The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

Low temperature properties of the metallic antiferromagnet ErZn12

The metallic antiferromagnet ErZn₁₂ orders magnetically at 2·76°K. We report measurements of the susceptibility, magnetisation, resistivity and specific heat of polycrystalline specimens of it in the vicinity of the magnetically ordered regime. We find evidence for a metamagnetic or spin-flop transition in fields of 3–7KOe. The resistivity increases monotonically from low temperature to the Néel point and has a power-law dependence on temperature of Tⁿ, where n～6·5. The specific heat shows a lambda anomaly at the Néel point and the divergences which appear above and below it are compared with those reported previously in the resistivity. The magnetic entropy indicates that the ordering process involves only one crystal-field doublet.     

铁电磁体Pb(Fe1/2Nb1/2)O3相变特征

借助与示差扫描量热法、磁化率测量、电子自旋共振、铁电与介电性质测量及电子衍射系统地研究了Pb(Fe_1/2Nb_1/2)O₃(PFN)的电、磁性质和相变特征．结果表明发生在380K附近的顺电－铁电转变和发生在145K附近的顺磁 反铁磁转变分别为一级相变和二级相变或弱一级相变．在室温下,PFN的剩余极化与矫顽场分别为11.5μC/cm²和3.04kV/cm.介电测量表明PFN的顺电-铁电相变为弥散型相变．其弥散指数为1.62.电子衍射表明Fe³⁺与Nb⁵⁺离子在B位置上是无序分布的,正是这种与无序分布相关联的成分涨落导致铁电相变的弥散性. 关键词：     

Neutron diffraction study of Ho(Rh0.3Ir0.7)4B4

Using powder neutron diffraction techniques, we have examined the magnetic order of the pseudoternary compound Ho(Rh_0.3Ir_0.7)₄B₄ below the Néel temperature T_N=2.7K. The magnetic structure consists of stacked antiferromagnetic basal plane sheets forming a body centered tetragonal unit cell, with a sublattice magnetization corresponding to 9.6±0.6μ_B per Ho³⁺ion at 1.5 K. Magnetic intensity versus temperature measurements indicate that the transition is second order and reveal no anomalous effects when the compound becomes superconducting at T_c=1.34K.     

The metal-nonmetal transition in NiS studied by the Mössbauer effect of 57Fe

Mössbauer spectra of ⁵⁷Fe impurities in α NiS show an abrupt collapse of the hyperfine splitting at T_t = 260 K, the nonmetal-metal transition; the extrapolated Néel point is ～420 K. Below T_t, it appears that iron adjacent to a vacancy is low-spin. There us evidence of substantial d-electrons delocalisation above T_t. β NiS does not order magnetically down to 4 K.     

Neutron diffraction study of the (1-x)αFe2O3xAl2O3 system

The magnetic structure of the (1-x) α Fe₂O₃xAl₂O₃ system with x= 0–0.1 has been investigated on polycrystalline samples by neutron diffraction method. The Morin transition temperature and the Néel temperature are observed to decrease on increasing x. The angle by which the magnetic moments turn out of the basal plane in the Morin transition also decreases with increasing x. The Morin transition does not occur above x = 0.09.     

Crystal and magnetic structure of KFeO2

The crystal and magnetic structures of KFeO₂ have been determined by neutron and X-ray powder-diffraction and Mössbauer-effect techniques. The crystal structure at 4.2 K and 300 K is orthorhombic and the magnetic space group is Pbca'. The Fe³⁺-ions in this structure are tetrahedrally coordinated by oxygen ions, and each Fe³⁺-ion has a magnetic moment which is antiferromagnetically coupled to the moments of four Fe³⁺-neighbours. The direction of the moments is parallel to the a-axis. A crystal phase transition has been observed near the Néel temperature?960 K.     

Antiferromagnetic resonance in α-RbMnCl3 · 2H2O

Antiferromagnetic resonance results on α-RbMnCl₃ · 2H₂O below the Néel temperature 4.56 K are reported. At T = 1.2K we find for the anisotropy fields H^c = 440 Oe and H^a = 1510 Oe. The spin flop field at T = 1.2K is 14.62 kOe.     

Ultrasonic attenuation anomalies at the two transitions in antiferromagnetic FeGe2

The attenuation of longitudinal and shear sound waves is measured through the Néel temperature, T_N≈286 K, and the lower transition temperature, T_K≈265 K, of FeGe₂. Longitudinal sound with wavevector q along the [100] axis of this tetragonal antiferromagnetic metal shows an attenuation peak at T_N, which is reversibly suppressed by compressive uniaxial stress σ along [010]. The estimated pressure dependence of T_N is dT_N/dp=(?2.8±0.3) mK bar^?1. The peak at T_N shows thermal and stress hysteresis, which suggests that it is associated with domain wall motion and that this transition is first order.     

Magnetic short-range order in the linear-chain antiferromagnet CsMnCl3 · 2H2O studied by optical birefringence

The linear optical birefringence of CsMnCl₃ · 2H₂O shows a magnetic contribution due to one-dimensional short-range order. The temperature derivative of this part is proportional to the magnetic specific heat. For the intrachain interaction we find |J|/k = 3.13 ± 0.15 K. Around the Néel temperature the interchain coupling causes a small birefringence change with the opposite sign.     

Crystallographic distortion in CrAs0.50Sb0.50

X-ray diffraction has shown that CrAs_0.50Sb_0.50 undergoes at the Néel temperature a crystallographic distortion (NiAs → MnP) accompanied by a discontinuous volume change. Neutron diffraction revealed in this compound a helimagnetic structure very similar to the ‘double spiral’ in CrAs discovered recently. The pressure dependence of the Néel temperature is estimated in the framework of the Bean Rodbell theory and compared with the CrAs value.     

Antiferromagnetic metallic state of NiS2

The neutron diffraction experiment we have made on NiS_1.91 single crystal under increasing pressure (0<P<30 kbar) indicates a constant increase of the Néel temperature of the sample. For high pressures (P>20 kbar) and low temperatures the sample is in an antiferromagnetic metallic state as suggested by theoretical considerations for a Mott-Hubbard insulator. The pressure dependence of the second magnetic transition in NiS₂ (T = 31 K for P = 0) is also reported.     

Magnetostriction and the two-spin correlation function in MnF2

The longitudinal magnetostriction along the [001] axis of MnF₂ was measured at temperatures 64 < T < 300 K in magnetic fields, H, up to 130 kOe. This magnetostriction is proportional to H² at low H, exhibits a λ anomaly near the Néel temperature T_N, and shows the field-induced transition from the antiferromagnetic phase to the paramagnetic phase for T just below T_N. The results are well described by a model which relates the magnetostriction to the two-spin correlation function.     

Specific features of local structural,valence, and magnetic states of iron ions in the perovskite Bi0.5Ca0.5FeO3

The perovskite Bi_0.5Ca_0.5FeO₃ has been investigated using the Mössbauer effect at temperatures of 295 and 675 K. The measured temperature of the magnetic phase transition (Néel temperature) is T _N = 640 ± 10 K. Above the Néel temperature, there are two nonequivalent structural states of iron ions. In the perovskite Bi_0.5Ca_0.5FeO₃ at room temperature, there are seven most probable nonequivalent magnetic states of iron ions with significantly different values of the hyperfine interaction parameters. Four iron states correspond to Fe³⁺ ions in the octahedral oxygen environment, and three iron states correspond to Fe³⁺ ions in the tetrahedral oxygen environment.     

Electrical resistivity and magnetic susceptibility studies of the system Mn5Si3-Fe5Si3

Electrical resistivity measurements have been performed on the system Mn_5?x,Fe_xSi₃ for the compounds x = 0,1,2,3,4 and 5 from 4.2 K to above room temperature. From the shape of the ρ(T) curves it can be inferred that the transition from antiferromagnetic to ferromagnetic ordering occurs in the concentration range 3 ? x ? 4; for Fe₅Si₃ and MnFe₄Si₃ the ρ(T) curves are characterized by a break in their slope, whereas for x = 1,2 and 3 a large minimum appears. Mn₅Si₃ exhibits two successive minima at 74 and 105 K. Magnetic susceptibility measurements for x = 1, 2 and 3 give confirmation of the Néel temperature for x = 1 and 2, whereas for x = 3 the behaviour is more complex.     

Synthesis and magnetic properties of InM2+CrS4(M2+ = Mn,Fe,Co, Ni)

The crystallographic and magnetic properties of the compounds InM²⁺CrS₄(M = Mn, Fe, Co, Ni) were studied. Three of these compounds except MnInCrS₄ were antiferromagnetic. The Néel termperature and paramagnetic Curie temperature were determined to be 20 and 82K for InFeCrS₄, 36 and 115K for InCoCrS₄ and 22 and 26K for InNiCrS₄, respectively.     

Magnetic properties and spin order of Sr7Fe10O22

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 T_N = 425 K; the magnetic susceptibility is constant from the lowest measuring temperature (78 K) up to T_N.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.     

Nouvelles donnes experimentales sur les proprietes magnetiques de Cr3 Se4 et des solutions solides Cr3 Set S4−t

Magnetic susceptibility and specific heat measurements and neutron diffraction experiments allow to locate Néel temperatures T_N and also two types of transitions below T_N. At T_t of order T_N/2 the susceptibility is maximum, and the magnetic structure is modified (for Se-rich compounds). The second transition takes place around 15 K and is characterized by a specific heat peak and a susceptibility anomaly but no modification of the magnetic structure, according to our powder diffraction data.     

The antiferromagnetic structure of TbB4

The magnetic structure of the rare earth tetraboride TbB₄ (crystallographic space group P4/mbm) has been determined by neutron diffraction on a polycrystalline sample. Below the experimentally determined Néel temperature of T_N = (43±1) K TbB₄ is ordered antiferromagnetically. The data refinement yielded a magnetic moment value of (7.7 ± 0.2) μ_B/Tb ion at 4.2 K which we interpret as Tb⁴⁺. The magnetic structure is antiferromagnetic collinear with the moments perpendicular to the tetragonal axis.     

On the nature of a new phase transition in α-MnS

Magnetic-susceptibility and X-ray-diffraction data of polycrystalline and oriented single crystal α-MnS reveal a new phase transition at T_tr = 131 K below the Néel temperature T_N = 148 K. The phase transition is characterized by an abrupt inversion of the rhombohedral distortion of the f.c.c. lattice along [111]. At T_tr there is a discontinuous change in the susceptibility of single crystals.