Magnetoelectric and magnetoelastic properties of easy-plane ferroborates with a small ionic radius

The magnetic, magnetoelectric, and magnetoelastic properties of RFe₃(BO₃)₄ ferroborates are studied. The measurement of the field dependences of the magnetoelectric polarization along the a axis in holmium ferroborate and in the mixed composition Ho_0.5Sm_0.5Fe₃(BO₃)₄ revealed the following dependences for easy-plane ferroborates: (a) the longitudinal and transverse magnetoelectric effects have the opposite signs and (b) magnetically induced polarization changes its sign in a field close to the field of exchange between rare-earth and iron ions. These dependences agree well with theoretical predictions based on the symmetry of the compounds. The relatively low f-d exchange field in holmium ferroborate (about 20 kOe), which magnetizes the rare-earth subsystem, causes smaller polarization jumps (about 30 ??C/m²) in fields lower than 10 kOe as compared to the jumps in other easy-plane ferroborates (R = Sm, Nd). The increase in the electric polarization induced in HoFe₃(BO₃)₄ in magnetic fields higher than 100 kOe (200?C300 ??C/m²) is found to be significantly smaller than in neodymium ferroborate, which indicates a substantial dependence of the magnetoelectric effects on the electronic structure of a rare-earth ion.     

Specificity of magnetoelectric effects in a new GdMnO<Subscript>3</Subscript> 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 (H_cr ～ 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.     

Magnetic and structural phase transitions in YMn<Subscript>2</Subscript>O<Subscript>5</Subscript> 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 < T_N=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.     

Nature of unusual spontaneous and field-induced phase transitions in multiferroics RMn2O5

Complex magnetic, magnetoelectric and magnetoelastic studies of spontaneous and field-induced phase transitions in TmMn₂O₅ were carried out. In the vicinity of spontaneous phase transition temperatures (35 and 25 K) the magnetoelectric and magnetoelastic dependences demonstrated the jumps of polarization and magnetostriction induced by the field ∼150 kOe. These anomalies can be attributed to the influence of magnetic field on the conditions of incommensurate-commensurate phase transition at 35 K and the reverse one at 25 K. In b-axis dependences the magnetic field-induced spin-reorientation phase transition was also observed below 20 K. Finally the magnetoelectric anomaly associated with metamagnetic transition is observed below the temperature of rare-earth subsystem ordering at relatively small critical fields of 5 kOe. This variety of spontaneous and induced phase transitions in RMn₂O₅ stems from the interplay of three magnetic subsystems: Mn³⁺, Mn⁴⁺, R³⁺. The comparison with YMn₂O₅ highlights the role of rare earth in low-temperature region (metamagnetic and spin-reorientation phase transitions), while the phase transition at higher temperatures between incommensurate and commensurate phases should be ascribed to the different temperature dependences of Mn³⁺ and Mn⁴⁺ ions. The strong correlation of magnetoelastic and magnetoelectric properties observed in the whole class of RMn₂O₅ highlights their multiferroic nature.     

Magnetoelectric effects in gadolinium iron borate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Magnetoelectric interactions have been investigated in a single crystal of gadolinium iron borate GdFe₃(BO₃)₄, whose macroscopic symmetry is characterized by the crystal class 32. Using the results of this study, the interplay of magnetic and electric orderings occurring in the system has been experimentally revealed and theoretically substantiated. The electric polarization and magnetostriction of this material that arise in spin-reorientation transitions induced by a magnetic field have been investigated experimentally. For H ‖ c and H ⊥ c, H-T phase diagrams have been constructed, and a strict correlation between the changes in the magnetoelectric and magnetoelastic properties in the observed phase transitions has been ascertained. A mechanism of specific noncollinear antiferroelectric ordering at the structural phase transition point was proposed to interpret the magnetoelectric behavior of the system within the framework of the symmetry approach in the entire temperature range. This ordering provides the conservation of the crystal class of the system when the temperature decreases to the antiferroelectric ordering point. The expressions that have been obtained for the magnetoelectric and magnetoelastic energy describe reasonably well the behavior of gadolinium iron borate observed experimentally.     

Peculiarities in the magnetic,magnetoelectric, and magnetoelastic properties of SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

Results of a complex investigation of the magnetic, magnetoelectric, and magnetoelastic properties of a SmFe₃(BO₃)₄ single crystal are presented. Samarium iron borate is similar to another easy-plane iron borate, NdFe₃(BO₃)₄, in that it possesses a large value of the magnetic-field-induced polarization (about 500 μC/m²), the sign of which changes when the field direction is changed between axes a and b of the crystal. However, the temperature dependence of the magnetic susceptibility and the field dependence of polarization and magnetostriction of the two compounds are significantly different, which is explained by the weak effect of external magnetic field on the ground-state multiplet of samarium ion, which is characterized by an extremely small value of its g-factor.     

Magnetoelectric effect in ytterbium aluminum borate YbAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The anisotropic magnetoelectric properties of an ytterbium aluminum borate YbAl (BO single crystal having noncentrosymmetric crystal structure (space group R32) are studied, including the orientational, field, and temperature dependences of the polarization in magnetic fields up to 5 T in the temperature range of 2–300 K. It has been shown experimentally for the first time that the symmetry of the observed magnetoelectric effects exactly corresponds to the trigonal structure of the crystal and is characterized by two quadratic magnetoelectric constants. The polarization in the basal plane P _{a, b} is a quadratic function of the field at low fields and reaches 250–300 μC/m² in a field of 5 T at a temperature of 2 K, almost an order of magnitude exceeding the previously reported values. A theoretical model based on the spin Hamiltonian of the ground Kramers doublet of Yb³⁺ ions in the crystal field is proposed including magnetoelectric interactions allowed by the symmetry. This model makes it possible to quantitatively describe all observed magnetic and magnetoelectric properties of YbAl₃(BO₃)₄.     

Influence of the ground state of the Pr<Superscript>3+</Superscript> ion on magnetic and magnetoelectric properties of the PrFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The magnetic, magnetoelectric, and magnetoelastic properties of a PrFe₃(BO₃)₄ single crystal and the phase transitions induced in this crystal by the magnetic field are studied both experimentally and theoretically. Unlike the previously investigated ferroborates, this material is characterized by a singlet ground state of the rare-earth ion. It is found that, below T _N = 32 K, the magnetic structure of the crystal in the absence of the magnetic field is uniaxial (l ∥ c), while, in a strong magnetic field H ∥ c (H _cr ～ 43 kOe at T = 4.2 K), a Fe³⁺ spin reorientation to the basal plane takes place. The reorientation is accompanied by anomalies in magnetization, magnetostriction, and electric polarization. The threshold field values determined in the temperature interval 2–32 K are used to plot an H-T phase diagram. The contribution of the Pr³⁺ ion ground state to the parameters under study is revealed, and the influence of the praseodymium ion on the magnetic and magnetoelectric properties of praseodymium ferroborate is analyzed.     

Effect of Gd-Mn exchange on phase transitions in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> 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.     

The possible mechanisms of phase transitions in RMn<Subscript>2</Subscript>O<Subscript>5</Subscript> oxide family

The magnetic, magnetoelectric, and magnetoelastic properties of ErMn₂O₅ single crystals have been studied at low temperatures and strong magnetic fields (up to 250 kOe) and compared to the analogous results obtained previously for YMn₂O₅. Based on these data, the possible mechanisms of various spontaneous and magnetic-field-induced phase transitions in these compounds are considered within the framework of the theory of representations of the Pbam-D _2h ⁹ space group. It is shown that a biquadratic exchange plays an important role in the formation (and mutual transformation) of magnetic structures revealed by neutron diffraction in the RMn₂O₅ oxide family.     

Magnetization,magnetoelectric polarization,and specific heat of HoGa3(BO3)4

A comprehensive experimental and theoretical study of magnetic, magnetoelectric, thermal, and spectroscopic characteristics of HoGa₃(BO₃)₄ gallium borate single crystals has been performed. A large magnetoelectric effect exceeding its values found in all iron and aluminum borates except HoAl₃(BO₃)₄ has been observed. The magnetoelectric polarization of HoGa₃(BO₃)₄ equals ΔP _ba (B _a ) ≈ ?1020 μC/m² at T = 5 K in a magnetic field of 9 T. The theoretical treatment based on the crystal field model for rare-earth ions provides a unified approach for the consistent interpretation of all measured characteristics. The crystal-field parameters are determined. The temperature (in the 3–300 K range) and magnetic field (up to 9 T) dependences of the magnetization, the Schottky anomaly in the temperature dependence of the specific heat, and its shift in the field B ‖ c are described. To compare the thermal properties of HoGa₃(BO₃)₄ with those of HoAl₃(BO₃)₄ exhibiting record values of the polarization, the specific heat of HoAl₃(BO₃)₄ at various B values and the temperature dependence of the polarization ΔP _b (T) in the applied magnetic field of 9 T have been measured.     

Thermal-expansion and magnetostriction anomalies in phase transitions in La1−x SrxMnO3

A study is reported on phase transitions in the La_1?x Sr_xMnO₃ system, both spontaneous and induced by a pulsed magnetic field of up to 250 kOe, accompanied by anomalies in magnetoelastic properties. The temperatures of the polaron (charge) and magnetic ordering, as well as those of structural transitions, are observed to be in good agreement with the results obtained by other methods. Jumps in the field dependence of longitudinal and transverse magnetostriction associated with field-induced orbital ordering have been found. A strong temperature dependence of the corresponding threshold fields is observed.     

Ferrimagnetism and hyperfine interactions in RFe5Al7(R = rare earth)

Magnetization and Mössbauer (⁵⁷Fe, ¹⁵⁵Gd) studies of RFe₅Al₇(R = Y, Sm to Lu, ThMn₁₂ crystal structure) in magnetic fields up to 50 kOe and temperatures 4.1 to 500 K have been performed. The Mössbauer studies yield the distribution of the iron ions among the various inequivalent crystallographic and magnetic sites, the hyperfine fields and their temperature dependence. The magnetization curves display a great variety of unusual magnetic phenomena. Among those; strong anisotropy, magnetic and thermal hysteresis (H_c = 24 kOe for DyFe₅Al₇ at 4.1 K), negative magnetization at low temperatures when the sample is cooled in a magnetic field (even in 1 Oe), compensation points, maxima points and time-dependent magnetization. Most of the phenomena observed can be explained in terms of a spin structure previously suggested for the RFe₆Al₆ compounds, composed of 4 magnetic sublattices. The rare earth moments lie antiparallel to the iron moments in the (j) site and to the ferromagnetic component of a canted antiferromagnetic structure of iron in the (f) site. Iron in the (i) site is nonmagnetic.     

Observation of spontaneous spin reorientation in Nd1 ? xDyxFe3(BO3)4 ferroborates with a competitive R-Fe exchange

The magnetic, magnetoelectric, and magnetoelastic properties of DyFe₃(BO₃)₄ single crystals and the Nd_0.75Dy_0.25Fe₃(BO₃)₄ substituted compound with the competitive Nd-Fe and Dy-Fe exchange interactions have been investigated in magnetic fields up to 210 kOe. It has been shown that the antiferromagnetic state in Nd_0.75Dy_0.25Fe₃(BO₃)₄ below the Néel temperature is easy-plane and a spontaneous spin-reorientation transition to a uniaxial state occurs near 25 K. Anomalies are observed in various physical characteristics in a spin-flop transition induced by the magnetic field along the trigonal axis. The H-T phase diagrams are constructed; they are in good agreement with a simple model taking into account the anisotropy of the exchange splitting and g factors of the ground doublets of the Dy³⁺ and Nd³⁺ ions. The features of the magnetoelectric effect in various magnetic phases have been studied. Spontaneous electric polarization has been revealed in the region of the existence of an easy-plane antiferromagnetic state in the substituted compound. A qualitative theoretical justification of the possibility of the appearance of the spontaneous spin-reorientation transition in the Nd_0.75Dy_0.25Fe₃(BO₃)₄ substituted compound is given. Original Russian Text ? Yu.F. Popov, A.M. Kadomtseva, G.P. Vorob’ev, A.A. Mukhin, V.Yu. Ivanov, A.M. Kuz’menko, A.S. Prokhorov, L.N. Bezmaternykh, V.L. Temerov, 2009, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2009, Vol. 89, No. 7, pp. 405–411.     

The nature of magnetoelastic anomalies in Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> upon suppression of charge ordering by a magnetic field

This paper reports on the results of investigations into the magnetic and magnetoelastic properties of Nd_0.5Sr_0.5MnO₃ 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.     

On the intensity of shock-initiated magnetoelastic oscillations arising in iron borate single crystals during pulsed magnetizing or magnetization-switching processes

The intensities of magnetoelastic oscillations accompanying pulsed 180° and 90° magnetization switching of iron borate single crystals as well as pulsed magnetization of the crystals from a demagnetized state (with zero total magnetic moment) are compared for the first time. The amplitude A ₁ of the oscillations of the signal obtained from the experimental sample by the induction method is adopted as a measure of the intensity of the magnetoelastic oscillations. It is found that for the same pulse heights of the magnetic field H exciting the magnetization-switching or magnetizing process, the amplitudes A ₁ of the oscillations observed in 90° magnetization-switching and initial-magnetization processes have practically the same value, which is $\sqrt 2 $ times smaller than the amplitude of the oscillations obtained in 180° magnetization switching (reversal). It is concluded on the basis of the result obtained that the intensity of the magnetoelastic oscillations is virtually independent of the initial state of the single crystal and is determined mainly by the energy density ΔM·H acquired by the magnetic subsystem of the crystal from the external field (ΔM is the change in magnetization). Hence it follows that when iron borate is used in fast modulators for Mössbauer γ rays it is preferable to use the 90° magnetization-switching regime rather than the magnetization regime as has been done until very recently.     

Features of the magnetic and magnetoelectric properties of HoAl3(BO3)4

The main features of the magnetic and record magnetoelectric properties of a HoAl₃(BO₃)₄ aluminoborate single crystal have been studied experimentally and theoretically. It has been found that the electric polarization that was previously detected in HoAl₃(BO₃)₄ and is record for multiferroics is significantly larger, ΔP _ba (B _a ) ≈ ?5240 μC/m², with an increase in the magnetic field to 9 T at T = 5 K. The measured magnetic properties and revealed features have been interpreted within a united theoretical approach based on the molecular field approximation and on calculations in the crystal field model for a rare-earth ion. The experimental temperature (from 3 to 300 K) and field (up to 9 T) dependences of the magnetization have been described. The parameters of the crystal field of trigonal symmetry for a Ho³⁺ ion in HoAl₃(BO₃)₄ have been determined from the interpretation of the experimental data.     

Ultralow-frequency magnetoelectric effect in a multilayer ferrite-piezoelectric structure

The magnetoelectric effect is experimentally studied in a multilayer nickel zinc ferrite-lead zirconate titanate structure at frequencies of 10^?3–10 Hz that is placed in a harmonically modulated magnetic field of amplitude to 1 kOe. The nonlinearity of the ferrite magnetostriction and the conductivity of the films are shown to double the frequency and distort the shape of the magnetoelectric voltage. The magnetoelectric signal amplitude decreases linearly with decreasing field modulation frequency. The feasibility of using the magnetoelectric effect to detect ultralow-frequency magnetic fields is demonstrated.     

Effect of the magnetic field on the 3d ordering and on the spin correlations of the linear antiferromagnet TMMC

The effect of an external magnetic field in the range 6–47 kOe on the low temperature proton spin-lattice relaxation rate in TMMC has been investigated. A peak in T₁^?1 at the 3d ordering temperature has been detected. Values of T_N for fields up to 47 kOe have been determined.