169Tm Mössbaüer spectroscopy and crystal field calculations in TmNi3 and TmCo3 intermetallics

I Phut. Chm Solidr Vol. 44. No. 4. pp 307-3W. Printed in Great Bream

1983

W22-3697/83/040307~l3$03,C@/0 Pergamon Press Ltd.

‘69Tm MGSSBAUER SPECTROSCOPY AND CRYSTAL FIELD CALCULATIONS IN TmNi3 AND TmCo3 INTERMETALLICW D. NIARCHOSS,P. J. VICCARO, G. K. SHENOYand B. D. DUNLAP Argonne National Laboratory, Argonne, IL 60439,U.S.A. and J. K. YAKINTHOS Demokritos University of Thrace, Xanthi, Greece (Receiued 19 April 1982;accepted 23 June 1982) Abstract-The values of the magnetic moments and the electric field gradient in TmNi, and TmCo, are obtained from ‘@TmMiissbauer spectroscopy. These values are compared to those from crystal field model based on point charge calculations for these intermetallics.

1.MTRODUCTION

3.RESUL'BANDDlSCIJSSION

Intermetallic compounds composed of rare-earth (R) and transition metal (T) elements show a variety of magnetic properties[l]. In general, the magnetism is driven by the magnetic exchange between the T element moments. The magnetic anisotropy on the other hand is determined by the crystalline electric field (CEF) interactions with localized 4f shell of the rare-earth. In RT, compounds, several attempts have been made to correlate measured anisotropies (i.e. spin directions) with those predicted from a simple point charge crystal field model. This approach has proven to be surprisingly successful, considering the approximations involved. The most extensive work involves RN&[21 and RCoj[3] intermetallic series. In these cases, the spin configuration can be accounted for by the point charge model. In addition, 16’Dy Miissbauer results for DyNi,[4] and DyCo,[S] appear to be in good agreement with the predictions of the model. In order to probe the validity of the point charge approach for other rare-earth elements, we have conducted 16’*TrnMassbauer experiments on TmNi, and TmCo,. A point charge calculation using the leading term, VZO,in the Hamiltonian is used to obtain the electronic levels for Tm in the two sites in the structure. The MGssbauer results, and the neutron diffraction measurements are compared to those predicted by the CEF calculations.

Both TmNi, and TmCo, crystallize with the rhombohedral PuNi, structure (R&n). There are two inequivalent Tm sites in the lattice with point symmetries 3m (site II) and %n (site I) and relative population 2: 1 respectively. Magnetic transition temperatures are 43 K for TmNi, and 270 K for TmCo3. The large difference in ordering temperatures is associated with the presence of a magnetic moment on Co and the apparent absence of a moment of Ni[3-4]. The ‘@Tm Miissbauer spectra taken at 4.2K for TmNi, and TmCo, are shown in Figs. l(a) and (b), respectively. Except for the magnitude of the splittings, the spectra are similar and are comprised of the superposition of two magnetically split component spectra,

1

1

I

I

I

1

99.e-

The details of the preparationof TmNi, and TmCo3 have been described previously[6]. The l/2’+ 3/2’ 8.4 keV transition in 16Trn was utilized in the Mijssbauer experiment. The source was a neutron irradiated alloy, ‘68Er,.,Alo.9at room temperature.

i

99.6 -

I

I

I

-60

-40

I

-20

I

1

I

I

0

20

40

60

I

VELOCITYcm/s

tWork supported by the U.S. Department of Energy. *Present address: Physics Department, Illinois Institute of Technology, Chicago, IL 60616,U.S.A.

Fig. 1. 16?rn Mijssbauer spectra at 4.2 K for (a) TmCo, and (b) TmNi,. The solid line represents the results of a least-square fit to the data assuming two magnetic sites and the hypefine Hamiltonian discussed in the text. 307

D. NIARCHOS et al.

308

V: = A:(?)( 1- g’).

representing the two sites, with an intensity ratio of approximately 1.2: 1. Saturation effects in the resonances are responsible for the deviation between this intensity ratio and the true population ratio. The hyperfine fields (H,,) and quadrupole interactions (e’qQ) for the two sites in TmCo3 and TmNi, are given in Table 1. The hyperfine field and quadrupole interaction for a rare-earth atom reflects the electronic structure of the 4f valence electrons. This structure in turn is the result of the CEF and magnetic exchange splitting of the rareearth Hund state. For Tm’+, the Hund state is the one with J = 6 and the full moment is developed when a jJ,I = 6 ground state occurs. The hyperfine field expected in this case is lH,I = 6700 kG. For site I in both TmCoz and TmNi?, the measured H, values at 4.2 K are close to this indicating that the ground state is consistent with a nearly pure lJ,I = 6 component. A smaller H, for site II would then correspond to a ground state composed of an admixture of allowed (J,) states. As shown previously[2], the Hamiltonian describing the crystal field interactions in both rare-earth sites is given by Z&F = cyvz”oz”t P(V,“O,” t V,‘V,‘) t y(v6”06,”t Vh30h3t v,“o,“,.

For a point charge model the crystal field potential AZ”is obtained through a sum over the neighboring lattice charges and u2 is the screening coefficient. For TmN& and TmCo3, XCEF determines the crystal field states and in principle, the anisotropy of the spin moment for a given state. In performing the lattice sum for TmNi,, a charge of zero was assigned to Ni[2]. The charge on Co is not known and lattice sums for TmCo, were performed for values of charges between 0 and 1. The results for the calculations including atoms within a radius of 30 A are given in Table 2. For both TmNi, and TmCol, V2”for site I and site II have opposite signs. The negative sign for site I results in the (26) state as the ground state and indicates the hexagonal c axis as the easy axis. The full free ion moment would be expected for this ground state with a magnetic exchange along with the c axis the hyperfine field associated with a \J,j = 6 state would be observed. The ground state for site II is IO)which implies an easy axis perpendicular to the c axis and a zero moment for the ground state in the absence of magnetic exchange. Neutron diffraction results for both TmNi, and TmCo3 indicated that in the magnetically ordered region, the Tm moments are collinear and parallel to the c axis. (The moment on Ni is zero and approximately 1 ~~ on Co.) From our Miissbauer measurements at 4.2K which for both intermetallics is below the magnetic transition temperature, the hyperfine field on ‘@Tm at site I is close to the value expected for a IJ,I = 6 ground state. This result is expected from the sign of the crystal field parameter VZ”,and an exchange field parallel to the c axis. For Tm atoms at site II, the measured hyperfine field is smaller than that obtained for site I in both intermetallics. The crystal field calculations predict a IJ,l= 0 ground state for site II and an overall splitting of the J = 6 manifold of approximately 200 cm-‘. With a magnetic exchange parallel to c, no mixing of the IJ,) states

(1)

Where (Y, /3 and y are the Stevens coefficients, 0” Stevens operators and V” the parameters of the crystalline field. In a previous analysis[2] it was shown that for the series RN& and RCo, the parameters, V4”,VR, Vd3,Vh3, Vh6are negligible, and an approximate Hamintonian with the hexagonal c axis as the quantization axis is given by se,,, = aVz”[3J*’ - J(J t l)]

(2)

where (Yis the Stevens coefficient for Tm” (O.Oll), and J, is the angular momentum operator. V2” is defined by

Table 1. Values of quadrupolesplittingsand hyperfinefields for TmCoi and TmNi? form Miissbauer measurements Site I

_

e2qQ(cn/sec)

Site II Heff(kC)

2 e qQ(cm/sec)

H.+ff(l;G)

TmCo3

26.5 f .5

6560 f 50

25.3 * 5

5930 * 50

ToNi

25.8 f

6490 f

11.3 f 5

4850 f 50

5

5

Table 2. Values of VI” in cm-’ for TmNi, and TmCoJ. By increasing the charge on the transition metal, the values of Vz”become smaller. The parameter 4 is the fraction of electron charge Tmb3

TM3

Site I site II

(3)

q-0

q-0

q -

-490

-542

-300

-203

195

206

188

181

0.5

q -

0.7

309

‘@TmMiissbauer spectroscopy and crystal field calculations Table 3. Values of the EFI in ergs/cm’ estimated using the point charge calculation_for TmNi3 and TmCoj. Also the experimental Miissbauer values are given for comparison. (Site I, 3m symmetry.) T&i

TmCo3

3

Site

I

site

q=o

1.79x106

(l-Y,k~y

-10.10x106

(I-RN;;

eV

==tot

ev=(tissba”er)

q =

q=O

1.98x106 -10.10x106

-8.31x106

-3.12~10~

-9.64x106

-9.91x106

I

0.6

1.09x106

q =

0.7

0.74x106

-10.10x106

-10.10x106

-9.01x106

-9.36x106

occurs and the moment on Tm at this site occurs through the lowering of higher angular momentum states by the exchange field. Given the magnitude of the crystal field splitting, the exchange field necessary to produce the observed hyperfine field in both intermetallics is found to be orders of magnitude larger than predicted by the magnetic transition temperatures of 370K for TmCo3 and 43 K for TmNi+ This would imply that the calculated value of V,” is too large for site II, assuming that the exchange field is collinear with the c axis. The sign of Vzo however appears to agree with that predicted by the point charge model. An independent although indirect evaluation of the CEF can be obtained from the value of the electric field gradient (EFG) at the Tm site. Neglecting the conduction electron contribution the (EFG) is a sum of two contributions:

\.l,J = 6 ground state, the values of the two contributions are compared to the experimental one in Table 3. As can be observed, the experimental values are larger than that predicted by the point charge model indicating a smaller lattice contributin to the EFG. This in turn would correspond to a smaller Vzo value for the site I. An analogous comparison is not possible for site II, since the exact nature of the ground state is not known. The above results imply smaller CEF interaction parameters at both Tm sites than those predicted from thepoint charge model for both TmNi, and TmCos. A similar conclusion is indicated by measurements in the S-state intermetallics, GdCo3 and GdNi, compounds by Tomola et al.[9]. A simple scaling of the measured VE between Gd nd Tm indicates that V:8:t should be smaller than that calculated by point charge lattice sum. The sign of the parameters appears to be predicted correctly by the point charge models.

eVzz = (1 - R)eVz: t (1 - y,)eV’“”

REFERENCES

(4)

where (1 - R)eVzi is the contribution from the 4f shell of the Tm3’ and is equal to - 10.1x lo6 erg/cm’ for a ]J,( = 6 state. The lattice term is given by eVF=-+

4v o (r )

where Vzo has been previously given (Table 2). The Sternheimer antishielding and shielding factors were taken to be y- = - 80 and R = 0.2, respectively. From the measured quadrupole interaction, e’qQ, and with Q = 1.2 barns for ‘@Tm, the value of eV,, = e’q can be determined experimentally. For site I where we can assume a

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