ARTICLE IN PRESS

0304-8853/$

doi:10.1016

�CorrespE-mail a

Journal of Magnetism and Magnetic Materials 310 (2007) 1645–1647

www.elsevier.com/locate/jmmm

Ferrimagnetic correlations in paramagnetic ErCo2

J. Herrero-Albillosa,�, F. Bartolomea, L.M. Garcıaa, J. Campoa,A.T. Youngb, T. Funkb, G.J. Cuelloc

aInstituto de Ciencias de Materiales de Aragon, CSIC-Universidad de Zaragoza, Dpto. Fısica de la Materia Condensada,

Pedro Cerbuna 12, 50009 Zaragoza, SpainbAdvanced Light Source, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA

cInstitut Laue Langevin, F-38042 Grenoble Cedex 9, France

Available online 17 November 2006

Abstract

We have performed X-ray magnetic circular dichroism experiments on ErCo2 in order to measure the Er and Co net magnetic

moments above and below its ferrimagnetic transition. The results demonstrate the occurrence of an unexpected antiparallel alignment of

Co and Er sublattices almost 30K above the magnetic transition. We attribute this antiparallel alignment to magnetic short-range order.

We have characterized the temperature dependence of the magnetic correlation length ðxÞ with small angle neutron scattering

measurements. We observe the expected divergence of x right above Tc and, a wide region well within the paramagnetic phase, where the

correlation length has an almost constant value of 7 A. We have further observed a direct correlation between Co magnetic moment and

x: at the onset of the long range order both magnitudes experience an abrupt increase, while the temperature range at which x has a

constant value coincides with the change of sign of the Co magnetic moment.

r 2006 Elsevier B.V. All rights reserved.

PACS: 61.10.Ht; 61.12.Ex

Keywords: Laves phases; X-ray magnetic circular dichroism; Small angle neutron scattering; Short-range order

ErCo2 is a ferrimagnet below the first order magneto-structural transition taking place at T c ¼ 32K. Er magneticmoments are predominately responsible for the magnetismin ErCo2, while Co moments are broadly thought to bemetamagnetically induced in the vicinity of Tc by the Erinternal field [1]. Although the Co-Laves phases have beenthoroughly studied during the last decades, the nature ofthe Co moment in the paramagnetic phase of ErCo2 is not afully clear question: Gignoux et al. [2] conclude from theirmagnetic susceptibility and neutron scattering experimentsthat there is a small Co induced moment in paramagneticphase, in contrast, Burzo et al. [3] assign an effectivemoment of 2mB to Co in the paramagnetic region from hightemperature magnetic susceptibility measurements. In thiswork we take advantage of the unique capability of X-raymagnetic circular dichroism (XMCD) to perform element

- see front matter r 2006 Elsevier B.V. All rights reserved.

/j.jmmm.2006.10.863

onding author. Tel.:+34 976 762 692; fax:+34 796 761 229.

ddress: Julia.Herrero@unizar.es (J. Herrero-Albillos).

specific magnetometry, directly measuring the Er and Conet magnetic moments (mEr and mCo, respectively) aboveand below T c [4].XMCD experiments were performed at ALS 4.0.2

beamline at the Co L2;3 and Er M4;5 absorption edgesfrom 5 to 80K on polycrystalline ErCo2 ingots under anapplied magnetic field of 1T, using total electron yielddetection mode.In Fig. 1 we show 2mCo and mEr as a function of

temperature as obtained from the XMCD signals, togetherwith the ErCo2 magnetization measured in a SQUIDmagnetometer. The XMCD signals have been scaled sothat the sum of mEr þ 2mCo reproduces the SQUIDmagnetization and the low temperature values of mCo

and mEr are those obtained from our neutron diffractionexperiments.The magnetic ordering transition is observed in both mCo

and mEr at T c ¼ 34K. As expected, most of the Comoment is developed at the magneto-structural transition.

ARTICLE IN PRESS

0 20 40 60 80-2

0

2

4

6

8

T (K)

M(T)

mEr

2mCo

2mCo+mEr

m (

μ B)

Fig. 1. (Color online) Selective magnetometry on ErCo2at 1 T. Squares

and triangles are 2mCo and mEr. Solid line is the SQUID magnetization

and open circles the sum mEr þ 2mCo.

0 40 80 120 160 200 240 280

0

5

10

15

20

0

40

80

120

160

T (K)

ξ (Å

)

ξ (Å

)

20 40 60 80

-0.8

-0.6

-0.4

-0.2

0.0

0.2

4

7

10

13

16

mC

o (

μ B)

I SA

NS(a

rb.

un

its)

T (K)

Fig. 2. (Color online) ISANS at q ¼ 0:12 A�1 (open diamonds) and

correlation length for ErCo2 (full circles) under an applied magnetic field

of 1T. Full squares on the inset are mCo.

J. Herrero-Albillos et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1645–16471646

However, Co has a magnetic moment in the wholetemperature range of about t0:2mB, and surprisingly, itchanges sign at a higher temperature, T f�60K, well withinthe paramagnetic regime: the antiparallel alignment of Coand Er sublattices occurs almost 30K above the ferrimag-netic transition.

The bulk properties of ErCo2 assure the absence of longrange order above Tc, therefore this ‘‘ferrimagneticalignment’’ must have its origin in magnetic short-rangeorder. In order to determine the temperature dependence ofthe magnetic correlation length ðxÞ above T c we havemeasured small angle neutron scattering (SANS) on ErCo2under an applied magnetic field of 1 T. The measurementswere performed at the D16 diffractometer at Institut LaueLangevin using the standard wavelength of the instrumentðl ¼ 4:54 AÞ and the detector positioned forming an angleg ¼ 6:23� with the neutron beam. Therefore, the exploredrange of momentum transfer (q) was 0:120:35 A�1.

In this range, the main contributions to the SANSintensity come from the coherent scattering of magneticstructures (of 2 A to some nanometers) and from theincoherent nuclear scattering. These two contributions canbe separated due to the fact that (i) the magneticcontribution is maximum for the q vectors perpendicularto the magnetization; (ii) the nuclear contribution isessentially temperature independent and can be estimatedfrom the high temperature measurements. To obtain themagnetic scattered neutron intensity ðIM

SANSÞ as a functionof q, we have integrated all the measured 2D diffracto-grams in a circular sector perpendicular to the applied fieldand centered on the neutron beam. Then we havesubtracted the signal at 275K and 1T, where magneticSANS is expected to be negligible.

A first approach to the analysis of the data can be themodel-free representation shown in Fig. 2, where IM

SANS at

q ¼ 0:12 A�1 is plotted as a function of the temperature ð�Þ.As expected, the scattering intensity strongly increases nearTc. Furthermore, a strong increase of IM

SANS around T ¼

120K evidences a strongly enhanced x indicating theformation of magnetic clusters in paramagnetic ErCo2, attemperatures as high as 4 times Tc.The observed IM

SANS curves follow a Lorentzian law:IðqÞ ¼ I0=ðq2 þ x�2Þ. Thus, we propose that the magneticcorrelations in ErCo2 are Ornstein–Zernike typehMð0Þ �MðrÞi� expð�r=xÞ=r, as it is usually done [5]. Allthe IðqÞ curves in paramagnetic phase were fit according tothis model and the obtained x values have been representedin Fig. 2 (full circles). We observe the expected divergenceof x right above T c and, a wide region in the vicinity of T f ,where the correlation length has an almost constant valueof 7� 1 A.The inset of Fig. 2 shows the temperature dependence of

mCo and x, evidencing a direct correlation between bothmagnitudes: at the onset of the long range order both x andmCo experience an abrupt increase, while the temperaturerange at which x has a constant value coincides with thechange of sign of mCo.To summarize, we have demonstrated the presence of net

magnetic moment on the Co sublattice in the paramagneticphase and that the mCo becomes negative 30K above theferrimagnetic transition, evidencing the occurrence ofshort-range order. Assuming that the magnetic correlationsin ErCo2 are Ornstein–Zernike type, we have characterizedthe temperature dependence of the correlation length fromour SANS measurements.

This work has been funded by the Spanish CICYTresearch Project MAT2005-02454 and the AragoneseCAMRADS research group. We thank E. Garcıa-Matres,A. Heinemann, N. Plugaru, M.J. Pastor and P. Cramer.

ARTICLE IN PRESSJ. Herrero-Albillos et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1645–1647 1647

References

[1] N.H. Duc, P.E. Brommer, in: K.H.J. Buschow (Ed.), Handbook of

Magnetic Materials, vol. 12, Elsevier Science, 1999.

[2] D. Gignoux, D. Givord, F. Givord, W.C. Koehler, R.M. Moon, Phys.

Rev. B 14 (1976) 162.

[3] E. Burzo, Phys. Rev. B 6 (1972) 2882.

[4] H. Ebert, S. Man’kovsky, Phys. Rev. Lett. 90 (2003) 077404.

[5] J.M. De Teresa, M.R. Ibarra, P. Algarabel, L. Morellon,

B. Garcıa-Landa, C. Marquina, C. Ritter, A. Maignan, C. Martin,

B. Raveau, A. Kurbakov, V. Trounov, Phys. Rev. B 65 (2002)

100403.