Torque Magnetometry Research Articles

Novel Pauli-paramagnetic quantum phase in a Mott insulator

In Mott insulators, the strong electron-electron Coulomb repulsion localizes electrons. In dimensions greater than one, their spins are usually ordered antiferromagnetically at low temperatures. Geometrical frustrations can destroy this long-range order, leading to exotic quantum spin liquid states. However, their magnetic ground states have been a long-standing mystery. Here we show that a quantum spin liquid state in the organic Mott insulator EtMe(3)Sb[Pd(dmit)(2)](2) (where Et is C(2)H(5)-, Me is CH(3)-, and dmit is 1,3-dithiole-2-thione-4,5-dithiolate) with two-dimensional triangular lattice has Pauli-paramagnetic-like low-energy excitations, which are a hallmark of itinerant fermions. Our torque magnetometry down to low temperatures (30 mK) up to high fields (32 T) reveals distinct residual paramagnetic susceptibility comparable to that in a half-filled two-dimensional metal, demonstrating the magnetically gapless nature of the ground state. Moreover, our results are robust against deuteration, pointing toward the emergence of an extended 'quantum critical phase', in which low-energy spin excitations behave as in paramagnetic metals with Fermi surface, despite the frozen charge degree of freedom.

Magnetization hysteresis and time decay measurements in FeSe0.50Te0.50: Evidence for fluctuation in mean free path induced pinning

We present results of magnetic measurements relating to vortex phase diagram in a single crystal of FeSe$_{0.5}$Te$_{0.5}$ which displays second magnetization peak anomaly for $H \parallel c$. The possible role of the crystalline anisotropy on vortex pinning is explored via magnetic torque magnetometry. We present evidence in favor of pinning related to spatial variations of the charge carrier mean free path leading to small bundle vortex pinning by randomly distributed (weak) pinning centers for both $H \parallel c$ and $H \perp c$. This is further corroborated using magnetization data for $H \parallel c$ in a single crystal of FeSe$_{0.35}$Te$_{0.65}$. Dynamical response across second magnetization peak (SMP) anomaly in FeSe$_{0.5}$Te$_{0.5}$ has been compared with that across the well researched phenomenon of peak effect (PE) in a single crystal of CeRu$_2$.

Coexistence of magnetic order and two-dimensional superconductivity at LaAlO3/SrTiO3 interfaces

A two dimensional electronic system with novel electronic properties forms at the interface between the insulators LaAlO$_3$ and SrTiO$_3$. Samples fabricated until now have been found to be either magnetic or superconducting, depending on growth conditions. We combine transport measurements with high-resolution magnetic torque magnetometry and report here evidence of magnetic ordering of the two-dimensional electron liquid at the interface. The magnetic ordering exists from well below the superconducting transition to up to 200 K, and is characterized by an in-plane magnetic moment. Our results suggest that there is either phase separation or coexistence between magnetic and superconducting states. The coexistence scenario would point to an unconventional superconducting phase in the ground state.

Cubic magnetic anisotropy of the antiferromagnetically ordered Cu 3TeO 6

A combination of highly sensitive torque magnetometry in low magnetic fields and a phenomenological approach to magnetic anisotropy is used to probe the symmetry of the antiferromagnetically ordered state of spin S = 1 / 2 system Cu 3TeO 6. The results show that the ordered state has four antiferromagnetic domains with spin axis in the 〈 1 ± 1 ± 1 〉 directions, in agreement with the previously reported neutron measurements. These results show that this approach, previously applied to ferromagnets and highly anisotropic antiferromagnets, is also successful in determining the symmetry of weakly anisotropic Heisenberg antiferromagnets with multiple spin domains. Possible microscopic origin of magnetic anisotropy is briefly discussed.

Magnetic torque study of the temperature-dependent anisotropy parameter in overdoped superconducting single-crystal YBa2Cu3O7

An overdoped YBa2Cu3O7 single crystal was studied by SQUID and torque magnetometry in order to investigate the temperature dependence of the anisotropy parameter close to the transition temperature T_c (0.87 T_c < T < T_c). Angle dependent torque measurements were performed and analyzed with the widely used Kogan model [Phys. Rev. B 38, 7049 (1988)] as well as with an extended model by Hao and Clem [Phys. Rev. Lett. 67, 2371 (1991)], taking into account the influence of the vortex cores on the magnetization. Both approaches yield similar results, with an out-of-plane anisotropy parameter around 6.5 which slightly increases with decreasing temperature, and a temperature independent in-plane anisotropy parameter gamma_ab}= 1.12(5).

Fabrication of L1-ordered FePt granular media using an in-line sputtering system

FePtAg-SiO2 granular films were fabricated using an in-line sputtering system with a composite alloy target. Two heating methods were investigated in consideration of practical manufacturing processes. Structural analyses showed that the FePtAg-SiO2 film had better L10 ordered structure and clearer grain boundaries when deposited on the preheated substrate than when annealed after deposition. Adding Ag improved both the L10 ordering and the face centered tetragonal (001) texture. Torque magnetometry indicated that the FePtAg-SiO2 film had a uniaxial perpendicular anisotropy of 1.4 × 107 (erg/cc) at an ordering parameter S of 0.72.

Nanomechanical torsional resonator torque magnetometry (invited)

Micromechanical resonators are very useful for detection of magnetic torque. We have developed nanoscale torsional resonators fabricated within silicon nitride membranes, as a platform for magnetometry of nanoscale magnetic elements. We describe the rotational magnetic hysteresis of a 10 nm thick film deposited on a resonator, and a study of magnetic hysteresis in a single, 1 μm diameter permalloy disk. The torsional resonator is patterned using a dual beam scanning electron/focused ion system. For the 1 μm diameter disk, it is found to be possible to tune the conditions such that an apparent magnetic supercooling of vortex nucleation is observed, as would be suggested by the modified Landau theory of the C- to vortex-state switch as a first-order phase transition. Complementary transmission electron and Lorentz microscopy of the same structures have also been performed.

Observation of magnetic supercooling of the transition to the vortex state

The magnetic hysteresis of an individual magnetic disc switching in and out of the vortex state has been exhaustively measured using nanomechanical torsional resonator torque magnetometry. Each individual hysteresis loop pinpoints two sharp events, a single vortex creation and an annihilation, with a bias field precision of 0.02 kA m− 1. Statistical analysis of thousands of hysteresis loops reveals a dramatic difference in the sensitivity of the vortex creation and annihilation field distributions to the measurement conditions. The data sets measured at different magnetic field sweep rates demonstrate that the transition from the high-field state to the vortex state is not well modeled as a conventional thermal activation process in which it is assumed that the ‘true’ nucleation field is lower than any of the observed switching fields. Instead, the results are suggestive of the classic supercooling signature of a first-order phase transition, or more specifically here, its magnetic equivalent. This phenomenological evidence is consistent with a theoretical picture of the vortex nucleation process as a modified Landau first-order phase transition.

Torque detected broad band electron spin resonance

We present a novel technique to measure high frequency electron spin resonance spectra in a broad frequency range (30-1440 GHz) with high sensitivity. We use a quasioptical setup with tunable frequency sources to induce magnetic resonance transitions. These transitions are detected by measuring the change in the magnetic torque signal by means of cantilever torque magnetometry. The setup allows tuning of the frequency, magnetic field, polarization, and the angle between the sample and the external magnetic field. We demonstrate the capabilities of this technique by showing preliminary results obtained on a single crystal of an Fe(4) molecular nanomagnet.

Evidence for a temperature dependent anisotropy of the superconducting state parameters in underdopedSmBa2Cu3Ox

The temperature dependence of the anisotropy of the superconducting state parameters, $\ensuremath{\gamma}$, was studied by torque magnetometry for the high-temperature superconductor ${\text{SmBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ in magnetic fields of up to 9 T. The measurements were performed on four underdoped single crystals with oxygen contents corresponding to ${T}_{\text{c}}$'s varying from 42.8 to 63.6 K. The anisotropy was found to be strongly temperature dependent while only a weak dependence on the magnetic field was observed. No evidence for a field dependent superfluid density was found. Possible origins of the temperature dependence of the anisotropy are discussed.

Dimensionality-driven spin-flop transition in quasi-one-dimensionalPrBa2Cu4O8

In the quasi-one-dimensional cuprate PrBa$_2$Cu$_4$O$_8$, the Pr cations order antiferromagnetically at 17 K in zero field. Through a combination of magnetic susceptibility, torque magnetometry, specific heat and interchain transport measurements, the anisotropic temperature-magnetic field phase diagram associated with this ordering has been mapped out. A low-temperature spin-flop transition in the Pr sub-lattice is found to occur at the same magnetic field strength and orientation as a dimensional crossover in the ground state of the metallic CuO chains. This coincidence suggests that the spin reorientation is driven by a change in the anisotropic Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction induced by a corresponding change in effective dimensionality of the conduction electrons.

Anisotropic superconducting properties of single-crystallineFeSe0.5Te0.5

Iron-chalcogenide single crystals with the nominal composition FeSe$_{0.5}$Te$_{0.5}$ and a transition temperature of $T_{c}\simeq14.6$ K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe_1.045Se_0.406Te_0.594. Additionally, a minor hexagonal Fe_7Se_8 impurity phase was identified. The magnetic penetration depth \lambda at zero temperature was found to be 491(8) nm in the ab-plane and 1320(14) nm along the c-axis. The zero-temperature value of the superfluid density \rho_s(0) \lambda^-2(0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron-pnictides. The temperature dependences of both \lambda_ab and \lambda_c are well described by a two-gap s+s-wave model with the zero-temperature gap values of \Delta_S(0)=0.51(3) meV and \Delta_L(0)=2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter \gamma_\lambda(T)=\lambda_c(T)/\lambda_{ab}(T) increases with decreasing temperature, in agreement with \gamma_\lambda(T) observed in the iron-pnictide superconductors.

Spin reorientation induced by a very high magnetic field in domain-structuredYFeO3films: Emergence of perpendicular anisotropy

We report experiments on the static magnetic properties of ${\text{YFeO}}_{3}$ under magnetic fields up to 28 T. We studied an untwined single crystal, a sintered powder and a film of ${\text{YFeO}}_{3}$ epitaxially grown by pulsed laser deposition. We proved the relevance of a little used technique, the torque magnetometry, for investigating films having Dzyaloshinsky-Moriya interaction under very high fields. A dedicated framework for analyzing the torque signal in this particular case is also given. We proposed an original model proposed to explain the irreversibility based on the comparison of the morphologies of an experimental study made on purpose. One of the consequences of the particular morphology of films is that the net magnetic moment of the film can be permanently set perpendicular to the substrate plane, giving rise to an effective perpendicular magnetic anisotropy with a coercive field as high as 3 T at room temperature.

Magnetism in a Mn modulation-doped InAs/InGaAs heterostructure with a two-dimensional hole system

For an InAs quantum well which is modulation-doped with Mn we measure directly and simultaneously the magnetization and magnetic anisotropy of, both, the two dimensional hole system (2DHS) and Mn dopants. Using highly sensitive micromechanical torque magnetometry at 400 mK we observe the de Haas-van Alphen effect of the 2DHS in a magnetic field B up to 14 T. Around B=0 we find a magnetic hysteresis which we attribute to the spontaneous ordering of magnetic moments in the interacting Mn-hole system. Tilted field experiments suggest a uniaxial magnetic anisotropy with the easy axis in (001) growth direction.

Anisotropy of the superconducting state parameters and intrinsic pinning in low-level Pr-dopedY Ba2Cu3O7−δ single crystals

The influence of low-level Pr substitution inY1 − xPrxBa2Cu3O7−δ single crystals on the anisotropy of the superconducting state parameters was investigatedby torque magnetometry in magnetic fields of up to 9 T. An averaged anisotropy parameter,γ, of 7.4 was found and no influence of the Pr ion concentration onγ was observed up to a Pr content of 2.4%. A pronounced maximum at angles between0.5° and1° out ofthe ab-plane was observed for all crystals in the irreversible angular dependenceof the torque. This maximum is attributed to intrinsic pinning associatedwith kinked vortices. The variation of the irreversible torque with thesubstitution level indicates the influence of the Pr content on pinning within theCuO2 planes, even though the anisotropy of the superconducting state parameters is not affected.

Possible Bose-Einstein condensate of magnons in single-crystallinePb2V3O9

We report the growth and the characterization of single crystals of the $S=1/2$ spin-dimer compound ${\text{Pb}}_{2}{\text{V}}_{3}{\text{O}}_{9}$. Magnetic-susceptibility, torque magnetometry, heat-capacity, and muon-spin-relaxation measurements provide evidence for a field-induced Bose-Einstein condensate of magnons in this system. At low temperatures, the field-dependent phase boundary between the dimerized and the condensed state is well described by the expression ${T}^{\ensuremath{\star}}\ensuremath{\propto}{(H\ensuremath{-}{H}_{c1})}^{1/\ensuremath{\phi}}$, with $\ensuremath{\phi}=(2.00\ifmmode\pm\else\textpm\fi{}0.04)$.

Helical instability of charged vortices in layered superconductors

It is shown that the electric charge of vortices can result in a helical instability of straight vortex lines in layered superconductors, particularly Bi-based cuprates or organic superconductors. This instability may result in a phase transition to a uniformly twisted vortex state, which could be detected by torque magnetometry, neutron diffraction, electromagnetic or calorimetric measurements.

Magnetic field induced quantum phase transition of theS=12antiferromagnetK2NaCrO8

The magnetic properties of alkali-metal peroxychromate K$_2$NaCrO$_8$ are governed by the $S = 1/2$ pentavalent chromium cation, Cr$^{5+}$. Specific heat, magnetocalorimetry, ac magnetic susceptibility, torque magnetometry, and inelastic neutron scattering data have been acquired over a wide range of temperature, down to 60 mK, and magnetic field, up to 18 T. The magnetic interactions are quasi-two-dimensional prior to long-range ordering, where $T_N = 1.66$ K in $H = 0$. In the $T \to 0$ limit, the magnetic field tuned antiferromagnetic-ferromagnetic phase transition suggests a critical field $H_c = 7.270$ T and a critical exponent $\alpha = 0.481 \pm 0.004$. The neutron data indicate the magnetic interactions may extend over intra-planar nearest-neighbors and inter-planar next-nearest-neighbor spins.

Easy plane anisotropy in Bi2CuO4

dc magnetic susceptibility measurements and the torque magnetometry were used toexperimentally probe the symmetry of the antiferromagnetically (AFM) ordered state ofBi2CuO4. A phenomenological approach to the anisotropy energy is used to modelthe angular dependence of torque for easy axis and easy plane anisotropy.Comparison of these results with the experimentally obtained curvesleads to the conclusion that the antiferromagnetically ordered state inBi2CuO4 has an easy planeanisotropy with the c plane as an easy plane. The estimated value of the critical field for the spin-flop in the easy plane isHc≈15–20 kOe giving the easy plane anisotropy energy constantK22≈27–47 × 103 erg mol−1. The resulting antiferromagnetic structure contains two equally populated mutuallyperpendicular AFM domains in zero magnetic field.

Fermi surface ofα-uranium at ambient pressure

We have performed de Haas--van Alphen measurements of the Fermi surface of $\ensuremath{\alpha}$-uranium single crystals at ambient pressure within the ${\ensuremath{\alpha}}_{3}$ charge-density wave (CDW) state from 0.020--10 K and magnetic fields to 35 T using torque magnetometry. The angular dependence of the resulting frequencies and the effective masses were measured. The observation of quantum oscillations within the ${\ensuremath{\alpha}}_{3}$ CDW state gives insight into the effect of the charge-density waves on the Fermi surface. We observed no signature of superconductivity in either transport or magnetization down to 0.020 K at ambient pressure.