Supramolecular spin valves

Magnetic molecules are potential building blocks for the design of spintronic devices. Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication. The development of solid-state spintronic devices based on the giant magnetoresistance, tunnel magnetoresistance and spin-valve effects has revolutionized magnetic memory applications. Recently, a significant improvement of the spin-relaxation time has been observed in organic semiconductor tunnel junctions, single non-magnetic molecules coupled to magnetic electrodes have shown giant magnetoresistance and hybrid devices exploiting the quantum tunnelling properties of single-molecule magnets have been proposed. Herein, we present an original spin-valve device in which a non-magnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc(2) single-molecule magnets (Pc=phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of new spintronic devices with quantum properties.     

A spin-valve-like magnetoresistance of an antiferromagnet-based tunnel junction

A spin valve is a microelectronic device in which high- and low-resistance states are realized by using both the charge and spin of carriers. Spin-valve structures used in modern hard-drive read heads and magnetic random access memoriescomprise two ferromagnetic electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the desired giant or tunnelling magnetoresistance effect. Here we demonstrate more than 100% spin-valve-like signal in a NiFe/IrMn/MgO/Pt stack with an antiferromagnet on one side and a non-magnetic metal on the other side of the tunnel barrier. Ferromagneticmoments in NiFe are reversed by external fields of approximately 50 mT or less, and the exchange-spring effect of NiFe on IrMn induces rotation of antiferromagnetic moments in IrMn, which is detected by the measured tunnelling anisotropic magnetoresistance. Our work demonstrates a spintronic element whose transport characteristics are governed by an antiferromagnet. It demonstrates that sensitivity to low magnetic fields can be combined with large, spin-orbit-coupling-induced magnetotransport anisotropy using a single magnetic electrode. The antiferromagnetic tunnelling anisotropic magnetoresistance provides a means to study magnetic characteristics of antiferromagnetic films by an electronic-transport measurement.     

Spin control by application of electric current and voltage in FeCo-MgO junctions

Efficient control and detection of spins are the most important tasks in spintronics. The current and voltage applied to a magnetic tunnel junction may exert a torque on the magnetic thin layer in the junction and cause its reversal or continuous precession. The discovery of the giant tunnelling magnetoresistance effect in ferromagnetic tunnelling junctions using an MgO barrier enabled us to obtain a large signal output from the magnetization reversal and precession. Also, the interplay of large spin configuration-electric conduction coupling provides highly nonlinear effects like the spin-torque diode effect. The negative resistance effect and amplification using it are predicted. A new discovery about a voltage-induced magnetic anisotropy change in Fe ultrathin films is also discussed.     

Magnetoresistance and electroresistance effects in Fe3O4 nanoparticle system

Nearly monodisperse spherical magnetite (Fe₃O₄) nanoparticles are prepared by colloidal chemistry route. Magnetic and electronic transport properties of the annealed pellets of these nanoparticles are reported. Effect of external magnetic and electric fields on the magnetic and transport properties of the material are studied as a function of temperature. We find that the highest resistance state of the ferromagnetic system occurs at a magnetic field which is approximately equal to its magnetic coercivity; this establishes the magnetoresistance (MR) in this system to be of the conventional tunnelling type MR as against the spin-valve type MR found more recently in some ferromagnetic oxide systems. The material also shows electroresistance (ER) property with its low-temperature resistance being strongly dependent on the excitation current that is used for the measurement. This ER effect is concluded to be intrinsic to the material and is attributed to the electric field-induced melting of the charge-order state in magnetite.     

Solid-state memories based on ferroelectric tunnel junctions

Ferroic-order parameters are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1?×?10(6)?A?cm(-2)). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ～1?×?10(4)?A?cm(-2) at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories, and have the advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism.     

Reversible electrical switching of spin polarization in multiferroic tunnel junctions

Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.     

Perpendicular transport in layered magnetic systems: ab initio study

We will review the results of the ab initio study of the current-perpendicular-to-plane magnetotransport of a sample sandwiched by two ideal leads. The ‘active’ part of the system is either a trilayer consisting of two magnetic slabs of a finite thickness separated by a non-magnetic spacer, or a multilayer formed by alternating magnetic and non-magnetic layers. We use the Kubo-Landauer formulation of the conductance based on surface Green functions as formulated by means of the tight-binding linear muffin-tin orbital method. The formalism is extended to the case of lateral supercells with random arrangements of atoms of two types, which in turn allows to deal with specular and diffusive scattering on equal footing. Applications refer to fcc-based Co/Cu/Co(0 0 1) multilayers including the transport through layers with non-collinear alignments of their magnetizations. We consider in detail the effect of substitutional alloying in the spacer as well as interdiffusion at the interfaces between magnetic and spacer layers.     

(La2/3Ca1/3)(Mn(3-x)/3)Fex/3)O3体系磁电阻行为的研究

通过系统地测量（La2/3Ca1/3）（Mn（3-x）／3Fex／3）O3（x＝0、0.1、0．2、0．3的体系样品的电阻率-温度关系以及一定温度下磁电阻率与磁场的关系,发现随x的变化其磁电阻率峰和电阻率峰均发生位移,磁电阻率峰值增大,并伴生磁电阻率峰展宽效应．作者认为由于Fe的替代,引起体系中Mn3 ／Mn4 比率及磁矩的变化,加之外场对磁有序结构的调制作用,从而影响了Mn3 －O－Mn4 的双交换作用,最终导致磁电阻行为发生变化．     

Switching the conductance of Dy nanocontacts by magnetostriction

The electrical conductance G of mechanical break-junctions fabricated from the rare-earth metal dysprosium has been investigated at 4.2 K where Dy is in the ferromagnetic state. In addition to the usual variation of the conductance while breaking the wire mechanically, the conductance can be changed reproducibly by variation of the magnetic field H, due to the large magnetostriction of Dy. For a number of contacts, we observe discrete changes in G(H) in the range of several G(0) = 2e(2)/h. The behavior of G(H) and its angular dependence can be quantitatively understood by taking into account the magnetostrictive properties of Dy. This realization of a magnetostrictive few-atom switch demonstrates the possibility of reproducibly tuning the conductance of magnetic nanocontacts by a magnetic field.     

Nanocontact magnetoresistance and quantized conductance

The behavior of the magnetoresistance (MR) effect in a ferromagnetic Ni nanocontact is investigated in the regime of conductance quantization (1/12900 /spl Omega//sup -1/). The giant MR effect is observed in the case that the nanocontact with quantized conductance is fabricated. We sometimes observe that the conductance discretely changes by the quantization unit of e/sup 2//h and that the quantized conductance is switched by the application of magnetic field. There exists a close relationship between nanocontact MR and quantized conductance.     

Ferromagnetic resonance driven by spin transfer torque

We study spin-torque-driven ferromagnetic resonance (ST-FMR) in point contacts. Point contacts as small as a few nanometers in size are used to inject microwave currents into F/N/F spin valves where two ferromagnetic (F) layers are separated by a nonmagnetic (N) metal spacer. High densities of injected currents produce the spin-transfer torque on magnetic moments and drive FMR in the F-layers. The resonance is detected electrically when a small rectified dc voltage appears across the point contact. Here we focus on the origin of this rectified signal and study ST-FMR in point contacts to spin valves with different ferromagnets (Py and Co) and single ferromagnetic (Py) films, as well as in spin-valve wires patterned by electron beam lithography. We find that this voltage can be explained by the resistance variations which originate from giant magnetoresistance in point contacts to spin valves and involve effects of anisotropic magnetoresistance and extraordinary Hall effect on the propagation of microwave currents in continuous F-films and microwires.     

Large magnetocaloric effect and magnetoresistance in ErNi single crystal

The magnetic properties,magnetocaloric effect and magnetoresistance in ErNi single crystal have been investigated in detail.With decreasing temperature,ErNi single crystal undergoes two successive mag-netic transitions:a paramagnetic to ferromagnetic transition at Tc=11 K and a spin-reorientation transition at TSR=5 K.Meanwhile,a sharp field-induced metamagnetic transition is observed below the Tc along the a axis.ErNi single crystal possesses a giant magnetocaloric effect around Tc.The maximum magnetic entropy change is-36.1 J(kg K)-1 along the a axis under the field change of 0-50 kOe.In par-ticular,the rotating magnetocaloric effect in ErNi single crystal reaches its maximum under a relatively low field,and the maximum rotating entropy change with a value of 9.3 J(kg K)-1 is obtained by rotat-ing the applied field from the[011]to[100]directions under 13 kOe.These results suggest that ErNi could be a promising candidate for magnetic refrigeration working at liquid-helium temperature region.Moreover,a complicated transport behavior is uncovered in ErNi single crystal,which is attributed to the complex magnetic states and magnetic polaronic effect.Both positive and negative magnetoresistance are observed.A considerable large magnetoresistance with the value of-34.5％is acquired at 8 K under 50 kOe when the field is along the[100]direction.     

(La2/3Ca1/3)(Mn(3-x)/3)Fex/3)O3体系磁电阻行为的研究

通过系统地测量（La_2/3Ca_1/3）（Mn_{（3-x）／3}Fe_x／3）O₃（x＝0、0.1、0．2、0．3的体系样品的电阻率-温度关系以及一定温度下磁电阻率与磁场的关系,发现随x的变化其磁电阻率峰和电阻率峰均发生位移,磁电阻率峰值增大,并伴生磁电阻率峰展宽效应．作者认为由于Fe的替代,引起体系中Mn³⁺／Mn⁴⁺比率及磁矩的变化,加之外场对磁有序结构的调制作用,从而影响了Mn³⁺－O^－Mn⁴⁺的双交换作用,最终导致磁电阻行为发生变化．     

Co0.525Fe0.475MnP化合物的室温磁热效应和磁电阻效应

采用多步骤固态烧结方法合成了具有单一Co₂P相的Co_0.525Fe_0.475MnP化合物,其反铁磁有序温度在室温附近。在升温过程中,这种化合物经历两个连续的磁转变：在285 K发生反铁磁到铁磁的一级相变,在375 K发生由铁磁到顺磁的二级相变。在0~5 T的外磁场中,两个相变点温度对应的最大磁熵变分别为1.1 J/(kg·K)(303 K)和-2.0 J/(kg·K)(383 K)。外磁场为零时,随着温度的降低电阻率曲线在铁磁到反铁磁转变温度附近出现极小值,是铁磁有序与反铁磁有序的竞争所致。在35 K再次出现的电阻率极小值,可归因于由Fe替代Co引起的自旋无序所导致的金属-绝缘体转变。在5 T磁场中磁电阻率的最大值对应温度为200 K时的-2.5%,在反铁磁温度以上磁电阻率迅速减小。这表明,这种化合物的磁电阻效应源于外磁场对反铁磁有序的影响。     

Single electron spintronics

Single electron electronics is now well developed, and allows the manipulation of electrons one-by-one as they tunnel on and off a nanoscale conducting island. In the past decade or so, there have been concerted efforts in several laboratories to construct single electron devices incorporating ferromagnetic components in order to introduce spin functionality. The use of ferromagnetic electrodes with a non-magnetic island can lead to spin accumulation on the island. On the other hand, making the dot also ferromagnetic introduces new physics such as tunnelling magnetoresistance enhancement in the cotunnelling regime and manifestations of the Kondo effect. Such nanoscale islands are also found to have long spin lifetimes. Conventional spintronics makes use of the average spin-polarization of a large ensemble of electrons: this new approach offers the prospect of accessing the quantum properties of the electron, and is a candidate approach to the construction of solid-state spin-based qubits.     

Quantized magnetoresistance in atomic-size contacts

When the dimensions of a metallic conductor are reduced so that they become comparable to the de Broglie wavelengths of the conduction electrons, the absence of scattering results in ballistic electron transport and the conductance becomes quantized. In ferromagnetic metals, the spin angular momentum of the electrons results in spin-dependent conductance quantization and various unusual magnetoresistive phenomena. Theorists have predicted a related phenomenon known as ballistic anisotropic magnetoresistance (BAMR). Here we report the first experimental evidence for BAMR by observing a stepwise variation in the ballistic conductance of cobalt nanocontacts as the direction of an applied magnetic field is varied. Our results show that BAMR can be positive and negative, and exhibits symmetric and asymmetric angular dependences, consistent with theoretical predictions.     

用FTMS方法制备Co／Cu巨磁阻超晶格多层薄膜

本文用双靶相对磁控溅射法（ＦＴＭＳ）在云母单晶基板上原位生长了巨磁阻Ｃｏ／Ｃｕ超晶格薄膜，研究了放电气压及背景真空对薄膜结构和电磁性质的影响。     

Magnetic and electrical properties of Zn3P2 + MnP materials

We have synthesized a new ferromagnetic material based on a Zn₃P₂ + MnP composite with a Curie temperature of 290 K. Experimental data demonstrate that its magnetic and electrical properties are determined by manganese phosphide nanoclusters. The composite has metallic conductivity. Its electrical resistance drops with increasing magnetic field, and its magnetoresistance in a magnetic field of 14 T at 7 K reaches 9%.     

Magnetic characterization of MnPt/CoFe bilayers using the MOKE technique

The performance of magnetoresistive devices (spin valves, tunnel junctions), made of two ferromagnetic (FM) layers and separated by a non-magnetic spacer, rely on the existence of two well separated resistance states. For this to occur, one of the FM layer is deposited just adjacent to an antiferromagnetic (AFM) layer. Due to the exchange interaction at the AFM/FM interface, the reversal of the magnetization (M) of such FM-pinned layer occurs at a high applied magnetic field. The magnetization of the other FM layer reverses almost freely when a small magnetic field is applied. Here we study the exchange bias effect in the MnPt (t)/CoFe (50 Å) system, using the Magneto-Optical Kerr Effect (MOKE) and domain imaging techniques. The exchange (H_E) and coercive (H_c) fields increase with increasing AFM thickness, saturating for t > 200 Å (H_E ≈ 670 Oe and H_c ≈ 315 Oe). Furthermore, we observe that the value of the exchange field is almost independent of the applied magnetic field sweeping rate (up to ≈ 300 kOe/s). Domain imaging allowed us to conclude that magnetization reversal in the studied system proceeds essentially by coherent magnetic moment rotation.     

Non-destructive evaluation of defects in ferromagnetic plates using a sensitive magnetic sensor based on second harmonic response of superconducting Bi1.6Pb0.4Sr4Ca2Cu3O10+δ pellet

The characteristics of a magnetic sensor, based on the non-linear electromagnetic response of the weak links present in polycrystalline BPSCCO superconductor are reported. The second harmonic response of the sensor in an alternating magnetic field at 40 kHz and at 77 K being a strong linear function of low d.c. field is utilized for magnetic field sensing. The noise limited resolution of the sensor is found to be 3.16 x 10⁻⁹ T/√Hz forH _a.c.= 16 G and frequency 40 kHz. The magnetic sensor has been applied for nondestructive detection of various types of flaws in ferromagnetic plates and also for detection of small magnetic inclusions in a non-magnetic matrix. Our results suggest that the 2f response based BPSCCO superconductive magnetometer has potential for its application in the area of non-destructive evaluation of defects in ferromagnetic materials.