Crystal field excitations in ErMn4Al8

~Solld State Communications, Vol. 72, No. 3, pp. 249-251, 1989. Printed in Great Britain.

Crystal

field excitations

0038-I098/8953.00+.00 Pergamon Press plc

in ErMn4Als

O. Moze, K. H. J. Buschow$, R. Osborn§, Z. Bowden§ and A. D. Taylor§

Istituto MASPEC del CNR, Via Chiavari 18/a, 43100, Parma, Italy. $ Philipps Research Laboratories, Eindhoven, The Netherlands. § Neutron Division, Rutherford Appleton Laboratory, Didcot, O X l l 0QX, United Kingdom.

( Received by R. Fieschi on 26 June 1989 )

Distinct crystal field excitations have been observed in tetragonal ErMn4Als by inelastic neutron scattering. The energy level sequence and crystal field parameters have been obtained from the observed crystal field transition peak positions and intensities. The crystal field parameters account for the basal plane orientation of the Er magnetic moment in ErFe4Als -and if extrapolated to the Fe rich compounds ErFenTi and ErFel0V~ also can account for the observed planar anisotropy.

1.

ErFet0V2 but may be rather comphcated due to the presence of large Er-Fe exchange fields. Thus the exchange field can be larger than the overall crystal field splitting and it would be difficult to observe well defined crystal field transitions. For this structure type it is possible, however, to directly observe the crystal field interaction by performing measurements on the compound ErMn4Als where the transition metal sublattice is non-magnetic above about 4K [10]. The magnetic properties of the Rare-Earth can therefore be studied in isolation and direct information about the crystalline electric field which is difficult to obtain by measurements of bulk properties can be obtained.

Introduction

The magnetic properties of ternary compounds of the type REFe4Als ( RE=Rare-earth ) have been extensively studied principally because they in general exhibit two phase transitions, with an antiferromagnetic ordering of the Fe atoms at temperatures above 150K followed by a ferromagnetic ordering of the RE moments at lower temperatures [1,2]. They hence display rather complex magnetic properties. These compounds crystallize in the ThMnn tetragonal stucture (I4/mmm) [3]. Ternary rare-earth compounds based on the ThMnl~ structure have recently shown great promise as materials complementary to the highly anisotropic compound Nd2Fe14B for possible permanent magnet apphcations. Compounds of the type REFe12_~T~ can be stabilized with Fe for x > l and with T = Ti, V, Si, Cr [4,5]. Neutron diffraction has shown that in this structure the T transition metal ions do not occupy all of the three available Fe sublattices 8i, 8j and 8f but only occupy in large a part of the 8i site [6,7]. The Curie points of these types of compounds are comparable to those of Nd2Fe14B. It would be expected that because of the positive value of the 2"J order Stevens coefficient a J the magnetic anisotropy of Er s+ would display an axial character. However the anisotropy of Er in this structure is anomalously low with the easy magnetization direction tilting away from the c-axis at low temperatures in ErFel0V2 and there is some considerable doubt and confusion as to the true values of the crystal field parameters [8,9]. In principle, a determination of the crystal field level scheme with neutron spectroscopy would be possible in

2.

Experimental

details

The ErMn4Als sample (20 gms) was prepared by argon arc melting followed by anneahng in an argon atmosphere at 850°C for 8 weeks. An X-ray diffraction measurement verified that the sample was single phase and crystallized in the ThMn12 structure with lattice parameters of a=8.829/~and c=5.096,~,. The measurements were performed on the direct geometry chopper spectrometer HET [11] at the U.K. spallation neutron source ISIS, Rutherford Appleton Laboratory with an incident neutron energy of 40meV. The scattered neutrons were detected by two arrays of SHe detectors lying at 2 and 4 in from the sample position and covering a scattering angle range q)=3-30 degrees. The measurements were performed at 20 and 100K. The array of detectors at 4m gives an energy resolution of approximately 2% at low scattering angles. 249

25O

3.

CRYSTAL FIELD EXCITATIONS IN ErMn4AI 8

CEF parameters in (2) from the energies of the CEF states. In Figure 1 are shown the measured spectra at 20K and 100K for the low angle (low Q) and high angle (high Q) scattering angles. The low angle bank is exclusively dominated by magnetic scattering whilst the high angle bank consists of phonon scattering. The incoherent phonon scattering appears to be surprisingly low given that it increases approximately as Q~. The low angle part of the spectrum contains at least three (at 2, 4.2, 6.5 mev) and possibly six distinct excitations (the others appearing as shoulders at 2.5, 3.5 and 5 meV) of magnetic character. These are ground state CEF transitions since at 20K the excited states are not populated. This is at least quantitatively confirmed in the 100K spectra as shown in figure l(b) where the intensities of the ground state transitions decrease and an excited CEF transition appears at approximately 11 meV. No further CEF splittings were observed. This could be due to much smaller ground state transition matrix elements and also the relatively large Er neutron absorption cross section. In spite of this and the relatively low neutron energy resolution some very useful results have been obtained as there is sufficient information to obtain at least the leading CEF parameters in equation (2). A preliminary fit of the neutron spectra by diagonalization of equation.2 gave a very reasonable fit for the following CEF parameters;

Results and discussion

The magnetic scattering law S(Q,w) for unpolarized neutrons and for small values of the momentum transfer Q from a system of N non- interacting ions is given by [12]; S(Q,w) = pNfZ(Q)~"~Pi < ilJ~lj >2 6(El - Ej -tiw)

(1)

i,j

where y=0.073 barn/str, ]iw is the neutron energy transfer, li> are the different crystal field eigenfunctions with energies Ei and thermal occupation probabilities Pi, J 1 is the total angular momentum component perpendicular to Q and f(Q) is the single ion magnetic form factor. The degeneracy of the J multiplets of the Er ion will be partly removed by the CEF potential produced by the charge distribution of the surrounding ions. In such a case the Hamiltonian for the tetragonal 2/a point symmetry of Er s+ in ErMn4Als with the z-axis as the quantization axis can be written as; H

=

0 0 B202 +

0 0 0 0 4 4 4 4 B 4 0 4 + B s O s + B 4 0 4 -F B s O s

(2)

where the B ~ are the CEF parameters and the O ~ are the Stevens operator equivalents built up of the spin operators. This CEF Hamiltonian will split the ground state 4Ils/2 (J=15/2) of the Er s+ ion into eight doublets. It should thus be possible in principle to derive the five independent

10

Vol. 72, No. 3

i

I

8 -+

i

(a) 2 0 K

: 2,

....... ,a.~.L

o

I ...-L2 |

5-

....

~.,at

.......

16

A . . . . . . . . . . I....

15

:L

20

J

r.~ 4

0

5

10

15

20

ho (meV) Fig.1. (a) Observed neutron spectra at 20K for ErMnaAls for both low (closed circles) and high angle (open circles)detector banks. The shaded region is the predominant contribution from phonon scattering. (b) Observed neutron spectra at 100K. Displayed in the inset are both the energy loss and energy gain sides of the low angle detector bank.

Vol. 72, No. 3

CRYSTAL FIELD EXCITATIONS IN ErMn4AI 8

B ° = -(2.5 ± 0.3) × lO-2meV

B ° = (7.0 ± 0.5) × lO-4rneV Bs° = (1.5 ± 0.1) × lO-SmeV B44 = -(2.1 -4-0.2) × lO-SmeV B64 = -(2.1 ± 0.2) × lO-SmeV

An intrinsic Lorentzian linewidth and an instrumental Gaussian linewidth were also included in the fit. Making an approximative extrapolation to the compounds ErFe4Als and ErFel0V~ these CEF parameters can account for the observed planar anisotropy. In ErFel0V2 a CEF-exchange model using terms in the crystal field Hamiltonian up to 4th order gave a satisfactory account of the temperature at which the easy magnetization direction commences to tilt away from the c-axis and demonstrated the importance of the higher order terms in the CEF Hamiltonian for Er in this particular structure type [8]. In this present work, using the fitted CEF parameters, one can gain some idea of the relative importance of the competing terms in the CEF Hamiltonian which can ultimately give

251

rise to the spin reorientations. For the present parameters, we obtain at T=0, for at least the leading (diagonal) terms in the CEF Hamiltonian, B° = -2.82meV, B4° = ll.47meV and S ° = 1.35meV ( where < > denotes the thermal average of the operator ). The large positive value of B° signifies the importance of this term for giving rise to the spin reorientation. The planar orientation of the Er moment in ErFe4Als can be approximately accounted for in this manner. The nonnegligible high order terms may also possibly account for the first order magnetic transitions that have also been observed in ErFel0V2 [8]. There is some disagreement between this present work and crystal field parameters obtained from high field magnetization and Mhssbauer measurements on ReFenTi compounds where sixth order terms are much larger [13]. In addition point charge calculations give a different sign for the parameter B° [14]. There will be of course some differences in CEF parameters obtained from ErMn4Als and ErFenTi compounds mainly due to the different environment around the Er ion in each case but nevertheless at least the signs of the CEF parameters should be in some agreement given the fact that the lattice parameters of ErMn4Als (a=8.829/~, c=5.096/~) and ErFellTi (a=8.471~, c=4.779/~) are not very noticeably different in size. Further measurements with a higher neutron energy resolution on the series of REMn4AIs and REFe4Als compounds are in progress and should enable rather consistent values of all the CEF parameters to be obtained for the series.

References [1]

[2] [3] [4] [5]

Bargouth. M. O., Will. G and Buschow. K. H. J, Journal Of Magnetism and Magnetic Materials, 6 129 (1977). Felner. I and Nowik. I, J. Phys.Chem. Solids, 39 951 (1978). Florio. J. V., Rundle. R. E and Snow. A. I, Acta. Cryst., 5 449 (1952). deMooij. D. B and Buschow. K. H. J, Phillips Journal of Research, 42 246 (1987). Buschow. K. H. J., deMooij. D. B., Brouha. M., Smit. H. H. A and Thiel. R. C., IEEE Trans. Magn., TM

1611 (1988). Moze. O., Pareti. L., Solzi. M and David. W. I. F., Solid State Communications, 66 465 (1988). [7] Helmholdt. R. B., Vleggar. J. J. M and Buschow. K. H. J., Journal Of The Less Common Metals, 138 L l l (1988). [8] Moze. O., Algarabel. P. A., Ibarra. M. R., Solzi. M and Pareti. L., Solid State Communications, ~ 711 (1988).

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[12] [13]

[6]

[14]

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