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MAGNETIC PROPERTIES OF Co-HEUSLER AND
RELATED MIXED ALLOYS
H. Ido, S. Yasuda
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplbment au no 12, Tome 49, decembre 1988
MAGNETIC PROPERTIES OF CO-HEUSLER AND RELATED MIXED ALLOYS
H. Ido and S. Yasuda
Faculty of Engineering, Tohoku Gakuin University, Tagajo, Miyagi, 985, Japan
Abstract.
-
Magnetic and crystallographic measurements were made for Coz-,Fe,MnM (M Si, Ge, Sn and Ga) and C O ~ - ~ M ~ , S ~ . In Co2MnM, the magnetic moment (pp) in the paramagnetic temperature region is generally much smaller than the ferromagnetic moment (pF) at T = 0 K, which implies that the Co-moment disappears above Tc. The other results are also discused.Ferromagnetic Co-Heusler alloys of the type Co2MnM (M = Si, Ge etc.) have been extensively studied by many authors [I-51. One of the present authors [2] pointed out by the susceptibility measure- ments of CoaMnM (M = Si, Ge) up t o 1300 K that the paramagnetic moment per formula unit, pp takes about one half value of the ferromagnetic moment per formula unit, p f , where pf and pp are defined as pf = 2 (2Sco
+
SM,) p~ and pp = 2SpB respectively. In these equations, Sc, and SMn are spin values ofCo-
and Mn-atoms respectively, and S is an averaged spin expressed by equation S ( S
+
1) = 2Sc0 (Sc,+
1)+
S M ~ (Sun
+
I ) , whose rigth hand appears in the Curie- Weiss constant of material which has two kinds of magnetic atoms. To confirm and extend the above results, we have studied CozMnGa, Co2-,Fe,MnM (M = Si, Ge and Sn) and Coz-,Mnl+,Si in addition to Co2MnM (M = Si, Ge and Sn). The Heusler type (L21) alloys in this work were prepared by the sinter- ing method [2], and confirmed t o have the good atomic ordering by X-ray diffraction. Saturation magnetiza- tion (0.) versus temperature curves in this work agree well with the data of Webster [I]. The a,-
T curves drop relatively rapidly in the temperature region just below the Curie temperatures (Tc) for M = Si and Ge. The paramagnetic susceptibilities for CozMnM (M = Si, Ge, Sn) obey well the Curie-Weiss law in the wide temperature regions except for the small parts of the X - r-
T curves as seen in figure 1. Similar results have been obtained also for Co2-,Fe,MnM (M = Si, Ge, Sn). As seen in figure 2, the data for x = 2 is somewhat different from the others. the magnetic pa- rameters obtained are summarized in table I and fig- ure 3. Data similar t o figure 3 are also obtained forFig. 1.
-
X-l vs. T curves for Co2MnM.Fig. 2.
-
X-l vs. T curves for Coz-,Fe,MnSi. M = Ge and Sn. The values of pf and pp are especially noted. Since the averaged spin S in pp (= 25) was defined in the equation S (S+
1) = 2Sco (Sco+
1)+
+SM, (SM,
+
I ) , if Sc, is not zero, the Mn-moment (= 2sMn) in the paramagnetic region becomes smaller than the experimental pp in table I and figure 3. As Mn atoms in the Heusler alloys generally have the mo- ment, 3 4pB, it seems very likely that the Co atoms in CozMnM lose their magnetic moment in the para- TableI.
- Magnetic and crystallog7ulphic data for CozMnM.PP ( m / f -u,) 2.03 2.61 3.35 3.28 pf ( m / f .u.) 5.10 4.66 5.37 4.09 Alloy CozMnSi CozMnGe CozMnSn ConMnGa
a
( A )
5.65 5.74 6.00 5.77 Tc (K) 1034 905 826 695 0, (K) 1044 890 870 770JOURNAL DE PHYSIQUE
Fig. 3. - Tc, ly and pp vs. x in Coz-,Fe,MnSi. magnetic temperature region. According to Webster [I], the Co atoms in ConMnM (M = Si, Ge etc.) have a magnetic moment of 0.75 /lB at T = 0 K. Therefore
the Co-moment would be induced by a moleculer field
of the Mn-sublattice moment below Tc. In figure 3, the pf value coincides with the pp value at x
=
1, which would mean that the condition for the A-site atoms to be magnetically polarized disappears by the change of averaged number of d electrons.The lattice parameter of Co2MnGa has been mea- sured by X-ray diffraction technique as shown in fig- ure 4. No anomaly was observed at Tc, which seems to contradict the polarization of the Co atoms below Tc. To investigate the effect of the atomic disorder such as as exchange of atoms between the A site(Co) and the B site(Mn), we prepared Coz-,Mnl+,Si al- loys. When the composition x is positive, the excess Mn atoms will occupy the A site and the situation will be reversed for the negative
x.
Magnetic data for Coz-,Mnl+,Si are shown in figure 5. Kinks ap- pear at a: = 0 on the curves of pf (2),
Tc
(x)
and the paramagnetic Curie point 0, (x),
which would mean that the atomic ordering of Co2MnSi is good and sta- ble. We also found that several Co2MnSi alloys which experienced various heat treatments, have the same magnetization within the limit of error a t T = 77 K. Therefore the atomic ordering is also stable for tem-Fig. 4.
-
Temperature dependence of lattice constants ofCozMnGa.
Fig. 5.
-
Tc, BP, pf and pp vs. x in Co2-,Mnl+,Si.perature variation in CozMnM. The ,uf us. x curve in figure 5 indicates that the excess Co atoms occupy the B-site ferromagnetically to the matrix magnetization and the excess Mn atoms occupy the A-site antiferro- magnetically t o the matrix magnetization.
[I] Webster, P. J.,
J .
Phys. Chem.Solids
32 (1971) 1221.[2] Ido,
H.,
J. Magn. Magn. Muter. 54-57 (1986) 937.131 Kawakami, M., Kasamatsu, Y., Ido, H., J. Magn. Mugn. Mater. 70 (1987) 265.
[4] Ishida, S., Akazawa,
