MAGNETIC NEUTRON SCATTERING FROM K2NiF4

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MAGNETIC NEUTRON SCATTERING FROM K2NiF4

J. Skalyo, Jr., G. Shirane, R. Birgeneau

To cite this version:

J. Skalyo, Jr., G. Shirane, R. Birgeneau. MAGNETIC NEUTRON SCATTERING FROM K2NiF4.

Journal de Physique Colloques, 1971, 32 (C1), pp.C1-882-C1-883. �10.1051/jphyscol:19711312�. �jpa-

00214344�

JOURNAL DE PHYSIQUE

Colloque C I, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 882

MAGNETIC NEUTRON SCATTERING FROM K,NiF4

J. SKALYO, Jr. and G. SHIRANE

Brookhaven National Laboratory (*), Upton, New York 11973, U. S. A.

and R. J. BIRGENEAU

Bell Telephone Laboratories, Murray Hill, New Jersey 07974, U. S. A.

R6sumC.

-

La diffusion magnttique des neutrons a Bte Ctuditk au-dessus et au-dessous de la temperature de tran- sition dans I'antiferromagnetique

a

deux dimensions K2NiF4. L'etude inklastique sur un spectromktre a trois axes a indique que pres de T N la forme de sl1(0,

E)

et SL(O,

E )

est trks differente de celle obtenue pour les systkmes

ii

trois dimensions avec l'kchange de Heisenberg. On a trouve que Sll(O,

E)

a une largeur extrkmement etroite comparee

i

Sl(O,

E)

dans le voisinage de TN. SI(O,

E)

montre un petit changement avec la temperature lorsque

T

est augmente au-dessus de

TN.

Abstract. - The magnetic scattering of neutrons has been investigated both above and below the transition tempe- rature in the two-dimensional antiferromagnet K2NiF4. Inelastic studies on a triple-axis spectrometer have indicated that near T N the behavior of Sll(0,

E),

and SI(O,

E )

differ considerably from results obtained forthree-dimensionalsystems with Heisenberg exchange. Sll(0,

E )

was found to have an extremely narrow line width compared to SL(O,

E),

in the vici- nity of

TN.

Sl(O,

E )

also showed little change with temperature as T was increased from

TN.

K,NiF4 is structurally composed of NiF, layers separated by two K F layers [I]. The relative isolation of the NiF, layers, as compared with the KNiF, structure, has given credence to Lines' [2] interpreta- tion of Srivastav'as [3] susceptibility data in terms of a two-dimensional antiferromagnet. A definitive deter- mination of the two-dimensional character of K2NiF4 has since been given by Birgeneau et al. [4] utilizing the quasielastic scattering of neutrons from a single crystal. The scattering occurred in the vicinity of magne-

tic

^<(

lines

^)>

in reciprocal space (parallel to the unor-

dered c-axis) and gave a qualitative measure of the length of planar spin correlations in the paramagnetic regime.

In particular, the quasielastic measurements give [5]

d a kT

^{A A}

-. - --

2

C Cam,

^{- Qa Q p l}

xap(Q)

⁽¹⁾

dQ g2

11, 10

where X a P ( ~ ) is the wavevector dependent susceptibility.

Subsequent measurements by Birgeneau et al. [6]

have served to point out the large disparity of X l l ( ~ ) and xL(Q) compared to observations of Schulhof et al. [7] on MnF,, a three-dimensional antiferromagnet with ,Heisenberg exchange. In the latter, $1(0) diverges at T, with ~ ~ ( 0 ) diverging at T slightly less than TN (the phase transformation precludes this divergence).

'The results for K,NiF, indicate that while x1I(0) diverges at TN, ,yl(0) is of small intensity and appears ignorant of the transition.

Spin-wave measurements at 4.2

O K

have been made by Skalyo et al. [8]. No measurable dispersion was observed in the [00(] direction resulting in an upper limit for the interplanar exchange that is 270 times smaller than intraplanar near neighbor exchange.

Combined with the antiferromagnetic resonance measurements of Birgetleau et a]. [9], the magnon dispersion is well-fitted with near neighbor intraplanar

(*)

Work performed

under the

auspices

of

the

U . S.

Atomic

Energy

Commission.

exchange and an anisotropy field 500 times smaller than the exchange field. The presence of magnons at finite q was also evidenced at temperatures apprecia- bly above TN [8]. Recent measurements of magnon- pair modes by Fleury and Guggenheim [lo] also demonstrate the persistence of magnons above Tx and show large differences in behavior compared to the pair mode spectrum of KNiF,.

In the present paper, we report the temperature dependence of the zone center spin-wave energy and an energy analysis of X"p(0) is made through TN.

Here, for E < k T one obtains [5]

where the magnetic Bragg contribution is a delta function in energy and reciprocal space and is zero above TN. A triple-axis spectrometer at the Brookha- ven High Flux Beam Reactor was utilized to measure SI1(0, E) and S1(O, E) through the transition region.

Pyrolytic graphite was used for both monochromator and analyzer. Initial neutron energy was 5.2 meV

;

as both Sl1(0, E) and S1(O, E) are measured simul- taneously, precise energy resolution is necessary to separate the components.

Typical energy scans at q

=

0 are shown in figure

1

for four temperatures near T,

=

97.230K. Below TN, the spin waves comprising S1(O,

E )

are clearly defined peaks separated from S1l(O, E) which is centered at E

=

0. The scans for S1(O, E) above TN indicate little change between 98 and 101 OK. It is also clear that the energy integrated intensity is little affected by the transition. The zone center magnon energy is renormalized toward zero as TN is approached.

Figure 2 depicts the q

=

0 magnon energy as a func- tion of temperature. Also shown are the AFMR measurements of Birgeneau et al. [9] and the scaled magnetization curve [6].

The width of the SL(O, E ) peaks is only partially accounted for by the instrumental resolution, whereas

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711312

MAGNETIC NEUTRON SCATTERING FROM K2NiF4 C 1

-

883

-4.0 -2.0 0.0 2.0

ENERGY (rneV) -4

FIG. 1.

-

Energy scans at the

3

^{= 0} position of the two- dimensional magnetic scattering in the vicinity of TN. The central peak is due to sII ^(0, E) and the broad peak is S1 (0,

E),

the zone center magnon below T s . The position of the scan avoids

the three-dimensional magnetic Rragg peak.

K2NiF4 ' q 20 MAGNON

0 (-1 I I 1 I 1 I 1 I I

I

0 10 20 30 40 50 60 70 8 0 90 100 TEMPERATURE ( O K )

FIG. 2.

-

The q = 0 magnon energy as a function of tempe- rature. The solid line is the scaled magnetization curve.

the width of the central S (0, E) peak is of instrumen- tal origin. An upper limit on the latter width below TN is 0.05 meV. It is noted that the integrated intensi- ties would appear to give ~ ~ ( 0 ) > X1l(0). This is due to the finite instrumental resolution mollifying the sharply peaked character of SIl(0, E) as compared t o increasing the visibility of the broadened SL(O, E).

The measurements indicated were made with 40 min collimation of the neutron beam. Similar measure- ments using 20 min collimation resulted in a resolution volume 8 times smaller in (Q, E) space. For this latter arrangement, the visibility of SI1(0, E) was little affected whereas SL(O, E) was no longer visible above background.

The results of the present paper will be includcd in a more detailed paper t o be published on K,NiF, which will also include extensive quasielastic measure- ments of Xll(c) [I 11.

References

BALZ

(D.)

and PLEITH

(K.), Z . Electrochem., 1955,

59, 545.

LINES

(M. E.), Phys. Rev., 1967, 164, 736.

SRIVASTAVA

(K. V.), Phys. Letters, 1963, 4 , 55.

BIRGENEAU (R.

J.),

GUGGENHEIM

(H. J.),

and SHI-

R A N E

(G.),

Phys. Rev. Letters, 1969, 22, 720 ; Phys. Rev., 1970, B 1, 221 1 .

MARSHALL

(W.)

and LOWDE (R.

D.), Rept. Prog.

Phvs.. 1968. 31. Dart 11. 705.

BIRGENEAU (R.

J.), SKALYO (J.,

JK), and SHIRANE (G.),

J . Appl. Plgvs.. 1970, 41, 1303.

171

SCHULHOF

(M. P.),

HELLER (P.), NATHANS (R.), and LINZ

(A.). PIZYS. Rev.. 1970. B 1. 2304.

[8]

SKALYO

(J.,

JR),

^SHIRANE

(G.), BIR'GENEAU (R. J.),

and

GUGGENHEIM (H. J.),

Phys. Rev. Letters, 1969. 23. 1394.

[9] BIRGENEAU

(k. J.),

DEROSA (F.),

and GUGGENHEIM (H. J.),

Solid State Comrn~uz., 1970, 8, 13.

1101

.- . FLEURY (P.

A.)

and

GUGGENHE~M (H.

J.).

^,, P ~ v s .

-

Rev.

~ e t t e r s , 1970, 24, 1346.

[ l l ]

BIRGENEAU (R. J.), SKALYO (J., JR), SHIRANE (G.),

to

be

published.