Effect of iron impurity on the magnetic susceptibility of gadolinium at elevated temperatures

JOURNAL OF THE LESS-COMMON METALS

336

EFFECT SUSCEPTIBILITY

OF IRON

IMPURITY

OF GADOLINIUM S. ARAJS

AND

ON THE

MAGNETIC

AT ELEVATED

TEMPERATURES

.R. V. COLVIN*

Edgar C. Bain Labora2ory for Fundamental Research, United States Steel Corporation, Research Center, Monroeville, Pa. (U.S.A (Received

.)

April moth, 1964)

SUMMARY Magnetic susceptibility of various gadolinium samples of different purities has been studied above 300’K as a function of temperature. All the samples, including those studied earlier, exhibit an anomaly in the magnetic susceptibility at about 8oo’K. It is suggested that this behavior results primarily from the ferromagnetic properties of FegGd which is present as an impurity. This inference is supported by susceptibility data on a sample of gadolinium doped with 0.5 wt.% of iron.

INTRODUCTION

During our investigation9 of the paramagnetic behavior of the rare earth metals at elevated temperatures, we found that various gadolinium samples exhibited an anomaly in the magnetic susceptibility at about 750°K~ An anamolous behavior in the susceptibility has also been noticed recently by CHECHERNIKOV~ at approximately 650°K. The present studies were undertaken to determine the origin of this behavior in gadolinium. EXPERIMENTAL Table

I gives the designations

and the electrical

resistivity

of samples values

CONSIDERATIONS used in this study,

TABLE SAMPLE

DESIGNATIONS,

Sample

SOURCES

OF GADOLINIUM,

designation

SE-r L-r M-I

M-3* RC-I * Sample M-3 is essentially tamination of 0.50 wt. %.

the sources of gadolinium,

at 42°K. I AND

RESIDUAL

ELECTRICAL

RESISTIVITIES

Source

Semi-Elements, Inc. Lindsay Chemical Div. Michigan Chemical Corp. Michigan Chemical Corp. Research Chemicals equivalent

5.2 5.7 5.9 11.4 2.4

to Sample M-I except for deliberate Fe con-

* Deceased March 26th, 1964. J. Less-Common

Metals, 7 (1964) 336-340

MAGNETIC

SUSCEPTIBILITY

OF GADOLINIUM

337

The sample SE-I was cut from a gadolinium bar, zone-melted by an electron beam, purchased from Semi-Elements, Inc. No general analysis of this material has been made. The content of iron in the sample SE-I, determined in this laboratory, was found to be 0.01 wt.%. Sample L-I for the susceptibility measurements was cut from a cast gadolinium obtained from Lindsay Chemical Division in as received condition. According to the supplier this material contained the following impurities expressed in wt.%: molybdenum 1.2, oxygen 0.15, nitrogen 0.03, and iron 0.02. Sample M-I was fabricated from gadolinium which had been arc-melted for about IO min. The original metal, according to the supplier, contained 0.35 wt.% nickel, 0.2 wt.% oxygen, 0.1 wt.% iron and silicon, respectively, 0.02 wt.% calcium, and less than 0.01 wt.% copper and tantalum. The content of iron on the arc-melted material was found to be 0.02 wt. o/O.The material from which Sample M-3 was machined was made from the arc-melted stock by remelting it together with 0.50 wt.% iron in an arc-melter. Chemical analysis confirmed this amount of iron in the sample and showed that the iron was uniformly distributed. The susceptibility sample RC-I was cut from a gadolinium rod which was used earlier for electrical resistivitys and thermal conductivity4 studies from liquid helium temperatures to about 300°K. The starting material was obtained from Research Chemicals. The details of preparation of the polycrystalline gadolinium rod and its analysis have been given elsewheres. The magnetic susceptibilities were determined with the apparatus described befores. Each susceptibility sample, wrapped in a tantalum foil, was sealed into a silica capsule containing argon at a pressure of 15 cm Hg at 298°K.

RESULTS AND DISCUSSION

Figure I shows the inverse magnetic susceptibility of the samples Se-r, L-I, and RC-I as a function of increasing temperatures. The data are completely consistent with the localized 4felectron theory1 except for the small anomalies between 700 and 800°K. Originally it was suspected that certain impurities may be responsible for these observations. However, the possibility that the anomalous behavior may be a characteristic of pure gadolinium could not be entirely neglected. Later, however, we realized that the most probably cause for these anomalies is the presence of small amounts of FezGd in gadolinium. The anomaly in the susceptibility arises because the Curie point of FezGd is in the range of the above-mentioned temperatures. According to studies at Research Chemicals6 the Curie temperature of FezGd is 743°K while WALLACE and his co-workers7 give it to be 825°K. The analyses of Samples L-I and M-I clearly indicate the presence of small amounts of iron. Apparently even quantities of order of a few hundredths wt.% iron do not dissolve in gadolinium, say, at room temperatures but form the intermetallic compound FezGd. This is consistent with the iron-gadolinium phase diagram established by COPELAND AND KATO~ but is not in accordance with the phase diagram proposed by NOVY et a1.9V10.If, indeed, the magnetic transition of FezGd is responsible for the small anomalies in the above-described gadolinium samples, then, by deliberately contaminating gadolinium with larger amounts of iron, we should anticipate considerably larger anomaly. For this reason, Sample M-3 was produced. The magnetic J. Less-Common

Metals,

7 (1964)

336-340

338

S. ARAJS, R. V. COLVIN

300 400 500 600 700 800 900 loo0 II00 12001300 1400 1500 7 PKl Fig. I. Inverse magnetic

susceptibility of Samples SE-I (lower curve), curve) as a function of temperature.

+

SAMPLE

L-r,

and RC-I

(upper

M-l

TEMPERATURES) 0

SAMPLE

M-3

(DECREASING

TEMPERATURES)

Fig, 2. Inverse magnetic

susceptibility

of Samples M-r and M-3 as a function J. Less-Common

Metals,

of temperature. 7 (1964) 336-340

MAGNETIC

SUSCEPTIBILITY

OF GADOLINIUM

339

susceptibilities of Samples M-I and M-3 are shown in Fig. 2. It is clearly observable that the small anomaly in Sample M-r due to 0.02 wt.% iron is indeed very noticeably increased in M-3 by the presence of 0.50 wt.% iron. It is significant that the anomalies in both samples occur at the same temperature, i.e., at about Soo”K, which is quite close to the Curie temperature of FezGd. The magnetic susceptibilities shown in Fig. I were obtained in a magnetic field H = 1359 Oe and with a magnetic field gradient dH/dz = 164.4 Oe cm-l, while those presented as Fig. 2 with H = 905.9 Oe and dH/dz = 109.6 Oe cm-l. The above interpretation about the role of FezGd upon the paramagnetic susceptibility of gadolinium is also supported by the electrical resistivity measurements on some of the susceptibility samples. The eutectic isotherm, which according to COPELANDAND KATO~ occurs at III~OK, exhibits a noticeable anomaly in the resistivity VS. temperature curve. However, practically no anomaly in the electrical resistivity is observed in the neighborhood of 800°K. It should be mentioned that the impurity effect also satisfactorily explains the anomalies observed in our earlier workl. There I/X VS.T curves for four gadolinium samples of different purities are presented. The largest anomaly, of very similar form to that of Sample M-3, occurs in the sample No. 4 with the largest iron content (0.06 wt.%). Nickel and cobalt also form intermetallic compounds with gadolinium. According to the presently available phase diagrams on gadolinium-nickel8~10~1~, the intermediate phase at the gadolinium-rich end is NiGds. The Curie point of NiGds is 283°K. The only known phase diagram of gadolinium-cobalt system is due to NOVY et al.10p12. Their studies show that the intermediate phase at the gadolinium-rich end is CoGds having the Curie point at 263°K. Since none of these intermediate phases have their Curie temperatures between 700~ and 8oo”K, the anomalies in our gadolinium samples cannot be due to possible small amounts of nickel and cobalt as impurities. After this investigation was completed, NIGH et al.l3 published the magnetization and electrical resistivity studies of gadolinium single crystals. Their susceptibility measurements along the c- and a-axes between 350” and 875°K show no anomalous behavior in this temperature range. The content of iron in their gadolinium is < 0.01 wt.“/,, i.e., lower than in our samples. This observation supports our suggestion that the above-described anomaly in the magnetic susceptibility of gadolinium at elevated temperatures is indeed an impurity-induced effect. ACKNOWLEDGEMENTS The authors are grateful to the following members of this Laboratory: G. WRAY for technical assistance, G. W. MOMEYERfor chemical analyses, and D. S. XILLER and R. P. SMITHfor helpful discussions. We also would like to acknowledge stimulating conversations with P. A. BECK of the University of Illinois.

REFERENCES S. ARAJS AND R. V. KLEBER (ed.), Rare

COLVIN,

Earth

Paramagnetism of polycrystalline rare earth metals. Research, MacMillan, New York, 1961, p. 178; J. A&%.

In E. V.

Phys., 32

(1961) 336s.

V. I. CHECHERNIKOV,

Phys. Metals Metallogr., 13 (3) (1962) 134. J.

Less-Common

Metals,

7 (1964)

336-340

340

S. ARAJS, R. V. COLVIN

s R. V. COLVIN AND S.ARAJS,Plays. Stat. Sol., 4 (1964) 37. 4 S. ARAJSAND R.V. COLVIN,J. A#. Phys.,35 (1964)1043. s S. ARAJS AND R. V. COLVIN,J. Phys. Chem. Solids, 24 (1963) 1233. 6 Research Chemicals, Division of Nuclear Corporation of America, Electr&zb asd Mcxgnstic Properties of Rare Earth Elements, Alloys, and Compounds, First Quarterly Report for thePeriod February 15 to May 15. 1960, Wright Air Development Division, Air Research and Development Command, Wright-Patterson Air Force Base, Contract No. AF 33(616)-7012, Task No. 73710. 7 K. A. GSCHNEIDNER,Jr., Rare Earth Alloys, Van Nostrand, New York, 1961, p. 186. 8 M. COPELAND AND H. KATO, Gadolinium alloys of iron, chromium, nickel, and stainless steel. In J. F. NACHMAN AND C.E.LUNDIN (eds.), Rare E&h Research, Gordon and Breach, New York, x962, p. 133. Q V. F.NovY, R.C. VICKERYAND E.V. KLEBER, Trans.Met.Soc.AIME, 221 (1961)580. 10C. E. LUNDIN, Rare-earth metal phase diagrams. In F. H. SPEDDINGAND A. H. DAANE (eds.), The Rare Earths, Wiley, New York, 1961, p. 224. 11 V. F. NOVY, R. C. VICKERY AND E. V. KLEBER, Trens. 1Met. Sot. AIME, 221 (1961) 585. 12 V. F.NovY, R. CVICKERYAND E.V. KLEBER, Trans. Met.Soc. AIME, 221 (1961)$38. 1s H.E.NIGH,S.LEGVOLD AND F.H.SPEDDING, Phys.Rev.,x32 (x963) 1092. J. Less-Common

Metals, 7 (1964) 336-340