Junji Iida

Magnetization and Spin Correlation of Two-Dimensional Triangular Antiferromagnet LuFe2O4

Magnetization, neutron diffraction and Mossbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.

MAGNETIZATION OF SINGLE CRYSTAL YFEMNO4

Magnetization measurements up to 143 kOe and susceptibility measurements have been carried out on a single crystal of hexagonal YFeMnO 4 . This oxide exhibits a two-dimensional antiferromagnetic character, and shows spin-glass-like properties at low temperatures. The Weiss temperature obtained from high temperature paramagnetic susceptibility is -800 K, while the critical temperature where the thermoremanent magnetization disappears is only 60 K. This result suggests that both the magnetic frustration due to competing exchange interaction and the random charge distribution of the Fe and Mn ions play important roles in this magnetism.

Effect of the Pressure and the Rare-Earth Substitution on the Verwey Transition of YFe2O4

Effects of the hydrostatic pressure as well as the rare earth substitution are examined on the two-step Verwey transition of the stoichiometric YFe 2 O 4 , by the electric and the magnetic measurements. The upper and the lower transition temperatures decrease with the compression by 2.6 and 10.5 K/kb, respectively. Substitution of Y by Lu decreases whereas that by Dy increases the transition temperature. At room temperature in the high conductivity phase, lattice constants in the hexagonal frame, a and c , are measured for substituted crystals and for the stoichiometric YFe 2 O 4 under pressure. It is shown that the upper and the lower transition temperatures are independent from c but decrease 10 and 42 K, respectively, by 10 -3 compression of the lattice along the a axis. The character of the transition is discussed briefly.

Crystal structure and charge distribution of YbFeMnO4.

The structure of synthetic YbFeMnO(4) has been refined by single-crystal X-ray diffraction. Space group R3m, a = 3.4580 (1), c = 25.647 (3) A, V = 265.59 (3) A(3), Z = 3. Yb is in octahedral coordination, whereas Fe and Mn are disordered on a single crystallographic type of trigonal bipyramid, in which the cation is off-centred from the basal plane. Assuming perfect stoichiometry, R(1) = 0.0195, but the charge distribution (CD) analysis suggests incomplete occupation of the Yb site. Refinement of the occupancy lowers R(1) to 0.0175, resulting in s.o.f.(Yb) = 0. 963 (3), with a significant improvement of the Fourier difference. The electroneutrality is likely preserved through incomplete occupancy of one of the two oxygen sites: the compound is thus non-stoichiometric, with the formula Yb(0.963)FeMnO(3.945). Another mechanism for preserving the electroneutrality is the oxidation of a small amount of Mn(2+) to Mn(3+), which is, however, less probable because of the reduction conditions in which the sample was synthesized. Both models give a satisfactorily CD result, but they cannot be definitively distinguished by X-ray data.

Crystal structure and Charge Distribution of ErFeMnO4

The structure of synthetic ErFeMnO4 has been refined by single-crystal X-ray diffraction. Space group is Rm, a=3.4831(1) A, c=25.609(4) A, V=269.07(5) A3, Z=3. Er is in octahedral coordination, whereas Fe and Mn occupy a single crystallographic type of distorted trigonal bipyramid, with one of the two apical bonds shorter and the other longer than the three basal bonds. Assuming Er in the centre of symmetry R1=0.031, but the Charge Distribution analysis suggests that the position of Er was not completely determined, and the Fourier difference indicates splitting of the Er cation around the centre of symmetry. Refinement in the split-atom model for Er lowers R1 to 0.016, reduces the displacement parameter of Er and removes the anomalies in the Fourier difference. A deviation from the mean structure in this kind of compounds seems regularly associated with the presence of distorted trigonal bipyramids.

Magnetic Properties of the Two Dimensional Antiferromagnets RFe2O4(R=Y, Er) at High Pressure

Pressure dependence of magnetic susceptibilities of YFe 2 O 4 and ErFe 2 O 4 has been measured up to 6 kbar above about 100 K. Two magnetic phase transitions are observed in the temperature region of 200–250 K at ambient pressure for both compounds. The transitions of YFe 2 O 4 are depressed with increasing pressure. Especially the lower transition rapidly decreases in temperature and then disappears at 6 kbar. The transitions of ErFe 2 O 4 show different pressure dependence, that is, the higher transition temperature increases and the lower one decreases with increasing pressure. The depression of the transition temperatures is explained as the volume effect on the first order phase transition, while the pressure dependence of the higher transition in ErFe 2 O 4 is interpreted as the 10/3 law for the volume dependence of superexchange interaction.

Magnetic property of single crystal Lu2Fe3O7

Abstract Magnetization, Mossbauer and neutron diffraction measurements were carried out on a single crystal of the two-dimensional antiferromagnet Lu 2 Fe 3 O 7 . Two magnetic orders with different magnetic properties corresponding to the ordering of two different layers were observed at around 60 and 230 K, respectively. The exchange interaction between the layers are revealed to be antiferromagnetic.

Magnetization and spin correlation in a single crystal of YFeMnO4

Magnetization and neutron‐diffraction measurements were carried out on a single crystal of hexagonal YFeMnO4, a two‐dimensional Heisenberg triangular antiferromagnet. The Fe3+ and Mn2+ are randomly distributed on equivalent triangular sites, producing the system with both frustration and randomness. The thermomagnetization curve in 50 Oe shows that a field‐cooling effect is observed below 60 K, where the susceptibility along the c axis has a cusplike peak. Neutron‐diffraction measurements reveal that a two‐dimensional (1/3,1/3,l) magnetic rod is observed in the whole temperature range instead of three‐dimensional Bragg points, and the magnetic correlation in the c plane does not become long range even at the lowest temperature. It also shows that spins fluctuate largely at room temperature, and this fluctuation still remains at low temperatures. This magnetic property is similar to that of spin‐glass system. Some type of spin freezing occurs below 60 K, and a slight increase of the correlation length down...

Crystal structures of the two dimensional antiferromagnets RFe2O4 (R = Y, Er) and their magnetic properties under pressure

Abstract The crystal structures of ErFe 2 O 4 and YFe 2 O 4 have been investigated at room temperature by means of time-of-flight (TOF) neutron diffraction. The structural parameters of ErFe 2 O 4 are found to be consistent with those of YFe 2 O 4 . Their magnetic properties have also been studied in relation to the structure. The pressure dependence of the magnetic phase transitions is found to be different in both oxides, even though their structures are similar.

Detection of a correlation between the orientation and the magnitude of the hyperfine field of57Fe in YFeMnO4

Overlapping Mössbauer spectra of a single crystal of YFeMnO4 are analyzed using the Hesse method extended to determine the distribution of two magnetic parameters: the orientation and the magnitude of the hyperfine field. A nearly linear correlation between them is detected with γ-rays incident parallel to thec-axis. A misfit of the spectrum around the central two lines may be attributed to a dynamical effect of the spins.