High-resolution Capacitance Dilatometry Research Articles

Strong uniaxial pressure dependencies evidencing spin-lattice coupling and spin fluctuations in Cr2Ge2Te6

Single crystals of Cr$_2$Ge$_2$Te$_6$ were studied by high-resolution capacitance dilatometry to obtain in-plane ($B\parallel ab$) and out-of-plane ($B\parallel c$) thermal expansion and magnetostriction at temperatures between 2 and 300 K and in magnetic fields up to 15 T. The anomalies in both response functions lead to the 'magnetoelastic' phase diagrams and separate the paramagnetic (PM), ferromagnetic low-temperature/low-field (LTF) and aligned ferromagnetic (FM) phases. The presence of two distinct thermal expansion anomalies at small fields $B\parallel ab$ of different magnetic field dependence clearly supports the scenario of an intermediate region separating PM and LTF phases and is indicative of a tricritical point. Simulations of the magnetostriction using the Stoner-Wohlfarth model for uniaxial anisotropy demonstrate that the observed quadratic-in-field behavior in the LTF phase is in line with a rotation of the spins from the preferred $c$ direction into the $ab$ plane. Both the LTF and the PM phase close to T$_{\rm C}$ exhibit very strong pressure dependencies of the magnetization, ${\partial}\ln{M_{\rm ab}}/{\partial}p_{\rm ab}$, of several hundred %/GPa and also the transition from the LTF to the FM phase strongly depends on $p_{\rm ab}$ ($\sim -$280%/GPa), indicating a strong decrease of the uniaxial anisotropy under applied in-plane pressure. Our data clearly demonstrate the relevance of critical fluctuations and magnetoelastic coupling in Cr$_2$Ge$_2$Te$_6$.

Role of magnetoelastic coupling and magnetic anisotropy in MnTiO3

We report the thermodynamic properties studied by thermal expansion, magnetostriction, magnetisation, and specific heat measurements as well as the low-energy magnetic excitations of \mto\ and investigate how magneto-elastic coupling and magnetic anisotropy affect the evolution of long-range order and the magnetic phase diagram. Specifically, we utilise high-resolution capacitance dilatometry and antiferromagnetic resonance (AFMR) studies by means of high-frequency electron spin resonance (HF-ESR) spectroscopy. The role of anisotropy is reflected by spin-reorientation at $B_{\rm{SF}}$~$\simeq 6$~T and a corresponding sign change in $\partial T_{\rm N}/\partial B$. Analysis of the AFMR modes enables us to establish the zero-field excitation gap $\Delta$ as well as its temperature dependence. We derive the effective anisotropy field $B_\mathrm{A} = 0.16(1)$~T which predominately originates from out-of-plane nearest-neighbour dipole-dipole interactions. Despite the nearly fully quenched orbital moment, our data show pronounced thermal expansion and magnetostriction anomalies at $T_{\rm N}$ and $B_{\rm{SF}}$ , respectively, which allows the experimental determination of sizable uniaxial pressure dependencies, i.e., $\partial B_{\rm SF}/\partial p_{\rm c} = -0.20(2)$~T/GPa, $\partial T_{\rm N}/ \partial p_b = 0.69(12)$~K/GPa, and $\partial T_{\rm N}/ \partial p_c = -2.0(4)$~K/GPa. Notably, short-range magnetic order appears up to at least 3$T_{\rm N}$ , as indicated by anisotropic lattice distortion, the violation of a constant Gr\"uneisen behavior, and the presence of local magnetic fields detected by HF-ESR.

Strong effects of uniaxial pressure and short-range correlations in Cr2Ge2Te6

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 and ${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{ab}}$ = $-$15.6 K/GPa) derived from thermodynamic relations. Moderate in-plane strain is thus sufficient to strongly enhance ferromagnetism in Cr$_2$Ge$_2$Te$_6$ up to room temperature. Moreover, unambiguous signs for short-range magnetic order up to 200~K are found.

Linear magnetoelastic coupling and magnetic phase diagrams of the buckled-kagomé antiferromagnet [Formula: see text

Single crystals of Cu_3Bi(SeO_3)_2O_2Cl were investigated using high-resolution capacitance dilatometry in magnetic fields up to 15 T. Pronounced magnetoelastic coupling is found upon evolution of long-range antiferromagnetic order at T_{mathrm {N}} = 26.4(3) K. Grüneisen analysis reveals moderate effects of uniaxial pressure on T_{mathrm {N}}, of 1.8(4) K/GPa, -0.62(15) K/GPa and 0.33(10) K/GPa for p parallel a, b, and c, respectively. Below 22 K Grüneisen scaling fails which implies the presence of competing interactions. The structural phase transition at T_{mathrm {S}} = 120.7(5) K is much more sensitive to uniaxial pressure than T_{mathrm {N}}, with strong effects of up to 27(3) K/GPa (p parallel c). Magnetostriction and magnetization measurements reveal a linear magnetoelastic coupling for Bparallel c below T_{mathrm {N}}, as well as a mixed phase behavior above the tricritical point around 0.4 T. An analysis of the critical behavior in zero-field points to three-dimensional (3D) Ising-like magnetic ordering. In addition, the magnetic phase diagrams for fields along the main crystalline axes are reported.

Magnetic phase diagram, magnetoelastic coupling, and Grüneisen scaling in CoTiO3

High-quality single crystals of ${\mathrm{CoTiO}}_{3}$ are grown and used to elucidate in detail structural and magnetostructural effects by means of high-resolution capacitance dilatometry studies in fields up to 15 T which are complemented by specific heat and magnetization measurements. In addition, we refine the single-crystal structure of the ilmenite ($R\overline{3}$) phase. At the antiferromagnetic ordering temperature ${T}_{\mathrm{N}}$ pronounced $\ensuremath{\lambda}$-shaped anomaly in the thermal expansion coefficients signals shrinking of both the $c$ and $b$ axes, indicating strong magnetoelastic coupling with uniaxial pressure along $c$ yielding six times larger effect on ${T}_{\mathrm{N}}$ than pressure applied in-plane. The hydrostatic pressure dependency derived by means of Gr\"uneisen analysis amounts to $\ensuremath{\partial}{T}_{\mathrm{N}}/\ensuremath{\partial}p\ensuremath{\approx}2.7(4)\phantom{\rule{4pt}{0ex}}\mathrm{K}/\mathrm{GPa}$. The high-field magnetization studies in static and pulsed magnetic fields up to 60 T along with high-field thermal expansion measurements facilitate in constructing the complete anisotropic magnetic phase diagram of ${\mathrm{CoTiO}}_{3}$. While the results confirm the presence of significant magnetodielectric coupling, our data show that magnetism drives the observed structural, dielectric, and magnetic changes both in the short-range ordered regime well above ${T}_{\mathrm{N}}$ as well as in the long-range magnetically ordered phase.

Magnetoelastic coupling and phases in the skyrmion lattice magnetGd2PdSi3discovered by high-resolution dilatometry

We report detailed thermodynamic studies on high-quality single crystals of the centrosymmetric skyrmion-hosting intermetallic Gd2PdSi3 by means of high-resolution capacitance dilatometry in fields up to 15 T which are complemented by specific heat and magnetization studies. Our dilatometric measurements show magnetoelastic effects associated with antiferromagnetic order at TN1 = 22.3 K and TN2 = 19.7 K, as well as strong field effects in an applied magnetic field of 15 Tup to 200 K (150 K) for B||c (B||a*, i.e. B perp c). The data allow us to complete the magneticphase diagram, including a new feature at T* approx 12 K below which a new degree of freedom becomesrelevant. For the first time, the magnetic B vs. T phase diagram for the a*-axis is also reported. Gr\"uneisen analysis shows the onset of magnetic contributions around 60 K, i.e., well above TN1. Uniaxial pressure dependencies of opposite sign, -1.3 K/GPa and 0.3 K/GPa, are extracted for the out-of-plane and in-plane directions at TN1. For T* we obtain dT*/dpc= 1.4 K/GPa. In particular we elucidate thermodynamic properties of the recently discovered skyrmion lattice phase and show that it is strongly enhanced by uniaxial pressure.

Revisiting the phase diagram of LaFe1−xCoxAsO in single crystals by thermodynamic methods

In this work we revisit the phase diagram of Co-doped LaFeAsO using single crystals and thermodynamic methods. From magnetic susceptibility studies we track the doping evolution of the antiferromagnetic phase, revealing a continuous suppression of $T_\mathrm{N}$ up to 5$\%$ Co doping. In order to study the evolution of the so-called nematic phase, the temperature dependence of the lengths changes along the $a$ and $b$ orthorhombic directions, $\Delta L/L_0$, was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion $\delta$ and of $T_\mathrm{S}$ with increasing Co content up to 4.5$\%$, while it is completely suppressed for 7.5$\%$ Co. Bulk superconductivity was found in a small doping region around 6$\%$ Co content, while both $T_\mathrm{c}$ and the superconducting volume fraction rapidly drop in the neighbouring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped BaFe$_2$As$_2$.

Magnetoelastic coupling and Grüneisen scaling in NdB4

We report high-resolution capacitance dilatometry studies on the uniaxial length changes in a NdB$_4$ single crystal. The evolution of magnetically ordered phases below $T_{\rm N}$= 17.2~K (commensurate antiferromagnetic phase, cAFM), $T_{\rm IT}$= 6.8~K (intermediate incommensurate phase, IT), and $T_{\rm LT}$= 4.8~K (low-temperature phase, LT) is associated with pronounced anomalies in the thermal expansion coefficients. The data imply significant magneto-elastic coupling and evidence of a structural phase transition at $T_{\rm LT}$ . While both cAFM and LT favor structural anisotropy $\delta$ between in-plane and out-of-plane length changes, it competes with the IT-type of order, i.e., $\delta$ is suppressed in that phase. Notably, finite anisotropy well above $T_{\rm N}$ indicates short-range correlations which are, however, of neither cAFM, IT, nor LT-type. Gr\"uneisen analysis of the ratio of thermal expansion coefficient and specific heat enables the derivation of uniaxial as well as hydrostatic pressure dependencies. While $\alpha$/$c_{\rm p}$ evidences a single dominant energy scale in LT, our data imply precursory fluctuations of a competing phase in IT and cAFM, respectively. Our results suggest the presence of orbital degrees of freedom competing with cAFM and successive evolution of a magnetically and orbitally ordered ground state.

High magnetic field phase diagram and failure of the magnetic Grüneisen scaling in LiFePO4

We report the magnetic phase diagram of single-crystalline LiFePO$_4$ in magnetic fields up to 58~T and present a detailed study of magneto-elastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at \tn\ in the thermal expansion coefficient $\alpha$ imply pronounced magneto-elastic coupling. Quantitative analysis yields the magnetic Gr\"uneisen parameter $\gamma_{\rm mag}=6.7(5)\cdot 10^{-7}$~mol/J. The positive hydrostatic pressure dependence $dT_{\rm N}/dp = 1.46(11)$~K/GPa is dominated by uniaxial effects along the $a$-axis. Failure of Gr\"uneisen scaling below $\approx 40$~K, i.e., below the peak temperature in the magneto-electric coupling coefficient [\onlinecite{toft2015anomalous}], implies several competing degrees of freedom and indicates relevance of recently observed hybrid excitations~[\onlinecite{yiu2017hybrid}]. A broad and strongly magnetic-field-dependent anomaly in $\alpha$ in this temperature regime highlight the relevance of structure changes. Upon application of magnetic fields $B||b$-axis, a pronounced jump in the magnetisation implies spin-reorientation at $B_{\rm SF} = 32$~T as well as a precursing phase at 29~T and $T=1.5$~K. In a two-sublattice mean-field model, the saturation field $B_{\rm sat,b} = 64(2)$~T enables the determination of the effective antiferromagnetic exchange interaction $J_{\rm af} = 2.68(5)$~meV as well as the anisotropies $D_{\rm b} = -0.53(4)$~meV and $D_{\rm c} = 0.44(8)$~meV.

Anisotropic Thermal Expansion of α-YbAlB4

Thermal expansion measurements were performed for α-YbAlB4 using powder X-ray diffraction and our high resolution capacitance dilatometry. Highly anisotropic thermal expansion, which is much larger along the c-axis compared to the a- and b-axes, was observed. In addition, high resolution capacitance dilatometry revealed negative thermal expansion appears below 90 K only along the a-axis, not along the c-axis. The observation is consistent with anisotropic hybridization stronger in the ab-plane, suggesting the formation of anisotripic heavy fermion well below the valence fluctuation temperature scale of 200 ∼ 300 K. Furthermore, linear thermal expansion coefficient along the a-axis starts to decrease below 115 K. The characteristic temperature scale of 90 - 115 K may correspond to the crystal electric field excited level. The in-plane negative thermal expansion can be regarded as a development of anisotropic hybridization due to |Jz = ±5/2 > crystal electric field ground states which becomes dominant on cooling below the excited states.

Structural Aspects of Metamagnetism inCa2−xSrxRuO4: Evidence for Field Tuning of Orbital Occupation

The crystal structure of Ca(2-x)Sr(x)RuO(4) with 0.2 < or = x < or = 1.0 has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the c direction and elongate in the a and b planes, whereas the opposite occurs upon cooling at high field (x = 0.2 and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.

Crystal and magnetic structure ofLa1−xSr1+xMnO4: Role of the orbital degree of freedom

The crystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_4 (0<x<0.7) has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.

High-resolution thermal expansion of MgB2

The thermal expansion of polycrystalline MgB 2 from 5-300 K is studied using high-resolution capacitance dilatometry. The thermal expansivity exhibits a small jump of - 5.8×10 -8 K -1 at T c (in accord with expectations from the Ehrenfest relationship and published specific heat and pressure data) and a negative peak-like feature close to 5 K. No indications of any structural instabilities are observed.

3D-XY Critical Fluctuations of the Thermal Expansivity in Detwinned YBa2Cu3O7-δ Single Crystals Near Optimal Doping

The strong coupling of superconductivity to the orthorhombic distortion in YBa2Cu3O7-d makes possible an analysis of the superconducting fluctuations without the necessity of subtracting any background. The present high-resolution capacitance dilatometry data unambiguously demonstrate the existence of critical, instead of Gaussian, fluctuations over a wide temperature region (+/- 10 K) around Tc. The values of the amplitude ratio A+/A-=0.9-1.1 and the leading scaling exponent |alpha|<0.018, determined via a least-squares fit of the data, are consistent with the 3D-XY universality class. Small deviations from pure 3D-XY behavior are discussed.

Structure, dynamics, and phase transitions in the fullerene derivatives C60O and C61H2.

The effect of perturbing the icosohedral symmetry of ${\mathrm{C}}_{60}$ by the addition of the side groups -O and -${\mathrm{CH}}_{2}$ upon orientational order-disorder and glass transitions in solid ${\mathrm{C}}_{60}$ has been studied by a combination of high-resolution capacitance dilatometry and single-crystal x-ray and powder inelastic neutron scattering. Both fullerene derivatives ${\mathrm{C}}_{60}$O (epoxide) and ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ (6,5-annulene) are shown to undergo a sequence of transitions similar to that found in pure ${\mathrm{C}}_{60}$, i.e., a first-order orientational ordering transition just below room temperature followed by an orientational glass transition at lower temperatures. Although the exact origin of the glass transition in ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ is unclear, the glass transition in ${\mathrm{C}}_{60}$O has the same origin as that in ${\mathrm{C}}_{60}$, with a significantly higher degree of order due to a larger energy difference between pentagon and hexagon orientations. The dilatometric data at the glass transition indicate that, in contrast to ${\mathrm{C}}_{60}$, the ground-state orientation of both ${\mathrm{C}}_{60}$O and ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ molecules is that with the smallest volume, also demonstrating a significant influence of the side groups upon the details of the structure. A possible explanation of these differences in terms of steric effects is proposed. \textcopyright{} 1996 The American Physical Society.

Structural phase transitions in ‘hcp’ C70 single-crystals

The thermal expansion of vapor-grownC70 single crystals ahs been investigated using high-resolution capacitance dilatometry from 5–380 K. Measurements were made both parallel and perpendicular to the hexagonalc-axis. Three first-order phase transitions which we associate with the consecutive disordering of theC70 molecules are observed upon heating at ≈280 K (long-axis spinning), ≈300 K (long-axis precession) and ≈355 K (quasi-free rotation), respectively. The highest-temperature transition exhibits a very large (≈50 K) thermal hysteresis. Powder and single-crystal X-ray diffraction show that the crystals are predominantly hexagonal-close-packed (HCP) with an idealc/a≈1.63 above 360 K andc/a≈1.84 at 295 K.

Glass transition in single-crystal C60 studied by high-resolution dilatometry.

The thermal expansion of single-crystalline ${\mathrm{C}}_{6}$ is studied using high-resolution capacitance dilatometry. In addition to the orientational-ordering transition at 261 K, we observe a transition at ${\mathit{T}}_{\mathit{g}}$\ensuremath{\approxeq}90 K due to the freezing in of orientational disorder, which has the character of a glass transition. The relaxation near ${\mathit{T}}_{\mathit{g}}$ is studied by varying the heating (cooling) rate over more than two decades. It is found to obey an Arrhenius behavior with ${\mathit{E}}_{\mathit{a}}$=288\ifmmode\pm\else\textpm\fi{}5 meV within a simple glass-transition model.

Anomalous thermal expansion at phase transitions of Ag 4RbI 5

High resolution capacitance dilatometry has been used to verify the first-order nature of the 208 K phase transition in Ag RbI and to measure the volume change at the 122 K first-order phase transition. The latter is in accord with prediction using the Clausius-Clapeyron equation. Problems relating to the chemical stability of the compound are discussed.