Masahiko Tokita

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...

Effective field theory with differential operator technique for dynamic phase transition in ferromagnetic Ising model

The non-equilibrium phase transition in a ferromagnetic Ising model is investigated by use of a new type of effective field theory (EFT) which correctly accounts for all the single-site kinematic relations by differential operator technique. In the presence of a time dependent oscillating external field, with decrease of the temperature the system undergoes a dynamic phase transition, which is characterized by the period averaged magnetization Q, from a dynamically disordered state Q = 0 to the dynamically ordered state Q ≠ 0. The results of the dynamic phase transition point Tc determined from the behavior of the dynamic magnetization and the Liapunov exponent provided by EFT are improved than that of the standard mean field theory (MFT), especially for the one dimensional lattice where the standard MFT gives incorrect result of Tc = 0 even in the case of zero external field.

Spin-Lattice Relaxation Time Due to the Quadrupole Moment of Au197 in Gold Metal

By use of the single-OPW, the expression for the spin-lattice relaxation time T 1 ( Q ) due to the quadrupole interaction is derived taking into account the electron interaction. The matrix element of the quadrupole interaction is calculated by use of the representation of the rotation group, and it is found that the effect of the core enhancement is represented by a function, α( q ), depending on the magnitude of the scattering wave vector q of an electron and α(2 k F ) agrees with the core enhancement factor by Kohn and Vosko. The numerical calculation leads to T 1 ( Q ) T =32 sec K which is considerably large in comparison with the value, 13 sec K, estimated from the deviation from the Korringa relation due to the electron interaction. When the value of α(2 k F ), for silver, estimated by Fukai by use of the single-pseudo-wave function is taken into account, a reasonable value is obtained.

NMR Study of Spin State in Geometrically Frustrated Compound Co2Cl(OH)3

NMR studies of a geometrically frustrated system, Co 2 Cl(OH) 3 , are carried out at low temperature. The NMR results of 59 Co and 1 H nuclei show that long-range ordered ferromagnetic moments on the triangular lattice plane coexist microscopically with disordered moments on the kagome lattice plane. The moment direction is not ordered. Although the disordered moments freeze within the NMR observation time (10 -4 s) at low temperature, the frozen moments begin to fluctuate with increasing temperature. Up to 4.2 K (0.4 T C ), the magnitude of the ferromagnetically ordered moments is independent of temperature.

NMR Linewidth of Au197 in Gold Metal

The Hamiltonian for the indirect interaction between nuclear spins, which arises from the Fermi contact, dipole and quadrupole interactions between nuclei and conduction electrons, is derived, in which cross-interaction terms of various types appear. It is shown that a part of the indirect interaction arising from the above quadrupole interaction just cancels the direct quadrupole interaction between nuclei. The second moment of the central line of Au 197 is calculated by taking account of the effect of the strained crystals on the quadrupole moment and by making use of the results obtained in our previous paper for the nuclear spin-lattice relaxation time. It is found that the main contributions to the line-width come from the exchange and pseudo-dipole interactions.

Proton NMR Study of Atacamite Cu2Cl(OH)3

The proton NMR spectrum of Cu2Cl(OH)3 at low temperature in zero applied field has two sharp peaks. This result means that the majority of magnetic moments order antiferromagnetically at low temperature. The magnetic structure determined by comparing the observed proton NMR spectrum with the numerical calculated ones in several magnetic models shows that the magnetic moments order “all-in all-out” orienting to body center. This magnetic structure is similar to antiferromagnetic pyrochlore with geometric frustration, and different from the one of atacamite Mn2Cl(OH)3, though having the same crystal structure. The magnitude of magnetic moments of Cu(I) is 1.12 \(\mu_{\text{B}}\) and Cu(II) 0.25 \(\mu_{\text{B}}\). In increasing temperature, the number of moments deviated from “all-in all-out” directions becomes large. However, the clear evidence of the disordering or spin-glass moments is not detected from observed NMR spectrum.

Proton NMR Study of Pyrochlore-Like Atacamite Mn2Cl(OH)3

The proton NMR spectrum of Mn 2 Cl(OH) 3 with zero applied field has rather sharp peaks, and a glassy property is not observed. The temperature dependence of NMR frequency shows that antiferromagnetic transition occurs at T N1 (=3.4 K), and that no anomaly appears at T N2 (=2.7 K). However, the change in frequency is considerably larger than the normal change caused by thermal fluctuation. The magnetic structure of Mn 2 Cl(OH) 3 determined by comparison with the observed proton NMR spectrum and the numerical calculated ones of various magnetic models shows that magnetic moments do not order in an “all-in all-out” orientation relative to the body center like antiferromagnetic pyrochlore, and not parallel to the local crystal field. The coexistence of antiferromagnetic and spin-glass moments does not reproduce the observed NMR spectrum. Our result suggests that the magnetic structure of atacamite Mn 2 Cl(OH) 3 is antiferromagnetic, but different from that of pyrochlore, although it is similar to the crystal...

Spin configuration of Mn(HCOO)2·2(NH2)2CO

Abstract The magnetic spin arrangement of Mn 2+ in the crystal of manganese formate di-urea, Mn(HCOO) 2 ·2(NH 2 ) 2 CO, is investigated within the Weiss molecular field approximation at T =0. This compound has the P4 1 2 1 2 tetragonal crystal structure and is a rather complicated antiferromagnet with eight sublattices. It is shown that this spin configuration is possible by taking into account the anisotropy energies due to the magnetic dipole field and the anisotropic exchange field, both caused by the small displacement of Mn 2+ from the perfect lattice sites, in addition to the superexchange interactions between the magnetic spins.

Temperature Dependence of ESR Lines Related to Phosphorus in Silicon

Temperature dependence of ESR in a silicon crystal containing 1.3×1017 phosphorus atoms/cm3 has been investigated at 4–40 K. The ESR signals depend strongly on the specimen temperature. Typical hyperfine structures of the phosphorus ground state were found in the low-temperature region. The hyperfine lines vanished at 20 K and a single line corresponding to the first excited state of phosphorus was observed in the high-temperature region. The amplitudes of the hyperfine lines and the single line show sharp maxima at 11 K and 30 K, respectively. The change of the signal amplitudes is due to the temperature dependence of spin relaxation time. The change of the linewidth for the hyperfine lines can be explained by the temperature dependence of spin relaxation time, and that for the single line is considered to be caused by motional narrowing.

Nuclear magnetic resonance of 197Au in gold metal

Abstract By the use of the single-OPW, the expressions for the spin-lattice relaxation time T 1 (Q) due to the quadrupole interaction and the second moment for the central resonance line are calculated, taking account of the electron interaction. The numerical calculation leads to T 1 (Q) T = 32 secK which is considerably large in comparison with the value, 13 secK, estimated from the deviation from the Korringa relation. When the single-pseudo-wave function is used, a reasonable value is obtained. The second moment of the central line of 197 Au is calculated by taking account of the effect of strained crystals for the quadrupole moment. It is found that main contributions to the line-width come from the exchange and pseudo-dipole interactions. Some examinations are made for the discrepancy of Stoners enhancement factor in noble metals between our analysis and the band calculation.