Hidenori Kubo

Magnetization of the Two-Dimensional Heisenberg Ferromagnet K2CuF4 with Small XY-Like Anisotropy

The temperature variation of magnetization in the quadratic layer ferromagnet K 2 CuF 4 has been investigated by measuring the NMR frequency of Cu 63 or out-of-layer F 19 nuclei at zero and applied fields in the temperature region of 0.27≦ T / T c ≦0.67. The results are analyzed by a two-dimensional spin wave theory. Under the external field the dipole interaction is approximated in a form of the effective field. The X Y -like anisotropy of exchange interaction is taken into consideration. Good agreement between the theory and the experimental results is obtained. The X Y -like anisotropy itself is not effective to the magnetization, but its contribution to the magnetization cannot be neglected when it cooperates with the external field. The parameter of the exchange interaction J within the layer is obtained to be 12.37 K. The NMR result at zero applied field is also discussed.

NMR Study of Nearly Two-Dimensional Ferromagnet (CnH2n+1NH3)2CuCl4(n=1∼4) at Low Temperature

NMR of Cl 35 nuclei (within layer) of (C n H 2 n +1 NH 3 ) 2 CuCl 4 is observed at the magnetically ordered state. Through the angular dependence of the NMR frequency, the crystallographic structure of the CuCl 4 layer is determined, which is slightly different from the known crystal structure at room temperature. The existence of various moment directions is shown in (C 3 H 7 NH 3 ) 2 CuCl 4 . The magnetic coupling between layers is discussed. The observed magnetization of (C n H 2 n +1 NH 3 ) 2 CuCl 4 ( n =1, 3 and 4) is compared with the renormalized spin wave theory. It is also shown that the magnetization follows the “T 3/2 -law” given as, /S=1-α(T/T c ) 3/2 , with α∼0.45. The field dependent behavior of the magnetization of (C 2 H 5 NH 3 ) 2 CuCl 4 is discussed at antiferromagnetic state.

Magnetic Phase Diagram of a Random Mixture with Competing Interactions: Co1-xMnxCl2·2H2O

Magnetic and thermal properties of Co 1- x Mn x Cl 2 ·2H 2 O with competing exchange interactions have been studied experimentally, For the system with x ∼0.5, the magnetic heat capacity have revealed only a broad maximum around 5 K, which is quite different from the heat capacity peak of λ-type for the pure systems and similar to that observed in metallic spin-glasses. The hysteresis in the magnetic susceptibility and thermoremanent magnetization have been observed in the mixtures with x ≤0.5, reflecting the frustration of magnetic interactions, while normal antiferromagnetic behavior has been seen in the region x >0.5. The magnetic phase diagram of this system is given, which contains four states: paramagnetic, antiferromagnetic, frustrated antiferromagnetic and anisotropic spin-glass states.

Proton NMR in a Random Mixture with Competing Anisotropies: CsMn1-xCoxCl3·2H2O

Proton NMR measurements were carried out to clarify the spin structure in the intermediate phase of CsMn 1- x Co x Cl 3 ·2H 2 O. Hyperfine fields at proton sites are compared with the calculated values of dipole fields. In Mn-rich antiferromagnetic phase, the moments including Co impurities point to the b -axis, while in Co-“rich” antiferromagnetic phase, most of the moments point to the c -axis. In the intermediate phase, three kinds of NMR lines were observed which are due to the moments parallel to the b -axis, c -axis and distributed in the b c -plane. The fraction of the moments parallel to the c -axis increases with increasing Co concentration. It is shown that in the intermediate phase the b - and c -axis domains coexist, and at the boundary of both domains the moments tilt continuously from the b - to c -axis. The tilting of the averaged moment is achieved by the change in the fraction of each domain. This model is supported by the fact that the relative intensities of the peaks change with an ext...

Study of Spin Configuration in Mn Formate Di-Urea by Proton NMR

Antiferromagnetic spin arrangement of Mn formate di-urea is studied by proton NMR measurements below T N (3.78 K). Observed spectra are compared with possible spin arrangements. The antiferromagnetic moments within the layer point to the direction rotating 15 degrees around the c -axis from the middle direction between the [001]- and [\bar100]-directions. The layer including antiferromagnetic moments rotates following to the stacking of four-fold screw axis along the c -axis. The origin of this spin arrangement is discussed.

Spin Freezing in a Reentrant Spin Glass Phase –Proton NMR in the Randomly Mixed System Co1-xMnxCl2 · 2H2O–

In proton NMR spectra of the title compound, the main and satellite lines are observed. The main line is due to the long range ordered moments of Co-type ( x 0.6) antiferromagnets. The satellite lines are intrinsic to the reentrant spin glass phase and are due to the moments in the spin glass state. The center frequencies strongly depend on the concentration and the line width has its maximum at about x =0.75. We show that a random freezing of the longitudinal component of moments occurs in the spin glass state, as expected in Ising systems. However, the freezing of the moments is not completely random. The coupling of the freezing two moments depends on the probabilities which are determined by the concentration. The probabilities reflect only the sign of the exchange interaction.

Nuclear Magnetic Resonance of Cu Nuclei in CsCuCl3 –A Screw Spin Arrangement in the Ordered State–

The NMR of copper nuclei in CsCuCl 3 has been studied at 1.77 K. NMR lines of Cu 63 nuclei were observed at 169.2 MHz with two satellite lines equally split by the quadrupole interaction. The analyses of the hyperfine field and the quadrupole splitting give that the spin direction of the Cu 2+ ions is parallel to the local z -axis. Since the local z -axes have a helical nature along the c -axis, a screw spin arrangement at the ordered state is verified from the NMR results.

Proton NMR study of nickel formate Di-hydrate, Ni(HCOO)2·2H2O

In nickel formate di-hydrate, Ni(HCOO) 2 ·2H 2 O, there are two nonequivalent Ni sites forming two magnetic subsystems, A- and B-layers. Heat capacity measurements suggest that the Ni ions in the B-layer are paramagnetic even below T N and the Ni ions in the A-layer are in antiferromagnetic order. On the other hand, the magnetization suggests the weak ferrimagnetic order of Ni ions in both A- and B-layers. To solve the inconsistency above, the spin system is investigated by proton NMR. The observed resonance frequencies are compared with calculations based on various magnetic structures. The comparison shows that the ferrimagnetic spins order with canting magnetic moments in the a – b plane. The magnitude of the magnetic moment of B-ions, 0.38µ 0 , is much smaller than that of A-ions, 2.38µ 0 , where µ 0 is the Bohr magneton. It is inferred that the paramagnetic B-ions are pulled into the antiferromagnetic order by the dipolar field due to A-ions. The magnetic moments are inferred to gradually incline par...

Existence of slowly fluctuating spin clusters at Néel temperature in the mixture Co1-xMnxCl2.2H2O

Proton NMR measurements are carried out on the mixture Co 1- x Mn x Cl 2 ·2H 2 O with various Mn concentrations in the region from 1 to 4.2 K. For x =0.54 proton NMR lines are observed near 17 MHz at just Neel temperature in zero applied field. This result shows the existence of slowly fluctuating spin clusters freezing for more than 10 -4 sec with the spin cluster of the diameter of more than 14 A near Neel temperature. The temperature dependence of a Bragg reflection by neutron diffraction is reinterpreted in relation to the amount of such spin clusters.

NMR Study of Oblique Antiferromagnetic Phase in Random Mixture CsMn1-xCoxCl3·2H2O

To clarify the spin structure in oblique antiferromagnetic (OAF) phase, proton NMR measurements are carried out at 1.7 K in zero applied field. The OAF phase is found in the region of 2.5%\( \lesssim x \lesssim \)11%, from the concentration dependence of NMR spectra. The spectra are interpreted by the model that in OAF phase, the moments surrounding Co impurity are drawn towards the impurity and then directed to other directions. Simultaneous tilting of all moments is denied by a little shift of the main line.