Ferromagnetic Fermi Liquid Research Articles

Quantum spin transport and dynamics through a ferromagnetic/normal metal junction

We study the spin transport in the low temperature regime (often referred to as the precession-dominated regime) between a ferromagnetic Fermi liquid (FFL) and a normal metal metallic Fermi liquid (NFL), also known as the F/N junction, which is considered as one of the most basic spintronic devices. In particular, we explore the propagation of spin waves and transport of magnetization through the interface of the F/N junction where nonequilibrium spin polarization is created on the normal metal side of the junction by electrical spin injection. We calculate the probable spin wave modes in the precession-dominated regime on both sides of the junction especially on the NFL side where the system is out of equilibrium. Proper boundary conditions at the interface are introduced to establish the transport of the spin properties through the F/N junction. A possible transmission conduction electron spin resonance (CESR) experiment is suggested on the F/N junction to see if the predicted spin wave modes could indeed propagate through the junction. Potential applications based on this novel spin transport feature of the F/N junction are proposed in the end.

High-Pressure Resistivity of YFe2Si2 and Magnetic Studies of Y1−y Ho y Fe2Si2 and YFe2(Si1−x Ge x )2 Systems

High-pressure resistivity measurements (up to 18.8 kbar) indicate that YFe2Si2 is not superconducting (SC) down to 1.2 K. On the other hand, a clear magnetic peak is observed at 232 K. This peak was recently proposed to a new nearly ferromagnetic Fermi-liquid (NFFL) state. By chemical substitutions of magnetic Ho ions in the nonmagnetic Y site and/or C or Ge in the Si sites of YFe2Si2, we show that this peak is affected and shifted to lower temperatures by three factors: (i) lattice parameters caused by chemical substitution, (ii) applied magnetic field, and (iii) disorder induced by substitution in the Si sites. While Ho substitution does not change much the peak position, both isovalent C and Ge substitutions shift the peak to lower temperature, e.g., in YFe2SiGe the peak position drops to 11 K, indicating that the dominant factor is the disorder induced in the Si site. None of investigated materials is SC down to 1.8 K. Apparently, in all compounds studied, the Fe ions are nonmagnetic, and it is assumed that all magnetic peaks reported here represent this new NFFL state previously proposed for YFe2Si2.

Nodal gap structure and order parameter symmetry of the unconventional superconductor UPt3

Spanning a broad range of physical systems, complex symmetry breaking is widely recognized as a hallmark of competing interactions. This is exemplified in superfluid 3He which has multiple thermodynamic phases with spin and orbital quantum numbers S = 1 and L = 1, that emerge on cooling from a nearly ferromagnetic Fermi liquid. The heavy fermion compound UPt3 exhibits similar behavior clearly manifest in its multiple superconducting phases. However, consensus as to its order parameter symmetry has remained elusive. Our small angle neutron scattering measurements indicate a linear temperature dependence of the London penetration depth characteristic of nodal structure of the order parameter. Our theoretical analysis is consistent with assignment of its symmetry to an L = 3 odd parity state for which one of the three thermodynamic phases in non-zero magnetic field is chiral.

The "Higgs" Amplitude Mode in Weak Ferromagnetic Metals

Using Ferromagnetic Fermi liquid theory, Bedell and Blagoev derived the collective low-energy excitations of a weak ferromagnet. They obtained the well-known magnon (Nambu-Goldstone) mode and found a new gapped mode that was never studied in weak ferromagnetic metals. In this article we have identified this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is identified as the Higgs since it can be show that it corresponds to a fluctuation of the amplitude of the order parameter. We use this model to describe the itinerant-electron ferromagnetic material MnSi. By fitting the model with the existing experimental results, we calculate the dynamical structure function and see well-defined peaks contributed from the magnon and the Higgs. Our estimates of the relative intensity of the Higgs amplitude mode suggest that it can be seen in neutron scattering experiments on MnSi.

Stoner Instability Revisited: Emergence of Local Quantum Criticality?

We revisit Stoner instability, an old problem but in a modern point of view. An idea is to extract out dynamics of directional fluctuations of spins explicitly, resorting to the CP$^{1}$ representation and integrating over their amplitude fluctuations. As a result, we derive an effective field theory for ferromagnetic quantum phase transitions in terms of bosonic spinons and fermionic holons. We show that this effective field theory reproduces overdamped spin dynamics in a paramagnetic Fermi liquid and magnon spectrum in a ferromagnetic Fermi liquid. An interesting observation is that the velocity of spinons becomes zero, approaching the ferromagnetic quantum critical point, which implies emergence of local quantum criticality. Based on this scenario, we predict the $\omega/T$ scaling behavior near ferromagnetic quantum criticality beyond the conventional scenario of the weak-coupling approach.

Phenomenological model of anomalous magnon softening and damping in half-metallic manganites

To describe anomalous zone-boundary softening and damping of magnons in manganites we present a phenomenological two-fluid model containing ferromagnetic Fermi-liquid and non-Fermi-liquid components associated with the itinerant and core electrons of Mn. The observed discontinuous increase of magnon damping is explained by the intersection of magnon dispersions with the electron-hole Stoner continuum arising due to the breakdown of the half-metallic ground state of manganites supported by the experiments and analysis of zero-point effects. Coupling of the Fermi-liquid and non-Fermi-liquid fluids yields conventional long wavelength magnons damped due to their interaction with longitudinal spin fluctuations.

Phases of the InfiniteUHubbard Model on Square Lattices

We apply the density matrix renormalization group (DMRG) to study the phase diagram of the infinite U Hubbard model on 2-, 4-, and 6-leg ladders. Where the results are largely insensitive to the ladder width, we consider the results representative of the 2D square lattice model. We find a fully polarized ferromagnetic Fermi liquid phase when n, the density of electrons per site, is in the range 1>n>n_F ~ 4/5. For n=3/4 we find an unexpected commensurate insulating "checkerboard" phase with coexisting bond density order with 4 sites per unit cell and block spin antiferromagnetic order with 8 sites per unit cell. For 3/4 > n, the wider ladders have unpolarized groundstates, which is suggestive that the same is true in 2D.

Physical properties of GdFe2(AlxZn1−x)20

The high ferromagnetic ordering temperature of the dilute, rare-earth-bearing, intermetallic compound ${\mathrm{GdFe}}_{2}{\mathrm{Zn}}_{20}$ has been understood as being the consequence of the Gd${}^{3+}$ moment being embedded in a nearly ferromagnetic Fermi liquid. To test this understanding in detail, single crystals of the pseudoternary series ${\mathrm{GdFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ ($x\ensuremath{\leqslant}0.122$) and ${\mathrm{YFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ ($x\ensuremath{\leqslant}0.121$) were grown out of Zn-rich solution. Magnetization, heat capacity, and resistivity measurements show that, with Al doping, the ferromagnetic phase transition temperatures of the ${\mathrm{GdFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ compounds decrease from 86 K ($x=$ 0) to 10 K ($x=$ 0.122); for the nonmagnetic analog, the ${\mathrm{YFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ series, the Stoner enhancement factor $Z$ decreases from 0.88 ($x=$ 0) to 0.35 ($x=$ 0.121) in a similar manner. Tight-binding linear-muffin-tin orbital atomic-sphere approximation band structure calculations are used to rationalize this trend. These results, together with the earlier studies of the $R({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{Zn}}_{20}$ ($R=$ Gd and Y) series, clearly highlight the importance of band filling and the applicability of even a simple, rigid-band model to these compounds.

Ground state of interacting quantum magnetic dipoles: Transition from a ferromagnetic Fermi liquid to an antiferromagnetic solid

We show that the ground state of a system of magnetic dipoles, with no electric charge, is a ferromagnetic quantum Fermi liquid at high densities, driven by the dipolar exchange energy. As in the system of classical point dipoles, the direct dipole energy is zero in this case. With decreasing density, there is a transition to an antiferromagnetic lattice state. An addition of short range hard core repulsive potential will arrest the infinite density collapse of the ferromagnetic state, and possible melting of the low density antiferromagnetic lattice state.

Lattice collapse and the magnetic phase diagram ofSr1−xCaxCo2P2

We report that the 122 type Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ solid solution undergoes an anomalous structural transition from the uncollapsed to the collapsed ThCr$_2$Si$_2$ structure at a distinct onset composition near $x=0.5$. Correlated with the structural changes, the electronic system evolves from a nearly ferromagnetic Fermi liquid to an antiferromagnetic metal, through a complex crossover regime. The structural collapse, driven by P-P bonding across the (Sr,Ca) layers, is much more pronounced in this system than it is in the analogous Fe-based system, indicating a strong sensitivity of structure to total electron count in the transition metal pnictide 122 family.

Magnetic properties ofRFe2Zn20andRCo2Zn20(R=Y,Nd,Sm,Gd–Lu)

Magnetization, resistivity, and specific heat measurements were performed on solution-grown single crystals of $R{\text{Fe}}_{2}{\text{Zn}}_{20}$ and $R{\text{Co}}_{2}{\text{Zn}}_{20}$ $(R=\text{Y},\text{Nd},\text{Sm},\text{Gd-Lu})$. Whereas ${\text{LuCo}}_{2}{\text{Zn}}_{20}$ and ${\text{YCo}}_{2}{\text{Zn}}_{20}$ manifest unremarkable metallic behavior, ${\text{LuFe}}_{2}{\text{Zn}}_{20}$ and ${\text{YFe}}_{2}{\text{Zn}}_{20}$ display behaviors such as characteristic of nearly ferromagnetic Fermi liquids. When the well-defined $4f$ local moments $({\text{Gd}}^{3+}{\text{-Tm}}^{3+})$ are embedded into this strongly polarizable host, they manifest enhanced ferromagnetic ordering and the values of ${T}_{\text{C}}$ for $R{\text{Fe}}_{2}{\text{Zn}}_{20}$ $(R=\text{Gd-Tm})$ scale with the de Gennes factor. In addition, data on the $R{\text{Fe}}_{2}{\text{Zn}}_{20}$ compounds indicate a small crystal electric field (CEF) effect compared with the interaction energy scale. On the other hand, the local moment bearing members of $R{\text{Co}}_{2}{\text{Zn}}_{20}$ $(R=\text{Nd},\text{Sm},\text{Gd-Tm})$ manifest weak magnetic interactions and the magnetic properties for $R=\text{Dy-Tm}$ members are strongly influenced by the CEF effect on the $R$ ions. The magnetic anisotropy and specific heat data for the Co series were used to determine the CEF coefficient of $R$ ion with its cubic point symmetry. These CEF coefficients, determined for the Co series, are consistent with the magnetic anisotropy and specific heat data for the Fe series, which indicates similar CEF effects for the Fe and Co series. Such analysis, combined with specific heat and resistivity data, indicates that for $R=\text{Tb-Ho}$, the CEF splitting scale is smaller than their ${T}_{\text{C}}$ values, whereas for ${\text{ErFe}}_{2}{\text{Zn}}_{20}$ and ${\text{TmFe}}_{2}{\text{Zn}}_{20}$ the $4f$ electrons lose part of their full Hund's rule ground state degeneracy above ${T}_{\text{C}}$. ${\text{YbFe}}_{2}{\text{Zn}}_{20}$ and ${\text{YbCo}}_{2}{\text{Zn}}_{20}$ manifest typical but distinct heavy fermion behaviors associated with different Kondo temperatures.

Variation of the magnetic ordering inGdT2Zn20(T=Fe, Ru, Os, Co, Rh and Ir) and its correlation with the electronic structure of isostructuralYT2Zn20

Magnetization, resistivity, and specific heat measurements were performed on solution-grown single crystals of six $\text{Gd}{T}_{2}{\text{Zn}}_{20}$ ($T=\text{Fe}$, Ru, Os, Co, Rh, and Ir) compounds, as well as on their Y analogs. For the Gd compounds, the Fe column members manifest a ferromagnetic (FM) ground state (with an enhanced Curie temperature ${T}_{C}$ for $T=\text{Fe}$ and Ru), whereas the Co column members manifest an antiferromagnetic (AFM) ground state. Thermodynamic measurements on $\text{Y}{T}_{2}{\text{Zn}}_{20}$ revealed that the enhanced ${T}_{C}$ for ${\text{GdFe}}_{2}{\text{Zn}}_{20}$ and ${\text{GdRu}}_{2}{\text{Zn}}_{20}$ can be understood within the framework of Heisenberg moments embedded in a nearly ferromagnetic Fermi liquid. Furthermore, electronic structure calculations indicate that this significant enhancement is due to a large transition metal partial density of states at the Fermi level that places these compounds close to the Stoner FM criterion. The change from FM to AFM ordering (between the Fe and Co column materials) is associated with the filling of electronic states with two additional electrons/f.u. The degree of this sensitivity is addressed by the studies of the pseudoternary compounds $\text{Gd}{({\text{Fe}}_{x}{\text{Co}}_{1\ensuremath{-}x})}_{2}{\text{Zn}}_{20}$ and $\text{Y}{({\text{Fe}}_{x}{\text{Co}}_{1\ensuremath{-}x})}_{2}{\text{Zn}}_{20}$, which clearly reveal the effect of $3d$-band filling on their magnetic properties.

Magnetic properties ofGdxY1−xFe2Zn20: Dilute, large-Smoments in a nearly ferromagnetic Fermi liquid

Single crystals of the dilute, rare earth bearing, pseudo-ternary series, Gd_xY_{1-x}Fe_2Zn_{20} were grown out of Zn-rich solution. Measurements of magnetization, resistivity and heat capacity on Gd_xY_{1-x}Fe_2Zn_{20} samples reveal ferromagnetic order of Gd^{3+} local moments across virtually the whole series ($x \geq 0.02$). The magnetic properties of this series, including the ferromagnetic ordering, the reduced saturated moments at base temperature, the deviation of the susceptibilities from Curie-Weiss law and the anomalies in the resistivity, are understood within the frame work of dilute, $\textbf {S}$ moments (Gd^{3+}) embedded in a nearly ferromagnetic Fermi liquid (YFe_2Zn_{20}). The s-d model is employed to further explain the variation of $T_{\mathrm{C}}$ with x as well as the temperature dependences of of the susceptibilities.

Myriad of correlated electron effects found in the [formula omitted] family

The dilute, rare earth bearing family of RT 2 Zn 20 (R = rare earth, T = transition metal) intermetallic compounds can be tuned to a nearly ferromagnetic Fermi liquid that is closer to the Stoner limit than Pd (for R = Y, Lu and T = Fe). The submersion of moment bearing rare earths (Gd–Tm) into this highly polarizable matrix gives rise to exceptionally high temperature ferromagnetism (e.g. T C = 86 K for GdFe 2 Zn 20 ) for a compound with only one moment bearing ion out of 23 in the formula unit. In addition to this d-shell correlated electron behavior, 4f hybridization gives rise to six, new, Yb-based heavy fermion compounds: YbT 2 Zn 20 (T = Fe, Ru, Os, Co, Rh, Ir). These half dozen compounds manifest a range of Yb ion degeneracy values ( N ) for T ∼ T K , ranging from N = 4 – 8 . Not only do these compounds offer a clear testing ground for the effects of low temperature degeneracy on the correlated electron state, they also present an exceptionally clear experimental confirmation of the generalized Kadowaki–Woods formalism.

Nearly ferromagnetic Fermi-liquid behaviour in YFe2Zn20 and high-temperature ferromagnetism of GdFe2Zn20

One of the historic goals of alchemy was to turn base elements into precious ones. Although the practice of alchemy has been superseded by chemistry and solid-state physics, the desire to dramatically change or tune the properties of a compound, preferably through small changes in stoichiometry or composition, remains. This desire becomes even more compelling for compounds that can be tuned to extremes in behaviour. Here, we report that the RT2Zn20 (R=rare earth and T=transition metal) family of compounds manifests exactly this type of versatility, even though they are more than 85% Zn. By tuning T, we find that YFe2Zn20 is closer to ferromagnetism than elemental Pd, the classic example of a nearly ferromagnetic Fermi liquid. By submerging Gd in this highly polarizable Fermi liquid, we tune the system to a remarkably high-temperature ferromagnetic (TC=86 K) state for a compound with less than 5% Gd. Although this is not quite turning lead into gold, it is essentially tuning Zn to become a variety of model compounds.

Quantum phase transition in the heavy-fermion compoundYbIr2Si2

We investigate the pressure-temperature phase diagram of YbIr$_2$Si$_2$ by measuring the electrical resistivity $\rho(T)$. In contrast to the widely investigated YbRh$_2$Si$_2$, YbIr$_2$Si$_2$ is a paramagnetic metal below $p_c\simeq 8$ GPa. Interestingly, a first-order, presumably ferromagnetic, transition develops at $p_c$. Similar magnetic properties were also observed in YbRh$_2$Si$_2$ and YbCu$_2$Si$_2$ at sufficiently high pressures, suggesting a uniform picture for these Yb compounds. The ground state of YbIr$_2$Si$_2$ under pressure can be described by Landau Fermi-liquid (LFL) theory, in agreement with the nearly ferromagnetic Fermi-liquid (NFFL) model. Moreover, evidence of a weak valence transition, characterized by a jump of the Kadowaki-Woods (KW) ratio as well as an enhancement of the residual resistivity $\rho_0$ and of the quasiparticle-quasiparticle scattering cross section, is observed around 6 GPa.

Heavy-mass fermi liquid near a ferromagnetic instability in layered ruthenates.

Low temperature magnetic, thermal, and transport measurements in Ca2-xSrxRuO4 clarify the appearance of a cluster glass phase, after the evolution of a nearly ferromagnetic heavy-mass Fermi liquid from the spin-triplet superconductor Sr2RuO4. As the Mott transition is approached across a 2nd-order structural transition, both the magnetization and specific heat decrease considerably while the transport scattering rate diverges. A metamagnetic transition to a highly spin polarized state, with a local moment S=1/2, is observed. We argue that an orbital rearrangement with Ca substitution changes itinerant ferromagnetism to antiferromagnetism of localized moments.

Non-Fermi liquid puzzle of MnSi at high pressure

The variation of the electrical resistivity of MnSi at low temperatures changes abruptly from T 2 to T 3/2 when the weak spin polarization is discontinuously suppressed at a hydrostatic pressure p c =14.6 kbar . The T 3/2 dependence of the non-polarized state prevails for p⪢ p c with a prefactor that is weakly pressure dependent and insensitive to sample purity. This contradicts the predictions of a nearly ferromagnetic Fermi liquid for p> p c anticipated from the weakly ferromagnetic state for p< p c. It highlights a novel aspect of the metallic state for p> p c, which most likely is related to unconventional dynamical properties of the spin density. Here I speculate on mechanisms that may form the basis for a future theoretical framework, e.g. incipient antiferromagnetism of itinerant ferromagnets, quantum instantons, changes of charge screening and quantum interference effects similar to frozen in disorder or related to the chirality of the spin fluctuations.

Non-Fermi-liquid nature of the normal state of itinerant-electron ferromagnets.

A century of research on magnetic phenomena had led to the view that the normal state of itinerant-electron ferromagnets such as Fe, Ni and Co could be described in terms of the standard model of the metallic state or its extension known as the nearly ferromagnetic Fermi liquid theory. In recent years, however, a large body of observations has accumulated from various complex intermetallic systems that raises the possibility that this assumption might be wrong. Here we examine this issue by means of high-precision measurements of the electrical transport and magnetic properties of pure ferromagnets-in particular, MnSi-in which the Curie temperature is tuned towards absolute zero by the application of hydrostatic pressure. With this method, it is possible for us to study the normal state over an extraordinarily large range of temperature of up to five orders of magnitude above the Curie temperature. Our results using MnSi reveal a particularly striking combination of properties-most notably a T3/2 power law for the resistivity-showing clearly that the normal state of this itinerant-electron ferromagnet cannot be described in terms of the standard model of metals.

Spin and charge instabilities in weak ferromagnetic metals

Weak ferromagnetic metals are modeled by a ferromagnetic local Fermi liquid. We study the response of the liquid at the quantum critical point where the magnetization vanishes at zero temperature. At small magnetizations we find an instability towards an s-wave superconducting state similar to the Larkin–Ovchinnikov–Fulde–Ferrell state. A non-Fermi liquid state is approached in the vicinity of the classical critical point. The phase diagram of a local Fermi liquid is described.