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The magnetic susceptibility of an MnO single crystal

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HAL Id: jpa-00236062

https://hal.archives-ouvertes.fr/jpa-00236062

Submitted on 1 Jan 1959

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The magnetic susceptibility of an MnO single crystal

T.R. Mcguire, R.J. Happel

To cite this version:

T.R. Mcguire, R.J. Happel. The magnetic susceptibility of an MnO single crystal. J. Phys. Radium,

1959, 20 (2-3), pp.424-426. �10.1051/jphysrad:01959002002-3042400�. �jpa-00236062�

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424

THE MAGNETIC SUSCEPTIBILITY OF AN MnO SINGLE CRYSTAL

By T. R. MCGUIRE and R. J. HAPPEL, Jr.,

United States Naval Ordnance Laboratory, White Oak, Maryland, U. S. A.

Résumé. 2014 On trouve que la susceptibilité magnétique d’un monocristal de MnO obtenu par fusion dans une flamme, est identique à celle de la substance en poudre. Pourtant, la variation de

susceptibilité avec le champ est plus prononcée et se prolonge dans la région paramagnétique.

Abstract. 2014 The magnetic susceptibility of an MnO single crystal grown by flame fusion was

found to be similar to the powder material ; however, the magnetic field dependence of the suscep-

tibility was larger than the dependence for the powder and in addition extended into the para-

magnetic region.

LE JOURNAL DE PHYSIQUE ET LE RADIUM TOME 20, FÉVRIER-MAns 1959,

Magnetic susceptibility measurements of MnO

powder made by Bizette, Squires and Tsaï [1]

showed strong magnetic field dependence of the susceptibility below 116 oK (the Néel temperature),

an effect which was interpreted by Néel [2] and

also Van Vleck [3] as one of the characteristics of

antiferromagnetism.

We have made measurements on a small pièce

of MnO single crystal which weighed 68.7 milli-

grams. The specimen was grown by Prof. W. H.

FIG. 1.

-

Magnetic susceptibility as a function of tempe-

rature. The dotted line for H

=

0 is the projection of

the slopes found in fig. 2.

Bauer of Rutgers University by the flame fusion method and made available to us by the M. J. T.

Lincoln Laboratory (1). Because of the small size

of the sample we were not able to determine the chemical composition but an X-ray Laue photo-

(1) We wish to thank Dr. S. Foner for sending us this specimen, and for the information that powder x-ray data

on the original boule showed that if higher oxide were present they were less than a few per cent,

graph by Miss S. W. Greenwald showed that the

sample is a single crystal. The agreement of its magnetic properties with powder data, both in magnitude and temperature dependence, lead us to believe that the composition is nearly stoichio-

metric.

The magnetic measurements were made by a

force method [4]. Preliminary data taken in the antiferromagnetic region indicated that the suscep-

tibility was isotropic. Fig. 1 and fig. 2 show the

FIG. 2.

-

The magnetic susceptibility

in fields 4 100 to 8 860 Oe.

susceptibility plotted as a function of temperature

and in fields up to 8 500 oersteds. The magnetic

field was along a [110] direction and the vertical

rotation axis was roughly a [110] direction.

After these initial measurements the sample was

annealed in helium in 200 OC for several hours and measured with the same orientation in fields up

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01959002002-3042400

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425 to 16 400 oersteds. The data are plotted in fig. 3

and in fig. 4. It is seen in fig. 4 that for the

lowest magnetic fields there is a slight falling off of

the curve. Since this effect was not observed in

FIG. 3.

-

Magnetic susceptibility as a function of tempe-

rature. The dotted line is the powder data of Bizette, Squires and Tsaï.

FiG. 4.

-

The magnetic susceptibility

in fields 6 800 to 16 400 Oe.

earlier measurements (fig. 2), we believe it is an experimental error in calibration against the plati-

num standard. Our relative error comparing data

at différent temperatures is estimated at + 1 %,

but absolute values compared over a period of time

are only good to ± 2 %.

The susceptibility of the MnO is isotropic and

this result is similar to that found by Singer [5] for

annealed NiO single crystals. The field depen-

dence of the susceptibility of the MnO crystal is larger than that found for the powder material and

there is some field dependence even in the para-

magnetic region. In fields up to 16 400 oersteds and at 77 °K the susceptibility is approximately

linear in H and there is no hystérésis effect.

From neutron diffraction studios by several investigators [6], [7], it has been determined that

spins lie in ferromagnetic (111) planes. Theore-

tical work by Kaplan [8], also Keffer and 0’Sul- livan [9], lead to the interpretation that a very

strong anisotropy K1 holds the spins in this plane,

but that a much smaller anisotropy K2 exists in

the (111) plane. Also applicable to MnO is the uniaxial case with a single anisotropy constant K

which Nagamiya [10] has discussed.

The above theories rgive an ’equation of the following form for low magnetic fields.

where X jj and xl are the susceptibilities parallel and perpendicular to the spin axis, Xp is the powder or

measured value, and h = H /He. The critical magnetic field H, causes the spins to switch to a crystallographic direction closer to X.L. *II For

a =2/15, He = [2K j(X, -tXjj )] 1(2 ; when a =1/45 H. = [6K2/(X-L

---

Xjj)]"£. Using our data

for 20 oR. we obtain, X"L= 87. H X 10- " emu/gm, Ha - 24 000 oersteds and K z-- 1. 1 X 105 ergs/ce

for Nagamiya’s uniaxial case, while

1.l. = 87,6 X 10- Il emu/gm, He =14 700 oersteds,

and K2:::= 1.5 X 104 ergs/ce for the Keffer-

O’Sullivan conditions. The values of the aniso-

tropy obtained from our data are smaller than obtained previously [7] from the powder data. It

is interesting that the calculated value of Xj,

(at 20 OK) is about 87 X 10-6 and this is the same

value used for the powder data by Keffer.

The isotropic susceptibility is interpreted to

mean that antiferromagnetic domains are present causing the spin axis to average out to the poly- crystalline value in a manner similar to the NiO case [5]. Néel [11] hp8 suggested that domain wall motion might contribute to the field dependence of

the magnetic susceptibility and it cannot be exclud-

ed in this case, especially since differences between

powder and single crystal must be accounted for.

The field dependence observed in the para-

magnetic region may be due to short range order,

which has been reported from neutron diffraction results [6]. According to the interpretation of Wangsness [12], short range order in this tempe-

rature region (116 OK to 250 OK) in MnO also has

the effect of decreasing microwave resonance

absorption [13].

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426

Further experiments are necessary before more

definite conclusions can be stated concerning the

most appropriate values of He and K.

Note added at time of presentation. It has

been found that the measurements made at 20 OK

are not reproduceable. This is now being studied.

REFERENCES

[1] BIZETTE, SQUIRE and TSAI, C. R. Acad. Sc., 1938, 207, 449.

[2] NÉEL (L.), Ann. Physique, 1936, 5, 256.

[3] VAN VLECK (J. H.), J. Chem. Phys., 1941, 9, 85.

[4] MCGUIRE (T. R.) and LANE (C. T.), Rev. Sc. Instr., 1949, 20, 489.

[5] SINGER (J. R.), Phys. Rev., 1956, 104, 929.

[6] SHULL, STRAUSER and WOLLAN, Phys. Rev., 1951, 83, 333.

[7] CORLISS, ELLIOTT and HASTINGS, Phys. Rev., 1956, 104, 924.

[8] KAPLAN (J. I.), J. Chem. Phys., 1954, 22, 1709.

[9] KEFFER (F.) and O’SULLIVAN (W.), Phys. Rev., 1957, 108, 637.

[10] NAGAMIYA (T.), Progr. Theor. Phys., Japan, 1951, 4,

342.

[11] NÉEL (L.), Proc. of the International Conference on

Theoretical Physics, Tokyo and Kyoto, 1954 (Science Council of Japan, Tokyo, 1954).

[12] WANGSNESS (R. K.), Phys. Rev., 1953, 89, 142.

[13] MAXWELL (L. R.) and MCGUIRE (T. R.), Rev. Mod.

Phys., 1953, 25, 279.

DISCUSSION

Question by S. Foner.

-

Although the purity of

of this MnO crystal appears to be better than

95 % MnO, this data should be taken with some

caution. Our measurements"on a part±of this crystal showed that the magnetic moment

at 4.2 OK was not a linear function of applied field.

Furthermore, it appeared that the susceptibility

increased as the temperature was decreased from about 70 OK to 4. 2 oK.

Such results disagree with the behavior of

"

simpler " antiferromagnets (see our contribution to this Colloque, p. 336). We have observed that

Mn304 becomes ferrimagnetic at about 50 OK. An impurity of a few percent of Mn304 in this material

would be sufficient to produce the observed effects.

Mr. McGuire.

-

We have tried to estimate the amount of ferromagnetic impurity in this sample

from measurements at liquid helium temperatures (fig. 5). The curve marked 1st run is afterthe

initial cooling and shows a steep positive slope

much greater than a 77,OK. All subsequent mea-

surements followed the curve marked 2nd run

which has the form typical of a ferromagnetic

Fie. 5. - The magnetic susceptibility

of MnO single crystal at 4.2 OK.

impurity. Using the standard Honda-Owen

method the impurity concentration was found to

be .01 gms per ems (assuming Mn3O4 as the impu- rity). This concentration value is probably some-

what low since the impurity might not be saturated and further there is the problem of separating the antiferromagnetic behavior from the effect of the

ferromagnetic impurity. For temperatures at

77 °K and above the susceptibility measurements

were reproducible and there was no evidence of a

ferromagnetic impurity.

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