MÖSSBAUER STUDY OF A VERY DILUTE Pt(Fe) ALLOY AT VERY LOW TEMPERATURES

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MÖSSBAUER STUDY OF A VERY DILUTE Pt(Fe) ALLOY AT VERY LOW TEMPERATURES

M. Scherg, E. Seidel, F. Litterst, W. Gierisch, G. Kalvius

To cite this version:

M. Scherg, E. Seidel, F. Litterst, W. Gierisch, G. Kalvius. MÖSSBAUER STUDY OF A VERY DILUTE Pt(Fe) ALLOY AT VERY LOW TEMPERATURES. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-527-C6-531. �10.1051/jphyscol:19746112�. �jpa-00215724�

JOURNAL DE PHYSIQUE Colloque C6, supplément au no 12, Tome 35, Décembre 1974, page C6-527

MOSSBAUER STUDY OF A VERY DILUTE Pt(Fe) ALLOY AT VERY LOW TEMPERATURES (*)

M. SCHERG, E. R. SEIDEL (**), F. J. LITTERST W. GIERISCH and G. M. KALVIUS Physik-Department der Technischen Universitat München

D 8046 Garching, James-Franck-Str., BRD

Résumé. - Le comportement magnétique d'un alliage Pt(Fe) très dilué ( < 50 ppm) a été étudié par spectroscopie Mossbauer jusqu'à ^0,06 K et dans des champs externes compris entre O et ⁵⁰ kOe.

A basse température, la susceptibilité locale initiale, ainsi que la variation du champ hyperfin à

saturation en fonction du champ appliqué, indiquent une faible réduction du moment du Fe. Un éclatement magnétique hyperfin est observé en dessous de ^0,2-0,l K, même en l'absence de champ appliqué. Ceci est attribué à une interaction hyperfine paramagnétique, observable à cause d'une relaxation électronique lente.

Abstract. - The magnetic behaviour of a very dilute ( < 50 ppm) Pt(Fe) alloy has been studied by Mossbauer spectroscopy down to ^0.06 K and in external fields between ^O and ⁵⁰ kOe. At IOW temperatures both the initial local susceptibility and the dependence of the saturation hyperfine field on applied field indicate a small reduction of the Fe moment. A magnetic hyperfine splitting is seen below ^0.2-0.1 K, even in the absence of applied field. This is interpreted as a paramagnetic hyperfine interaction observable because of slow electronic relaxation.

1. Introduction. - The existence of giant magnetic moments localized at the site of the Fe atom in dilute Pt(Fe) alloys is well established experimentally. It is explained by a ferromagnetic exchange coupling of the impurity moments with localized Pt moments via the conduction electrons. In this way a rather sizeable cloud of polarized spins is formed around the impu- rity [l-31. Similar observations were made in other alloy systems such as Pt(Co) [2, 41, Pd(Fe) [l, 21 and Pd(Co) [l, 2, 51. Even at relatively low Fe concentra- tions the regions of polarized spins may overlap leading to a ferromagnetic order for these alloys below the critical temperature which depends on the impurity concentration [6].

Recent investigations of the magnetoresistance and of the hyperfine (hf) field at the impurity nucleus [7, 81 in 500-1 000 ppm Pt(Fe) alloys show free spin behaviour of the giant magnetic moments for T > 1.5 K.

In the very low temperature regime, however, charac- teristic deviations from free spin behaviour were observed by Mossbauer spectroscopy [3] down to 0.3 K. The observed data were grossly similar to those obtained with the classical Cu(Fe) Kondo alloy system 19, IO].

The purpose of this work is to extend these measure- ments to still lower temperatures in the millidegree

(*) Work supported in part by Bundesministerium für Forschung ^und Technologie.

(**) Now at the Max-Planck-Institut für Plasmaphysik, D 8046 Garching, BRD.

region using samples of even higher dilution ratios in order to study the local susceptibility and the impurity moment. At these low temperatures the observation of paramagnetic hf splittings in the absence of an applied magnetic field is also expected. One may estimate [ I l ] that for 3d impurities in metals the relaxation frequency will roughly equal the hf inter- action frequency around 0.1 K.

2. Experimental. - In order to achieve sufficient high dilution ratios the Pt(Fe) alloy was prepared as the source in a Mossbauer transmission experiment.

Radioactive 57Co was deposited in the form of carrier-free 57CoC12 onto a Pt foil (4N). The activated foil was then reduced in a purified H2 Stream for 112 hour at 800 OC and subsequently annealed for 16 hours at 1.100 OC under pure hydrogen atmosphere.

The foil was then quenched to room temperature at a rate of 100 OC/hour. The estimated mean magnetic impurity content was 8 ppm 57Co and 33 ppm 57Fe which was largely contained as impurity in the original Pt foil. This source was attached to a piece of very pure copper at the bottom of the mixing chamber of a 3He/4He-dilution refrigerator using a GaIn solder.

A magnetic field up to 60 kOe could be applied parallel to the y-ray direction by a superconducting coil. There- fore, the AI, = O hf transitions will not be observed.

A single line absorber of potassiumhexaferrocyanide (0.25 mg Fe/cm2) vibrated sinusoidally outside the cryostat at room temperature, is used for the detection

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746112

C6-528 M. SCHERG, E. R. SEIDEL, F. J. LITTERST, W. GIERISCH AND G. M. KALVIUS of the Mossbauer spectrum of the alloy source. The

magnetic stray field at the absorber is less than 3 kOe at the largest external fields used thus leading to only a small line broadening.

3. Results. - Without an external magnetic field (He,,) applied, the spectrum consists of a single Lorentzian line of FWHM = 0.302 & 0.002 mm/s at room temperature. This line broadens continuously on approaching low temperatures reaching

FWHM = 0.361 f 0.002 mm/s

at 4.2 K. Satellite lines with increasing amplitudes and resolution appear below 0.2-0.1 K in addition to a further broadening of the center peak. The spectrum ends with a complex hf pattern at lowest temperatures.

An example is shown in the top spectrum of figure 1.

In the very low temperature region a substantial hf field at the site of the Fe ion is detected upon applica- tion of rather small external fields. The spectra taken

FIG. 1. - Set of spectra of Pt(Fe) taken at the lowest tempera- tures and various external fields H.

above 0.2-0.1 K consist of 4 well-resolved Lorentzian lines as expected in Our geometry. Below the tempera- ture of approximately 0.1 K two additional lines appear near the expected positions of the forbidden AIz = O transitions at external fields below a few kOe.

These lines are rapidly reduced in intensity upon increasing the external field into the range of - ^{2.5 kOe.}

Typical examples are shown in figure 1.

In the whole temperature range from 4.2-0.06 K a strong dependence of experimental linewidths on temperature and magnetic field is observed which is substantially more pronounced for the two outer resonance lines corresponding to the 1, = 4 + 4

and - 9 ^-+ - 3 nuclear transitions. For constant field the outer linewidth increases with increasing Hext/T and reaches a well-defined maximum at

-

Most spectra could be fitted with a normal 4-line magnetic hf pattern with different linewidths for the inner and the outer line pairs. Al1 other spectra were analyzed using a pattern of six Lorentzians, again allowing for different linewidths for the different pairs of lines. The spectra for He,, = O in the very low temperature regime could not be fitted at all.

The line broadenings, just described, are explained by spin-lattice relaxation : fast rates exist for small Hext/T values and slow rates for large Hex,/T values (static regime). Both cause little broadening in contrast to the intermediate region where the relaxation rates are comparable to the Larmor frequency of the Fe nuclear moment. This relaxation behaviour exists even at very low ternperatures (see Fig. 1). Such a behaviour has already been observed earlier in Mo(Fe) alloys, but with much poorer resolution [12]. It can be explained within the framework of common relaxation theories 1131 (Gotze, W., private communication).

In figure 2 the hf fields (the external field has been subtracted) are given as a function of external field He,, for various temperatures.

The hf field data at 4.2 K are well described by a Brillouin function :

Hhf/Hsat = < ^Sz >lS ⁼ BS(~Hext/kT) with H,,,, S, and ⁼ gSpB left as free parameters in a computer fit. Note that < ^S, >, the time average component of S along He,,, is only sufficiently defined at rather high spin-lattice relaxation rates. Our results for H,,, ⁼ 314.5 ^f 0.3 kOe, S = 2.34 f 0.25, and

p = 6.31 + ^{0.24 p,} are in good agreement with those of reference [3] for a 1 000 ppm Pt(Fe) alloy. Generally these values should be considered with care since full saturation of the Fe moment could not be achieved at 4,2 K. The reciprocal initial susceptibility

is plotted in figure 3. Data below T = 0.3 K were not included since non-linear effects did not allow a

MOSSBAUER STUDY OF A VERY DILUTE Pt(Fe) ALLOY AT VERY LOW TEMPERATURES C6-529

FIG. 2. - Hyperfine fields at Fe site in Pt(Fe) as a function of external field H for various temperatures.

_ 1 ^{P t (Fe) '} 1

FIG. 3. ^- Reciprocal initial local susceptibility

xz ^{= -} i ~ ~ ~ n l ~ l H ' , o of the Fe moments in Pt(Fe).

reliable deduction of the initial susceptibility even for very weak external fields. The initial susceptibilities of figure 3 follow closely a Curie-Weiss law with a para- magnetic Néel temperature of 8 = 0.10 f 0.04 K.

Curie-Weiss behaviour may arise either through antiferromagnetic order of iron clusters in the Pt matrix or through Kondo-like behaviour of the impu- rity spins [14-161. In figure 4 the hf field is plotted as a function of He,,/T for various values of He,,. The destruction of a spin compensated state is clearly visible, but, even for very small fields, the deviations from free spin behaviour are much smaller for Pt(Fe)

FIG. 4. - The hyperfine field for Pt(Fe) as a function of H / T for various external fields H. The dashed lines represent some of the experimentally determined initial slopes of the impurity magnetization which shows strong nonlinear effects for rather

low H.

P t (Fe)

FIG. 5. ^- Very low temperature saturation hyperfine field at the Fe nucleus in Pt(Fe) as a function of the extemal magnetic

field H.

than for other Kondo systems [9, 10, 161. Figure 5 finally shows the low temperature saturation hf field (H,,,/T large) as a function of H ,,,. Since H,,, is consi- dered to be proportional to the moment localized on the Fe atom itself, these results imply that this moment is reduced by lowering the temperature.

Nuclear orientation measurements [17] on 57Co down to 12 mK were conducted on a small piece of the Pt(Fe) foil used as the source in the present Mossbauer experiments.

These investigations yielded hf fields at the Co site which agree well with earlier results [18] on a Pt foil containing only 1 ppm of 57Co. NO indication of

C6-570 M. SCHERG, E. R. SEIDEL, F. J. LITTERST, W. GIERISCH AND G. M. KALVIUS magnetic order could be detected ! The external applied

field was between 2.6 and 29.7 kOe.

4. Discussion. - Our results for the initial local susceptibility as compared to typical Kondo systems like Cu(Fe) [9, 10, 14, 151 show that the Kondo temperature in very dilute Pt(Fe) is certainly far below 0.1 K. The Curie-Weiss temperature 6 ^N 100 It 40 mK derived from the local susceptibilities (see Fig. 3) should be considered an upper limit since a light curvature in the magnetization curve for small values He,, (see Fig. 2) will result in too big a value of theinitial 1 / ~ , ~ , . Measurements of the initial local susceptibility of a 70 ppm Pt(Fe) alloy by Bishop et. al. [19] in exter- na1 fields of only 250 Oe, 10 Oe, and 0.26 Oe down to 10 mK yielded a Curie-Weiss law with û = 38 mK, 5 mK, and 2 mK, respectively. These data are basi- cally in good agreement with Our findings and also predict a Kondo-like behaviour with an extremely low Kondo temperature. One could argue that a strong ferromagnetic polarization around the impurity moment lowers the Kondo temperature when compared with a normal alloy system like Cu(Fe) [9, 101. Such a dependence of Kondo temperatures has been observed earlier for NiRh(Fe) ailoys [20] where an increasing Ni content produces a larger Fe giant moment and correspondingly smaller deviations from free spin behaviour.

A logarithmic dependence of H,,, vs. He,, is obtained by a perturbation calculation of Giovannini 1211 and Ting [22] for S = 5 and a half filled, simple conduction band well above the Kondo temperature and for pHex, 4 kT. This behaviour is in qualitative agree- ment with the present data for Pt(Fe) for He,, bet- ween 2.5 kOe and 10 kOe. It should be kept in mind, however, that this mode1 is based on largely oversimpli- fied assumptions.

The proof for the existence of a Kondo ground state is intimately connected with the question whether the present sample actually represents a paramagnetic alloy containing only non-interacting magnetic impu- rities. Antiferromagnetic alignment has been deduced by resistivity measurements in more concentrated alloys (1 000-10 000 ppm Fe) 1231. On first sight, the splittings observed in the present spectra below

0.2-0.1 K for He,, = O kOe and the additional reso- nance lines in small external fields below the same temperature region might also be interpreted as arising from magnetic order of clusters of Fe impu- rities. In fact, we cannot unambiguously exclude this possibility. However, impurity interaction effects are expected to be small in our sample because of the rather low Fe and Co concentration. Assuming a linear dependence of the critical temperature [6, 241 on impurity concentration below 10 000 ppm Fe, the present sample should order far below 20 mK. Fur- thermore the spectra below 0.2 K are rather consistent with the assumption of paramagnetic relaxation effects. In particular, the behaviour seen in figure 1, namely the marked variation of linewidth with He,, and the clearly visible increase of H,, with He,, is untypical for magnetically ordered materials. In addi- tion, the above mentioned nuclear orientation measure- ments support the claim of paramagnetic behaviour.

For He,, > 100 Oe the effective field approximation is valid and the electronic spin splits up into 2 S + 1 equally spaced components. Taking S = 3 (fully aligned d shell) we should observe in the limit of long relaxation times i. e. for T < 0.1 K a second 4-line pattern (S, = - $1 with its outer line positions roughly in the position of the AI, ⁼ O transitions of the main spectrum (S, ⁼ - 5) and finally a third spectrum due to S, = - 4 and so forth with decreasing intensity. With increasing He,, these lines disappear due to vanishing thermal population.

For He,, = O kOe, the electronic spin is degenerate but hf interactions may still occur [Il]. In the limit of long relaxation times a complex hf pattern due to a A . 1. S interaction results. The spectrum could become even more complicated by a small residual magnetic field at the site of the Fe atom due to polarization of its surrounding. More experimental work is required to decide whether we are really dealing with para- magnetic relaxation or magnetic order due to an inhomogeneous distribution of Fe atoms.

Acknowledgment. - We wish to thank W. Koch for help in preparing the source. Helpful discussions with Prof. A. J. Freeman and J. Stohr are gratefully acknowledged.

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