Pressure-induced Magnetic Phase Transition Research Articles

Pressure-induced successive magnetic and structural phase transitions in Fe1.087Te

The crystal and magnetic structures of Fe1.087Te have been studied by neutron powder diffraction in the temperature range from 5 to 170 K at pressures in the range from ≈0.8 to ≈7 GPa. The p, T-phase diagram contains three phases with monoclinic, orthorhombic and tetragonal symmetry. The monoclinic and orthorhombic phases are both antiferromagnetically ordered and stable at temperatures below ≈69 K while the non-magnetic tetragonal phase is stable above this temperature. The monoclinic phase is stable for p ≲ 1.2 GPa while the orthorhombic phase is stable for 1.2 ≲ p ≲ 1.7 GPa and the tetragonal phase becomes stable at higher pressures at the lowest measured temperatures. The magnetic ordering is antiferromagnetic bicollinear and commensurate in the monoclinic phase but incommensurate and pressure dependent in the orthorhombic phase. The pressure-induced collapse of magnetic order at ≈1.7 GPa is accompanied by abrupt changes in volume and compressibility, suggestive of a spin state change of the Fe2+ ions in the FeTe layers.

Pressure-induced magnetic, structural, and electronic phase transitions in LaFeO3: A density functional theory (generalized gradient approximation) + U study

We have investigated the behavior of orthoferrite LaFeO3 at ambient conditions and under pressure using DFT (generalized gradient approximation (GGA)) + U approach. Ground state electronic (band gap) and magnetic properties are considerably improved due to the Hubbard correction. Moreover, the experimentally observed pressure-driven phase transition, namely, the simultaneous occurrence of spin crossover, isostructural volume collapse, and drastic reduction in electrical resistance (electronic phase transition) is nicely described by GGA + U calculations. In particular, despite a sharp drop in resistance, a small band gap still remains in the low spin state indicating an insulator to semiconductor phase transition, in good agreement with the experiments but in contrast to GGA, which predicts metallic behavior in low spin state. We discuss the origin of variation in electronic structure of LaFeO3 in low spin state as obtained from GGA to GGA + U methods. These results emphasize the importance of correlation effects in describing the pressure-driven phase transition in LaFeO3 and other rare-earth orthoferrites.

Pressure-induced polar phases in multiferroic delafossiteCuFeO2

Using a unique experimental setup allowing neutron diffraction measurements at pressures up to 10 GPa, the collaboration of authors from Japan and UK investigate pressure-induced magnetic phase transitions in CuFeO${}_{2}$ -- a triangular lattice antiferromagnet that exhibits multiferroic behavior.

Pressure-induced structural and magnetic phase transitions in ordered and disordered equiatomic FeCo

The magnetic and structural phase diagram of equiatomic FeCo has been studied up to 45 GPa using $K$-edge x-ray magnetic circular dichroism, x-ray absorption near edge spectroscopy, x-ray diffraction, and supporting density-functional-theory-based calculations. FeCo foils with different degrees of chemical order were obtained by magnetron sputtering. Our results show that Fe${}_{0.5}$Co${}_{0.5}$ undergo the bcc ferromagnetic to hcp nonferromagnetic transition in the 30--45 GPa pressure range. Interestingly, the chemical order, i.e., the relative arrangements of Fe and Co atoms, plays a major role in affecting the high-pressure structural and magnetic phase diagram of these alloys. This result is confirmed by first-principles modeling of different structures of equiatomic FeCo alloy. Moreover, the total-energy analysis reveals a strong competition between different magnetic hcp states upon compression. A possible emergence of antiferromagnetism is emphasized and requires further experimental investigation.

First-Principles Study of Pressure-Induced Structural and Magnetic Phase Transitions of Binary Ferromagnets: MnSn and MnSb

First-principles calculations within density functional theory have been performed to study the structural and magnetic phase transitions of two 3d compounds (MnSn and MnSb) under pressure. We consider the rocksalt (B1), cesium chloride (B2), zinc-blende (B3), nickel arsenide (B81) modifications of MnSn and MnSb. Both spin-polarized and nonspin-polarized calculations are carried out for ferromagnetic (FM) and paramagnetic (PM) states, which can be used to distinguish the ground-state magnetic configuration with increasing pressure. The crystal structure preferences and the possible phase transitions among them have been studied. We report FM B81⟶14GPaFM B2⟶145GPaPM B2 phase transition for MnSn, FM B81⟶23GPaPM B2 for MnSb. Phonon band dispersions, density of phonon states, and elastic stiffness constants are given and used to analyze the lattice dynamical and mechanical stability for the pressure-induced phase. Electronic band structures and density of states at different pressures are given to discuss the detailed electronic and magnetic properties.

Emergence of pressure-induced metamagnetic-like state in Mn-doped CdGeAs2 chalcopyrite

The effect of hydrostatic pressure on resistivity and magnetic ac susceptibility has been studied in Mn-doped CdGeAs2 room-temperature (RT) ferromagnetic chalcopyrite with two types of MnAs micro-clusters. The slight increase of temperature by about 30 K in the region between RT and Curie temperature TC causes a significant change in the positions of pressure-induced semiconductor-metal transition and magnetic phase transitions in low pressure area. By conducting measurements of the anomalous Hall resistance in the field H \leq 5 kOe, we present experimental evidence for pressure-induced metamagnetic-like state during the paramagnetic phase at pressure P = 5 GPa.

Pressure-induced magnetic phase transition in Cr2O3 determined by second harmonic generation measurements

We demonstrate that pressure-induced magnetic phase transitions can be detected by optical second harmonic generation (SHG) measurements in diamond anvil cells and show that the transition from an acentric to a centrosymmetric magnetic structure occurs in Cr2O3 at 10(1) GPa. The pressure dependence of the Néel temperature, dTN/dP = −1.0(5) K/GPa from our SHG measurements, which differs significantly from earlier results where dTN/dP ranged from −16 K/GPa to +15 K/GPa.

First-Principles Studies on Magnetic Stability of SrC and BaC in Rocksalt, Zincblende, and Nickel Arsenide Phases Under Pressure

Using the first-principles method based on the density functional theory, we investigated the magnetic stability of sp half-metal ferromagnets SrC and BaC in rocksalt (RS), zincblende (ZB), and nickel arsenide (NA) structures under external pressure. The magnetic moments, total energy of magnetic and nonmagnetic phases, and lattice constants are calculated as a function of the applied pressure. The calculations show the occurrence of pressure-induced magnetic phase transitions which are mainly resulted from the band widening of anion p states. It is also confirmed that for both SrC and BaC, the rocksalt structure is the most stable phase among the three phases.

First-principles studies on magnetic properties of rocksalt structure MC (M=Ca, Sr, and Ba) under pressure

Magnetic properties of sp-electron half-metallic ferromagnets (HMFs) MC (M=Ca, Sr, and Ba) in rocksalt structure at ambient and elevated pressure have been investigated using first-principles within the generalized gradient approximation in the scheme of Perdew et al. [Phys. Rev. Lett. 77, 3865 (1996)]. Magnetic moments, lattice constants, and total energies are calculated as a function of external pressure. The calculations predict the occurrence of pressure-induced magnetic phase transitions and find that the main reason for the magnetic transitions of sp-electron HMFs is the band widening of anion p states.

First principles study of pressure-induced magnetic transition in CrN

First principles calculation was performed using tight-binding LMTO method with local density approximation (LDA) and atomic sphere approximation (ASA) to understand the electronic properties of chromium nitride. The equilibrium geometries, the magnetic moment, the electronic band structure, the total and partial DOS are obtained under various pressures and are analyzed in comparison with the available experimental data. The most stable structure of CrN is NaCl structure in the FM state. A pressure-induced second order magnetic phase transition from ferromagnetic (FM) to non-magnetic (NM) at very high pressure of 0.5549Mbar is predicted. Our results indicate that CrN can be used as a hydrogen storage material.

Volume and structural study of Fe64Mn36 anti-ferromagnetic Invar alloy under high pressure

We have investigated the pressure variation of the volume and structure of an FCC Fe64Mn36 anti-ferromagnetic Invar alloy. The inclination of the pressure–volume (P–V) curve of the FCC structure becomes discontinuous at a pressure of 4GPa. According to the bulk modulus at zero pressure estimated by the Birch–Murnaghan equation of state, the pressure between 4 and 10GPa is 33GPa larger than that at a pressure below 4GPa. Considering previous experiments on magnetism at high pressure the Neel temperature at 4GPa almost decreases to room temperature. These results suggest that the increase in the bulk modulus by 33GPa can be attributed to the pressure-induced magnetic phase transition from anti-ferromagnetism to paramagnetism. Volume at zero pressure was estimated using the Birch–Murnaghan equation of state. The volume of FCC structure in the anti-ferromagnetic state was 1.17% larger than the volume in the paramagnetic state, namely, the spontaneous magnetostriction was 1.17%. Pressure-induced structural transition from FCC to HCP occurs with an increase in the pressure, especially at up to 5GPa. The value of c/a is 1.62; this value almost corresponds to that of an ideal HCP structure. The bulk modulus of the HCP structure estimated by the Birch–Murnaghan equation of state is larger than that of the FCC structure, and the volume/atom ratio is smaller than that of the FCC structure.

Effect of pressure on thermopower and resistivity of EuCo2P2

The measurements of electrical resistivity ρ and thermopower S of the single-crystalline EuCo2P2 have been performed at temperatures from 2 K to 300 K under hydrostatic pressures up to 3 GPa. The temperature dependence of ρ and S show drastic changes at the critical pressure Pc, indicating a large modification of electronic structure around the Fermi level due to a pressure-induced structural and magnetic phase transition. The magnetic phase transition temperature increases linearly with increasing pressure, and shows a sudden increase at the critical pressure Pc, which correspond to the change of magnetic state from the localized Eu(4f) sub-lattice magnetism into the itinerant Co(3d) sub-lattice magnetism.

Development of an energy-domain57Fe-Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh-pressure studies with a diamond anvil cell

An energy-domain (57)Fe-Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh-pressure measurements. The main optical system consists of a single-line pure nuclear Bragg reflection from an oscillating (57)FeBO(3) single crystal near the Néel temperature and an X-ray focusing device. The developed spectrometer can filter the Doppler-shifted single-line (57)Fe-Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X-rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR (57)Fe-Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure-induced magnetic phase transition of polycrystalline (57)Fe(3)BO(6) and an unknown high-pressure phase of Gd(57)Fe(2) alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earth's core.

Pressure-induced structural, electronic, and magnetic phase transitions inFeCl2studied by x-ray diffraction and resistivity measurements

High-pressure (HP) synchrotron x-ray diffraction (XRD) studies were carried out in ${\text{FeCl}}_{2}$ $({T}_{N}\ensuremath{\approx}24\text{ }\text{K})$ together with resistivity $(R)$ studies at various temperatures and pressures to 65 GPa using diamond-anvil cells. This work follows a previous HP $^{57}\text{F}\text{e}$ M\"ossbauer study in which two pressure-induced (PI) electronic transitions were found interpreted as: (i) quenching of the orbital-term contribution to the hyperfine field concurring with a tilting of the magnetic moment by $55\ifmmode^\circ\else\textdegree\fi{}$, and (ii) collapse of the magnetism concurring with a sharp decrease in the isomer shift. The $R(P,T)$ studies affirm that the cause of the collapse of the magnetism is a PI $p\text{\ensuremath{-}}d$ correlation breakdown, leading to an insulator-metal transition at $\ensuremath{\sim}45\text{ }\text{GPa}$ and is not due to a spin crossover $(S=2\ensuremath{\rightarrow}S=0)$. The structure response to the pressure evolution of the two electronic phase transitions starting at low pressures (LP), through an intermediate phase (IP) 30--57 GPa, and culminating in a high-pressure phase, $P>32\text{ }\text{GPa}$, can clearly be quantified. The IP-HP phases coexist through the 32--57 GPa range in which the HP abundance increases monotonically at the expense of the IP phase. At the LP-IP interface no volume change is detected, yet the $c$ axis increases and the $a$ axis shrinks by 0.21 and $0.13\text{ }\text{\AA{}}$, respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus ${K}_{0}=35.3(1.8)\text{ }\text{GPa}$. The intralayer Cl-Cl distances increase but no change is observed in Fe-Cl bond length nor are there substantial changes in the interlayer spacing. The pressure-induced electronic IP-HP transition leads to a first-order structural phase transition characterized by a decrease in Fe-Cl bond length and an abrupt drop in $V(P)$ by $\ensuremath{\sim}3.5%$ accompanying the correlation breakdown. In this transition no symmetry change is detected and the XRD data could be satisfactorily fitted with the ${\text{CdI}}_{2}$ structure. The bulk modulus of the HP phase is practically the same as that of the LP-IP phases suggesting negligible changes in the phonon density of state.

Erratum: “Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3”

Erratum: “Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl₃”

Pressure-Induced Magnetic Phase Transitions in Yb-Based Heavy-Fermion Compounds

We report a detailed study of the pressure–temperature ( P – T ) phase diagrams of two Yb-based heavy-fermion compounds, namely, the antiferromagnet YbAgGe and weak ferromagnet YbRhSb. YbAgGe with a quasikagome lattice of Yb ions undergoes two magnetic phase transitions at T M1 = 0.8 K (second order) and T M2 =0.65 K (first order). With increasing pressure ( P ) up to 0.5 GPa, the transitions at T M1 and T M2 merge into one transition. For 0.5 < P < 1.6 GPa, T M2 remains constant and abruptly rises above 1.6 GPa. This rise occurs with a significant change in the specific heat anomaly from a small cusp at T M2 into a large peak at T M3 . This finding suggests that the suppressed magnetic entropy is released due to geometrical frustration at ambient pressure. On the other hand, YbRhSb undergoes a transition into an unusual magnetic state at T M1 = 2.7 K, which is associated with a small spontaneous moment of 3 ×10 -3 µ B /Yb. Measurements of the resistivity, specific heat, and ac magnetic susceptibility χ ac reveal that the application of pressure above 0.9 GPa induces another magnetic transition at T M2 below T M1 . T M2 ( P ) exhibits a V-shaped minimum of 1.8 K at P C = 1.6 GPa. With increasing pressure up to 2 GPa, the significant enhancement of the peak height of χ ac suggests that the canted antiferromagnetic state below P C changes to a ferromagnetic state with a large magnetic moment above P C . These complex P – T phase diagrams for YbAgGe and YbRhSb are explained in terms of the competition among distinct types of magnetic interactions, magnetic frustration, and crystal field anisotropy.

Pressure-Induced Magnetic Quantum Phase Transition in KCuCl3

Magnetization and neutron elastic scattering measurements under hydrostatic pressure were performed on KCuCl 3 , which is a three-dimensionally coupled spin dimer system with a gapped ground state. The spin gap decreases with increasing pressure. A quantum phase transition from a gapped singlet state to an antiferromagnetic state occurs at P c ≃8.2 kbar. For P > P c , the Neel temperature and the ordered moment increase with pressure. It was found that this quantum phase transition is caused by decrease in an intradimer interaction and increase in the sum of interdimer interactions with increasing pressure.

Isotropic magnetoelastic interaction and the pressure-induced first-order magnetic phase transition in CoS2

The change in the order of the magnetic phase transition in ferromagnetic CoS2 under pressure recently revealed by Goto et al. [Phys. Rev. B56, 14019 (1997)] is explained in terms of an exchange-magnetostriction model that takes into account the magnetoelastic interactions (MEI) of the first and second order with respect to volume deformations. Using numerical values of MEI constants determined from experimental data, the equilibrium and nonequilibrium magnetic phase diagrams have been calculated for CoS2 in the space of variables T (temperature), P (pressure), and H (magnetic field). The temperature dependence of magnetization at various pressures and the magnetization isotherms have been calculated. The results of the calculations show satisfactory agreement with experiment. Analytical expressions for the critical magnetic field and temperature of the first-order magnetic phase transition (FOMPT) and for the FOMPT curve in a magnetic field at a constant pressure have been obtained. The effect of an applied magnetic field on the characteristics of the FOMPT are analyzed.

Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3

Magnetization and neutron elastic scattering measurements under a hydrostatic pressure were performed on KCuCl3, which is a three-dimensionally coupled spin dimer system with a gapped ground state. It was found that an intradimer interaction decreases with increasing pressure, while the sum of interdimer interactions increases. This leads to the shrinkage of spin gap. A quantum phase transition from a gapped state to an antiferromagnetic state occurs at Pc ? 8.2 kbar. For P > P c, magnetic Bragg reflections were observed at reciprocal lattice points equivalent to those for the lowest magnetic excitation at zero pressure. This confirms that the spin gap decreases and closes under applied pressure.

Magnetic behaviour of the two-dimensional Heisenberg ferromagnetCs2CuF4 under high pressure: a sensitive magnetic measurement in a diamond-anvil cell to26 GPa

The isomorphous compounds A2CuF4 (A = K,Rb, Cs) are two-dimensional Heisenberg ferromagnets with the Curie temperatures of 6.25, 6.05 and4.55 K, respectively. Previous studies showed that pressure-induced magnetic phase transitions occurin K2CuF4 and Rb2CuF4 at around 10 GPa, above which the ferromagnetic behaviour disappears as a resultof the structural change from antiferrodistortive to ferrodistortive order of theCuF6 octahedra in thebasal plane. For Cs2CuF4 it was revealed that the interlayer exchange interaction changes fromferromagnetic to antiferromagnetic at a certain pressure below 2.2 GPa. Motivatedby this unexpected result, we have carried out magnetic measurements onCs2CuF4 in a diamond-anvil cell at various pressures up to 26 GPa by using asuperconducting quantum interference device vibrating coil magnetometer.It was found that the Néel temperature increases with pressure at a rate of0.3 K GPa−1 up to 17 GPa, reaches a maximum of about 9 K at around 17 GPa and thendecreases rapidly to zero. At pressures above 21 GPa the magnetic susceptibilitydecreases to the background level, strongly suggesting that a structuralchange similar to those observed for the other two compounds occurs forCs2CuF4 at around this pressure.