Turnbuckle diamond anvil cell for high-pressure measurements in a superconducting quantum interference device magnetometer

Pressure-Induced Structural, Optical and Magnetic Modifications in Lanthanide Single-Molecule Magnets

Lanthanide Single-Molecule Magnets are fascinating objects that break magnetic performance records with observable magnetic bistability at the boiling temperature of liquid nitrogen, paving the way for potential applications in high-density data storage. The switching of lanthanide SMM has been successfully achieved using several external stimuli such as redox reaction, pH titration, light irradiation or solvation/desolvation thanks to the high sensitivity of the magnetic anisotropy to any structural change in the lanthanide surrounding. Nevertheless, the use of applied high pressure as an external stimulus is largely underused, especially considering that it can be combined with high pressure X-ray diffraction to establish a complementary structure-property relationship. This Concept article summarizes the few relevant examples of investigations of lanthanide SMMs under applied high pressure, provides conclusions on the effect of such stimulus on molecular structures and magnetic anisotropy, and finally draws perspective on the future development of magnetic measurements under applied pressure.

Putting the Squeeze on Molecule-Based Magnets: Exploiting Pressure to Develop Magneto-Structural Correlations in Paramagnetic Coordination Compounds

The cornerstone of molecular magnetism is a detailed understanding of the relationship between structure and magnetic behaviour, i.e., the development of magneto-structural correlations. Traditionally, the synthetic chemist approaches this challenge by making multiple compounds that share a similar magnetic core but differ in peripheral ligation. Changes in the ligand framework induce changes in the bond angles and distances around the metal ions, which are manifested in changes to magnetic susceptibility and magnetisation data. This approach requires the synthesis of a series of different ligands and assumes that the chemical/electronic nature of the ligands and their coordination to the metal, the nature and number of counter ions and how they are positioned in the crystal lattice, and the molecular and crystallographic symmetry have no effect on the measured magnetic properties. In short, the assumption is that everything outwith the magnetic core is inconsequential, which is a huge oversimplification. The ideal scenario would be to have the same complex available in multiple structural conformations, and this is something that can be achieved through the application of external hydrostatic pressure, correlating structural changes observed through high-pressure single crystal X-ray crystallography with changes observed in high-pressure magnetometry, in tandem with high-pressure inelastic neutron scattering (INS), high-pressure electron paramagnetic resonance (EPR) spectroscopy, and high-pressure absorption/emission/Raman spectroscopy. In this review, which summarises our work in this area over the last 15 years, we show that the application of pressure to molecule-based magnets can (reversibly) (1) lead to changes in bond angles, distances, and Jahn–Teller orientations; (2) break and form bonds; (3) induce polymerisation/depolymerisation; (4) enforce multiple phase transitions; (5) instigate piezochromism; (6) change the magnitude and sign of pairwise exchange interactions and magnetic anisotropy, and (7) lead to significant increases in magnetic ordering temperatures.

Imaging stress and magnetism at high pressures using a nanoscale quantum sensor

Pressure alters the physical, chemical, and electronic properties of matter. The diamond anvil cell enables tabletop experiments to investigate a diverse landscape of high-pressure phenomena. Here, we introduce and use a nanoscale sensing platform that integrates nitrogen-vacancy (NV) color centers directly into the culet of diamond anvils. We demonstrate the versatility of this platform by performing diffraction-limited imaging of both stress fields and magnetism as a function of pressure and temperature. We quantify all normal and shear stress components and demonstrate vector magnetic field imaging, enabling measurement of the pressure-driven α↔ϵ phase transition in iron and the complex pressure-temperature phase diagram of gadolinium. A complementary NV-sensing modality using noise spectroscopy enables the characterization of phase transitions even in the absence of static magnetic signatures.

The Effect of Pressure on Magnetic Properties of Prussian Blue Analogues

We present the review of pressure effect on the crystal structure and magnetic properties of Cr(CN)6-based Prussian blue analogues (PBs). The lattice volume of the fcc crystal structure space group Fm 3 ¯ m in the Mn-Cr-CN-PBs linearly decreases for p ≤ 1.7 GPa, the change of lattice size levels off at 3.2 GPa, and above 4.2 GPa an amorphous-like structure appears. The crystal structure recovers after removal of pressure as high as 4.5 GPa. The effect of pressure on magnetic properties follows the non-monotonous pressure dependence of the crystal lattice. The amorphous like structure is accompanied with reduction of the Curie temperature (TC) to zero and a corresponding collapse of the ferrimagnetic moment at 10 GPa. The cell volume of Ni-Cr-CN-PBs decreases linearly and is isotropic in the range of 0–3.1 GPa. The Raman spectra can indicate a weak linkage isomerisation induced by pressure. The Curie temperature in Mn2+-CrIII-PBs and Cr2+-CrIII-PBs with dominant antiferromagnetic super-exchange interaction increases with pressure in comparison with decrease of TC in Ni2+-CrIII-PBs and Co2+-CrIII-PBs ferromagnets. TC increases with increasing pressure for ferrimagnetic systems due to the strengthening of magnetic interaction because pressure, which enlarges the monoelectronic overlap integral S and energy gap ∆ between the mixed molecular orbitals. The reduction of bonding angles between magnetic ions connected by the CN group leads to a small decrease of magnetic coupling. Such a reduction can be expected on both compounds with ferromagnetic and ferrimagnetic ordering. In the second case this effect is masked by the increase of coupling caused by the enlarged overlap between magnetic orbitals. In the case of mixed ferro–ferromagnetic systems, pressure affects μ(T) by a different method in Mn2+–N≡C–CrIII subsystem and CrIII–C≡N–Ni2+ subsystem, and as a consequence Tcomp decreases when the pressure is applied. The pressure changes magnetization processes in both systems, but we expect that spontaneous magnetization is not affected in Mn2+-CrIII-PBs, Ni2+-CrIII-PBs, and Co2+-CrIII-PBs. Pressure-induced magnetic hardening is attributed to a change in magneto-crystalline anisotropy induced by pressure. The applied pressure reduces saturated magnetization of Cr2+-CrIII-PBs. The applied pressure p = 0.84 GPa induces high spin–low spin transition of cca 4.5% of high spin Cr2+. The pressure effect on magnetic properties of PBs nano powders and core–shell heterostructures follows tendencies known from bulk parent PBs.

Magnetic and structural changes in LaCo0.9Mn0.1O3 at high pressure

The structural and magnetic properties of LaCo_0.9Mn_0.1O₃ have been studied as a function of pressure by neutron powder diffraction and DC magnetometry. The material is confirmed to exhibit rhombohedral R [Formula: see text] c symmetry between ambient pressure and 6 GPa. We have determined the bulk modulus B ₀ of the sample using a second-order Birch-Murnaghan equation of state which yielded: B ₀  =  140(9) GPa and V [Formula: see text]. We report a non-linear increase of the Curie temperature T _C from an ambient pressure value of 224.7 K to  ∼236 K at a pressure of 4 GPa. Finally, we confirm the glassy-like nature of the magnetism in LaCo_0.9Mn_0.1O₃, which is maintained throughout the pressure range explored.

A novel diamond anvil cell for x-ray diffraction at cryogenic temperatures manufactured by 3D printing

A new miniature high-pressure diamond anvil cell was designed and constructed using 3D micro laser sintering technology. This is the first application of the use of rapid prototyping technology to construct high-pressure apparatus. The cell is specifically designed for use as an X-ray diffraction cell that can be used with commercially available diffractometers and open-flow cryogenic equipment to collect data at low temperature and high pressure. The cell is constructed from stainless steel 316L and is about 9 mm in diameter and 7 mm in height, giving it both small dimensions and low thermal mass, and it will fit into the cooling envelope of a standard Cryostream^TM cooling system. The cell is clamped using a customized miniature buttress thread of diameter 7 mm and pitch of 0.5 mm enabled by 3D micro laser sintering technology; such dimensions are not attainable using conventional machining. The buttress thread was used as it has favourable uniaxial load properties allowing for higher pressure and better anvil alignment. The clamp can support the load of at least 1.5 kN according to finite element analysis (FEA) simulations. FEA simulations were also used to compare the performance of the standard thread and the buttress thread, and demonstrate that stress is distributed more uniformly in the latter. Rapid prototyping of the pressure cell by the laser sintering resulted in a substantially higher tensile yield strength of the 316L stainless steel (675 MPa compared to 220 MPa for the wrought type of the same material), which increased the upper pressure limit of the cell. The cell is capable of reaching pressures of up to 15 GPa with 600 μm diameter culets of diamond anvils. Sample temperature and pressure changes on cooling were assessed using X-ray diffraction on samples of NaCl and HMT-d₁₂.

Pressure induced enhancement of the magnetic ordering temperature in rhenium(IV) monomers

Materials that demonstrate long-range magnetic order are synonymous with information storage and the electronics industry, with the phenomenon commonly associated with metals, metal alloys or metal oxides and sulfides. A lesser known family of magnetically ordered complexes are the monometallic compounds of highly anisotropic d-block transition metals; the ‘transformation' from isolated zero-dimensional molecule to ordered, spin-canted, three-dimensional lattice being the result of through-space interactions arising from the combination of large magnetic anisotropy and spin-delocalization from metal to ligand which induces important intermolecular contacts. Here we report the effect of pressure on two such mononuclear rhenium(IV) compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, with T_c controlled by the strength of the intermolecular interactions. As these are determined by intermolecular distance, ‘squeezing' the molecules closer together generates remarkable enhancements in ordering temperatures, with a linear dependence of T_c with pressure.

Materials that demonstrate long-range magnetic order are synonymous with information storage. Here, the authors report the effect of pressure on two mononuclear rhenium compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, where T_c is proportional to pressure.

Enforcing Multifunctionality: A Pressure-Induced Spin-Crossover Photomagnet

Photomagnetic compounds are usually achieved by assembling preorganized individual molecules into rationally designed molecular architectures via the bottom-up approach. Here we show that a magnetic response to light can also be enforced in a nonphotomagnetic compound by applying mechanical stress. The nonphotomagnetic cyano-bridged Fe(II)-Nb(IV) coordination polymer {[Fe(II)(pyrazole)4]2[Nb(IV)(CN)8]·4H2O}n (FeNb) has been subjected to high-pressure structural, magnetic and photomagnetic studies at low temperature, which revealed a wide spectrum of pressure-related functionalities including the light-induced magnetization. The multifunctionality of FeNb is compared with a simple structural and magnetic pressure response of its analog {[Mn(II)(pyrazole)4]2[Nb(IV)(CN)8]·4H2O}n (MnNb). The FeNb coordination polymer is the first pressure-induced spin-crossover photomagnet.

High-pressure crystallography of periodic and aperiodic crystals

More than five decades have passed since the first single-crystal X-ray diffraction experiments at high pressure were performed. These studies were applied historically to geochemical processes occurring in the Earth and other planets, but high-pressure crystallography has spread across different fields of science including chemistry, physics, biology, materials science and pharmacy. With each passing year, high-pressure studies have become more precise and comprehensive because of the development of instrumentation and software, and the systems investigated have also become more complicated. Starting with crystals of simple minerals and inorganic compounds, the interests of researchers have shifted to complicated metal-organic frameworks, aperiodic crystals and quasicrystals, molecular crystals, and even proteins and viruses. Inspired by contributions to the microsymposium 'High-Pressure Crystallography of Periodic and Aperiodic Crystals' presented at the 23rd IUCr Congress and General Assembly, the authors have tried to summarize certain recent results of single-crystal studies of molecular and aperiodic structures under high pressure. While the selected contributions do not cover the whole spectrum of high-pressure research, they demonstrate the broad diversity of novel and fascinating results and may awaken the reader's interest in this topic.

A compact bellows-driven diamond anvil cell for high-pressure, low-temperature magnetic measurements

We present the design of an efficient bellows-controlled diamond anvil cell that is optimized for use inside the bores of high-field superconducting magnets in helium-3 cryostats, dilution refrigerators, and commercial physical property measurement systems. Design of this non-magnetic pressure cell focuses on in situ pressure tuning and measurement by means of a helium-filled bellows actuator and fiber-coupled ruby fluorescence spectroscopy, respectively. We demonstrate the utility of this pressure cell with ac susceptibility measurements of superconducting, ferromagnetic, and antiferromagnetic phase transitions to pressures exceeding 8 GPa. This cell provides an opportunity to probe charge and magnetic order continuously and with high resolution in the three-dimensional Magnetic Field-Pressure-Temperature parameter space.

Nonlinear pressure dependence of TN in almost multiferroic EuTiO3

The antiferromagnetic (AFM) phase transition temperature TN of EuTiO3 has been studied as a function of pressure p. The data reveal a nonlinear dependence of TN on p with TN increasing with increasing pressure. The exchange interactions exhibit an analogous dependence on p as TN (if the absolute value of the nearest neighbor interaction is considered) and there is evidence that the AFM transition is robust with increasing pressure. The corresponding Weiss temperature ΘW remains anomalous since it always exhibits positive values. The data are analyzed within the Bloch power law model and provide excellent agreement with experiment.

Numerical simulation and experiment on multilayer stagger-split die

A novel ultra-high pressure device, multilayer stagger-split die, has been constructed based on the principle of "dividing dies before cracking." Multilayer stagger-split die includes an encircling ring and multilayer assemblages, and the mating surfaces of the multilayer assemblages are mutually staggered between adjacent layers. In this paper, we investigated the stressing features of this structure through finite element techniques, and the results were compared with those of the belt type die and single split die. The contrast experiments were also carried out to test the bearing pressure performance of multilayer stagger-split die. It is concluded that the stress distributions are reasonable and the materials are utilized effectively for multilayer stagger-split die. And experiments indicate that the multilayer stagger-split die can bear the greatest pressure.

From quantum disorder to magnetic order in an s=1/2 kagome lattice: a structural and magnetic study of herbertsmithite at high pressure

The structural and magnetic properties of deuterated herbertsmithite have been studied by means of neutron powder diffraction and magnetic susceptibility measurements in a wide range of temperatures and pressures. The experimental data demonstrate that a phase transition from the quantum-disordered spin-liquid phase to the long-range ordered antiferromagnetic phase with the Néel temperature T(N)=6  K is induced at P=2.5  GPa. The observed decrease of T(N) upon compression correlates with the anomalies in pressure behavior of Cu-O bond length and Cu-O-Cu bond angles. The reasons for the observed spin-freezing transition are discussed within the framework of the available theoretical models and the recent observation of the field-induced spin freezing.

Magnetic measurements at pressures above 10 GPa in a miniature ceramic anvil cell for a superconducting quantum interference device magnetometer

A miniature ceramic anvil high pressure cell (mCAC) was earlier designed by us for magnetic measurements at pressures up to 7.6 GPa in a commercial superconducting quantum interference magnetometer [N. Tateiwa et al., Rev. Sci. Instrum. 82, 053906 (2011)]. Here, we describe methods to generate pressures above 10 GPa in the mCAC. The efficiency of the pressure generation is sharply improved when the Cu-Be gasket is sufficiently preindented. The maximum pressure for the 0.6 mm culet anvils is 12.6 GPa when the Cu-Be gasket is preindented from the initial thickness of 300-60 μm. The 0.5 mm culet anvils were also tested with a rhenium gasket. The maximum pressure attainable in the mCAC is about 13 GPa. The present cell was used to study YbCu(2)Si(2) which shows a pressure induced transition from the non-magnetic to magnetic phases at 8 GPa. We confirm a ferromagnetic transition from the dc magnetization measurement at high pressure. The mCAC can detect the ferromagnetic ordered state whose spontaneous magnetic moment is smaller than 1 μ(B) per unit cell. The high sensitivity for magnetic measurements in the mCAC may result from the simplicity of cell structure. The present study shows the availability of the mCAC for precise magnetic measurements at pressures above 10 GPa.

Miniature ceramic-anvil high-pressure cell for magnetic measurements in a commercial superconducting quantum interference device magnetometer

A miniature opposed-anvil high-pressure cell has been developed for magnetic measurement in a commercial superconducting quantum interference device magnetometer. Non-magnetic anvils made of composite ceramic material were used to generate high-pressure with a Cu-Be gasket. We have examined anvils with different culet sizes (1.8, 1.6, 1.4, 1.2, 1.0, 0.8, and 0.6 mm). The pressure generated at low temperature was determined by the pressure dependence of the superconducting transition of lead (Pb). The maximum pressure P(max) depends on the culet size of the anvil: the values of P(max) are 2.4 and 7.6 GPa for 1.8 and 0.6 mm culet anvils, respectively. We revealed that the composite ceramic anvil has potential to generate high-pressure above 5 GPa. The background magnetization of the Cu-Be gasket is generally two orders of magnitude smaller than the Ni-Cr-Al gasket for the indenter cell. The present cell can be used not only with ferromagnetic and superconducting materials with large magnetization but also with antiferromagnetic compounds with smaller magnetization. The production cost of the present pressure cell is about one tenth of that of a diamond anvil cell. The anvil alignment mechanism is not necessary in the present pressure cell because of the strong fracture toughness (6.5 MPa m(1∕2)) of the composite ceramic anvil. The simplified pressure cell is easy-to-use for researchers who are not familiar with high-pressure technology. Representative results on the magnetization of superconducting MgB(2) and antiferromagnet CePd(5)Al(2) are reported.