Controlled temperature broadening of colossal magnetoresistance in a manganite heterostructure
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Abstract:
An intrinsic characteristic of the “colossal” magnetoresistance manganite compounds is that the resistance and the magnetoresistance vary strongly with temperature over the small temperature regime in which the magnetoresistance is exceptionally large. We propose a heterostructure constructed of layers of varying composition manganites which extends the regime of large magnetoresistance and greatly broadens the sharp peak in resistance. Data from a prototype heterostructure are presented that demonstrate the effectiveness of this method.Keywords:
Manganite
Colossal Magnetoresistance
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The low-field magnetoresistance (MR) properties of polycrystalline La0.67Sr0.33MnO3 and La0.67CaO33MnO3 thin films with different grain sizes have been investigated and compared with epitaxial films. MR as high as 15% has been observed in the polycrystalline films at a field of 1500 Oe at low temperatures, whereas the MR of the epitaxial films is less than 0.3% in the same field range. Based on the magnetization dependence of the MR, the current-voltage characteristics, and the temperature dependence of the resistivity, we attribute the low-field MR to spin-dependent scattering of polarized electrons at the grain boundaries which serve as pinning centers for the magnetic domain walls.
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It is well established that the resistivity of the manganites is a strong function of the magnetization. Near the ferromagnetic ordering temperature, colossal changes in the resistivity are seen in applied fields of several Tesla; such fields are too large for a number of potential applications. An alternative approach is to change the state of magnetization by injecting spin polarized carriers into manganite/classical ferromagnet heterostructures. In this work, results on manganite/normal-metal heterostructures in current perpendicular-to-plane geometry are reported. We observe a colossal magnetoresistance in fields of the order of 1 T which we attribute to magnetic interface scattering. The magnitude of this magnetoresistive effect can be controlled by the applied voltage, i.e., the heterostructures act as magnetic sensors with variable sensitivity. Implications of the interface resistance on spin injection from classical ferromagnets into manganites are discussed.
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Lanthanum
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Magnetic field induced properties of manganite perovskites with colossal magnetoresistance (invited)
We present a systematic study of the magnetotransport and magnetic properties of the half-doped La0.5Ca0.5MnO3+δ system. The solid is a metamagnet which undergoes a first-order antiferromagnet (AFM) to ferromagnet (FM) phase transition under a field or by changing temperature. Associated with the AFM–FM transition is an insulator to metal transition. A maximum 109-fold magnetoresistance ratio has been observed at 4.2 K between the least and the most conductive states. At low T (⩽50 K), we have also observed two additional metastable electronic states in the canted AFM state at certain fields. The resistivity of each state differs from one another by at least one order of magnitude. The existence of these multiple states may be related to the unique charge- and spin-ordered state of the half-doped manganite.
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Charge ordering
Metal–insulator transition
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An n-n heterojunction composed of La0.33Ca0.67MnO3 and Nb-doped SrTiO3 was fabricated, and it shows good rectifying property. The temperature variation of junction resistance for high reverse voltage exhibits a metal-insulator-like transition that shifts to high temperatures with further increasing voltage. The heterojunction presents a remarkable positive magnetoresistance under the reverse bias voltage at low temperatures, and the maximum of magnetoresistance can even reach ∼400% under a field of 1T. A qualitative explanation is given based on the analysis of the electron filling near the interface and its tunable feature under the bias voltage and magnetic field. This result can be helpful for both the understanding of the manganites and the future applications of the manganite-based devices.
Manganite
Colossal Magnetoresistance
Metal–insulator transition
Lanthanum manganite
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Magnetic refrigeration
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A very large positive magnetoresistance (MR) has been discovered in a Fe3O4/SrTiO3/ La0.7Sr0.3MnO3 heterostructure for the transport perpendicular to the layer planes and applied magnetic field in the film plane. The observed MR features do not show any obvious correlation with the hysteresis behavior of the ferromagnetic bilayers. A possible explanation of these results is given in terms of the relative differences in the majority and minority spin bands of the two ferromagnetic layers and the field induced modifications of domain structures therein.
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We report the observation of a large magnetoresistance (83%) at low magnetic fields of tens of Oe at 4.2 K in the epitaxial trilayer junction structure, La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3. The spin-polarization parameter of the manganite has been determined from the magnetoresistance value. The switching fields of the two magnetic layers were designed by using the magnetic shape anisotropy. By limiting the sweeping field in a low field range (∼100 Oe), we have achieved bistable resistive states at zero field, which is of potential interest for magnetoelectronic applications.
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Colossal Magnetoresistance
Bistability
Resistive touchscreen
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Large magnetoresistive materials are of immense interest for a number of spintronic applications by developing high density magnetic memory devices, magnetic sensors and magnetic switches. Colossal magnetoresistance, for which resistivity changes several order of magnitude (${\sim10^4 \%}$) in an external magnetic field, occurs mainly in phase separated oxide materials, namely manganites, due to the phase competition between the ferromagnetic metallic and the antiferromagnetic insulating regions. Can one further enhance the magnetoresistance by tuning the volume fraction of the two phases? In this work, we report a huge colossal magnetoresistance along with the ultra-sharp metamagnetic transition in half doped ${Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3}$ manganite compound by suitably tuning the volume fraction of the competing phases. The obtained magnetoresistance value at 10 K is as large as $\sim10^{13}\%$ in a 30 kOe external magnetic field and $\sim10^{15}\%$ in 90 kOe external magnetic field and is several orders of magnitude higher than any other observed magnetoresistance value reported so far. Using model Hamiltonian calculations we have shown that the inhomogeneous disorder, deduced from tunneling electron microscopy, suppresses the CE-type phase and seeds the ferromagnetic metal in an external magnetic field.
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In this letter, an oxide heterostructure has been fabricated by successively growing La0.7Sr0.3MnO3 and ZnO layers on a LaAlO3 (100) substrate using pulsed laser deposition. The ZnO∕La0.7Sr0.3MnO3 heterostructure exhibits good rectifying behavior and a positive colossal magnetoresistance (MR) effect over a temperature range of 77–280K. The maximum MR values are determined to be about 53.9% at H=0.5T and 36.4% at H=0.3T. A possible explanation is given in terms of the effect of magnetic fields on the depletion layer and the capture carriers effect at the interface.
Colossal Magnetoresistance
Pulsed Laser Deposition
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