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2015: Investigation of magnetic and magnetocaloric properties of 'heavy' lanthanide metals led to the discovery of new phases at the magnetic phase diagrams

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The scientific group of Prof. Alexander Tishin and Dr. V.I. Zverev (Department of General Physics and Physics of Condensed Matter, Faculty of Physics, Moscow State University), which is engaged in research of new functional materials, in 2015 completed a series of complex investigations of magnetic and magnetocaloric properties of a number of high-purity single-crystalline 'heavy' lanthanide metals in the temperature range 4.2 -350 K in magnetic fields up to 10 T.

Rare earth metals (REM) and their alloys cause continuing interest of researchers for more than fifty years. The unique magnetic properties, namely, the highest among all elements of the Periodic Table values ​​of magnetic moment per atom combined with a high practical perspective cause active investigation of their properties: the number of publications dealing with rare earth materials and their alloys is steadily increasing since 1950-s. The study of their properties is also of the great practical importance since these metals, their alloys and compounds are widely used in all areas of modern technology.

Detailed study of magnetothermal properties of these metals using a series of different experimental methods in the case of magnetic fields applied along different crystallographic directions on high-purity single crystals has not yet been performed. A similar study of the magnetic and thermal properties of high-purity single crystals is of fundamental importance, since the nature of the magnetic phase diagram is significantly affected by the concentration of H, C, O, N and F, when it exceeds a few hundred ppm.

A group of scientists led by Prof. Alexander Tishin and Dr. Vladimir Zverev (which is currently composed of students, graduate students and staff of the Faculty of Physics, as well as the staff AMT&C Group www.amtc.ru) has studied new functional materials for many years (together with the Ames Lab, Iowa State University). The aim of a systematic study of high-purity rare earth metals and their compounds is the practical application of research results in the field of magnetic refrigeration technology and biomedical magnetic hyperthermia and controlled drug release from the surface of the implant.

A number of new important results obtained by the scientific group, published in 1998-2015 in high-rating journals.

As a result of experimental studies of the magnetic and thermal properties of four different samples of Gd we confirm that purity plays an important role in obtaining the intrinsic properties. The large amount of impurities typically found in commercially designated 99.9 wt. % purity Gd, lowers the Curie temperature and also broadens the temperature range of the paramagnetic-ferromagnetic transition, while masking the spin reorientation transition which is obvious in much higher-quality single-crystalline samples. [1].

As a result of measuring magnetization, ac magnetic susceptibility, heat capacity, and the magnetocaloric effect of single crystals Dy, we confirmed the majority of previous findings but also report some notable differences in the behaviors of the magnetothermal properties, especially in the vicinities of Curie, Néel, and tricritical temperatures. When the magnetic field vector is parallel to the a axis of a crystal, the refined H-T phase diagram of Dy is more complicated than previously thought, and it contains several new phases. The appearance of some of these new phases has been explained by considering the Landau-Ginsburg theory of phase transitions. [2].

The phase diagram of a single crystal terbium in the basal plane has been refined. It was found that the critical field of the existence of a helical antiferromagnetic (HAFM) ordering is 155 Oe, which is less than 300 - 800 Oe, published earlier. It is established that there exists HAFM structure in terbium exists in the range of temperatures from 221 to 228 K. The magnetic phase of the fan type is observed in the same temperature range in the magnetic field range from 5 Oe to 155 kOe [3]

The anomalous behavior of spin polarization parameter P and the contact surface of the superconductor/ferromagnet Z depending on the contact resistance in the magnetically heterogeneous systems - the single crystal and the thin film holmium has been demonstrated. Using the experimental method of Andreev spectroscopy the existence of the cone-like type structure in holmium at temperatures below 20 K in weak magnetic fields has been established [4]

Detailed heat capacity and magnetization measurements of a high purity (99.92 at.%) Ho single crystal in magnetic fields from 0 to 100 kOe applied parallel to the b-axis have been carried out from 1.5 to 350 K. These measurements led to the refinement of the magnetic phase diagram of Ho. Together with the already known phases such as the helical AFM phase between 20 and 131K and the ferromagnetic cone type structure observed below 20K in low magnetic fields, we observed three regions of spinslip structures in the intervals 2035, 3542, and 95110 K. The boundaries of the spinslip structures have been clarified. The existence of spinslip III is broader than reported in previous studies. In addition to the known helifan and fan phases observed in Ho, we observed another phase of the ferro+fan type, which is located between 40120K and 2080 kOe. [5].

The results of this work have been published in the following papers:

  1. S.Yu. Dan'kov, A.M. Tishin, K.A. Gschneidner, Jr., V.K. Pecharsky, Magnetic phase transitions and the magnetothermal properties of gadolinium, Phys. Rev. B 54, 3478 (1998).
  2. A.S. Chernyshov, .O. Tsokol, A.M. Tishin, K.A. Gschneidner, Jr., V.K. Pecharsky, Magnetic and magnetocaloric properties and the magnetic phase diagram of single-crystal dysprosium, Phys. Rev. B 71, 184410 (2005).
  3. V.I. Zverev, A.M. Tishin, A.S. Chernyshov, Ya. Mudryk, K.A. Gschneidner, Jr., V.K. Pecharsky, Magnetic and magnetothermal properties of single crystalline terbium along the easy crystallographic axis, J. Phys.: Cond. Matter 26, 066001 (2014)].
  4. I. T. M. Usman, K. A. Yates, J. D. Moore, K. Morrison, V. K. Pecharsky, Gschneidner, T. Verhagen, J. Aarts, V. I. Zverev, J. W. A. Robinson, J. D. S. Witt, M. G. Blamire, L. F. Cohen, Evidence for spin mixing in holmium thin film and crystal samples, Phys. Rev. B 83, 144518 (2011).
  5. V.I. Zverev, A.M. Tishin, Zou Min, Ya. Mudryk, K.A. Gschneidner, Jr., V.K. Pecharsky, Magnetic and magnetothermal properties of single crystalline terbium along the easy magnetization direction, J. Phys.: Cond. Matter 27, 146002 (2015).