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Non-uniform magnetic system driven by non-magnetic ion substitution in CaRu1-xScxO3:Two-component analysis

We have studied magnetic and transport properties in polycrystalline CaRu1-xScxO3 for 0 =<x =<0.20 in order to clarify the substitution effects of a non-magnetic trivalent ion. We find that a ferromagnetic transition with Tc = 30 K is observed in Sc-substituted samples. The composition dependence of the Curie-Weiss temperature implies that the magnetic susceptibility has a paramagnetic contribution with negative theta and a ferromagnetic contribution with positive theta. The field dependence of magnetization at 2 K is also understood as a summation of the ferromagnetic and paramagnetic components. These results suggest that CaRu1-xScxO3 is a non-uniform magnetic system. The relationship between the ferromagnetic ordering and the transport properties is also discussed.


I. INTRODUCTION
The physical properties of CaRuO 3 and SrRuO 3 have been extensively studied because of their contrasting magnetism in the same structure. Both compounds crystallize in an orthorhombic perovskite structure and show a metallic conduction. On the other hand, they exhibit completely different magnetic properties. While SrRuO 3 is well known as an itinerant ferromagnet with the transition temperature T c = 165 K, 1,2 CaRuO 3 shows paramagnetic behavior, in which the magnetic susceptibility obeys the Curie-Weiss law. Due to a high Curie-Weiss temperature (-140 K), one can expect an antiferromagnetic ordering in this compound below around 100 K, but a neutron diffraction and a Mössbauer spectroscopy have revealed that CaRuO 3 has no long-range magnetic ordering down to 1.5 K. 3,4 As such, the magnetic ground state of CaRuO 3 has been controversial, and there are many suggestions in the literature. Felner et al. have found the irreversibility between zero-field-cooled and field-cooled dc magnetization curves and a magnetic sextet in the Mössbauer study for 57 Fe-substituted CaRuO 3 at 4.1 K. 5 From these results, they have proposed that CaRuO 3 is not paramagnetic, but rather in a spin-glass state. However, the AC susceptibility measurements could not give clear evidence of the spin-glass transition. 6 CaRuO 3 is believed to be on the verge of a magnetic ordering. In other words, this compound is readily in a magnetically ordered state resulting from the change of electronic states. Indeed, 5%-Sr and 5%-Na substitutions for Ca induce a spin-glass ordering and an antiferromagnetic ordering, respectively. 7,8 The substitutions for the Ru site in CaRuO 3 have attracted much interest due to their anomalous effects on magnetism. It has been reported that the partial substitution of transition metal ions for Ru induces a magnetic ordering. Surprisingly, this phenomenon occurs regardless of the magnetism of the substituent ions; a ferromagnetic ordering is reported for M = Fe, Mn, Cr, and Ti and a spin-glass state for M = Sn, Co, and Cu. [9][10][11][12][13][14][15] It should be noted here that ferromagnetism is induced by the substitution of a non-magnetic Ti 4+ ion. This result is incompatible with a simple magnetic dilution effect. Since this ferromagnetism is induced by only 2 % of isovalent substitution, He and Cava have suggested that CaRuO 3 is poised at a critical point between ferromagnetism and paramagnetism at low temperature. 12 Recently, Hardy et al. have proposed that Ti-substituted CaRuO 3 is a heterogeneous itinerant ferromagnetic system, 16 but the mechanism of the ferromagnetism is still unclear.
A comparison with other ion substitutions would be useful for clarifying the origin of the ferromagnetism. We focus on how the magnetism changes with the formal Ru valence. A trivalent ion substitution creates a pentavalent Ru 5+ to increase the formal Ru valence, which should be compared with the Ti 4+ substitution. To see this effect clearly, the substitution of a non-magnetic trivalent ion is desirable, because one can exclude a magnetic impurity effects. However, to our knowledge, there are no such studies up to now.
In this paper, we have measured the magnetization, the electrical resistivity, and the Seebeck We have found a ferromagnetic order with T c ∼ 30 K in Sc-substituted samples. Based on the assumption that two magnetic components exist in CaRu 1−x Sc x O 3 , we discuss a possible picture of the ferromagnetic order.  The magnetization measurements were performed by a superconducting quantum interference device magnetometer (Quantum Design MPMS). Magnetization (M) data were collected from 2 to 300 K in the DC field (H) of 0.1 T on field cooling and zero field cooling. The field dependence of magnetization was measured at 2 K in a magnetic field range from -7 to 7 T. Transport properties were measured from 4.2 to 300 K using homemade probes. The electrical resistivity and the Seebeck coefficient were measured using the four-probe technique and the steady-state two-probe method, respectively.

III. RESULTS AND DISCUSSION
Powder X-ray diffraction patterns of CaRuO 3 and CaRu 0.8 Sc 0.2 O 3 are shown representatively in Fig. 1(a), with a calculated pattern of CaRuO 3 obtained with RIETAN-FP. 17,18 The diffraction patterns show that all the prepared samples belong to the orthorhombic perovskite structure of the space group Pnma. Figures 1(b) and 1(c) show the x dependence of the lattice constants and the lattice volume V of CaRu 1−x Sc x O 3 at room temperature, respectively. The change in the lattice parameter a is small, while b and c increase gradually with Sc content x. Correspondingly, the lattice volume continuously increases with increasing x. We expect that Ru 5+ ions are generated by Sc substitution and calculate the lattice volume expected from the formula Ca[Ru 4+ 1−2x Ru 5+ x ]Sc 3+ x O 3 , using the expressions given by where r m is the average ionic radius and r 4+ V I (0.62 ), r 5+ V I (0.565 ), and r 3+ V I (0.745 ) are the ionic radius of Ru 4+ , Ru 5+ , and Sc 3+ , respectively. 19 As shown in Fig. 1(c), the experimentally-observed where C is the Curie constant and θ CW is the Curie-Weiss temperature. According to this expression, an extrapolation of the linear part to H/M = 0 gives a value of θ CW . The extrapolations are shown as the solid lines for x = 0 and 0.20 in Fig. 2(b). As can be seen in Fig. 2(b), θ CW shifts to positive with increasing x, and the sign finally changes at x = 0.20. We should emphasize that this x dependence of θ CW is seriously incompatible with weakly x-dependent T c , because T c is proportional to θ CW within mean-field approximations in a uniform magnet. Thus, this result implies that the system is not uniform; the susceptibility should be understood as an average of a paramagnetic part with negative θ CW and a ferromagnetic part with positive θ CW . Then the sign change in θ CW corresponds to the increase in the volume fraction of the ferromagnetic part. A similar change in θ CW is seen in Ca 1−x La x MnO 3 , in which a ferromagnetic interaction is induced by La substitution. 20 Neumeier and Cohn have pointed out that La substitution causes the magnetic phase segregation in this system because local ferromagnetic regions appears within the antiferromagnetic phase of Based on these speculations, we propose that the magnetic susceptibility for CaRu 1−x Sc x O 3 has two contributions above T c , given by where χ p (T ) and χ f (T ) are the paramagnetic and ferromagnetic parts, respectively. We assume that the ferromagnetic part consists of the generated Ru 5+ ions and the paramagnetic one is originated from the Ru 4+ ions. We further assume that the volume fraction of the two components is identical to that of the Ru 5+ and Ru 4+ ions. Since the chemical formula is Ca[Ru 4+ 1−2x Ru 5+ x ]Sc 3+ x O 3 , we consider that the paramagnetic phase of CaRuO 3 is essentially unaffected with the volume fraction of 1 − 2x. In this assumption, we can regard the paramagnetic part as the magnetic susceptibility of CaRuO 3 , i.e., χ p (T ) = χ(0, T ). Then we can extract the ferromagnetic part χ f (T ) by subtracting the experimentally-observed χ(T) of CaRuO 3 from that of the Sc-substituted samples, using an expression given by Figure 3 shows the temperature dependence of thus obtained χ f . All the data fall into a single curve, supporting the validity the two-component model described by Eq. (4). A broken curve depicts the calculation given by where C f is the Curie constant of the ferromagnetic component and T c is the ferromagnetic transition temperature. The calculated curve with C f = 2.08 emu K/mol and T c = 30 K explains well the experimental data. We employed the value of T c from Fig. 2(a). From the value of C f , we obtain an effective magnetic moment µ eff to be 4.08µ B per formula unit, corresponding to S ∼ 3/2.
This result implies that Ru 5 + ions with S = 3/2 constitute the ferromagnetic component.
To further investigate the two-component analysis, the field dependence of magnetization was measured. We have examined that a demagnetization correction is negligible for all the measure-ments. Figure 4(a)  With increasing Sc, the ferromagnetic hysteresis loop becomes more remarkable and the magnetization increases, reflecting the development of the ferromagnetic order. Note that M increases linearly above 60 kOe without any sign of saturation. This feature is also observed in previous studies of other transition metal ion substitution for Ru. 11,16,22 We assume that M-H curve can be understood as a summation of the ferromagnetic and paramagnetic contribution, as was already discussed above. Then the experimentally-observed magnetization M(x, H) is given by where M f (H) and M p (H) = χ p H are the ferromagnetic and paramagnetic parts, respectively.

Accordingly, the ferromagnetic component M f (H) is given by
The field dependence of M f is shown in Fig. 4  shows metallic conduction (dρ/dT > 0) for CaRuO 3 , while it systematically changes to nonmetallic one (dρ/dT < 0) with increasing Sc content. For example, ρ continues to increase by two orders of magnitude from 300 down to 4.2 K for CaRu 0.80 Sc 0.20 O 3 . We should note that the resistivity increases sharply below 50 K, as seen in the change in the slope of the solid lines in Fig.   5(a). One can attribute the rapid increase in ρ to magnetic domain boundaries. 23,24 When spinpolarized conduction electrons in one domain move to the neighboring domains, they undergo the spin-dependent scattering at the boundaries between domains with different magnetization directions. The temperature dependence of the Seebeck coefficient S is shown in Fig. 5(b). In contrast to the resistivity, the Seebeck coefficient retains metallic against Sc substitution. Since the diffusive term of the Seebeck coefficient is predominantly determined by carrier concentration, the Sc substitution mainly affects not the carrier concentration but the mobility. Moreover, one can see that S increases with x more remarkably at low temperatures, while it is almost independent near room temperature.
To see this clearly, we plot ∆S = S (x)−S (x = 0) in Fig. 6, where ∆S takes a maximum near the transition temperature T c and the maximum increases with increasing Sc content x. The shape of ∆S reminds us of the magnetic susceptibility near T c , suggesting that the spin fluctuation affects the transport properties. It has been theoretically suggested that the spin fluctuation influences the transport coefficients in high-T c copper oxides. 25 The relationship between the ferromagnetic order and the transport properties is indicated in a qualitative manner, and a further study such as magneto-transport needs to be performed. However, the domain size, the distribution pattern, and the segregation mechanism are to be explored. The value of µ eff above T c indicates that Ru 5+ ions generate the ferromagnetic component.
One open issue is that the saturation magnetization is only one-third of S = 3/2. One possibility is that only one-third of Ru 5+ ions contribute to the ferromagnetic order. In this case, the rest of Ru 5+ ions would show paramagnetic behavior and result in the linearly increase of M f at high field.
A second possibility is that the ferromagnetism is of itinerant nature. The discrepancy between µ eff and M s is often observed in itinerant ferromagnets, 26 which is consistent with the moderately conductive behavior. At least we can say that the ferromagnetism observed in the present work is different from that in SrRuO 3 in the sense that the generated Ru 5+ ions are responsible for the order.
At the present stage, the magnetism of CaRu 1−x Sc x O 3 is still enigmatic in some respects. To clarify the picture of the magnetism in more detail, the microscopic investigations such as transmission microscope in the real space and neutron scattering in the momentum space are indispensable.

IV. SUMMARY
We have measured magnetization and the transport coefficients of polycrystalline 20). The ferromagnetic order with T c ∼ 30 K is found in all the Scsubstituted samples. The Curie-Weiss temperature θ CW shows the anomalous composition dependence, implying that the susceptibility has a paramagnetic contribution with negative θ CW and a ferromagnetic contribution with positive θ CW . We actually find that the M/H -T and M-H curves can be understood as a summation of the ferromagnetic and paramagnetic components. Based on these results, we conclude that CaRu 1−x Sc x O 3 is a non-uniform magnetic system consisting of the paramagnetic component originated from CaRuO 3 and the ferromagnetic one driven by Sc substitution. Furthermore, we have found that the transport properties are substantially affected by the ferromagnetic order.