Itinerant Helimagnet Research Articles

Electric current control of spin helicity in an itinerant helimagnet

A helimagnet is a chiral magnet in which the direction of the magnetic moment spatially rotates in a plane perpendicular to the propagation vector. The sense of the rotation known as spin helicity is a robust degree of freedom of matter and may provide a new concept of magnetic memory if it can be electrically controlled and detected. Here we show that the helicity can be controlled by magnetic fields and electric currents in an itinerant helimagnet MnP. Second-harmonic resistivity measurements allow us to read out the controlled helicity. In contract to an insulating multiferroic magnet, in which spin rotation was shown to be controllable by an electric field, we achieve helicity manipulation by using an electric current in the conducting helimagnet. The controllability of the spin helicity may pave the way to new method of realizing magnetic memories based on the spin internal degrees of freedom.

Low Temperature Heat Capacity of polycrystalline MnSi

The itinerant helimagnet MnSi has been investigated by heat capacity using relaxation calorimetry in H=0. The paramagnetic to helimagnetic transition temperature Tc is found to be 29.6 K. Low temperature heat capacity data in the range 2-18 K is fitted with a usual electronic and phonon contribution. However, an additional magnetic term T 3/2 needs to be added to witness the role of spin wave excitations in the magnetically ordered state below Tc. As well the energy gap on the order A=1.29 meV in the spin wave spectrum is evidenced. Small value of magnetic coefficient δ = 1.66 mJmol-1K-5/2 hints the itinerant nature of the magnetism in MnSi. Obtained coefficients gives the Debye temperature θD = 534 K and the Sommerfeld coefficient of specific heat γ = 0.0367 Jmol-1K-2. Noteworthy observation is the negative trend in the magnetic part of the entropy after Tc.

Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe

Chiral magnetic interactions induce complex spin textures including helical and conical spin spirals, as well as particle-like objects such as magnetic skyrmions and merons. These spin textures are the basis for innovative device paradigms and give rise to exotic topological phenomena, thus being of interest for both applied and fundamental sciences. Present key questions address the dynamics of the spin system and emergent topological defects. Here we analyse the micromagnetic dynamics in the helimagnetic phase of FeGe. By combining magnetic force microscopy, single-spin magnetometry and Landau–Lifschitz–Gilbert simulations we show that the nanoscale dynamics are governed by the depinning and subsequent motion of magnetic edge dislocations. The motion of these topologically stable objects triggers perturbations that can propagate over mesoscopic length scales. The observation of stochastic instabilities in the micromagnetic structure provides insight to the spatio-temporal dynamics of itinerant helimagnets and topological defects, and discloses open challenges regarding their technological usage.

Observation of Coherent Helimagnons and Gilbert Damping in an Itinerant Magnet

We study the magnetic excitations of itinerant helimagnets by applying time-resolved optical spectroscopy to Fe(0.8)Co(0.2)Si. Optically excited oscillations of the magnetization in the helical state are found to disperse to lower frequency as the applied magnetic field is increased; the fingerprint of collective modes unique to helimagnets, known as helimagnons. The use of time-resolved spectroscopy allows us to address the fundamental magnetic relaxation processes by directly measuring the Gilbert damping, revealing the versatility of spin dynamics in chiral magnets.

Emergence of topological Hall effect from fanlike spin structure as modified by Dzyaloshinsky-Moriya interaction in MnP

A nontrivial Hall effect has been observed as an anomaly of the Hall conductivity in the magnetization process in a typical itinerant helimagnet MnP. Since the Dzyaloshinsky-Moriya interaction slightly modulates the spin structure, the anomaly is identified only in the specific magnetic (fanlike) phase that can have finite scalar spin chirality. The decrease in the magnitude of the anomaly as observed by doping Co is well accounted for in terms of the topological Hall effect arising from the spin-chirality-induced Berry phase in real space. A fictitious magnetic flux in real space is estimated to be $\ensuremath{\sim}1$ T, which is consistent with the small modulation of the spin structure.

Phase diagram of the itinerant helimagnet MnSi from high-pressure resistivity measurements and the quantum criticality problem

We performed a series of resistivity measurements on a MnSi single crystal at high pressures, created by a piston-cylinder device with a liquid pressure medium. The form of the resistivity curve at ambient pressure clearly indicates a first-order nature of the magnetic phase transition in MnSi. Application of high pressure rapidly degrades the first-order features of the phase transition. The temperature derivative of resistivity demonstrates two notable features of the phase transition that disappear on increasing pressure: a sharp peak marking the first-order phase transition and a shallow maximum situated slightly above the critical temperature and pointing to prominent helical fluctuations. The current experimental data rule out any strong first-order phase transition in MnSi at high pressures and low temperatures, which would prevent development of a quantum critical region. On the contrary, there should exist true quantum critical phenomena in MnSi at high pressures because a weak first-order transition, if it survives at high pressures to the lowest temperatures, should not suppress the entire quantum critical region.

Quantum Order-by-Disorder Near Criticality and the Secret of Partial Order in MnSi

The vicinity of quantum phase transitions has proven fertile ground in the search for new quantum phases. We propose a physically motivated and unifying description of phase reconstruction near metallic quantum-critical points using the idea of quantum order by disorder. Certain deformations of the Fermi surface associated with the onset of competing order enhance the phase space available for low-energy, particle-hole fluctuations and self-consistently lower the free energy. Applying the notion of quantum order by disorder to the itinerant helimagnet MnSi, we show that, near the quantum critical point, fluctuations lead to an increase of the spiral ordering wave vector and a reorientation away from the lattice-favored directions. The magnetic ordering pattern in this fluctuation-driven phase is found to be in excellent agreement with the neutron-scattering data in the partially ordered phase of MnSi.

Au4V – Moment Stability and Spin Fluctuations in the Ordered Phase

Although neither gold nor vanadium generally possess a magnetic moment, the intermetallic compound Au4V is found to be ferromagnetic below 42K. In this paper we report the results of a muon spin relaxation study of the itinerant electron moment fluctuations in Au4V above the Curie temperature. The temperature dependence of the muon spin relaxation rate is found to be similar to that of the weak itinerant helimagnet, MnSi.

Search for Electronic Phase Separation at Quantum Phase Transitions

Phase separation and extreme sensitivity to disorder and defects are key features of electronic order near quantum phase transitions. Neutron depolarization imaging and neutron Larmor diffraction are new experimental techniques capable of providing detailed real-space and reciprocal-space information, respectively, on the existence and nature of phase separations. Proof-of-principle depolarization imaging in Pd1−x Ni x , CePd1−x Rh x and NbFe2 suggests distinct differences of the real-space distribution of ferromagnetic moments and Curie temperatures in materials at ferromagnetic quantum phase transitions. This compares with neutron Larmor diffraction which provides high-resolution reciprocal-space information of phase separation and the absence of quantum criticality in the itinerant helimagnet MnSi or the parasitic nature of small moment antiferromagnetism in URu2Si2.

Absence of magnetic phase separation in MnSi under pressure

We report muon spin spectroscopy data (muSR) obtained under hydrostatic pressure on a large single crystal of the itinerant helimagnet MnSi, and recorded down to 0.235 K and up to 15.1 kbar. Up to the critical pressure p_c = 14.9 (2) kbar, where the magnetic order is suppressed, the muSR data unambiguously demonstrate that the ground state of the system is magnetic with no indication of any phase separation.

Magnetic and Transport Properties of FeAs Single Crystals

Magnetic susceptibility and transport properties of an itinerant helimagnet FeAs are studied in single-crystalline samples of this binary compound. A kink due to the magnetic transition is observed...

New angles on the border of antiferromagnetism inNiS2andURu2Si2

Abstract Following the border of antiferromagnetism (AF) to zero temperature is a promising route to unconventional metallic and superconducting phases. Many interesting examples of antiferromagnetic quantum phase transitions can only be reached by pressure tuning. The range of quantitative experimental probes, which can be realised in a high-pressure environment is limited. However, advances have recently been made in neutron scattering, where elliptically shaped neutron guides now increase the beam intensity directed to mm size sample for high pressure studies. This has been demonstrated on the simple antiferromagnet NiS 2 . Neutron scattering also allows highly accurate measurements of the lattice constant via the Larmor diffraction technique, which proved extremely useful in studying the high-pressure phase diagram of the itinerant helimagnet MnSi. We now combined Larmor diffraction with conventional diffraction measurements to investigate the pressure–temperature phase diagram of URu 2 Si 2 up to 20 kbar. URu 2 Si 2 offers a further spectacular example for the presence of unconventional phases in the vicinity of antiferromagnetism. In this compound, antiferromagnetism is replaced below approximately 5 kbar by the mysterious “hidden order” (HO) and unconventional superconductivity. Our measurements allow the observation of magnetic order and changes in the a - and c -axis lattice constants across the phase transitions in the same experiment. The results contain clear indications of a first-order transition and strong differences between the AF phase and the HO phase in the coupling to the lattice.

Theory of helimagnons in itinerant quantum systems. III. Quasiparticle description

In two previous papers, we studied the problem of electronic properties in a system with long-ranged helimagnetic order caused by itinerant electrons. A standard many-fermion formalism was used. The calculations were quite tedious because different spin projections were coupled in the action and because of the inhomogeneous nature of a system with long-ranged helimagnetic order. Here we introduce a canonical transformation that diagonalizes the action in spin space and maps the problem onto a homogeneous fermion problem. This transformation to quasiparticle degrees of freedom greatly simplifies the calculations. We use the quasiparticle action to calculate single-particle properties, in particular, the single-particle relaxation rate. We first reproduce our previous results for clean systems in a simpler fashion, and then study the much more complicated problem of three-dimensional itinerant helimagnets in the presence of an elastic relaxation rate $1/\ensuremath{\tau}$ due to nonmagnetic quenched disorder. Our most important result involves the temperature dependence of the single-particle relaxation rate in the ballistic limit ${\ensuremath{\tau}}^{2}T{ϵ}_{F}>1$ for which we find a linear temperature dependence. We show how this result is related to a similar result found in nonmagnetic two-dimensional disordered metals.

Chirality induced anomalous-Hall effect in helical spin crystals

Under pressure, the itinerant helimagnet MnSi displays unusual magnetic properties. We have previously discussed a bcc helical spin crystal as a promising starting point for describing the high pressure phenomenology. This state has topologically non-trivial configurations of the magnetization field. Here we note the consequences for magneto-transport that arise generally from such spin textures. In particular, a skyrmion density induced ‘topological’ Hall effect, with unusual field dependence, is described.

Theory of helical spin crystals: Phases, textures, and properties

Motivated by recent experiments on the itinerant helimagnet MnSi, we study general properties of helical spin crystals - magnetic structures obtained by superposing distinct spin spirals. An effective Landau description of helical spin crystals is introduced and simple rules for stabilizing various spin crystal structures over single spirals are established. Curious properties of the magnetic structures so obtained, such as symmetry stabilized topological textures and missing Bragg reflections are pointed out. The response of helical spin crystals to crystalline anisotropy, magnetic field and non-magnetic disorder are studied, with special reference to the bcc1 spin structure, a promising starting point for discussing the 'partial order' phases seen at high pressure in MnSi. Similar approaches may be applied to other crystallization problems such as Larkin-Ovchinnikov-Fulde-Ferrel states in spin-imbalanced superconductors.

Blue Quantum Fog: Chiral Condensation in Quantum Helimagnets

It is shown that a condensation transition involving a chiral order parameter can occur in itinerant helimagnets, in analogy to the transition between the isotropic phase and the phase known as blue fog or blue phase III in cholesteric liquid crystals. It is proposed that such a transition is the explanation for recent neutron scattering results in MnSi. Predictions are made that will allow for experimental tests of this proposal.

Nonanalytic corrections to Fermi-liquid behavior in helimagnets

The Goldstone mode in the ordered phase of itinerant helimagnets, such as MnSi or FeGe, is determined and shown to have a strongly anisotropic dispersion relation. The softness of this mode is, in a well-defined sense, in between that of ferromagnetic and antiferromagnetic magnons, respectively. It is shown that this soft mode leads to nonanalytic corrections to Fermi-liquid behavior, with a $T$ contribution to the specific heat coefficient, and a ${T\phantom{\rule{0.1em}{0ex}}}^{5∕2}$ contribution to the resistivity. The quasiparticle inelastic lifetime shows anisotropic behavior in momentum space.

Thermodynamical study under hydrostatic pressure of MnSi

We report a study under pressure on the field dependence of magnetic susceptibility χ( H) and magnetoresistivity ϱ( H) on single crystals of the weakly itinerant helimagnet system MnSi. Close to the critical pressure P c = 14.8 kbar, the shapes of χ( H) and ϱ( H) indicate some metamagnetic behaviors induced by the external magnetic field.

Metamagnetic behavior induced by hydrostatic pressure in MnSi

We report a study under pressure on the field dependence of magnetic susceptibility χ( H) and magnetoresistivity ϱ( H) on single crystals of the weakly itinerant helimagnet system MnSi. Close to the critical pressure P c = 14.8 kbar, the shapes of χ( H) and ϱ( H) indicate some metamagnetic behaviors induced by the external magnetic field.

Spin dynamics of the itinerant helimagnet MnSi studied by positive muon spin relaxation.

The local magnetic fields and spin dynamics of the itinerant helimagnet MnSi (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}29.5 K) have been studied experimentally using positive muon spin rotation and relaxation (${\mathrm{\ensuremath{\mu}}}^{+}$SR) methods. In the ordered phase (T${\mathit{T}}_{\mathit{c}}$), zero-field \ensuremath{\mu}SR was used to measure the hyperfine fields at the muon sites as well as the muon spin-lattice relaxation time ${\mathit{T}}_{1}^{\mathrm{\ensuremath{\mu}}}$. Two magnetically inequivalent interstitial ${\mathrm{\ensuremath{\mu}}}^{+}$ sites were found with hyperfine coupling constants ${\mathit{A}}_{\mathrm{hf}}^{(1)}$=-3.94 kOe/${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ and ${\mathit{A}}_{\mathrm{hf}}^{(2)}$=-6.94 kOe/${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$, respectively. In the paramagnetic phase (T>${\mathit{T}}_{\mathit{c}}$), the muon--nuclear-spin double-relaxation technique was used to simultaneously but independently determine the spin-lattice relaxation time ${\mathit{T}}_{1}^{\mathrm{Mn}}$ of $^{55}\mathrm{Mn}$ spins and that of positive muons (${\mathit{T}}_{1}^{\mathrm{\ensuremath{\mu}}}$) over a wide temperature range (${\mathit{T}}_{\mathit{c}}$T\ensuremath{\le}150 K). The temperature dependence of ${\mathit{T}}_{1}^{\mathrm{Mn}}$ and ${\mathit{T}}_{1}^{\mathrm{\ensuremath{\mu}}}$ in both phases shows systematic deviations from the predictions of self-consistent renormalization theory.