ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 758–759
Enhanced orbital magnetic moments in the Heusler compounds Co2 CrAl; Co2 Cr0:6Fe0:4Al; Co2FeAl a . H.J. Elmersa,*, S. Wurmehlb, G.H. Fechera, G. Jakoba, C. Felserb, G. Schonhense b
a Institut fur . Physik, Johannes Gutenberg-Universitat . Mainz, Staudingerweg 7 Mainz D-55099, Germany Institut fur . Anorganische und Analytische Chemie, Johannes Gutenberg-Universitat . Mainz, Mainz D-55099, Germany
Abstract Using the magnetic circular dichroism in X-ray absorption at the L2;3 -edges of the 3d-transition metals, we determined the element speciﬁc ratio between orbital and spin magnetic moment of the Heusler compounds Co2 CrAl; Co2 Cr0:6 Fe0:4 Al and Co2 FeAl: The orbital magnetic moment per spin is large (0.1–0.2) compared to bulk values of Fe and Co metals. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Cc; 71.20.Lp; 78.40.Kc Keywords: Heusler alloy; Orbital moment; X-ray absorption; Magnetic circular dichroism
The orbital magnetic momentum is related to the magneto-optical Kerr effect and provides a direct link to the magnetic anisotropy [1,2]. In an ionic compound, the orbital moment originates from the spin–orbit coupling and may reach large values of more than 1 mB per atom. In bulk metals the strong crystalline ﬁeld interactions quench the orbital moments to 0:08 mB for FCC Co  and 0:05 mB for Fe . However, localization of 3d electrons at interfaces is known to increase this value [3,5]. A similar electron localization arises in the intermetallic Heusler compounds which show similarities to ionic compounds with respect to their physical properties [6,7]. We focus on the Al based Heusler compounds Co2 Cr1x Fex Al that are halfmetallic ferromagnets for xp0:5 according to band structure calculations [7–9]. For the mixed compound ðx ¼ 0:4Þ large magnetoresistance effects were observed in powder pellets at room temperature . The Heusler compounds were prepared by arc melting under an argon atmosphere . Structural properties were measured using X-ray diffraction as a standard *Corresponding author. Tel.: +49-6131-392-4150; fax: +496131-392-3807. E-mail address: [email protected]
method. The homogeneous cubic phase with a lattice ( was conﬁrmed for all specimens. constant of a ¼ 5:74 A Magnetic circular dichroism (MCD) in the 2p-3d X-ray absorption spectroscopy (XAS) was measured at the firstDragon-beamline at the SRRC (Hsinchu/Taiwan). The XAS spectrum was obtained by the total electron yield method. The helicity of the light was ﬁxed while two spectra with opposite directions of the external ﬁeld, deﬁned as mþ and m ; were acquired consecutively. The magnetic ﬁeld applied to the sample ð0:8 TÞ was aligned with the surface normal and at an angle of 30 with respect to the incident photon direction. Before the MCD measurement the surfaces were cleaned in situ in ultrahigh vacuum in order to remove the surface oxide layer. Differences between the Fe MCD signal of the Heusler compound and that of the pure Fe reference sample (see Fig. 1) are most pronounced at the L3 edge. While the energy position of the MCD maximum is shifted towards lower energy by only DE ¼ 150750 meV; it is obvious that the peak is much narrower for the Heusler compound compared to the reference sample. This can be attributed to a narrow 3d peak in the PDOS of minority electrons predicted by theory. It directly reﬂects the higher degree of localization of d electrons.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1086
ARTICLE IN PRESS H.J. Elmers et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 758–759
Co2Cr0.6Fe0.4Al Fe Reference
µL / µS
MCD Signal (arb. units)
-0.6 -0.8 -1.0
710 720 Photon Energy (eV)
Fig. 1. A solid line indicates the Fe 2p-3d MCD spectra ðmþ m Þ for Fe in the Co2 Cr0:6 Fe0:4 Al Heusler compound (solid) and its integrated value. The MCD spectra for a pure Fe reference sample (dashed) is shown for comparison. The inset shows the expanded MCD spectra at the L3 edge.
Using the sum rules for the integrated MCD signals A, B at the L3 ; L2 edges, orbital ðmL Þ and spin ðmS Þ magnetic moments can be separated . We determine the orbital magnetic moment per spin according to r ¼ mL =ðmS þ mT Þ ¼ 23 ðA þ BÞ=ðA 2BÞ: The magnetic dipole term mT ; that is usually less than 20% of mS will be neglected in the following . For the mixed compound shown in Fig. 1 we obtain r ¼ 0:0770:02; being larger than the Fe bulk value ðr ¼ 0:05 ). We note that due to the MCD mean probing depth of about 1 nm  the bulk properties are augmented by surface effects that are due to a loss of symmetry at the surface , as well as to strain induced by the cleaning. Surprisingly, the orbital momentum contribution does not change monotonically with the total number of 3d electrons (see Fig. 2). While all measured values of r are large compared to bulk values, the orbital momentum contribution is smallest for the mixed Co2 Cr0:6 Fe0:4 Al compound. Usually the orbital contribution becomes larger when the 3d states are more localized. Therefore, the minimum of r values for the mixed compound suggests that the 3d states are most localized for the pure ternary compounds. It is interesting to note that a maximum magnetoresistance effect was found for
X Fig. 2. Orbital magnetic moment per spin mL =mS for the elements Cr, Fe and Co in Co2 Cr1x Fex Al:
pressed powder pellets with decreasing values when the composition of Co2 Cr1x Fex Al deviates from x ¼ 0:4: More detailed studies might answer the question whether these two observations are related. We thank Y. Hwu, P.-C. Hsu, W.-L. Tsai (Academia Sinica, Taipei) and H.-M. Lin (Tatung University, Taipei) for their support during the experiments in Taiwan.
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