Pulsed NMRON measurements on insulators

Selective single and double quantum excitation pulsed NMRON has been utilized to obtain rotation patterns, free induction decays, and spin echoes in antiferromagnetic⁵⁴Mn−MnCl₂·4H₂O and the quasi-2 dimensional ferromagnet⁵⁴Mn−Mn(COOCH₃)₂·4H₂O. The pulsed technique is well suited to observing fast spin-lattice relaxation and T₁ values down to 100 ms have been measured in these systems. These short times may make magnetic insulators viable hosts for on-line experiments. The dependence on field and temperature of the⁵⁴Mn spin-lattice relaxation time T₁ has been investigated and a T₁ minimum at high fields observed in both systems. A spin-spin relaxation time T₂≈50 μs is measured in⁵⁴Mn−MnCl₂·4H₂O. The observation of NMRON in the paramagnetic phase of MnCl₂·4H₂O allows the resonant frequencies in the antiferromagnetic and paramagnetic phases to be compared, yielding a value for the zero point spin deviation in the former phase.     

NMRON spin echo in a slowly varying magnetic field

Nuclear spin-spin relaxation of⁶⁰Co and⁵⁶Co in iron single crystals has been studied, using the three-pulse NMRON spin echo. A previously reported rapidT ₂ in⁶⁰CoFe is shown to have arisen from a modulation of the echo amplitude, caused by variations in the phase of the Larmor precession relative to the applied rf field. A lower limit ofT ₂∼0.2s is found in⁵⁶Co⁵⁶ Fe. Extension of this result to other CoFe samples is discussed.     

Hyperfine Fields of Sr and Y in Ferromagnetic Hosts, and Magnetic Moment of 93Y

Recent studies of hyperfine fields B _HF of Sr and Y in Fe are briefly reviewed. The B _HF value of Y in Ni is given by NMRON of ^91mYNi. The nuclear magnetic moment of the ground state of ⁹³Y is extracted from the β-ray detected NMRON of ⁹³YFe.     

Contribution of electron-nuclear interaction to the residual resistivity of metals

In nonmagnetic metals the spin-spin interaction of the electrons and nuclei makes a strongly magnetic field and temperature T dependent contribution to the residual resistivity. The nuclei act as magnetic impurities. For magnetic metals (Tb, Ho, Dy) with a high internal magnetic field, the nuclear contribution to the resistivity vanishes at low temperatures T, where the nuclear spins are ordered, and saturates at high temperatures T, where the nuclear spins are disordered—the analog of the Schottky effect for the nuclear specific heat. The electron-nuclear interaction can destroy superconductivity in metals with low critical magnetic fields under conditions of ferromagnetic ordering of the nuclear spins. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 193–197 (10 August 1996)     

Nuclear Orientation Studies of Magnetic Materials

Low temperature nuclear orientation (LTNO) and nuclear magnetic resonance on oriented nuclei (NMRON) can be utilized to investigate the magnetic properties of solids, and are especially useful when high sensitivity is required, for example in the study of small or dilute systems. Spins of stable isotopes can also be studied using NMR thermally detected by NO (NMR-TDNO) of the radioactive nuclei. An effect of nuclear spin coupling in ordered magnets is frequency pulling of the abundant spins, and this has been investigated in the quasi-2-dimensional ferromagnet ⁵⁴Mn–Mn(COOCH₃)₂4H₂O by comparing the NMRON of the dilute radioactive nuclei with the NMR-TDNO of the abundant host nuclei. A structure in the spectra is observed that is yet to be explained. Recent LTNO experiments on magnetic multilayers are described. Experiments implanting radioactive ions into insulating magnets show that the implanted nuclei experience sizeable hyperfine fields, and this may be useful for probing magnetic materials that are difficult to dope by conventional means.     

Effect of the 16O → 18O isotopic substitution and a magnetic field on the spin dynamics in the (La0.25Pr0.75)0.7Ca0.3MnO3 manganite according to the 139La NMR data

¹³⁹La nuclear magnetic resonance spectra have been recorded and the spin-spin relaxation times in the paramagnetic region in external magnetic fields of 5 and 9.4 T have been measured for two types of manganites: initial (La_0.25Pr_0.75)_0.7Ca_0.3Mn¹⁶O₃ and enriched with the ¹⁸O isotope ((La_0.25Pr_0.75)_0.7Ca_0.3Mn¹⁸O₃). The manganite enriched with the heavier oxygen isotope exhibits disappearance of a signal in the charge-ordering region (T < T _co) in an external field of 5 T. This effect is related to the anomalous increase in the spin-spin relaxation rate. With an increase in the external magnetic field to 9.4 T, the difference in the behavior of the relaxation rate for the two samples in the charge-ordering region remains pronounced, although the magnitude of this effect becomes much smaller. The observed giant isotope effect is a bright evidence of the important role of oxygen motion in the formation of long-range magnetic order in the manganites under consideration. Original Russian Text ? K.N. Mikhalev, I.E. Litvinov, Z.N. Nigamat’yanova, A.Yu. Yakubovskii, A.R. Kaul’, O.Yu. Gorbenko, K. Kumagai, Y. Furukawa, 2007, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2007, Vol. 71, No. 5, pp. 726–729.     

NMRON studies of the antiferromagnetic and paramagnetic phases of54Mn-MnCl2 · 4H2O

Pulsed NMRON, CW NMRON and thermal NMR-NO methods have been utilized to study⁵⁴Mn-MnCl₂ · 4H₂O. The⁵⁴Mn spin-lattice relaxation timeT ₁ in zero applied field has been measured between 35 and 90 mK in the antiferromagnetic phase. Above 65 mK the dominant relaxation mechanism is a Raman process with the electronic magnons, but at lower temperatures a direct process takes over. NMRON has been observed for the first time in the paramagnetic phase, and a line width of 300 kHz, with both homogeneous and inhomogeneous contributions, is observed. In the antiferromagnetic phase the line width is 35 kHz, and there are also homogeneous and inhomogeneous contributions. The dependence ofT ₁ for the⁵⁴Mn spins on field and temperature was studied in the paramagnetic phase. AT ₁ minimum centred atB ₀=2.64 T was observed. The hyperfine parameter <⁵⁴ AS>/h=−513.6(3) MHz in the paramagnetic phase, and comparison with the value in the antiferromagnetic phase gives 0.013(1) for the zero point spin deviation.     

Nuclear spin-lattice and nuclear spin-spin relaxation time measurements in EuB6 at low temperatures using the spin-echo technique

Experimental results on the nuclear spin-lattice and nuclear spin-spin relaxation times in the ferromagnetic EuB₆ at temperatures below 4·2 K are presented using the external magnetic field,H _ext, in the range of 0 ⩽H _ext ⩽ 10 kG. Nuclear spin-spin relaxation time computed on the basis of the Suhl-Nakamura process turns out to be 3·2μs, which compares well with the experimental value 11·1μs obtained with the 10 kG magnetic field at 1·7 K. It is found that in the ferromagnetic EuB₆,T ₁ is approximately 5 × 10³ times larger thanT ₂ at 1·7 K with the 10 kG magnetic field. Thus the effect ofT ₁ onT ₂ can be neglected. From the experimental value ofT ₂, the value of the homogeneous line broadening is found to be 14 kHz. The corresponding value obtained from the cw method is 175 kHz. This evidently shows the presence of the inhomogeneous line broadening in the cw NMR.     

Irreducible tensor operators and selective/hard single and double resonance experiments in insulators: Applications to NMRON and MQ-NMR

Recently, pulsed NMRON experiments have been carried out on trace amounts of radioactive⁵⁴Mn in the antiferromagnet MnCl₂4H₂O at 500 MHz (Le Gros et al. [1]). In this compound, the quadrupole splitting between the two lowest NMR transitions is ≈3 MHz, which precludes the use of non-selective (hard) rf pulses. Yet within the restricted 2*2 manifold, associated with a given transition, the nuclear rotation is “hard”. In this paper, the theory of “selective-hard” NMRON and MQ-NMR experiments is developed within the framework of irreducible tensor operators. In essence, the theory extends the early work of Jaynes [4] to deal with the higher-order multipolar states created during the course of a given NMR experiment. Several new pulsed NMRON and MQ-NMR experiments are proposed. For example, it is demonstrated how “ouble resonance”, “selective-hard” experiments on the pseudo spin-1 manifold spanned by |±1> and |0> Zeeman states of any integer spinI could be used to extract small chemical shifts in the face of very large quadrupole splittings.     

In situ 54Mn NMRON Studies of the Mixed Halide Mn(Br X Cl1−X )2 · 4H2O in Applied Magnetic Fields

⁵⁴Mn NMRON in the mixed halide antiferromagnet, Mn(Br _X Cl_1−X )₂ · 4H₂O, for varying external field, is reported. Significant qualitative differences are found between these NMRON transitions and those of the two terminal compounds, especially in respect to line widths. A tentative assignment is made to most of the observed NMRON transitions out to 0.8 T. An unidentified lower frequency inhomogeneous signal, present only at the lowest temperatures, with no counterpart in the terminal compounds, is also reported.     

Dynamic NMR investigation of plastic crystals in bulk and confined in porous materials

In this work, the molecular dynamics of four organic compounds confined in silica pores of nominal diameter 6 and 20 nm are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substances. By using organic compounds forming soft plastic crystals on freezing as adsorbates, damage to the pore structures can be avoided. NMR lineshapes, spin-lattice relaxation times (T ₁), spin-spin relaxation timesT ₂ and diffusivities are reported as a function of temperature. Since the porous grains are much greater than the distance travelled by the molecules during the experiment, intracrystalline NMR parameters were obtained. However, the shortT ₂ (∼1 ms) encountered in both the bulk and confined samples prohibited measurements ofT ₂ and the diffusivity in the low-temperature ordered phases. The confinement in the pores gives rise to substantial changes in the phase behavior and molecular dynamics. Thus, the¹H lineshape observations of the confined samples clearly reveal a narrow-line component superimposed on a broad resonance at temperatures well below the transition point of the bulk material. In the freezing region, the narrow-line component is attributed to the surface layer and the undercooled liquid in the smaller pores that remains unfrozen. In the two-component, low-temperature region, the narrow component corresponds to the surface layer, while the broad component originates from the crystalline phase at the center of the pores.     

Quadrupole spin-echo envelopes for Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> single crystals doped with the atoms of transition and rare-earth elements

The influence of a weak (below 50 Oe) constant magnetic field on a quadrupole spin-echo envelope was studied for an undoped single crystal Bi₄Ge₃O₁₂, in which local magnetic fields on the order of 20–30 G were previously found, as well as for single Bi₄Ge₃O₁₂ crystals doped with the atoms of transition and rare-earth elements. In all of these cases, the spin-echo envelopes were strongly influenced. A considerable increase in the nuclear spin-spin relaxation time T ₂ was observed for the undoped sample upon the switching of weak external magnetic fields. For the doped samples, the spin-echo envelope decay became much slower already in the zero field. The external magnetic fields exhibited a markedly weaker influence on the spin-echo envelope for the doped samples. The text was submitted by the authors in English.     

Spin-wave contributions to nuclear magnetic relaxation in magnetic metals

The longitudinal and transverse nuclear magnetic relaxation rates 1/T ₁(T) and 1/T ₂(T) are calculated for three- and two-dimensional (3D and 2D) metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic moments in the spin-wave temperature region. The contribution of the one-magnon decay processes is strongly enhanced in comparison with the standard T-linear Korringa term, especially for the FM case. For the 3D AFM case this contribution diverges logarithmically, the divergence being cut at the magnon gap ω due to magnetic anisotropy, and for the 2D AFM case as ω^-1. The electron-magnon scattering processes yield T ²ln(T/ω) and T ²/ω^1/2-terms in 1/T ₁ for the 3D AFM and 2D FM cases, respectively. The two-magnon (“Raman”) contributions are investigated and demonstrated to be large in the 2D FM case. Peculiarities of the isotropic 2D limit (where the correlation length is very large) are analyzed. Received 29 November 1999 and Received in final form 6 June 2000     

Observation of magnetic Mn sites in cubic and hexagonal HoMn2: 55Mn NMR study

⁵⁵Mn nuclear magnetic resonance has been measured for both cubic C15 and hexagonal C14 HoMn₂. In the ordered state, we found a high-frequency signal, which can be assigned to magnetic Mn atoms, for both C15 and C14 phases together with a low-frequency signal from non-magnetic Mn atoms. The results of the spin-spin relaxation time T₂ in the ordered state and the NMR spectra in the paramagnetic state are also given to discuss the magnetic instability and the magnetic structure.     

Nuclear orientation and nuclear spin-lattice relaxation in insulating magnetically ordered crystals

The mechanism of nuclear spin-lattice relaxation in insulating magnetically ordered crystals at low temperatures is discussed. In 3-dimensional systems the relaxation time T₁ is long, but in the lower dimensional systems it can be quite short making them possible candidates for hosts in on-line experiments. An experiment in 2-dimensional⁵⁴Mn−Mn (COOCH₃)₂·4H₂O is discussed with particular reference to the relatively short value of T₁ in low applied fields. A preliminary experiment in the 1-dimensional system⁵⁴Mn−(CH₃)₄NMnCl₃ (TMMC) is also described. Pulsed NMRON measurements in⁵⁴Mn−MnCl₂·4H₂O are outlined and the advant-systems with fast relaxation emphasized.     

Proton Spin–Lattice Relaxation Study of the Setting Reaction in Conventional and Resin-Modified Glass-Ionomer Dental Cements

The setting processes in KetacCem, AquaCem and Fuji I glass-ionomer dental cements (GIC) as well as in the resin-modified glass-ionomer dental cement (RM-GIC) Fuji Plus have been studied by proton spin–lattice, T ₁, and spin–spin, T ₂, relaxation. The setting time dependence of the degree of hydration was studied as well. In contrast to zinc oxide dental cements, the changes in T ₁ and T ₂ are determined by the interactions of the water with the internal surface. Proton nuclear magnetic resonance relaxation is thus suitable to follow the setting of GIC and RM-GIC and gives valuable information on the GIC hardening dynamics. Authors' address: Tomaz Apih, J. Stefan Institute, Jamova 39, Ljubljana 1000, Slovenia     

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu₂Si₂, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu₂Si₂. In YbRh₂Si₂, superconductivity appears to be suppressed at T???10?mK by AF order (T_N?=?70?mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at T_A slightly above 2?mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at T_c?=?2?mK. Like the pressure – induced QCP in CeRhIn₅, the magnetic field – induced one in YbRh₂Si₂ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher T_cs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.     

High precision54Mn NMRON in antiferromagnetic MnCl2.4H2O,using a new method of nuclear spin cooling in the mK region

A practical method of nuclear spin cooling in the mK region, based on dramatic reduction in the spin-lattice relaxation time T₁ at the antiferromagnetic-spin flop and spin flopparamagnetic phase transitions, is described. Far lower temperatures (down to 13 mK) than have been obtained to date (～ 90 mK) are achieved in reasonable times, greatly increasing the sensitivity of NMRON. Following resonance, active isotope spins can be rapidly reset (in times ～ 1 second) to thermal equilibrium. For⁵⁴Mn in MnCl₂.4H₂O, hyperfine parameters A=?203.75 MHz, P=0.155 MHz and the exchange/ crystal field parameter combination B_ex + 4¦D¦/gμ_B=1.355 T have been determined from good agreement between perturbation theory to second order and detailed NMRON results in applied fields parallel to the easy axis in the range 0-0.7 T. Splitting of observed lines in zero applied field (? 3.05 MHz) is mainly due to the pseudo-quadrupole (second order magnetic) interaction. Small third order effects (～0.05 MHz) have been resolved.     

Electronic states of boron in superconducting MgB2 studied by11B NMR

Nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation rateT ₁⁻¹ of¹¹B have been measured in superconducting polycrystalline MgB₂ with 7_c^ons = 39.5 K. It is shown that (T ₁T⁻¹ and the Knight shiftK _s are independent of temperature and nearly isotropic aboveT _c. Both of these quantities are decreased gradually in going to the superconducting state. According to NMR data the density of states near the Fermi level is flat at the scale of about 500 K. Some conclusions on the orbital content of the density of states at the Fermi level were drawn and compared with the results of the band structure calculations.     

On the possibility of studying crystal field levels and excitations by μ+SR spectroscopy

We point out that ⁺SR relaxation timesT ₁ andT ₂ measured in metallic magnetic materials can sometimes be expressed in terms of the spin-spin correlation functions of the magnetic ions. We calculate these functions in a random phase approximation and notice they can strongly depend on the crystal field levels and excitations of the magnetic ions. The shortcomings of this approximation are discussed.Part of this work was done at the French Atomic Commission at Grenoble.