Successive Magnetic Transitions Research Articles

Pressure-Temperature Magnetic Phase Diagram of TbNiSn.

The electrical resistivity of TbNiSn for the b-axis has been measured under hydrostatic pressures up to 22 kbar. Resistivity anomalies associated with the successive magnetic transitions are observed. At P=Okbar, the transitions at TN (=18. 5 K) and T3 (=7. 6 K) are second order whereas the transition at T2 (=6. 0 K) is first order. We observe that the resistivity drops discontinuously at T2 and accompanies thermal hysteresis. There is not appreciable change in the resistivity at T1 (=2. 2 K). The pressure-temperature magnetic phase diagram has been constructed. All the transition temperatures first decrease, take minima at 3 kbar and then increase with increasing pressure. It should be also noted that the T3 line is merged in the T2 line above 17 kbar. Total spin disorder resistivity is pressure independent.

Magnetic Phases in UNiGa under Pressure.

The linear thermal expansion of single crystalline UNiGa has been measured in the range of pressure up to 2. 2 GPa and magnetic field to 1. 5 T to investigate the stability of magnetic phases of UNiGa at high pressure. UNiGa shows several successive magnetic phase transitions at low temperature. Each magnetic phase transition is accompanied by the thermal expansion anomalies. Two of four magnetic phases in zero magnetic field are destabilized by pressure. A new antiferromagnetic phase induced by pressure is found above 1. 8 GPa. Using these results, we construct p-T magnetic phase diagrams for fields up to 1. 5 T and B-T phase diagrams for pressure up to 1. 8 GPa.

Neutron diffraction and Mössbauer measurements for magnetism of BCC Cr-Fe-Mn alloys

The successive magnetic transition of BCC Cr 65.0Fe 20.0Mn 15.0 alloy has been studied by neutron diffraction and Mössbauer effect measurements. An antiferromagnetic 1 0 0 scattering and the hyperfine field at Fe sites were confirmed to appear just below T N by neutron diffraction and Mössbauer effect, respectively. The hyperfine field distribution estimated from the Mössbauer spectra has two peaks below 150 K. The amount of Fe atoms for the peak of the high-field part increases with decreasing temperature at around T c. The position of the high-field peak shifts remarkably to higher field below T g with almost keeping the fractional area for this peak.

Successive magnetic phase transitions in Cu cCo 1− cCl 2FeCl 3 GBICs

Cu c Co 1− c Cl 2FeCl 3 GBICs (0 ≤ c ≤ 1) undergo magnetic phase transitions at T c1 , T c2 and T c3 ( T c3 < T c2 < T c1 ) depending on the Cu concentration. The phase transitions at T c1 and T c3 show spin glass behaviors. The phase transitions at T c2 and T c3 are related to the in-plane spin orders of Cu c Co 1− c Cl 2 layers and FeCl 3 layers, respectively. The spin orders in the Cu c Co 1− c Cl 2 layers and the FeCl 3 layers are coupled to each other through an interplanar exchange interaction, leading to the appearance of phase transition at T c1 and the decrease of T c2 with increasing Cu concentration.

Successive magnetic phase transitions of TbRu 2Ge 2

Successive magnetic phase transitions of the tetragonal compound TbRu 2Ge 2 have been studied by neutron diffraction using a single crystal. Three magnetic phase transitions at 4, 30 and 37 K (= T N) were confirmed. These magnetic structures are an anti-phase structure with the propagation vector k 1 = (0.235, 0, 0) (0.235 = 4 17 ) , an amplitude-modulated structure with k 1 and a sinusoidal modulated structure with k 1 and k 2 = (0.247, 0, 0) , respectively, for low, middle and high temperatures.

Mössbauer and neutron diffraction studies of BCC CrFeMn ternary alloys

Successive magnetic phase transitions of disordered BCC Cr 65Fe 20Mn 15 alloy have been studied by Mössbauer effect and neutron diffraction measurements. The (1 0 0) antiferromagnetic peak was observed below T N by neutron diffraction, and it still exists at the temperature of the ferromagnetic phase, indicating that the antiferromagnetic phase should coexist with the ferromagnetism. The mean hyperfine field by Mössbauer effect begins to grow below T N with decreasing temperature and increases rapidly below T C and T g. The hyperfine field distribution at 13 K shows two distinct peaks in an agreement with Cr 80Fe 20 alloy.

Magnetic structure of TbNiSn single crystals

Measurements of magnetization and neutron diffraction have been performed on TbNiSn single crystals with an orthorhombic (Pnma) TiNiSi-type structure. Magnetic measurements indicate that this compound exhibits the four successive magnetic phase transitions at T 1 = 2.0 K, T 2 = 6.0 K, T 3 = 7.6 K and T N = 18.5 K. There is little difference in neutron diffraction patterns between above and below T 1. The magnetic structure is of a square-wave with Q = (0.600, 0.345, 0) for T < T 2, Q = (0.665, 0.33, 0) for T 2 < T < T 3 and of a sine-modulated type with Q = (0.685, 0.33, 0) for T 3 < T < T N.

Magnetic properties of NdPtAl

Magnetic properties of the ternary compound NdPtAl with orthorhombic TiNiSi-type structure have been investigated by powder neutron diffraction experiments. NdPtAl shows ferromagnetic order below T C = 19.2 K and no further phase transition could be detected down to 1.5 K. The ordered magnetic Nd moments of 2.08(2) μ B at saturation are parallel to the b axis. The ferromagnetism of NdPtAl is in contrast to the complex magnetic structures found in CePtAl with three successive magnetic phase transitions between 2.2 and 5.9 K.

Single-crystal neutron diffraction studies onCeCu2Ge2sandCeCu1.9Ni0.1Ge2s

We have performed single-crystal neutron diffraction studies on ${\mathrm{CeCu}}_{2}$ ${\mathrm{Ge}}_{2}$ and ${\mathrm{CeCu}}_{1.9}$ ${\mathrm{Ni}}_{0.1}$ ${\mathrm{Ge}}_{2}$ to get insight into the very unusual magnetic properties of these compounds. ${\mathrm{CeCu}}_{2}$ ${\mathrm{Ge}}_{2}$ orders antiferromagnetically below ${\mathrm{T}}_{\mathrm{N}}$ =4.15 K in an incommensurate sinusoidal amplitude modulated structure. ${\mathrm{CeCu}}_{1.9}$ ${\mathrm{Ni}}_{0.1}$ ${\mathrm{Ge}}_{2}$ shows two successive magnetic phase transitions at ${\mathrm{T}}_{\mathrm{N}1}$ =3.65 K and ${\mathrm{T}}_{\mathrm{N}2}$ =2.65 K, respectively. The corresponding magnetic structures are very similar to each other as well as to the magnetic structure of pure ${\mathrm{CeCu}}_{2}$ ${\mathrm{Ge}}_{2}$ . Based on the temperature dependence of the intensities of the principal magnetic reflections, the two magnetic phases seem to superimpose independently of each other.

Observation of Successive Phase Transitions in Kagomé Lattice Antiferromagnets RFe3(OH)6(SO4)2[R=NH4, Na, K

Successive magnetic phase transitions were found in the Kagome lattice antiferromagnets RFe 3 (OH) 6 (SO 4 ) 2 [ R=NH 4 , Na, K]. The temperature dependence of the susceptibility has two cusps around 60 K for each compound. The X-ray diffraction patterns and the quadrupole resonance spectra show that the transitions are not caused by a structural change. The presence of these transitions was confirmed by NMR experiments. The NMR spectra show the development of magnetic ordering below the upper transition temperature. In the low-temperature phase the nuclear magnetization does not relax exponentially, which may suggest a domain structure of 120° spin structure due to degeneracy on the Kagome lattice. The successive transitions may be caused by a small Ising-like anisotropy in the Heisenberg antiferromagnet.

Crystal growth and characterization of cuprous ferrite (CuFeO2)

The growth of bulk cuprous ferrite (CuFeO2) single crystals by the travelling-solvent, floating-zone technique is reported. Crystals 5 to 8 mm in diameter by 10 to 30 mm have been grown from two sets of starting materials: Cu2O + Fe2O3 and 2CuO + Fe2O3, respectively. The trigonal unit-cell parameters determined by X-ray diffraction are ah = 3.035(1) Å and ch = 17.161(4) Å in the hexagonal description. Mössbauer studies reveal an isomer shift for 57Fe at room temperature of 0.40 mm/s with respect to metallic iron, confirming that iron is in the trivalent state and that CuFeO2 is cuprous metaferrite (Cu+Fe3+O2). Magnetic susceptibility indicates that CuFeO2 undergoes two successive magnetic transition temperatures around TN1 = 13.5 K and TN2 = 9.5 K, respectively. Thermal analysis reveals that, under an argon atmosphere, CuFeO2 partially decomposes at 1180°C to form Fe2O3 (or Fe3O4) and Cu2O.

Crossover effect in triangular-lattice antiferromagnetic mixed crystals CsNi1−xFexCl3

Heat capacity measurements have been made on triangular-lattice antiferromagnetic mixed crystals CsNi1−xFexCl3. CsNiCl3 has two successive magnetic phase transitions at TNi=4.74K and TN2=4.29K as a result of the spin frustration effect in triangular lattice. Substituting Ni atoms for Fe atoms makes TNi lower and eventually only one transition occurs for x=0.007 samples at TN=4.23K. As x increases further TN becomes higher, e.g., TN becomes 4.59K for x=0.03. These results are reasonably explained by the idea that the interaction between Ni2+ ions is drastically modified and the crossover occurs from Ising-like Heisenberg type to XY type interactions by the substitution of Fe2+ ions. The value of the critical exponents for specific heat near the phase transition also changes according to the crossover which is qualitatively in agreement with recent theoretical predictions.

Growth and characterization of CuFeO 2 single crystals

In this paper, the first successful growth of CuFeO 2 single crystals by applying the traveling solvent floating-zone technique is reported. Typical crystals of 5–8 mm in diameter and 10–30 mm in length have been obtained from the starting materials Cu 2O and Fe 2O 3 with the molar ratio 1:1. The as-grown crystal is homogeneous and of the trigonal system with cell parameters a h = 3.04Å and c h = 17.17Å in the hexagonal description. In Mössbauer effect (ME) measurements, the isomer shift for 57Fe in CuFeO 2 at room temperature is 0.40 mm/s with respect to metallic iron, which offers experimental confirmation that the iron is present in the trivalent state. Magnetic susceptibility measurements indicate that CuFeO 2 has two successive magnetic transition temperatures at T N = 13.5 K and T N = 9.5 K, respectively. Thermal analysis reveals that CuFeO 2 partially decomposes into Fe 2O 3 and Cu 2O at about 1180°C.

Successive phase transitions in the hexagonal-layered Heisenberg antiferromagnets MnX 2 (X = Br, I)

Successive magnetic phase transitions in the Heisenberg antiferromagnets MnX 2 (X = Br, 1) have been studied by neutron diffraction. Two ( T N1 = 2.32 K and T N2 = 2.17 K) and three ( T N1 = 3.95 K, T N2 = 3.8 K and T N3 = 3.45 K) transitions were observed in the bromide and the iodide, respectively. In both compounds, transverse sinusoidally modulated structures occur in the intermediate phases (MnBr 2: T N2 < T < T N1 MnI 2: T N3 < T < T N1), whereas the(MnBr 2) and helical (MnI 2) structures occur through first-order transitions in the lowest-temperature phases. The successive transitions are brought about by several competing exchange interactions and the dipole interaction.

Raman study of pressure-induced magnetic phase transition in EuSe

Raman spectra of EuSe have been measured under hydrostatic pressure up to 2.1 GPa at 2 K by using a diamond anvil. With increasing pressure, distinct spectral changes occur twice, which can be interpreted in terms of successive magnetic phase transition: ferri → antiferro (AF-I) → ferro. The Raman peaks are attributed to phonons at special points in the Brillouin zone taking account of the folding effect on the phonon dispersion. This makes it possible to draw overall features of the dispersion curve of LO phonon along the [111] direction under high pressure.

Magnetoelastic effect in dense Kondo compound CePtSn

Six elastic constants Cii of orthorhombic CePtSn were measured between 1.8 and 300 K by an ultrasonic technique. The crystalline electric field (CEF) parameters were determined from the elastic softening due to the CEF effect. Anisotropic elastic anomalies around the successive magnetic transitions well reflect the subtle change in the spin configuration.

Structural, magnetic and electrical properties of new uranium intermetallics: U 3Cu 4Si 4 and U 3Cu 4Ge 4

Two new uranium compounds U 3Cu 4Si 4 and U 3Cu 4Ge 4 have been prepared for the first time. Their crystal structures have been refined from single-crystal X-ray diffraction data and their physical properties were in investigated on polycrystalline specimens by means of magnetization, magnetic susceptibility and electrical resistivity measurements. U 3Cu 4Si 4 crystallizes in an orthorhombic structure of the U 3Ni 4Si 4-type. It orders antiferromagnetically at T N = 135 K, but then undergoes a successive magnetic phase transition at about 70 K, probably to a canted antiferromagnetic state. In turn U 3Cu 4Ge 4 is isostructural with the orthorhombic compound Gd 3Cu 4Ge 4, and becomes ferromagnetic below T c = 67 K.

Partially disordered phase and critical behavior in RbVBr3

Successive magnetic phase transitions of the planar antiferromagnet RbVBr3 have been investigated by neutron scattering and susceptibility measurements. The spin ordering in the intermediate phase is a partially disordered structure and that of the low-temperature phase is a triangular one. The critical exponent β and the field dependence of the phase boundaries are reported.

Specific heat of NdCu2 at very low temperatures

NdCu2 undergoes successive magnetic transitions at 6.5 and 4.1 K, respectively. We extend the previous specific-heat measurements to below 1 K and to magnetic fields up to 6 T. The specific heat C below 3 K decreases exponentially, indicating a gap of 10.3 K·kB in the magnetic excitation spectrum and hence a rather strong magnetic anisotropy. Below 0.8 K C increases towards low T because of the hyperfine contribution of Nd. In a magnetic field of 6 T applied parallel to the b direction of orthorhombic NdCu2, the gap increases to 12.8 K·kB. Determination of the magnetic transitions by measurements of C in a magnetic field and by the magnetocaloric effect complements the (B, T) phase diagram of NdCu2.

Neutron Diffraction Study of Successive Phase Transitions in the Heisenberg Antiferromagnet MnBr2

Neutron diffraction experiments on the Heisenberg antiferromagnet MnBr 2 were performed to study the successive magnetic phase transitions ( T N1 =2.32 K, T N2 =2.17 K). The magnetic structure in the intermediate phase is found to be a transverse sinusoidally-modulated structure with an incommensurate modulation vector q ic ≃(0.207, 0, 0.387), which varies as a function of temperature. At T N2 a first-order transition takes place from the incommensurate structure to the commensurate ( q c =(1/4 0 1/4)) structure. These phase transitions are successfully accounted for using mean-field theory based on a Hamiltonian which consists of six exchange interactions and the dipole interaction.