SURFACE-ENHANCED MAGNETIC ORDER AND CRITICAL BEHAVIOR OF Tb(0001) FILMS ON W(110)

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SURFACE-ENHANCED MAGNETIC ORDER AND

CRITICAL BEHAVIOR OF Tb(0001) FILMS ON

W(110)

C. Rau, C. Jin

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, SupplQment au no 12, Tome 49, dQcembre 1988

SURFACE-ENHANCED MAGNETIC ORDER AND CRITICAL BEHAVIOR OF

Tb(0001) FILMS

ON

W(110)

C. Rau and C. Jin

Department of Physics, Rice University, Houston, TX 77251, U.S.A.

Abstract.

-

The magnetic properties of uncoated, epitaxial Tb(0001) films on W(110) substrates are studied using electron capture spectroscopy. The topmost layer orders ferromagnetically below a surface Curie temperature Tc. =

249.96 f 0.02 K, which lies above the bulk Curie and NCI temperatures Tcb=220 K and TNb = 228 K. Near Tcs, novel critical behavior is observed, suggesting the existence of strong surface anisotropies.

Introduction

Phase transitions at magnetic surfaces are much more diverse than those that occur in the bulk [I-31. This is mainly due to various anisotropies associated to the material, its lattice structure, and to the pres- ence of the surface itself. Surface anisotropies have recently been shown to be of paramount importance in determining the magnetic properties of surfaces [2, 4, 51 and thin films [6-111.

In this paper, we report on a study of the magnetic properties of the free (uncoated) surface of thin (50

A

thick) films of Tb(0001) on W(110) substrates. It is

found that the topmost Tb-layer orders ferromagneti-

cally at a surface Curie temperature Tc, = 249.96 K, which lies above both the bulk Curie and NBel temperatures Tcb = 220 K and T N ~ = 228 K, respec-

tively. The magnetization of the topmost Tb layer is found to vary in a strongly non-monotonic fashion close to 240 K. Near Tc,, the observed critical behavior of the magnetization is consistent with that of the anisotropic special transition recently predicted by Diehl and Eisenriegler [5].

(AES), the residual 0 and C contaminations are found to be less than 1 % of a monolayer.

The single-crystalline state of the W(110) surfaces is detected by means of low-energy electron diffraction (LEED). The atomic flatness of the substrate surfaces is investigated by means of a scanning tunneling micro- scope and by means of grazing-angle ion reflection of deuterons a t the W(110) surfaces. The ion reflectivity I (reflected beam intensity per solid angle/incoming beam intensity per solid angle) amounts to 95 %

,

es- tablishing the atomic flatness of the surfaces [12].

For the deposition of 50

A

thin Tb(0001) films, we use electron-beam evaporation as described in refer- ences [13, 11, 141. The hexagonal phase of the de- posited Tb(0001) films is confirmed by LEED, the atomic flatness is revealed by ion reflectivity measurements, and the absence of surface contaminations is detected by AES.

Results a n d discussion

In figure 1, the T-dependence of P at Tb(0001)/W(110) surfaces is given for

H

= 250 Oe. Experimental

Magnetic order at surfaces of ultra-thin Tb(001) z5 films is measured by electron capture spectroscopy

(ECS) which is able to probe the long-ranged ferro- Zo

magnetic order at the topmost surface layer of a mag-

netic material [12].

-

The electron spin polarization

2

l5

P = (nf

-

n - )

/

(nf

+

n-) is defined along the direc-

2

tion of the magnetic field H applied at the target, and 3 10

nf and n- are the numbers of majority- and minority-

z

spin electrons [12]. 5 -

Atomically clean and %at W(110) substrate crystals are prepared under ultra-high vacuum conditions [12].

The crystals are cleaned by extensive heating cycles a t ' ' ' ' ' ' I

I I ,.

120 150 180 210 2 4 0 270 3 0 0

1 770 K in oxygen followed by flashing off at 2 270 K TEMPERATURE ( K I

the adsorbed Oxygen which after removal of _{Fig. 1.}_-_{Electron spin polarization}_P_{(% )}_{of the topmost}

[I3]. After further annealing and _{surface layer of 50 A-thin Tb(0001)/W(110) films as func-} cycles at 1 300 K, using Auger electron spectroscopy tion of temperature.

' ' I ' I I I ' ' ' ' I ' ' Tb (0001)/W 1110)

-

- - - - - Tcb +a -

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C8 - 1628 JOURNAL DE PHYSIQUE

H was varied between 25 Oe and 600 Oe, and no re- markable influence on the polarization data was detected. Nonzero P values establish that the surfaces of the films are ferromagnetically ordered up to 248 K, which lies above both TCb and TNb as indicated in figure 1. Similar behavior was recently found in ECS experiments a t surfaces of polycrystalline Tb [15]. In these experiments, using ferromagnetic induction and Kerr effect measurements, T c ~ was found t o be located at 220 K as shown in figure 1.

With increasing temperature, P decreases from 22 % a t 146 K, until it reaches a value of 7 % at about 240 K, which lies slightly above Tcb and T N ~ . AS T increases further, P increases very steeply t o 21 % at 243 K, a t which it drops suddenly to zero at a surface Curie temperature Tcs = 249.96 K.

In figure 2, the temperature variation of P in the

neighborhood of Tc, is shown in a log-log plot. This provides a direct determination of the critical ex- ponent @ which is determined simultaneously with Tcs by a linear least-square fit of the P data. For (Tcs

-

T)

/

Tc, ranging between 2 x

lov2

and we obtain @ = 0.348 f 0.01.

I

_{, , , , , , . ,} _{, , , < , , , , ,} _{, , , , , , ( , ,} _{, , , , , , ,}

1

'JlI

16' 1 6 ' lo-= I 0.' I

(Tea-T) /Tc,

Fig. 2. - Log-log plot of the electron spin polarization data shown in figure 1.

Previous magnetic studies of surface critical behavior have been restricted to the case Tc, = Tcb, for which @ is measured to be 0.75 f 0.05 [8, 91 for sev- eral systems. The only previous experimental determination of @ for the case Tc, larger than T c ~ was for ultra-thin films of V(100) on Ag(100) substrates [Ill; but for V, unlike for Tb, TCb = 0, and the exact value

@ = 1/8 for the two-dimensional (2D) Ising ferromag- net was recently confirmed by ECS.

In the present case, Tc. lies close to Tcb, and we can no longer expect, as we did for V, to observe 2D Ising critical behavior. The fact that Tcs

#

TCb indicates that the magnetic couplings between the surface spins are strongly anisotropic suggesting that spin isotropy, which leads to a large value of @ = 0.75 may not be valid for the present system.

Taking surface anisotropies into account, it is recently found [5] that @ can deviate from 0.75 2c 0.05. Along an axis of easy (hard) magnetization /3 is 0.35 (0.93). The value ~3 = 0.35 agrees well with our experimental result /J = 0.348 measured along an easy

axis.

Acknowledgments

The authors are very grateful to H. W. Diehl, M. E. Fisher, M. Robert and G. T. Trammel1 for stimulating discussions. This work was supported by the National Science Foundation and the Robert A. Welch Founda- tion.

[l] Fisher, M. E., Int. J. Quantum. Chem. Quantum

Chem. Symp. 14 (1982) 671.

[2] Diehl, H. W., Phase Transitions and Critical Phenomena, vol. 10, Eds. C. Domb and J. L. Lebowitz (Academic, London) 1986.

[3] Binder, K., Phase Transitions and Critical Phe- nomena, vol. 8, Eds. C . Domb and J. L. Lebowitz (Academic, London) 1983.

[4] Selzer, S. and Majlis, Phys. Rev. B 27 (1983) 544.

[5] Diehl, H. W. and Eisenriegler, E., Phys. Rev. B 30 (1984) 300.

[6] Gay, J. G. and Richter, Roy, Phys. Rev. Lett. 56

(1986) 2728.

[7] Jonker, B. T., Walker, K.-H., Kisker, E., Prinz, G. A. and Carbone, C., Phys. Rev. Lett. 57

(1986) 142;

Stampanoni, M., Vaterlaus, A., Aeschlimann, M. and Meier, F., Phys. Rev. Lett. 59 (1987) 2483. [8] Weller, D., Alvarado, S. F., Gudat, W., Schroder,

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[lo]

Rau, C. and Eichner, S., Nucl. Meth. Mater.

Res., Eds. H . Bethge, H. Baumann, H. Jex and

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