Internal Hyperfine Field Research Articles

Sublattice spin reversal and field induced Fe3+ spin-canting across the magnetic compensation temperature in Y1.5Gd1.5Fe5O12 rare-earth iron garnet

In the present work Fe3+ sublattice spin reversal and Fe3+ spin-canting across the magnetic compensation temperature (T) are demonstrated in polycrystalline Y1.5Gd1.5Fe5O12 (YGdIG) by means of in-field Mössbauer spectroscopy measurements. Corroborating in-field Mössbauer measurements, both Fe3+ & Gd3+ sublattice spin reversal has also been manifested in hard x-ray magnetic circular dichroism (XMCD) measurements. From in-field 57 Fe Mössbauer measurements, estimation and analysis of effective internal hyperfine field (H), relative intensity of absorption lines in a sextet elucidated unambiguously the signatures of Fe3+ spin reversal and field induced spin-canting of Fe3+ sublattices across T. Gd L3-edge XMCD signal is observed to consist of an additional spectral feature, identified as Fe3+ magnetic contribution to XMCD spectra of Gd L3-edge, enabling us the extraction of both the sublattices (Fe3+ & Gd3+) information from a single absorption edge analysis. The evolution of the XMCD amplitudes, which is proportional to magnetic moments, as a function of temperature for both magnetic sublattices extracted at the Gd L3-edge reasonably match with values that are extracted from bulk magnetization data of YGdIG and YIG (Y3Fe5O12) and corresponding Fe K-edge XMCD amplitudes for Fe contribution. These measurements pave new avenues to investigate how the magnetic behavior of such complex system acts across the compensation point.

On the anisotropic weak magnetic field effect in radical-pair reactions.

For more than 60 years, scientists have been fascinated by the fact that magnetic fields even weaker than internal hyperfine fields can markedly affect spin-selective radical-pair reactions. This weak magnetic field effect has been found to arise from the removal of degeneracies in the zero-field spin Hamiltonian. Here, I investigated the anisotropic effect of a weak magnetic field on a model radical pair with an axially symmetric hyperfine interaction. I found that S-T± and T0-T± interconversions driven by the smaller x and y-components of the hyperfine interaction can be hindered or enhanced by a weak external magnetic field, depending on its direction. Additional isotropically hyperfine-coupled nuclear spins preserve this conclusion, although the S → T± and T0 → T± transitions become asymmetric. These results are supported by simulating reaction yields of a more biologically plausible, flavin-based radical pair.

Low-Coordinate Iron(II) Amido Half-Sandwich Complexes with Large Internal Magnetic Hyperfine Fields.

The half-sandwich complex [Cp'Fe{N(dipp)(SiMe3)}] (Fe-dipp; Cp' = 1,2,4-tri-tert-butylcyclopentadienyl and dipp = 2,6-diisopropylphenyl) and the mixed metallocene [Cp'Fe{(η5-C6H3iPr2)═N(SiMe3)}] (Fe-chd) formed in the reaction between [{Cp'Fe(μ-I)}2] and [Li{N(dipp)(SiMe3)}]2 were characterized by NMR spectroscopy and X-ray diffraction analysis. Fe-dipp complements the series of low-coordinate, quasi-linear iron amido half-sandwich complexes [Cp'Fe{N(tBu)(SiMe3)}] (Fe-tBu) and [Cp'Fe{N(SiMe3)2}] (Fe-tms) reported earlier, and all three compounds were characterized in the solid state by zero-field 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements, confirming their S = 2 electronic ground state. Moreover, the Mössbauer absorption spectra reveal slow paramagnetic relaxation at low temperatures with large internal magnetic hyperfine fields of Bhf = 96.4 T (Fe-dipp, 20 K), Bhf = 101.3 T (Fe-tBu, 15 K), and Bhf = 96.9 T (Fe-tms, 20 K). The magnetic measurements further confirm that the presence of significant axial zero-field splitting and slow relaxation of magnetization is detected, which is revealed even in the absence of a static magnetic field in the case of Fe-tBu. Supplementary ab initio and density functional theory calculations were performed and support the experimental data.

P31 NMR investigation of quasi-two-dimensional magnetic correlations in T2P2S6 ( T =Mn, Ni)

We report the anomalous breakdown in the scaling of the microscopic magnetic susceptibility---as measured via the $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) shift $K$---with the bulk magnetic susceptibility $\ensuremath{\chi}$ in the paramagnetic state of ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$. This anomaly occurs near ${T}_{\mathrm{max}}\ensuremath{\sim}117$ K the maximum in $\ensuremath{\chi}(T)$ and is therefore associated with the onset of quasi-two-dimensional (quasi-2D) magnetic correlations. The spin-lattice relaxation rate divided by temperature ${({T}_{1}T)}^{\ensuremath{-}1}$ in ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ exhibits broad peaklike behavior as a function of temperature, qualitatively following $\ensuremath{\chi}$, but displaying no evidence of critical slowing down above the N\'eel temperature ${T}_{N}$. In the magnetic state of ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, NMR spectra provide good evidence for ${60}^{\ensuremath{\circ}}$ rotation of stacking-fault-induced magnetic domains, as well as observation of the spin-flop transition that onsets at 4 T. The temperature-dependent critical behavior of the internal hyperfine field at the P site in ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ is consistent with previous measurements and the two-dimensional anisotropic Heisenberg model. In a sample of ${\mathrm{Ni}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, we observe only two magnetically split resonances in the magnetic state, demonstrating that the multiple-peaked NMR spectra previously associated with ${60}^{\ensuremath{\circ}}$ rotation of stacking faults is sample-dependent. Finally, we report the observation of a spin-flop-induced splitting of the NMR spectra in ${\mathrm{Ni}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, with an onset spin-flop field of ${\ensuremath{\mu}}_{0}{H}_{\mathrm{sf}}=14$ T.

Magnetic Friedel Oscillation at the Fe(001) Surface: Direct Observation by Atomic-Layer-Resolved Synchrotron Radiation ^{57}Fe Mössbauer Spectroscopy.

The surface magnetism of Fe(001) was studied in an atomic layer-by-layer fashion by using the insitu iron-57 probe layer method with a synchrotron Mössbauer source. The observed internal hyperfine field H_{int} exhibits a marked decrease at the surface and an oscillatory behavior with increasing depth in the individual upper four layers below the surface. The calculated layer-depth dependencies of the effective hyperfine field |H_{eff}|, isomer shift δ, and quadrupole shift 2ϵ agree well with the observed experimental parameters. These results provide the first experimental evidence for the magnetic Friedel oscillations, which penetrate several layers from the Fe(001) surface.

Spin correlations in the S = 1 armchair chain Ni2NbBO6 as seen from NMR* *Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0401802), the National Natural Science Foundation of China (Grant Nos. 11874057, 11504377, 11574288, 11874158, U1732273, and 21927814), and the Users with Excellence Program of Hefei Science Center CAS (Grant No. 2019HSC-UE008). A

We report our nuclear magnetic resonance (NMR) study on the structurally spin chain compound Ni2NbBO6 with complex magnetic coupling. The antiferromagnetic transition is monitored by the line splitting resulting from the staggered internal hyperfine field. The magnetic coupling configuration proposed by the first-principle density functional theory (DFT) is supported by NMR spectral analysis. For the spin dynamics, a prominent peak at T ∼ 35 K well above the N éel temperature (T N ∼ 20 K at μ 0 H = 10 T) is observed from the spin-lattice relaxation data. As compared with the dc-susceptibility, this behavior indicates an antiferromagnetic coupling with the typical energy scale of ∼3 meV. Thus, the Ni2NbBO6 compound can be viewed as strongly ferromagnetically coupled armchair spin chains along the crystalline b-axis. These facts place strong constraints on the theoretical model for this compound.

Study of spin-phonon coupling and magnetic field induced spin reorientation in polycrystalline multiferroic [formula omitted

The present work reports the preparation of polycrystalline multiferroic GdFeO3 (GdFO) and characterization with x-ray diffraction (XRD), magnetization, temperature dependent Raman spectroscopy, temperature and magnetic field dependent F57e Mössbauer spectroscopy measurements. The sample is found to be phase pure from Rietveld refinement of XRD pattern. The Mössbauer spectra measured in the presence of external magnetic field show the signatures of field induced spin reorientation transition, which are corroborated by magnetization measurements. From the temperature dependent variation of internal hyperfine field, Neel transition temperature (TN,Fe) of 672.5 ± 0.2 K and critical exponent (β) of 0.333 ± 0.003 is obtained. Temperature dependent (300–760 K) Raman spectroscopy measurements show the signatures of spin-phonon coupling and local structural re-arrangement across TN,Fe.

Magnetic-field-induced changes in superparamagnetic cluster dynamics in the martensitic phase of Ni43Co7Mn40Sn10

The off-stoichiometric Heusler alloys, such as Ni50Mn25+yX25−y (X = Sn, In, Ga, etc.), have been extensively investigated using a variety of experimental techniques to probe their interesting and potentially useful magnetic properties. Recent 55Mn nuclear magnetic resonance (NMR) experiments, carried out largely in zero field (ZF) and making use of the large internal hyperfine field at the nuclear sites, have demonstrated the power of this approach in determining the ground state magnetic characteristics of these materials. In particular, the results reveal that distinct nanoscale ferromagnetic and antiferromagnetic phases coexist. A key parameter used in interpreting the NMR data is the transverse relaxation time T2 which, inter alia, determines the NMR blocking temperature TBNMR of magnetic regions. The present experiments on a polycrystalline sample of a specific illustrative alloy, Ni43Co7Mn40Sn10, which has received considerable attention, show that the application of relatively small external fields, comparable to or greater than the local anisotropy field in the ferromagnetic cluster regions, produces dramatic changes in T2 and hence TBNMR. The experimental findings are discussed using an extended version of a recently proposed nanocluster model for superparamagnetic systems. It is demonstrated that the field and temperature induced changes in T2 provide a significant test of the model and lead to a notable advance in applying the NMR technique to the investigation of the magnetic properties of this type of alloy.

Nuclear-Magnetic-Resonance Properties of the Staircase Kagomé Antiferromagnet PbCu3TeO7**Supported by the National Natural Science Foundation of China under Grant Nos 11374364 and 11222433, the National Basic Research Program of China under Grant No 2011CBA00112, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry.

We report the first nuclear magnetic resonance (NMR) study on single crystals of staircase Kagomé antiferromagnet PbCu3TeO7 (TN1 ∼36 K). A Curie constant Θ ∼ −140 K is obtained by a Curie–Weiss fit to the high-temperature Knight shift of 125Te. The hyperfine coupling constant is estimated to be 125Ahf = −67 kOe/μB, and a strong interlayer coupling among staircase Kagomé planes is suggested with such a large hyperfine coupling, according to the lattice structure. The 63,65Cu NMR spectra are found by the zero-field (ZF) NMR at T = 2 K, and the internal hyperfine fields are estimated to be 10.3 T and 9.6 T, for Cu(1) and Cu(2) sites, respectively, in the lattice. A second type of ZF NMR signal with a large rf enhancement is also seen after field-cycling through a high magnetic field.

Structural, electrical, magnetic and 57Fe Mössbauer study of polycrystalline multiferroic DyFeO3

Structural, Raman spectroscopy, leakage current density, temperature dependent magnetization and Mössbauer measurements of polycrystalline DyFeO3 (DFO) prepared through sol–gel route are reported in this paper. Phase purity and structure of the prepared sample is confirmed from x-ray diffraction and Raman spectroscopy measurements. The room temperature leakage current density (J–E) measurements indicate that Ohmic contribution and space charge limited conduction are the dominating mechanisms at low and high applied electric fields respectively. Signatures of Fe3+ spin reorientation transition (TSR) and the antiferromagnetic ordering of Dy3+ ions are observed from the temperature dependent (10–350K) magnetization data. The M–H data measured at 2K shows the field induced metamagnetic transition. Internal hyperfine field obtained from temperature dependent (5–300K) 57Fe Mössbauer measurements is observed to decrease below the TSR and further found to increase till 5K indicating the contribution of Dy3+ magnetic ordering on the hyperfine field of Fe nucleus.

57Fe Mössbauer studies across the spin density wave transition in BaFe2−xRuxAs2

Mössbauer measurements have been carried out in powdered single crystalline samples of BaFe2−xRuxAs2, for Ru concentration in the x = 0.0–0.5 range. The internal hyperfine field (Bhf) measured at 5 K is found to decrease with an increase in Ru concentration, consistent with the disappearance of a magnetic ground state with Ru substitution. The temperature dependent Mössbauer measurements have been used to study the evolution of magnetic ordering at the Fe sites, in samples with a Ru fraction of x = 0.1 and 0.5. From the analysis of the data, it is surmised that the isomer shift increases with a decrease in temperature, with a characteristic slope change at the structural transition in both samples studied. In both the x = 0.1 and x = 0.5 samples, a low Bhf centred around 0.5 Tesla is seen to occur well above the structural transition temperature, the contribution from which is suppressed with a decrease in temperature. Below the structural transition temperature, a bimodal distribution of Bhf centred at about 3 Tesla and 5 Tesla emerges, the contribution from which increases with a further decrease in temperature. Spin polarized density functional calculations suggest the occurrence of different magnetic moments at the Fe sites in the Ru substituted compounds, and provide a rationale for the experimentally observed multimodel Bhf.

Spin relaxation in antiferromagnetic Fe–Fe dimers slowed down by anisotropic DyIII ions

By using Mössbauer spectroscopy in combination with susceptibility measurements it was possible to identify the supertransferred hyperfine field through the oxygen bridges between DyIII and FeIII in a {Fe4Dy2} coordination cluster. The presence of the dysprosium ions provides enough magnetic anisotropy to “block” the hyperfine field that is experienced by the iron nuclei. This has resulted in magnetic spectra with internal hyperfine fields of the iron nuclei of about 23 T. The set of data permitted us to conclude that the direction of the anisotropy in lanthanide nanosize molecular clusters is associated with the single ion and crystal field contributions and 57Fe Mössbauer spectroscopy may be informative with regard to the the anisotropy not only of the studied isotope, but also of elements interacting with this isotope.

Iron oxide nanoparticles in NaA zeolite cages

Zeolite NaA samples with varying concentration of Fe3+ ions have been prepared by wet chemical method. Based on powder X-ray diffraction, 29Si and 27Al MAS NMR and Fe3+ EPR investigations, the formation of nano-sized ferric oxide particles inside the larger α-cages of zeolite NaA has been established. Both Mössbauer effect and magnetization measurements carried out down to 4.5 K established the superparamagnetic behaviour of these Fe2O3 particles with a blocking temperature of ≈20 K, where the magnetization values showed deviation for the zero field cooled and field cooled samples and the appearance of a very narrow magnetic hysteresis loop below this temperature. For all Fe3+ containing samples the room temperature Mössbauer spectrum is a broad quadrupole doublet with chemical shift, δ ≈ 0.33 mm/s and quadrupole splitting, ΔEq ≈ 0.68 mm/s. Variable temperature 57Fe Mössbauer effect measurements exhibited magnetic features below the blocking temperature and at 4.5 K, the observed spectrum is a broad magnetic sextet characterized by an internal hyperfine field value of ≈504 kOe along with a very weak central superparamagnetic quadrupole doublet.

Antiferromagnetic order in Ca10(Pt3As8)(Fe2As2)5 observed by 75As NMR

75As nuclear magnetic resonance (NMR) measurements carried out on underdoped, non-superconducting Ca10(Pt3As8)(Fe2As2)5 reveal physical properties that are similar but not identical to 122 superconductor parent compounds such as BaFe2As2. Results from the single crystal study indicate a phase transition to an antiferromagnetic (AF) state on cooling through T ∼ 100 K, albeit nonuniformly. Specifically, the NMR lineshape reflects the presence of staggered hyperfine fields on the As sites associated with a striped AF order. The variation of the internal hyperfine field with temperature suggests that the phase transition to the AF state is discontinuous, and therefore likely coincident with the structural transition inferred from transport experiments.

Symmetry of the Hidden Order in URu2Si2from Nuclear Magnetic Resonance Studies

The hidden order (HO) in URu 2 Si 2 has been investigated by both 29 Si and 99 Ru NMR on a single crystal. Tiny, but finite additional broadening of the spectrum in the HO phase in both 29 Si and 99 Ru NMR with external magnetic field applied along the crystallographic [100] or [110] direction, and its persistence as H ext →0 in 29 Si NMR clearly indicate the existence of internal hyperfine field, and the breaking of the time-reversal symmetry in the HO phase can therefore be concluded. We discuss which symmetry of the D 4 h symmetry group of the URu 2 Si 2 crystal structure is compatible with the observed dependence on the crystal-axis direction of the existence or nonexistence of the internal hyperfine field. Then, by imposing a strong constraint that the wave-vector of the HO is also Q 0 =[0,0,1], the wave-vector of the pressure-induced antiferromagnetism, we show that the two-dimensional representation E - remains as the only possible candidate for the HO symmetry. Since dipoles are obviously excluded...

Effects of Substituting Se with Te in the FeSe Compound: Structural, Magnetization and Mössbauer Studies

Polycrystalline samples of FeSe1−xTex (x=0.00, 0.25, 0.50, 0.75 and 1.00) were synthesized by solid-state reaction to study the effects of substituting Se with Te in the system. The magnetization properties of the resulting compounds were investigated and the crystallographic structures of the samples analyzed through X-ray diffraction. Mossbauer spectroscopy was used to determine the ionic state of the Fe ions and the hyperfine fields. The magnetic susceptibility curves of the samples with x=0.25, 0.50 and 0.75 show superconducting behavior. The lattice parameters and the cell volume increase monotonically with increasing Te concentration and the Mossbauer spectra reveal the absence of internal magnetic hyperfine fields.

Two inequivalent sublattices and orbital ordering inMnV2O4studied byV51NMR

We report detailed $^{51}\text{V}$ NMR spectra in a single crystal of ${\text{MnV}}_{2}{\text{O}}_{4}$. The vanadium spectrum reveals two peaks in the orbitally ordered state, which arise from different internal hyperfine fields at two different V sublattices. These internal fields evolve smoothly with externally applied field, and show no change in structure that would suggest a change of the orbital ordering. The result is consistent with the orbital ordering model recently proposed by Sarkar et al. [Phys. Rev. Lett. 102, 216405 (2009)] in which the same orbital that is a mixture of ${t}_{2g}$ orbitals rotates by about $45\ifmmode^\circ\else\textdegree\fi{}$ alternately within and between orbital chains in the $I{4}_{1}/a$ tetragonal space group.

Low-energy spin dynamics in the antiferromagnetic phase of CaFe2As2

We present 75As nuclear magnetic resonance data in the paramagnetic and magnetic states of single-crystal CaFe2As2. The electric field gradient and the internal magnetic field at the As sites change discontinuously below the first-order structural transition at T0=169 K. In the magnetic state, we find a single value of the internal hyperfine field consistent with commensurate antiferromagnetic order of Fe moments pointing in the ab plane. The spin lattice relaxation rate shows Korringa behavior for T≲T0/3, reflecting the metallic nature of the ordered state. Surprisingly, T1-1 exhibits a small peak at 10 K, revealing the presence of slow spin fluctuations that may be associated with domain wall motion.

POLAREX

POLAREX (POLARization of EXotic nuclei) is a new facility for the study of nuclear magnetic moments and decay modes of exotic nuclei using the established On-Line Nuclear Orientation (OLNO) method. A radioactive beam of interest is implanted into a ferromagnetic host foil held at a temperature of order 10mK in a 3 He - 4 He dilution refrigerator. The foil is magnetized by an applied magnetic field and the nuclear spins become polarized through the internal hyperfine field. The angular distribution of decay products from the polarized sample is measured. Accurate values of nuclear moment are obtained by NMR. The new facility will have access to neutron-rich nuclides produced at the ALTO facility (Linear Accelerator at Orsay Tandem) by fission induced by electrons from the linear electron accelerator. Basic concepts and initial tests are outlined.

Ironed straight

A linear two-coordinate iron complex with an extremely large magnetic moment and internal hyperfine field