Kazuko Zenmyo

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.

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.

Site Dilution Study of a Square Lattice Heisenberg Antiferromagnet with S= 5/ 2 Covering the Percolation Threshold

The site dilution problems on a square lattice antiferromagnet with spin S =5/2, Mn(HCOO) 2 2(NH 2 ) 2 CO ( T N =3.77 K), have been studied for a wide range of magnetic concentration x , covering the percolation threshold x p =0.59 for the square lattice, by the measurements of magnetic heat capacity and susceptibility. The N\({\rm \acute{e}}\)el temperature T N ( x ) has been found to decrease as d/ d x { T N ( x )/ T N (1)}= R =2.7 just below x =1 for the three kinds of non-magnetic impurities, Mg 2+ (2p 6 ), Zn 2+ (3d 10 ) and Cd 2+ (4d 10 ), at almost the same reduction rate as in the cases of the antiferromagnet K 2 MnF 4 with S =5/2 and the ferromagnet K 2 CuF 4 with S =1/2. The present value R =2.7, however, makes a contrast to R =3.1 for the diluted La 2 CuO 4 , an S =1/2 Heisenberg antiferromagnet. The extrapolation of T N ( x ), with R =2.7(3.1) down to T N ( x )=0 gives a pseudo-critical concentration x = x c =0.63(0.68) deviating from x p =0.59. This indicates that T N ( x ) must draw concavel...

NMR Studies of Reentrant Spin Glass Phase in Random Mixture Co1-xMnxCl2·2H2O with Competing Exchange Interactions

The detailed magnetic phase boundary between antiferromagnetic and reentrant spin glass phases is determined by the (dis) appearance of a satellite line in NMR spectra caused by protons in a spin glass state. Transition temperatures for compositions around x =0.6 are found to be lower than the transition temperatures in other concentration regions. In the intermediate concentration region between x =0.47 and x =0.73, a Co-type antiferromagnetic ordering coexists with a Mn-type ordering. The quantity of spins in a spin glass state is estimated from relative intensities of a main line and its satellite lines. This is found to be about 40±10% of the total spins at 1.45 K, for 0.47≤ x ≤0.73.

Phase separation in A-site-ordered perovskite manganite LaBaMn2O6 probed by 139La and 55Mn NMR.

139La- and 55Mn-NMR spectra demonstrate that the ground state of the A-site-ordered perovskite manganite LaBaMn2O6 is a spatial mixture of the ferromagnetic and antiferromagnetic regions, which are assigned to the metallic and the insulating charge ordered state, respectively. This exotic coexisting state appears below 200 K via a first-order-like formation of the antiferromagnetic charge ordered state inside the ferromagnetic metal one. The Mn spin-spin relaxation rate indicates that the ferromagnetic region coexisting with the antiferromagnetic one in LaBaMn2O6 is identical to the bulk ferromagnetic metal phase of the disordered form La0.5Ba0.5MnO3 in spite of the absence of A-site disorder. This suggests a mesoscopic rather than nanoscopic nature of the ferromagnetic region in LaBaMn2O6.

Phase Separation inA-Site-Ordered Perovskite ManganiteLaBaMn2O6Probed byLa139andMn55NMR

139La- and 55Mn-NMR spectra demonstrate that the ground state of the A-site-ordered perovskite manganite LaBaMn2O6 is a spatial mixture of the ferromagnetic and antiferromagnetic regions, which are assigned to the metallic and the insulating charge ordered state, respectively. This exotic coexisting state appears below 200 K via a first-order-like formation of the antiferromagnetic charge ordered state inside the ferromagnetic metal one. The Mn spin-spin relaxation rate indicates that the ferromagnetic region coexisting with the antiferromagnetic one in LaBaMn2O6 is identical to the bulk ferromagnetic metal phase of the disordered form La0.5Ba0.5MnO3 in spite of the absence of A-site disorder. This suggests a mesoscopic rather than nanoscopic nature of the ferromagnetic region in LaBaMn2O6.

NMR Study of the Oblique Antiferromagnetism in the Random Mixture Fe1-xCoxCl2·2H2O with Competing Anisotropies

Proton NMR measurements are carried out in both of the antiferromagnetic (AF) and oblique antiferromagnetic (OAF) phases in the random mixture Fe 1- x Co x Cl 2 ·2H 2 O with competing anisotropies. NMR spectra show that in the Fe-rich and Co-rich antiferromagnetic phases all of Fe and Co magnetic moments point parallel to their easy axes, the α-axis for FeCl 2 ·2H 2 O (Fe-type AF) and the b -axis for CoCl 2 ·2H 2 O (Co-type AF), respectively. Some part of Fe spins tilt from b -axis near the boundary concentration region between the Co-type AF and OAF phases. In the OAF phase, both α- and b -components of the moments order independently. Then Fe and Co moments tilt from their easy axes with larger magnitudes of magnetic moments than that in the antiferromagnetic phase. These tilting moments coexist with the moments which are parallel to either α- or b -axis.

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.