The quantum spin-liquid compound (C4H12N2)Cu2Cl6 is studied by µSR under hydrostatic pressures up to 23.6 kbar. At low temperatures, pressure-induced incommensurate magnetic order is detected beyond a quantum critical point at Pc ∼ 4.3 kbar. An additional phase transition to a different ordered phase is observed at P1 ∼ 13.4 kbar. The data indicate that the high-pressure phase may be a commensurate one. The established (P −T ) phase diagram reveals the corresponding pressure-induced multicritical point at P1, T1 = 2.0 K.Traditionally, magnetic insulators have been the most important prototype systems for testing concepts and theories of phase transitions, universality and scaling [1,2]. They owe this to their well-defined short-range interactions, a broad range of interaction topologies and dimensionalities, and their amenability to numerical modeling. With a more recent interest in quantum phase transitions [3,4], magnetic insulators have become the prototypes of choice to study quantum critical points (QCPs). Realizations of such important QCPs as BoseEinstein condensation (BEC) [5], deconfinement in one dimension [6,7], and the Ising model in a transverse field [8] have been found in quantum magnets in applied magnetic fields. Magnetic BEC, for example, occurs in gapped quantum antiferromagnets (AFMs) with a spin singlet ground state, when an external field drives the energy gap to zero by virtue of Zeeman effect. The result in spontaneous long-range magnetic order in the perpendicular direction, and thus a breaking of SO(2) symmetry [5]. At the QCP, the soft mode has a parabolic dispersion, so that the dynamical critical exponent is z = 2. By now, this transition has been extensively studied experimentally and theoretically [5].A qualitatively different type of soft mode transition in gapped quantum AFMs may occur if the spin gap is driven to zero by varying the ratio of exchange constants. The resulting spontaneous long-range magnetic order breaks SO(3) symmetry, and the spectrum is expected to be linear at the QCP (z = 1). In practice, the only way to continuously tweak the exchange interactions is by applying external pressure. Closing the spin gap with pressure in quantum Heisenberg AFMs has been attempted in experiments [9][10][11]. However, only one good realization of pressure-induced ordering in such systems has been found to date, namely, that in TlCuCl 3 [12][13][14]. Further studies of this QCP brought fascinating new insights [15], particularly the observation of a longitudinal mode, which is a magnetic analog of the celebrated Higgs boson [4]. In the present work, we report the observation of pressure-induced ordering in the S = 1/2 frustrated gapped quantum AFM (C 4 H 12 N 2 )Cu 2 Cl 6 (abbreviated PHCC), and use muon spin rotation (µSR) experiments to map out the P − T phase diagram. We show that the pressure-driven transition leads to an incommensurate magnetic order. At still higher pressures, we detect an additional transition and multicritical point. The indication is that these are an inc...

The disordered antiferromagnet PbFe 1/2 Nb 1/2 O3 (PFN ) is investigated in a wide temperature range by combining Mössbauer spectroscopy and neutron diffraction experiments. It is demonstrated that the magnetic ground state is a microscopic coexistence of antiferromagnetic and a spin-glass orders. This speromagnet-like phase features frozen-in short-range fluctuations of the Fe 3+ magnetic moments that are transverse to the long-range ordered antiferromagnetic spin component.Phase transitions in the presence of disorder and/or competing interactions are one of the central unresolved problems in modern condensed matter physics 1-4 . With both effects present, one may encounter a freezing of microscopic degrees of freedom without conventional longrange order. In magnetic systems, the corresponding phenomenon is referred to as a spin-glass (SG) transition 5 . By now, spin glasses are reasonably well understood for models with discrete (Ising) symmetries and long-range interactions 6,7 . In contrast, for continuous (Heisenberg and XY) symmetries with short-range coupling, the properties and sometimes the very existence of the SG phase remain a matter of debate [8][9][10][11][12] . An important outstanding question is whether the SG phase can coexist with true long-range order (LRO) 12,13 ? Theory 14-16 and numerical studies 17-20 have consistently provide an affirmative answer; see Ref.21 for a review. Both ferromagnetic (FM) 15 and AF 16 models demonstrate a SG freezing of spin components transverse to the long range order parameter. The problem gained a particular urgency in the context of cuprate superconductors, where SG and AF phases are adjacent on the concentrationtemperature phase diagram but appear to be mutually exclusive 22,23 .On the experimental side though, the situation is much less clear-cut and hotly debated. Most hurdles on this route are the known measurement issues endemic to spin glasses 1,24,25 . In addition, even if long range order and SG are shown to appear simultaneously, it may be extremely difficult to establish their co-existence on the microscopic scale, as opposed to an inhomogeneous phase separation. A great deal of work was done on amorphous, ferromagnetic Fe X Zr 100−X alloys. While strong support for uniformly coexisting SG and LRO in these systems have been presented 20,[26][27][28] , evidence pointing to a cluster-based scenario also exist 29 . In crystalline materials, simultaneous antiferromagnetic (AF) and SG states have been observed in Fe 0.6 Mn 0.4 TiO 3 30,31 and Co 2 (OH)PO 4 32 . However, even in these Ising systems, the microscopic nature of such coexistence is not unambiguous 31,32 .A solid experimental proof of microscopic SG and LRO coexistence in a crystalline material remains elu- sive. Besides finding an appropriate model compound, one has to strategically choose the experimental techniques. Momentum-resolved (scattering) experiments are well-suited to probe microscopic quantities averaged over the entire sample, but do not provide spatiallyresolved information. I...

The influence of bond randomness on long range magnetic ordering in the weakly coupled S = 1/2 antiferromagnetic spin chain materials Cu(py)2(Cl1-xBrx)2 is studied by muon spin rotation and bulk measurements. Disorder is found to have a strong effect on the ordering temperature TN, and an even stronger one on the saturation magnetization m0, but considerably more so in the effectively lower-dimensional Br-rich materials. The observed behavior is attributed to Random Singlet ground states of individual spin chains, but remains in contradiction with chain mean field theory predictions. In this context, we discuss the possibility of a universal distribution of ordered moments in the weakly coupled Random Singlet chains model