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NMR evidence for a Peierls transition in the layered square-net compound LaAgSb$_2$
Authors:
Seung-Ho Baek,
Sergey L. Bud'ko,
Paul C. Canfield,
F. Borsa,
Byoung Jin Suh
Abstract:
We measured the central ($1/2\leftrightarrow -1/2$) and first satellite ($\pm3/2\leftrightarrow \pm1/2$) lines of the \la\ NMR spectra as a function of temperature in LaAgSb$_2$, in order to elucidate the origin and nature of the charge-density-wave (CDW) transitions at $T_\text{CDW1}=207$ K and $T_\text{CDW2}=186$ K. In the normal state, the Knight shift K reveals a fairly linear relationship wit…
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We measured the central ($1/2\leftrightarrow -1/2$) and first satellite ($\pm3/2\leftrightarrow \pm1/2$) lines of the \la\ NMR spectra as a function of temperature in LaAgSb$_2$, in order to elucidate the origin and nature of the charge-density-wave (CDW) transitions at $T_\text{CDW1}=207$ K and $T_\text{CDW2}=186$ K. In the normal state, the Knight shift K reveals a fairly linear relationship with decreasing temperature, which is ascribed to a pseudogap in the spin excitation spectrum, pointing towards the material being an unconventional metal. Upon further cooling, K decreases more steeply below $T_\text{CDW1}$, indicative of the partial Fermi surface gap opening on top of the pseudogap. The most remarkable finding in our study is a clear splitting of the satellite lines at $T_\text{CDW1}$ observed for $H\parallel c$, whose temperature dependence behaves as the BCS order parameter in the weak-coupling limit, evidencing that the CDW transition induces the periodic lattice distortion. Our NMR findings therefore demonstrate that the CDW transition in LaAgSb$_2$ is of Peierls type, being driven by the electronic instability in the vicinity of the Fermi level.
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Submitted 30 November, 2022;
originally announced November 2022.
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Dynamic effect of electron-number parity in metal nanoparticles
Authors:
K. Son,
D. Park,
C. Lee,
A. Lascialfari,
S. H. Yoon,
K. Y. Choi,
A. Reyes,
J. Oh,
M. Kim,
F. Borsa,
G. Scheutz,
Y. G. Yoon,
Z. H. Jang
Abstract:
Parity is a ubiquitous notion in science and serves as a fundamental principle for describing a physical system. Nanometer-scale metal objects are predicted to show dramatic differences in physical properties depending on the electron-number parity. However, the identification of the electron-number parity effects in real metal nanoparticles has remained elusive because of the variations in variou…
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Parity is a ubiquitous notion in science and serves as a fundamental principle for describing a physical system. Nanometer-scale metal objects are predicted to show dramatic differences in physical properties depending on the electron-number parity. However, the identification of the electron-number parity effects in real metal nanoparticles has remained elusive because of the variations in various features of nanoparticles. Here we report the nuclear magnetic resonance (NMR) detection of the dynamic effect of the electron-number parity in silver nanoparticles. With theoretical modeling of the NMR relaxation in silver nanoparticles, the measured nuclear spin-lattice relaxation rate is found to be proportional to the electron-number-parity-dependent susceptibility and to the temperature. This observation demonstrates the electron-number-parity-governed spin dynamics in silver nanoparticles.
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Submitted 23 November, 2022;
originally announced November 2022.
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NMR and $μ^{+}$SR detection of unconventional spin dynamics in Er(trensal) and Dy(trensal) molecular magnets
Authors:
E. Lucaccini,
L. Sorace,
F. Adelnia,
S. Sanna,
P. Arosio,
M. Mariani,
S. Carretta,
Z. Salman,
F. Borsa,
A. Lascialfari
Abstract:
Measurements of proton Nuclear Magnetic Resonance (1H NMR) spectra and relaxation and of Muon Spin Relaxation ($μ^{+}$SR) have been performed as a function of temperature and external magnetic field on two isostructural lanthanide complexes, Er(trensal) and Dy(trensal) featuring crystallographically imposed trigonal symmetry. Both the nuclear 1/T1 and muon $λ$ longitudinal relaxation rates, LRR, e…
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Measurements of proton Nuclear Magnetic Resonance (1H NMR) spectra and relaxation and of Muon Spin Relaxation ($μ^{+}$SR) have been performed as a function of temperature and external magnetic field on two isostructural lanthanide complexes, Er(trensal) and Dy(trensal) featuring crystallographically imposed trigonal symmetry. Both the nuclear 1/T1 and muon $λ$ longitudinal relaxation rates, LRR, exhibit a peak for temperatures T lower than 30K, associated to the slowing down of the spin dynamics, and the width of the NMR absorption spectra starts to increase significantly at T ca. 50K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks have a field and temperature dependence different from those previously reported for all Molecular Nanomagnets. They do not follow the Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature and field and thus cannot be explained in terms of a single dominating correlation time $τ$c determined by the spin slowing down at low temperature. Further, for T lower than 50K the spectral width does not follow the temperature behavior of the magnetic susceptibility chi. We suggest, using simple qualitative considerations, that the observed behavior is due to a combination of two different relaxation processes characterized by the correlation times $τ$LT and $τ$HT, dominating for T lower than 30K and T higher than 50K, respectively. Finally, the observed flattening of LRR for T lower than 5K is suggested to have a quantum origin.
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Submitted 25 October, 2019; v1 submitted 20 May, 2019;
originally announced May 2019.
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Magnetic properties and spin dynamics of 3d-4f molecular complexes
Authors:
P. Khuntia,
M. Mariani,
A. V. Mahajan,
A. Lascialfari,
F. Borsa,
T. D. Pasatoiu,
M. Andruh
Abstract:
We present the magnetic properties of three recently synthesized binuclear molecular complexes [NiNd], [NiGd] and [ZnGd] investigated by dc magnetization and proton nuclear magnetic resonance (NMR) measurements. The high-temperature magnetic properties are related to the independent paramagnetic behavior of the two magnetic metal ions within the binuclear entities both in [NiNd] and [NiGd]. On low…
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We present the magnetic properties of three recently synthesized binuclear molecular complexes [NiNd], [NiGd] and [ZnGd] investigated by dc magnetization and proton nuclear magnetic resonance (NMR) measurements. The high-temperature magnetic properties are related to the independent paramagnetic behavior of the two magnetic metal ions within the binuclear entities both in [NiNd] and [NiGd]. On lowering the temperature, the formation of a magnetic dimer, with a low-spin ground state due to antiferromagnetic interaction (J/kB = -25 K) between Ni2+ and Nd3+, is found in the case of [NiNd], while in [NiGd] a ferromagnetic interaction (J/kB = 3.31 K) between the magnetic ions leads to a high-spin (S = 9/2) ground state. The temperature dependence of the proton nuclear spin lattice relaxation rate 1/T1 in [NiNd] is driven by the fluctuation of the hyperfine field at the nuclear site due to relaxation of the magnetization. At high temperature the independent Ni2+ and Nd3+ spins fluctuate fast while at low temperature we observe a slowing down of the fluctuation of the total magnetization of the dimer because of the insurgence of antiferromagnetic spin correlations. The relaxation mechanism in [NiNd] at low temperature is interpreted by a single, temperature dependent, correlation frequency wc\simT^3.5, which reflects the life time broadening of the exchange coupled spins via spin-phonon interaction. The proton NMR signal in [NiGd] could be detected just at room temperature, due to the shortening of relaxation times when T is decreased. The magnetic properties of [ZnGd] are the ones expected from a weakly interacting assembly of isolated moments except for anomalies in the susceptibility and NMR results below 15 K which currently cannot be explained.
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Submitted 29 November, 2011;
originally announced November 2011.
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Magnetic properties and spin dynamics in single molecule paramagnets Cu6Fe and Cu6Co
Authors:
P. Khuntia,
M. Mariani,
M. C. Mozzati,
L. Sorace,
F. Orsini,
A. Lascialfari,
F. Borsa,
M. Andruh,
C. Maxim
Abstract:
The magnetic properties and the spin dynamics of two molecular magnets have been investigated by magnetization and d.c. susceptibility measurements, Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance (NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields, H=0.5 and 1.5 Tesla. The two molecular magnets consist of CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO…
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The magnetic properties and the spin dynamics of two molecular magnets have been investigated by magnetization and d.c. susceptibility measurements, Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance (NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields, H=0.5 and 1.5 Tesla. The two molecular magnets consist of CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO4)38H2O in short Cu6Fe and the analog compound with cobalt, Cu6Co. It is found that in Cu6Fe whose magnetic core is constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion with J = 0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the weak interaction is antiferromagnetic with J = -1.12 K. The NMR spectra show the presence of non equivalent groups of protons with a measurable contact hyperfine interaction consistent with a small admixture of s-wave function with the d-function of the magnetic ion. The NMR relaxation results are explained in terms of a single ion (Cu2+, Fe3+, Co3+) uncorrelated spin dynamics with an almost temperature independent correlation time due to the weak magnetic exchange interaction. We conclude that the two molecular magnets studied here behave as single molecule paramagnets with a very weak intramolecular interaction, almost of the order of the dipolar intermolecular interaction. Thus they represent a new class of molecular magnets which differ from the single molecule magnets investigated up to now, where the intramolecular interaction is much larger than the intermolecular one.
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Submitted 28 September, 2009;
originally announced September 2009.
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Single Crystal 31P NMR Studies of the Frustrated Square Lattice Compound Pb2(VO)(PO4)2
Authors:
R. Nath,
Y. Furukawa,
F. Borsa,
E. E. Kaul,
M. Baenitz,
C. Geibel,
D. C. Johnston
Abstract:
The static and dynamic properties of V^{4+} spins (S = 1/2) in the frustrated square lattice compound Pb2(VO)(PO4)2 were investigated by means of magnetic susceptibility chi and 31P nuclear magnetic resonance (NMR) shift (K) and 31P nuclear spin-lattice relaxation rate 1/T1 measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below TN \simeq 3.65 K. NMR spe…
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The static and dynamic properties of V^{4+} spins (S = 1/2) in the frustrated square lattice compound Pb2(VO)(PO4)2 were investigated by means of magnetic susceptibility chi and 31P nuclear magnetic resonance (NMR) shift (K) and 31P nuclear spin-lattice relaxation rate 1/T1 measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below TN \simeq 3.65 K. NMR spectra above TN show two distinct lines corresponding to two inequivalent P sites present in the crystal structure. The observed asymmetry in hyperfine coupling constant for the in-plane (P1) P site directly points towards a distortion in the square lattice at the microscopic level, consistent with the monoclinic crystal structure. The nearest- and next-nearest-neighbor exchange couplings were estimated to be J1/kB = (-5.4 \pm 0.5) K (ferromagnetic) and J2/kB = (9.3 \pm 0.6) K (antiferromagnetic), respectively. 1/(T1 T chi) is almost T-independent at high temperatures due to random fluctuation of spin moments. Below 20 K, the compound shows an enhancement of 1/(T1 T chi) which arises from a growth of antiferromagnetic spin correlations above TN. Below TN and for the field applied along the c-axis, the NMR spectrum for the P1 site splits into two satellites and the spacing between them increases monotonically with decreasing T which is a direct evidence of a columnar antiferromagnetic ordering with spins lying in the ab-plane. This type of magnetic ordering is consistent with expectation from the J2/J1 \simeq -1.72 ratio. The critical exponent beta = 0.25 \pm 0.02 estimated from the temperature dependence of sublattice magnetization as measured by 31P NMR at 11.13 MHz is close to the value (0.231) predicted for the two-dimensional XY model.
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Submitted 4 December, 2009; v1 submitted 18 September, 2009;
originally announced September 2009.
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Theory of severe slowdown in the relaxation of rings and clusters with antiferromagnetic interactions
Authors:
Ioannis Rousochatzakis,
Andreas Laeuchli,
Ferdinando Borsa,
Marshall Luban
Abstract:
We show that in the severe slowing down temperature regime the relaxation of antiferromagnetic rings and similar magnetic nanoclusters is governed by the quasi-continuum portion of their quadrupolar fluctuation spectrum and not by the lowest excitation lines. This is at the heart of the intriguing near-universal power-law temperature dependence of the electronic correlation frequency $ω_c$ with…
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We show that in the severe slowing down temperature regime the relaxation of antiferromagnetic rings and similar magnetic nanoclusters is governed by the quasi-continuum portion of their quadrupolar fluctuation spectrum and not by the lowest excitation lines. This is at the heart of the intriguing near-universal power-law temperature dependence of the electronic correlation frequency $ω_c$ with an exponent close to 4. The onset of this behavior is defined by an energy scale which is fixed by the lowest spin gap $Δ_0$. This explains why experimental curves of $ω_c$ for different cluster sizes and spins nearly coincide when $T$ is rescaled by $Δ_0$.
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Submitted 31 January, 2009; v1 submitted 3 November, 2008;
originally announced November 2008.
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Magnetization and spin dynamics of a Cr-based magnetic cluster: Cr$_{7}$Ni
Authors:
A. Bianchi,
S. Carretta,
P. Santini,
G. Amoretti,
Y. Furukawa,
K. Kiuchi,
Y. Ajiro,
Y. Narumi,
K. Kindo,
J. Lago,
E. Micotti,
P. Arosio,
A. Lascialfari,
F. Borsa,
G. Timco,
R. E. P. Winpenny
Abstract:
We study the magnetization and the spin dynamics of the Cr$_7$Ni ring-shaped magnetic cluster. Measurements of the magnetization at high pulsed fields and low temperature are compared to calculations and show that the spin Hamiltonian approach provides a good description of Cr$_7$Ni magnetic molecule. In addition, the phonon-induced relaxation dynamics of molecular observables has been investiga…
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We study the magnetization and the spin dynamics of the Cr$_7$Ni ring-shaped magnetic cluster. Measurements of the magnetization at high pulsed fields and low temperature are compared to calculations and show that the spin Hamiltonian approach provides a good description of Cr$_7$Ni magnetic molecule. In addition, the phonon-induced relaxation dynamics of molecular observables has been investigated. By assuming the spin-phonon coupling to take place through the modulation of the local crystal fields, it is possible to evaluate the decay of fluctuations of two generic molecular observables. The nuclear spin-lattice relaxation rate $1/T_1$ directly probes such fluctuations, and allows to determine the magnetoelastic coupling strength.
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Submitted 19 August, 2008;
originally announced August 2008.
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19F nuclear spin relaxation and spin diffusion effects in the single ion magnet LiYF4:Ho3+
Authors:
B. Z. Malkin,
M. V. Vanyunin,
M. J. Graf,
J. Lago,
F. Borsa,
A. Lascialfari,
A. M. Tkachuk,
B. Barbara
Abstract:
Temperature and magnetic field dependences of the 19F nuclear spin-lattice relaxation in a single crystal of LiYF4 doped with holmium are described by an approach based on a detailed consideration of the magnetic dipole-dipole interactions between nuclei and impurity paramagnetic ions and nuclear spin diffusion processes. The observed non-exponential long time recovery of the nuclear magnetizati…
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Temperature and magnetic field dependences of the 19F nuclear spin-lattice relaxation in a single crystal of LiYF4 doped with holmium are described by an approach based on a detailed consideration of the magnetic dipole-dipole interactions between nuclei and impurity paramagnetic ions and nuclear spin diffusion processes. The observed non-exponential long time recovery of the nuclear magnetization after saturation at intermediate temperatures is in agreement with predictions of the spin-diffusion theory in a case of the diffusion limited relaxation. At avoided level crossings in the spectrum of electron-nuclear states of the Ho3+ ion, rates of nuclear spin-lattice relaxation increase due to quasi-resonant energy exchange between nuclei and paramagnetic ions, in contrast to the predominant role played by electronic cross-relaxation processes in the low-frequency ac-susceptibility.
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Submitted 15 August, 2008;
originally announced August 2008.
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^{7}Li NMR Study of Heavy Fermion LiV2O4 Containing Magnetic Defects
Authors:
X. Zong,
S. Das,
F. Borsa,
M. D. Vannette,
R. Prozorov,
J. Schmalian,
D. C. Johnston
Abstract:
We present a systematic study of the variations of the ^{7}Li NMR properties versus magnetic defect concentration up to 0.83 mol% within the spinel structure of polycrystalline powder samples and a collection of small single crystals of LiV2O4 in the temperature range from 0.5 to 4.2 K. We also report static magnetization measurements and ac magnetic susceptibility measurements at 14 MHz on the…
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We present a systematic study of the variations of the ^{7}Li NMR properties versus magnetic defect concentration up to 0.83 mol% within the spinel structure of polycrystalline powder samples and a collection of small single crystals of LiV2O4 in the temperature range from 0.5 to 4.2 K. We also report static magnetization measurements and ac magnetic susceptibility measurements at 14 MHz on the samples at low temperatures. Both the NMR spectrum and nuclear spin-lattice relaxation rate are inhomogeneous in the presence of the magnetic defects. The NMR data for the powders are well explained by assuming that (i) there is a random distribution of magnetic point defects, (ii) the same heavy Fermi liquid is present in the samples containing the magnetic defects as in magnetically pure LiV2O4, and (iii) the influences of the magnetic defects and of the Fermi liquid on the magnetization and NMR properties are separable. In the single crystals, somewhat different behaviors are observed. Remarkably, the magnetic defects in the powder samples show evidence of spin freezing below T ~ 1.0 K, whereas in the single crystals with similar magnetic defect concentration no spin freezing was found down to 0.5 K. Thus different types of magnetic defects and/or interactions between them appear to arise in the powders versus the crystals, possibly due to the substantially different synthesis conditions of the powders and crystals.
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Submitted 9 February, 2008;
originally announced February 2008.
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Probing spin dynamics and quantum relaxation in LiY0.998Ho0.002F4 via 19F NMR
Authors:
M. J. Graf,
A. Lascialfari,
F. Borsa,
A. M. Tkachuk,
B. Barbara
Abstract:
We report measurements of 19F nuclear spin-lattice relaxation 1/T1 as a function of temperature and external magnetic field in LiY0.998Ho0.002F4 single crystal, a single-ion magnet exhibiting interesting quantum effects. The 19F 1/T1 is found to depend on the coupling with the diluted rare-earth (RE) moments. Depending on the temperature range, a fast spin diffusion regime or a diffusion limited…
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We report measurements of 19F nuclear spin-lattice relaxation 1/T1 as a function of temperature and external magnetic field in LiY0.998Ho0.002F4 single crystal, a single-ion magnet exhibiting interesting quantum effects. The 19F 1/T1 is found to depend on the coupling with the diluted rare-earth (RE) moments. Depending on the temperature range, a fast spin diffusion regime or a diffusion limited regime is encountered. In both cases we find it possible to use the 19F nucleus as a probe of the rare-earth spin dynamics. The results for 1/T1 show a behavior similar to that observed in molecular nanomagnets, a result which we attribute to the discreteness of the energy levels in both cases. At intermediate temperatures the lifetime broadening of the crystal field split RE magnetic levels follows a T3 power law. At low temperature the field dependence of 1/T1 shows peaks in correspondence to the critical magnetic fields for energy level crossings (LC). The results can be explained by inelastic scattering between the fluorine nuclear spins and the RE magnetic levels. A key result of this study is that the broadening of the levels at LC is found to be become extremely small at low temperatures, about 1.7 mT, a value which is comparable to the weak dipolar fields at the RE lattice positions. Thus, unlike the molecular magnets, decoherence effects are strongly suppressed, and it may be possible to measure directly the level repulsions at avoided level crossings.
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Submitted 17 August, 2005;
originally announced August 2005.
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Dynamics of Magnetic Defects in Heavy Fermion LiV2O4 from Stretched Exponential 7Li NMR Relaxation
Authors:
D. C. Johnston,
S. -H. Baek,
X. Zong,
F. Borsa,
J. Schmalian,
S. Kondo
Abstract:
7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small concentration of magnetic defects within the structure drastically changes the 7Li nuclear magnetization relaxation versus time from a pure exponential as in pure LiV2O4 to a stretched exponential, indicating glassy behavior of the magnetic defects. The stretched exponential function is described as arising from a distributio…
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7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small concentration of magnetic defects within the structure drastically changes the 7Li nuclear magnetization relaxation versus time from a pure exponential as in pure LiV2O4 to a stretched exponential, indicating glassy behavior of the magnetic defects. The stretched exponential function is described as arising from a distribution of 7Li nuclear spin-lattice relaxation rates and we present a model for the distribution in terms of the dynamics of the magnetic defects. Our results explain the origin of recent puzzling 7Li NMR literature data on LiV2O4 and our model is likely applicable to other glassy systems.
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Submitted 30 March, 2005;
originally announced March 2005.
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Direct measurement of the tunneling rate of the magnetization in Fe8 via 57Fe nuclear spin-lattice relaxation by strong collision
Authors:
S. H. Baek,
F. Borsa,
Y. Furukawa,
Y. Hatanaka,
S. Kawakami,
K. Kumagai
Abstract:
$^{57}$Fe and $^1$H relaxation measurements have been performed in single crystal and oriented powder of enriched $^{57}$Fe8 molecular cluster in the temperature range 0.05--1.7 K in zero external field and with small perturbing longitudinal field ($< 1$ T). On the basis of the experimental results it is argued that in zero external field the nuclear spin-lattice relaxation ($1/T_1$) mechanism i…
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$^{57}$Fe and $^1$H relaxation measurements have been performed in single crystal and oriented powder of enriched $^{57}$Fe8 molecular cluster in the temperature range 0.05--1.7 K in zero external field and with small perturbing longitudinal field ($< 1$ T). On the basis of the experimental results it is argued that in zero external field the nuclear spin-lattice relaxation ($1/T_1$) mechanism is driven by a strong collision mechanism whereby $1/T_1$ is a direct measure of the incoherent tunneling probability in the low lying magnetic energy states of the molecular nanomagnet. The approximate value of the effective tunneling rate vs $T$ and $H$ derived directly from $1/T_1$ is shown to be consistent with theoretical estimates based on known parameters of the Hamiltonian.
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Submitted 28 October, 2004; v1 submitted 24 September, 2004;
originally announced September 2004.
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NMR in magnetic molecular rings and clusters (II)
Authors:
F. Borsa,
A. Lascialfari,
Y. Furukawa
Abstract:
In the last years there has been a great interest in magnetic systems formed by a cluster of transition metal ions covalently bonded via superexchange bridges, embedded in a large organic molecule. Following the synthesis and the structural and magnetic characterization of these magnetic molecules by chemists, the physicists realized the great interest of these systems as a practical realization…
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In the last years there has been a great interest in magnetic systems formed by a cluster of transition metal ions covalently bonded via superexchange bridges, embedded in a large organic molecule. Following the synthesis and the structural and magnetic characterization of these magnetic molecules by chemists, the physicists realized the great interest of these systems as a practical realization of zero-dimensional model magnetic systems. In fact the magnetic molecules can be synthesized in crystalline form whereby each molecule is magnetically independent since the intramolecular exchange interaction among the transition metal ions is dominant over the weak intermolecular, usually dipolar, magnetic interaction. We have undertaken a systematic NMR investigation of molecular nanomagnets since back in 1996. The present review tries to give an account of the main results obtained so far and of the many exciting projects that still lie ahead. The work was done through a continuous very fruitful collaboration among three NMR laboratories: at the University of Pavia, Italy, at Iowa State University and Ames Laboratory, Ames, IA, USA and at Hokkaido University, Sapporo, Japan with occasional very useful collaborations with the high field NMR lab. in Grenoble, France. None of the work could be done without the precious collaboration and help of our colleagues in chemistry at the University of Florence and of Modena, Italy and at Ames Laboratory in USA who synthesized and characterized the samples used in the NMR work.
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Submitted 16 April, 2004;
originally announced April 2004.
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NMR in magnetic molecular rings and clusters (I)
Authors:
F. Borsa,
A. Lascialfari,
Y. Furukawa
Abstract:
In the last years there has been a great interest in magnetic systems formed by a cluster of transition metal ions covalently bonded via superexchange bridges, embedded in a large organic molecule. Following the synthesis and the structural and magnetic characterization of these magnetic molecules by chemists, the physicists realized the great interest of these systems as a practical realization…
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In the last years there has been a great interest in magnetic systems formed by a cluster of transition metal ions covalently bonded via superexchange bridges, embedded in a large organic molecule. Following the synthesis and the structural and magnetic characterization of these magnetic molecules by chemists, the physicists realized the great interest of these systems as a practical realization of zero-dimensional model magnetic systems. In fact the magnetic molecules can be synthesized in crystalline form whereby each molecule is magnetically independent since the intramolecular exchange interaction among the transition metal ions is dominant over the weak intermolecular, usually dipolar, magnetic interaction. We have undertaken a systematic NMR investigation of molecular nanomagnets since back in 1996. The present review tries to give an account of the main results obtained so far and of the many exciting projects that still lie ahead. The work was done through a continuous very fruitful collaboration among three NMR laboratories: at the University of Pavia, Italy, at Iowa State University and Ames Laboratory, Ames, IA, USA and at Hokkaido University, Sapporo, Japan with occasional very useful collaborations with the high field NMR lab. in Grenoble, France. None of the work could be done without the precious collaboration and help of our colleagues in chemistry at the University of Florence and of Modena, Italy and at Ames Laboratory in USA who synthesized and characterized the samples used in the NMR work.
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Submitted 16 April, 2004;
originally announced April 2004.
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Tunneling splitting of magnetic levels in Fe8 detected by 1H NMR cross relaxation
Authors:
Y. Furukawa,
K. Aizawa,
K. Kumagai,
R. Ullu,
A. Lascialfari,
F. Borsa
Abstract:
Measurements of proton NMR and the spin lattice relaxation rate 1/T1 in the octanuclear iron (III) cluster [Fe8(N3C6H15)6O2(OH)12][Br8 9H2O], in short Fe8, have been performed at 1.5 K in a powder sample aligned along the main anisotropy z axis, as a function of a transverse magnetic field (i.e., perpendicular to the main easy axis z). A big enhancement of 1/T1 is observed over a wide range of f…
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Measurements of proton NMR and the spin lattice relaxation rate 1/T1 in the octanuclear iron (III) cluster [Fe8(N3C6H15)6O2(OH)12][Br8 9H2O], in short Fe8, have been performed at 1.5 K in a powder sample aligned along the main anisotropy z axis, as a function of a transverse magnetic field (i.e., perpendicular to the main easy axis z). A big enhancement of 1/T1 is observed over a wide range of fields (2.5-5 T), which can be attributed to the tunneling dynamics; in fact, when the tunneling splitting of the pairwise degenerate m=+-10 states of the Fe8 molecule becomes equal to the proton Larmor frequency a very effective spin lattice relaxation channel for the nuclei is opened. The experimental results are explained satisfactorily by considering the distribution of tunneling splitting resulting from the distribution of the angles in the hard xy plane for the aligned powder, and the results of the direct diagonalization of the model Hamiltonian.
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Submitted 22 January, 2003;
originally announced January 2003.
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NMR in the normal and in the superconducting state of MgB2 and comparison with AlB2
Authors:
S. H. Baek,
B. J. Suh,
E. Pavarini,
F. Borsa,
R. G. Barnes,
S. L. Budko,
P. C. Canfield
Abstract:
$^{11}$B NMR measurements have been performed in $^{11}$B enriched MgB$_2$ powder samples in external fields of 0.813, 1.55, 4.7 and 7.2 T both in the normal phase and in the superconducting phase. A previously unreported dipolar Pake doublet has been observed in the quadrupole perturbed NMR spectrum. The Knight shift can thus be accurately determined by narrowing the line with the Magic Angle S…
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$^{11}$B NMR measurements have been performed in $^{11}$B enriched MgB$_2$ powder samples in external fields of 0.813, 1.55, 4.7 and 7.2 T both in the normal phase and in the superconducting phase. A previously unreported dipolar Pake doublet has been observed in the quadrupole perturbed NMR spectrum. The Knight shift can thus be accurately determined by narrowing the line with the Magic Angle Spinning (MAS) technique. Results of Knight shift ($K$) and relaxation rates ($1/T_1$) for both $^{11}$B and $^{27}$Al nuclei are reported also for AlB$_2$. The comparison of the data in the two compounds shows the dramatic drop of the density of states at the boron site in AlB$_2$ with respect to MgB$_2$. The experimental values for $K$ and $1/T_1$ are in most cases in good agreement with the theoretical values obtained from first principles calculations. The recovery of the nuclear magnetization below $T_c$ in random powder samples is non-exponential due to the anisotropy of the upper critical field. The exponential drop of $1/T_1$ in the superconducting phase observed by Kotegawa et al. is confirmed here but not the coherence peak.
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Submitted 24 January, 2002; v1 submitted 24 January, 2002;
originally announced January 2002.
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$^{11}B$ NMR and Relaxation in $MgB_2$ Superconductor
Authors:
J. K. Jung,
Seung Ho Baek,
F. Borsa,
S. L. Bud'ko,
G. Lapertot,
P. C. Canfield
Abstract:
$^{11}B$ NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at 7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound $MgB_2$ with superconducting transition temperature in zero field $T_c$ = 39.2 K. From the first order quadrupole perturbed NMR specrum a quadrupole coupling frequency of 835 $\pm$ 5 kHz is obtained. The Knight shift is very small and it decreases t…
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$^{11}B$ NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at 7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound $MgB_2$ with superconducting transition temperature in zero field $T_c$ = 39.2 K. From the first order quadrupole perturbed NMR specrum a quadrupole coupling frequency of 835 $\pm$ 5 kHz is obtained. The Knight shift is very small and it decreases to zero in the superconducting phase. The NSLR follows a linear law with $T_1T$ = 165 $\pm$ 10 (sec K) . The results in the normal phase indicate a negligible $s$-character of the wave function of the conduction electrons at the Fermi level. Below $T_c$ the NSLR is strongly field dependent indicating the presence of an important contribution related to the density and the thermal motion of flux lines. No coherence peak is observed at the lower field investigated (1.4 T).
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Submitted 1 March, 2001;
originally announced March 2001.
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Glassy Spin Freezing and NMR Wipeout Effect in the High-T_c Superconductor La1.90Sr0.10CuO4: What is the Relationship With Stripes ?
Authors:
M. -H. Julien,
A. Campana,
A. Rigamonti,
P. Carretta,
F. Borsa,
P. Kuhns,
A. P. Reyes,
W. G. Moulton,
M. Horvatic,
C. Berthier,
A. Vietkin,
A. Revcolevschi
Abstract:
We report on 139La and 63Ccu NMR/NQR measurements in the high-Tc superconductor La1.90Sr0.10CuO4 with Tc=26.5 K. Spin fluctuations probed by 139La spin-lattice relaxation (T1), continuously slow down on cooling through Tc. We argue that spin-freezing and superconductivity are bulk effects in this sample. Thus, both phenomena have to coexist microscopically. The distribution of 139La T1 values at…
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We report on 139La and 63Ccu NMR/NQR measurements in the high-Tc superconductor La1.90Sr0.10CuO4 with Tc=26.5 K. Spin fluctuations probed by 139La spin-lattice relaxation (T1), continuously slow down on cooling through Tc. We argue that spin-freezing and superconductivity are bulk effects in this sample. Thus, both phenomena have to coexist microscopically. The distribution of 139La T1 values at low temperature reveals a wide spread of spin fluctuation frequencies in CuO2 planes. A simple estimate shows that Cu nuclei at sites where electronic fluctuations are the slowest are not observable because of too short relaxation times (wipeout effect). This means that the Cu NQR wipeout, observed in this sample, can be explained primarily by slow magnetic, rather than charge, fluctuations. This result does not rule out the connection between wipeout effect and charge stripe order [Hunt et al., Phys. Rev. Lett. 82, 4300 (1999)], but it indicates that the relationship between both phenomena is not straightforward. We argue that the wipeout fraction cannot define a proper order parameter for a stripe phase, and cannot be used alone as a criterion for its existence.
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Submitted 24 October, 2000;
originally announced October 2000.
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NQR Study of Spin-Freezing in Superconducting La2-xSrxCuO4: The Example of x=0.06
Authors:
M. -H. Julien,
P. Carretta,
F. Borsa
Abstract:
NQR 139La and 63Cu spin-lattice relaxation rate (1/T_1) measurements in a La1.94Sr0.06CuO4 single crystal are described. Slowing-down of Cu2+ spin fluctuations is evidenced through a dramatic increase of 139La 1/T_1 on cooling. While the onset of diamagnetism occurs at Tc=8 K, 1/^{139}T_1 has a peak at Tg~5 K, when the characteristic frequency of magnetic fluctuations reaches the NQR frequency n…
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NQR 139La and 63Cu spin-lattice relaxation rate (1/T_1) measurements in a La1.94Sr0.06CuO4 single crystal are described. Slowing-down of Cu2+ spin fluctuations is evidenced through a dramatic increase of 139La 1/T_1 on cooling. While the onset of diamagnetism occurs at Tc=8 K, 1/^{139}T_1 has a peak at Tg~5 K, when the characteristic frequency of magnetic fluctuations reaches the NQR frequency nu_Q~19 MHz. In agreement with a number of previous studies, these results show that the so-called "cluster spin-glass" phase persists in the superconducting regime. Issues concerning the coexistence of the two phases are discussed.
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Submitted 23 September, 1999;
originally announced September 1999.
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Proton NMR for Measuring Quantum-Level Crossing in the Magnetic Molecular Ring Fe10
Authors:
M. -H. Julien,
Z. H. Jang,
A. Lascialfari,
F. Borsa,
M. Horvatic,
A. Caneschi,
D. Gatteschi
Abstract:
The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a function of temperature and magnetic field (up to 15 T) in the molecular magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic field values corresponding to a crossing between the ground state and the excited states of the molecule. We propose that this is due to a cross-relaxation effect between t…
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The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a function of temperature and magnetic field (up to 15 T) in the molecular magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic field values corresponding to a crossing between the ground state and the excited states of the molecule. We propose that this is due to a cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir of the Zeeman levels of the molecule. This effect provides a powerful tool to investigate quantum dynamical phenomena at level crossing.
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Submitted 24 May, 1999;
originally announced May 1999.
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Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for $x\leq 0.04$
Authors:
P. Carretta,
F. Tedoldi,
A. Rigamonti,
F. Galli,
F. Borsa,
J. H. Cho,
D. C. Johnston
Abstract:
The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation time T_1 measurements. The results are discussed in the framework of the connection between T_1 and the in-plane magnetic correlation length $ξ_{2D}(x,T)$. It is found that at h…
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The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation time T_1 measurements. The results are discussed in the framework of the connection between T_1 and the in-plane magnetic correlation length $ξ_{2D}(x,T)$. It is found that at high temperatures the system remains in the renormalized classical regime, with a spin stiffness constant $ρ_s(x)$ reduced by small doping to an extent larger than the one due to Zn doping. For $x\gtrsim 0.02$ the effect of doping on $ρ_s(x)$ appears to level off. The values for $ρ_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger than the ones estimated from the temperature behavior of sublattice magnetization in the ordered phase ($T\leq T_N$). It is argued that these features are consistent with the hypothesis of formation of stripes of microsegregated holes.
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Submitted 31 March, 1999;
originally announced March 1999.
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Charge Segregation, Cluster Spin-Glass and Superconductivity in La1.94Sr0.06CuO4
Authors:
M. -H. Julien,
F. Borsa,
P. Carretta,
M. Horvatic,
C. Berthier,
C. T. Lin
Abstract:
A 63Cu and 139La NMR/NQR study of superconducting (Tc=7 K) La1.94Sr0.06CuO4 single crystal is reported. Coexistence of spin-glass and superconducting phases is found below ~5 K from 139La NMR relaxation. 63Cu and 139La NMR spectra show that, upon cooling, CuO2 planes progressively separate into two magnetic phases, one of them having enhanced antiferromagnetic correlations. These results establi…
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A 63Cu and 139La NMR/NQR study of superconducting (Tc=7 K) La1.94Sr0.06CuO4 single crystal is reported. Coexistence of spin-glass and superconducting phases is found below ~5 K from 139La NMR relaxation. 63Cu and 139La NMR spectra show that, upon cooling, CuO2 planes progressively separate into two magnetic phases, one of them having enhanced antiferromagnetic correlations. These results establish the AF-cluster nature of the spin-glass. We discuss how this phase can be related to the microsegregation of mobile holes and to the possible pinning of charge-stripes.
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Submitted 15 April, 1999; v1 submitted 27 February, 1999;
originally announced March 1999.