ANTIFERROMAGNETISM OF α-Mn AND ITS DILUTE ALLOYS

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ANTIFERROMAGNETISM OF α-Mn AND ITS

DILUTE ALLOYS

G. de Doncker, J. van Cauteren, M. Rots

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, dkembre 1988

ANTIFERROMAGNETISM

O F

crMn

AND

ITS

DILUTE

ALLOYS

G. De Doncker, J. Van Cauteren and M. Rots

Iwtituut voor Kern- en Strdingsfysika, CeIestijnenlaan 200 D, 8-3030 Leuven, Belgium

Abstract.

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A coexistence of local moment and itinerant antiferromagnetism in a-Mn is proposed because two hyperfine fields were observed, which behave differently as function of temperature or external field. In addition, this magnetic coexistence as well as the crystal phase stability will be discussed on its dependence on impurity concentration.

In this study we present the first measurements on very diluted solute hyperfine fielddistributions in a-Mn. The method used is the perturbed angular correlation technique applied on the 173-247 keV 7- 7 cascade in the decay of 'llln t o ' " ~ d . FkOm me% surements as a function of temperature and external field, we believe t o offer evidence that the localized moment picture is not sufficient to describe the magnetic nature. The starting material was c~lnmercially available Mn-flakes of purity 99.9

+

but also flakes purified by vacuum distillation [I]. The "'1n activity concentration remains in the 0.5 ppb range and is well incorporated in the material.

1. Paramagnetic phase

Measuring on the Mn-flakes, we observe at ,T = 150 K a small electric quadrupole interaction WQ = 1.13 (5) Nrad/s, corresponding to a field gradient

We have to conclude that the indium, because of the

nearly cubic symmetry reflected, occupies site-I loca-

tions only. We consider the small field gradient as

being due to residual lattice imperfections. Indeed, purifying the material by distillation reduces the interaction strength and in the limit gives a constant anisotropy, revealing the cubic environment of site-I positions.

2. Antiferromagnetic phase

Upon cooling the magnetic ordering sets in and two distinct sites appear. In figure 1 some typical PAC spectra are displayed together with the frequency distribution. Over the whole temperature range down to

4.2 K, the frequencies corresponding to the two sites remain in a ratio close t o 1:2, while the relative popul* tion of the highest interaction site remains at 63(3)%

.

At 4.2 K, the values are 6.42(2)T (width 6 = 0.29(5)T) and 3.24(4)T (6 = O.!2!2(3)T). Figure 2 shows the temperature dependence of both hyperfine fieIds, which

Fig. 1.

-

Time dierential PAC spectra for ' " ~ d in CY-

Mn and the derived frequency distributions as a function of temperature. Below the magnetic ordering temperature TN=120 K the frequency distribution is double peaked in- dicating two hf-field sites.

Fig. 2. - The temperature dependence of the hyperfine field at the h- and 1-site. Insert: hf-field jump around T = 17 K

in the 1-site.

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C8

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182 JOURNAL DE PHYSIQUE

reveals two features: i) the ordering temperature for both fields is To = 120.14 (5) K, which is definitely higher than the NBel temperature TN = 95 K obtained with several other techniques [2,3]; ii) the insert shows below 20 K a small, but significant jump, only for the low field site. Although the average probe concentration amounts less than 0.5 ppb, cadmium influences the ordering temperature drastically.

3. Impurity dependence of the ordering tem- perature To

Resistivity-[4] and NMR-data [5] have indicated several features on this subject. Changes in N&l temperature axe of the order of 10-20 K/at%

.

We have checked these features with three diluted alloys. Mea- suring on a-Mn flakes gives us an ordering temperature To of 120(1)K. Dilute a-&Cr, -Fe, -Ni alloys give To =113(1)K, 128(1)K, (120-125 K). These results are in agreement with [45] because Cr(Fe, Ni) is to be found on the left (right) of Mn in the periodic table. In

order t o check the influence of residual impurities, we performed experiments on purified material. The ordering temperature reduces drastically, resulting in one

case in TO =95(1)K which corresponds to the bulk NBel temperature. In a second run, the influence of the indium concentration is investigated. Changing the concentration, in Mn-flakes, within the range 0.1-10 ppb, results in To =116(1)K up to To =145(1)K. Moreover we observe that the low field site is unstable for high In-concentrations, and transforms to ,f3-Mn. Further- more, a t a certain concentration, the a-Mn formation is inhibited while dilution restores the a-Mn phase. In a third run, the In-concentration dependence of To for vacuum destillated samples is checked. Varying the concentration between 0.04-0.2 ppb results in To = 100.3(1)K-116.0(1)K.

4. Discussion of the results

The question thus arises why indium probe locations are magnetically different, while crystallographically identical. The local moment feature in a-Mn is well documented [4] but here we suggest some features rem- inescent to the magnetism in chromium. For example the broad hyperfine field distribution for the low field site may be due to the Overhauser distribution. F'ur- thermore the hf-field jump around T = 20 K ressem- bles the spin-flip transition of the spin-density-wave as observed [6] in chromium with the same technique. Finally measurements in an external field evenmore underline the essentially different nature of both field sites observed. Indeed while the hyperfine field values measured remain almost unchanged, the hyperfine field distribution for the low field site sharpens drastically by field cooling. Further experiments were performed at constant temperature (4.2 K, 77 K) as

a function of external field (see Fig. 3). For a local moment magnetism, one expects a gradual evolution towards a spin-flip state with the spins perpendicular t o the applied field. This is what is observed at the high field site (full line) but not for the low field site. The dotted line in figure 3 represents the behaviour assunGng a random orientation of the hf-field relative t o the external field. Finally the low field site shows a decrease of total observed field below 20 K in zero- field-cooled samples but not in field-cooled samples.

d i

i

3 4

Bext IT1

Fig. 3. - Total observed field versw external field for h-

and 1-site at T = 4.2 K and 77 K. (o, 0 ) : high field site

for 4.2 K, resp. 77 K. (A, x): low field site for 4.2 K,

resp. 77 K. Random geometry (dotted curve). Full line: perpendicular geometry between hf-field and external field.

[I] Kunitomi, N., Yamada, T., Nakai, Y. and Fujii, Y., J. Appl. Phys. 40 3 (1969) 1965.

[2] Yamada, T., Kunitomi, N., Nakai, Y., Cox, D. E. and Shirane, G., J. Phys. Soc. J p n 28 (1970) 615.

[3] Nasagawa, H. and Senba, M., J. Phys. Soc. Jpn 39 (1975) 70.

[4] Williams, W. and Stanford, J. L., Phys. Rev. B

7 (1973) 3244.

[5] Kohara, T., J. Phys. Soc. J p n 37 2 (1974) 393. [6] Venegas, R., Peretto, P., W, G. N. and Trabut,