M. Gospodinov

Field-Induced Reentrant Novel Phase and a Ferroelectric-Magnetic Order Coupling in HoMnO 3

A reentrant novel phase is observed in the hexagonal ferroelectric HoMnO3 in the presence of magnetic fields in the temperature range defined by a plateau of the dielectric constant anomaly. The plateau evolves with fields from a narrow dielectric peak at the Mn-spin rotation transition at 32.8 K in zero field. The anomaly appears both as a function of temperature and as a function of magnetic field without detectable hysteresis. This is attributed to the indirect coupling between the ferroelectric (FE) and antiferromagnetic (AFM) orders, arising from an FE-AFM domain wall effect.

Short-range B -site ordering in the inverse spinel ferrite NiFe 2 O 4

. The numberof experimentally observed Raman modes exceeds significantly that expected for a normal spinelstructure and the polarization properties of most of the Raman lines provide evidence for a micro-scopic symmetry lower than that given by the Fd¯3m space group. We argue that the experimentalresults can be explained by considering the short range 1:1 ordering of Ni

Magnetic frustration in an iron-based Cairo pentagonal lattice.

The Fe3+ lattice in the Bi2Fe4O9 compound is found to materialize the first analogue of a magnetic pentagonal lattice. Because of its odd number of bonds per elemental brick, this lattice, subject to first neighbor antiferromagnetic interactions, is prone to geometric frustration. The Bi2Fe4O9 magnetic properties have been investigated by macroscopic magnetic measurements and neutron diffraction. The observed noncollinear magnetic arrangement is related to the one stabilized on a perfect tiling as obtained from a mean field analysis with direct space magnetic configuration calculations. The peculiarity of this structure arises from the complex connectivity of the pentagonal lattice, a novel feature compared to the well-known case of triangle-based lattices.

Anisotropy-enhanced giant reversible rotating magnetocaloric effect in HoMn2O5 single crystals

Magnetic and magnetocaloric properties of HoMn2O5 single crystals were investigated. HoMn2O5 undergoes a large conventional magnetocaloric effect around 10 K. The magnetocaloric effect was found to present a giant anisotropy. Consequently, a large magnetocaloric effect (−ΔSR,max= 12.43 J/kg K for 7 T) can be obtained simply by rotating the single crystal HoMn2O5 within the cb plane in constant magnetic field instead of moving it in and out of the magnetic field zone. This can open the way for the implementation of compact, simplified, and efficient rotary magnetic refrigerators.

Raman and infrared-active phonons in hexagonal HoMnO3 single crystals: magnetic ordering effects

Polarized first- and second-order Raman scattering and infrared reflection spectra of hexagonal HoMnO3 single crystals in the temperature range 10–300 K are reported. Based on the symmetry analysis and comparison with the results of lattice dynamics calculations the observed lines are assigned to the lattice eigenmodes. The magnetic ordering of Mn ions, which occurs below TN = 76 K, is shown to affect Raman- and infrared-active phonons, which modulate Mn–O–Mn bonds and, consequently, the Mn–Mn exchange interaction.

Field-induced phases inHoMnO3at low temperatures

The novel field-induced re-entrant phase in multiferroic hexagonal HoMnO3 is investigated to lower temperatures by dc magnetization, ac susceptibility, and specific heat measurements at various magnetic fields. Two new phases have been unambiguously identified below the Neel transition temperature, TN=76 K, for magnetic fields up to 50 kOe. The existence of an intermediate phase between the P[6]_3[c]m and P[6]_3c[m] magnetic structures (previously predicted from dielectric measurements) was confirmed and the magnetic properties of this phase have been investigated. At low temperatures (T<5 K) a dome shaped phase boundary characterized by a magnetization jump and a narrow heat capacity peak was detected between the magnetic fields of 5 kOe and 18 kOe. The transition across this phase boundary is of first order and the magnetization and entropy jumps obey the magnetic analogue of the Clausius-Clapeyron relation. Four of the five low-temperature phases coexist at a tetracritical point at 2 K and 18 kOe. The complex magnetic phase diagram so derived provides an informative basis for unraveling the underlying driving forces for the occurrence of the various phases and the coupling between the different orders.

Structural anomalies at the magnetic and ferroelectric transitions in RMn 2 O 5 (R=Tb,Dy,Ho)

Strong anomalies of the thermal expansion coefficients at the magnetic and ferroelectric transitions have been detected in multiferroic

Magnetic phase diagrams of multiferroic hexagonal RMnO3 (R = Er, Yb, Tm, and Ho)

RMn_2O_5

Pressure-induced enhancement of ferroelectricity in multiferroic R Mn 2 O 5 ( R = Tb , Dy , Ho )

. Their correlation with anomalies of the specific heat and the dielectric constant is discussed. The results provide evidence for the magnetic origin of the ferroelectricity mediated by strong spin-lattice coupling in the compounds. Neutron scattering data for

Growth, magnetic properties, and Raman scattering of La2NiMnO6 single crystals

HoMn_2O_5