Copiapite-group minerals are among the most common hydrated iron sulfate minerals in a variety of geological environments on Earth. They are also believed to be widespread on the Martian surface. The transformation and stability of the copiapite-group minerals are examined in this study using a diverse array of methods, including low- (LT) and high-temperature (HT) single-crystal X-ray diffraction (SCXRD), LT- and HT-powder X-ray diffraction (PXRD), vacuum powder X-ray diffraction, HT-Raman spectroscopy, magnetization and heat capacity measurements. The research is conducted over a broad temperature range (−175–740 °C) and under vacuum (∼ 600 Pa) conditions that are partially similar to those found on the Martian surface (from −153 °C to over 20 °C and ∼ 600 Pa). The obtained results indicate that aluminocopiapite, (Al0.54Fe3+0.13)Σ0.67Fe3+4(SO4)6(OH)2(H2O)20, is unstable under low vacuum conditions and undergoes a structural transition to a post-aluminocopiapite phase, (Al0.63Fe3+0.04)Σ0.67Fe3+4(SO4)6(OH)2(H2O)12.44, with a significantly lower water and iron content and a higher aluminum content. Schwertmannite, Fe3+16O16(OH)9.6(SO4)3.2·10H2O is formed as a film/shell on the crystal surface of post-aluminocopiapite via a single crystal-to-single crystal (SC-SC) topotactic transformation and exsolution. After 14 days of exposure to air, the post-aluminocopiapite crystal with schwertmannite shell undergoes a reversible process, reverting to its initial aluminocopiapite state. A closely analogous transformation, involving partial dehydration, was observed for copiapite, Fe2+Fe3+4(SO4)6(OH)2(H2O)20, demonstrating that this behavior is a general feature of the copiapite group.It is therefore unlikely that copiapite-group minerals would exist on the surface of Mars and in comparable extraterrestrial environments in their initial form. Instead, under Mars surface conditions with low vacuum, post-copiapites and schwertmannite may be among the most common minerals in hydrated iron sulfate mineral associations. This is relevant for decoding past geo- and climatic environments on Mars and for selecting the conditions for the return of intact samples collected by rovers to Earth.

Hydrated iron sulfate minerals have received considerable attention from the standpoint of environmental science, as well as due to extensive studies on the mineralogy of Mars. In this paper, we report on the thermal evolution of coquimbite AlFe33+SO46H2O12 center dot 6H2O by single-crystal X-ray diffraction (SCXRD) from -173 to 77 degrees C. Powder X-ray diffraction (PXRD) was performed in the temperature range of -180 to 740 degrees C and low vacuum of 600 Pa. Magnetic properties for coquimbite are reported in the range of -271 to 7 degrees C. It was observed that coquimbite is stable between -180 and + 145 degrees C and at a low vacuum of 600 Pa. We observed a gradual transition from coquimbite to the amorphous phase at 150 degrees C, followed by a transition to mikasaite at 225 degrees C, and a second amorphization at 575 degrees C, with afterward crystallization to hematite. SCXRD shows that the behavior of coquimbite with increasing temperature can be divided into two stages, with negative and strongly anisotropic thermal expansion at Stage I (-173 to -143 degrees C) and only positive thermal expansion at Stage II (-133 to 77 degrees C). All the O-H center dot center dot center dot O bonds remain virtually intact during Stage I, except for Ow2-H3 center dot center dot center dot O2. The negative thermal expansion observed along the c-axis in the LT range is a result of the simultaneous reduction of several bond lengths and angular distortions: (1) decrease of Ow2-H3 center dot center dot center dot O2 hydrogen bonds oriented approximately along the c-axis; and (2) shrinkage of M3O3(H2O)3 octahedra, evidenced by the decrease in M3-O3 and M3-Ow3 bonds. The nature of the expansion of the coquimbite structure during Stage II is better understood in terms of the orientation of [M2M32(SO4)6(H2O)6]3- clusters along the c-axis. M-O and S-O bonds are only slightly affected by the temperature rise at Stage II, whereas O-H center dot center dot center dot O angular transformations seem to be the main driving force for the expansion of the coquimbite structure along the alpha 11 direction upon heating.Coquimbite exhibits distinct magnetic properties compared to other iron sulfates, driven by antiferromagnetic interactions within its M3-M2-M3 trimeric clusters of Fe3+. The presence of Al3+-Fe3+ site mixing in coquimbite introduces structural disorder, partially disrupting its magnetic ordering and contributing to magnetic entropy and magnetization features, such as a 1/3 magnetization plateau.

Hydrated iron sulfate minerals have received considerable attention from the standpoint of environmental science, as well as due to extensive studies on the mineralogy of Mars. In this paper, we report on the thermal evolution of coquimbite AlFe33+(SO4)6(H2O)12·6H2O by single-crystal X-ray diffraction (SCXRD) from −173 to 77 °C. Powder X-ray diffraction (PXRD) was performed in the temperature range of −180 to 740 °C and low vacuum of 600 Pa. Magnetic properties for coquimbite are reported in the range of −271 to 7 °C. It was observed that coquimbite is stable between −180 and +145 °C and at a low vacuum of 600 Pa. We observed a gradual transition from coquimbite to the amorphous phase at 150 °C, followed by a transition to mikasaite at 225 °C, and a second amorphization at 575 °C, with afterward crystallization to hematite. SCXRD shows that the behavior of coquimbite with increasing temperature can be divided into two stages, with negative and strongly anisotropic thermal expansion at Stage I (−173 to −143 °C) and only positive thermal expansion at Stage II (−133 to 77 °C). All the O-H···O bonds remain virtually intact during Stage I, except for Ow2-H3···O2. The negative thermal expansion observed along the c-axis in the LT range is a result of the simultaneous reduction of several bond lengths and angular distortions: (1) decrease of Ow2-H3···O2 hydrogen bonds oriented approximately along the c-axis; and (2) shrinkage of M3O3(H2O)3 octahedra, evidenced by the decrease in M3-O3 and M3-Ow3 bonds. The nature of the expansion of the coquimbite structure during Stage II is better understood in terms of the orientation of [M2M32(SO4)6(H2O)6]3− clusters along the c-axis. M-O and S-O bonds are only slightly affected by the temperature rise at Stage II, whereas O-H···O angular transformations seem to be the main driving force for the expansion of the coquimbite structure along the α11 direction upon heating. Coquimbite exhibits distinct magnetic properties compared to other iron sulfates, driven by antiferromagnetic interactions within its M3-M2-M3 trimeric clusters of Fe3+. The presence of Al3+-Fe3+ site mixing in coquimbite introduces structural disorder, partially disrupting its magnetic ordering and contributing to magnetic entropy and magnetization features, such as a 1/3 magnetization plateau.

A number of hydrous iron sulfate minerals have been detected on the surface of Mars under extraterrestrial conditions. Nonetheless, certain inquiries regarding the properties and phase evolution of hydrous iron sulfate minerals remain unresolved and subject to debate at present. In our research, the behavior of römerite, Fe 2+ Fe 3+ 2 (SO 4 ) 4 (H 2 O) 14 , was examined by utilizing in situ single-crystal and powder X-ray diffraction while simultaneously acquiring data upon heating. Römerite is stable under low-vacuum conditions. It exhibits a significant negative thermal expansion in the α 33 direction throughout the entire temperature range from −173 to 77°C and on up to decomposition. There is a cooperative interaction between the rotation of the sulfate tetrahedra in the [Fe 3+ (SO 4 ) 2 (H 2 O) 4 ] − clusters and the features of the hydrogen-bond system that determines the thermal expansion of römerite. The structure of römerite shows that the sulfate tetrahedra are the most rigid complexes, followed by the Fe2 3+ O 2 (H 2 O) 4 octahedra, and the Fe1 2+ (H 2 O) 6 octahedra are the most flexible. High-temperature powder X-ray diffraction, thermogravimetry and differential scanning calorimetry were used to determine the phase transformations and the eventual decomposition of römerite at higher temperatures up to 740°C. The decomposition of römerite at 60°C is followed by an amorphization, a transformation into a mikasaite-like phase at ∼275°C and a further decomposition into a hematite-like phase above 550°C, associated with the high-temperature form of magnetite, Fe 3 O 4 , above 575°C. The magnetic behavior of römerite reveals weak interactions between the Fe 2+ and Fe 3+ centers, in line with the large spatial separation between these ions.

Two new dimorphic spin-1/2 quantum magnets, alpha- and beta-Cu4O2(VO4)Cl, were synthesized via a chemical vapor transport method that emulates mineral formation in volcanic fumaroles. alpha-Cu4O2(VO4)Cl (1) is a pure vanadate analogue of the coparsite mineral characterized by [O2Cu4]4+ 1D single rods, whereas beta-Cu4O2(VO4)Cl (2) adopts a new structure type with the [O2Cu4]4+ 2D layered topology. The thermal expansions of both 1 and 2 studied by high-temperature single-crystal X-ray diffraction are reported. Using ab initio calculations, we infer the presence of antiferromagnetic Cu1-Cu3 units with strong couplings on the order of 200-400 K forming chains (1) and layers (2). The Cu2 atoms are weakly coupled to such units. Magnetic susceptibility measurements corroborate this scenario by showing deviations from the paramagnetic behavior even at 300 K. Moreover, 1 reveals an antiferromagnetic ordering below T N = 24 K with a weak uncompensated magnetic moment.

Manipulation and control of defects triggered by an electron beam allow us to conduct defect engineering on layered materials. We investigate topologically stable helices within a [Dy(10 nm)/Tb(10 nm)](30) multilayer subjected to MeV electron(e)-irradiation up to a maximum fluence of 9.58 x 10(18) e/cm(2). As electrons can go through the sample homogeneously and with high penetration depth, they produce defects without doping. Our e-irradiation results indicate defect induced magnetic manipulation, which increases the blocking/freezing temperature of spin-frustrated interfaces by 4%. This increase implies an increase in the spin-cluster volume. Consequently, the reduced uncompensated pinning centres decrease the interfacial exchange bias coupling by 45%. Direct manipulation of pinning centres would thereby allow us to tailor spintronic devices in a clean way.

A new guanidinium-templated hydrated iron sulfate, [CN3H6][FeIIFeIII(SO4)3(H2O)3] (1), was prepared from strongly acidic aqueous solutions. Its crystal structure is comprised from FeIIIO6 and FeIIO3(H2O)3 octahedra linked by sulfate bridges forming a [FeIIFeII(SO4)3(H2O)3]- 3D framework with a layer-by-layer ordering of ferric and ferrous cations. The structural topology of the framework is related to the anhydrous rhombohedral mikasaite Fe2(SO4)3. The removal of part of the sulfate tetrahedra and the partial replacement of the Fe3+ cations in the [Fe3+2(SO4)3]0 framework by Fe2+ provide a negative charge and allow the incorporation of the protonated organic species in the voids. The compound 1 has been characterized by single-crystal X-ray diffraction, TG and DSC analyses, UV-vis-NIR spectroscopy, magnetic susceptibility, Mössbauer spectroscopy, IR and Raman spectroscopy, and density functional band-structure calculations. The magnetic behavior of 1 shows an interplay of FeII (S = 2) and FeIII (S = 5/2) sublattices that exhibit different types of antiferromagnetic couplings, one FeIII-FeIII (J1 ∼ 6.1 K) and two FeII-FeIII couplings (J2 ∼ 1 K, J3 ∼ 5.9 K) within corrugated honeycomb layers. These ferrimagnetic layers are coupled antiparallel to each other, resulting in an overall antiferromagnetic order below TN = 31 K.

In this work, we demonstrate that cutting diamond crystals with a laser (532 nm wavelength, 0.5 mJ energy, 200 ns pulse duration at 15 kHz) produced a ≲20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface.

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so‐called C or P1 centers, are irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen‐doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered.

We have studied the magnetization of a recently synthesized CuS compound and found two phase transitions around room temperature. The phase transitions in the crystalline structure, characterized by XRD studies, are accompanied by changes also in the electrical resistivity. A hysteretic first-order phase transition has been found between 260 and 320K, from a low-temperature paramagnetic anilite phase to a diamagnetic high-temperature low-digenite phase. A second order phase transition was recognized at similar or equal to 352K from low digenite to a paramagnetic high-digenite structure at high temperatures.

In the last 43 years several hints were reported suggesting the existence of granular superconductivity above room temperature in different graphite-based systems. In this paper, some of the results are reviewed, giving special attention to those obtained in water and n-heptane treated graphite powders, commercial and natural bulk graphite samples with different characteristics as well as transmission electron microscope lamellae. The overall results indicate that superconducting regions exist and are localized at certain internal interfaces of the graphite structure. The existence of the rhombohedral graphite phase in all samples with superconducting-like properties suggests its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical calculations predict. High precision electrical resistance and magnetization measurements were used to identify a transition at \(T_\mathrm{c} \gtrsim 350~\)K. To check for the existence of true zero resistance paths in the samples we used local magnetic measurements, which results support the existence of superconducting regions at such high temperatures.

Measuring with high precision the electrical resistance of highly ordered natural graphite samples from a Brazil mine, we have identified a transition at ∼350 K with ∼40 K transition width. The step-like change in temperature of the resistance, its magnetic irreversibility and time dependence after a field change, consistent with trapped flux and flux creep, and the partial magnetic flux expulsion obtained by magnetization measurements, suggest the existence of granular superconductivity below 350 K. The zero-field virgin state can only be reached again after zero field cooling the sample from above the transition. Paradoxically, the extraordinarily high transition temperature we found for this and several other graphite samples is the reason why this transition remained undetected so far. The existence of well ordered rhombohedral graphite phase in all measured samples has been proved by x-rays diffraction measurements, suggesting its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical studies predicted. The localization of the granular superconductivity at these two dimensional interfaces prevents the observation of a zero resistance state or of a full Meissner state.

We report depth sensitive investigations of the magnetic interaction between exchange-coupled stacked CoO and ferromagnetic Co bilayers (separated by thick Au layers) as we explore the degree of recovery of the untrained state after the first two field cycles. Such a recovery is expected by field cycling a reorientation field (H-RE) along a direction (Omega(RE)) away from the initial field cooling direction. Measurements as a function of Omega(RE) and the strength of H-RE (along each direction) map the influence of Omega(RE) on the reversal mechanism in the layers and thereby the degree of recovery. Our results are consistent with the earlier observations in similar systems that was realized with Omega(RE) = 90 degrees. We ascribe these partial and/or significant recoveries to the unchanged sense of rotation after initial field cooling of the ferromagnetic magnetization upon each field cycling. Furthermore, in our system, we find that this recovery can be regulated by choosing various other H-RE and Omega(RE) values without changing the rotational sense. The best recipe for recovery is identified for Omega(RE) = 45 degrees, that can be achieved partially with H-RE = 3.0 kOe and remain significant even with H-RE = 10.0 kOe. In this study we not only understand the fundamental mechanism in the recovery of training, but also instigate its technological prospects by lifting the directional restrictions of the reorientation field.

Fe layers with thicknesses between 5 and 100 nm were sputtered on mesoporous nanostructured anatase TiO2 templates. The morphology of these hybrid films was probed with grazing-incidence small-angle X-ray scattering and X-ray reflectivity, complemented with magnetic measurements. Three different stages of growth were found, which are characterized by different correlation lengths for each stage. The magnetic behavior correlates with the different growth regimes. At very small thicknesses the TiO2 template is coated and a porous Fe film results, with in-plane and out-of-plane magnetization components. With increasing thickness, agglomeration of Fe occurs and the magnetization gradually turns mostly in plane. At large thicknesses, the iron grows independently of the template and the magnetization is predominantly in plane with a bulk-like characteristic.

The study of spatially confined complex oxides is of wide interest, since correlated electrons at interfaces might form new states of matter. Here La0.7Sr0.3MnO3/SrRuO3 superlattices with coherently grown interfaces and layer thicknesses down to one unit cell were fabricated by pulsed laser deposition. The superlattices were studied by X-ray, HRTEM, magnetization and magnetotransport measurements. For such small thicknesses La0.7Sr0.3MnO3 films are antiferromagnetic and insulating. Despite the small layer thickness, the LaSrMnO layers in the superlattices were ferromagnetic with Curie temperatures close to room temperature. Whereas the resistivity of the superlattices showed metallic behaviour and was dominated by the conducting SrRuO3 layers, the off-diagonal resistivity showed an anomalous Hall effect with ferromagnetic loop shape even far above the Curie temperature of the SrRuO3 layers as well as a positive high field slope. This indicates the presence of a highly conducting, ferromagnetically ordered hole gas at the interfaces that might be formed by a charge-transfer process. This result opens up an alternative route for the fabrication of quasi-two-dimensional systems.

In the last years the number of nominally non-magnetic solids showing magnetic order induced by some kind of defects has increased continuously. From the single element material graphite to several covalently bonded non-magnetic compounds, the influence of defects like vacancies and/or non-magnetic ad-atoms on triggering magnetic order has attracted the interest of experimentalists and theoreticians. We review and discuss the main theoretical approach as well as recently obtained experimental evidence based on different experimental methods that supports the existence of defect-induced magnetism (DIM) in non-magnetic as well as in magnetic materials.

In recent years, the number of nominally nonmagnetic solids showing magnetic order induced by some kind of defect has increased continuously. From the single element material graphite to several covalently bonded nonmagnetic compounds, the influence of defects like vacancies and/or nonmagnetic ad-atoms on triggering magnetic order has attracted the interest of experimentalists and theoreticians. We review and discuss the main theoretical approach as well as recently obtained experimental evidence based on different experimental methods that support the existence of defect-induced magnetism in nonmagnetic as well as in magnetic materials.

Substrate‐attached and freestanding single crystalline Fe70Pd30 ferromagnetic shape memory alloy membranes, which were synthesized by molecular beam epitaxy on MgO (001) and later released from their substrates, are characterized with respect to their structural, thermal and magnetic properties. Residing in the two‐phase region of austenite and the correct martensite phase with face centered tetragonal (fct) structure at room temperature, they reveal martensite transition with little hysteresis at 326 K and 320 K, respectively. Comparing substrate‐attached with freestanding films, which show fundamentally different magnetic fingerprints, it is proposed that domain structure is capable of posing a bias on the austenite → fct‐martensite phase transition by favoring martensite variants with their easy axis aligned along the field – just as the substrate constitutes a mechanical constraint on the transition. If confirmed, this would suggest thermo‐magnetic actuation as an alternative where only moderate magnetic fields are feasible, but moderate temperature changes are possible.