정재호 교수

Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr2O4, undergoes an elongation along the c axis (c > a = b) at its spin-Peierls-like phase transition at T(N) = 7.8 K. The Néel phase (T < T(N)) has an incommensurate spin structure with a characteristic wave vector Q(M) = (0, delta,1) with delta approximately 0.09 and with spins lying on the ac plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr2O4, that undergoes a c-axis contraction and a co

The nature of the magnetic state of $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$ has been controversial. In this compound Ni spins $(S=1∕2)$ form a triangular lattice with the possibility of magnetic frustration, but the exact state of spin correlation has not yet been known in spite of the extensive research work. A factor that complicates understanding of the magnetic state is the orbital state of ${\mathrm{Ni}}^{3+}$ which is a Jahn-Teller (JT) ion. While there is no signature of long-range Jahn-

Inelastic pulsed neutron scattering measurements on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.95}$ single crystals indicate that the sample has a distinct a-$b$ plane anisotropy in the oxygen vibrations. The Cu-O bond-stretching-type phonons, which are suspected to interact strongly with charge, are simultaneously observed along the a and b directions due to a 7-meV splitting arising from the orthorhombicity, even though the sample is twinned. The bond-stretching LO branch with the pol

Using neutron scattering techniques, we have investigated spin wave excitations in noncollinear ferrimagnetic spinels $\mathrm{Mn}{B}_{2}{\mathrm{O}}_{4}$ $(B=\mathrm{Mn},\mathrm{V})$ with ${e}_{g}$ and ${t}_{2g}$ orbital degeneracies, respectively, that lead to tetragonal distortions along opposite directions. Linear spin wave analysis of the excitations yields spatially inhomogeneous nearest neighbor interactions in both tetragonal spinels. We find the ratio ${J}^{c}∕{J}^{ab}\ensuremath{\appro

We report the antiferromagnetic (AFM) interlayer exchange coupling between Ga0.97Mn0.03As ferromagnetic semiconductor layers separated by Be-doped GaAs spacers. Polarized neutron reflectivity reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers is AFM. When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the

We report the observation of the giant magnetoresistance effect in semiconductor-based GaMnAs/GaAs:Be multilayers. Clear transitions between low-field-high-resistance and high-field-low-resistance states are observed in selected samples with Be-doped nonmagnetic spacers. These samples also show negative coercive fields in their magnetic hysteresis and antiferromagnetic (AFM) splittings in polarized neutron reflectivity. Our data indicate that the AFM interlayer exchange couplings in this system

Abstract Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show

Tetragonal ferrimagnetic spinel Mn3O4 shows complex magnetic phase transitions due to competing anti ferromagnetic interactions involving Mn2+/3+ ions with difference valences. Using magnetization and neutron diffraction measurements, we propose that its tetragonal lattice possesses an instability towards orthorhombic distortion in zero field well below \(T_{\text{N}}\). The direction of the observed instability suggests that the anomaly is related to lifting of the persistent geometrical frustr

A robust long-range antiferromagnetic coupling between ferromagnetic Ga0.97Mn0.03As layers has previously been realized via insertion of nonmagnetic Be-doped GaAs spacers between the magnetic layers. In this paper we report the observation of weak antiferromagnetic coupling between Ga0.97Mn0.03As layers through undoped GaAs spacers with thicknesses as large as 25 monolayers. The field and the temperature dependences of the sample magnetization suggest that the interlayer coupling in these system

The process of magnetization reorientation in a ferromagnetic semiconductor GaMnAs films was investigated using planar Hall effect measurements. In addition to the well-known two-step switching behavior that occurs during this process, we have observed two additional distinct features in field scan data of the planar Hall resistance (PHR). First, the region of the external field required to begin and complete the reorientation of magnetization from one easy axis to another strongly depends on th

Using neutron-scattering techniques, we have investigated the nuclear and the magnetic structures of the triangular antiferromagnet $3R{\text{-AgNiO}}_{2}$. The symmetry analysis based on the group theory suggests that the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ charge order proposed for $2H{\text{-AgNiO}}_{2}$ [E. Wawrzy\ifmmode \acute{n}\else \'{n}\fi{}ska et al., Phys. Rev. Lett. 99, 157204 (2007)] will have a monoclinic symmetry if present in the trigonal lattice of $3R{\text{-A