Simulation of the magnetic properties of manganese oxide Pb<Subscript>3</Subscript>Mn<Subscript>7</Subscript>O<Subscript>15</Subscript>

The magnetic susceptibility, heat capacity, and spin-spin correlation functions of manganese oxide Pb₃Mn₇O₁₅ are calculated by the Monte Carlo method. Two critical temperatures are determined: T ₁ ≈ 20 K, above which a modulated structure along the hexagonal axis is formed, and T ₂ ≈ 70 K, at which the long-range magnetic order disappears. The antiferromagnetic exchange interaction constant in a hexagonal plane is estimated to be J ₁ ～ 7 K, and the antiferromagnetic and ferromagnetic exchange interaction constants between hexagonal planes are calculated to be J ₂ ～ 3 K and K ～ 50 K, respectively.     

Magnetic and crystallographic study of Tb0.75Y0.25Co3B2

Tb_0.75Y_0.25Co₃B₂ was studied as a function of temperature by neutron powder diffraction, ac susceptibility and SQUID magnetization measurements. The solid solution, which is of hexagonal symmetry and is paramagnetic at 300 K, undergoes a magnetic Co–Co ordering transition at ∼150 K, and a second magnetic Tb–Tb ordering transition at ∼17 K. The latter induces a spin-reorientation transition, in which the magnetic axis rotates from the c-axis toward the basal plane. The component of the magnetic axis, which is perpendicular to c, leads to a crystal symmetry reduction from hexagonal to monoclinic. The observed magnitude of the magnetic moment of the Tb ion is 1.5 μ_B, unusually small relative to the free ion and parent compound (TbCo₃B₂) values. These magnetic and crystal properties are discussed and compared with what was previously published for the parent compound.     

Mechanism of the formation of an uncompensated magnetic moment in bacterial ferrihydrite nanoparticles

The magnetic properties of antiferromagnetic nanoparticles of FeOOH · nH₂O with sizes of 3–7 nm, which are products of vital functions of Klebsiella oxytoca bacteria, have been studied. Particles exhibit a superparamagnetic behavior. The characteristic blocking temperature is 23 K. Analysis of the magnetization curves shows that the mechanism of the formation of the uncompensated magnetic moment of particles is the random decompensation of magnetic moments of Fe³⁺ ions both on the surface and in the bulk of the antiferromagnetic particle. In this mechanism, the exchange coupling between the uncompensated magnetic moment of the particle and its antiferromagnetic “core” is implemented. It has been found that the temperature dependence of the uncompensated magnetic moment has the form 1 — constT ².     

High-pressure effect on the crystal and magnetic structure of LuMnO3: Correlation between a distortion of the triangular lattice and the symmetry of the magnetic state in hexagonal frustrated manganites

The effect of a high pressure (up to 6 GPa) on the crystal and magnetic structure of the hexagonal manganite LuMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. It is found that, as the pressure increases, the ordered magnetic moment of Mn ions at T = 10 K decreases noticeably from 2.48 (0 GPa) to 1.98 μ_B (6 GPa). This decrease is due to an enhancement of the geometrical frustration effects on the triangular lattice. At the same time, the symmetry of the triangular antiferromagnetic state (the irreducible representation Γ₂) remains unchanged. A correlation is revealed between the distortion parameter of the triangular lattice formed by Mn ions and the symmetry of the antiferromagnetic state of hexagonal manganites RMnO₃. Based on this correlation, a generalized magnetic phase diagram of these compounds is constructed. The obtained phase diagram provides an explanation for the changes observed in the magnetic state of hexagonal manganites caused by high pressure and chemical substitution.     

Magnetic order in the ternary compound NdRu2Ge2

Heat capacity, electrical resistivity and neutron diffraction studies have been performed on the tetragonal ternary compound NdRu₂Ge₂. Between temperatures of 17 and 10 K, the compound exhibits two types of sine-wave modulated magnetic structures, having wave vectors k = (0.19, 0.05, 0.125) and k = (0.12, 0.12, 0.0) with the amplitude of the moment along the c axis. Below 10 K, a first-order transition occurs to a ferromagnetic state with a moment of 3.64(13)μ_B aligned along the tetragonal axis.     

Specific features of the formation of the ground state in PrB6

The temperature dependences of the thermal conductivity and magnetization of a PrB₆ single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ～ 7 and ～4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB₆ at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.     

Magnetic and dielectric properties of orthorhombic and hexagonal multiferroics Tb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript>

The transition from a stable orthorhombic structure to a hexagonal structure has been revealed in Tb_1−x Y _x MnO₃ multiferroics at x = 0.2–0.4. It has been shown that almost single-phase crystals with an orthorhombic or hexagonal structure can be obtained by choosing the growth conditions. It has been found that the magnetic and dielectric properties of orthorhombic single crystals with x = 0.2–0.3 are similar to the properties of pure TbMnO₃ and are characterized by a strong anisotropy of the magnetic susceptibility at low temperatures and by the presence of a number of magnetic phase transitions, including those to the ferroelectric state. New spontaneous (T ≤ 15 K) and magnetic-field induced (H | C ₆) phase transitions accompanied by the appearance of an uncompensated rare-earth magnetic moment ∼1 μ_B/mole have been observed in hexagonal single crystals with x = 0.3–0.5.     

151Eu Mössbauer Spectroscopy in La0.38Eu0.29Ca0.33MnO3

Electrical conductivity with and without magnetic field, d.c. magnetization and ¹⁵¹Eu Mössbauer studies were carried out in La_0.38Eu_0.29Ca_0.33MnO₃ perovskite manganite system. An insulating ground state is found throughout the temperature range with charge ordered (CO) state emerging at T _CO ～ 140 K, where as an external magnetic field of 6 T induces metal-insulator transition at ～120 K. D.C. magnetization measurements show the antiferromagnetic (AFM) transition occurring at T _N ≈ 48 K. The temperature dependent ¹⁵¹Eu Mössbauer measurements showed that the substituted Eu replaces La³⁺ in the 3+ charge state and a small magnetic moment gets induced at the Eu nucleus at low temperatures. The anomalous variation of the f- factor with temperature occurring around T _N and T _CO corroborates the occurrence of antiferromagnetic (AFM) and charge ordering (CO) transition, respectively.     

Magnetic properties of Europium-graphite intercalation compound C6Eu

An ordered state with a complex magnetic structure has been observed below 4CK. The magnetization for H ⊥ c (c; the c-axis) shows a field-induced phase transition from a small magnetic moment state ( ～ 0.6μ_B) at lower fields to a larger moment one ( 2.2 - 2.5μ_B) above 25kG. The transition field depends on temperature and is found to be 5 - 2kG. The magnetization for H//c has only a sublinear field dependence. Above 40K the magnetic susceptibility obeys the Curie-Weiss law with θ = +1.3K and the molar Curie constant C_m = 6.78, which suggests that Eu ion is divalent in C₆Eu.     

Magnetic structure and thermodynamic properties of TmPtIn

Physical properties of TmPtIn have been investigated by means of magnetic, electrical transport, calorimetric as well as neutron diffraction measurements. The compound crystallizes in the hexagonal ZrNiAl-type crystal structure. It orders antiferromagnetically below T_N = 3.5 K with the Tm magnetic moments confined to the basal hexagonal plane. They form a non-collinear “triangular” magnetic structure that may be described by the propagation vector . At 1.6 K, the Tm magnetic moment is equal to 5.59(9)μ_B. The antiferromagnetic character of the electronic ground state is reflected in the low temperature behaviors of the magnetic susceptibility and the specific heat, which may be described by spin-wave theory of antiferromagnetic magnons with linear dispersion relation. The compound exhibits metallic character of electrical conduction.     

Magnetic phase transition in TbMn2Ge2

The crystal and magnetic stucture of TbMn₂Ge₂ are determined by neutron diffraction using a powder sample. The crystal structure of this compound is of the ThCr₂Si₂ type with small mixing of Mn and Ge atoms between 4(d) and 4(e) positions. At RT the antiferromagnetic collinear structure consist of a+?+? sequence of ferromagnetic layers of Mn atoms with the magnetic moment parallel to the c-axis. At 85 K, the ferromagnetic ordering within the Tb sublattice is observed. The magnetic moment (～7.7 μ_B) is parallel to the c-axis. At 4.2 K additional reflections are observed, which correspond to antiferromagnetic components in a monoclinic unit cell.     

Magnetization anisotropy of the Tm- and Fe-subsystems in Tm2Fe17

Magnetic and neutron diffraction measurements were carried out in order to study the spontaneous and induced spin-reorientation (SR) transition of the “easy axis–easy plane” type in the poly and single-crystalline samples of the hexagonal Tm₂Fe₁₇. We have determined the temperature dependence of the lattice parameters and the angle between the c-axis and the magnetic moment of the Tm-subsystem. We also find that the SR transition is accompanied by a large (about 20%) magnetization change of the Tm subsystem. In order to induce such a SR transition with the external magnetic field, μ₀H_cr=5 T is necessary to be applied along the hard-magnetization direction (the a-axis) at 4.2 K. The H_cr value decreases with an increasing temperature. The magnetization measurements demonstrate that at 10 K the saturation magnetization along the easy-magnetization direction (the c-axis) is smaller than that along the hard-magnetization direction. Based on this observation, we believe that Fe-subsystem of Tm₂Fe₁₇ is likely to have magnetization anisotropy.     

High-pressure effects on the crystal and magnetic structure of the Nd0.78Ba0.22CoO3 cobaltite

The crystal and magnetic structure of the Nd_0.78Ba_0.22CoO₃ cobaltite is studied by neutron diffraction at high pressures up to 4.2 GPa in the temperature range 10–300 K. The pressure dependences of structural parameters are obtained. Ferromagnetic ordering of the Co sublattice is observed at normal pressure below T _C ～ 140 K, and ferrimagnetic ordering of the Co and Nd sublattices with an antiparallel direction of magnetic moments appears at T _F ～ 40 K. The magnetic moment of Co and the temperature T _C change slightly under pressure, which points to the stability of the initial intermediate-spin (S = 1) state of Co³⁺ ions. This behavior differs considerably from the characteristic behavior of cobaltites that are close in chemical composition and structure and exhibit ferromagnetic ordering of only the Co sublattice. In these cobaltites, the magnetic moment of Co is substantially suppressed and T _C decreases under pressure, which is related to the change in the state of Co³⁺ ions from the intermediate spin state to the nonmagnetic low-spin state (S = 0). The interplay between the appearance of the magnetic interaction of the R-Co sublattices and the stability of the spin state of Co³⁺ ions in the Nd_0.78Ba_0.22CoO₃ cobaltite is discussed.     

Thermal and magnetic studies of the nearly one-dimensional antiferromagnetic system CsNiBr3

Magnetic susceptibility, specific heat and ¹³³Cs magnetic resonance measurements in a single crystal of CsNiBr₃ are reported. The data reveal two magnetic transitions separating the paramagnetic phase from the antiferromagnetic ground state. At the higher transition temperature T_N2 = (14.25 ± 0.05)K a net magnetic moment is observed only along the hexagonal c-axis, while only below the lower transition temperature T_N1 = (11.75 ± 0.05)K a perpendicular component of the magnetic moment appears also. Above T_N2 CsNiBr₃ can be described as a one-dimensional antiferromagnet with intrachain exchange interaction $\text{J}\text{k}{}_{\mathrm{B}}\text{= ?(17.0 ± 0.2)}\text{K}$ and single-ion anisotropy constant $\text{D}\text{k}{}_{\mathrm{B}}\text{? ?1.5}\text{K}$. Below T_N1, the data are consistent with the non-colinear triangular structure of the Ni²⁺ moments proposed previously for the isomorphic crystal CsNiCl₃. A reduced value of the zero-temperature susceptibility over the classical value is found and atrributed to the zero point deviations.     

Simultaneous gamma, x-ray, and electron Mössbauer spectroscopy of the volume and surface magnetic structure of hexagonal M-ferrites

Direct comparative studies are made between the magnetic structures of a surface layer of thickness ～40 nm and the bulk magnetic structure of ferromagnetic single crystals of hexagonal M ferrites (BaFe₁₂O₁₉, SrFe₁₂O₁₉, PbFe₁₂O₁₉) with a magneto-plumbite structure. Measurements are made by simultaneous gamma, x-ray, and electron Mössbauer spectroscopy in order to investigate the properties of the surface layer and the bulk crystal simultaneously. Experimental data obtained with a depth resolution of ～ 10 nm show that the orientation of the magnetic moments of the iron ions (along the crystallographic c axis) does not change on approaching the surface from the crystal volume. Thus, to within an experimental error of ～ 10 nm, single crystals of the hexagonal ferrites BaFe₁₂O₁₉, SrFe₁₂O₁₉, and PbFe₁₂O₁₉ with a ferromagnetic structure do not have a “ transition” surface layer whose magnetic structure differs from that of the bulk crystal such as that which exists, with a depth of several hundred nm, in antiferromagnetic materials with weak ferromagnetism.     

Successive structural phase transitions in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> in the range 10–1120 K

The crystal and magnetic structures of the Pr_0.5Sr_0.5CoO₃ metallic ferromagnet have been studied using neutron diffraction and synchrotron radiation. Successive structural transitions with the reduction of the crystal symmetry from cubic (space group Pm3m) to rhombohedral (\(R\bar 3c\), ～800 K), orthorhombic (Imma, ～300 K) and, then, to triclinic at ～120 K are detected during cooling from 1120 K. The transition from the orthorhombic system to a phase with a lower symmetry is characterized by a sharp change in the anisotropy of the unit cell, which indicates the partial ordering of the e _g orbitals of cobalt. The accompanying change in the interatomic distances and valence angles give rise to an anomaly in the temperature dependence of the magnetic susceptibility at T ≈ 120 K. The ordered magnetic moment μ_Co ≈ 2μ_B corresponds to the assumption of the intermediate spin state of Co³⁺ ions and the mixture of low- and intermediate-spin states of Co⁴⁺ ions.     

Staggered spin order of localized π-electrons in the insulating state of the organic conductor κ-(BETS)<Subscript>2</Subscript>Mn[N(CN)<Subscript>2</Subscript>]<Subscript>3</Subscript>

The magnetic properties of the conduction π-electron system of κ-(BETS)₂Mn[N(CN)₂]₃ have been probed using ¹³C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below T ? 23 K is shown to be accompanied by ordering of the π-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of ～0.5 kbar, the π-spin system maintains the properties of the metallic state down to 5 K.     

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.     

Magnetic properties and the mechanism of formation of the uncompensated magnetic moment of antiferromagnetic ferrihydrite nanoparticles of a bacterial origin

The magnetic properties of the superparamagnetic ferrihydrite nanoparticles that form as a result of the vital activity of Klebsiella oxytoca bacteria are studied. Both an initial powder with an average number of iron atoms N _Fe ～ 2000–2500 in a particle and this powder after annealing at 140°C for 3 h in air are investigated. The following substantial modifications of the magnetic properties of the ferrihydrite nanoparticles are detected after annealing: the superparamagnetic blocking temperature increases from 23 to 49.5 K, and the average magnetic moment of a particle increases (as follows from the results of processing of magnetization curves). The particles have antiferromagnetic ordering, and the magnetic moment resulting in the superparamagnetism of the system appears due to random spin decompensation inside the particle. For this mechanism, the number of uncompensated spins is proportional to the number of magnetically active atoms raised to the one-half power, and this relation holds true for the samples under study at a good accuracy. The possible causes of the detected shift of magnetic hysteresis loops at low temperatures upon field cooling are discussed.     

Crystal structure and magnetic state of the LaMn1−x VxO3 perovskites

The crystal structure and the magnetic state of polycrystalline LaMn_1?x V_xO₃ (0.1<x<0.9) compounds have been studied by x-ray and neutron diffraction methods, as well as by magnetization and ac susceptibility measurements. It is shown that substitution of vanadium for manganese ions leaves the orthorhombic crystal structure of the compounds (space group Pnma) unchanged. The magnetic structure is observed to change from a canted antiferromagnetic ordering (wavevector k=[0, 0, 0], with the antiferromagnetic moments aligned with the a axis and the ferromagnetic component of the magnetic moment parallel to the b axis) at vanadium concentrations x<0.4 to a collinear antiferromagnetic ordering (with the magnetic moments parallel to the b axis) at x>0.8; at this transition occurs through an intermediate state exhibiting spin-glass properties.