MAGNETIC CRITICAL SCATTERING FROM AN ITINERANT ANTIFERROMAGNET OF γ-Fe0.5Mn0.5 ALLOY

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MAGNETIC CRITICAL SCATTERING FROM AN ITINERANT ANTIFERROMAGNET OF γ-Fe0.5Mn0.5

ALLOY

Y. Ishikawa, Y. Endoh

To cite this version:

Y. Ishikawa, Y. Endoh. MAGNETIC CRITICAL SCATTERING FROM AN ITINERANT ANTI-

FERROMAGNET OF γ-Fe0.5Mn0.5 ALLOY. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-

1017-C1-1019. �10.1051/jphyscol:19711363�. �jpa-00214400�

JOURNAL DE PHYSIQUE Colloque C

I ,

- ¹⁰¹⁷

MAGNETIC CRITICAL SCATTERING

FROM AN ITINERANT ANTIFERROMAGNET OF Y - F ~ ~ . ~ M ~ ~ . ~ ALLOY

by Y. ISHIKAWA and Y. ENDOH Department of Physics, Tohoku University, Sendai

R4sum6. -

On Btudie la diffusion critique des neutrons par un monocristal de y-Fe0,~37Mn0,463 au voisinage des points reciproques du reseau

et

Les rksultats montrent que les fluctuations de spins dans la gamme de tempb ratures -

TNITN

s'interpretent suffisamment par la theorie de diffusion. Les correlations longitudinales

des spins tendent vers infini

TN. La susceptibilit6 parallele

suit B. la loi, xl

a(T- T N ) - ~ . ~ ~ ~ ~ . ~ ~ . Cepen- dant les correlations transversales ont une portee limitCe a TN avec l'inverse de la portCe des corr6lations

et I'inClasticite r

MeV. On trouve qu'un mode collectif apparait nouvellement dans un spectre de neutrons

a

la tempbature

au-dessus de TN, oh la susceptibilite statique a la valeur maximale.

Abstract.

- The magnetic critical scattering from a single crystal of y-Fe0.537 Mn0.463 has been studied around the

and

reciprocal lattice points. The spin fluctuations in the temperature range

T- TN/TN

was found to be interpreted in terms of the diffusion theory. The longitudinal spin correlation becomes divergent at TN, the paraIIeI staggered susceptibility

obeysa power law of u(T-TN)-~,

The transverse spin correlation, however, remains finite at TN with the inverse of the correlation length

and the inelasticity r

MeV.

A new diffuse collective mode was found to appear above a temperature 120

above TN, where the static susceptibility becomes maximum.

This paper reports the first observation of the neu- tron critical scattering from an itinerant antiferro- magnet

y-Fe,.,Mn0.,. The Fe-Mn alloy with the f. c. c. structure is an antiferromagnet with an isotropic spin structure which consists of four sublattices, each containing one spin aligned in one of the four different [ I l l ] directions [I]. The iron atom was suggested to have a localized moment coupled weakly with that of the manganese atom which is mainly responsible for the itinerant antiferromagnetism [2]. The critical behavior in this alloy is quite different from those of the typical antiferromagnetic substances. The maxi- mum value of the static susceptibility x0 occurs at a temperature To about 150 higher than the temperature where the specific heat exhibits a peak

xo is inde- pendent of temperature above To, suggesting that the moment of manganese disappears above it.

The critical scattering from a single crystal of Feo.53,Mno,,,3 approximately 2 cm3 in volume was measured near the (001) and (110) reciprocal lattice points using both double- and triple-axis neutron spectrometers of Institute for Solid State Physics. The spatial correlation of spins was studied by

(8 - 2 8 scans) of the double-axis spectrometer. The specimen was embedded in a quartz tube filled with helium gas in order to minimize the temperature gradient in the crystal and the temperature was controlled to f 0.03O. The elastic 001 scattering at room temperature has suggested that neither

contamination nor the nuclear scattering due to the short range order between iron and manganese atoms are present at the (001) point. In figure 1, the peak values of the 001 and 110 scatterings are plotted against the temperature. The former is approximately proportional to the perpendicular staggered suscepti- bility x,(Q), while the latter to the square of the sublattice magnetization. The maximum of x,(Q) was found to occur at the Nkel temperature TN of 478 + 0.5 OK where the sublattice magnetization disappears. In the figure is also shown the static

9 I I 1

480 490 500 5'0 (OK)

Temperature

FIG. 1 .

-Temperature dependences of peak values of the

001

and

scatterings. The static susceptibility measured for the same specimen is also plotted against temperature by cross

marks.

susceptibility x0 measured for the same specimen. It is to be noted that the maximum of xo occurs about 150 higher than TN. The Nkel temperature just corresponds to the temperature where the specific heat shows a peak, in good agreement with the thermodynamical expectation.

If the spin fluctuations near the NBel temperature are governed by diffusion processes, the critical scattering cross section around the (001) and (110) reciprocal lattice points are given, in the static appro- ximation, by [4]

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711363

C 1

-

where

and

are the inverse of the correlation

length of the longitudinal spin correlation which becomes divergent at TN and that of the transverse correlation respectively. t is a constant which is not necessary unity. c,/Ic?~ and

are proportional to the parallel and perpendicular staggered susceptibi- lities x,,(Q) and x,(Q) respectively. The theoretical curve calculated by convoluting the cross section (1) with the resolution function of the spectrometer [5] was found to make a satisfactory fit to the observed 001 scattering in the temperature range

Both Co and

were determined by least meant squares fits to the data. The perpendicular staggered susceptibility x,(Q) does not diverge at TN. u, as TN is equal to 0.032 A-'. The theoretical curve deviates substantially from the observed scattering for the temperatures outside of this temperature range, suggesting that a collective mode is superimposed on the diffusive mode in the 001 scattering. The 110 scattering above TN was analyzed using rc, determined from the 001 scattering and assuming t

1. The parallel staggered susceptibility x,,(Q) was found to diverge at TN and it obeys a power law,

with y

1.35 , 0.05, which is the value expected for the Heisenberg system. The fact that I/Ic, is finite at TN indicates that the effective magnetic interaction is anisotropic in the alloy [4]. The difference in the inter- action parameter between parallel and perpendicular components, AJIJ estimated from

at TN is of order of which can be attributed to the magnetic anisotropy. Such an anisotropy in the correlation length has also been found in the critical scattering from a tetragonal antiferromagnet MnF, [6, 71.

The dynamical behavior of the spin fluctuations has been studied by the triple-axis spectrometer with an incident energy of 53.6 meV. Figure 2 is the energy spectra of the 001 scattering at q

0 measured employing the constant Q technique. A broken line in the uppermost figure is the resolution function of the spectrometer estimated from the elastic scattering. The upper two figures indicate that the dynamical spin fluctuations are well described by diffusion processes, the neutron cross section of which is given by [4]

The solid lines are convolution of the cross section (3) with the resolution function. The inelasticity determined are indicated in the figure. The large inelasticity of

r,

1.9 meV found at TN is consistent with the fact that x,(Q) does not diverge at TN. The theory predicts that r, is proportional to l/x,(Q) in the first approxi- mation [4].

When the temperature is higher than

the scattering profile distinctly deviates from Lorent- zian as shown in the figure, suggesting that a diffuse

FIG. 2.

-

function shown in the uppermost figure.

collective mode appears in the 001 scattering. A pre- liminary studies of this collective mode by the constant E technique indicates that it has a dispersion relation

D I q I with D an order of magnitude of 35 t 15 m e ~ / A - I at T - TN/TN

6. The similar dispersion relation has been found in a para- magnetic chromium alloy [8, 91, but the spin wave velocity D in the Fe-Mn alloy is clearly much smaller than that in the Cr alloys.

The critical fluctuation of spins observed in the temperature range

0.02 < T - TN/TN < 0.02 are well interpreted by the diffusion theory. The critical scattering is presumably due to the localized moment of the iron atom. However. the fact that a new diffuse collective mode appears slightly above TN has never been found for other systems. It is quite suggestive that the temperature above which the relaxation mechanism is changed is very close to To where both staticsuscep- tibility and thermoelectric power show a singularity.

We suggest that the band gap due to the magnetic origin disappears in the short range order

above To-,, which modifies the energy spectra of the scattering.

Acknowledgments. -

The authors are indebted

to Dr. S. Asano for computer calculation. They thanks

to Prof. M. Tachiki for fruitful discussion.

MAGNETIC CRITICAL SCATTERING FROM AN ITINERANT ANTIFERROMAGNET C 1

-

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