Yoshiko Someya

Microscopic evidence for a new type of ordering of spin components in a random mixture with competing magnetic anisotropies

Abstract The Mossbauer spectroscopy was applied on a random mixture of two kinds of magnetic ions with competing anisotropies, Fe 1−x Co x Cl 2 ·2H 2 O. The Mossbauer spectra observed in the Co(Fe)-rich antiferromagnetic (AF) phase near the tetracritical point show that, although the m 2 (m 1 ) spin component parallel to the easy axis of pure FeCl 2 ·2H 2 O(CoCl 2 ·2H 2 O) has no long-range order in this phase (from the neutron diffraction experiment), the m 2 (m 1 ) spin component contributes to the magnetic hyperfine field at 57 Fe nuclei as well as the m 1 (m 2 ) spin component (which has a long-range order). This fact indicates that the m 2 (m 1 ) spin components exists in the Co(Fe)-rich AF phase near the tetra-critical point at least in a time scale of ∼ 10 −8 s. This result predicts that the antiferromagnetic ordering in the Co(Fe)-rich AF phase is a new type one. The new phase observed in the intermediate concentration region is demonstrated as the “mixed ordering” phase.

Mössbauer and Neutron Scattering Studies of Magnetic Properties of Random Mixtures with Competing Spin Anisotropies: FexCo1-xTiO3

The physics of random mixtures with competing spin anisotropies is found to be much more complex than ever considered. According to our neutron scattering study of Fe x Co 1- x TiO 3 , orderings of the orthogonal spin components S // and \(\vec{\textbf{\itshape S}}_{\bot}\) seem to be decoupled with each other though not complete; two antiferromagnetic (AF) phases and a mixed ordering phase appear on the concentration vs temperature phase diagram. However, according to our Mossbauer study, S // and \(\vec{S}_{\bot}\) seem to be closely coupled in the AF phases; the boundary between the AF and the mixed ordering phase is not found. This apparently contradictory result is interpreted as follows. In the \(S_{//}(\vec{\textbf{\itshape S}}_{\bot})\) ordered AF phase, orientational distribution of \(\vec{\textbf{\itshape S}}_{\bot}(S_{//})\)s is such that it does not produce coherent scatterings of neutrons, but each \(\vec{\textbf{\itshape S}}_{\bot}(S_{//})\) stays statically at least in the time scale of th...

Mössbauer Study of a Random Mixture with Orthogonal Spin Anisotropies: Fe1-xCoxCl2·2H2O

Mossbauer spectroscopy has been applied to a random mixture with orthogonal spin anisotropies, Fe 1- x Co x Cl 2 ·2H 2 O. The direction of an “individual Fe spin” of Co(Fe)-rich samples determined from the Mossbauer spectra was tilted even in the antiferromagnetic (AF) phase as well as in the mixed ordering phase. This means that, in the Co(Fe)-rich AF phase, although the spin component parallel to the easy axis of FeCl 2 ·2H 2 O (CoCl 2 ·2H 2 O) has no long-range order according to the neutron scattering measurement, the component exists on each lattice site statically at least in the time scale of 57 Fe Mossbauer spectroscopy (∼10 -8 s). The direction of an “individual Fe spin” has been shown to be strongly affected by the local environment around the Fe spin.

Mössbauer and neutron diffraction studies of competing magnetic orderings in random mixtures: Co1−xFexTiO3

Combined Mossbauer and neutron diffraction studies of Co1−xFexTiO3 show that the physics of random mixtures with competing orthogonal spin anisotropies is much more complex than ever considered. These studies give evidence that the orthogonal spin components are closely coupled with each other even in the antiferromagnetic phases. It is also shown that the transition temperature from the antiferromagnetic to the mixed-ordering phase is characteristic for the neutron diffraction measurement but not for the Mossbauer measurement.

Phase Diagram in a Random Mixture of Two Antiferromagnets with Competing Spin Anisotropies. I

The phase diagram of a random mixture of two antiferromagnets with competing spin anisotropies (A 1- x B x ) has been analyzed by extending the theory of Matsubara and Inawashiro, 1) and Oguchi and Ishikawa. 2) In the model assumed, the anisotropy energies are expressed by the anisotropic exchange interactions. According to this formulation, it has been shown that the concentration dependence of T N becomes a function of \(\mathscr{I}_{\text{PQ}}^{\eta}(\mathscr{I}_{\text{PQ}}^{\eta} \equiv (2zS_{\text{P}}(S_{\text{P}}+1)\cdot 2zS_{\text{Q}}(S_{\text{Q}}+1))^{1/2}\cdot J_{\text{PQ}}^{\eta}/3\), where P, Q=A, B; S P is a magnitude of P-spin, and J PQ η is a η component of exchange integral between P- and Q-spin). Further, the phase boundary between an AF phase and an OAF (oblique antiferromagnetic) phase at T =0 K has been shown to be determined by α(≡ S B / S A ), if \(\mathscr{I}_{\text{PQ}}^{\eta}\) are given. The obtained phase diagrams for Fe 1- x Co x Cl 2 , K 2 Mn 1- x Fe x F 4 and Fe 1- x Co x Cl 2...

Mössbauer Study of the Perovskite Compound RbFeF3: Magnetic Structure in the Intermediate Temperature Region, 39 K<T<87 K

The Mossbauer spectroscopy is applied to the perovskite compound, RbFeF 3 , in the phase which is realized between T 1 (39 K) and T 2 (87 K). The Mossbauer spectra clearly indicate the existence of two kinds of Fe 2+ sites. We label one of the sites as the site I and the other as the site II. From the Mossbauer spectra observed for a stress-free single crystal sample in an applied magnetic field, it is concluded that the spontaneous magnetization along a pseudo-cubic [100] axis originates in a ferrimagnetic spin arrangement; the magnetic moment of an Fe 2+ ion at the site II is larger than that at the site I. It is also concluded that the spin easy axis coincides with a pseudo-cubic [100] axis along which the lattice constant is the smallest. From the Mossbauer parameters determined, the electronic state of an Fe 2+ ion is inferred.

Competing magnetic orderings in random mixtures: FexNi1−xCl2

Abstract Neutron scattering and Mossbauer absorption techniques are applied to the mixture Fe x Ni 1− x Cl 2 in which the strong Ising-type anisotropy of Fe and the weak XY-type anisotropy of Ni complete with each other. A mixed ordering phase is found to exist in a narrow range of concentrations around x ≈ 0.08. It is found that if a small amount of Fe, ≈2% replaces Ni in NiCl 2 the directions of majority spins of Ni rise up from the c -plane (the spin easy axis of NiCl 2 ) as a whole Diffuse scattering with concentrations is observed in the S ‖ -ordered antiferromagnetic (AF) phase over a wide range of temperature for the samples with concentrations near the crossing point. It might be one of the distinctive properties of the ordered phase in the mixture with competing spin anisotropies.

Mössbauer study of the magnetic axis in RbFeF3 in the interval T2<T<TN

Abstract The antiferromagnetic axis in RbFeF 3 in the interval T 2 T T N has been expected to be along [111] from powder Mossbauer studies and along [001] from an X-ray diffraction study. We report our direct observation of the magnetic axis by Mossbauer spectroscopy using stressed single crystal samples. The results show that the magnetic axis is along [111].

Mössbauer Study of the Perovskite Compound RbFeF3: Magnetic Structure in the Phase below 39 K and Temperature Dependence of Hyperfine Parameters below the Néel Temperature

The Mossbauer spectroscopy is applied to the perovskite compound RbFeF 3 . It is concluded that in the phase realized below T 1 (39 K) the spontaneous magnetization originates in a canting of the spins, i.e. the spins are almost along a pseudo-cubic [011] axis but cant by about 6° from the [011] axis, yielding spontaneous magnetization along a pseudo-cubic [0 \bar11] axis. The temperature dependence of the Mossbauer parameters obtained for the three temperature regions below T N (101 K), T 2 (87 K)< T < T N , T 1 < T < T 2 , T < T 1 , is given.