Ferromagnetic quantum phase transition in Sr_1−xCa_xRuO₃ thin films

Abstract

The ferromagnetic quantum phase transition in the perovskite ruthenate Sr 1−x Ca x RuO 3 is studied by lowtemperature magnetization and electrical resistivity measurements on thin films. The films were grown epitaxially on SrTiO 3 substrates using metalorganic aerosol deposition and characterized by X-ray diffraction and room temperature scanning tunneling microscopy. High residual resistivity ratios of 29 and 16 for x = 0 and x = 1, respectively, prove the high quality of the investigated samples.We observe a continuous suppression of the ferromagnetic Curie temperature from T C = 160 K at x = 0 towards T C → 0 at x c ≈ 0.8. The analysis of the electrical resistivity between 2 and 10 K reveals T 2 and T 3/2 behavior at x ≤ 0.6 and x ≥ 0.7, respectively. For undoped CaRuO 3 , the measurement has been extended down to 60 mK, revealing a crossover to T 2 behavior around 2 K, which suggests a Fermi-liquid ground state in this system. Copyright line will be provided by the publisher Introduction Quantum phase transitions (QPTs) in weak itinerant ferromagnets have recently attracted much interest due to the discovery of exciting low-temperature states such as unconventional superconductivity [1], partial magnetic order [2] or non-Fermi liquid (NFL) phases [3]. The system Sr 1−x Ca x RuO 3 represents a rare example of an oxide NFL metal that displays an itinerant electron ferromagnetic (FM) QPT [4,5,6]. Although the general trend of a continuous suppression of ferromagnetism with increasing Ca concentration is well established, details close to the QPT differ substantially in previous reports. Early studies on sintered polycrystals have suggested a FM quantum critical point at x = 0.7 [5]. Subsequent Muon-spin rotation measurements on the same crystals have proven that the magnetically ordered volume fraction at low-T decreases from 100% upon increasing Ca content x well before T C → 0 [7], indicating magnetic phase separation. Most interestingly, similar behavior has also been found near the pressure-driven first-order QPT in MnSi [7]. On the other hand, magnetization measurements on flux grown single crystals have shown FM order beyond x = 0.8 with a smooth crossover to spin-glass-like behavior at larger x, extending towards x = 1 [4]. Electrical resistivity measurements on thin films, prepared by pulsed-

Metallic quantum ferromagnets

“…Demko et al (2012) measured the magnetization and susceptibility using a magnetooptical technique on a composition-spread epitaxial film of 200 nm thickness. They found a phase diagram that differs markedly from previous results, including those by Schneider et al (2010), namely, a pronounced tail with an onset around x = 0.4, see Fig. 35.…”

“…s For x = 0 (Schneider et al, 2010). RRR values varied from a high of 28.9 for x = 0 to a low of 2.9 for x = 0.5. t High ρ0 not intrinsic, but due to granularity of the polycrystalline sample.…”

Polaronic Emergent Phases in Manganite-based Heterostructures

“…The MAD technique was originally developed to grow epitaxial films of high-T C superconductors YBa 2 Cu 3 O 7−x [36]. Later on, other functional oxides, such as manganites, cobaltates, titanites, ruthenates, manganite/titanite superlattices, and nanocomposite films [22,[37][38][39][40][41][42], as well as single-oxide films of ZnO, MgO, Al 2 O 3 , have been successfully obtained by MAD. The essence of a vacuum-free MAD, operating at a high partial pressure of oxygen, pO 2~0 .2 bar, is the use of aerosols of solutions of metalorganic precursors, containing all necessary chemical ingredients, e.g., La-, Caand Mn-acetylacetonates, in a molecular form.…”

Strong-disorder magnetic quantum phase transitions: Status and new developments

“…By randomly substituting the smaller calcium atoms for the strontium atoms, one can thus tune the material through a ferromagnetic quantum phase transition. This system was extensively studied in the past, but different experiments [38,39,40,41,42,43,44] showed large variations of the phase boundary and especially in the critical calcium concentration x c beyond which long-range magnetic order vanishes.…”