Ferromagnetic Metallic Ground State Research Articles

The influence of substrate orientation on the magnetic behavior of nearly-stoichiometric Mn5Ge3 thin films: DFT calculations and experimental analysis

The Mn5Ge3 exhibits a wide range of fascinating properties such as high spin polarization up to 42 %, large spin diffusion lengths, near-room Curie temperature, and possible integration with Ge, GaAs, and Si substrates. The influence of substrate orientation on magnetic behavior has been investigated. The thin films studied present an in-plane magnetic reversal which is isotropic in samples grown on GaAs(111) and anisotropic in samples grown on GaAs(001). Another difference observed is a linear trend to saturation on the out-of-plane hysteresis loop in samples grown on GaAs(111), comparatively to samples grown on GaAs(001). To better understand this scenario, Density Functional Theory (DFT) calculations were conducted. The results indicate a ferromagnetic metallic ground state with properties that are consistent with the experimental results. The projected densities of states indicate that the main responsible for the magnetic moment observed in the Mn5Ge3 is the Mn-Mn interaction. The magnetic field distribution isosurfaces obtained from the DFT calculations offer a satisfactory explanation of the magnetic reversal experimentally observed, especially the in-plane magnetic behavior in samples grown on both GaAs(111) and GaAs(001) substrates. This work provides a theoretical-experimental approach that allows a deeper understanding of the magnetic anisotropy behavior under the influence of the crystallographic orientation of the substrate observed in nearly-stoichiometric Mn5Ge3 thin films, which is a good candidate for rare-earth free magnet in spintronic applications.

Magnetic-Competition-Induced Colossal Magnetoresistance in n -Type HgCr2Se4 under High Pressure

The n-type HgCr_{2}Se_{4} exhibits a sharp semiconductor-to-metal transition (SMT) in resistivity accompanying the ferromagnetic order at T_{C}=106 K. Here, we investigate the effects of pressure and magnetic field on the concomitant SMT and ferromagnetic order by measuring resistivity, dc and ac magnetic susceptibility, as well as single-crystal neutron diffraction under various pressures up to 8GPa and magnetic fields up to 8T. Our results demonstrate that the ferromagnetic metallic ground state of n-type HgCr_{2}Se_{4} is destabilized and gradually replaced by an antiferromagnetic, most likely a spiral magnetic, and insulating ground state upon the application of high pressure. On the other hand, the application of external magnetic fields can restore the ferromagnetic metallic state again at high pressures, resulting in a colossal magnetoresistance (CMR) as high as ∼ 3×10^{11}% under 5T and 2K at 4GPa. The present study demonstrates that n-type HgCr_{2}Se_{4} is located at a peculiar critical point where the balance of competition between ferromagnetic and antiferromagnetic interactions can be easily tipped by external stimuli, providing a new platform for achieving CMR in a single-valent system.

Growth and electrical transport properties of La0.7Sr0.3MnO3 thin films on Sr2IrO4 single crystals

We report the physical properties of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ thin films on ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ single crystals. The manganite films are deposited using oxide molecular beam epitaxy on flux-grown (001)-oriented iridate crystals. Temperature-dependent magnetotransport and x-ray magnetic circular dichroism measurements reveal the presence of a ferromagnetic metallic ground state in the films, consistent with films grown on ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{La}}_{0.3}{\mathrm{Sr}}_{0.7}{\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}{\mathrm{O}}_{3}$. A parallel resistance model is used to separate conduction effects within the ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ substrate and the ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ thin films, revealing that the measured resistance maximum does not correspond to the manganite Curie temperature but results from a convolution of properties of the near-insulating substrate and metallic film. The ability to grow and characterize epitaxial perovskites on ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ crystals enables a new route for studying magnetism at oxide interfaces in the presence of strong spin-orbit interactions.

Effects of Dy sub lattice dilution on transport and magnetic properties in Dy1-xKxMnO3

Interaction of multiple oxidation states of manganese ions with rare earth ions in manganites leads to the observation of various magnetic ground states. To understand the effect of average ionic size on electrical conductivity and magnetic ground state properties in Dy1-xKxMnO3 (x=0.0, 0.1, 0.2 & 0.3), we have investigated electron transport as a function of temperature and magnetic properties as a function of temperature, frequency and magnetic field of these compounds. Although mixed valent manganese ions can facilitate a double exchange interaction via oxygen ion leading to a ferromagnetic metallic ground state, no insulator-metal transition was detected. On the other hand, in the compounds with x=0.0 to 0.2, transport properties suggested an adiabatic small polaron hopping conduction mechanism. However, x=0.3 compound followed a variable range hopping conduction. Temperature variation of magnetization data exhibited three different types of transitions involving Mn and Dy ions. The bifurcation between zero field cooled and field cooled magnetization data was observed in all the compounds. Field-dependent magnetization of all the compounds showed hysteresis loops for temperatures less than 10 K. Although large irreversibility between zero field cooled and field cooled magnetization data was observed with a peak for x=0.3, the peak temperatures did not alter with frequency ruling out the possibility of spin glass behavior. The presence of hysteresis loops and lack of saturation magnetization implied the simultaneous presence of ferromagnetic and antiferromagnetic exchange interactions well within the antiferromagnetic ordering of Mn ions.

Controlling the sharpness of metal-insulator transition in epitaxial (La1−xPrx)0.67Ca0.33MnO3 (0 ≤ x ≤ 0.35) films

We report that epitaxial strain and chemical doping can be used cooperatively to tune the sharpness of metal-insulator transition (MIT) in epitaxial (La1−xPrx)0.67Ca0.33MnO3 (LPCMO) films. Compared to multiple MITs in anisotropically strained LPCMO/(LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7(001)C (LSAT) films with a phase-separated ground state, the lattice-matched LPCMO/NdGaO3(110)Or (NGO) films show a sharp MIT near the Curie temperature (TC), with a ferromagnetic-metallic ground state. The sharpness of MIT, as evaluated by the temperature coefficient of resistance (TCR), can be two times larger in LPCMO/NGO films than in LPCMO/LSAT films. Moreover, for LPCMO/NGO films, TCR greatly relies on the Pr doping level x, where a maximum TCR value of 88.17% K−1 can be obtained at x = 0.25, but shows less dependence on the film thicknesses. These results suggest that the combination of epitaxial strain and chemical doping could be employed to control not only the ground state of the manganite films, but the sharpness of MIT at various TC, providing the feasibility to design manganite-based infrared devices in a broad temperature range.

Magnon spectrum in the domain ferromagnetic state of antisite-disordered double perovskites

In their ideal structure, double perovskites like Sr_2FeMoO_6 have alternating Fe and Mo along each cubic axes, and a homogeneous ferromagnetic metallic ground state. Imperfect annealing leads to the formation of structural domains. The moments on mislocated Fe atoms that adjoin each other across the domain boundary have an antiferromagnetic coupling between them. This leads to a peculiar magnetic state, with ferromagnetic domains coupled antiferromagnetically. At short distance the system exhibits ferromagnetic correlation while at large lengthscales the net moment is strongly suppressed due to inter-domain cancellation. We provide a detailed description of the spin wave excitations of this complex magnetic state, obtained within a 1/S expansion, for progressively higher degree of mislocation, i.e., antisite disorder. At a given wavevector the magnons propagate at multiple energies, related, crudely, to `domain confined' modes with which they have large overlap. We provide a qualitative understanding of the trend observed with growing antisite disorder, and contrast these results to the much broader spectrum that one obtains for uncorrelated antisites.

High-TC ferromagnetic order in CaRuO3/La2/3Ca1/3MnO3 superlattices

Ferromagnetic-metallic ground state with high Curie temperature (TC) of 200–258 K has been observed in CaRuO3/La2/3Ca1/3MnO3 (CRO/LCMO) superlattices with the ultrathin LCMO layer of 0.8–3.2 nm thick. This contradicts the antiferromagnetic or low-TC insulating ground state observed in single-layer LCMO thin-films. TC and the saturated magnetization of the superlattices are determined dominantly by the LCMO layer thicknesses, indicating no direct magnetic contribution from the CRO layers or the interfaces. Also, they are less sensitive to the growth oxygen pressure as compared to the pure LCMO films. We ascribe the stabilized, bulklike ferromagnetism in the ultrathin LCMO layer to charge transfer from CRO at the interfaces, which could enhance the double-exchange and meanwhile suppress the phase separation, contrary to the case for LCMO thin-films. This interface engineering that can greatly depress the notorious “dead layer” in manganites might be significant in designing the correlated spintronic devices.

Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

Bulk La0.67Ca0.33MnO3 (LCMO) and NdGaO3 (NGO) have the same Pbnm symmetry but different orthorhombic lattice distortions, yielding an anisotropic strain state in the LCMO epitaxial film grown on the NGO(001) substrate. The films are optimally doped in a ferromagnetic-metal ground state, after being ex-situ annealed in oxygen atmosphere, however, they show strikingly an antiferromagnetic-insulating (AFI) transition near 250 K, leading to a phase separation state with tunable phase instability at the temperatures below. To explain this drastic strain effect, the films with various thicknesses were ex-situ annealed under various annealing parameters. We demonstrate that the ex-situ annealing can surprisingly improve the epitaxial quality, resulting in the films with true substrate coherency and the AFI ground state. And the close linkage between the film morphology and electronic phase evolution implies that the strain-mediated octahedral deformation and rotation could be assisted by ex-situ annealing, and moreover, play a key role in controlling the properties of oxide heterostructures.

Tuning the ground state of La0.67Ca0.33MnO3films via coherent growth on orthorhombic NdGaO3substrates with different orientations

The commensurate La${}_{0.67}$Ca${}_{0.33}$MnO${}_{3}$ films, of which the bulk is a ferromagnetic-metal (FM), can show strikingly different ground states when grown on NdGaO${}_{3}$ substrates of various orientations. With a larger orthorhombic lattice distortion in the substrates, the (110) films can grow essentially strain-free, but the (100) and (001) films are anisotropically strained, accompanied by an increased orthorhombicity. As a result, while the (110) films show constantly a bulklike FM ground state, the others behave dramatically different, i.e., they show a tunable antiferromagnetic-insulator (AFI) phase below \ensuremath{\sim}250 K with strong phase instability in the wide temperature range and at low magnetic fields. Considering that in La${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$ system the AFI phase is highly involved with the orthorhombic lattice distortion, the different ground state in (100) and (001) films could be ascribed to the epitaxial anisotropic strain, which enhances the orthorhombicity of the films via increasing the rotation and deformation of the MnO${}_{6}$ octahedra.

Phase evolution and the multiple metal-insulator transitions in epitaxially shear-strained La0.67Ca0.33MnO3/NdGaO3(001) films

The ferromagnetic-metal (FM) to antiferromagnetic-insulator (AFI) transition, near 250 K, has been induced in epitaxially shear-strained La0.67Ca0.33MnO3/NdGaO3(001) films, although they are doped for a FM ground state. We showed that for these films the phase diagram can feature the five regions of paramagnetic-insulator, FM, AFI dominated, FM dominated, and the frozen state, evolving with decreasing temperatures. And the phase instabilities in the temperature/magnetic-field regime can lead to multiple metal-insulator transitions over the various phase boundaries, in addition to a huge low-field magnetoresistance in the wide temperature range. The results underline that with the elastic-driven phase separation and consequently the complex phase evolution, this epitaxial system could be employed for further understanding of the manganites, and also for thin-film devices applications.

Anisotropic-strain-induced antiferromagnetic-insulating state with strong phase instability in epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films

We grow epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films simultaneously on the lattice-closely-matched substrates, cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 [LSAT(001)] and orthorhombic NdGaO3 [NGO(001) and NGO(110)]. While all as-grown films show a ferromagnetic-metallic (FM) ground state as observed for the bulk target, the annealed films show quite different magnetotransport behavior as follows: on NGO(110) they show a robust FM ground state, on LSAT(001) and NGO(001) however, they show surprisingly a coexisted antiferromagnetic insulating state with high phase instability in a wide temperature range. The phase coexistence being easily induced via the control of anisotropic epitaxial strain suggests that the phase separation in manganites could be elastically driven, and thus can be strain-engineered for devices applications.

Control of the charge-ordering-insulating phase in epitaxial La1−xCaxMnO3 (x=0.30–0.45) thin films under the anisotropic strain

The control of charge-ordering-insulating (COI) phase in epitaxial La1−xCaxMnO3/NdGaO3(001) (x=0.30–0.45) thin films with essentially the ferromagnetic metal ground state as observed for the bulk counterparts has been realized via the anisotropic strain relaxation. This epitaxial system is special in that there is a negligible average lattice mismatch but a large anisotropic strain in between the film and the substrate. By changing the film thickness, postannealing temperature, along with the doping level for strain relaxation, the COI phase in the films can be tuned to either melt completely under 1 T, producing a huge low-field magnetoresistance (MR) in a wide temperature range (e.g., for the 20 nm film with x=0.33 and annealed at 780 °C, the MR can be over 70% at 0.2 T and 97% at 0.5 T in 10–200 K), or survive under a high magnetic field of 6 T. The results demonstrate the crucial role of anisotropic strain relaxation in inducing the inhomogeneity in manganites films, thus providing a forward understanding of the strain field in manganite physics.

Ti substituted La 0.67Ca 0.33MnO 3 ortho-perovskites: Dominant role of local structure on the electrical transport and magnetic properties

We report the effects of local structure on the electrical transport and magnetic properties of La 0.67Ca 0.33Mn 1− x Ti x O 3 (0⩽ x⩽0.10) system. Linear increase in the lattice parameters, consequent expansion of unit cell and a monotonic decrease in the relative Mn 4+ concentration with x suggest that Ti 4+ predominantly replaces Mn 4+ in La 0.67Ca 0.33Mn 1− x Ti x O 3 ( x⩽0.10). The ferromagnetic-metallic ground state modifies to a glassy insulator for x⩾0.05. No field induced metallic state could be discerned for x=0.10 even at a field of 8 T and at as low a temperature as 4.2 K. Ti substitution significantly enhances the colossal magnetoresistance (CMR) effect. Both the metal to insulator transition temperature and Curie temperature ( T c) decrease at a rate of ∼26 K/at% up to x=0.05. T c levels off for higher compositions. Modification of the major carrier concentration (decreased Mn 4+ concentration) seems to be insufficient to account for the observed reduction in the transition temperatures. This in turn emphasizes the significance of local structural effects: systematic elongation of the Mn–O bond lengths and decrease of Mn–O–Mn angles leading to strong suppression of itinerant ferromagnetism and metallicity of the compounds. The additional features in the ac susceptibility, viz., a broad shoulder just below T c followed by a sharp decrease in the susceptibility signal at low temperatures and a non-closure of MR at zero field as high as ∼75% at 5 K indicate a frustrated magnetic ground state for x=0.07. Based on the inter-comparison of the structural, electrical transport and magnetic properties of the Mn site substituted La 0.67Ca 0.33MnO 3 with iso-valent diamagnetic and paramagnetic ions, we argue that local structural effects have a decisive role to play, compared to the local spin coupling effects, in the ferromagnetic-metallic ground state of the CMR manganites.

Electron spin resonance study of bulk and nanosized La 0.875Sr 0.125MnO 3

The structural, transport and electron spin resonance properties of bulk and nanosized La 0.875Sr 0.125MnO 3 prepared by a sol–gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T C . In addition to a sharp peak in the vicinity of T C , another sharp peak close to T OO is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.

Growth of manganese silicide films by co-deposition of Mn and Si on Si(1 1 1): A spectroscopic and morphological investigation

MnSi is a ferromagnetic compound with a Curie temperature of 29 K. Recent theoretical studies predict that 2 ML of MnSi epitaxially grown on Si show a ferromagnetic metallic ground state with spin polarization of about 50%. This would allow the development of spintronic devices based on the injection of spin-polarized current from a ferromagnetic metal into a semiconductor. In this context the possibility of growing in situ MnSi on the Si(1 1 1) surface has been explored. Thermal reaction, crystalline structure and electronic properties of the grown films have been studied in situ by photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), and low energy electron diffraction (LEED). Depositing a thin film of Mn on Si(1 1 1) the formation of ordered islands (with dimensions dependent on the amount of deposited Mn) is driven by annealing at selected temperatures, as already observed. Our preliminary studies show that by simultaneously depositing Mn and Si in 1:1 stoichiometry on Si(1 1 1) a large improvement in the homogeneity of the MnSi films is achieved.

Structure, transport and magnetism of La 0.67Ca 0.33Mn 1−xTa xO 3

For the first time, the effect of Ta 5+ substitution at the Mn site of La 0.67Ca 0.33MnO 3 compound is reported. A significant increase in lattice parameters and unit cell expansion indicate larger structural modification upon substitution. The ferromagnetic-metallic ground state is modified to a cluster glass insulator for x > 0.03 . The observed suppression of the ferromagnetic transition temperature T c of ∼39 K/at% might be the highest reported in Mn-site substituted La 0.67Ca 0.33MnO 3. The well-known effects of changed carrier density, larger average ionic radius at Mn site and dilution of Mn sublattice appear insufficient to explain this strong suppression of T C.

Low temperature specific heat of bi-layered manganites La 2−2 xSr 1+2 xMn 2O 7 ( x=0.3 and 0.5)

The specific heat ( C) of bi-layered manganites La 2−2 x Sr 1+2 x Mn 2O 7 ( x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T 3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T 2 term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6–10 K) and magnetic field (6 T).

Magnetic State of Ba-Doped Manganites Depending on the Oxygen Vacancy Concentration

Magnetization and electrical transport properties of La0.60Ba0.40MnO3—γ (0 ≤ γ ≤ 0.20) anion-deficient manganites have been studied. Magnetic ordering temperature decreases insignificantly with γ in contrast to spontaneous magnetization. Ferromagnetic metallic ground state of stoichiometric samples (γ = 0) is destroyed gradually with increasing oxygen vacancy concentration (to γ = 0.20). It is shown that the long-range ferromagnetic order remains for γ ≤ 0.10 while the cluster spin glass properties appear for 0.10 < γ ≤ 0.20 with temperature of cluster moment freezing of 40 K. Electrical resistivity behavior of the samples with 0 ≤ γ ≤ 0.10 correlates with magnetization. Below the magnetic ordering temperature the resistivity becomes of metallic type and a peak of magnitoresistance is observed. The strongly reduced samples (0.10 < γ ≤ 0.20) have the activated type resistivity and exhibit a large magnetoresistance below the temperature at which the spontaneous magnetization occurs. For the explanation of the obtained results it is assumed that both the Mn3+–O–Mn3+ and the Mn3+–O–Mn4+ magnetic interactions become negative near the oxygen vacancies. The magnetic state of the anion-deficient samples is interpreted basing on the superexchange interaction model.

Low-energy charge dynamics of La 0.7Ca 0.3MnO 3 thin films

Direct estimations of the low-energy (0.5–6 meV) complex optical spectrum in La 0.7Ca 0.3MnO 3 thin films have been investigated using terahertz time-domain spectroscopy. At low temperature, a clear Drude-term appears. Using the simple Drude model, we have estimated the scattering rate and the plasma frequency in the ferromagnetic metallic ground-state. Low-energy charge dynamics are briefly discussed on the basis of present results.

Layered Magnetic Manganites

▪ Abstract We review the recent progress in the study of layered magnetic manganites La2−2xSr1+2xMn2O7, which have many unique features of charge transport and magnetism inherent to the quasi-two-dimensional electronic structure. The system shows a wide variety of magnetic-field-induced phenomena due to the layered crystal and magnetic structure, such as the highly anisotropic ferromagnetic metallic ground state, the colossal magnetoresistance (CMR) effect, and the tunneling-type magnetoresistance (TMR). The charge transport properties, as well as the magnetic ones, strongly depend on the carrier-doping level and the applied pressure, which reflects the variation of the orbital-dependent occupancy of itinerant eg-like electrons. Although the layered manganite is one of a new class of CMR materials, the study of this system may also reveal some of the key issues for understanding the CMR effect in mixed-valent manganese oxide.