Copper Metaborate Research Articles

New magnetic states in copper metaborate CuB2O4

The static and resonance properties of copper metaborate CuB2O4 were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.

Quasi-one-dimensional excitations of copper metaborate in the commensurate phase [formula omitted

The low-energy branch of the spin-wave spectrum of CuB 2O 4 was investigated theoretically by means of linear spin-wave theory in the commensurate magnetic phase. We have found that these excitations are associated with the magnetic subsystem Cu(B). The exchange interactions in this subsystem between nearest and next nearest neighbours, which forms the quasi-one-dimensional “zig-zag” ladder structure, were determined.

Complex magnetic ground state ofCuB2O4

The magnetic ground-state of copper metaborate ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$ was investigated with unpolarized and polarized neutron scattering. A phase transition was found at ${T}_{N}=21\mathrm{K}$ to a commensurate weakly ferromagnetic state followed by a second transition at ${T}^{*}=10\mathrm{K}$ to an incommensurate magnetic structure. Neutron diffraction revealed a continuously changing magnetic propagation vector below ${T}^{*},$ and unusually asymmetric magnetic satellite reflections. Additionally, diffuse scattering is observed in the temperature range $1.5\mathrm{K}l~Tl~30\mathrm{K}.$ The magnetic structure determined in both phases are shown to be consistent with results of symmetry analysis. In particular, we find that only one of the two inequivalent ${\mathrm{Cu}}^{2+}$ sublattice fully orders down to the lowest temperature. Our results show that the complex magnetic behavior of copper metaborate is a consequence of mutual interaction between the two ${\mathrm{Cu}}^{2+}$ sublattices with different ordering temperatures.

Magnetic Properties of Copper Metaborate CuB2O4

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electromechanical properties and anisotropy of acoustic wave propagation in CuB2O4 copper metaborate

The propagation of bulk acoustic waves in single-crystal CuB2O4 copper metaborate is studied. The elastic, piezoelectric, and dielectric constants are calculated. The anisotropy in the parameters of bulk acoustic wave propagation in this crystal are determined.

Soliton lattice in copper metaborate, CuB 2 O 4 , in the presence of an external magnetic field

The spontaneous formation of a magnetic soliton lattice in copper metaborate, CuB2O4, is also stable at a temperature of 4.2 K in the presence of an external magnetic field up to 1.3 T applied along the tetragonal [110] direction similar to the undisturbed case, where it is stable up to a temperature of 10 K. Increasing the external magnetic field induces a phase transition from the incommensurate to the commensurate structure similar to increasing the temperature at zero field as reported earlier. The formation of domain walls (solitons) can therefore be associated with combined effects of Dzyaloshinskii interaction and anisotropy. The T/H phase diagrams show a coexistence region of commensurate and incommensurate phases, in agreement with the theory of a solition lattice at fields up to 1.3 T.

Magnetic properties of copper metaborate CuB2O4

The results of experimental research on the magnetic and resonance properties, heat capacity, muon spin relaxation, and neutron scattering of single-crystal copper metaborate CuB2O4 are reviewed. The results of a symmetry analysis and of modeling by the method of the phenomenological thermodynamic potential are presented. The magnetic structure of the crystal in various temperature intervals of the magnetic ordering region is discussed.

Spin-wave spectrum of copper metaborate in the commensurate phase [formula omitted

We have investigated the spin-wave spectrum of copper metaborate, CuB 2O 4, by means of inelastic neutron scattering in the commensurate magnetic phase. We have found two branches of spin-wave excitations associated with the two magnetic sublattices Cu(A) and Cu(B), respectively. In the temperature regime 10 K⩽T⩽21 K , where only the Cu(A) magnetic moments are ordered, the interaction between the two sublattices is found to be negligible. With this approximation we have determined the ‘easy plane’ exchange parameters of the Cu(A) subsystem within standard spin-wave theory.

A neutron scattering and μSR investigation of the magnetic phase transitions of CuB 2O 4

We have investigated the magnetic ground state in CuB 2O 4, copper metaborate, by means of neutron diffraction and μSR measurements. At T N=21 K CuB 2O 4 undergoes a second-order phase transition from a paramagnetic to an antiferromagnetic commensurate state followed by a second transition at T*=10 K where the two magnetic sublattices form a soliton lattice, as reported earlier. We give a detailed analysis of the magnetic structure in the commensurate phase which is found to be different from the one published recently. New experimental results are presented which show the existence of a third transition below T≈1.8 K. The magnetic structure remains incommensurate at very low temperatures, but a re-distribution of the magnetic intensities is observed in the neutron diffraction data set which suggests that the phases between the Cu 2+ spins are changed.

Incommensurate magnetic structure in copper metaborate

Based on the experimental data on copper metaborate single crystals obtained in X-ray and neutron diffraction studies and heat capacity, magnetic susceptibility, and muon spin relaxation measurements, a phenomenological theory of the incommensurate magnetic structure of this crystal was developed. Considering the space group of the crystal, $$I\bar 4 2d$$ , Lifshits invariants were included into its thermodynamic potential. An analysis showed that magnetic structure formation at 10–20 K was dominated by the subsystem of copper spins in 4b unit cell sites. Below 10 K, the role played by the magnetic subsystem of copper spins in 8d unit cell sites in the formation of the magnetic structure of copper metaborate substantially increased. This caused a sharp increase in the wave vector of the incommensurate structure as temperature lowered. Numerical simulation of the temperature dependence of the wave vector of the helix and the heat capacity of the crystal gave a satisfactory description of the experimental data. This simulation was used to estimate the parameters of the phenomenological thermodynamic potential of the magnetic system of copper metaborate.

Formation of a magnetic soliton lattice in copper metaborate.

The magnetic ground state of CuB2O4 is incommensurate at T = 1.8 K and undergoes a continuous phase transition to a noncollinear commensurate antiferromagnetic state at T(small star), filled approximately 10 K. Close to T(small star), filled higher-order magnetic satellites are observed. Coexistence of long- and short-range magnetic order is observed in both magnetic phases. This suggests that the association of the Dzyaloshinskii-Moriya interaction and anisotropy leads to the formation of a magnetic soliton lattice.

Optical absorption of copper metaborate CuB2O4

High-quality single crystals of copper metaborate CuB2O4 with a volume exceeding 1 cm3 are grown. The optical absorption spectra of these crystals are recorded for the first time owing to their sufficiently large size. The spectra exhibit a broad transmission window in the visible region and an intense absorption peak around ∼2.0 eV, which is already split into two bands at room temperature. A weak anisotropy is revealed in unpolarized light upon light beam propagation along the a and c axes. The spectra are compared with the absorption spectrum of CuGeO3.

Synthesis and magnetic properties of copper metaborate single crystals, CuB2O4

Large high-quality single crystals of copper metaborate are grown on the basis of the phase diagram of the ternary Li2O-CuO-B2O3 system. Bright blue crystals with a volume of about 1 cm3 were grown by the method of spontaneous crystallization while slowly cooling the melt. The magnetic susceptibility and electron spin resonance were measured. It is shown that the effective magnetic moment of a Cu2+ ion is equal to 1.6 μB and the g-factor, to 2.170 and 2.133 for the magnetic field oriented parallel and perpendicular to the fourfold axis, respectively. At 21 and 10 K, sharp anomalies of magnetic susceptibility are observed.

Magnetic susceptibility and magnetic-field behavior of CuB2O4 copper metaborate

An experimental study is reported regarding the temperature dependence of the magnetic susceptibility of a CuB2O4 tetragonal single crystal within the 4.2–200-K range. It has been established that the magnetic susceptibility exhibits anomalies at 21 and 10 K and depends strongly on crystal orientation in the magnetic field. A study has been carried out of the field dependences of the magnetization of CuB2O4 at various temperatures and crystal orientations. It is shown that for T>21 K, the crystal is in a paramagnetic state determined by Cu2+ copper ions with an effective magnetic moment of 1.77 µB. Within the 10–21 K interval, the field dependence of the magnetization is typical of a weak ferromagnet with magnetic moments of the two antiferromagnetically coupled sublattices lying in the tetragonal plane of the crystal. The spontaneous weakly-ferromagnetic moment is 0.56 emu/g at 10 K. The canting angle of the sublattice magnetic moments, determined by the Dzyaloshinski-Moriya interaction, is 0.49°. It is believed that below 10 K, the CuB2O4 crystal retains its easy-plane magnetic structure, but with a zero spontaneous magnetic moment.

Refined crystal structure of copper metaborate CuB2O4

Refined crystal structure of copper metaborate CuB2O4

The crystal structure of copper metaborate, CuB2O4

This work forms part of the Ph. D. Thesis of one of us (M.M.R.) who also acknowledges the research grant given by Ministerio de Educacion y Ciencia, Spain.