Spintronics – starting with Giant MagnetoResistance (GMR)

Basics electrical conduction in a ferromagnet majority minority 4s 3d EF Basics electrical conduction in a ferromagnet The density of states (DOS) in a ferromagnet is split into majority and minority bands due to the exchange interaction s- and d-electrons contribute to electrical conduction. The mobility of 3d-electrons is smaller (flat energy bands → low velocity/high effective mass) than for 4s-electrons. Only electron-states close to the Fermi energy of importance. In the example above, ↓-electrons have more empty states to scatter to, the resistivity will be higher for these electrons; , two independent parallel electron conduction channels. Neglecting sd, the resistivity of ↑-electrons (majority electrons) will be , while the resistivity of ↓-electrons (minority electrons) can be written as , in most cases .

GMR FM Me E, J H e- CPP (Current Perpendicular Plane) multilayers CIP (Current In Plane) multilayers FM = metallic ferromagnetic layer; Me = metallic non-magnetic layer GMR Spin dependent scattering, two current channels, one for majority spins (low resistivity) and one for minority spins (high resistivity) Systems/geometries displaying GMR

(FM tFM / Me tMe)n multilayers FM = Fe, Ni, Co or some 3d alloy, Me = Cu, Ag, V, Cr, tFM (Me) = layer thickness ≈ a few monolayers (some nm thickness) High resistance for antiferromagnetically (AF) aligned layers, low resistance for ferromagnetically (F) aligned layers. The microscopic origin of the AF coupling can be explained by the RKKY (Ruderman-Kittel-Kasuya-Yosida) model, indirect type of interaction between two FM layers, FM1 polarizes the conduction electron spins and the spin polarization propagates across the Me spacer layer and interacts with FM2, spin polarization oscillates in sign and decays with distance as r-2 CIP geometry HS

M H F coupling AF coupling

Mechanism of GMR Two current channels with different resistivities, the difference is mainly explained by the electronic structure and differences in the DOS for the majority and minority conduction electrons. In addition, we need to consider scattering centers, here we distinguish between bulk scattering and interface (FM/Me) Scattering centers at interfaces may be surface roughness, regions of interdiffusion, etc., while scattering in the bulk of a layer is due to impurity atoms. interface bulk FM elements FM alloys e-

Important length scales; spin diffusion length lsdl , should be , lsdl ~ 102 – 103 nm for magnetic 3d elements; mean free path lmfp , should be , lmfp ~ 10 – 30 nm for magnetic 3d elements and only a few nm for alloys like permalloy. If these conditions hold, one would expect similar results for the CIP and CPP geometries. The resistance of the different current channels can be described using simple resistor models. AF configuration where R+ (R_ ) is the resistance for electrons with S=+1/2 (S=-1/2 ), while are the resistances for the two conduction channels (minority and majority carriers, respectively). F configuration

The magnetoresistance thus is In the absence of AF coupling, there are (at least) two possible ways to obtain different relative orientation of the magnetization in successive FM layers in a field interval: Use two ferromagnetic materials exhibiting different coercivities, either as building block in a multilayer or as part of a sandwich structure. Use two FM layers separated by a Me layer in a sandwhich structure, one FM layer will be constrained by coupling to an adjacent antiferromagnetic layer (exchange anisotropy); FeMn (TN ≈ 430 K), IrMn (TN ≈ 470 K), ... FM1 Me FM2 AF uni-directional anisotropy!

How to prepare the layer so that it exhibits exchange anisotropy TN = Neel temperature, ordering temperature for the antiferromagnetic layer

Multilayered or sandwich structures in applications? Si /(60 Å Ni80Fe20 / 22 Å Cu / 40 Å Ni80Fe20 / 70 Å FeMn) / 50 Å Ta)

SPINTRONICS The acronym was originally used as the name for a research program at DARPA (Defence Advanced Research Project Agency) Overall goals To produce a new generation of electronic devices where the spin of the carriers should play a crucial role in addition to or in place of the charge To produce such materials that can be integrated with existing semiconductor materials One example Magnetoresistive Random Access Memory – MRAM

Magnetic tunnel junctions IBM – Infineon in 2005 demonstrated a 16-Mbit magnetoresistive random access memory (MRAM) prototype Freescale Semiconductor Inc. summer 2006 introduced its first 4-Mbit MRAM Magnetic tunnel junctions Quantum mechanics dictates that an electron in a metallic electrode has a certain probability to tunnel through an (insulating) potential barrier to another metallic electrode. Important parameters – thickness of barrier, height of potential barrier and density of states (DOS) in the metallic electrodes, . In ferromagnets like Fe, Ni and Co, the DOS for spin-up and spin-down 3d electrons are exchange-split.

Tunnelling between two ferromagnetic electrodes parallel alignment electrons keep their 3d- character during tunnel- ling and spin- conservation → spin dependent transport, important parameter – spin polarization antiparallel alignment Tunnelling between two ferromagnetic electrodes electrode 1 barrier electrode 2

Jullière’s model (M. Julliere, Physics Letters 54A, 225 (1975)) spin conservation the conductance is proportional to products like Relation between conductivity and resistivity changes ( conductivity (resistivity) for parallel magnetizations …) Conductance when the magnetizations in the two FM electrodes are parallel ( Fermi-Dirac distribution) and the corresponding result when the magnetizations are in opposite directions insulator FM1 FM2

Using the definition of spin polarization, we obtain and can be generalized to two different FM materials with two different spin polarizations; P1 and P2 Theory by Slonczewski (Phys. Rev. B 39, 6995 (1989)) describes how the tunneling conductance depends on barrier height. Typical dimensions FM electrodes of lateral size mm2 or smaller, thickness 10-20 nm Oxide tunnel barrier one or a few nm thick, barrier height 2-3 eV Materials FM electrodes Co, Fe, Ni, NiFe, CoFe, CoFeB Amorphous oxide tunnel barrier NiO, CoO, Al2O3, or crystalline tunneling barrier MgO Junction resistance from < 100 W to tens of kW, depends exponentially on barrier thickness

Important Sharp interfaces without interdiffusion, minimum of spin-flip scattering at interfaces Measured spin polarizations P (Meservey and Tedrow, Phys. Rep. 238, 173 (1994)) How to prepare a junction in low / high resistance states Material Ni Co Fe NiFe CoFe P +23% +35% +40 +32% ~50% one FM electrode may be pinned by an AF layer or the FM layers may have different Hc Non-volatile memory! H -H3 -H2 H1 R M H2

TMR-results for Co/Al2O3/NiFe junctions 20.2% 27.1% 27.3% RT 77K 4.2K

‘Recent’ advances – epitaxial barriers or barriers with texture First-principles based calculations of the tunneling conductance in Fe(001) | MgO(001) | Fe(001) junctions, W.H. Butler et al., Phys. Rev. B 63, 054416 (2001) Experimental confirmation Stuart S.P. Parkin et al, Nature Materials 3, 862 (2004); Shinji Yuasa et al, Nature Materials 3, 868 (2004) Good results both for epitaxial junctions and polycrystalline but highly (001) oriented junctions.

Fe | MgO | Fe | IrMn epitaxial junctions at 293 K and at 20K.

Writing by currents in one word and one bit line (the total field is large enough to switch the magnetization in one electrode) Reading by probing the resistance over one TMR junction

Comparison of memory technologies for the Year 2011 (forecast done ~10 years ago) CMOS Technology DRAM Flash SRAM MRAM Reference SIA 1999 Generation at introduction 64 GB 180 MB/cm2 Circuit speed 150 MHz 913 MHz >500 MHz Feature size 50 nm 35 nm <50 nm Access time 10 ns 1.1 ns <2 ns Write time <10 ns Erase time <1 ns 10 ms Retention time 10 years Infinite Endurance cycles 105 Operating voltage 0.5-0.6 V 5 V <1 V Voltage to switch state 0.2 V <50 mV SIA – Securities Industry Association Access time – defined by how quickly the memory can respond to a read or write request Retention time – memory is retention of information over this period of time

2007 R&D moving to spin transfer torque RAM (SPRAM) Spin torque transfer switching Manipulation of magnetic moments in a nano-scale ferromagnet by a current is one of the most important techniques for the future spintronics devices. Especially, current induced magnetization switching in magnetic tunnel junctions (MTJs) is expected to be the method for writing in high density magnetoresistive random access memory (MRAM). The necessary critical current for spin transfer switching decreases as l2 (where l is the in-plane dimension of the TMR structure) as the free-layer volume decreases. From this scalability, the spin transfer switching can reduce the writing current in MRAM and avoid excessive heating, while the conventional writing method using the magnetic field generated by currents needs larger writing current. The spin torque transfer switching will allow for higher density MRAM memory units. Switching current (*): a = magnetic damping constant; Hk = anisotropy field; V = volume of ’free’ layer; Eb = energy barrier * J. Slonczewski, JMMM 159, L1-7 (1996); L. Berger, PRB 54, 9353-9358 (1996)

Electrons flowing through a magnetic layer in a magnetoresistive device are spin polarized along the magnetization of F1. When these spin-polarized electrons pass through another magnetic layer (F2), the polarization direction may have to change depending on the relative orientation of F1 and F2. In this repolarization process, the magnet experiences a torque (spin torque) associated with the transfer of spin angular momentum from the electrons. For large current, the spin torque amplifies the spin precession and magnetization switching occurs. The magnetization of the free layer is controllable by the current direction. Fig. 1 Schematic illustration of spin torque transfer Fig. 2 Typical TMR loop driven by a pulse current.

Some recent news 2007 R&D moving to spin transfer torque RAM (SPRAM) February - Tohoku University and Hitachi developed a prototype 2 Mbit Non-Volatile RAM Chip employing spin-transfer torque switching August - IBM, TDK Partner In Magnetic Memory Research on Spin Transfer Torque Switching to lower the cost and boost performance of MRAM November - Toshiba applied and proved the spin transfer torque switching with perpendicular magnetic anisotropy MTJ device November - NEC Develops World's Fastest SRAM-Compatible MRAM With Operation Speed of 250MHz 2008 Japanese satellite, Sprite Sat, to use Freescale MRAM to replace SRAM and FLASH components August - Scientists in Germany have developed next-generation MRAM that is said to operate as fast as fundamental performance limits allow, with write cycles under 1 nanosecond. 2009 Hitachi and Tohoku University demonstrated a 32-Mbit spin-transfer torque MRAM in June 2010 A perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction S. Ikeda et al., Nature Materials 9, 721-724 (2010)

40 nm TMR device low switching current < Ic > = 50 mA; 120% TMR ratio; high thermal stabilty factor Eb / kBT = 43

Mål Känna till att det finns 2 strömkanaler för ledningselektroner i ett metalliskt ferromagnetiskt material, en kanal för majoritetselektroner och en annan för minoritetselektroner Känna till hur ett magnetiskt multilager som uppvisar GMR effekten är uppbyggt och de magnetiska tillstånden (hur magnetiseringen i olika skikt är riktade) för hög respektive låg resistans Kunna förklara GMR effekten m.h.a. strömkanalerna för majoritets- och minoritetselektroner Känna till vad som menas med ”exchange anisotropy” Känna till hur magnetiska tunnelövergångar är uppbyggda och kvalitativt kunna redogöra för tunnelmagnetoresistans (TMR) m.h.a. DOS för majoritets– och minoritetselektronerna Kunna beskriva hur resistansen för ett TMR element varierar med magnetiseringen för elementet. Hur ”sätter” man elementet i ett högresistanstillstånd? Känna till tunnelövergångar som utnyttjar MgO barriären Kunna beskriva utformningen av ett MRAM med ”bit” och ”word” ledare Kvalitativt kunna beskriva hur man byter riktning på det fria lagrets magnetisering om man använder en spinnpolariserad ström i ett TMR element