Specific heat of (Ca1−xSrx)3Ru2O7 single crystals

Abstract

We have measured the specific heat of crystals of (Ca 1-x Sr x ) 3 Ru 2 O 7 using ac-and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Néel (T N =56 K) and structural (T c = 48 K) phase transitions in the pure, x=0 material.While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L ~ 0.3 R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-fieldlike step at T N , but its magntitude (∆c P ~ R) is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with full ordering of localized spins. The T N transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T c transition decreases rapidly with Sr substitution and is not observable for our x=0.58 sample. PACS: 71.27.+a, 75.40.Cx, 65.40 The spin-structure of the double-layer, n =2, compounds have been especially unusual. In the pure strontium (i.e. x=1) salt, there is no resulting long range spin order at ambient pressure, but longitudinal strain can induce a ferromagnetic state [5,10]. In the pure calcium (x=0) salt, the spins order ferromagnetically within the bilayers, but the interlayer (c-axis) interactions are predominantly antiferromagnetic, leading to a Néel transition at T N = 56 K with spin polarization along the b-axis [6,[11][12][13]. With further cooling, the spin polarization rotates within the plane toward the a-axis [13,14]. There is a second, more unusual, phase transition at T c = 48 K; here all components of the susceptibility drop abruptly upon cooling through T c [11,14], which has been suggested to be due to formation of a charge-density-wave [15], while the lattice contracts along c by [3,11,20]. All crystals studied were characterized by single crystal or powder x-ray diffraction, EDS and TEM,indicating good crystal quality with no impurities or intergrowths or significant clustering of strontium ions. Magnetic and transport properties were measured using a Quantum Design MPMS XL 7T magnetometer. (Since the shape of all crystals studied in this work is essentially cubic, the demagnetizing factor for all samples and all three principal crystallographic axes is expected to be the same, and is not included in our the susceptibility is almost isotropic in the ab-plane (suggesting that the RuO 6 octahedra are almost "untilted", as in the pure Sr compound) with peaks in M at T p ~ 50 K followed by weak minima (T~ 40 K) and relatively high susceptibilities at lower temperatures.Only small anomalies are observed in the resistance at these temperatures, and the shape of the magnetic anomalies [4] suggests that spin ordering is complex but incomplete. A more complete discussion of the substitution-dependence of the resistance and magnetization, including their field dependence...

Interplay between the lattice and spin degrees of freedom in(Sr1−xCax)3Jin1, Qu2, Qian3 et al. 2010Phys. Rev. B264280View full textAdd to dashboardCite

“…For x = 0.6, 0.7, and 1.0 samples, however, we observe anomalies near the AFM and MI transition temperature, consistent with the result previously reported. 38 Neutron-scattering studies on Ca 3 Ru 2 O 7 ͑Ref. 37͒ reveal that although the lattice parameters exhibit jumps near T MI , the space group of the system does not change across the MI transition, remaining at Bb2 1 m below T MI .…”

Magnetic structure and orbital state ofCa3Ru2O7investigated by resonant x-ray diffraction

“…The phase diagram of Ca 3 Ru 2 O 7 is particularly interesting as multiple phase transitions are induced by external magnetic fields, [3][4][5] uniaxial or hydrostatic pressure, [6][7][8] and doping. 9,10 Most of these transitions have been attributed to rearrangements of the magnetic and/or orbital ordering pattern. As recently demonstrated for Ca 2 RuO 4 , resonant x-ray diffraction ͑RXD͒ is a highly sensitive probe of magnetic and orbital ordering phenomena in the ruthenates.…”

Ca 3 ( Ru 1 − x Cr x ) 2 O 7 : A new paradigm for spin valves

“…The specific heat C͑T͒ and the magnetizations for applied magnetic field B ʈ b-͑M b ͒ and a-axes ͑M a ͒ at B = 0.5 T are shown in Fig. The measured value of ⌬␥ / R therefore suggests that the anomaly at T N is inconsistent with both conventional itinerant and localized pictures of antiferromagnetism, 19 and thus indicates a more complex spin ordering. The transition anomaly at T MI is sharper with an approximate jump ⌬C ϳ 0.31 R ͑the gas constant R = 8.314 J / mole K͒; and the other near T =83 K ͑slightly lower than T N =86 K͒ is broader with a smaller ⌬C ϳ 0.28 R. A fit of the low-T data to C = ␥T + ␤T 3 for 1.7 K Ͻ T Ͻ 30 K yields coefficients of the electronic and phonon contributions, ␥ ϳ 31 mJ/ mol K and ␤ ϳ 3.0 ϫ 10 −4 mJ/ mol K 3 , respectively, which are comparable to the values for x =0.…”