- Email: [email protected]
Magnetic properties of UPd2Al3 thin films investigated by resonant magnetic X-ray scattering
Physica B 259—261 (1999) 631—633
Magnetic properties of UPd Al thin films investigated by resonant magnetic X-ray scattering A. Hiess *, N. Bernhoeft , S. Langridge, C. Vettier, M. Jourdan, M. Huth, H. Adrian, G.H. Lander Institut Laue-Langevin, BP 156, F-38042 Grenoble, France Rutherford Appleton Laboratory, ISIS facility, Chilton, Didcot OX11 0QX, UK European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France Joh.-Gutenberg-Universita( t, Inst. fu( r Physik, D-55099 Mainz, Germany European Commission, JRC, Institute for Transuranium Elements, D-76125 Karlsruhe, Germany
Abstract Thin films with thicknesses varying from 100 to 1600 As of the magnetic heavy-fermion superconductor UPd Al have been investigated by resonant magnetic X-ray scattering. All films studied show the same antiferromagnetic structure as the bulk material. The magnetic order seems to develop initially on the surface of the film and penetrates into the film on lowering the temperature. In the hexagonal c direction (film growth direction) the magnetic correlations extend over the complete film thickness at low temperature. The magnetic correlation lengths in the plane of the film are approximately equal to those along the c-axis. 1999 Elsevier Science B.V. All rights reserved. Keywords: Heavy fermions; Magnetic X-ray scattering; Thin films
UPd Al belongs to the class of magnetic heavy-fer mion superconductors in which, at low temperatures, ¹, antiferromagnetic order (¹ (bulk)"14 K, k"0.85 k ) , coexists with superconductivity (¹ "2 K) [1]. The mag netic structure of ferromagnetic (0 0 1) layers, with moments in the basal plane, coupled #!#! along the hexagonal c direction is described by the magnetic propagation vector (0 0 ). An important question relates to the dimensionality of the magnetic interactions and their interaction with superconductivity. This was the motivation to investigate the magnetic properties of UPd Al thin films using resonant magnetic X-ray scat tering. The use of the technique relies on the enhancement of the magnetic cross section as the photon energy is tuned to the uranium M absorption edge (E - "3.73 keV) [2]. The experiments discussed here 6
* Corresponding author. Tel.: 33-476-496855; fax: 33-476483906; e-mail: [email protected].
were performed at the magnetic scattering beamline ID20 of the European Synchrotron Radiation Facility. The samples, with nominal thicknesses of 100, 200, 400, 800, and 1600 As , were epitaxially grown on the (1 1 1) surface of LaAlO by electron beam co-evaporation of the elements [3] in Mainz. The films grow with their c-axis perpendicular to the surface, the mosaic spread ranges from 1.2 for the thinnest film to about 0.3 for the thickest. All films show a characteristic kink in the resistivity, which has been attributed to the antiferromagnetic order, and all films — except the thinnest — become superconducting with ¹ +2 K. The effective thickness of the films has been determined by atomic force microscopy and small angle X-ray scattering in Mainz and by analyzing reflectivity fringes close to a structural Bragg reflection in the present magnetic X-ray scattering experiment. The observed values obtained from all techniques are consistent within a few As . The magnetic scattering observed at the antiferromagnetic Bragg positions (0 0 ) (Fig. 1) and (0 0 ) shows that the magnetic ordering is the same in the films as in the
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 7 4 - 0
632
A. Hiess et al. / Physica B 259–261 (1999) 631—633
Fig. 2. Energy width, (c/*¸) and (2c/*H) at ¹"4 K versus film thickness. The dotted line is a result of a model calculation.
Fig. 1. Temperature dependence of the magnetic intensity, energy width, (c/*¸) and (2c/*H) as deduced from the (0 0 ) magnetic reflection measured on a 1600 As thick film. To extract the effective widths *¸ and *H, the observed full-widths of the magnetic peaks have been deconvoluted from resolution effects and the mosaicity of the sample based on the (0 0 1) charge reflection.
bulk [4] and that the ordering temperature ¹ +12 K is , slightly reduced. Even for the thinnest film of 100 As the observed magnetic peak intensity reached about 500 cts/s making detailed investigations of the magnetism possible. As a preliminary data analysis, *H (the FWHM of
H-scans around the specular (0 0 l) antiferromagnetic peaks) and *¸ (the FWHM of ¸-scans) have been
extracted. (c/*¸) is a measure for the magnetic correlation length in the direction of the film growth (hexagonal c-axis). *¸ is constant for ¹(12 K and corresponds
to *¸ of the structural reflections, showing that the magnetic order penetrates the whole film and is restricted by the film thickness only (Fig. 2). On the other hand, in the transverse direction to the surface normal, *H of the magnetic peaks remains
wider at all ¹ than *H of the (0 0 1) charge peak. This demonstrates a finite magnetic correlation length in the plane of the films that is considerably smaller than the in-plane charge correlation length of 1 lm as determined by X-ray and electron microscopy experiments. We take *H to reflect the resolution effects and the mosaicity of the sample. Using a simple quadratic relationship for deconvolution we extract (2c/*H) as a length in As that can be used to characterise the in-plane magnetic correlations. Surprisingly, (2c/*H) is approximately equal to the film thickness in all films at low ¹.
Due to the comparable length scale of the photon absorption, the film thickness and the coherence of the X-ray beam, the energy width of a magnetic reflection depends on both the thickness of the magnetic layer and the location of the magnetic order in real space [5]. Model calculations reproduce well the observed energy widths (Fig. 2), confirming that at low ¹ the magnetic order penetrates the whole film. The energy width is smallest for the thinnest film, whilst for the thickest films the observed values become similar to those observed in the bulk samples. Turning now to the ¹ dependence of the magnetic signal (Fig. 1). On warming, the energy widths, (2c/*H) and (c/*¸ ) decrease, whilst the ratio *¸/*H is approx imately independent of ¹. Such changes are consistent with the idea that the magnetic order develops initially on the surface of the film and penetrates into the film on lowering ¹. In conclusion, we have established a microscopic model for the magnetic order in thin films of UPd Al down to 100 As : The magnetic order develops at the surface of the film and for ¹(¹ penetrates into the , film. The ratio *¸/*H is approximately independent of film thickness. This suggests that the correspondence of (2c/*H) and the film thickness at low ¹ is intrinsic, and not related to film quality or interfacial disturbance. Numerous experiments, such as inelastic neutron scattering [6], muon spin rotation [7] and resistivity under high pressure [8], indicate that the phenomena of superconductivity and antiferromagnetism in UPd Al are insep arable. Given the superconducting correlation length is about 100 As , this suggests that the lack of superconductivity in the thinnest film of this compound is related to the small magnetic correlation lengths (2c/*H) and (c/*¸) imposed by the film thickness.
A. Hiess et al. / Physica B 259–261 (1999) 631—633
References [1] C. Geibel et al., Z. Phys. B 84 (1991) 1. [2] J.P. Hill, D.F. McMorrow, Acta Crystallogr. A 52 (1996) 236. [3] M. Huth et al., Physica B 199—200 (1994) 116.
633
[4] B. Gaulin et al., Phys. Rev. Lett. 73 (1994) 8909. [5] N. Bernhoeft, Acta Crystallogr. (1998), in press. [6] N. Bernhoeft et al., in these Proceedings (SCES’98), Physica B 259—261 (1999). [7] R. Feyerherm et al., Phys. Rev. Lett. 73 (1994) 1849. [8] P. Link et al., J. Phys. CM 7 (1995) 373.
Magnetic properties of UPd Al thin films investigated by resonant magnetic X-ray scattering A. Hiess *, N. Bernhoeft , S. Langridge, C. Vettier, M. Jourdan, M. Huth, H. Adrian, G.H. Lander Institut Laue-Langevin, BP 156, F-38042 Grenoble, France Rutherford Appleton Laboratory, ISIS facility, Chilton, Didcot OX11 0QX, UK European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France Joh.-Gutenberg-Universita( t, Inst. fu( r Physik, D-55099 Mainz, Germany European Commission, JRC, Institute for Transuranium Elements, D-76125 Karlsruhe, Germany
Abstract Thin films with thicknesses varying from 100 to 1600 As of the magnetic heavy-fermion superconductor UPd Al have been investigated by resonant magnetic X-ray scattering. All films studied show the same antiferromagnetic structure as the bulk material. The magnetic order seems to develop initially on the surface of the film and penetrates into the film on lowering the temperature. In the hexagonal c direction (film growth direction) the magnetic correlations extend over the complete film thickness at low temperature. The magnetic correlation lengths in the plane of the film are approximately equal to those along the c-axis. 1999 Elsevier Science B.V. All rights reserved. Keywords: Heavy fermions; Magnetic X-ray scattering; Thin films
UPd Al belongs to the class of magnetic heavy-fer mion superconductors in which, at low temperatures, ¹, antiferromagnetic order (¹ (bulk)"14 K, k"0.85 k ) , coexists with superconductivity (¹ "2 K) [1]. The mag netic structure of ferromagnetic (0 0 1) layers, with moments in the basal plane, coupled #!#! along the hexagonal c direction is described by the magnetic propagation vector (0 0 ). An important question relates to the dimensionality of the magnetic interactions and their interaction with superconductivity. This was the motivation to investigate the magnetic properties of UPd Al thin films using resonant magnetic X-ray scat tering. The use of the technique relies on the enhancement of the magnetic cross section as the photon energy is tuned to the uranium M absorption edge (E - "3.73 keV) [2]. The experiments discussed here 6
* Corresponding author. Tel.: 33-476-496855; fax: 33-476483906; e-mail: [email protected].
were performed at the magnetic scattering beamline ID20 of the European Synchrotron Radiation Facility. The samples, with nominal thicknesses of 100, 200, 400, 800, and 1600 As , were epitaxially grown on the (1 1 1) surface of LaAlO by electron beam co-evaporation of the elements [3] in Mainz. The films grow with their c-axis perpendicular to the surface, the mosaic spread ranges from 1.2 for the thinnest film to about 0.3 for the thickest. All films show a characteristic kink in the resistivity, which has been attributed to the antiferromagnetic order, and all films — except the thinnest — become superconducting with ¹ +2 K. The effective thickness of the films has been determined by atomic force microscopy and small angle X-ray scattering in Mainz and by analyzing reflectivity fringes close to a structural Bragg reflection in the present magnetic X-ray scattering experiment. The observed values obtained from all techniques are consistent within a few As . The magnetic scattering observed at the antiferromagnetic Bragg positions (0 0 ) (Fig. 1) and (0 0 ) shows that the magnetic ordering is the same in the films as in the
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 7 4 - 0
632
A. Hiess et al. / Physica B 259–261 (1999) 631—633
Fig. 2. Energy width, (c/*¸) and (2c/*H) at ¹"4 K versus film thickness. The dotted line is a result of a model calculation.
Fig. 1. Temperature dependence of the magnetic intensity, energy width, (c/*¸) and (2c/*H) as deduced from the (0 0 ) magnetic reflection measured on a 1600 As thick film. To extract the effective widths *¸ and *H, the observed full-widths of the magnetic peaks have been deconvoluted from resolution effects and the mosaicity of the sample based on the (0 0 1) charge reflection.
bulk [4] and that the ordering temperature ¹ +12 K is , slightly reduced. Even for the thinnest film of 100 As the observed magnetic peak intensity reached about 500 cts/s making detailed investigations of the magnetism possible. As a preliminary data analysis, *H (the FWHM of
H-scans around the specular (0 0 l) antiferromagnetic peaks) and *¸ (the FWHM of ¸-scans) have been
extracted. (c/*¸) is a measure for the magnetic correlation length in the direction of the film growth (hexagonal c-axis). *¸ is constant for ¹(12 K and corresponds
to *¸ of the structural reflections, showing that the magnetic order penetrates the whole film and is restricted by the film thickness only (Fig. 2). On the other hand, in the transverse direction to the surface normal, *H of the magnetic peaks remains
wider at all ¹ than *H of the (0 0 1) charge peak. This demonstrates a finite magnetic correlation length in the plane of the films that is considerably smaller than the in-plane charge correlation length of 1 lm as determined by X-ray and electron microscopy experiments. We take *H to reflect the resolution effects and the mosaicity of the sample. Using a simple quadratic relationship for deconvolution we extract (2c/*H) as a length in As that can be used to characterise the in-plane magnetic correlations. Surprisingly, (2c/*H) is approximately equal to the film thickness in all films at low ¹.
Due to the comparable length scale of the photon absorption, the film thickness and the coherence of the X-ray beam, the energy width of a magnetic reflection depends on both the thickness of the magnetic layer and the location of the magnetic order in real space [5]. Model calculations reproduce well the observed energy widths (Fig. 2), confirming that at low ¹ the magnetic order penetrates the whole film. The energy width is smallest for the thinnest film, whilst for the thickest films the observed values become similar to those observed in the bulk samples. Turning now to the ¹ dependence of the magnetic signal (Fig. 1). On warming, the energy widths, (2c/*H) and (c/*¸ ) decrease, whilst the ratio *¸/*H is approx imately independent of ¹. Such changes are consistent with the idea that the magnetic order develops initially on the surface of the film and penetrates into the film on lowering ¹. In conclusion, we have established a microscopic model for the magnetic order in thin films of UPd Al down to 100 As : The magnetic order develops at the surface of the film and for ¹(¹ penetrates into the , film. The ratio *¸/*H is approximately independent of film thickness. This suggests that the correspondence of (2c/*H) and the film thickness at low ¹ is intrinsic, and not related to film quality or interfacial disturbance. Numerous experiments, such as inelastic neutron scattering [6], muon spin rotation [7] and resistivity under high pressure [8], indicate that the phenomena of superconductivity and antiferromagnetism in UPd Al are insep arable. Given the superconducting correlation length is about 100 As , this suggests that the lack of superconductivity in the thinnest film of this compound is related to the small magnetic correlation lengths (2c/*H) and (c/*¸) imposed by the film thickness.
A. Hiess et al. / Physica B 259–261 (1999) 631—633
References [1] C. Geibel et al., Z. Phys. B 84 (1991) 1. [2] J.P. Hill, D.F. McMorrow, Acta Crystallogr. A 52 (1996) 236. [3] M. Huth et al., Physica B 199—200 (1994) 116.
633
[4] B. Gaulin et al., Phys. Rev. Lett. 73 (1994) 8909. [5] N. Bernhoeft, Acta Crystallogr. (1998), in press. [6] N. Bernhoeft et al., in these Proceedings (SCES’98), Physica B 259—261 (1999). [7] R. Feyerherm et al., Phys. Rev. Lett. 73 (1994) 1849. [8] P. Link et al., J. Phys. CM 7 (1995) 373.