Ferromagnetic Host Research Articles

Sn hyperfine field trends in the Heusler alloys Co2Mn(Al, Si)

The Sn hyperfine field in the series of Heusler alloys Co2MnAl0.98−xSixSn0.02 (x=0.00, 0.20, 0.40, 0.58, 0.78, 0.98) has been measured at 77 K using119Sn Mossbauer effect. The measurements show an approximately linear decrease in field across the series from a value of +40 kOe in Co2MnAl0.98Sn0.98 to a value of −16 kOe in Co2MnSi0.9xSn0.9x. These results are discussed in terms of previous results for Sn hyperfine fields in Co-based Heusler alloys and theoretical predictions for non-magnetic site hyperfine fields in ferromagnetic hosts.

Calculation of the volume dependence of the electron density at the nucleus for the elements Li through Am. Relationship of this volume dependence to the hyperfine field for the elements dissolved in a ferromagnetic host

Calculation of the volume dependence of the electron density at the nucleus for the elements Li through Am. Relationship of this volume dependence to the hyperfine field for the elements dissolved in a ferromagnetic host

Hyperfine fields at impurities in ferromagnetic metals

A model has been developed which describes the hyperfine field at nonmagnetic impurities in ferromagnetic metallic hosts as arising from conduction-electron polarization. The problem is approached by considering the effect of both the screened electrostatic potential of the impurity and a spin-dependent potential arising from the exchange scattering of the conduction electrons at the localized magnetic moments of the host lattice. The influence of zero-point motion and thermal vibrations is explicitly taken into account. The model is applied to study the spin density at muons in Gd, Dy, Ni, Fe, and Co, and the predictions of the model are compared with experiment. The effect of various physical mechanisms on the temperature, pressure, and site dependence of the hyperfine field is discussed in detail.

Nuclear orientation of on-line separated isotopes

Abstract On-line techniques have proven to be successful for the study of nuclei far from stability in performing spectroscopy. Further information on these short-lived nuclei can be obtained by measuring anisotopy coefficients of β- and γ-transitions. These measurements permit the clarifying of decay schemes and the study of nuclear moments. We thus propose the combination of on-line mass-separation with low temperature nuclear orientation. A system for this is being set up at the LISOL facility. The main part consists of a 3 He- 4 He dilution cryostat with high cooling power. It features a side-access port to allow direct implantation on the cold finger. A liquid helium cooled transport line with adjustable baffles makes the connection to the isotope separator. Nuclei with a lifetime down to a few minutes can be continuously implanted into a ferromagnetic host and oriented by the hyperfine field at temperatures lower than 30 mK. The limit of the lifetime is set by the spin-lattice relaxation time. In order to study shorter lived nuclei an “on-line” immediate orienting mechanism is installed: the beam line includes a UHV-chamber, where polarization is achieved by scattering at grazing incidence on a magnetized nickel single crystal. For preserving the obtained orientation during measurement the low temperature implantation is still required. Both mentioned orienting mechanisms completement each other for a nuclear study of short-lived isotopes. Sets of deflection plates and a movable part of the beam line allow a quick transition from one mode to another. Furthermore the cooled collecting foils can be changed in a short time without breaking the cooling cycle it removal of the daughter activities is requested.

Hyperfine field at an impurity in a ferromagnetic host

A simple model is developed for calculating the hyperfine field at an impurity site by describing the host magnetic ions in terms of a spin-dependent spherical potential around the impurity. The model is used to estimate the effects of zero-point motion and lattice relaxation around the impurity on the hyperfine field. Both these effects are found to be important. The temperature dependence of the hyperfine field is investigated by including the non-linear dependence of the hyperfine field on the average bulk magnetisation, lattice vibrations and thermal expansion. Application to the case of muon spin rotation in ferromagnetic Gd and Dy gives results in qualitative agreement with experiments.

Scaling of the transient magnetic field with the polarization density of the host

The transient magnetic field strength has been determined accurately for 28Si(2~) at an initial recoil velocity of vi = 1.7 vo (vo = c/137) in magnetized Fe, Co, Ni and Gd. The first-excited state of 28Si was produced with the 28Si(~,~') reaction at E~ = 7.50 MeV. Time-integral rotations of the ~-u angular correlation were measured, which led to transient-field strengths for 2esi in the four ferromagnetic hosts. These strengths were found to scale with the polarization density of the host rather than with the number of polarized electrons per host atom.

Theory of impurity hyperfine field systematics in ferromagnetic hosts

Theory of impurity hyperfine field systematics in ferromagnetic hosts

Self-consistent calculation of hyperfine fields at impurity sites in ferromagnetic host

The spin-density functional formalism is used to study the hyperfine-field systematics of interstitial and substitutional impurities in a ferromagnetic host. Both the core- and conduction-electron contributions to the spin density at the impurity site are treated fully self-consistently. The unperturbed conduction electrons of the host metal are approximated by plane waves, the initial polarization of which is generated by smoothing the host-ion magnetic moments into a homogeneous magnetic field outside the impurity cell. A good qualitative agreement with experimental results is obtained for the hyperfine-field systematics of both light impurities (H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}Ne) and heavy impurities (Cd\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}Cs) in Ni. The inability of the mere jellium model to calculate the hyperfine field of ${\ensuremath{\mu}}^{+}$ in magnetic materials is demonstrated. It is shown that the hyperfine field of ${\ensuremath{\mu}}^{+}$ does not depend entirely on the local polarization of the host conduction electrons at the interstitial regions. The effect of the muon zero-point motion on the hyperfine field is discussed.

Temperature dependence of hyperfine fields at nonmagnetic impurities in ferromagnetic metals

A model for the spin densities at dilute non-magnetic impurities in ferromagnetic host metals is presented which predicts generally a non-linear dependence of the hyperfine field on the host magnetization. The influence of lattice vibrations on the transferred hyperfine field is considered and the results of specific calulations are presented.

A localized magnon mode in the extended Wolff-Clogston model

The Wolff-Clogston model for a dilute alloy with interactions between electrons in the host metal is considered. A localized magnon mode is suggested to appear above the magnon band for a fcc structure, provided the impurity is “less magnetic” (δ U<0) than a strongly ferromagnetic host.

Hyperfine fields and magnetic interactions in Heusler alloys

The authors have studied transition and noble metal site NMR spectra of stoichiometric, off-stoichiometric and substituted Heusler alloys. The hyperfine fields can be analysed consistently in terms of local and transferred contributions. The transferred fields are negative at near neighbour and second neighbour to a Mn site, and positive at third neighbour. In many cases excess Mn atoms substituted onto Z sites form antiferromagnetic good local moments in these ferromagnetic hosts with strongly anomalous hyperfine field temperature dependences. Both the transferred fields and the Mn-Mn coupling follow the predictions of a Friedel-Anderson d resonance model correctly treated in the preasymptotic region. The local hyperfine fields at Mn sites are very similar to fields in dilute Mn alloys, while the local fields at Co sites are positive indicating unquenched local orbital moments on the Co atoms.

Hyperfine field systematics of nonmagnetic ions in ferromagnetic alloys

Hyperfine fields at interstitial and substitutional nonmagnetic ion sites in ferromagnetically ordered binary and tenary alloys have been studied within the framework of an extension of the phenomenological model due to Daniel and Friedel. It is concluded that the effects associated with the local Hartree potential of the impurity and the spin-split conduction band of the host are largely responsible for the observed systematic change of sign of the hyperfine field as a function of the impurity valence. The strong correlation noted by Balabanov and Delyagin between hyperfine fields and the atomic number of the impurity, in a fixed column of the periodic table, is shown to be largely due to the orthogonalization of the conduction electron wave functions to the occupied impurity core orbitals. Theoretical results for hyperfine fields at a large number of impurity sites in several ferromagnetic hosts are compared with the most recent experimental data and a number of predictions are given. The present charge screening model is critically compared with other theoretical model calculations and improvements that should be made on the present calculations are outlined.

Nuclear spin lattice relaxation of165Ho in magnetically ordered rare earth metals

The longitudinal nuclear relaxation of 165Ho as a dilute substituent in the ferromagnetic hosts Gd, Tb and Dy has been studied by the spin echo method at liquid helium temperatures. The data have been interpreted on the basis of the Weger mechanism for the passage of nuclear magnetic energy to the conduction electrons via spin waves. The relaxation rates are found to be proportional to temperature; the nuclear magnetisation recovery follows a superposition of seven exponential functions with time constants ranging from about 1 to 30 mu s at 4.2K. The relaxation rates of Ho in Gd are found to differ by a factor of two when the direction of the Gd host magnetisation is changed by doping with Tb and Dy. The results are compared with earlier measurements on the pure metals Tb and Dy.

Resistivity and magnetoresistance study of various s,p impurities in ferromagnetic hosts

We have measured the electrical resistivity between 4.2 K and 300 K and longitudinal and transverse magnetoresistance at 4.2 K of several Ni, Co and Fe based alloys containing various s,p impurities such as Sn and Sb. The deviations from Matthiessen’s Rule (DMR) derived from the resistivity data have been fitted to the two current models of Campbell et al. and values for α (the ratio of spin up to spin down resistivities at the impurity site), μ (the same ratio at host sites) and D (the coefficient of spin mixing scattering events) estimated. These data yield values of α close to one, as expected from a simple non-magnetic s,p screening state around such impurities. The magnetoresistance data also yield values of α close to 1, but there are consistent differences between these data and the two current model predictions with α=1, especially for the Ni and Co based systems.

Studies of hyperfine fields of some 4d and 5d impurities in ferromagnets

Hyperfine fields for some impurity elements of the 4d and 5d series have been studied in ferromagnetic iron, cobalt, nickel and gadolinium host lattices. The temperature dependences of the reduced hyperfine fields Bhf(T)/Bhf(0) are compared to that of the reduced magnetisation M(T)/M(0). Large deviations between the two curves are found for T>or=0.5 Tc for one particular impurity element in each transition series, whereas the others show only minor irregularities. The phenomena are discussed in terms of the concept of a critical screening charge in each series. Values of hyperfine fields at low temperature for a number of impurity-host combinations are also reported: FePd, -547+or-38 kG; CoPd, -395+or-27 kG; GdPd, +64+or-6 kG; PdNi, -170+or-13 kG; MoNi, -39.5+or-1.7 kG; FeOs, -1095+or-95 kG; IrNi, -479+or-33 kG.

Determination of HF fields of radon in ferromagnets

The α-γ angular correlation method has been applied to226Ra and224Ra decays to investigate the hyperfine fields of radon in iron, cobalt and nickel. A random, static quadrupole interaction combined with an aligned magnetic interaction was assumed in the analysis. Gadolinium has also been investigated as a ferromagnetic host, but the oxide layer proved to be too thick.

Hyperfine fields at nonmagnetic elements in ferromagnetic metal hosts

It is shown that there is strong experimental evidence that the hyperfine fields at nonmagnetic solute atoms in ferromagnetic metal hosts depend on the atomic volume of the impurities. Charge-perturbation models are discussed and shown to be in disagreement with the hyperfine-field data.

Hyperfine fields on nonmagnetic elements in ferromagnetic metal hosts

Hyperfine fields on nonmagnetic elements in ferromagnetic metal hosts

Gadolinium as a ferromagnetic host for IMPAC experiments

The properties of ferromagnetic Gd as a host for IMPAC measurements have been investigated. The transient and internal magnetic fields at Cd, Nd, Sm, Dy, Er, Yb and Hf nuclei recoil implanted into polarized Gd at 80 K have been studied by the IMPAC technique. All available experimental transient field data for Gd have been analysed in the framework of the Lindhard-Winther theory. Empirical values of the parametersv p andC ion C atom have been deduced which give good agreement between experiments and theory. Internal magnetic fields at rare-earth nuclei in magnetized Gd at 80 K have been deduced. The results areH h.f. (NdGd)=−1370±440 kG,H h.f.(SmGd)=−1440±120 kG,H h.f.(DyGd)=1410±400 kG,H h.f.(ErGd)=2310±420 kG andH h.f.(YbGd)=−216±32 kG. The signs of these fields are, except for Yb which is in a 2+ ionic state, consistent with a ferromagnetic coupling between the 4f spins of the implanted ion and the Gd host. The deduced internal field at Hf in Gd is −440±90 kG. The observed time-dependent interactions for rare-earth nuclei in ferromagnetic Gd are consistent with the Abragam-Pound theory. For the Cd isotopes,g-factors of the first 2+ states were deduced from the experiments. The results areg(110Cd)=0.49±0.11,g(114Cd)=0.34±0.09 andg(116Cd)=0.41±0.11. The use of transient magnetic fields forg-factor measurements on high-spin rotational states is discussed.

Magnetic hyperfine fields at Ta in Co and Fe hosts measured by the e?-? time-differential perturbedangular-correlation method

The radioactivity181Hf was implanted into pure Co and Fe hosts with the help of an isotope separator. The ferromagnetic hosts produce very strong magnetic hyperfine interactions at the nuclear site of Ta atoms. These hyperfine interactions were studied by time-differential measurements of the 133 keV conversion electron — 482 keV γ angular correlation. It is found that in both hosts an appreciable fraction of Hf atoms occupies regular lattice sites after the implantation. Using the knowng-factor of the 482 keV state of181Ta the magnetic hyperfine fieldsHhf=±362.4(5.0) and ±596(18) kG for the Co and Fe hosts, respectively, were deduced These fields fit nicely into the systematics of the hyperfine fields for the 5d transition elements but are not well accounted by the existing theoretical models.