金属间化合物RMn2Ge2(R=La,Pr,Nd,Sm,Gd,Tb和Y)中的自发磁相变 及相变时的磁弹性异常

在10—800K的温度范围内用X射线衍射方法测量了RMn₂Ge₂(R=La,Pr ,Nd,Sm,Gd,Tb和Y)的晶格常数与温度的变化关系.在各种类型的自发磁相变观察到晶格常数 的磁弹性异常现象.实验得出,自发磁相变时的磁弹性异常主要由Mn次晶格引起,并且Mn-Mn 交换相互作用能不仅与晶格常数a有关,而且与晶格常数c有关.用Kittle的交换反转模型讨 论了低温时的铁磁—反铁磁一阶相变. 关键词： 稀土金属间化合物 磁相变 磁弹性     

Interrelation between the soliton lattice and electric polarization in RMn<Subscript>2</Subscript>O<Subscript>5</Subscript> oxides

Magnetic transitions from the paramagnetic state to an incommensurate magnetic structure and then to an ordered phase with long-range antiferromagnetic order in RMn₂O₅ (R is a rare-earth ion) oxides are analyzed. It is shown that a transition from the paramagnetic to the incommensurate phase is associated with exchange as well as relativistic interactions and can be described, apart from the basic magnetic order parameter, by an associated order parameter (viz., electric polarization along the y axis of the crystal). As a result of such a transition, the emergence of electric polarization in the crystal is not accompanied by a change in crystal symmetry.     

Magnetic structure and physical properties of the multiferroic compound PrMn2O5

RMn₂O₅ (R=lanthanide, Bi, Y) multiferroic compounds are intensively studied for their potential application in the spintronic field. In these systems, the key issue is to understand the origin of the strong coupling between the ferroelectric and magnetic orders and to investigate the influence of the nature of the R ions in this coupling. While the phase diagram of RMn₂O₅ compounds with small R size is well established, this of large R size compounds is missing due to the lack of samples originating with difficulties of synthesis. We present in this paper the first investigation of the thermodynamic, structural and magnetic properties of high quality polycrystalline PrMn₂O₅ samples. Our work shows that PrMn₂O₅ presents two magnetic transitions corresponding to commensurate magnetic orderings. We also evidence a weak lattice effect coupled to the magnetic order. Our results point out that the physical properties of PrMn₂O₅ differ from those of the parent compounds with magnetic R ions.     

Enhanced exchange bias in magnetron-sputtered Ni–Mn–Sb–Al ferromagnetic shape memory alloy thin films

In the present study, the influence of aluminium (Al) addition on the martensite-austenite phase transformation and exchange bias of Ni–Mn–Sb films have been investigated. Ni–Mn–Sb–Al films with different Al concentration (∼0–5.6%) were deposited by co-sputtering of Ni–Mn–Sb and Al targets. Experimental results revealed the decrease in martensitic transformation temperature with increasing Al content upto a certain extent (3.3%) beyond which martensitic transformation was suppressed. Paramagnetic to ferromagnetic transition temperature (T_C) also decreased with increasing Al concentration. Ni₅₀Mn_36.3Sb_10.4Al_3.3 thin film showed significant improvement in exchange bias field as compared to pure Ni_50.3Mn_36.9Sb_12.8 thin film. This enhancement in the exchange bias field H_EB = 611 Oe at 10 K is attributed to the increase of AFM-FM interactions that result from the decrease of Mn–Mn distance due to the incorporation of Al atoms. This behaviour is an additional property of the FSMA thin films apart from various other multifunctional properties and therefore, is of technological importance for their applications in magnetic storage devices.     

X-ray studies of magnetic phase transitions in the intermetallic compounds RMn2Ge2 (R=La, Sm, Gd, Nd, Tb, and Y)

The lattice parameters a and c of the tetragonal intermetallic compounds RMn₂Ge₂ (R=La, Sm, Gd, Nd, Tb, and Y) have been measured by x-ray diffraction in the temperature interval 10–800 K. Anomalies are observed in the temperature dependence of a and c due to phase transitions from the paramagnetic to the magnetically ordered state in the Mn subsystem, transitions between various magnetically ordered phases due to a change in the magnitude and sign of the Mn-Mn exchange interaction, and magnetic transitions caused by ordering of the rare-earth subsystem leading to a rearrangement of the magnetic structure of the Mn subsystem. It is concluded that, along with the lattice parameter a, the lattice parameter c also has an influence on the Mn-Mn exchange interaction. Fiz. Tverd. Tela (St. Petersburg) 41, 2053–2058 (November 1999)     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

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.     

Magnetic phase transition and the corresponding magnetostriction of intermetallic compounds RMnsGe2（R=Sm,Gd）

The temperature dependence of lattice constants a and c of intermetallic compounds RMn₂Ge₂ (R=Sm, Gd) is measured in the temperature range 10－800K by using the x-ray diffraction method. The magnetoelastic anomalies of lattice constants are found at the different kinds of spontaneous magnetic transitions. The transversal and longitudinal magnetostrictions of polycrystalline samples are measured in the pulse magnetic field up to 25T. In the external magnetic field there occurs a first-order field-induced antiferromagnetism－ferromagnetism transition in the Mn sublattice, which gives rise to a large magnetostriction. The magnitude of magnetostrictions is as large as 10^-3. The transversal and longitudinal magnetostrictions have the same sign and are almost equal. This indicates that the magnetostriction is isotropic and mainly caused by the interlayer Mn－Mn exchange interaction. The experimental results are explained in the framework of a two-sublattice ferrimagnet with the negative exchange interaction in one of the sublattices by taking into account the lattice constant dependence of interlayer Mn－Mn exchange interaction.     

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 coupling in RMn2O5 multiferroic: a Monte Carlo simulation

Magnetoelectric coupling in RMn₂O₅ (with R?=?non magnetic) multiferroics have been studied using the Monte Carlo simulation. The variation of magnetization and the polarization of RMn₂O₅ multiferroic have been determined. The system undergoes a magnetic transition at T_N and a further reduction of the temperature leads to a ferroelectric transition at T_C?<_? T_N depending on the coupling strength. Magnetic and ferroelectric hysteresis loops are obtained for several temperatures values. Variation of polarization with the external magnetic field of RMn₂O₅ has been given. Variation of polarization and magnetization with the electric field of RMn₂O₅ has been obtained.     

Magnetic properties of RNi2Si2 compounds (R= rre earth)

We report an extensive study of the magnetic properties of tetragonal RNi₂Si₂ compounds (R=Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm), through resistivity, neutron diffraction, susceptibility and magnetization experiments. All compounds exhibit complex incommensurate antiferromagnetic structures, while a transition occurs in TbNi₂Si₂ between a modulated phase and a simple antiferromagnetic structure, stable at low temperature. The magnitude of the bilinear exchange interactions deviates from the Gennes law and the direction of the ordered magnetic moments presents anomalies across the series, including the probable existence of other types of interactions between the rare earth ions.     

Magnetic dynamics of phase separation domains in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> and Gd<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ are compared with the properties of the previously investigated RMn₂O₅ (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn₂O₅ compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn₂O₅ is built.     

Magnetic and luminescent properties of manganese-doped ZnSe crystals

Magnetic and photoluminescent properties of manganese-doped ZnSe crystals with different impurity concentrations were investigated. The concentration of Mn²⁺ ions in ZnSe crystals has been varied from 0.01 to 0.3 at%. Magnetic and photoluminescent studies have confirmed the introduction of Mn in ZnSe crystals. It was established that Mn²⁺ ions are responsible for the emission bands with maximum at 616 nm and 633 nm, which correspond to ⁴T₂→⁶A₁ and ⁴T₁→⁶A₁ intracentre transitions of Mn²⁺ ions respectively. It was found that the concentration quenching of the photoluminescent bands is associated with Mn²⁺ ions, which are due to the formation of Mn–Mn clusters. Magnetic properties studies have shown that at high doping levels the manganese atoms form Mn–Mn clusters in ZnSe. From the temperature dependence of magnetic susceptibility of ZnSe:Mn crystals that follows the Curie–Weiss law, it was possible to estimate the Curie–Weiss temperature Θ(x) and the effective Mn–Mn antiferromagnetic exchange constant (J₁).     

Magnetic Structures and Magnetic Phase Transitions in Rare-Earth <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>Si<Subscript>2</Subscript> Intermetallic Compounds (<Emphasis Type="Italic">R</Emphasis> = Sm,Tb)

The magnetic structures that form in La_1–xR_xMn₂Si₂ (R = Sm, Tb) layered compounds with various concentrations x have been determined by magnetic neutron diffraction and magnetic measurements, and the magnetic phase diagrams have been built. It is shown that the formation of the magnetic structures is dependent not only on exchange interactions, but also on the type of the magnetic anisotropy of a rare-earth atom. It is found that, in La_1–xTb_xMn₂Si₂ compounds with 0.2 < x < 0.5, the competition of the Tb–Mn and Mn–Mn interlayer exchange interactions and the existence of a strong uniaxial magnetic anisotropy in the Mn and Tb sublattices leads to the frustrated magnetic state and prevents the formation of the long-range magnetic order in the Tb sublattice.     

Magnetic phase transitions and phase diagrams of DyMn2Ge2

The experimental studies of magnetic phase transitions in the layered tetragonal intermetallic compound DyMn₂Ge₂ are continued. The existence of spontaneous phase transitions is confirmed by the results of measurements of the temperature dependences of lattice parameters and the initial magnetic susceptibility. The measurements in strong (up to 50 T) and ultrastrong (up to 150 T) fields revealed two new field-induced magnetic transitions. The inclusion of the exchange interaction between next-to-nearest layers of manganese and the crystal field effects for the rare-earth subsystem along with the antiferromagnetic exchange interaction of nearest Mn layers has made it possible to describe the magnetic properties of DyMn₂Ge₂ in a wide range of magnetic fields. The parameters of these interactions are determined from a comparison of the experimental and theoretical magnetization curves and H-T phase diagrams.     

Magnetic and magnetocaloric properties of Pr0.6−xEuxSr0.4MnO3 manganese oxides

Studies of the structural, magnetic and magnetocaloric properties of polycrystalline Pr_0.6−xEu_xSr_0.4MnO₃ (0≤x≤0.15) perovskite manganites were carried out. Substitution for praseodymium with europium, with smaller ionic radius, induces local distortion in the 〈Mn–O–Mn〉 bonds and consequently causes a random distribution in the magnetic exchange interactions. The competition between magnetic interactions leads to the appearance of an inhomogeneous magnetic state in our samples. Pr_0.6−xEu_xSr_0.4MnO₃ (x=0, 0.05, 0.1 and 0.15) polycrystalline samples were prepared using the solid–solid reaction method at high temperature. The compounds yielded are single phase and crystallize in the orthorhombic system with the Pnma space group. The substitution of Eu for Pr leads to a decrease of the Curie temperature T_C from 303 K for x=0.00 to 260 K for x=0.15. All of our compounds exhibit a large magnetic entropy change with a maximum around 2.2 J/kg K under a magnetic applied field change of 2 T for all compounds.     

Intrinsic magnetic inhomogeneity of Eu substituted La0.7Pb0.3Mno3 single crystals

The study of magnetic and magnetotransport properties of the crystals of (La_1−yEu_y)_0.7Pb_0.3MnO₃ system has been carried out. Eu ions enter the crystals being in trivalent nonmagnetic state. Europium ions possessing of smaller ionic radius in comparison with La ions, induce local distortions of Mn–O–Mn bonds in the system that cause random distribution of magnetic exchange interactions in magnitude and, probably, in sign. The competition of magnetic interactions leads to the appearance of the inhomogeneous magnetic state in the crystals. The enhancement of concentration of Eu ions results in decrease of the Curie temperature and broadening of the inhomogeneous magnetic state area. At y=0–0.4 the coexistence of the paramagnetic phase with conductivity of the polaronic type and the ferromagnetic metallic phase is observed in a bounded temperature interval both above and below T_C. Below T_C the increasing of y up to 0.6 induces the magnetic state representing the coexistence of two different FM phases. These phases are spatially separated due to frustration of FM and AFM exchange interactions on phase boundaries. Above T_C, up to 1.6T_C ferromagnetic clusters exist in a paramagnetic matrix similar to the case of samples with y=0–0.4. Concerning electric properties, the samples with y=0–0.4 reveal the metal–insulator transition at temperature that practically coincides with T_C. The sample with y=0.6 has conductivity of insulator character up to the lowest temperatures. For all investigated compositions y=0–0.6 the CMR effect is observed in the area where the inhomogeneous magnetic state exists. The effect is determined by different conductivity of the coexisting phases, as well as by sensitivity of the inhomogeneous state to external magnetic field.     

Magnetostriction of SmMn2Ge2 and GdMn2Ge2 intermetallic compounds

Longitudinal and transverse magnetostrictions of polycrystalline samples of intermetallic compounds RMn₂Ge₂ (R=Sm or Gd) are measured in pulsed magnetic fields up to 250 kOe. It is found that linear magnetostrictive strains of about 10^?3 arise in a temperature range in which the magnetic field causes a change in the magnetic state of a manganese magnetic subsystem. The results obtained are described within the model of a two-sublattice ferrimagnet with a negative exchange interaction in the manganese subsystem in terms of a strong dependence of this interaction on interatomic distances.     

Magnetic transition from the paramagnetic to long-period structure in RMn2O5 multiferroics: Renormalization group analysis of critical behavior

A transition from the paramagnetic state to a long-period magnetic structure with an incommensurate wave vector along one crystallographic axis in RMn₂O₅ multiferroics is considered. An effective Hamiltonian for these oxides is constructed with allowance for spin fluctuations. Critical points are found, and their stability is analyzed using the renormalization group approach. It is shown that critical fluctuations in these compounds admit a second-order phase transition with respect to a multicomponent order parameter.     

Magnetic transitions in layered triangular antiferromagnets

Magnetic states and phase transitions of the layered triangular antiferromagnets in an applied field are studied. It is shown that in compounds like VBr₂ and VCl₂ quantum effects change the ground-state structure and cause successive phase transitions as the magnetic field increases. Coplanar structures of different spin configuration are realized far from the saturation field and a noncoplanar structure of umbrella-type configuration is realized near this field. The ground-state phase diagram is constructed, and a finite region of fields where the collinear phase is also possible is indicated.