ELECTRONIC STRUCTURE AND 121Sb HYPERFINE FIELDS IN THE HEUSLER ALLOYS Ni1-xCuxMnSb

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ELECTRONIC STRUCTURE AND 121Sb

HYPERFINE FIELDS IN THE HEUSLER ALLOYS Ni1-xCuxMnSb

L. Swartzendruber, B. Evans

To cite this version:

L. Swartzendruber, B. Evans. ELECTRONIC STRUCTURE AND 121Sb HYPERFINE FIELDS IN

THE HEUSLER ALLOYS Ni1-xCuxMnSb. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-

265-C6-268. �10.1051/jphyscol:1974639�. �jpa-00215796�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 35, Dicembre 1974, page C6-265

ELECTRONIC STRUCTURE AND 121Sb HYPERFINE FIELDS IN THE HEUSLER ALLOYS Nil

,Cu,MnSb

L. J. SWARTZENDRUBER

Institute for Materials Research, National Bureau of Standards, Gaithersburg, Maryland 20760, U. S. A.

and B. 3. EVANS (*)

Department of Geology and Mineralogy

The University of Michigan, Ann Arbor, Michigan 48104, U. S. A.

R6sumB. - Le champ magnetique hyperfin Hn mesurC a 4,2 K au site de Sb dans les alliages Nil-,Cu,MnSb (0 < x < 1) de structure Clb augmente de 296 kG pour x = 0 a 504 kG pour x = 0,6. Hn diminue rapidement pour x < 0,7 et est alors caracterise par une large distribution gaussienne

de Hn. Pour x = 0,8 par exemple, < Hn > vaut 327 kG et

130 kG. En utilisant le modkle recent de Blandin et Campbell base sur I'interaction RKKY, on peut montrer que I'aug- mentation de Hn avec x pour x < 0,7 provient essentiellement de I'augmentation de la concentration electronique. Une explication analogue pourrait Bgalement rendre compte de la valeur de 600 kG de Hn au site de Sb dans PdzMnSb si l'on admet une contribution de 0,25 electron du Pd a la bande de conduction. Les champs hyperfins eleves au site de Sb dans ces alliages sont en accord avec les previsions de modkles thkoriques connus.

Abstract. - At 4.2 K the magnetic hyperfine field, Hn, at Sb in the C~I, structure alloys Nil-,Cu,MnSb, 0 < x c 1, increases from 296 kG at x = 0 to 504 kG at x = 0.6. For x < 0.7, Hn decreases rapidly and is characterized by a broad Gaussian distribution. For example, at x = 0.8 the average Hn is 327 kG and the width of the distribution,

is 130 kG. Using the recent model of Blandin and Campbell based on the RKKY interaction, the increase of Hn with x for x < 0.7 can be shown to arise primarily from the increase in electron concentration. A similar explanation might also apply to the 600 kG Hn at Sb in PdzMnSb if it is assumed that Pd contributes 0.25 electron to the conduction band. The large hyperfine fields at Sb in the above alloy systems appear to be consis- tent with the predictions of extant theoretical models.

1. Introduction. - Heusler alloys, L2, structure type, and alloys having the related C1, structure type have received considerable attention in connection with experimental and theoretical studies of the origin of magnetic hyperfine fields at non-magnetic impurities in magnetic hosts [l-61. These alloys continue to be of interest, firstly, because until very recently most theore- tical attempts have been unable to account for either the magnitude or the sign of the magnetic hyperfine field, Hn, and secondly, because recent experimental determinations of Hn have changed by almost a factor of two the range of values for H,, that must be account- ed for theoretically.

Swartzendruber and Evans [7] have reported earlier the observation of an anomalously large Hn of 580 kG at the Sb site in Pd,MnSb. Hn at Sb in PdzMnSb is almost a factor of two larger than that observed in other Heusler alloys, X2MnSb, and in the related C1, alloys, XMnSb. Before the present study, it was not

(*) Alfred P. Sloan Research Fellow.

certain whether the large Hn at Sb in Pd2MnSb was due to some idiosyncrasy of the electronic structure of Pd2MnSb or if it was due to some systematic variation in electronic structure of which Pd,MnSb represented one of the extrema. It was also apparent that an Hn of this magnitude for a 5sp impurity was beyond the pre- dictive scope of extant theoretical models at the time of its discovery and that it signalled some fundamental changes in theory if it were part of some general systematics.

Following indications from the NMR measurements of Endo, Shinogi, and Kimura [8] that the substitution of Cu for Ni in NiMnSb leads to large increases in Hn at Sb, the magnetic hyperfine field at Sb has been deter- mined for alloys with composition Nil-,Cu,MnSb.

We confirm the previous NMR measurements for x < 0.5, find a different dependence of Hn on x for x > 0.5, and extend the range of measurements to higher x values. Again, hyperfine fields greater than 500 kG are observed. We propose that these large fields at Sb in Nil -,Cu,MnSb are due primarily to the larger

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974639

C6-266 L. J. SWARTZENDRUBER AND B. J. EVANS

electron concentration (as measured by the elec- tron/atom ratio) of these alloys relative to other Heusler alloys. A similar explanation is proffered for the large Hn at Sb in Pd,MnSb. Further, under the assumption that the electron concentration strongly influences the magnitude of H n , we demonstrate that the large hyperfine fields at Sb in the above-mentioned alloys are qualitatively consistent with a recent formu- lation of the RKKY interaction by Blandin and Campbell [9].

2. Experimental. - Alloys in the solid solution series Nil -,Cu,MnSb were prepared for x = 0.0,0.50, 0.60,0.70,0.80,0.85,0.90, and 1.0 by induction melting in a gettered argon atmosphere of quantities of Ni, Cu, Mn, and Sb corresponding to the desired stoichiometry.

The resulting ingots were placed in evacuated and sealed vycor tubes and homogenized at 873 K for 144 hours and allowed to cool in the furnace.

All alloys were examined metallographically and determined to be single phase. X-ray analyses were performed using Ni-filtered CUKE radiation and a Norelco powder diffractometer. The single phase character of the alloys was verified and all samples exhibited powder diffraction patterns consistent with the C1, structure. The lattice constants were in reaso- nable agreement with those previously reported by Endo [lo]. Indications of a small amount of Cu-Sb or Cu-Mn disorder for 0.5 < x < 0.7 were provided by the Ioss of the a, a, resolution for the (31 1) diffraction peak.

Mossbauer samples were prepared by mounting 300 mg of the powdered alloy in a 2.5 cm plastic disc.

Transmission spectra of all alloys were obtained at 4.2 K using a 1 2 1 ~ n / ~ a S n 0 , source which was also cooled to 4.2 K. The spectrometer, which has been described previously [ll], was calibrated using a 5 7 ~ o / P d source and pure iron foil. Isomer shifts were determined relative to an InSb absorber. The data were analyzed by means of computer-assisted least-mean- squares fitting techniques. The lines in the spectra were assumed to be Lorentzian in shape and their relative intensities were constra'ined to those appropriate in the thin absorber limit. The fitted linewidths varied from 3.5 to 4.5 mm/s for the alloys and the linewidth for InSb was 3.6 mm/s.

3. Results. - For 0 < x < 0.7, satisfactory fits could be obtained using a magnetic hyperfine pattern with a single value for H,. For x = 0.8 and x = 0.85, satisfactory least-squares fits could not be obtained with a single value for H, ; and the spectra were fitted to a Gaussian distribution of hyperfine fields with a width

b ~ , using techniques developed earlier [12]. Spectra typical of these cases are shown in figures l a and 1b.

For x = 0.9 and x ⁼ 1.0 the spectra are not suffi- ciently resolved to distinguish between a single valued hyperfine field and a hyperfine field distribution. For these two samples, a linewidth of 3.5 mm/s was

L-

-

I

'

VELOCITY ( mrn/s)

FIG. la. - 12lSb Mossbauer spectrum of Nio.4Cuo.sMnSb at 4.2 K. The points are the experimental spectrum and the solid line is the result of a least squares fit of magnetic hyperfine

pattern with a single value for Hn.

VELOCITY

( m m h )

FIG. lb. - 121Sb Mossbauer spectrum of Ni0.2Cuo.sMnSb at 4.2 K. The points are the experimental spectrum and the solid line is the result of a least squares fit of a magnetic hyperfine

pattern with a gaussian distribution for Hn.

assumed and a fit to a single valued hyperfine field was made. Values for the fitted parameters are pre- sented in table I. The linewidths of the fitted spectra are observed to increase from 3.5 mm/s for NiMnSb to 4.6 mm/s for Nio ,,Cue .,MnSb ; this line broadening correlates with the disorder indicated in the X-ray diffraction pattern. The isomer shift as shown in figure 2 has neither a significant nor systematic dependence on the value of x.

12'Sb MijSSbauer parameters of Ni, -, Cu,MnSb at 4.2 K

x Hn 6 rexp ^OH,

(w. r. t. InSb)

(kc) (mmls) (mmls) (kG)

(") Constrained to this value during fitting.

ELECTRONIC STRUCTURE AND lzlSb HYPERFINE FIELDS IN THE HEUSLER ALLOYS Nil -%CuzMnSb C6-267

4. Discussion. - As shown in table I and figure 3, Hn reaches a maximum value of 504 kG at x = 0.6, remaining essentially unchanged up to x ⁼ 0.7, and decreasing rapidly thereafter. This decrease, as well as the distribution in fields for x > 0.7, is believed to be due to complex magnetic structures arising during the transition [lo] from ferromagnetic order to the anti- ferromagnetic order found in CuMnSb.

1 4 - 1 3 -

13

v, c 1 2 . -

+ ; ^{1 1 -}

-

0 -

\

E 9 - E ud

8 7

only about twenty percent. It is not expected that a calculation using the model of Blandin and Campbell would be applicable for x > 0.7 since no provision has been made for the appearance of antiferroma- gnetism.

These results suggest that the large increase in Hn on going from PdMnSb to Pd,MnSb [7] might also be due to an increase in the electron concentration. In order to account for the magnitude of Hn in Pd,MnSb / * with the assumption that the electron concentration or

/ / electron-atom ratio plays a decisive role in determining

/ / the magnitude of H,, at lZ1Sb in these alloys. The elec-

/

/ tron to atom ratio is 1.5 for NiMnSb and 1.75 for

/ /

. CuMnSb, assuming the Hume-Rothery valences for Ni,

/ Cu, and Sb and a moment of 4 ^p, for Mn. The depen-

I;</:'* dence of Hn on the electron concentration can be quantified using the model of Blandin and Camp-

/ bell [9] for the RKKY interaction in Heusler alloys.

/ / The dashed line in figure 3 is generated by employing

-

eq. (8) of ref. [9] and normalizing it to 300 kG in

, , , a x , , , ,

on this basis, each Pd atom should contribute about 0.25 electron to the conduction band. The non-zero hyperfine field observed for CuMnSb is not compatible with the antiferromagnetic structure proposed by Forster et al. [14]. The non-zero value could arise either from a small amount of disorder or from a slightly canted spin structure. The observed spectrum for CuMnSb can be fitted equally well by a single hyper- fine field with slightly broadened lines or by a distri- bution of fields with an average value of zero.

0 1 2 3 4 5 6 7 8 9 1 NiMnSb. In addition to the assumptions of ref. [9], we

x ~n C U ~ _ ~ N I ~ M ~ Sb further assume that the Ni contributes no electron to the conduction band ; that Cu contributes one electron

2. - 121sb isomer shifts, 6, for Nil-zCuzMnSb at 4.2 K. to the conduction band and that the electron concen-

The dashed line simply connects the isomer shifts at x = 0 tration varies linearly with x. The functional form of the

and x = 1. dependence of Hn on x is qualitatively reproduced by this calculation and the quantitative disagreement is

Concentration, x

FIG. 3. - Magnetic hyperfine field at 121Sb as a function of x in Nil-,CuzMnSb at 4.2 K. Filled circles represent Mijssbauer results found in this work and the squares the NMR results of reference [8]. The shaded region indicates the width of the hyperfine field distribution. The dashed line represents the results calculated using the theory of reference [9] (normalized to give the

observed value at x

0).

The increase in Hn with increasing x is not due to changes occurring exclusively in the local electronic structure of the Sb site since the isomer shift, cf.

figure 2, shows no significant dependence on x . I t is unlikely that there are significant variations in the isomer shift that are less than the precision of our measurements since the lZ1Sb isomer shift is rather sensitive to small changes in electron density and has been observed to vary by as much as + I mm/s for the same nominal oxidation state of Sb in very similar compounds [13].

The above results for Nil -,Cu,MnSb are consistent

5. Conclusion. -The increase in Hn at Sb with increasing x in Nil -,Cu,MnSb is due to an increase in the electron concentration arising from the contri- butions of electrons to the conduction band by Cu.

The magnitude of Hn and its functional dependence on x are in qualitative agreement with the Blandin and Campbell RKKY model in which the domi- nant contribution to Hn at the Sb site is due to conduc- tion electron polarization. A similar explanation might also apply to Hn at Sb in Pdz-,MnSb. For x > 0.7 in Nil-,Cu,MnSb comparison with theory is difficult since variations in H, are due primarily to the transition from ferromagnetic to antiferromagnetic order.

Acknowledgments. - B. J. Evans gratefully acknow-

ledges the partial support of his effort in this study

by the Alfred P. Sloan Foundation. The able assistance

of Dave Fickle with sample preparations is cheerfully

acknowledged. R. D. Robbins provided technical

assistance.

L. J. SWARTZENDRUBER AND B. J. EVANS

References

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