the hydride became insulating. The results agree with conclusions drawn from detailed measurements of the conductance on this multilayer system [15]. The time scale for the uptake of hydrogen is essentially a function of its pressure and the thickness of the LaHx sublayers. In any case the kinetics of the absorption and desorption processes for hydrogen in these heterostructures are very different: for a multilayer with insulating lanthanum hydride placed into vacuum it takes typically at least 2 weeks until the additional hydrogen has been released and the initial stable dihydride phase is restored. This permits to perform X-ray diffraction (and other experiments) on the additionally charged samples ex situ. The high-angle spectra reveal that the fcc lattice symmetry and the lattice parameter of LaHx do not change across the metal-to-insulator transition, in agreement with the behavior of the bulk hydride. In fact, the lattice parameter of bulk LaHx is B1.5% smaller in the trihydride than in the dihydride phase [9], which is within the error bars of our experiment. It turns out that the structural roughness at the interfaces of the multilayers measured with X-ray reflectometry ( in the insulating phase increases only by B0.2 A of the hydride. In spite of the preservation of the interatomic distances in the LaHx layers upon additional hydrogen uptake, as indicated by X-ray diffraction, the low-angle neutron reflectivity spectra recorded in situ under a hydrogen atmosphere (see Section 4 below) reveal an increase of the bilayer thickness (tFe þ tLaH ) which amounts to 4% in the insulating phase of LaHx. A similar effect was observed in polycrystalline Fe/NbHx [16] and epitaxial W/NbHx [17] multilayers for which the hydrogen-induced relative out-of-plane expansion
80
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
of the Nb layers is considerably larger than the increase of the Nb interplanar spacing. The anomalous increase of the Nb layer thickness is attributed to hydrogen incorporated in imperfections of the Nb lattice, like grain boundaries, dislocations, voids, or at the interfaces of the heterostructures. These defects add to the normal interstitial sites for hydrogen absorption. The similar observations on the Fe/LaHx multilayers suggests that a similar trapping mechanism is effective in this system, too. The effect is reversible: after desorption of the hydrogen added after preparation the original bilayer thickness is restored.
3. MOKE magnetometry The magnetization of the multilayers was studied by measuring the Kerr rotation of the polarized beam of a He–Ne laser reflected by the sample. The measurements were performed in the longitudinal configuration using a photoelastic modulator [18]. Rotation angles down to 2 1041could be resolved which corresponds in the case of Fe layers to a magnetization of 40 emu/ cm3. For the wavelength of the light used (lL ¼ 632:8 nm) the depth sensitivity is about ( in Fe. In the Fe/LaHx multilayers the 300 A probed depth depends on the hydrogen concentration in the LaHx spacer since its transmittance increases as the LaHx layers become insulating. Therefore the displayed MOKE hysteresis curves have been normalized to the signal obtained at saturation. The samples were mounted into a highvacuum hydrogenation chamber that was placed between the two coils of a Helmholtz magnet. Magnetic fields up 900 Oe could be applied. Glass windows were used to introduce the laser beam into the chamber. Hydrogen pressures up to 1000 mbar were applied while recording the magnetic hysteresis loops. For all samples the easy axis of magnetization was found to be in the layer plane without any preferred direction. In the same setup the transmittance of the samples could be measured. All measurements were performed at room temperature, as already indicated.
After preparation, when the hydride is in the metallic phase LaHx, the multilayers show interlayer exchange coupling for spacer thickness ( with AFM Fe-layer magnetization tLaH o30 A, ( and FM alignment for tLaH between 16 and 23 A, alignment in all other cases. The Fe-layer thickness ( Representative examples for the was always 15 A. hysteresis curves are displayed in Fig. 2a and b for ( thick LaHx two multilayers with 20 and 24 A ( layers, respectively. For tLaH ¼ 24 A (Fig. 2b), the hysteresis curve essentially has a rectangular shape with a large remanence and a low saturation field. This points to an FM Fe-layer orientation. For the ( (Fig. 2a), lower spacer-layer thickness, tLaH ¼ 20 A the curve rises more slowly with the applied magnetic field, the saturation field is considerably higher and the remanence is lower. This suggests an AFM coupling of neighboring Fe layers as it was confirmed by neutron reflectometry. The complex shape of the magnetization curve, its pronounced hysteresis and the deviation from linearity for applied fields up to a few 10 Oe indicates that effects from domain nucleation and/ or domain-wall displacements in the multilayers are superposed to the linear M2H dependence expected for normal spin-flop AFM in a magnetic field. In the case of AFM interlayer exchange coupling the coupling energy J can be estimated from the magnetic saturation fields HS using the relation [19] J ¼ HS MS;Fe tFe =4;
ð1Þ
where MS;Fe denotes the saturation magnetization of the Fe layers; measurements with a vibrating sample magnetometer reveal that it is close to the literature value of bulk BCC Fe (1704 emu/cm3). HS was taken as a technical saturation field at 95% of the saturation magnetization from the average of the hysteresis branches for increasing and decreasing field, respectively. The resulting energies for AFM coupling are plotted in Fig. 3 (solid circles). The exchange energy for FM coupling was not accessible by magnetometry. For an overview, the multilayers concerned are represented in Fig. 3 with J set to zero (crosses) in case of an FM hysteresis. As can be seen, AFM interlayer coupling in the as-prepared multilayers Fe/LaHx
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
81
( ( ( ( Fig. 2. MOKE-hysteresis curves of the multilayers [Fe(15 A)/LaH x(20 A)] 35 and [Fe(15 A)/LaHx(24 A)] 32. The top row (a) and (b) displays the hysteresis curves obtained after preparation (xB1:5); the subsequent rows in each column show their evolution with time after 1 mbar H2 had been introduced into the sample chamber. Note the different scale of the magnetic field in the two rows.
( is restricted to spacer-layer thickness 16 Ap ( Only one AFM peak is observed, tLaH p23 A. there is no oscillatory behavior as tLaH varies. The ( amounts to maximum value of jJj at tLaH ¼ 17 A B0.014 erg/cm2. This value is of the order of the coupling strength observed for maximum AFM coupling in Fe/V superlattices [8] if compared at
the same spacer-layer thickness, assuming asymptotic scaling as 1=t2spacer [4]. Also displayed in Fig. 2 are the hysteresis curves ( recorded at of the samples with tLaH ¼ 20 and 24 A several stages on the indicated time scale after the introduction of 1 mbar of hydrogen gas into the measuring chamber. These experiments were
82
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
Fig. 3. Strength of the AFM exchange coupling, J; as a function of the thickness of the metallic LaHx interlayers. Fe-layer thickness: ( Solid circles: values obtained after preparation. In the regimes with FM Fe-layer orientation J has been set to zero (crosses). 15 A. Open diamonds: values obtained after additional hydrogen absorption (see text). The continuous line is a guide to the eye only.
possible due to the slow kinetics of the hydrogen uptake in small applied gas pressures. As an increasing amount of atomic hydrogen is absorbed in the spacer layers the curves change their shape. The sample coupled antiferromagnetically after ( Fig. 2a) shows an inpreparation (tLaH ¼ 20 A, creasingly less sheared, more narrow hysteresis curve (Fig. 2c, e and g). The curve recorded at 100 min after the beginning of additional hydrogen uptake is squared with a high remanent magnetization and low saturation field. These characteristics remain unchanged even after 24 h and upon further increase (up to 1000 mbar) of the hydrogen pressure in the chamber. Obviously, the absorption of additional hydrogen into LaHx essentially leads to an FM orientation of the Fe-layer magnetizations in this sample, with a very soft hysteresis curve. Comparison with the transmission experiments confirms that the LaHx layers are insulating in the final loading stage. The altered magnetic properties cannot result from a lattice expansion of the LaHx spacer layer since AFM ( prior to coupling was observed up to tLaH ¼ 23 A additional hydrogen absorption. The change of the magnetic configuration must be due to a modification of the electronic structure of the LaHx sublayers induced by the additionally dissolved hydrogen on interstitial sites.
A different behavior in the course of hydrogen absorption can be recognized for the sample with ( It shows an FM hysteresis curve for tLaH ¼ 24 A. the stable La dihydride phase (Fig. 2b) that becomes increasingly sheared and looses remanence until it becomes AFM after 100 min (Fig. 2f). In an intermediate state, after 45 min, the hysteresis curve exhibits two steps (Fig. 2d). This indicates that in the intermediate loading stage domains with different hydrogen concentrations and hence different magnetic exchange interactions are present. The smooth shape observed after 100 min (Fig. 2f) reveals that at this stage the coupling is AFM in the entire sample. As more time elapses and the hydrogen content of the spacer increases further the AFM structure decays and eventually a very soft hysteresis curve is observed, with a saturation field HS E4 Oe and a coercive field HC o0:5 Oe (Fig. 2h). Again, as for ( LaHx, an increase of the applied hydrogen 20 A pressure (up to 1000 mbar) does not affect the final magnetic configuration and transmittance measurements confirm insulating LaHx-spacer layers. To illustrate the evolution of the interlayer exchange coupling during hydrogen uptake in more detail we have plotted in Fig. 4 the saturation field HS ; the squareness MR =MS (i.e. the remanence MR referred to the saturation magne-
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
83
Fig. 4. Temporal evolution of the magnetic field at 95% saturation, HS95, the remanence referred to the saturation magnetization, ( ( ( ( MR =MS ; and the coercive field HC of the multilayers [Fe(15 A)/LaH x(20 A)] 35 and [Fe(15 A)/LaHx(24 A)] 32 exposed to 1 mbar H2.
tization MS ) and the coercive field HC extracted from MOKE magnetometry on two multilayers exposed to 1 mbar of hydrogen as a function of loading time. For the sample with spacer thickness ( that initially displays AFM coupling tLaH ¼ 20 A there is a jump-like change of these magnetic quantities almost immediately when hydrogen charging is started. The decreasing values of the saturation field MS95 confirm that the energy needed to align the Fe layers parallel is reduced and hence indicates a decreasing AFM coupling strength. Simultaneously, the ratio MR =MS is close to unity as expected for ferromagnetic alignment in zero field. This demonstrates that the absorption even of a minimum amount of additional hydrogen atoms has a dramatic effect on the interlayer coupling in this multilayer. With further loading, both the saturation field and the coercive field take on very small values (HS95 o4 Oe, HC o0:5 Oe) that is characteristic for multilayers with insulating spacer. The sample ( behaves differently. Here the with tLaH ¼ 24 A transition from FM to AFM coupling occurs gradually as can be seen from the continuous
increase of the saturation field with the charging time. The maximum strength of AFM coupling is reached after charging for about 100 min. However, the MR =MS ratio shows initially a sharp drop and then exhibits a broad maximum as more hydrogen is absorbed by the LaHx sublayers. As the absorption of hydrogen continues the AFM configuration decays gradually to be replaced by a magnetic structure with a narrow, squared hysteresis curve in the final state (see Fig. 2h). The multilayers with different spacer layer thicknesses included in this study can be classified to follow either of the two examples decribed above for the additional hydrogen uptake after ( that preparation. Samples with tLaH ¼ 16223 A initially are antiferromagntically coupled show a transition to FM alignment whereas for tLaH ¼ 15; ( AFM coupling is induced during the 24 and 26 A loading process. But as seen in Fig. 2, the latter samples are characterized by an FM soft magnetic hysteresis loop when saturated with hydrogen. The AFM coupling energy J evaluated from the maximum magnetic saturation field of these multilayers is included in Fig. 3. Note that for
84
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
( the coupling energy J is very small. tLaH ¼ 24 A ( For the multilayers with tLaH ¼ 10; 11 or >26 A the sign of the initially FM coupling is not inverted by additional hydrogen uptake. However, as all the other samples, these multilayers become magnetically soft as saturation of hydrogen absorption is approached. For the thinnest LaHx sublayers the time scale for hydrogen charging is significantly larger than for the thicker ones. The absorption of hydrogen in the spacer layers proceeds more rapidly at higher pressures of the surrounding atmosphere, but the same states of interlayer coupling are reached.
4. Neutron reflectivity The magnetic structure of the multilayers was identified by neutron reflectometry at grazing incidence in y=2y geometry. The measurements were performed at the reflectometer V6 of the Hahn-Meitner-Institut in Berlin in zero magnetic field using unpolarized monochromatic neutrons ( [20]. This setup with with wavelength l ¼ 4:7 A momentum transfer q ¼ 4p sin y=l parallel to the growth direction of the multilayers is sensitive to the chemical periodicity of the layered stack and to its magnetic configuration. To cover the metal-toinsulator transition in the LaHx spacer hydrogen loading to above x ¼ 2 d was performed in situ in the measuring chamber, and the reflectivity spectra were recorded while the samples were exposed to hydrogen gas. Fig. 5 presents the neutron reflectivity spectra of ( ( the multilayer [Fe(15 A)/LaH x(20 A)] 35, with metallic LaHx after preparation (x ¼ 2 d) and with insulating LaHx after hydrogenation at 5.4 mbar hydrogen pressure, respectively. At the left-hand side, the edge of total reflection is visible. At the right-hand side, at a momentum transfer qB ; is the first-order Bragg reflection from the artificial multilayer periodicity. Its intensity is due to the chemical order and, in part to the magnetic order in case of ferromagnetically aligned Fe-layers. For the as-prepared multilayer with metallic LaHx a reflection peak at qB =2 appears which is of magnetic origin. It indicates a doubled magnetic periodicity compared to the chemical one and thus
Fig. 5. Neutron reflectivity spectra in zero field of the multi( ( layer [Fe(15 A)/LaH x(20 A)] 35 with metallic LaHx after preparation (xB1:5) and with insulating LaHx in a H2 atmosphere of 5.4 mbar after hydrogenation to saturation. Solid lines: reflectivities fitted by an algorithm based on optical theory [21].
AFM coupling. The fit of the reflectivity spectrum based on an optical theory [21] yields a magnetic moment for Fe which amounts only to about 60% of the value resulting from vibrating sample magnetometry. The apparent low value of the Fe moment is due to the fact that the reflectivity is not entirely specular [22]: the magnetic structure of the sample consists of domains which leads to diffuse scattering around the AFM peak and may be responsible for the complexity of the hysteresis loop in Fig. 2a. For the insulating spacer, at 5.4 mbar of hydrogen gas, the AFM reflection has vanished and the Fe layers are ferromagnetically aligned. These findings confirm the conclusions drawn from the profiles of the magnetic hysteresis curves measured by MOKE (Fig. 2). The hysteresis loops reveal that the transition to the FM Fe-layer orientation occurs already when
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
LaHx is still metallic. With the additional hydrogen uptake in LaHx the structural Bragg peak in the neutron reflectivity spectrum is shifted towards smaller q values and its intensity is slightly increased. In the insulating state of the spacer the shift corresponds to an expansion of the bilayer ( which may be attributed to an thickness of 0.8 A increase of the LaHx-layer thickness of 4%. However, as shown by high-angle X-ray diffraction this effect is not connected with an increased lattice parameter of LaHx and possibly results from hydrogen absorption by defects in the volume or at the interfaces of the hydride. From the MOKE results it is clear that a 4% increase of ( after prethe spacer-layer thickness (here 20 A paration) cannot be at the origin of the observed reversal of the interlayer coupling from AFM to FM. The results obtained from neutron reflectometry ( ( on the multilayer [Fe(15 A)/LaH x(24 A)] 32 are presented in Fig. 6. After preparation, the sample exhibits a ferromagnetically aligned structure, only the Bragg reflection from the multilayer periodicity is observed at qB in the spectrum. Loading of the sample at 0.1 mbar hydrogen for 2 h results in an additional half order reflection at qB =2 which proves antiparallel alignment of the Fe-layer magnetization vectors. The considerable width of the magnetic reflection peak as compared to the upper spectrum in Fig. 5 indicates that AFM order is rather imperfect. This must be due to the small interlayer coupling strength at this spacer-layer thickness (see Fig. 3). The graph at the bottom of Fig. 6 represents the reflectivity spectrum for the insulating state of the LaHx spacer (after loading in 1 mbar H2 for a few hours). The structural Bragg peak is shifted to a somewhat lower q value, ( similarly as for the multilayer with the 20-A-thick LaHx spacer. The AFM signature in the reflectivity spectrum is reduced to a very broad residual peak near qB =2: This feature is irreversibly erased after the sample has been cycled in an external magnetic field. At this spacer thickness interlayer exchange coupling is weak (Fig. 3), it is likely to vanish in the insulating state. AFM order may be slowly destroyed then by thermal fluctuations. The corresponding hysteresis curve in this final state shows a soft step-like signature (Fig. 2), similar as
85
Fig. 6. Neutron reflectivity spectra in zero field of the multi( ( layer [Fe(15 A)/LaH x(24 A)] 32 with metallic LaHx after preparation (xB1:5) and under 0.1 mbar H2 after 2 h. The spectrum at the bottom was obtained for insulating LaHx under 1 mbar H2. Solid lines: reflectivities fitted by an algorithm based on optical theory [21].
( thick in the case of the multilayer with 20 A insulating LaHx spacers. The hydrogen-induced alterations in the multilayers are completely reversible. As indicated in Section 2, the additionally dissolved hydrogen is released and the initial stable dihydride phase is reestablished if the samples are stored under vacuum for at least 2 weeks (at room temperature). As a result, the initial magnetic configuration is recovered, the structural Bragg reflections in the neutron reflectivity spectra adopt their initial positions and widths.
5. Discussion Our results reveal that the interlayer exchange coupling in Fe/LaHx multilayers changes signifi-
86
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
cantly if the hydrogen content x is varied. Even small amounts of additional hydrogen atoms dissolved in metallic LaHx may invert the sign of the coupling (Fig. 4). Since the interatomic distances in the spacer are preserved this must be related to modifications of its electronic structure. Changes of the magnetic interlayer coupling induced by alloying a second component to the metallic spacer material have been attributed to modifications of the Fermi surface and to disorder at the interfaces which change the periodicity and attenuate the strength of the coupling, respectively (see Ref. [4]). Recent theoretical work [23–27] has lead to considerable progress in the understanding of the RE hydrides: of the metallic dihydride state, the insulating trihydride state, and of the nonstoichiometric regime in between, where the metal–insulator transition occurs. A key aspect for the electronic structure is the hybridization of the electron states of the host metal and the hydrogen s states: bands form which result from the RE–hydrogen and hydrogen–hydrogen interaction. They are situated at energies below a set of bands of essentially RE–metal 5d character. Models to explain the gap opening in REH3 include a Peierls-like approach where symmetry lowering changes in the hydrogen positions are sufficient to explain the observed band gap [26,27]. Ng et al. [23,24] considered strong electron correlations and treated the problem in a manybody theory based on a lattice of H ions to study the electronic structure of lanthanum hydride as a prototype of REHx systems. In LaH2, there are two low-lying hydrogen-derived bands which accommodate four of the five valence electrons (supplied by La with the valence configuration 5d16s2 and the two H atoms). The remaining electron occupies the bottom of a lanthanumderived band that contains the Fermi energy, accordingly the dihydride is a metal. In insulating LaH3 an additional band joins the low-lying hydrogen-derived valence bands. These three Hderived bands accommodate all six valence electrons, and owing to strong electron correlations a gap of B2 eV opens. The substoichiometric phase LaH3g becomes metallic when the electron states centered at the octahedral hydrogen vacancies
overlap. This occurs only at a notable vacancy content, gE0:25; since these states are highly localized due to the strong hybridization between the hydrogen 1s and neighboring lanthanum 5deg orbitals. Experiments of photoemission [28,29], inverse photoemission [30], and X-ray emission at the L3 edge of La [31] confirm largely the theoretical results. Hydrogen-derived states were observed at some 5 eV binding energy, the density of states at the Fermi energy is reduced in the dihydride as compared to the pure metal. This reduction continuous as the hydrogen concentration is raised from x ¼ 2 to 3. Obviously, the hydride forms at the expense of the conduction-band population, the metal-derived 5d states appear to be depopulated in favor of the low-lying bands. A recent study of X-ray absorption on the La–L2,3 edges that is dominated by the 2p-to-5d dipole transition has confirmed these findings for LaHx in the multilayer system [15]. Measurements of magnetic circular dicroism (XMCD) in these experiments revealed the existence of an extended spin polarization of the itinerant 5d states of La in the metallic hydride that is due to hybridization with the 3d states of Fe [15]. This suggests that these states mediate the observed exchange coupling between the Fe layers. The length scale of the 5d polarization (decay length from the interface: ( is in agreement with the upper limit of B15 A) the LaHx layer thickness for which coupling has been observed.2 As the hydrogen content is increased the XMCD signals are diminished in amplitude and modified in shape. Let us note that in the insulating phase of the LaHx hydride the dichroic signals remain finite, possibly due to the presence of spin-polarized 5d states in the gap of the LaHx layers induced by the adjacent Fe layers [15]. 2
The present experiments provide a direct proof for interlayer coupling for the AFM case only. For the as-prepared multilayers this covers the spacer-layer thicknesses ( ( 16 Apt LaH p23 A. There is no doubt then that coupling must be also effective and even stronger in the FM case for tLaH ( For tLaH > 23 A, ( where magnetic alignment is ferroo16 A. ( magnetic again, the presence of coupling up to at least 26 A follows indirectly from the hydrogen-induced change to AFM magnetization orientation.
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
The Fermi surface of La dihydride is much simpler than that of the pure metal [32]. It is a multiply connected hole surface formed by a warped cube around the G point in the Brillouin zone with necks around the L points along the [1 1 1] axes. According to the model of Ng et al. [23,24] each additionally dissolved hydrogen atom in the spacer material attracts an electron. This will alter the Fermi surface due to the reduced density of itinerant 5d electrons. It will also modify the potential step that the conduction electrons in the spacer experience upon crossing the interface with Fe [33], and thus the spin dependent reflection coefficient for the conduction electrons. The result is a change of the phase and strength of the exchange coupling. Both effects are observed here in the Fe/LaHx multilayers. Unfortunately, there is only one AFM maximum in the coupling energy but no oscillatory behavior. Apparently, the osci( due to llations cannot develop beyond tLaH B30 A the weak coupling strength. This makes it difficult to relate the interlayer coupling and the H-induced changes to the Fermi surface of the spacer material. At sufficiently high hydrogen concentration, when the LaHx spacer becomes insulating, conduction electrons are no longer available and exchange coupling between the Fe layers is disrupted. In this final state the multilayers are always characterized by a narrow step-like hysteresis loop. Due to the mutual perturbation between the Fe and LaH3g electronic states in the interfacial region, the hydrogen concentration in the spacer layers may be smaller at the interfaces than in the volume. This would not be in conflict with an overall insulating behavior of the hydride which is insensitive to relatively large deviations g from stoichiometry [23,24]. There have been reports on thermally induced exchange coupling between Fe layers across amorphous semiconducting interlayers. But the coupling energy derived is considerably smaller compared to metallic spacers (including the present ones) and the effect is thought to be closely related to the amorphous structure [34,35]. Bruno has predicted non-oscillatory exchange coupling across a crystalline insulating barrier within the quantum interference model [36]. But since the strength of the coupling is relatively small and its amplitude decays exponen-
87
tially with the spacer-layer thickness his result is not relevant for the present multilayer system. In the absence of exchange coupling in the multilayers with insulating LaHx magnetic dipolar interaction may provide a coupling between adjacent Fe layers. In equilibrium, it would align their magnetization vectors antiparallel. We do not observe antiparallel alignment of the Fe layers for insulating LaHx neither with MOKE magnetometry nor with neutron reflectometry. This points to a very weak magnetic dipolar interaction that has no relevance for our investigations.
6. Summary and conclusions While the Fe layers in Fe/La multilayers are magnetically decoupled (See footnote 1) they become coupled by exchange interaction when La is hydrogenated. This must be related to the profound differences in the electronic band structure of the interlayer in the two cases. FM and AFM coupling is observed depending on the thickness of hydride layers, but without oscillatory behavior predicted by all of the existing models. Starting from the metallic La dihydride the effect of dissolving increasingly more hydrogen was examined. Sign and value of the exchange coupling across the interlayers depend crucially on their hydrogen content. We find that the coupling can be inverted from AFM to FM and from FM to AFM, and the important changes take place at rather low additional hydrogen uptake of the spacers when they are still metallic. It is emphasized that these alterations are not due to any structural changes in the multilayers but to modifications of the electronic structure of LaHx, since the interatomic distances and the quality of the interfaces are maintained. Recent measurements of X-ray absorption and XMCD [15] suggest that the interlayer coupling is transmitted by the itinerant 5d band electrons of the spacer which are spin polarized by the adjacent Fe, with a ( Their number is increasdecay length of B15 A. ingly reduced with increasing hydrogen absorption. When the absorption is saturated the hydride layers are insulating and the exchange coupling is likely to disappear. In this final state the multi-
88
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
layers are always characterized by a very soft FM rectangular hysteresis curve, with low saturation and coercive fields that is thought to characterize the individual magnetically decoupled Fe layers. The corresponding neutron reflectivity spectrum does not show a magnetic superstructure. Magnetic dipolar interactions that remain as a possibility for coupling apparently are too weak to be effective. The changes in the magnetic properties are completely reversible when the initial hydrogen concentration of the as-prepared state is restored. Even though the hydrogen-induced modification of the magnetic interlayer coupling must be of electronic origin the mechanism is not entirely clear. According to Stiles [4] there is no good model for exchange coupling in multilayer systems that are not well lattice matched, as in the present case of the structurally non-coherent heterostructures. We propose that the observed changes in sign and strength of the coupling in the metallic state of the LaHx spacer is due to altered spin dependent reflection coefficients at the interface. In fact changes of the degree of hybridization of the Fe-3d and La-derived 5d band states at the interfaces have been directly observed in the XMCD experiments [15]. A possible influence of modifications of the Fermi surface of LaHx on the coupling cannot be excluded. They would alter the period for the oscillation with the hydride-layer thickness. However, since only one AFM coupling peak was observed it is impossible to discuss oscillatory coupling. Acknowledgements It is a pleasure to acknowledge stimulating discussions with Markus Munzenberg. . We appreciate the financial support of the Deutsche Forschungsgemeinschaft within SFB 345. The experiments at BENSC, Hahn-Meitner Institut Berlin were supported by the European Commission through the PECO 93 Action (contract: ERB CIPD CT 940088). References [1] P. Grunberg, . R. Schreiber, Y. Pang, M.B. Brodsky, H. Sowers, Phys. Rev. Lett. 57 (1986) 2442.
[2] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures, Vol. 2, Springer, Berlin, 1994. [4] M.D. Stiles, J. Magn. Magn. Mater. 200 (1999) 322. [5] P. Bruno, Phys. Rev. B 52 (1995) 411. [6] F. Klose, C. Rehm, D. Nagengast, H. Maletta, A. Weidinger, Phys. Rev. Lett. 78 (1997) 1150. . [7] B. Hjorvarsson, J.A. Dura, P. Isberg, T. Watanabe, T.J. Udovic, G. Andersson, C.F. Majkrzak, Phys. Rev. Lett. 79 (1997) 901. . [8] D. Labergerie, C. Sutter, H. Zabel, B. Hjorvarsson, J. Magn. Magn. Mater. 192 (1999) 238. [9] P. Vajda, in: K.A. Gschneidner, L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earth, Vol. 20, Elsevier Science, Amsterdam, 1995. [10] J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Dekker, D.G.d. Groot, N.J. Koeman, Nature 380 (1996) 231. [11] G.P. Felcher, W. Lohstroh, H. Fritzsche, M. Munzenberg, . H. Maletta, W. Felsch, Appl. Phys. Lett. 72 (1998) 2894. [12] J. Krauser, Hahn-Meitner-Institut Berlin, private communication. [13] L.G. Parratt, Phys. Rev. 95 (1954) 359. [14] L. Nevot, P. Croce, Rev. Phys. Appl. 15 (1980) 761. . [15] M. Munzenberg, . Dissertation, Universit.at Gottingen, 2000. [16] C. Rehm, H. Fritzsche, H. Maletta, F. Klose, Phys. Rev. B 59 (1999) 3142. [17] F. Klose, Ch. Rehm, M. Fieber-Erdmann, E. Holub. Krappe, H.J. Bleif, H. Sowers, R. Goyette, L. Troger, H. Maletta, Physica B 283 (2000) 184. . [18] M. Munzenberg, . Diplomarbeit, Universit.at Gottingen, 1997. [19] A. Bart!el!emy, A. Fert, M.N. Baibich, S. Hadjoudi, F. Petroff, P. Etienne, R. Cabanel, S. Lequin, F.N.v. Dau, G. Creuzet, J. Appl. Phys. 67 (1990) 5908. [20] F. Mezei, R. Golub, F. Klose, H. Toews, Physica B 213&214 (1995) 898. [21] V.-O.d. Haan, G.G. Drijkoningen, Physica B 198 (1994) 24. [22] G.P. Felcher, Physica B 192 (1993) 137. [23] K.K. Ng, F.C. Zhang, V.I. Anisimov, T.M. Rice, Phys. Rev. Lett. 78 (1997) 1311. [24] K.K. Ng, F.C. Zhang, V.I. Anisimov, T.M. Rice, Phys. Rev. B 59 (1999) 5398. [25] R. Eder, H.F. Pen, G.A. Sawatzky, Phys. Rev. B 56 (1997) 10115. [26] P.J. Kelly, J.P. Dekker, R. Stumpf, Phys. Rev. Lett. 78 (1997) 1315. [27] P. van Gelderen, P. Kelly, G. Brocks, Phys. Rev. Lett. 85 (2000) 2989. [28] D.J. Peterman, J.H. Weaver, M. Croft, D.T. Peterson, Phys. Rev. B 27 (1983) 808. [29] L. Schlapbach, J. Osterwalder, T. Riesterer, J. Less Common Met. 103 (1984) 295. [30] J. Osterwalder, Z. Phys. B 61 (1985) 113.
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89 [31] A.C. Sarma, W.G. Bos, J. Phys. Chem. Solids 32 (1971) 1423. [32] M. Gupta, J.P. Burger, Phys. Rev. B 22 (1980) 6074. [33] J. Mathon, M. Villeret, D.M. Edwards, R.B. Muniz, J. Magn. Magn. Mater. 121 (1993) 242.
89
[34] S. Toscano, B. Briner, H. Hopster, M. Landolt, J. Magn. Magn. Mater. 114 (1992) L6. [35] P. Walser, M. Hunziker, M. Landolt, J. Magn. Magn. Mater. 200 (1999) 95. [36] P. Bruno, Phys. Rev. B 49 (1994) 13231.
Hydrogen-controlled interlayer exchange coupling in Fe/LaHx multilayers W. Lohstroha,*, F. Leuenbergera, W. Felscha, H. Fritzscheb, H. Malettab a
I. Physikalisches Institut, Universitat Bunsenstrae 9, 37073 Gottingen, Germany . Gottingen, . . b Hahn-Meitner-Institut Berlin, Glienicker Str. 100, 14109 Berlin, Germany Received 11 May 2001; received in revised form 26 July 2001
Abstract Magneto-optic Kerr magnetometry and neutron reflectometry reveal that Fe layers exhibit magnetic exchange coupling through LaHx spacer layers. Ferromagnetic and antiferromagnetic coupling is observed on multilayers of these materials depending on the thickness of the hydride layers, but without oscillatory behavior. Starting from metallic La dihydride spacer layers the effect of dissolving increasingly more hydrogen was examined. Sign and value of the coupling depend crucially on the hydrogen content x: The coupling can be inverted from antiferromagnetic to ferromagnetic and vice versa. These alterations are due to modifications of the electronic structure of the hydride. When the hydrogen absorption saturates the hydride layers become insulating and the exchange coupling is likely to disappear. In this final state the multilayers are always characterized by a very soft ferromagnetic rectangular hysteresis curve. Upon removal of the hydrogen to the initial concentration the original magnetic structure is restored. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Metallic multilayers; Interlayer exchange coupling; Spin structures; Rare-earth hydrides; Hydrogen in thin films
1. Introduction Interlayer exchange coupling is a property common to many multilayer systems [1–3]. Mediated by the spin polarization of the conduction electrons in a non-magnetic metallic spacer material the magnetization vectors of neighboring ferromagnetic layers are aligned either parallel or antiparallel. The relative orientation generally
*Corresponding author. Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK. Tel.: +44-1865-272-367; fax: +44-1865-282-221. E-mail address: [email protected] (W. Lohstroh).
oscillates as a function of the thickness of the spacer layers. Various theoretical models have been proposed to explain the effect, for an overview see Ref. [3]. They all agree in that the periods of the oscillatory coupling are determined by critical spanning vectors of the Fermi surface of the material that makes up the spacer [4]. Furthermore, it is now widely accepted that the different models can be unified into an approach based upon quantum interference due to spindependent electron confinement in the spacer layers caused by the magnetic layers [5]. This means that matching of the energy bands at the interfaces is important. In this more general model interlayer exchange coupling is described in terms
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 4 9 1 - 7
78
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
of spin-dependent reflection of electrons at the interfaces. Strength and phase of the coupling are sensitive to details of the interfacial structure. Recent experiments on Fe/Nb [6] and Fe/V [7,8] multilayers have shown that the introduction of hydrogen into the Nb or V spacers opens a new way for tuning the exchange coupling between the Fe layers. By simply changing the hydrogen pressure surrounding the sample the magnetic structure can be reversibly inverted from antiferromagnetic (AFM) to ferromagnetic (FM) and vice versa. The change of interlayer coupling energy is not simply due to the variation of the interatomic distances in the Nb or V spacer layers with their hydrogen content but to a modification of the Fermi surface. In view of these observations multilayers comprising rare-earth (RE) metals are of particular interest. The electronic structure of these elements and their metallic character changes drastically upon hydrogen absorption. The light RE metals like La form a cubic dihydride REH2 (CaF2-type structure) with hydrogen atoms on tetrahedrally co-ordinated sites (b-phase), and dissolve in a single phase further hydrogen on octahedral sites (g-phase) up to the cubic trihydride phase REH3 (BiF3-type structure) [9]. Both phases have almost the same lattice parameter, but a very different electronic structure: the dihydride is metallic whereas the trihydride is insulating. The transition is accompanied by a change of the optical properties from metallic reflecting to transparent for photon energies below B2 eV. The binding energies for hydrogen are very different at the tetrahedral and octahedral positions: it is an exciting recent discovery that for RE hydrides in thin film geometry the transition between the di- and trihydride phases is completely reversible in a relatively short time, whereas in the dihydride the hydrogen is irreversibly bound. This lends considerable technological importance to these materials, permitting to construct switchable mirrors for visible light [10]. Quite generally, it has caused renewed interest in RE hydride systems among both experimentalists and theoreticians. We have studied and discuss here the magnetic structures in Fe/LaHx multilayers, xX2 d; and how they are altered by a variation of the
hydrogen content x: The experimental observations were carried out by magneto-optic Kerr (MOKE) magnetometry and by neutron reflectometry at room temperature. In contrast to the pure Fe/La system1 these heterostructures exhibit an interlayer exchange coupling for the metallic phase of the LaHx spacer. The addition of small amounts of atomic hydrogen is sufficient to alter the coupling between the Fe layers, from AFM to FM and vice versa. Upon further hydrogen uptake, LaHx looses its metallic character and the magnetic layers are thought to decouple. When the hydrogen content is reduced to the initial concentration, the magnetic structures are restored. Hence interlayer coupling may be reversibly switched off and on in this multilayer system, which is quite unique.
2. Sample characteristics ( A series of multilayer samples [Fe(15 A)/ ( LaHx(tLaH )] n (tLaH ¼ 10260 A) was grown at room temperature by reactive ion beam sputtering with argon in an ultra-high vacuum chamber (base pressure o5 1010 mbar), after hydrogen had been introduced (partial pressure 8 106 mbar). The number of bilayers, n; was chosen so that the ( High-purity total thickness was close to 2000 A. gases Ar(6N), H2(6N) and target metals La(3N), Fe(4N8) were used. During the layer deposition the partial pressure of reactive impurity gases (O2, N2 and H2O) was below 1010 mbar. Si(1 0 0) ( Cr buffer layer served wafers covered with a 40 A as substrates. For the measurements of the optical transmittance the samples were deposited on thin Cr-coated glass plates. Unlike the La layers the Fe layers do not dissolve hydrogen since the heat of solution is endothermic for this metal, whereas it is strongly exothermic for La. The samples were ( thick Pd cover layer to terminated with a 100 A prevent oxidation and to ensure additional hydrogen uptake and release after preparation. 1 In Fe/La multilayers exchange coupling between the Fe layers is absent, but artificial magnetic configurations may be induced by a small magnetric field acting on the samples during growth (see Ref. [11]).
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
Analysis of selective samples by the resonant nuclear reaction 1H(15N, ag)12C revealed a stable lanthanum hydride in the multilayers after preparation, with composition LaHx where dE0:5 [12]. Such deviation from the perfect stoichiometry of a b-phase rare-earth hydride is not uncommon and has been attributed to effects from the interfaces and/or lattice disorder in the thin-film structure [9]. Structural characterization of the multilayers was performed by X-ray diffraction in y=2ygeometry using Cu-Ka radiation. The small-angle spectra (Fig. 1) reveal layered stacks with good periodicity and sharp interfaces. Fits to the data (solid line) employing an algorithm based on optical theory [13,14] yield a RMS roughness ( (FWHM) at the interfaces. The values of 2.5–3 A high-angle spectra show that both components are crystalline: Fe grows in the a-phase BCC structure, LaHx in the b-phase CaF2-like FCC structure with (1 1 1) texture. Rocking scans around the (1 1 1) reflection show a width of B51 at half maximum. No satellites around the main reflections are observed which indicates the absence of coherent growth in the multilayers, in spite of the small mismatch (B3%) of the component-layer lattices. The exposure of the as-prepared multilayer samples (with x ¼ 2 d) to an atmosphere of
( Fig. 1. Low-angle X-ray reflections of the multilayer [Fe(15 A)/ ( 35, xE1:5: The sharp peaks are the Bragg LaHx(20 A)] reflectivities of the mutilayer; the broader interference fringes in between are due to the Pd capping layer. Solid line: fit to the data based on optical theory.
79
hydrogen gas at room temperature in a previously evacuated chamber leads to additional uptake of atomic hydrogen in the LaHx sublayers. In this way the insulating state of the hydride (x > 2:8) can be reached even for the thinnest layers at moderate hydrogen pressures (p1000 mbar). This is ascertained by measurements of the transmittance of visible light that were carried out in parallel with the MOKE magnetometry: a steep increase of the transmitted light was observed when the hydride became insulating. The results agree with conclusions drawn from detailed measurements of the conductance on this multilayer system [15]. The time scale for the uptake of hydrogen is essentially a function of its pressure and the thickness of the LaHx sublayers. In any case the kinetics of the absorption and desorption processes for hydrogen in these heterostructures are very different: for a multilayer with insulating lanthanum hydride placed into vacuum it takes typically at least 2 weeks until the additional hydrogen has been released and the initial stable dihydride phase is restored. This permits to perform X-ray diffraction (and other experiments) on the additionally charged samples ex situ. The high-angle spectra reveal that the fcc lattice symmetry and the lattice parameter of LaHx do not change across the metal-to-insulator transition, in agreement with the behavior of the bulk hydride. In fact, the lattice parameter of bulk LaHx is B1.5% smaller in the trihydride than in the dihydride phase [9], which is within the error bars of our experiment. It turns out that the structural roughness at the interfaces of the multilayers measured with X-ray reflectometry ( in the insulating phase increases only by B0.2 A of the hydride. In spite of the preservation of the interatomic distances in the LaHx layers upon additional hydrogen uptake, as indicated by X-ray diffraction, the low-angle neutron reflectivity spectra recorded in situ under a hydrogen atmosphere (see Section 4 below) reveal an increase of the bilayer thickness (tFe þ tLaH ) which amounts to 4% in the insulating phase of LaHx. A similar effect was observed in polycrystalline Fe/NbHx [16] and epitaxial W/NbHx [17] multilayers for which the hydrogen-induced relative out-of-plane expansion
80
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
of the Nb layers is considerably larger than the increase of the Nb interplanar spacing. The anomalous increase of the Nb layer thickness is attributed to hydrogen incorporated in imperfections of the Nb lattice, like grain boundaries, dislocations, voids, or at the interfaces of the heterostructures. These defects add to the normal interstitial sites for hydrogen absorption. The similar observations on the Fe/LaHx multilayers suggests that a similar trapping mechanism is effective in this system, too. The effect is reversible: after desorption of the hydrogen added after preparation the original bilayer thickness is restored.
3. MOKE magnetometry The magnetization of the multilayers was studied by measuring the Kerr rotation of the polarized beam of a He–Ne laser reflected by the sample. The measurements were performed in the longitudinal configuration using a photoelastic modulator [18]. Rotation angles down to 2 1041could be resolved which corresponds in the case of Fe layers to a magnetization of 40 emu/ cm3. For the wavelength of the light used (lL ¼ 632:8 nm) the depth sensitivity is about ( in Fe. In the Fe/LaHx multilayers the 300 A probed depth depends on the hydrogen concentration in the LaHx spacer since its transmittance increases as the LaHx layers become insulating. Therefore the displayed MOKE hysteresis curves have been normalized to the signal obtained at saturation. The samples were mounted into a highvacuum hydrogenation chamber that was placed between the two coils of a Helmholtz magnet. Magnetic fields up 900 Oe could be applied. Glass windows were used to introduce the laser beam into the chamber. Hydrogen pressures up to 1000 mbar were applied while recording the magnetic hysteresis loops. For all samples the easy axis of magnetization was found to be in the layer plane without any preferred direction. In the same setup the transmittance of the samples could be measured. All measurements were performed at room temperature, as already indicated.
After preparation, when the hydride is in the metallic phase LaHx, the multilayers show interlayer exchange coupling for spacer thickness ( with AFM Fe-layer magnetization tLaH o30 A, ( and FM alignment for tLaH between 16 and 23 A, alignment in all other cases. The Fe-layer thickness ( Representative examples for the was always 15 A. hysteresis curves are displayed in Fig. 2a and b for ( thick LaHx two multilayers with 20 and 24 A ( layers, respectively. For tLaH ¼ 24 A (Fig. 2b), the hysteresis curve essentially has a rectangular shape with a large remanence and a low saturation field. This points to an FM Fe-layer orientation. For the ( (Fig. 2a), lower spacer-layer thickness, tLaH ¼ 20 A the curve rises more slowly with the applied magnetic field, the saturation field is considerably higher and the remanence is lower. This suggests an AFM coupling of neighboring Fe layers as it was confirmed by neutron reflectometry. The complex shape of the magnetization curve, its pronounced hysteresis and the deviation from linearity for applied fields up to a few 10 Oe indicates that effects from domain nucleation and/ or domain-wall displacements in the multilayers are superposed to the linear M2H dependence expected for normal spin-flop AFM in a magnetic field. In the case of AFM interlayer exchange coupling the coupling energy J can be estimated from the magnetic saturation fields HS using the relation [19] J ¼ HS MS;Fe tFe =4;
ð1Þ
where MS;Fe denotes the saturation magnetization of the Fe layers; measurements with a vibrating sample magnetometer reveal that it is close to the literature value of bulk BCC Fe (1704 emu/cm3). HS was taken as a technical saturation field at 95% of the saturation magnetization from the average of the hysteresis branches for increasing and decreasing field, respectively. The resulting energies for AFM coupling are plotted in Fig. 3 (solid circles). The exchange energy for FM coupling was not accessible by magnetometry. For an overview, the multilayers concerned are represented in Fig. 3 with J set to zero (crosses) in case of an FM hysteresis. As can be seen, AFM interlayer coupling in the as-prepared multilayers Fe/LaHx
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
81
( ( ( ( Fig. 2. MOKE-hysteresis curves of the multilayers [Fe(15 A)/LaH x(20 A)] 35 and [Fe(15 A)/LaHx(24 A)] 32. The top row (a) and (b) displays the hysteresis curves obtained after preparation (xB1:5); the subsequent rows in each column show their evolution with time after 1 mbar H2 had been introduced into the sample chamber. Note the different scale of the magnetic field in the two rows.
( is restricted to spacer-layer thickness 16 Ap ( Only one AFM peak is observed, tLaH p23 A. there is no oscillatory behavior as tLaH varies. The ( amounts to maximum value of jJj at tLaH ¼ 17 A B0.014 erg/cm2. This value is of the order of the coupling strength observed for maximum AFM coupling in Fe/V superlattices [8] if compared at
the same spacer-layer thickness, assuming asymptotic scaling as 1=t2spacer [4]. Also displayed in Fig. 2 are the hysteresis curves ( recorded at of the samples with tLaH ¼ 20 and 24 A several stages on the indicated time scale after the introduction of 1 mbar of hydrogen gas into the measuring chamber. These experiments were
82
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
Fig. 3. Strength of the AFM exchange coupling, J; as a function of the thickness of the metallic LaHx interlayers. Fe-layer thickness: ( Solid circles: values obtained after preparation. In the regimes with FM Fe-layer orientation J has been set to zero (crosses). 15 A. Open diamonds: values obtained after additional hydrogen absorption (see text). The continuous line is a guide to the eye only.
possible due to the slow kinetics of the hydrogen uptake in small applied gas pressures. As an increasing amount of atomic hydrogen is absorbed in the spacer layers the curves change their shape. The sample coupled antiferromagnetically after ( Fig. 2a) shows an inpreparation (tLaH ¼ 20 A, creasingly less sheared, more narrow hysteresis curve (Fig. 2c, e and g). The curve recorded at 100 min after the beginning of additional hydrogen uptake is squared with a high remanent magnetization and low saturation field. These characteristics remain unchanged even after 24 h and upon further increase (up to 1000 mbar) of the hydrogen pressure in the chamber. Obviously, the absorption of additional hydrogen into LaHx essentially leads to an FM orientation of the Fe-layer magnetizations in this sample, with a very soft hysteresis curve. Comparison with the transmission experiments confirms that the LaHx layers are insulating in the final loading stage. The altered magnetic properties cannot result from a lattice expansion of the LaHx spacer layer since AFM ( prior to coupling was observed up to tLaH ¼ 23 A additional hydrogen absorption. The change of the magnetic configuration must be due to a modification of the electronic structure of the LaHx sublayers induced by the additionally dissolved hydrogen on interstitial sites.
A different behavior in the course of hydrogen absorption can be recognized for the sample with ( It shows an FM hysteresis curve for tLaH ¼ 24 A. the stable La dihydride phase (Fig. 2b) that becomes increasingly sheared and looses remanence until it becomes AFM after 100 min (Fig. 2f). In an intermediate state, after 45 min, the hysteresis curve exhibits two steps (Fig. 2d). This indicates that in the intermediate loading stage domains with different hydrogen concentrations and hence different magnetic exchange interactions are present. The smooth shape observed after 100 min (Fig. 2f) reveals that at this stage the coupling is AFM in the entire sample. As more time elapses and the hydrogen content of the spacer increases further the AFM structure decays and eventually a very soft hysteresis curve is observed, with a saturation field HS E4 Oe and a coercive field HC o0:5 Oe (Fig. 2h). Again, as for ( LaHx, an increase of the applied hydrogen 20 A pressure (up to 1000 mbar) does not affect the final magnetic configuration and transmittance measurements confirm insulating LaHx-spacer layers. To illustrate the evolution of the interlayer exchange coupling during hydrogen uptake in more detail we have plotted in Fig. 4 the saturation field HS ; the squareness MR =MS (i.e. the remanence MR referred to the saturation magne-
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
83
Fig. 4. Temporal evolution of the magnetic field at 95% saturation, HS95, the remanence referred to the saturation magnetization, ( ( ( ( MR =MS ; and the coercive field HC of the multilayers [Fe(15 A)/LaH x(20 A)] 35 and [Fe(15 A)/LaHx(24 A)] 32 exposed to 1 mbar H2.
tization MS ) and the coercive field HC extracted from MOKE magnetometry on two multilayers exposed to 1 mbar of hydrogen as a function of loading time. For the sample with spacer thickness ( that initially displays AFM coupling tLaH ¼ 20 A there is a jump-like change of these magnetic quantities almost immediately when hydrogen charging is started. The decreasing values of the saturation field MS95 confirm that the energy needed to align the Fe layers parallel is reduced and hence indicates a decreasing AFM coupling strength. Simultaneously, the ratio MR =MS is close to unity as expected for ferromagnetic alignment in zero field. This demonstrates that the absorption even of a minimum amount of additional hydrogen atoms has a dramatic effect on the interlayer coupling in this multilayer. With further loading, both the saturation field and the coercive field take on very small values (HS95 o4 Oe, HC o0:5 Oe) that is characteristic for multilayers with insulating spacer. The sample ( behaves differently. Here the with tLaH ¼ 24 A transition from FM to AFM coupling occurs gradually as can be seen from the continuous
increase of the saturation field with the charging time. The maximum strength of AFM coupling is reached after charging for about 100 min. However, the MR =MS ratio shows initially a sharp drop and then exhibits a broad maximum as more hydrogen is absorbed by the LaHx sublayers. As the absorption of hydrogen continues the AFM configuration decays gradually to be replaced by a magnetic structure with a narrow, squared hysteresis curve in the final state (see Fig. 2h). The multilayers with different spacer layer thicknesses included in this study can be classified to follow either of the two examples decribed above for the additional hydrogen uptake after ( that preparation. Samples with tLaH ¼ 16223 A initially are antiferromagntically coupled show a transition to FM alignment whereas for tLaH ¼ 15; ( AFM coupling is induced during the 24 and 26 A loading process. But as seen in Fig. 2, the latter samples are characterized by an FM soft magnetic hysteresis loop when saturated with hydrogen. The AFM coupling energy J evaluated from the maximum magnetic saturation field of these multilayers is included in Fig. 3. Note that for
84
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
( the coupling energy J is very small. tLaH ¼ 24 A ( For the multilayers with tLaH ¼ 10; 11 or >26 A the sign of the initially FM coupling is not inverted by additional hydrogen uptake. However, as all the other samples, these multilayers become magnetically soft as saturation of hydrogen absorption is approached. For the thinnest LaHx sublayers the time scale for hydrogen charging is significantly larger than for the thicker ones. The absorption of hydrogen in the spacer layers proceeds more rapidly at higher pressures of the surrounding atmosphere, but the same states of interlayer coupling are reached.
4. Neutron reflectivity The magnetic structure of the multilayers was identified by neutron reflectometry at grazing incidence in y=2y geometry. The measurements were performed at the reflectometer V6 of the Hahn-Meitner-Institut in Berlin in zero magnetic field using unpolarized monochromatic neutrons ( [20]. This setup with with wavelength l ¼ 4:7 A momentum transfer q ¼ 4p sin y=l parallel to the growth direction of the multilayers is sensitive to the chemical periodicity of the layered stack and to its magnetic configuration. To cover the metal-toinsulator transition in the LaHx spacer hydrogen loading to above x ¼ 2 d was performed in situ in the measuring chamber, and the reflectivity spectra were recorded while the samples were exposed to hydrogen gas. Fig. 5 presents the neutron reflectivity spectra of ( ( the multilayer [Fe(15 A)/LaH x(20 A)] 35, with metallic LaHx after preparation (x ¼ 2 d) and with insulating LaHx after hydrogenation at 5.4 mbar hydrogen pressure, respectively. At the left-hand side, the edge of total reflection is visible. At the right-hand side, at a momentum transfer qB ; is the first-order Bragg reflection from the artificial multilayer periodicity. Its intensity is due to the chemical order and, in part to the magnetic order in case of ferromagnetically aligned Fe-layers. For the as-prepared multilayer with metallic LaHx a reflection peak at qB =2 appears which is of magnetic origin. It indicates a doubled magnetic periodicity compared to the chemical one and thus
Fig. 5. Neutron reflectivity spectra in zero field of the multi( ( layer [Fe(15 A)/LaH x(20 A)] 35 with metallic LaHx after preparation (xB1:5) and with insulating LaHx in a H2 atmosphere of 5.4 mbar after hydrogenation to saturation. Solid lines: reflectivities fitted by an algorithm based on optical theory [21].
AFM coupling. The fit of the reflectivity spectrum based on an optical theory [21] yields a magnetic moment for Fe which amounts only to about 60% of the value resulting from vibrating sample magnetometry. The apparent low value of the Fe moment is due to the fact that the reflectivity is not entirely specular [22]: the magnetic structure of the sample consists of domains which leads to diffuse scattering around the AFM peak and may be responsible for the complexity of the hysteresis loop in Fig. 2a. For the insulating spacer, at 5.4 mbar of hydrogen gas, the AFM reflection has vanished and the Fe layers are ferromagnetically aligned. These findings confirm the conclusions drawn from the profiles of the magnetic hysteresis curves measured by MOKE (Fig. 2). The hysteresis loops reveal that the transition to the FM Fe-layer orientation occurs already when
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
LaHx is still metallic. With the additional hydrogen uptake in LaHx the structural Bragg peak in the neutron reflectivity spectrum is shifted towards smaller q values and its intensity is slightly increased. In the insulating state of the spacer the shift corresponds to an expansion of the bilayer ( which may be attributed to an thickness of 0.8 A increase of the LaHx-layer thickness of 4%. However, as shown by high-angle X-ray diffraction this effect is not connected with an increased lattice parameter of LaHx and possibly results from hydrogen absorption by defects in the volume or at the interfaces of the hydride. From the MOKE results it is clear that a 4% increase of ( after prethe spacer-layer thickness (here 20 A paration) cannot be at the origin of the observed reversal of the interlayer coupling from AFM to FM. The results obtained from neutron reflectometry ( ( on the multilayer [Fe(15 A)/LaH x(24 A)] 32 are presented in Fig. 6. After preparation, the sample exhibits a ferromagnetically aligned structure, only the Bragg reflection from the multilayer periodicity is observed at qB in the spectrum. Loading of the sample at 0.1 mbar hydrogen for 2 h results in an additional half order reflection at qB =2 which proves antiparallel alignment of the Fe-layer magnetization vectors. The considerable width of the magnetic reflection peak as compared to the upper spectrum in Fig. 5 indicates that AFM order is rather imperfect. This must be due to the small interlayer coupling strength at this spacer-layer thickness (see Fig. 3). The graph at the bottom of Fig. 6 represents the reflectivity spectrum for the insulating state of the LaHx spacer (after loading in 1 mbar H2 for a few hours). The structural Bragg peak is shifted to a somewhat lower q value, ( similarly as for the multilayer with the 20-A-thick LaHx spacer. The AFM signature in the reflectivity spectrum is reduced to a very broad residual peak near qB =2: This feature is irreversibly erased after the sample has been cycled in an external magnetic field. At this spacer thickness interlayer exchange coupling is weak (Fig. 3), it is likely to vanish in the insulating state. AFM order may be slowly destroyed then by thermal fluctuations. The corresponding hysteresis curve in this final state shows a soft step-like signature (Fig. 2), similar as
85
Fig. 6. Neutron reflectivity spectra in zero field of the multi( ( layer [Fe(15 A)/LaH x(24 A)] 32 with metallic LaHx after preparation (xB1:5) and under 0.1 mbar H2 after 2 h. The spectrum at the bottom was obtained for insulating LaHx under 1 mbar H2. Solid lines: reflectivities fitted by an algorithm based on optical theory [21].
( thick in the case of the multilayer with 20 A insulating LaHx spacers. The hydrogen-induced alterations in the multilayers are completely reversible. As indicated in Section 2, the additionally dissolved hydrogen is released and the initial stable dihydride phase is reestablished if the samples are stored under vacuum for at least 2 weeks (at room temperature). As a result, the initial magnetic configuration is recovered, the structural Bragg reflections in the neutron reflectivity spectra adopt their initial positions and widths.
5. Discussion Our results reveal that the interlayer exchange coupling in Fe/LaHx multilayers changes signifi-
86
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
cantly if the hydrogen content x is varied. Even small amounts of additional hydrogen atoms dissolved in metallic LaHx may invert the sign of the coupling (Fig. 4). Since the interatomic distances in the spacer are preserved this must be related to modifications of its electronic structure. Changes of the magnetic interlayer coupling induced by alloying a second component to the metallic spacer material have been attributed to modifications of the Fermi surface and to disorder at the interfaces which change the periodicity and attenuate the strength of the coupling, respectively (see Ref. [4]). Recent theoretical work [23–27] has lead to considerable progress in the understanding of the RE hydrides: of the metallic dihydride state, the insulating trihydride state, and of the nonstoichiometric regime in between, where the metal–insulator transition occurs. A key aspect for the electronic structure is the hybridization of the electron states of the host metal and the hydrogen s states: bands form which result from the RE–hydrogen and hydrogen–hydrogen interaction. They are situated at energies below a set of bands of essentially RE–metal 5d character. Models to explain the gap opening in REH3 include a Peierls-like approach where symmetry lowering changes in the hydrogen positions are sufficient to explain the observed band gap [26,27]. Ng et al. [23,24] considered strong electron correlations and treated the problem in a manybody theory based on a lattice of H ions to study the electronic structure of lanthanum hydride as a prototype of REHx systems. In LaH2, there are two low-lying hydrogen-derived bands which accommodate four of the five valence electrons (supplied by La with the valence configuration 5d16s2 and the two H atoms). The remaining electron occupies the bottom of a lanthanumderived band that contains the Fermi energy, accordingly the dihydride is a metal. In insulating LaH3 an additional band joins the low-lying hydrogen-derived valence bands. These three Hderived bands accommodate all six valence electrons, and owing to strong electron correlations a gap of B2 eV opens. The substoichiometric phase LaH3g becomes metallic when the electron states centered at the octahedral hydrogen vacancies
overlap. This occurs only at a notable vacancy content, gE0:25; since these states are highly localized due to the strong hybridization between the hydrogen 1s and neighboring lanthanum 5deg orbitals. Experiments of photoemission [28,29], inverse photoemission [30], and X-ray emission at the L3 edge of La [31] confirm largely the theoretical results. Hydrogen-derived states were observed at some 5 eV binding energy, the density of states at the Fermi energy is reduced in the dihydride as compared to the pure metal. This reduction continuous as the hydrogen concentration is raised from x ¼ 2 to 3. Obviously, the hydride forms at the expense of the conduction-band population, the metal-derived 5d states appear to be depopulated in favor of the low-lying bands. A recent study of X-ray absorption on the La–L2,3 edges that is dominated by the 2p-to-5d dipole transition has confirmed these findings for LaHx in the multilayer system [15]. Measurements of magnetic circular dicroism (XMCD) in these experiments revealed the existence of an extended spin polarization of the itinerant 5d states of La in the metallic hydride that is due to hybridization with the 3d states of Fe [15]. This suggests that these states mediate the observed exchange coupling between the Fe layers. The length scale of the 5d polarization (decay length from the interface: ( is in agreement with the upper limit of B15 A) the LaHx layer thickness for which coupling has been observed.2 As the hydrogen content is increased the XMCD signals are diminished in amplitude and modified in shape. Let us note that in the insulating phase of the LaHx hydride the dichroic signals remain finite, possibly due to the presence of spin-polarized 5d states in the gap of the LaHx layers induced by the adjacent Fe layers [15]. 2
The present experiments provide a direct proof for interlayer coupling for the AFM case only. For the as-prepared multilayers this covers the spacer-layer thicknesses ( ( 16 Apt LaH p23 A. There is no doubt then that coupling must be also effective and even stronger in the FM case for tLaH ( For tLaH > 23 A, ( where magnetic alignment is ferroo16 A. ( magnetic again, the presence of coupling up to at least 26 A follows indirectly from the hydrogen-induced change to AFM magnetization orientation.
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
The Fermi surface of La dihydride is much simpler than that of the pure metal [32]. It is a multiply connected hole surface formed by a warped cube around the G point in the Brillouin zone with necks around the L points along the [1 1 1] axes. According to the model of Ng et al. [23,24] each additionally dissolved hydrogen atom in the spacer material attracts an electron. This will alter the Fermi surface due to the reduced density of itinerant 5d electrons. It will also modify the potential step that the conduction electrons in the spacer experience upon crossing the interface with Fe [33], and thus the spin dependent reflection coefficient for the conduction electrons. The result is a change of the phase and strength of the exchange coupling. Both effects are observed here in the Fe/LaHx multilayers. Unfortunately, there is only one AFM maximum in the coupling energy but no oscillatory behavior. Apparently, the osci( due to llations cannot develop beyond tLaH B30 A the weak coupling strength. This makes it difficult to relate the interlayer coupling and the H-induced changes to the Fermi surface of the spacer material. At sufficiently high hydrogen concentration, when the LaHx spacer becomes insulating, conduction electrons are no longer available and exchange coupling between the Fe layers is disrupted. In this final state the multilayers are always characterized by a narrow step-like hysteresis loop. Due to the mutual perturbation between the Fe and LaH3g electronic states in the interfacial region, the hydrogen concentration in the spacer layers may be smaller at the interfaces than in the volume. This would not be in conflict with an overall insulating behavior of the hydride which is insensitive to relatively large deviations g from stoichiometry [23,24]. There have been reports on thermally induced exchange coupling between Fe layers across amorphous semiconducting interlayers. But the coupling energy derived is considerably smaller compared to metallic spacers (including the present ones) and the effect is thought to be closely related to the amorphous structure [34,35]. Bruno has predicted non-oscillatory exchange coupling across a crystalline insulating barrier within the quantum interference model [36]. But since the strength of the coupling is relatively small and its amplitude decays exponen-
87
tially with the spacer-layer thickness his result is not relevant for the present multilayer system. In the absence of exchange coupling in the multilayers with insulating LaHx magnetic dipolar interaction may provide a coupling between adjacent Fe layers. In equilibrium, it would align their magnetization vectors antiparallel. We do not observe antiparallel alignment of the Fe layers for insulating LaHx neither with MOKE magnetometry nor with neutron reflectometry. This points to a very weak magnetic dipolar interaction that has no relevance for our investigations.
6. Summary and conclusions While the Fe layers in Fe/La multilayers are magnetically decoupled (See footnote 1) they become coupled by exchange interaction when La is hydrogenated. This must be related to the profound differences in the electronic band structure of the interlayer in the two cases. FM and AFM coupling is observed depending on the thickness of hydride layers, but without oscillatory behavior predicted by all of the existing models. Starting from the metallic La dihydride the effect of dissolving increasingly more hydrogen was examined. Sign and value of the exchange coupling across the interlayers depend crucially on their hydrogen content. We find that the coupling can be inverted from AFM to FM and from FM to AFM, and the important changes take place at rather low additional hydrogen uptake of the spacers when they are still metallic. It is emphasized that these alterations are not due to any structural changes in the multilayers but to modifications of the electronic structure of LaHx, since the interatomic distances and the quality of the interfaces are maintained. Recent measurements of X-ray absorption and XMCD [15] suggest that the interlayer coupling is transmitted by the itinerant 5d band electrons of the spacer which are spin polarized by the adjacent Fe, with a ( Their number is increasdecay length of B15 A. ingly reduced with increasing hydrogen absorption. When the absorption is saturated the hydride layers are insulating and the exchange coupling is likely to disappear. In this final state the multi-
88
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89
layers are always characterized by a very soft FM rectangular hysteresis curve, with low saturation and coercive fields that is thought to characterize the individual magnetically decoupled Fe layers. The corresponding neutron reflectivity spectrum does not show a magnetic superstructure. Magnetic dipolar interactions that remain as a possibility for coupling apparently are too weak to be effective. The changes in the magnetic properties are completely reversible when the initial hydrogen concentration of the as-prepared state is restored. Even though the hydrogen-induced modification of the magnetic interlayer coupling must be of electronic origin the mechanism is not entirely clear. According to Stiles [4] there is no good model for exchange coupling in multilayer systems that are not well lattice matched, as in the present case of the structurally non-coherent heterostructures. We propose that the observed changes in sign and strength of the coupling in the metallic state of the LaHx spacer is due to altered spin dependent reflection coefficients at the interface. In fact changes of the degree of hybridization of the Fe-3d and La-derived 5d band states at the interfaces have been directly observed in the XMCD experiments [15]. A possible influence of modifications of the Fermi surface of LaHx on the coupling cannot be excluded. They would alter the period for the oscillation with the hydride-layer thickness. However, since only one AFM coupling peak was observed it is impossible to discuss oscillatory coupling. Acknowledgements It is a pleasure to acknowledge stimulating discussions with Markus Munzenberg. . We appreciate the financial support of the Deutsche Forschungsgemeinschaft within SFB 345. The experiments at BENSC, Hahn-Meitner Institut Berlin were supported by the European Commission through the PECO 93 Action (contract: ERB CIPD CT 940088). References [1] P. Grunberg, . R. Schreiber, Y. Pang, M.B. Brodsky, H. Sowers, Phys. Rev. Lett. 57 (1986) 2442.
[2] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures, Vol. 2, Springer, Berlin, 1994. [4] M.D. Stiles, J. Magn. Magn. Mater. 200 (1999) 322. [5] P. Bruno, Phys. Rev. B 52 (1995) 411. [6] F. Klose, C. Rehm, D. Nagengast, H. Maletta, A. Weidinger, Phys. Rev. Lett. 78 (1997) 1150. . [7] B. Hjorvarsson, J.A. Dura, P. Isberg, T. Watanabe, T.J. Udovic, G. Andersson, C.F. Majkrzak, Phys. Rev. Lett. 79 (1997) 901. . [8] D. Labergerie, C. Sutter, H. Zabel, B. Hjorvarsson, J. Magn. Magn. Mater. 192 (1999) 238. [9] P. Vajda, in: K.A. Gschneidner, L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earth, Vol. 20, Elsevier Science, Amsterdam, 1995. [10] J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Dekker, D.G.d. Groot, N.J. Koeman, Nature 380 (1996) 231. [11] G.P. Felcher, W. Lohstroh, H. Fritzsche, M. Munzenberg, . H. Maletta, W. Felsch, Appl. Phys. Lett. 72 (1998) 2894. [12] J. Krauser, Hahn-Meitner-Institut Berlin, private communication. [13] L.G. Parratt, Phys. Rev. 95 (1954) 359. [14] L. Nevot, P. Croce, Rev. Phys. Appl. 15 (1980) 761. . [15] M. Munzenberg, . Dissertation, Universit.at Gottingen, 2000. [16] C. Rehm, H. Fritzsche, H. Maletta, F. Klose, Phys. Rev. B 59 (1999) 3142. [17] F. Klose, Ch. Rehm, M. Fieber-Erdmann, E. Holub. Krappe, H.J. Bleif, H. Sowers, R. Goyette, L. Troger, H. Maletta, Physica B 283 (2000) 184. . [18] M. Munzenberg, . Diplomarbeit, Universit.at Gottingen, 1997. [19] A. Bart!el!emy, A. Fert, M.N. Baibich, S. Hadjoudi, F. Petroff, P. Etienne, R. Cabanel, S. Lequin, F.N.v. Dau, G. Creuzet, J. Appl. Phys. 67 (1990) 5908. [20] F. Mezei, R. Golub, F. Klose, H. Toews, Physica B 213&214 (1995) 898. [21] V.-O.d. Haan, G.G. Drijkoningen, Physica B 198 (1994) 24. [22] G.P. Felcher, Physica B 192 (1993) 137. [23] K.K. Ng, F.C. Zhang, V.I. Anisimov, T.M. Rice, Phys. Rev. Lett. 78 (1997) 1311. [24] K.K. Ng, F.C. Zhang, V.I. Anisimov, T.M. Rice, Phys. Rev. B 59 (1999) 5398. [25] R. Eder, H.F. Pen, G.A. Sawatzky, Phys. Rev. B 56 (1997) 10115. [26] P.J. Kelly, J.P. Dekker, R. Stumpf, Phys. Rev. Lett. 78 (1997) 1315. [27] P. van Gelderen, P. Kelly, G. Brocks, Phys. Rev. Lett. 85 (2000) 2989. [28] D.J. Peterman, J.H. Weaver, M. Croft, D.T. Peterson, Phys. Rev. B 27 (1983) 808. [29] L. Schlapbach, J. Osterwalder, T. Riesterer, J. Less Common Met. 103 (1984) 295. [30] J. Osterwalder, Z. Phys. B 61 (1985) 113.
W. Lohstroh et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 77–89 [31] A.C. Sarma, W.G. Bos, J. Phys. Chem. Solids 32 (1971) 1423. [32] M. Gupta, J.P. Burger, Phys. Rev. B 22 (1980) 6074. [33] J. Mathon, M. Villeret, D.M. Edwards, R.B. Muniz, J. Magn. Magn. Mater. 121 (1993) 242.
89
[34] S. Toscano, B. Briner, H. Hopster, M. Landolt, J. Magn. Magn. Mater. 114 (1992) L6. [35] P. Walser, M. Hunziker, M. Landolt, J. Magn. Magn. Mater. 200 (1999) 95. [36] P. Bruno, Phys. Rev. B 49 (1994) 13231.