Local-moment Ferromagnetism Research Articles

Effect of Controlled Artificial Disorder on the Magnetic Properties of EuFe2(As1-xPx)2 Ferromagnetic Superconductor.

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, , shows an extraordinarily large decrease. It starts at in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered lattice act as efficient pairbreakers leading to a significant reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of (less than 1 K) after the irradiation drives to below . Our results suggest that FM and SC phases coexist microscopically in the same volume.

Ab-initio calculation of effective exchange interactions, spin waves, and Curie temperature in L21- and L12-type local moment ferromagnets

Employing first-principles electronic structure calculations in conjunction with the frozen-magnon method, we study the effective exchange interactions and spin waves in local moment ferromagnets. As prototypes we have chosen three L21-type full Heusler alloys Cu2MnAl, Ni2MnSn, and Pd2MnSn, and the L12-type XPt3 compounds with X = V, Cr, and Mn. We have also included CoPt3 which is a usual ferromagnet. In all compounds due to the large spatial separation (∼4 Å) of the magnetic transition-metal atoms, the 3d states belonging to different atoms overlap weakly and as a consequence the exchange coupling is indirect, mediated by the sp electrons. Calculated effective exchange parameters are long range and show RKKY-type oscillations. The spin-wave dispersion curves are in reasonable agreement with available experimental data. Using the calculated exchange parameters, we have estimated the Curie temperatures within both the mean-field and the random-phase approximations. In local moment ferromagents deviations of the estimated Curie temperature with respect to the available experimental data occur when the ground-state electronic structure calculations overestimate the values of the spin magnetic moments as in VPt3.

Exchange interactions, spin waves, and Curie temperature inzincblende half-metallic sp-electron ferromagnets: the case of CaZ (Z = N, P, As, Sb)

Using first-principle calculations in conjunction with the frozen-magnontechnique we have calculated the exchange interactions and spin-wavedispersions in the series of the zincblende half-metallic II–V (CaZ,Z = N, P, As, Sb) ferromagnets. The calculated exchange constants are used to estimate theCurie temperature within the random phase approximation. The large Stoner gap in thesealloys gives rise to well-defined undamped spin waves throughout the Brillouin zone.Moreover we show that the spin-wave stiffness constants for the considered systems areamong the largest available for local moment ferromagnets. The predicted Curietemperature of half-metallic CaN is noticeably higher than the room temperature withrespect to the other compounds, and thus we propose CaN as a promising candidate forfuture applications in spintronic devices.

Competition of local-moment ferromagnetism and superconductivity in Co-substituted EuFe2As2

In contrast to SrFe2As2, where only the iron possesses a magnetic moment, in EuFe2As2 an additional large, local magnetic moment is carried by Eu2+. Like SrFe2As2, EuFe2As2 exhibits a spin-density wave transition at high temperatures, but in addition, the magnetic moments of the Eu2+ order at around 20 K. The interplay of pressure-induced superconductivity and the Eu2+ order leads to a behavior which is reminiscent of re-entrant superconductivity as it was observed, for example, in the ternary Chevrel phases or in the rare-earth nickel borocarbides. Here, we study the delicate interplay of the ordering of the Eu2+ moments and superconductivity in EuFe1.9Co0.1As2, where application of external pressure makes it possible to sensitively tune the ratio of the magnetic (TC) and the superconducting (Tc,onset) critical temperatures. We find that superconductivity disappears once TC > Tc,onset.

Unusual signatures of the ferromagnetic transition in the heavy fermion compoundUMn2Al20

Magnetic susceptibility results for single crystals of the new cubic compounds UT$_2$Al$_{20}$ (T=Mn, V, and Mo) are reported. Magnetization, specific heat, resistivity, and neutron diffraction results for a single crystal and neutron diffraction and inelastic spectra for a powder sample are reported for UMn$_2$Al$_{20}$. For T = V and Mo, temperature independent Pauli paramagnetism is observed. For UMn$_2$Al$_{20}$, a ferromagnetic transition is observed in the magnetic susceptibility at $T_c$ = 20 K. The specific heat anomaly at $T_c$ is very weak while no anomaly in the resistivity is seen at $T_c$. We discuss two possible origins for this behavior of UMn$_2$Al$_{20}$: moderately small moment itinerant ferromagnetism, or induced local moment ferromagnetism.

Local-moment ferromagnetism and unusual magnetic domains inFe1/4TaS2crystals

Single crystals of ${\text{Fe}}_{1/4}{\text{TaS}}_{2}$ have been studied by using magneto-optical (MO) imaging and radio-frequency magnetic susceptibility, $\ensuremath{\chi}$. Real-time MO images reveal unusual slow dynamics of dendritic domain formation, the details of which are strongly dependent upon magnetic and thermal history. Measurements of $\ensuremath{\chi}(T)$ show well-defined local-moment ferromagnetic transition at $T\ensuremath{\approx}155\text{ }\text{K}$ as well as thermal hysteresis for $50lTl60\text{ }\text{K}$. This temperature range corresponds to the domain-formation temperature as determined by MO. Together these observations provide strong evidence for local-moment ferromagnetism in ${\text{Fe}}_{1/4}{\text{TaS}}_{2}$ crystals with large temperature-dependent magnetic anisotropy.

Effect of chemical disorder on NiMnSb investigated by appearance potential spectroscopy: an experimental and theoretical study

The half-Heusler alloy NiMnSb is one of the local-moment ferromagnets with unique properties for future applications. Band structure calculations exclusively predict majority bands at the Fermi level, thus indicating 100% spin polarization there. For the development of functional materials, chemical disorder has to be considered quantitatively. The magnetization, spin polarization and electronic structure are expected to be sensitive to structural and stoichiometric changes. We studied the influence of chemical disorder on the unoccupied electronic density of states of NiMnSb(0 0 1) by the use of the ab initio coherent potential approximation method and results are discussed along with corresponding measurements.

Coexistence of long-range magnetic order and superconductivity from Campbellpenetration depth measurements

Application of a tunnel-diode resonator (TDR) technique for studies of the vortex responsein magnetic superconductors is described. Operating at very small excitation fields and asufficiently high frequency, the TDR was used to probe the small-amplitude linearAC response in several types of single crystals where long-range magnetic ordercoexists with bulk superconductivity. Full local-moment ferromagnetism destroyssuperconductivity and can coexist with it only in a narrow temperature range(∼0.3 K). In contrast, weak ferromagnetic as well as antiferromagnetic orders can coexistwith bulk superconductivity and may even lead to an enhancement of vortexpinning. By analyzing the Campbell penetration depth we find a sharp increase ofthe true critical current in the vicinity of the magnetic phase transitions. Weconclude that critical magnetic fluctuations are responsible for this enhancement.

Distinguishing local moment versus itinerant ferromagnets: Dynamic magnetic susceptibility

Radio-frequency measurements of dynamic magnetic susceptibility of various ferromagnets show striking differences between local-moment ferromagnetism (LFM) and weak itinerant ferromagnetism (IFM) ferromagnetic systems. LFMs show a very sharp peak in susceptibility in the vicinity of the Curie temperature TC that rapidly decreases in amplitude and shifts to higher temperature with the application of a weak dc bias field. In stark contrast, the generally accepted IFM systems show no peak, but rather a broad maximum well below TC. The temperature of this maximum shifts to lower values and the amplitude is suppressed with an applied dc field.

Precise measurements of radio-frequency magnetic susceptibility in ferromagnetic and antiferromagnetic materials

Dynamic magnetic susceptibility, χ , was studied in several intermetallic materials exhibiting ferromagnetic, antiferromagnetic and metamagnetic transitions. Precise measurements by using a 14 MHz tunnel diode oscillator (TDO) allow detailed insight into the field and temperature dependence of χ . In particular, local moment ferromagnets show a sharp peak in χ ( T ) near the Curie temperature, T C . The peak amplitude decreases and shifts to higher temperatures with very small applied dc fields. Anisotropic measurements of CeVSb 3 show that this peak is present provided the magnetic easy axis is aligned with the excitation field. In a striking contrast, small moment, itinerant ferromagnets (i.e., ZrZn 2 ) show a broad maximum in χ ( T ) that responds differently to applied field. We believe that TDO measurements provide a very sensitive way to distinguish between local and itinerant moment magnetic orders. Local moment antiferromagnets do not show a peak at the Néel temperature, T N , but only a sharp decrease of χ below T N due to the loss of spin-disorder scattering changing the penetration depth of the ac excitation field. Furthermore, we show that the TDO is capable of detecting changes in spin order as well as metamagnetic transitions. Finally, critical scaling of χ ( T , H ) in the vicinity of T C is discussed in CeVSb 3 and CeAgSb 2 .

Crystal structure, magnetism and bonding of the hexagonal compounds Pd1.63Mn0.37Si and Pd1.82Mn0.18Ge related to the Fe2P structure

We have used single crystal x-ray diffraction methods to establish the crystal structures of a compound in the Pd–Mn–Si system and in the Pd–Mn–Ge system. The title compounds have structures related to the Fe2P structure type and are ferromagnetic with Curie temperatures above the room temperature. Density functional electronic structure calculations help to understand the nature of the local moment ferromagnetism in these compounds. However, neither the electronic structure calculations nor the magnetic measurements provide any evidence of half-metallic behaviour.

Temperature-dependent electronic structure and magnetic stability of thin ferromagnetic films

We study correlation effects and temperature dependencies in the electronic structure of thin ferromagnetic local-moment films. In a first step the Kondo-lattice model is investigated as a candidate for a proper representation of local-moment ferromagnets. Magnetic and electronic key quantities as the Curie temperature and the quasiparticle density of states are derived with previously tested many-body procedures. It is shown that the magnetic properties can be interpreted exclusively in terms of the temperature-dependent electronic quasiparticle structure. An extended RKKY theory leads to effective Heisenberg exchange integrals, which turn out to be functionals of the conduction electron self-energy, getting therewith a remarkable temperature and band occupation dependence. In a second step, the model studies are combined with tight-binding-LMTO bandstructure calculations in order to get for real ferromagnetic films quasiparticle densities of states and quasiparticle bandstructures. The proposed method avoids the double-counting of relevant interactions and takes into account the correct symmetry of the atomic orbitals. Special results are given for thin ferromagnetic EuO (1 0 0) films. The Curie temperature T C of the EuO film turns out to be strongly thickness-dependent, starting from a very low value (≃15 K) for the monolayer and reaching the bulk value at about 30 layers. For a 20-layer film we predict the existence of a surface state, the temperature behaviour of which can lead to a surface halfmetal–insulator transition.

Structural, transport, magnetic, and thermal properties ofEu8Ga16Ge30

${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ is the only clathrate known so far where the guest positions are fully occupied by a rare-earth element. Our investigations show that, in addition to the previously synthesized ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ modification with clathrate-I structure, there exists a second modification with clathrate-VIII structure. Polycrystalline samples of both phases behave as local-moment ferromagnets with relatively low Curie temperatures (10.5 and 36 K). The charge-carrier concentrations are rather small (3.8 and $12.5\ifmmode\times\else\texttimes\fi{}{10}^{20} {\mathrm{cm}}^{\ensuremath{-}3}$ at 2 K) and, together with the low Curie temperatures, point to a semimetallic behavior. Both the specific heat and the thermal conductivity are consistent with the concept of guest atoms ``rattling'' in oversized host cages, leading to low thermal conductivities (``phonon glasses''). However, the electron mobilities are quite low, which, if intrinsic, would question the properties of an ``electron crystal'', commonly presumed in ``filled-cage'' materials. The dimensionless thermoelectric figure of merit reaches values of 0.01 at 100 K.

The transport properties of Heusler alloys: 'ideal' local moment ferromagnets

The authors have made resistivity, magnetoresistance, Hall effect and thermoelectric power measurements on the Heusler alloys Pd2MnSn, Ni2MnSn and Cu2MnAl. Results are characteristic of well-ordered spin-only local-moment ferromagnets. The thermoelectric power curves all show a unique behaviour which the authors suggest is that characteristic of any local-moment ferromagnet.

Magnetic moments in heusler alloys

The Heusler alloys are a series of local moment ferromagnets of composition X 2Mn Y, with a magnetization of ∼4 mu B per Mn atom. Magnetic measurements on the alloy series Ni 2Mn x T 1− x Sn, where T is Ti, V or Cr, indicate that the T site moment changes from −mu B (for Ti), through zero (for V), to +mu B (for CR). A simple physical interpretation is proposed for this sign change in the present work. This approach facilities the interpretation of several other features observed in Heusler alloys.