We have investigated the magnetoresistive behavior of Dirac semi-metal Cd3As2 down to low temperatures and in high magnetic fields. A positive and linear magnetoresistance (LMR) as large as 3100% is observed in a magnetic field of 14 T, on high-quality single crystals of Cd3As2 with ultralow electron density and large Lande g factor. Such a large LMR occurs when the magnetic field is applied perpendicular to both the current and the (100) surface, and when the temperature is low such that the thermal energy is smaller than the Zeeman splitting energy. Tilting the magnetic field or raising the temperature all degrade the LMR, leading to a less pronounced quadratic behavior. We propose that the phenomenon of LMR is related to the peculiar field-induced shifting/distortion of the helical electrons' Fermi surfaces in momentum space.Compared with those negative magnetoresistive behaviors such as giant magnetoresistance [1] and colossal magnetoresistance [2] whose mechanisms have been well understood, positive large LMR was also reported in past decades but its mechanism is not fully clarified. Such behavior was found in highly disordered nonmagnetic narrow-band semiconductors such as Ag 2±δ Te and Ag 2±δ Se [3], in bismuth thin films [4], and in Dirac electron systems such as epitaxial graphene [5] and topological insulators-related materials [6][7][8][9][10]. Large LMR was also observed in InSb [11], a material with very small electron effective mass and very large electron Lande g factor. Several theories have been proposed to explain the phenomenon. Abrikosov proposed that the LMR is a quantum magnetoresistance of linearly dispersed electron systems, arising when all the electrons are filled in the first Landau level (LL), i.e., in the extreme quantum limit [12,13]. Wang and Lei proposed that the LMR can still arise when the LLs are smeared, if with a positive g factor [14]. There are also pictures involving no LLs. Parish and Littlewood explained the LMR in Ag 2±δ Te and Ag 2±δ Se by modeling the materials as a network due to disorder-induced mobility fluctuation [15]. So far the mechanism of LMR is still waiting to be clarified.In this work, we revisit the LMR issue by investigating the magnetoresistive behavior of Cd 3 As 2 single crystals. Cd 3 As 2 is predicted to be a three-dimensional (3D) Dirac semimetal [16], with linearly dispersed electron states in the bulk, and Fermi arcs at the surface which connect the bulk Dirac cones. The existence of 3D Dirac cones has been confirmed by angular resolved photoemission spectroscopy study [17]. And a LMR behavior has recently been observed on samples with Fermi level well above the Dirac point [9, 10], i.e., with carrier density of the order 10 18 cm −3 . Here, we report our investigations on single crystals of Cd 3 As 2 with a much lower carrier density, such that the Fermi surfaces are small spheres very close to the Dirac points in the momentum space, rather than near the Lifshitz transition (i.e., touching with each other). We observed a positive, very large an...

MgO-barrier magnetic tunnel junction sensors with both CoFeB layers pinned by IrMn have been fabricated, which show a tunneling magnetoresistance (TMR) of up to 255% at room temperature. The perpendicular configuration for magnetic field sensing is set using a two-step field annealing process. The linear TMR field range and sensitivity are tuned by inserting an ultrathin Ru layer between the upper IrMn and the top-pinned CoFeB layer. The field sensitivity reaches 26%/mT, while the noise detectivity is about 90 nT/Hz at 10 Hz for a 0.3 nm Ru insertion layer. The bias dependence of the noise suggests that this is a useful design for sensor applications.

The key physics of the spin valve involves spin-polarized conduction electrons propagating between two magnetic layers such that the device conductance is controlled by the relative magnetization orientation of two magnetic layers. Here, we report the effect of a magnon valve which is made of two ferromagnetic insulators (YIG) separated by a nonmagnetic spacer layer (Au). When a thermal gradient is applied perpendicular to the layers, the inverse spin Hall voltage output detected by a Pt bar placed on top of the magnon valve depends on the relative orientation of the magnetization of two YIG layers, indicating the magnon current induced by the spin Seebeck effect at one layer affects the magnon current in the other layer separated by Au. We interpret the magnon valve effect by the angular momentum conversion and propagation between magnons in two YIG layers and conduction electrons in the Au layer. The temperature dependence of the magnon valve ratio shows approximately a power law, supporting the above magnon-electron spin conversion mechanism. This work opens a new class of valve structures beyond the conventional spin valves.

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α′-FeRh-based junctions driven by the magnetic phase transition of α′-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α′-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α′-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.

We compare low frequency noise in magnetic tunnel junctions with MgO barriers prepared by electron-beam evaporation with those prepared by radiofrequency sputtering, both showing a high tunneling magnetoresistance. The normalized noise parameter in the parallel state of junctions with evaporated barriers is at least one order of magnitude lower than that in junctions with sputtered barriers, and exhibits a weaker bias dependence. The lowest normalized noise is in the 10 −11 m 2 range. A lower density of oxygen vacancies acting as charge trap states in the evaporated MgO is responsible for the lower noise. © 2010 American Institute of Physics. ͓doi:10.1063/1.3431620͔Magnetic tunnel junctions ͑MTJs͒ have attracted a great deal of attention since the demonstration of the tunneling magnetoresistance ͑TMR͒ effect at room temperature. 1,2 Following theoretical predictions, 3,4 a large TMR ratio of up to 200% at room temperature in MTJs with CoFeB electrodes and MgO tunnel barriers was achieved. 5,6 A record room temperature TMR of 604% has since been reported in a pseudospin valve stack, 7 which is close to the theoretical maximum. 3,4 Major advances in MTJs have led to important applications in hard-disk read heads, 8 sensors, 9 and magnetic random access memory. 10 For magnetic field sensing applications, both signal and noise in the MTJ are of equal significance. It has been found previously that 1 / f noise usually dominates other kinds of noise, e.g., thermal noise and shot noise, at frequencies up to a few kilohertz in an MTJ device, and it limits the low-frequency sensitivity. [11][12][13][14][15][16] The 1 / f noise in these devices is normally quantified by the Hooge parameter ␣, defined as AfS V / V 2 , in which A is the junction area, f the frequency, S V the noise power spectrum density, and V the voltage applied to the MTJ. 12,17 In conventional radio frequency ͑rf͒ sputtered MTJs with CoFeB electrodes and MgO tunnel barriers, ␣ was reported to decrease as a function of annealing time and reach a minimum value of 2 ϫ 10 −10 m 2 . 18 The lowest ␣ value ever observed in MTJs, between ͑1-5͒ ϫ 10 −11 m 2 , was achieved in fully epitaxial Fe͑100͒/MgO͑100͒/Fe͑100͒ junctions grown by molecular beam epitaxy ͑MBE͒. 19 Recently, some of us suggested electron-beam evaporation as an alternative to MBE or rf-sputtering for growing high quality MgO tunnel barriers. The junctions fabricated in this way have been shown to possess high TMR ratios of ϳ240% at room temperature and a reduced density of oxygen vacancies, compared to counterparts grown entirely by rf-sputtering. 20 In this work, we report on low frequency noise in the MTJs with electron-beam evaporated MgO barriers of different thicknesses ͑EB-MTJs͒, comparing it to that measured in our MTJs with rf-sputtered MgO tunnel barriers ͑rf-MTJs͒. We show that the Hooge parameter ␣ in the EB-MTJs in their parallel state is at least one order of magnitude lower than that in the rf-MTJs, across a large range of resistance-area ͑RA͒ product 5 ϫ 10 2 Ͻ RAϽ 1 ϫ 10 6 ⍀ m 2 ...

Inverted tunneling magnetoresistance, where resistance decreases as the free layer in a magnetic tunnel junction flips its direction of magnetization after saturation, has been observed at zero bias in magnetic tunnel junctions with a thin CoFeB layer in the pinned synthetic antiferromagnetic CoFe/ Ru/ CoFeB stack. Magnetoresistance values as high as −55% at room temperature are measured in MgO-based tunnel junctions when the thickness of the pinned CoFeB layer is 1.5 nm. The inverted magnetoresistance is associated with imbalance of the synthetic antiferromagnetic pinned layer. Asymmetric bias dependence with a magnetoresistance sign change is observed for a 0.5 nm pinned CoFeB layer. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2779241͔MgO-based magnetic tunnel junctions ͑MTJs͒ have transformed the prospects for spin electronic devices due to their remarkably high magnetoresistance at room temperature. [1][2][3][4] Tunneling magnetoresistance ͑TMR͒, defined as ͑R AP − R P ͒ / R P , can be as large as 360% in exchangebiased MTJs ͑Ref. 5͒ and 405% in unpinned pseudospinvalve MTJs. 6 Here R P and R AP are the resistances of the tunnel junctions when the magnetizations of the ferromagnetic electrodes in contact with tunnel barrier are parallel and antiparallel, respectively. High magnetoresistance in these devices is due to the spin filter effect of the crystalline MgO barrier, which is relatively transparent for majority spin electrons injected from an oriented bcc Fe or Fe-Co electrode but attenuates minority spin electrons, as result of the different symmetries of the ↑ and ↓ wave functions. Besides high TMR values, linear and hysteresis-free switching has also been observed in MgO-based MTJs when the thickness of the free CoFeB layer is less than 1.0 nm. 7 Moreover, an interfacial resonance state located in the minority band of Fe ͑001͒, which has been probed by spin-polarized tunneling in epitaxial Fe/ MgO / Fe MTJs, causes the TMR to change sign from positive to negative above a critical voltage. 8 Here we report the observation of a high inverted magnetoresistance when the free layer in MgO-based MTJs switches at zero bias. A switch in the sign of the resistance change near zero applied field occurs as a function of the thickness of the pinned CoFeB layer next to the MgO barrier in an MTJ stack with a synthetic antiferromagnet. The effect is not found when a single pinned layer is used.MTJs with a complete layer sequence Si/ SiO 2 ͑sub-strate͒ /Ta͑5͒ /Ru͑50͒ /Ta͑5͒ /Ni 81 Fe 19 ͑5͒ /Ir 22 Mn 78 ͑10͒ /Co 90 Fe 10 ͑2͒ / Ru͑0.85͒ / CoFeB͑t͒ / MgO͑2.5͒ / CoFeB͑3͒ /Ta͑5͒ / Ru͑5͒ were grown using a Shamrock sputtering tool, where the numbers in parentheses are layer thicknesses in nanometers. The thickness ͑t͒ of the pinned CoFeB ͑Co 40 Fe 40 B 20 ͒ layer was varied from 0 to 3.0 nm. The MgO was grown in a separate zone of our sputtering system, from a target-facingtarget source. Similar MTJs with AlO x barriers were also prepared for a comparative test. Well-oriented ͑001͒ MgO barrier layers were confir...

The strength of the exchange bias field is found to influence the low-frequency magnetoresistive noise associated with the magnetic reference layer in sputtered-deposited and electron-beam evaporated CoFeB/MgO/CoFeB tunnel junctions. The noise is due to magnetic losses arising in the reference layer. The losses are parameterized by a phase lag ε which exhibits a non-trivial dependence on the externally applied field. The general trend found amongst all devices is that the losses are largest in the antiparallel state. The effect of exchange bias on the reference's layers noise is investigated at a field corresponding to maximum resistance susceptibility, H ref. Higher values for the phase lag at H ref , ε ref , are found in devices having a large exchange bias field. We also observed that H ref and ε ref are larger in devices having thicker seed layers. This characteristic is also evident in double-barrier magnetic tunnel junctions. Prolonged thermal annealing is found to decrease ε ref , reduce H ref , and alter the field profile of the resistance susceptibility of the reference layer to resemble that of a more magnetically soft behavior. In addition to its impact on the magnetoresistive noise, the incorporation of exchange bias layers into the materials stack also affects the tunneling magnetoresistance ratio with higher values found at smaller exchange bias fields. We attribute the magnitude of the magnetic losses, and hence the magnetoresistive noise, from the reference layer to disorder in its magnetic microstructure. Our results indicate that the nature and degree

All electrical manipulation of magnetization in an Y3Fe5O12 (YIG)/Pt system was crucial to develop magnon-based spintronic devices. In this study, we realized spin–orbit torque (SOT) switching in perpendicular YIG/Pt films. Perpendicular magnetic anisotropy of YIG was induced by strain from the bottom substrate and also influenced by the capping Pt layer and subsequent annealing. Besides, SOT efficiency of the YIG/Pt system was also measured. Damping-like torque with an efficiency of 0.98 Oe/(MA/cm2) was the dominating term to contribute to the SOT switching. This work was beneficial to construct electrically controllable magnon devices.