The 2007 Nobel Prize in Physics The 2007 Nobel Prize in Physics The Discovery of Giant Magnetoresistance Compiled by the Class for Physics of the Royal Swedish Academy of Sciences The Royal Swedish Academy of Sciences (Kungl. Vetenskapsakademien) Information Department P. O. Box 50005 SE-104 05 Stockholm, Sweden E-mail: [email protected] Website: www.kva.se 1. INTRODUCTION fields. The most useful material has been discovered materials showing a very The phenomenon called magnetoresis- an alloy between iron and nickel, Fe20Ni80 large magnetoresistance, now known as tance (MR) is the change of resistance of (permalloy). In general, however, there giant magnetoresistance (GMR). These a conductor when it is placed in an external was hardly any improvement of the per- materials are so called magnetic multilay- magnetic field. For ferromagnets like iron, formance of magnetoresistive materials ers, where layers of ferromagnetic and cobalt and nickel this property will also since the work of Kelvin. The general non-magnetic metals are stacked on each depend on the direction of the external consensus in the 1980s was that it was not other [Fig. 1]. The widths of the individual field relative to the direction of the current possible to significantly improve on the layers are of nanometre size — i.e. only through the magnet. Exactly 150 years ago performance of magnetic sensors based a few atomic layers thick. In the original W. Thomson [1] (Lord Kelvin) measured on magnetoresistance. experiments leading to the discovery of the behaviour of the resistance of iron GMR one group, led by Peter Grünberg and nickel in the presence of a magnetic Therefore it was a great surprise when in [3], used a trilayer system Fe/Cr/Fe, field. He wrote “I found that iron, when 1988 two research groups independently while the other group, led by Albert Fert subjected to a magnetic force, acquires an increase of resistance to the conduction of electricity along, and a diminution of resistance to the conduction of electric- ity across, the lines of magnetization”. This difference in resistance between the parallel and perpendicular case is called anisotropic magnetoresistance (AMR) [2]. It is now known that this property originates from the electron spin-orbit coupling. In general magnetoresistance effects are very small, at most of the order of a few per cent. The MR effect has been of substantial © CNRS Photolibrary, photo: C. Lebedinsky © Forschungszentrum Jülich importance technologically, especially in Allbert Fert Peter Grünberg connection with readout heads for mag- netic disks and as sensors of magnetic 1/2 of the prize 1/2 of the prize France Germany The content of this Highlight on “The 2007 Nobel Prize in Physics” was re- Université Paris-Sud Forschungszentrum Jülich leased originally by the Nobel Founda- Unité Mixte de Physique Jülich, Germany tion on the website Nobel e-Museum. CNRS/THALES Permission for reprinting in the Bulletin Orsay, France this news release was requested and granted. b. 1938 b. 1939 2 AAPPS Bulletin December 2007, Vol. 17, No. 6 The Discovery of Giant Magnetoresistance Fig. 1: Schematic figure of magnetic multilay- Fig. 2: After Refs. [3] and [4]. Left: Magnetoresistance measurements [3] (room temperature) ers. Nanometre thick layers of iron (green) are for the trilayer system Fe/Cr/Fe. To the far right as well as to the far left the magnetizations of separated by nanometre thick spacer layers the two iron layers are both parallel to the external magnetic field. In the intermediate region of a second metal (for example chromium or the magnetizations of the two iron layers are antiparallel. The experiments also show a hys- copper). The top figure illustrates the trilayer terisis behaviour (difference 1 and 4 (2 and 3)) typical for magnetization measurements. Right: Fe/Cr/Fe used by Grünberg’s group [3], and Magnetoresistance measurements [4] (4.2K) for the multilayer system (Fe/Cr)n . To the far right the bottom the multilayer (Fe/Cr)n , with n as (>HS , where HS is the saturation field) as well as to the far left (< – HS ) the magnetizations of high as 60, used by Fert’s group [4]. all iron layers are parallel to the external magnetic field. In the low field region every second iron layer is magnetized antiparallel to the external magnetic field. 10 kG = 1 Tesla. [4], used multilayers of the form (Fe/Cr)n from Fert’s group already referred to the 3d elements that will be discussed. where n could be as high as 60. observed effect as “Giant Magnetoresis- tance.” Grünberg also realized at once In the free atoms, the 3d and 4s atomic In Fig. 2 the measurements of Grün- the new possibilities for technical appli- energy levels of the 3d transition elements berg’s group are displayed (left) together cations and patented the discovery. From are hosts for the valence electrons. In the with those of Fert’s group (right). The this very moment the area of thin film metallic state these 3d and 4s levels are y-axis and x-axis represent the resistance magnetism research completely changed broadened into energy bands. Since the 4s change and external magnetic field, re- direction into magnetoelectronics. orbitals are rather extended in space there spectively. The experiments show a most will be a considerable overlap between 4s significant negative magnetoresistance The discovery of giant magnetoresis- orbitals belonging to neighbouring atoms, for the trilayer as well as the multilayers. tance immediately opened the door to a and therefore the corresponding 4s band is The systems to the right, involving large wealth of new scientific and technological spread out over a wide energy range (15- stacks of layers, show a decrease of resis- possibilities, including a tremendous influ- 20 eV). In contrast to this, the 3d orbitals tance by almost 50% when subjected to a ence on the technique of data storage and are much less extended in space. Therefore magnetic field. The effect is much smaller magnetic sensors. Thousands of scientists the energy width of the associated 3d en- for the system to the left, not only because all around the world are today working on ergy band is comparatively narrow (4-7 the system is merely a trilayer but also magnetoelectronic phenomena and their eV). In practice one cannot make a clear because the experiments led by Grünberg exploration. The story of the GMR effect is distinction between the 3d and 4s orbitals were made at room temperature, while a very good demonstration of how a totally since they will hybridize strongly with the experiments reported by Fert and unexpected scientific discovery can give each other in the solid. Nevertheless for co-workers were performed at very low rise to completely new technologies and simplicity this two band picture will be temperature (4.2K). commercial products. used here and the 3d electrons will be con- sidered as metallic — i.e. they are itinerant Grünberg [3] also reported low tempera- 2. BACKGROUND electrons and can carry current through the ture magnetoresistance measurements for 2.1. Ferromagnetic Metals system, although they are still much less a system with three iron layers separated Among the d transition metals (Sc…Cu, mobile than the 4s electrons. by two chromium layers and found a re- Y…Ag, Lu…Au, i.e. 3d, 4d, and 5d sistance decrease of 10%. transition elements), the 3d metals iron, A useful concept in the theory of solids cobalt and nickel are well-known to be is the electron density of states (DOS), Not only did Fert and Grünberg measure ferromagnets. Among the lanthanides (the n(E), which represents the number of elec- strongly enhanced magnetoresistivities, 4f elements, La-Lu) gadolinium is also a trons in the system having energy within but they also identified these observations ferromagnet. The origin of magnetism in the interval (E, E+dE). According to the as a new phenomenon, where the origin these metals lies in the behaviour of the exclusion principle for fermions (in this of the magnetoresistance was of a totally 3d and 4f electrons, respectively. In the case electrons), only one electron can oc- new type. The title of the original paper following it is mainly the magnetism in the cupy a particular state. However each state AAPPS Bulletin December 2007, Vol. 17, No. 6 3 The 2007 Nobel Prize in Physics the density of states at the Fermi energy N(EF) can now be very different for the two spin bands. This also means that for a ferromagnet the character of the state at the Fermi energy is quite different for spin up and spin down electrons. This is an important observation in connection with the GMR effect. This picture of 3d energy bands [Fig. 3 to the right] for the ferromagnetic metals is often referred to Fig. 3: To the left a schematic plot is shown for the energy band structure of a d transition as the itinerant model [6], also known as metal. The density of states N(E) is shown separately for the spin up and down electrons and the Stoner-Wohlfarth model [5]. where a simplified separation has been made between the 4s and 3d band energies. For the non-magnetic state these are identical for the two spins. All energy levels below the Fermi energy are occupied states (orange and blue). The coloured area (orange + blue) corresponds One important property of ferromagnets to the total number of valence electrons in the metal. To the right the corresponding picture is is that at high temperature their magnetism illustrated for a ferromagnetic state, with a spin-polarization chosen to be in the up direction is lost. This happens at a well defined tem- (N↑ > N↓; blue area > orange area).