# ferrimagnetic correlations in paramagnetic

Post on 26-Jun-2016

214 views

Embed Size (px)

TRANSCRIPT

Journal of Magnetism and Magnetic Mate

s

omu

ersi

5000

ora

804

17

structural transition taking place at T c 32K. Er magnetic

temperature magnetic susceptibility measurements. In thiswork we take advantage of the unique capability of X-raymagnetic circular dichroism (XMCD) to perform element

net magnetic moments (mEr and mCo, respectively) above

magnetization and the low temperature values of mCoand mEr are those obtained from our neutron diffractionexperiments.

ARTICLE IN PRESSThe magnetic ordering transition is observed in both mCoand mEr at T c 34K. As expected, most of the Comoment is developed at the magneto-structural transition.

0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmmm.2006.10.863

Corresponding author. Tel.:+34 976 762 692; fax:+34 796 761 229.E-mail address: Julia.Herrero@unizar.es (J. Herrero-Albillos).moments are predominately responsible for the magnetismin ErCo2, while Co moments are broadly thought to bemetamagnetically induced in the vicinity of Tc by the Erinternal eld [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 high

and 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 eld 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 2mCo reproduces the SQUIDmoments 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 x with small angle neutron scatteringmeasurements. We observe the expected divergence of x right above Tc and, a wide region well within the paramagnetic phase, where thecorrelation 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 aconstant 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 rst order magneto- specic magnetometry, directly measuring the Er and CoWe have performed X-ray magnetic circular dichroism experiments on ErCo2 in order to measure the Er and Co net magneticFerrimagnetic correlation

J. Herrero-Albillosa,, F. BartolA.T. Youngb, T. F

aInstituto de Ciencias de Materiales de Aragon, CSIC-Univ

Pedro Cerbuna 12,bAdvanced Light Source, Lawrence Berkeley National Lab

cInstitut Laue Langevin, F-3

Available online

Abstractrials 310 (2007) 16451647

in paramagnetic ErCo2

ea, L.M. Garcaa, J. Campoa,nkb, G.J. Cuelloc

dad de Zaragoza, Dpto. Fsica de la Materia Condensada,

9 Zaragoza, Spain

tory, University of California, Berkeley, CA 94720, USA

2 Grenoble Cedex 9, France

November 2006

www.elsevier.com/locate/jmmm

ARTICLE IN PRESSm aHowever, Co has a magnetic moment in the wholetemperature range of about t0:2mB, and surprisingly, itchanges sign at a higher temperature, T f60K, 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 long

range 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 x above T c we have

0 20 40 60 80-2

0

2

4

6

8

T (K)

M(T)

mEr

2mCo

2mCo+mEr

m (

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 2mCo.

J. Herrero-Albillos et al. / Journal of Magnetis1646measured small angle neutron scattering (SANS) on ErCo2under an applied magnetic eld of 1 T. The measurementswere performed at the D16 diffractometer at Institut LaueLangevin using the standard wavelength of the instrumentl 4:54 A and the detector positioned forming an angleg 6:23 with the neutron beam. Therefore, the exploredrange of momentum transfer (q) was 0:120:35 A1.In this range, the main contributions to the SANS

intensity 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 IMSANS as a functionof q, we have integrated all the measured 2D diffracto-grams in a circular sector perpendicular to the applied eldand centered on the neutron beam. Then we havesubtracted the signal at 275K and 1T, where magneticSANS is expected to be negligible.A rst approach to the analysis of the data can be the

model-free representation shown in Fig. 2, where IMSANS atq 0:12 A1 is plotted as a function of the temperature .As expected, the scattering intensity strongly increases nearTc. Furthermore, a strong increase of I

MSANS around T

120K evidences a strongly enhanced x indicating theformation of magnetic clusters in paramagnetic ErCo2, attemperatures as high as 4 times Tc.The observed IMSANS curves follow a Lorentzian law:

Iq I0=q2 x2. Thus, we propose that the magneticcorrelations in ErCo2 are OrnsteinZernike typehM0 Mri expr=x=r, as it is usually done [5]. Allthe Iq curves in paramagnetic phase were t according to

0 40 80 120 160 200 240 280

0

5

10

15

20

0

40

80

120

160

T (K)

(

)

(

)

20 40 60 80

-0.8

-0.6

-0.4

-0.2

0.0

0.2

4

7

10

13

16

mC

o (

B)

I SA

NS(a

rb.

un

its)

T (K)

Fig. 2. (Color online) ISANS at q 0:12 A1 (open diamonds) andcorrelation length for ErCo2 (full circles) under an applied magnetic eld

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

nd Magnetic Materials 310 (2007) 16451647this 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 OrnsteinZernike 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. Garca-Matres,A. Heinemann, N. Plugaru, M.J. Pastor and P. Cramer.

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. Mankovsky, Phys. Rev. Lett. 90 (2003) 077404.

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

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

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

100403.

ARTICLE IN PRESSJ. Herrero-Albillos et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 16451647 1647

Ferrimagnetic correlations in paramagnetic ErCo2References

Recommended