Properties of molecular beam epitaxy grown Eux ...

Properties of molecular beam epitaxy grown Eux(transition metal)y films (transition metals: Mn, Cr) K. Balin, A. Nowak, A. Gibaud, J. Szade, and Z. Celinski Citation: Journal of Applied Physics 109, 07E323 (2011); doi: 10.1063/1.3559528 View online: http://dx.doi.org/10.1063/1.3559528 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/109/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Sharp chemical interface in epitaxial Fe3O4 thin films Appl. Phys. Lett. 105, 241603 (2014); 10.1063/1.4904459 Structure and magnetism in strained Ge1− x − y Sn x Mn y films grown on Ge(001) by low temperature molecular beam epitaxy Appl. Phys. Lett. 103, 012403 (2013); 10.1063/1.4813117 Electronic structure of CuCrO2 thin films grown on Al2O3(001) by oxygen plasma assisted molecular beam epitaxy J. Appl. Phys. 112, 113718 (2012); 10.1063/1.4768726 Metal/semiconductor phase transition in chromium nitride(001) grown by rf-plasma-assisted molecular-beam epitaxy Appl. Phys. Lett. 85, 6371 (2004); 10.1063/1.1836878 Structural and thermoelectric transport properties of Sb 2 Te 3 thin films grown by molecular beam epitaxy J. Appl. Phys. 91, 715 (2002); 10.1063/1.1424056

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 155.158.129.39 On: Thu, 24 Sep 2015 05:54:02

JOURNAL OF APPLIED PHYSICS 109, 07E323 (2011)

Properties of molecular beam epitaxy grown Eux(transition metal)y films (transition metals: Mn, Cr) K. Balin,1,2,a) A. Nowak,1,3 A. Gibaud,3 J. Szade,1 and Z. Celinski2 1

A. Chełkowski Institute of Physics, University of Silesia, Katowice, 40-007, Poland Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918, USA 3 Laboratoire de Physique de l’Etat Condense´, University du Maine, Le Mans Cedex, 72085, France 2

(Presented 18 November 2010; received 4 October 2010; accepted 30 November 2010; published online 1 April 2011) The electronic and crystallographic structures, as well as the magnetic properties, of Eux(transition metal)y (transition metals: Mn, Cr) thin films grown by molecular beam epitaxy were studied. Relative changes of the Eu/Mn and Eu/Cr ratios derived from the XPS lines, as well as x-ray reflectivity, indicate mixing of the Eu/Mn and Eu/Cr layers. Valency transitions from Eu2þ to Eu3þ were observed in both systems for most studied stoichiometries. A transition to a magnetically ordered phase was observed at 15 K, 40 K, and 62 K for selected films in the Eu–Mn system, C 2011 American Institute of Physics. and at 50 K for the film with a Eu/Cr ratio of 0.5. V [doi:10.1063/1.3559528]

The formation of RExTMy (RE, rare earth; TM, transition metal) binary compounds with the stoichiometries RE2TM, RETM, RETM2, RETM3, RETM5, RE2TM17, RETM11, and RETM13 is well known.1 However, for rather chemically active europium, stable bulk compounds are formed only with Ni, Cu, and Zn.1 There are some very limited reports on a EuFe2 compound.2 Recently, we demonstrated the existence of a EuMn2 ordered alloy in a thin film form.3 The aim of this work is to examine the existence of Eu–Mn and Eu–Cr binary alloys in thin film form. Our studies include the characterization of the electronic and crystallographic structures, as well as of the magnetic and transport properties, of such films. In this work we focus on a comparative analysis of Eu–Mn and Eu–Cr films grown by molecular beam epitaxy (MBE). The presented analysis is based on films with selected concentrations of Eu and Mn or Cr. In intermetallic alloys, Eu can occur not only in the two valency states Eu3þ and Eu2þ, but also in a mixed valency state.4–6 There is a noticeable difference in the magnetic properties of europium in the two different valency states Eu3þ and Eu2þ. Eu3þ is nonmagnetic (J ¼ 0), while Eu2þ has a large pure spin moment (J ¼ [72]). This difference in magnetic properties motivated our study of Eu based intermetallic alloys. The magnetic properties of REMn2 intermetallic compounds are especially interesting. Binary REMn2 systems exhibit different magnetic properties depending on the type of RE. Compounds with the stoichiometry REMn2 are paramagnetic for RE ¼ Y, Yb, or Lu; antiferromagnetic for RE ¼ Pr, Nd, Sm, Gd, or Tb; and ferromagnetic at low temperatures for RE ¼ Gd, Tb, Dy, Ho, Er, or Tm.7,8 In our stud-

a)

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0021-8979/2011/109(7)/07E323/3/$30.00

ies, we examined EuMn2 and EuCr2, as well as other films with varying concentrations of Eu and the TM. Eu–Mn and Eu–Cr systems 10 to 30 nm thick were grown by a molecular beam epitaxy system9 equipped with a SPECS GmbH XPS system and reflection high energy electron diffraction (RHEED). A series of films with different concentrations of Eu and Mn or Cr were grown on MgO, Si, and GaAs substrates (with a 50 nm buffer layer of Mo deposited by sputtering). Growth of Eu–Mn films was performed by deposition of (Eu/TM)10 layers, with the thickness of the individual Eu and Mn or Cr layers being dependent on the assumed concentration. To ensure the formation of Eu–Mn and Eu–Cr alloys, the films were annealed up to 550 K for about 24 h. In order to confirm the reaction between elements, in situ RHEED and XPS measurements were carried out. XPS allowed us to analyze the shape and chemical shifts of the core levels of Eu (3d and 4d), Mn (2p), and Cr (2p), as well as the valence transitions of europium. A superconducting quantum interference device (SQUID) magnetometer was used to characterize the magnetic properties of the Eu–Mn and Eu–Cr films. X-ray diffraction and x-ray reflectivity (XRR) measurements were carried out using a Philips Expert XRD system. With XPS, we monitored the area ratio of the relative changes in the photoemission of the Eu 4d/Mn 2p and Eu 4d/ Cr 2p levels by integrating the photoemission lines, as well as the chemical shifts of the core levels. Figure 1 shows the spectra of the Eu 4d levels for selected compositions of Eu– Mn and Eu–Cr films taken just after deposition, and after 12 or 24 h of annealing. Initially, the position and well-defined exchange splitting in the Eu 4d line are observed, indicating that the Eu layers exhibit the properties of pure metallic Eu.10 During the annealing process, a slight change in the Eu spectra could be detected in the case of films with a 0.5 Eu/ TM ratio, and a strong chemical shift was observed for other

109, 07E323-1

C 2011 American Institute of Physics V


07E323-2

Balin et al.

FIG. 1. (Color online) XPS spectra of the Eu 4d level obtained just after deposition of the Eu–Tm/Mo/Si film and after annealing at 480 K for 12 h. (a) Eu/Mn: 0.5. (b) Eu/Cr: 0.5. (c) Eu/Mn: 1.43. (d) Eu/Cr: 0.27.

concentrations of Eu and TM (there was a 9.7 eV difference in binding energy between the final and initial states of Eu 3d5/2 for Eu2Mn17). Valence transitions from Eu2þ to Eu3þ were observed in both systems for the examined stoichiometries (Eu2TM, EuTM, EuTM3, EuTM5, and Eu2TM17). The exceptions, where europium was in the divalent state, were the EuMn2 compound3 and a film rich in europium with a Eu/Mn ratio of 6.7. For the Eu–Cr system, among all of the examined concentrations of Eu and Cr, the europium was divalent only in films with a stochiometry close to EuCr2. The Mn 2p and Cr 2p lines showed no significant shift. To monitor contamination of the films, the photoemission lines of oxygen and carbon were monitored during the annealing process. An increase of the oxygen content on the sample surface was observed in the survey spectra after long annealing; thin layers of EuO and/or Eu2O3 are thought to be formed on the Eu–Mn film surface. The presence of the EuO phase was confirmed in the ex situ x-ray diffraction and magnetization versus temperature measurements for selected Eu–Mn films, but no trace of Eu2O3 was observed in the XRD patterns. Changes in the crystalline structure before and after annealing were observed using RHEED. After deposition, the RHEED pattern for both the Eu–Mn and the Eu–Cr system consisted only of weak rings, which represent a polycrystalline structure of the films. The diffraction patterns collected after annealing for the Eu–Mn system showed a different set of rings in addition to diffused spots of a much higher intensity. This indicates a polycrystalline structure with at least partial texture. There are small changes in the

J. Appl. Phys. 109, 07E323 (2011)

polycrystalline structure of the Eu–Cr system before and after the annealing process. Additionally, x-ray reflectivity studies were performed. Well-defined oscillations in the x-ray patterns for almost all of the examined concentrations of Eu and Mn allowed for calculations of an electron density profile. The calculations indicate the formation of a uniform Eu–Mn layer covering the Mo/Si substrate for all concentrations. For the Eu–Cr system, only the calculations for the film with a Eu/Cr ratio of 0.29 indicate the formation of a uniform Eu–Cr layer. We determined the crystallographic structure of the EuMn2 composition,3 and our data indicate the possible formation of other compounds with a higher Mn concentration. However, a limited number of new peaks in the diffractograms make a unique determination of the crystallographic structures impossible at this time. Diffraction patterns obtained from the XRD measurements for the Eu–Cr system indicate that Eu–Cr layers do not create a well-defined crystallographic structure, even if a uniform layer of Eu–Cr is formed, as in the case of the film with the Eu/Cr ratio of 0.29. Temperature-dependent SQUID measurements were performed in both zero-field cooled (ZFC) and field-cooled (FC) modes with the magnetic field parallel to the surface of the films. For selected films, the magnetic moment measured as a function of temperature exhibits a significant difference in the FC and ZFC modes. Figure 2 presents a comparison of the temperature dependence of the magnetic moment in the FC and ZFC modes of Eu–Mn films with a thickness of 30.7 nm. The films were grown under the same conditions as the (Eu/Mn)24 multilayers. The difference between the films was in their postdeposition treatment. One of the samples was covered with a Mo/Au coating after deposition, and a second was annealed for about 24 h before being coated with Mo/ Au. The valency of europium in the film that was not annealed is purely divalent after deposition, whereas in the annealed sample it is in the Eu3þ valency state. Thus the origins of the magnetic properties of the annealed film cannot be associated with the EuO phase, because it contains divalent Eu. In contrast to the EuMn2 film, where we observed3 a

FIG. 2. (Color online) Temperature dependence of the magnetic moment with the field applied in the plane of the film, obtained in the ZFC and FC modes for Eu–Mn/Mo/Si films with a thickness of 30.7 nm.


07E323-3

Balin et al.

FIG. 3. Temperature dependence of the magnetic moment with the field applied in the plane of the film, obtained in the ZFC and FC modes for the Eu–Cr sample with a ratio of 0.5.

J. Appl. Phys. 109, 07E323 (2011)

behavior of EuCr2 is an effect of frustration in the magnetic ordering. A similar effect was observed in EuMn2 and was discussed in a previous paper.3 In conclusion, XRD, XRR, and RHEED showed that the mixing of Eu–TM layers was the most efficient for the Eu–Mn system where ordered phases are formed. For Eu–Cr alloys, we did not detect the existence of crystallographically ordered phases. Relative changes in the photoemission of Eu 4d/Mn 2p and Eu 4d/Cr 2p levels, area ratios, and chemical shifts of core levels monitored by XPS also indicate mixing between europium and manganese or chromium. Valence transitions from Eu2þ to Eu3þ were observed in all examined concentrations, with the exception of those with a Eu/Mn or Eu/Cr ratio of 0.5, where Eu is divalent. SQUID measurements indicate the existence of magnetically ordered phases for EuMn2 (with a Eu2þ valency state), compunds with a Eu/ Mn ratio of 1.35 (with Eu3þ states), and EuCr2 (with Eu2þ states). This work was supported by NSF grant DMR0907053.

strong influence of the EuO on the temperature dependence of the magnetic moment, the transition at 62 K may be associated with the existence of an unknown Eu–Mn phase. The transition at 12 K observed for the annealed sample can be linked to the EuMn2 phase, which was determined in our previous studies.3 The measurements of the hysteresis loops revealed a decreasing value of coercive field with increasing temperature. Figure 3 shows the magnetic moment measurements of a film with a Eu/Cr ratio of 0.5. The ZFC measurements indicate the existence of a transition at 50 K. There is no sign of a europium oxide contribution to the measured moment. For this composition, europium is in the Eu2þ valency state [see Fig. 1(b)]. Magnetic moment measurements of the film with a Eu/Cr ratio of 0.29 show no sign of magnetically ordered phases. In this film, the europium was in the Eu3þ valency state [see Fig. 1(d)]. These results indicate that the magnetic behavior is associated with the divalency of Eu and that the

1

B. Predel and O. Madelung, Landolt-Bornstein, Group IV Physical Chemistry—Phase Equilibria, Crystallographic and Thermodynamic Data of Binary Alloys, Volume 5—Electronic Materials and Semiconductors (Springer-Verlag, Berlin, 1998). 2 Y.-T. Ning, X.-M. Zhou, Y. Zhen, N.-Y. Chen, H. Xu, and J.-Z. Zhu, J. Less-Common Met. 147, 167 (1989). 3 K. Balin, J. Szade, A. J. Hutchison, A.Nowak, A. Gibaud, and Z. Celinski, J Appl. Phys. 107, 09E154 (2010). 4 A. R. Miedema, J. Less-Common Met. 46, 167 (1976). 5 E.-J. Cho, S.-J. Oh, S. Suga, T. Suzuki, and T. Kasuya, J. Electron Spectrosc. Relat. Phenom. 77, 173 (1996). 6 C. Laubschat, B. Perscheid, and W.-D. Schneider, Physical Review B 28 (8), 4342 (1983). 7 K. Inoue, Y. Nakamura, and A.V. Tsvyashchenko, J. Phys. Soc. Jpn. 64 (6), 2175 (1995). 8 H. Nakamura, M. Moriwaki, M. Shiga, K. Inoue, T. Nakamura, A. V. Tsvyashchenko, and L. Fomicheva, J. Magn. Magn. Mater. 150, LI37 (1995). 9 Z. Celinski, J. Vac. Sci. Techn. A. 19, 383 (2001). 10 H. Lee, J.-Y. Kim, K.-J. Rho, B.-G. Park, and J.-H. Park, J. Appl. Phys. 102, 053903 (2007).
