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Magnetic structures of α-MnS and MnSe from 57Fe Mossbauer spectroscopy

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1983 J. Phys. C: Solid State Phys. 16 345

(http://iopscience.iop.org/0022-3719/16/2/017)

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J. Phys. C: Solid State Phys., 16 (1983) 345-353. Printed in Great Britain

Magnetic structures of a-MnS and MnSe from "Fe Mossbauer spectroscopy?

R J Pollard$, V H McCann and J B Ward Department of Physics, University of Canterbury, Christchurch. New Zealand

Received 2 July 1982, in final form 2 August 1982

Abstract. 57Fe Mossbauerspectroscopyisused to investigate the low-temperature behaviours of a-MnS and MnSe, and the results are successfully interpreted in terms of a crystal field model. In a-MnS there are at least two magnetic spin arrangements. A t 131 K < T < TN the spins are along a cubic (110) direction, whereas at T < 117 K a multiaxial arrangement is likely. MnSe is found to exhibit two distinct crystallographic structures which coexist at certain temperatures. In a NaCl form the spin arrangement is multiaxial. The other form is a NiAs modification, in which the spins are along a hexagonal (210) direction.

1. Introduction

Both MnS and MnSe exist in cubic (NaC1-type) and hexagonal forms. The cubic forms were studied by neutron diffraction over three decades ago, and there was much dis- cussion in the literature concerning their magnetic spin arrangements. At T < TN a-MnS undergoes a small rhombohedral distortion along (1 11) (Heikens et a1 1977). The spins order within the plane perpendicular to this direction, and adjacent planes are coupled antiferromagnetically (Corliss er a1 1956). The spin direction in the plane is unknown, however it has been realised that both collinear and non-collinear spin arrangements are compatible with the neutron diffraction results (Roth 1958).

Mossbauer spectroscopy can be used to measure the hyperfine pattern resulting from each inequivalent site, and this information may be used in determining the spin arrange- ment for each structure. Siegwarth (1967) and Ok and Mullen (1968) showed that their Mossbauer spectra were consistent with uniaxial directions of (172) in MnO and NiO, and (113) for COO. Techniques similar to those of these authors are employed here.

Heikens et a1 (1977) observed a new phase transition in a-MnS at 131 K ( T x = 148 K) which is characterised by an abrupt inversion of the rhombohedral distortion. The microscopic nature of this transition was unknown, and is discussed here.

Studies of MnSe are complicated by crystallographic transformations at tempera- tures below 300 K. These have led to contradictory interpretations of results. Authors have reported that at low temperatures MnSe consists of a cubic form only (It0 et a1 1978), a hexagonal form exhibiting stacking faults (Andresen and R ~ t t e r u d 1969), and

+ Part of a PhD thesis by R J Pollard. $ Present address: Department of Physics, University of Manitoba. Winnipeg. Manitoba R3T 2N2, Canada.

0022-3719/83/020345 + 09 $02.25 @ 1983 The Institute of Physics 345

346 R J Pollard, V H McCann and J B Ward

coexisting cubic and hexagonal forms (van der Heide er a f 1980). The cubic form has been reported to have a Nee1 temperature which lies in the range from 140 to 250 K and which depends on the sample's thermal history (It0 er a1 1978). Its magnetic structure is thought to be the same as that of a-MnS (Pickart er a1 1961).

Neutron diffraction patterns of the hexagonal modification have been interpreted in terms of a NiAs structure consisting of ferromagnetic sheets perpendicular to the c axis, stacked antiferromagnetically. The spin direction could be within the sheets (Andresen and Rotterud 1969), or perpendicular to the sheets (Jacobson and Fender 1970). These authors report that the modification disappears on warming above 280 or 245K respectively.

2. Experimental

The Mossbauer spectrometer has been described previously (Pollard er a1 1982). Powder samples doped with "Fe were prepared by heating the elements in a sealed evacuated silica tube at 600°C (a-MnS) and 750°C (MnSe). In the case of a-MnS, the product from one heating was pressed to 0.6 GPa under vacuum before subsequent heatings, following the technique of Acampora et a f (1956). In all cases the samples were succes- sively ground and reheated until their Mossbauer spectra exhibited a single line. The linewidth of the final a-MnS sample was slightly broader than usual, because of the presence of a very small amount of 57FeSz. This was neglected in the analyses at low temperatures since it could not be resolved.

The compound present in each sample was confirmed by x-ray diffraction. X-ray fluorescence was used to determine the atomic constitution by the technique of Lee and McConchie (1982). This confirmed that the samples were stoichiometric to within 20 .2 a t .%, and that the Fe concentrations were not more than k0.05 at.% different from the nominal compositions quoted later.

Absorbers were prepared by pressing a sample mixed with boric acid as a binder. They were sufficiently thin that thickness effects could be neglected.

3. Results

Calculated line positions and intensities of Mossbauer spectra were computed by dia- gonalising the Hamiltonian for combined electric quadrupole and magnetic dipole hyperfine interactions, and by summing Lorentzian shaped lines (Pollard er a1 1982). The experimental data were then fitted by a least-squares fitting programme to determine the hyperfine magnetic field B and its orientation angles Q;hf, hf with respect to the principal axes of the electric field gradient (EFG), as well as the components of the EFG

Spectra of a-MnS + 0.5 at .% 57Fe are shown in figure 1. At high temperatures a single line is observed, which is expected from the cubic symmetry. At 117 K < T < 156 i 2 K (TN), magnetic spectra are observed. This value of TN is higher than the value quoted by Heikens er a1 (1977). From the asymmetry of the spectra, it is clear that V,, # Oat T < TN. It wasfoundthattheparametersobtainedbycurvefittingthemagnetic spectra were ambiguous (not even the sign of V,, could be deduced). Therefore only the

( V Z Z and 4.

Magnetic structures of a-MnS and MnSe 347

values of B , p , q and r were determined, where

A4 +- pA2 +- q A -+ r = O

is the secular equation for the 57Fe excited state (Pollard et a1 1982). At temperatures below 117 K the spectra broaden considerably. Satisfactory curve

fits assuming a single inequivalent 57Fesite could not be obtained. These spectra therefore result from more than one site; this is the result expected for a multiaxial spin arrangement.

I 3608K

I 151 7 K 1

I

1L2 5 K

I 1 '8 0 K

L- _ _ _ ~ -1 0 1 2 . 3 L

V e l o c i t y i m m s - )

Figure 1. Mossbauer spectra of wMnS. A phase transition occurs at 131 K (Heikens et al 1977). The full curves represent fitted curves.

Spectra of MnSe + 1 at. 7~ "Fe are shown in figure 2. A sample with 2 at. % "Fe gave identical patterns. On cooling the sample from room temperature, a narrow single line (from the cubic structure) was observed until 155 K. At this temperature extra lines appeared, and increased in intensity until 122 K. These spectra were best interpreted as resulting from two "Fe sites: one is from the paramagnetic cubic form, and the other arises from a magnetic NiAs modification. Parameters from the magnetic site were not significantly ambiguous, and q+,f = 90". At T < 122 K the cubic phase itself became magnetically ordered, however its spectrum could not be fitted assuming a single Fe site. Complex spectra were observed down to low temperatures.

On warming the absorber. two forms were present until 300 K. The relative propor- tions of the forms are shown in figure 3. The NiAs form vanished at 300 K , but was magnetic up to this temperature. After thermally cycling the sample six times to 4.2 K. there was little change in the behaviour.

To further investigate the cubic form, a sample of MnSe doped with 15 at. 7c MgSe and 2 at .% 57FeSe was prepared. This concentration of Mg suppresses this NiAs form

348 R J Pollard, V H McCann and J B Ward

1 4 0 1 K

-1 0 1 2 3 4 Velocity [mm S- ' I

Figure 2. Mossbauer spectra of MnSe taken during a cycle of cooling from room temperature to 4 K, and on re-heating to room temperature. They are shown in chronological order from top to bottom. Calculated spectra for each structure are shown, as well as the sum of these. In the spectrum at 199.5 K, the NiAs component has B = 0 whilst still magnetically ordered.

(van der Heide et a1 1980). Representative spectra are shown in figure 4. Complex patterns were again observed below TN = 101 ? 2 K . The spectra of all three cubic compounds (a-MnS, MnSe, and MnSe + MgSe) near 6 K shows similar features, hence it is reasonable to suppose that their magnetic arrangements are similar.

" " I 1 I

O i 100

Temperature I K 1

Figure 3. Fraction of the spectrum area attributed to the NiAs modification of MnSe. Points with broken error bars were obtained during a cooling cycle, and points with full error bars were obtained on warming.

Magnetic structures of a-MnS and MnSe

2 9 5 0 K

- 1 0 1 2 3

V e l o c i t y imm S - ' I

Figure 4. Mossbauer spectra of MnSe + 15 a t .% MgSe.

349

4. Analysis

A crystal field analysis of the Mossbauer parameters was performed following the procedure described previously (Pollard et a1 1982). Briefly, the 'D term of Fe2+ is split by a cubic field to give a 'T2, orbital triplet. The 5T2g wavefunctions are perturbed by

X = A[LZ - +L(L + l)] + AL * S + JefdSi)(S, COS v e x sin 8 e x

+ S, sin qex sin e,, + S , cos OeX)

where A , A and Jeff are trigonal field, spin-orbit, and exchange interaction constants to be determined. The direction of magnetisation is given by the polar angles vex, e,, with respect to the trigonal axes. Xis diagonalised, and Mossbauer parameters are calculated from the Boltzmann expectation values of the magnetic hyperfine field B where

x [+L(L * S ) + $(L * S)L - L ( L + 1)S]

and from the components of the valence contribution to the EFG (- v ' ) w h e r e

In addition to v', a small lattice contribution to the EFG was also included. A least-squares fitting programme was used to determine A , A, J e f f , vex, e,,, ( F 3 ) , B,

and (re3) by comparing calculated and experimental hyperfine parameters. For a-MnS the parameter A was set equal to zero, since the lattice is only very slightly

distorted from cubic symmetry (Heikens et a1 1977). The values determined for 131 K < T < TN are shown in table 1. The angles vex = 30" and e,, = 90" obtained

350 R J Pollard, V H McCann and J B Ward

-5M 1 1 I8 1 , 1 ,

Table 1. Crystal field parameters of "Fe in cr-MnS and MnSe. The number in parentheses is the uncertaintyin the last digit, e.g., -53(5) = -53 k 5 .

Compound q;,,(deg) B,,(deg) J,ti(cm-') B,(T) V3)(au) (rC3)(au) A("') A(cm-')

I 1 1 1

wMnS 30(10) 90(10) -53(5) -39.8(5) 3.70(5) 3.70(5) 0 -81(1) MnSe(NiAs) O(18) 90(5) -56 (5 ) -41.3(5) 4.56(8) 2.4(3) 60(6) -78(3)

la1 z I

Figure 5. ( a ) Orientations of the direction of magnetisation ( M ) , B , and the EFG principal axis system with respect to trigonal crystal axes in cr-MnSe at 131 K < T < TN. ( b ) Spin arrangement in cr-MnS for the same temperature range. The open circles represent S ions.

3Gt 1

Magnetic structures of a-MnS and MnSe

0- b

40

20-

35 1

-

a-MnS

0 103 120 140

I

1

t \ /

I ,

1

i

I , , v, , I 160 180 200 220 240 260 280 300

T e m p e r n t u r e i K 1

Figure 7. Temperature variations of B for wMnS and MnSe (NiAs).

- 0.51 i

- 1.5 :bo 120 140 160 180 2 0 0 220 240 260 280 300

I 1 I I I

T e m p e r n t u r e IK I

Figure 8. Temperature variations of EQ = teQV,, and q for MnSe (NiAs).

O h loo 120 140

Tempernture I K I

Figure 9. Temperature variation of ehi for MnSe (NiAs).

352 R J Pollard, V H McCann and J B Ward

I ,

Y

Figure 10. (a ) Orientations of M , B , and the EFG principal axes with respect to trigonal axes in MnSe(NiAs). The angle mchanges smoothly from 7.4"at 100 K to 1.5"at 300 K. The angle Pchanges from 4.5" at 100 K to 90" at 195 K, and to 179.6" at 300 K. ( b ) Spin arrangement in MnSe(NiAs). The open circles represent Se ions. The spins point in a hexagonal (210) direction.

correspond to a cubic spin direction of (110) as shown in figure 5. Figures 6 and 7 show reasonable agreement between the experimental points and the fitted (full) curves. The discrepancies are attributed to the small impurity in this sample.

Attemptstofitthetrendsinthedataat 117K < T < 131 Kfailedtoobtainsatisfactory agreement, even though the spectra in this range are well approximated by a single 57Fe site. Crystal field calculations to represent multiaxial ordering showed that the broad- ened spectra observed at T < 117K are the expected result, although the exact spin arrangement has not been deduced.

Excellent agreement was obtained from a crystal field fit to the data from MnSe(NiAs) at 122 K < T < 300 K, as shown by the match between the measured points and the calculated curves in figures 7 to 9. A similar analysis of MnSe + 2 at .% 57Fe spectra yielded concurrent values. A remarkable feature of the B and hf values is that they are nearly zero at 195 K. This is caused by the cancellation of BL and Bs; at 195 K the only non-zero field arises from the much smaller BD term which is along the trigonal z axis. A spectrum taken near this temperature is shown in figure 2. The axis systems and the spin arrangement are shown in figure 10.

Finally, results from a Debye model analysis of the isomer shifts are reported in table 2.

Table 2. Isomer shift values (IS) extrapolated to 0 K and effective Debye temperatures.

Compound rs(O)(mm s-l) M K )

cu-MnS 1.12(4) 240(50) MnSe(NaC1) 1.09(6) 300( 150) MnSe(NiAs) 1.10(8) 220( 100)

5. Conclusion

The results from a-MnS are consistent with the phase transition at TI, = 131 K reported by Heikens er a1 (1977). It has been found that at T > TI, the spins are aligned in a cubic

Magnetic structures of a-MnS and MnSe 353

(110) direction. At T < 117K the results are consistent with a multiaxial spin arrange- ment. Although previous authors were aware that such an arrangement could be present, this is the first experimental evidence to be reported.

MnSe exhibits two distinct crystallographic structures. On cooling through 155 K part of the sample converts to a magnetically ordered NiAs modification; the spins in this form point along a hexagonal (210) direction. Some cubic form remains at all temperatures, and has a NCel temperature of 122K which is independent of thermal history. Its spin arrangement appears to be multiaxial.

On warming, the NiAs modification converts to the cubic form; the transformation is completed at 300 K. It remains magnetically ordered up to this temperature, however its internal magnetic field shows an unusual local minimum caused by the fortuitous cancellation of the two largest hyperfine field components.

Acknowledgments

We are grateful to D M McConchie for performing x-ray fluorescence analyses, to the Department of Geology, University of Canterbury for use of an x-ray diffractometer, and to the University of Canterbury for a research grant.

References

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