Power-Law Dependence of Low-Temperature Magnetic Specific Heat for Hole-Doped Delafossite CuCr1-xMgxO2

Abstract

Delafossite CuCrO2 is one of the canonical frustrated compounds having an S 1⁄4 3=2 antiferromagnetic triangular lattice (ATL), wherein a nonmagnetic Cu layer (Cuþ; S 1⁄4 0) and a magnetic CrO2 layer (Cr 3þ; S 1⁄4 3=2) are alternatively stacked. Because of various application possibilities, such as in transparent conductor, as thermoelectric material, and as multiferroic material, its magnetic, transport, thermal, structural, and ferroelectric properties have been widely studied. To explore novel phenomena in geometrically frustrated compounds, the effects of various substitutions on the physical properties have also been investigated. Among them, the substitution of nonmagnetic Mg2þ for magnetic Cr3þ is intriguing, because it produces itinerant holes and dramatically reduces the resistivity, which nontrivially affects the 120 Neel state. In spite of the distinct disorder being introduced into the magnetic state and the antiferromagnetic (AF) correlation being suppressed by the Mg substitution, the Neel temperature (TN) increases and the peak of magnetic specific heat (Cmag) at TN becomes sharper (Fig. 1), indicating the promotion of the AF transition. These effects of the Mg substitution on Cmag around TN are in marked contrast to those of the isovalent substitution, such as Al substitution for Cr and Ag for Cu, which clearly suppresses the Neel state owing to the introduced chemical randomness and the enhanced two-dimensional (2D) nature. Although the recent elastic and inelastic neutron scattering measurements seem to indicate that the Mg substitution slightly modulates the longer range magnetic interactions, the realized 120 Neel state below TN for CuCr1 xMgxO2 is almost identical to that for CuCrO2. On the other hand, the measurements of magnetization and magnetoresistance indicate that the itinerant holes doped by the Mg substitution seem to produce some ferromagnetic (FM) interaction between the localized spins. The perturbated FM interaction should compete with the AF exchange interaction, which should suppress the Neel order. However, the AF transition is clearly promoted, as shown in Fig. 1. According to the recent PES and XAS measurements, the spectra of Cr4þ are not observed but the spectrum of Cr3þ is modulated by the Mg substitution, indicating that the Cr ion for CuCr0:97Mg0:03O2 is trivalent. Then, CuCr1 xMgxO2 with x 0:03 is regarded as approximately an S 1⁄4 3=2 spin system so that the orbital degree of freedom of Cr4þ is absent. Therefore, although the microscopic mechanism has not been established, the unconventional promotion of the AF transition should be attributed to the doped itinerant holes having FM interaction with localized spins, since the spin dilution simply blurs the AF transition. To elucidate the origin of the unconventional promotion of the AF transition by the hole doping more clearly, it is necessary to understand the magnetic ground state correctly. In this short note, we report on the power-law dependence of the low-temperature (low-T ) Cmag of CuCr1 xMgxO2 without an enhancement of low-T Cmag by Mg substitution. These behaviors are due to the mixture of the effect of chemical randomness producing the 2D diffuse liquid like component and that of hole doping suppressing the low-energy excitation of long-range anisotropic 3D AF magnons. Figure 2(a) shows a summary of Cmag=T vs ðT=5Þ 1 at low temperatures below 5K for CuCr1 xMgxO2 together with those for the isovalent substituted compounds such as Cu1 xAgxCrO2 and CuCr1 xAlxO2, where is the coefficient of the T -linear component and is the exponent of the power-law dependence of Cmag summarized in Fig. 2(b). Although the values are much enhanced by the heavy Al substitution, those for CuCrO2, 7) CuCr1 xMgxO2, 7) and Cu1 xAgxCrO2 are about 1mJ/K mol, which is comparable to the experimental and fitting errors. The observed finite values are perhaps due to the spin glass component. It should be emphasized that, as shown in Fig. 2(b), for CuCr1 xMgxO2 gradually decreases from 3 with an increase in x, as observed in Cu1 xAgxCrO2 and CuCr1 xAlxO2, although the change around TN with the Mg substitution is quite different from those for the isovalent substituted compounds (Fig. 1). The decrease in seems to indicate the appearance of the same 2D diffuse spin liquid like state as those of Cu1 xAgxCrO2 and CuCr1 xAlxO2. However, in contrast to the isovalent compounds, the low-T Cmag for CuCr1 xMgxO2 seems to be not enhanced but rather suppressed by the Mg substitution. Furthermore, as shown in Fig. 2(b), the change in upon the Mg substitution is more dramatic than that upon the Al substitution in spite of the introduction of similar spin defects. 0 5 10 15