ELECTRICAL RESISTANCE NEAR FERROMAGNETIC CURIE POINTS

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ELECTRICAL RESISTANCE NEAR FERROMAGNETIC CURIE POINTS

F. Zumsteg, R. Parks

To cite this version:

F. Zumsteg, R. Parks. ELECTRICAL RESISTANCE NEAR FERROMAGNETIC CURIE POINTS.

Journal de Physique Colloques, 1971, 32 (C1), pp.C1-534-C1-535. �10.1051/jphyscol:19711179�. �jpa-

00214001�

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JOURNAL

DE PHYSIQUE

Colloque C 1, supplément au n° 2-3, Tome 32, Février-Mars 1971, page C l - 534

ELECTRICAL RESISTANCE NEAR FERROMAGNETIC CURIE POINTS

F. C. ZUMSTEG and R. D. PARKS

Department of Physics and Astronomy University of Rochester, Rochester, New York 14627, U. S. A.

Résumé. — Mesures de haute précision sont effectuées pour la résistivité électrique près de T

a

de Ni, GdNi2 et des monocristaux orientés de Gd. Dans les trois cas le caractère général des anomalies de la résistivité observée peut être expliqué en considérant tant la dispersion de désordre de spin, dans le système des modèles de de Gennes-Friedel et de Fisner-Langer, et l'effet de couplage spin-orbite.

Abstract. — Precision measurements of the electrical resistance near T have been made on Ni, GdNi2 and oriented single crystals of Gd. The general character of the observed resistance anomalies in all three cases is explained by consi- dering both spin disorder scattering, within the frame-work of the de Gennes-Friedel and Fisher-Langer models, and the effect of spin lattice coupling.

The resistivity of metallic ferromagnets is generally characterized by anomalous behavior in the vicinity of the Curie temperature T

c

. Attempts to understand this behavior have been hampered by the fact that the anomalies are qualitatively different in different mate- rials. For example the resistance R of Ni, [1] Fe, [2] Gd [3] in the a direction, and GdCo

2

[4] (Group I) is monotonic with temperature near T

^e

with a singularity in di?/dr at T

c

. On the other hand R(T) of another group of materials, e. g., Gd [3] in the c direction, GdNi

2

[5] and GdPt

2

[4] (Group II), displays a maximum in the vicinity of T

^c

. It has been generally assumed that spin disorder scattering plays a major role in these anomalies.

De Gennes and Friedel [6] have derived the contri- bution to the resistance from spin disorder scatter- ing within the mean field model: the calculation implicitly incorporates the Ornstein-Zernike form of the spin-spin correlation function. This approach leads to a cusp in R(T) at T

^c

.

Fisher and Langer [7] have reformulated the pro- blem using a more realistic form for the two particle correlation function suggested by neutron scattering experiments. They demonstrate that the short range spin-spin correlations above T

c

give the principle contributions to both the energy U

^m

^

^g

(T) and the spin disorder contribution to resistance, i?

mag

(7

y

). It fol- lows that the singular parts of dU

^mag

fdT(= C

^mag

) and di?

mag

/dT should have identical temperature depen- dences sufficiently close to T

^a

[7]. This implies a mono- tonic dependence of R(T) near T

Q

and a singularity in dRfdT at T

^e

—just the behavior exhibited by Ni and other Group I materials. Zumsteg and Parks [1]

have shown that in Ni the temperature dependence of the specific heat anomaly is the same as that of the dR/dT anomaly in the temperature range

1(T

3

< e_ =$ 5 x 1 0

- 2

and

2 x 1 0

^{- 4}

< e

⁺

< 2 x H T

¹

,

where e_ = (T

^e

- T)/T

^c

and e

⁺

= (T - T

^c

)/T

^c

. It has been suggested that the behavior exhibited by Group II materials can be accounted for by the mean field theory of de Gennes and Friedel. Kawatra

[5] and coworkers found that they could fit their results on the resistivity of GdNi

2

above T

c

to a two parameter formula derived by Kim [8] from the de Gennes-Friedel theory. It was then suggested that GdNi

²

behaves differently than Ni because the magne- tic spins are localized in the former and not in the latter.

In order to explore this question further Zumsteg et al. [3] studied the case of c-axis Gd, which exhibits a strong peak in R(T) near T

c

. This work led to the surprising discovery that the major cause of the resis- tance anomaly was not spin-disorder scattering but rather the anomalous lattice contraction near T

^c

. When the component of R(T) due to the latter effect was subtracted out, the remaining resistance (due presumably to spin disorder scattering) was found to have the temperature dependence characteristic of Group I materials.

The obvious question arose : can anomalous lat- tice effects explain the resistance anomalies in all Group II materials ? In an attempt to answer this question we made resistivity, magnetic susceptibility and expansion coefficient measurements on polycrys- talline GdNi

²

. As seen in figure 1, there is not the strong lattice anomaly that occurs in Gd. The purpose of the magnetic susceptibility % (actually, the induc- tance of a coil wound around the sample) measure- ments was to determine T

^c

in an independent manner.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711179

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ELECTRICAL RESISTANCE NEAR FERROMAGNETIC CURIE POINTS C

1

-

⁵³⁵

The results for ~(7') and R(T) are shown in figure 2. ,

The method of sliding averages was used to derive a plot dR/dT from the data shown in figure 2. This

is shown in figure 3 together with a theoretical plot of the de Gennes-Friedel prediction. As seen the fit is good in the region 10

O K

< (T - Tc) < 2000K, but fails for T - Tc < 10 OK, suggesting the breakdown the mean field approach for T - T, S 10

OK

(corres- ponding to

^E

-- 0.1), and the gradual change over to the Fisher-Langer behavior as T

^-,

T,. Our results suggest that lattice effects do not make the dominant contribution to the resistance anomaly in GdNi,.

In the case of c-axis Gd it is diEcult to determine the region of breakdown of the mean field theory since the resistance anomaly is dominated by the lattice effect and since the latter [i.e., L(T) where L is the unit cell length in the c direction] has the same tempe-

FIG.

3. rature dependence above Tc [viz., ( A ~ l n s t BE)] as does the spin disorder scattering in the de Gennes- Friedel model. The observed range of temperature over which R(T) follows the Fisher-Langer behavior in a- axis Gd [3] and Ni [I], suggests that in these cases the switch over to mean field behavior must be outside the region

E+

2 0.2.

Thus we see that several different mechanisms must be considered to explain the resistance anomaly at magnetic Curie points. In at least one case (c-axis Gd) the anomaly is dominated by lattice effects. But, even in the case where the anomaly is dominated by spin disorder scattering the qualitative appearance of the anomaly depends upon how close to T, the mean field approach provides an adequate description.

There is no definitive evidence that the basic character of the anomaly depends on whether or not the magnetic spins are localized.

References [I] ZUMSTEG (F. C.) and PARKS (R. D.), Phys. Rev. Let-

ters, 1970, 24, 520.

[2] WALLACE (D. C.), SIDLES (P. H.) and DANIELSON (G. C.), J. Appl. Phys., 1960, 31, 168.

131 ZUMSTEG (Fi. C.), CADIEU (F. J.), MARCELJA (S.) and PARKS (R. D.), Phys. Rev. Letters (to be published).

[4] KAWATRA (M. P.), MYDOSH (J. A.) and BUDNICK (J. I.), Phys. Rev. B (to be published).

[5] KAWATRA (M. P.), SKALSKI (S.), MYDOSH (J. A.) and BUDNICK (J. I.), Phys. Rev. Letters, 1969, 23. 83.

[6] DE

GENNES fJ?.