Specific-heat study for ferromagnetic and antiferromagnetic phases in SrRuMnO
Low-temperature electronic states in SrRuMnO for have been investigated by means of specific-heat measurements. We have found that a jump anomaly observed in at the ferromagnetic (FM) transition temperature for SrRuO changes into a broad peak by only 5% substitution of Mn for Ru. With further doping Mn, the low-temperature electronic specific-heat coefficient is markedly reduced from the value at (33 mJ/K mol), in connection with the suppression of the FM phase as well as the enhancement of the resistivity. For , approaches to mJ/K mol or less, where the antiferromagnetic order with an insulating feature in resistivity is generated. We suggest from these results that both disorder and reconstruction of the electronic states induced by doping Mn are coupled with the magnetic ground states and transport properties.
Faculty of Science, Ibaraki University, Mito 310-8512, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Institute for Solid State Physics, The University of Tokyo, Kashiwa 270-8581, Japan
E-mail: email@example.com, firstname.lastname@example.org
The interplay of magnetism and charge transport in the vicinity of metal-insulator transition is one of the most intriguing issues in the physics of strongly correlated electron systems. The distorted perovskite compound SrRuO shows a ferromagnetic (FM) order below , in which the itinerant 4d electrons are considered to be responsible for the spontaneous spin polarization [rf:Callaghan66, rf:Kanbayasi76, rf:Allen96]. The electrical resistivity exhibits unusual metallic behavior called “bad metal”, characterized by an absence of suppression in and a very small mean-free pass comparable to lattice constants at high temperatures [rf:Allen96]. In addition, optical conductivity [rf:Kostic98] and photoemission [rf:Okamoto99] studies indicate the enhancements of anomalous electronic states originating from many-body correlation effects.
In the mixed compounds SrRuMnO [rf:Sahu2002, rf:Cao2005, rf:Yokoyama2005, rf:Han2006, rf:Zhang2006, rf:Woodward2008, rf:Kolesnik2008], it is revealed that the substitution of Mn for Ru suppresses the FM phase, and then induces the C-type antiferromagnetic (AFM) phase above . The structural transition from orthorombic to tetragonal symmetries also occurs in connection with the variation of magnetic ground state. Furthermore, the Mn substitution changes the characteristic of from metallic to insulating ones. These features suggest that doping Mn into SrRuO modifies the itinerant electronic states due to the effects of strong correlations between Ru 4d and Mn 3d electrons, and it significantly affects the magnetic, transport and lattice properties. It is therefore interesting to investigate the relationship between the electronic states and these properties. To clarify this, we have performed specific-heat measurements on SrRuMnO in the low and intermediate ranges.
2 Experimental Procedure
Polycrystalline samples of SrRuMnO with were prepared by means of the conventional solid-state method. The mixtures of appropriate amounts of SrCO, RuO and MnO are first calcined at 750 C for 4 hours. They were shaped into pellets after careful mixing, and then sintered at 1300 C for 24 hours. This sinter process was iterated 10 times to achieve homogeneity of the samples. The details on the sample preparation are presented elsewhere [rf:Yokoyama2005]. Specific-heat was measured between 2 K and 275 K with a thermal-relaxation technique. To check an effect of thermal conductance in the sintered samples on the data, we performed the experiments using both a commercial system (PPMS: Quantum Design) and a hand-made equipment, where we used the plate-shaped samples with the mass of about 4 mg and 80 mg, respectively. The thermal-relaxation curves for all the measurements were well definitive, and the data obtained from both the equipments were consistent within the experimental accuracy. Electrical resistivity measurements were performed using a standard four-wire dc technique from 3.5 K to 300 K. Ac-susceptibility was measured in the temperature range of for to estimate the FM transition temperature . Frequency and amplitude of the applied ac field were 180 Hz and .