Peter Grünberg (1939-2018) Physicist Who Revolutionized Data Storage with Work on Magnetism in Nanomaterials

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COMMENT OBITUARY Peter Grünberg (1939-2018) Physicist who revolutionized data storage with work on magnetism in nanomaterials.

eter Grünberg was one of the first smart GMR-based devices, including the physicists to understand the potential spin-valve concept that he patented and was of nascent nanotechnologies for fun- later developed at IBM for use in hard disks. Pdamental research. He discovered giant The first commercial GMR-based hard disks magnetoresistance, or GMR: a large change in appeared in 1997. Since then, their data- electrical resistance induced by a small mag- storage capacity has risen by almost three netic field in stacks of ultrathin magnetic and orders of magnitude. Grünberg conceived of non-magnetic layers. For this, he won a share many other devices, from magnetic sensors of the 2007 Nobel Prize in Physics (as did I; to the compass used in smartphones today. OLIVER BERG/PICTURE-ALLIANCE/DPA we independently discovered the same effect). The discovery of GMR kicked off an Ultimately, his work led to the development intense period of research activity, including of hard-disk drives and greatly increased data experiments on GMR and interlayer storage. It also kicked off the field of spintron- exchange coupling in a great diversity of mag- ics. Grünberg died on 7 April 2018, aged 78. netic multilayers. At the same time, the theory Grünberg was born in 1939 in Pilsen, behind GMR was developed. Grünberg’s Bohemia, then a German protectorate, now Jülich team took a semi-classical approach; part of the Czech Republic. In 1945, his family I worked in collaboration with New York left for West Germany. There, at 19, Grünberg University on a quantum approach. went to study physics at the Goethe University The burgeoning field of spintronics has Frankfurt; he then did a PhD at the Technical yielded fascinating results. In 1995, Terunobu University of Darmstadt. first demonstration of a quantum effect in Miyazaki and Nobuki Tezuka in Japan, and a For his PhD, he used optical spectroscopy magnetism. The coupling results from the US group led by Jagadeesh Moodera, inde- to determine the energy levels of rare-earth interference between electronic wave func- pendently showed that quantum tunnelling ions in magnetic garnet crystals. In his post- tions reflected at the surface of the magnetic of electrons between magnetic layers gives doc, he turned another spectroscopy tech- layers. For me, it was also the revelation of a rise to much larger magnetoresistances nique — Raman scattering — on garnets, nanostructure in which I could test some of than with GMR (T. Miyazaki and N. Tezuka this time at Carleton University in Ottawa, my own ideas about magnetoresistance. J. Magn. Magn. Mat. 139, L231–L234; 1995; Canada, from 1969 to 1971. And in 1972, Soon after, in early 1988, our two teams J. S. Moodera et al. Phys. Rev. Lett. 74, 3273; thanks to his expertise in the spectroscopic independently discovered GMR. Grünberg’s 1995). In 1996, John Slonczewski at IBM in study of magnetic materials, Grünberg was group showed it in their iron–chromium– Yorktown Heights, New York, and Luc Berger offered a post at the newly founded Institute iron sandwich; my team in France, in a stack at Carnegie Mellon University in Pittsburgh, for Magnetism at the Jülich Research Centre of 40 layers of iron and chromium. Pennsylvania, introduced the concept that in Germany. It was clear from the outset that GMR transferring spins between magnetic materi- Here, Grünberg quickly demonstrated his would have vast applications — especially als could create torque on their magnetization pioneering spirit, developing the spectros- because it could happen at room temperature. direction (J. C. Slonczewski J. Magn. Magn. copy technique of Brillouin light-scattering It could be used to detect small magnetic Mat. 159, L1–L7; 1996; L. Berger Phys. Rev. spectroscopy (BLS). BLS examines the inelas- fields easily, which proved useful for mag- B 54, 9353; 1996). Today, this mechanism tic scattering of light; it can probe both the netic sensors and, in particular, for reading is exploited to write non-volatile magnetic ground state of magnetic materials and their magnetic hard disks. It was also the first memories. Grünberg and his team contrib- excited states. In the 1970s, physicists were example of electronics that exploit both the uted much to these research fields. struggling to pick up the specific excitation spin and charge of electrons — a field today Grünberg was warmly esteemed by modes expected to occur at the surface of called spintronics. colleagues around the world for his great crea- magnetic materials. Grünberg singled out I met Grünberg in August 1988 at the tive talent in physics, and for his integrity and these modes, and identified them as spin International Colloquium on Magnetic modesty. To me, he was also a good friend. waves of the Damon–Eshbach type. Films and Surfaces in Le Creusot, a small And I liked his sense of humour. Having During a sabbatical at Argonne National town in the middle of France. We presented shared emotional and amusing moments in Laboratory in Illinois in 1985, Grünberg our respective results, and came to the conclu- Stockholm, we often recalled our procession used an emerging technique of growing sion that we had made the same discovery. to the Nobel banquet, when neither of us was metals on single crystals to extend his BLS To our delight, most participants seemed to completely successful in our attempts not to experiments to layers of magnetic materi- think it was important: we celebrated with a tread on the trains of the Royal Princesses. als less than 1 nanometre thick. This led to couple of glasses of local wine, a Pommard Peter was a great physicist, and a gentle and his first major discovery. In a sandwich of burgundy. That night, we had the feeling that sincere person. The spintronics and nano- magnetic iron, non-magnetic chromium the conference concert, with pieces played on magnetism community will miss him sorely. ■ and more iron, he and his co-workers dem- piano and violin by some of our colleagues, onstrated the existence of antiferromag- was in celebration of our work. Grünberg, a Albert Fert is a physicist at the University netic exchange coupling between the iron skilled guitarist, did not play that time, but I Paris-Sud in Orsay and at UMPh, a joint layers across chromium (P. Grünberg et al. had several opportunities to hear him play. laboratory of CNRS and Company Thales. Phys. Rev. Lett. 57, 2442; 1986). This was the Grünberg had an impressive vision for e-mail: [email protected]