LOW TEMPERATURE MAGNETORESISTANCE AND ELECTRON SPIN RESONANCE IN Ge-Mn-Te

(1)

HAL Id: jpa-00213989

https://hal.archives-ouvertes.fr/jpa-00213989

Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

LOW TEMPERATURE MAGNETORESISTANCE AND ELECTRON SPIN RESONANCE IN Ge-Mn-Te

F. Hedgcock, J. Lass, T. Raudorf

To cite this version:

F. Hedgcock, J. Lass, T. Raudorf. LOW TEMPERATURE MAGNETORESISTANCE AND ELEC- TRON SPIN RESONANCE IN Ge-Mn-Te. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-506- C1-507. �10.1051/jphyscol:19711168�. �jpa-00213989�

(2)

LOW TEMPERATURE MAGNETORE SI STANCE AND ELECTRON SPIN RESONANCE IN Ge-Mn-Te

F. T. HEDGCOCK, J. S. LASS, and T. W. RAUDORF

Eaton Electronics Research Laboratory McGill University Montreal, Quebec, Canada

RBsum6. - Nous avons mesure, a basse temperature, la magn6torksistance, la magnetisation et l'absorption E. S. R.

pour un alliage Ge-Te (0,9 at. % Mn) ayant une concentration de porteurs negatifs de 9 x 1020/cc. I1 y a correlation entre la composante nkgative de la magn6tor8sistance et le carre de la magnetisation. Cette caracteristique identifie un metal ferromagnetique, a une temperature inferieure, a la temperature de Curie. La valeur de saturation de la composante negative de la magnktorksistance augmente pour une diminution de tempkrature et peut 6tre reliee & Byintensit& du signal E. S . R. pour le mgme compos6.

Abstract. - Low temperature measurements of the magnetoresistance, magnetization and E. S. R. absorption are reported for a sample of Ge-Te with a hole concentration of 9 x lOzo/cc and containing 0.9 at. % Mn. The magnetoresistance exhibits a negative component which correlates with the square of the experimentally measured magnetization.

This behaviour is characteristic of a ferromagnetic metal below the Curie temperature. The saturation value of the negative component of the magnetoresistance increases with decreasing temperature and can be correlated with the E. S. R.

line intensity on the same sample.

Negative magnetoresistance has been observed in heavily doped elemental semiconductors and ascribed by numerous workers to the possible presence of localised magnetic moments resulting from the existence of a non-periodic impurity potential [I, 21.

The purpose of the present research was to inves- tigate the magnetoresistance of a semiconductor with a known concentration of magnetic ions.

Numerous workers [I] have ascribed the occurrence of negative magnetoresistance in heavily doped elemental semiconductors to the presence of localized magnetic moments (unionized carriers) resulting from the existence of a non-periodic impurity potential. An exchange interaction between the mobile charge carriers and the localized magnetic moments is considered responsible for the electron transport anomalies [2], the exchange interaction being similar t o that occurring in dilute paramagnetic alloys exhi- biting a Kondo effect [3].

We wish to report measurements on the low temperature magnetoresistance, magnetization and E. S. R.

on a Ge-Mn-Te sample containing nominally 2 at. %

Mn. The electron microprobe concentration deter- mination yielded a value of 0.9 % Mn. The hole density deduced from the high field Hall effect [3]

at 4.2 OK is 9 x loz0 holesjcc and the electrical resistivity at this temperature is 1.0 x a-cm. At 4.20K the material is ferromagnetic with a magnetisation at 16 kilogauss equal t o 1.44 e.m.u. per gram, which is equivalent to a moment of 5.8 + ^{0.6 Bohr}

magnetons per manganese ion. The coercive force at 4.2 OK is approximately 600 gauss and is zero, within the experimental error (50 gauss), at approximately 45 OK. These results are in good agreement with previous workers [4, 51 where it was deduced that the Mn ion is in a M n + + state and that the material exhibits a broad paramagnetic-ferromagnetic transition below the Curie temperature.

The E. S. R. data on the same sample at the temperature of liquid nitrogen exhibited an absorption line, Lorentzian in character, centered at a g value of approximately 2.0, with a peak to peak line width of 1 300 gauss. Upon decreasing the temperature the

line broadened to the point of disappearing at approximately 25 OK and shifted progressively toward the low field section of the spectrum. In the liquid helium temperature range another line appeared which increased in intensity as the temperature was lowered. This asymmetric line had a half power line width of approximately 2 900 gauss and the line position, which corresponded to a g value of approximately 2.2, exhibited no temperature dependence.

Low temperature magnetoresistance data for the 0.9 at. % Mn sample was obtained using a 4 terminal A. C. method and typical results are shown in figure la.

It is at once evident that a negative magnetoresistance component is present in this material. Shown in

FIG. 1 (a). - Total magnetoresistance as a function of magnetic field for the 0.9 at % Mn sample measured at 4.2 %.

FIG. ¹(b). -Total magnetoresistance as a function of the square of the magnetic field showing the separation of the negative component from the positive component which varies

linearly with HZ.

FIG. 1 (c). - The separated negative component of the magnetoresistance ^(Aplp),as a function of magnetic field.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711168

(3)

LOW TEMPZRATURE MAGNETORESISTANCE AND ELECTRON SPIN RESONANCE C 1 - 507

figure 16 and l c is the separation of the measured rature ESR line intensity increased rapidly with decreas- magnetoresistance into two components, one posi- ing temperature. In figure 3b the two effects are shown tive, quadratic in H, and one negative which becomes to scale together as the temperature varies. If the constant for fields above 25 kG. E. S. R. line intensity is interpreted as a measure of Shown in figure 2a is the magnetization of the the manganese ion magnetization then the magnitude sample measured at 4.20K. The hysteresis present

in the magnetoresistance is again evident in the magne-

I

FIG. 2. - Magnetization as a function of magnetic field for the 0.9. at % Mn sample measured at 4.2 OK.

Inset. The negative magnetoresistance component ( A ~ l p ) ~ , as a function of the experimentally determined magnetization

at 4.2 OK.

tization. For a ferromagnetic material the magnetoresistance has been related t o the magnetization through the empirical law [6] :

~p = A ( M ~ - M;) (1) where A is a function of temperature, M is the magnetization and M,, is a constant magnetization which in the case of ferromagnetic metals has been related to the magnetization arising from domain wall displacement.

Such displacements reverse the spontaneous magnetization without affecting the resistivity. Figure 26 shows the experimentally separated quantity (Aplp), as a function of the experimentally measured magnetization. It can be seen t o have the relationship expec- ted from equation (1) apart from the low-field region of hysteresis.

The magnitude of the magnetoresistance was observed to vary rapidly with temperature below 4.2 OK,

see figure 3a. In this temperature range we do not expect any significant temperature dependence of the bulk magnetization. However, the low tempe-

FIG. 3 (a). - The temperature dependence of the saturation value of ( A p l ~ ) ~ as a function of temperature. The values of

(Ap/p),,t have been normalized to the value a t 4.2 OK.

FIG. 3 (6). - Relative E. S. R. line intensity as a function of the relative value of (Ap/p)sat. Both parameters have been nor-

malized to the value at 4.2 OK.

of the negative magnetoresistance appears to reflect this magnetization. A more definite conclusion will require a detailed interpretation of the E. S. R. data which is at present underway.

Acknowledgements. - The authors would like to thank Professor J. Lewis for providing the samples, Professor J. MacKinnon who kindly allowed us to use his Foner magnetometer, and Professors W. B.

Muir and J. 0. Strom-Olsen for their assistance in setting up the A. C. resistance measuring device. This research was sponsored jointly by the National Research Council of Canada and the Defence Research Board of Canada. One of the authors (J. S. Lass) would like to acknowledge financial support received through a N. R. C. Postdoctoral Fellowship.

References

[I] ALEXANDER (M. N.) and HOLCOMB (N. F.), Rev. Mod. ^{4]CHAMENTAWSKI (M.), RODOT (H.), VILLERS (G.) and Phys., 1968, 48, 815. SASAKI (W.), J . Phys. Soc. RODOT (M.), C. R. Ac. Sci., Paris, 1965,261,2198.

Japan, Supplement, 1966, 21, 543. [S] RODOT (M.), LAWIS (J.), RODOT (H.), VILLERS (G.), [2] TOYOZAWA N.), J . e y s . Soc. Japan, 1962, 17, 986. COHEN (J.) and MOLLARD (P.), J. Phys. Soc.

131 This value was used in order to avoid compltcations ^Japan,Supplement, 1966, 21, 627.

from the extraordinary Hall effect term. ^[6]JAN (J. P.), Solid State Physics, 1957, 5 , 70 (Academic Press Inc., New York, New York).