Biographical Sketch of R. D. Taylor
Reprinted from the October 2003 edition of the Mössbauer Spectroscopy Newsletter, published as part of Volume 26, Issue 8 of the Mössbauer Effect Reference and Data Journal

Dr. R. Dean Taylor has spent most of his professional career working on various aspects of the Mössbauer effect (ME). At Los Alamos National Laboratory, the huge initial enthusiasm for this new discipline in the early 1960s dwindled, until by the 1970s Dean was the only staff member left pursuing ME research. From the mid-1960s until his retirement in 1990, he benefited from an almost continuous series of able post-doc collaborators and students. An even bigger impact on his activities began in 1983, with his active collaboration with Professor Moshe P. Pasternak of Tel Aviv University, a collaboration that continues even now. Retirement has not interrupted Dean’s work at Los Alamos. A 30-year career in ME has been followed by 13 years as a non-salaried consultant, with the Laboratory providing a lab and taking care of all necessary expenses for sources, supplies, travel, and visitors.

Dean was born in 1928 in Okemah, Oklahoma, the youngest of the five sons of Ray and Vera Taylor. His parents had received only a high school education, but they worked hard to provide a college education for all of their sons and (finally) one daughter. Dean received a B.S. in 1950 from Pittsburg State University and a Ph.D. in Physical Chemistry in 1954 from Rice University. He then joined the Low Temperature Physics Group at Los Alamos, where the main interests were the liquid state and flow properties of the new 3He isotope and its mixtures with 4He. These were the exciting days of superfluidity, two-fluid flow, and finding observable consequences of quantum mechanics in a pristine system. Enough 3He soon became available to use it for cryopumping, pushing the refrigeration limit down to below ~0.4 K, after an adiabatic demagnetization cryostat had been built to study a rare earth nuclear specific heat anomaly below 1K. In the early 1960s Dean built a dilution refrigerator achieving steady state temperatures of about 0.02 K.

In 1959 researchers at Los Alamos first heard of Rudolf Mössbauer’s discovery of the unexpected nuclear resonance fluorescence and absorption in 191Ir – exciting physics, to confirm or disprove. The necessary gear was readily available at LANL, along with physicists, radiochemists, cryogenists, and theorists, who were eager to proceed. With confirmation of the Mössbauer effect, the race was on to find other isotopes and new experimental challenges for this new discipline. Multichannel analyzers barely existed, so EPUT devices were used to determine the Mössbauer spectrum from a series of constant velocity data. Doppler drives using lathe beds or hydraulically driven pistons and a single channel data acquisition required heroic patience to get data. By the early 1960s, multichannel analyzers and electromechanical drives with feedback stabilization certainly accelerated the interest and progress in the ME field. The discovery of the 57Fe and 119Sn ME isotopes really got this new discipline jumping.

Dean Taylor (left) and the late W. A. Steyert with one of the first Westinghouse superconducting magnets used for Mössbauer studies (1963).

What exciting times these were! The various backgrounds of the entirely new group of researchers to ME added a bit of zip to the community. Everything was new and novel and was generally publishable. Oftentimes a new idea became a paper ready for publication in only a few days or weeks. The competition was fierce, but friendly.

Dean’s first major interest in ME was to demonstrate the low temperature “brute force” nuclear polarization of magnetized 57Fe. His collaborator, Greg Dash, recognized that the effect was miniscule at 1K because the Boltzmann factor controlling the nuclear ground state levels of 57Fe in metallic Fe was only 0.0022 K. However, the ground state moment of 57Co is over 50 times larger than that of 57Fe, so a cooled source was used assuming that at least part of the parent polarization would be retained even through the decay processes and the lifetime of the Mössbauer level. An anisotropy in corresponding Mössbauer lines analyzed by a room temperature Fe absorber was seen in 1960, using adiabatic demagnetization refrigeration. A single line absorber and 3He refrigeration improved the statistics of results, but it was hardly a thermometer. By 1971 Dean had results in a dilution refrigerator at 0.024 K. 54Mn and 60Co γ-ray anisotropy thermometers were also incorporated for a temperature comparison. Agreement was satisfactory, indicating that the expected nuclear polarization of the 57Co in iron was indeed retained through to the Mössbauer level. The best summary may be found in Temperature - Its Measurement and Control in Science and Industry (Reinhold Publishing Corp., New York, NY, 1972), Vol. 4, pp. 1259-1265.

Some of the early “quickie” experiments included the temperature dependence of the hyperfine field in iron, the ME using the 6.3 keV internal conversion x-rays, elliptical polarization of 57Fe γ-rays, and the first definitive observation of the 181Ta ME. Some of the early collaborators were D. Nagle, P. Craig, D. Cochran, W. Visscher, and H. Frauenfelder.

Professor R. L. Mössbauer (left) after introduction by Dean Taylor at ICAME '93 in Vancouver, Canada.
Almost all of the ME studies at Los Alamos during the first 20 years were source studies with the 57Co (57Fe) present as a very dilute impurity. The Mössbauer parameters should certainly be influenced by the host environment, and measurements of the temperature dependence of the absolute ƒ-value of 57Fe in various elements demonstrated several lattice dynamical features. Paramagnetic systems showed interesting properties when subjected to low temperatures and high magnetic fields newly made available via superconducting magnets.

A systematic study of ten elemental ME sources showed a variety of localized moments, zero to 14 μB, and a low temperature saturation effect, later associated with the Kondo effect. Collaborators were T. Kitchens, the first post-doc at Los Alamos, M. Maley, and the late W. Steyert.

Lattice dynamics studies included finding anharmonicity in Nb3Sn and discovering correlations between the ƒ-values and the isomer shift of impurity atoms. When a system becomes superconducting, magnetic susceptibility no longer provides any information about the magnetization, if any. But superconductivity does not shield the ME, and Dean reported the first case of coexistence of magnetic order and superconductivity in 1973.

Research funding was tight in the “energy crisis” years of the 1970s, and Dean became a co-director of the new Los Alamos Superconducting Power Transmission Line project. Nevertheless, the ME interests did not die. A. Migliori and he used a moveable Pd(57Co) source to measure the local magnetic field profile around a superconducting tape carrying large persistent currents. SPTL ended in 1980, allowing more time for pure research and various other administrative positions.

Spin glasses, more magnetic superconductors, and giant moment experiments were soon superceded by a new field, ME at high pressures using diamond anvil cells. The small sample size (<100mg) and substantial attenuation of the diamonds, even for the 14.4 keV γ-ray of 57Fe, seemed formidable. Attenuation was reduced by choosing a pressurized source experiment. Cubic α-Fe subjected to pressures above about 18 GPa becomes hexagonal ε-Fe, which had been shown to be non-magnetic down to 2.2 K. In fact, no hyperfine field (magnetism) was found at LANL in ε-Fe cooled to 0.020 K, and no localized moment behavior was seen in applied fields of up to 5 T. 151Eu studies in Eu and in EuO soon followed, showing valence (isomer shift) and (moment) hyperfine field changes with pressure. During this time, Prof. M. Pasternak arrived for a sabbatical, and their first project became the effect of pressure on iodine using the ME of 129I.

A series of pressure experiments evolved during Prof. Pasternak’s summer visits and Dean’s visits to Tel Aviv, including SnI4, GeI4, NiI2, FeI2, andVI2. These showed features of metallization, amorphization, quadrupole distortion, isomer shift, and magnetization changes. The collapse of the magnetic hyperfine field with pressure in NiI2 has become the classic demonstration of the Mott transition. Resistivity and XRD measurements soon supplemented the ME results. After the Brookhaven reactor was shut down, 129mTe sources were unavailable, but new “point sources” allowed 57Fe and 119Sn pressure experiments instead.

Dean Taylor (left) and Professor Moshe Pasternak of Tel Aviv University pose in Taylor's lab at LANL.

A small foray into gamma ray lasers with Professor G. Hoy prompted another attempt to see the ME in ultranarrow linewidth 109Ag. The count rate of a 109Cd in Ag source as a function of temperature should reflect the (small) temperature-dependent Mössbauer self-absorption. Concomitant Ag x-rays were counted for normalization. A statistically significant positive result was seen at 4K and later confirmed at Old Dominion. Caught up in the new excitement of high temperature superconductors, Dean and student M. Smith used primarily iron doping in a sequence of evolving samples. Lots of samples, lots of results, lots of publications, and some fun.

New cell designs and better anvils have allowed Mössbauer spectroscopy at ever higher pressures. Recently, almost all high-pressure experiments with Prof. Pasternak have involved iron-bearing compounds, including some of interest to the geosciences (R. Jeanloz). Pressure-induced transformations abound in the various systems – metallization, amorphization, structural changes, magnetization collapse, coordination changes, spin crossover, valence modification, and self-oxidation – and the ME is the primary tool. Most of these results have also been discussed in a series of review papers.

Dean was married to Janis Dexter in 1961, and they have two children, Scott and Kay. The children (and their spouses) have blessed the grandparents with three offspring. Global distances have not prevented some great visits, including skiing and safari. Dean has served on the National Ski Patrol for 27 years, mostly at the local ski area. His hobbies include skiing, hiking, fishing, photography, travel, and his latest…Mössbauer spectroscopy. By 1961 he had climbed 35 of the 54 mountains in Colorado whose elevation top 14,000 feet – and spent his 30th birthday atop the Matterhorn.

Taylor has served as the US representative on the International Board on the Applications of the Mössbauer Effect (IBAME), and helped organize ICAME-93 in Vancouver. Since 1994 he has been an associate editor of the Mössbauer Effect Reference and Data Journal.

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