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Lu superlattices
T
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1915-1917 North-Holland
Magnetic order in Dy/Lu superlattices R.S. Beach a, J.A. Borchers b, R.W. Erwin b, C.P. Flynn J.J. Rhyne b and M.B. Salamon a
a,
A. Matheny a,
a Departma~t of Physics, Unieersity of Illinois, Urbana, IL 61801, USA b National Institute of Standards and Technology., Gaithersburg, MD 20899, USA Several superlattices containing alternate layers of the rare earth elements dysprosium and lutetium were grown by molecular beam epitaxy. Neutron diffraction shows that these samples develop a helical phase in the ( -.- 40 ,~,) Dy layers at TN = 178 K which propagates coherently across the nonmagnetic Lu (20-55 A thick). The Dy layers order ferromagnetically at temperatures which vary from 140 to 160 K (the ferromagnetic phase in the bulk material appears at 85 K). Below Tc the ferromagnetic Dy layers may be either aligned or anti-aligned. The transition is accompanied by a distortion of the superlattice basal plane comparable to that which occurs in bulk Dy. We also observe an approximately 80 K increase in Tc in thin Lu/Dy ( < 150 A)/Lu films. We discuss how the observed high Tc may be related to the elastic coupling of the Dy to Lu.
The evolution of molecular beam epitaxy (MBE) has led to the ability to tailor materials on the atomic scale. The one-dimensional periodic confinement of alternating materials in a superlattice is the simplest step in that direction. Since the first metallic superlattices were constructed in the early 1980s, a variety of superlattices containing magnetic metals has been studied. In this paper we present results on a superlattice system composed of the rare earth metals dysprosium and lutetium. In bulk Dy, at T N = 178 K the moments in a given sheet perpendicular to the c-axis align ferromagnetically in the basal plane. Along the c-axis the structure is helical; the spins rotate from one sheet to the next by a turn angle of 43 ° The turn angle decreases to 26.5 ° at 85 K, where a spontaneous first order transition to ferromagnetism occurs. This is accompanied by a 0.5% orthorhombic distortion of the basal plane. The transition is driven by a gain of 3.2 K / a t o m in the magnetoelastic energy [1]. Earlier neutron diffraction data from a set of c-axis superlattices with alternating layers of yttrium and Dy showed that the Dy spiral order propagates across the intervening nonmagnetic Y with the same chiral sense [2]. In addition, in that system the ferromagnetic phase of Dy is completely suppressed, apparently by the epitaxial clamping of the Dy to the nonmagnetostrictive Y layers and substrate. The 4f shell fills at lutetium. Like Y, Lu is nonmagnetic and takes the hcp structure; it is a logical substitute for Y in these superlattices. While Dy stretches to meet Y epitaxially, the 1.2% lattice mismatch between Dy and Lu compresses the Dy in the basal plane. To date we have fabricated a D y / L u superlattice series, and sets of Dy and Erbium films grown on Lu seed layers. The samples were all grown on [1120] single crystal
polished sapphire substrates. A 630 ,~, [110] Nb buffer layer was first deposited. This was followed by a 630 ~, [0002] Lu base layer over which defects introduced at the Nb interface are cleared. The film or superlattice was then grown onto this surface with the growth plane normal to the hcp c-axis. The samples were characterized structurally by standard X-ray diffraction techniques. They are single crystals, with coherence lengths of 700 and 300 A along the c-axis and in the basal plane respectively. The magnetic structure of the superlattices was studied primarily by neutron diffraction. For a superlattice of wavelength ,,1, consisting of a nonmagnctic material and one that orders helically such as D y / Y . the scattering occurs along c* at values of K = n ( 2 r r / A ) + dp/A, where qb is the magnetic phase advance across one bilayer. The peaks are modulated by a broad envelope function that selects those about K = m G + Q. G is a reciprocal lattice vector in the magnetic element, and Q is the magnetic ordering wave vector of the rare earth. Fig. 1 shows the results of neutron scattering scans along the growth direction for a superlattice [Dye4 ILu20]s0 of 80 bilayers, each with 14 atomic planes of Dy and 20 planes of Lu, in the paramagnetic phase, just below T N at 170 K, and at low temperature. The data were obtained at the National Institute of Standards and Technology reactor on a triple axis spectrometer. The high temperature peaks are structural; the first order satellite lies immediately left of the main Bragg peak found at 2.24 A-~. The peaks that emerge in the scans below TN are due to magnetic scattering. Note that the width of these peaks is comparable to that of the Bragg peak: the magnetic order is coherent over several bilayer periods. The 170 K data, taken just below T N, weakly display the signature of the Dy spiral phase. In samples with lower ferromagnetic transition
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. Ah rights reserved
1916
R.S. Beach et al. / Magnetic order 01 Dy / Lu superlattices
1
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1
1
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~
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1
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1
0
0
¢D i
t
i
l
~
I
I
!
t
l
1OK
170 K
195 K
K/A" Fig. 1. Intensity of scattered neutrons vs. K for superlattice [Dyt4 ILu.,o]80. The ripples around 2.0 ,~-I in the 170 K scan (inset) indicate the presence of a coupled spiral phase. The magnetic peak widths are roughly 0.048 ,~,-i. At 160 K tht spiral gives way to anti-aligned ferromagnetic Dy layers. Tighter collimation gives better resolution in the 10 K scan.
I
C) O-
l
0
I= 0
(5
C'4
i
4.06
4 07
4.0B
4.09
410
4..11
4.12
Fig. 2. X-ray scans in the basal plane through (202.0) of superlattice [Dy:l ILulo]70. The data were obtained at the NSLS. The splitting displays a distortion of the hexagonally symmetric basal plane below Tc.
R.S. Beach et al. / Magnetic order in Dy / Lu superlattices
temperatures the spiral intensity is greater, and the long range nature of the spiral phase is clearly seen. In this supcrlattice this phase yields to intralayer ferromagnetic order at 160 K (in bulk Dy T c = 85 K). The 10 K dat~ show unambiguously the presence of magnetic peaks between the structural superlattice peaks. This corresponds to a doubling of the chemical unit cell: each Dy layer is ferromagnetic, but alternating layers are anti-aligned with a coherence of several superlattice periods. Three superlattices have been thoroughly characterized. All three display long range spiral order at T N -=175 K as is observed in D y / Y . While that system is never spontaneously ferromagnetic, two D y / L u superlattices order ferromagnetically within anti-aligned Dy layers at 160 K ([Dyl41Lu2o]s0) and 145 K ([Dy2t ILUlo]7o), The third ([Dy21 ILus]7s) exhibits a spiral phase as well, and is ferromagnetic below 140 K, but the Dy layers in the low temperature phase are ferromagnetically coupled. Both ferromagnetic and antiferromagnetic interlayer coupling was found in superlattices composed of Y and gadolinium [3]. The helix in all three superlattices advances in the Dy at no more that~ 3 1 ° / l a y e r , with 26 ° the smallest turn angle observed at the ferromagnetic transition. The turn angle is smaller at any given temperature than in D y / Y , or in elemental Dy. The spiral order propagates coherently through the Lu with a phase advance of roughly 45 ° per Lu layer. This value is close to the maximum (at 48 o in Lu) in :he generalized susceptibility of the nonmagnetic element [4], as is the case in the D y / Y superlattice system. Despite the fact that the system is epitaxial, in fig. 2 it can be seen that the spiral-ferromagnetic transition is acconapanied b,,, a distortion of the cntirc supcrlatrice basal plane. Thc magnitude of the (20-5-0) peak splitting reveals a distortion approximately 85% that of the bulk Dy transition. The data suggest that the basal plane breaks into domains no more than 500 ~, across, as it must to stay fixed to the substrate. Whether this distortion is actually orthorhombic is currently under investigation. We also observe high transition temperatures in very thin L u / D y / L u films. Experiments in progress show a sudden increase in Tc for Dy films grown on Lu seed layers as thc Dy thickness falls below 100 A.
1917
Where a 150 ,~, Dy film sandwiched between 500 .~ Lu layers is ferromagnetic below 100 K, one with 40 ,~ Dy has a T c of 170 K. A smaller increase in thc Dy Tc has been observed in E r / D y / E r sandwich structures [5]. The D y / L u systcm demonstrates interlayer coupling similar to D y / Y above T c , and to G d / Y below it. The coincidence of both coupled phases in one superlattice supports the conclusion that interlayer coupling in the two previous systems has the same physical source. The form and range of the coupling as a function of Lu thickness have not yet been determined. An equally compelling question is the source of the enhancement of Tc. In one superlattice Tc is up by more than 80 K. A magnetoelastic energy model [6] which successfully predicted the critical field values in the D y / Y system was also able to account for the transition temperature as it related to strain in a series of Er films grown onto Lu seed layers [7]. The same model, applied to the D y / L u superlattice series, predicts a jump in T¢ from the bulk value much smaller than observed. The ferromagnetic transition does not seem to be first order in the superlattices (or the films). With high resolution X-rays we are currently measuring basal plane lattice parameters through the transition to determine if the Dy lattice is clamped or continuously distorts at Tc. References
[1] M. Rosen and H. Mimer, Phys. Rev. B 1 (1970) 3748. [2] R.W. Erwin, J.J. Rhyne, M.B. Salamon, J.A. Borchers. S. Sinha, R. Du, J.E. Cunningham and C.P. Flynn, Phys. Rev. B. 35 (1987) 6808. [3] J. Kwo, M. Hong, F.J. DiSalvo. J.V. Waszczak and C.F. Majkrzak, Phys. Rev. B. 35 (1987) 7295. [4] H.R. Child, W.C. Koehler, E.D. Wollan and J.W. Cable, Phys. Rev. 138 (1965) A1655. [5] R.F.C. Farrow, S.S.P. Parkin, V.S. Speriousu, A. Bezinge and A.P. Segmuller, Mater. Res. Soc. Syrup, Proc. Voi. 151 (1989) 203. [6] R.W. Er~vin, ,~.J. Rhyne, J.A. Borchers, M.B. Salamon, R. Du and C.P. Flynn, J. Appl. Phys. 63 (1988) 3461. [7] R.S. Beach, J.A. Borchers, R.W. Erwin, J.J. Rhyne, A. Matheny, C.P. Flynn, and M.B. Salamon, J. Appl. Phys. 69 (1991) 4535.
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1915-1917 North-Holland
Magnetic order in Dy/Lu superlattices R.S. Beach a, J.A. Borchers b, R.W. Erwin b, C.P. Flynn J.J. Rhyne b and M.B. Salamon a
a,
A. Matheny a,
a Departma~t of Physics, Unieersity of Illinois, Urbana, IL 61801, USA b National Institute of Standards and Technology., Gaithersburg, MD 20899, USA Several superlattices containing alternate layers of the rare earth elements dysprosium and lutetium were grown by molecular beam epitaxy. Neutron diffraction shows that these samples develop a helical phase in the ( -.- 40 ,~,) Dy layers at TN = 178 K which propagates coherently across the nonmagnetic Lu (20-55 A thick). The Dy layers order ferromagnetically at temperatures which vary from 140 to 160 K (the ferromagnetic phase in the bulk material appears at 85 K). Below Tc the ferromagnetic Dy layers may be either aligned or anti-aligned. The transition is accompanied by a distortion of the superlattice basal plane comparable to that which occurs in bulk Dy. We also observe an approximately 80 K increase in Tc in thin Lu/Dy ( < 150 A)/Lu films. We discuss how the observed high Tc may be related to the elastic coupling of the Dy to Lu.
The evolution of molecular beam epitaxy (MBE) has led to the ability to tailor materials on the atomic scale. The one-dimensional periodic confinement of alternating materials in a superlattice is the simplest step in that direction. Since the first metallic superlattices were constructed in the early 1980s, a variety of superlattices containing magnetic metals has been studied. In this paper we present results on a superlattice system composed of the rare earth metals dysprosium and lutetium. In bulk Dy, at T N = 178 K the moments in a given sheet perpendicular to the c-axis align ferromagnetically in the basal plane. Along the c-axis the structure is helical; the spins rotate from one sheet to the next by a turn angle of 43 ° The turn angle decreases to 26.5 ° at 85 K, where a spontaneous first order transition to ferromagnetism occurs. This is accompanied by a 0.5% orthorhombic distortion of the basal plane. The transition is driven by a gain of 3.2 K / a t o m in the magnetoelastic energy [1]. Earlier neutron diffraction data from a set of c-axis superlattices with alternating layers of yttrium and Dy showed that the Dy spiral order propagates across the intervening nonmagnetic Y with the same chiral sense [2]. In addition, in that system the ferromagnetic phase of Dy is completely suppressed, apparently by the epitaxial clamping of the Dy to the nonmagnetostrictive Y layers and substrate. The 4f shell fills at lutetium. Like Y, Lu is nonmagnetic and takes the hcp structure; it is a logical substitute for Y in these superlattices. While Dy stretches to meet Y epitaxially, the 1.2% lattice mismatch between Dy and Lu compresses the Dy in the basal plane. To date we have fabricated a D y / L u superlattice series, and sets of Dy and Erbium films grown on Lu seed layers. The samples were all grown on [1120] single crystal
polished sapphire substrates. A 630 ,~, [110] Nb buffer layer was first deposited. This was followed by a 630 ~, [0002] Lu base layer over which defects introduced at the Nb interface are cleared. The film or superlattice was then grown onto this surface with the growth plane normal to the hcp c-axis. The samples were characterized structurally by standard X-ray diffraction techniques. They are single crystals, with coherence lengths of 700 and 300 A along the c-axis and in the basal plane respectively. The magnetic structure of the superlattices was studied primarily by neutron diffraction. For a superlattice of wavelength ,,1, consisting of a nonmagnctic material and one that orders helically such as D y / Y . the scattering occurs along c* at values of K = n ( 2 r r / A ) + dp/A, where qb is the magnetic phase advance across one bilayer. The peaks are modulated by a broad envelope function that selects those about K = m G + Q. G is a reciprocal lattice vector in the magnetic element, and Q is the magnetic ordering wave vector of the rare earth. Fig. 1 shows the results of neutron scattering scans along the growth direction for a superlattice [Dye4 ILu20]s0 of 80 bilayers, each with 14 atomic planes of Dy and 20 planes of Lu, in the paramagnetic phase, just below T N at 170 K, and at low temperature. The data were obtained at the National Institute of Standards and Technology reactor on a triple axis spectrometer. The high temperature peaks are structural; the first order satellite lies immediately left of the main Bragg peak found at 2.24 A-~. The peaks that emerge in the scans below TN are due to magnetic scattering. Note that the width of these peaks is comparable to that of the Bragg peak: the magnetic order is coherent over several bilayer periods. The 170 K data, taken just below T N, weakly display the signature of the Dy spiral phase. In samples with lower ferromagnetic transition
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. Ah rights reserved
1916
R.S. Beach et al. / Magnetic order 01 Dy / Lu superlattices
1
|
1
1
'
~
|
|
1
|
1
0
0
¢D i
t
i
l
~
I
I
!
t
l
1OK
170 K
195 K
K/A" Fig. 1. Intensity of scattered neutrons vs. K for superlattice [Dyt4 ILu.,o]80. The ripples around 2.0 ,~-I in the 170 K scan (inset) indicate the presence of a coupled spiral phase. The magnetic peak widths are roughly 0.048 ,~,-i. At 160 K tht spiral gives way to anti-aligned ferromagnetic Dy layers. Tighter collimation gives better resolution in the 10 K scan.
I
C) O-
l
0
I= 0
(5
C'4
i
4.06
4 07
4.0B
4.09
410
4..11
4.12
Fig. 2. X-ray scans in the basal plane through (202.0) of superlattice [Dy:l ILulo]70. The data were obtained at the NSLS. The splitting displays a distortion of the hexagonally symmetric basal plane below Tc.
R.S. Beach et al. / Magnetic order in Dy / Lu superlattices
temperatures the spiral intensity is greater, and the long range nature of the spiral phase is clearly seen. In this supcrlattice this phase yields to intralayer ferromagnetic order at 160 K (in bulk Dy T c = 85 K). The 10 K dat~ show unambiguously the presence of magnetic peaks between the structural superlattice peaks. This corresponds to a doubling of the chemical unit cell: each Dy layer is ferromagnetic, but alternating layers are anti-aligned with a coherence of several superlattice periods. Three superlattices have been thoroughly characterized. All three display long range spiral order at T N -=175 K as is observed in D y / Y . While that system is never spontaneously ferromagnetic, two D y / L u superlattices order ferromagnetically within anti-aligned Dy layers at 160 K ([Dyl41Lu2o]s0) and 145 K ([Dy2t ILUlo]7o), The third ([Dy21 ILus]7s) exhibits a spiral phase as well, and is ferromagnetic below 140 K, but the Dy layers in the low temperature phase are ferromagnetically coupled. Both ferromagnetic and antiferromagnetic interlayer coupling was found in superlattices composed of Y and gadolinium [3]. The helix in all three superlattices advances in the Dy at no more that~ 3 1 ° / l a y e r , with 26 ° the smallest turn angle observed at the ferromagnetic transition. The turn angle is smaller at any given temperature than in D y / Y , or in elemental Dy. The spiral order propagates coherently through the Lu with a phase advance of roughly 45 ° per Lu layer. This value is close to the maximum (at 48 o in Lu) in :he generalized susceptibility of the nonmagnetic element [4], as is the case in the D y / Y superlattice system. Despite the fact that the system is epitaxial, in fig. 2 it can be seen that the spiral-ferromagnetic transition is acconapanied b,,, a distortion of the cntirc supcrlatrice basal plane. Thc magnitude of the (20-5-0) peak splitting reveals a distortion approximately 85% that of the bulk Dy transition. The data suggest that the basal plane breaks into domains no more than 500 ~, across, as it must to stay fixed to the substrate. Whether this distortion is actually orthorhombic is currently under investigation. We also observe high transition temperatures in very thin L u / D y / L u films. Experiments in progress show a sudden increase in Tc for Dy films grown on Lu seed layers as thc Dy thickness falls below 100 A.
1917
Where a 150 ,~, Dy film sandwiched between 500 .~ Lu layers is ferromagnetic below 100 K, one with 40 ,~ Dy has a T c of 170 K. A smaller increase in thc Dy Tc has been observed in E r / D y / E r sandwich structures [5]. The D y / L u systcm demonstrates interlayer coupling similar to D y / Y above T c , and to G d / Y below it. The coincidence of both coupled phases in one superlattice supports the conclusion that interlayer coupling in the two previous systems has the same physical source. The form and range of the coupling as a function of Lu thickness have not yet been determined. An equally compelling question is the source of the enhancement of Tc. In one superlattice Tc is up by more than 80 K. A magnetoelastic energy model [6] which successfully predicted the critical field values in the D y / Y system was also able to account for the transition temperature as it related to strain in a series of Er films grown onto Lu seed layers [7]. The same model, applied to the D y / L u superlattice series, predicts a jump in T¢ from the bulk value much smaller than observed. The ferromagnetic transition does not seem to be first order in the superlattices (or the films). With high resolution X-rays we are currently measuring basal plane lattice parameters through the transition to determine if the Dy lattice is clamped or continuously distorts at Tc. References
[1] M. Rosen and H. Mimer, Phys. Rev. B 1 (1970) 3748. [2] R.W. Erwin, J.J. Rhyne, M.B. Salamon, J.A. Borchers. S. Sinha, R. Du, J.E. Cunningham and C.P. Flynn, Phys. Rev. B. 35 (1987) 6808. [3] J. Kwo, M. Hong, F.J. DiSalvo. J.V. Waszczak and C.F. Majkrzak, Phys. Rev. B. 35 (1987) 7295. [4] H.R. Child, W.C. Koehler, E.D. Wollan and J.W. Cable, Phys. Rev. 138 (1965) A1655. [5] R.F.C. Farrow, S.S.P. Parkin, V.S. Speriousu, A. Bezinge and A.P. Segmuller, Mater. Res. Soc. Syrup, Proc. Voi. 151 (1989) 203. [6] R.W. Er~vin, ,~.J. Rhyne, J.A. Borchers, M.B. Salamon, R. Du and C.P. Flynn, J. Appl. Phys. 63 (1988) 3461. [7] R.S. Beach, J.A. Borchers, R.W. Erwin, J.J. Rhyne, A. Matheny, C.P. Flynn, and M.B. Salamon, J. Appl. Phys. 69 (1991) 4535.