We report a detailed study of UGe$_{2}$ single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition $T_C = 53 K$. The characteristic changes are exhibited most strongly at an energy scale of around 12 meV (100 cm$^{-1}$). They recover their unrenormalized value above $T_C$ and for $ω>$ 40 meV. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature $T_x \sim$ 30 K, observed in transport and thermodynamic experiments. In the ferromagnetic state we find signatures of a strong coupling to the longitudinal magnetic excitations that have been proposed to mediate unconventional superconductivity in this compound.
We present thermoelectric power and resistivity measurements in the ferromagnet UGe$_2$ as a function of temperature and magnetic field. At low temperature, huge quantum oscillations are observed in the thermoelectric power as a function of the magnetic field applied along the $a$ axis. The frequencies of the extreme orbits are determined and an analysis of the cyclotron masses is performed following different theoretical approaches for quantum oscillations detected in the thermoelectric power. They are compared to those obtained by Shubnikov-de Haas experiments on the same crystal and previou…
The phase diagram of several itinerant ferromagnets reveals the common feature. The phase transition temperature decreases with pressure increase and reaches zero value at some critical pressure $P_c$ such that at low enough temperatures one can expect critical behavior specific for quantum phase transition. It is not the case, however. Being the second order at ambient pressure the transition from paramagnetic to ferromagnetic state at high pressures - low temperatures is transformed to the discontinuous jump. We discuss the magneto-elastic mechanism of development of the first order type ins…
We discuss a mechanism of real-space spin-triplet pairing, alternative to that due to quantum paramagnon excitations, and demonstrate its applicability to $\mathrm{UGe_2}$. Both the Hund's rule ferromagnetic exchange and inter-electronic correlations contribute to the same extent to the equal-spin pairing, particularly in the regime in which the weak-coupling solution does not provide any. The theoretical results, obtained within the orbitally-degenerate Anderson lattice model, match excellently the observed phase diagram for $\mathrm{UGe_2}$ with the coexistent ferromagnetic (FM1) and superco…
We provide a microscopic description of the magnetic properties of UGe$_2$ and in particular, of its both classical and quantum critical behavior. Namely, we account for all the critical points: the critical ending point (CEP) at the metamagnetic phase transition, the tricritical point, and the quantum critical end point at the ferromagnetic to paramagnetic phase transition. Their position agrees quantitatively with experiment. Additionally, we predict that the metamagnetic CEP can be traced down to zero temperature and becomes quantum critical point by a small decrease of both the total elect…
Inelastic neutron scattering was used to study the low energy magnetic excitations of the ferromagnetic superconductor UGe$_{2}$. The ferromagnetic fluctuations are of Ising nature with a non-conserved magnetization and have an intermediate behavior between localized and itinerant magnetism.
In strongly correlated materials, cooperative behavior of the electrons causes a variety of quantum ordered states that may, in some cases, coexist. It has long been believed, however, that such coexistence among ferromagnetic ordering, superconductivity and heavy-fermion behavior is impossible, as the first supports parallel spin alignment while the conventional understanding of the latter two phenomena assumes spin-singlet or anti-parallel spins. This understanding has recently been challenged by an increasing number of observations in uranium systems (UGe2, URhGe, UIr and UCoGe) in which su…
Recently co-existence of the ferromagnetism and superconductivity was reported in the high-pressure region (1.0-1.6 GPa) in UGe$_2$. We performed the heat capacity measurement on UGe$_2$ under high pressure. At 1.13 GPa, we found a peak corresponding to the superconducting transition. The superconducting temperature $T_{SC}$ and ${\itΔ} C/(γT_{SC})$ are 0.6 K and 0.25, respectively. The superconducting transition was also confirmed by the appearance of the Meissner effect in the {\it ac} susceptibility. From these results, we confirm a bulk nature of the superconductivity in UGe$_2$. The value…
It is assumed that U atoms in $UGe_2$ have a number of $f$ electrons appropriate to give them each a spin $s=1$ as well as one extra itinerant electron which may equally well be on one or other U atom. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1/2 fermi operators of itinerant electrons. Applying hydrostatic pressure changes the bandwidths of spin-up and spin-down itinerant electrons in different way, which leads to decreasing of the contribution of the fermions to the magnetization keeping the spin-fermion interaction unchanged. In turn the local spin-fe…
The recent discovery that superconductivity occurs in several clean itinerant ferromagnets close to low temperature magnetic instabilities naturally invites an interpretation based on a proximity to quantum criticality. Here we report measurements of the pressure dependence of the low temperature magnetisation in one of these materials, UGe_2. Our results show that both of the magnetic transitions observed in this material as a function of pressure are first order transitions and do not therefore correspond to quantum critical points. Further we find that the known pressure dependence of the s…
We report on the itinerant ferromagnetic superconductor UGe_2 through ^73Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum signals a first-order transition from the low-temperature (T) and low-P ferromagnetic phase (FM2) to high-T and high-P one (FM1) around a critical pressure of P_x ~ 1.2 GPa. The superconductivity exhibiting a maximum value of T_sc=0.7 K at P_x ~ 1.2 GPa, was found to take place in connection with the P-induced first-order transition. The nuclear spin-lattice relaxation rate 1/T_1 has probed the ferromagnetic transition, exhibiting a peak at the C…
We report on Hall resistivity and electrical resistivity measurements under pressure and magnetic field in UGe_2 with ferromagnetic (FM) tricriticality. The Hall resistivity sensitively detects the first order metamagnetic transition from a paramagnetic (PM) phase to a FM phase in a large pressure range almost up to the quantum critical end point (QCEP). The drastic change in the Fermi surface at the PM-FM transition is detected up to the vicinity of the QCEP, while a strong modification in the field variation of the inelastic scattering between electrons is observed toward the QCEP. The compa…
We study the nuclear relaxation rate 1/T1 as a function of temperature for a superconducting-ferromagnetic coexistent system using a p-wave triplet model for the superconducting pairing symmetry. This calculation is contrasted with a singlet s-wave one done previously, and we see for the s-wave case that there is a Hebel-Slichter peak, albeit reduced due to the magnetization, and no peak for the p-wave case. We then compare these results to a nuclear relaxation rate experiment on UGe2 to determine the possible pairing symmetry signatures in that material. It is seen that the experimental data …
Studying the prototypical ferromagnetic superconductor UGe$_2$ we demonstrate the potential of the Modulated IntEnsity by Zero Effort (MIEZE) technique---a novel neutron spectroscopy method with ultra-high energy resolution of at least 1~$μ$eV---for the study of quantum matter. We reveal purely longitudinal spin fluctuations in UGe$_2$ with a dual nature arising from $5f$ electrons that are hybridized with the conduction electrons. Local spin fluctuations are perfectly described by the Ising universality class in three dimensions, whereas itinerant spin fluctuations occur over length scales co…
Motivated by the recent discovery of a continuous ferromagnetic quantum phase transition in CeRh$_6$Ge$_4$ and its distinction from other U-based heavy fermion metals such as UGe$_2$, we develop a unified explanation of their different ground state properties based on an anisotropic ferromagnetic Kondo-Heisenberg model. We employ an improved large-$N$ Schwinger boson approach and predict a full phase diagram containing both a continuous ferromagnetic quantum phase transition for large magnetic anisotropy and first-order transitions for relatively small anisotropy. Our calculations reveal three…
Three-dimensional stationary precession solitons with nonzero Hopf indices are found numerically by solving the Landau-Lifshitz equation. The structure and existence domain of the solitons are found.
The feasibility of a single-domain ferromagnet based on uniaxial magnetic ions was examined. For a noncentrosymmetric uniaxial magnetic ion of magnetic moment at a site of local electric dipole moment p, it is unknown to date whether the magnetic moment prefers to be parallel or antiparallel to the electric dipole moment. The nature of this magnetoelectric interaction was probed in terms of analogical reasoning based on the Rashba effect and density functional calculations. We show that the magnetic and electric dipole moments prefer an antiparallel arrangement, predict that Fe-doped CaZnOS is…
We report the novel critical behavior of magnetization in low carrier concentration systems UTeS and USeS that exhibit the large negative magnetoresistance around the ferromagnetic transition temperatures T_C ~ 85 and 23 K, respectively. UTeS and USeS crystallize in the same orthorhombic TiNiSi-type crystal structure as those of uranium ferromagnetic superconductors URhGe and UCoGe. We determine the critical exponents, beta for the spontaneous magnetization M_s, gamma for the magnetic susceptibility chi, and delta for the magnetization isotherm at T_C with several methods. The ferromagnetic st…
Three different cases of magnetic field reentrant ground states in heavy fermion systems, URhGe, UGe$_2$ and CeRhIn$_5$ recently studied in Grenoble, are discussed. URhGe is a ferromagnetic superconductor with reentrace of superconductivity under magnetic field ($H$) which is associated to the spin reorientation field. UGe$_2$ is a ferromagnetic superconductor with an enhancement of the upper critical field $H_{c2}$ of superconductivity at its metamagnetic transition between 2 ferromagnetic phases. CeRhIn$_5$ is a superconductor with $H$ reentrant antiferromagnetism. We analyze the links betwe…
It is argued that the phase transition in low-T_c clean itinerant ferromagnets is generically of first order, due to correlation effects that lead to a nonanalytic term in the free energy. A tricritical point separates the line of first order transitions from Heisenberg critical behavior at higher temperatures. Sufficiently strong quenched disorder suppresses the first order transition via the appearance of a critical endpoint. A semi-quantitative discussion is given in terms of recent experiments on MnSi and UGe_2. It is then shown that the critical temperature for spin-triplet, p-wave superc…
Magnetoelastic coupling and uniaxial pressure dependencies of the ferromagnetic ordering temperature in the quasi-two-dimensional layered van-der-Waals material CrI$_3$ are experimentally studied and quantified by high-resolution dilatometry. Clear anomalies in the thermal expansion coefficients at $T_{\rm C}$ imply positive (negative) pressure dependencies $\partial T_{\rm C}/\partial p_{\rm i}$ for pressure applied along (perpendicular to) the $c$ axis. The experimental results are backed up by numerical studies showing that the dominant, intra-layer magnetic coupling increases upon compress…
Magnetic biskyrmions are observed in experiments but their existences are still under debate. In this work, we present the existence of biskyrmions in a magnetic film with tilted uniaxial anisotropy via micromagnetic simulations. We find biskyrmions and bubbles share a unified three-dimensional structure, in which the relative position of two intrinsic Bloch points dominates the two-dimensional topological property in the film middle. The film edge can drive Bloch points and transform bubbles into biskyrmions via the demagnetizing field. This mechanism is found in the formation process of bisk…
An earlier theory of the quantum phase transition in metallic ferromagnets is revisited and generalized in three ways. It is shown that the mechanism that leads to a fluctuation-induced first-order transition in metallic ferromagnets with a low Curie temperature is valid, (1) irrespective of whether the magnetic moments are supplied by the conduction electrons or by electrons in another band, (2) for ferromagnets in the XY and Ising universality classes as well as for Heisenberg ferromagnets, and (3) for ferrimagnets as well as for ferromagnets. This vastly expands the class of materials for w…
It is shown that the peculiar features observed in the low-temperature phase diagrams of ZrZn_2, UGe_2, and MnSi can be understood in terms of a simple mean-field theory. The nature of the ferromagnetic transition changes from second order to first order at a tricritical point, and in a small external magnetic field surfaces of first-order transitions emerge which terminate in quantum critical points. This field dependence of the phase diagram follows directly from the existence of the tricritical point. The quantum critical behavior in a nonzero field is calculated exactly.
We study theoretically the variation of the magnetic and the structural transition temperatures of the iron arsenide superconductors with external uniaxial stress. We show that the increase of the transition temperatures reported in recent experiments is compatible with a simple magneto-elastic model in which the physical origin of this variation is linked to the fact that antiferromagnetic bonds are longer than the ferromagnetic ones in the magnetic phase. We make predictions which can be verified in order to test the relevance of this model.
In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu$_2$Ga$_2$B. We demonstrate a feasible control both globally and…
Surface magnetic domains of a UCoGa single crystal during magnetization/demagnetization processes in increasing/decreasing magnetic fields were investigated by means of magnetic-force-microscopy (MFM) images at low temperatures. The observed domain structure is typical for a ferromagnet with strong uniaxial anisotropy. The evolution of magnetic domains during cooling of the crystal below TC has also been manifested by MFM images. Analysis of the available data reveals that the high uniaxial magnetocrystalline energy in combination with the relatively small ferromagnetic exchange interaction in…
Cr$_2$Ge$_2$Te$_6$ is a quasi-2D semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr$_2$Ge$_2$Te$_6$ single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{c}}$ = 24.7 K/GPa an…
Decoupling the effects of geometry and nature of strain in LaMnO$_3$: Interplay of dynamic correlations and uniaxial strain driving magnetic phase transitions
Magnetic skyrmions in chiral-lattice ferromagnets are currently attracting enormous research interest because of their potential applications in spintronic devices. However, they emerge in bulk specimens only in a narrow window of temperature and magnetic field. This limited stability regime is recognized as an obstacle to technical applications. Recent experiments demonstrated that the thermodynamic stability of magnetic skyrmions is enhanced or suppressed by the application of a uniaxial strain depending on its axial direction in bulk chiral-lattice ferromagnets MnSi [Y. Nii et al., Nat. Com…
One dimensional S=1 XXZ chains with uniaxial single-ion-type anisotropy are studied by numerical exact diagonalization of finite size systems. The numerical data are analyzed using conformal field theory, the level spectroscopy, phenomenological renormalization group and finite size scaling method. We thus present the first quantitatively reliable ground state phase diagram of this model. The ground states of this model contain the Haldane phase, large-D phase, Néel phase, two XY phases and the ferromagnetic phase. There are four different types of transitions between these phases: the Brezins…
