Abstract: The aim of this study is to investigate magnetic properties of an example of half-metallic and ferromagnetic compounds, namely, the half Heusler PtMnSb. From the magnetic measurements, it can be shown a change from a localized magnetism due to the spin waves thermally excited, to an itinerant magnetism with spin fluctuations is to reduce the moments in this compound. Furthermore, the electrical and thermal resistivity measurements were performed. The resistivity results with respect to the magnetic properties are confirmed and a transition of a half-metallic state to a metallic state at 100 K is observed.
INTRODUCTION
The electric transport in the magnetic materials is often strongly modified by the application of a magnetic field. The spin electronics i.e., the phenomena of electronic spin-dependent transport became in some years a very active research field in magnetism. Conduction electrons behaviour depends on the orientation of their spin in relation to the local magnetization of the material. The physical quantities characterizing the transport become dependent on the majority or minority character of the spin. The fundamental studies in the spin electronics domain are generally focused on two themes. One is concerned with the highly spin-polarized materials as the half-metallic ferromagnets. The half-metals discovered by de Groot et al. (1983) are the materials whose band-structure shows a metallic state for up-spin electrons whereas an energy gap occurs at the Fermi level for down-spin electrons. Hence, the conduction electrons are highly spin-polarized at the Fermi level. These systems are therefore exemplary materials for the study of the spin electronics (Pierre and Karla, 2000; Borca et al., 2000; Komesu et al., 2000). The other is about the manganites whose study with colossal magnetoresistance (CMR) permitted the electronic correlations comprehension and the role of the constraints, for epitaxial manganites films (Tokura and Tomioka, 1999; Ramos et al., 2002; Garcia-Munoz et al., 2002; Souza-Neto et al., 2004; Singh et al., 2006).
In this present study, the magnetic properties of a half-metallic compound example are to be investigated in relation with the first theme that we are described, half Heusler PtMnSb. Moreover, electrical and thermal resistivity measurements were performed. These measurements have allowed investigating the effects of spin fluctuations in this compound.
MATERIALS AND METHODS
Preparation and crystallographic properties: PtMnSb polycrystalline samples were prepared by melting of stoichiometric amounts of pure constituents in induction arc furnace under argon atmosphere. The resulting buttons were annealed at 650 to 800°C in argon-filled quartz tubes during 14 days under high vacuum and cooled down to room temperature in a furnace, in order to obtain a good crystallographic order. During arc-melting, the weight losses were less than 0.2% of the total mass. The final achieved sample that we used was a bar of 1.9 mm width and 1.5 mm height with 0.35605 g weight. After annealing, the crystal structure of the samples was examined by X-ray powder diffraction (XRD) using Cu Kα radiation. The X-ray diffraction data show that the ternary equiatomic stannides contained no second phases in investigated 2θ range (20-120°) within the detection limit of the conventional XRD techniques. Hence, this XRD (Fig. 1) shows the existence of one cubic phase with superlattice lines characteristic of half Heusler C1b MgAgAs-type structure. The lattice parameter a = 6.153 Å obtained is in conformity with the results found by another authors (Masumoto and Watanabe, 1970; Hames and Crangle, 1971; Otto et al.,1989a).
| Fig. 1: | X-ray diffraction diagram of PtMnSb at 300 K |
Experimental methods: Magnetic measurements were performed between 5 and 300 K with a supra-conductive bobbin in field below 10 T. For each temperature, a complete magnetization curve M(H) was recorded. Arrott-Below plots (M2 versus H/M) were generally not linear and a quadratic fit was used to obtain the spontaneous magnetization.
The electrical and thermal resistivity (in null field), was measured from 5 to 300 K using the conventional four-probe method with AC current.
RESULTS AND DISCUSSION
Magnetic properties: The PtMnSb alloy is a half Heusler ferromagnetic
phase (Otto et al., 1989a) and half-metallic (de Groot et al.,
1984; de Groot and Buschow, 1986) of witch the Curie temperature is 572 K and
the saturation moment, 4 μB per formula. The half Heusler phases
with MgAgAs-type structure (Fig. 2) have a face-centered cubic
cell, except that one of the X sites is empty, giving a formula XYZ and crystallize
in the C1b structure (space group F
| Fig. 2: | Crystal structure of half-Heusler compounds |
| Fig. 3: | Temperature variation of spontaneous magnetization for PtMnSb.
The continuous line is a T2 fit between 90 and 300 K |
The temperature dependence of the spontaneous magnetization, reported on Fig. 3, presents an anomalous behaviour at low temperature. The spontaneous magnetization Msp witch reaches 4.25 μB per formula at 2 K (Fig. 3) varies linearly as T3/2 below 90 K, like in Heisenberg ferromagnets at low temperature, as one can observe it on Fig. 4. Above 100 K, the square of the spontaneous magnetization obeys to a T2 law which is the classical behaviour for itinerant ferromagnets. Such anomalous behaviour has already been observed in the NiMnSb case (Hordequin et al., 1996, 2000; Pierre et al., 1997; Ristoiu et al., 2000) and has been interpreted as the passage from a localized magnetism due to the spin waves excited thermally to an itinerant ferromagnetism in which the spin fluctuations reduce the moments. This transition can be associated with the half-metallic character of PtMnSb at low temperature, knowing that the Stoner-type spin fluctuations cannot appear in the half-metals. Thus, the magnetic measurements are in conformity with the previous studies (Otto et al., 1989a; Hordequin et al., 1996, 2000; Pierre et al., 1997; Ristoiu et al., 2000). However, the magnetic moment, 4.25 μB per Mn atom, found is somewhat higher than the expected value, 4.0 μB per Mn atom determined from band structure calculations (Hames and Crangle, 1971; Otto et al., 1989a; van Engen et al., 1983; de Groot et al., 1983). The discrepancy could be due to the asymmetry of the compound stoichiometric composition in which Mn could be in higher quantity than the other components of the PtMnSb alloy.
| Fig. 4: | Spontaneous magnetization dependant Temperature for PtMnSb (a) above 100 K and (b) below 90 K |
Electrical resistivity measurements: The PtMnSb resistivity thermal variation is represented in Fig. 5. The anomalous behaviour observed below 70 K confirms the magnetization measurements discussed in the above section. For temperatures lower than 70 K, the resistivity follows a T2 function variation, ρ(T) = A01 + B1T2, with A01 = 22.15 μΩ.cm (residual resistivity) and B1 = 1.43 10-3 μΩ.cm/K2 whereas, ρ(T) = A02 + B2T1.67, with A02 = 24.45 μΩ.cm and B2 = 2.37 10-3 μΩ.cm/K1.67 at temperatures above 100 K. This discontinuity, already carried out in the case of NiMnSb (Hordequin et al., 1996, 2000; Pierre et al., 1997; Ristoiu et al., 2000) was related to a transition from a half-metallic state due to the spin waves below 70 K to a metallic state above 100 K due to the spin fluctuations. The transition temperature seems to be 100 K. Generally, the electrical resistivity of a ferromagnetic material is characterized at T = 0 K by the residual resistivity due to the impurities, crystallographic disorder or crystal defects and above 0 K the magnetic contribution due to spin-disorder scattering (with the spin fluctuations and/or with the spin waves) and electron-photon scattering appears. For spin waves scattering, a conduction electron conserves its spin because the spin waves energy is not sufficient to upturn a single spin. Instead, for spin fluctuations scattering, the spin inversion is possible. At low temperature, the resistivity due to the magnetic spin-disorder contribution varies thermally as T2 function (Mills and Lederer, 1966) while at more high temperature the spin waves give a linear thermal variation and the spin fluctuations give a T5/3 function variation according to Ueda and Moriya (1975). In the present case, at low temperatures, the process of scattering with spin inversion being forbidden, only spin wave contributions to the resistivity can be noticed (the photons contribution being negligible) while above the transition temperature, in addition to the spin waves, one must take into account the spin fluctuations and photons contributions. However, the value 1.67 of the critical exponent β found at T > 100 K, slightly higher than 1.35 found by Hordequin et al. (2000), but in good agreement with the typical spin fluctuations exponent (β = 1.66), indicates that the spin fluctuations contribution is predominant. Furthermore, the residual resistivity 22.15 μΩ.cm obtained, higher than the value found by Otto et al. (1989b) 6.8 μΩ.cm could indicate a likely atomic disorder in our sample.
| Fig. 5: | Resistivity versus temperature for PtMnSb. The lines are T2
fit below 70 K and T1.67 fit above 100 K |
CONCLUSION
The magnetization and electrical resistivity measurements performed on PtMnSb show the same features as the half-metal NiMnSb and prove spin fluctuations existence. These spin fluctuations cause the transition from a localized magnetism in which collective spin-wave excitations are predominant to a high temperature itinerant-like ferromagnetism in which spin fluctuations have the main contributions. On the other hand, this compound exhibits a transition from a half-metallic state to a metallic state between 70 and 100 K. However, complementary measurements of magnetoresistance, anisotropic magnetoresistance (AMR), Hall resistivity and thermoelectrical power are required to better characterize this compound transport properties.
ACKNOWLEDGMENTS
We wish to thank L. Ranno to have permitted the measurements in laboratory of magnetism Louis Néel, CNRS Grenoble, France. Thanks are also given to the director, the staff of this laboratory and Kouacou thesis supervisor J. Pierre for their help.