It has generally been believed that, within the context of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, the conduction electrons in a metal cannot be both ferromagnetically ordered and superconducting. Even when the superconductivity has been interpreted as arising from magnetic mediation of the paired electrons, it was thought that the superconducting state occurs in the paramagnetic phase. Here we report the observation of superconductivity in the ferromagnetically ordered phase of the d-electron compound ZrZn2. The specific heat anomaly associated with the superconducting transition in this material appears to be absent, and the superconducting state is very sensitive to defects, occurring only in very pure samples. Under hydrostatic pressure superconductivity and ferromagnetism disappear at the same pressure, so the ferromagnetic state appears to be a prerequisite for superconductivity. When combined with the recent observation of superconductivity in UGe2 (ref. 4), our results suggest that metallic ferromagnets may universally become superconducting when the magnetization is small.

We report a study of the ferromagnetism of ZrZn2, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures T as function of pressure p. We find that the ordered ferromagnetic moment disappears discontinuously at pc=16.5 kbar. Thus a tricritical point separates a line of first order ferromagnetic transitions from second order (continuous) transitions at higher temperature. We also identify two lines of transitions of the magnetisation isotherms up to 12 T in the p − T plane where the derivative of the magnetization changes rapidly. These quantum phase transitions (QPT) establish a high sensitivity to local minima in the free energy in ZrZn2, thus strongly suggesting that QPT in itinerant ferromagnets are always first order.PACS numbers: 71.27.+a, 75.50.Cc, 74.70.Tx The transition of a ferromagnet to a paramagnet with increasing temperature is regarded as a canonical example of a continuous (second order) phase transition. This type of behavior has been well established in many materials ranging from nickel [1] to chromium tribromide [2]. The detailed variation of the order parameter near the critical point, in this case the Curie temperature, has been analyzed in a wide variety of systems using classical statistical-mechanical models for the case when the Curie temperature is not too small. Classical statistics are appropriate when all fluctuating modes have energies much less than k B T c . It was pointed out by Hertz [3] that the system undergoes a 'quantum' phase transition (QPT) when the transition is driven by non-thermal fluctuations whose statistics are in the quantum limit.The search for a second order (critical) QPT in itinerant electron systems, which are believed to be responsible for enigmatic quantum phases like magnetically mediated superconductivity and non-Fermi liquid behavior, has become of particular interest in recent years. Experimental studies have thereby revealed notable differences from "standard" second order behaviour in all materials investigated to date. For example, in MnSi [4] and UGe 2 [5], itinerant-electron magnetism disappears at a first order transition as pressure is applied. The bilayer ruthenate Sr 3 Ru 2 O 7 , undergoes a field induced QPT with multiple first-order metamagnetic transitions [6] and associated non-Fermi liquid behavior in the resistivity [7]. However, these materials have complicating factors: the zero-field ground state of MnSi is a helical spin spiral; UGe 2 is a strongly uniaxial (Ising) system; Sr 3 Ru 2 O 7 is a strongly two-dimensional metal. In fact, theoretical studies suggest [8, 9, 10, 11] that ferromagnetic transitions in clean three-dimensional (3D) itinerant ferromagnets at T = 0 are always first order.In this Letter we address the nature of the ferromagnetic QPT experimentally. The system we have chosen is the itinerant ferromagnet ZrZn 2 , which is a straight forward itinerant ferromagnet with a cubic (C15) structure and small magnetic anisotropy. ZrZn 2 has a small ordered moment (M = 0.17µ B f.u. −1 ...

In biomedicine, magnetic nanoparticles provide some attractive possibilities because they possess peculiar physical properties that permit their use in a wide range of applications. The concept of magnetic guidance basically spans from drug delivery and hyperthermia treatment of tumours, to tissue engineering, such as magneto-mechanical stimulation/activation of cell constructs and mechanosensitive ion channels, magnetic cell-seeding procedures, and controlled cell proliferation and differentiation. Accordingly, the aim of this study was to develop fully biodegradable and magnetic nanocomposite substrates for bone tissue engineering by embedding irondoped hydroxyapatite (FeHA) nanoparticles in a poly(1-caprolactone) (PCL) matrix. X-ray diffraction analyses enabled the demonstration that the phase composition and crystallinity of the magnetic FeHA were not affected by the process used to develop the nanocomposite substrates. The mechanical characterization performed through small punch tests has evidenced that inclusion of 10 per cent by weight of FeHA would represent an effective reinforcement. The inclusion of nanoparticles also improves the hydrophilicity of the substrates as evidenced by the lower values of water contact angle in comparison with those of neat PCL. The results from magnetic measurements confirmed the superparamagnetic character of the nanocomposite substrates, indicated by a very low coercive field, a saturation magnetization strictly proportional to the FeHA content and a strong history dependence in temperature sweeps. Regarding the biological performances, confocal laser scanning microscopy and AlamarBlue assay have provided qualitative and quantitative information on human mesenchymal stem cell adhesion and viability/proliferation, respectively, whereas the obtained ALP/DNA values have shown the ability of the nanocomposite substrates to support osteogenic differentiation.

The Weyl semimetal NbP was found to exhibit topological Fermi arcs and exotic magnetotransport properties. Here, we report on magnetic quantum-oscillation measurements on NbP and construct the three-dimensional Fermi surface with the help of band-structure calculations. We reveal a pair of spin-orbit-split electron pockets at the Fermi energy and a similar pair of hole pockets, all of which are strongly anisotropic. The Weyl points that are located in the k(z) approximate to pi/c plane are found to exist 5 meV above the Fermi energy. Therefore, we predict that the chiral anomaly effect can be realized in NbP by electron doping to drive the Fermi energy to the Weyl points

We present a combined neutron diffraction and bulk thermodynamic study of the natural mineral linarite PbCuSO4(OH)2, this way establishing the nature of the ground-state magnetic order. An incommensurate magnetic ordering with a propagation vector k=(0,0.186,1/2) was found below T(N)=2.8 K in a zero magnetic field. The analysis of the neutron diffraction data yields an elliptical helical structure, where one component (0.638μ(B)) is in the monoclinic ac plane forming an angle with the a axis of 27(2)°, while the other component (0.833μ(B)) points along the b axis. From a detailed thermodynamic study of bulk linarite in magnetic fields up to 12 T, applied along the chain direction, a very rich magnetic phase diagram is established, with multiple field-induced phases, and possibly short-range-order effects occurring in high fields. Our data establish linarite as a model compound of the frustrated one-dimensional spin chain, with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions. Long-range magnetic order is brought about by interchain coupling 1 order of magnitude smaller than the intrachain coupling.