Verwey Transition Research Articles

Studies of electrical and magnetic properties across the Verwey transition in epitaxial magnetite thin films

The magnetic and transport properties of magnetite (Fe3O4) films grown on isostructural substrates (MgAl2O4, MgGa2O4, and CoGa2O4), with varying degrees of lattice mismatches (3.8%, −1.4%, and −0.8%, respectively), have been investigated. A significant reduction in the density of antiphase boundary defects is observed for the Fe3O4 films grown under optimal process conditions on smaller lattice mismatch substrates (MgGa2O4 and CoGa2O4) as compared to MgAl2O4. Correspondingly, films on these substrates show much improved magnetic properties and sharper transition in the resistance values at the Verwey transition. Room temperature magnetic hysteresis and ferromagnetic resonance measurement studies indicate the presence of uniaxial magnetic anisotropy induced by substrate-induced strain in the film. Temperature-dependent transport measurements confirm that film thickness has a larger effect on the sharpness of the Verwey transition temperature than the degree of lattice mismatch with substrates.

Magnetic hysteresis of 0.6–110 μm magnetites across the Verwey transition

We report saturation magnetization, Ms, saturation remanence, Mrs, coercive force, Hc, and remanence coercivity, Hcr, as a function of grain size, d, and temperature, T, for 0.6–135 μm magnetites. Five annealed and four unannealed samples were measured at 5–10 K intervals from 300 to 20 K. Mrs and Hc increase by factors of 1.5–4 in cooling through the Verwey transition (TV ≈ 120 K) and by smaller amounts around 50 K. Hysteresis properties change continuously over ≈20 K below TV or for annealed 0.6, 3, and 6 μm grains, within ≈10 K below TV. Hc(d) changes for annealed magnetites from ∼d–0.5 at 300 K to ∼d–0.6–d–0.7 at 120–130 K to ∼d–0.3 at 80–100 K. Day plots of Mrs(T)/Ms(T) versus Hcr(T)/Hc(T) indicate major domain structure changes with T, e.g., 6 μm grains change from large pseudo-single-domain (PSD) at 300 K to multidomain (MD) just above TV and return to PSD below TV, evolving to higher Mrs and Hc down to 20 K. Hysteresis loops change from normal at 300 K to slightly constricted near TV to severely constricted below 50 K. We interpret these results in the light of electron microscopic observations by Kasama et al. (2010 , 2012) . Hardening of magnetic hysteresis below TV and the evolution from MD to PSD, and even to single-domain in the finest grains, results from subdivision of grains by monoclinic twinning, reduced magnetic domain sizes in monoclinic magnetite, and confinement of magnetic domains within twin domains. Constricted hysteresis loops indicate coexisting magnetically hard and soft phases, initially growing monoclinic regions and residual cubic magnetite.

Selective growth of α-Fe2O3, γ-Fe2O3 and Fe3O4 at low temperatures and under ambient pressure

We fabricated thin films of α- and γ-Fe2O3 and Fe3O4 at 400 °C under ambient (non-vacuum) conditions by using a mist chemical vapor deposition (mist CVD) method and investigated their structural and physical properties. We found that the structural phases of the films with preferred orientations depend on surface structures of substrate crystals and that the valence states of Fe in the films depend on solvents in which the Fe precursor (Iron (III) acetylacetonate) was dissolved. When films are grown by atomizing aqueous solutions of the precursor, (001) α-Fe2O3 and (001) γ- Fe2O3, both of which have only Fe3+, are grown on (001) Al2O3 and (001) SrTiO3 substrates, respectively. On the other hand, when methanol-rich solutions (98% methanol and 2% water in volume) are atomized, (111) and (001)-oriented Fe3O4 films which accommodate Fe3+ and Fe2+ were grown on the Al2O3 and SrTiO3 substrates, respectively. Magnetization measurements show that the γ-Fe2O3 and Fe3O4 films have room temperature saturated magnetizations as large as 200 and 500 emu/cc. Furthermore, the Fe3O4 films exhibit drastic decreases in the magnetization at 110 K, which are attributed to the Verwey transition. These results indicate that mist CVD is a nice technique that can enable one to control both crystal structures and Fe valence states in iron oxide films.

Preparation and characterization of amorphous Cr2O3 nanoparticles obtained by solution plasma discharge

Amorphous chromium (III) oxide (Cr2O3) nanoparticles were synthesized by a solution plasma discharge procedure in CrCl3 aqueous solution. X–ray diffraction, Fourier transform infrared spectroscopy, and thermal analysis were conducted to analyze the nanoparticles. Nanoscale Cr2O3 with good magnetic properties can be employed in information storage and spintronics. Therefore, the magnetic properties of the product were measured to assess their potential application. The results showed that the obtained Cr2O3 product was amorphous with a sheet-like morphology, in which small nanoparticles were attached to each other and wrapped by the water layer. Transmission electron microscopy results indicated that the covered particles produced in the CrCl3 solution were spherical with a mean diameter of 50 nm. The sheet-like morphology was converted into thinner slices when discharge electrolysis was conducted in the solution containing H2SO4. The amorphous phase could be transferred to nanocrystalline Cr2O3 with an average grain size of 100 nm after calcination at 600 °C. The Cr2O3 particles obtained in the H2SO4 solution became more uniform after heat treatment. The results of the magnetic property measurement showed that the Verwey transition and blocking temperatures of the produced Cr2O3 nanoparticles were 16 K and 66 K, respectively, and the separation of the zero-field-cooled curve and field-cooled curve occurred at 300 K. The magnetic properties of the nanoparticles were shown to be irreversible due to hysteresis. The formation process of amorphous Cr2O3 was proposed as 2Cr + 3H2O → Cr2O3 + 3H2 and 4Cr3+ + 6OH− + 6e– → 2Cr2O3 + 3H2.

Variation in Magnetic Fabrics at Low Shock Pressure Due to Experimental Impact Cratering

AbstractMagnetic fabrics provide important clues for understanding impact cratering processes. However, only a few magnetic fabric studies for experimentally shocked material have been reported so far. In the framework of MEMIN (Multidisciplinary Experimental and Modeling Impact Research Network), we conducted two impact experiments on blocks of Maggia gneiss with the foliation oriented perpendicular (A38) and parallel (A37) to the target surface. Maggia gneiss has plenty of biotite bands forming a strong rock foliation. The bulk magnetic susceptibility varies from 0.376 × 10−3 to 1.298 × 10−3 SI in unshocked and from 0.443 × 10−3 to 3.940 × 10−3 SI in shocked gneiss. The thermomagnetic curves reveal a Verwey transition at −147 °C and a Curie temperature between 576 and 579 °C in unshocked and shocked samples, indicating nearly pure magnetite, which carries the magnetic fabrics. In A37 and A38 kinking is prominent from the point source down to a depth of 2 and 4.2 dp (projectile diameter) or 1 and 2.1 cm, respectively. Kinking, folding, and fracturing changed the position of magnetite grains with respect to each other to reorient the magnetic fabrics. Reorientation of magnetic fabrics is conspicuous down to 20 dp (10 cm) in A38, where no other impact‐related deformation is visible. The reorientation of magnetic fabrics may, therefore, aid in identifying impact processes at very low pressures, starting at 0.1 GPa, when other common indicators are absent.

Three-Dimensional Nanoconfinement Supports Verwey Transition in Fe3O4 Nanowire at 10 nm Length Scale.

Herein, we construct three-dimensional (3D) Fe3O4 epitaxial nanowires at a 10 nm length scale on a 3D MgO nanotemplate using an original nanofabrication technique that mainly comprises nanoimprint lithography and inclined thin-film deposition. Despite the high density of inevitable nanoscale defects, the ultrasmall Fe3O4 nanowires exhibit a prominent Verwey transition at about 112 K with a maximum relative change in resistance of 9.5, which is 6 times larger than that of the thin-film configuration. Numerous measurements on a large number of Fe3O4 nanowires grown concurrently on the same 3D MgO nanotemplate reveal a dramatic difference in their electrical transport property with the presence/absence of the Verwey transition. A comparative study of Fe3O4 wires of increasing volume and a thin film reveals that a profound change in the Verwey transition is observed only for wires with a volume on the order of 10 nm3. Moreover, a significant decrease in the sharpness of the resistance jump and the transition temperature of the Verwey response is noticed with an increasing volume of Fe3O4. This indicates the potency of the 3D nanofabrication technique in controlling nanoscale defects, which is further reconfirmed through magnetoresistance measurement. A feature of the magnetoresistance curve identifies the antiphase boundaries as a major source of defects. The occurrence of the smallest magnetoresistance in the ultrasmall nanowire with the highest Verwey transition temperature and resistance change ratio proves that 3D isotropic spatial confinement into a length scale comparable to the average spacing between two antiphase boundaries enables the favorable control over nanoscale defects. A simple statistical model satisfactorily illustrates the dependence of electrical transport properties on the volume of Fe3O4 from the macroscale down to the nanoscale. Finally, an ultrasmall nanowire with a low defect concentration allows the estimation of the true coherence length of the fundamental quasiparticle, the trimeron, responsible for the Verwey transition.

Co-emergence of magnetic order and structural fluctuations in magnetite

The nature of the Verwey transition occurring at TV ≈ 125 K in magnetite (Fe3O4) has been an outstanding problem over many decades. A complex low temperature electronic order was recently discovered and associated structural fluctuations persisting above TV are widely reported, but the origin of the underlying correlations and hence of the Verwey transition remains unclear. Here we show that local structural fluctuations in magnetite emerge below the Curie transition at TC ≈ 850 K, through X-ray pair distribution function analysis. Around 80% of the low temperature correlations emerge in proportion to magnetization below TC. This confirms that fluctuations in Fe-Fe bonding arising from magnetic order are the primary electronic instability and hence the origin of the Verwey transition. Such hidden instabilities may be important to other spin-polarised conductors and orbitally degenerate materials.

The influence of strain on the Verwey transition as a function of dopant concentration: towards a geobarometer for magnetite-bearing rocks

SUMMARY Magnetite is an abundant magnetic mineral that commonly records the ancient magnetic field in a wide variety of rock types. When cooled below ≈124 K, magnetite undergoes a phase transition, called the Verwey transition, whose characteristics are highly sensitive to grain size and stoichiometry. Studying the Verwey transition thus yields information on the formation conditions and compositions of rocks. The transition is also stress sensitive, thereby opening an avenue to understanding a rock’s strain history; however, the reason for the stress sensitivity is poorly understood. In particular, the temperature of the transition decreases when measured under pressure, yet mostly increases upon pressure release. Moreover, the stress sensitivity of the transition as a function of dopant concentration, especially after pressure cycling, was never systematically tested. We addressed these issues in order to further develop magnetite as a pressure gauge. Multidomain magnetite samples were pressure cycled up to maximum pressures of ∼5 GPa at room temperature to measure the influence of strain on the Verwey transition temperature as a function of dopant concentration after full decompression. The transition temperature measured via changes in magnetic remanence ($T_{\rm V}^{M}$) systematically increased with respect to pressure (P) in more doped samples, where domain wall pinning from impurities dominates $\mathrm{d}T_{\rm V} ^{\rm M}/\mathrm{d}P$. In less doped samples, no to only moderate pressure cycling dependence on $T_{\rm V}^{\rm M}$ was observed. Bulk coercive force (Bc) and magnetic remanence after saturation (Mrs) measured above or below the transition also increased with respect to pressure, but here effects related to permanent strain of the lattice structure prevail, and Bc versus P is steeper for less doped samples. Bc versus P increases in all cases, with a difference in slope dictated by dopant concentrations segregating the first to second-order nature of the transition. Thus, strain developed during pressure cycling controls $T_{\rm V}^{\rm M}$ and coercivity by a mechanism based on pinning of magnetic domains by both interstitial cations and structural lattice distortions. The combined observables, $T_{\rm V}^{\rm M}$ and Bc−Mrs, reflect both the dopant level and strain state of magnetite, which can quantify the pressure multidomain magnetite has experienced, especially in the range between 1 and 5 GPa. Based on these new results, we present a model that distinguishes between electronic versus defect-driven processes explaining the strain-related influences on the transition. Magnetite’s use as a geobarometer is thus a measure of its defect state, which is expressed through two somewhat independent mechanisms when sensed by magnetic observations.

Magnetic Properties and Electronic Conductivity of Fe3O4 Magnetite Nanowires

We have separately assessed the contributions of isolated Fe3+ ions and the ferrimagnetic subsystem to the total magnetization of an aligned magnetite (Fe3O4) nanowire array grown in pores of an anodized alumina membrane and evaluated the magnetic anisotropy field of the nanowires, which has been found to be an order of magnitude weaker than the expected shape anisotropy field. The reduction in magnetic anisotropy in the nanowires can be accounted for by dipole–dipole interaction between individual nanowires in the array. In electron spin resonance spectra of the nanowires, we have identified a phase-inverted line, corresponding to their microwave magnetoresistance. The Verwey transition in the magnetite nanowires has been shown to be suppressed due to deviations from stoichiometry and size effects.

Magnetite and the Verwey transition, from γ-rays to low-energy electrons

Magnetite, a semiconducting ferrimagnetic iron spinel with a metal-insulator phase transition, the Verwey transition, has long been the subject of Mossbauer spectroscopy studies, which continue today. We review the current status of the understanding of the Mossbauer spectra of magnetite. Furthermore, magnetite is a very attractive material in current topics such as spintronics. In this particular subject, to determine the behavior of magnetic domains is paramount, and the changes ocurring on the near surface region upon undergoing the Verwey transition are relevant. In order to advance in this area, we have incorporated some new techniques, namely microscopy observations made with low-energy electrons. These observations can be performed upon changing the temperature, and can provide magnetic contrast through the use of spin-polarized electrons. By this means, we have observed the ferroelastic transformation associated with the Verwey transition, discovered an order-disorder transition of the (001) surface of magnetite and observed the changes in the magnetic domains on the same surface by changing the temperature. Low-energy electrons also are the key to the Mossbauer experiments of magnetite films and surfaces, with the promise of providing surface-sensitive spatially resolved Mossbauer spectra.

XRD, internal field-NMR and Mössbauer spectroscopy study of composition, structure and magnetic properties of iron oxide phases in iron ores

We report the phase-composition, structure and magnetic properties of two representative samples of naturally available iron-oxide containing ores/soils collected from two different regions in Karnataka, India. Presence of elements such as Fe, Si, Al and O were identified in both the samples using energy dispersive analysis of X-rays (EDAX). X-ray diffraction results confirmed that different ceramic phases such as crystalline iron oxide phases (Fe3O4, γ-Fe2O3 and α-Fe2O3), aluminosilicates (Al2SiO5) and low-quartz (SiO2) phases constitute the soil. The presence of ferrimagnetic phases (Fe3O4, γ-Fe2O3) makes the soil respond to a permanent magnet. Separation between the magnetic and non-magnetic phases was performed using a permanent magnet. The non-magnetic part of the sample contains a high amount of α-Fe2O3 phase along with aluminosilicates and low-quartz. The magnetic phases were further characterized and quantified. The presence of Fe3O4 phase in the samples was confirmed from the Verwey transition observed by internal-field NMR spectroscopy. Our results demonstrate that internal-field NMR and Mössbauer spectroscopy are complementary tools for characterizing iron containing soils. Furthermore, we found that the soil collected from the low temperature region (Sandur) contains more amounts of ferrimagnetic oxide (Fe3O4, γ-Fe2O3) phases, whereas the high temperature region (Hospete) contains more α-Fe2O3 phase. Hence, our results confirm that the phase composition of the soil is intimately related to the local daily temperature.

Large magnetoresistance effects in Fe3O4

We investigated the magnetoresistance (MR) of a single crystal of magnetite, Fe3O4. In an effort to distinguish between different contributions to the MR the samples were prepared in two different initial magnetic states, i.e. by either zero-field or by field cooling from room temperature. The different magnetic structures in this sample have a dramatic effect on the magnetoresistance: for initially zero-field-cooled conditions a negative MR of about −20% is observed just below the Verwey transition at K. For decreasing temperature the MR increases, changes sign at ∼78 K and reaches a record positive value of ∼45% at around 50 K. This behavior is completely absent in the field-cooled sample. Magnetization measurements corroborate an alignment of the easy magnetization direction in applied magnetic fields below as a cause of the strong effects observed in both, magnetization and MR. Our results point to a complex interplay of structural and magnetocrystalline effects taking place upon cooling Fe3O4 through .

A Short Review on Verwey Transition in Nanostructured Fe3O4 Materials

Verwey transition (VT) of Fe3O4 has been extensively investigated as this results in sharp changes in its physical properties. Exploitation of VT for potential applications in spin/charge transport, multiferroicity, exchange bias, and spin Seebeck effect-based devices has attracted researchers recently. Although hundreds of reports have been published, the origin of VT is still debatable. Besides, not only the size effects have a significant impact on VT in Fe3O4, even the conditions of synthesis of Fe3O4 nanostructures mostly affect the changes in VT. Here, we review not only the effects of scaling but also the growth conditions of the Fe3O4 nanostructures on the VT and their novel applications in spintronics and nanotechnology.

Gold-decorated magnetic nanoparticles design for hyperthermia applications and as a potential platform for their surface-functionalization

The integration of noble metal and magnetic nanoparticles with controlled structures that can couple various specific effects to the different nanocomposite in multifunctional nanosystems have been found interesting in the field of medicine. In this work, we show synthesis route to prepare small Au nanoparticles of sizes <d> = 3.9 ± 0.2 nm attached to Fe3O4 nanoparticle cores (<d> = 49.2 ± 3.5 nm) in aqueous medium for potential application as a nano-heater. Remarkably, the resulted Au decorated PEI-Fe3O4 (Au@PEI-Fe3O4) nanoparticles are able to retain bulk magnetic moment MS = 82–84 Am2/kgFe3O4, with the Verwey transition observed at TV = 98 K. In addition, the in vitro cytotoxicity analysis of the nanosystem microglial BV2 cells showed high viability (>97.5%) to concentrate up to 100 µg/mL in comparison to the control samples. In vitro heating experiments on microglial BV2 cells under an ac magnetic field (H0 = 23.87 kA/m; f = 571 kHz) yielded specific power absorption (SPA) values of SPA = 43 ± 3 and 49 ± 1 μW/cell for PEI-Fe3O4 and Au@PEI-Fe3O4 NPs, respectively. These similar intracellular SPA values imply that functionalization of the magnetic particles with Au did not change the heating efficiency, providing at the same time a more flexible platform for multifunctional functionalization.

Verwey transition temperature distribution in magnetic nanocomposites containing polydisperse magnetite nanoparticles

Polymeric nanocomposites containing Fe3O4 nanoparticles were prepared through a chemical route under different precursor-to-solvent ratios and were submitted to structural and morphologic characterization. The embedded nanoparticles, containing pure magnetite and characterized by considerable polydispersity, are rather homogeneously dispersed in the matrix. The magnetic properties of two representative samples were analyzed in detail between T = 5 K and room temperature. Magnetic effects clearly associated with the Verwey monoclinic to cubic transition with transition temperatures distributed in the interval 95–120 K were put in evidence. On heating through this region, the coercive field and the maximum susceptibility of hysteresis loops display marked downward/upward steps, respectively, while the high-field magnetization is not affected at all; a comparable upward step is measured in the FC/ZFC curves. Reporting the maximum susceptibility as a function of the reciprocal of the coercive field in the interval from T = 95 to T = 120 K, and using the predictions for single-domain nanoparticles with randomly distributed axes of uniaxial and cubic anisotropy (the former/latter case being applicable below/above the Verwey transition, respectively), the evolution of the transformed cubic-anisotropy fraction upon heating has been studied, and the distribution of Verwey transition temperatures related to the sample polydispersity has been accurately determined. The low-temperature value of the uniaxial anisotropy constant is obtained from coercive field measurements and found to be comparable to, albeit slightly higher than the corresponding quantity measured in bulk crystalline magnetite.

Magnetically tunable iron oxide nanotubes for multifunctional biomedical applications

Design of a multifunctional magnetic bionanosystem has become increasingly important towards advancing the future of clinical medicine. While hollow iron oxide nanoparticles with enhanced surface areas allow for more drug molecules to be attached to the particles, their relatively low saturation magnetization (MS) hinders their practicality in medicinal applications such as drug delivery and hyperthermia therapy. We demonstrate that this limitation can be overcome by utilizing 1D magnetic nanotubes that possess both enhanced surface areas and high MS. In this study, highly crystalline, tunable aspect ratio Fe3O4 nanotubes have been successfully synthesized using a hydrothermal method. Magnetic measurements showed a clear Verwey transition (∼120 K) and high MS (∼75 emu/g) at 300 K, confirming the high quality of the synthesized Fe3O4 nanotubes. Calorimetric experiments on randomly dispersed Fe3O4 nanotubes in water with concentration of 1 mg/mL showed a large Specific Absorption Rate (SAR) value of 400 W/g for an AC magnetic field of 800 Oe, which increased to 500 W/g when the nanotubes were aligned parallel to the DC magnetic field and suspended in a 2% agar solution. Our study shows the possibility of using the Fe3O4 nanotubes as a highly effective multifunctional nanoscale tool for targeted hyperthermia and on-demand drug delivery.

Magnetic properties of magnetite nanowire arrays

The contributions of isolated Fe3+ ions and the ferrimagnetic subsystem to the total magnetization of Fe3O4 nanowires were separated. The magnetic anisotropy of nanowires was determined. The value of the magnetic anisotropy is smaller than the expected value of the shape anisotropy. The decrease in the magnetic anisotropy in nanowires can be explained by the dipole-dipole interaction between individual nanowires in the array. The Verwey transition suppression was found. A phase-inverted line corresponding to the microwave magnetoresistance was identified in the electron spin resonance spectra of nanowires.

Temperature dependence of magnetic first-order-reversal-curves for hollow Fe3O4 submicron particles

We report results of temperature dependence of first-order-reversal curves (FORCs) for hollow submicron particles with different outer diameter ranging from 400 to 700 nm. At low temperatures below the Verwey transition temperature, Tv, the FORC distribution exhibits a butterfly-like feature, associated with two pronounced FORC peaks, indicating the formation of a vortex structure for hollow Fe3O4 submicron particles. With increasing temperature from T = 10 K, the intensity of the two peaks steeply decreases and the peaks merge at T ∼ 130 K close to Tv. The results suggest a change of stability of the vortex state with temperature and were explained as due to a change of magnetic anisotropy associated with a structural transition at Tv.

The Effect of the Superlattice on the Magnetic and Transport Properties of Epitaxial Magnetite Thin Films

The surface structural, transport, and magnetic properties of superlattice films were examined, focusing on the effects of the superlattice periodicity of (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO) layers. The superlattice structures of (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO)n ultrathin films were fabricated using molecular-beam epitaxy on MgO(001) substrates, where n is the number of layers. It was observed that these oxide films have a single crystalline quality with the same orientation as the substrates for Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> as thin as approximately three unit cells. The transport and magnetic results showed that the conductivity and magnetization of the superlattice increase as the number of (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO) layers is increased. Furthermore, a sharp Verwey transition was found in the (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO)n superlattices with thickness layer smaller than seen in an earlier study for a single or superlattice film. These results give the indication that Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> superlattice ultrathin films have better properties than single layers of the same thickness. This artificial growth of superlattice ultrathin films will provide a useful method to realize novel properties for other complex oxide systems.

Tunable Magnetic Phases at Fe3O4/SrTiO3 Oxide Interfaces.

We demonstrate the emergence and control of magnetic phases between magnetite (Fe3O4), a ferrimagnetic halfmetal, and SrTiO3, a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition ( TV) was detected to persist from bulk-like down to ultrathin Fe3O4 films, decreasing from 117 ± 4 K (38 nm) to 25 ± 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent hard X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism techniques. We observe a reduction of Fe2+ ions with decreasing film thickness, accompanied by an increase of Fe3+ ions in both tetrahedral and octahedral sites and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe2O3 intralayer. To manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the γ-Fe2O3 that finally leads to stoichiometric and ferrimagnetic Fe3O4/SrTiO3(001) heterointerfaces.