Resistivity, specific heat and magnetic susceptibility measurements performed on SrFe2As2 samples evidence a behavior very similar to that observed in LaFeAsO and BaFe2As2 with the difference that the formation of the SDW and the lattice deformation occur in a pronounced first order transition at T0 = 205 K. Comparing further data evidences that the Fe-magnetism is stronger in SrFe2As2 and in EuFe2As2 than in the other layered FeAs systems investigated up to now. Full potential LDA band structure calculations confirm the large similarity between the compounds, especially for the relevant low energy Fe 3d states. The relation between structural details and magnetic order is analyzed.The discovery of superconductivity in doped LaFeAsO [1] and the subsequent raising of the superconducting (SC) transition temperature T c to 56 K [2, 3, 4] initiated a surge of interest in layered FeAs systems. Undoped RFeAsO compounds (R = La -Gd) present a structural transition at T 0 ∼ 150 K followed by the formation of a spin density wave (SDW) at a slightly lower temperature T N ∼ 140 K [5,6]. Electron or hole doping by substituting O by F [7], or R by a tetravalent or divalent cation [8,9], or by reducing the O-content [10] leads to the suppression of the SDW and to the onset of superconductivity. This connection between a vanishing magnetic transition and the simultaneous formation of a SC state is reminiscent of the behavior in the cuprates and in the heavy fermion systems, and therefore suggests the SC state in these doped RFeAsO systems to be of unconventional nature, too. While this has to be confirmed by further studies, there seems to be a general belief that the intriguing properties of these compounds are connected with very peculiar properties of the FeAs layers. The tetragonal ZrCuSiAs structure type in which these compounds crystallize [11] results in a square Fe lattice with As in the center of the square but being alternately shifted above and below the Fe-plane. It is well known that the ThCr 2 Si 2 structure type presents a very similar arrangement of the transition metal and p-element. Thus it was natural to look for appropriate candidates within the huge amount of compounds crystallizing in this structure type. In a very primitive approach, the RFeAsO compounds can be rationalized as a stack of alternating (Fe 2 As 2 ) 2− and (2+ layer by a layer with a single large atom A leads to the ThCr 2 Si 2 structure type. In order to keep the same electron counts as in the RFeAsO materials, A has to be a divalent atom. Therefore, appropriate candidates are A 2+ Fe 2 As 2 compounds. To the best of our knowledge, three of the possible candidates have already been investigated. M. Pfisterer and G. Nagorsen have re- * Electronic address: geibel@cpfs.mpg.de ported the synthesis, the structure as well as preliminary susceptibility data of SrFe 2 As 2 and BaFe 2 As 2 [12, 13]. From their susceptibility data they concluded the occurrence of a magnetic phase transition around 200 K and 130 K, respectively and suggested it to be ...

X-ray and Neutron diffraction as well as muon spin relaxation and Mössbauer experiments performed on SrFe 2 As 2 polycrystalls confirm a sharp first order transition at T 0 = 205 K corresponding to an orthorhombic phase distortion and to a columnar antiferromagnetic Fe ordering with a propagation vector (1,0,1), and a larger distortion and larger size of the ordered moment than reported for BaFe 2 As 2 . The structural and the magnetic order parameters present an remarkable similarity in their temperature dependence from T 0 down to low temperatures, showing that both phenomena are intimately connected. Accordingly, the size of the ordered Fe moments scale with the lattice distortion when going from SrFe 2 As 2 to BaFe 2 As 2 . Full-potential band structure calculations confirm that the columnar magnetic order and the orthorhombic lattice distortion are intrinsically tied to each other.

The orthorhombic CePd 1−x Rh x system exhibits a continuous evolution from ferromagnetic order in CePd (T C = 6.6 K) to an intermediate-valence ground state in CeRh. Here we report low-temperature (T 0.08 K) specific-heat measurements for concentrations 0.80 x 0.95 close to the disappearance of magnetic order (∼0.87). In contrast to x = 0.8, still demonstrating a smeared phase transition signature at 0.27 K, non-Fermi-liquid behaviour C(T )/T ∝ log T is observed in the electronic specific-heat coefficient at x = 0.85. For higher Rh concentrations a weak power-law dependence C(T )/T ∝ T −α with α ≈ 0.4 is found. Upon applying magnetic fields Fermi-liquid-like behaviour C(T )/T ∼ const is recovered in all the different samples. The magnetic entropy as well as the concentration of unscreened magnetic moments at 2 K indicate a broad distribution of local T K values ranging from well below 2 K to far above 50 K.

The CePd 1−x Rh x alloy exhibits a continuous evolution from ferromagnetism ͑T C = 6.5 K͒ at x = 0 to a mixedvalence ͑MV͒ state at x = 1. We have performed a detailed investigation on the suppression of the ferromagnetic ͑F͒ phase in this alloy using dc ͑ dc ͒ and ac ͑ ac ͒ susceptibility, specific heat ͑C m ͒, resistivity ͑͒, and thermal expansion ͑␤͒ techniques. Our results show a continuous decrease of T C ͑x͒ with negative curvature down to T C =3 K at x * = 0.65, where a positive curvature takes over. Beyond x * , a cusp in ac is traced down to T C * =25 mK at x = 0.87, locating the critical concentration between x = 0.87 and 0.90. The quantum criticality of this region is recognized by the −log͑T / T 0 ͒ dependence of C m / T, which transforms into a T −q ͑q Ϸ 0.5͒ one at x = 0.87. At high temperature, this system shows the onset of valence instability revealed by a deviation from Vegard's law ͑at x V Ϸ 0.75͒ and increasing hybridization effects on high-temperature dc and ͑T͒. Coincidentally, a Fermi liquid contribution to the specific heat ͑␥͒ arises from the MV component, which becomes dominant at the CeRh limit. In contrast to antiferromagnetic systems, no C m / T flattening is observed for x Ͼ x cr but, rather, the mentioned power-law divergence, which coincides with a change of sign of ␤͑T͒. The coexistence of F and MV components and the sudden changes in the T dependencies are discussed in the context of randomly distributed magnetic and Kondo couplings.