- Email: [email protected]
dysprosium superlattices
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1700}1701
Magnetic phase diagram of holmium/dysprosium superlattices A. del Moral *, C. de la Fuente , J.I. Arnaudas , M. Ciria , L. Benito , M.R. Wells, R.C.C. Ward Lab. de Magnetismo, Depto. de Fn& sica de la Materia Condensada and Inst. de Cienca. de Mat. de Arago& n, Universidad de Zaragoza and C.S.I.C. 50009 Zaragoza, Spain Clarendon Laboratory, Oxford University, Parks Road, Oxford OX1 3PU, UK
Abstract We report on the magnetisation measurements with a magnetic "eld applied along the HCP-structure b-axis for Ho/Dy magnetic superlattices (SLs). The zero-"eld-cool susceptibility, , shows the magnetic-order coexistence of the 8$! Ho and Dy layers, below their respective ordering temperatures. The magnetic-phase diagrams (MPD) of Ho/Dy SLs have been obtained from magnetisation measurements, and we found that the magnetic phases of the Ho and Dy layers are very similar to the bulk ones. Although the analysis of the high-"eld magnetisation indicates that Ho and Dy do not interact magnetically, this coupling exists at low "eld. The increment of the NeH el temperature in the Ho layers with respect to the bulk could re#ect some kind of polarisation between Ho and Dy layers. In the MPD that we have proposed for Ho/Dy SLs, a huge region exists where an helifan phase remains stable for both rare-earth layers, especially for the Ho /Dy SL. This could represent the "rst time that the helifan phase of Ho layers stabilises a similar phase for Dy layers, due to the coherent propagation of conduction band electrons through Ho and Dy layers. However, at low temperatures, we do not expect to "nd the ferro-cone phase in Ho layers as in the bulk, because of the epitaxial strain in between the Ho and Dy layers of the superlattice. 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin "lms*epitaxial; Magnetization*thin "lms; Magnetic structure
The layered magnetic materials have recently attracted great interest. During the last few years, the study of the magnetic phases of the two magnetic rare-earth (RE) superlattices (SLs) has received much attention [1]. However, most of these studies were done in the absence of any magnetic-"eld applied. As a part of the systematic studies we made about the magnetic and magnetoelastic (MEL) properties in RE SLs [2], we here present a study of the magnetic phases diagram of Ho/Dy SLs, by using low- and high-"eld magnetisation measurements. These magnetic phases have been suggested, by using the results previously obtained in the bulk REs [3]. The Ho/Dy SLs studied in this work have been grown by LaMBE facility, at the Clarendon Laboratory. The growth process was
* Corresponding author. Fax: #34-976-761-229. E-mail address: [email protected] (A. del Moral).
similar to that described in [1]. Nominally the samples were Ho /Dy and Ho /Dy , where the subscripts refer to the number of atomic planes of each element within the bi-layer repeat, which was repeated 60 times. The SL structure was investigated in situ by re#ection high-energy electron di!raction (RHEED) and ex situ by X-ray di!raction. For these SLs, the crystalline coherence length is 2000 As , the mosaic spread 0.53, and the average interdi!usion at the interfaces is estimated from X-ray di!raction as $2 atomic planes [4]. The measurements were performed by using a SQUID 8$! magnetometer with the magnetic "eld applied along the b-axis direction, in order to provide the magnetic phases existing at zero "eld. The high-"eld magnetisation, M, measurements were performed using a VSM magnetometer (up to 12 T, and between 10 K and the ordering temperature of Dy layers). For getting the right magnetisation values in units of emu/cm, the volume of the magnetic sample was calculated from the measurements of the thin-"lm surface and the thickness of Ho/Dy
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 7 6 0 - 5
A. del Moral et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1700}1701
1701
Fig. 2. The H}¹ phase diagram of Ho /Dy SL for applied magnetic "eld along the b-axis as obtained from the magnetization and ZFC-susceptibility. Lines are visual guides. (P: Paramagnetic; F: Ferromagnetic; H: Helix; FN: Fan; HFN: Helifan.) Fig. 1. (a) Iso"elds and (b) isotherms for applied magnetic "eld H, parallel to the b-axis for Ho /Dy SL. The inset shows the ZFC susceptibility at H"0.1 T.
SLs. In Fig. 1(a) and (b), we present the iso"elds and isotherm, respectivly, for Ho Dy SL. In these "gures we "nd several features in the M vs. ¹ and M vs. H curves, which were identi"ed as meta-magnetic transitions. These transitions can be seen in the stepchanges and slope-changes in the curves. We also present in the inset of Fig. 1(a) the plot of vs. ¹ with an 8$! applied "eld of 0.1 T along the b-axis. Arrows mark the magnetic transitions. The zero-"eld-cool susceptibility shows the coexistence of Ho and Dy magnetic orders below their respective ordering temperatures (for the Ho /Dy SL, we found ¹ 140 K for Ho and , ¹ 180 K for Dy layers). The MPD of Ho/Dy SLs have , been obtained from the and the magnetisation 8$! measurements. The appearance of steps or changes of slope in the data determines magnetic transition temperatures or "elds. By tabulating the transition temperatures at a given "eld and the transition "elds at a given temperature, we are able to construct a H}¹ phase diagram (MPD) for the Ho/Dy SLs. In Fig. 2 we show the MPD for the Ho /Dy SL, but we do not reproduce here the one for Ho /Dy because it is very close to the former. The magnetic phases we propose seem rather similar to the bulk ones [3]. So apparently, the magnetic ordering of the Ho and Dy layers seems to be uncoupled for Ho/Dy SLs. M can easily be interpreted as the addition of the thickness weighted holmium and dysprosium magnetisation layer contributions. Although this indicates that Ho and Dy do not interact magnetically at high "elds, we suspect that this interaction exists but at lower "elds. In fact the small increment in ¹ of Ho layers, ,
10 K, re#ects an exchange polarisation of the Ho layers by the Dy ones. Besides, this exchange coupling should be coherent along the SL structure. As mentioned above,
the MPD of both Ho/Dy SLs are very similar. In the MPD proposed here for Ho /Dy SL (see Fig. 2), a huge region exists where we could "nd an helifan phase stable for both RE blocks. In fact, the small energy di!erences between both helifan phases could allow their coexistence within such a region. This could merely imply the existence of an helifan phase in Dy, which would be due to the coherent propagation of conduction band electrons through Ho and Dy blocks. In fact, from band calculations, Dy and Ho have close band structures. This is not so strange, because Ho/Dy and Ho/Y SLs are similar from the epitaxial-strain point of view (the basalplane of Ho layers is expanded), and Ho/Y SLs show the helifan phase [5]. However, at low temperatures the situation is completely di!erent, because the anisotropy within the easy basal-plane is strongly increased. The magnetic moments of Ho and Dy layers are kept within the basal plane, which favours a dipolar}dipolar interaction along the c-axis. Although such an interaction gives rise to a ferro-cone phase in bulk holmium, we do not expect to "nd such a structure in our Ho/Dy SLs as it is well proved that the epitaxial strain fully suppresses the cone phase at low temperatures [6].
References [1] R.A. Cowley, J.A. Simpson, R.C.C. Ward, M.R. Wells, D.F. McMorrow, J. Phys.: Condens. Matter 10 (1998) 2115. [2] M. Ciria, Ph.D. Thesis, University of Zaragoza, 1997. [3] R.J. Elliot, Magnetic Properties of Rare-Earth Metals, Plenum, New York, 1972. [4] R.C.C. Ward, private communication. [5] C. de la Fuente, R.A. Cowley, J.P. Go!, R.C.C. Ward, M.R. Wells, D.F. McMorrow, J. Phys.: Condens. Matter 11 (1999) 6529. [6] D.A. Jehan, D.F. McMorrow, R.A. Cowley, R.C.C. Ward, M.R. Wells, N. Hagmann, Phys. Rev. B 48 (1993) 5594.
Magnetic phase diagram of holmium/dysprosium superlattices A. del Moral *, C. de la Fuente , J.I. Arnaudas , M. Ciria , L. Benito , M.R. Wells, R.C.C. Ward Lab. de Magnetismo, Depto. de Fn& sica de la Materia Condensada and Inst. de Cienca. de Mat. de Arago& n, Universidad de Zaragoza and C.S.I.C. 50009 Zaragoza, Spain Clarendon Laboratory, Oxford University, Parks Road, Oxford OX1 3PU, UK
Abstract We report on the magnetisation measurements with a magnetic "eld applied along the HCP-structure b-axis for Ho/Dy magnetic superlattices (SLs). The zero-"eld-cool susceptibility, , shows the magnetic-order coexistence of the 8$! Ho and Dy layers, below their respective ordering temperatures. The magnetic-phase diagrams (MPD) of Ho/Dy SLs have been obtained from magnetisation measurements, and we found that the magnetic phases of the Ho and Dy layers are very similar to the bulk ones. Although the analysis of the high-"eld magnetisation indicates that Ho and Dy do not interact magnetically, this coupling exists at low "eld. The increment of the NeH el temperature in the Ho layers with respect to the bulk could re#ect some kind of polarisation between Ho and Dy layers. In the MPD that we have proposed for Ho/Dy SLs, a huge region exists where an helifan phase remains stable for both rare-earth layers, especially for the Ho /Dy SL. This could represent the "rst time that the helifan phase of Ho layers stabilises a similar phase for Dy layers, due to the coherent propagation of conduction band electrons through Ho and Dy layers. However, at low temperatures, we do not expect to "nd the ferro-cone phase in Ho layers as in the bulk, because of the epitaxial strain in between the Ho and Dy layers of the superlattice. 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin "lms*epitaxial; Magnetization*thin "lms; Magnetic structure
The layered magnetic materials have recently attracted great interest. During the last few years, the study of the magnetic phases of the two magnetic rare-earth (RE) superlattices (SLs) has received much attention [1]. However, most of these studies were done in the absence of any magnetic-"eld applied. As a part of the systematic studies we made about the magnetic and magnetoelastic (MEL) properties in RE SLs [2], we here present a study of the magnetic phases diagram of Ho/Dy SLs, by using low- and high-"eld magnetisation measurements. These magnetic phases have been suggested, by using the results previously obtained in the bulk REs [3]. The Ho/Dy SLs studied in this work have been grown by LaMBE facility, at the Clarendon Laboratory. The growth process was
* Corresponding author. Fax: #34-976-761-229. E-mail address: [email protected] (A. del Moral).
similar to that described in [1]. Nominally the samples were Ho /Dy and Ho /Dy , where the subscripts refer to the number of atomic planes of each element within the bi-layer repeat, which was repeated 60 times. The SL structure was investigated in situ by re#ection high-energy electron di!raction (RHEED) and ex situ by X-ray di!raction. For these SLs, the crystalline coherence length is 2000 As , the mosaic spread 0.53, and the average interdi!usion at the interfaces is estimated from X-ray di!raction as $2 atomic planes [4]. The measurements were performed by using a SQUID 8$! magnetometer with the magnetic "eld applied along the b-axis direction, in order to provide the magnetic phases existing at zero "eld. The high-"eld magnetisation, M, measurements were performed using a VSM magnetometer (up to 12 T, and between 10 K and the ordering temperature of Dy layers). For getting the right magnetisation values in units of emu/cm, the volume of the magnetic sample was calculated from the measurements of the thin-"lm surface and the thickness of Ho/Dy
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 7 6 0 - 5
A. del Moral et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1700}1701
1701
Fig. 2. The H}¹ phase diagram of Ho /Dy SL for applied magnetic "eld along the b-axis as obtained from the magnetization and ZFC-susceptibility. Lines are visual guides. (P: Paramagnetic; F: Ferromagnetic; H: Helix; FN: Fan; HFN: Helifan.) Fig. 1. (a) Iso"elds and (b) isotherms for applied magnetic "eld H, parallel to the b-axis for Ho /Dy SL. The inset shows the ZFC susceptibility at H"0.1 T.
SLs. In Fig. 1(a) and (b), we present the iso"elds and isotherm, respectivly, for Ho Dy SL. In these "gures we "nd several features in the M vs. ¹ and M vs. H curves, which were identi"ed as meta-magnetic transitions. These transitions can be seen in the stepchanges and slope-changes in the curves. We also present in the inset of Fig. 1(a) the plot of vs. ¹ with an 8$! applied "eld of 0.1 T along the b-axis. Arrows mark the magnetic transitions. The zero-"eld-cool susceptibility shows the coexistence of Ho and Dy magnetic orders below their respective ordering temperatures (for the Ho /Dy SL, we found ¹ 140 K for Ho and , ¹ 180 K for Dy layers). The MPD of Ho/Dy SLs have , been obtained from the and the magnetisation 8$! measurements. The appearance of steps or changes of slope in the data determines magnetic transition temperatures or "elds. By tabulating the transition temperatures at a given "eld and the transition "elds at a given temperature, we are able to construct a H}¹ phase diagram (MPD) for the Ho/Dy SLs. In Fig. 2 we show the MPD for the Ho /Dy SL, but we do not reproduce here the one for Ho /Dy because it is very close to the former. The magnetic phases we propose seem rather similar to the bulk ones [3]. So apparently, the magnetic ordering of the Ho and Dy layers seems to be uncoupled for Ho/Dy SLs. M can easily be interpreted as the addition of the thickness weighted holmium and dysprosium magnetisation layer contributions. Although this indicates that Ho and Dy do not interact magnetically at high "elds, we suspect that this interaction exists but at lower "elds. In fact the small increment in ¹ of Ho layers, ,
10 K, re#ects an exchange polarisation of the Ho layers by the Dy ones. Besides, this exchange coupling should be coherent along the SL structure. As mentioned above,
the MPD of both Ho/Dy SLs are very similar. In the MPD proposed here for Ho /Dy SL (see Fig. 2), a huge region exists where we could "nd an helifan phase stable for both RE blocks. In fact, the small energy di!erences between both helifan phases could allow their coexistence within such a region. This could merely imply the existence of an helifan phase in Dy, which would be due to the coherent propagation of conduction band electrons through Ho and Dy blocks. In fact, from band calculations, Dy and Ho have close band structures. This is not so strange, because Ho/Dy and Ho/Y SLs are similar from the epitaxial-strain point of view (the basalplane of Ho layers is expanded), and Ho/Y SLs show the helifan phase [5]. However, at low temperatures the situation is completely di!erent, because the anisotropy within the easy basal-plane is strongly increased. The magnetic moments of Ho and Dy layers are kept within the basal plane, which favours a dipolar}dipolar interaction along the c-axis. Although such an interaction gives rise to a ferro-cone phase in bulk holmium, we do not expect to "nd such a structure in our Ho/Dy SLs as it is well proved that the epitaxial strain fully suppresses the cone phase at low temperatures [6].
References [1] R.A. Cowley, J.A. Simpson, R.C.C. Ward, M.R. Wells, D.F. McMorrow, J. Phys.: Condens. Matter 10 (1998) 2115. [2] M. Ciria, Ph.D. Thesis, University of Zaragoza, 1997. [3] R.J. Elliot, Magnetic Properties of Rare-Earth Metals, Plenum, New York, 1972. [4] R.C.C. Ward, private communication. [5] C. de la Fuente, R.A. Cowley, J.P. Go!, R.C.C. Ward, M.R. Wells, D.F. McMorrow, J. Phys.: Condens. Matter 11 (1999) 6529. [6] D.A. Jehan, D.F. McMorrow, R.A. Cowley, R.C.C. Ward, M.R. Wells, N. Hagmann, Phys. Rev. B 48 (1993) 5594.