Giant heat dissipation at the low-temperature reversible-irreversible transition inGd5Ge4

Abstract

The heat exchanged at the low-temperature first-order magnetostructural transition is directly measured in Gd 5 Ge 4 . Results show that the origin and the temperature dependence of the heat exchanged varies with the reversible/irreversible character of the first-order transition. In the reversible regime, the heat exchanged by the sample is mostly due to the latent heat at the transition and decreases with decreasing temperature, while in the irreversible regime, the heat is irreversibly dissipated and increases strongly with decreasing temperature, reaching a value of 237 J / kg at 4 K. 12 In order to enhance MCE at low temperatures, materials with a second-order phase transition have been investigated, such as ErAl 2 ͑Ref. 13͒ and ErNi 2 , 14 but as far as we know, MCE associated with a first-order phase transition below 20 K has not been reported.Gd 5 ͑Si x Ge 1−x ͒ 4 alloys have been extensively studied since the discovery of giant MCE in x ഛ 0.5 compounds. 3 Ge-rich compounds ͑x ഛ 0.2͒ present a first-order structural transition ranging from ϳ20 K ͑x =0͒ to ϳ120 K ͑x = 0.2͒, accompanied by a change in the magnetic ordering between antiferromagnetic ͑AFM͒ and ferromagnetic ͑FM͒ state, 3-5 which gives rise to a large entropy change. In particular, the unusual magnetic behavior shown by the end-compound Gd 5 Ge 4 at low temperatures has lately attracted a lot of interest. [15][16][17][18][19][20][21][22][23][24] In this alloy, the nature of the AFM ordering related to the high-temperature phase is complex, with competing AFM ͑between layers͒ and FM interactions ͑within layers͒. 5,[15][16][17][18] Moreover, due to this anisotropy in the exchange interactions, the magnetic ordering remains AFM without undergoing the structural transition after zero-field-cooling ͑ZFC͒ down to ϳ2 K. 15,19 The application of a certain magnetic field, whose value depends on the temperature, induces the first-order AFM-to-FM transition, which is irreversible at temperatures below ϳ10 K, partially reversible from ϳ10 to ϳ20 K and fully reversible above ϳ20 K ͑Refs. 15, 17, 20, and 21͒ as in the rest of Ge-rich compounds. The irreversibility of the transition might be due to a hindrance of the kinetics of the reversed AFM-FM transition, which at sufficiently low temperatures and high fields becomes arrested ͑i.e., structural relaxation time is larger than experimental time scales͒. 21,24 In this work, a study of the heat absorbed or released at the low-temperature irreversible and reversible first-order field-induced phase transitions is carried out in Gd 5 Ge 4 . Results show that the origin and the temperature dependence of the exchanged heat varies with the reversible/irreversible character of the first-order transition. In detail, the heat exchanged by the sample in the reversible regime is majorly due to the latent heat at the magnetostructural transition, while the heat dissipation associated with the magnetic work is much smaller. In contrast, in the irreversible regime, giant heat dissipation much larger than the latent heat...

Correlating the local magnetic properties of the magnetic phase transition inGd5Ge4using scanning Hall probe imaging

“…3͒ than the previously reported ͑ϳ2-3 T͒ 1,3,17,18 critical magnetic fields taken at the onsets of the AFM→ FM transitions at the same temperature. Some of the discrepancies may also be related to intrinsic difficulties in determining the onsets, differences in purities of samples employed in various studies, as well as to the magnetocrystalline anisotropy of Gd 5 Ge 4 19 coupled with complex preferred orientations of as-solidified arcmelted buttons. Finally, the M-H loop hysteresis width is defined at the normalized moment midpoint of the curve and is labeled as the full width at half maximum ͑FWHM͒.…”

“…15 Studies of the magnetic state in Gd 5 Ge 4 have focused on measuring the bulk magnetization. [1][2][3][4][5][6][7][8][9]11,[16][17][18][19] Here we investigate the spatially local magnetic behavior using a Hall probe technique. We present isothermal Hall images taken at constant fields throughout the reversible AFM-FM transition 16 in Gd 5 Ge 4 at 35 K. The images show phase coexistence on length scales from 30 to 300 m. We extract magnetization-field ͑M-H͒ loops from 30ϫ 30 m 2 pixel areas-similar to the phase coexistence length-repeating this over the complete Hall image, and in this way we describe the distribution of local magnetic properties of the phase coexistence regime through the transitional state.…”

Griffiths-like phase and magnetic correlations at high fields in Gd5Ge4

“…4,10,11 The transition can be induced not only by magnetic field or temperature, but also by Si substitution 12 or pressure. 13 In addition, Gd 5 Ge 4 shows the presence of a Griffiths-like (GL) phase in the paramagnetic (PM) region, 14 short-range FM correlations in the AFM and PM regions at high fields, 9 or a spin-flop transition in the AFM phase. 15 The competing FM and AFM interactions in Gd 5 Ge 4 arise from its naturally layered crystal structure.…”

“…The intermetallic compound Gd 5 Ge 4 has recently received much attention [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] due to the unusual magnetic behavior originating from the competition of ferromagnetic (FM) and antiferromagnetic (AFM) interactions present in its layered crystal structure. 1 One of the most interesting features is the field-induced metamagnetic transition between AFM and FM states, which is coupled to a structural change 2 that gives the first-order nature of the transition.…”

Dissipative heat and exchange dispersion on metastability of Gd5Si4

“…The high electrical resistivity is reported to be due to a large entropy change deduced from a change in FM (T c ¼340 K for Gd 5 Si 4 ) to antiferromagnetic (AFM) state (T N ¼15 K of Gd 5 Ge 4 ) [18]. Recently two very interesting characters that are more convenient to consider are reported as exchange heat and surprising massive magnetic field on Gd 5 Ge 4 [19,20]. In relation to ''dissipative heat'' and ''exchange heat'' two main thoughts of experimental work are investigated to understand the effective parameter by which the displacement of atomic position by magnetic field is reported [20].…”