arXiv subjects

T. T. M. Palstra

Publications and source records attributed to T. T. M. Palstra.

At least 19 recordsLinked to original sources

arXiv2020

All-electrical detection of skyrmion lattice state and chiral surface twists

We study the high-temperature phase diagram of the chiral magnetic insulator Cu$_2$OSeO$_3$ by measuring the spin-Hall magnetoresistance (SMR) in a thin Pt electrode. We find distinct changes in the phase and amplitude of the SMR signal at critical lines separating different magnetic phases of bulk Cu$_2$OSeO$_3$. The skyrmion lattice state appears as a strong dip in the SMR phase. A strong enhancement of the SMR amplitude is observed in the conical spiral state, which we explain by an additional symmetry-allowed contribution to the SMR present in non-collinear magnets. We demonstrate that the SMR can be used as an all-electrical probe of chiral surface twists and skyrmions in magnetic insulators.

cond-mat.mes-hall↗

arXiv2018

Gate-Controlled Magnetoresistance of a Paramagnetic Insulator|Platinum Interface

We report an electric field-induced in-plane magnetoresistance of an atomically flat paramagnetic insulator|platinum (Pt) interface at low temperatures with an ionic liquid gate. Transport experiments as a function of applied magnetic field strength and direction obey the spin Hall magnetoresistance phenomenology with perpendicular magnetic anisotropy. Our results establish the utility of ionic gating as an alternative method to control spintronic devices without using ferromagnets.

cond-mat.mes-hall↗

arXiv2018

Inducing ferromagnetism and Kondo effect in platinum by paramagnetic ionic gating

Electrically controllable magnetism, which requires the field-effect manipulation of both charge and spin degrees of freedom, has attracted growing interests since the emergence of spintronics. In this work, we report the reversible electrical switching of ferromagnetic (FM) states in platinum (Pt) thin films by introducing paramagnetic ionic liquid (PIL) as the gating media. The paramagnetic ionic gating controls the movement of ions with magnetic moments, which induces itinerant ferromagnetism on the surface of Pt films with large coercivity and perpendicular anisotropy mimicking the ideal two-dimensional Ising-type FM state. The electrical transport of the induced FM state shows Kondo effect at low temperature suggesting spatially separated coexistence of Kondo scattering beneath the FM interface. The tunable FM state indicates that paramagnetic ionic gating could serve as a versatile method to induce rich transport phenomena combining field effect and magnetism at PIL-gated interfaces.

physics.app-ph↗

arXiv2018

New magnetic phase of the chiral skyrmion material Cu2OSeO3

The lack of inversion symmetry in the crystal lattice of magnetic materials gives rise to complex non-collinear spin orders through interactions of relativistic nature, resulting in interesting physical phenomena, such as emergent electromagnetism. Studies of cubic chiral magnets revealed a universal magnetic phase diagram, composed of helical spiral, conical spiral and skyrmion crystal phases. Here, we report a remarkable deviation from this universal behavior. By combining neutron diffraction with magnetization measurements we observe a new multi-domain state in Cu2OSeO3. Just below the upper critical field at which the conical spiral state disappears, the spiral wave vector rotates away from the magnetic field direction. This transition gives rise to large magnetic fluctuations. We clarify physical origin of the new state and discuss its multiferroic properties.

cond-mat.mtrl-sci↗

arXiv2016

Dissipation phenomena and magnetic phase diagram of Cu$_2$OSeO$_3$ below 50 K

The magnetic phase diagram of the chiral magnet Cu$_2$OSeO$_3$ has been investigated by dc magnetisation and ac susceptibility over a very large frequency range from 0.1 Hz up to 1 kHz and temperatures below 50 K. Qualitatively different phase diagrams have been obtained by applying the magnetic field along the easy $\langle100\rangle$ or the hard $\langle110\rangle$ crystallographic directions. Strong $χ"$ appears close to $B_{c2}$, but only below 30 K and is more pronounced when $B$ is applied along $\langle100\rangle$. In addition, the transition from the helical to the conical phase leads to two adjacent $B_{c1}$ lines below 50 K. The frequency dependence of $χ"$ indicates the existence of very broad distributions of relaxation times both at $B_{c1}$ and $B_{c2}$. The associated very long characteristic times eventually prevent the system from reaching thermal equilibrium at low temperatures and lead to thermal hysteresis.

cond-mat.str-el↗

arXiv2016

Probing current-induced magnetic fields in Au|YIG heterostructure with low-energy muon spectroscopy

We investigated the depth dependence of current-induced magnetic fields in a bilayer of a normal metal (Au) and a ferrimagnetic insulator (Yttrium Iron Garnet - YIG) by using low energy muon spectroscopy (LE-muSR). This allows us to explore how these fields vary from the Au surface down to the buried Au|YIG interface, which is relevant to study physics like the spin-Hall effect. We observed a maximum shift of 0.4 G in the internal field of muons at the surface of Au film which is in close agreement to the value expected for Oersted fields. As muons are implanted closer to the Au|YIG interface the shift is strongly suppressed, which we attribute to the dipolar fields present at the Au|YIG interface. Combining our measurements with modelling, we show that dipolar fields caused by the finite roughness of the Au|YIG interface consistently explains our observations. Our results, therefore, gauge the limits on the spatial resolution and the sensitivity of LE-muSR to the roughness of the buried magnetic interfaces, a prerequisite for future studies addressing current induced fields caused by the spin-Hall effect.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic relaxation phenomena in Cu$_2$OSeO$_3$ and phase diagram

We present an investigation of the magnetic field-temperature phase diagram of Cu$_2$OSeO$_3$ based on DC magnetisation and AC susceptibility measurements covering a broad frequency range of four orders of magnitude, from very low frequencies reaching 0.1 Hz up to 1 kHz. The experiments were performed in the vicinity of $T_C=58.2$ K and around the skyrmion lattice A-phase. At the borders between the different phases the characteristic relaxation times reach several milliseconds and the relaxation is non-exponential. Consequently the borders between the different phases depend on the specific criteria and frequency used and an unambiguous determination is not possible.

cond-mat.str-el↗

arXiv2016

Electrical detection of spiral spin structures in Pt|Cu_2OSeO_3 heterostructures

The interaction between the itinerant spins in metals and localized spins in magnetic insulators thus far has only been explored in collinear spin systems, such as garnets. Here, we report the spin-Hall magnetoresistance (SMR) sensitive to the surface magnetization of the spin-spiral material, Cu_2OSeO_3. We experimentally demonstrate that the angular dependence of the SMR changes drastically at the transition between the helical spiral and the conical spiral phases. Furthermore, the sign and magnitude of the SMR in the conical spiral state are controlled by the cone angle. We show that this complex behaviour can be qualitatively explained within the SMR theory initially developed for collinear magnets. In addition, we studied the spin Seebeck effect (SSE), which is sensitive to the bulk magnetization. It originates from the conversion of thermally excited low-energy spin waves in the magnet, known as magnons, into the spin current in the adjacent metal contact (Pt). The SSE displays unconventional behavior where not only the magnitude but also the phase of the SSE vary with the applied magnetic field.

cond-mat.mtrl-sci↗

arXiv2016

Optically probing symmetry breaking in the chiral magnet Cu2OSeO3

We report on the linear optical properties of the chiral magnet Cu2OSeO3, specifically associated with the absence of inversion symmetry, the chiral crystallographic structure, and magnetic order. Through spectroscopic ellipsometry, we observe local crystal-field excitations below the charge-transfer gap. These crystal-field excitations are optically allowed due to the lack of inversion symmetry at the Cu sites. Optical polarization rotation measurements were used to study the structural chirality and magnetic order. The temperature dependence of the natural optical rotation, originating in the chiral crystal structure, provides evidence for a finite magneto-electric effect in the helimagnetic phase. We find a large magneto-optical susceptibility on the order of V(540nm)~10^4 rad/(T*m) in the helimagnetic phase and a maximum Faraday rotation of ~165deg/mm in the ferrimagnetic phase. The large value of V can be explained by considering spin cluster formation and the relative ease of domain reorientation in this metamagnetic material. The magneto-optical activity allows us to map the magnetic phase diagram, including the skyrmion lattice phase. In addition to this, we probe and discuss the nature of the various magnetic phase transitions in Cu2OSeO3.

cond-mat.mtrl-sci↗

arXiv2013

Orbital superexchange and crystal field simultaneously at play in YVO3: resonant inelastic x-ray scattering at the V L edge and the O K edge

We report on the observation of orbital excitations in YVO3 by means of resonant inelastic x-ray scattering (RIXS) at energies across the vanadium L3 and oxygen K absorption edges. Due to the excellent experimental resolution we are able to resolve the intra-t2g excitations at 0.1-0.2 eV, 1.07 eV, and 1.28 eV, the lowest excitations from the t2g into the eg levels at 1.86 eV, and further excitations above 2.2 eV. For the intra-t2g excitations at 0.1-0.2 eV, the RIXS peaks show small shifts of the order of 10-40 meV as a function of temperature and of about 13-20 meV as a function of the transferred momentum q||a. We argue that the latter reflects a finite dispersion of the orbital excitations. For incident energies tuned to the oxygen K edge, RIXS is more sensitive to intersite excitations. We observe excitations across the Mott-Hubbard gap and find an additional feature at 0.4 eV which we attribute to two-orbiton scattering, i.e., an exchange of orbitals between adjacent sites. Altogether, these results indicate that both superexchange interactions and the coupling to the lattice are important for a quantitative understanding of the orbital excitations in YVO3.

cond-mat.str-el↗

arXiv2013

Surface-enhanced charge-density-wave instability in underdoped Bi2201

Neutron and x-ray scattering experiments have provided mounting evidence for spin and charge ordering phenomena in underdoped cuprates. These range from early work on stripe correlations in Nd-LSCO to the latest discovery of charge-density-waves in YBCO. Both phenomena are characterized by a pronounced dependence on doping, temperature, and an externally applied magnetic field. Here we show that these electron-lattice instabilities exhibit also a previously unrecognized bulk-surface dichotomy. Surface-sensitive electronic and structural probes uncover a temperature-dependent evolution of the CuO2 plane band dispersion and apparent Fermi pockets in underdoped Bi2201, which is directly associated with an hitherto-undetected strong temperature dependence of the incommensurate superstructure periodicity below 130K. In stark contrast, the structural modulation revealed by bulk-sensitive probes is temperature independent. These findings point to a surface-enhanced incipient charge-density-wave instability, driven by Fermi surface nesting. This discovery is of critical importance in the interpretation of single-particle spectroscopy data and establishes the surface of cuprates and other complex oxides as a rich playground for the study of electronically soft phases.

cond-mat.supr-con↗

arXiv2012

Probing orbital fluctuations in RVO3 (R = Y, Gd, or Ce) by ellipsometry

We study optical excitations across the Mott gap in the multi-orbital Mott-Hubbard insulators RVO3. The multi-peak structure observed in the optical conductivity can be described consistently in terms of the different 3d^3 multiplets or upper Hubbard bands. The spectral weight is very sensitive to nearest-neighbor spin-spin and orbital-orbital correlations and thus shows a pronounced dependence on both temperature and polarization. Comparison with theoretical predictions based on either rigid orbital order or strong orbital fluctuations clearly rules out the latter. Both, the line shape and the temperature dependence give clear evidence for the importance of excitonic effects.

cond-mat.str-el↗

arXiv2012

Hubbard exciton revealed by time-domain optical spectroscopy

We use broadband ultra-fast pump-probe spectroscopy in the visible range to study the lowest excitations across the Mott-Hubbard gap in the orbitally ordered insulator YVO3. Separating thermal and non-thermal contributions to the optical transients, we show that the total spectral weight of the two lowest peaks is conserved, demonstrating that both excitations correspond to the same multiplet. The pump-induced transfer of spectral weight between the two peaks reveals that the low-energy one is a Hubbard exciton, i.e. a resonance or bound state between a doublon and a holon. Finally, we speculate that the pump-driven spin-disorder can be used to quantify the kinetic energy gain of the excitons in the ferromagnetic phase.

cond-mat.str-el↗

arXiv2010

Structural anomalies associated with the electronic and spin transitions in LnCoO3

A powder X-ray diffraction study, combined with the magnetic susceptibility and electric transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100 - 1000 K. A non-standard temperature dependence of the observed thermal expansion was modelled as a sum of three contributions: (1) Weighted sum of lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate (IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of excited (IS or HS) states of Co3+ ions at the course of the diamagnetic-paramagnetic transition. (3) An anomalous expansion due to excitations of Co3+ ions to another paramagnetic state accompanied by an insulator-metal transition. The anomalous expansion is governed by parameters that are found to vary linearly with the Ln ionic radius. In the case of the first magnetic transition it is the energy splitting E between the ground low spin state and the excited state, presumably the intermediate spin state. The energy splitting E, determined by a fit of magnetic susceptibility, decreases with temperature. The values of E determined for LaCoO3 and YCoO3 at T = 0 K as 164 K and 2875 K, respectively, fall to zero at T = 230 K for LaCoO3 and 860 K for YCoO3. The second anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is characterized by its center at T = 535 K for LaCoO3 and 800 K for YCoO3. The change of the unit cell volume during each transition is independent on the Ln cation and is about 1% in both cases.

cond-mat.mtrl-sci↗

arXiv2008

Anomaly in Nonlinear Magnetoelectric Response of YbMnO3

We observe a seemingly complex magnetic field dependence of dielectric constant of hexagonal YbMnO3 near the spin ordering temperature. After rescaling, the data taken at different temperatures and magnetic fields collapse on a single curve describing the sharp anomaly in nonlinear magnetoelectric response at the magnetic transition. We show that this anomaly is a result of the competition between two magnetic phases. The scaling and the shape of the anomaly are explained using the phenomenological Landau description of the competing phases in hexagonal manganites.

cond-mat.mtrl-sci↗

arXiv2008

Magnetic and dielectric properties of YbMnO3 perovskite thin films

Metastable manganite perovskites displaying the antiferromagnetic so-called E-phase are predicted to be multiferroic. Due to the need of high-pressures for the synthesis of this phase, this prediction has only been confirmed in bulk HoMnO3. Here we report on the growth and characterization of YbMnO3 perovskite thin films grown under epitaxial strain. Highly-oriented thin films, with thickness down to ~30nm, can be obtained showing magneto-dielectric coupling and magnetic responses as those expected for the E-phase. We observe that the magnetic properties depart from the bulk behavior only in the case of ultrathin films (d< 30nm), which display a glassy magnetic behavior. We show that strain effects alone cannot account for this difference and that the film morphology plays, instead, a crucial role.

cond-mat.mtrl-sci↗

arXiv2008

Collective orbital excitations in orbitally ordered YVO3 and HoVO3

We study orbital excitations in the optical absorption spectra of YVO3 and HoVO3. We focus on an orbital absorption band observed at 0.4 eV for polarization E parallel c. This feature is only observed in the intermediate, monoclinic phase. By comparison with the local crystal-field excitations in VOCl and with recent theoretical predictions for the crystal-field levels we show that this absorption band cannot be interpreted in terms of a local crystal-field excitation. We discuss a microscopic model which attributes this absorption band to the exchange of two orbitals on adjacent sites, i.e., to the direct excitation of two orbitons. This model is strongly supported by the observed dependence on polarization and temperature. Moreover, the calculated spectral weight is in good agreement with the experimental result.

cond-mat.str-el↗

arXiv2007

Continuous first order orbital order-disorder transition in Nd1-xCaxMnO3

The nature of the cooperative Jahn-Teller (JT) transition accompanied by orbital order-disorder in Nd1-xCaxMnO3 has been studied by high temperature (300 - 1200K) synchrotron and laboratory X-ray powder diffraction in the low doping region (0<x<0.1). For very low doping a large temperature range of phase coexistence associated with a first-order transition has been observed, resembling a martensitic transformation. The transition appears continuous because of a gradual evolution of the volume fractions of the phases over a broad temperature interval. The first-order nature of the transition and the phase coexistence is suppressed with increasing of doping.

cond-mat.mtrl-sci↗