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1.
A simple classical model for nuclear spin-lattice relaxation due to solitons in XY one-dimensional magnetic chains in an external magnetic field is discussed. The results show that one should expect opposite behavior for the H/T dependence of T-11 for the ferromagnetic and the antiferromagnetic case.  相似文献   

2.
The magnetic and magnetoelastic properties of single crystals of electron-doped rare-earth manganites La1?x Sr x MnO3 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?x Sr x MnO3 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.  相似文献   

3.
The thermopower α and magnetothermopower Δα/α have been studied on single-crystal Sm0.55Sr0.45MnO3 samples consisting of three types of magnetic clusters (ferromagnetic clusters with the Curie temperature T C = 134 K, the A-type antiferromagnetic clusters with the Néel temperature T NAT C, and the CE-type antiferromagnetic clusters with T NCE = 240 K). The temperature dependences of α and Δα/α have extremes in the vicinity of T NCE, namely, a wide maximum in curve α(T) and a sharp minimum in curve{Δα/α}(T). The negative magnetothermopower in the minimum has a giant value of 50% in the magnetic field of 13.2 kOe. The thermopower is shown to be mainly due to the existence of the CE-type antiferromagnetic clusters, in which there is a charge-orbital ordering that displaces the oxygen atoms. The changed crystal lattice inside these clusters changes the value of the thermopower inside them. This thermopower influences the voltage drop across the sample during measuring the thermopower and, thus, the effective value of α of the whole sample. The application of a magnetic field near T NCE accelerating the destruction of the CE-type antiferromagnetic order causes the sharp decrease in the thermopower of the whole sample. This implies that the CE-type antiferromagnetic clusters with the charge-orbital ordering make main contribution to the thermopower of the whole sample.  相似文献   

4.
This paper reports on the results of x-ray diffraction and magnetic studies of manganites in the Nd0.9MnOx system with an oxygen content varying in the range 2.84 < x < 2.93. A sample with an oxygen content x = 2.84 undergoes a first-order phase transition at a temperature close to T = 1050 K, whereas an anomaly observed in the elastic properties for a sample with x = 2.93 indicates a phase transition near T ≈ 500 K. It is assumed that these transitions are governed by cooperative ordering of Mn e g orbitals of the same type as in stoichiometric NdMnO3. The manganite at an oxygen content x = 2.85 is an antiferromagnet with a Néel temperature T N = 85 K, whereas the magnetic properties of the manganites at x = 2.90 and 2.93 suggest that an antiferromagnetic component coexists with a ferromagnetic component. The magnetic interaction between the ferromagnetic and antiferromagnetic components manifests itself in the fact that the magnetic moment becomes opposite in direction to the external magnetic field. The properties of the samples are consistent with the hypothesis that part of the neodymium ions (up to 5%) can be substituted for manganese ions.  相似文献   

5.
The structural stability and physical properties of CrNCN were studied using density functional theory with explicit electronic correlation (GGA+U). Calculated results indicate that the title compound, similar to MNCN (M=Mn, Fe, Co, Ni), is thermodynamically stable but mechanically unstable. Analysis of electronic and magnetic structures reveals that CrNCN is an antiferromagnetic semiconductor. However, the exact magnetic structure of CrNCN consists of an antiferromagnetic intralayer and a ferromagnetic interlayer, which differs from that of the type-II antiferromagnetic semiconductor MNCN (M=Mn, Fe, Co, Ni), which consists of a ferromagnetic intralayer and an antiferromagnetic interlayer.  相似文献   

6.
The magnetic state of a CaMnO3 ? δ crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn4+ and Mn3+ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn3+-Mn4+ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO2.75 crystal are governed directly by the competition of the Mn3+-Mn4+ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.  相似文献   

7.
The compound BaCaFe4O8 crystallizes in the trigonal space group P31m with one formula unit per unit cell with lattice constants a = 5.4059 A and c = 7.7023 A Neutron diffraction measurements carried out on a powder sample over the temperature range 300–900 K showed that the compound undergoes a magnetic transition to an antiferromagnetic state at a Néel temperature TN = 680 ± 5 K. Analysis of the room temperature neutron diffraction pattern gave a magnetic unit cell that has the same periodicty as the crystallographic one. An antiferromagnetic model is proposed with the iron spin magnetic moments parallel to the c-axis of the unit cell. The magnetic moment of the Fe3+ ion was found to be (4.5 ± 0.1)μB  相似文献   

8.
We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies κ = J′a 2 c/(g L μ B )2. For increasing values of κ, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120° structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF3, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are seperated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction.  相似文献   

9.
10.
The temperature dependence of the EPR spectrum for the α-phase of iron tungstate has been investigated in the temperature range of 40–260 K. At temperatures betweenT 1 ≈ 250 K andT 2 ≈ 205 K where the antiferromagnetic phase transition occurs, a relatively narrow EPR line arising from the dominant iron(III) species has emerged, gaining intensity with the temperature increase. Its linewidth temperature evolution could be described by Huber equation, with TN = 200 K, which is consistent with the peak seen in magnetic susceptibility measurements, while the correspondingg-factor shifts to higher fields reflecting the build-up of internal field emerging from increasing shortrange order in the spin system. At temperatures lower than T2, a very broad and distorted EPR line with temperature dependentg-factor and linewidth has been observed reflecting the corresponding rise of the magnetic susceptibility below the antiferromagnetic phase transition, presumably arising from magnetic clusters embedded in the antiferromagnetic background.  相似文献   

11.
The magnetic properties of Sr7Fe10O22 have been investigated by means of magnetic susceptibility and Mössbauer absorption measurements. This compound proved to be antiferromagnetic with a Néel temperature TN = 425 K; the magnetic susceptibility is constant from the lowest measuring temperature (78 K) up to TN.The Mössbauer measurements and the analogies with “brownmillerite” type compounds indicate that iron ions occupy one octahedral and two tetrahedral different sites. An antiferromagnetic spin configuration with moments lying in the ab plane appears to be consistent with the experimental results. A small spontaneous magnetic moment was observed at room temperature with features resembling those of strontium hexaferrite; a weak ferromagnetic behavior can not however be excluded taking into account the aforementioned susceptibility behaviour.  相似文献   

12.
The magnetic and electrical properties of the Al-doped polycrystalline spinels ZnxCryAlzSe4 (0.13≤z≤0.55) with the antiferromagnetic (AFM) order and semiconducting behavior were investigated. A complex antiferromagnetic structure below a Néel temperature TN≈23 K for the samples with z up to 0.4 contrasting with the strong ferromagnetic (FM) interactions evidenced by a large positive Curie-Weiss temperature θCW decreasing from 62.2 K for z=0.13 to 37.5 K for z=0.55 was observed. Detailed investigations revealed a divergence between the zero-field-cooling (ZFC) and field-cooling (FC) susceptibilities at temperature less than TN suggesting bond frustration due to competing ferromagnetic and antiferromagnetic exchange interactions in the compositional range 0.13≤z≤0.4. Meanwhile, for z=0.55 a spin-glass-like behavior of cluster type with randomly oriented magnetic moments is observed as the ZFC-FC splitting goes up to the freezing temperature Tf=11.5 K and the critical fields connected both with a transformation of the antiferromagnetic spin spiral via conical magnetic structure into ferromagnetic phase disappear.  相似文献   

13.
New metal oxide pyroxene compound NaVGe2O6 containing isolated edge-sharing VO6 (S=1) chains undergoes transition into a long-range antiferromagnetic state at T N =16 K. The broad maximum in the temperature dependence of magnetic susceptibility at T M =26 K indicates the low-dimensional character of the magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Néel temperature.  相似文献   

14.
In a magnetic field parallel to the magnetization axis of an antiferromagnetic Fe Br2 single crystal, a caracteristic metamagnetic behaviour is observed. The transition from an antiferromagnetic phase to a paramagnetic phase is studied by help of magnetization measurements in a steady field (H < 60 kOe). The measurement precision has allowed a detailed study of the magnetization isotherms, caracteristic of a first order magnetization phase transition (T < Tc = 4, 7 K) and of a second order phase transition (Tc < T < TN = 14, 2 K).We have observed an original phase diagram. In a certain temperature and field range, the ordered phase is stable on the high temperature side of the transition point. Some theoretical studies in an Ising model, or in the hypothesis of a strong magnetoelastic coupling forecast the existence of such a magnetic phase diagram.At present, we proceed to a theoretical study, in a molecular field approximation, of the magnetic phase diagram of compounds similar to Fe Br2 where we take into account the relative values of parameters J1, J2 and D associated with ferromagnetic and antiferromagnetic interactions and crystalline anisotropy.  相似文献   

15.
MnAlF5 is antiferromagnetic with TN = 2.35 ± 0.10 K. The magnetic cell requires doubling of the b and c parameters of the crystallographic unit cell. The directions of the moments are close to the b axis. The magnetic structure is discussed.  相似文献   

16.
17.
Magnetic properties of the Ce1-xLaxMn2Si2 system were investigated by means of neutron diffraction and magnetometry. The samples with low La concentration (x?0.5) have antiferromagnetic properties. A transition from an antiferromagnetic to a ferromagnetic state can be observed for x=0.6 (for increasing temperature). More La leads to the samples being ferromagnetic. A collinear magnetic structure is seen from the neutron diffraction spectra. From all the results known up to now it follows, that type of magnetic ordering, i.e. antiferro- or ferro-depends on the Mn-Mn interatomic distances in the basal plane.  相似文献   

18.
The mpx3 phases (M = Mn, Fe, Ni; X = S, Se) with sheet structures are insulators with localized moments. They show antiferromagnetic ordering at low temperatures. From the magnetic structures determined by neutron diffraction the magnetic interactions are considered and shown to be characteristic of 2D behavior.  相似文献   

19.
The magnetic, electrical, and optical properties of Ca1 ? x LaxMnO3 ? δ(x ≤ 0.12) manganite single crystals have been studied. The state with a spatially inhomogeneous electron distribution has been found. Interrelations between the electric and magnetic subsystems are analyzed. The obtained magnetic data show evidence for the formation of a G-type antiferromagnetic (G-AFM) phase with a spin-canted structure in the crystal with x = 0.05, for which the Curie and Néel temperatures are T C = T N(G) = 115 K. On cooling from the paramagnetic state, the crystals with x = 0.10 and 0.12 exhibit transitions from the paramagnetic to a C-type antiferromagnetic (C-AFM) phase in a part of the volume at T N(C) = 150 and 200 K, and from the paramagnetic to the G-type antiferromagnetic (G-AFM) phase in the remaining volume at T N(G) = 110 and 108 K, respectively. The onset of the C-type magnetic phase nucleation in crystals is observed at lower dopant (La) concentrations than in polycrystalline samples, which is explained by the deviation of single crystals from the stoichiometry with respect to oxygen. The magnetic phase transitions are manifested by anomalies in the electric resistance and magnetoresistance of doped crystals. An analysis of the electrical and optical properties of the samples shows evidence of (i) the formation of a charge energy gap in the C-AFM phase with retained paramagnetic metallic regions and (ii) the presence of ferromagnetic “metallic” droplets in the insulating G-AFM phase. The multiphase state of Ca1 ? x LaxMnO3 ? δ manganite single crystals featuring the coexistence of two magnetic phases, the regions with orbital/charge ordering, and the FM “metallic” droplets is related to a competition of exchange interactions by the superexchange and double exchange mechanisms.  相似文献   

20.
By means of inalastic neutron scattering we have determined the dispersion relation of the magnetic excitations in CsFeCl3 at different temperatures.The dispersion in c-direction, along the Fe-chains is typically ferromagnetic and in the hexagonal plane antiferromagnetic. Due to the lack of an applicable theory the data were parametrized by the simple heuristic formula:?ω = [2J[1 - cos πqc] [A + 2J(1 - cos π)qc)] + [C + J' (1.5 + γ(q))]2]12The gap was found to be C = 0.148 THz, the easy plane anisotropy A = 0.308 THz, the ferromagnetic interaction J = 0.148 THz and the antiferromagnetic interaction J' = -0.04 THz. At 1.25 K all excitations had a width smaller than the instrumental resolution ΔE = 0.025 THz. These results can be interpreted as follows: CsFeCl3 is a singlet ground state system with strong ferromagnetic interaction J along the crystallographic c-axis and weak antiferromagnetic interaction J' in the plane perpendicular to c.In addition we have measured the influence of a magnetic field along the hexagonal c-axis. The splitting found agrees with the assumed level scheme yielding g = 2.5 for the first excited level.  相似文献   

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