English subtitles for clip: File:Magnetic orders.webm
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1 00:00:00,050 --> 00:00:02,094 Magnetic orders 2 00:00:05,998 --> 00:00:07,941 In some materials, atoms carry spins 3 00:00:07,964 --> 00:00:10,980 which are small quantum magnets due to some of their electrons. 4 00:00:12,103 --> 00:00:17,752 In ferromagnets, the spins interact with their neighbors and align. 5 00:00:22,006 --> 00:00:25,268 At low temperature, all the spins are aligned over great distances. 6 00:00:25,301 --> 00:00:27,223 This is ferromagnetic order. 7 00:00:27,245 --> 00:00:29,795 The material then exhibits a magnetization. 8 00:00:29,856 --> 00:00:31,984 [Ferromagnetism] 9 00:00:32,037 --> 00:00:33,938 But there are other magnetic orders. 10 00:00:34,779 --> 00:00:40,552 For example, spins sometimes prefer to be anti-aligned with their neighbors. 11 00:00:44,401 --> 00:00:47,929 Here too, a long range order appears. It's antiferromagnet. 12 00:00:47,959 --> 00:00:50,793 Yet the total magnetization remains zero. 13 00:00:50,834 --> 00:00:52,890 [Antiferromagnetism] 14 00:00:52,943 --> 00:00:54,988 There are even more subtle orders. 15 00:00:55,015 --> 00:00:57,119 For example, two types of atom can alternate, 16 00:00:57,131 --> 00:00:59,270 one with a larger spin than the other. 17 00:01:01,513 --> 00:01:04,563 A ferrimagnetic order then appears. 18 00:01:04,609 --> 00:01:06,660 [Ferrimagnetism] 19 00:01:07,935 --> 00:01:11,852 Finally, in some cases, the situation can be completely random. 20 00:01:12,536 --> 00:01:15,555 Imagine a ferromagnet. 21 00:01:15,593 --> 00:01:18,988 Then, suppose that, randomly, some links are antiferromagnetic instead. 22 00:01:19,008 --> 00:01:21,828 Spins can no longer all align or anti-align. 23 00:01:21,858 --> 00:01:24,649 They are frozen but in a random configuration. 24 00:01:27,343 --> 00:01:30,951 It's a spin glass. 25 00:01:35,284 --> 00:01:41,269 When these different systems are heated, they remain rigid up to a certain temperature. 26 00:01:47,743 --> 00:01:51,176 At the critical temperature Tc, all the spins undergo a phase transition, 27 00:01:51,206 --> 00:01:52,654 like water boiling at 100 °C. 28 00:01:52,676 --> 00:01:56,941 The materials then suddenly lose their order and become paramagnetic again.