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Register a new userQuantum percolation phase transition and magneto-electric dipole glass in hexagonal ferrites
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Hexagonal ferrites do not only have enormous commercial impact ({pounds}2 billion/year in sales) due to applications that include ultra-high density memories, credit card stripes, magnetic bar codes, small motors and low-loss microwave devices, they also have fascinating magnetic and ferroelectric quantum properties at low temperatures. Here we report the results of tuning the magnetic ordering temperature in PbFe$_{12-x}$Ga$_x$O$_{19}$ to zero by chemical substitution $x$. The phase transition boundary is found to vary as $T_N sim (1-x/x_c)^{2/3}$ with $x_c$ very close to the calculated spin percolation threshold which we determine by Monte Carlo simulations, indicating that the zero-temperature phase transition is geometrically driven. We find that this produces a form of compositionally-tuned, insulating, ferrimagnetic quantum criticality. Close to the zero temperature phase transition we observe the emergence of an electric-dipole glass induced by magneto-electric coupling. The strong frequency behaviour of the glass freezing temperature $T_m$ has a Vogel-Fulcher dependence with $T_m$ finite, or suppressed below zero in the zero frequency limit, depending on composition $x$. These quantum-mechanical properties, along with the multiplicity of low-lying modes near to the zero-temperature phase transition, are likely to greatly extend applications of hexaferrites into the realm of quantum and cryogenic technologies.
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We report a new peculiar effect of the interaction between a sublattice of frustrated quantum spin-1/2 chains and a sublattice of pseudospin-1/2 centers (quantum electric dipoles) uniquely co-existing in the complex oxide Li2ZrCuO4. 7Li nuclear magnetic-, Cu2+ electron spin resonance and a complex dielectric constant data reveal that the sublattice of Li+-derived electric dipoles orders glass like at Tg ~ 70 K yielding a spin site nonequivalency in the CuO2 chains. We suggest that such a remarkable interplay between electrical and spin degrees of freedom might strongly influence the properties of the spiral spin state in Li2ZrCuO4 that is close to a quantum ferromagnetic critical point. In particular that strong quantum fluctuations and/or the glassy behavior of electric dipoles might renormalize the exchange integrals affecting this way the pitch angle of the spiral as well as be responsible for the missing multiferroicity present in other helicoidal magnets.
We consider phase separated states in magnetic oxides (MO) thin films. We show that these states have a non-zero electric polarization. Moreover, the polarization is intimately related to a spatial distribution of magnetization in the film. Polarized states with opposite polarization and opposite magnetic configuration are degenerate. An external electric field removes the degeneracy and allows to switch between the two states. So, one can control electric polarization and magnetic configuration of the phase separated MO thin film with the external electric field.
The structural and magnetic properties of the hexagonal four-layer form of SrMnO$_3$ have been investigated by combining magnetization measurements, electron diffraction and high-resolution synchrotron X-ray and neutron powder diffraction. Below 350K, there is subtle structural phase transition from hexagonal symmetry (space group $P6_3/mmc$) to orthorhombic symmetry (space group $C222_1$) where the hexagonal metric is preserved. The second-order phase transition involves a slight tilting of the corner-sharing Mn$_{2}$O$_{9}$ units composed of 2 face-sharing MnO$_6$ octahedra and the associated displacement of Sr$^{2+}$ cations. The phase transition is described in terms of symmetry-adapted displacement modes of the high symmetry phase. Upon further cooling, long range magnetic order with propagation vector $mathbf{k}=(0,0,0)$ sets in below 300K. The antiferromagnetic structure, analyzed using representation theory, shows a considerably reduced magnetic moment indicating the crucial role played by direct exchange between Mn centers of the Mn$_{2}$O$_{9}$ units.
We report an ultrasonic investigation of the elastic moduli on a single crystal of hexagonal YMnO_3 as a function of temperature. Stiffening anomalies in the antiferromagnetic Neel state below T_N = 72.4 K are observed on all the four elastic moduli C_{ii}. The anomalies are the most important on C_{11} and C_{66} for in-plane elastic deformations; this is consistent with a strong coupling of the lattice with the in-plane exchange interactions. We use a Landau free energy model to account for these elastic anomalies. We derive an expression which relates the temperature profile of the anomaly to the order parameter; the critical exponent associated to this parameter $beta$ = 0.42 is not consistent with a chiral XY or 3D Heisenberg universality class, but more in agreement with a conventional antiferromagnetic long range order. A tiny softening anomaly on C_{11} for which hysteresis effects are observed could be indicative of an interaction between ferroelectric and magnetic domains at T_N. Moreover, magnetic fluctuations effects both above and below T_N are identified through abnormal temperature and magnetic field effects.
We submit that the magnetic space-group Cac (#9.41) is consistent with the established magnetic structure of BaFe2Se3, with magnetic dipole moments in a motif that uses two ladders [Caron J M et al 2011 Phys. Rev. B 84 180409(R)]. The corresponding crystal class m1 allows axial and polar dipoles and forbids bulk ferromagnetism. The compound supports magneto-electric multipoles, including a magnetic charge (monopole) and an anapole (magnetic toroidal dipole) visible in the Bragg diffraction of x-rays and neutrons. Our comprehensive simulation of neutron Bragg diffraction by BaFe2Se3 exploits expressions of a general nature that can be of use with other magnetic materials. Electric toroidal moments are also allowed in BaFe2Se3. A discussion of our findings for resonant x-ray Bragg diffraction illustrates the benefit of a common platform for neutron and x-ray diffraction.
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