Low-temperature Ferromagnetism Research Articles

Crystal, ferromagnetism, and magnetoresistance with sign reversal in a EuAgP semiconductor

We synthesized the ferromagnetic EuAgP semiconductor and conducted a comprehensive study of its crystalline, magnetic, heat capacity, band gap, and magnetoresistance properties. Our investigation utilized a combination of X-ray diffraction, optical, and PPMS DynaCool measurements. EuAgP adopts a hexagonal structure with the P63/mmc space group. As the temperature decreases, it undergoes a magnetic phase transition from high-temperature paramagnetism to low-temperature ferromagnetism. We determined the ferromagnetic transition temperature to be TC = 16.45(1) K by fitting the measured magnetic susceptibility using a Curie-Weiss law. Heat capacity analysis of EuAgP considered contributions from electrons, phonons, and magnons, revealing η = 0.03 J/(mol·K2), indicative of semiconducting behavior. Additionally, we calculated a band gap of ∼1.324(4) eV based on absorption spectrum measurements. The resistivity versus temperature of EuAgP measured in the absence of an applied magnetic field shows a pronounced peak around TC, which diminishes rapidly with increasing applied magnetic fields, ranging from 1 to 14 T. An intriguing phenomenon emerges in the form of a distinct magnetoresistance transition, shifting from positive (e.g., 1.95% at 300 K and 14 T) to negative (e.g., −30.73% at 14.25 K and 14 T) as the temperature decreases. This behavior could be attributed to spin-disordered scattering.

Ferroelasticity, thermochromism, semi-conductivity, and ferromagnetism in a new layered perovskite: (4-fluorophenethylaminium)2[CuCl4

A new A2BX4-type layered perovskite, (4-fluorophenethylaminium)2[CuCl4], exhibits multi-channel properties including above-room-temperature ferroelasticity, reversible thermochromism, indirect semi-conductivity, and low-temperature ferromagnetism.

Spontaneous spin magnetism of donor conduction electrons of hybridized states of a crystal formed by a system of 3d-elements impurity atoms of low concentration (<1 at.%)

The given work is devoted to the experimental proof of existing the spontaneous spin polarization of the donor electron system of 3d-transition element impurity atoms of low concentration (<1 at.%) in a mercury selenide crystal. For this purpose there have been measured the dependences of the magnetization on the magnetic field strength at low temperatures (T=5 K). As a result of the analysis of the obtained dependences, there were extracted the impurity contributions, which are described by the magnetization curves typical of the ferromagnets, and by the magnetic parameters conforming to the spontaneous magnetism of the systems under study, which are unambiguously related to the donor conduction electrons of the outer d-shells of impurity atoms. By its nature, according to the developed theoretical concepts, the spontaneous spin polarization manifests itself in exchange interaction, taking place in hybridizing the electronic states of the impurity atom and the conduction band ones of the crystal. Keywords: 3d-impurities of the low concentration, gapless semiconductors, hybridized electronic states, spontaneous spin polarization, low-temperature ferromagnetism

Magnetic transition and magnetocaloric effect of R5(Si, Sn)3 (R = Pr, Nd) alloys

The magnetic transition and magnetocaloric effect of R5(Si, Sn)3 (R = Pr, Nd) alloys have been investigated. The single phase with Cr5B3-type tetragonal structure is observed for x(y) ≤ 1 in Pr5Si3-xSnx and Nd5Si3-ySny alloys. The magnetic phase transition temperature of these alloys is elevated with increased the substitution amount. All phase transitions in these alloys are of second-order nature undergoing low-temperature ferromagnetism to high-temperature paramagnetism. For the field change of 7 T, the maximum of entropy change (− ΔSMmax) of 7.0 J/kgK and 4.4 J/kgK have been achieved in Pr5Si2Sn and Nd5Si2Sn alloys compared with 12.1 J/kgK and 4.3 J/kgK in Pr5Si3 and Nd5Si3 alloys. The relative cooling power (RCP) is enhanced to 359.2 J kg-1 and 382.4 J kg-1 in Pr5Si2Sn and Nd5Si2Sn alloys compared with 283.2 J kg-1 and 216.5 J kg-1 in Pr5Si3 and Nd5Si3 alloys. The excellent magnetocaloric properties suggest that R5(Si, Sn)3 (R = Pr, Nd) alloys could be the promising candidates for magnetic cooling.

Hydrazine intercalated iron sulfide (N2H4)1-xFe2+δS2: Synthesis and characterization

The discovery of superconductivity in anti-PbO-type FeS has attracted great interest in the intercalation compounds of FeS. However, it is much more difficult to synthesize intercalates of FeS than to synthesize intercalates of FeSe. Here, we report a novel intercalation compound of FeS with chemical composition of (N2H4)1-xFe2+δS2 (x ​~ ​0.25, δ ​~ ​0.1), which is synthesized via a combination of self-flux solid state method and hydrothermal route. The successful intercalation of hydrazine was confirmed by X-ray diffraction, thermogravimetric analyses and attenuated total internal reflection-Fourier transform infrared spectroscopy, while the maintenance of anti-PbO-type FeS layer in the structure was verified by transmission electron microscopy and Raman spectroscopy. Magnetic and resistivity measurements show that (N2H4)1-xFe2+δS2 is a ferromagnetic metal rather than a superconductor. The low-temperature ferromagnetism may be caused by the intercalated hydrazine molecules or interlayer iron ions and the absence of superconductivity should be closely related to the ferromagnetic ordering.

Twist-angle-disorder and sulfur-induced annihilation of percolative magnetism in exfoliated lamellae of highly oriented pyrolytic graphite

The existence of low temperature ferromagnetism in twisted graphene systems and locally twisted-sublattices of highly-oriented-pyrolytic-graphite (HOPG) has recently attracted significant attention. Despite important progress, additional investigations are needed to evaluate the role of twist-angle-disorder on the magnetism arising from those locally-twisted-interfaces. Doping these systems with sulfur has been previously shown to induce a direct modification of their magnetic properties together with formation of competing ferromagnetic/superconductive ordering. Being interested in investigating the role of doping-temperature on carbon-sulfur interactions, we performed a novel investigation on the structural-magnetic-relationships of these systems. TEM of undoped-HOPG-lamellae revealed locally-twisted-sublattices, with hexagonal-moiré-superlattices exhibiting periodicities corresponding to θrot ~ 2.35°, 1.7°, 1.6°, 1.5°, 1.45°, 1.05°, 0.408°, 0.38°, 0.35° and 0.34°. Significant variation in the period of the moiré-superlattices was found, thus indicating the existence of twist-angle-gradients ∇θ. Sulfur-doping was achieved at T ~ 550 , ~ 900 and ~ 990 °C. The characteristics of the doped-samples were investigated by Raman-spectroscopy. The appearance of broad bands in the region of 1200 and 1400 cm−1 of the Raman-spectrum is shown. SQUID ZFC/FC magnetic curves acquired from 2 to 300 K together with T-dependent magnetization vs field signals revealed an almost-complete sulfur-induced annihilation of the ferromagnetic-ordering. Significant differences were further found between exfoliated- and unexfoliated-samples. Particularly, we analysed the evolution of the ZFC and FC curves in the region of T ~ 25 K. where a magnetic transition possibly arising from the locally twisted moiré superlattices is present. Furthermore, an unusual increase of the diamagnetic-component with the decrease of the temperature is discussed.

Low temperature ferromagnetism in CaCu3Ti4O12 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 11504227 and 51971128), the Program of Shanghai Academic/Technology Research Leader, China (Grant No. 20XD1401800), and the Project of the Science and Technology Commission of Shanghai Municipality, China (Grant No. 19020501000).

The low-temperature magnetic order behaviors of perovskite oxide CaCu3Ti4O12 (CCTO) ceramics prepared by different methods are discussed. X-ray diffraction, scanning electron microscope, x-ray photoelectron spectroscopy, and direct current (DC) magnetization are used to characterize the structures, microscopic morphologies, valence states, and magnetic properties of the samples. The results show that the magnetic behaviors of CCTO ceramics are very sensitive to the preparation process. The quenched CCTO ceramic and CCTO powders grown in a molten salt crystal, which contain much more oxygen vacancies and Ti3+, show the coexistence of weak ferromagnetic order and antiferromagnetic order below the Neel temperature. It suggests that the bound magnetopolaron formed by oxygen vacancies and Ti3+ ion composite defects are responsible for the weak ferromagnetic order at low temperature.

Improved-sensitivity integral SQUID magnetometry of (Ga,Mn)N thin films in proximity to Mg-doped GaN

Nominally 45 nm GaN:Mg / 5 nm (Ga,Mn)N / 45 nm GaN:Mg trilayers structures prepared by molecular beam epitaxy on GaN-buffered Al2O3 substrates are investigated to verify whether the indirect co-doping by holes from the cladding layers can alter the spin-spin interaction in (Ga,Mn)N. The four investigated structures, differing with the Mg doping level, are carefully characterized at a nanoscale by high-resolution transition electron microscopy (HRTEM), energy-dispersive x-ray spectroscopy, and secondary ion mass spectrometry. Importantly, HRTEM decisively excluded a presence of foreign Mn-rich phases. Magnetic studies of these structures are aided by the employment of a dedicated experimental approach of the in situ compensation of the magnetic contribution from the substrate, allowing an up to about fifty-fold reduction of this contribution. This technique, dedicated to these structures, simultaneously provides a tenfold reduction of long-term instabilities of the output of the magnetometer and lowers the experimental jitter to merely 5 × 10−7 emu at 70 kOe, vastly increasing the precision and the credibility of the results of the standard integral (volume) superconducting quantum interference device magnetometry, particularly in high magnetic fields. The magnetic characteristics of the trilayer structures established here prove identical with the already known properties of the thick (Ga,Mn)N single layers, namely (i) the low temperature ferromagnetism among Mn3+ ions driven by superexchange and (ii) purely paramagnetic response at higher temperatures. The possible cause of the lack of any effects brought about by the adjacent Mg-doping is a presence of residual Mn in the cladding layers, resulting in the deactivation of the p-type doping intended there. This finding points out that a more intensive technological effort has to be exerted to promote the co-doping-driven carrier-mediated ferromagnetic coupling in Mn-enriched GaN, especially at elevated temperatures.

Removal of Ferrimagnetic ZnFe2O4 Impurity Phase by Li Co-Doping in Fe Doped ZnO

A simple approach is presented to remove the impurity phase of ZnFe2O4 from Fe doped ZnO. Gradual incorporation of non-magnetic Li ions reduced 5.6% phase contamination of ZnFe2O4 to negligible limit (<0.01%) from 4 at% (atomic %) Fe doped ZnO. X-ray absorption near edge spectroscopy (XANES) revealed that the most of the Fe ions were in Fe3+ state. The optical absorption spectra of Fe doped ZnO samples exhibited d-d transition between Fe ions. It also indicated the signature of ZnFe2O4 phase, which reduced gradually on Li increment. ZnFe2O4 contained samples exhibited strong magnetic ordering with spin-glass character at low temperature and weak ferromagnetism (FM) at the room temperature with disordered inverse spinel structure. However, 6 at% Li incorporated sample did not show any significant FM in the entire temperature range of 2–300 K. ZnFe2O4 is supposed to be the possible source of the observed strong magnetic ordering i.e., FM has extrinsic origin. The gradual removal of impurity phase ZnFe2O4 by increasing Li doping without damaging any intrinsic property of Fe doped ZnO is primarily due to gradual increase of tetrahedral coordination of Fe ions.

Investigation of optical and magnetic properties of Mn-doped tetragonal ZrO2 nanocrystals

Pure and Mn-doped ZrO2 nanocrystals (Zr1-xMnxO2) with varying Mn concentrations (x ​= ​0.02, 0.04, 0.06, and 0.08) have been synthesized using sol-gel method. The effect of Mn doping concentration on structural, optical and magnetic properties has been investigated. X-ray diffraction data and SAED analysis revealed that all Zr1-xMnxO2 nanocrystals have a tetragonal structure without any secondary phase. The average crystallite size obtained from XRD decreases (within the experimental uncertainty) with increased Mn concentration. Magnetic measurements have revealed that Zr0.98Mn0.02O2 nanocrystalline sample exhibits both low and room temperature ferromagnetism. The origin of ferromagnetism is attributed to the anionic vacancies created due to Mn doping in ZrO2. Higher Mn concentrations shows superparamagnetism whereas pure ZrO2 displayed diamagnetic behavior. The UV–Vis absorption spectra showed a wide absorption peak at 200–330 ​nm, and a redshift was observed in the bandgap with an increase in the concentration of Mn2+.

Electronic and magnetic properties of α−FeGe2 films embedded in vertical spin valve devices

We studied metastable $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$, a layered tetragonal material, embedded as a spacer layer in spin valve structures with ferromagnetic ${\mathrm{Fe}}_{3}\mathrm{Si}$ and ${\mathrm{Co}}_{2}\mathrm{Fe}\mathrm{Si}$ electrodes. For both types of electrodes, spin valve operation is demonstrated with A metallic transport behavior of the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$ spacer layer. The spin valve signals are found to increase with both temperature and spacer thickness, which is discussed in terms of a decreasing magnetic coupling strength between the ferromagnetic bottom and top electrodes. The temperature-dependent resistances of the spin valve structures exhibit characteristic features, which are explained by ferromagnetic phase transitions between 55 and 110 K. The metallic transport characteristics as well as the low-temperature ferromagnetism are found to be consistent with the results of first-principles calculations.

Interstitial Effects on the Magnetic Phase Transition and Magnetocaloric Effects in (Hf, Ta)Fe2 Kagome Phase

The effects of Ta substitution on the first-order phase transition have been firstly studied in Hf1-xTaxFe2 (x = 0.15, 0.165, 0.18) alloys with kagome-type lattice. Higher Ta substitution leads to the reduced metamagnetic transition temperature (Tt) from low-temperature ferromagnetism to high-temperature antiferromagnetism and paramagnetism. By doping C atoms, the Tt is gradually reduced and thermal hysteresis becomes widened in the alloys with higher Ta substitution. The critical field which triggers the metamagnetic phase transition is not substantially affected by interstitial C atoms. With increased Ta substitution, for the field change of 7 T, the maximum of magnetic entropy change (ΔSmax) of 3.7 J kg−1 K−1 is obtained at x = 0.15 and enhanced to 4 J kg−1 K−1 at x = 0.165 but reduced to ~ 3.21 J kg−1 K−1 in Hf0.82Ta0.18Fe2 alloy. By C addition, both Tt and the temperature where ΔS is maximized are shifted to a lower temperature, especially in the higher Ta-contained alloys.

Magnetic and transport properties of a ferromagnetic layered semiconductor MnIn2Se4

Magnetic two-dimensional (2D) materials hold considerable promise for the next generation of spintronic devices. In this study, a ternary 2D layered semiconductor compound, MnIn2Se4, with good photoelectric properties and low-temperature ferromagnetism is introduced. Field-effect transistors based on few-layer MnIn2Se4 exhibit n-type characteristics, with a high on-off ratio of up to 450 times, and exhibit a good photoresponse with an on-off ratio of 25 times. Magnetic measurements show that few-layer MnIn2Se4 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ∼7 K. This study suggests that MnIn2Se4 has potential in applications involving magnetic and optoelectronic devices.

Evidence of ferromagnetic ordering at low temperatures in epitaxial TbMnO3 thin films grown by means of DC magnetron sputtering

Thin films of the archetypal multiferroic TbMnO3 were epitaxially deposited on (0 0 1)-oriented SrTiO3 substrates by means of DC magnetron sputtering. A substrate temperature of 1053 K and an oxygen pressure of 300 Pa were determined to be appropriate growing conditions for obtaining high-quality, epitaxial TbMnO3 films using this physical growing technique. The structural and magnetic properties of ~100 nm thin TbMnO3 films were investigated. X-ray diffraction patterns showed that the films are single-phase and (00ℓ)-oriented. Rocking curves of the (0 0 2) peak confirm the good crystalline quality of the films. X-ray photoelectron spectroscopy analysis confirmed that the nominal valence of the Mn ions is 3+. The films grown on the SrTiO3 substrates displayed substantial epitaxial strain, which decidedly changed the magnetic ground state of the bulk-like counterpart. In particular, the films displayed ferromagnetic-like interactions below a temperature close to the magnetic ordering temperature of the Mn3+ spins (TN ≈ 42 K). Well-defined hysteresis loops with coercive fields as high as 0.1 T were observed at 5 K. The variation of the coercive field with the temperature clearly indicated the presence of low-temperature ferromagnetism in the TMO films. The origin of the anomalous ferromagnetism in the films is discussed in terms of the strain-induced distortion generated by the lattice mismatch between the film and substrate. The reported results can give insight into accurate strategies for controlling single-phase spin-driven multiferroic states.

Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure.

In the bulk, LaCoO_{3} (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO_{6} octahedra in combination with the ferroelastic order parameter.

Exchange Spring in a Heterostructure with Pd0.96Fe0.04 Low-Temperature Ferromagnet

The exchange bias of the magnetic hysteresis loop and the exchange-spring effect have been observed in thin-film heterostructures comprising layers of Pd0.96Fe0.04 low-temperature ferromagnet, metallic cobalt, and cobalt monoxide (CoO) grown on single-crystalline sapphire by methods of magnetron sputtering and molecular beam epitaxy in an ultrahigh-vacuum system. It is established that the exchange anisotropy of Pd0.96Fe0.04 layer in Pd0.96Fe0.04/CoO/Co/Al2O3 thin-film heterostructure is absent and the exchange-spring effect in this system is not manifested. In the case of a Pd0.96Fe0.04/Co/CoO/Al2O3 structure, it is shown that, according to the results of magnetometric measurements, the inversion of the order of antiferromagnetic and ferromagnetic layers leads to pinning of the magnetic moment of Pd0.96Fe0.04 layer at the interface with cobalt and to realization of the exchange-spring effect. Additional evidence in favor of this interpretation was obtained using the micromagnetic modeling of magnetic hysteresis loops.

Study on the structural and magnetic properties of Fe/Co co-doped Bi4Ti3O12 ceramics

Polycrystalline Bi4(Ti3-xFex/2Cox/2)O12 (0 ≤ x ≤ 0.4) ceramics are prepared by solid state reaction method. The low temperature ferromagnetism is successfully induced by Fe/Co co-doping. Particularly, the x = 0.3 ceramics possess the largest remnant magnetization (2Mr) of about 0.272 emu/g at 100 K. Furthermore, all the doped ceramics undergo a ferromagnetic to paramagnetic transition, and show typical spin glass behavior, which is confirmed by the isothermal remnant magnetization and Memory Effect. The magnetic mechanisms probably originate from the coexistence and competition between ferromagnetic double exchange interactions and antiferromagnetic superexchange interactions, which lead to the appearance of the spin glass behavior in the doped ceramics and the strongest ferromagnetism in the x = 0.3 ceramics. The present work provides basic data and theoretical reference for the exploitation of the new multifunctional materials.

Обменная пружина в гетероструктуре с низкотемпературным ферромагнетиком Pd-=SUB=-0.96-=/SUB=-Fe-=SUB=-0.04-=/SUB=-

AbstractThe exchange bias of the magnetic hysteresis loop and the exchange-spring effect have been observed in thin-film heterostructures comprising layers of Pd_0.96Fe_0.04 low-temperature ferromagnet, metallic cobalt, and cobalt monoxide (CoO) grown on single-crystalline sapphire by methods of magnetron sputtering and molecular beam epitaxy in an ultrahigh-vacuum system. It is established that the exchange anisotropy of Pd_0.96Fe_0.04 layer in Pd_0.96Fe_0.04/CoO/Co/Al_2O_3 thin-film heterostructure is absent and the exchange-spring effect in this system is not manifested. In the case of a Pd_0.96Fe_0.04/Co/CoO/Al_2O_3 structure, it is shown that, according to the results of magnetometric measurements, the inversion of the order of antiferromagnetic and ferromagnetic layers leads to pinning of the magnetic moment of Pd_0.96Fe_0.04 layer at the interface with cobalt and to realization of the exchange-spring effect. Additional evidence in favor of this interpretation was obtained using the micromagnetic modeling of magnetic hysteresis loops.

Engineering of high-temperature ferromagnetic Si1–xMnx (x ≈ 0.5) alloyed films by pulsed laser deposition: Effect of laser fluence

Thin films of Si1−xMnx were deposited on Al2O3 substrate using pulsed laser deposition(PLD)technique employing time-of-flight separation of the ejected particles with different laser fluence (E = 3.8–7.6 J/cm2). Magnetic properties of these films reveal that the variation in the laser fluence has engineered several magnetic phases in Si1−xMnx films: high temperature (HT) ferromagnetic (FM) phase Si1−xMnx(x = 0.51–0.52) with uniformly distributed Si vacancies, and the low-temperature (LT) FM phases MnSi and Mn4Si7 with Curie temperature ≤40 K. At small fluence, the mesoscopic size MnSi and Mn4Si7 crystallites are formed in the upper layer of Si1−xMnx films with Si excess (x ≈ 0.4–0.5), yielding to inhomogeneous magnetic phase distribution along the film growth direction, while the high and optimal fluence has resulted in uniform distribution of the HT FM phase during the film growth. The XPS, magnetotransport and ferromagnetic resonance (FMR) measurements confirm the presence of several FM phases in the samples and the amplifying role of LT FM for films grown at E ≤ 4.4 J/cm2.

Charge transfer effect for the La0.7Ca0.3MnO3/NiO heterostructure and novel interfacial ferromagnetism

We report the formation of new ferromagnetic (FM) states in antiferromagnetic (AFM) NiO at the interface with FM La0.7Ca0.3MnO3 (LCMO). The LCMO/NiO heterostructures exhibit an exchange bias field of 209Oe that vanishes as the temperature rises above 90K. A new magnetization temperature at 90K is observed and can be ascribed to Ni3+-O-Mn3+ superexchange interactions. Mn 3s and Ni 3p core-level spectra, measured by X-ray photoelectron spectroscopy, show a direct evidence of charge transfer effects of the type Mn4+-Ni2+→Mn3+-Ni3+ at the interface region. The valence band offset (VBO) at the LCMO/NiO interface can be determined to be ΔEVBO∼0.77eV. The valence band of LCMO is shifted to higher binding energy compared with NiO. Thus, charge transfer occurred because of the shifting of the valence band edge at the heterostructure interfaces. We speculate that the superexchange interactions of the Ni3+ and Mn3+ at the interface give rise to the novel low-temperature FM order and the magnetic regions that pin the FM LCMO layer as the temperature decreases.