We have investigated the magnetic ground state of the multiferroic relaxor ferroelectric Pb͑Fe 1/2 Nb 1/2 ͒O 3 with SR spectroscopy and neutron scattering. We find that a transition to a partially disordered phase occurs below T = 20 K that coexists with long-range antiferromagnetic ordering. The disordered phase resembles a spin glass. No clustering of magnetic ions could be evidenced by SR so that the coexistence appears homogeneous in the sample.

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 relaxor ferroelectric PbMg1/3Ta2/3O3 was studied by single‐crystal neutron and synchrotron X‐ray diffraction, and its detailed atomic structure modelled in terms of static Pb displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb‐based relaxor family like PbMg1/3Nb2/3O3 or PbZn1/3Nb2/3O3 the diffuse scattering in the [H00]/[0K0] scattering plane has a butterfly shape around the h00 Bragg reflections and is orthogonal to the scattering vector for hh0 peaks. In the [HH0]/[00L] plane the diffuse scattering is elongated along the 〈112〉 directions and is orthogonal to the scattering vector for hhh reflections. It is found that a model consisting of correlated Pb displacements along the 〈111〉 directions reproduces adequately the main features of the diffuse scattering in PbMg1/3Ta2/3O3 when the correlation lengths between the Pb‐ion displacement vectors are longest along the 〈111〉 and shortest along the 〈11〉 and 〈10〉 directions.

Magnetic properties of PbFe1/2Nb1/2O3 single crystal showed onset of glasslike behavior at 120 K, saturation of onset at 28 K, and freezing of glasslike state at 27.6 K. Zero field cooled (ZFC) and field cooled magnetization curves displayed sharp cusp in ZFC. Almeida and Toulouse line illustrated stability over 1000 Oe with unstable onset for low field. Magnetic hysteresis was observed below glasslike freezing temperature having steplike behavior and low coercive field ∼296 Oe, which may be due to chemical inhomogeneity, strong anisotropy, and weak dipole interaction.

Neutron scattering investigations were carried out in PbMg 1/3 Ta 2/3 O3 and BaMg 1/3 Ta 2/3 O3 complex perovskites. The crystal structure of both compounds does not show any phase transition in the temperature range 1.5 -730 K. Whereas the temperature dependence of the lattice parameter of BaMg 1/3 Ta 2/3 O3 follows the classical expectations, the lattice parameter of relaxor ferroelectric PbMg 1/3 Ta 2/3 O3 exhibits anomalies. One of these anomalies is observed in the same temperature range as the peak in the dielectric susceptibility. We find that in PbMg 1/3 Ta 2/3 O3, lead ions are displaced from the ideal positions in the perovskite structure at all temperatures. Consequently short-range order is present. This induces strong diffuse scattering with an anisotropic shape in wavevector space. The temperature dependences of the diffuse neutron scattering intensity and of the amplitude of the lead displacements are similar.