Mamoru Mekata

Magnetic Study on Mn4N and its Related Compounds

In order to obtain some informations on the electronic structure of Mn 4 N, the magnetic properties of Mn 4 N compounds, partially substituted by In and Sn respectively, are studied by a newly devised magnetic torsion balance. These compounds show very complicated temperature- and composition-dependences of saturation magnetization and have the magnetic susceptibilities nearly independent of temperature at far above the Curie temperatures. Moreover, the composition dependence of the magnetic moment varies remarkably with the species of substituent metals. These facts may be explained in terms of an intuitive band scheme which leads to the variation of the magnetic moment on each sublattice with temperature. The band scheme of γ-Mn is also discussed in connection with that of Mn 4 N

Magnetic Study on Hexagonal Nitrides of 3d Transition Metals

Magnetic and Mossbauer effect measurements on the nitrides with hcp metal lattice of V, Cr, Mn and Fe are carried out as a function of composition. Hexagonal VN x and hexagonal CrM x are Pauli paramagnetic. Hexagonal MnN x shows antiferromagnetism, while both hexagonal and orthorhombic FeN x show ferromagnetism at low temperatures. In these Phases, nitrogen atoms suppress magnetic ordering in consequence of the reduction of magnetic moments. As a whole, the observed types of magnetism of hexagonal nitrides resemble those of their metals in spite of the difference in crystal structures. Type II superconductivity is found in hexagonal VN x although it is not yet conclusive.

Helical Magnetic Structure in CsCuCl3

Neutron diffraction measurements were made on single crystals of CsCuCl 3 . The sense of Cu 2+ ion helix was found from nuclear reflections to be predominantly right-handed. The magnetic reflection data could be interpreted satisfactorily by considering a triangular spin structure of helically modulated ferromagnetic chains: The magnetic moments lie in the c-plane and rotate by 5.1±0.1° along the chain or the c-axis, while the moments rotate by 120° in the c-plane. The magnitude of the magnetic moment is 0.58±0.01 µ B at 4.2 K. The critical exponent of the magnetization, β was found to be 0.358±0.015 around the Neel temperature, 10.70±0.05 K. We present a model that the helical modulation along the c-axis is caused by the antisymmetric exchange (Dzyaloshinsky-Moriya) interaction characteristic of the crystal symmetry of CsCuCl 3 . The model predicts a right-handed magnetic helix in the present case.

Successive magnetic ordering in CuFeO2 : a new type of partially disordered phase in a triangular lattice antiferromagnet

Magnetic ordering in a rhombohedrally stacked triangular lattice antiferromagnet, CuFeO 2 was investigated on powder and single crystals by measurements of neutron diffraction, magnetic susceptibility, Mossbauer effect together with Monte Carlo simulations. CuFeO 2 was found to exhibit transitions at T N1 =16±0.5 K and T N2 =11±0.5 K. The low temperature phase has an orthorhombic magnetic unit cell with collinear moments along the c -axis, whereas the intermediate temperature phase is a partially disordered phase with a monoclinic magnetic unit cell where 1/5 of moments remain paramagnetic. The Monte Carlo simulations for the Ising spin tiangular lattice antiferromagnet can reproduce the observed successive transition in the case of the exchange parameters 4 J 3 <2 J 2 ∼ J 1 <0. This combination of exchange parameters gives an account for the stability of the collinear magnetic structure in CuFeO 2 .

Antiferromagnetism in hcp Iron-Manganese Alloys

Antiferromagnetism in hcp Fe-Mn alloys (17.8, 25.9 and 28.6 at %Mn) was investigated by means of Mossbauer effect and neutron diffraction measurements. Neel temperatures and internal magnetic fields at 0°K were determined to be 230°K and 16 kG, respectively, over the composition range of the measured alloys. Consequently, hcp alloys have weaker internal magnetic fields and lower Neel temperatures than the fcc modifications. In the neutron diffractions on powder and sheets with preferred orientation of hcp Fe 0.714 Mn 0.289 , only (001) magnetic reflection was found. The averaged magnetic moment was evaluated to be 0.25 µ B assuming the antiferromagnetic stacking of ferromagnetic (001) sheets. Isomer shifts relative to bcc Fe and quadrupole splittings of hcp alloys are extrapolated to the zero Mn concentration to be -0.20 mm/sec and 0.14 mm/sec, respectively. The results are discussed in connection with magnetism in hcp Fe.

Neutron diffraction study of CuFeO2

Neutron diffraction study revealed that a powdered CuFeO 2 , a quasi-two-dimensional antiferromagnet on a triangular lattice (AFT), has two successive magnetic phase transitions at low temperatures. In the high-temperature phase below T N1 =16 K, it has a monoclinic magnetic unit cell with five spins in a layer (\(\sqrt{7}a\times\sqrt{7}a\times2{c}\), γ=141.78°). At T N2 =10 K, it shows a discontinuous transition and enters the low-temperature phase with an orthorhombic magnetic unit cell with four spins in a layer (\(\sqrt{3}a\times2{a}\times2{c}\)). In both magnetic structures, spins are collinear and parallel to the c axis.

Magnetic Ordering in a Partially Frustrated Triangular Antiferromagnet RbFeBr3

Specific heat of quasi-one-dimensional RbFeBr 3 has been measured in the temperature range 1.1∼10 K in applied magnetic fields. Two anomalies with slight field dependences have been found at T N =5.61±0.02 K and T t =2.00±0.04 K in zero field. Below 120 K, the spin frustration is partially released by a lattice distorsion. Analysis by molecular field approximation for magnetic ordering process in the triangular spin ( S =1) system with frustration partially relased by lattice modulation is presented. The partial release of the frustration enables us to predict from the analysis that the high temperature anomaly corresponds to the transition from paramagnetic phase with mutually uncorrelated magnetic chains to a partially disordered phase in which one third of magnetic chains remains uncorrelated with the others and the low temperature one to the transition from the partially disorded phase to the triangular phase in the c -plane.

New Critical Exponent β of the XY Antiferromagnet on Stacked Triangular Lattice, CsMnBr3

The phase transition of the XY antiferromagnet on a stacked triangular lattice, CsMnBr 3 is studied by neutron diffraction. The critical exponent β is found to be β=0.25±0.01. The value is in excellent agreement with the predicted value by Kawamura for a new universality class, giving convincing experimental support to the phase transition characterized by the Z 2 ×S 1 or O(2) symmetry.

Neutron Diffraction Study of γ-Phase Mn-Ir Single Crystals

Neutron diffraction was measured on single crystals of the disordered γ-phase Mn 1- x Ir x ( x =0.128, 0.170, 0.204 and 0.256). Diffraction patterns were interpreted by an antiferromagnetic two-sublattice structure with tetragonal symmetry. The (110) magnetic reflection was measured up to about 800 K for the determination of the temperature dependence of sublattice magnetization and the Neel temperature. The critical index for magnetization was found to be 0.38 for the x =0.170 alloy. The Neel temperature increases remarkably with increasing Ir at a rate of about 9 K/at%Ir. The obtained magnetic form factors are well expressed by f =(1/0.9)( f 3d -0.1 f 4s ) constructed from the scattering functions calculated by Freeman and Watson. The average magnetic moment per Mn atom increases from 2.2µ B to 2.6µ B with increasing Ir from x =0 to x =0.204. Diffuse scattering due to partial atomic order of Mn 3 Ir was observed in the x =0.256 alloys.

New Partially Disordered Phases with Commensurate Spin Density Wave in Frustrated Triangular Lattice

We have studied magnetic ordering of frustrated classical Ising spins with S =1/2 on the antiferromagnetic triangular lattice taking into account of the 2nd and the 3rd neighbor interactions by means of Monte Carlo simulations and the mean field approximation. The antiferromagnetic second and third neighbor interactions give rise to the first order phase transitions from the ground state to partially disorder states characterized by quasi-long range order with Kosterlitz-Thouless character due to the spin frustration. These partially disordered phases with 5- and 9-sublattices have commensurate sinusoidal spin density waves and related to 10- and 18-clock XY spin models, respectively.