We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in mang...We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb：SrTio3 p-n junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature of manganites; and the effect of particle size on magnetic properties in manganite nanoparticles.展开更多
The extraordinary colossal magnetoresistance (CMR) behavior in Mn-site doped system La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15) was reported. It was found that the substitution with Cr on Mn sites introduces an addi...The extraordinary colossal magnetoresistance (CMR) behavior in Mn-site doped system La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15) was reported. It was found that the substitution with Cr on Mn sites introduces an additional bump in zero-field resistivity. With increasing Cr content, this additional bump grows up drastically while the original resistivity peak associated with magnetic order transition diminishes gradually. Under the applied magnetic field, both bumps of resistivity are deeply compressed, which leads to the appearance of two peaks in CMR response. As a result, the temperature range of CMR response is significantly broadened, spanned from the lowest to near room temperature. These results suggest that Mn-site element substitution could be a potent way of tuning CMR response.展开更多
The representative sample La0.58Dy0.09Ca0.33MnO3 of Dy doped La0.67Ca0.33MnO3 rare-earth manganites was investigated. The most important effect of Dy doping is to introduce the magnetoimpurity and form the spin cluste...The representative sample La0.58Dy0.09Ca0.33MnO3 of Dy doped La0.67Ca0.33MnO3 rare-earth manganites was investigated. The most important effect of Dy doping is to introduce the magnetoimpurity and form the spin clusters which induce dramatically large CMR in La0.58Dy0.09Ca0.33MnO3. The fitting results of field-induced resistivity decrease to the Brillouin function indicate that the CMR is caused by the spin dependent hopping between spin clusters. It is the magnetic field that reduces the size of spin clusters and induces a field-induced irreversible CMR behaviour.展开更多
Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and pa...Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and paramagenetic ferromagnetic-transitions as well as colossal magnetoresistance is due to the formation of ferromagnetic clusters in the paramagnetic background. The transition to metallic state is resulted from percolation of ferromagnetic metallic clusters, while the colossal magnetoresistance is due to the application of magnetic field, which accelerates the growth of ferromagnetic metallic clusters and causes the shift of the onset temperature for the metallic percolation to higher temperature. Based on the random resistor network model, the zero-field resistivity versus temperature dependence is simulated by using experimental parameters, and experimental data well agree with those in whole temperature range, giving a strong support to our approach.展开更多
A phenomenological model based on phase separation between ferromagnetic metallic and paramagnetic insulating domains was applied to analyze the electrical transport and colossal magnetoresistance for mixed-valent man...A phenomenological model based on phase separation between ferromagnetic metallic and paramagnetic insulating domains was applied to analyze the electrical transport and colossal magnetoresistance for mixed-valent manganites of RE_(2/3)AE_(1/3)MnO_3. The results show that the model can yield results in agreement with experimental observations in these manganites. The present approach provides a simple picture to visualize the reason that the temperature dependence of resistance (with and without applied magnetic fields) in these compounds has the peculiar shape, without invoking any complicated concept.展开更多
We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb,a new member of“111”type of diluted magnetic materials.The material crystallizes into Cu2Sb-type structure w...We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb,a new member of“111”type of diluted magnetic materials.The material crystallizes into Cu2Sb-type structure which is isostructural to“111”type Fe-based superconductors.With suitable carrier and spin doping,the Na(Zn,Mn)Sb establishes spin-glass ordering with freezing temperature(Tf)below 15 K.Despite lack of long-range ferromagnetic ordering,Na(Zn,Mn)Sb single crystal still shows sizeable anomalous Hall effect below Tf.Carrier concentration determined by Hall effect measurements is over 1019 cm-3.More significantly,we observe colossal negative magnetoresistance(MR≡[ρ(H)−ρ(0)]/ρ(0))of-94%in the single crystal sample.展开更多
Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 compound have been studied.A broad peak of magnetic entropy change （-△S M） is found above the Curie temperature...Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 compound have been studied.A broad peak of magnetic entropy change （-△S M） is found above the Curie temperature （T C =120 K）,which can be associated with the existence of two-dimensional short range ferromagnetic order.Additionally,the curvilinear shape of-△S M for layered perovskite is quite different from that of the Ln 1-x A x MnO 3 probably arising from magnetocrystalline anisotropy.At the same time,a wide peak of colossal magnetoresitance effect near T C is found in the layered provskite La 1.4 Sr 1.6 Mn 2 O 7.展开更多
Ferromagnetic semiconductor Ga_(1–x)Mn_(x)As_(1–y)P_(y) thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers.In this regime,...Ferromagnetic semiconductor Ga_(1–x)Mn_(x)As_(1–y)P_(y) thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers.In this regime,we report a colossal negative magnetoresistance(CNMR)coexisting with a saturated magnetic moment,unlike in the traditional magnetic semiconductor Ga_(1–x)Mn_(x)As.By analyzing the temperature dependence of the resistivity at fixed magnetic field,we demonstrate that the CNMR can be consistently described by the field dependence of the localization length,which relates to a field dependent mobility edge.This dependence is likely due to the random environment of Mn atoms in Ga_(1-x)Mn_(x)As_(1-y)P_(y) which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.展开更多
Electrical properties and magnetoresistance have been studied in two series of xAg-La0.67（Ca0.65Ba0.35）0.33MnO3 and xPd-La0.67（Ca0.65Ba0.35）0.33MnO3 （abbreviated by xAg-LCBMO and xPd-LCBMO） composites. Both Pd a...Electrical properties and magnetoresistance have been studied in two series of xAg-La0.67（Ca0.65Ba0.35）0.33MnO3 and xPd-La0.67（Ca0.65Ba0.35）0.33MnO3 （abbreviated by xAg-LCBMO and xPd-LCBMO） composites. Both Pd and Ag addition induce a decrease in resistivity and an increase in temperature at which the resistivity reaches its maximum. This is mainly due to the improvement of grain boundaries caused by the segregation of good conductive metal grains on the grain boundaries/surfaces. In addition, both Pd and Ag addition induce a large enhancement of room temperature magnetoresistance （RTMR）. Note that 27% molar ratio of Ag addition induces a large RTMR of about 70%, about ten times larger than pure LCBMO, whereas 27% molar ratio Pd addition brings a much larger RTMR of about 170%. The large enhancements of MR can be attributed to the decrease in resistivity of the samples caused by the good conductive metal. On the other hand, the polarization of Pd atoms near the Mn ions on the grain surfaces/boundaries plays a very im-portant role in the increase in MR, which induces a large number of spin clusters in Pd-added samples.展开更多
In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide ...In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide （AAO） membranes, The magnetic and electronic transport properties are investigated by using the Quantum Design physics properties measurement system （PPMS） and magnetic properties measurement system （MPMS）. The resistance peak temperature （Tp） is about 85 K and the Curie temperature （To） is about 250 K for the LCMO film on an AAO membrane with a pore diameter of 20nm. Large magnetoresistance ratio （MR） is observed near Tp. The MR is as high as 85% under 1 T magnetic field. The great enhancement of MR at low magnetic fields could be attributed to the lattice distortion and the grain boundary that are induced by the nanopores on the AAO membrane.展开更多
La_(0.67)Ca_(0.33)MnO_3)_x /(ZrO_2)_(1-x) (x is the volume fraction ratio) percolated composites were fabricated by combining La_(0.67)Ca_(0.33)MnO_3 (LCMO) powders with ZrO_2 particles, where LCMO powders were prepar...La_(0.67)Ca_(0.33)MnO_3)_x /(ZrO_2)_(1-x) (x is the volume fraction ratio) percolated composites were fabricated by combining La_(0.67)Ca_(0.33)MnO_3 (LCMO) powders with ZrO_2 particles, where LCMO powders were prepared by sol-gel process. Low field magnetoresistance ( LFMR ) is greatly enhanced at low temperature when the system is close to the metallic percolation threshold of x=0.4. The magnetoresistance ratio of ( LCMO)_(0.4) /( ZrO_2 )_(0.6) in a 10 mT magnetic field at 77 K is 7.8 %, about 7.12 times larger than that of pure LCMO compound. The enhancement of spin-dependent tunneling of electrons at the interfaces between LCMO and ZrO_2 grains is responsible for the enhanced LFMR. With increasing ZrO_2 addition, Curie temperature T_C decreases firstly and then remains constant at about 220 K when (x<60%), showing limited substitution effect of Zr ions on B sites.展开更多
The Gd substituting effects for La in La0.67Ca0.33MnO3 has been studied. With increasing the substituting amount of Gd, the phase transition temperature of metal-isolator for the samples decreases, the corresponding p...The Gd substituting effects for La in La0.67Ca0.33MnO3 has been studied. With increasing the substituting amount of Gd, the phase transition temperature of metal-isolator for the samples decreases, the corresponding peak resistivity increases, the Curie temperature decreases monotonically. The substitution of La-Ca-Mn-O with 11% Gd for La improved the magnetoresistance ratio by an order of magnitude. The effects of substituting Gd can be explained in terms of the lattice effects. An irreversible MR behaviour was observed in Gd-substituting compounds. This effect became marked when the substituting amount of Gd was greater than 7%. A maximum irreversible increment of MR ratio as large as 91% was obtained when Gd substituting amount was 11%.展开更多
The electronic and magnetic properties of Ce doped SrMnO3 have been investigated us- ing the pseudo-potential plane wave method within the generalized gradient approximation method by first principles. The different M...The electronic and magnetic properties of Ce doped SrMnO3 have been investigated us- ing the pseudo-potential plane wave method within the generalized gradient approximation method by first principles. The different Mn-O bond lengths indicate that there is a strong Jahn-Teller distortion of the MnO6 octahedron, which associates with a structural phase transition from cubic symmetry （Pm3m） to tetragonal symmetry （I4/mcm）, and the Jahn- Teller ordering stabilizes a chain like （C-type） antiferromagnetie ground state. The electronic structures indicate that SrMnO3 and Sr1-xCexMnO3 （z=0.125 and 0.25） are semiconductor and metallic, respectively. The doping of SrMnO3 with cerium induces simultaneously a decrease in the electrical resistivity, which can be attributed to the formation of Mn3＋ as a result of charge compensation. The density of states and charge density map present that hybridization exists between some of O bands with those of Mn and Ce bands, the bonding between Sr and O is mainly ionic. Density of states and magnetic moment calculations show that the formal valence state of the Ce ion is trivalence.展开更多
A series of La0.7-xSmxSr0.3MnO3, La0.7-xGdxSr0.3MnO3, and La0.7-xDyxSr0.3MnO3 （x=0.00, 0.10, 0.20, 0.30） samples were prepared by the solid-state reaction method. The influence of the substitution of Sm, Gd, and Dy ...A series of La0.7-xSmxSr0.3MnO3, La0.7-xGdxSr0.3MnO3, and La0.7-xDyxSr0.3MnO3 （x=0.00, 0.10, 0.20, 0.30） samples were prepared by the solid-state reaction method. The influence of the substitution of Sm, Gd, and Dy for La on the magnetic and electric properties and on the magnetoresistance （MR） was studied through measurements of M-T curves and p-T curves. The results showed that： lattice distortion induced by substitution of Sm, Gd, and Dy for La and extra magnetism of substitution had great influence on the magnetic and electric properties of pcrovskite manganites; substitution of magnetic rare earth element for La was an effective way to change Curie temperature and to strengthen MR in perovskite manganites; and appropriate substitution proportion would generate large MR near room temperature.展开更多
An empirical formula of the critical temperature that is concentration dependent for polycrystalline （La1-xPrx）0.7Ca0.3MnO3 is presented in this paper. With this formula, the temperature dependence of resistance is ...An empirical formula of the critical temperature that is concentration dependent for polycrystalline （La1-xPrx）0.7Ca0.3MnO3 is presented in this paper. With this formula, the temperature dependence of resistance is simulated for various values of x by using the random resistor network model and the Monte Carlo method. The hysteresis effect in p-T curves is reasonably explained. The simulation results are in good agreement with the relevant experimental measurements.展开更多
By finely controlling the deposition parameters in the pulsed electron deposition process, granular La 2/3 Ca 1/3 MnO 3 （LCMO） film was grown on silicon substrates. The substrate temperature, ambient pressure in the...By finely controlling the deposition parameters in the pulsed electron deposition process, granular La 2/3 Ca 1/3 MnO 3 （LCMO） film was grown on silicon substrates. The substrate temperature, ambient pressure in the deposition chamber and acceleration potential for the electron beam were all found to affect the grain size of the film, resulting in different morphologies of the samples. Transport properties of the obtained granular films, especially the magnetoresistance （MR）, were studied. Prominent low-field MR was observed in all samples, indicating the forming of grain boundaries in the sample. The low-field MR show great sensitive to the morphology evolution, which reaches the highest value of about 40% for the sample with the grain size of about 250 nm. More interestingly, positive-MR （p-MR） was also detected above 300 K when low magnetic field applying, whereas it disappeared with higher magnetic field applied up to 1.5 and 2 Tesla. Instead of the spin- polarized tunneling process being commonly regarded as a responsible reason, lattice mismatch between LCMO film and silicon substrate appears to be the origin of the p-MR展开更多
The resistivity of the heavy-doped La1/3Ca2/3MnO3 （LCMO） is simulated using a random resistor network model, based on a phase separation scenario. The simulated results agree well with the reported experimental data...The resistivity of the heavy-doped La1/3Ca2/3MnO3 （LCMO） is simulated using a random resistor network model, based on a phase separation scenario. The simulated results agree well with the reported experimental data, showing a transition from a charge-disordered （CDO） state embedded with a few ferromagnetic （FM） metallic clusters to a charge-ordered （CO） state, corresponding to the transition from a high-temperature paramagnetic （PM） insulating state to a low-temperature antiferromagnetic （AF） insulating state. Furthermore, we find that the number of AF/CO clusters increases with decreasing temperature, and the clusters start to connect to each other around 250K, which causes percolating in the system. The results further verify that phase separation plays a crucial role in the electrical conductivity of LCMO.展开更多
The infrared spectra of La_(0.65)Ba_xMnO_(3-|?￡(c) (x = 0.35, 0.33 and 0.30) were investigated experimentally. The result shows that the sample La_(0.65)Ba_xMnO_(3-|?￡(c) has the largest Curie temperature and the sm...The infrared spectra of La_(0.65)Ba_xMnO_(3-|?￡(c) (x = 0.35, 0.33 and 0.30) were investigated experimentally. The result shows that the sample La_(0.65)Ba_xMnO_(3-|?￡(c) has the largest Curie temperature and the smallest resistivity and wave number of the stretching vibration mode of MnO_6 octaheUron at 300 K among the investigated samples. However, the absorption strength for the stretching vibration mode of Mn0_6 octahedron in La_(0.65)Ba_xMnO_(3-|?￡(c) is stronger for parainagnetic phase than that for ferromagnelic phase, which may be connected with the reducing of the dynamic in- coherent Jahn-Teller distortion below Curie temperature. In addition, the large shift of wave number for the stretching mode at the temperatures from 293 to 423 K has been observed in La_(0.65)Ba_xMnO_(3-|?￡(c), which may be due to the in- crease of the Mn-O bond length with temperature increasing.展开更多
Phase structures, the transport ana magnetic properties of the Perovskite-type manganite （La0.8-x CexSr0.2）0.97 MnO3（x = 0 - 0. 26） prepared by La2O3 containing CeO2 with different contents were studied. Experimen...Phase structures, the transport ana magnetic properties of the Perovskite-type manganite （La0.8-x CexSr0.2）0.97 MnO3（x = 0 - 0. 26） prepared by La2O3 containing CeO2 with different contents were studied. Experiments show that the compounds consist of a magnetic perovskite phase and non-magnetic CeO2 and Mn3O4. The resistivity and magnetoresistance ratio （MR） of the samples vary with changing x. Their room-temperature MR reaches -3% - - 14% at the magnetic field of 1 T. For x =0; x =0.037 and x = 0.26 samples, the conductance keeps unchanged basically in a relatively wide temperature range above 600 K, and the result shows that it is feasible for producing SOFC cathode materials with these samples.展开更多
The perovskite La_(0.67)Ca_(0.33)MnO_3/La_(0.67)Sr_(0.33)CoO_3/La_(0.67)Ca_(0.33)MnO_3 trilayers were fabricated by a facing-target sputtering technique and their magnetotransport properties were investigated. The mag...The perovskite La_(0.67)Ca_(0.33)MnO_3/La_(0.67)Sr_(0.33)CoO_3/La_(0.67)Ca_(0.33)MnO_3 trilayers were fabricated by a facing-target sputtering technique and their magnetotransport properties were investigated. The magnetoresistance is dependent on spacer thickness and dramatically decreases when La_(0.67)Sr_(0.33)CoO_3 layer is thick enough because of its short-circuiting effect. Different from La_(0.67)Ca_(0.33)MnO_3 single layer, trilayer films with thin La_(0.67)Sr_(0.33)CoO_3 spacer have the enhanced metal-semiconductor transition temperature (T_(MS)) of La_(0.67)Ca_(0.33)MnO_3 layers. The magnetic coercivity H_C shows a nonmonotonic behavior with changing the spacer layer thickness at 230 K. The waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom La_(0.67)Ca_(0.33)MnO_3 layers across the spacer La_(0.67)Sr_(0.33)CoO_3 layer.展开更多
基金supported by the National Natural Science Foundation of Chinathe National Basic Research Program of China(Grant Nos.2012CB922003,2011CBA00102,and 2009CB929502)
文摘We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb：SrTio3 p-n junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature of manganites; and the effect of particle size on magnetic properties in manganite nanoparticles.
文摘The extraordinary colossal magnetoresistance (CMR) behavior in Mn-site doped system La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15) was reported. It was found that the substitution with Cr on Mn sites introduces an additional bump in zero-field resistivity. With increasing Cr content, this additional bump grows up drastically while the original resistivity peak associated with magnetic order transition diminishes gradually. Under the applied magnetic field, both bumps of resistivity are deeply compressed, which leads to the appearance of two peaks in CMR response. As a result, the temperature range of CMR response is significantly broadened, spanned from the lowest to near room temperature. These results suggest that Mn-site element substitution could be a potent way of tuning CMR response.
文摘The representative sample La0.58Dy0.09Ca0.33MnO3 of Dy doped La0.67Ca0.33MnO3 rare-earth manganites was investigated. The most important effect of Dy doping is to introduce the magnetoimpurity and form the spin clusters which induce dramatically large CMR in La0.58Dy0.09Ca0.33MnO3. The fitting results of field-induced resistivity decrease to the Brillouin function indicate that the CMR is caused by the spin dependent hopping between spin clusters. It is the magnetic field that reduces the size of spin clusters and induces a field-induced irreversible CMR behaviour.
基金Funded by the National "973" Project of China (No.2006CB921606)the Foundation from the Ministry of the National Education (Nos. 309020and 20060487011)
文摘Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and paramagenetic ferromagnetic-transitions as well as colossal magnetoresistance is due to the formation of ferromagnetic clusters in the paramagnetic background. The transition to metallic state is resulted from percolation of ferromagnetic metallic clusters, while the colossal magnetoresistance is due to the application of magnetic field, which accelerates the growth of ferromagnetic metallic clusters and causes the shift of the onset temperature for the metallic percolation to higher temperature. Based on the random resistor network model, the zero-field resistivity versus temperature dependence is simulated by using experimental parameters, and experimental data well agree with those in whole temperature range, giving a strong support to our approach.
文摘A phenomenological model based on phase separation between ferromagnetic metallic and paramagnetic insulating domains was applied to analyze the electrical transport and colossal magnetoresistance for mixed-valent manganites of RE_(2/3)AE_(1/3)MnO_3. The results show that the model can yield results in agreement with experimental observations in these manganites. The present approach provides a simple picture to visualize the reason that the temperature dependence of resistance (with and without applied magnetic fields) in these compounds has the peculiar shape, without invoking any complicated concept.
基金financially supported by the Ministry of Science and Technology(MOST)NSF of China through the research projects(2018YFA03057001,11820101003)+2 种基金CAS Project for Young Scientists in Basic Research(YSBR-030)support of Beijing Nova program(2020133)the Youth Innovation Promotion Association of CAS(2020007).
文摘We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb,a new member of“111”type of diluted magnetic materials.The material crystallizes into Cu2Sb-type structure which is isostructural to“111”type Fe-based superconductors.With suitable carrier and spin doping,the Na(Zn,Mn)Sb establishes spin-glass ordering with freezing temperature(Tf)below 15 K.Despite lack of long-range ferromagnetic ordering,Na(Zn,Mn)Sb single crystal still shows sizeable anomalous Hall effect below Tf.Carrier concentration determined by Hall effect measurements is over 1019 cm-3.More significantly,we observe colossal negative magnetoresistance(MR≡[ρ(H)−ρ(0)]/ρ(0))of-94%in the single crystal sample.
基金supported by the National Natural Science Foundation of China (Nos. 50331040 and 60171043)
文摘Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 compound have been studied.A broad peak of magnetic entropy change （-△S M） is found above the Curie temperature （T C =120 K）,which can be associated with the existence of two-dimensional short range ferromagnetic order.Additionally,the curvilinear shape of-△S M for layered perovskite is quite different from that of the Ln 1-x A x MnO 3 probably arising from magnetocrystalline anisotropy.At the same time,a wide peak of colossal magnetoresitance effect near T C is found in the layered provskite La 1.4 Sr 1.6 Mn 2 O 7.
基金This work was supported by the National Science Foundation Grant No.DMR 1905277.
文摘Ferromagnetic semiconductor Ga_(1–x)Mn_(x)As_(1–y)P_(y) thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers.In this regime,we report a colossal negative magnetoresistance(CNMR)coexisting with a saturated magnetic moment,unlike in the traditional magnetic semiconductor Ga_(1–x)Mn_(x)As.By analyzing the temperature dependence of the resistivity at fixed magnetic field,we demonstrate that the CNMR can be consistently described by the field dependence of the localization length,which relates to a field dependent mobility edge.This dependence is likely due to the random environment of Mn atoms in Ga_(1-x)Mn_(x)As_(1-y)P_(y) which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.
文摘Electrical properties and magnetoresistance have been studied in two series of xAg-La0.67（Ca0.65Ba0.35）0.33MnO3 and xPd-La0.67（Ca0.65Ba0.35）0.33MnO3 （abbreviated by xAg-LCBMO and xPd-LCBMO） composites. Both Pd and Ag addition induce a decrease in resistivity and an increase in temperature at which the resistivity reaches its maximum. This is mainly due to the improvement of grain boundaries caused by the segregation of good conductive metal grains on the grain boundaries/surfaces. In addition, both Pd and Ag addition induce a large enhancement of room temperature magnetoresistance （RTMR）. Note that 27% molar ratio of Ag addition induces a large RTMR of about 70%, about ten times larger than pure LCBMO, whereas 27% molar ratio Pd addition brings a much larger RTMR of about 170%. The large enhancements of MR can be attributed to the decrease in resistivity of the samples caused by the good conductive metal. On the other hand, the polarization of Pd atoms near the Mn ions on the grain surfaces/boundaries plays a very im-portant role in the increase in MR, which induces a large number of spin clusters in Pd-added samples.
基金Project supported by the key program of Natural Science Foundation of Zhejiang Provincial, China （Grant No Z605131）, National Natural Science Foundation of China （Grant No 60571029）, the ‘Hundred Talents Project＇ of Chinese Academy of Sciences, China, the Creative Research Group of the National Natural Science Foundation of China （Grant No 60321001）.
文摘In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide （AAO） membranes, The magnetic and electronic transport properties are investigated by using the Quantum Design physics properties measurement system （PPMS） and magnetic properties measurement system （MPMS）. The resistance peak temperature （Tp） is about 85 K and the Curie temperature （To） is about 250 K for the LCMO film on an AAO membrane with a pore diameter of 20nm. Large magnetoresistance ratio （MR） is observed near Tp. The MR is as high as 85% under 1 T magnetic field. The great enhancement of MR at low magnetic fields could be attributed to the lattice distortion and the grain boundary that are induced by the nanopores on the AAO membrane.
文摘La_(0.67)Ca_(0.33)MnO_3)_x /(ZrO_2)_(1-x) (x is the volume fraction ratio) percolated composites were fabricated by combining La_(0.67)Ca_(0.33)MnO_3 (LCMO) powders with ZrO_2 particles, where LCMO powders were prepared by sol-gel process. Low field magnetoresistance ( LFMR ) is greatly enhanced at low temperature when the system is close to the metallic percolation threshold of x=0.4. The magnetoresistance ratio of ( LCMO)_(0.4) /( ZrO_2 )_(0.6) in a 10 mT magnetic field at 77 K is 7.8 %, about 7.12 times larger than that of pure LCMO compound. The enhancement of spin-dependent tunneling of electrons at the interfaces between LCMO and ZrO_2 grains is responsible for the enhanced LFMR. With increasing ZrO_2 addition, Curie temperature T_C decreases firstly and then remains constant at about 220 K when (x<60%), showing limited substitution effect of Zr ions on B sites.
基金This work was supported by the State Key Project of Funda-mental Research (G19980613010) the National Scientific and Technological Department of China.
文摘The Gd substituting effects for La in La0.67Ca0.33MnO3 has been studied. With increasing the substituting amount of Gd, the phase transition temperature of metal-isolator for the samples decreases, the corresponding peak resistivity increases, the Curie temperature decreases monotonically. The substitution of La-Ca-Mn-O with 11% Gd for La improved the magnetoresistance ratio by an order of magnitude. The effects of substituting Gd can be explained in terms of the lattice effects. An irreversible MR behaviour was observed in Gd-substituting compounds. This effect became marked when the substituting amount of Gd was greater than 7%. A maximum irreversible increment of MR ratio as large as 91% was obtained when Gd substituting amount was 11%.
文摘The electronic and magnetic properties of Ce doped SrMnO3 have been investigated us- ing the pseudo-potential plane wave method within the generalized gradient approximation method by first principles. The different Mn-O bond lengths indicate that there is a strong Jahn-Teller distortion of the MnO6 octahedron, which associates with a structural phase transition from cubic symmetry （Pm3m） to tetragonal symmetry （I4/mcm）, and the Jahn- Teller ordering stabilizes a chain like （C-type） antiferromagnetie ground state. The electronic structures indicate that SrMnO3 and Sr1-xCexMnO3 （z=0.125 and 0.25） are semiconductor and metallic, respectively. The doping of SrMnO3 with cerium induces simultaneously a decrease in the electrical resistivity, which can be attributed to the formation of Mn3＋ as a result of charge compensation. The density of states and charge density map present that hybridization exists between some of O bands with those of Mn and Ce bands, the bonding between Sr and O is mainly ionic. Density of states and magnetic moment calculations show that the formal valence state of the Ce ion is trivalence.
基金supported by the Key Programme of the National Natural Science Foundation of China (19934003)the Grand Programme of the Natu-ral Science Research of Education Bureau of Anhui Province (ZD2007003-1)+1 种基金 Natural Science Research Programme of Colleges and Universi-ties of Anhui Province (KJ2008A19ZC) Professors’ and Doctors’ Research Foundation of Suzhou College (2006jb02)
文摘A series of La0.7-xSmxSr0.3MnO3, La0.7-xGdxSr0.3MnO3, and La0.7-xDyxSr0.3MnO3 （x=0.00, 0.10, 0.20, 0.30） samples were prepared by the solid-state reaction method. The influence of the substitution of Sm, Gd, and Dy for La on the magnetic and electric properties and on the magnetoresistance （MR） was studied through measurements of M-T curves and p-T curves. The results showed that： lattice distortion induced by substitution of Sm, Gd, and Dy for La and extra magnetism of substitution had great influence on the magnetic and electric properties of pcrovskite manganites; substitution of magnetic rare earth element for La was an effective way to change Curie temperature and to strengthen MR in perovskite manganites; and appropriate substitution proportion would generate large MR near room temperature.
基金Project supported by the National Natural Science Foundation of China（Grant No 60476047） and the Natural Science Foundation of Henan Province （Grant No 0411011700）.
文摘An empirical formula of the critical temperature that is concentration dependent for polycrystalline （La1-xPrx）0.7Ca0.3MnO3 is presented in this paper. With this formula, the temperature dependence of resistance is simulated for various values of x by using the random resistor network model and the Monte Carlo method. The hysteresis effect in p-T curves is reasonably explained. The simulation results are in good agreement with the relevant experimental measurements.
基金Funded by the National Natural Science Foundation of China(No.10875107)the Aeronautical Science Foundation(No.2010ZF55013)+1 种基金the Basic and Advanced Technology Program of Henan Province (No.112300410229)the Foundation for University Young Key Teacher by Henan Province (No. 2010GGJS-146)
文摘By finely controlling the deposition parameters in the pulsed electron deposition process, granular La 2/3 Ca 1/3 MnO 3 （LCMO） film was grown on silicon substrates. The substrate temperature, ambient pressure in the deposition chamber and acceleration potential for the electron beam were all found to affect the grain size of the film, resulting in different morphologies of the samples. Transport properties of the obtained granular films, especially the magnetoresistance （MR）, were studied. Prominent low-field MR was observed in all samples, indicating the forming of grain boundaries in the sample. The low-field MR show great sensitive to the morphology evolution, which reaches the highest value of about 40% for the sample with the grain size of about 250 nm. More interestingly, positive-MR （p-MR） was also detected above 300 K when low magnetic field applying, whereas it disappeared with higher magnetic field applied up to 1.5 and 2 Tesla. Instead of the spin- polarized tunneling process being commonly regarded as a responsible reason, lattice mismatch between LCMO film and silicon substrate appears to be the origin of the p-MR
基金Supported by the National Natural Science Foundation of China under Grant No 10334070. We would like to thank Professor Jin K. J. for her helpful discussion.
文摘The resistivity of the heavy-doped La1/3Ca2/3MnO3 （LCMO） is simulated using a random resistor network model, based on a phase separation scenario. The simulated results agree well with the reported experimental data, showing a transition from a charge-disordered （CDO） state embedded with a few ferromagnetic （FM） metallic clusters to a charge-ordered （CO） state, corresponding to the transition from a high-temperature paramagnetic （PM） insulating state to a low-temperature antiferromagnetic （AF） insulating state. Furthermore, we find that the number of AF/CO clusters increases with decreasing temperature, and the clusters start to connect to each other around 250K, which causes percolating in the system. The results further verify that phase separation plays a crucial role in the electrical conductivity of LCMO.
基金This work was financially supported by the Trans-Century Foundation of the Educational Ministry of China and by the National Nat
文摘The infrared spectra of La_(0.65)Ba_xMnO_(3-|?￡(c) (x = 0.35, 0.33 and 0.30) were investigated experimentally. The result shows that the sample La_(0.65)Ba_xMnO_(3-|?￡(c) has the largest Curie temperature and the smallest resistivity and wave number of the stretching vibration mode of MnO_6 octaheUron at 300 K among the investigated samples. However, the absorption strength for the stretching vibration mode of Mn0_6 octahedron in La_(0.65)Ba_xMnO_(3-|?￡(c) is stronger for parainagnetic phase than that for ferromagnelic phase, which may be connected with the reducing of the dynamic in- coherent Jahn-Teller distortion below Curie temperature. In addition, the large shift of wave number for the stretching mode at the temperatures from 293 to 423 K has been observed in La_(0.65)Ba_xMnO_(3-|?￡(c), which may be due to the in- crease of the Mn-O bond length with temperature increasing.
文摘Phase structures, the transport ana magnetic properties of the Perovskite-type manganite （La0.8-x CexSr0.2）0.97 MnO3（x = 0 - 0. 26） prepared by La2O3 containing CeO2 with different contents were studied. Experiments show that the compounds consist of a magnetic perovskite phase and non-magnetic CeO2 and Mn3O4. The resistivity and magnetoresistance ratio （MR） of the samples vary with changing x. Their room-temperature MR reaches -3% - - 14% at the magnetic field of 1 T. For x =0; x =0.037 and x = 0.26 samples, the conductance keeps unchanged basically in a relatively wide temperature range above 600 K, and the result shows that it is feasible for producing SOFC cathode materials with these samples.
文摘The perovskite La_(0.67)Ca_(0.33)MnO_3/La_(0.67)Sr_(0.33)CoO_3/La_(0.67)Ca_(0.33)MnO_3 trilayers were fabricated by a facing-target sputtering technique and their magnetotransport properties were investigated. The magnetoresistance is dependent on spacer thickness and dramatically decreases when La_(0.67)Sr_(0.33)CoO_3 layer is thick enough because of its short-circuiting effect. Different from La_(0.67)Ca_(0.33)MnO_3 single layer, trilayer films with thin La_(0.67)Sr_(0.33)CoO_3 spacer have the enhanced metal-semiconductor transition temperature (T_(MS)) of La_(0.67)Ca_(0.33)MnO_3 layers. The magnetic coercivity H_C shows a nonmonotonic behavior with changing the spacer layer thickness at 230 K. The waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom La_(0.67)Ca_(0.33)MnO_3 layers across the spacer La_(0.67)Sr_(0.33)CoO_3 layer.
