J. Z. Sun

Spin torque switching of perpendicular Ta∣CoFeB∣MgO-based magnetic tunnel junctions

Spin torque switching is investigated in perpendicular magnetic tunnel junctions using Ta∣CoFeB∣MgO free layers and a synthetic antiferromagnet reference layer. We show that the Ta∣CoFeB interface makes a key contribution to the perpendicular anisotropy. The quasistatic phase diagram for switching under applied field and voltage is reported. Low switching voltages, Vc 50 ns=290 mV are obtained, in the range required for spin torque magnetic random access memory. Switching down to 1 ns is reported, with a rise in switching speed from increased overdrive that is eight times greater than for comparable in-plane devices, consistent with expectations from a single-domain model.

Observation of large low‐field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current‐perpendicular‐to‐plane transport. These results show that low‐field spin‐dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

Current-driven magnetic switching in manganite trilayer junctions

Abstract A current-driven switching of magnetic state is observed in manganite trilayer junctions. The switching current threshold I c depends on applied magnetic field. The switching is due to the transfer of spin-momentum from spin-polarized carriers to a ferromagnetic cluster situated between the electrodes. A model developed based on the spin-momentum transfer process quantitatively describes the experimental observation.

THICKNESS-DEPENDENT MAGNETOTRANSPORT IN ULTRATHIN MANGANITE FILMS

To understand the near-interface magnetism in manganites, ultrathin films of La0.67Sr0.33MnO3 were grown epitaxially on single-crystal (001) LaAlO3 and (110) NdGaO3 substrates. The temperature and magnetic field-dependent film resistance is used to probe the film’s structural and magnetic properties. A surface and/or interface related dead layer is inferred from the thickness-dependent resistance and magnetoresistance. The total thickness of the dead layer is estimated to be ∼30 A for films on NdGaO3 and ∼50 A for films on LaAlO3.

High‐resolution scanning SQUID microscope

We have combined a novel low temperature positioning mechanism with a single‐chip miniature superconducting quantum interference device (SQUID) magnetometer to form an extremely sensitive new magnetic microscope, with a demonstrated spatial resolution of ∼10 μm. The design and operation of this scanning SQUID microscope will be described. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, will be presented, and a representative application will be discussed.

Spin-polarized transport and magnetoresistance in magnetic oxides

Abstract Magnetic oxide materials possessing a high degree of spin polarization have been found to exhibit enhanced spin-dependent transport properties. For example, interfaces created by naturally occurring or artificial grain boundaries in the perovskite manganites, La 1− x A x MnO 3 , result in a large magnetoresistance (MR) at low fields. Extrinsic grain boundary MR has also been reported in other conducting magnetic oxides, such as chromium dioxide (CrO 2 ), pyrochlore Tl 2 Mn 2 O 7 , magnetite (Fe 3 O 4 ), and the ordered double-perovskite Sr 2 FeMoO 6 . Even larger changes in resistances at low fields are obtained in the layered manganite, La 2−2 x Sr 1+2 x Mn 2 O 7 for x =0.3, which possesses a natural tunnel junction structure in the c -axis direction. But, by far, the largest MR effect to date has been observed in thin film ferromagnetic/insulating/ferromagnetic tunnel junction structures, where resistance changes as high as a factor of 10 at low fields has been reported for the strontium-doped perovskite manganites. In most cases, the enhanced MR effect in these magnetic oxides is limited to rather low temperatures and decreases rapidly with increasing temperature. With a better understanding of the nature of the boundaries in these junctions, it might be possible to enhance the MR at higher temperatures.

Enhanced magnetoresistance in sintered granular manganite/insulator systems

We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.

Spin torque switching of 20 nm magnetic tunnel junctions with perpendicular anisotropy

Spin-transfer torque magnetic random access memory (STT-MRAM) is one of the most promising emerging non-volatile memory technologies. MRAM has so far been demonstrated with a unique combination of density, speed, and non-volatility in a single chip, however, without the capability to replace any single mainstream memory. In this paper, we demonstrate the basic physics of spin torque switching in 20 nm diameter magnetic tunnel junctions with perpendicular magnetic anisotropy materials. This deep scaling capability clearly indicates the STT MRAM device itself may be suitable for integration at much higher densities than previously proven.

Growth and giant magnetoresistance properties of La‐deficient LaxMnO3−δ (0.67≤x≤1) films

Epitaxial thin films of lanthanum‐deficient LaxMnO3−δ (0.67≤x≤1) have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The as‐deposited films exhibit a ferromagnetic transition at temperatures ranging from 115 to 240 K, with the transition temperature (Tc) increasing with higher La deficiency. A sharp drop in resistivity and negative magnetoresistance is observed close to Tc, a behavior similar to that observed in divalent substituted La1−xMxMnO3−δ (M=Ba, Sr, Ca, Pb) films. Postannealing the films in O2 reduces the resistivity and raises the Tc to values close to room temperature. A magnetoresistance value of 130% (Δρ/ρH) has been obtained at 300 K at 4 T for a post‐annealed film with x=0.75.

Batch-fabricated spin-injection magnetic switches

A method is developed for the fabrication of sub-100 nm current-perpendicular spin-valve junctions with low contact resistance. The approach is to use a batch-fabricated trilayer template with the junction features defined by a metal stencil layer and an undercut in the insulator. The spin-valve thin film stack is deposited afterwards into the stencil, with the insulator undercut providing the necessary magnetic isolation. Using this approach, reproducible spin-current-induced magnetic switching is demonstrated for junctions down to 50 nm×100 nm in size.