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MAGNETIC STRUCTURE AND EXCITATIONS IN
THE INTERMETALLIC COMPOUND MnNi
K. Mikke, V. Udovenko, J. Milczarek, E. Vintaikin, S. Makushev, V. Dmitriev
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, SupplBment au no 12, Tome 49, d6cembre 1988
MAGNETIC STRUCTURE AND EXCITATIONS IN THE INTERMETALLIC
COMPOUND MnNi
K. Mikke ( I ) , V. A. Udovenko (2), J. J. Milczarek (I), E. Z. Vintaikin (2), S. Y. Makushev (2)
and V. B. Dmitriev (2)
( I ) Institute of Atomic Energy, Swierk, 05-400 Otwock, Poland (2) Bardin Institute of Iron and Steel, Moscow, U.S.S.R.
Abstract. - Neutron diffraction studies were made of the magnetic structure and spin wave excitations in a single-domain crystal of intermetallic compound MnNi. It was shown that the magnetic moments of manganese atoms are arranged in (001) planes and are oriented along [I101 direction. Spin wave velocities are not significantly different from those in a number of 7-Mn alloys in spite of very high Nee1 temperature of MnNi and the energy gap, if any, is not bigger than 3 meV.
Our earlier studies of Mn-based itinerant electron antiferromagnets were now extended to the intermetal- lic compound MnNi. This is a very interesting case with the highest, among 3d metal alloys, value of
TN
(1080 K), accompanied by a very high value of mag- netic moment on Mn atoms (3.8 , u ~ ).
Studied over nearly three decades, its magnetic structure and spin dynamics were not known since only polycrystalline samples were available [I]. The aim of this report is t o present the results for the single domain single crystal of MnNi.The sequence of the high temperature phase transi- tions prevents the growth of single crystals of MnNi with strictly stoichiometric composition, but by al- lowing a small deviation from stoichiometry this diffi- culty was overcome and a good single crystal was ob-, tained. In a prolonged aging process the crystal de- composed and 90 % of its volume became the MnNi compound, while the remaining volume was a 7-phase with C M ~ N 70 % i.e. close to the "zero matrix" for neutron scattering. The crystal had the fct structure with c / a = 0.947 and three types of domains with c axes oriented along the main cubic axes of the origi- nal fcc phase. By a subsequent cooling under uniax- ial stress 88 % of the domains were oriented along the applied stress direction, while the remaining 12 % were distributed nearly equally over the other two di- rections. Neutron diffraction data for this single do- main crystal allowed an unambiguous determination of its magnetic structure [2]. From previous data it was established that the magnetic structure is a layer- type structure. The Mn atoms are arranged in an- tiferromagnetic layers separated by non-magnetic (or weakly magnetic) layers of nickel atoms, but the ex- act orientation of spins could not be determined. The required distinction was between the Kouvel-type B structure with spins oriented along the
[loo]
direction and Kouvel-type C structure with spin directions along [llO]-type directions in the layer planes [I]. The deci-sive feature are the intensities of (210)-type magnetic reflections. Figure l a shows the computed intensities of magnetic superstructure lines in the (001) reciprocal lattice plane, and figure l b - the measured ones after all essential corrections, except the corrections for the Mn magnetic form-factor. Considering the fact that in both B and C,structures two types of magnetic do- mains are formed with perpendicular spin directions, even in the atomic single domain, and the equality of intensities of all (210) type reflections in the B struc- ture, then the observed differences between intensities in [110] directions against those in [110] act in favour of the C-type structure. This conclusion is supported further by the effect of uniaxial stress. When applied along the [I101 it resulted in redistribution of intensi- ties of (210) type reflections - strong enhancement of those along [110] and reduction to negligible values of those along [110].
Magnetic inelastic neutron scattering was measured close to the magnetic (100) reflection. Constant en- ergy transfer scans I (q) were made in the [loo] direc- tion for the energy transfer range from 0 to 16 meV at the incident neutron energy Eo = 50 meV. The mea-
Type C structure
t M l r n l pdtm 1.0 Expamfntalpttm
h k h
-3 -2 -1
01
2 31
-3 -2-10,
2 3Fig. 1.
-
Computed magnetic structure form-factor for Kouvel C type structure (a) and measured (b) intensity distributions of magnetic superstructure in (001) reciprocal lattice plane. The arrows indicate the magnetic moment directions.C8 - 186 JOURNAL DE PHYSIQUE
L Y I
- 01 00 03 q17,oo
WAVEVECTOR
Fig. 2. - I (q) scan for A E = 16 meV with computed curves for three indicated values of v and for Eg=2 meV. Single domain sample.
sured I (q) distributions were fitted t o those computed as a convolution of the spin-wave dispersion relation
b =
Js
vq) with the four dimensional resolu- tion function of the neutron spectrometer, E, and v de- noting the energy gap in the SW spectrum and the SW velocity, correspondingly. An example of the measured and calculated I (q) distributions for AE = 16 meV is given in figure 2 (with background subtracted). The background itself was high-
it amounted-
70 % of the total intensity at q = 0. From I (q) scans for sev- eral values of AE the estimated value of SW velocityis v = 180 f 30 meV
A.
The energy distributions I (E) a t q = 0 were measured for incident neutron energies 25.9, 35 and 50 meV. There is some indication of the presence of the energy gap not bigger, if any, than 2 meV. The accuracy of the I ( E ) scans was strongly affected by a very small effectjbackground ratio caused by the small size of the single domain sample. The ex- tension of these studies was made therefore by using a polydomain sample of a much bigger size allowing t o extend the energy transfers up to 34 meV with es- sentially better effect/background ratio. An example of the obtained results is shown in fi&re 3. The ex- perimental distribution indicates clearly a reduction of the SW lifetimes which is however much smaller than that reported for the Mn(27 % Ni)[q.
Although in our calculations the SW lifetime was not included, the comparison with the lifetime corrections obtGned for fcc Mn-Fe alloys [3] justifies the expectation that our estimates of v would not suffer essential changes if SW lifetime is included into the cross-section. The analysis of all the I (q) and I (E) scans leads to the following values of the SW velocity: v = 180 f 20 meVA
at 300 K and v = 230 f 30 meVA
a t 100 K. The I (E) scans at q = 0 show that Eg is not bigger than 3 meV, even at 100 K. The observed values of v are close t o those observed in most r-Mn alloys [3-61. This is aFig. 3.
-
I (q) scan for A E = 30 meV with Eo = 70 meV and T = 300 K, together with distributions for indicated values of v and Eg. Polydomain sample.very puzzling fact in view of the exceptionally high value of TN in MnNi which leads to expectation, jus- tified e.g. by the data for Cr and fcc y-Mn-Fe alloys, that v should be much larger. On the other hand the strong, although much smaller than in Mn(27 % Ni) [7], damping of spin waves is analogous to the effects observed in Mn(38 % Ni) [6] fcc y-Mn-Fe [3] and Mn- Cu alloys [5] and in much lesser degree in fct yMn-Fe alloys [4]. They were explained in terms of the specific features of electronic band structure of y-Mn [3, 71 favouring strong interaction of spin waves with single- particle excitations. Since band structure calculations for MnNi are not yet available it is not possible to de- cide whether a similar explanation might be applied for intermetallic MnNi compound.
Acknowledgement
This work was performed under the contract CPBP 01.09.
[I] PAl, L., Tarnbczi, T. and Konczos, G., Phys. State Solidi 42 (1970) 49.
[2] Vintaikin, E. Z., Dmitriev, V. B., Makushev, S. Yu. and Udovenko, V. A.,
Fiz.
Met. Metalloved.63 (1987) 577.
[3] Tajima, K., Ishikawa, Y., Endoh, Y. and Noda, Y., J. Phys. Soc. Jpn 41 (1976) 1195.
[4] Mikke, K., Jankowska, J. and Jaworska, E., Phys-
ica B 120 (1983) 156.
[5] Mikke, K., Jankowska, J. and Jaworska, E., J.
Magn. Magn. Mater. 31-34 (1983) 125.
[6] Mikke, K., Milczarek, J. J., Udovenko, V. A., Jaworska, E. and Vintaikin, E. Z., Proc. Cod. ICNS'88, Grenoble (1988) to be published. [7] Hennion, B., Hutchings, M. T., Lowde, R. D.,
