K. I. Kamilov

Magnetoelectric and magnetoelastic properties of rare-earth ferroborates

The magnetic, electric, magnetoelectric, and magnetoelastic properties of rare-earth ferroborates RFe3(BO3)4 (R=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) as well as yttrium ferroborate YFe3(BO3)4 have been studied comprehensively. A strong dependence not only of the magnetic but also magnetoelectric properties on the type of rare-earth ion, specifically, on its anisotropy, which determines the magnetic structure and the large contribution to the electric polarization, has been found. This is manifested in the strong temperature dependence of the polarization below the Neel point TN and its specific field dependence, which is determined by the competition between the external and exchange f-d fields. A close correlation has been found between the magnetoelastic properties of ferroborates and the magnetoelastic and magnetic anomalies at magnetic-field induced phase transitions. It is found that in easy-plane ferroborates, together with magnetic-field induced electric polarization spontaneous polarization also ari...

Magnetocaloric properties of La0.7Ca0.3Mn16O3 and La0.7Ca0.3Mn18O3 manganites and their “sandwich”

The effect of 16О → 18О isotope substitution on specific heat and magnetocaloric effect of polycrystalline La0.7Ca0.3MnO3 manganite is studied. Mainly the effect of isotope substitution for the specific heat and magnetocaloric effect is only the reduction of temperatures of anomalies. ΔTad values at magnetic field change ΔH = 18 kOe are equal to ΔTad = 2.41 K and 2.60 K for La0.7Ca0.3Mn16O3 (LCMO16) and La0.7Ca0.3Mn18O3 (LCMO18), respectively. The sandwich of the LCMO16 and LCMO18 samples was produced for direct measurement of ΔTad. The use of sandwich from materials with near similar magnetocaloric properties increases the relative cooling power by about 20%.

Magnetic and structural phase transitions in YMn2O5 ferromagnetoelectric crystals induced by a strong magnetic field

Magnetic, magnetoelectric, and magnetoelastic properties of YMn2O5 ferromagnetoelectric single-crystals are investigated in strong pulsed magnetic fields of up to 250 kOe and in static magnetic fields of up to 12 kOe. It is found that, in YMn2O5 at T < TN=42 K, a transverse weakly ferromagnetic moment of σ 0=0.8 G cm3/g exists that is oriented along axis a and is attributed to the magnetoelectric interaction. When a magnetic field is directed along axis b, which is likely to be the axis of antiferromagnetism, a spin-flop transition is observed that is accompanied by jumps in magnetostriction and electric polarization. When a magnetic field is directed along axis a, the temperature of ferroelectric transition shifts from 20 to 25 K at H≈200 kOe. A theoretical analysis of the experimental results is given within phenomenological theory with regard to the fact that a YMn2O5 compound belongs to noncollinear antiferromagnetic crystals even in the exchange approximation.

Specificity of magnetoelectric effects in a new GdMnO3 magnetic ferroelectric

A complex study of the magnetic, electric, magnetoelectric, and magnetoelastic properties of GdMnO3 single crystals has been performed in the low-temperature region in strong pulsed magnetic fields up to 200 kOe. An anomaly of the dielectric constant along the a axis of a crystal has been found at 20 K, where a transition from an incommensurate modulated phase to a canted antiferromagnetic phase, as well as electric polarization along the a and b axes of the crystal induced by the magnetic field H‖ b (Hcr ∼ 40 kOe), is observed. Upon cooling the crystal in an electric field, the magnetic-field-induced electric polarization changes its sign depending on the sign of the electric field. The occurrence of the electric polarization is accompanied by anisotropic magnetostriction, which points to a correlation between the magnetoelectric and magnetoelastic properties. Based on these results, it has been stated that GdMnO3 belongs to a new family of magnetoelectric materials with the perovskite structure.

Effect of Gd-Mn exchange on phase transitions in GdMn2O5 induced by a strong magnetic field

Complex studies of the magnetic, magnetoelectric, and magnetoelastic properties of GdMn2O5 single crystals in strong pulsed magnetic fields are carried out in order to obtain additional indirect information on the character of the rare-earth and manganese spin ordering. It is shown that magnetic ordering of Gd3+ spins affects the manganese sublattice spin orientation and initiates new magnetic phase transitions. The observed magnetoelectric properties of the GdMn2O5 system are interpreted in terms of the theory of phase transitions.

Induced by magnetic field spin reorientation in YMn2O5

Abstract In ferroelectric YMn2O5 weak ferromagnetism was found along a-axis of rhombic crystal. It was shown that the nature of ferromagnetic sublattices cant is determined by magnetoelectric ordering. Reorientation of magnetic moment was observed under strong magnetic field applied along b-axis of crystal, that was accompanied by sharp increase of magnetostriction and jumps of electric polarization. In EuMn2O5, as in YMn2O5, spontaneous and magnetic field induced phase transitions from incommensurate to commensurate phase were observed, accompanied by cardinal changing of magnetoelectrical properties.

The nature of magnetoelastic anomalies in Nd0.5Sr0.5MnO3 upon suppression of charge ordering by a magnetic field

This paper reports on the results of investigations into the magnetic and magnetoelastic properties of Nd0.5Sr0.5MnO3 single crystals in pulsed magnetic fields up to 250 kOe, the magnetic and kinetic properties of these crystals in weak static magnetic fields, and their thermal expansion. It is demonstrated that the studied properties exhibit a number of anomalies due to a transition from the antiferromagnetic semiconducting state to the ferromagnetic metallic state upon suppression of charge ordering.

Anomalies of magnetic and magnetoelastic properties in Nd1−xCaxMnO3

Abstract Magnetic, anisotropic, and magnetoelastic properties have been investigated for Nd1−xCaxMnO3 monocrystals (x=0; 0.25; 0.05; 0.3) in pulsed magnetic fields up to 250 kOe in temperature range 10– 100 K . Jumps of magnetization and magnetostriction induced by the strong magnetic field H||b have been observed at H>100 kOe in the x=0, 0.25, 0.05 compounds. We attribute this phenomena to a reorientation of the weak ferromagnetic moment from c- to b-axis. Corresponding H–T phase diagrams were obtained.

Magnetoelectric phenomena in manganites R0.6Ca0.4MnO3(R = Pr, Nd) with charge ordering suppressed by a magnetic field

A change in electric polarization (up to 300 μC/m2) upon magnetic-field suppression of a charge-ordered antiferromagnetic state upon a transition to the ferromagnetic conducting phase (Hcr ∼ 65–80 kOe at 4.2 K) is discovered in Pr0.6Ca0.4MnO3 and Nd0.6Ca0.4MnO3 single crystals. The transition is also accompanied by a jump in magnetization and magnetostriction. The dependence of the induced polarization sign on the polarity of the electric field in which the sample was preliminarily cooled indicates the existence of spontaneous electric polarization. The effect is the strongest in Nd0.6Ca0.4MnO3 and is weaker by a factor of 5–10 in Pr0.6Ca0.4MnO3, for which the tolerance factor is higher. The observed effect may be associated with recently predicted noncentrosymmetric structures in doped manganites with x ∼ 0.5 (see D.V. Efremov, J. van den Brink, and D.I. Khomskii, Nature Materials 3, 853 (2004)), in which eg electrons are not localized upon charge and orbital ordering at one manganese ion, but are distributed among neighboring ions, thus forming an ordered polar dimer structure.

Phase transitions induced by a strong magnetic field in electron-doped manganites

The magnetic and magnetoelastic properties of single crystals of electron-doped rare-earth manganites La1−xSrxMnO3 are studied. Phase transitions from the A-type antiferromagnetic phase to the C-type anti-ferromagnetic phase in a strong magnetic field are revealed in La1−xSrxMnO3 manganites with a strontium content x = 0.65. A similar phase transition is observed in manganites with a strontium content x = 0.8, at which the La0.2Sr0.8MnO3 manganite is assumed to transform from the C-type antiferromagnetic phase to the G-type antiferromagnetic phase.